Reusable adhesive works underwater

IOM3
,
4 Mar 2019

A new reusable adhesive developed from shape memory polymers can work underwater. Shardell Joseph reports. 

A new dry adhesive has been introduced by researchers from the University of Illinois, USA, which includes high adhesion strength, is reusable, and can be submerged without losing its effectiveness.

Using shape memory polymers (SMPs), these reusable dry adhesives have the capacity to activate in seconds, they fully function in and out of water, and are strong enough to deadlift 11lb of weight.

Dry adhesives are known to be poor in achieving high adhesion to wet or submerged surfaces. Commercial glues and pressure-sensitive adhesives perform inadequately due to the liquid layers intercalated at the contact surface. However, there has been continued demand for underwater adhesives due to potential functions for biomedical, industrial and household applications.

The research team – Associate Professor in Mechanical Science and Engineering Seok Kim, graduate student Jun Kyu Park, and former graduate student Jeffrey D Eisenhaure – managed to manipulate SMPs in such a way that they can retain dry adhesion properties.

‘Our solution uses the shape memory effect of SMPs to enable dry adhesives with high adhesion strength, high adhesion switchability and repeatable use, which is unique over others such as commercial adhesive strips and bio-inspired fibrillar ones,’ says Kim.

The study, Reversible underwater dry adhesion of a shape memory polymer, outlines the oceanic inspiration for submerging adhesives and the processes by using SMPs.

Sticking with nature

Biomimicry has frequently inspired underwater adhesive design. Marine creatures such as mussels, oysters and sandcastle worms that habitually adhere to submarine surfaces produce adhesive proteins that are eventually cured into rigid cements. These proteins are commonly rich in dihydroxyphenylalanine (DOPA), a chemical compound which dehydrates the contact interface and promotes the adhesion caused by covalent and noncovalent interactions.

Mimicking this process, the team developed synthetic underwater adhesives by combining DOPA with functional chemicals.

‘Dry adhesives, such as those inspired by gecko feet, are believed to be inadequate to achieve high adhesion to a wet or submerged surface,’ Kim said. ‘However, this belief has been nullified because of our SMP reversible dry adhesives.’

SMPs are an emerging class of active polymers because of their dual-shape capability, changing from a temporary state back to the permanent shape. They are a class of external-stimuli responsive polymers with the ability to memorise a permanent shape, which can be recovered from a deformed shape.

In the study, the research team found that applying pressure to the SMP in its compliant rubbery condition enables it to squeeze out the liquid from its contact interface. Under sufficient pressure, the SMP then transitions to a glassy state, which essentially creates a hermetic contact condition that maintains highly strong dry adhesion.

The SMP’s shape recovery properties allow for reversal of the adhesion, so as the material can transition fluently between both states, the adhesive is reusable.

In terms of the interaction between a rubber and a wet rigid surface, it has been proven that the rubber can squeeze out the intercalated liquid monolayers between itself and a rigid surface when external pressure is applied. The dry adhesion of the SMP under water is measured to be much higher then adhesives based on passive suction, such as DOPA-based adhesives.

‘Existing dry adhesives natively do not adhere to wet or submerged surfaces because water easily flows in and ruins the adhesive contact interface after preloading and unloading,’ Kim says. ‘However, our SMP can form and hold the strong adhesive contact after preloading and unloading due to the shape memory effect.’

Shape memory polymers

The underwater capabilities of the SMPs have shown potential for the creation of inexpensive, reversible, dry adhesives for use in both dry and wet conditions for biomedical, industrial and domestic applications. The research team has so far demonstrated one household application.

The demonstration depicted the reusable adhesive hook having an SMP adhesive with embedded wires on a backing and a separate hook slide, which was bonded to a glass wall with water flowing down it. The hook slide was installed to the bonded SMP adhesive, followed by a 3kg backpack hung onto the hook. Successfully holding the weight, the SMP adhesive is reheated, and easily peeled with no damage to the glass wall.

Ready for household applications, Kim plans to commercialise this adhesive technology through his start-up Respondic, Inc. The company will have two product types – reusable, adhesive fasteners and reversible adhesive grippers – which will be marketed as command strips, temporal holders, bath suction cups and pick-and-place tools.

The team will continue developing this technology, focusing on quicker response time and lowering manufacturing costs. In addition, Kim and his colleagues are committed to further research into using the dry adhesive as a footpad for a four-legged robot, allowing it to walk on a vertical wall or ceiling.

The paper was published in Advanced Materials Interfaces, and can be read here: bit.ly/2S9BJXx