We report the use of Two-Photon-Polymerization (2PP) lithography to manufacture precision microneedles for the purpose of intracochlear drug delivery.
The cochlea, or inner ear, is a space fully enclosed within the temporal bone of the skull, except for two membrane-covered portals connecting it to the middle ear space. One of these portals is the round window, which is covered by the Round Window Membrane (RWM). A longstanding clinical goal is to deliver therapeutics into the cochlea to treat a plethora of auditory and vestibular disorders. Standard of care for several difficult to treat diseases calls for injection of a therapeutic substance through the tympanic membrane into the middle ear space, after which a portion of the substance diffuses across the RWM into the cochlea. The efficacy of this technique is limited by an inconsistent rate of molecular transport across the RWM.
A solution to this problem involves the introduction of one or more microscopic perforations through the RWM to enhance the rate and reliability of diffusive transport. Hence, ultra-sharp polymer microneedles specifically designed to perforate the RWM are made using direct 3D printing via 2PP lithography.
The needles are 3D printed, developed and mounted on sterile 23 Gauge blunt syringe tips for practical use. The needles are then used to perforate freshly excised guinea pig membranes. The perforation force is collected, and the resulting holes are analyzed via confocal microscopy, which has the benefit of visualizing the fibers that give the RWM its mechanical properties.
The microneedle has tip radius of curvature of 500 nm and shank radius of 50 µm. It perforates the RWM with a mean force of 1.19 mN. The resulting perforations performed in-vitro are lens-shaped with major axis equal to the microneedle shank diameter and minor axis about 25% of the major axis, with mean area 1670 µm2. The major axis is aligned with the direction of the connective fibers within the RWM. The fibers were separated along their axes without ripping or tearing of the RWM suggesting the main failure mechanism to be fiber-to-fiber decohesion.
The needles are imaged using a Scanning Electron Microscope (SEM) after use, and it is seen that the tips of these microneedles are bent to some extent, limiting their reusability. Therefore, radii of curvature of the tips are systematically changed in order to find an optimal shape for the needles with the purpose of enhancing the mechanical strength and preventing blunting.
These results establish a foundation for the use of 2PP as a means to fabricate microneedles to perforate the RWM and other similar membranes requiring precision manufacturing of complex geometries. The small perforation area along with fiber-to-fiber decohesion are promising indicators that the perforations would heal readily following in-vivo experiments. An optimal needle geometry is currently being researched for the purpose of RWM perforation.