글리신 보충제 탐구 - 문치연 추천 보충제!!

[문형철]

글리신은

인체 아미노산의 11.5%를 차지하며,

단백질 질소의 20%

https://kr.iherb.com/pr/now-foods-glycine-pure-powder-1-lb-454-g/615

글리신은 

단순한 비필수 아미노산이 아니라, 

비타민·보조인자 의존 대사 네트워크의 허브로서 

항산화, 1‑탄소 대사, 해독, 신경전달, 면역·대사 조절에 관여합니다.​

https://pmc.ncbi.nlm.nih.gov/articles/PMC11510825/

글리신은 
단순한 아미노산으로, 근육 대사의 핵심 구성 요소이며 
입증된 세포 보호 효과와 치료 영양소로서의 가설적 이점을 지닙니다. 

세포, 시험관 내 및 동물 연구에 따르면 
글리신은 동화 작용 경로를 활성화하고 
단백질 분해 유전자 발현을 억제함으로써 
근육 소모에 대한 보호 효과를 강화합니다. 

일부 증거는 
글리신 보충이 최대 출력 향상에 기여하고 
고강도 운동 중 젖산 축적을 감소시키며 
수면의 질과 회복을 개선할 수 있음을 시사합니다. 

본 문헌 고찰은 글리신의 운동 능력 향상제(에르고제닉 에이드)로서의 잠재력과 근육 재생, 근력, 지구력 운동 수행 능력, 수면의 질과의 관련성을 비판적으로 탐구한다. 

또한 향후 연구의 핵심 분야를 강조한다. 결론적으로, 글리신이 운동 능력 향상제로서 근육 기능, 회복 및 전반적인 운동 능력을 지원하는 식이 보충제로서의 잠재력을 확인하고 운동 수행을 위한 영양 권장 사항을 확립하기 위해서는 인간 대상의 무작위 대조 임상 시험이 더 필요하다고 판단된다. 


또한 
고용량(체중 kg당 500mg 초과)은 세포독성 효과를 유발하고 
급성 글루타메이트 독성에 기여할 수 있다는 점을 고려하는 것이 필수적이다.

1. 1 탄소 대사와 B 비타민

글리신은 

세린·트레오닌 등으로부터 유래해 glycine cleavage system(GCS)을 통해 

테트라하이드로폴산(THF)에 메틸렌기를 공급하며, 

이는 엽산(B9)·비타민 B12·B6 의존 1‑탄소 대사의 핵심입니다.​

원카본 메타볼리즘은 
세포가 포도당, 지방 같은 걸 분해하면서 나오는 1탄소 단위—메틸렌, 포름일 같은—를 재활용하는 경로.

거기서 글리신은 핵심 플레이어.
글리신이 세린으로 바뀌면 1탄소 단위가 풀려나와서 DNA 메틸화나 뉴클레오티드 합성에 쓰임.
반대로 글리신 합성도 원카본 사이클에 의존.
그러니까 서로 떼려야 뗄 수 없는 관계

이 경로에서 메틸렌‑THF는 티미딘·푸린 합성(DNA/RNA), 메티오닌–SAM 합성을 통한 메틸화 반응(에피제네틱 조절), 항산화 글루타티온 합성에 사용되며, B9/B12/B6 결핍 시 글리신/세린 흐름 이상과 함께 DNA 합성 장애·저메틸화·산화스트레스 증가가 동반됩니다.​

https://www.frontiersin.org/journals/aging-neuroscience/articles/10.3389/fnagi.2023.1322419/full

https://www.frontiersin.org/journals/epigenetics-and-epigenomics/articles/10.3389/freae.2024.1451971/full

2. 항산화·세포보호 기능과 보조인자
글리신은 

글루타티온(GSH)의 세 구성 아미노산 중 하나로, 

cysteine·glutamate와 함께 

NADPH, 비타민 B2/B3 의존 경로와 연동해

 ROS 제거·적색‑환원 항상성 유지에 필수적입니다.​

최신 리뷰에 따르면 글리신 보충은 ERK1/2, Akt‑mTOR‑FOXO1 신호를 조절하고, 막 안정화 및 염소 이온 유입 억제를 통해 저산소·허혈‑재관류·ATP 고갈 상황에서 세포사·염증을 감소시키는 세포 보호 효과를 보이며, 이는 비타민 의존 항산화·에너지 대사와 기능적으로 연결됩니다.​

3. 해독·글리신 결합(glycine conjugation)과 CoA 의존성
간에서 글리신은 

glycine‑N‑acyltransferase(GLYAT) 반응을 통해

 벤조산, 살리실산, 중쇄지방산 등 xenobiotic·지방산을 acyl‑CoA 형태에서 acylglycine으로 전환해 배설시키는 

주요 해독 경로에 사용됩니다.​

이 경로는 ATP·CoA 의존적이며, 미토콘드리아 복합체 I 기능 저하 모델에서 Acsm1/2와 Glyat 발현 감소로 글리신 결합 능력이 떨어지면, 약물·지방산 해독 실패와 에너지 대사 장애가 악화되는 것이 보고되어, 글리신–CoA–비타민 의존 에너지 대사의 긴밀한 연계를 보여줍니다.​

https://pmc.ncbi.nlm.nih.gov/articles/PMC9932296/

4. 신경전달과 비타민 연계 대사
글리신은 

척수·뇌간에서 억제성 신경전달물질로 작용하며, 

동시에 NMDA 수용체의 공작용(co‑agonist)으로 시냅스 가소성에 관여합니다.​

글리신 수송체(GLYT1/2) 기능과 시냅스 내 글리신 농도는 에너지 대사·글루타티온 합성·1‑탄소 대사와 상호작용하며, 이러한 경로는 비타민 B군(특히 B2/B3에 의존하는 미토콘드리아 탈수소효소, B6 의존 아미노산 대사)과 기능적으로 연결됩니다.​

https://pmc.ncbi.nlm.nih.gov/articles/PMC5350494/

초록 및 배경: 글리신은 인간, 동물, 포유류에서 비필수 아미노산으로, 콜린, 세린, 하이드록시프롤린, 트레오닌으로부터 간과 신장의 상호작용을 통해 합성됩니다. 그러나 일반적인 식이 조건에서 충분히 합성되지 않아 성장, 건강, 웰빙에 필수적입니다. 글리신은 크레아틴, 글루타치온, 헴, 퓨린, 포르피린 등의 전구체로 작용하며, 암 예방, 대사 장애(심혈관 질환, 염증, 비만, 암, 당뇨) 치료, 수면 개선, 신경 기능 강화에 유익합니다. 리뷰는 글리신 대사의 최근 발견과 질환 상태에서의 보호 효과를 중점으로 합니다.

서론: 글리신은 1820년에 발견된 가장 단순한 아미노산으로, 단백질 산 가수분해물에서 분리되었습니다. 포유류(돼지, 설치류, 인간)에서 비필수적이지만, 내생 합성이 불충분해 면역 저하, 성장 저하, 영양 대사 이상을 유발할 수 있습니다. 조건부 필수 아미노산으로, 특히 신생아와 태아 성장에 중요하며, 새끼 새에서는 필수적입니다.

생리적 기능: 글리신은 인체 아미노산의 11.5%를 차지하며, 단백질 질소의 20%입니다. 콜라겐에서 매 세 번째 위치를 차지해 삼중 나선 구조를 형성합니다. 효소 활성 부위에 유연성을 제공하며, 중추신경계에서 신경전달물질로 작용해 식이 섭취, 행동, 항상성을 조절합니다. 면역 기능(세포 내 Ca²⁺ 수준 변화, superoxide 생산, 사이토카인 합성)을 규제하며, 담즙산 결합으로 지질/비타민 흡수를 돕습니다. RNA, DNA, 크레아틴, 세린, 헴의 전구체로 세포 보호, 면역 반응, 성장, 대사, 생존에 기여합니다.

글리신 합성: 트레오닌(트레오닌 데히드로게나아제 경로), 콜린(사르코신 경로, 쥐에서 40-70% 변환), 세린(SHMT 경로, 미토콘드리아/세포질), 하이드록시프롤린/글리옥실레이트로부터 합성. 조직/발달 단계/종에 따라 차이 있음 (예: 미토콘드리아 SHMT가 주요).

글리신 분해: 장내 세균에 의해 30% 분해. D-아미노산 산화효소(글리옥실레이트로), SHMT(세린으로), GCS(미토콘드리아, 주요 경로)로 분해됩니다. GCS 결함은 글리신 뇌증 유발.

이점: 간독성(알코올 손상 방지), 위장 장애(허혈 재관류 손상 보호), 장기 이식 실패(허혈-재관류 손상 감소), 출혈/내독소 쇼크(생존율 향상), 위궤양(산 분비 억제), 관절염(염증 감소), 암 치료(종양 성장 억제), 혈관 건강(혈소판 응집 감소).

• 토론: 글리신은 항산화, 항염증, 냉동 보호, 면역 조절제로 작용하며, 에피제네틱스에 영향. 대사 장애 치료에 유용하나, 고용량 독성 주의 필요. 사이토카인/염증/자유라디칼 관련 질환 메커니즘 연구 요구.
• 함의: 글리신 보충(식품: 콩류, 생선, 유제품, 육류)이 건강 증진. 조건부 필수성 강조, 정맥 투여로 쇼크 사망률 감소, 식이로 수면/신경/혈관 건강 개선.

5. 대사·염증 조절과 B 비타민 관련 시스템

종합 리뷰 및 2024–2025년 업데이트에 따르면, 

글리신은 

항산화·항염증·면역조절·인슐린 감수성 개선·미토콘드리아 기능 향상·수명 연장과 연관된 대사 효과를 보이며, 

특히 저등급 만성 염증·대사증후군·심혈관질환 등에서 보호적 역할이 제시됩니다.​

최근 다중 오믹스 연구에서는 암 악액질 모델에서 니아신·비타민 B6·글리신 관련 1‑탄소 효소가 일관되게 감소하고, 이에 상응하는 대사체 변화가 관찰되어, B 비타민 의존 효소와 글리신 대사가 전신 에너지·단백질 수식(예: 말로닐화) 조절의 축이라는 점이 부각됩니다.​

https://www.mdpi.com/1422-0067/24/14/11236

주어진 논문 요약: Glycine: The Smallest Anti-Inflammatory Micronutrient (MDPI, 2023)

이 논문은 2023년 7월 8일 International Journal of Molecular Sciences (Volume 24, Issue 14, Article 11236)에 게재된 리뷰 논문으로, 글리신의 항염증 효과를 중점으로 다룹니다. 저자는 Karla Aidee Aguayo-Cerón 등 9명이며, MDPI 출판사입니다. (저널 임팩트 팩터: 약 5.6, 중간 수준.)

초록 및 배경

글리신은 비필수 아미노산으로, 다양한 세포에 발현된 수용체(GlyR, NMDA)와 수송체(GlyT1/2)에 결합합니다. 항염증 효과가 다수 연구에서 확인되었으며, 프로염증 사이토카인 감소, 유리지방산 저하, 인슐린 반응 개선 등이 주요합니다. 메커니즘은 명확하지 않으나, 핵인자 kappa B (NF-κB) 조절이 핵심으로, 비필수적이지만 식이 보충이 만성염증 예방에 필요합니다.

주요 메커니즘

• NF-κB 억제: GlyR 활성화로 Cl- 유입 → 과분극 → Ca2+ 저하 → TNF-α 경로 억제 → 프로염증 사이토카인/케모카인 감소.
• 항염증 촉진: IL-10, 아디포넥틴 증가; NRF2 신호로 NF-κB 다운; GlyR 독립 경로로 세포 보호(괴사/파이롭토시스 방지).
• 세포 수준: 대식세포, 단핵구, T 림프구, 내피세포 등에서 프로염증 매개체 감소.

연구 결과 및 만성염증 치료 역할

• 동물 모델: 식이 0.5% 글리신 보충으로 감염/패혈증/출혈 쇼크에서 염증 저하, 생존율 향상; LPS 유발 염증 억제(Kupffer 세포, 폐포 세포).
• 인간 연구: 고글리신 혈중 수준이 제2형 당뇨(T2DM) 위험 감소; 5g/일 보충으로 비만 시 포도당 내성 개선.
• 질환별: 비만/T2DM(저등급 만성염증 치료, 인슐린 분비 촉진), 알코올성 간질환(간독성 저하), 뇌허혈(미세아교세포 M1→M2 전환), 암 악액질(산화/염증 환경 개선), 급성 췌장염/관절염/패혈증(손상/부종 감소).
• 보충제 함의: 만성염증(저등급) 치료제로 제안; 동물/인간 연구에서 항산화, 신경보호, 장 보호 효과.

결론

글리신은

면역조절제로 작용해 만성염증 관련 질환(비만, 당뇨, 신경퇴행 등)에서 잠재력 있으나,

장기 인간 연구 부족.

보충이 항상성 유지에 유용하나, 세포/질환별 메커니즘 더 연구 필요.

추가 최신 논문 요약 (2023-2025, 고영향력/고인용지수 중심)

최신(2023-2025), 고영향력(임팩트 팩터 5+ 저널), 고인용(출판 후 빠른 인용 추세) 논문을 선별했습니다. 웹 검색 결과에서 글리신 보충의 만성염증 치료 관련 리뷰/연구를 우선. 각 논문의 초점은 글리신 보충의 항염증 메커니즘, 임상 증거, 함의입니다.

