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This work presents a hybrid tissue-anchoring mechanism that combines a pre-compressed microspring for autonomous actuation and a barbed microneedle (MN) for robust tissue anchoring with minimized preloading. The spring-microneedle unit (SMU) allows facile integration with potential ingestible resident systems to enable prolonged operation in the gastrointestinal (GI) tract (e.g. biomarker monitoring and therapeutic drug delivery). Utilizing high-precision direct laser writing (DLW), the SMU is directly fabricated on a flexible polyimide substrate, which that can be conveniently attached onto the exterior packages of ingestible devices to reduce the form factor and therefore to save room for multifunctional system components. The 3-D printed microspring can be compressed and embedded within a thin layer of dissolvable polymer (polyethylene glycol) and autonomously actuated with a ~140- $\mu \text{m}$ displacement upon polymer dissolution in aqueous media at 37°C. Mechanical calibration of the SMU demonstrated an average release force of 8 mN for a 100- $\mu \text{m}$ actuation. According to our previous research into barbed MNs, this actuation force provides a large enough preload for a MN to achieve robust tissue-anchoring. Above all, the novel and effective tissue-anchoring mechanism has a great potential for enabling long-term operation of miniature biosensing and drug delivery systems. [2020-0169] [ABSTRACT FROM AUTHOR]

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] [ABSTRACT FROM AUTHOR]