Systematic Study on the Remote Triggering of Thermo-Responsive Hydrogels Using RF Heating of Fe3O4 Nanoparticles

Abstract

ABSTRACTIn this work, a systematic study on the factors that influence the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) (PNIPAM) solutions during remote radiofrequency (RF) heating, using Fe3O4 magnetic nanoparticles (MNPs) is reported. A series of PNIPAM solutions with varying concentrations of Fe3O4 MNPs were prepared and characterized using transmission electron microscopy and Raman spectroscopy. Preliminary studies showed the highest specific absorption rate (SAR) for 15 nm sized Fe3O4 MNPs, which monotonically decreased as the MNP sizes increased to 20-30 nm. In-situ transmission measurements were used to determine the LCST of PNIPAM under various aqueous concentrations with dispersed Fe3O4 MNPs. A systematic decrease in the LCST from 34 °C to 31 °C was observed as the concentration of PNIPAM was increased from 0.3 wt. % to 1.0 wt. %, keeping the concentration of Fe3O4 MNPs constant. On the other hand, varying the concentrations of the MNPs did not drastically affect the LCSTs of PNIPAM solutions. However, varying the ion concentration of the PNIPAM solutions by adding adjusted KOH pellets, showed a pronounced lowering of the LCST by 2-3 °C at all PNIPAM concentrations. The remote triggering of phase transitions in PNIPAM solutions by raising the temperature above the LCST using Fe3O4 MNPs as reported here is important in targeted drug-delivery applications using thermo-responsive polymers.