1. Glycine and aging: Evidence and mechanisms (2023)

• 저자: Sofie Lautrup et al.
• 저널: Ageing Research Reviews (Elsevier, IF ≈ 13.1, 고영향력; 인용: 50+회, 고인용 추세).
• 주요 발견: 노화 관련 만성염증에서 글리신 보충이 메티오닌 제한 모방, 오토파지 활성화로 항염증 효과. 쥐 모델에서 8% 식이 글리신(인간 등가 10-15g/일)이 염증 마커(NF-κB, IL-6) 감소, 수명 연장(4-6%). 인간 코호트에서 글리신 수준 저하가 만성염증(비만, 심혈관)과 연관; 보충(3-5g/일)으로 인슐린 민감성 향상. 메커니즘: GCS 경로를 통한 one-carbon 대사 조절, 산화 스트레스 저하.
• 함의: 노화/만성염증 치료 보충제로 추천; 임상 시험 확장 필요. (염증-노화 연계 강조, 원 논문의 항염증 역할 확장.)

https://www.sciencedirect.com/science/article/pii/S1568163723000818

2. Metabolic impact of dietary glycine supplementation in individuals with type 2 diabetes: a randomized, double-blind, placebo-controlled trial (2025)

• 저자: (구체적 저자 미상, Nature Scientific Reports 기반).
• 저널: Scientific Reports (Nature Portfolio, IF ≈ 4.6, 중고영향력; 최신 출판으로 인용 증가 예상).
• 주요 발견: T2DM 환자(80명)에서 12주 글리신 보충(5g/일)으로 만성염증 마커(Hs-CRP, IL-6) 20-30% 감소, 혈당 조절 개선. 플라시보 대비 글리신 그룹에서 isobutyrylglycine 배설 증가(대사 활성화 지표). 메커니즘: 간 해독 경로 강화, 프로염증 사이토카인 억제. 부작용 없음.
• 함의: T2DM 만성염증 치료 보조제로 안전/효과적; 더 큰 코호트 연구 제안. (임상 증거 중심, 원 논문의 T2DM 역할 업데이트.)

https://www.nature.com/articles/s41598-025-20511-x

3. An Update of the Promise of Glycine Supplementation for Enhancing Skeletal Muscle Health Across the Lifespan (2024)

• 저자: Daniel McCarthy, Mark C. Turner.
• 저널: Nutrients (MDPI, IF ≈ 5.9, 중고영향력; 인용: 20+회).
• 주요 발견: 근육 관련 만성염증(근감소증)에서 글리신 보충(5-15g/일)이 항염증 효과로 근력/재생 촉진. 고강도 운동 후 젖산 축적 감소, 염증 마커 저하. 노인 코호트에서 3개월 보충으로 근육 기능 개선. 메커니즘: 크레아틴/글루타치온 합성 증가, NF-κB 억제.
• 함의: 평생 근육 건강(염증 관련) 보충제로 잠재력; RCT 추가 필요. (근육-염증 초점, 원 논문의 면역조절 확장.)

4. The potential impact of glycine supplementation on the process of inflammaging (2025)

• 저자: (구체적 저자 미상, Quality in Sport 기반).
• 저널: Quality in Sport (UMK 출판, IF 낮음 but 최신; 인용 초기 단계).
• 주요 발견: inflammaging(노화 관련 만성염증)에서 글리신 보충(3-10g/일)이 산화 스트레스 저하, 인슐린 민감성 향상, 사이토카인 균형 조절. 연구 검토: 동물/인간에서 IL-6/TNF-α 감소. 메커니즘: GlyR 활성화, NRF2 경로.
• 함의: inflammaging 치료 보충제로 유망; 영양 개입 연구 강조. (노화-염증 특화, 원 논문 결론 보강.)

이 논문들은 글리신 보충의 안전성과 항염증 잠재력을 강조하나, 장기 효과/용량 최적화 연구가 더 필요합니다. 고임팩트 저널(Nature, Elsevier) 중심으로 선별했으며, 인용지수는 출판 시기상 초기지만 빠른 증가 추세입니다. 추가 질문 있으시면 말씀해주세요.

1개의 웹페이지 11개의 웹페이지

6. 핵심 메시지(비타민·보조인자 관점)
글리신은 

엽산‑메티오닌 사이클(1‑탄소 대사)에서 B9, B12, B6, B2에 의존하는 반응과 결합해 

DNA 합성·메틸화·항산화·에너지 대사를 통합 조절합니다.​

CoA, NAD(P)H, B 비타민 의존 효소들과 함께 해독(glycine conjugation), 글루타티온 합성, 미토콘드리아 기능을 매개하며, 이 축의 붕괴는 대사질환·암·악액질·만성염증과 연관된다는 점이 최근 고영향력·고인용 리뷰와 오리지널 논문들에서 반복적으로 제시되고 있습니다.

https://pmc.ncbi.nlm.nih.gov/articles/PMC3806315/

https://www.nature.com/articles/s41467-023-41952-w

Review

The hidden power of glycine: A small amino acid with huge potential for piezoelectric and piezo-triboelectric nanogenerators

Author links open overlay panelLuís Nascimento a b, Gavin Richardson c, Priscila Melo d, Nathalie Barroca a b

Show more

Add to Mendeley

Share

Cite

https://doi.org/10.1016/j.cej.2025.161514Get rights and content

Under a Creative Commons license

Open access

Highlights

• •
• •
• •
• •

Abstract

Glycine is the smallest nonessential amino acid in humans and animals, a basic building block with a crucial role on several physiological events. Despite its simplicity, glycine presents three polymorphs – α, β, and γ- with distinct functional properties. While α-glycine is widely used in pharmaceuticals, β- and γ-glycine have found a place in energy harvesting due to their non-centrosymmetric structure and therefore notable piezoelectricity. β-glycine polymorph shows remarkable out-of-plane piezoelectricity (d16 = 178 pm.V−1), comparable to traditional piezoceramics like barium titanate or lead zirconate, whereas γ-glycine demonstrates superior in-plane piezoelectricity (d33 = 10.4 pm.V−1) comparable to biological materials (0.1–10 pm.V−1). Advances in polymorph synthesis and stabilization enabled the fabrication of glycine-based piezoelectric (PENGs) and piezo-triboelectric nanogenerators (PTENGs) with favourable features such as enhanced flexibility, integrability, and electromechanical coupling. In this review, we delineate glycine polymorphism, relative stability, and crystallization methods, with a special focus on strategies to stabilize specific polymorphs in view of enhancing its piezoelectric activity. Furthermore, the interaction of glycine with hydrophilic polymers is explored to develop biodegradable nanogenerators. The development of glycine-based PENGs and PTENGs is presented, with detailed examination of the piezoelectric and triboelectric mechanisms illustrating the beneficial effects of incorporating glycine. These approaches bring the performance of organic-based PTENGs closer to ceramic-based nanogenerators. Emergent applications encompassing sensors for physiological processes and electromechanical energy conversion devices such as ultrasound-induced drug delivery and electrotherapy are reviewed along their performance in vitro and in vivo. Finally, we discuss the advantages, limitations, and future progresses for these devices. Overall, this review provides a comprehensive overview of glycine research, offering a solid foundation to further its utilization in the field of green energy.

Graphical abstract

1. Download: Download high-res image (138KB)
2. Download: Download full-size image

• Previous article
• Next article

Keywords

Organic

Piezoelectric

Triboelectric

Glycine

Amino acid

Energy-harvesting

1. Introduction

The pursuit of innovative materials capable of mimicking the diverse characteristics of living organisms represents a significant frontier in societal progress. In the quest for materials suitable for crafting implantable devices with sensing, delivery or actuation capabilities, properties such as biocompatibility, biodegradability and biosafety hold a paramount importance [[1], [2], [3]].

Traditionally regarded as a fundamental component in cell and molecular biology, certain molecules have transitioned into pivotal roles within the biomedical engineering applications. Amino acids, the building block of peptides and proteins, have long been acknowledged as essential for the structural integrity and optimal functionality of cells, organs, tissues, organisms, and organ systems. Moreover, these play a significant role in facilitating the transportation of biomolecules and catalysing bio-transformations crucial for sustaining cellular processes [4]. However, amino acid research has surpassed its traditional focus on cellular processes, emerging as a notably promising field within material science. This is primarily attributed to the ability of certain amino acids to form intricate three-dimensional (3D) crystalline structures, no less complex than polymers [3].

In 1970, Vasilescu reported that 17 out of the 20 naturally occurring amino acids (excluding glutamine, phenylalanine, and tryptophan) exhibit the capacity to convert mechanical energy into electrical energy, presenting piezoelectric properties [5]. Thus, the presence of piezoelectricity in aminoacids easily explains why many organs like bone, tendons, skin and hair also shows piezoelectric moieties. From a structural perspective, ceramic and aminoacid piezoelectricity is only generated in crystals without inversion symmetry, known as noncentrosymmetric crystalline structures [3,6]. When applying a mechanical energy to a piezoelectric material, its crystalline structure deforms, originating uncompensated dipoles and consequently an electrical moment (i.e., direct piezoelectric effect). The polarization charge density induced by the electrical moment is proportional to the mechanical stress applied. The contrary can also be found, characterized by an electrical moment applied to a piezoelectric material that induces a mechanical deformation of the crystalline arrangement (i.e., inverse piezoelectric effect). [[7], [8], [9]].

Glycine was the first amino acid for which polymorphism and piezoelectricity was reported, firstly at ambient conditions and later at higher pressures [7]. Presenting a chemical structure characterized by a central carbon bound to an amino group (–NH2), carboxyl group (–COOH), and two hydrogens, its simplicity is countered by its complex role in body functioning and its applicability for the treatment of various diseases, including ischemic stroke, anxiety, insomnia, schizophrenia, senescence, etc [[10], [11], [12]]. Due to its simplicity, glycine is frequently used as a model of amino acid piezoelectric behaviour revealing high piezoelectric outputs (10–178 pm.V−1, depending on polarization direction). Functional physical properties largely depend on glycine supramolecular packing into three main polymorphs (α-GC, β-GC and γ-GC). β- and γ-GC phases of glycine present a non-centrosymmetric crystalline structure (space group P21 and P31, respectively), whereas α-GC shows a centrosymmetric structure (space group space group P21/n), being non-piezoelectric [12,13]. Interestingly, β- and γ-GC phases have shown similar piezoelectric coefficients to barium titanate (BTO) and potassium sodium niobate (KNN), suggesting its use for sensor, actuator and even as a mechano-electrical additive [14]. It was observed that a glycine sensor yielded a piezoelectric voltage constant, of 60 mV m/N (g33), which surpasses commercial piezoelectric lead zirconium titanate (PZT) [15]. Nonetheless, the engineering of devices that use glycine still faces major difficulties. The control of glycine crystallization into a desirable electroactive polymorph and its combination with other materials is still difficult due to γ- and β-GC readily converting to non-piezoelectric α-GC. Nonetheless, the emerging glycine-based devices present remarkable piezoelectric and triboelectric mechano-electrical conversion, and thus, scientific investment is needed towards the stabilization of glycine electroactive phases making this aminoacid an important player in the development of energy conversion systems.

A typical triboelectric nanogenerator (TENG) device is composed by four components, namely, a charge generating, charge trapping, charge collecting, and charge storage layers. In a triboelectric nanogenerator, charge generation is mainly limited to charge density of the contacting layers which is dependent on various factors like the area and contact between the chosen pair of materials [16]. In a perfect triboelectric nanogenerator, the generated charges can be trapped by physical and chemical defects, preventing their recombination with opposite charges and consequential loss. Charges flow to charge collecting layers and are converted from AC to DC kept in storage layers. As such, the two main merits to increase TENG performance is the increase of both charge density and charge trapping [16,17]. However, charges can drift and combine with induced opposite charges interfering with charge accumulation and leading to a sharp decrease in triboelectric potential [16,18]. Opposite charges can be present in the triboelectric nanogenerator, but triboelectric charges are also capable of attracting opposite charges from the atmosphere, resulting in charge loss and decreased induced charge density, in a process called air-breakdown discharging [16,19].

Whilst mechanisms of triboelectrification are still under investigation and debate, little is known about glycine. Nevertheless, it has been reported that, in glycine, an interplay between chemical and structural effects can help explain the reported increases in triboelectric potential. The presence of –NH2 groups on glycine backbone leads to a tendency to lose electrons, thus becoming a positive triboelectric material [20]. However, it is worth mentioning that glycine also presents a carboxyl group, belonging to the electronegative group which can hinder triboelectric generation [20]. Moreover, glycine crystals exhibit a high surface-to-volume ratio, enabling a micro-capacitor effect that stores or traps electrons after contact electrification, thereby reducing air breakdown effects. This decreases static electron density on the negative triboelectric surface and increases the gap between positive and negative layers resulting in a larger electrical output [19,21].

Besides its potential as a triboelectric layer, glycine can contribute to energy harvesting devices exploiting also its piezoelectric character, giving origin to piezoelectric-triboelectric nanogenerators (PTENGs). Briefly, in a piezoelectric-triboelectric hybrid the piezoelectric effect takes place instantly due to initial deformation. The piezoelectricity-induced dipoles generate charges which can support triboelectric charges at the friction material interface. In early development, the integration of piezoelectric materials like BaTiO3 [22], ZnO nanorods [23] or PZT [24] shown enhanced triboelectric output efficiency. However, they present major concern related to toxicity effects. The utilization of glycine represents an advance in the development of biocompatible and biodegradable nanogenerators. For instance, optimizing glycine concentration has shown to enhance triboelectric performance. At optimal content and appropriate distribution within the device, a piezoelectric potential gradient is generated and can facilitate triboelectric charge transfer through external load. [21].

Although current reviews on the future of piezoelectric actuation highlight glycine as a promising biomaterial, most significant developments in glycine-based devices have emerged only in the past five years. This work aims to address the progress and current limitations of glycine-based piezo- and triboelectric nanogenerators. We first present a detailed overview of glycine's physicochemical characteristics, from polymorph stability to innovative strategies for obtaining mechano-electrically active polymorphs. Furthermore, we compile the research that focus on exploiting glycine as a building block for tissue engineering scaffolds, integrate glycine into piezoelectric and triboelectric actuation, with emphasis on its applications in piezoelectric and triboelectric nanogenerators.

2. Glycine: The simplest aminoacid in nature2.1. Overview on glycine crystal polymorphism

Generally, amino acids are not prone to exhibit polymorphic behaviour at ambient conditions [25]. Glycine contradicts this tendency exhibiting various crystalline structures, depending on temperature and pressure of crystallization. Zwitterionic glycine provides the structure-forming motif from which crystal reorganization and packing happens, characterized by a head-to-tail hydrogen bonded chain. The distance characterizing the hydrogen bridge differs among glycine polymorphs, although having similar topology. The structural arrangement of glycine crystals resembles peptide chains, except that it is the NH-O hydrogen bond that holds the amino acids together, rather than a true peptide bond. Glycine crystals show different shapes and optical activity which massively hinder the standardization of a true glycine structure [6,25]. Nonetheless there have been three well studied polymorphic structures in glycine, namely alpha (α-GC), beta (β-GC), and gamma (γ-GC) glycine [13,26,27]. The structure of γ-GC can be compared with α-chains of peptides, while α-GC and β-GC resemble the β-sheet conformation of proteins. Additionally, glycine can also exist in its dihydrate form, where glycine chains are not present, being kept instead by bridges formed of intermolecular forces [28,29]. Interestingly, although glycine crystallization has been known for the last 50 years, literature still reports variations in β (β’), δ and ζ phases. Since there is not enough knowledge on these phases to identify possible applications, they will not be explored in this review.

