Multifunctional roles of TEMPO-oxidized cellulose nanofibrils on the enhancement of mechanical and conductive properties of acrylic-based hydrogels for temperature response and human motion sensing

Simultaneously having competitive mechanical properties, fatigue-resistant stability, temperature adaptability, excellent conductivity and sensitivity has been a challenge in the design of acrylic-based conductive hydrogels if they are meant for application in electronic skin mimics capable of sensory and stimulus responses. In this work, an acrylic-based conductive hydrogel with both excellent mechanical and electrical properties was prepared based on an integrated strategy employing the multifunctionality of TEMPO-oxidized CNFs (TOCNFs) and metal ion interaction. The TOCNFs with their abundant –COOH groups played a key role in the hydrogel by a) well-dispersing polyacrylic acid (PAA) chains in the hydrogel to form a homogeneous and porous multi-network, b) furnishing more –COOH to interact with Fe3+, c) forming more hydrogen bonds with PAA and glycerol, and d) offering high modulus to the final hydrogel. The obtained optimal hydrogel showed competitive mechanical properties (0.88 MPa at 70 % compressive strain; 0.24 MPa at 873 % tensile strain) and fatigue-resistant properties (56.5 % strength retention after 500 cycles of 50 % compression; 51.7 % strength retention after 20 cycles of 200 % tensile), high electrical conductivity (2.45 S m−1) and sensitivity (GF up to 2.62 for tensile strains over 100 %), while still maintaining a high electrical conductivity (1.67 S m−1) at −25 °C. Accordingly, the hydrogel prepared in this work was able to act as an electronic skin that responds to temperature variations and detect human body movement of pressing, writing, stretching and bending with highly sensitive conductive signals, which endows it great potential for applications in humanoid robotics and multi-scenario strain sensing.