Visual, non-invasive monitoring of body temperature of patients without using a thermometer may become a reality soon, thanks to the work carried out by a team of scientists led by John Philip, head of the smart materials 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° C. The study was published in the journal Optical Materials .
The emulsion has iron oxide nanoparticles containing oil droplets dispersed in water. “Till now ferrofluid was used as a magnetic stimuli-responsive material. We now found that in the presence of a temperature-sensitive polymer — poly(N-isopropylacrylamide), also known as 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 thermo-responsive polymers. To our surprise, we found that the adsorbed polymer swells and collapses upon changing the temperature between 32° and 36° C. This change was clearly manifested as colour change. From this observation came the novel idea of using PNIPAM-stabilised 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° C, the polymer is highly hydrated and swollen due to repulsive interaction between individual monomer segments.
But when the temperature crosses 34° C, the polymer becomes dehydrated leading to a collapsed state.
The polymer will once again become hydrated and swollen when the temperature falls below 34° 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 at IGCAR and the first author of the paper.
Using magnetic fields, the scientists first achieved a particular ordering (spacing between the arrays of emulsion droplets) of emulsion and got a particular colour.
When 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 the normal temperature is fixed at yellow, the change will be to green when the temperature increases.
Colour with higher wavelength is produced at lower temperature and colour of lower wavelength at higher temperature.