A high-performance pressure sensor enabled by a sodium alginate bilayer hydrogel via tailored unit ring-opening

Inspired by human skin, gradient-structured hydrogels are critical for converting mechanical stimuli into precise electrical signals. However, designing such hydrogels remains challenging. This study presents a novel bilayer hydrogel fabricated via a layer-by-layer strategy using two forms of sodium alginate (SA): unmodified SA in the bottom layer acts as crosslinking reinforcement, due to its rigid β-D-mannuronic acid (M) and α-L-guluronic acid (G) units, imparting high toughness and modulus; whereas oxidized SA (OSA) in the top layer undergoes ring opening, increasing its softness and flexibility. The resulting hydrogel functions as a high-performance motion sensor, featuring a simple fabrication process, high sensitivity (46.3 Pa^-1, 0-1 kPa), broad sensing range (up to 120 kPa), excellent stability (1000 cycles at 40 % strain), and programmable design. Importantly, both sensitivity and detection range can be precisely tuned by adjusting the oxidation degree of OSA. This work provides a feasible strategy for designing gradient hydrogels and opens new avenues for advanced sensing applications.