Nipaam monomer

N-Isopropylacrylamide (NIPAAm) (700 mM) monomer and N,N’-Methylenebis (acrylamide) (BIS) (8.6 mM) crosslinker were dissolved in deionized water. Fe₃O₄ (2.5 wt% of monomer solution) was prepared in water and blended with the monomer solution. The mixture was stirred for 15 min to obtain a homogeneous dispersion of the magnetite nanoparticles. The radical initiator Ammonium Persulfate (APS) (0.1 wt% of mixture) and accelerator Tetramethylethylenediamine (TEMED) (2.25 wt% of mixture) were then added and mixed to initiate the redox reaction. After mixing, the solution was injected into a cylindrical mold that was 400 μm in diameter and allowed to stand overnight at room temperature to achieve the desired cylindrical iron-encapsulated PNIPAAm hydrogel particles. In order to prevent the particle from sticking to the substrates during experiment, we formed an intercalating lubricating layer by immersing the particles in Tween 20 solution until fully swollen^{21 (link)}. The diameters of hydrogel particles were 700 μm at the swollen state.
The chemicals, including NIPAAm monomer, BIS cross-linker, TEMED, APS were obtained from Sigma Aldrich (St. Louis, MI). The magnetic Fe₃O₄ nanoparticles (15–30 nm diameter) were obtained from US Research Nanomaterials, Inc (Houston, TX).