Mechanically robust and highly conductive bacterial cellulose hydrogels through synergy of directional freeze–thawing and salting-out for wearable sensors

Abstract

It is urgent to develop high-performance conductive hydrogels that simultaneously exhibit high biocompatibility, mechanical robustness, exceptional conductivity and frost resistance to fulfil the requirements of wearable flexible sensors. Herein, we present a simple and integrated approach for designing iso/anisotropic bacterial cellulose (BC) hydrogels by immersing the as-prepared polyvinyl alcohol (PVA)–BC–Borax hydrogels in saturated NaCl solutions using a salting-out effect to assist the non–/directional freeze–thawing strategy. Benefitting from the synergistic strengthening of molecular interactions, including hydrogen, coordination, and boron ester bonds, as well as the enhanced PVA and BC crystallinity resulting from the salting-out effect, the tensile strength (0.8–3.2 MPa) and high ionic conductivity (61.5–136.2 mS/cm) of isotropic hydrogels (PVA–BC–Borax/NaCl) can be well tuned by adjusting the feeding mass fractions of PVA, borax and BC. The optimised hydrogel, PVA7.5%–BC0.45%–Borax0.375%/NaCl, demonstrated remarkable mechanical properties (3.2 MPa, 295.1 %), ionic conductivity (72.6 mS/cm) and good anti-freezing properties (–40.8 °C), maintaining excellent conductivity (61.8 mS/cm) even at –20 °C due to the introduction of NaCl. When subjected to directional freeze–thawing cycles, the D(Directional)-PVA7.5%–BC0.45%–Borax0.375%/NaCl hydrogel, with its organised layers and larger pore–sized structures, exhibited higher breaking elongation (609.2 %) and ionic conductivity (81.6 mS/cm) than its isotropic counterpart (295.1 %, 72.6 mS/cm). The D-PVA7.5%–BC0.45%–Borax0.375%/NaCl hydrogel was successfully used as a strain sensor for detecting a variety of human movements, showing excellent responsiveness to external stimuli, thereby demonstrating its potential applications in human–machine interaction, flexible sensing and bioelectronics.