Anatomical and Functional Consequences of Microneedle Perforation of Round Window Membrane

Contrast Enhancement of Cochlea after Direct Microneedle Intracochlear Injection of Gadodiamide through the Round Window Membrane with Minimal Dosage

RATIONALE AND OBJECTIVES

The potential of contrast-enhanced MRI for diagnosing endolymphatic hydrops is limited by long wait times following intravenous (IV) or intratympanic (IT) delivery, high contrast dosages, and inconsistent signal intensity enhancements. This study investigates microneedle-mediated intracochlear (IC) gadodiamide injection for consistent and efficient contrast delivery with minimal contrast dosage.

MATERIALS AND METHODS

A 100 µm diameter microneedle with 35 µm lumen was used to inject 1 µL of diluted gadodiamide (17.4 mM) into a guinea pig cochlea via the round window membrane. Serial MRI imaging was performed in a post-mortem animal using a 9.4 T small-animal MRI. Maximum intensity projections of MRI scans were generated to visualize diffusion of contrast within cochlea over time; mean intensities in defined regions of interest (ROIs) were calculated. Contrast diffusion time and intensity enhancements were determined.

RESULTS

Contrast was observed in the basal turn of scala tympani (ST) and scala vestibuli (SV) in the first MRI scan for all subjects which was acquired as early as 35 min after injection. Two-tailed paired t-tests confirmed that contrast reached the first two turns of ST and SV within 60 min, and the second half of third turns and apical turns of ST and SV within 90 min (p < 0.05). Intensity enhancements, defined as the percentage increase of the ROI mean intensity in the injection side compared to the contralateral side, exceeded 100% in the first turn and ranged from 12% to 32% in the third and apical turns of ST and SV at 90 min after injection.

CONCLUSIONS

IC gadodiamide enables controllable and efficient contrast delivery with significantly lower contrast dosage, making it a viable alternative for contrast-enhanced cochlear MRI.

Microneedle-mediated intracochlear injection safely achieves higher perilymphatic dexamethasone concentration than intratympanic delivery in guinea pig

Intracochlear injection through the round window membrane (RWM) has been proposed to overcome imprecise drug delivery into the inner ear. Using a novel ultrasharp microneedle, we compared the perilymphatic dexamethasone (DEX) concentration achieved after intratympanic vs. intracochlear injection at two different time points and assessed its safety in guinea pigs. For this purpose, DEX sodium phosphate (10 mg/mL) was administered either in the right middle ear space via continuous intratympanic injection or in the right scala tympani of the cochlea with microneedle-mediated injection (1 µL at 1 µL/min) across the RWM. Both groups were evaluated at 1-hour or 3-hour time points. Perilymph from both cochleae was sampled for liquid chromatography-mass spectrometry, and bilateral cochleae were harvested for immunofluorescence. Eighteen guinea pigs were included. The mean DEX concentration was higher in the intracochlear delivery group than in the intratympanic delivery group at 1-hour time point (mean difference 67,863 ng/mL, 95% CI (8,352-127,374 ng/mL), p = 0.03). No difference was found at 3-hour time point. In every animal on both cochleae, no disruption in hair and supportive cells of the organ of Corti and utricle was observed. Significant middle ear inflammation was observed with the intratympanic delivery method compared to intracochlear. In conclusion, microneedle-mediated intracochlear injection achieves higher perilymphatic DEX concentration than the intratympanic route by a factor of 7 while preserving the cochlear architecture and inducing significantly less middle ear inflammation. In this new era of inner ear therapeutics, the potential for translational application is tangible and promising.

Advanced transdermal drug delivery system: A comprehensive review of microneedle technologies, novel designs, diverse applications, and critical challenges

