Microneedle Insertion into Visco-Hyperelastic Model for Skin for Healthcare Application

Recently, microneedle patches have been explored for extracting interstitial fluid with the goal of extracting temporally relevant, clinical-grade information for human health monitoring. As compared to traditional hypodermic needles, the sub-millimeter scale of microneedles allows for the creation of micropores providing access into human skin interstitial fluid while minimizing interactions with blood vessels and nerves, leading to painless insertion with little to no bleeding. An essential sub-component is the actuator, responsible for driving the microneedle into the skin with a precise force and velocity to ensure reliable insertion. Reliability, in this case, consists of two criteria: the ability of the microneedle to 1) penetrate the skin, and 2) withstand penetration forces without mechanical failure. Evaluation of these criteria requires a thorough understanding of the non-linear, time-dependent interactions between the microneedle and the skin during insertion, including rupture and tearing of the skin on the micron scale, and the resultant stresses on the microneedle. To this end, a comprehensive finite-element model was developed to simulate the microneedle insertion process. This analysis yielded a prediction of complete microneedle insertion without failure of the microneedle and an estimated insertion force of 0.055 N per microneedle, well within the capability of the actuator system considered. This insertion force was validated using experimental data obtained through microneedle insertion in whole skin samples. The model was then used to perform several parametric studies, yielding valuable insights for possible future design improvements.