Pyrolyzed Ultra-Sharp Glassy Carbon Microneedles

We report glassy carbon ultra-sharp microneedles pyrolyzed from a polymeric precursor structure fabricated via Two-Photon Polymerization lithography (2PP). The glassy carbon microneedles have tip radius of curvature of 1 µm, diameters ranging from 22 µm to 26 µm, and lengths ranging from 106 µm to 126 µm. They are capable of safely penetrating the 1.5 mm wide and 35 µm thick round window membrane in a guinea pig model that serves as a barrier between the middle and inner ear spaces. The precursor needle has a tip diameter of 7.5 µm, a base diameter of 100 µm and a length of 600 µm. To maintain structural integrity during pyrolysis, we show that the exposed surface area to volume ratio of the polymeric precursor needs to be within a certain range. Upon designing the polymeric precursor structure under these constraints, the structures can shrink by up to 81% during pyrolysis while maintaining structural integrity. The pyrolyzed glassy carbon is elastic with a brittle failure mechanism with a Young’s modulus of 9.0 GPa and characteristic strength of 710 MPa. Furthermore, glassy carbon is known to be biocompatible. We show that the pyrolyzed ultra-sharp glassy carbon microneedles perforate the round window membrane safely and can serve to enable safe direct delivery of therapeutics into the inner ear delivery. In addition to glassy carbon microneedles, this pyrolysis process can be adapted to fabricate many other biomedical devices.