A dual wedge microneedle for sampling of perilymph solution via round window membrane

Precision medicine for inner-ear disease is hampered by the absence of a methodology to sample inner-ear fluid atraumatically. The round window membrane (RWM) is an attractive portal for accessing cochlear fluids as it heals spontaneously. In this study, we report on the development of a microneedle for perilymph sampling that minimizes the size of RWM perforation, facilitates quick aspiration, and provides precise volume control. Here, considering the mechanical anisotropy of the RWM and hydrodynamics through a microneedle, a 31G stainless steel pipe was machined into wedge-shaped design via electrical discharge machining. The sharpness of the needle was evaluated via a surface profilometer. Guinea pig RWM was penetrated in vitro, and 1 μL of perilymph was sampled and analyzed via UV-vis spectroscopy. The prototype wedge shaped needle was successfully fabricated with the tip curvature of 4.5 μm and the surface roughness of 3.66 μm in root mean square. The needle created oval perforation with minor and major diameter of 143 and 344 μm (n = 6). The sampling duration and standard deviation of aspirated volume were 3 s and 6.8 % respectively. The protein concentration was 1.74 mg/mL. The prototype needle facilitated precise perforation of RWMs and rapid aspiration of cochlear fluid with precise volume control. The needle design is promising and requires testing in human cadaveric temporal bone and further optimization to become clinically viable.