Biomedical Engineering Reference
In-Depth Information
Introduction
Retinitis Pigmentosa (RP) and Age-related Macular Degeneration (AMD) are
retinal diseases that cause a slowly progressing loss of vision due to a degen-
eration of the light sensitive cells (rods and cones) in the retina. In the end
state, RP and AMD can lead to complete blindness due to the loss of photore-
ceptors, while the connected ganglion and bipolar cells in the retina as well
as the optical nerve remain largely intact. The aim of a number of retinal
prostheses efforts is to partially restore vision to patients with severe cases of
RP and AMD by stimulating the ganglion cells with electrical pulses emitted
through an array of electrodes attached to the retina. Various clinical trials
have already demonstrated the stimulation of visual sensation using single
electrodes and electrode arrays. In light of these successful results, research is
focused on increasing the number of electrodes in the array, which is likely
to increase the capability to perform useful tasks for patients receiving the
implant.
While a number of efforts toward the realization of a retinal prosthesis are
ongoing [1-3], we will focus on a dual-unit prosthesis approach consisting of
an external unit, with a camera and wireless transmitter, and a unit implanted
in the eye consisting of a wireless receiver, implanted microelectronics, and a
stimulating electrode array [4, 5]. The wireless transmission of data and power
to a chronically implanted prosthesis is necessary to avoid percutaneous wire
connections. Biocompatibility and safe operation of the prosthesis components
has to be maintained with minimal deposition of electromagnetic power and
heat, setting stringent design conditions.
This chapter is organized in three sections, respectively addressing aspects of
the inductively coupled telemetry link, the thermal heat simulations of the retinal
prosthesis components, and simulations of the retinal electrode array.
Inductively Coupled Links for a Dual-Unit
Retinal Prosthesis
In the dual-unit retinal prosthesis under consideration here, like in numerous other
biomedical applications, inductive coupling is the preferred method for transcu-
taneous power transfer. Inductive links can carry data to and from implanted
biomedical devices without the need of wires piercing the skin, therefore reducing
the risk of infection. The effectiveness of the inductive link and compliance with
safety standards are of critical importance for these applications. In general, skin
mobility and variations in the thickness of subcutaneous fatty tissue can cause
misalignment of the coils, leading to a change of transmission characteristics.
In the retinal prosthesis, eye movement can obviously cause substantial changes
in the relative positions of external and internal coils. Thus, numerical studies
are necessary to assess the performance of the coupling between external and
internal coils in advance to clinical trials. Different coil geometries are considered
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