Hydrogel Adhesion

Abstract

Hydrogel adhesion, integrating hydrogels with a variety of materials, has advanced emerging technologies in the fields of functional materials, soft ionotronics and electronics, and biomedical applications. However, achieving strong adhesion between hydrogels and other materials is fundamentally challenging. This thesis explores various ways to develop chemistry of bonds, mechanics of dissipation, and topology of connecting materials to create strong hydrogel adhesion with diverse materials, and demonstrates potential applications in medicine and engineering. In medical practices, strong adhesion between hydrogel and biological tissues is important. Existing adhesives are either cytotoxic, adhere weakly to tissues, or cannot be used in wet environments. We design a bio-inspired adhesives consisting of two layers: an adhesive layer made of polymer chains and a dissipative layer made of a hydrogel capable of dissipation. The polymer chains can interpenetrate into both hydrogel and tissue, and form electrostatic interactions, covalent bonds, and physical interpenetration with the polymer networks of the hydrogel and the tissue. The two layers synergistically lead to higher adhesion energy on various tissues than existing tissue adhesives. Adhesion occurs within minutes, independent of blood exposure, and compatible with in vivo dynamic movements. This class of tough adhesive is promising in tissue adhesion, but the adhesion restricts to specific functional groups from both materials. To address this issue, we develop a topological adhesion that uses bio-compatible polymer chains to form a network, in topological entanglement with the two polymer networks of the hydrogel and the tissue, stitching them together like a suture at the molecular scale. This approach does not require functional groups from both materials, and the interface is mechanically compatible with soft hydrogels and tissues. To illustrate the principle, pH is used to trigger several polymers to form networks, and strong adhesion can be created between hydrogels in full range of pH, and between hydrogels and tissues. The molecular suture can be further designed to be permanent, transient, or removable on-demand. In developing the bonding methods so far, we realize the ways to achieve hydrogel adhesion multiply considerably by examining another aspect of adhesion: the topology of connecting materials. Topologies in existing methods are limited. Topologies of numerous varieties are possible, but have not been explored. To illustrate the potential, we delve into a specific bond-stitch topology, study its chemistry and physics, and highlight the synergy of chemistry, mechanics, and topology can enable strong adhesion between any hydrogels and any materials. The diversity of topologies benefits the adhesion to accommodate different materials and manufacturing processes, and also facilitates functional adhesion for various purposes. We study the mechanics of adhesion. We conduct slow crack tests to identify the failure mechanism in the debonding of stitch-stitch topology. We also show that the hysteresis in the hydrogels greatly amplifies the energy release rate at high crack speed, but contributes negligibly to the energy release rate at low crack speed. Hydrogel adhesion brings unprecedented capabilities and new opportunities. The field of innovation is wide open.