Colloid-loaded microneedles for transdermal delivery: formulation stability, redispersion, biointerfacial transport,...

Colloid-loaded microneedles combine the barrier-bypassing capability of microneedle arrays with the solubilizing, stabilizing, depot-forming, and cell-interactive functions of colloidal carriers. Nanocrystals, nanosuspensions, lipid vesicles, polymeric nanoparticles, nanogels, extracellular vesicles, and lipid nanoparticles have been integrated with coated, dissolving, hollow, and hydrogel-forming microneedles for local and systemic delivery. Here, integrated colloid-loaded microneedles refer to systems in which colloidal carriers are coated onto, incorporated into, infused through, or reservoir-coupled with microneedle platforms, whereas solid microneedle pretreatment followed by topical colloid application is discussed only as a microneedle-assisted comparator. For integrated colloid-loaded microneedles, performance is not determined solely by microneedle geometry, insertion efficiency, or drug loading, but rather by whether the carrier survives conversion into a microneedle product and remains functionally relevant after hydration and release in skin. Existing reviews mainly emphasize microneedle materials, fabrication strategies, therapeutic applications, or smart devices, whereas the colloidal determinants of performance remain less integrated. Unlike application-centered or platform-centered reviews, this review focuses on the state transitions and functional persistence of colloidal carriers across fabrication, storage, insertion, hydration, redispersion, and dermal biointerfacial transport. We therefore reframe these products as formulation-device-biointerface systems, with emphasis on carrier integrity, matrix compatibility, redispersion, and post-insertion fate as determinants of therapeutic performance. To support mechanism-based evaluation, we further propose a study-level evidence hierarchy and practical analytical/QbD framework for assessing intact carrier delivery, redispersibility, bioactivity, and translational quality attributes. Issues related to biologic stability, sterilization, storage, manufacturing scalability, regulatory complexity, and critical quality attributes are also discussed within a quality-by-design framework.