Modeling, characterization, and fabrication of bell-tip microneedle array by diffraction and self-aligned lens effects

Abstract

Microneedle arrays have been proposed in a wide range of biomedical applications, such as transdermal drug delivery and sensing. However, a scalable manufacturing process of precise microneedle fabrication of the microneedle has been challenged. This paper demonstrates UV-lithography-based one-step fabrication of fine-tuned bell-tip microneedles using a combination of light diffraction and the self-aligned lens effect. Microscale photopatterns can derive the predictive diffraction patterns where the higher light intensity at the center of the photopattern solidifies the liquid photoresist and forms a microlens shape in a self-aligned manner. The light through the microlens focuses down to a sharp point to form a conical shape for the body of the microneedle. Then light propagation through the vertex of the cone causes light emission, creating a fine bell-tip. The described light propagation behavior was characterized and explained in terms of the light intensity distribution from the diffraction based on the extended Fresnel–Kirchhoff diffraction model. The optics finite element analysis software was used to verify the light propagation and the intensity distribution. The step-by-step fabrication process was demonstrated using biocompatible photosensitive resins and validated the light attenuation and the cross-linking energy. The 20 × 20 bell-tip microneedles' array was able to be fabricated from the predicted model. Finally, a microneedle array with various shapes and heights on the same substrate was fabricated by single light exposure, demonstrating numerous achievable shapes using the proposed microneedle fabrication method.