Recent advances in bio-inspired microfibrillar adhesives have resulted in technologies that allow reliable attachment to a variety of surfaces. Because capillary and van der Waals forces are considerably weakened underwater, fibrillar adhesives are however far less effective in wet environments. Although various strategies have been proposed to achieve strong reversible underwater adhesion, strong adhesives that work both in air and underwater without additional surface treatments have yet to be developed. In this study, we report a novel design—cupped microstructures (CM)—that generates strong controllable adhesion in air and underwater. We measured the adhesive performance of cupped polyurethane microstructures with three different cup angles (15, 30, and 45°) and the same cup diameter of 100 μm in dry and wet conditions in comparison to standard mushroom-shaped microstructures (MSMs) of the same dimensions. In air, 15°CM performed comparably to the flat MSM of the same size with an adhesion strength (force per real contact area) of up to 1.3 MPa, but underwater, 15°CM achieved 20 times stronger adhesion than MSM (∼1 MPa versus ∼0.05 MPa). Furthermore, the cupped microstructures exhibit self-sealing properties, whereby stronger pulls lead to longer stable attachment and much higher adhesion through the formation of a better seal.

Background Suction organs provide powerful yet dynamic attachments for many aquatic animals, including octopus, squid, remora, and clingfish. While the functional morphology of suction organs from some cephalopods and fishes has been investigated in detail, there are only few studies on such attachment devices in insects. Here we characterise the morphology and ultrastructure of the suction attachment organs of net-winged midge larvae (genus Liponeura; Diptera: Blephariceridae) – aquatic insects that live on rocks in rapid alpine waterways where flow speeds can reach 3 m s− 1 – using scanning electron microscopy, confocal laser scanning microscopy, and X-ray computed micro-tomography (micro-CT). Furthermore, we study the function of these organs in vivo using interference reflection microscopy. Results We identified structural adaptations important for the function of the suction attachment organs in L. cinerascens and L. cordata. First, a dense array of spine-like microtrichia covering each suction disc comes into contact with the substrate upon attachment, analogous to hairy structures on suction organs from octopus, clingfish, and remora fish. These spine-like microtrichia may contribute to the seal and provide increased shear force resistance in high-drag environments. Second, specialised rim microtrichia at the suction disc periphery were found to form a continuous ring in close contact and may serve as a seal on a variety of surfaces. Third, a V-shaped cut on the suction disc (“V-notch“) is actively opened via two cuticular apodemes inserting on its flanks. The apodemes are attached to dedicated V-notch opening muscles, thereby providing a unique detachment mechanism. The complex cuticular design of the suction organs, along with specialised muscles that attach to them, allows blepharicerid larvae to generate powerful attachments which can withstand strong hydrodynamic forces and quickly detach for locomotion. Conclusion The suction organs from Liponeura are underwater attachment devices specialised for resisting extremely fast flows. Structural adaptations from these suction organs could translate into future bioinspired attachment systems that perform well on a wide range of surfaces.

are an opportunity to improve adhesion in wet conditions. [13,19] However, these bonds inevitably require functional groups at the target surface and are subject to wear during repeated attachment-detachment cycles as in pick-and-place handling. Microstructured elastomer surfaces are capable of reliable and switchable adhesion in dry environments. [20-23] Several groups have demonstrated their potential underwater, provided the water can be expelled from the contact region. [24,25] In particular, hydrophobic microstructures or microstructures with reentrant geometry have the ability to trap air in between the structures when immersed in water. [26-28] Such air bubbles can improve adhesion through the presence of capillary forces, even when the contact is fully immersed. [29,30] The microstructure tips can be further modified by introducing chemical bonds [3] or water absorbers such as hydrogels; [19] however, switchability by external stimuli remains elusive. In nature, suction cups have evolved for temporary underwater adhesion during locomotion or when catching prey. [31,32] Many species, such as octopus, [31] clingfish, [32,33] and netwinged midge larvae [34] utilize muscular actuation to reduce the hydrostatic pressure in the contact and, therefore, to control the adhesive force. This principle has been translated to synthetic macroscopic grippers working in dry environments by adding pumps to control the air pressure. On the microscale, recent reports demonstrate the fabrication of microsucker arrays by micromachining or optical lithography combined with replica molding. [35-37] The reported adhesion to smooth silicon surfaces is in the range of 50-100 kPa in air and underwater. In a previous study, [38] we presented cupped microstructures (CMs) created by two-photon lithography and replica molding. Adhesion strengths of individual structures were about 1 MPa in air and underwater. Despite similar adhesive strengths in both media, adhesion mechanisms were attributed to suction under water and van der Waals interactions in dry conditions. The present article explores the potential of deformable cupped microstructures, reminiscent of suction cups, for switchable adhesion in wet conditions. Underwater adhesion tests are systematically performed with constant retraction velocities ranging from 0.1 to 100 µm s-1 until detachment. Finally, we demonstrate underwater manipulation (pick-and-place) of a submerged object using an array of cupped microstructures. 2. Results and Discussion Cupped microstructures were generated by two-photon lithography using standard (meth)acrylate-based resin (Figure 1a). Switchable underwater adhesion can be useful for numerous applications, but is extremely challenging due to the presence of water at the contact interface. Here, deformable cupped microstructures (diameter typically 100 µm, rim thickness 5 µm) are reported that can switch between high (≈1 MPa) and low (<0.2 MPa) adhesion strength by adjusting the retraction velocity from 100 to 0.1 µm s-1. The velocity at which the swi...

Limpets ( Patella vulgata L.) are renowned for their powerful attachments to rocks on wave-swept seashores. Unlike adult barnacles and mussels, limpets do not adhere permanently; instead, they repeatedly transition between long-term adhesion and locomotive adhesion depending on the tide. Recent studies on the adhesive secretions (bio-adhesives) of marine invertebrates have expanded our knowledge on the composition and function of temporary and permanent bio-adhesives. In comparison, our understanding of the limpets' transitory adhesion remains limited. In this study, we demonstrate that suction is not the primary attachment mechanism in P. vulgata ; rather, they secrete specialized pedal mucus for glue-like adhesion. Through combined transcriptomics and proteomics, we identified 171 protein sequences from the pedal mucus. Several of these proteins contain conserved domains found in temporary bio-adhesives from sea stars, sea urchins, marine flatworms and sea anemones. Many of these proteins share homology with fibrous gel-forming glycoproteins, including fibrillin, hemolectin and SCO-spondin. Moreover, proteins with potential protein- and glycan-degrading domains could have an immune defence role or assist degrading adhesive mucus to facilitate the transition from stationary to locomotive states. We also discovered glycosylation patterns unique to the pedal mucus, indicating that specific sugars may be involved in transitory adhesion. Our findings elucidate the mechanisms underlying P. vulgata adhesion and provide opportunities for future studies on bio-adhesives that form strong attachments and resist degradation until necessary for locomotion.

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