Photo-Switchable Cross-Linking in Polymer Gels: Effects on Surface Creasing and Network Relaxation during Swelling

TL;DR

This study addresses how photo-responsive coumarin cross-links can program the mechanics and surface topography of PEG-based hydrogels during swelling. By integrating permanent PEGDA with dynamic CoumAc cross-links, the authors enable in situ tuning of cross-link density using wavelength-specific UV: 365 nm induces dimerization and stiffening, while 254 nm can cleave dimers, though bulk effects are limited by light attenuation. Real-time RT-FTIR and DMA reveal that 365 nm post-cure raises the storage modulus by up to 69% with rapid onset, and surface imaging shows swelling-induced creases are steered by the CoumAc state and photopolymerization conditions. The work establishes design principles for programmable hydrogels with adaptive surface morphologies, enabling smart coatings, actuators, and responsive biomaterials where spatiotemporal control of stiffness and surface patterns is desirable.

Abstract

Polymer gels with photo-responsive cross-links enable tunable mechanics and surface morphologies, making them promising for adaptive materials. While prior work on coumarin cross-linked gels has focused on photo-mediated events in dilute solution, their network-level mechanical responses remain unclear. Here, we design PEG hydrogels with both permanent covalent and dynamic coumarin cross-links, allowing in situ modulation of cross-linking under wavelength specific UV light. Real-time FTIR and dynamic mechanical analysis show that post-cure 365 nm irradiation drives rapid dimerization, increasing storage modulus by up to 69\%, whereas cleavage of coumarin cross-links via 254 nm post-cure irradiation has a more limited effect due to attenuation in bulk samples. Surface imaging reveals that dynamic cross-linking governs swelling-induced crease formation and evolution. Together, these results establish design principles for hydrogels with programmable mechanics and adaptive surface topographies, advancing application in smart coatings, actuators, and responsive biomaterials.

Figures (20)

• Figure 1: Molecular structure of a) PEGMA, b) CoumAc, c) PEGDA, and d) DMPA.
• Figure 1: Reaction scheme for the synthesis of 7,2-(acryloyloxyethoxy)-4-methylcoumarin.
• Figure 2: Design of photoresponsive network. Schematic depicting how this dual-cross-linked network is designed to switch between a more constrained state and a less constrained state upon post-cure irradiation with UV light.
• Figure 2: H-NMR spectrum of intermediate product of synthesis
• Figure 3: The addition of CoumAc cross-linking, as well as the wavelength of UV light used during photo-polymerization, significantly changes crease patterns. Microscope images measuring 3.328 x 3.328 mm compare the surface crease patterns on the surface of previously polymerized gels after 320 seconds of swelling in water for the a) 1:99 PEG formulation, as well as the 0.5:4.5:95 PEGCoumAc formulation b) polymerized at 365 nm and c) polymerized at 254 nm. In-focus creases are traced in blue to clarify the patterns of interest. Original microscope images are included in Fig. S\ref{['fig:micro images wo tracing']}.