Glycine has been investigated to obtain detailed information on different aspects with emphasis on the geometry of hydrogen bonds, which control polymorph struct, atomic displacement, and other features of the structure [[30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43]]. Additionally, Boldyreva et al. reported that glycine crystal structure was extensively analysed − 100 entries for glycine polymorphs in the last 3 years (Cambridge structural database), a testimony on how intricate and complex glycine manufacturing and isolation is [25]. From these, 32 belong to α-GC, 11 to β-polymorph and 28 to γ-GC, with the remaining belonging to high pressure stabilized polymorphs [44].

2.2. Piezoelectricity in glycine

Among the competitive advances in piezoelectric and triboelectric nanogenerators, developing novel functional materials plays a crucial role to push forward these technologies. γ-GC and β-GC have been recognized as promising alternatives due to their low permittivity and their superb piezoelectric behaviour, which rivals with conventional piezopolymers and piezoceramics (Table 1) [3,6]. When comparing with top-performing perovskites, while glycine has smaller piezoelectric constants, it is nevertheless a tantalizing material owing to i) its non-toxic nature, glycine as a building block of proteins, is by nature biocompatible and environmentally friendly [45]; ii) effective piezoelectric response at low frequencies [46] which is particularly interesting for applications in wearable or implantable devices where low-frequency biomechanical events are exploited; iii) cost-effective and environmentally friendly synthesis and processing [47]: whereas ceramics requires complex fabrication techniques and high temperatures and polymers may require organic solvents, glycine can readily be synthesized under milder conditions [3]. Although some investigation has been done in the last decade on the piezoelectric behaviour of β-GC and γ-GC phases, if one considers that the crystal symmetry of these two phases allows eight and thirteen non-zero piezoelectric coefficients, respectively, there may exist crystallographic directions along which piezoelectricity can be higher than previously reported in literature [48]. Thus, the understanding of piezoelectric properties and crystalline structure for the three main polymorphs of glycine (α-GC, β-GC and γ-GC) is fundamental to understand its promise as an electro-mechanical material.

Table 1. Piezoelectric constants of distinct materials.

Piezoelectric materialsPiezoelectric ConstantReferencesNormal PiezoelectricShear Piezoelectric

Ceramics
PZT-5H	d33 = 593 pC/N; d31 = −274 pC/N	d15 = 741 pC/N	[60]
ZnO	d33 = 6––13 pC/N; d31 = −5 pC/N	d15 = −8.3 pC/N	[58], [61]
Barium Titanate	d31 = −34.1 pC/N; d33 = 85.6 pC/N	d15 = 395 pmV−1	[62]
KNN-based material	d33 = 200 pC/N	----------------	[63]
Synthetic Polymers
PVDF	d33 = −3 pC/N; d31 = 23 pC/N	----------------	[64]
PVDF-TrFE	d33 = −25 to −40 pC/N; d31 = 12–25 pC/N	----------------	[65], [66]
PVDF-HPF	d33 = −24 pC/N; d31 = 30 pC/N	----------------	[67], [68]
PVDF-CTFE	d33 = −140 pC/N	----------------	[69]
PLLA	----------------	d14 = 6––12 pC/N	[53]
Proteins
Collagen	----------------	d14 = 0.2 – 2.0 pC/N	[70]
Natural polymers
Silk	d33 = 0.36 pm/V	d14 = −1.5 pC/N	[71], [72]
Peptides
Diphenylalanine nanotubes	d33 = ∼17.9 pm/V	d15 = 45 – 60 pm/V	[51]
Virus
M13 bacteriophage	d33 = −7.8 – 26.4 pm/V	d14 = 3.9 pm V−1	[73], [74], [75]
Amino-acids
γ-GC [computed]	d33 = 10.4 pm V−1; d11 = 1.7 pm V−1; d22 = 1.1 pm V−1	----------------	[48]
β-GC [computed]	d22 = −5.7 pm V−1	d16 = 178 pm V−1	[48]

PZT-5H – Lead Zirconate; KNN – Potassium Sodium Niobate; PVDF-TrFE – poly(vinylidene fluoride-trifluoroethylene; PVDF-HFP – Poly(vinylidene fluoride-co-hexafluoropropylene); PVDF-CTFE – polyvinylidene fluoride-co-trifluoroethylene.

2.3. The three “faces” of glycine

α-Glycine: α-GC crystallizes in a centrosymmetric non-polar space group #14 (P21/n) and is formed by a hydrogen-bonded double layer of glycine zwitterions to give an ABCD packing (Fig. 1A and D). Thus, the molecular dipoles in α-GC sum to zero and do not produce a net polarization [48].

1. Download: Download high-res image (518KB)
2. Download: Download full-size image

Fig. 1. Schematic representation of glycine unit cells of α- (A.), β- (B.), and γ- (C.) GC. Arrows indicate the x-, y-, and z- directions in which e-field was applied. Reproduced with permission [59]; Morphology of glycine microcrystals of different polymorphs: (D.) α- GC, (E.) β- GC, (F.) γ- GC. PFM observation of different types of as-grown ferroelectric domains in β- GC: (G.) stripe domains with charged domain walls, (H) quasi-periodic ensembles of needle-like domains, and (I.) irregular shaped domains with segmental step-like domain walls. White arrows show the direction of spontaneous polarization. Reproduced with permission [12].

β-Glycine: β-GC has a polar structure described by the space group #4 (P21; monoclinic; Fig. 1B). β-GC molecular dipoles sum to a spontaneous polarization along 2-axis contributing to one longitudinal d22 (−5.7 pm V−1), two transverse, and five shear non-zero coefficients, yielding an experimentally confirmed superb shear piezoelectric behaviour (d16 = 178 pm V−1) [48]. For reference (Table 1), this value is about two orders of magnitude higher than the maximum piezoelectricity reported for collagen [49], almost 20 times higher than the experimentally measured magnitude of γ-GC [50], three times higher than diphenylalanine nanotubes [51], six times higher than any form of PVDF polymers [52], about half of classical perovskite ceramic BaTiO3 and roughly 15 times higher than shear piezoelectricity of poly (L-lactic acid) (PLLA) [53]. This unusually high d16 has been attributed to the interplay between molecular packing and stiffness which lowers the resistance to deformation. In this polymorph, high polarization over a relatively small area − in which molecules are loosely packed around a monoclinic angle − lowers the shear stiffness [48].

Additionally, recent reports confirm‎ that β-GC is also ferroelectric (Fig. 1H) with switchable polarization and diverse domain structures [[54], [55], [56], [57]]. Advanced techniques such as piezoresponse force microscopy (PFM) and Raman spectroscopy have shed light on the domain structure of β-GC on nonpolar surface crystals. Domain structures on β-GC can exist as stripe-like with flat charged domain walls (Fig. 1E and G) attributed to growth-related defects, quasi-periodic ensembles of needle-like domains induced by pyroelectric effects during cooling (Fig. 1H), and irregularly shaped domains with segmental stripe-like domain walls (Fig. 1I) [55,56]. The magnificent piezo and ferroelectric properties of β-GC allied to its stabilization on Pt substrates, and nonlinear susceptibility higher than γ-GC, make β-glycine a promising functional material for sensors, energy harvesting, and nonlinear optical devices.

γ-Glycine: γ-GC spatial orientation is very different from all other polymorphs presenting a head-to-tail hydrogen bonded chains linked by additional hydrogens into triple helices, forming a 3D hydrogen-bonded chiral crystal structure with trigonal P31 (or P32; Fig. 1C) space symmetry. This spatial distribution allows three longitudinal (d11, d22, and d33), four transverse and six shear non-zero coefficients. Based on predicted values, γ-GC d33 was 10.4 pm V−1 (along the [001] direction; Table 1), which is 2-fold higher than longitudinal piezoelectric coefficient in β-GC, and comparable to the value of zinc oxide (5–13 pC m−1) [58]. The computed values for γ-GC have been confirmed experimentally through piezometer and PFM measurements [48].

Regarding its ferroelectricity, although polarization switching is feasible, it requires electric fields approximately four to eight times higher than those employed for β-GC [50]. Noteworthy, near the apex of a PFM tip, an applied voltage of 100 V only generates a maximum electric field of 3 V/nm. Exceeding these voltages result in electric breakdown and limits practical single molecule switching in γ-GC. Distinguishing ferroelectric characteristics between γ-GC and β-GC extends beyond mere differences in polarization reversal mechanisms. In β-GC, spontaneous polarization arises from alignment of –NH3+ groups and carboxyl groups, whereas in γ-GC reversing polarization necessitates a 180° rotation perpendicularly to the molecule axis making the process much more difficult [12]. As such, single molecule switching in γ-GC remains a theoretical possibility.

2.4. Polymorph growth and synthesis for enhancing piezoelectric response

Harnessing specific properties in glycine is highly dependent on molecular packing, thereby exponentiating the importance of controlling specific crystal growth and its stabilization. In fact, small variations within a specific process can lead to completely different polymorphs. One of the current challenges in glycine-based devices – that may hinder further developments − is the obtention and stabilization of singular electro-active polymorphs and the engineering of such devices considering that manufacturing may destabilize the piezoelectric polymorph. As such, the different methods to synthesize each polymorph is discussed, with particular emphasis on their relationship with glycine piezoelectric properties:

α-Glycine: α-GC is the most representative polymorph in living tissues, with implications in cellular growth and health, as well as prevention of many pathologies including cancer [45]. Obtaining metastable α-glycine is the easiest compared with the other two polymorphs. α-GC can be easily obtained from pure aqueous solutions at pH between 3.8 and 8.9 [[76], [77], [78]]. Standard pharmaceutical protocols demonstrate that, if an aqueous solution containing α, β and γ-GC is stored for long time, α-GC dominates, thereby pointing to its higher thermodynamical stability. Furthermore, ionic liquid-mediated crystallization processes and the combination of anti-solvents and ultrasounds have been used to obtain almost pure α-GC [45,79,80].

γ-Glycine: Early investigations show that α-GC nucleates much faster than the stable monomer-based γ-GC from pure (additive-free) aqueous glycine solutions under usual conditions despite their thermodynamic stability [81]. Some contradictions have been found with reports stating γ-GC crystallization under conditions previously reported to have induce α-GC crystallization. Boldyreva et al. [25] found that glycine crystallization presents a phenomenon called “solution memory”, in which the polymorph that crystallizes from a solution is the one originally dissolved, unless the solution is stored for a long time (a couple of weeks at least), or frozen and then unfrozen, to “erase the memory”. This phenomenon coupled with batch-to-batch differences in commercial glycine can explain in part the discrepancy in reported outcomes of crystallization from solutions prepared in − otherwise – identical conditions [25,82]. Nonetheless, single γ-GC are currently grown by methods such as slow cooling, slow evaporation and gel methods in the presence of various salts, all of them bearing a higher level of complexity compared to the ones used for the α-GC polymorph [83].

Under ambient conditions and during spray drying, γ-GC was crystallized from initial acidic (pH < 3.8) as well as basic (pH > 8.9) conditions. Additionally, γ-GC has also been obtained from salt-containing aqueous solutions. Specifically, monovalent cation salts, such as potassium nitrate (KNO3), sodium chloride (NaCl), and ammonium sulphate ((NH4)2SO4), have been found to inhibit α – GC primary nucleation while simultaneously promoting γ – GC primary nucleation and growth [84,85]. Divalent cation ions, such as magnesium sulphate (MgSO4) and calcium nitrate (Ca(NO3)2) both encourage γ – GC nucleation and inhibit α-GC primary nucleation but not sufficiently to induce γ–GC growth. To explain how salts impact glycine nucleation, theories pertaining to pH changes were proposed. Although it is well known that a large change in solution pH benefits γ–GC nucleation, the fact that NaCl and KNO3 do not alter solution pH makes this an unlikely explanation for why these monovalent cation salts both encourage γ–GC primary nucleation and inhibit α–GC primary nucleation [85]. It has been pointed out that the dissolution of salts into ions can form ion-glycine interactions, expected to be stronger than the dipole–dipole interactions (e.g., water-glycine or glycine-glycine interactions). These complexes induce glycine head-to-tail chains structurally similar to γ–GC packing, which can serve as nucleation precursors of γ–GC. This mechanism gains strength considering that a similar theory has been postulated to interpret an observation that inorganic salts generally promote the nucleation of DL-alanine crystal which is structurally akin to γ–GC. Moreover, the inhibiting effect of most these salts on α - GC was attributed to the fact that salts tend to destroy cyclic dimers of glycine, responsible for α–GC [86].

Rather than attempting the isolation of γ-GC as a single crystal followed by methodologies aimed at decreasing its size, inducing the specific crystallization of the target mechano-electric active phase has been achieved by the nucleation of glycine within hydrophilic matrices. These methodologies mainly contemplate the development of 2D films by solvent casting with different polymers and nucleation surfaces explored to control nucleation planes of glycine. These strategies have found considerable success in achieving piezoelectric coefficients close to the theorized d33 (d33 = 10.4 pC/N−1 along the [001] plane) for γ-GC which, due to the random crystalline orientation provided by traditional crystallization techniques, were still far from being accomplished. This presents a unique opportunity for the formulation of 2D and 3D devices that can be used as piezoelectric nanogenerators (PENGs), triboelectric nanogenerators (TENGs) or sensors. γ-GC crystallization has been investigated in a wide array of hydrophilic polymers, such as polyvinyl alcohol (PVA), sodium hyaluronic acid (sodium HA), pullunan, polyacrylamide, gelatin and poly (ethylene oxide) (PEO), wherein crystallization is governed by hydrogen interactions between the hydrophilic groups of glycine and these polymers [21,87,88].

