An injectable hyaluronic acid/PEG hydrogel for cartilage tissue engineering formed by integrating enzymatic crosslinking and Diels-Alder "click chemistry"

The application of enzymatic crosslinking and Diels-Alder (DA) click chemistry for hydrogel formation has recently bloomed, because of the efficient chemical bonding and the mild biological reaction conditions. In this study, an injectable hyaluronic acid/PEG (HA/PEG) hydrogel was successfully fabricated for the first time through integrating two cross-linking processes, including firstly enzymatic crosslinking and subsequent DA click chemistry. The enzymatic crosslinking resulted in a fast gelation of HA/PEG in 5 min, leading to the formation of an injectable material. In addition, the DA click reaction crosslinking made a hydrogel that has outstanding shape memory and anti-fatigue properties. The storage modulus and breakage strength of the hydrogels were close to 27 kPa and 109.4 kPa, respectively. The compressive strain could reach up to 81.9%. After 10 cycles of a loading and unloading test, the hydrogel still could be loaded by 80 kPa for 1 min and the corresponding deformation could be completely recovered in 1 min after unloading. The ATDC-5 cells were capsulated into the hydrogel bulk in situ and showed high metabolic viability and proliferation. All of these results suggest that the HA/PEG injectable hydrogel formed by integrating the two cross-linking processes has a great potential in cartilage tissue engineering.