Controlling the geometry of laser ablated microneedle cavities in different mould materials and assessing the replication fidelity within polymer injection moulding

Microneedle arrays are minimally invasive devices which enable self-administration of drugs in a straightforward and efficient way. Recently, a low-cost mass production method to produce solid polymer microneedles using laser ablated moulds in an injection moulding process has been developed. In this context, it is key to understand how the geometry of the microneedle cavities can be changed and what parameters affect the replication fidelity during injection moulding. In this study, two laser scanning parameters, being the programmed base diameter and the number of layers were varied in four levels and their effects on the geometry of the microneedle cavities were evaluated in tool steel, copper alloy, and aluminium alloy inserts. Afterwards, the microneedle cavities were replicated in polypropylene and polycarbonate using injection moulding. The study showed how the depth and base diameter of the microneedle cavity can be changed by varying laser scanning parameters. Moreover, the moulds in the three materials exhibited different microneedle cavity shapes and dimensions, due to differences in material properties. As to the injection moulding process, strong linear relationships were found between the aspect ratio of the microneedle cavities and the replicated thermoplastic microneedles, where a higher aspect ratio resulted in a lower replication. The replication fidelity for the polypropylene microneedles was in all cases higher compared to the polycarbonate needles. Between the three mould materials, a comparable replication fidelity was found for tool steel and copper alloy and a lower replication for the aluminium alloy, due to differences in thermal and wetting properties.