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Photoresponsive hydrogels are an emerging class of stimuli-responsive materials that exhibit changes in physical or chemical properties in response to light. Previous investigations have leveraged photothermal mechanisms to achieve reversible changes in hydrogel friction, although few have focused on photochemical means. To date, the tribological properties of photoswitchable hydrogels ( e.g. , friction and lubrication) have remained underexplored. In this work, we incorporated photoresponsive methoxy-spiropyran-methacrylate monomers (methoxy-SP-MA) into a hydrogel network to form a copolymerized system of poly( N -isopropylacrylamide- co -2-acrylamido-2-methylpropane sulfonic acid- co -methoxy-spiropyran-methacrylate) (p(NIPAAm- co -AMPS- co -SP)). We demonstrated repeatable photoresponsive changes to swelling, friction, and stiffness over three light cycles. Our findings suggest that volume changes driven by the decreased hydrophilicity of the methoxy-SP-MA upon light irradiation are responsible for differences in the mechanical and tribological properties of our photoresponsive hydrogels. Our results could inform future designs of photoswitchable hydrogels for applications ranging from biomedical applications to soft robotics.

Performing conventional mechanical characterization techniques on soft materials can be challenging due to issues such as limited sample volumes and clamping difficulties. Deep indentation and puncture is a promising alternative as it is an information-rich measurement with the potential to be performed in a high-throughput manner. Despite its promise, the method lacks standardized protocols, and open questions remain about its possible limitations. Addressing these shortcomings is vital to ensure consistent methodology, measurements, and interpretation across samples and labs. To fill this gap, we examine the role of finite sample dimensions (and by extension, volume) on measured forces to determine the sample geometry needed to perform and unambiguously interpret puncture tests. Through measurements of puncture on a well-characterized elastomer using systematically varied sample dimensions, we show that the apparent mechanical response of a material is in fact sensitive to near-wall effects, and that additional properties, such as the sliding friction coefficient, can only be extracted in the larger dimension case where such effects are negligible.

The ability to control adhesion is critical in various technologies including wearable electronics, pressure sensitive adhesives, and robotic systems. Biomimetic fibrillar structures, random surface roughness, and chemical surface treatments have been employed to modify the adhesion energy of materials used in these applications. However, polymer thin film dewetting has not been investigated as a surface modification tool to control adhesion. In this work, polystyrene thin films are thermally annealed on a polydimethylsiloxane substrate, causing them to dewet and form stiff, microscopic asperities on the soft substrate. The size of the asperities increases with increasing pre-annealing film thickness. Adhesion is quantified by flat-punch normal indentation testing. The largest asperities exhibited a decrease in adhesion to below the sensitivity of the instrument. More interestingly, the surfaces covered with the smallest asperities displayed a pressure-dependent adhesive response. By increasing the normal compressive stress applied prior to separation, the total debonding energy increased monotonically on the smallest asperity-covered surfaces.

The influence of poroelasticity on the contact mechanics of thin polyacrylamide films was investigated with a surface forces apparatus (SFA). A model based on a thin film approximation described compression forces for hydrated gels; polymer scaling theory explained the effects of gel dehydration. The results demonstrate that fluid flow dictates the apparent stiffness of highly confined poroelastic films.

Understanding the adhesion of rigid thin films to compliant substrates is critical for the development and implementation of flexible electronic devices and wearable sensor technologies. Quantifying the strength of a film-substrate interface can be challenging due to the brittleness of glassy films which can greatly complicate sample preparation, handling, and testing. Here, a method for measuring the adhesion of glassy thin films to soft elastomeric substrates is explored that exploits an understanding of surface buckling instabilities, specifically the transition from wrinkling to delamination. The adhesion (given by the critical strain energy release rate (G c )) for two model materials' interfaces is quantified by determining the critical delamination strain for thin glassy polymer films (polystyrene (PS) and poly(methyl methacrylate) (PMMA)) from an elastomeric substrate (poly(dimethyl siloxane) (PDMS)). By accounting for edge defects that greatly reduce the critical strain for delamination, reasonable adjusted G c values of 21.0 ± 5.1 mJ m -2 and 32.2 ± 4.9 mJ m -2 are found for PS-PDMS and PMMA-PDMS interfaces, respectively. The utilization of this method to characterize film modulus and adhesion could be used as a facile measurement technique for more applied polymer thin film systems.