Challenges in Realizing a Chronic High-Resolution Retinal Prosthesis - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 135

Table 7.1. Process technologies and their characteristics.

Vendor CMOS Feature size

(micrometers)

Number of metal
layers

Operating
voltage

AMIS 035 4 3.3

05 3 5

07 3 100

15 2 5

TSMC 018 6 1.8, 3.3

025 5 2.5, 3.3

035 4 3.3, 5

IBM 013 8 1.2, 2.5

018 6 1.8, 3.3

025 5 2.5, 3.3

austriamicrosystems 035 4 2.5, 50

08 2 5, 50

area. Operating at low voltages not only helps in choosing a smaller and denser
process technology, but also reduces the power consumption of the circuits.
It is obvious that the supply voltages are directly determined by the maximum
voltage appearing at the electrode during stimulation. Usually additional voltage
of around 0.5 V over the electrode voltage is required for the current generators to
function properly. From the circuit design point of view, using a supply voltage
higher than the specified operating voltage introduces the problem of long-term
reliability of the circuit components. This requires careful design to avoid any high
voltages appearing across the devices at any point of time during circuit operation.
Circuit techniques that stack transistors to divide the high voltage into smaller
voltages can be used to alleviate this problem [14]. It is clear that reducing the
electrode voltage will help the design of low power, miniaturized IC along with
ensuring the reliability of the circuit components. Reducing the electrode voltage is
a challenge since this requires reduction of the stimulation current and/or reduction
of the electrode impedance. For a given surface area of the retina to place the
electrodes, migrating from a low-resolution device to a high-resolution device
requires reducing the size and area of the electrodes. A reduction in size is often
accompanied with an increase in the impedance of the electrodes. So making low-
impedance electrodes even when reducing the electrode size is one of the major
challenges. Placing the electrodes closer to the retina has been shown to reduce the
stimulus current threshold [17]. These efforts are necessary in order to overcome
the challenges of power consumption and area.

Stimulation Flexibility

There are two ways of connecting the current generators and the electrodes.
One biphasic current generator can be dedicated to one electrode or a number
of electrodes can be grouped together and they can be assigned to one current