Remotely Controlled Micromanipulation by Buckling Instabilities in Fe3O4 Nanoparticle Embedded Poly(N-isopropylacrylamide) Surface Arrays.

The micromanipulation of biological samples is important for microbiology, pharmaceutical science, and related bioengineering fields. In this work, we report the fabrication and characterization of surface-attached microbeam arrays of 20 µm width and 25 µm height made of poly(N-isopropylacrylamide), a thermoresponsive polymer, with embedded spherical or octopod Fe3O4 nanoparticles. Below 32 °C, the microbeams imbibe water and buckle with an amplitude of approximately 20 µm. Turning on an AC-magnetic field induces the microbeam array to expel water due to the heating effect of the nanoparticles (magnetic hyperthermia), leading to a reversible transition from a buckled to nonbuckled state. It is observed that the octopod nanoparticles have a heating rate 30% greater (specific absorption rate, SAR) than that of the spherical nanoparticles, which shortens the time scale of the transition from the buckled and nonbuckled state. The return of the microbeams to the buckled state is accomplished by turning off the AC magnetic field, the rate of which is dictated by dissipation of heat and is independent of the type of nanoparticle. It is further demonstrated that this transition can be used to propel 50 µm spherical objects along a surface. While the motion is random, this study shows the promise of harnessing shape-shifting patterns in microfluidics for object manipulation.

Poly(N-isopropylacrylamide) cross-linked coatings with phototunable swelling.

A series of terpolymers were synthesized comprising the following monomers: N-isopropylacrylamide (NIPAAm), a stimuli-responsive structural unit that swells and collapses in response to temperature; methacryloxybenzophenone (MaBP), a photo-cross-linking unit that is activated at a wavelength of 365 nm; and phenacyl methacrylate (PHEm), a photolabile protected carboxyl group that can be deprotected at a wavelength of 254 nm. It is shown that the terpolymers can be photo-cross-linked at long UV wavelength light (λ = 365 nm) to establish surface-attached, cross-linked coatings and subsequently photochemically cleaved at short UV wavelength light (λ = 254 nm), which is found to be consistent with first-order kinetics. The photocleavage reaction produces free carboxylic groups, which can be used to locally tune the swelling characteristics and transition temperature of the coating, which depends on both the irradiation exposure and the overall PHEm content. For instance, for a terpolymer with 7.1 mol % PHEm, the transition temperature between the swollen and collapsed states increased from 20 to 50 °C at a pH of 8.5 with an exposure dose of 0.52 J/cm(2) at 254 nm. Finally, photocleavage can be used to create chemically patterned regions to provide a basis by which to conjugate cationic markers, proteins, or nanoparticles to the terpolymer coating.