Visual, non-invasive monitoring of body temperature of patients in hospitals without using a thermometer may become a reality thanks to the work carried out by a team of scientists led by Dr. John Philip, Head of the SMART section at the Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam near Chennai. The concept is based on ferrofluid emulsion contained in a thin film that changes colour with rise in temperature within a narrow range — 30-40 degree C. The results were published in the journal Optical Materials.
The emulsion has iron oxide nanoparticles-containing oil droplets dispersed in water. The stimuli-responsive materials change in their properties to stimulus such as stress, temperature, moisture, or magnetism. “Till now ferrofluid was used as a magnetic stimuli-responsive material and we have come up with several applications such as hermetic seal, optical filters and defect detection. We now found that in the presence of a temperature-sensitive polymer — poly(N-isopropylacrylamide or PNIPAM) — the ferrofluid emulsion can be used as a thermally tunable grating to produce different colours,” says Dr. Philip.
“Recently, we were looking at the interaction forces between droplets covered with thermoresponsive polymers. To our surprise, we found that the adsorbed polymer swells and collapse upon changing the temperature between 32 and 36 degree C. This change was clearly manifested as colour change. From this observation came the novel idea of using PNIPAM-stabilized emulsions as a multistimulii grating. This is a first-of-its-kind approach where the grating spacing can be tuned either by changing the temperature or by changing the magnetic field strength,” says Dr. Philip.
Up to about 34 degree C, the polymer is highly hydrated and swollen due to repulsive interaction between individual monomer segments. But when the temperature crosses 34 degree C, the polymer becomes dehydrated leading to a collapsed state (due to inter and intra attractive forces between monomers). The polymer can once again become hydrated and swollen when the temperature falls below 34 degree C. “By using certain additives, we can tune the collapse of the polymer to higher temperature to reflect fever conditions,” clarifies A.W. Zaibudeen, senior research fellow and the first author of the paper.
Using magnetic field, the scientists first achieved a particular ordering (spacing between the arrays of emulsion droplets) of emulsion and got a specific colour. When the polymer is added as a stabiliser and the temperature is increased the grating spacing of the polymer changes and gives rise to a different colour or spacing.
“The colour given off at normal temperature can be fixed by changing the emulsion property and magnetic field strength,” Dr. Philip says. If yellow is chosen to represent normal temperature, it will change to green when the temperature increases. Colour with higher wavelength is produced at lower temperature and colour of lower wavelength at higher temperature.
The researchers see numerous applications for their gratings — visual manifestation of environmental conditions (temperature and humidity) and selection of a particular colour from white light. In addition, there other potential specialised applications such as calorimetric sensors, photonic materials, optical devices and drug delivery systems. “I believe that once the proof of concept is demonstrated, the scientific community would come up with many more new ideas for practical applications,” Dr. Philip says.