Shutters-Inspired metal ions coordination hydrogel Strain/Pressure sensor for joint behavior evaluation and flatfeet correction

The metal ions coordination hydrogels (MI-HG) that polymer gels serve as flexible substrates and metal ions as conductive media have emerged as flexible electronic sensor materials. Nevertheless, fabricating MI-HG by a traditional soaking method is difficult to guarantee the good permeation of metal ions, manifesting as the "core–shell" structure of MI-HG, which undoubtedly reduces their mechanical property, conductivity, and sensitivity. Inspired by the shutter opening and closing regulation mechanism, a strategy that completely opening the backbone layers of polymer gels facilitates metal ions permeation thus improving the whole performance of MI-HG, especially the mechanical property is proposed. Herein, poly(vinyl alcohol)(poly(acrylic acid)-polyacrylamide) (PVA(PAA-PAM)) is used as the typical polymer hydrogel (HG), trivalent aluminum ion (Al3+) as the metal salt, and N,N'-methylenebisacrylamide (MBAA) as the important regulation 'rod', fabricating a series of aluminum coordination hydrogels (Al-HGx%). The results indicate that the mechanical performances of Al-HG2% are significantly improved. For example, the toughness of Al-HG2% reaches 996.12 KJ/m3, approximately three times that of the Al-HG0% sample (354.45 KJ/m3). The combination of structural characterization as well as finite element analysis, the mechanism for the enhanced mechanical performances based on shutter's completely open effect is elucidated, the product of which (Al-HG2%) has potential application in flexible strain/pressure sensor for joint behavior evaluation and flatfeet correction.