Photo-crosslinking speckle patterns for large deformation measurement of hydrogels using digital image correlation

Hydrogels often undergo large or inhomogeneous deformation when they are used in soft electronic devices, adhesives, or biological implants. To avoid the potential risk of damage and failure in service, the mechanical response of hydrogels, especially subjected to large deformation, requires meticulous evaluation. Digital image correlation (DIC) has been increasingly employed in the mechanical tests of hydrogels due to non-contact measuring the deformation field by tracking speckle patterns motion on the specimen. However, measuring large deformation of hydrogels using DIC is challenging because the speckle patterns painted on the wet surface suffer various issues, such as bleeding when water is squeezed out, fragmentation or debonding if the stress transferred from hydrogel exceeds the strength or adhesion of painting. In this work, we developed a UV lithography-based speckle pattern preparation method to overcome these difficulties. Speckle patterns are generated by curing a polymer on the surface of hydrogels through chemical-crosslinked bonds, making them an integral part of the hydrogel surface. Experiments indicate that the speckle patterns work as reliable information carrier for DIC to measure large deformation up to strain of 580% and highly concentrated localized strain field within specimen. The speckle patterns show good durability in cyclic loading tests with peak strain up to 150%, achieving low relative deviation (<6%) of the measured deformation field in different cycles. Furthermore, our method allows the optimization of speckle patterns by controlling the shape, size, and coverage of speckles through well designed masks, which guarantees the accuracy and robustness of DIC measurement.