Simulations of artificial vision suggest that 1000 electrodes may be required to restore vision to individuals with diseases of the outer retina. In order to achieve such an implant, new technology is needed, since the state-of-the-art implantable neural stimulator has at most 22 contacts with neural tissue. Considerable progress has been made towards that goal with the development of image processing, microelectronics, and polymer based MEMS. An image processing system has been realized that is capable of real-time implementation of image decimation and filtering (for example, edge detection). Application specific integrated circuits (ASICs) have been designed and tested to demonstrate closed loop power control and efficient microstimulation. A novel packaging process has been developed that is capable of simultaneously forming communication coils, interconnects, and stimulating electrodes

Fink, Wolfgang

Humayun, Mark S.

Li, Wen

Liu, Wentai

Sivaprakasam, Mohanasankar

Tai, Yu-Chong

Tarbell, Mark A.

Weiland, James D.

Caltech Authors - Main

Systems Design of a High Resolution Retinal Prosthesis 
 
James D. Weiland1, Wolfgang Fink2,1, Mark S. Humayun1, Wentai Liu3,Wen Li4, Mohanasankar 
Sivaprakasam3, Yu-Chong Tai4, Mark A. Tarbell2  
1Doheny Eye Institute, Department of Ophthalmology, Keck School of Medicine, University of Southern California, 
Los Angeles, CA 
2Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA, USA 
3Department of Electrical Engineering, University of California, Santa Cruz, CA, USA 
4Electrical Engineering, Division of Engineering and Applied Science, California Institute of Technology,  
Pasadena, CA, USA 
 
Abstract 
Simulations of artificial vision suggest that 1000 electrodes 
may be required to restore vision to individuals with diseases 
of the outer retina. In order to achieve such an implant, new 
technology is needed, since the state-of-the-art implantable 
neural stimulator has at most 22 contacts with neural tissue. 
Considerable progress has been made towards that goal with 
the development of image processing, microelectronics, and 
polymer based MEMS. An image processing system has 
been realized that is capable of real-time implementation of 
image decimation and filtering (for example, edge detection). 
Application specific integrated circuits (ASICs) have been 
designed and tested to demonstrate closed loop power 
control and efficient microstimulation. A novel packaging 
process has been developed that is capable of simultaneously 
forming communication coils, interconnects, and stimulating 
electrodes. 
Introduction 
Several incurable eye diseases result in blindness for 
100,000’s of individuals each year.[1] One proposed 
treatment for these conditions is a retinal prosthesis that will 
stimulate the retina at many distinct locations to create a 
pattern of neural activation and thus a visual perception.[2,3] 
In order to function properly, a retinal prosthesis will require 
the presence of cells in the retina. Therefore, diseases 
primarily limited to the outer retina are potentially treatable 
with a retinal prosthesis. The two most common outer retinal 
degenerative diseases are age-related macular degeneration 
(AMD) and retinitis pigmentosa (RP). AMD is more 
prevalent but RP is more severe. Electrical stimulation in 
human test subjects with these conditions, has demonstrated 
the feasibility of an electronic retinal prosthesis as a means 
of providing some degree of vision.[4] This paper describes 
progress towards a high-resolution retinal prosthesis 
designed to restore visual functions such as face recognition, 
mobility, and navigation. 
  
Background 
To design a high resolution retinal prosthesis, it is important 
to consider the system requirements with respect to the 
diseased tissue, the number of pixels needed for the patient 
to have useful vision, and the output requirements to elicit 
the perception of light with an electrode on the retinal 
surface. 
Post-mortem evaluations of retinae with RP or AMD have 
shown that, in spite of photoreceptor degeneration, 
significant numbers of inner retinal cells remain. However, 
the neural circuitry is altered compared to normal retina. 
Psychophysical tests with the first prototypes show that 
phosphenes (artificially created sensation of light) can be 
reliably elicited in humans with severe RP.[5,6] 
Simulations of prosthetic vision with a retinal implant have 
predicted the number of electrodes needed to perform 
certain visual tasks. A number of studies have been 
performed that assess the visual task performance of 
normally sighted individuals while immersed in a simulated 
environment. These studies have been recently 
reviewed.[7] The combined results suggest that a retinal 
prosthesis must have 600-1000 individual pixels to restore 
function such as face recognition, reading, and unaided 
mobility.  
 
