Scientists from Japan have developed a new glue that can be turned on and off at will. The glue is made using caffeic acid, which forms and breaks cross-links when exposed to light of different wavelengths. When irradiated with ultraviolet light with a wavelength of 365 nanometers, the glue turns into a strong film that holds at room temperature with a shear bond strength of up to 7.2 MPa.
However, when irradiated with 254-nm ultraviolet light, the cross-links break down and the adhesive reverts to its original state, allowing it to be reused like new. The researchers embedded magnetic nanoparticles in the adhesive, which heat up when a magnetic field is applied, bonding the adhesive to the substrate. This material could find a wide range of applications, making it easier to disassemble products into their component parts at the end of their useful life and turn them into new products.
Developing adhesives is tricky because it involves balancing two conflicting properties: how well they stick together and how easily they separate from each other. In enhancing one property, the other is usually sacrificed. The ideal adhesive should be one that holds firmly during use, but can be separated on demand to correct mistakes or when the product is no longer needed.
Researchers at Japan’s National Institute of Materials Science (NIMS) have developed an adhesive that does just that. The key component is caffeic acid, which is able to form and break down cross-links when exposed to light of different wavelengths. In this case, the experts made a polymer containing caffeic acid, applied it to a surface and irradiated it with ultraviolet light with a wavelength of 365 nanometers (nm). As a result, the polymer hardens into a strong film that holds at room temperature with a shear bond strength of up to 7.2 MPa.
When adhesion is no longer needed, the film can be exposed to 254 nm UV light, which breaks down the crosslinks and returns it to its original state. The film leaves no residue on the surface and does not lose its adhesion properties, allowing it to be reused as a new film.
The researchers subjected the adhesive to a series of tests, including repeatedly bending samples and lifting a 40 kg (88 lb) weight, which it withstood for 72 hours without any signs of failure. In other cases, the glue was used to repair cracked silicone tubing, then high-pressure water was run through it and there were no leaks.
In subsequent tests, the team demonstrated that the material could be used even underwater. Magnetic nanoparticles were embedded in the adhesive, which heat up when a magnetic field is applied, bonding the adhesive to the substrate.
According to experts, this material could find a wide range of applications, making it easier to disassemble products into their component parts at the end of their useful life and turn them into new products.
According to Dr. Emre Kizilcan, a postdoctoral fellow in the Department of Mechanical Engineering at Imperial College London, “The development of a light-sensitive adhesive that can be activated and deactivated on demand is a significant breakthrough as it provides a reversible bonding mechanism that can be used in a variety of applications such as manufacturing, electronics and biomedicine.”
The study is published in the journal Advanced Functional Materials.
