Bruce Lee, Associate Professor of Biomedical Engineering at Michigan Technological University, and his team have developed a process whereby an adhesive can be deactivated by applying a small voltage.
Tech Briefs: How did you get the idea for this project?
Professor Bruce Lee: When I was in my PhD program, my advisor was interested in learning from mussels. He wanted to learn how they adhere to surfaces and then to try designing a synthetic adhesive based on them. When I became a professor, I continued to work on some of these projects. We found that under some conditions the chemical was less adhesive. So, we started questioning whether there was a way to control it to the point where we could turn it on and then turn it back off.
Tech Briefs: Is this kind of adhesion different from other adhesives?
Professor Lee: Yes, for example, our adhesive and epoxy glues are both polymers, but the key difference is that we add something to the chemical group so that it mimics a specific amino acid found in mussel-adhesive proteins. What is unique about this is that it can adhere to surfaces under water, while most commercially available adhesives do not bond well to wet surfaces.
Tech Briefs: In your experimental setup, you have the adhesive against a sphere, and you apply current to it through a platinum wire in the presence of saltwater? Is there a chemical significance to using the combination of platinum and seawater?
Professor Lee: Sure, the sphere is titanium and the wire is platinum. In order to apply electricity, you need two electrodes: the titanium sphere is one and the platinum wire is the other — positive and negative. You need the saltwater to conduct the electricity to complete the circuit.
Tech Briefs: Wouldn’t the current flow even if the saltwater wasn’t there?
Professor Lee: It would, but the adhesive, at least the version we have, is not very conductive, so we used the saltwater as a more conductive medium. What we’re trying to go for in the future, is to improve the conductivity of the adhesive. This is really just the first experiment.
Tech Briefs: Is the circuit wired back to a voltage source?
Professor Lee: Yes. The power source applies a voltage across the electrodes.
Tech Briefs: Could you give me some idea of the amplitude of the voltage and current.
Professor Lee: The voltage range we used was one to nine volts. Nine volts was much faster than one volt — nine volts would take less than a minute to completely deactivate the adhesive, one volt, significantly longer. The current is on the order of tens of milliamps.
Tech Briefs: How do you measure the stickiness?
Professor Lee: The sphere acts as the surface of contact, as well as the conducting electrode. The sphere, which is attached to the adhesive is connected to a load cell. We can pull away in the same direction as the load cell so we can measure the force required to retract the surface.
Tech Briefs: What sort of mechanical apparatus do you use to pull on the sphere?
Professor Lee: We have an actuator that moves up and down connected in series with the load cell to measure the force of adhesion and compressive force, and at the end it’s attached to the metal sphere. We can push against the adhesive and then pull away. That will give us the force that’s necessary to detach the sphere from the adhesive.
Tech Briefs: Can you give me an idea of some possible applications.
Professor Lee: This project is funded by the Office of Naval Research. They’re interested in an adhesive that can be used to attach and detach sensors or other devices underwater, with the push of a button — that’s their long-term goal.
There are also possible biomedical uses for temporary application of prostheses, wearable sensors — things like that. Also, removing wound dressings often damages the skin. So, it would be desirable to deactivate, then remove that adhesive.
In manufacturing, it could be used for attaching and detaching parts with complex geometries. Different geometries that are hard to attach/detach could be attached more easily — you could first position the part and then activate the adhesive.
Tech Briefs: Are you now working on being able to reverse the process — to be able to activate the adhesive electrically?
Professor Lee: Yes. We demonstrated in the past, being able to use pH to turn on the adhesion. With electricity, it’s slightly more complicated. We just renewed our Navy grant to work on that now.
Tech Briefs: Could you explain how electricity affects the adhesive properties.
Professor Lee: The key chemistry that’s involved is what’s called oxidation. For example, when an apple is exposed to oxygen it turns brown — that’s an oxidation process — It’s similar to that. Our chemical oxidizes and turns brown as well. The pH is more basic when it’s oxidized and with an acidic pH, it returns to a reduced form. So, you can use pH to control the oxidation state. What’s novel about our chemical is if you oxidize it, it becomes less adhesive. What we did before was to use pH to make it more basic to reduce the adhesion and then use the acidic pH to reverse that. Now we’re using electricity, essentially electrochemical oxidation — the electricity oxidizes the adhesive and reduces its adhesive property. The complicating aspect is that it also leads to cross-linking, which makes it less reversible — that’s the part we’re trying to control.
Tech Briefs: What would you change?
Professor Lee: Part of the issue is that nine volts is too high, so we’re trying to deactivate the adhesive with less voltage and current. Hopefully, that would prevent the irreversible cross-linking we’re observing. But to do that we might have to make it more conductive. We want to control the chemistry to the extent that when we deactivate, we don’t push it so far that it would become irreversible. That’s the balance of the things we have to work with — we haven’t found the right conditions yet.
Tech Briefs: Will this be able to work without using saltwater as a conductor?
Professor Lee: Ultimately, that’s what we want to do. We have to make the adhesive more conductive. Then it would be more independent of the environmental.
Tech Briefs: What excites you most about this project?
Professor Lee: This is a completely new and different area of research. So, what’s interesting, is that there are a lot of unknowns in the chemistry, and lots of things we can study. The challenge and opportunity in that are very exciting.
An edited version of this interview appeared in the July 2020 issue of Tech Briefs.