Arrays of Microneedles (MNs) are devices consisting in a plurality of micro-projection with a common support. They are used as medical microdevices for diagnostic (sampling) or therapeutic applications, and in particular they can be used as drug delivery system, either as drug eluting bodies or as drug injection devices. In the latter case, MNs create a miniaturised transport pathway through the stratum corneum (SC) into viable epidermis (VE) without stimulating nerve reside in the dermal layer. This offers the promise of painless, effective drug delivery, but mass production of cost effective MNs arrays is challenging.
In this paper, laser micromachining is used to produce a mould for pyramidal MNs through injection moulding (IM). A cavity to produce an array of MNs (500µm wide and 1000µm high) was directly micro-fabricated on 1.2311 grade steel mould using femtosecond laser. Direct micro-fabrication with and without post -processing was used to produce the cavity. The former approach generated bulky residues on the micromachined cavity surface, leading to coarse surface finish. Dispersive x-ray spectroscopy (EDS) data show that the residues found in the cavities has high oxygen content and suggested that laser irradiation in the presence of atmospheric oxygen induced unavoidable oxidation at the irradiated region.
Hence, the surface finish could be improved by eliminating the oxide layer from MNs surface via laser post-processing. The removal of the oxide layer also facilitate laser access to the finer features, and hence enhanced tip sharpness in the replicated MNs. Nevertheless, excessive laser post-processing caused undesired shape distortion at the tip of the micro-needles, due to multiple reflections being absorbed preferentially in the bottom of the machined cavity.