Mechanical strengthening of fiberoptic microneedles using an elastomeric support

Abstract

Background and objectives: Microneedles made from silica fiberoptics permit transmission and collection of light, which is an important functional advantage over metal or silicon microneedles. This added functionality may enhance or even enable new percutaneous light-based clinical diagnostic and therapeutic procedures. Micron-diameter fiberoptic microneedles, created from solid fibers capable of light emission and detection, are designed to penetrate several millimeters into tissue while minimizing tissue invasion and disruption. The mechanical strength (critical buckling force) of high aspect ratio (length to diameter) microneedles is a potential problem, which has motivated our invention of an elastomeric support device. In this study, we have tested our hypothesis that embedding the microneedles in an elastomeric support medium may increase microneedle critical buckling force.

Materials and methods: The critical buckling force of silica microneedles with 55, 70, and 110 µm diameters and 3 mm lengths were measured with and without a surrounding elastomeric support (PDMS, polydimethylsiloxane). These experimental results were compared to theoretical calculations generated by the Rayleigh-Ritz buckling model. The insertion force required to penetrate ex vivo porcine skin was measured for microneedles with 55 and 70 µm diameters.

Results: Use of the PDMS support increased critical buckling force for microneedles of 55, 70, and 110 µm diameters by an average of 610%, 290%, and 33%, respectively. Theoretical calculations by the Rayleigh-Ritz model consistently overestimated the experimentally determined strengthening, but correlated highly with the greater enhancement offered to thinner microneedles. Aided by mechanical strengthening, microneedles 55 µm in diameter were able to repeatedly penetrate.

Conclusions: The critical buckling force of microneedles can be increased substantially to allow extremely high-aspect ratio microneedles, 55-110 µm in diameter and 3 mm in length, to penetrate ex vivo porcine skin. By this strengthening method, the safety and reliability of microneedles in potential clinical applications can be considerably enhanced.