Glue with an On-and-Off Switch
Researchers can make an adhesive either stick or come undone by changing its acidity.
Researchers at the University of Sheffield, in the UK, have made an adhesive that can be turned on and off with an external switch. The switch is the acidity of the solution surrounding the glue: two different kinds of polymers in the glue attract each other based on the solution’s acidity. By repeatedly making the solution less or more acidic, Mark Geoghegan and his colleagues can switch the adhesive’s stickiness, making two surfaces bond together and then come apart up to five times.
Scientists are developing several different reusable glues, such as tapes that mimic gecko feet and mussel adhesives. (See “Climbing Walls with Carbon Nanotubes” and “Nanoglue Sticks Underwater.”) These are like sticky tape: they need pressure to make them adhere to a surface and then require force to be pulled off. There have also been efforts to develop reusable adhesives whose tackiness can be turned on and off with heat or electricity, says Manoj Chaudhury, a chemical-engineering professor at Lehigh University who was not involved in the current research.
The new work, published online last week in Angewandte Chemie, shows that acidity– the pH level of a liquid–could be used to reverse a material’s stickiness. “I think that trying to control adhesion using pH is an excellent idea,” Chaudhury says.
The new adhesive consists of two different parts. One is a 20-nanometer-thick layer of an alkaline polymer on a silicon substrate. The other is an acidic hydrogel, a network of polymer chains scattered in water. The researchers attach the ends of the alkaline-polymer chains to the silicon substrate so that the polymers stick out of the surface “like bristles on a brush,” Geoghegan says.
When the arrangement is placed in a slightly acidic solution, the oppositely charged polymer and the hydrogel attract each other and form a tight hydrogen bond. When the researchers increase the acidity of the solution to pH 1–about the range of battery acid–the alkaline polymer loses its charge, and the hydrogen bonds break. The researchers can then peel the two surfaces apart.
The adhesive could prove useful for medical applications such as wound dressing, says Costantino Creton, a polymer-adhesion expert at the Industrial Physics and Chemistry Higher Educational Institution, in Paris. You could apply the adhesive to a wound until it has healed, he says; then, when you “put your finger in a solution with the right kind of pH, [the adhesive] comes off.” However, Creton cautions that for medical use, the researchers would need to develop a similar system that works at a less acidic pH.
According to Geoghegan, the adhesive system could also be used for drug delivery by incorporating it along with drugs into a pill. Because the stomach and the intestines have different acidity levels, researchers could design the system to let go of its drug cargo in either organ, he says.
To test the strength of the adhesive, Geoghegan and his colleagues put two different forces on top of the gel to press the surfaces together. Under the heavier load, the surfaces stick together tighter and take longer to separate–three days, as opposed to seven hours with the lighter load. Geoghegan speculates that the reason the adhesion is stronger under the heavier load could be a “Velcro effect,” in which the polymer brush penetrates the hydrogel. The researchers are now trying to find out if the brush and gel do indeed act like Velcro.
Even the few hours it takes to separate the surfaces is too long for practical use, Creton says. For use in dressing wounds or other applications, the adhesion should switch off faster.