Fabrication of Wet-Responsive Bioinspired Adhesives and Their Applications

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Fabrication of Wet-Responsive Bioinspired Adhesives and Their Applications
Yi, Hoon
Jeong, Hoon Eui
Bioinspired adhesive; Adhesion; Adaptable adhesive; Switchable adhesive; Wet-responsive adhesive; Stimuli-responsive material; Wearable device; Transfer printing
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Graduate School of UNIST
Inspired by the fascinating adhesion properties of various creatures in nature, various bioinspired adhesives have been developed. Since the bioinspired adhesives exhibit excellent adhesion strength and reversible adhesion, they have strong potential for a wide variety of applications including wearable devices, nanoscale manufacturing techniques, and soft robotics. However, the bioinspired adhesives made of conventional elastomeric materials have limited adhesion strengths to rough surfaces and limited controllability on adhesion strengths, which limits their practical applications. As the challenges mainly result from the fixed physical property (e.g. elastic modulus of material) of the elastomer-based adhesives, utilization of stimuli-responsive materials that enable active modulation of their mechanical properties on demand is expected to be an effective solution overcoming the limitations. Wet-responsive hydrogels are tunable in their shape, volume, and mechanical properties based on hydration/dehydration in an active and reversible manner. Therefore, it is expected that bioinspired adhesives made of the wet-responsive hydrogels could overcome the aforementioned challenges. In this dissertation, we propose wet-responsive bioinspired adhesives made of hydroxypropyl cellulose (HPC) hydrogel and polyethylene dimethacrylate (PEGDMA) hydrogel that exhibit superior surface adaptability and high adhesion-on/off switchability, respectively. For superior adaptability, a bioinspired adhesive comprised of wet-responsive HPC is proposed as it enables adaptation to a rough surface due to its controllable swelling behavior. By hydration/dehydration, the elastic modulus of the HPC hydrogel can be modulated on demand. In the presence of a small amount of water, the individual bioinspired HPC microstructures in the adhesive can be easily deformed along the rough surface with the decreased elastic modulus of the HPC. As dehydrated, the elastic modulus of HPC microstructures is recovered with maintaining the deformed morphology. Through these processes, the surface roughness-adapted HPC adhesive exhibits strong adhesion strength. Furthermore, the adaptable HPC adhesive is reusable as the deformed microstructures can recover their original shapes based on a shape-memory capability of HPC. In order to develop the bioinspired adhesive that exhibits actively controllable and switchable adhesion on demand, PEGDMA hydrogel with swelling behavior is utilized as it has shape-reconfigurable property. The prepared PEGDMA adhesive shows high adhesion strengths against substrates with the aid of bioinspired nano‐ or microstructure array in the dry state (adhesion-on state). When the adhesive is exposed to water, a hydration‐induced shape transformation of the array and macroscopic film bending occur, switching the adhesion off with an extremely high adhesion switching ratio. Also, the switchable adhesion behavior of the adhesive is maintained over repeating cycles of hydration and dehydration, indicating their ability to be used repetitively. As the rough surface adaptation and adhesion on/off properties of the developed adhesives only require water droplets, they have a wide range of applications in diverse fields. Specifically, the adhesives have a strong potential for use in a biomedical field as the HPC and PEGDMA hydrogels are biocompatible. Accordingly, we demonstrate several unique biomedical practical applications of the developed adhesives. Firstly, with the adaptable HPC adhesive, an attachable photonic skin is developed as a wearable skin-like sensor. The photonic skin consisting of an HPC mechanochromic sensor and the adaptable adhesive can firmly laminate to diverse substrates including human skins, detecting mechanical signals from various target objects. Secondly, the adhesion-switchable PEGDMA adhesive is utilized for a nanotransfer printing (nTP). We demonstrate that diverse metallic and semiconducting nanomembranes can be transferred from donor substrates to either rigid or flexible surfaces including biological tissues with the PEGDMA adhesive in a reproducible and robust fashion. In total, this dissertation presents the fabrication of wet-responsive bioinspired adhesives and their applications. The overall contents consist of three main themes, that are as follow: (1) fabrication of bioinspired adhesives with optimized geometries, (2) rough surface-adaptable adhesive made of wet-responsive HPC hydrogel and (3) adhesion-switchable adhesive made of wet-responsive PEGDMA hydrogel.
Department of Mechanical Engineering
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