Silk fibroin-based stretchable electronics

• Main Author: Cui, Yajing
• Other Authors: Chen, Xiaodong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/146461
• Institution: Nanyang Technological University

In this thesis, silk as an ancient commercial textile material has been successfully rendered with properties that meet the requirements for on-skin and implantable bioelectronics applications. Previously, instant water solubility, limited stretchability, and lack of suitable electronics fabrication methods are main factors that hindered the stable and wide application of silk material in soft bioelectronics. This thesis covers a complete process spanning from material modification, device fabrication to practical application of devices. Thin film silk material with aqueously stable, soft and stretchable properties are obtained by PEGylation method. Dual effect of newly introduced crosslinks by poly(ethylene glycol) diglycidyl ether and softening effect of water contribute to aqueous softness and stretchability. Besides, physical interaction of PEG chain with silk fibroin promotes the stability of silk material in bioenvironments. The Young’s modulus of PEGylated silk was down to 1.51 MPa and stretchability can reach up to > 400%. Apart from softness and stretchability, self-healing smartness that mimic the real biological tissues is also important for durable and sustainable silk material towards potential damage. Crosslinks and dynamic hydrogen bonds introduced to silk molecular chain provide enhanced elasticity as well as self-healing property. Rapid self-healing property with 100% healing efficiency (<10 min) and high elastic stretchability (>200%) was realized in silk materials, while many of previously reported self-healing polymers require external stimuli (heat, light etc.) and longer time (>48 h) to reach high healing efficiency (>90%). Exposed in ambient environment, skin is susceptible to abrasion, sharp objects and etc. Self-healing silk that can heal itself from damage is thus highly suitable for on-skin electronics application. The integration of conductive material can be accomplished by simple thermal deposition of gold thin film. The as-prepared electrodes exhibit conformal contact with skin with a stretchability of 120% and a heal-after-cut stretchability to 60%, which is enough for on-skin application. EMG measurements demonstrate comparable signal-to-noise ratio between self-healing silk-based electrode and commercial gel electrode. While in implantable bioelectronics application, the necessity to introduce more modality such as optical modality for precise characterization and modulation of neuronal activity, makes transparent PEGylated SF a competitive material option. As such, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) was robustly integrated with PEGylated SF by simply pouring as-reacted silk fibroin solution on PEDOT:PSS layer for the formation of interpenetrated layer. The transparency of the as-fabricated electrodes maintains at > 90 % in UV-Vis region, which facilitates simultaneous usage of optical technique. It also shows stretchability >200%, long-term electronic performance stability in PBS (4 months) as well as good cyclic stability. The bioelectrode was applied to photothrombosis model, where acute infarction is induced underneath electrode and spatially gradient ischemia are recorded to provide information for stroke prediction. Additionally, negligible delay between electrical stimulation and cerebral vessel oxygenation is observed with no obstruction to imaging cerebral perfusion compared to gold electrodes. In conclusion, this thesis leverages the versatility of natural biomaterials and their application in bioelectronics as a substitute to the elastomer counterpart. The thin-film format of silk materials with novel properties can also pave the way for a variety of functional next-generation bioelectronics.