SINGAPORE - Taking inspiration from dried snail mucus and gecko feet, scientists in Singapore have developed a reusable adhesive that can adhere strongly to rough surfaces, unlike conventional adhesives.

In tests, they found that a palm-sized amount of the adhesive could support a weight of 60kg from a ceiling hook. It also proved to be 10 times stronger than a gecko’s feet clinging to a wall. 

And despite its strength, this adhesive can be detached easily just by heating it with a hair dryer – unlike strong adhesives, which are usually difficult to detach and may cause damage to the surface as they are removed.

The technology opens new possibilities for robotic grippers that could allow a person to scale walls “like a real-life Spider-Man” or climbing robots that can cling onto ceilings for survey or repair applications, said Professor K. Jimmy Hsia, who led the research team at Nanyang Technological University (NTU).

To create the adhesive, the team of scientists used shape-memory polymers, a kind of smart material that can readily adhere to and be detached from a surface.

These polymers can be deformed but are able to return to their original shape after applying an external stimulus such as heat and electric currents.

The shape-memory polymers mimic the versatility of snail mucus, said Dr Linghu Changhong, the research paper’s first author and NTU Research Fellow.

When their surrounding environment becomes too warm, snails secrete mucus that dries up and hardens, allowing them to cement themselves onto a surface and become dormant for long periods of time to conserve water and energy.

When the air cools, the hardened mucus softens and the snail can move freely again.

Drawing inspiration from this, the team manipulated the polymer to soften into a “rubber” state that conforms to the microscopic nooks and crevices found on surfaces when it is heated.

As it cools to room temperature, the material stiffens into a “glassy” state, locking the adhesive onto the surface.

Currently, it takes about three minutes for the material to be cooled thoroughly and be locked in place, but the researchers hope to reduce the time needed in the future.

In their tests, the scientists set the temperature at which the polymer detaches to 60 deg C, a temperature that falls outside most real-world conditions.

The adhesive also overcomes the “adhesion paradox”, which material scientists have been puzzling over, said Dr Changhong who began the research in 2020.

Adhesives are weaker on rough surfaces, even though their bumps and ridges provide more surface area for molecules to adhere to.

This is because surface roughness reduces the points of contact between the object and the surface to be adhered to, said Dr Changhong.

But their tests showed that the adhesive’s strength increases along with surface roughness when in a solid state and decreases when in the “rubber” state, overcoming the paradox. The more bumps and ridges there are, the more nooks and crannies the adhesive has, to conform to in its “rubber” state.

The researchers also found that adhesion was most effective when they designed the polymer to have an arrangement of fibrils – the structural building blocks of the adhesive – that resembled the arrangement of microscopic hair on a gecko’s feet. Geckos can cling to almost any surface.

In their experiments, the researchers found that one fibril with a 19.6 mm2 cross-section could support loads up to 1.56 kg. Every additional fibril allows for more weight to be supported.

A palm-sized array of 37 fibrils weighing about 30g can hold a weight of 60kg – an adult human’s weight.

With a metal bar attached to the ceiling using two of the adhesives, the researchers were able to do multiple pull-ups during a lab demonstration at NTU.

And as the shape-memory polymer used is E44 epoxy, an “off-the-shelf product” that is “not very expensive”, costs of producing the material are lower, said Prof Hsia, who is also President’s Chair in Mechanical Engineering, NTU School of Mechanical & Aerospace Engineering and School of Chemistry, Chemical Engineering and Biotechnology.

Professor Kevin Turner, Professor and Department Chair of Mechanical Engineering and Applied Mechanics at the University of Pennsylvania, said the innovative design of the adhesive gives it impressive adhesion strength and switchability – the ability to switch between low and high levels of stickiness, distinguishing it from other adhesives.

“The researchers achieved this impressive performance by exploiting an understanding of the mechanics of adhesion rather than by developing new materials,” he said, adding that the strength and switchability of the adhesive will make it well-suited for applications where temporary attachment is required.

The team’s research was published last month in the scientific journal National Science Review and was funded by the Ministry of Education under the Academic Research Fund Tier 2.