Microfabricated Polysilicon Microneedles for Minimally Invasive Biomedical Devices

A two-wafer polysilicon micromolding process has been developed for the fabrication of hollow tubes useful for microfluidic applications. These small tubes can be fabricated with a pointed end, resulting in a micro hypodermic injection needle. Microneedles are desired because they reduce both insertion pain and tissue damage in the patient. Such microneedles may be used for low flow rate, continuous drug delivery, such as the continuous delivery of insulin to a diabetic patient. The needles would be integrated into a short term drug delivery device capable of delivering therapeutics intradermally for about 24 hours. In addition, microneedles can be used for sample collection for biological analysis, delivery of cell or cellular extract based vaccines, and sample handling providing interconnection between the microscopic and macroscopic world. The strength of microneedles was examined analytically, experimentally and by finite element analysis. Metal coatings provide significant increases in the achievable bending moments before failure in the needles. For example, a 10 mu m platinum coating increased the median bending moment of a 160 mu m wide, 110 mu m high microneedle with a 20 mu m wall from 0.25 to 0.43 mNm. In addition, fluid flow in microneedles was studied experimentally. Microneedles 192 mu m wide, 110 mu m high and 7mm long have flow rates of 0.7 ml/sec under a 138 kPa inlet pressure. This flow capacity exceeds previous microneedle capacities by an order of magnitude.