JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.17, NO.1, FEBRUARY, 2017 ISSN(Print) 1598-1657 https://doi.org/10.5573/JSTS.2017.17.1.035 ISSN(Online) 2233-4866

Manuscript received Oct. 14, 2016; accepted Jan. 31, 2017 Department of Electrical and Computer Engineering, Seoul National University, Seoul, Korea E-mail : kimsj@snu.ac.kr

A Multi-photodiode Array-based Retinal Implant IC with On/off Stimulation Strategy to Improve Spatial

Resolution

Jeong Hoan Park, Shinyong Shim, Joonsoo Jeong, and Sung June Kim*

Abstract—We propose a novel multi-photodiode array (MPDA) based retinal implant IC with on/off stimulation strategy for a visual prosthesis with improved spatial resolution. An active pixel sensor combined with a comparator enables generation of biphasic current pulses when light intensity meets a threshold condition. The threshold is tuned by changing the discharging time of the active pixel sensor for various light intensity environments. A prototype of the 30-channel retinal implant IC was fabricated with a unit pixel area of 0.021 mm2, and the stimulus level up to 354 µA was measured with the threshold ranging from 400 lx to 13120 lx. Index Terms—Photodiode, neural stimulator, retinal implant, on/off stimulation, spatial resolution

I. INTRODUCTION

In conventional retinal implants, designed to restore of blind patients by electrical stimulation of surviving inner retinal neurons, the main focus has been on providing a large number of stimulation channels with detailed visual information [1-3]. To achieve many stimulation channels, the data rate between the stimulation IC and the electrode array must be maintained high with many interconnection lines. In such a design, the image processor unit and the stimulation pulse generation circuit are often separated [4,

5]. On the other hand, a multi-photodiode array (MPDA) based subretinal implant that employs photodiode array and image processor are integrated into the same IC with stimulation circuitry. Recently clinical trials using the subretinal implant with more than 1000 channels have been demonstrated where patients could perceive consistent light with basic tasks such as distinguishing grating patterns and characters [6, 7].

Fig. 1. Explanation of the proposed MPDA based retinal implant architecture and its input and expected output processed by MATLAB.

36 JEONG HOAN PARK et al : A MULTI-PHOTODIODE ARRAY-BASED RETINAL IMPLANT IC WITH ON/OFF STIMULATION …

However, despite a large number of stimulation channels employed, visual acuity of the blind patients using the 1500-channel MPDA-based sub-retinal implant did not increase dramatically, even compared with that obtained from using epiretinal implant having only 60 channels [1]. It may be due to the current spread elicited by the electrodes with increasing level of electrical stimulation. Maintaining the stimulus level at a moderate level is preferred to achieve higher spatial resolution [8, 9].

In this paper, we propose a novel MPDA-based retinal implant IC with an on/off stimulation strategy which generates biphasic current pulses in response to incident light only above a designated intensity threshold, thus allowing excitation of retinal neurons with minimum current level to prevent unnecessary spread. Fig. 1 explains the on/off stimulation strategy. The electrical signal obtained by conversion of light intensity information is input to a comparator with a threshold value that is obtained using the average background light intensity throughout the array. Thus only the light intensity that surpasses background intensity can elicit biphasic current pulses. This allows the stimulus level to be set at a minimum value, and the benefits mentioned above can be achieved.

Section II describes the detailed circuit of the proposed MPDA-based retinal stimulator. Experimental results and discussion follow in section III and IV, respectively.

II. CIRCUIT DESCRIPTION

Fig. 2 schematically represents the MPDA-based retinal implant IC and the flow of signals. The IC consists of a bias generator, a digital controller, and an array of multi-channel pixels. Each pixel has a photo sensor, a voltage-controlled current source (VCCS), and a biphasic current generator (BCG).

A 4-transistor active pixel sensor (4-tr APS) combined with a low power comparator (COMP) is adopted for the on/off stimulation strategy of the photo sensor [10, 11]. The 4-tr APS allows the change of photo-sensitivity via discharging time (TX) and provides excellent noise characteristics as compared with other types of CMOS image sensors. The output voltage of the 4-tr APS (VOUT,APS) decreases during TX, and this relation can be expressed as

,OUT APS DD photo TH

DD TH

V V V VV V k L TX

= - -

= - - × × (1)

Fig. 2. The schematic of the proposed MPDA-based retinal implant IC and the flow of digital signals.

