Researchers at MIT have developed a new gel they think will be useful as a highly sensitive, quickly responding, and inexpensive environmental, or chemical sensor to indicate when pressure, temperature, or humidity levels have changed.
The new gel responds almost instantly to stimuli and changes color when exposed to different temperatures, pressure changes, salt concentration levels, and different humidity levels making it a highly sensitive environmental sensor.
One place it would be useful is in a food plant. If the food is supposed to be kept dry, the gel would quickly indicate that the food has been exposed to humidity by quickly changing colors after having been exposed to the humidity.
The gel is made of material that expands and contracts, or changes in thickness, when it is exposed to different environmental stimuli. The change in the gel’s thickness makes the gel change color. The gel changes into colors from the entire range of spectrum light the eye can see.
The gel is unique in the entire world because it is the only thing existing that changes colors because its properties can continually be changed. Objects are already being manufactured that change colors when you look at them from different directions, but their properties always remain the same. The MIT research team wanted to develop something whose properties would change in accordance to the different stimuli it was exposed to.
The gel was first made of a thin film of a layer of polystyrene and poly-2-vinyl-pyridine. Polystyrene is a clear plastic that can be molded into different shapes, or it can also be used to make foam and poly-2-vinyl-pyridine forms gels. The idea was that the two substances together will refract different colors of light depending on how thick the substances are and how well they refract light.
The researchers found that if they kept the layer of polystyrene constant in thickness, but altered the thickness of the poly-2-vinyl-pyridine by stimulating it with pH levels and salt concentrations, the substance changed color within fractions of a second.
The researchers tested the gel by adding salt and found that the salt’s ions makes the poly-2-vinyl-pyridine collapse in on itself, changing the volume of the substance and changing the color. The poly-2-vinyl-pyridine expands again when the salt ions are washed away.
They also saw that the gel responds very sensitively to changes in pressure, humidity, and temperature.
One thing that distinguishes this color changing gel from other color changing gel is that it was made in a one-dimensional structure as opposed to the others that were made in a 3D lattice structure. Because the new gel is one dimensional, the gel can only expand in one dimension so more color wavelengths are reflected. The shifts in color in the new gel are different by 575 percent. The three dimensional gels have a much smaller range of colors that they change to.
Elizabeth Thomson, “MIT gel changes color on demand,” Massachusetts Institute of Technology.