Beyond Material Selection: Function-Driven Design of Polymer-Based Microneedles for Transdermal Drug Delivery

Polymeric microneedles have emerged as promising platforms for minimally invasive drug delivery, yet their clinical translation remains limited despite extensive proof-of-concept research. This review critically examines polymeric microneedles from a function-driven design perspective, linking classification, fabrication, and material selection to the principal determinants of performance and translational success. Particular attention is given to the interplay among microneedle geometry, polymer properties, skin biomechanics, and application conditions, which collectively define the mechanical window required for reliable insertion. Drug release is considered a programmable design function that should be aligned with dose and pharmacological objectives, while polymer dissolution, degradation, persistence, and potential accumulation are evaluated in relation to safety and regulatory acceptance. Stimuli-responsive and hybrid systems are also assessed by weighing their functional benefits against the additional complexity they introduce. Recurring mechanical, pharmacokinetic, manufacturing, and clinical design–performance mismatches are identified as major contributors to the gap between laboratory success and clinical implementation. Based on these findings, a function-driven framework is proposed that integrates therapeutic objectives, insertion performance, release kinetics, manufacturability, and safety from the earliest stages of development, thereby supporting the rational design of clinically viable next-generation microneedle systems.