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where Vphoto is a voltage induced by the photodiode, L is a light intensity, k is a responsivity (V/lx·s), and VTH is the threshold voltage of the MOSFET M1. At the comparator (COMP), if VOUT.APS is below the reference voltage (VREF1), digital signals (AN, CA) that control the switching of BCG pass through the SW[1:0], enabling biphasic pulse generation. Using this property and (1), the threshold light intensity (Lthrehold) for generating current stimulus can be written as (2) and (3)

1 ,REF OUT APS

DD TH threshold

V VV V k L TX

=

= - - × × (2)

1( )( )

DD REF THthreshold

V V VL k TX- -= × (3)

From (3), it is found that Lthreshold is inversely

proportional to TX. Thus Lthreshold can be adjusted by varying TX, making this type of photo sensor uniquely applicable to various light intensity environments.

Furthermore, we propose an automatic TX controller to control the light intensity threshold for efficient image processing of on/off stimulation strategy. In Fig. 2, it is also shown that the distributed photodiodes discharge the capacitor, decreasing the voltage. As the voltage is below VREF2, the output of Schmitt trigger is changed to VSS. The value of TX is calculated with RST and the capacitor voltage via the AND gate. Thus the circuit in Fig. 2 can perform automatic control of the TX value according to the average light intensity.

The VCCS based on the MOS resistor is used to control the current stimulus level efficiently [12]. An OTA retains drain voltage of M1 to a fixed voltage of VFIX forcing M1 to remain in triode region for acting as the MOS resistor. The telescopic OTA optimized using gm/ID methodology is operated in weak inversion region [13].

Simultaneous monopolar stimulation is used for reduction of the number of electrodes and for ease of scaling up the stimulation channels. The N-channel monopolar stimulation needs N+1 electrodes, compared with bipolar stimulation that needs 2·N electrodes. As most MPDA-based retinal implant IC’s include the stimulating electrodes [6, 7], reduction of the number of electrodes increases area an electrode can have. The electrode impedance is reduced to decrease the circuit compliance voltage for reduced power consumption.

As control signals are shared by all the pixels during

simultaneous stimulation, it is possible to increase stimulation channels without modification of the digital circuit.

The BGA is designed to have two independent current sources, one for anodic phase and the other for cathodic phase, for safety reason [14]. In a structure that employs single current source, the monopolar, simultaneous multichannel stimulation, amplitudes of the biphasic stimulation pulses can be affected by the electrode-cell impedance. This can result in unsafe retinal stimulation due to excess charge used. Two independent current sources are designed to deliver predictable and safe stimulation to retinal neurons regardless of electrode-cell impedance.

III. EXPERIMENTAL RESULTS

For verification of our design, a prototype of 30-channel MPDA-based retinal implant IC was fabricated using Towerjazz 0.18 µm CIS process. Fig. 3 shows the microphotograph of the fabricated prototype and the layout of a unit pixel. The pixel area is 0.021 mm2, implying that 470-cahnnel pixels can be integrated into a macular area of 10 mm2. Characteristics of the 4-tr APS are listed in Table 1.

Fig. 3. A microphotograph of the fabricated 30-channel MPDA-based retinal implant IC and layout of unit pixel.

Table 1. The characteristics of the 4-tr APS

Parameters Units Values Size μm2 2.2 ×2.2

Conversion Gain μV/e- 60

Responsivity,Green V/ lx·s 0.6 full well capacity Ke- 10.0 Pixel noise floor μV 300 Dynamic Range dB 66

38 JEONG HOAN PARK et al : A MULTI-PHOTODIODE ARRAY-BASED RETINAL IMPLANT IC WITH ON/OFF STIMULATION …

A customized bench top test setup was configured to measure the IC performance, as shown in Fig. 4(a). The IC was connected with a PCB using wire-bonding and protected by transparent epoxy for light transmission. The digital controller was implemented with an FPGA (Spartan 3A, Xilinx Corp., USA). The light source was a digitally-controlled quartz tungsten halogen light lamp (66884, Newport, USA), and light intensity was

measured by the light meter (TES 1336A, TES Corp., Taiwan). A resistor was connected as a load to the channel output. And then, current pulses as shown in Fig. 4(b) were measured using the oscilloscope (DPO4034, Tektronix Inc., USA) in various light condition.

Fig. 4(c) shows the measurement of the stimulation current pulse burst was measured while light with supra-threshold intensity was illuminated. Otherwise, no

(a) (b)

L thre

shol

d (lx)

(c) (d)

(e)

Fig. 4. (a) Block diagram and a photograph of the test setup, (b) Oscilloscope screen images showing the measured biphasic current pulses, (c) The channel output when the light source is switched on and off, (d) Dependency of the threshold light intensity (Lthreshold) on the TX, (e) The amplitude of anodic and cathodic current.

