Protein Gel Phase Transition: Toward Superiorly Transparent and Hysteresis‐Free Wearable Electronics

Abstract

The next generation of wearable electronics for health monitoring, Internet‐of‐Things system, “interface‐on‐invisible,” and green energy harvesting require electrically conductive material that is superiorly transparent, negligibly hysteretic, industrially feasible, and highly stretchable. The practical potential of ionic hydrogel is challenged with obvious hysteresis and a limited sensing range due to relative delamination and viscoelastic performance. Herein, a novel liquid conductor, termed as egg white liquid, is developed from self‐liquidation of egg white hydrogel, and the liquid not only inherits the designed architecture from a hydrogel predecessor but also achieves comparable conductivity (20.4 S m−1) to the ionic hydrogel and ultrahigh transparency (up to 99.8%) . Moreover, the 3D‐printed liquid–elastomer hybrid exhibits excellent conformability, remarkable sensitivity with negligible hysteresis (0.77%), and the capability of monitoring human motions and dynamic moduli is further demonstrated. The liquid nature inspires a gesture‐controlled touchless user interface for front‐end electronic systems. Furthermore, mechanical energy harvesting and pressure sensing are evidenced by exploiting this liquid conductor into a triboelectric nanogenerator. Notably, the as‐prepared liquid via subsequent phase transition possessing superior transparency, ultralow hysteresis, economic benefit, and unique liquid phase may potentially fuel the development of a new class of wearable electronics, human–machine interface, and clean energy.

3D Printing of Hydrogels for Stretchable Ionotronic Devices

“…Xing and co-workers utilized a novel liquid conductor, the egg white liquid, as the rheological modifier to address such challenges. [169] The initial conductivity of the liquid-elastomer hybrids was 16.9 S m −1 , whereas it increased to 20.4 S m −1 after storage for 5 h. More importantly, the egg white liquid showed negligible influence on the transparency (99.8%) and mechanical hysteresis. To date, other materials, such as CNT, [170] PEDOT:PSS, [171] polypyrrole (PPy), [172] and hydrated salt, [173] were also embedded into the ink for improved conductivity of the printed hydrogels.…”

“…[206] Xing and co-workers utilized the egg white liquid (EWL) as a novel liquid conductor to print a liquid-elastomer hybrid sensor. [169] Sensing performances of the printed sensor could be tuned by controlling the internal dimensions of the EWL. Sensors with inner widths of 1, 2, and 3 mm exhibited electrical hysteresis of 12.13%, 4.21%, and 0.77%, respectively.…”

Wearable triboelectric sensors for biomedical monitoring and human-machine interface

“…The overwhelming majority of HMIs relies on tactile interactions ( Cao et al., 2018a ; Guo et al., 2020 ; Huang et al., 2020 ; Kang et al., 2019 ; Meng et al., 2018 ; Wu et al., 2018b , 2020 ; Xue et al., 2016 ; Yuan et al., 2017 ). From the traditional keyboards ( Ahmed et al., 2017 ; Chen et al., 2015 ; Jeon et al., 2018 ; Wang et al., 2018a ; Wu et al., 2018a ; Yang et al., 2013 ) and touch pads ( Chen et al., 2018 ; Dong et al., 2018 ; Shi et al., 2019a ) to the rising electronic skin ( Chang et al., 2020 ; Lai et al., 2016 , 2019 ; Wu et al., 2017 ), the tactile sensors are developed to be more flexible, sensitive, efficient, and multi-functional, even with human-like intelligence. In this part, six examples of TENG-based tactile sensors are reviewed: a high-resolution pressure-sensitive TS matrix for tactile mapping ( Wang et al., 2016 ); an elastic metal-free tactile sensor for detecting both normal and tangential forces ( Ren et al., 2018 ); a transparent and attachable ionic hydrogel-based pressure sensor for coded communication ( Lee et al., 2018 ); a flexible touch pad with subdivided units for tactile XY positioning ( Pu et al., 2020 ); a user-interactive electronic skin for touch track mapping based on the triboelectric-optical model ( Zhao et al., 2020 ); and a triboelectric tactile sensor producing various amplitudes of signals based on the history of pressure stimulations for mimicking neuromorphic functions of synaptic potentiation and memory ( Wu et al., 2020 ).…”

Recent Advances in Aptasensor for Cytokine Detection: A Review

“…Additionally, these human biofluids contain many cytokines, making them attractive samples [125]. Moreover, because a wearable sensor must be attached to the surface of the human body, the substrate of the sensor must be flexible, and the signal must be constant despite deformation [126].…”