Three aspects of MEMS microneedle technology are developed in this work. First, a new two-wafer polysilicon micromolding process for the fabrication of multifunctional microneedles is developed. Second, microneedle strength is addressed including the investigation of strengthening methods. Lastly, the fluid-flow properties of microneedles are studied analytically and experimentally.
Polysilicon microneedles of smaller size and greater sharpness than stainless steel needles have been fabricated using a two-way polycrystalline silicon (polysilicon) micro molding process called Polymolding. Polymolded needles from 80 to 200 um in diameter and 3 to 8 mm in length with submicrometer-tip radii have been fabricated. In contrast with stainless steel needle machining techniques, Polymolding permits the incorporation of complex features into needles. Microneedles with features such as multi-stage filters, multiple channels, multiple outlet ports, barbs that limit the insertion distance, barbs that inhibit removal of the microneedles, and bends of arbitrary angles have been fabricated with Polymolding. Considerable process development, particularly with the deep-trench etcher, was carried out in order to produce these complex features. Substantial process development was required too achieve sharp needle tips. Initially, the tip tapered in one dimension to produce a chisel-like point that is not conducive to barrier penetration. Other problems that are addressed are polysilicon delimitation, mold surface roughness and oxide conformity.
The strength of Polymolded needles was studied analytically, experimentally and by finite-element analysis. We found that metal coatings provide significant increases in the achievable bending-moments before failure in the needles. For example, a 10um platinum coating increased the maximum bending moment of a 160um-wide, 110 um-high microneedle with a 20 um polysilicon wall thickness from 0.25 to 0.43 mNm.
Fluid flow in Polymolded microneedles was studied analytically and experimentally. Polymolded microneedles 192 um-wide, 110 um-high and 7mm-long can carry flow rates of 0.7 cc/sec under a 138 kPa inlet pressure. This flow capacity exceeds that of previous microneedle technologies by more than an order of magnitude. The fluid-flow in dual-channel microneedles and the effects of the constricted outlet ports were also studied.