Intrinsically Conductive Bifunctional Nanocellulose-Reinforced Robust and Self-Healable Electronic Skin: Deep Insights into Multiple Bonding Network, Property Reinforcement, and Sensing Mechanism

Abstract

The rapid development of intelligent electronic skin with skin-like protein structures and noninvasive adhesion to the skin has attracted attention in the field of wearable electronics. However, poor mechanical strength, narrow sensing range, and low adhesion hinder its practical applications. Herein, a multifunctional composite sensor (SCGC) was achieved by incorporating an intrinsically conductive bifunctional nanocellulose (CNFene) into a Ca 2+ /glycerin (GL)-modified silk fibroin (SF) matrix. The SCGC film showed robust mechanical strength (tensile strength = 76.1 MPa, elongation at break = 144.2%), wide sensing range, and excellent self-adhesion ability (strong adhesion at low temperatures even in liquid nitrogen). Especially, the bifunctional nanocellulosereinforced electronic skin can sensitively detect small-and largescale human motion and has high-temperature change sensitivity (the temperature sensitivity = −5.2%/°C) due to multiple interactions of the double hydrogen-bonding network and calcium ion chelation among SF/CNFene/GL/Ca 2+ . Therefore, this strategy has potential applications in flexible electrodes, biomimetic sensors, and temperature biosensors.

Highly Sensitive, Self-Healable, and Robust Polyvinyl Alcohol/Nanocellulose Composite Electronic Skin by Single-Step Marangoni Self-Assembly for Multipoint Sensing

“…(a) ( R – R 0 )/ R 0 change of PVA/CP 40 under different strains, (b) mechanism illustration of change of conductive paths in PVA/CP 40 at different states, (c) comparison of GF with this work: CNCs–polyanilin/polyvinyl alcohol (CNC–PANI/PVA), poly [2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl), and acrylamide ionogel fibers [P­(SBMA- co -AM) IGFs], silkworm cocoon glycerin-2% cellulose nanofiber graphene (SCGC-2% CNFene), polyacrylamide/carboxymethylcellulose (PAAm/CMC), cellulose nanofiber pyrrole iron­(III)/polyvinyl alcohol (CNF Fe /PVA), 0.8 wt % multi-branched CNCs/polyvinyl alcohol (0.8 wt % MPVA/CNC), CNCs/methyl methacrylate/polyvinyl alcohol (CNC/MMA/PVA), thermoplastic elastomer and carbon nanotubes film (TPE/CNTs film), silk-crosslinked polyelectrolyte hydrogels (SCPEHs), conductive silk fibroin hydrogel (CSFH), and thermoplastic polyurethane-0.6 wt % graphene (TPU-0.6G), (d) response time and relaxation time of PVA/CP 40 , and (e) ( R – R 0 )/ R 0 change of PVA/CP 40 with 1600 stretching–releasing cycles from 0 to 40% at 40%/s, and the insets exhibited enlarged images of 256–265, 748–757, and 1460–1469 cycles, respectively.…”

“…Among them, the insets showed that the response time and relaxation time of the composite film under instantaneous tension were 172 and 183 ms, respectively. This demonstrated when the film was stimulated by rapidly changing external motion, the response signal could be quickly and accurately captured, which was 54 silkworm cocoon glycerin-2% cellulose nanofiber graphene (SCGC-2% CNFene), 6 polyacrylamide/carboxymethylcellulose (PAAm/CMC), 55 cellulose nanofiber pyrrole iron(III)/polyvinyl alcohol (CNF Fe /PVA), 29 0.8 wt % multi-branched CNCs/polyvinyl alcohol (0.8 wt % MPVA/CNC), 33 CNCs/methyl methacrylate/polyvinyl alcohol (CNC/MMA/PVA), 56 thermoplastic elastomer and carbon nanotubes film (TPE/CNTs film), 57 silk-crosslinked polyelectrolyte hydrogels (SCPEHs), 58 conductive silk fibroin hydrogel (CSFH), 59 and thermoplastic polyurethane-0.6 wt % graphene (TPU-0.6G), 60 crucial for practical applications. In order to further evaluate the long-term dynamic stability and durability of PVA/CP 40 , it was subjected to 1600 stretching−releasing cycles of 40% strain at a rate of 40%/s (Figure 5e).…”

“…Electronic skin (E-skin), with the similar features and functions of natural skin including stretchability, self-healing, tactile sensitivity, multiple sensing, and intelligent barrier, − has received increasing attention and been widely used in wearable sensing devices, , health monitoring, − bionic prostheses, soft robotics, , and human–machine interfaces. , E-skin needs to fit seamlessly with human skin or other irregular shapes, and will be exposed to various long-term stresses, converting external stimuli such as temperature, humidity, pressure, and tension into regular electrical signals. Therefore, the current E-skin mainly contained multi-functional composite gel and film with self-healing, biocompatibility, and tactile sensing ability. − Compared with the general problems of gel (water loss and fragility), the film based E-skin has been believed as an ideal sensing material.…”

Degradable Elastomeric Silk Biomaterial for Flexible Bioelectronics

“…56 The addition of CaCl 2 in plasticized silk films has also been shown to decrease Young's modulus with the increasing CaCl 2 concentration and increase elongation at break. 57 Molecular simulations demonstrated that while Ca ions had a negligible effect on hydrogen bonds in the silk matrix and secondary structure, the increased water uptake in silk films due to plasticizers decreased the crystalline strength while introducing structures conducive to stretchability. 36 Incorporating methacryloxypropyl-terminated PDMS (P-PDMS) with PF not only doubles tensile strength but also increases film brittleness (note that no plasticizer is used).…”

Novel Lignin Hydrogel Sensors with Antiswelling, Antifreezing, and Anticreep Properties

“…Flexible wearable electronic devices have attracted a significant amount of interest in the fields of human motion detection, health monitoring, electronic skins, flexible sensors, and soft robotics because of their large mechanical flexibility and their ability to adapt to different working environments to a certain extent and meet different deformation requirements. − Among a wide variety of sensing materials, hydrogels show great application potential due to their functionality, formability, and chemical editability. − Although the application value of hydrogel sensors in detecting body movement and vital signs has been proven, their practical application is still significantly limited, which is mainly caused by the inherent properties of hydrogels. − The first inherent property that hinders the practical application of hydrogels as flexible wearable electronic devices is swelling behavior. Hydrogel is a kind of soft material composed of a three-dimensional polymer framework with high water content. − For obtaining good biocompatibility, this three-dimensional polymer framework is generally designed as hydrophilic polymer segments, which will lead to the swelling behavior of the hydrogel in the aqueous environment. − Swelling behavior is usually accompanied by the introduction of a large number of solvent substances and the dilution or loss of functional molecules. − Meanwhile, the decrease of hydrogel density caused by swelling behavior also has a significant negative impact on the mechanical properties of hydrogels. − The second inherent property is the low-temperature phase transition behavior of hydrogels.…”