Electrical and Electronic Engineering - Theses
Permanent URI for this collection
Search Results
1 - 1 of 1
-
ItemSafety of a wide-field suprachoroidal retinal prosthesis( 2012)Retinal prostheses have emerged as a promising therapy for blindness arising from retinal degenerative diseases; the leading cause of untreatable visual impairment. Implantation in the suprachoroidal space is a potentially safer alternative to other modes of intraocular implantation because the choroid provides isolation from the neuroretina. This work aimed to characterise the safety of a suprachoroidal retinal prosthesis covering a wide visual field. For this, surgical implantation trauma was assessed, an implant developed, then the pathological response to chronic implantation was characterised and the efficacy of retinal stimulation evaluated. The initial study with flexible circuit electrode arrays assessed surgical implantation trauma. The 8 mm × 100 mm arrays were implanted suprachoroidally in cats, by vitreoretinal surgeons, through a temporal sclerotomy and then advanced towards area centralis. Subsequent histopathological analysis revealed that the implants were consistently located in the suprachoroidal space, without breaching retina, choroid or sclera. The risks identified were: scleral deformation due to implant stiffness, choroidal incarceration through the wide sclerotomy and subclinical retinal folding as the implant approached the optic disc. A medical grade implant substrate was developed for improved safety and simpler surgery. An array of 21 platinum electrodes (ø 600 μm) was built in a spherically contoured silicone substrate (8 mm × 19 mm). The electrode array was designed to be reliably implanted suprachoroidally, conform to the sclera, minimise the trauma found in the first study and be robust for long-term implantation. The implant had a transscleral cable which terminated in a subcutaneous implantable connector. These contoured wide-field electrode arrays were unilaterally implanted in cats for 3 months to evaluate long term safety. Fundus examination revealed that the implants stabilised and initial oedema resolved within 2 weeks. The retinas were unharmed except for small hyperpigmented regions near the optic disc. Electroretinographic assessment showed hypersensitivity in the implanted eyes at 2 weeks, but retinal function was normal after 3 months. Following 3 months of implantation, efficacy of neural stimulation was characterised by recording cortical potentials. For this, the subcutaneous connector was exposed and the electrodes stimulated with biphasic current pulses. Evoked cortical potentials were reliably recorded on the epidural surface of V1 for 98 out of 100 stimulated electrodes, with a median threshold of 150 nC/phase. The average electrode impedance in vivo of 12.5 kΩ was higher than the preimplantation impedance in saline of 3.13 kΩ, which indicated a stable tissue interface. After termination the eyes were processed for histopathological assessment. Tissue samples from the implant tract revealed a thin layer of fibrous tissue on the implant surface (1–2 cells thick) with sporadic macrophages and giant cells. Presence of active inflammatory cells was scarce. The retina over the electrode array was not altered significantly compared to the control eye. The suprachoroidal electrode array was safe, reliable and well tolerated in the eye during 3 months of implantation. The tissue reaction to the implant was minimal and the undamaged retina was reliably stimulated to evoke cortical activity. The spherically contoured electrode array showed promise for safe clinical use.