Team creates LEDs, photovoltaic cells, and light detectors using novel one-molecule-thick material.
CAMBRIDGE, Mass.-Steve Leeb proudly holds up to the light a tiny vial containing a cylinder of clear gel speckled with flecks of metal. "This represents 18 months of work," said Dr. Leeb, the Carl Richard Soderberg Assistant Professor in Power Engineering at MIT.
The gel and two similar ones are the first of their kind: they contract in response to a magnetic field, and expand to their original size when the field is removed. One potential application is in the controlled release of drugs. When loaded with a pharmaceutical and placed under the skin, the gel could be triggered to contract, releasing the drug. This could be quicker and more efficient than the oral administration of drugs, and easier than injections.
"I envision a watch-sized device containing a power supply and coil [to generate a magnetic field]. When the individual needed to receive a dose of drug he or she would simply position the device over the implanted gel, press a button, and activate the magnetic field," said Professor Leeb, who holds appointments in the Department of Electrical Engineering and Computer Science and the Laboratory for Electromagnetic and Electronic Systems.
Professor Leeb and colleagues will be presenting a paper on the magnetically activated gels at a June 24-27 conference in Italy held by the Institute of Electrical and Electronics Engineers.
Gels are composed of long polymer chains that are cross-linked to one another and suspended in a solvent like water. Jello is a common example. The MIT work, however, is based on a special kind of gel with a unique behavior. In response to a small change in temperature, pH, or another environmental stimulus, the gel will dramatically contract or expand.
These gels were discovered in 1978 by Professor Toyoichi Tanaka of the Department of Physics, who has worked with them ever since. On June 1, he won a 1996 Discover Magazine Award for Technological Innovation for "smart hydrogels," one class of the gels.
Professor Leeb became intrigued by the gels after taking an undergraduate course taught by Professor Tanaka. Now the two are collaborators. "In my lab we're trying to develop practical applications for the gels. Toyo's lab is our resource for the fundamental physics and understanding of the material," Professor Leeb said.
Work on the new gels that can be activated by a magnetic field began when it occurred to Professor Leeb and colleagues that such a material might be useful. In the end, they devised three different versions.
For the first, the researchers inserted a tiny nickel needle into a preformed gel. The idea is that when a magnetic field is applied, the nickel heats up. That in turn heats the gel, which contracts. When the field is removed, the gel cools and goes back to its original size.
Though this version works, its applications are limited. "You might not want a little needle in an implant that's under the skin," said Professor Leeb, referring to the potential device for the controlled release of drugs.
So, with graduate students Deron K. Jackson and Ahmed H. Mitwalli of the Department of Electrical Engineering and Computer Science, he went back to the lab and came up with another idea. Why not create a gel with tiny flakes of metal interspersed throughout?
This turned out to be more difficult than the researchers expected. "Because the flakes are so small, we couldn't put them in after the gel was formed, as we did with the needle," Professor Leeb said. So they decided to add the metal to the recipe for the gel itself. That initially didn't work because the metal deactivated the gelation process. "It was very discouraging. We just ended up with a metal goo," remembers Professor Leeb.
The solution? The researchers coated the metal particles with the material poly (vinyl alcohol), or PVA, before adding them to the recipe. That let the gel formation proceed.
In the third demonstration, the researchers replaced the water in the gel with a ferrofluid containing sub-microscopic particles of metal. The metal fluid seeps into the gel, and the gel swells to its full size. But when a magnetic field is applied, the ferrofluid heats, causing the gel to contract. This system might not be practical for use in the body because of the metal fluid. However, Professor Leeb is exploring other applications that could take advantage of the gel/ferrofluid mixture's change in viscosity.
Authors of the paper to be presented in Italy are Professor Leeb; Elmer C. Lupton '65 of Gel Sciences, Inc.; Mr. Jackson; Mr. Mitwalli; Pablo Narvaez, who received the SB in EECS last week, and Dahlene Fusco, a Wellesley senior who is participating in the work through MIT's Undergraduate Research Opportunities Program.
The work is funded by a seed grant from the Center for Materials Science and Engineering (through the NSF), MIT's Carl Richard Soderberg Career Development Chair, and AT&T.