Fe3+-induced oxidation and coordination cross-linking in catechol–chitosan (CCS) hydrogels under acidic pH conditions; EDTA was used to disintegrate the coordination cross-linking system.

The development of new biomaterial inks with good structural formability and mechanical strength is critical to the fabrication of 3D tissue engineering scaffolds. For extrusion-based 3D printing, the resulting 3D constructs are essentially a sequential assembly of 1D filaments into 3D constructs. Inspired by this process, this paper reports the recent study on 3D printing of nanoclay-incorporated double-network (NIDN) hydrogels for the fabrication of 1D filaments and 3D constructs without extra assistance of support bath. The frequently used "house-of-cards" architectures formed by nanoclay are disintegrated in the NIDN hydrogels. However, nanoclay can act as physical crosslinkers to interact with polymer chains of methacrylated hyaluronic acid (HAMA) and alginate (Alg), which endows the hydrogel precursors with good structural formability. Various straight filaments, spring-like loops, and complex 3D constructs with high shape-fidelity and good mechanical strength are fabricated successfully. In addition, the NIDN hydrogel system can easily be transformed into a new type of magnetic responsive hydrogel used for 3D printing. The NIDN hydrogels also supported the growth of bone marrow mesenchymal stem cells and displayed potential calvarial defect repair functions.

As promising candidates for tissue engineering, hydrogels possess great potential, especially in bioadhesives and load-bearing tissue scaffolds. However, a strategy for synthesizing hydrogels that could achieve the above requirements remains a challenge. Here, a mussel-inspired naturally derived double-network (DN) hydrogel composed of a special combination of two well-characterized natural polymers, hyaluronic acid and alginate, is presented. The key features are its two-step synthesis strategy, which generates injectable and adhesive properties in the first step and then transforms into a DN hydrogel with high mechanical strength and good resilient properties. Based on this strategy, the DN hydrogel could be tamed into a self-supporting three-dimensional (3D) printable bioink. As a rheological modifier, alginate was used to lubricate the covalent cross-linking hydrogels for better extrusion performance. The incorporation of alginate also enhanced the mechanical performance of the soft covalent network by forming reversible alginate–Ca2+ ionic cross-links, which interpenetrate through the outer water-retention scaffold with delicate weblike structures. In vitro cell culture data indicated that our bioink formulation and printing strategy are compatible with human umbilical vein endothelial cells (HUVECs).

Compared with hydrogel-like biological tissues such as cartilage, muscles, and blood vessels, current hyaluronic acid hydrogels often suffer from poor toughness and limited self-healing properties. Herein, a facile and generalizable strategy inspired by mussel cuticles is presented to fabricate tough and self-healing double-network hyaluronic acid hydrogels. These hydrogels are composed of ductile, reversible Fe 3+ -catechol interaction primary networks, and secondarily formed brittle, irreversible covalent networks. Based on this design strategy, the hyaluronic acid hydrogels are demonstrated to exhibit reinforced mechanical strength while maintaining a rapid self-healing property. In addition, by simply regulating pH or UV irradiation time, the mechanical properties of the hydrogels can be regulated conveniently through variations between the primary and secondary networks.