Transparent, stretchable and degradable protein electronic skin for biomechanical energy scavenging and wireless sensing

https://doi.org/10.1016/j.bios.2020.112567Get rights and content

Highlights

  • Conductive and ultrastretchable (520%) silk protein film was prepared.

  • Hollow silver nanofibers were constructed on silk film to enhance conductivity.

  • Bio-TENG was air permeable, biocompatible, stretchable and degradable.

  • Bio-TENG was applied for energy harvesting and wirelessly controlling Internet of Things.

Abstract

Self-powered flexible sensors play an increasingly important role in wearable and even implantable electronic devices. Silk protein is an ideal material for flexible sensors because of its terrific biocompatibility and controllable degradation rate. Here, we overcome the problem of mechanical flexibility and poor electrical conductivity of proteins, and develop a highly transparent, biocompatible, full-degradable and flexible triboelectric nanogenerator (Bio-TENG) for energy harvesting and wireless sensing. First, the mechanical flexibility of the silk protein film is greatly enhanced by the mesoscopic functionalization of regenerated silk fibroin (RSF) via adding glycerol and polyurethane (PU). Second, hollow silver nanofibers are constructed on the silk film to form an air-permeable, stretchable, biocompatible and degradable thin layer and utilized as friction electrode. The obtained Bio-TENG demonstrates high transparency (83% by one Ag gird layer), stretchability (Ɛ = 520%) and an instantaneous peak power density of 0.8 W m−2 that can drive wearable electronics. Besides, the Bio-TENG can work as artificial electronic skin for touch/pressure perception, and also for wirelessly controlling Internet of Things as a switch.

Introduction

The evolution of Internet of Things (IoT) technology has vastly promoted electronic equipment and sensors flourished in our life. Inevitably, majorities of them are powered with batteries that need to be frequently charged or regularly replaced (Liu et al. 2017b, 2018a; Xu et al., 2019; Zhang et al., 2020b). Triboelectric nanogenerators (TENGs) can harvest energy through human movement efficiently (Chen et al. 2016, 2020; Chen and Wang, 2017; Jin et al., 2020; Pu et al., 2018; Yan et al., 2020), which can provide feasible options for powering electronics (Jiang et al., 2018), wound healing (Long et al., 2018), health care (Lin et al., 2017; Yang et al., 2015), visualization (Zhang et al., 2020a), intelligent interaction (Cao et al., 2018; Shi and Lee, 2019) and sensors (Chu et al., 2020; Guo et al., 2018; Meng et al., 2020; Su et al., 2020; Zhang et al. 2017, 2018; Zhou et al., 2020b), such as wearable devices (Bai et al., 2014; Chen et al., 2018c; Dong et al., 2018; Lin et al., 2018; Meng et al., 2019; Pu et al., 2017a; Zhou et al., 2020a), implantable devices (Ouyang et al., 2019; Zheng et al., 2014) and tactile sensors (Chen et al., 2015; Lee et al., 2018; Wu et al., 2018; Yuan et al., 2017).

To achieve long-term monitoring of human health, sensors are generally required to be close-fitting to human bodies, and thus mechanical flexibility and skin comfort are significant for on-skin TENGs (Chen et al. 2017, 2018b). Synthetic polymers, such as polyethylene terephthalate (PET), Polydimethylsiloxane (PDMS) and polyimide (PI), are often used in flexible TENGs due to their flexibility and high cost-effective (Chen et al., 2018a; Yu and Wang, 2016). However, some issues remain unresolved for such polymer-based substrates. For example, most of these polymers are nondegradable that they would produce a lot of electronic waste. More importantly, for in vivo applications, the patients have to undergo a second operation to remove the devices. In addition, most of the polymer-based TENGs are neither air/water permeable nor biocompatible, suggesting they are not suitable for long-term pasting on human skin.

