Solid silicon microneedles for drug delivery applications

In this work, solid silicon (Si) microneedles with a higher aspect ratio and sharper tips are fabricated. Tetramethylammonium Hydroxide (TMAH) etching factors have been optimized and used to fabricate long and tapered microneedles. The needles thus fabricated are found to be suitable for transdermal drug delivery applications. The optimized etching factors include varying the concentration of TMAH, etching time, etching rates, temperature, and window size of optical mask. It is found that by increasing the window size, the etch rates in both vertical and lateral directions increase extensively. However, on increasing the temperature beyond 90 °C, etching becomes rapid and uncontrollable. In order to obtain microneedles with high aspect ratio, sample placement in the glass boat of TMAH setup and TMAH concentration should be manipulated to attain a higher etch rate in vertical direction compared to the lateral one. Solid silicon microneedles with an average height of 158 μm, base width of 110.5 μm, aspect ratio of 1.43, tip angle of 19.4° and tip diameter of 0.40 μm are successfully fabricated. A microhardness value of 44.4 (HRC) was obtained for the fabricated Si microneedles. This is 52.2 times higher than the skin Ultimate Tensile Strength (UTS), which makes insertion of microneedles through the skin safer and easier without any breakage.