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biphasic current pulse was measured. The dependency of the threshold light intensity

(Lthreshold) on the TX value was measured. Fig. 4(d) shows the measured data (in red lines) together with the design values (in blue lines). The device was designed to operate in the living room lighting condition of under 400 lx. However, the measured values were significantly higher than the design values. The discrepancy was found to be due to the abnormally large offset voltage in the comparator. This fault is under investigation and will be corrected in the next design. We can further increase the light sensitivity by increasing geometrical area used by APS for [15], perhaps using a 3-dimentional structure [16].

The biphasic currents in anodic and cathodic phases decreased for the increased LV (input voltage) values, as expected, and are shown in Fig. 4(e). Amplitudes of current in the anodic and cathodic phase were measured to be within the range of 0-354 µA and 0-332 µA, respectively. The average current mismatch was 4.85 % with a standard deviation of 3.52 %. We thought that main reason for the current mismatch is in process variation, as well as in the channel length modulation that is caused by the drain voltage of the MOSFETs in the mirroring circuit. Thus, for safety concern, we added a short switch between CH and REF so we can remove the unwanted residual charge in the resting state.

The performance of the chip is summarized in Table 2.

IV. DISCUSSION

In this study, we designed and verified a prototype of 30-channel MPDA-based retinal implant IC. On/off stimulation strategy with moderate current stimulus can reduce the stimulation level for a given incident light intensity. This can help reduce the inter-channel crosstalk which otherwise prohibits achieving desired spatial

resolution in the retinal stimulation. The BCG with two independent current sources is used for monopolar, simultaneous multi-channel stimulation, which features the following advantages: 1) We can increase the number of stimulation channels without modifying the digital controller, 2) We can adjust the magnitude of biphasic current pulses regardless of the electrode-cell impedance, and 3) We can afford giving more area per pixel to achieve lower electrode impedance.

V. CONCLUSIONS

A novel multi-photodiode array (MPDA) based retinal implant IC was fabricated with on/off stimulation strategy and tunable light intensity threshold for the various light environments. The prototype of retinal implant IC was fabricated with an area of 0.021 mm2 per unit pixel which can expand up to 470 channels in the macular area of 10 mm2. Biphasic current pulse ranging from 0 µA to 354 µA was measured with the light intensity threshold from 400 lx to 13120 lx.

ACKNOWLEDGMENTS

This work was supported in part by the National Research Foundation of Korea within the Ministry of Education, Science and Technology through the Public Welfare and Safety Research Program under Grant NRF-2010-0020851, in part by the Defense Acquisition Program Administration through the Project entitled Control of Animal Brain using MEMS Chip under Grant UD140069ID, in part by IDEC, and in part by the Department of Electrical and Computer Engineering at Seoul National University through the Brain Korea 21 plus Project in 2015.

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Table 2. Summary of chip performance

Process 0.18 μm CIS process

Stimulation strategy Monopolar and simultaneous biphasic current stimulation

Supply voltage VDD = 1.65 V VSS = -1.65 V

Light intensity Threshold 400 lx @TX=12.5 ms 13120 lx @TX=0.79 ms

Stimulus current level 0-354 μA (ICA), 0-332 μA (IAN) Unit pixel area 0.021 mm2

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Jeong Hoan Park received the B.S. degree in the Department of Elec- trical Engineering, Seoul National University, in 2011. He is currently pursuing the Ph.D. degree in the Department of Electrical and Com- puter Engineering, Seoul National

University. His interests include IC design of low noise amplifier, electrical stimulator, artificial retina using photo-diode for neural prosthesis.

Shinyong Shim received the B.S. degrees in the Department of Electrical and Computer Engineering, Seoul National University, Seoul, in 2015. He is currently pursuing the Ph.D. degree in the Department of Electrical and Computer Engineering,

Seoul National University. His interests include artificial retina and artificial eye for the blind animal.

Joonsoo Jeong received the B.S., M.S. and Ph.D. degrees from the School of Electrical Engineering and Computer Science, Seoul National University, Seoul, in 2009, 2011, and 2015 respectively. He is currently a postdoctoral research associate at

School of Engineering, Brown University, RI, USA. His main research topic is the development of chronically implantable electronics using a new material of liquid crystal polymer, especially for the retinal prosthesis. His research interests include polymer-based microfabri- cation, long-term reliability of polymer encapsulation, and stretchable and deformable electronics.

JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.17, NO.1, FEBRUARY, 2017 41

Sung June Kim (S’79–M’84–SM’06) received the B.S. degree in electronics engineering from Seoul National University, Seoul, in 1978, and the M.S. and Ph.D. degrees in electrical engineering from Cornell University, Ithaca, NY, in 1981, and

1983, respectively. He is currently a Professor in the Department of Electrical and Computer Engineering, Seoul National University. He is interested in neural prosthesis, bioelectronics, biosensors, optoelectronics, and semiconductor devices and fabrication methods.