Microneedle-Based Transdermal Drug Delivery: Innovations in Vaccines, Biologics, and Chronic Disease Management

Transdermal delivery offers a non-invasive, patient-friendly alternative to oral and parenteral administration, but the barrier function of the stratum corneum restricts conventional patches to small, lipophilic, potent molecules. Microneedles (MNs) are micron-scale projections, typically 150 to 1,500 µm long, that create transient microchannels through the stratum corneum and deliver drugs to the epidermis and upper dermis with minimal pain and little to no bleeding. Since the first demonstrations in the 1990s, MN technology has expanded from solid silicon arrays to coated, dissolving, hydrogel-forming, and hollow designs, enabling delivery of vaccines, peptides, proteins, nucleic acids, and small molecules. This review summarizes MN types, materials, and fabrication; delivery mechanisms and pharmacokinetics; applications in vaccination, biologic therapy, and chronic disease management; and the translational, regulatory, and manufacturing challenges that remain. We also outline emerging directions, including closed-loop and smart systems, MN-based diagnostics, and integration with wearable electronics.