Nonswellable and highly sensitive hydrogel for underwater sensing

Because of the serious swelling issue and the lack of regular conductive channels, designing underwater sensors from conductive hydrogel (CHs) with high sensitivity remains challenging. Herein, an anisotropic conductive hydrogel (A-PCT-P) was obtained from polyvinyl alcohol (PVA), carbon nanotubes (CNTs), and tannic (TA) by a four-step strategy to address this issue. First, the PVA aqueous solutions were frozen-thawed to form precursor hydrogels crosslinked by crystalline regions. Next, the precursor hydrogels were pre-stretched at 60 °C to form regular conductive channels. Later, the pre-stretched hydrogels were frozen at refrigerator to fix the orientation structures. Finally, the dialysis process promoted the hydrogel to reach swelling equilibrium. Owing to the synergy of these strategies, the obtained hydrogel displayed excellent swelling-resistance. It barely expanded after immersing in deionized water with stable mechanical performances and volume. Meanwhile, the hydrogel-based sensor showed high sensitivity (gauge factor of 69.3), due to the CNTs and ordered structures. Remarkably, there was no baseline drift even after 1000 stretch-release repeats. All these advantages have promising applications in underwater sensing.