Bridging the electrode-neuron gap: finite element modeling of in vitro neurotrophin gradients to optimize neuroelectronic interfaces in the inner ear

Filamented hydrogels as tunable conduits for guiding neurite outgrowth

Anisotropic scaffolds with unidirectionally aligned fibers present an optimal solution for nerve tissue engineering and graft repair. This study investigates the application of filamented light (FLight) biofabrication to create hydrogel matrices featuring highly aligned microfilaments, facilitating neurite guidance and outgrowth from encapsulated chicken dorsal root ganglion (DRG) cells. FLight employs optical modulation instability (OMI) to rapidly and safely (<5 s) fabricate hydrogel constructs with precise microfilament alignment. The tunability of FLight matrices was demonstrated by adjusting four key parameters: stiffness, porosity, growth factor release, and incorporation of biological cues. Matrix stiffness was fine-tuned by varying the projection light dose, yielding matrices with stiffness ranging from 0.6 to 5.7 kPa. Optimal neurite outgrowth occurred at a stiffness of 0.6 kPa, achieving an outgrowth of 2.5 mm over 4 days. Matrix porosity was modified using diffraction gratings in the optical setup. While significant differences in neurite outgrowth and alignment were observed between bulk and FLight gels, further increases in porosity from 40 % to 70 % enhanced cell migration and axon bundling without significantly affecting maximal outgrowth. The incorporation of protein microcrystals containing nerve growth factor (NGF) into the photoresin enabled sustained neurite outgrowth without the need for additional NGF in the media. Finally, laminin was added to the resin to enhance the bioactivity of the biomaterial, resulting in a further increase in maximum neurite outgrowth to 3.5 mm after 4 days of culture in softer matrices. Overall, the varied matrix properties achieved through FLight significantly enhance neurite outgrowth, highlighting the importance of adaptable scaffold characteristics for guiding neurite development. This demonstrates the potential of FLight as a versatile platform for creating ideal matrices for clinical applications in nerve repair and tissue engineering.

Decreasing the physical gap in the neural-electrode interface and related concepts to improve cochlear implant performance

Cochlear implants (CI) represent incredible devices that restore hearing perception for those with moderate to profound sensorineural hearing loss. However, the ability of a CI to restore complex auditory function is limited by the number of perceptually independent spectral channels provided. A major contributor to this limitation is the physical gap between the CI electrodes and the target spiral ganglion neurons (SGNs). In order for CI electrodes to stimulate SGNs more precisely, and thus better approximate natural hearing, new methodologies need to be developed to decrease this gap, (i.e., transitioning CIs from a far-field to near-field device). In this review, strategies aimed at improving the neural-electrode interface are discussed in terms of the magnitude of impact they could have and the work needed to implement them. Ongoing research suggests current clinical efforts to limit the CI-related immune response holds great potential for improving device performance. This could eradicate the dense, fibrous capsule surrounding the electrode and enhance preservation of natural cochlear architecture, including SGNs. In the long term, however, optimized future devices will likely need to induce and guide the outgrowth of the peripheral process of SGNs to be in closer proximity to the CI electrode in order to better approximate natural hearing. This research is in its infancy; it remains to be seen which strategies (surface patterning, small molecule release, hydrogel coating, etc.) will be enable this approach. Additionally, these efforts aimed at optimizing CI function will likely translate to other neural prostheses, which face similar issues.

Investigating Internalization of Reporter-Protein-Functionalized Polyhedrin Particles by Brain Immune Cells

Achieving sustained drug delivery to the central nervous system (CNS) is a major challenge for neurological injury and disease, and various delivery vehicles are being developed to achieve this. Self-assembling polyhedrin crystals (POlyhedrin Delivery System; PODS) are being exploited for the delivery of therapeutic protein cargo, with demonstrated efficacy in vivo. However, to establish the utility of PODS for neural applications, their handling by neural immune cells (microglia) must be documented, as these cells process and degrade many biomaterials, often preventing therapeutic efficacy. Here, primary mouse cortical microglia were cultured with a GFP-functionalized PODS for 24 h. Cell counts, cell morphology and Iba1 expression were all unaltered in treated cultures, indicating a lack of acute toxicity or microglial activation. Microglia exhibited internalisation of the PODS, with both cytosolic and perinuclear localisation. No evidence of adverse effects on cellular morphology was observed. Overall, 20-40% of microglia exhibited uptake of the PODS, but extracellular/non-internalised PODS were routinely present after 24 h, suggesting that extracellular drug delivery may persist for at least 24 h.

Current advances in biomaterials for inner ear cell regeneration

Inner ear cell regeneration from stem/progenitor cells provides potential therapeutic strategies for the restoration of sensorineural hearing loss (SNHL), however, the efficiency of regeneration is low and the functions of differentiated cells are not yet mature. Biomaterials have been used in inner ear cell regeneration to construct a more physiologically relevant 3D culture system which mimics the stem cell microenvironment and facilitates cellular interactions. Currently, these biomaterials include hydrogel, conductive materials, magneto-responsive materials, photo-responsive materials, etc. We analyzed the characteristics and described the advantages and limitations of these materials. Furthermore, we reviewed the mechanisms by which biomaterials with different physicochemical properties act on the inner ear cell regeneration and depicted the current status of the material selection based on their characteristics to achieve the reconstruction of the auditory circuits. The application of biomaterials in inner ear cell regeneration offers promising opportunities for the reconstruction of the auditory circuits and the restoration of hearing, yet biomaterials should be strategically explored and combined according to the obstacles to be solved in the inner ear cell regeneration research.