Design, fabrication, and characterisation of a silicon microneedle array for transdermal therapeutic delivery using a single step wet etch process

The fabrication of silicon in-plane microneedle arrays from a simple single wet etch step is presented. The characteristic 54.7 degrees sidewall etch angle obtained via KOH etching of (100) orientation silicon wafers has been used to create a novel microneedle design. The KOH simultaneously etches both the front and back sides of the wafer to produce V shaped grooves, that intersect to form a sharp pyramidal six-sided microneedle tip. This method allows fabrication of solid microneedles with different geometries to determine the optimal microneedle length and width for effective penetration and minimally invasive drug delivery. A modified grooved microneedle design can also be used to create a hollow microneedle, via bonding of two grooved microneedles together, creating an enclosed hollow channel. The microneedle arrays developed, effectively penetrate the skin without significant indentation, thereby enabling effective delivery of active ingredients via either a poke and patch application using solid microneedles or direct injection using hollow microneedles. This simple, scalable and cost effective method utilises KOH to etch the silicon wafer in-plane, allowing microneedles with variable length of several mm to be fabricated, as opposed to out-of-plane MNs, which are geometrically restricted to dimensions less than the thickness of the wafer. These microneedle arrays have been used to demonstrate effective delivery of insulin and hyaluronic acid into the skin.