P. Buckley, D. Maitland, T. Wilson, W. Small, J. Bearinger, W. Benett, Lawrence Livermore National Laboratory, Livermore, CA; G. McKinley, MIT, Cambridge, MA
Presently there is interest in making medical devices such as expandable stents and intravascular microactuators from shape memory polymer (SMP). One of the key challenges in realizing SMP medical devices is the implementation of a safe and effective method of thermally actuating various device geometries in vivo. A novel scheme of actuation by Curie-thermoregulated inductive heating is presented. Prototype medical devices made from SMP loaded with Nickel Zinc ferrite ferromagnetic particles were actuated in air by applying an alternating magnetic field to induce heating. Dynamic mechanical thermal analysis was performed on both the particle-loaded and neat SMP materials to assess the impact of the ferrite particles on the mechanical properties of the samples. Calorimetry was used to quantify the rate of heat generation as a function of particle size and volumetric loading of ferrite particles in the SMP. These tests demonstrated the feasibility of SMP actuation by inductive heating. Rapid and uniform heating was achieved in complex device geometries and particle loading up to 10% volume content did not interfere with the shape recovery of the SMP.
Summary: We have demonstrated the feasibility of fabricating inductively-heated SMP devices employing dispersed nickel zinc ferrite ferromagnetic particles and using a magnetic field to trigger actuation. Elimination of a physical power connection removes restrictions on device geometry imposed by laser and resistive heating modalities. Furthermore, as a result of the predominant hysteresis loss heating mechanism, self-thermoregulation can be achieved by tailoring the particle Curie temperature to prevent overheating in medical applications. Initial actuation testing indicated that the addition of 10% volume content of particles provides sufficient heating for SMP actuation in air and does not interfere significantly with the shape memory properties of the material. Preliminary DMTA results show an increase in the modulus of the SMP composite with the addition of 10% particles, that is in accordance with standard composite theory as well as an increase in the glass transition temperature of the soft phase of the SMP. With further optimization of particle material to provide clinically acceptable Curie temperatures and magnetic field frequencies, inductive heating may provide an effective means of deployment of SMP medical devices.