Glycine crystallization studies in PVA have provided insights into the interaction between γ-GC and PVA during solvent casting that have been extended to other hydrophilic polymers [21,88]. Initial density function theory (DFT) calculations show that when glycine molecule has its two O atoms bound with the hydroxyl groups (–OH) on PVA chains, the overall system energy reaches a minimum (Fig. 2A2−A3), becoming more stable. Because PVA has its OH groups available, it can guide glycine packing and direct its nucleation and growth. Without PVA to balance the dipoles in glycine, the dipole direction would exhibit an alternating distribution to minimize the internal electrostatic energy, and α-GC would dominate [87]. During solvent cast, these molecular interactions govern film formation through the sequential precipitation of both PVA and glycine. Briefly, in solvent cast, PVA precipitates first and accumulates at the interfaces owing to its amphiphilic nature. As the concentration continuously increases, salting out of PVA (Fig. 2A1) is activated at a certain point because of competition for water hydration between the polymer and electrolyte molecules (glycine). Most of PVA precipitates out at both water–air and water–solid interfaces, leaving a glycine-rich solution in between [87]. Similar mechanisms have been found in a systematic comparison between PEO, PVA, gelatin, sodium HA, pullulan and polyacrylamide (PAM), revealing preferences in glycine crystallization planes depending on the polymer. While all produced films revealed a sandwiched glycine layer, only PEO yielded clear γ-GC crystallization, with characteristic crystal planes ([101], [110], and [102] peaks) demonstrating that the alternating ester bonds and short alkane chains have a very good induction on the formation of γ-GC. Moreover, PVA and gelatin-induced γ-GC crystallization exhibited [110] but no [102], an indication that these polymers are not beneficial for facets to form parallel to the film length [88]. The rest of the tested polymers (PAM, pullulan and sodium HA) showed fewer XRD peaks for γ-GC, providing key observations such as: (1) the amine side groups of PAM do not help the formation of γ-GC, since the difference between PAM and PVA is mainly the side groups, and (2) the main chain of the polysaccharide does not offer advantages for the formation of γ-GC since polysaccharides and PVA both present abundant hydroxyl groups [88]. Nonetheless, efficient crystallization of γ-GC has been reported when coupled with another polysaccharide, i.e. chitosan. In this case, the “solution memory” effect was exploited, and pre-crystallized γ-GC was introduced and then recrystallized via slow evaporation. This, combined with the functional groups in chitosan, significantly enhanced the crystallization of γ-GC [21].

1. Download: Download high-res image (1MB)
2. Download: Download full-size image

Fig. 2. Strategies for glycine crystallization. (A and B) Synthesis approach of different piezoelectric glycine-PVA films. (A1) Schematic crystallization process of glycine-PVA sandwich thin films. The inset shows the orientation alignment of glycine molecules at the PVA surface during nucleation, leading to long-range crystal alignment. (A2) Schematic of three possible ways for glycine molecules to bind with PVA chains. (A3) DFT-calculated binding energies for the three binding situations shown in A2. From [89]. Reprinted with permission from AAAS; (B1) Schematic illustration of nucleation site at the center flat of the film, inset depicts as-grown film showing nucleation spot and wafer scale size. (B2) Schematic glycine crystal structure showing the direction relationship of [001] and [102] crystallization planes. From [87]. Reprinted with permission from Royal Society of Chemistry; (C1) γ-GC-PEO crystallization on different substrates, their respective contact angles on glass, PS and PTFE interfaces; (C2) Schematic illustration of different interface-induced orientations of the bottom PEO layer and γ-GC; (C3) Schematic illustration of the evaporation and drying process of glycine-PEO aqueous solutions on PTFE interfaced. From [88]. Reprinted with permission from Elsevier. (D1) Fabrication of piezoelectric β-GC nanocrystalline film and active self-assembly via synergistic nanoconfinement and in-situ poling. (D2) Photographs of glycine − silicon wafer (left) and a film on a flexible gold-coated polyethylene terephthalate (PET) substrate (right) illustrating 2D film flexibility. (D3) Cross-section SEM image of as-obtained β-GC film. Adapted from [109]. Reprinted with permission from Springer Nature under Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/); Synthesis and growth mechanism of piezoelectric Glycine-Nb2C-PANI, (E1) Schematic illustration of the dipping and pulling progress of Gly-Nb2C-PANI. (E2) Growth mechanism of glycine molecules on Nb2CTx nanosheets. (E3)Schematic illustration and SEM images of the surface changes on a single PAN nanofiber after co-crystallization process. From [101]. Reprinted with permission from John Wiley and Sons.

The crystallization of γ-GC within polymers is insufficient to create constructs that fully realize the piezoelectric potential of this polymorph. Glycine-polymer interactions alone cannot effectively modulate crystalline orientation, which is crucial for optimal piezoelectric performance. When glycine is oriented in the [001] planes parallel to the film orientation, it exhibits the highest out-of-plane (OOP) piezoelectric coefficient (d33). As such, methodologies that influence glycine nucleation planes and crystal orientation within the polymer can have a significant impact on the piezoelectric behaviour of the resulting composites. For example, the hydrophobic-hydrophilic properties of different film-forming interfaces (glass, polytetrafluoroethylene (PTFE) and polystyrene (PS)) have shown to impact the crystallization and piezoelectric properties of γ-GC-PEO mixtures. The increased contacting angle (Fig. 2C1) from glass, PS and PTFE modulate film formation and crystallization with glycine-PEO composite films prepared on more hydrophobic PTFE interface displaying the largest OOP coefficient (≈ 8.2 pC/N) followed by glass (≈ 5 pC/N) and PS (≈ 4 pC/N). Mechanistically, during evaporation and drying of the composite solution, especially in the later stages, the hydrophilic part of PEO (rich in −O- bonds) of the bottom layer is more concentrated and faces the glass interface, which leads the hydrophobic part to contact with glycine (Fig. 2C2–C3), resulting in moderate d33 for the composite film. Comparatively, the PS interface displays moderate hydrophobic properties, making PEO in contact with the PS interface to have both hydrophilic and hydrophobic parts (Fig. 2C2–C2), and therefore not conducive to guiding the growth of glycine crystals in the same direction, resulting in a reduced d33. On the other hand, PTFE has the most potent hydrophobic properties. PEO's hydrophilic portion (−O-bond) can form hydrogen bonds with the amino or carboxyl groups of glycine, which helps to orient γ-GC development (Fig. 2C2–C3), resulting in the highest d33 [87,88].

Manipulating film-forming interfaces can also allow the control of nucleation planes (Fig. 2A1–B1). For instance, the utilization of a glass wall around a substrate changes Gibbs Free Energy from positive to negative, reducing the local evaporation rate and slow down the supersaturation build up stage. Consequently, in PVA-glycine mixture, the nucleation site shifted from the edge to the centre of the film, as shown in Fig. 2. This shift in nucleation site also affected crystal orientation, with glycine exhibiting a significantly enhanced [102] plane and reduced [110] orientation compared to films grown on standard polystyrene (PS) substrates. In the modified films, the [102] facets were more parallel to film plane, while [110] facets were more perpendicular to film surface (Fig. 2A1 and B1–B2). Notably, [102] and [001] facets are only 24.2° apart, while the [110] and [001] are perpendicular to each other. Although [001] cannot be observed in XRD spectra, the presence of strong [102] peak and almost negligible [110] peak indicated that the [001] plane of glycine was predominantly parallel to the film surface, promoting OOP piezoelectricity. When compared with non-tuned substrates like polymethyl methacrylate (PMMA), polystyrene (PS), polydimethylsiloxane (PDMS) that produced misoriented alignments, a substantially enhanced d33 was observed (PS + wall = 6.13 pC N−1 vs PS ≈ 4 pC N−1; PDMS ≈ 2 pC N−1; PMMA ≈ 4 pC N−1) [87,89]. Novel strategies are emerging to modulate and strengthen [102] planes by combining the established understanding of γ-GC/PVA nucleation with alternative techniques like ultrasound. Ultrasound can lower the energy barrier for nucleation, accelerating the crystallization process and promoting [102] alignment while also preventing agglomerate formation. As a result, γ-GC/PVA films produced by ultrasonic-assisted mixing-solidification reached a maximum value of 10.4 pC/N standing out among other piezoelectric biomaterials including silk, collagen, DL-alanine, and virus, further indicating the enormous potential of γ-GC as a building block for devices based on electromechanical energy conversion (Table 1) [90]. Moreover, Liwei Dong et al. developed a model to take into consideration frequency-dependent performance in the design of piezoelectric devices. The model accurately predicted electromechanical performance of γ-GC/PVA biofilm across a wide frequency range, establishing key design principles for film optimization that enhance the piezoelectric performance at specific stimuli frequencies [91].

β-Glycine: Although hard to produce stable β-GC crystal, its high shear piezoelectricity (178 pm V−1) and marvellous piezoelectric voltage coefficients (8 Vm N−1) larger than any currently used ceramic or polymer, makes it a very promising material for energy applications [48]. Due to its metastability, many reports show that even when β-GC is produced it eventually transforms to α-GC under ambient conditions hindering its applicability [48]. However, when grown to big enough sizes, β-GC can be stored for up to one year [92]. Typical procedures to isolate single β-GC species rely on the comprehension of the Ostwald rule of selectivity, which describes the formation of polymorphic species. The rule states that during polymorph crystallization the less stable specie crystallizes first, however when enough time and suitable conditions are given, meta-stable species eventually transform into more stable phases, in this case β → α or β → γ transitions [93].

Methodologies to obtain single β-GC involve the addition of antisolvent mixtures onto concentrated aqueous glycine solutions, spray methodologies and even freeze-drying techniques. Anti-solvent techniques are widely used in the production of crystalline polymorphs in the pharmaceutical, food, and chemical industries. By introducing an antisolvent that efficiently decreases the solute's initial solubility and raises the supersaturation in the bulk solution, this method can produce crystalline polymorphs from the solution. A wide range of alcohols have been investigated as anti-solvents for glycine crystallization, with ethanol and methanol emerging as the most commonly used [94]. However, because of its high boiling point and azeotropic (when a composition of a separate mixture in vapor and liquid phases is the same) behaviour, ethanol antisolvent still requires high temperature to be separated from water after crystallization. As such, methanol, liquid dimethyl ether, 2-propanol and acetone have been used as substitute [95,96]. While it is possible to obtain stable β-GC using these antisolvents, the approach has significant drawbacks. These include narrow and poorly understood metastable zones, the need for large quantities of antisolvent, and very low isolation yields. This limitation arises because the technique relies on establishing an equilibrium between metastable and stable species in both solvents [80].

Early focus on glycine crystallization dynamics aimed at understanding how glycine crystallizes at low temperatures and low pressures using the lyophilization technique. Lyophilization is widely utilized for manufacturing pharmaceutical proteins, diagnostic agents and other therapeutic agents [97]. From a neutral aqueous solution, frozen glycine undergoes rapid secondary crystallization, forming β-GC with a eutectic melting temperature of −3.4 °C [98]. In frozen solutions, β-GC was detected when the initial pH was ≥4. Although it is an interesting approach, it is generally avoided since the crystals formed are long and fragile, which hinders further processability [99,100].

Recent stabilization attempts rely on exploiting chemical interactions (hydrogen bonding) and electrically mediated nanoconfinement approaches. When grown in highly crystalline and chemically active Pt [111] substrates, the control of glycine concentration in the aqueous solution yielded β-GC crystallization. Although needle-like β-GC was present on the substrate, the polymorph could not be purely obtained [57]. Nonetheless, this approach offered valuable insights into activated surfaces featuring functional groups, such as hydroxyl or amines, which can enhance and stabilize β-GC through hydrogen bonding interactions. Chitosan has emerged as an excellent and widely used substrate due to its N-acetyl and hydroxyl groups, which act as anchors for glycine hydrogen bonding [46]. Additionally, acetic acid, often used to dissolve chitosan, has also shown to facilitate the crystallization of β-GC on solvent casted chitosan films [37]. Moreover, functional groups present in materials like chitosan, PVA, Mxene and alginate can enhance glycine dipole alignment along the [001] plane, contributing to the strong OOP piezoelectricity displayed by β-GC through the formation of weak ionic interactions [102].

Advances in nanotechnology have shown that the dynamics and stability of materials undergo significant changes when confined to nanoscale spaces [102]. Thus, nanoconfinement approaches have gained considerable attention in various fields, including pharmaceuticals [103] and organic semiconductors developments [104]. This technique involves restricting the crystallization process within nanoscale spaces, which dramatically alters the thermodynamics and kinetics of polymorph formation. In essence, nanoconfinement causes shifts in relative Gibbs free energies due to the increased surface area-to-volume ratio, making metastable forms more energetically favourable within these confined spaces [105]. Additionally, the confined space slows down molecular diffusion and rearrangement, enabling the capture of metastable forms before they transition into more thermodynamically stable polymorphs, thus deviating from Ostwald’s rule of stages typically observed in bulk crystallization [106]. For example, when using nanometer-scale alumina pores with average size of 55 nm and porous channels with diameters of less than 24 nm, β-GC was successfully stabilized. Remarkably, the nanocrystals confined within the 24 nm pores remained in the β-GC phase for over a year [107]. In contrast, β-GC crystals grown in larger pore sizes transformed into more stable forms within a few days [92].

Alternative approaches explore electric field assisted manufacturing techniques like electrospray or electrospinning for β-GC crystallization combining these with molecular interactions [108,109]. Initial electrospray of glucose and glycine yielded stable β-GC [110]. More recently, the synergy of in-situ electric field and nanoconfinement effectively produced self-assembled β-GC films [108,109]. Mechanistically, during the electrospray process, the rapid water evaporation and large surface-area-to-volume ratio of the produced nano-micro droplets leads to glycine nucleation and the formation of a dense nanocrystalline β-GC film with 4.5 µm in thickness, and compact nanograin structure (Fig. 2D1–D3.) [109]. During the electrospray process, β-GC crystallization is nearly completed before depositing on the substrate, with numerous nanocrystals still carrying a thin water shell, clustering together to form compact films. Additionally, the in-situ electric fields induced poling, i.e. domain alignment of β-GC nanocrystals, suggesting that the net polarization direction [020] is parallel to the electric field (optimal piezoelectric direction; Fig. 2D1). The resulting monocrystalline film (Fig. 2D3) exhibited OOP around 11.2 pm V−1, consistent with reported values (15 pm V−1). Comparatively, the electric strength was superior to most reported bio-organic films (Table 1). Due to its low relative permittivity of approximately 5, the β-GC film exhibited impressive piezoelectric voltage coefficient (g33 = 252 x 10−3 Vm N−1), with a melting temperature of 192 °C, higher than the Curie temperature (Tc) of most piezoelectric materials. Furthermore, this g33 value is comparable to PVDF, but with much higher Tc, demonstrating a stable structure with excellent performance over a broad temperature range [109]. Similarly, a remarkable g33 of 190 x 10−3 Vm N−1, along with high Curie temperature (∼185 °C) and an effective piezoelectric coefficient of 10.8 pm V−1 were recently obtained for β-glycine/polyvinylpyrrolidone films manufactured via a thermal-electric aerosol printing technology. This study confirmed that nanoconfinement induced β-polymorph nucleation with predominant net polarization direction [020] parallel to the electric field. Also, it gave additional inputs on temperature dependent crystallization. When the printing process was done at temperature >60 °C only α-polymorph was formed, whereas in the absence of a thermal field, crystalline β-glycine was mainly aligned in [001] orientation [111].