Transdermal drug delivery presents numerous advantages over conventional administration routes, including non-invasiveness, enhanced patient adherence, circumvention of hepatic first-pass metabolism, self-administration capabilities, controlled release, and increased bioavailability. Nevertheless, the barrier function of stratum corneum limits this strategy to molecules possessing requisite physicochemical attributes. To expand the field of transdermal delivery, researchers have pioneered physical enhancement techniques, with micron-sized needles emerging as a particularly promising platform for the transdermal and intradermal delivery of therapeutic agents across a spectrum of molecular sizes. Microneedles function by disrupting the skin's integrity, generating microchannels that facilitate efficient drug permeation. This innovative technology boasts a captivating profile characterized by non-invasive drug delivery, enhanced efficacy and onset time, improved patient acceptability, self-administration possibilities, and precise dosing capabilities. Consequently, both academic institutions and industry have invested substantial resources in the development of microneedle systems for pharmaceutical delivery. This comprehensive review elucidates the multifaceted aspects of microneedle technology, encompassing its historical evolution, diverse materials, innovative designs, fabrication methodologies, and characterization techniques. The review extends to various microneedle types, including solid, hollow, coated, dissolving, swelling, and porous microneedles, as well as cutting-edge designs such as stimulus-responsive, iontophoresis-assisted, and bionic microneedles. Furthermore, we explore microneedle applications in vaccination, targeted delivery, and the administration of biologics, long-acting therapeutic agents, and cosmetics. Critical challenges in microneedle development, including dimensional considerations, safety concerns, acceptability factors, production scalability, regulatory hurdles, and sustainability issues, are thoroughly addressed, alongside a presentation of future prospects in this rapidly evolving field.

Progression of photoresin-based microneedles: From established drug delivery to emerging biosensing technologies

Microneedles have emerged as a highly promising technology for advancing chemical biosensing and drug delivery applications, offering a minimally invasive, efficient, and versatile approach to healthcare innovation. This review provides a comprehensive analysis of photoresin-based microneedles, with a particular focus on SU-8 photoresin due to its favorable mechanical properties, biocompatibility, and ease of fabrication. Advanced techniques for surface modification are discussed to enhance the functionality of microneedles, enabling their application in precise biochemical diagnostics and effective drug therapy. Additionally, a concise overview of the two-photon polymerization technology is presented, emphasizing its remarkable potential in the production of microneedle arrays. By examining the various types of resins employed in the production of microneedles and their integration with nanostructures, this review offers valuable insights into the development and optimization of microneedle-based systems for diverse healthcare purposes.

Microneedle-Mediated Delivery of siRNA via Liposomal-Based Transfection for Inner Ear Gene Therapy

HYPOTHESIS

Microneedle-mediated intracochlear injection of siRNA-Lipofectamine through the round window membrane (RWM) can be used to transfect cells within the cochlea.

BACKGROUND

Our laboratory has developed 100-μm diameter hollow microneedles for intracochlear injection through the guinea pig RWM. In this study, we test the feasibility of microneedle-mediated injection of siRNA and Lipofectamine, a commonly used reagent with known cellular toxicity, through the RWM for cochlear transfection.

METHODS

Fluorescently labeled scramble siRNA was diluted into Lipofectamine RNAiMax and OptiMEM. One microliter of 5 μM siRNA was injected through the RWM of Hartley guinea pigs at a rate of 1 μl/min (n = 22). In a control group, 1.0 μl of Lipofectamine, with no siRNA, was diluted into OptiMEM and injected in a similar fashion (n = 5). Hearing tests were performed before and either at 24 hours, 48 hours, or 5 days after injection. Afterward, animals were euthanized, and cochleae were harvested for imaging. Control cochleae were processed in parallel to untreated guinea pigs.

RESULTS

Fluorescence, indicating successful transfection, was observed within the basal and middle turns of the cochlea with limited distribution in the apex at 24 and 48 hours. Signal was most intense in the organ of Corti, spiral ligament, and spiral ganglion. Little to no fluorescence was observed at 5 days post-injection. No significant changes in auditory brainstem response (ABR) were noted post-perforation at 5 days, suggesting that siRNA-Lipofectamine at low doses does not cause cochlear toxicity.

CONCLUSIONS

Small volumes of siRNA and Lipofectamine can be effectively delivered to cochlear structures using microneedles, paving the way for atraumatic cochlear gene therapy.

A Benchtop Round Window Model for Studying Magnetic Nanoparticle Transport to the Inner Ear

INTRODUCTION

The round window membrane (RWM) presents a significant barrier to the local application of therapeutics to the inner ear. We demonstrate a benchtop preclinical RWM model and evaluate superparamagnetic iron oxide nanoparticles (SPIONs) as vehicles for magnetically assisted drug delivery.

