MedE PhD Thesis: Luizetta V. Elliott
Stimuli responsive micro-architected materials
Shape memory polymers (SMPs) respond to heat by generating programmable movement useful in devices that require substantial deformation and operate at transient temperatures, including stents, embolization coils and robotic grippers. Transitioning these materials to the micro-scale can result in expanded potential applications, such as clot removal from retinal vasculature, neural probe delivery and responsive metamaterials. To achieve these goals, shape transformation must occur in SMPs with complex 3D geometries and nanoscale features.
This thesis describes the synthesis and sculpting of a benzyl methacrylate-based SMP into 3D structures with <800nm characteristic critical dimensions via two photon lithography. The glass transition based shape memory mechanism of these materials is explored through dynamic nanomechanical analysis of 8µm-diameter cylindrical pillars, which revealed the initiation of a tunable glass transition at 60⁰C not present in highly crosslinked materials. Shape memory programming of the characterized pillars as well as complex 3D architectures, including flowers with 500nm thick petals and cubic lattices with 2.5µm unit cells and overall dimensions of 4.5µm x 4.5µm x 10µm, demonstrated an 86 +/- 4% characteristic shape recovery ratio. These results reveal a pathway towards SMP devices with nanoscale features and arbitrary 3D geometries changing shape in response to temperature.
This thesis then focuses on a particular potential application for such materials: neural probes designed for deployment in primate brains. Architected shape memory structures have the potential to create favorable long-term recording environments through softening triggered by biological conditions, deployment to beyond tissue damage during initial electrode positioning and architectural features designed for optimal scaffold-tissue interactions. This thesis addresses one of the barriers to the deployment of such structures: the high loading during centimeter scale insertions required for primate brain targeting is incompatible with buckling free insertion of low stiffness and/or cross sectional area probes required for minimizing the foreign body response.
Lamb brain tissue model experiments with 280µm diameter platinum coated carbon fiber probes demonstrate that 59+/- 3% of the work during 3cm probe insertion is attributable to friction, suggesting that friction reduction is a favorable approach to load minimization. A phosphorylcholine based ~100nm low friction coating is used to reduce the shear stress at the probe-brain interface by 20+/-7 %, demonstrating a facile method for friction reduction that has minimal impacts on probe cross sectional area. Surgical validation of probe insertion in a porcine head model reveals that these probes are suitable for whole brain penetration of brains at the primate scale (~102g). These results show that loading requirements during whole brain penetration can be reduced through addressing the contribution of friction and introduce a viable vehicle for recording electrode delivery to large scale brains.
In summary, this thesis provides the foundation for developing stimuli responsive micro scale devices and materials and, in the case of deep brain neural recording, the building blocks for the design of an integrated shape memory/ low friction carbon fiber electrode delivery device. Future research on the scalable fabrication of architected shape memory polymers could enable the wide-spread application of such materials.