Microneedles for Vaccination: Mechanistic Foundations, Materials Innovation, Clinical Translation, and Global Health...

Vaccination ranks among the most consequential interventions in medicine, yet cold-chain dependence, sharps hazards, needle phobia, and reliance on trained vaccinators limit coverage where vaccine-preventable mortality is highest. Microneedle patches deposit antigen into the antigen-presenting-cell-rich epidermis and upper dermis through projections that penetrate the stratum corneum without reaching dermal nociceptors. Such targeting yields immunogenicity matching or exceeding intramuscular injection at a fraction of the antigen mass, with dose-sparing up to six-fold recorded for influenza, polio, and SARS-CoV-2 antigens. Solid-state formulation converts that immunological advantage into a logistical one, since polymeric matrices preserve potency for as long as two years at ambient temperature, removing the refrigeration infrastructure that consumes significant delivery cost. Six microneedle types have reached preclinical or clinical maturity, with dissolving microneedle patches the most advanced. Two trials provide the clinical evidence: a Phase I influenza study showing non-inferior antibody responses and successful self-application, and a Phase I/II measles-rubella trial in The Gambia reaching 93% measles and 100% rubella seroconversion in infants without related serious adverse events. Engineering advances currently include an automated printer producing thermostable mRNA-lipid nanoparticle patches that retain bioactivity for six months at ambient temperature, the first intradermal self-amplifying RNA patch, and quantum-dot on-body immunization records. Sterility assurance, dose uniformity, nucleic acid integrity within solid matrices, and fragmented regulatory guidance remain the challenging obstacles, alongside a clinical pipeline concentrated on few antigens. This review integrates the mechanistic, materials, manufacturing, clinical, regulatory, and global health dimensions of the field to guide translation and equitable deployment.