In this paper, feasibility studies are presented on microneedle fabrication by using a multiple-pulse laser microhole drilling technique to drill an axial hole into a thin nitinol wire of 150 μm in diameter. Nitinol is chosen for its biocompatibility and excellent super-elasticity to eliminate breakage risk. One potential use of this microneedle is for drawing a small amount of blood for glucose monitoring. The critical factor for drilling microholes into a thin nitinol wire axially is the restoration of the semi-infinite material condition, which is the key to prevent the thin wire from being melt away by the laser due to excessive heat transfer in the radial direction. The results show that holes, up to 607 μmin depth, can be drilled into a thin nitinol wire of 150 μm in diameter using 18 repetitions of a 3-pulse group with 13 μs pluses. However, hole quality is poor. The challenges for improving hole qualities, such as centering, hole blockage, through holes, and process parameters, are discussed.

Laser drilling is an important industrial process for the production of various sizes of holes. In this paper, we investigate rapid, high aspect ratio microhole drilling using multiple microsecond pulses based on the single pulse drilling technique reported in [17, 18]. It was established that there would be a synergistic effect if a subsequent pulse is irradiated at the target within 100 μs of the previous pulse before the melt solidifies. However, the peak power values of subsequent pulses decrease with higher repetition rates. The results show that the synergistic effect could outweigh the reduction in laser power. Another contributing factor of the synergistic effect is related to the melt ejection efficiency. As the hole deepens, the melt ejection becomes less effective to eject the melt completely out of the hole, resulting in a partially blocked hole. A subsequent laser pulse needs to reopen the hole before the hole can be deepened further. To overcome this hole blocking problem, shooting a subsequent pulse at a higher repetition rate also ensures that the energy absorption is more efficient when a subsequent laser pulse is irradiating at the hole blocking melt which is not yet solidified. This multiple-pulse drilling techniquewas applied for through-hole drilling. It was found that the total drilling times through an 800 μm plate were found to be 634 ms and 21.9 ms at 13 kHz and 20 kHz, respectively. The drilling efficiency at the 20 kHz repetition rate is drastically higher, needing only 428 shots, compared with 8240 shots at the 13 kHz, an improvement of nearly 200 times. It is confirmed that this multiple-pulse drilling technique with microsecond pulses using a 300Wsingle mode fiber laser is a viable technique to produce high aspect ratio through holes with a simple and robust setup for the production environment.