Instead of using electrospray to produce glycine films, electrospinning can be employed to create glycine-polymer nanofibrous mats. Both hydrophilic PVA and hydrophobic poly(ε-caprolactone) (PCL) [112,113] were used to stabilize glycine phase while electric fields are beneficial to orient crystalline plane orientation. Interestingly, whereas alignment is favoured in the [020] plane for electrosprayed β-GC (parallel to the electric field), in electrospun PVA/glycine fibers, alignment was found in the [001] plane, a clear indication of physical interactions between glycine and the polymer [112]. Additionally, highly porous polyacrylonitrile (PANI) nanofibers were used as a co-crystallization substrate for glycine and Nb2CTx (Mxene) through dip coating. Chemical interactions between glycine and Mxene nanosheets allied to the confinement provided by the nano-porosity of the PANI fibers effectively modulated glycine crystallization (Fig. 2E1–E3). Weak ion bonds formed between O atoms of glycine and Nb atoms of Nb2CTx further induced the directional growth of glycine along preferential [001] axis. This reflected on increases in both piezoelectric coefficient (5.0 pC N−1 at 0.5 wt% Nb2CTx) and piezoelectric voltage constant (g33 = 129 x 10−3 Vm N−1) higher than many piezoelectric materials [101].

2.5. Dependence of polymorph stability on temperature and pressure

Glycine polymorph stability is conventionally ranked as γ > α > β. However, this can vary depending on temperature and pressure. At standard temperature and pressure, the α-GC polymorph is thermodynamically favoured only at temperatures exceeding 440 K. Below this threshold, γ-GC becomes predominant despite its lower density. Expectedly, β-GC remains metastable across the entire temperature spectrum [110]. Moreover, α-GC tends to crystallize most frequently from aqueous solutions, particularly through slow evaporation of a saturated solution, however, it can be challenging to crystallize this polymorph without seeding. When seeding is not used, γ and β-GC can also co-crystallize with the α-GC polymorph. Furthermore, there are documented cases of “solution memory”, where the crystallizing polymorph mirrors the dissolved original form, that occurs unless specific conditions are used to disrupt this memory (such as prolonged storage and freeze–thaw cycles) [25]. In the presence of vapours, β and γ-GC in aqueous slurries at room temperature are converted into dense α-GC whereas wet NH3 facilitates stable conversion of both β and α-GC into γ-GC. Under CH3COOH vapor however, no transition between polymorphs is observed owing to interactions of glycine with the carboxy groups of the gas, hindering recrystallization into alternative polymorphs [114]. During heating, phase transformation occurs, such as metastable β-GC transforming into α-GC and γ-GC, wherein incomplete transformations can lead to crystals featuring coexisting domains of both polymorphs. This phenomenon has been attributed to kinetic factors, although the coexistence of domains may indicate a stationary or equilibrium state [114,115].

Pressure, although understudied, has demonstrated its influence on the relative stability of glycine. When subjected to immersion in a pressure transmitting fluid followed by compression, a previously stable crystal can undergo various modes of recrystallization [44]. However, none of these newly formed polymorphs can endure for extended periods, and none of these high-pressure phases correspond to the formation of the densest polymorph, α-GC. At ambient pressure, the kinetic barrier of the transformations between the glycine polymorphs in the solid state are substantial enough to maintain metastable polymorphs. However, at elevated pressures, structural rearrangements occur wherein the hydrogen-bonded triple helices within the polar chiral structures of γ-GC transform into the non-polar centrosymmetric structure of α-GC, characterized by hydrogen-bonded double layers [25,116].

2.6. Glycine in-vivo degradation

Although the use of glycine in energy conversion systems is relatively recent, its research in the pharmaceutical field dates back to 1820 when it was first isolated from acid hydrolysates of protein (i.e. gelatin) by the French chemist H. Braconnot [117]. To fulfil its promise as a key player in biodegradable devices, it is then required to understand how glycine will be degraded by the human body.

After implantation, in vivo aqueous microenvironment will eventually lead to glycine leashing from the device and enter the bloodstream. To understand glycine pathway in the body, two key processes must be examined: its catabolism and its role in physiologically relevant functions.

In humans, glycine catabolism occurs through three different pathways. The most preval‎ent is glycine conversion into serine through serine hydroxymethyltransferase (SHMT). For the SHMT pathway to function, a co-factor named tetrahydrofolate (THF) is necessary. This co-factor is produced through the glycine cleavage enzyme system (GCS), which produces ammonia, CO2 and THF. Additionally, glycine can also be metabolized through conversion into glyoxylate by D-amino acid oxidase [118]. Fig. 3 schematically represents the SHMT and GCS metabolic pathways at the hepatic mitochondria. The GCS pathway is composed of three proteins (P,T, and L protein) and a carrier protein (H protein). Briefly, the GCS catalysis a reversible reaction, with the first step being the decarboxylation of glycine by P protein using the H protein as a co-substrate. Then the aminoethyl moiety bounded to the lipoic acid of H protein acts as an intermediate that is degraded to THF and ammonia through a reaction catalysed by the T protein. Finally, the dithiol form of the H protein is desulfurized by the catalysis of L protein with NAD+ as a co-substrate [119]. Noteworthy, during GCS, CH2-THF is produced, a physiologically relevant molecule involved in the biosynthesis of compounds such as purines and methionine [120].

1. Download: Download high-res image (207KB)
2. Download: Download full-size image

Fig. 3. Schematic representation of biodegradable glycine composite nanogenerator. Glycine molecules are metabolized in the mitochondria through glycine cleavage system (GCS) and its utilization for the formation of serine [118], [119].

Glycine can also be eliminated from the body through a glycine deportation system (GDS). This pathway is responsible for the removal of 400 to 800 mg per day of glycine in the form of urinary hippuric acid. Briefly, glycine located at various tissues (like brain and muscle), freely diffuses from glycinergic neuronal vesicles and glia into a glycine central pool that includes the peripheral circulation [121]. These molecules enter the liver and are taken up by the mitochondria. In contrast to the GCS pathway, the GDS leads to an irreversible reaction with benzoic acid which leads to the formation of hippuric acid. Once the hippuric acid is formed, it does not again liberate glycine being excreted into urine by the kidneys [122].

In-vivo glycine is a major player in many physiologically relevant processes. Glycine participates in conjugation reactions with endogenous and xenobiotic metabolites leading to detoxification pathways. These metabolites, when present in certain concentrations, can be toxic [123]. In adult humans, glycine conjugates with bile acids with a role in lipid absorption and regulation of cholesterol homeostasis. After food intake, glycine and bile acids can be reabsorbed in the intestine, while degraded molecules can be secreted in the feces [124]. Additionally, the amino acid also participates in the biosynthesis of collagen, in which at every third position of the molecule only the side group (−H) of glycine fits. In addition to serine, glycine also participates in creatine, glutathione and purine formation [125]. The first participates in muscle and nerve energy metabolism, whereas glutathione is the most important antioxidant in cells, and purines are very important for protein synthesis and cell proliferation. Early in the decade, evidence also shown that glycine is a major anti-inflammatory and immunomodulatory agent, with protective effect in cells from ischemic-reperfusion injury in various organs including the liver, kidney, lung, intestine and skeletal muscle [126,127].

3. Glycine potential for energy harvesting and tissue engineering applications

While glycine is a relatively simple molecule, its application is mainly restricted to its pharmaceutical application playing a role as an active ingredient of many drugs. The translation of glycine into biomedical solutions is currently limited. In this section we highlight the use of glycine polymorphs in advancing engineering solutions, such as piezoelectric nanogenerators (PENGs), triboelectric nanogenerators (TENGs) or as electroactive component in tissue engineering scaffolds [12,25].

3.1. Glycine as a part of scaffolds

The primary role of glycine is related to body metabolism. Being a common motif on arginylglycylaspartic acid (RGD) groups, this amino acid has important effects on cell adhesion. Thus, RGD groups are widely utilized to enhance cell attachment, proliferation, and differentiation on various scaffold materials [130], [131], [132]. Contrarily, glycine use as a solo additive is scarce, owing to its dissolution in aqueous environments. Still, the glycine incorporation into polymeric scaffolds has been responsible for changes on overall scaffold mechanical, hydrophilic and in vitro performance. For example, the incorporation of glycine in polylactic nanofibrous scaffolds produced by thermal-induced phase-separation, increased its hydrophilic properties while maintaining enough pressure resistance to meet the requirements of engineered cartilage tissue scaffold materials. Alternatively, glycine has also been used to mitigate glutaraldehyde cytotoxicity through reactions between –NH2 group of glycine and free –CHO groups of the crosslinker. This has found success in increasing the in vitro performance of chitosan–gelatin-alginate foams [133], [134] Owing to its cytoprotective and biological effects, the impregnation of glycine into polysaccharide-based biomaterials is accounted to further develop neuron attachment and neurite outgrowth direction [135], [136], [137].

3.2. Glycine as building block for piezoelectric- and triboelectric-based devices

Primarily explored for its physiological roles, the understanding of glycine properties opened its use for advanced applications. As one of the simplest building-block of life and featuring impressive piezoelectric properties, glycine makes up for a promising material to develop modern wearable and implantable devices for sensing and energy harvesting applications.

Highly piezoelectric materials necessitate only small amounts of pressure to produce a detectable electrical signal, thereby exhibiting high sensitivities (10–100 mV N−1). When compared to traditional piezopolymer-based sensors, like PVDF films (12 mV kPa−1) [138] and electrospun PVDF-TrFE nanofibrous membranes (109 mV N−1) [139], glycine/chitosan film displayed only moderate sensitivity (2.82 ± 0.2 mV kPa−1). Nevertheless, this can be mitigated by coupling with a graphene field transistor (7.56 x 10−4 kPa−1) [46]. Combined with a graphene transistor, the higher piezopotential generated at larger pressures increments glycine conductance, aiding proton-ion hopping and grain/bulk conductance, eventually resulting in higher sensitivities [46], [140]. By modulating film conductance and sensitivity, these 2D chitosan/β-GC films can produce a high and stable signal even after 9000 cycles [46]. Recently, a γ-GC/PVA sensor was developed with a performance surpassing the previously mentioned PVDF films and PVDF-TrfE nanofibrous membranes. This sensor exhibits a voltage output of 1.12 V, with a fitted sensitivity of 139 mV kPa−1 within the 0 – 4 kPa pressure range. Herein, the introduction of a conductive steel ball inside the γ-GC/PVA film generates localized stresses on a single-point contact effectively increasing transverse piezoelectric performance (d33). Finite element methods shows that this single-point design delivers up to 27 times higher voltage output compared with conventional configuration [141].

In glycine, each molecule has a local dipole moment directed from the amino- to the carboxy-terminal direction and spontaneous polarization of these crystal comes from the summation of permanent dipole moments. Energy harvesting devices based on glycine are emerging and rely on both PENGs or PTENGs energy generation mechanisms. Although in both cases there is generation of charges and thus electrical outputs, mechanistically these actuation types differ on how charges are generated. To understand the efficiency of these devices, intrinsic parameters like open-circuit voltage (VOC), short-circuit current (ISC) and power output or power density are commonly used to qualify both PENGs and TENGs in terms of electrical power generation and transduction. Under mechanical stimulation, the open-circuit voltage (VOC) is the difference of electric potential between two terminals of a device when disconnected from the circuit and in the absence of external load and current flow [142]. The short-circuit current (ISC) provides a measure of the maximum current the device can deliver when the external load resistance is zero, indicating the capacity of the device to convert mechanical into electrical energy [143], [144]. Transversely, power density is a comprehensive measure of energy conversion efficiency quantifying the amount of electric power generated per unit volume or area of the nanogenerator [145]. For piezoelectricity-driven devices, when an external pressure is applied to the piezoelectric material, each crystal undergoes deformation, and a net polarization is created due to the movement of dipoles inside the crystalline structure. This movement generates a potential difference that can be collected as a piezoelectric voltage between a top and a bottom electrode (Fig. 4A1−A2.).

1. Download: Download high-res image (2MB)
2. Download: Download full-size image

Fig. 4. Glycine based PTENGs and TENG systems. (A1.) Schematic representation of γ-GC and chitosan interactions on PENG/PTENG devices; (A2) Schematic of PENG device structure and working principle of PENG and instantaneous maximum output power (Pmax) of γ-GC/CS 50 wt% composite PENG with various external load resistance; (A3) Schematic of PTENG device structure and working principle of PTENG and instantaneous maximum output power (Pmax) of γ-GC/CS 50 wt% composite PTENG with various external load resistance; (A4) Schematic diagram of electron-cloud-potential-well (ECPW) model for explaining charge transfer between two friction materials; (A5-A6) 100 driven LEDs and driven watch and sustained by PTENG power generation; (A7) Chemical crystallographic polarization of γ-GC P31 structure and their longitudinal polarization aligning within CS matrix. Adapted from [21]. Reprinted with permission from Elsevier. (B1 – B2) Digital photograph of pristine SF film and SF/30% γ-GC composite film; (B3 – B4) Flexibility of SF/γ-GC composite films through mechanical folding and rolling; (B5 – B8) The maximum output power of (B5) pristine SF film and (B6 – B8) SF/7.5, 15 and 30% γ-GC composite films. Adapted from [154]. Reprinted with permission Taylor & Francis. (C1) Device architecture of the FB-HNG and insets of PLGA nanofibers and PVA/γ-Gly/PVA film with surface nanostructures; (C2-C3) Cross-section image of PVA/γ-Gly/PVA film and optical image of wafer-scale composite; (C4) Working principle of FB-HNG in positive (red dash line box) and negative (black dash line box) coupling of triboelectric and piezoelectric dual effects; (C5 – C6) Output voltage and current in different cases including only tribo/piezo− (positive coupling), tribo/piezo0 (triboelectric alone), tribo/piezo+ (negative coupling) and piezo (piezoelectric alone). From [157]. Reprinted with permission from Elsevier. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

As a standalone material, piezoelectric β-GC films produced maximum open-circuit voltages of 14.5 V, about one order of magnitude larger than most reported piezoelectric biomaterials. These β-GC films remained stable for 24.000 compressing cycles displaying an output performance large enough to light up three LEDs simultaneously [109]. Additionally, emerging research on glycine-based PENGs highlights that the isolation of electro-active glycine inside hydrophilic polymers (such as, PVA, PEO, chitosan) can yield powerful harvesting systems. When crystallizing γ-GC within a PEO matrix, the resulting film produced a Voc about 5 V, suitable for sensing applications. When attached to the back of the finger or hand, movement signals for different bending angles or different fingers could be monitored and recognized [88]. Comparatively, the introduction of γ-GC onto chitosan yielded a PENG with superior Voc (approximately 7 V), a major increase in comparison with pure chitosan (0.66 V) [21].

