Nowadays, due to the rapid development of electronic technology, flexible wearable electronic devices are widely used in various intelligent applications, such as remote medical diagnosis, electronic skin, and human-machine interface. As the largest sensory organ in the human body, the skin has flexibility and stretchability, and is sensitive to various external signals such as tension, pressure, temperature, or humidity.The design and manufacturing of electronic skin that simulates the sensation and stimulus response of natural skin, inspired by medical needs and challenges posed by these unique characteristics, has attracted great attention from researchers around the world.Conductive hydrogels have been recognized as one of the most ideal materials for assembling flexible and wearable electronic skin due to their many promising properties, such as excellent flexibility and skin affinity, flexible function adjustability, reliable conductivity or ease of manufacture.

 Hydrogels are typical soft and moist materials with three-dimensional network structure, which are formed by cross-linking polymer chains through physical, ionic or covalent interactions. The hydrogel can accommodate a large amount of fluid while still maintaining stability and insolubility. Due to its good hydrophilicity, permeability and low friction coefficient, hydrogels have great potential applications in medical foam or sponge, soft tissue engineering, fluid absorbents and biosensor membranes, to name just a few examples related to medicine.Generally, hydrogels are mainly made of petroleum based synthetic polymers, such as polymethacrylic acid 2-hydroxyethyl ester (pHEMA), polyethylene glycol (PEG), polyvinyl alcohol (PVA), polyacrylic acid (PAA) and polyacrylamide (PAM). Among these synthetic polymers, PAA polymerized from acrylic acid (AA) not only has good biocompatibility, but also carries a large number of hydrophilic COOH groups, which can provide a cross-linked network rich in water for ionic conductive hydrogels, giving PAA based ionic conductive hydrogels high conductivity.Therefore, PAA based hydrogels are considered as ideal candidates for flexible electronic skin. However, pure PAA hydrogels usually have weak compressive strength and fatigue resistance, so they are often damaged or destroyed when a strong external compressive force is applied to them. This greatly suppresses their application as strain and pressure sensors for pressure detection, such as in the detection of human diseases, such as preliminary screening of patients with flat feet.In addition, the practical application of hydrogel e-skin may involve significantly changing thermal environment and hydration environment (such as cold, hot, dry or wet conditions), which poses a major challenge to hydrogel e-skin from the perspective of mechanics, function and human device integration [36].In addition, traditional conductive hydrogels containing a large amount of water often inevitably freeze at temperatures below zero, and lose flexibility in the icing state, which greatly limits the use of such hydrogels in cold environments. This also leads to the failure of the conductive hydrogel sensor to respond consistently to temperature stimuli.Therefore, it is very important to develop hydrogel e-skin that can respond to various stimuli below zero. In general, it is a major challenge for hydrogel e-skin to have good biocompatibility, sufficient strength, fatigue resistance, excellent conductivity and environmental compatibility at both low and high temperatures.

Cellulose nanofibers (CNFs) are an important type of nano cellulose with excellent biocompatibility. As a special additive, they have been widely used, especially to enhance the mechanical properties of hydrogels through their inherent high Young's modulus, high aspect ratio and high surface activity.In addition, in recent years, it is reported that CNFs also have the ability to act as effective dispersants in different systems, such as Pickering lotion, graphene dispersion and PEDOT.This inspired us that CNFs can actually play a dual role as dispersants (used to reduce the agglomeration of PAA polymer chains) and mechanical reinforcements (used to enhance the mechanical properties of AA based hydrogels). In addition, in order to improve the freezing tolerance, ionic compounds can be introduced into the hydrogel network as cryoprotectants. However, hydrogels based on ionic compounds (such as NaCl, LiCl and CaCl2) have some limitations. For example, due to the inevitable evaporation of water under dehydration conditions, they have poor stability during long-term use, which hinders their practical application.Compared with ionic compounds, glycerol (1,2,3-dipantriol) can form hydrogen bonds with water molecules and disrupt their natural arrangement, hindering the formation of ice crystals (i.e. inducing freezing at lower temperatures) and water evaporation (i.e. achieving dehydration at higher temperatures or for longer periods of time).The antifreeze effect of glycerol-H2O system has been proven in food preservation and automotive antifreeze formulations. It is reported that through the introduction of glycerol-H2O binary solvent system, ionic conductive organic hydrogels can maintain good mechanical and conductive (~8.2 S m − 1) properties in a wide temperature range (− 20 to 60 ° C).In addition, the organic hydrogel can be stored at 25 ° C for 30 days with minimal weight loss. This prompted us to use glycerol-H2O binary solvent in the PAA-CNFs system, which can not only maintain the high conductivity of the hydrogel, but also improve the antifreeze and water retention properties of the hydrogel.

Highlights

1. Based on the comprehensive strategy of the versatility of TEMPO oxidized CNFs (TOCNFs) and the interaction of metal ions, this work prepared an acrylic based conductive hydrogel with excellent mechanical and electrical properties.

2. TOCNFs with rich – COOH groups play a key role in hydrogels by a) dispersing polyacrylic acid (PAA) chains well in the hydrogel to form a uniform porous multi network, b) providing more – COOH and Fe3+interactions, c) forming more hydrogen bonds with PAA and glycerol, and d) providing high modulus for the final hydrogel.

3. The obtained optimal hydrogel shows competitive mechanical properties (0.88 MPa at 70% compression strain and 0.24 MPa at 873% tensile strain) and fatigue resistance (56.5% strength retention after 500 50% compression cycles and 51.7% strength retention after 20 200 °% tensile cycles), high conductivity (2.45 S m − 1) and sensitivity (GF is as high as 2.62 for tensile strains over 100 °%), while maintaining high conductivity (1.67 S m − 2) at − 25 ° C.

Source: Sensor Expert Network