Design and characterization of hollow microneedles for localized intrascleral drug delivery of ocular formulations

Effective drug delivery to the posterior segment of the eye remains a challenge owing to the limitations of conventional methods such as intravitreal injections, which are associated with significant side effects. This study explored the use of hollow microneedles (HMNs) for localized intrascleral drug delivery as a minimally invasive alternative. Stainless steel HMNs with bevel angles of 30°, 45°, 60°, and 75° were fabricated using wire electron discharge machining. The penetration force of these HMNs in ex vivo porcine sclera was assessed using a texture analyser, revealing that the 60° bevel angle required the lowest force (<2N), making it optimal for scleral penetration. To ensure precision in drug delivery, 3D-printed adapters were developed to control the injection angles and volumes. The distribution of a model dye, rhodamine B, was studied via digital imaging, multiphoton microscopy, and confocal microscopy. The results showed that HMNs with a 60° bevel angle could penetrate the sclera to a depth of approximately 450 µm at a 45° injection angle, providing enhanced distribution within the scleral layers. This study confirmed that the use of HMNs enables effective and controlled intrascleral drug delivery, resulting in the formation of localized depots with minimal tissue damage. This research demonstrates the potential of HMNs as a promising alternative to traditional ocular drug delivery methods, offering improved bioavailability and the potential to reduce patient discomfort.