We demonstrate a micromilled microneedle flexible patch array (μNFPA) fabricated using stainless steel (SS) that has the potential to serve as a next generation bioelectronic interface. Micromilling (μM), a rapid, cost-effective, scalable, and subtractive ‘makerspace microfabrication’ technique, has been optimized for machining SS of thickness of 50.8 μm such that the μNFPA has a pristine surface finish and the ability to realize complex microscale geometries. Microfabrication of 5 × 5 micromilled, μNFPA is reported, and three distinct microneedle (μN) geometries have been realized. A slotted μN with a ∼200 μm drilled hole has also been demonstrated for potential delivery of therapeutic cargo and extraction of histological samples. The target height and base width of the μNs were chosen to be 400 μm for them to be minimally invasive. The μM tool, a pointed tool with two cutting lips was studied in detail such that an optimized feed rate and spindle speed can be determined for achieving the highest material removal rates with ideal values of chip load. It is observed that a feed rate, spindle speed, depth of cut, and air pressure of 2.5 mm s−1, 1000 Hz, 70 μm and 25 psi respectively can optimally micromill the desired geometries onto the SS substrate. The micromilled μNFPA was additionally acid pickled to sharpen the tips for facilitating the entry and exit through the epidermal layers of our porcine skin model. It was observed that μM-μNFPA that were sharpened after μM offer easier penetration compared to unsharpened μM-μNFPAs. As an initial demonstration of a bioelectronic application for μM-μNFPA arrays, we used them to monitor the change in impedance as a function of time and corresponding dehydration of a porcine skin coupon. A future iteration of the device could serve as the basis for a wearable dehydration monitor for athletes, soldiers and/or hospital patients.

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