Three-Dimensional Printed Hydrogels with High Elasticity, High Toughness, and Ionic Conductivity for Multifunctional Applications.

Hydrogels have drawn extensive attention due to their unique physical and biological properties. However, the relatively low mechanical strength and poor processability of hydrogels limit their applications. Especially, the emerging 3D printing technology for nontoxic hydrogels requires proper formability and controllable mechanical behaviors. In this study, a new strategy to construct a novel double-network biocompatible hydrogel from poly(ethylene glycol) diacrylate (PEGDA) and short-chain chitosan (CS) via ionic-covalent cross-linking is by a two-step method involving UV curing followed by immersion in an anionic solution. The CS-based ionic network and PEGDA-based covalent network as well as the hydrogen bonds between them jointly induce excellent mechanical properties, which can be regulated by changing the PEGDA/CS content and ionic cross-linking time. Compared with conventional hydrogels, this mechanically optimized hydrogel exhibits a superior elastic modulus (3.84 ± 0.4 MPa), higher tensile strength (7.23 ± 0.2 MPa), and higher tensile strain (162 ± 7%). Notably, its excellent printing capability through the citrate anionic solution adjustment enables 3D printing with precision, flexibility, and a complex inner structure by extrusion in air at room temperature. In addition, a number of citrate ions existed in the ionic network, giving the hydrogels good electrical conductivity. Therefore, this printable, conductive, and tough hydrogel exhibits potential for vascular engineering, cartilage tissue engineering, and wearable device applications.