The self-healing hydrogel and conductive hydrogel have attracted extensive attention in tissue engineering. The selfhealing hydrogel can restore its original structure and functionality after damage. The conductive hydrogel is beneficial to the differentiation and proliferation of electrical-stimuli-responsive cells. It is significant to integrate the self-healing ability and the electrical conductivity into a single hydrogel system. Herein we present polypyrrole-grafted gelatin-based hydrogels with combined conductive, self-healing and injectable properties. Methacrylic anhydride was first grafted onto gelatin to form double-bond-functionalized gelatin. Then, the commonly used conductive polymer polypyrrole was grafted onto gelatin by reacting with the double bond. Finally, the polypyrrole-grafted gelatin was mixed with ferric ions to construct the hydrogels. As revealed by the results, the hydrogels possess good conductivity owing to the incorporated polypyrrole and ferric ions. The reversible ionic interactions of ferric ions with gelatin and polypyrrole endow the hydrogels with selfhealing abilities. It is interesting that the hydrogels exhibit good injectable properties attributed to their self-healing abilities. Moreover, the hydrogels show a controllable porous structure, an inhibited swelling ability, and good cytocompatibility and blood compatibility.

Heterogeneous three-layer scaffolds were fabricated by mimicking the biochemical composition and structure of the hyaline cartilage, calcified cartilage, and subchondral bone of the osteochondral tissue for the repair of osteochondral defects. The hyaline cartilage layer was composed of collagen I (50.0 wt %) and sodium hyaluronate (50.0 wt %). The calcified cartilage layer and subchondral bone layer were composed of collagen I, sodium hyaluronate, and nanohydroxyapatite with different proportions. N-Hydroxysuccinimide/N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride was used to mediate the crosslinking reaction of the amine groups of collagen with carboxyl groups of sodium hyaluronate. The hyaline cartilage layer and calcified cartilage layer were designed as dense structures, while the subchondral bone layer was designed as a relatively loose structure by adjusting the crosslinking degree. The scaffolds displayed a uniform and interconnected porous structure and possessed a high porosity over 85%, which were conducive to cellular adhesion and proliferation. The scaffolds could remain at 50−75% after 30 days of degradation owing to crosslinking, providing enough time for the regeneration of the osteochondral tissue. Especially, the hyaline cartilage layer and calcified cartilage layer preferred to induce the proliferation of chondrocytes, while the subchondral bone layer was more conducive to the proliferation of osteoblasts. In conclusion, the heterogeneous multilayer scaffolds could serve as implant materials for osteochondral reconstruction.

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