A new type of reversible adhesion changes stickiness at the flip of a pH switch

Researchers from the UK and Germany have developed a novel system of reversible adhesion, where two surfaces bind tightly or loosen completely depending on the prevailing pH1.

Mark Geoghegan from the University of Sheffield, UK, and colleagues created a polymer ’brush’ consisting of chains of the polybase poly[2-(dimethyl amino)ethyl methacrylate] attached to a silicon surface. They presented the brush to a gel consisting of poly(methacrylic acid), in which the polymer chains are three-dimensionally cross-linked.

At around neutral or slightly acidic pH, the acidic groups of the gel carry a negative charge while the basic groups of the brush ’hairs’ are positively charged. In these circumstances there is strong electrostatic attraction between the two and the brush and the gel, in Geoghegan’s words, ’stick like mad’. 

"The technology is so outrageously simple that I cannot believe it will not find a use" - Mark Geoghegan, University of Sheffield, UK

If the pH of the surrounding solution is then dropped to around 1, the basic groups lose their charge and the surfaces come apart without any damage. Significantly, the process is reversible.   Geoghegan does not envisage immediate applications for the discovery, but is confident that it will prove useful. "The key thing is that when you start to talk about switching in a molecular environment, you don’t want to have to make complex wiring arrangements. You want something that will respond automatically to its environment and is self-regulating," Geoghegan told Chemistry World. ’I see this kind of technology as forming an important part of the concept of a molecular toolbox for soft nanotechnology - the development of nanotechnological devices that can mediate macroscopic phenomena. In this case adhesion. I will leave it to the engineers to think about how these things can be incorporated into devices, but the technology is so outrageously simple that I cannot believe it will not find a use.’ Geoghegan is now looking to characterise in more detail the nature of the forces involved in the adhesion, which also appear to involve hydrogen bonding.

Bi-min Zhang Newby, a surface scientist at the University of Akron, US, said that the work was ’interesting because it demonstrates the reversible switching adhesion of polyelectrolytes by simply tuning the pH of the aqueous solution.’ She added, ’More importantly, the work is scientifically significant because it is the first time anyone has attempted to quantify the adhesion strength and to determine the origin of the adhesion of responsive materials, in this case, polyelectrolytes.’

The new system comes hot on the heels of the demonstration of ’geckel’, a new type of adhesive technology developed by scientists in the US.2 This borrows two adhesive approaches from nature - the use of nanoscale hairs, as found on the feet of geckos, coated with a synthetic polymer based upon 3,4-L-dihydroxyphenylalanine, which is similar to one of the polymers that mussels use to anchor themselves to surfaces. The resulting surface shows high adhesion in water and can be peeled off and re-stuck many times over.

Simon Hadlington

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