Modulation of double-network hydrogels via seeding calcium carbonate microparticles for the engineering of ultrasensitive wearable sensors

Abstract

Double-network (DN) hydrogels with high strength and toughness have shown their potential for applications in materials science and biomedical engineering. Biocompatible sodium alginate (SA)/polyacrylamide (PAM) hydrogels are a promising class of DN hydrogels, which are typically cross-linked with calcium ions (Ca²⁺). However, the use of calcium salts typically induces structural inhomogeneity and reduces the mechanical properties of the resultant hydrogels, which limit their application in tissue scaffolds, actuators, and wearable devices. Herein, we fabricate a homogeneous polymer DN hydrogel by pre-seeding calcium carbonate (CaCO₃) microparticles into SA/PAM hydrogels, followed by the triggered release of Ca²⁺ from the microparticles in acidic solution. The acid-triggered cross-linking generates sacrificial ionic bonds capable of dissipating energy, which endows the Ca²⁺/SA/PAM DN hydrogels with high tensile strength (0.85 MPa), stretchability (1850%), and fracture toughness (6.4 MJ m⁻³). These properties can be easily adjusted by controlling the trigger time as well as the concentration of the CaCO₃ microparticles. In addition, the Ca²⁺/SA/PAM DN hydrogel exhibits high strain sensitivity with a gauge factor of ∼8.9, a wide strain detection range (0.03–1800%), and excellent durability (500 cycles at a strain of 50%), which can be used as a strain sensor to monitor human motions with a fast response (∼0.02 s). Furthermore, the Ca²⁺/SA/PAM DN hydrogel as a sensor can monitor the pain signal induced by an in situ cascade reaction at a wound site in a diabetic rat model. This study provides a controllable strategy to engineer stretchable and tough DN hydrogels for potential applications in flexible devices.