Complementary Capillary System Integrated Microneedles for Autonomously Localized Therapeutics Loading

This work presents novel solid microneedles (MNs) integrated with a complementary capillary system (CCS) that enables autonomous localization of therapeutic loading. The CCS-MN, realized by 3-D direct-laser-writing (DLW), displays a double-stacked cone structure with a unique top cone consisting of opposing capillary actions: 1) external/internal capillary channels for rapid wicking/loading of liquid contents, respectively, and 2) a hydrophobic doubly-reentrant structure located at the bottom circumference for autonomous liquid confinement at the MN tip. A $2\times2$ CCS-MN array achieved robust skin penetration, withstanding 90 mN force while penetrating $300~\mu \text{m}$ into porcine skin. Significantly enhanced localized liquid loading was confirmed with the integrated CCS-MN when compared to a conventional conical MN, showing a 3.3-fold increase in loaded liquid volume (10 nL vs. 3 nL) - in good agreement with the qualitative staining test on the porcine skin from the delivered dye. Combined, this work provides a unique MN design solution realized by the DLW technology that will significantly advance minimally invasive transdermal delivery platforms. [2020-0166]