Scientists develop strong but reusable glue from smart materials

Scientists at Nanyang Technological University, Singapore (NTU Singapore) have developed a smart, reusable glue that is more than 10 times stronger than the bond of a gecko’s feet, paving the way for the development of reusable super glue and grippers that can hold heavy can hold weights on rough and smooth surfaces.

The NTU research team, led by Professor K Jimmy Hsia, has found a way to maximize the adhesion of the smart adhesives by using shape memory polymers, which can easily stick and release when needed, simply by heating them .

Writing in the diary last month National Science Reviewthe team describes their breakthrough in adhesion by designing the shape memory polymer material in the form of hair-like fibrils.

This smart adhesive can support extremely heavy weights, opening up new possibilities for robotic grippers that allow humans to effortlessly climb walls, or for climbing robots that can cling to ceilings for research or repair applications.

Professor Hsia, President’s Chair in Mechanical Engineering, NTU School of Mechanical & Aerospace Engineering (MAE) and School of Chemistry, Chemical Engineering and Biotechnology, said: “This research is based on a fundamental understanding of the mechanisms of adhesion forces on rough surfaces. could help us develop very strong, yet easily removable, adhesives that can conform to rough surfaces. The technology will be very useful in adhesive grippers and climbing robots and could one day allow humans to climb walls like a real Spider-Man. climb.’

Shape memory polymers are materials that can retain “memories” of their previous shape and return to their original shape after being deformed by the application of external stimuli such as heat, light or electric current. These properties make them ideal for use as switchable adhesives that can adapt to different surfaces.

In their tests, the researchers used a shape memory polymer called E44 epoxy, a stiff and glass-like plastic at room temperature. When heated, the material turns into a soft, rubbery state that can conform and adhere to microscopic nooks and crevices. As it cools, it becomes glassy, ​​creating extra strong adhesive bonds thanks to a form-fitting effect.

When the material is reheated, it returns to its rubbery state so it can be pulled away and easily separated from the surface it was clinging to.

The researchers found that the most effective adhesion came from designing the shape memory polymer into an array of hair-like fibrils. Each fibril had to be carefully designed: larger fibrils had weaker adhesion, while the smaller fibrils were difficult to fabricate and prone to collapse and breakdown. The sweet spot had a radius between 0.5mm and 3mm, pushing the limits of adhesion while maintaining structural integrity.

In their experiments, the researchers discovered that one fibril with a diameter of 19.6 mm² cross section can carry loads up to 1.56 kg. Each additional fibril allows more weight to be supported. An array of 37 palm-sized fibrils, weighing about 30 grams, can support a weight of 60 kg: the weight of an adult human.

The first author of the research article, NTU Research Fellow Dr. Linghu Changhong, said: “Our smart adhesive is an example of how shape memory polymers can maintain and even improve adhesion as surface roughness increases. This overcomes the ‘adhesion paradox’ that scientists are concerned about, where there is a decrease is of the adhesion strength on rough surfaces, despite there being more surface area for molecules to adhere to.

“Our tests showed that the polymer adhesion strength increases along with surface roughness in the solid state and decreases in the rubbery state.”

Co-corresponding author Professor Gao Huajian, formerly a Distinguished University Professor from NTU’s School of MAE and currently Xinghua University Professor at Tsinghua University, said: “For practical gripping purposes, the adhesive should be strong enough to stick to a surface, but still easy to release when necessary. Switching between the two modes is vital for practical applications. Stronger adhesives can withstand heavier loads, but are typically more difficult to release – this is what we call a ‘switchability conflict’.

“Our research into shape memory polymers has resulted in an adhesive that can easily harden to stick to surfaces, and just as easily soften to release, all while supporting heavy weights, including that of a human.”

Professor Hsia added: “The shape memory polymer adhesives we designed have overcome both the adhesion paradox and the switchability conflict, providing guidance for the development of stronger and more switchable adhesives that are adaptable to rough surfaces.”

Paving the way for sticky climbing gear

Detaching the shape memory polymer while it is attached to a glass surface requires less than a minute of heating with a hair dryer to bring temperatures up to 60°C. Conversely, when attaching, it takes about three minutes for the material to completely cool and lock into place.

The temperature at which the polymer changes state can be controlled by adjusting the proportions of the components used to form the polymer. This allows the polymer to be used in extreme environments, such as hot weather conditions. During their tests, the researchers set the polymer release temperature at 60°C, a temperature outside the most comfortable real-world conditions.

The material’s ability to bond and detach with heat alone allows it to act as a reusable super glue that won’t leave a sticky residue on walls. It can also be used as soft grippers that can hold objects with different surface textures and hold them reliably for long periods of time.

Dr. Changhong said: “At this current stage, the heating and cooling times, as well as the switching temperature, limit the number of practical situations. However, our findings show that it is possible to reduce the waiting times to just a few seconds, and the switching temperatures can be reduced to almost body temperature, which dramatically opens up the application possibilities.

“The stimuli to move the material from one state to another can also be different, such as the use of electric current or light.”

In the future, the research team wants to shorten the cooling time required for bonding. The team envisions that the adhesive could eventually be used in climbing gear, such as gloves and boots, that allow climbers to adhere to and climb walls. Robots can also be equipped with the material to create wall climbing robots, which are useful in many industries such as construction and surveying.