Systems Design 
The system is designed to restore a functional level of 
vision to blind individuals so it will have 1000 individual 
electrodes. Each electrode will have a dedicated current 
driver. While demultiplexing is possible, to reduce the 
number of current drivers, it is not yet known if stimulus 
pulses should be simultaneous or interleaved, so maximum 
flexibility should be maintained at this development stage.  
Systems power and data are delivered inductively via a 
dual band telemetry system. The receiver coils are 
positioned in the front half of the eye, an electronics 
module in the vitreous cavity, and an electrode array on the 
epiretinal surface at the back of the eye. The electronics 
module has ASICs and passive components for AC/DC 
power conversion, data demodulation, and stimulus current 
generation based on received data. The implant coils, 
interconnects, and electrodes are formed during a single 
parylene micromachining process described below. The 
implanted system is controlled by an external system, 
which includes a camera for image acquisition, image 
processing hardware, a power amplifier and data 
modulation. 
Our progress to date supports the feasibility of the overall 
system (Fig. 1). We have demonstrated real-time image 
processing, dual-band telemetry, high-voltage circuit 
drivers, and parylene substrate electrodes, interconnects, and 
coils. We have also performed biological experiments to 
assess the feasibility of the implantation of this device. 
Fig. 1 – Conceptual diagram of retinal prosthesis system. A 
video camera on a pair of glasses captures information. A 
wearable computer (not shown) processes video information 
and transmits it to the implant. Power is also sent wirelessly via 
inductive coils. The implanted electronics decode the 
information and produces DC chip power from the induced AC 
voltage on the power coil. The implanted chip produces a 
pattern of stimulation based on the received data. The stimulus 
pulse pattern is applied to the retina via a microelectrode array. 
Real Time Image Processing - AVS 
To enhance and optimize the visual perception of retinal 
implant patients, we have created a comprehensive software 
package: the Artificial Vision Simulator (AVS) [8, 9]. AVS 
interfaces with standard video cameras and presents the 
captured video stream in a user-defined pixelation. AVS 
employs standard image processing algorithms (e.g., 
[10,11]), as well as newly developed, customized image 
manipulation algorithms, such as gray scale and contrast 
enhancement, to modify the captured video stream (Fig. 2). 
All algorithms are implemented to be executable in real time 
(30 fps) and can be engaged in arbitrary, user-defined order. 
These features afford an unprecedented flexibility in 
manipulating the video stream of the external camera in real 
time for the benefit of retinal implant patients, both by a 
technician when fitting the system to the patients or, within 
limits, by the patients themselves. We have begun migrating 
the currently laptop-based AVS processing system onto a 
portable, battery-powered micro-processor platform for 
enhanced user mobility. 
Power and Data Telemetry 
In order to achieve high power efficiency and large data rate, 
the system has dual-band architecture with two pairs of coils 
to transmit power and data with two separate carriers (Fig. 
3). This allows independent optimization of the power and 
data telemetry links. The data clock and power clock are 
synchronized at the transmitter side and received at the 
implant side. The dual band telemetry system has been 
validated for fully wireless recovery of power, data, and 
system clock at the implant side.  
A reverse data telemetry link transmits implant power 
information using load shift keying to the external unit that 
uses the information to transmit the ‘just-needed’ power to 
increase efficiency and battery life [12]. The forward data 
telemetry employs differential phase shift keying (DPSK), 
where ‘1’ is coded as a phase shift of 180º and ‘0’ is coded 
as zero phase shift. The data signal is modulated on a 
20MHz carrier at a data rate up to 2 Mbps. The 
demodulation uses subsampling and analog demodulation 
to recover data at the implant that was verified through a 
custom integrated circuit [13]. 
 
