Design and analysis of a contact-aided compliant force-limiting applicator mechanism for microneedle patches

Microneedle patches are emerging minimally invasive devices that deliver drugs using micron-scale needles to penetrate the stratum corneum of the skin, offering significant advantages such as minimal pain, convenient administration, and low risk of infection. However, microneedle patches are typically applied by thumb pressing, making it difficult to limit the maximum input force, which can cause needle damage or even nerve contact causing pain. This article proposes a force-limiting applicator for microneedle patches, which limits the maximum input force through a sheet-based contact-aided compliant force-limiting mechanism (CCFM). The sheet-based CCFM consists of a compliant sheet and a contact rod (with smooth contact surface). As the contact surface contacts the compliant sheet, causing it to deform, the reaction force gradually increases. When the contact surface separates from the compliant sheet, the deformation disappears, leading to a sudden reduction in the reaction force. The sheet-based CCFM contains no electronics, making the applicator easy to miniaturize, reusable, and cost-effective. In addition, by designing different dimensions of the compliant sheet and contact surface, the applicator can satisfy various force-limiting requirements. The chained pseudo-rigid-body model (CPRBM) is used to predict the deformation of the compliant sheet in contact with linear and curved contact surfaces. The model is validated using finite element analysis (FEA), with maximum relative errors of 6.61% and 6.79%, respectively. The reaction force is obtained using the Lagrange multiplier method. For the two different contact surfaces, the maximum relative errors between the analytical and FEA results are 9.3% and 6.8%. In addition, physical prototypes are manufactured and experimental tests are performed to further verify the feasibility of the design. Finally, the design procedures, the limitation of the analytical model, and future work are discussed.