METHODS

Guinea pig RWM explants were inset into a 3D-printed dual chamber benchtop device. Custom-synthesized 7-nm iron core nanoparticles were modified with different polyethylene glycol chains to yield two sizes of SPIONs (NP-PEG600 and NP-PEG3000) and applied to the benchtop model with and without a magnetic field. Histologic analysis of the RWM was performed using transmission electron microscopy (TEM) and confocal microscopy.

RESULTS

Over a 4-h period, 19.5 ± 1.9% of NP-PEG3000 and 14.6 ± 1.9% of NP-PEG600 were transported across the guinea pig RWM. The overall transport increased by 1.45× to 28.4 ± 5.8% and 21.0 ± 2.0%, respectively, when a magnetic field was applied. Paraformaldehyde fixation of the RWM decreased transport significantly (NP-PEG3000: 7.6 ± 1.5%; NP-PEG600: 7.0 ± 1.6%). Confocal and electron microscopy analysis demonstrated nanoparticle localization throughout all cellular layers and layer-specific transport characteristics within RWM.

CONCLUSION

The guinea pig RWM explant benchtop model allows for targeted and practical investigations of transmembrane transport in the development of nanoparticle drug delivery vehicles. The presence of a magnetic field increases SPION delivery by 45%-50% in a nanoparticle size- and cellular layer-dependent manner.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:3355-3362, 2024.

Precision medicine: a new era for inner ear diseases

The inner ear is the organ responsible for hearing and balance. Inner ear dysfunction can be the result of infection, trauma, ototoxic drugs, genetic mutation or predisposition. Often, like for Ménière disease, the cause is unknown. Due to the complex access to the inner ear as a fluid-filled cavity within the temporal bone of the skull, effective diagnosis of inner ear pathologies and targeted drug delivery pose significant challenges. Samples of inner ear fluids can only be collected during surgery because the available procedures damage the tiny and fragile structures of the inner ear. Concerning drug administration, the final dose, kinetics, and targets cannot be controlled. Overcoming these limitations is crucial for successful inner ear precision medicine. Recently, notable advancements in microneedle technologies offer the potential for safe sampling of inner ear fluids and local treatment. Ultrasharp microneedles can reach the inner ear fluids with minimal damage to the organ, collect μl amounts of perilymph, and deliver therapeutic agents in loco. This review highlights the potential of ultrasharp microneedles, combined with nano vectors and gene therapy, to effectively treat inner ear diseases of different etiology on an individual basis. Though further research is necessary to translate these innovative approaches into clinical practice, these technologies may represent a true breakthrough in the clinical approach to inner ear diseases, ushering in a new era of personalized medicine.

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

OBJECTIVES

Oral or intratympanic corticosteroids are commonly used to treat sudden sensorineural hearing loss (SSHL), tinnitus, and Meniere disease. Direct intracochlear delivery has been proposed to overcome the variability in bioavailability and efficacy of systemic or middle ear delivery. In this study, we aim to characterize the physiologic consequences of microneedle-mediated direct intracochlear injection of dexamethasone through the round window membrane (RWM).

METHODS

In Hartley guinea pigs (n = 5), a post-auricular incision followed by bullostomy was made to access the round window membrane. Using 100 μm diameter hollow microneedles, 1.0 μl of 10 mg/ml dexamethasone was injected through the RWM over 1 min. Compound action potential (CAP) and distortion product otoacoustic action emissions (DPOAE) were measured before perforation, at 1 h, and at 5 h following injection. CAP hearing thresholds were measured from 0.5 to 40 kHz, and DPOAE f2 frequencies ranged from 1.0 and 32 kHz. Repeated measures ANOVA followed by pairwise t-tests were used for statistical analysis.

RESULTS

ANOVA identified significant CAP threshold shifts at four frequencies (4, 16, 36, and 40 kHz) and differences in DPOAE at 1 frequency (6 kHz). Paired t-tests revealed differences between the pre-perforation and 1 h time point. By 5 h post injection, both CAP hearing thresholds and DPOAE recover and are not significantly different from baseline thresholds.

CONCLUSION

Direct intracochlear delivery of dexamethasone via microneedles results in temporary shifts in hearing thresholds that resolve by 5 hours, thus supporting microneedle technology for the treatment of inner ear disorders.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:388-392, 2024.

