Bio-Inspired and Stimuli-Responsive Adhesive Hydrogel Structure Fabricated by Single-Exposure Photolithography

[Introduction] Since adhesive structures that can be attached by specific morphology do not require special chemical properties, diverse materials can be adopted, and numerous researchers have applied bio-inspired frames in nature. Mainly, the mimicked adhesive structures consist of complex three-dimensional (3D) shapes rather than simple 2D shapes, and, in order to fabricate these structures, a series of rather complex processes are involved. Herein, we present the 3D bio-inspired (male diving beetle foot) adhesive structure fabrication utilizing a simple single-exposure photolithography system. The adhesive structure made of photocurable hydrogel cannot be attached in a contracted environment such as dried and ethanol, but the structures will be deformed into a concave adhesive shape by local expansion in an underwater environment. In addition, it is confirmed that adhesion can be controlled by stimuli according to changes in the surrounding environment.<br/><br/>[Materials and Methods] A photolithography system, based on a mercury−xenon lamp ultraviolet (UV) light source (365 nm, Hamamatsu) with a fiber-based light guide to solidify photocurable polymers, a digital micromirror device (DMD, Texas Instruments) to reflect desired UV light patterns, and objective lens (Olympus) to adjust the pattern size, was utilized in order to create 3D adhesive hydrogel structures. The photocurable hydrogel material was prepared by mixing Poly(ethylene glycol) diacrylate (Mn 700, Sigma-Aldrich) and Irgacure 1173 (photoinitiator, BASF) by a vortex mixer (Scientific Industries).<br/><br/>[Results and Discussion] The 3D hydrogel structure fabricated through the single-exposure photolithography process was expected to maintain different polymeric network densities locally, and through Raman spectroscopy analysis, the local carbon double bonding (1635 cm^-1) intensity was demonstrated to be different. The hydrogel structures exposed to dry or ethanol solution were not able to adhere due to the convex surface based on shrinkage. But, the hydrogels soaked in water environment were transformed into a concave shape due to local expansion based on difference of polymeric densities, enabling adhesion. The expanded and adhesive hydrogel structure immersed in water was saturated with pull-off force under the pre-loading condition of about 70 kPa, and the force was about 130 mN. Additionally, the change of hydrogel structures according to temperature change was analyzed. The hydrogel structures could not be adhered due to shrinkage on the basis of water temperature increase around 50 degrees Celsius. Contrary to the previous case, the hydrogel structures could be adhered in accordance with expansion based on ethanol temperature increase nearly 40 degrees Celsius.<br/><br/>[Conclusion] We have shown bio-inspired and stimuli-responsive adhesive hydrogel structures fabricated by utilizing a single-exposure photolithography process. The created hydrogel structures are transformed into an adhesible form by local expansion in water, and in addition, the frame is transformed according to the change of the surrounding thermal environment. Thus, the adhesion states can be controlled. We believe that the manufactured adhesive hydrogel structure will contribute to soft actuation and robotics fields.