Silk-based stretchable and functional devices

• Main Author: Chen, Geng
• Other Authors: Chen Xiaodong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Science::Biological sciences::Biochemistry
• Online Access: https://hdl.handle.net/10356/136954
• Institution: Nanyang Technological University

Silk fibroin is a type of biomaterials which attracted a lot of research efforts due to its nature biocompatibility and unique mechanical strength. The application of silk fibroin in flexible electronics has rendered excellent conformability, good flexibility and dielectric property. However, there is a lack of stretchable silk fibroin-based electronics which possess skin-like softness and stretchability, also the multifunctionality was not considered. In this thesis, silk fibroin has been modified into soft and stretchable by two methods. One is by addition of CaCl2 and the other is addition of glycerol. Both can realize high stretchability (>250%) and skin-like softness (< 4 MPa). In the first case, by adding various ratio of CaCl2, silk fibroin can obtain a tunable Young’s modulus under various relative humidity. This plasticization was further explained by molecular dynamics simulations which revealed that the CaCl2 and incorporated water would decrease the strength of crystallites and increase the ratio of extensible secondary structures. This plasticized silk fibroin films were further integrated with wrinkled gold surface which naturally formed at ambient environments. The feasibility of silk-based electrodes with high skin conformability can improve the signals fidelity of on-skin electrophysiological signals. Beside planar silk fibroin thin film, mesh-structured silk fibroin possesses high gas permeability and low evaporative resistant was fabricated. The nanomesh structures were prepared by electrospinning of silk solution and followed by a stabilization with glycerol, which equipped the fiber mats with solution processability. After that, a solution coating process of PEDOT: PSS with 15 vol% glycerol which plays the role to conduct electrical signals. The silk electrode is highly stretchable with good breathability (low thermal insulation, low evaporative resistance, and high water-vapor transmission rate) compared with commercial gel and polymer electrodes. In sweaty status after exercise, our silk electrodes show better electrocardiography (ECG) signals quality than commercial gel electrodes, and they did not disturb the heat dissipation during sweat evaporation. Furthermore, although on-skin electrodes have been realized based on plasticization of silk fibroin and mesh structures, there is missing point of multifunctionality. A mechanically heterogeneous silk fibroin film was proposed to act as the integration platform for hybrid-integrated sensors so that the multifunctionality can be obtained based on traditional complementary metal oxide semiconductor (CMOS) technology. This mechanically heterogeneous silk film was achieved based on the plasticized silk fibroin and selective methanol treatments at specific patterns. The contrast of modulus can be tuned by varying the ratio of CaCl2 as well. Stretchable electrodes/circuits (>120%) were screen printed at the soft region and rigid active devices were placed at the rigid region. Both experimental results and finite-element analysis (FEA) proved that the mechanically heterogeneous silk fibroin film can protect the rigid devices from mechanical failure, thus improving the stretchability of whole system (>50%). Based on this, a wireless powered sensor and passive RFID were integrated to achieve communication and data processing functions, which is beyond single sensor unit. To conclude, silk fibroin-based stretchable electronics pave the way for next-generation sustainable and environment-friendly electronics.