Anatomic, Physiologic, and Proteomic Consequences of Repeated Microneedle-Mediated Perforations of the Round Window Membrane

BACKGROUND

We have developed 3D-printed microneedle technology for diagnostic aspiration of perilymph and intracochlear delivery of therapeutic agents. Single microneedle-mediated round window membrane (RWM) perforation does not cause hearing loss, heals within 48-72 h, and yields sufficient perilymph for proteomic analysis. In this study, we investigate the anatomic, physiologic, and proteomic consequences of repeated microneedle-mediated perforations of the same RWM at different timepoints.

METHODS

100-μm-diameter hollow microneedles were fabricated using two-photon polymerization (2PP) lithography. The tympanic bullae of Hartley guinea pigs (n = 8) were opened with adequate exposure of the RWM. Distortion product otoacoustic emissions (DPOAE) and compound action potential (CAP) were recorded to assess hearing. The hollow microneedle was introduced into the bulla and the RWM was perforated; 1 μL of perilymph was aspirated from the cochlea over the course of 45 s. 72 h later, the above procedure was repeated with aspiration of an additional 1 μL of perilymph. 72 h after the second perforation, RWMs were harvested for confocal imaging. Perilymph proteomic analysis was completed using liquid chromatography-tandem mass spectrometry (LC-MS/MS).

RESULTS

Two perforations and aspirations were performed in 8 guinea pigs. In six, CAP, DPOAE, and proteomic analysis were obtained; in one, only CAP and DPOAE results were obtained; and in one, only proteomics results were obtained. Hearing tests demonstrated mild hearing loss at 1-4 kHz and 28 kHz, most consistent with conductive hearing loss. Confocal microscopy demonstrated complete healing of all perforations with full reconstitution of the RWM. Perilymph proteomic analysis identified 1855 proteins across 14 samples. The inner ear protein cochlin was observed in all samples, indicating successful aspiration of perilymph. Non-adjusted paired t-tests with p < 0.01 revealed significant changes in 13 of 1855 identified proteins (0.7%) between the first and second aspirations.

CONCLUSIONS

We demonstrate that repeated microneedle perforation of the RWM is feasible, allows for complete healing of the RWM, and minimally changes the proteomic expression profile. Thus, microneedle-mediated repeated aspirations in a single animal can be used to monitor the response to inner ear treatments over time.

Pyrolyzed Ultrasharp Glassy Carbon Microneedles

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.

Inner Ear Diagnostics and Drug Delivery via Microneedles

Objectives: Precision medicine for inner ear disorders has seen significant advances in recent years. However, unreliable access to the inner ear has impeded diagnostics and therapeutic delivery. The purpose of this review is to describe the development, production, and utility of novel microneedles for intracochlear access. Methods: We summarize the current work on microneedles developed using two-photon polymerization (2PP) lithography for perforation of the round window membrane (RWM). We contextualize our findings with the existing literature in intracochlear diagnostics and delivery. Results: Two-photon polymerization lithography produces microneedles capable of perforating human and guinea pig RWMs without structural or functional damage. Solid microneedles may be used to perforate guinea pig RWMs in vivo with full reconstitution of the membrane in 48–72 h, and hollow microneedles may be used to aspirate perilymph or inject therapeutics into the inner ear. Microneedles produced with two-photon templated electrodeposition (2PTE) have greater strength and biocompatibility and may be used to perforate human RWMs. Conclusions: Microneedles produced with 2PP lithography and 2PTE can safely and reliably perforate the RWM for intracochlear access. This technology is groundbreaking and enabling in the field of inner ear precision medicine.

Rationally Designed Magnetic Nanoparticles for Cochlear Drug Delivery: Synthesis, Characterization, and In Vitro Biocompatibility in a Murine Model

Hypothesis

Magnetic nanoparticles (MNPs) for cochlear drug delivery can be precisely engineered for biocompatibility in the cochlea.

Background

MNPs are promising drug delivery vehicles that can enhance the penetration of both small and macromolecular therapeutics into the cochlea. However, concerns exist regarding the application of oxidative, metal-based nanomaterials to delicate sensory tissues of the inner ear. Translational development of MNPs for cochlear drug deliver requires specifically tuned nanoparticles that are not cytotoxic to inner ear tissues. We describe the synthesis and characterization of precisely tuned MNP vehicles, and their in vitro biocompatibility in murine organ of Corti organotypic cultures.