Natural, biocompatible (Parida et al., 2019) and degradable materials (Wu et al., 2019) are widely used to fabricate electrification layers, such as chitosan (Wang et al., 2018), cellulose (Cui et al., 2017), starch (Ccorahua et al., 2019) and protein (Hou et al., 2019; Liu et al., 2017a). As a natural, biocompatible and degradable material, silk fibroin (SF) protein has a strong ability to lose electrons in the triboelectric series (Kim et al., 2016). Besides, flexible, transparent, and controllable degradation properties enable its potential applications (Gogurla et al., 2019), especially for flexible, stretchable, and transparent wearable electronic devices in micro-systems (Pu et al., 2017b). Nevertheless, the applications of pure SFF in electronic devices remain a significant challenge due to its brittleness. Herein, the breakthrough of this challenge will generate a ready craft for large-scale production of TENGs.

In this study, two strategies were adopted to fabricate transparent, stretchable and degradable protein TENG. First, the mechanical flexibility of the silk protein film was markedly enhanced (strain Ɛ = 520%) by mesoscopic doping of RSF via adding glycerol and PU. Second, hollow silver nanofibers were constructed on the silk film to form the layer of air-permeable, stretchable, biocompatible and degradable friction electrode. In addition to the advantages of SF and hollow silver, the devices demonstrate superior biocompatibility in cell culture tests. Moreover, the devices can be completely and controllably degraded in papain and acetic acid. Attributed to the aforementioned excellent performance, the devices can act as electronic skins pasting on the human body to wirelessly control the electronic equipment, such as unmanned aerial vehicles, home appliances and robots.

Section snippets

Materials

All chemical reagents were in analytical grade and used as received without further treatment. Glycerol, ethyl alcohol, and sodium carbonate (Na2CO3) were purchased from Sinopharm Chemical Reagent Co., Ltd; polyurethane (PU) was obtained from EZ·Brush Corporation and lithium bromide (LiBr) was purchased from Aladdin Industrial Corporation. Bombyx mori silkworm cocoons were bought from Guangxi Sericulture Technology Co., Ltd.

Fabrication of the silk fibroin film (SFF)

The silk fibroin solution was prepared using the popular Alkali

Preparation of the SFF and property characterization

According to the previous reports, the secondary structures of silk fibroin mainly consist of α-helix, β-sheet and random coil (Nguyen et al., 2015). Specifically, a few β-sheet subunits in proximity can stack with each other and transform to β-crystallites during the vaporization process. Here, the percentages of the secondary structures were calculated by fitting the FTIR spectra, ranging from 1575 to 1725 cm−1 (Fig. 1c and Fig. S3). The calculated result shows that the percentage of α-helix,

Conclusions

In summary, we have fabricated a flexible, transparent, stretchable and biocompatible STENG from conductive SFF, and used it as electronic skin for biomechanical energy collecting and tactile sensing. The STENG consists of Ag/NFs as its flexible conductive electrode and SFF as a triboelectrification layer. The device demonstrates high transparency (83% for one Ag gird layer), high stretchability (up to λ = 6.2 or strain Ɛ = 520%), and can light more than 25 commercial LED bulbs per square

CRediT authorship contribution statement

Hao Gong: Investigation, Visualization, Writing - original draft. Zijie Xu: Investigation, Visualization. Yun Yang: Cell experiment Technique support. Qingchi Xu: Technique support. Xuyi Li: Film Technique support. Xing Cheng: Drone technique support. Yaoran Huang: Drone technique support. Fan Zhang: Investigation. Jizhong Zhao: Investigation. Shengyou Li: Investigation. Xiangyang Liu: Resources. Qiaoling Huang: Conceptualization, Supervision, Discussion, Writing - review & editing. Wenxi Guo:

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.

Acknowledgments

This work was supported by the Shenzhen Basic Research Program (JCYJ20180306173007696), National Natural Science Foundation of China (11904301), Natural Science Foundation of Fujian Province of China2016J05135, 2015J01557)the Fundamental Research Funds for the Central Universities of China (No. 20720180013), the Opening Foundation of State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University (201704). NUS AcRF Tier 1 (R-144-000-367-112), the “111” Project (B16029) and the

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