*Article* **The Effect of Crosslinking Degree of Hydrogels on Hydrogel Adhesion**

**Zhangkang Li <sup>1</sup> , Cheng Yu <sup>2</sup> , Hitendra Kumar <sup>3</sup> , Xiao He <sup>4</sup> , Qingye Lu <sup>4</sup> , Huiyu Bai 2,\*, Keekyoung Kim 1,3,\* and Jinguang Hu 1,4,\***


**Abstract:** The development of adhesive hydrogel materials has brought numerous advances to biomedical engineering. Hydrogel adhesion has drawn much attention in research and applications. In this paper, the study of hydrogel adhesion is no longer limited to the surface of hydrogels. Here, the effect of the internal crosslinking degree of hydrogels prepared by different methods on hydrogel adhesion was explored to find the generality. The results show that with the increase in crosslinking degree, the hydrogel adhesion decreased significantly due to the limitation of segment mobility. Moreover, two simple strategies to improve hydrogel adhesion generated by hydrogen bonding were proposed. One was to keep the functional groups used for hydrogel adhesion and the other was to enhance the flexibility of polymer chains that make up hydrogels. We hope this study can provide another approach for improving the hydrogel adhesion generated by hydrogen bonding.

**Keywords:** hydrogel; adhesive strength; crosslinking; hydrogen bonding