Methods

MNPs were synthesized via 2-phase ligand transfer process with precise control of nanoparticle size. Core and hydrodynamic sizes of nanoparticles were characterized using electron microscopy and dynamic light scattering, respectively. In vitro biocompatibility was assayed via mouse organ of Corti organotypic cultures with and without an external magnetic field gradient. Imaging was performed using immunohistochemical labeling and confocal microscopy. Outer hair cell, inner hair cell, and spiral ganglion neurites were individually quantified.

Results

Monocore PEG-MNPs of 45 and 148 nm (mean hydrodynamic diameter) were synthesized. Organ of Corti cultures demonstrated preserved outer hair cell, inner hair cell, and neurite counts across 2 MNP sizes and doses, and irrespective of external magnetic field gradient.

Conclusion

MNPs can be custom-synthesized with precise coating, size, and charge properties specific for cochlear drug delivery while also demonstrating biocompatibility in vitro.

Membrane curvature and connective fiber alignment in guinea pig round window membrane

The round window membrane (RWM) covers an opening between the perilymph fluid-filled inner ear space and the air-filled middle ear space. As the only non-osseous barrier between these two spaces, the RWM is an ideal candidate for aspiration of perilymph for diagnostics purposes and delivery of medication for treatment of inner ear disorders. Routine access across the RWM requires the development of new surgical tools whose design can only be optimized with a thorough understanding of the RWM's structure and properties. The RWM possesses a layer of collagen and elastic fibers so characterization of the distribution and orientation of these fibers is essential. Confocal and two-photon microscopy were conducted on intact RWMs in a guinea pig model to characterize the distribution of collagen and elastic fibers. The fibers were imaged via second-harmonic-generation, autofluorescence, and Rhodamine B staining. Quantitative analyses of both fiber orientation and geometrical properties of the RWM uncovered a significant correlation between mean fiber orientations and directions of zero curvature in some portions of the RWM, with an even more significant correlation between the mean fiber orientations and linear distance along the RWM in a direction approximately parallel to the cochlear axis. The measured mean fiber directions and dispersions can be incorporated into a generalized structure tensor for use in the development of continuum anisotropic mechanical constitutive models that in turn will enable optimization of surgical tools to access the cochlea. STATEMENT OF SIGNIFICANCE: The Round Window Membrane (RWM) is the only non-osseous barrier separating the middle and inner ear spaces, and thus is an ideal portal for medical access to the cochlea. An understanding of RWM structure and mechanical response is necessary to optimize the design of surgical tools for this purpose. The RWM geometry and the connective fiber orientation and dispersion are measured via confocal and 2-photon microscopy. A region of the RWM geometry is characterized as a hyperbolic paraboloid and another region as a tapered parabolic cylinder. Predominant fiber directions correlate well with directions of zero curvature in the hyperbolic paraboloid region. Overall fiber directions correlate well with position along a line approximately parallel to the central axis of the cochlea's spiral.

A Novel 3D-Printed Head Holder for Guinea Pig Ear Surgery

BACKGROUND

Otologic surgery in guinea pig requires head immobilization for microscopic manipulation. Existing commercially available stereotaxic frames are expensive and impede access to the ear as they rely on ear bars or mouthpieces to secure the head.

METHOD

Prototype head holders were designed using the Solidworks 2019 software and 3D-printed using Formlabs Form 2 Printers with photopolymer resin. The head holder consists of a C-shaped brace with adjustable radial inserts of 1/4-20 UNC standard screws with cone point tips providing head fixation for animals of various sizes. The C-shaped brace is attached to a rod that can be secured to a commercially available micromanipulator. The head holder design was tested during in vivo guinea pig experiments where their head motion with (n = 22) and without the head holder (n = 2) was evaluated visually through a stereotaxic microscope at 24× magnification during surgery.

RESULTS

The head holder design was easy to use and allowed for both nose cone administration of anesthesia and access to the ear for intraoperative auditory testing and manipulation. Functionally, the head holder successfully minimized head movement. Furthermore, harvested round window membranes evaluated at 72 hours following surgery showed precise perforations with the use of head holder.

CONCLUSION

The novel 3D-printed head holder enables simultaneous access for nose cone administration of anesthesia and surgical manipulation of the ear and brain. Moreover, it provides a modular, intuitive, and economical alternative to commercial stereotaxic devices for minimizing head motion during small animal surgery.

