Safety and efficacy of explanting or replacing suprachoroidal electrode arrays in a feline model.

BACKGROUND: A key requirement for retinal prostheses is the ability for safe removal or replacement. We examined whether suprachoroidal electrode arrays can be removed or replaced after implantation. METHODS: Suprachoroidal electrode arrays were unilaterally implanted into 13 adult felines. After 1 month, arrays were surgically explanted (n = 6), replaced (n = 5) or undisturbed (n = 2). The retina was assessed periodically using fundus photography and optical coherence tomography. Three months after the initial implantation, the function of replaced or undisturbed arrays was assessed by measuring the responses of the visual cortex to retinal electrical stimulation. The histopathology of tissues surrounding the implant was examined. RESULTS: Array explantation or replacement was successful in all cases. Fundus photography showed localized disruption to the tapetum lucidum near the implant's tip in seven subjects following implantation. Although optical coherence tomography showed localized retinal changes, there were no widespread statistically significant differences in the thickness of the retinal layers or choroid. The distance between the electrodes and retina increased after device replacement but returned to control values within eight weeks (P < 0.03). Staphylomas developed near the scleral wound in five animals after device explantation. Device replacement did not alter the cortical evoked potential threshold. Histopathology showed localized outer nuclear layer thinning, tapetal disruption and pseudo-rosette formation, but the overall retinal morphology was preserved. CONCLUSIONS: It is feasible to remove or replace conformable medical grade silicone electrode arrays implanted suprachoroidally. The scleral wound requires careful closure to minimize the risk of staphylomas.

In vivo and in vitro comparison of the charge injection capacity of platinum macroelectrodes.

Platinum (Pt) is the most commonly used metal for stimulating electrodes. This study aims to determine the amount of charge that can be delivered without causing irreversible electrochemical reactions (charge injection capacity, Q inj) of Pt macroelectrodes (geometric surface area >0.001 cm(2)) in vitro and in vivo using voltage transient measurements. Pt macroelectrodes were stimulated with biphasic charge-balanced cathodic-first constant-current pulses in phosphate buffered saline. Potential excursions were measured (versus Ag/AgCl electrode) and used to determine Qinj. The in vitro Qinj were compared to those measured in vivo following: acute and chronic implantation close to the retina; chronic intracochlear implantation; and acute subdural implantation, in the cat. Qinj increased with pulsewidth from 35 to 54 µC/cm(2) for respective pulse widths of 100 to 3200 µs per phase in vitro. Qinj was significantly less in vivo. There was no significant difference in Qinj between acutely (3.84 to 16.6 µC/cm(2) with pulsewidths of 100 to 3200 µs) and chronically (6.99 to 15.8 µC/cm(2) with pulsewidths of 200 to 3200 µs) implanted suprachoroidal electrodes. Intracochlear Qinj was not different to suprachoroidal Qinj, while subdural Qinj was significantly less than the suprachoroidal Q inj (p < 0.05). These results have important implications in providing guidelines on Qinj for the safe use of Pt stimulating macroelectrodes and question the relevance of measuring Qinj in vivo using voltage transients.

Electrical stimulation of retinal ganglion cells with diamond and the development of an all diamond retinal prosthesis.

Electronic retinal implants for the blind are already a market reality. A world wide effort is underway to find the technology that offers the best combination of performance and safety for potential patients. Our approach is to construct an epi-retinally targeted device entirely encapsulated in diamond to maximise longevity and biocompatibility. The stimulating array of our device comprises a monolith of electrically insulating diamond with thousands of hermetic, microscale nitrogen doped ultra-nanocrystalline diamond (N-UNCD) feedthroughs. Here we seek to establish whether the conducting diamond feedthroughs of the array can be used as stimulating electrodes without further modification with a more traditional neural stimulation material. Efficacious stimulation of retinal ganglion cells was established using single N-UNCD microelectrodes in contact with perfused, explanted, rat retina. Evoked rat retinal ganglion cell action potentials were recorded by patch clamp recording from single ganglion cells, adjacent to the N-UNCD stimulating electrode. Separately, excellent electrochemical stability of N-UNCD was established by prolonged pulsing in phosphate buffered saline at increasing charge density up to the measured charge injection limit for the material.

Biocompatibility of immobilized aligned carbon nanotubes.

In vivo host responses to an electrode-like array of aligned carbon nanotubes (ACNTs) embedded within a biopolymer sheet are reported. This biocompatibility study assesses the suitability of immobilized carbon nanotubes for bionic devices. Inflammatory responses and foreign-body histiocytic reactions are not substantially elevated when compared to negative controls following 12 weeks implantation. A fibrous capsule isolates the implanted ACNTs from the surrounding muscle tissue. Filamentous nanotube fragments are engulfed by macrophages, and globular debris is incorporated into the fibrous capsule with no further reaction. Scattered leukocytes are observed, adherent to the ACNT surface. These data indicate that there is a minimal local foreign-body response to immobilized ACNTs, that detached fragments are phagocytosed into an inert material, and that ACNTs do not attract high levels of surface fouling. Collectively, these results suggest that immobilized nanotube structures should be considered for further investigation as bionic components.