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In This Issue

Commercial adhesives are either strong and irreversible, such as superglues, or weak and reusable, such as pressure-sensitive tapes. Some biologically inspired adhesives such as Velcro are both strong and reversible, but they require interlocking structures on opposing surfaces for adhesion. To create a strong and reversible adhesive that works on an array of surfaces, Hyesung Cho, Gaoxiang Wu, Jason Christopher Jolly, et al. (pp. 13774–13779) fashioned a hydrogel that combines the desirable properties of liquid and dry adhesives, drawing inspiration from snails. Snails glide along rough and uneven surfaces, secreting mucus, which provides traction. As the mucus dries and hardens, its shear modulus rises from 100 Pa to 1 GPa, resulting in a structure called an epiphragm, which cements the snail to the surface. The authors developed a reversible hydrogel made of poly(2-hydroxyethyl methacrylate) (PHEMA) that works on smooth and rough surfaces, with adhesive strengths reaching 892 N/cm2, comparable to superglues. Like the snail’s epiphragm, the gel deforms in its soft, wet state to adapt and adhere to the target surface. Upon drying, the gel shrinks, turns glassy, and interlocks with the surface, its shear modulus increasing by 3 orders of magnitude to around 2.3 GPa. Rehydrating the gel liberates the stored energy, restores the gel’s original shape, and quickens detachment—a physical attribute termed shape adaptation and memory. The authors scaled up the adhesive by layering the hydrogel atop a silicon backbone attached to a tendon made of Kevlar tape. Using only a pair of 2 cm2 samples of the hydrogel, the authors demonstrate that an adult human weighing 87 kg can be suspended from the adhesive rig. Additionally, the authors used a structured PHEMA gel pad to detach the wing scales of morpho butterflies without damaging the scales’ gossamer microstructures—a task too delicate for liquid glues. According to the authors, the PHEMA hydrogel can be scaled up and chemically modified for an array of applications. — P.N.