A 16-channel stimulator ASIC for use in an

intracortical visual prosthesis.

Philip R. Troyk¹, David E.A. Detlefsen¹, Glenn A. DeMichele²

¹ Illinois Institute of Technology, IIT Center, Chicago, IL

² Sigenics, Inc., Lincolnshire, IL

troyk@iit.edu

Abstract

A conservative estimate for the number of electrodes to be used in a first generation intracortical visual prosthesis is approximately 1000 individual electrodes. Our recent surgical experiences with animal models have led us to the conclusion that even if grouping the electrodes in 16-electrode arrays, the 60 cables would have to cross the dura may pose considerable risk of infection or damaging tethering of the electrodes. Therefore we are pursuing a design for a wireless array module that contains power, communication, and elecrode drivers within an application specific integrated circuit (ASIC).

1. INTRODUCTION

Figure 1 shows an implantation of 96 electrodes, in the form of six 16-electrode arrays, in the area V1 of a macaque; note the cables. Although each cable is smaller than 1mm diameter, their routing and crossing seem awkward and highly undesirable. Considering that we anticipate the use of ~1000 electrodes for a future human implantation, ten times the number of cables shown in Figure 1 would be required.

In our opinion, such extensive cabling may not be surgically feasible for a first generation system. Therefore, we envision a wireless modular system similar that that shown diagrammatically in Figure 2. Sixteen AIROF electrodes are driven by the backside ASIC. Here we report on the design of the first generation ASIC for this module.

2. METHODS

Figure 3 shows a CAD drawing for the ASIC. Each of the 16 electrodes electrode is driven by a dedicated compliance supply limited constant current driver [1].

A single coil, mounted near the integrated circuit chip is used to transfer power, inward telemetry, and outward telemetry. The magnetic link carrier frequency is 4.8Mhz. On-chip capacitors provide the resonant tuning for the input coil. Using FSK modulation of a Class-E converter [2], the inward command telemetry bandwidth is 1.2Mbits/sec. Power supply rectification is accomplished using a bipolar/NMOS bridge circuit. The bipolar transistors are connected as diodes, and used in the upper portion of the bridge, thus avoiding the parasitic bipolar conduction normally associated with upper PMOS rectifiers. A shunt clamp circuit limits the power supply voltage to approximately 3.8Volts. Outward telemetry, used to remotely monitor an individual electrode voltage, is accomplished by AM modulation of the 4.8 MHz carrier at 130 kHz.

The state machine processes commands for defining the stimulation parameters for each electrode channel, as well as for global chip functions.

During assembly of the module, we anticipate that temperatures as high as 150°C will be required for the processing and curing of the polymeric packaging. Therefore, it will not be possible to electrochemically activate the AIROF electrodes prior to module sealing. Rather, the electrodes will be activated after the final module assembly and polymer curing. In order to accommodate this function, a special global command has been implemented that allows for activation of the electrodes via the magnetic link.

During activation, the clock that normally operates the stimulus timing counters in each of the sixteen electrode cells is suspended. The duration of the “stimulus” pulse, as used for activation, is determined by a pair of stimulus on-off commands over the inductive link.

For normal operation, the stimulus polarity is cathodic first, with a subsequent recharge and biasing of the electrodes. Stimulus parameters can be varied as follows:

Pulse amplitude (constant current):

64 uA maximum in 0.5 uA steps.

Pulsewidth

750 usec maximum in 50 usec steps

Each electrode cell can be switched on or off, completely disconnecting any individual electrode. Normally pulse width and amplitude information are loaded into each electrode cell using a single command. In an alternate mode of operation, the amplitude information for each electrode cell can be continuously varied, thus allowing for an arbitrary stimulation waveshape to be applied to the electrode during the cathodic pulse.

We have fabricated this design, in the XFab CX08 BICOMS process. Prototype chips have been returned and are presently In test within our laboratory.

4. DISCUSSION AND CONCLUSIONS

In order to implement the compliance supply limited constant current drivers, the compliance supplies must be generated relative to a reference electrode. In this ASIC design, thse supplies are produced by special regulators controlled by a buffered electrode input. This requires the inclusion of a reference electrode within the module. Since stabilizer pins are normally used to maintain the position of the array, (see Figure 2) considering the normally short (~2mm) electrodes, these pins can also function as the reference electrode and the counter electrode (one pin for each function).

References

[1] Srivastava, N.R., Troyk, P.R., Cogan, S.F., “A laboratory testing and driving system for AIROF microelectrodes,” Proc. 26^th IEEE EMBS, San Francisco, CA, Sept 1-5, pp. 4271 – 4274, 2004.

[2] Troyk, P.R., DeMichele, G.A., (2003) Inductively-Coupled Power and Data Link for Neural Prostheses using a Class-E Oscillator and FSK Modulation, Proceedings of EMBS Conference, Cancun, Mexico, September 17-21, pp 3376 – 3379.

Acknowledgements

Funding by the Brain Research Foundation, private donations, and NIH grant R01 EB002184