Impact of Systemic versus Intratympanic Dexamethasone Administration on the Perilymph Proteome

Glucocorticoids are the first-line treatment for sensorineural hearing loss, but little is known about the mechanism of their protective effect or the impact of route of administration. The recent development of hollow microneedles enables safe and reliable sampling of perilymph for proteomic analysis. Using these microneedles, we investigate the effect of intratympanic (IT) versus intraperitoneal (IP) dexamethasone administration on guinea pig perilymph proteome. Guinea pigs were treated with IT dexamethasone (n = 6), IP dexamethasone (n = 8), or untreated for control (n = 8) 6 h prior to aspiration. The round window membrane (RWM) was accessed via a postauricular approach, and hollow microneedles were used to perforate the RWM and aspirate 1 μL of perilymph. Perilymph samples were analyzed by liquid chromatography-mass spectrometry-based label-free quantitative proteomics. Mass spectrometry raw data files have been deposited in an international public repository (MassIVE proteomics repository at https://massive.ucsd.edu/) under data set # MSV000086887. In the 22 samples of perilymph analyzed, 632 proteins were detected, including the inner ear protein cochlin, a perilymph marker. Of these, 14 proteins were modulated by IP, and three proteins were modulated by IT dexamethasone. In both IP and IT dexamethasone groups, VGF nerve growth factor inducible was significantly upregulated compared to control. The remaining adjusted proteins modulate neurons, inflammation, or protein synthesis. Proteome analysis facilitated by the use of hollow microneedles shows that route of dexamethasone administration impacts changes seen in perilymph proteome. Compared to IT administration, the IP route was associated with greater changes in protein expression, including proteins involved in neuroprotection, inflammatory pathway, and protein synthesis. Our findings show that microneedles can mediate safe and effective intracochlear sampling and hold promise for inner ear diagnostics.

Simulation assisted design for microneedle manufacturing: Computational modeling of two-photon templated electrodeposition

Fully metallic micrometer-scale 3D architectures can be fabricated via a hybrid additive methodology combining multi-photon lithography with electrochemical deposition of metals. The methodology - referred to as two-photon templated electrodeposition (2PTE) - has significant design freedom that enables the creation of complicated, traditionally difficult-to-make, high aspect ratio metallic structures such as microneedles. These complicated geometries, combined with their fully metallic nature, can enable precision surgical applications such as inner ear drug delivery or fluid sampling. However, the process involves electrochemical deposition of metals into complicated 3D lithography patterns thicker than 500 μm. This causes potential and chemical gradients to develop within the 3D template, creating limitations to what can be designed. These limitations can be explored, understood, and overcome via numerical modeling. Herein we introduce a numerical model as a design tool that can predict growth for manufacturing complicated 3D metallic geometries. The model is successful in predicting the geometric result of 2PTE, and enables extraction of insights about geometric constraints through exploration of its mechanics.

Hearing Preservation in Patients Who Undergo Labyrinthectomy and Translabyrinthine Procedures: A Case Report and Systematic Review

Importance

Preservation of endolymphatic fluids, maintenance of a fluid-filled vestibule, and preservation of the cochlear nerve and its vasculature are believed to be necessary to retaining hearing after an inner-ear operation. However, some studies have reported no hearing loss despite the violation of the vestibule, questioning the importance of maintaining a fluid-filled vestibule in preserving hearing.

Objective

To report on the preservation of hearing after a complete labyrinthectomy for Meniere disease and after disruption of the vestibule.

Evidence Review

This systematic review adhered to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) reporting guidelines. A search of PubMed, Scopus, Ovid, and Cochrane Library databases was conducted to identify English-language articles on hearing preservation after labyrinthectomy, published from 1947 through December 11, 2019. The search strategy used a combination of boolean operators and included the following Medical Subject Heading terms and keywords: hearing preservation, labyrinth surgery, labyrinthectomy, vestibule violation, vestibule disruption, translabyrinthine approach, schwannoma removal, and semicircular canal ablation. Studies that included disruption of the vestibule and hearing preservation were included.

