Designing adhesives means balancing two conflicting properties – how well it sticks together, and how easily it comes apart. Obviously, boosting one usually sacrifices the other. An ideal glue would be one that holds strong during use, but can be released on demand to adjust for mistakes or when a product is no longer useful.
Now, scientists at Japan’s National Institute for Materials Science (NIMS) have developed an adhesive that can do just that. The key ingredient is caffeic acid, which can form and break cross-links under different wavelengths of light. In this case, the team made a polymer containing caffeic acid, applied it to a surface, and exposed it to UV light with a wavelength of 365 nanometers (nm). This cures it into a firm film that holds strong at room temperature, with a shear adhesion strength of up to 7.2 MPa.
When that adhesion is no longer needed, the film can be exposed to 254-nm UV light, which breaks the cross-links and returns it to its original state. In doing so it doesn’t leave any residue behind on the surface and doesn’t lose any of its adhesive properties, essentially allowing it to be reused like new.
The researchers subjected the adhesive to a series of tests, including bending samples repeatedly and lifting a 40-kg (88-lb) weight, which it was able to do for 72 hours with no sign of breaking. In others, they used it to repair cracked silicon tubes then ran high-pressure water through them, finding no leaks.
In follow-up tests, the team demonstrated that it can even be used underwater. Magnetic nanoparticles were embedded into the adhesive, which heat up when a magnetic field is applied, fusing the adhesive to the substrate.
The team says this material could have a wide range of applications, allowing products to be more easily dismantled into components at the end of their useful lives and converted into new products.
The research was published in the journal Advanced Functional Materials.
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