In-vitro perforation of the round window membrane via direct 3-D printed microneedles

Abstract

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 reliably and precisely 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. This paper reports the use of direct 3D printing via Two-Photon Polymerization (2PP) lithography to fabricate ultra-sharp polymer microneedles specifically designed to perforate the RWM. The microneedle has tip radius of 500 nm and shank radius of 50 μ m, and perforates the guinea pig 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 μ m. 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 small perforation area along with fiber-to-fiber decohesion are promising indicators that the perforations would heal readily following in vivo experiments. These results establish a foundation for the use of Two-Photon Polymerization lithography as a means to fabricate microneedles to perforate the RWM and other similar membranes.

3D Printed Multi-Functional Hydrogel Microneedles Based on High-Precision Digital Light Processing

“…Solid MNs is relatively rough and could cause irreversible wound and dosage of coated MNs is limited, besides, it is difficult to control and quantify [10,11]. Photosensitive resin used in SLA has poor biocompatibility [19,20]. Combining template driven fabrication with UV curing materials is a complicated process [21,22].…”

“…Moreover, SLS requires high power supply and printing temperature, which would raise the cost of fabrication. Ultraviolet (UV) photosensitive resin is commonly used in SLA, but photosensitive resin has poor biocompatibility [19,20]. At the same time, there are some methods combining template driven fabrication with UV curing materials, which make materials cross-linking into strong structure [21,22].…”

Inner ear delivery: Challenges and opportunities

“…Microneedles are designed with the microanatomic and mechanical properties of the RWM in mind, to create controlled and precise perforations in the membrane with minimal trauma (Figure 3). 40,41 To date, several needle designs have been fabricated with an assortment of unique characteristics. 39,[41][42][43][44][45] Microneedles fabricated using two-photon polymerization (2PP) are able to perforate guinea pig RWM reliably.…”

“…40,41 To date, several needle designs have been fabricated with an assortment of unique characteristics. 39,[41][42][43][44][45] Microneedles fabricated using two-photon polymerization (2PP) are able to perforate guinea pig RWM reliably. 41,45 In the guinea pig, these perforations are not associated with any notable lasting effects on hearing, and the perforations also heal within a week after perforation.…”

“…Thus, the ability to introduce microperforations in a controlled manner leads to the precise and reliable enhancement of RWM permeability. Microneedles are designed with the microanatomic and mechanical properties of the RWM in mind, to create controlled and precise perforations in the membrane with minimal trauma (Figure ) . To date, several needle designs have been fabricated with an assortment of unique characteristics .…”