Findings

This systematic review identified 10 studies with 10 patients who underwent surgical removal of cholesteatoma or vestibular schwannoma and displayed postoperative hearing preservation. This study also reported on 1 patient with Meniere disease who retained hearing after undergoing a complete labyrinthectomy. Among these 11 patients, the mean (range) age was 45.1 (27-55) years, and 8 patients (73%) were women. Multiple theories exist that explain the mechanism behind hearing preservation, such as sealing of the ductus reuniens or closure of the remaining vestibule.

Conclusions and Relevance

This systematic review describes a set of patients who did not experience hearing loss after a labyrinthectomy or surgical violation of the vestibule, which seems to contradict prevailing principles for retaining hearing after inner-ear surgical procedures. This finding suggests that hearing preservation is possible after labyrinthine destruction despite the absence of a fluid-filled vestibule and that other mechanisms, such as occlusion of the ductus reuniens with granulation tissue or bone dust, may be sufficient to achieve that outcome.

[Development of novel drug carrier via round window membrane]

As isolated anatomical position, limited labyrinthine artery supply, and blood-labyrinth barrier hampers systemic drug delivery to the inner ear. The efficient concentration of drug treatment is unsatisfactory and there's possible side effects after systemic administration. Intratympanic injection of drug can bypass the blood-labyrinth and permeated to the hair cells or synaptic area via the round-or oval window of the cochlea. Efficacy and safety of pharmacotherapy has become increasingly relied on the inner ear delivery carrier system. The goal of this review focus on the anatomical barrier that need to be overcome in the intratympanic applications, the improvement of drug retention and specific targets, and the safety of novel drug carriers, these emerging strategies of local drug delivery promise novel and better guidance for the clinical application.

Drug delivery device for the inner ear: ultra-sharp fully metallic microneedles

Drug delivery into the inner ear is a significant challenge due to its inaccessibility as a fluid-filled cavity within the temporal bone of the skull. The round window membrane (RWM) is the only delivery portal from the middle ear to the inner ear that does not require perforation of bone. Recent advances in microneedle fabrication enable the RWM to be perforated safely with polymeric microneedles as a means to enhance the rate of drug delivery from the middle ear to the inner ear. However, the polymeric material is not biocompatible and also lacks the strength of other materials. Herein we describe the design and development of gold-coated metallic microneedles suitable for RWM perforation. When developing microneedle technology for drug delivery, we considered three important general attributes: (1) high strength and ductility material, (2) high accuracy and precision of fabrication, and (3) broad design freedom. We developed a hybrid additive manufacturing method using two-photon lithography and electrochemical deposition to fabricate ultra-sharp gold-coated copper microneedles with these attributes. We refer to the microneedle fabrication methodology as two-photon templated electrodeposition (2PTE). We demonstrate the use of these microneedles by inducing a perforation with a minimal degree of trauma in a guinea pig RWM while the microneedle itself remains undamaged. Thus, this microneedle has the potential literally of opening the RWM for enhanced drug delivery into the inner ear. Finally, the 2PTE methodology can be applied to many different classes of microneedles for other drug delivery purposes as well the fabrication of small scale structures and devices for non-medical applications. Graphical Abstract Fully metallic ultra-sharp microneedle mounted at end of a 24-gauge stainless steel blunt syringe needle tip: (left) Size of microneedle shown relative to date stamp on U.S. one-cent coin; (right) Perforation through guinea pig round window membrane introduced with microneedle.

Novel 3D-printed hollow microneedles facilitate safe, reliable, and informative sampling of perilymph from guinea pigs

BACKGROUND

Inner ear diagnostics is limited by the inability to atraumatically obtain samples of inner ear fluid. The round window membrane (RWM) is an attractive portal for accessing perilymph samples as it has been shown to heal within one week after the introduction of microperforations. A 1 µL volume of perilymph is adequate for proteome analysis, yet the total volume of perilymph within the scala tympani of the guinea pig is limited to less than 5 µL. This study investigates the safety and reliability of a novel hollow microneedle device to aspirate perilymph samples adequate for proteomic analysis.

METHODS

The guinea pig RWM was accessed via a postauricular surgical approach. 3D-printed hollow microneedles with an outer diameter of 100 µm and an inner diameter of 35 µm were used to perforate the RWM and aspirate 1 µL of perilymph. Two perilymph samples were analyzed by liquid chromatography-mass spectrometry-based quantitative proteomics as part of a preliminary study. Hearing was assessed before and after aspiration using compound action potential (CAP) and distortion product otoacoustic emissions (DPOAE). RWMs were harvested 72 h after aspiration and evaluated for healing using confocal microscopy.

