Preparation of gradient hydrogel for pressure sensing by combining freezing and directional diffusion processes

Gradient ionic gels have received extensive attention in the field of pressure sensing due to their gradient variation of structure and elastic moduli, which can achieve a broad-range pressure response. However, designing and fabricating gradient gels with good mechanical and sensing properties remains a challenge. In this study, a physically cross-linked PVA ionic hydrogel with gradient structure is successfully synthesized by a directional ion diffusion process in NaCl solution at low temperature (–15 °C). Along the direction of ion diffusion, the gradient gel exhibits a continuous change from dense to porous structure, which is attributed to the synergy of freezing and salt-out effect. Cyclic compression test proves the good fatigue resistance of the gradient gel. The gradient ionic gel is applicated as a flexible capacitive pressure sensor, which exhibits high sensitivity of 1.09 kPa−1 (<100 kPa) and a broad sensing range (400 Pa to 0.8 MPa), while the response times can achieve to 92 ms. The gradient gel sensor demonstrates the signal stability in a long-term loading-unloading process and independence to the test frequency, as well as high sensitivity and instantaneous response under repeated tiny compressive strains and water droplets. Moreover, the excellent sensing capability enable the gradient hydrogel to be a versatile platform for flexible wearable sensors to monitor subtle vital signal and human movement. This strategy provides an innovative route to fabricate gradient hydrogels and develops the applications in flexible pressure-responsive smart materials.