Fabrication of a Master Mold for Microneedles with a Micron-sized Air-vent Hole

Currently, advanced microfabrication, surgical micromachining, vacuum-assisted molding, and high-resolution microscopy are used to fabricate and characterize microneedles. The major challenges are to prevent air entrapment during viscous polymer casting and achieve consistent microneedle tip sharpness. To begin, aspirate nine milliliters of distilled water into a 10 milliliter disposable syringe and add it into a 50 milliliter conical centrifuge tube.

Using an S/T spoon and greaseproof paper, weigh one gram of gelatin powder on a precision balance and transfer it into the conical tube containing water. With the S/T spoon, break apart any clumps of gelatin by stirring the mixture in a clockwise direction for five minutes, ensuring that the rotating solution does not spill. Now, fill a water bath with four liters of distilled water.

Set the water bath temperature to 60 degrees Celsius and wait for approximately 30 minutes until it reaches the target temperature. Now, place the tube containing the gelatin solution in the water bath and incubate for 30 minutes to dissolve the gelatin. Stick double-sided non-woven tape on two microscope slides and lay them flat inside a Petri dish, spacing them seven millimeters apart.

Then, place the fabricated silicon master mold on top of the two microscope slides with its backside containing the air-vent hole facing downward. Adjust the position of the mold so that the air vent hole is centered over the gap between the two slides. Now, place the Petri dish containing the silicon master mold at the center of a vacuum desiccator measuring approximately 260 by 260 by 100 millimeters.

Then, using a one milliliter disposable syringe, apply 0.3 milliliters of methanol to the center of the silicon master mold, and gently move the syringe tip along the surface of the mold to distribute the methanol evenly toward the edges. Seal the vacuum desiccator and use a vacuum pump to apply a vacuum of minus 80 kilopascal for two minutes to let the methanol fully infiltrate the silicon master mold. Once done, release the vacuum to remove remaining bubbles in the methanol.

Next, remove the prepared gelatin solution from the water bath and wipe water from the outside of the tube using a cleanroom wipe. For solid microneedles, use a 20 to 200 microliter micropipette to dispense 100 microliters of the gelatin solution onto the center of the silicon master mold. Then, seal the vacuum desiccator and apply a vacuum of minus 80 kilopascal for two minutes.

After releasing the vacuum, incubate the Petri dish flat in a 60 degrees Celsius water bath for two minutes. Then, carefully remove the Petri dish from the water bath without tilting it. Finally, using a pair of tweezers, touch the surface of the gelatin solution and spread the solution evenly to the edges of the mold, ensuring the entire mold surface is coated with a thin layer.

The silicon master mold with an air-vent hole was successfully fabricated using anisotropic wet etching, forming a five by five microneedle array layout with distinct V grooves meeting at the cavity tip, and the anisotropic etch profile was confirmed via a cross-sectional scanning electron microscopy image showing upper and lower etch depths. In the air-vent-hole-equipped master mold, the hole was successfully etched at the center of the cavity tip as shown in the top view scanning electron microscopy image. These molds produced a 100%cavity yield with clearly defined pyramidal cavities and a distinct air-vent hole at the tip across all 25 cavities.

Microneedles fabricated using a mold without an air-vent hole showed blunt tips due to trapped air, despite vacuum application for over 10 minutes. When using a mold with an air-vent hole, sharp microneedle tips formed after only two minutes under vacuum due to successful air venting during casting. A five by five microneedle array fabricated using the AVH-equipped mold showed a 100%fabrication yield with bubble-free, sharply defined tips.

When casting 40 microliters of gelatin into molds with an air-vent hole, microneedles with hollow interiors were formed due to gelatin flowing toward the venting point during curing. We fabricated air-vent assisted master molds that are defect-free. These sharp-tipped, solid and hollow microneedles are efficient and offered 100%yield.

Our protocol overcomes the hurdle of uncontrolled air entrapment during viscous polymer casting that limits microneedle tip fidelity and yield. Our method also enables reliable tip formation without complex equipment, using air venting and simple vacuum-assisted casting.