Development of an optical microneedle device embedding sub-nanoliter volumes of boronic acid-based fluorescent hydrogel

Although the diagnostic utility of interstitial fluid (ISF) has been extensively explored, collecting and sensing ISF remains challenging. Microneedle devices offer a promising approach as a minimally invasive method to obtain ISF in a small volume or to sense biomolecules in the ISF within the body. However, conventional enzymatic measurements consume target molecules, compromising sensing reliability, especially in a small volume. To overcome the above issue, we developed fluorescence-based optical microneedles for non-consumptive molecular sensing within tiny ISF samples. The optical microneedle was functionalized with a small-volume fluorescent hydrogel block at its tip. The hydrogel block measured 100 µm in diameter and 100 µm in length, with a total volume of 0.79 nL. The microneedle made of amorphous poly-L-lactide (PLLA) had a high-aspect ratio shape (500 µm in base diameter, 200 µm in top diameter, and 2 mm in length), reaching the ISF near the vascular plexus in the reticular layer. In addition, the fluorescent hydrogel was functionalized with boronic acid, which reversibly binds to D-glucose. As proof of our technology, we conducted D-glucose sensing using an optical microneedle. The average value of measurement errors from actual D-glucose concentrations was calculated to be 5.6% in the range of 6.1 to 37.5 mM. Therefore, it was confirmed that the microneedle device is useful for ISF measurements.