Design and fabrication of an adhesive hydrogel-forming microneedle patch with an auxetic baseplate

Conventional microneedle patches exhibit low adhesion to joints affected by rheumatoid arthritis due to the intrinsic characteristics of joint tissue, such as curved skin and consistent muscle movement as well as rigidity of the MN’s baseplate not matching the elastic nature of the skin. This results in weak conformal contact between the patch and the skin, leading to disrupted transdermal drug delivery. To address these challenges, we have developed a hydrogel-forming microneedle (HFMN) patch loaded with methotrexate (MTX), physically trapped within an auxetic hyperelastic baseplate. The baseplate’s auxeticity imparts synclastic curvature properties to the patch, alongside a uniform tension distribution with reduced intensity across the entire patch. The hydrogel matrix reached an equilibrium swelling ratio of approximately 600% within 50 minutes, with an equilibrium water content of 85.7%, supporting rapid interstitial fluid uptake upon skin insertion. Additionally, the room temperature vulcanizing silicone rubber (RTV-SR) based baseplate, with its mechanical similarity to human skin and self-adhesiveness, further enhances microneedle patch adhesion to the damaged joint. Microneedles achieved an average insertion depth of up to approximately 420 µm in excised rat skin, sufficient to fully penetrate the stratum corneum barrier. The adhesion test demonstrated that a peeling force of 4.95 N/cm² was required for the RTV-SR-based baseplate which is significantly higher than those reported in previous studies. Ex-vivo permeation studies through excised rat skin demonstrated a rapid, swelling-driven burst release followed by a diffusion-limited plateau, with the Weibull model providing the best overall fit to the release profile (R² = 0.988), eaching a cumulative permeation of 46.54 ± 1.07 µg/cm², delivering approximately 53.6% of the loaded methotrexate within 8 hours. Additionally, formability tests demonstrate the superior shape-fitting capability of the auxetic patch during continuous joint movement, with 95% less height of separation, suggesting its potential for long-term drug delivery applications. Statistically significant differences (p < 0.05) were observed between auxetic and non-auxetic configurations across the mechanical and formability outcomes, supporting the functional benefit of the auxetic design. It is believed that the proposed HFMN patch’s multi-aspected adhesion mechanism could provide a unique new approach towards strengthening the adhesion of transdermal patches.