Design, fabrication, and penetration assessment of polymeric hollow microneedles with different geometries

Hollow microneedles (HMNs) are minimally invasive needle-like microfeatures usually arranged in arrays designed for drug delivery and body fluid collection in a painless manner. In a recent work, we demonstrated a novel methodology to mass produce hollow polymer microneedles at a low cost. This methodology combines ultrashort pulse laser ablation to create inverse needle shapes in moulds and replication through polymer injection moulding. For a HMN to be functional, it should effectively pierce the skin at a low force and enable fluidic passage through the skin without leakage. This study investigates the impact of different laser scanning strategies on the cavity morphology and analyses how the various geometrical characteristics of the needle influence the penetration efficacy. To assess the penetration behaviour of the replicated HMNs, a combination of agarose gel and Parafilm® is employed as an in vitro testing platform. Furthermore, a correlation between HMN geometry, penetration performance, and modification of polymer material and holding pressure during injection moulding is established. The results indicate that a certain needle length is essential for effective penetration. Moreover, minimising the tip area, a factor significantly affecting penetration force, can be achieved by increasing the eccentricity of the scan-free area and expanding the scanning diameter. However, it is important to consider other functional needle features such as the ridge height or full lumen, which come at a cost to the tip sharpness. This work highlights the multiple interactions between the scanning strategy, the injection moulding process parameters, the needle geometry, and the penetration force. This study provides insights into optimization of the HMN design and the fabrication for enhanced penetration efficacy of functional injection-moulded polymeric HMNs.