Electrically switched underwater capillary adhesion

Nature-Inspired Wet Drug Delivery Platforms

Nature has created various organisms with unique chemical components and multi-scale structures (e.g., foot proteins, toe pads, suckers, setose gill lamellae) to achieve wet adhesion functions to adapt to their complex living environments. These organisms can provide inspirations for designing wet adhesives with mediated drug release behaviors in target locations of biological surfaces. They exhibit conformal and enhanced wet adhesion, addressing the bottleneck of weaker tissue interface adhesion in the presence of body fluids. Herein, it is focused on the research progress of different wet adhesion and bioinspired fabrications, including adhesive protein-based adhesion and inspired adhesives (e.g., mussel adhesion); capillarity and Stefan adhesion and inspired adhesive surfaces (e.g., tree frog adhesion); suction-based adhesion and inspired suckers (e.g., octopus' adhesion); interlocking and friction-based adhesion and potential inspirations (e.g., mayfly larva and teleost adhesion). Other secreted protein-induced wet adhesion is also reviewed and various suckers for other organisms and their inspirations. Notably, one representative application scenario of these bioinspired wet adhesives is highlighted, where they function as efficient drug delivery platforms on target tissues and/or organs with requirements of both controllable wet adhesion and optimized drug release. Finally, the challenges of these bioinspired wet drug delivery platforms in the future is presented.

Detachment forces during parallel-plate gap separation mediated by a simple yield-stress fluid

In this work we have monitored the multiple stages of the normal traction force response of a yield-stress fluid confined between two circular parallel plates that are separated at constant velocity. At narrow initial gaps, the air-fluid interface suffers from the Saffman-Taylor instability, confirmed by visual inspection of fingering patterns imprinted on the fluid. At larger initial gaps, the fluid preserves the initially imposed circular symmetry of the confining plates, indicating the absence of instability. Due to the system characteristics and experimental environment, the multiple traction force contributions occurred in cascade, permitting us to isolate the adhesion responses associated with viscosity, capillarity, and yield stress. Employing a standard Herschel-Bulkley model, we assessed the scaling of the traction force in multiple regimes-specifically, evaluating the dependencies of the fingering to yield-stress transitions.

Electro-capillary peeling of thin films

Thin films are widely-used functional materials that have attracted much interest in academic and industrial applications. With thin films becoming micro/nanoscale, developing a simple and nondestructive peeling method for transferring and reusing the films remains a major challenge. Here, we develop an electro-capillary peeling strategy that achieves thin film detachment by driving liquid to percolate and spread into the bonding layer under electric fields, immensely reducing the deformation and strain of the film compared with traditional methods (reaching 86%). Our approach is evaluated via various applied voltages and films, showing active control characterizations and being appropriate for a broad range of films. Theoretically, electro-capillary peeling is achieved by utilizing the Maxwell stress to compete with the film's adhesion stress and tension stress. This work shows the great potential of the electro-capillary peeling method to provide a simple way to transfer films and facilitates valid avenues for reusing soft materials.

A Photocured Bio-based Shape Memory Thermoplastics for Reversible Wet Adhesion

Development of reversible wet or underwater adhesives remains a grand challenge. Because weakened intermolecular interactions by water molecules or/and low effective contact area cause poor interface to the wet surfaces, which significantly decreases adhesive strength. Herein, a new photocured, bio-based shape memory polymer (SMP) that shows both chemical and structural wet adhesion to various types of surfaces is developed. The SMP is polymerized from three monomers mainly from bio-sources to form linear polymer chains dangled with hydrophobic side chains. The hydrogen acceptor and donor groups in the chains form hydrogen bonding with the surfaces, which is protected by the hydrophobic chains in the interface. The SMP shows tunable phase transition temperature (T_g) of 17-38 °C. In a rubbery state above T_g, the adhesive forms conformable contact with the targeted surfaces. Below T_g, a transition to a glassy state locks the conformed shapes to largely increase the effective contact area. As a result, the adhesive exhibits long-term underwater adhesion of > 15 days with the best adhesion strength of ~ 0.9 MPa. Its applications in leak repair, underwater on-skin sensors were demonstrated. This new, general strategy would pave avenues to designing bio-based, long-lasting, and reversible adhesives from renewable feedstocks for widespread applications.

Getting glued in the sea

Inspired by ocean organisms, scientists have been developing adhesives for application in the marine environment. However, water and high salinity, which not only weaken the interfacial bonding by the hydration layer but also induce the deterioration of adhesives by erosion, swelling, hydrolysis, or plasticization, are detrimental to adhesion, resulting in specific challenges in the development of under-seawater adhesives. In this focus review, current adhesives that are capable of macroscopic adhesion in seawater were summarized. The design strategies and performance of these adhesives were reviewed based on their bonding methods. Finally, some future research directions and perspectives for under-seawater adhesives were discussed.