Exploring Photo-Induced Polymerization and Localized Degradation in Dynamic Disulfide-Containing Hydrogels

Abstract

Photodegradable hydrogels have emerged as a versatile platform for research on tissue engineering, cell function and cell delivery because of the unique property of dynamically altering physical and chemical material properties by the use of light. The ability of modifying extracellular matrix (ECM)-mimicking networks is intriguing as mammalian cells are surrounded by a three-dimensional (3D) environment that is remodelled over time. Frequently, photoresponsive o-nitrobenzyl (oNB) derivatives are incorporated into such networks. Degradation of such hydrogel systems can be either induced by one-photon irradiation using UV light or by a two-photon process applying pulsed IR-laser light. However, the inherent light-sensitivity of the incorporated photo-triggers requires handling under light protection for all experimental steps. Furthermore, high light intensities and irradiation times are necessary for the photoscission that can ultimately damage living cells encapsulated into the material. A striking but rarely used alternative is the utilization of disulfide linkages which exclusively become photocleavable by the use of a radical photoinitiator at cytocompatible doses of light. However, previous studies showed that disulfide-containing hydrogels suffer from limited network stability and uncontrollable swelling. Thus, we introduce a disulfide-containing, thiol-terminated crosslinker that can be synthesized in a 1-step procedure from readily available resources. We observed high photoreactivity with norbornene-modified poly(vinyl alcohol) (PVA-NB) and radical concentration-dependent network scission, however, accompanied with tremendous swelling. Herein, we highlight the importance of the addition of native and norbornene-derived gelatin (Gel-NB) to the well-established thiol-crosslinked PVA-NB hydrogel platform. By adding native gelatin, high photoreactivity was combined with higher network stability via physical crosslinks. However, only the chemical incorporation of the gelatin macormers (Gel-NB) resulted in increased long-term network integrity, controllable swelling and in-vitro degradation. Finally, we demonstrate an innovative method for the localized and mild disulfide cleavage via two-photon micropatterning. The developed dynamic hydrogels provide various opportunities for the design of tunable materials that can be used for numerous applications including tissue engineering and microfluidics.