Fig. 2 - Artificial Vision Simulator (AVS) [8, 9]: user-defined 
pixelation and real time image processing of captured camera 
video stream, (resolution here: 16 x 16): raw camera image of 
door (left), gray scale enhancement (middle), and additional 
contrast enhancement (right). 
To integrate the Artificial Vision Simulator with the high 
resolution retinal prosthesis system, an extensible Ethernet-
based (i.e., TCP/IP) Data Exchange Protocol has been 
developed to facilitate the transfer of the AVS-manipulated 
(video) output data to the Power & Data Telemetry system. 
This Ethernet-based protocol allows for maximum 
flexibility between systems, and is capable of two-way data 
transfer as well as status message transmission. 
Contingency measures are integrated into the protocol, 
providing for negative acknowledgements and data resend 
requests, should the need arise in which data is required to 
be resent. Fig. 3 shows demonstration data from the dual 
band telemetry systems, where AVS is used to provide data 
to the telemetry system and a MEMS coil (described 
below) is used to receive data. 
Stimulator Microelectronics 
The semiconductor process for the stimulator design is 
mainly governed by the stimulation voltage at the 
electrode. Based on biological data, we estimate the 
maximum stimulation compliance voltage to be 9V. Given 
the biphasic current pulses and monopolar mode of 
stimulation, the microelectronics should account for twice 
the stimulation voltage in addition to overhead incurred by 
the stimulation circuit, power recovery and regulation. This 
requires a high voltage semiconductor process rated for up 
to 30V. The choice of process is also dictated by the need 
for small feature size of transistors to achieve a highly 
miniaturized chip. This is against the general trend of 
semiconductor fabrication processes where reduction of 
feature size is accompanied by reduction of operating 
voltage.  
To accommodate the special requirements of high voltage 
and high density microstimulator, a high voltage deep sub 
micron CMOS process is employed. This enables a high 
density microstimulator by taking advantage of small feature 
size for digital circuitry and still achieving high voltage 
compliance [14]. Because this particular process also 
provides low voltage transistors, the digital controller is 
integrated with high voltage current drivers and power 
regulators on a single chip. A high density microstimulator 
consisting of power receiver, forward data receiver, reverse 
data transmitter and stimulation, analog driver and digital 
controller is currently under fabrication. 
L2_p
skin
L2_d
L1_p
L1_dData 
driver
Power
Driver
&
Back telemetry 
receiver
Power
Receiver
&
Back telemetry 
transmitter
Data
receiver
Power at fpwr
Data at fdata
Primary side Secondary  side
Back data at fpwr
VL1_p
VL2_p
VL1_d
VL2_d
 
Fig. 3 – (Top) Dual band telemetry block diagram (Bottom) 
Demonstration of simultaneous power and data transmission. 
Note a MEMS coil is used for data receive. 
Packaging, Coil, Interconnects, and Electrodes 
We have developed a chip-level integrated interconnect (CL-
I2) packaging technology to assemble foundry-fabricated 
chips with RF coils, multi-electrode array and other external 
components, and have now extended it to incorporate silicon 
housings and functional parylene-based devices. 
Prefabricated chips and passive components can be 
embedded in a carrier silicon wafer and sealed with parylene. 
Subsequent fabrication of other surface MEMS components, 
such as electrode arrays and coils, can be performed on the 
same substrate using a parylene-based skin technology. Fig. 
4 shows the detailed process flow, which starts with a 
standard 4 inch wafer coated with a layer of sacrificial 
photoresist.  A 5 μm layer of parylene C is deposited on top 
of the photoresist.  To secure parylene on the substrate, 
parylene anchors surrounding the chip housings are etched 
into the substrate using deep reactive-ion etching (DRIE). 
After parylene deposition, a 500 nm layer of gold is 
deposited using an E-beam evaporator, and patterned to 
form the first layer of coil wires.  Next, cavities matching 
the chip dimensions are etched into the parylene and silicon 
using oxygen plasma reactive-ion etching (RIE) and Bosch 
process, and the chips are then dropped into the cavities to 
self-align with lateral displacements less than 10 μm. 
Epoxy is applied to the bottom of the cavities as needed in 
order to compensate for cavity depth inaccuracy and to fill 
the gaps surrounding the chips. Parylene deposition is 
performed again to seal the chips and to form an insulation 
layer between two metal layers, followed by oxygen 
plasma etching to properly open interconnection vias. A 
second 500 nm layer of gold is then deposited and 
patterned to form the top layer of coil wires and 
interconnects. Another 5 μm layer of parylene C is coated 
on top and patterned using oxygen plasma in the RIE to 
define the profile of devices. Through-wafer trenches 
surrounding the chips are then etched from the backside 
using Bosch process, and the flexible skin is released from 
the substrate, carrying the chips encapsulated in silicon 
housings.  Lastly, the entire device can be sealed with 
parylene as a protection layer.  
 
