Thermo-responsive adhesion properties of composite hydrogel microcapsules

The present work describes the synthesis and characterisation of thermo-responsive microcapsules consisting of poly-N-isopropylacrylamide (PNIPAM) hydrogel, hydrophilic citrate-stabilized superparamagnetic iron oxide nanoparticles (SPIONs) that perform the function of a local heat source and hydrophobic nanoparticles (either SiO(2) or oleic acid-coated SPIONs) that stabilize the microcapsule surface. The microcapsules have been synthesized by the inverse Pickering emulsion polymerisation method. The effect of iron oxide concentration in the hydrogel on the shape, size, shrinkage ratio and heating rate of the microcapsules has been systematically investigated. The adhesion properties of the composite microcapsules towards a range of substrates have been characterised in a microfluidic flow cell and it was found that the temperature-induced change of the microcapsule size is an effective way of controlling adhesion. The induction heating of the embedded SPIONs makes it possible to control adhesion remotely in environments where macroscopic heating is not possible.