Light-degradable hydrogels have emerged as useful platforms for use in many diverse applications, including cell and drug delivery, tissue engineering, and photo-pattering. The light-induced degradation of photo-cleavable hydrogels relies on the incorporation of photo-labile groups either on the polymer backbone or at the cross-link points. Ortho-nitrobenzyl ester derivatives, coumarins, disulfide bonds, and ruthenium complexes are the most commonly used photo-labile moieties, which have been incorporated within hydrogels to confer them with photo-degradable properties. Lately, covalent adaptable hydrogels, which can undergo reversible bond dissociation in response to one or more external stimuli, have attracted great attention as recyclable materials sealants, tissue adhesives, soft robotics, and others. Herein, we present a new class of reversible hydrogels, which undergo cleavage of the cross-link bonds by UV light irradiation and a subsequent sol-gel transition upon heating (Figure 1). The photo-degradable hydrogels bearing thioacetal bonds at the cross-link points were synthesized via an acid-catalyzed step-growth polymerization of an α,ω-chain-end-functionalized poly(ethylene glycol) (PEG), with thiol groups, (PEG(SH)2), and an aromatic dialdehyde. Hydrogels with an Mn of the PEG chains of 1,500 and 4,000 g mole–1 were synthesized and studied in terms of their photo-degradation behavior and reversible gel formation. The chemical structure of the synthesized hydrogels was confirmed by solid-state 1H and 1H-13C HSQC NMR experiments and FTIR spectroscopy, while their morphology and swelling behavior were determined by scanning electron microscopy and swelling experiments. The disruption and reformation of the hydrogels following UV irradiation and heating, respectively, were investigated by dynamic rheology.

Light-degradable hydrogels with temperature-assisted reversible properties

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