“Dismantlable” Adhesives: Easy on Materials, Indispensable to Modern Society

My research project, the Development of Photoreversible Adhesives Based on Thermally Stable Molecular Switches, which was selected for the Japan Science and Technology Agency’s PRESTO Strategic Basic Research Program, focuses on molecular switches. These are molecules that can switch between two or more stable states, and I use these to develop adhesives that can repeatedly bond and debond materials by light.

Adhesives are used in all kinds of everyday products, but we don’t actually have a clear understanding of why they stick. Nevertheless, polymeric materials like plastic and rubber cannot be welded or bolted together, so joining by adhesives is the only viable method. In recent years, there has been an increased spotlight in the automotive field and elsewhere on multi-materials, which combine different materials to suit the applications, and this has led to a growing need for technologies to bond different materials together. It is also important that adhesives can break down products into their constituent parts to allow for recycling of the materials, and thus adhesives must come off without damaging the materials. To date, this has meant adhesives debondable through the application of heat, but because the whole of the adherends is heated, the heat sometimes damages the materials, so although they peel off they do not remain fit for recycling.

My aim is to create an adhesive that can dismantle products using a specific wavelength of light, such as ultraviolet. Unlike heat, light can stimulate only the targeted area and only when needed, and its wavelength and irradiance can be easily and precisely controlled, thus minimizing damage to the adherends.

How are such adhesives made? This is where the functional dyes and polymers that my research focuses on come into play. Polymer is a general term for materials like plastic, rubber, and resin, and adhesives are mainly composed of polymers, too. Functional dyes are molecules that change color or other properties in reaction to external stimuli such as light, force, or electricity. The molecular switches mentioned at the start of this article are also functional dyes. By combining polymers and functional dyes, it is possible to create adhesives that gain and lose adhesiveness in response to specific stimuli.

Working with Functional Dyes and Polymers at Vastly Different Scales

In our PRESTO project, we are striving to develop an adhesive that can change from solid to liquid and back again based on photoinduced changes of molecular switches. When solid, the adhesive sticks strongly; when exposed to ultraviolet light, it turns into a liquid and peels off. When the liquid is exposed to visible light, it solidifies and becomes sticky again.

The core focus of our work is thermal stability, which means that the molecular switches are not “switched” unless exposed to precisely the right kind of light, and that the solid and liquid states of the adhesive remain stable.

One well-known molecular switch is azobenzene. It can also be used to switch a material between solid and liquid states, but has one drawback: it is thermally unstable. Specifically, it cannot maintain its liquid state at room temperature and immediately reverts to solid. One of the molecular switches I am studying is hindered stiff stilbene (HSS). Although HSS is not permanently stable and cannot sustain its state indefinitely, still it has a 1,000-year half-life (i.e., the time it takes for one half to return to its original state), so I believe it could be useful in practice.

I am often asked that if ultraviolet light can be used to dismantle materials, surely then it must be unusable outdoors. The answer is no. This is not the case. Certainly, sunlight contains ultraviolet rays, but they occupy a much smaller fraction than visible light. Most adhesives we developed become solid when exposed to visible light, and are therefore able to maintain their solid state outdoors.

Also, it is possible to change the wavelength of light a molecular switch responds to, and to determine whether or not a material changes states, or whether it goes from one state to another but not the other way around, by altering the design of the molecular switches, polymers, and materials. This design is simultaneously the most fascinating and challenging part of my research. Creating something that induces visible changes in state such as sticking and unsticking requires not only the precise design of molecular structures on a microscopic scale, but also the appropriate design of material structures on a scale between those two. Moving back and forth from one wildly different scale to the other makes designing the structures a challenge, but my ability to do so has become one of my strengths.

Boldly Pursuing Interesting Research Wherever It Leads

The concept of using light to debond adherends has been proven to work, so now we would like to commercialize this and put it to practical use through joint research with the private sector. This will require further improvements of the adhesive strength and reversibility. For now, we are working to achieve adhesives that can bond and debond materials not just once, but again and again.

For me, it is all about the fun of research; that was the policy that led me to decide on the theme of the project that got picked up by PRESTO. I dream of overturning common sense and rewriting the textbooks, but coming up with the ideas to make this happen requires movement—I can’t stay stuck in the same place. That’s why I strive to pursue interesting research into whatever new fields it may lead me. Indeed, I believe that this willingness to follow my research into new fields is another of my strengths. At the moment, I am looking into harnessing machine learning to make the synthesis and design of dyes and polymers smoother, and there are many other things I would like to try. For instance, in addition to the PRESTO project, I am also working on soft actuators (i.e., artificial muscles), so I would like to try my hand at developing soft robots.