Fig. 4 – Process flow for CL-I2 
A parylene electrode array has been implanted in an animal 
model to demonstrate its biocompatibility.[15] Fig. 5 shows 
an array of 1000 individual electrodes on the surface of the 
retina (as photographed through a dilated pupil). The 
implants were retrieved after 6 months. Examination of the 
retina showed only minimal response to the implant, well 
within acceptable limits. 
The coil design is highly dependent on the application. 
Data telemetry and low-power system can use thin-film 
metal coils, but the power requirements for a retinal 
prosthesis (10-100 mW) will require a high quality, low 
resistance coil. Progress towards microfabrication of coils 
within the overall packaging process is demonstrated 
through integration of an EM4100 read-only RFID chip 
(Fig. 6). A square planar coil is designed for wireless power 
and data transmission. The coil consists of two metal layers 
with 22 turns on each layer (Fig. 7). The overall coil size is 
2 cm by 2 cm.  Due to the low power consumption of the 
wireless chip, only the coil is needed to power the circuit, 
and no extra capacitor is required. An RFID reader module 
verified communication to the chip via the microfabricated 
coil (Fig. 8). For applications that require higher power and 
greater operating ranges, the power transfer efficiency can be 
enhanced by increasing the number of turns, the metal 
thickness, or the metal layers.  
Accelerated-lifetime testing has been performed to evaluate 
the durability of the parylene packaging. RFID chips and 
coils are separately coated with 10 μm of parylene-C, and 
then annealed at 200 °C for 2 days in a vacuum oven. 
Samples are tested in hot saline under different conditions, 
and their failure mechanisms are monitored at distinct time 
intervals using both optical and electron-beam metrologies. 
For the parylene encapsulated chips, unpowered testing is 
performed at 77 °C, and the chips still function well after 90 
days. Parylene coated coils are tested under both passive and 
active electrical stressing at two different temperatures, and 
their MTTF at body temperature is extrapolated using an 
Arrhenius model. The preliminary results estimate that 
parylene packaging can remain intact at 37 °C for over 60 
years, which is very promising for biomedical implantations. 
Conclusions 
Image processing, telemetry, and microfabrication 
technology is being advanced to support a 1000-channel 
retinal prosthesis. Integration of a final system is the next 
step in development. 
Acknowledgment 
This material is based on work supported by the National 
Science Foundation under Grant No. EEC-0310723. 
 
Fig. 5 – 1000 electrode, parylene substrate array implanted on 
retinal surface. Electrodes covers 5x6 mm area. 
 
 
Fig. 6 - Diagram of chip with coil packaged using CL-I2 
technology 
 
Fig. 7 – Close of MEMS coil, showing two overlapping metal 
layers, separated by parylene insulating layer 
 
Fig. 8 – Package and Coil testing used an RFID chip. (left) 
RFID with MEMS coil. (right) Test data from RFID reader. 
 
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Systems design of a high resolution retinal prosthesis

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