Microfabricated Parylene Microneedles and Pneumatic/Hydraulic Actuators for Use in Interventional, Transvascular Drug Delivery

A method for performing interventional surgery has been developed. Transvascular delivery of therapeutic and diagnostic agents into perivascular space is desirable for various cardiovascular treatments and applications. The dosing of perivascular space requires the perpendicular actuation of a microneedle to slide through the layers of tissue comprising a blood vessel wall. The microneedle must be long enough to pass through the arterial or venous wall and narrow enough to limit damage and injury to the vessel or the blood around the injury.
A fabrication process has been developed for three-dimensional hollow parylene microstructures. This process has been applied to the creation of hollow parylene microneedles. Parylene is a less brittle and less rigid material than more common structural MEMS materials like polysilicon. The beneficial toughness of parylene microneedles enables their application to interventional surgery, where there exists concern over leaving behind foreign bodies. The lack of rigidity of parylene is compensated by tapering the microneedle tip with a slimmer angle than is possible with brittle materials and by stiffening the needle structure by widening the shaft.  
Parylene microneedles were fabricated with lengths of 1.125, 1.625, and 2.250 mm and widths of 100 and 150 um. Tip angles were varied to determine the effect of geometry on tissue insertion forces. Microneedles were tested in bending and buckling to determine their structural integrity. Needles of 1.625 mm length, 150 um width and 10 um wall thickness were displaced out of plane with a cantilever stiffness of about 5 mN1mm. Needles with similar geometry  resulted in 100 mN of column strength when tested in buckling. Needle tips, however, buckled against as little as 20 mN of applied force in a rigid test setup.
Parylene microneedles were inserted into chicken tissue without failure for the 1.125 mm long microneedles and with some buckling for the 1.625 and 2.250 mm long microneedles. Resulting data indicated that a 15 degree bevel of the needle tip resulted in markedly lower insertion forces than those of a 30 degree tip.
A hydraulic actuator was developed and made with a second parylene molding process. The actuator resembles a balloon that is flattened out and rolled into the shape of a "C". A microneedle is sheathed within the body of the actuator, perpendicular to the axis of the actuator. The actuator is attached to the distal tip of a catheter. Upon pressurization, the actuator unfurls and expands to a diameter greater than 150% of its original diameter. In the process, the actuator pushes the microneedle out to puncture through a vessel wall for the delivery of therapeutic or diagnostic agents
Publication date: 
December 31, 2001
Publication type: 
Ph.D. Dissertation
Seward, K. P. (2001). Microfabricated Parylene Microneedles and Pneumatic/hydraulic Actuators for Use in Interventional, Transvascular Drug Delivery. United States: University of California, Berkeley.

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