RESULTS

There was no permanent damage to hearing at 72 h after perforation as assessed by CAP (n = 7) and DPOAE (n = 8), and all perforations healed completely within 72 h (n = 8). In the two samples of perilymph analyzed, 620 proteins were detected, including the inner ear protein cochlin, widely recognized as a perilymph marker.

CONCLUSION

Hollow microneedles can facilitate aspiration of perilymph across the RWM at a quality and volume adequate for proteomic analysis without causing permanent anatomic or physiologic dysfunction. Microneedles can mediate safe and effective intracochlear sampling and show great promise for inner ear diagnostics.

High-resolution two-photon polymerization: the most versatile technique for the fabrication of microneedle arrays

Microneedle patches have received much interest in the last two decades as drug/vaccine delivery or fluid sampling systems for diagnostic and monitoring purposes. Microneedles are manufactured using a variety of additive and subtractive micromanufacturing techniques. In the last decade, much attention has been paid to using additive manufacturing techniques in both research and industry, such as 3D printing, fused deposition modeling, inkjet printing, and two-photon polymerization (2PP), with 2PP being the most flexible method for the fabrication of microneedle arrays. 2PP is one of the most versatile and precise additive manufacturing processes, which enables the fabrication of arbitrary three-dimensional (3D) prototypes directly from computer-aided-design (CAD) models with a resolution down to 100 nm. Due to its unprecedented flexibility and high spatial resolution, the use of this technology has been widespread for the fabrication of bio-microdevices and bio-nanodevices such as microneedles and microfluidic devices. This is a pioneering transformative technology that facilitates the fabrication of complex miniaturized structures that cannot be fabricated with established multistep manufacturing methods such as injection molding, photolithography, and etching. Thus, microstructures are designed according to structural and fluid dynamics considerations rather than the manufacturing constraints imposed by methods such as machining or etching processes. This article presents the fundamentals of 2PP and the recent development of microneedle array fabrication through 2PP as a precise and unique method for the manufacture of microstructures, which may overcome the shortcomings of conventional manufacturing processes.

Inner Ear Gene Delivery: Vectors and Routes

Objectives

Current treatments for hearing loss offer some functional improvements in hearing, but do not restore normal hearing. The aim of this review is to highlight recent advances in viral and non-viral vectors for gene therapy and to discuss approaches for overcoming barriers inherent to inner ear delivery of gene products.

Data Sources

The databases used were Medline, EMBASE, Web of Science, and Google Scholar. Search terms were [("cochlea*" or "inner ear" or "transtympanic" or "intratympanic" or "intracochlear" or "hair cells" or "spiral ganglia" or "Organ of Corti") and ("gene therapy" or "gene delivery")]. The references section of resulting articles was also used to identify relevant studies.

Results

Both viral and non-viral vectors play important roles in advancing gene delivery to the inner ear. The round window membrane is one significant barrier to gene delivery that intratympanic delivery methods attempt to overcome through diffusion and intracochlear delivery methods bypass completely.

Conclusions

Gene therapy for hearing loss is a promising treatment for restoring hearing function by addressing innate defects. Recent technological advances in inner ear drug delivery techniques pose exciting opportunities for progress in gene therapy.

Inner ear delivery: Challenges and opportunities

OBJECTIVES

The treatment of inner ear disorders remains challenging due to anatomic barriers intrinsic to the bony labyrinth. The purpose of this review is to highlight recent advances and strategies for overcoming these barriers and to discuss promising future avenues for investigation.

DATA SOURCES

The databases used were PubMed, EMBASE, and Web of Science.

RESULTS

Although some studies aimed to improve systemic delivery using nanoparticle systems, the majority enhanced local delivery using hydrogels, nanoparticles, and microneedles. Developments in direct intracochlear delivery include intracochlear injection and intracochlear implants.

CONCLUSIONS

In the absence of a systemic drug that targets only the inner ear, the best alternative is local delivery that harnesses a combination of new strategies to overcome anatomic barriers. The combination of microneedle technology with hydrogel and nanoparticle delivery is a promising area for future investigation.

LEVEL OF EVIDENCE

NA.