Hydrogel-Forming Microneedle Arrays Allow Detection of Drugs and Glucose In Vivo: Potential for Use in Diagnosis and Therapeutic Drug Monitoring

Iron Oxide Nanoparticle-Powered Micro-Optical Coherence Tomography for in Situ Imaging the Penetration and Swelling of Polymeric Microneedles in the Skin

In recent years, polymeric microneedles (MNs) have attracted keen interests among researchers because of their applicability in transdermal drug delivery and interstitial skin fluid (ISF) extraction. When designing and characterizing such devices, it is critical to monitor their real-time in vitro and in vivo performances to optimize the desired effects, yet most of the existing methods are incapable of such functions. To address this unmet need, we develop a real-time noninvasive imaging methodology by integrating iron oxide (Fe₃O₄) nanoparticles into polymeric MNs to enhance image contrast for micro-optical coherence tomography (μOCT) imaging. Using the Fe₃O₄-integrated polystyrene-block-poly(acrylic acid) (PS-b-PAA) MNs as an example, we evaluate the influences of Fe₃O₄ concentrations on contrast enhancement in μOCT imaging and visualize the real-time swelling process of polymeric MNs in biological samples for the first time. Our results show that a concentration of ∼4-5 wt % Fe₃O₄ nanoparticles not only helps achieve the best contrast-to-noise ratio in μOCT imaging, which is 10 times higher than that without Fe₃O₄ nanoparticles in air and hydrogel, but also enables the real-time changes in the profile of MNs to be observed clearly in their swelling process in skin tissues. On the basis of such findings, we utilize the optimized concentration of Fe₃O₄ nanoparticles to further quantitatively study the swelling kinetics of PS-b-PAA MNs in agarose hydrogel and fresh skin tissues, which lasts ∼20 and ∼30-35 s, respectively. The suitability of such a methodology for enhancing μOCT imaging would greatly facilitate the development and clinical translation of MN-based medical technologies.

Recent advances in the design of polymeric microneedles for transdermal drug delivery and biosensing

Microneedles are an efficient and minimally invasive approach to transdermal drug delivery and extraction of skin interstitial fluid. Compared to solid microneedles made of silicon, metals and ceramics, polymeric microneedles have attracted extensive attention due to their excellent biocompatibility, biodegradability and nontoxicity. They are easy to fabricate in large scale and can load drugs in high amounts. More importantly, polymers with different degradation profiles, swelling properties, and responses to biological/physical stimuli can be employed to fabricate polymeric microneedles with different mechanical properties and performance. This review provides a guideline for the selection of polymers and the corresponding fabrication methods for polymeric microneedles while summarizing their recent application in drug delivery and fluid extraction. It should be noted that although polymeric microneedles can achieve efficient transdermal delivery of drugs, their wide applications were limited by their unsatisfactory transdermal therapeutic efficiency. Delivery of nanomedicines that incorporate drugs into functional nanoparticles/capsules can address this problem and thus may be an interesting direction in the future.

Evaluation of a minimally invasive glucose biosensor for continuous tissue monitoring

We describe here a minimally invasive glucose biosensor based on a microneedle array electrode fabricated from an epoxy-based negative photoresist (SU8 50) and designed for continuous measurement in the dermal compartment with minimal pain. These minimally invasive, continuous monitoring sensor devices (MICoMS) were produced by casting the structures in SU8 50, crosslinking and then metallising them with platinum or silver to obtain the working and reference electrodes, respectively. The metallised microneedle array electrodes were subsequently functionalised by entrapping glucose oxidase in electropolymerised polyphenol (PP) film. Sensor performance in vitro showed that glucose concentrations down to 0.5 mM could be measured with a response times (T₉₀) of 15 s. The effect of sterilisation by Co60 irradiation was evaluated. In preparation for further clinical studies, these sensors were tested in vivo in a healthy volunteer for a period of 3-6 h. The sensor currents were compared against point measurements obtained with a commercial capillary blood glucometer. The epoxy MICoMS devices showed currents values that could be correlated with these. Graphical Abstract Microneedle arrays for continuous glucose monitoring in dermal interstitial fluid.

Porous polymer microneedles with interconnecting microchannels for rapid fluid transport

We report a porous polymer microneedle array with continuous micropores that has high mechanical strength and water absorption speed.