By partially retracting microneedles after insertion and other methods to overcome flow resistance of dense dermal tissue, protocols can be designed for hollow microneedles to microinfuse fluid at therapeutically relevant rates.

Conventional micromolding provides rapid and low-cost methods to fabricate polymer microstructures, but has limitations when producing sophisticated designs. To provide more versatile micromolding techniques, we developed methods based on filling micromolds with polymer microparticles, as opposed to polymer melts, to produce microstructures composed of multiple materials, having complex geometries, and made using mild processing conditions. Polymer microparticles of 1 to 30 µm in size were made from PLA, PGA and PLGA using established spray drying and emulsion techniques either with or without encapsulating model drug compounds. These polymer microparticles were filled into PDMS micromolds at room temperature and melted or bonded together to form microstructures according to different protocols. Porous microstructures were fabricated by ultrasonically welding microparticles together in the mold while maintaining the voids inherent in their packing structure. Multi-layered microstructures were fabricated to have different compositions of polymers and encapsulated compounds located in different regions of the microstructures. More complex arrowhead microstructures were fabricated in a two-step process using a single mold. To assess possible applications, microstructures were designed as microneedles for minimally invasive drug delivery. Multilayer microneedles were shown to insert into cadaver tissue and, according to design, detach from their base substrate and remain embedded in the tissue for controlled release drug delivery over time. We conclude that polymer particlebased micromolding can encapsulate compounds within microstructures composed of multiple materials, having complex geometries, and made using mild processing conditions.