Investigation of thermo-responsive optical properties of a composite hydrogel

Poly(N-isopropylacrylamide) (PNIPAM) hydrogels crosslinked by methylene bisacrylamide (MBA) are known to exhibit a transition from hydrophilic to hydrophobic character at the lower critical solution temperature (LCST) of approximately 32 °C. This transition is normally accompanied by a volume change of the hydrogel. In this paper we describe a novel method of synthesizing composite PNIPAM/alginate particles that change their optical properties (transparency) in response to a temperature change but keep both their original shape and size. The method is based on loading the NIPAM monomer into a calcium alginate hydrogel and carrying out the polymerization above the LCST. The thermo-responsive optical properties of the composite particles were characterised in detail by optical microscopy and image analysis. The change in transparency as function of temperature was found to be fully reversible but revealed some hysteresis. An optimum value of PNIPAM concentration in the composite hydrogel was found, for which the difference in transparency above and below the LCST is maximised.