Pyrolyzed Ultrasharp Glassy Carbon Microneedles

Abstract

Polymeric microneedles fabricated via two‐photon polymerization (2PP) lithography enable safe medical access to the inner ear. Herein, the material class for 2PP‐lithography‐based microneedles is expanded by pyrolyzing 2PP‐fabricated polymeric microneedles, resulting in glassy carbon microneedles. During pyrolysis the microneedles shrink up to 81% while maintaining their complex shape when the exposed surface‐area‐to‐volume ratio (SVR) is 0.025<SVR<0.04, for the temperature history protocol used herein. The derived glassy carbon is confirmed with energy‐dispersive X‐ray spectroscopy and Raman spectroscopy. The pyrolyzed glassy carbon has Young's modulus 9.0 GPa. As a brittle material, the strength is stochastic. Using the two‐parameter Weibull distribution, the glassy carbon has Weibull modulus of 3.1 and characteristic strength of 710 MPa. The viscoelastic response has characteristic time scale of about 10000 s. In vitro experiments demonstrate that the glassy carbon microneedles introduce controlled perforations across the guinea pig round window membrane (RWM) from the middle ear space into the inner ear, without damaging the microneedle. The resultant controlled perforation of RWM is known to enhance diffusion of therapeutics across the RWM in a predictable fashion. Hence, the glassy carbon microneedles can be deployed for mediating inner ear delivery.

Physiologic Effects of Microneedle‐Mediated Intracochlear Dexamethasone Injection in the Guinea Pig

“…To circumvent the current issues apparent in intracochlear administration, our laboratory has developed microneedles that allow for perforation of the RWM with minimal trauma and full reconstitution of the RWM structure within 72 h. [19][20][21][22][23][24] Several of these studies used an in vivo guinea pig model to assess the physiological consequences of perforation, aspiration, and injection through the RWM with follow-up hearing studies between 0 and2 h, 48 h, 72 h, and 1 week post-perforation. Between 0 and 2 h post-perforation, our laboratory has found threshold shifts of 5-10 dB in the 22-40 kHz frequency range at 0-2 h post-perforation and resolution of these findings within 1 week.…”