Highly adhesive chitosan/poly(vinyl alcohol) hydrogels via the synergy of phytic acid and boric acid and their application as highly sensitive and widely linear strain sensors

In recent years, flexible strain sensors have attracted increasing interest, and accurate sensing and comfortable wearables are highly demanded. However, current flexible strain sensors fail to have wide linearity and high sensitivity simultaneously, and their adhesion is insufficient for convenient wear and precise motion monitoring. Herein, chitosan/poly(vinyl alcohol) hydrogels with phytic acid (PA) and boric acid (BA) as crosslinkers (CS/PVA-PA-BA hydrogels) were fabricated. The synergy of phytic acid and boric acid not only improved the mechanical properties of the obtained hydrogels (1070% of fracture strain and 0.83 MPa of fracture stress), but also provided them with outstandingly strong adhesion. Their adhesive strength was up to 527 kPa for a variety of materials, including glass, silica rubber, steel, polytetrafluoroethylene (PTFE), and skin. In addition, the hydrogel-based strain sensor demonstrated high sensitivity (gauge factor = 4.61), a wide linear strain range (up to 1000%, R-2 = 0.996), fast response time (90 ms), and good stability. A flexible strain sensor with such high sensitivity and wide linear range simultaneously, to the best of our knowledge, has never been reported before. The development of CS/PVA-PA-BA hydrogels is expected to inspire a novel method for high-adhesive and high-sensing-performance wearable electronics.