In the last two decades, many reviews have categorized advances in piezoelectric nanogenerators [146], approaching ceramic-based nanogenerators, to flexible and naturally based nanogenerators for a wide range of applications. Table 3 present examples on biodegradable PENG devices. Manufacturing, material combinations and even different testing conditions yield PENGs with a wide range of electrical performances making the comparison between these difficult. Nonetheless, 2D β-GC films, γGC-PVA films and γGC/Nylon-11 fibers can electrically outperform PENGs based on silk nanofibers [147], silk films with ferroelectric particles [148], PLLA/cellulose film composites [149] and even diphenylalanine arrays on PLLA substrates [150] showing similar performance to PVA/cellulose nanocrystal composites [151]. As highlighted in this review, glycine research has been focused on the careful control of crystallization to favour specific polymorphs. Although this represents a necessity to avoid α-GC crystallization, it also translates into devices with good piezoelectric performances while using flexible and insulator polymers, thus expanding material diversity. Moreover, when combined with piezopolymers, such as Nylon-11, glycine can increase the piezoelectric δ-phase of the polymer further boosting the performance [152], [153]. More examples on glycine ability to potentiate crystallization of other polymers will be given.

On the other hand, triboelectric devices are based on contact-separation of two different charged dielectric triboelectric layers (Fig. 4A3.). Contact-separation leads to the generation of a potential difference between two layers that drive free charges in the electrodes. High power density, direct conversion, durability and flexibility in material selection makes triboelectric coupling a powerful option for electrical generation, with higher outputs compared to typical PENGs. Glycine application in TENGs is still a new topic with resulting devices not considered as pure TENGs but rather as PTENG hybrids since piezoelectric contribution cannot be decoupled (Fig. 4A2–A3) [154], [155]. Polymer/glycine PTENGs show remarkable electrical output generation, depicted in Table 2. Combined with hydrophilic polymers as a filler, glycine can be utilized as a positive triboelectric material for energy harvesting due to electron donation by its –NH2 groups [21], [154]. Additionally, depending on the strategy utilized for the development of these PTENGs different energy conversion mechanisms can happen. The electron-cloud-potential-well (ECPW) model is usually utilized to explain charge transfer between two friction materials, before, during and after contacting process. Prior to contact, materials are separated, and their respective electrons are confined within the well due to the presence of energy barriers (Fig. 4A4. A). The addition of glycine generates electrons in trap states near the lowest unoccupied orbitals (Fig. 4A4. B) and upon contact, electrons from glycine can hop to unoccupied levels of triboelectrical negative materials (like PTFE) via electron cloud overlapping, as illustrated in Fig. 3A4 C. The hopped electrons remain confined within the well, where they either recombine or emit to the air via the air breakdown process. The residual charges are then transferred to the load following wire connection, as shown in Fig. 4A4. D [21].

Table 2. Strategies to crystallize specific glycine polymorphs, effect on glycine crystallographic orientation and resulting piezoelectric outputs (reference values of other piezoelectric polymers and ceramics are also presented).

Glycine polymorphCrystallization strategyEffectpiezoelectric outputRefs

γ-GC	Heterostructure Glycine-PVA film	Increased concentration (2 to 3 wt%) exposed larger portion of [001] polar surface toward OOP	d33 = 5 – 6 pC/N g33 = 157.5 x 10−3 Vm/N	[89]
	Solvent evaporation of aqueous solutions (PVA/Glycine) onto different substrates (PS; PMMA; PDMS)	Glycine crystallization along [001] direction through surface curvature tuning	d33 = 4.24 pC/N PS d33 = 3.55 pC/N PMMA d33 = 1.56 pC/N PDMS	[87]
	Solvent evaporation of aqueous solutions of PEO on different substrates	Hydrophobic interface-induced effect of PTFE on the molecular orientation regulation of bottom layer PEO and γ-GC crystals causes the increase of d33	Average d33 = ∼8.2 pC/N Maximum d33 = 13 pC/N	[88]
	Ultrasound-assisted process, guided by density functional theory	Enhanced orientation in [102] planes	Maximum d33 = 10.4 pC/N g33 = 324 x 10−3 Vm N−1	[90]
β-GC	Computed β-GC	------------	d16 = 178 pC/N	[48]
	Electrospray 2D β-GC film	Electrospray allied to nanoconfinement enhances β phase OOP [020] Absence of electric field: dominant OOP orientation [001] perpendicular to [020]Without nanoconfinement [001] characteristic of non-piezo α-GC	deff = 11.2 pm V−1 d33 = ∼13.3 pm V−1 g33 = 252 x 10−3 Vm N−1	[109]
	Co-Axial Electrospun PVA/β-GC nanofibers	Electrospun fiber with peak intensity of [001] > [110] indicating glycine alignment inside fiber	deff = 12.5 pm V−1 (2.0 fold increase to bulk γ-GC) with coercive bias of 10 V	[112]
	Electrospun PCL//β-GC nanofibers	------------	deff = 18.91 pC/N displacement of ∼ 12 µm for 1 – 4 Hz	[113]
	β-GC/Nb2CTx Piezoelectric Nanofibers	Micro-nanoscale capillary confinement bolsters [001] crystal plane	deff = 360 pC/N under 20 N d33 = 18.4 to 20 pC/N g33 = 129 x 10−3 Vm N−1	[101]
	Glycine/alginate-based film	------------	d33 = 7.2 pC/N g33 = 52x10−3 mV/N	[128]
	β-glycine/polyvinylpyrrolidone films	External DC e-field induces net polarization direction [020] parallel to e-field	d33 = 10.8 pm.V−1 g33 = 190 x10−3 V.m/N	[111]
	Co-Axial Electrospun PVA/β-GC nanofibers	------------	d33 = 4.15 pm. V−1	[129]

The introduction of γ-GC into insulating chitosan led to an increase in VOC up to 78.9 V and ISC ∼ 64 µA (Fig. 4A2–A3.; Table 3), an almost two-fold increase compared with pristine chitosan. Noteworthy, triboelectric electrical generation was able to produce up to 11 times more output current, compared to piezoelectric generation (Fig. 4A2.–A3.; Table 3). The resulting γ-GC/chitosan-based PENG/TENG hybrid could generate sufficient power to light up 100 LEDs, one Casio calculator and a watch, separately (Fig. 4A5. – A6) [21]. Coupled with ECPW theory, glycine molecular orientation also participates effectively on electrical generation through piezoelectric and triboelectric phenomena. Stable γ-GC can exist in P31 and P32 forms (Sohncke space group; Fig. 4A8.). Upon external forces (compression and friction), charges in the GC structure can be displaced from their centres, resulting in the formation of an electrical polarization. Moreover, the hydrogen interaction between GC and chitosan improves GC particle alignment along the c-axis direction (Fig. 4A7.) [87]. Thus, longitudinal piezoelectricity occurs along γ-GC molecules introducing additional surface charges and a piezoelectric potential gradient, thus increasing the performance of the PTENG device (Fig. 4A8.) [156]. Similarly, the introduction of glycine inside silk fibroin matrices yielded 2D flexible films (Fig. 4B1. – B4.) with a very high electrical performance. Compared to pristine silk fibroin (VOC = 68 V and ISC = 88 µA; Fig. 4B5.), the introduction of γ-GC increased the VOC up to 81 V and ISC up to 121 µA for 15 % wt of γ-GC. Among all the composites tested, the silk fibroin/15 % γ-Gly exhibited highest Pmax reaching 205 µW with a power density of 22.8 µW/cm2 (Fig. 4B6 – B8.). The high electrical output of the hybrid system was primarily attributed to the synergistic combination of triboelectric and piezoelectric modes. In this system, the mixed β- and γ-GC phases generate additional charges within the composite through piezoelectric dipoles activated by stress induced during the contact process [21], [154]. Modulating glycine concentration in the composite is thus a fundamental parameter for optimal mechano-electrical performance. At higher concentrations, γ-GC particles tend to agglomerate, resulting in a non-uniform distribution and poorly aligned dipoles that partially counteract each other. This leads to weaker inductive charges and reduced electrical output. This has been reported in various other works [21], [101], [128], [157], [158], [159].

Table 3. Comparison between glycine-based piezoelectric and piezo/triboelectric devices and recent PENGs and TENGs found in literature.

MechanismCompositionDielectric properties loss factorSensitivityVOC (V)ISC (µA)Output power (µW/cm2)Refs

Glycine-based PENGs	β-GC/Chitosan	3.5 for 1 MHz 7.7 for 100 MHz 0.18 and 0.08 in frequency from 100 Hz to 1 MHz	2.82 ± 0.2 mV kPa−1	190 mV	−	−	[202]
	β-GC/Chitosan and Graphene GTEF	− −	2.70 x 10−4 (5 – 20 kPa) and 7.56 x 10−4 (20–––35 kPa)	−	−	−	[140]
	2D β-GC film	− −	−	14.5 V	4 µA	3.61 µW/cm2	[109]
	γ-GC wafer-scale heterostructured piezoelectric bio-organic thin films	5 −	−	4.1 V	360nA	−	[89]
	γ-GC/PEO films	− −	−	5 V	−	−	[88]
	β-GC/alginate-based films	− −	1.95 mV/kPa	500 mV at 40 N	−	--	[128]
	γ-GC/PVA Film	5	−	18 V	2.2	11.3	[90]
	Nylon 11/γ-GC	−	−	11.2 V	11.57	−	[153]
Glycine-based PTENGs	γ-GC/Chitosan	∼27 at 50 wt% at 103 Hz −	−	7 V 12 V at 10 Hz	12.5 µA 28 µA at 10 Hz	∼9.59 µW from 100 to 10 MΩ	[21]
				∼78.9 V	∼64 µA	79 µW at 1 MΩ
				VDC = 70 V	IDC = 50 µA	−
	γ-GC/Silk Fibroin	− −	−	81 V	121 µA	205 µW at 5 MΩ	[154]
	PVA- γ-GC −PVA/PLGA	−	−	94 V	2.3 µA	−	[157]
Other Biodegradable PENGs	Silk Fibroin Nanofibers	−	0.15 V kPa−1	8 V	−	5	[147]
	Cellulose Nanocrystals/PVA	−	4.2 V/kPa	25	0.850	0.6	[151]
	PHBV/PLLA/KNN	−	−	6 V	0.6	−	[203]
	Silk Fibroin composite with biocompatible ferroelectric nanoparticles	−	−	30 wt% KNN:Mn nanoparticles: 2.2 VAg nanowires (1 wt%) and PVPV (12 wt%): 2.2 V	.	30 wt% KNN:Mn nanoparticles: 0.12Ag nanowires (1 wt%) and PVPV (12 wt%): 0.12	[148]
	Cellulose/PLLA	−	−	10.3	0.2618	0.45	[149]
	Diphenylalanine/PLA	−	−	1.78	0.070	0.156	[150]
	Wood Sponge	−	−	0.69 V	0.0071	0.6 nW/	[204]
Other TENGs	CS/Ag NWs & PVDF	−	−	47.9	4.1	13.76	[205]
	CS/Acetic Acid & Ecoflex	−	−	13.5	0.042	1.75	[206]
	CS/CaCl2 & PTFE	−	−	149	15	44	[207]
	CS/Natural Rubber/Cellulose nanocrystal &PTFE	−	−	101.7	10.6	15.6	[208]
	CS/ZnO & PDMS	−	−	0.97	49	2.72	[160]
	CS/Silk Fiber & PTFE	−	−	77	13	22.44	[158]
	Textured ZnO NW film grown on carbon fibers, 1000 carbon fibers	−	−	3	0.2	−	[209]
	ZnO NW arrays covered by PMMA layer	−	−	58	134	0.78	[163]
	Silver nanowire/PLGA- PVA	−	0.011	90	1.5	130	[161]
	ZnO Nanorods in Silk Fibroin Hydrogel	−	−	25 V	0.08	6.2	[162]
	PDMS/ZnSnO3/MWCNT	−	−	475 V	36	0.062 mC cm−2	[210]
Other PTENGs	PVDF/Silicon Rubber/Gold/Al	−	3.65 µW/g	25.8 V	8.82	−	[211]
	BTO NPs/PDMS/Al	−	−	60 V	1	−	[164]
	BTO/PDMS/Copper	−	−	55 V	−	−	[212]
	BT nanorods/chitosan	−	−	111 V	22	1568 µW/cm2	[167]
	Poled PVDF/PDMS/Al	−	−	5.2 V	500nA	−	[165]
	PVA/PVDF NFs/CA NFs	−	−	220 V	1.1	80 mW/m2	[166]

GTEF – Graphene Field-Effect Transistors; Ag NWs – Silver Nanowires; PVA- polyvinyl alchool; PHBV – Poly(3-hydroxybutyrate-co-3-hydroxyvalerate); PLGA – poly(lactic-co-glycolic); ZnO – Zinc Oxide; PTFE – Polytetrafluoroethylene; CaCl2 – Calcium Chloride; PDMS – Polydimethylsiloxane; ZnSnO3 – Zinc Stanoate; MWCNT – Multi-Walled Carbon Nanotubes; NFs – Nanoflower; Al – Aluminum; BT – Barium Titanate; PVDF – polyvinylidene fluoride.

Noteworthy, fine tuning of glycine crystallization using an electric field assisted evaporation yielded a well-encapsulated layer of glycine in between thinner PVA layers (Fig. 4C1–C3). Unlike previous composites where glycine directly contributes to the triboelectric effect, in this configuration, only PVA contributes to the triboelectric conversion within the final multilayered hybrid nanogenerator (FB-HNGs). The layered film interfaces with PLGA that serves as the negative triboelectric layer (Fig. 4C1). The FB-HNG combines triboelectric (TENG) and piezoelectric (PENG) effects, whose phase differences can either enhance (positive coupling; Fig. 4C4. Red dashed) or reduce (negative coupling; Fig. 4C4. Black dashed) the overall output. In positive coupling, the triboelectric and piezoelectric currents flow in the same direction when an external force is applied or released, leading to an increased output (Fig. 4C5) of 49.5 V, surpassing the sum of individual TENG (33.2 V) and PENG (9.4 V) components due to enhanced dielectric constant of the PVA/γ-Gly/PVA layer. The dielectric constant of the composite is a good indicator about the successful dispersion of the polymorph. If GC is well dispersed and present proper interfacial adhesion with the chosen matrix then a high dielectric constant can be obtained, while poor dispersion and agglomeration of fillers will result in the deterioration of these dielectric properties. For example, for γ-GC crystallized in chitosan foams, dielectric constant reached a maximum of 27 for 50 wt% of γ-GC, whereas for 70 wt% a reduction down to 20 was measured influencing its electrical properties (VOC = 40 V and ISC = 40 µA for 70 wt% vs VOC = 78.9 V and ISC = 64 µA for 50 wt%) [21]. Conversely, the negative coupling generated by PVA/γ-Gly/PVA layer happens when the currents flow in opposite directions because glycine’s polarization is oriented towards an opposite electrode (Fig. 4C4 Black dashed), resulting in a lower Voc (24.5 V).

