Highly Robust, Sensitive, Antifreezing, and Drying-Tolerant Polyacrylamide/Gelatin/Zr⁴⁺ Hydrogels as Flexible Strain Sensors

Abstract

The immersion method in which metal ions were used to toughen macromolecular chains to avoid the failure of hydrogel synthesis due to the rapid formation of coordination bonds in the gel precursor was often used to develop a robust gel. However, the uneven distribution of ions inside the hydrogel was harmful for its sensing performance by soaking. Herein, a robust polyacrylamide/gelatin/ZrOCl 2 •8H 2 O (PG-Zr 4+ ) hydrogel was developed by a straightforward strategy to toughen gelatin chains with Zr 4+ . The PG-Zr 4+ hydrogel held not only a high transparency (73%), robust fracture strength (0.842 MPa), and outstanding toughness (4 MJ m −3 ) but also fast self-recovery and satisfactory conductivity. Furthermore, the crystallization peak of the PG-Zr 4+ hydrogel disappeared at −60 to 25 °C with the introduction of dimethyl sulfoxide (DMSO) as a cryoprotectant; it could still exhibit stable electrical responses to weak stimuli of 1% strain at −20 °C for 24 h. Surprisingly, when simply assembled into a strain sensor with a gauge factor (GF) of 5.817, it was capable of precise real-time monitoring of both subtle pronunciation signals and drastic joint movements. This work opened the door to develop flexible wearable devices with multiple advantages.

A Facile Strategy to Fabricate Tough and Adhesive Elastomers by In Situ Formation of Coordination Complexes as Physical Crosslinks

“…[29][30][31] This strategy has been applied by other groups to develop hydrogels with high strength and toughness. [32][33][34] Although these hydrogels will become rigid and fragile plastics after evaporating the solvent, they can be converted to elastomers after incorporating segments with a low glass transition temperature (T g ) as the majority of the matrix. Therefore, we envision to develop tough elastomers by direct solvent evaporation of metallosupramolecular hydrogels.…”

Janus-type ionic conductive gels based on poly(N,N-dimethyl)acrylamide for strain/pressure sensors

“…Inspired by the preparation of Janus-type gels using the coating diffusion method, we assume that if metal ions that can form coordination bonds with the polymer network are introduced into the gel, the metal-coupling interaction between the polymer network and the metal ions can not only weaken the adhesion between the polymer network and the solid substrate but also improve the mechanical properties and electrical conductivity of the gel. 27 However, the d-d transitions of most metal ions, such as Fe(III) and Cu(II), 28,29 usually make the gel show an obvious color, and Fe(III) is easily converted into Fe(II) with a poor coordination ability under redox conditions, which reduces the mechanical properties of the gel. 30,31 As a transition element with stable valency, high charge density, and relatively small radius (0.72 Å), Zr(IV) contains multiple empty electron orbitals and can form stable coordination bonds with polymer networks.…”