Gel puts sensors in a new light

A thin film of a novel gel can serve as a fast and inexpensive sensor for pressure, temperature, pH, humidity and other variables, believe researchers at the Massachusetts Institute of Technology, Cambridge, Mass. The material responds quickly to changes and provides a far greater color range than other gels, says Edwin Thomas, professor and head of the school’s Department of Materials Science and Engineering.

Pressure variation causes instantaneous change in color, he notes, while response to temperature shift takes only sub-seconds, too fast for the eye to see.  The material is unique, what he calls the “world champion” in color change — it boasts up to a 600% change in reflectivity, far greater than possible with other color-changing gels, claims Thomas. “You can use mechanical or chemical forces to get really big responses, going through the entire range of light from ultraviolet to infrared,” he explains. Calibrating of color against the values of the particular variable allows the gel to provide high resolution.

The gel consists of 10 to 15 pairs of alternating layers of hard and soft polymers, polystyrene and poly-2-vinyl-pyridine (P2VP), respectively, and is about 1 micron thick. The material is self-assembling — the separate layers develop on their own as the gel forms, notes Thomas.

“We wanted to develop something that was ‘tunable’,” he says, adding “The gel is very versatile.”

Key to the greater color range is that the film structure restricts physical change to a single dimension — thickness — unlike other gels that expand in three dimensions.
External stimuli like pressure or temperature alter the thickness of the softer P2VP layers or the film’s index of refraction. Increasing pressure doesn’t affect the index of refraction but compresses the layers, shifting the color towards blue, notes Thomas.

Temperature response is more complex because the index of refraction goes down while thickness goes up as temperature rises.

Thomas has tested the gel for 500 cycles and has found no deterioration in its performance. The mechanisms that cause the color change have no obvious reason to fatigue, he notes.

So far, the researchers have made 4 in. by 4 in. gels. However, there’s no size restriction, says Thomas. Basically, low-viscosity block copolymer is dissolved in a solvent and then sprayed or painted onto a surface, which can be irregular.

Evaporation creates the gel. The resultant thin film is soft and so might scratch or peel but can be protected by coating with a clear film.

Sensors should be low cost, Thomas believes, because the polymers are inexpensive and easily processed, plus not much material is needed and application is easy.

Potential plant roles include to visibly warn personnel that process equipment or piping is at high temperature. In addition, the gel offers benefits for research, such as on large-area airfoils, to replace embedded sensors now used. The researchers also are working on a gel that changes color in response to applied voltages, which should be good for study of electrical fields, he says. “No ‘killer’ applications are obvious yet, though,” admits Thomas. The school is thinking of licensing the technology, and several companies already have approached the researchers.

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