There are various methods to improve triboelectric layer performance ranging from the introduction of hydroxyl, amine, or fluorine groups (to increase charge affinity), change the surface of the triboelectric layer through texturing methods, add dielectric layers that act has insulating layer and enhances charge retention [16]. This leads to a wide range of TENGs developed in the last decade, with a wide range of electrical performance, as depicted in Table 3. Combined with chitosan, silk and PVA, γ-GC effectively increased the electrical output of the system through the conjugation of glycine electronegativity (–NH2 groups) with changes in permittivity and contact between triboelectric layers [21], [154], [157]. These systems are comparable to CS/Silk fiber/PTFE [160], silver nanowire/PLGA-PVA [161], surpassing ZnO nanorods on fibroin hydrogels [162] or ZnO nanoarrays covered by PMMA layers [163], while still maintaining a completely degradable triboelectric system. However, these increases are not only attributable to glycine role as a nanofiller, but also due to its piezoelectric role, turning a triboelectric system into a piezoelectric-triboelectric hybrid. These systems are relatively new, and the ones found (Table 3.) mostly use non-degradable materials like BTO nanoparticles [164], and poled PVDF-films [165]. Compared to film-based structures, glycine can outperform these, however falling short when compared to fibrous PVDF [166] and BT nanorods [167].

3.3. Glycine used as a phase stabilizer for other piezoelectric polymers

Hydrogen bonding drives the interaction between glycine and the polymer, enabling glycine to act as a heterogeneous nucleating agent. Leveraging amphiphilic – hydrophobic combinations, a new world of applications can be envisioned for glycine. More specifically, combined with PLLA, MoS2 and Nb2CTx, in-situ co-crystallization mediated by glycine endowed the enhancement of PLLA and PVDF β-piezo active phases [101], [168], [169].

PLLA is a biocompatible and biodegradable medical polymer used extensively by the U.S. Food and Drug Administration polymers. PLLA piezoelectricity stems from the electrical polarity of the carbon–oxygen double bonds (C=O) branching out from its backbone [170]. The co-crystallization of PLLA with glycine has shown to bolster β- piezo active crystallization improving its performance as a sensor (Fig. 5A1). Glycine comprises a backbone structure with a primary NH2 group and an acidic COOH group. Molecular dynamic simulations have shown that the C=O bonds on PLLA preferably bond to the –OH groups on glycine. This has been complemented with DFT calculations displaying that when –OH groups bound with the C=O groups on PLLA chains, the overall system energy reaches a minimum which can potentially guide and anchor the orientation of C=O groups and potentiate β-phase. In spin coated PLLA films, the addition of glycine increased β-phase nucleation up to 54.1 %. Mechanistically, the self-powered, durable sensor exhibited notable response times (10 ms) with suitable sensitivity (13.2 mV kPa−1), comparable to a commercialized piezoelectric quartz force sensor (PCB Piezotronics; 208C01) and higher than pre-built PEDOT:PSS/PTFE/PZT (5.2 mV kPa−1) [171], Au nanowires/carbon nanotubes/PDMS (5.7 mV kPa−1) [172], and Nylon-11 (0.13 mV kPa−1) [152] sensors, with capacity to monitor diaphragmatic contraction, pulse pressure and systolic augmentation (Fig. 5A2–A3) [169]. Electrospinning is an efficient technique for fabricating self-polarized piezoelectric nanofibers with high orientation, due to stretching forces exerted on electrified solution jets [173], [174], [175], [176]. For PLLA, electrospinning has been a gold-standard technique to create electro-mechanical devices, however, the self-polarized structure of PLLA tends to be partially rearranged under mechanical stretching or temperature fluctuations, tending to relax and depolarize back to thermodynamically non-piezoelectric phases [177]. To address this issue, glycine has been used has a nucleating agent of PLLA β- phase. The high positive e-field makes the positively charge –NH2 groups to be repelled to the surface of the fiber, whereas –OH groups of glycine preferentially interact with C=O groups on PLLA chain via hydrogen bonding forming stable C=O--–HO dipoles and a PLLA core and glycine shell fiber. This intermolecular anchoring effect directs the nucleation of β – PLLA with ordered C=O orientation chains. The PLLA/Glycine core/shell successfully addressed the long-term stability issue of piezoelectric PLLA. Pure PLLA nanofibers (NFs) dropped from 0.32 to 0.23 V on day 7 and completely lost its piezoelectricity in day 14. The creation of the glycine shell could enhance PLLA crystallinity (up to 64 %) and crystal orientation, retaining ∼ 78 % of their initial piezoelectric output and remaining stable even after 102 days at room conditions. In aqueous medium, Vpp of pure PLLA NF fabrics completely lost their piezoelectricity within 7 days while core/shell PLLA/Gly NFs while the core/shell NFs were nearly intact throughout the 56-day testing period [178].

1. Download: Download high-res image (1MB)
2. Download: Download full-size image

Fig. 5. Schematic demonstration of the (A1) fabrication process for biodegradable piezoelectric PLLA/Gly film and demonstration of the real-time applications of the sensor in healthcare monitoring; (A2) Image of piezoelectric sensor conformally affixed to human wrist, radial artery pulse, and throat; (A3) and respective signal depicted from movement of blood vessel, pulse pressure and late systolic augmentation, carotid artery. Adapted from [169]. Reprinted with permission from John Wiley and Sons. (B1) Scheme of noncovalent assembly network and hierarchically anisotropic structure, with insets illustrating self-polarized phase transition (i – iii). (B2 – B4) Large human movements monitoring including finger bending (B2), expression‎ recognition (B3) and speech recognition (B4). Adapted from [168]. Reprinted with permission from Elsevier.

A PVDF/Gly-MoS2 PENG was also developed utilizing a one-step solid-state drawing method. Glycine was used as a stabilizer to exfoliate bulk molybdenum disulfide into monolayer or a few-layer MoS2. Driven by van der Waals interactions between hydrophobic MoS2 and amphiphilic glycine, the glycine molecules are inclined to be adsorbed on the surface of nanosheets to weaken their interlayer interactions. Then, crystallization of water endowed by lyophilisation process, further promotes the formation of cocrystals to stabilize the exfoliated nanosheets. The co-crystallization of glycine on MoS2 endow the functionalized nanosheets with activated OOP and abundant polar surface groups that favour noncovalent assembly in polymer matrix. These aligned 2D nanosheets interact and lock PVDF dipoles through hydrogen bonding and ion–dipole interactions, leading to well-aligned electroactive β-phase within the polymer matrix (Fig. 5B1. (i) – (iii)). The locking of these dipoles without the need of an electrical treatment step yielded a composite with a piezoelectric coefficient comparable to and even greater than most reported PVDF-based piezoelectric materials. Interactions contributed for the formation of PENGs with high sensitivity (4.73 and 1.63 nA kPa−1), good response times (15.7 ms) and high stability. PENG allowed for clear detection as a pedometer to monitor human walking, detect subtle human motions such as facial expression‎, speech, breath produced clear detection (Fig. 5B2–B4) [101], [168].

3.4. In vitro and in vivo assessment of glycine piezo/tribo

Glycine presence on the body makes it an inherently biocompatible material. Additionally, its combination with hydrophilic and usually biosafe polymers deems the engineered solutions optimal for internal and external body applications. Hybrid nanogenerators (HGs) based on sandwich-like piezoelectric films are promising body energy harvesters by synergistically exploiting piezoelectric and triboelectric mechanisms.

The hydrophilic properties of glycine contribute to the development of highly soluble and compatible systems in physiological fluids hindering their use as long term energy harvesting devices. To fill this gap, these composites need to be encapsulated within slowly degrading hydrophobic layers, utilizing materials like PLA, PLGA or PCL [89], [101], [128], [157]. For example, PLGA was used as an encapsulating layer of a PVA/γ-Gly/PVA transient hybrid nanogenerator. However, PBS degradation tests (37 °C, pH about 7.4) revealed that the device could only maintain transient power for only 3 days, with significant degradation by day 9 due to increase moisture and swelling of the encapsulating layers (Fig. 6A1), completely degrading by day 11 (Fig. 6A2–A3). In vitro cell cytotoxicity of the hybrid nanogenerator was screened by showing the absence of cytotoxicity in L-929 and 3 T3-L1 fibroblasts when in contact with different quantities of dissolved fully biodegradable hybrid nanogenerator (FB-HNG) (Fig. 6A4–A5) [157]. PLGA and beeswax were also explored to encapsulate PVA/γ-GC films. At 37 °C PLGA encapsulated devices degraded completely by day 44, whereas the introduction of beeswax and molybdenum sheets increased degradation time to 97 days. 3T3 cells biocompatibility was assessed under various concentration of molybdenum powder and glycine-PVA mixture. Additionally, survival rates were assessed in different concentration of PLGA, beeswax and glycine-PVA solution. While these materials maintained low cytotoxicity, increasing concentrations gradually increased adverse effects on cells [141]. A similar methodology was also used to access the cytocompatibility of Gly-Alg-Glycerol pressure sensor in rat primary bone-derived mesenchymal stem cells when contacting with different amounts of dissolved Gly-Alg-Glycerol (from 0.01 to 1 mg ml−1). When using the intact Gly-Alg-Glycerol film, cells exhibited a spread morphology with elongated actin cytoskeleton, and viability comparable to those cultured on tissue culture plates. Moreover, the film showed blood compatibility via haemolysis assessment [128]. On the other hand, Meysam T. Chorsi exploited PCL a hydrophobic polymer for the encapsulation of glycine towards the development of a blood–brain barrier (BBB) ultrasonic transducer. The glycine-PCL nanofibers showed suitable capacity to maintain mouse adipose-derived stem cell viability and proliferation [113]. Recently, PC12 cells were cultured on the core/shell NF films without and with PDMS encapsulation. Cell viability studies confirmed that the PLLA/Gly nanofibers were biocompatible and nontoxic with packaged and unpackaged nanofibers showing similar viability levels [178].

1. Download: Download high-res image (1MB)
2. Download: Download full-size image

Fig. 6. In vitro output properties and cell toxicity of FB-HNG. (A1) and (A2) Output current of the FB-HNG in a simulated in vivo environment by immersing it in water for varying durations. (A3) Photographs of FB-HNG at various stages of the degradation in phosphate-buffered saline (37 °C, pH = 7.4), (A4) and (A5) Live/Dead fluorescent staining images of L-929 and 3T3-L1 cells co-incubated with different concentrations of the FB-HNG materials after 24 h, respectively. Scale bars are 100 µm. From [157]. Reprinted with permission from Elsevier; Schematic of implementation routes of glycine in SD rats on thigh and chest areas with (B1) inset showing Gly-Nb2C-PANI implantation; (B2) Piezoelectric voltage outputs of the Gly-Nb2C-PANI driven by respiration when implanted on the pectoralis major muscle in the chest, and the corresponding respiratory curve below; (B3 and B4) Output of a FB-HNG implanted on the chest area and tigh area measured by DMM 6500 (internal resistance of 10 MΩ); (B2 and B5) Piezoelectric voltage output of Gly-Nb2C-PANI and Gly-PANI implanted on the quadriceps femoris muscle at the thigh area during gentle stretching; Adapted from [101] and [157]. Reprinted with permission from John Wiley and Sons and Elsevier; In-vitro and in-vivo assessment of β-GC/PCL nanofibrous ultrasonic transducer (C1) Model, experiment setup and x-ray image of the glycine-PCL-based ultrasonic transducer in enhancing the delivery of chemotherapeutic drug to the brain for the treatment of GBM tumor. (C2) Representative confocal fluorescence images of the superficial and deep regions show the signal of dextran (bright green). The images are immunostained for CD31 to detect blood vessels (red). Dashed line indicates the site of the implant; (C3) Bioluminescence images of GBM tumor growth in live animals (day 27) and ex vivo images of GBM-bearing brains (mice were euthanized on day 31). From [113]. Reprinted with permission from AAAS; Validation of continuously accelerated wound healing by b-WPUE in mice. (D1) Digital images of back wound healing in mice with different treatments. The boundaries of wounded areas were highlighted by white dotted lines; (D2) Quantitative analysis of wound healing from 0 to 10 days; (D3) Summary of the complete wound closure times. From [90]. Reprinted with permission from AAAS. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

In vivo, experiments include the eval‎uation of the PENGs/TENGs: i) biocompatibility tests such as hematoxylin and eosin (HE) staining for a comprehensive picture of the microanatomy of tissues [128]; ii) sensing applications [89], [101], [128], [157], [178]; iii) performance as electromechanical energy conversion devices as ultrasound-induced drug deliver [113], and iv) ultrasound-induced electrotherapy [90]. These studies are mostly made on Sprague-Dawley (SD) rats due to their genetic diversity and ease of handling with the location of implantation varying depending on the role and function of the electro-mechanical device.

The electromechanical energy conversion of these hybrid nanogenerators as sensing devices have been tested on Sprague-Dawley rats due to their genetic diversity and ease of handling. For that purpose, different body parts were assessed, such as the top of the pectoralis major muscle (rat respiration; Fig. 6B1–B3) and the rat quadriceps femoris muscle (leg stretching; frequency ∼1 Hz; Fig. 6B1; B3–B4) but also the wrinkled surface of the gastrointestinal tract [89], [101], [128], [157]. Encapsulated PVA/Glycine/PVA heterostructured piezoelectric film produced high and consistent outputs when implanted on top of quadriceps femoris muscle and pectoralis major muscle (>4 V and ∼5 V, respectively; Fig. 6B3 and B4). Biocompatibility testing demonstrate that HE staining of major organs do not show differences between controls and implanted groups (Fig. 6B6) [157]. Moreover, when implanting γ-GC/PVA bio-organic films, leg stretching (frequency ∼ 1 Hz) and respiration, could be easily perceived and generated a consistent and stable Vpp of over 150 mV and 20 mV, respectively (Fig. 6B2 and B5) [89]. In further attempts to promote γ-GC/PVA bio-organic films piezoelectricity, a conductive rigid ball was introduced to induce single point contact and improve d33 polarization. Importantly, when implanted in-vivo the careful consideration of encapsulating layers (PLGA + beeswax) retarded degradation from 7 days to 28 days allowing for higher term actuation. Nonetheless, piezoelectric response diminished as implantation time increased, remaining above 2 pC/N, and 1 pC/N at day 14–21 and 21–28, respectively. To test the suitability as a piezoelectric sensor, devices were subcutaneously implanted in the chest and thigh displaying clear sensing capacities for breathing and leg movement, respectively. Additionally, time-domain response indicated stable piezoelectric detection during respiratory and leg movement, and frequency domain analysis demonstrated spectral consistency, suggesting the possibility for deeper understanding of respiratory dynamics and irregularity detection [141].

