(503c) Controlled, Wireless Heating of Iron Oxide Nanoparticle Composites

Magnetic induction heating of superparamagnetic iron oxide nanoparticles (SPIONs) has been proposed to wirelessly power polymer composite films for in situthermal sterilization of implanted medical device infections. Wide variations in device application demand coatings with a wide variety of mechanical and chemical properties. For example, orthopedic implant coatings must withstand large mechanical stresses while heart valves require significantly more elasticity. To determine the effect of the composite matrix and fabrication processes on the coating's ability to deliver wireless heating, we investigated the synthesis, characterization, and performance of SPIONs in hydrophobic (polystyrene (PS)) and hydrophilic (poly(vinyl alcohol) (PVA)) polymer films and their application to controlled surface heating in an alternating magnetic field (AMF). Magnetic composites were characterized for particle size, agglomeration, and dispersion (scanning electron microscopy); crystal structure and oxide state (x-ray diffraction); and magnetic susceptibility to an AMF (specific absorption heating rate (SAR) measurements).

SPION suspensions were synthesized in-house via an aqueous co-precipitation method and were shown to exhibit good dispersion and particle size (10-20nm) as commonly reported in literature. Aqueous suspensions were further prepared for hydrophobic applications with retained particle size and dispersion from the original hydrophilic colloid. PVA and PS films were fabricated via solvent casting to produce uniformly-thick, dry films with minimum particle loadings of 10dry-wt% iron oxide. PVA sols in water were crosslinked to produce stable, hydrated films up to 80°C. Normalized for variations in the magnitude of the strength of the AMF, a film’s heat flux was characterized for uniformity via infrared thermal imaging. A film’s intrinsic power output was analyzed via SAR measurements using a five-turn, 50mm ID coil and a 5kW AMF generator at 350kHz. Films were fabricated for various maximum SAR values by controlling the particle concentration and film thickness. Due to the low pH requirement of the PVA crosslinking mechanism, iron oxide particles exhibited noticeable oxidation at concentrations above 10dry-wt%, thereby decreasing the intrinsic SAR value. Ultimately, PS and PVA films were produced with similar SAR values but drastically different mechanical properties and utility.

For SPION films with known SAR, temperature control at the composite’s surface was implemented with PID control of the magnetic field power output and fiber-optic temperature probe measurements; transient heating times en route to a steady state surface temperature are a function of the heat-sink condition and controller tuning parameters. Maximum surface temperatures on both PVA and PS composites targeted an 80°C setpoint for a conduction-only heat-sink.