Achieving good adhesion between surfaces under wet conditions is often difficult. Nevertheless, it is an important requirement for a variety of applications. The performance of commercially available wet adhesives deteriorates with time due to hydration-induced softening and dissolution. Researchers at Department of Mechanical Engineering, Ulsan National Institute of Science and Technology (UNIST) and Department of Mechanical Engineering, Incheon National University, Republic of Korea have demonstrated a flexible, wet-responsive, and shape reconfigurable hydrogel adhesive with microhook arrays that is capable of exhibiting a strong and reversible interlocking adhesion in wet or underwater conditions. The interlocking adhesion mechanism is based on the hydration-triggered swelling behavior of the hydrogel polymers (Hyun-Ha Park et al., ACS Macro Lett., 2017, 6 (12), pp 1325–1330).
Polyethylene glycol dimethacrylate (PEGDMA) (molecular weight: 550) was used to fabricate the microscale hook arrays (Fig. 1(a)). The choice of PEGDMA hydrogel is based on its ability to strike a balance between the degree of swelling and the mechanical strength, absorb large quantities of water or physiological solutions, amenability for UV curing. The PEGDMA microscale hook arrays were designed with protruding heads that enables an effective interlocking between two mating arrays (Fig. 1(b)). When two identical hydrogel microhook arrays are brought into contact with each other, only a relatively weak adhesion between them could be realized under dry conditions. However, a significant volume expansion and shape transformation of the hydrogel microhooks occurs upon exposure to water (Fig. 1(c)). This is due to the anisotropic swelling of the hydrogel that provides a higher structural bending strength, contact surface, and friction with the neighboring microstructures. The adhesion strength is further increased with an increase in swelling time due to swelling-induced shape changes in the microhook arrays. During swelling, the size of the microhooks is increased in the direction normal to the surface rather than in the lateral direction due to the mechanical constraint provided by the substrate. The PEGDMA adhesives with smaller pitches offered a higher shear and normal strengths since they could exert a larger overlapping areas and provide tighter contact with the neighboring structures. This water-responsive shape change of the hydrogel adhesive is highly reversible upon removal of water by drying (de-swelling). During repeated cycles of swelling and de-swelling, the PEGDMA hydrogel adhesive exhibit excellent performance without any notable degradation in the extent of adhesion.
Fig. 1 (a) Fabrication protocol of the microhook arrays by photolithography using two layers of the photoresist (i.e., LOR30B and AZ 4330); (b) illustration of the reversible swelling and de-swelling process of the PEGDMA microhooks; and (c) illustration of the reversible interlocking of the PEGDMA microhook arrays via the hydration-induced shape reconfiguration of the array for high adhesion under wet conditions
T.S.N. Sankara Narayanan
Recent Advancements in Science 