Weiying Zheng et al. presented one of the few studies which uses β-GC instead of γ-GC for sensing applications. β-Gly-Nb2C-PANI biocompatibility was validated by implantation in the subcutaneous muscle area of the back of mice. Histological analysis showed collagenous fibers perceived by Masson staining and inflammatory factors (macrophage/monocyte) stained by CD68. Mild fibrosis was found in the 2nd week, followed by return to basal levels of CD86 and a decrease in the percentage of collagen fibers at week 4 symbolizing a gradual decrease in cellular inflammatory response in damaged tissue (Fig. 6B7). When implanted in the quadriceps femoral muscle Mxene-glycine electromechanical conversion yielded consistent Vpp > 300 mV in β-Gly-Nb2C-PANI compared to approximately 100 mV for Gly-PANI, reinforcing the importance of stabilizing agents of glycine meta-stable phase. Further implantation in the pectoralis major muscle (rat respiration) resulted in the generation of a Vpp > 20 mV [101]. In vivo biocompatibility and biodegradability of Gly-Alg-Glycerol films were assessed by implanting them into dorsal subcutaneous SD rats. The films dissolved almost entirely within 2 h, with no tissue damage and mild inflammatory reactions that healed within 7 days. When encapsulated with PLA and beeswax, the degradation time extended to 5 days before a sharp decline in output voltage, indicating liquid infiltration. The piezoelectric sensor showed functionality by generating rhythmic pulse voltage signals corresponding to cardiac pulsations and respiration when tested on various body parts, such as the outer wall of the heart, diaphragm base, and thigh muscle. During cyclic stretching and releasing of the leg at 0.6 Hz, it produced a stable output voltage of approximately 200 mV [128].

Glycine-based sensors have been developed for in vivo monitoring of gut processes. A nanofibrous core/shell (PLLA/glycine) mat, encapsulated with PDMS, was implanted onto the intestinal wall of the transverse colon, conforming seamlessly to the surface without detachment. The device successfully tracked peristalsis patterns. When tested with 0.2 Hz enemas in rats, the sensor produced a consistent voltage (Vpp > 2 V) per peristalsis cycle. The device demonstrated diagnostic capability by accurately tracking the onset and remission of acute colitis induced by dextran sulfate sodium (DSS) in rats. Colitis-positive rats exhibited heightened propulsive activity, comparable to patients with diarrhea-predominant irritable bowel syndrome. Biocompatibility assessments, including histological and pathological tests, showed mild immune reactions at week 1, resolving by week 5, with no signs of inflammation, cellular toxicity, or organ damage throughout the study (heart, liver, spleen, lung, and kidney). These findings confirm‎ the device's safety and potential for non-invasive gastrointestinal diagnostics [178].

Implanted US transducers have recently emerged as an effective and safe method to facilitate the cyclic opening of the blood–brain barrier (BBB) for drug delivery. However, dramatic attenuation of the US in the skull, tedious processes for tunning the US transducer and the inability of extracorporeal US devices to repeatedly induce powerful sonication at precise location are still challenges to be addressed [179], [180]. Implanted devices rely then on traditional piezoelectric materials like PZT that contains toxic elements such as lead and require invasive brain removal surgeries [181]. Glycine-encapsulated PCL nanofibers have been developed as biodegradable ultrasound transducers for delivering the chemotherapeutic drug paclitaxel (PTX) to the brain, targeting glioma treatment. In vivo biocompatibility studies demonstrated successful device integration within the blood–brain barrier (BBB) and significant dextran leakage around microvessels, surpassing both PCL (negative) and PLLA (positive) controls, as confirmed by immunofluorescence (Fig. 6C1−C2). The device exhibited a degradation period of 25 days, enabling effective therapeutic access to brain tissue (Fig. 6C3). In orthotopic glioblastoma mouse models, the system achieved significant tumour reduction – eval‎uated by a genetically modified cell bioluminescence intensity −and doubled the survival time of treated animals compared to controls [113]. Although showing promising electro-mechanical behaviour, issues related with the functional lifetime of the device which in many cases need to be from several months to several years still need to be addressed. Envisaging a distinct application, a PVA/γ-GC thin film was tested as biodegradable ultrasonic wireless electrotherapy device for wound healing. When stimulated via ultrasound, the mechano-electrical stimulation provided by PVA/γ-GC outperforms unstimulated control by completely closing the wound after 10 days (Fig. 6D1-D3), whereas the control lesion needed 15 days. The rapid healing was accompanied by an increase in angiogenesis and superior macrophage polarization, with fewer M1 and more M2 macrophages, observed at days 6 and 12, respectively. Moreover, immunofluorescence staining displayed a reduction of pro-inflammatory cytokines (interleukin-1β, IL-6 and tumour necrosis-α), suggesting that the electrical stimulation effectively reduced inflammatory response while intensifying anti-inflammatory effects over the treatment course [90]. Within a different outlook, PDMS encapsulated glycine/PVP films were investigated as an external charging system for electronic transcutaneous implants. Under US stimulation, the biofilm generated a voltage of ∼3.6 V, current of ∼10 µA, and a power density of ∼35 µW cm−1 enough to recharge some implants like pacemakers and cardioverter-defibrillators [111].

4. Final remarks and perspectives

Glycine-based piezoelectric and piezo/triboelectric nanogenerators offer a promising opportunity for advancing sustainable and biocompatible energy harvesting systems. Piezoelectric and triboelectric device development have undergone three different generations. In the first, materials like ZnO, PZT and BT were used, however drawbacks related to power output and flexibility hindered their application. Synthetic electroactive polymers like PVDF and Nylon 11 were then used due to their flexible nature, which allowed for energy harvesting from body movements. The third generation of piezoelectric and triboelectric devices explore synthetic biodegradable polymers and natural materials, like cellulose, chitosan and now glycine, however at the potential expense of output. One can envisage that, by using FDA-approved biodegradable polymers and tuning their degradation profiles for specific applications, these TENGs and PTENGs constructs can advance personalized medicine, bringing wearable or implantable devices closer to compliance and market readiness. Tuning glycine electromechanical properties through blending with other polymers or nanomaterials can improve strength, flexibility, and efficiency, enabling robust and high-performance nanogenerators suitable for diverse applications. These show remarkable electro-mechanical capacity, sometimes rivalling with ceramics and surpassing polymer based PENGs. Nevertheless, key challenges remained:

4.1. Polymorph stability and crystalline rigidity

While stable γ-GC and its integration into polymeric matrices using simple techniques such as solvent casting has been achieved, there are still major difficulties in achieving the same for β-GC. Given the promising DFT-calculated shear piezoelectricity of β-GC, innovative strategies must be developed to design efficient β-GC-based piezoelectric devices. The metastable and fragile nature of this polymorph, along with its tendency to transform into more stable forms − due to changes in ambient temperature, humidity, energy-intensive processes (e.g., ball milling), or interactions with different polymers – call for the development of processes that can stabilize it. Key manufacturing strategies can be considered to obtain this phase in a stable form:

• •
• •

The flexibility of biodegradable materials is often challenged by the need to maintain mechanical durability and stability. In the case of glycine crystals, their inherent rigidity is sometimes mentioned as an obstacle for its use on flexible devices. However, this can be easily surpassed by the careful optimization of glycine concentration within the polymeric matrices [111].

4.2. Device sterilization, device degradation and biocompatibility

Device sterilization is a crucial factor, particularly due to the polymorphic transitions glycine undergoes in aqueous environments, under pressure, and at elevated temperatures—conditions commonly encountered during sterilization. Therefore, future studies should include sterilization testing, prioritizing techniques suitable for heat-sensitive materials, such as low-temperature plasma, and those compatible with both heat- and moisture-sensitive compounds, like ethylene oxide sterilization. Conversely, methods such as dry heat sterilization and gamma irradiation should be avoided to establish standardized protocols that preserve polymorph stability.

While some glycine-based biodegradable implantable devices have demonstrated functional lifespan up to several weeks, the inherent fast glycine degradation in moist or biological environments may hinder performance for applications requiring functionality for several months. Thus, it is crucial to investigate the design of encapsulation layers that targets the intended application.

• •

For implantable devices, emphasis should be placed on eval‎uating degradation products, their impact on surrounding tissues, and long-term stability through histological and pathological assessments of individual organs. Specifically, while FDA does not hold glycine as GRAS (Generally Recognized As Safe), it approves its use as a food additive under specific FDA regulations, it permits its use as a nutrient in foods, with maximum glycine content of 3.5 % of total protein and daily intake limit of 6.5 g (21 CFR 172.320) and also as a flavouring agent or adjuvant (21 CFR 172.812). Moreover, glycine is FDA-approved as a drug for specific medical applications like parenteral nutrition solutions (e.g. Aminosyn II, Clinisol) and surgical irrigation (e.g., Glycine Irrigation Solution). Importantly, while the FDA does not approve dietary supplementation for safety or efficacy, glycine is marketed as a dietary supplement targeting conditions with low-grade inflammation, such as obesity [190]. Additionally, in humans, glycine (3 g) before bedtime has found to improve sleep quality, daytime sleepiness, and fatigue [191]. More positive effects of glycine administration in various physiological systems can be found. Only mild digestive symptoms (soft stool and abdominal pain) have occurred with bedtime ingestion of 9 g of glycine [192]. Conversely, non-degradable and wearable devices, on the other hand, should be examined for wear-and-tear effects, and interactions with the external environment [193], [194].

4.3. Electrical performance trade-offs

In the case of degradable glycine-based PTENGs, optimal electric performance requires a delicate balance between glycine crystallization and device biodegradability. The challenge lies in designing materials that degrade predictably without compromising the efficiency of the energy harvesting process.

Piezoelectric and triboelectric devices exploring conventional materials such as silicones, PVDF, PDMS, PTFE, and ZnO often outperform glycine-based devices. While achieving fully degradable devices requires certain trade-offs, advancements in β-GC-based systems and the optimization of existing γ-based nanogenerators offer promising pathways to enhance energy harvesting efficiency. Additionally, although inferior to ceramic based tribo- and piezoelectric devices, glycine-based ones surpass degradable-based nanogenerators and sometimes compete with traditional materials [Table 3].

Given β-GC excellent out-of-plane (d16) piezoelectric coefficient, future designs should exploit out-of-plane energy harvesting. So far, only a few systems have considered it for implantable devices [195]. In the human-body, a couple of movements are susceptible to inducing a shear actuation. These encompass elbow movement, blood flow, and heartbeat with the latter a delicate balance between in-plane and out-of-plane movements. To potentiate out-of-plane contribution, some strategies are suggested:

• •

Combination of materials, design factors and predictive strategies: So far, research on glycine has largely focused on its integration with polymers that offer chemically favourable cues to promote its crystallization into piezoelectric polymorphs. While this approach is effective for PENGs, triboelectric nanogenerators require more complex combinations due to the need for specific positive and negative triboelectric layers. As a result, material selection for TENGs demands more research on: i) materials suitable as counter triboelectric layer for glycine and ii) the effect of glycine in these systems. For that purpose, α-glycine could be used to study its role in triboelectric-based devices by excluding the piezoelectric contribution when using this specific polymorph.

Furthermore, there are a lot of strategies known to enhance triboelectricity that haven’t been applied to glycine-based TENGs. Specifically, increasing surface-to-volume ratio [197] which provides more charge trapping sites, increase film roughness [198] and surface functionalization [199] to further enhance surface charge or facilitate charge transfer [200], [201].

To optimize these systems, artificial intelligence, combined with DFT calculations and triboelectric simulations could be employed to predict the interactions between glycine and other material layers. Additionally, predictive modelling could provide insights into glycine crystallization behaviour and optimal device architectures, accelerating the development of more efficient glycine-based energy harvesters.

4.4. Future directions: Transient tribo/piezoelectric therapeutic nanogenerator

Overall, glycine natural biocompatibility makes it an excellent candidate for biomedical applications, ranging from safe implantable sensors, nanogenerators, actuators or even part of therapeutic drug delivery systems. In addition, the ecological nature of glycine and standardized synthesis methodologies also position the resulting devices as sustainable alternatives to conventional nanogenerators, answering to the increasing demand for green energy solution in low-power, disposable electronics. Notably, β-GC/PCL nanofibers have shown potential for integration within the blood–brain barrier, enabling targeted drug delivery to brain tissue [113]– an exciting prospect for neurological disorder treatments, within a fully biodegradable device.

Thus, the versatility of glycine-based materials also opens the door to composite strategies like the development of transient tribo/piezoelectric energy generators with therapeutic biodegradation. These devices address key limitations of existing technologies as they offer biodegradability and flexibility superior to ceramic-based nanogenerators, while also overcoming the low output power of polymer alternatives and introducing therapeutic benefits throughout their lifecycle. However, achieving fully functional and market-ready devices will require more than just glycine. Significant challenges remain, including the integration of FDA-approved polymers with well-characterized degradation profiles, the development of advanced designs to maximize output, further research on sterilization and device life cycle, and the establishment of standardized protocols. Addressing these hurdles is essential to advancing glycine-based devices toward personalized medicine and market-ready compliance.

CRediT authorship contribution statement

Luís Nascimento: Writing – review & editing, Writing – original draft, Conceptualization. Gavin Richardson: Writing – review & editing, Supervision. Priscila Melo: Writing – review & editing, Supervision. Nathalie Barroca: Writing – review & editing, Supervision, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Luís Nascimento thanks FCT for the Ph.D. grant (10.54499/2023.01401.BD). This work was supported by the project Flexobone 2022.02424.PTDC, supported by the Foundation for Science and Technology, in its State Budget component (OE) (DOI:10.54499/2022.02424.PTDC), and by the projects UIDB/00481/2020 and UIDP/00481/2020 - Fundação para a Ciência e a Tecnologia, DOI:10.54499/UIDB/00481/2020 and DOI:10.54499/UIDP/00481/2020.

Data availability

No data was used for the research described in the article.

References

1. [1]
2. [2]
3. [3]
4. [4]