Development and Functional Evaluation of Unobstructing Magnetic Microactuators for Self-Clearing Implantable Catheters

Hydrocephalus is a debilitating neurological disorder for which there is no cure. A typical treatment for hydrocephalus requires the chronic implantation of a shunt system designed to manage introcranial pressure. Unfortunately, these vital medical devices have an unacceptably high failure rate (40%) within the first year of implantation. One of the leading causes of shunt failure is attributed to obstructions of the ventricular catheter. Although the issue of frequent shunt failure has been recognized for the past several decades, only a limited amount of advances in shunt technology have been made to alleviate the problem.;This dissertation chronicles our latest efforts toward engineering a self-clearing ventricular catheter with integrated microfabricated magnetic microactuators. Using an earlier version of the torsion-beam-based magnetic microactuators, we first examined the mechanical robustness of the microscale devices. To quantify the effects of long-term actuation, we subjected the microdevices to an accelerated testing protocol to quantify the effects of long-term actuation and observed minimum change (Delta < 2%) in resonant frequency of the microdevices. To alleviate the concerns of magnetic resonance safety of using magnetic microdevices as an implant, we subjected the devices to a 7-T magnetic resonance imaging scanner and demonstrated no significant changes to the mechanical properties of the microdevices.;Next, we introduced a novel cantilever-based unobstructing magnetic microactuator design to alleviate the concerns of "self-occlusion" of the earlier torsion-beam-based devices. By using well-known theories on bimorph bending, we designed and fabricated a novel cantilever-based magnetic microactuator that remained clear of the pore at its resting state. The static and dynamic responses of the new generation of microactuators were predicted using analytical and numerical models, and were subsequently confirmed with experimental results.;We then created a physiologically relevant experimental setup to evaluate the obstruction-clearing capabilities of our magnetic microactuators. Initially, we attempted to occlude the silicon-based catheter pores using a circulating in-vitro cell-culture system without success. To circumvent this issue, we attempted to create a cellular analogue by immobilizing proteins involved in cellular-adhesion mechanisms onto the surfaces of our device and cellular-scale microbeads. Subsequent evaluation of cell adhesion showed ubiquitous non-specific binding, which led us to further simplify the microbead-to-surface binding mechanism. Using a photoactivated hetero-bifunctional chemical crosslinker, ASBA, we successfully created a cellular analogue with similar cell adhesion properties. Jet impingement studies were used to determine that a shear stress of 1981-2284 dyn/cm² is needed to remove the ASBA conjugated microbeads bound to parylene surface.;A 2D numerical analysis based on the Navier-Stokes equation for incompressible Newtonian fluids was performed using a finite element modeling software to understand the fluid dynamic properties of our magnetic microactuators operating in CSF. The fluid dynamic analysis generates shear stress profiles of the magnetic microactuator at various angular deflection states. Using the shear stress profile, we determined the amount of area that experiences shear stress > 2500 dyn/cm² and reported that the shear stress increases with increasing angular deflection.;Finally, we describe in detail on the development and evaluation of a circulating-flow occlusion system, which was designed to quantify the obstruction-clearing capabilities of our magnetic microactuators. The system consisted of three separate components: (1) electromagnet, (2) fluidic circuit, and (3) microscope-based optical measurement system. The optical measurement system was chosen to allow for parallel examination of multiple devices for greater experimental throughput. The circulating-flow occlusion system was then put to test by using the ASBA-conjugated cellular analogue in physiological relevant fluidic conditions. We report complete occlusion of the magnetic microactuator after 11 h of continuous circulation. Subsequently, the magnetic microactuator driven at its resonant frequency successfully removed the obstruction accumulated over the 11-h-period.;The progress reported in this dissertation may be used as a guide to further refine the magnetic microactuator design. For example, the fluid dynamic analysis described in this work may be used as a design tool to determine appropriate device geometry and dimensions. In addition, the circulating-flow occlusion system may be used to quantify the occlusion-clearing capabilities of various device designs in a controlled manner without the large variability that comes with biological samples. With the methods described in this work, it may be possible to isolate the optimum device design and actuation protocols that may be useful in the in-vivo evaluation, which will bring us one step closer to the realization of MEMS-enabled self-clearing implantable catheters.