A Light Amplitude Modulated Neural Stimulator Design with Photodiode

Transcription

1 A Light Amplitude Modulated Neural Stimulator Design with Photodiode for Visual Prostheses Ji-Hoon Kim, Choul-Young Kim, and Hyoungho Ko* Department of Electronics, Chungnam National University, Daejeon, Korea * Abstract The neural stimulators have been employed to the visual prostheses system based on the functional electrical stimulation (FES). Due to the size limitation of the implantable device, the smaller area of the stimulator IC is highly desired. This paper presents a neural stimulator with photodiode for visual prostheses. The proposed stimulator provides the light amplitude modulated stimulation current using photodiode. In conventional concept of visual prostheses, the image information is captured by external camera, and the converted electronic signal is sent to the implanted chip, therefore, the digital analog converter (DAC) is required in the implanted chip. The proposed stimulation scheme with onchip photodiodes is expected to be fully implantable and to have smaller size than previous external camera system 1. Introduction Keywords: Neural stimulator, Photodiode, Visual prostheses Recently, neural stimulation technology based on functional electrical stimulation (FES) has been brought to great public attention in many application areas including visual prosthesis and artificial cochlear implants. In case of the people blinded by retinitis pigmentosa (RP) or agerelated macular degeneration (AMD), the photoreceptors, which change the light signal to the electrical signal, are degenerated, however, the others retinal nerve cells are still remained such as bipolar cell and ganglion cell. Therefore, the visual prostheses system research aims to stimulate the remained retinal nerve cells directly [1]. The neural stimulator has size limitation because it is implanted directly in the eyes. Moreover, the large size of the implanted device causes to feel uncomfortable when the operated patients move their eyes. Thus, the small size is the one of the most important design issue in the neural stimulator. Many neural stimulator ICs were reported for the visual prostheses. In the previous neural stimulation system [2-5], the external camera generally captures the image information of light. The image information is sent to the implanted chip in retina by wireless links after converted to electronic signal. The implanted chip offers the artificial vision by electronically stimulating the optic nerves. The proposed stimulator provides the light amplitude modulated stimulation current using photodiode. Because the photocurrent directly determines the stimulation current amplitude, the external camera and the digital analog converter (DAC) can be removed. With the proposed scheme, the fully implantable neural stimulator system with smaller size can be implemented. 2. Proposed Neural Stimulator Structure The top level architecture of the proposed stimulator IC is shown in Figure 1. The light is received by the photodiode, and the photocurrent is generated depend on the light intensity. The photocurrent amplifier provides the amplified photocurrent to the biphasic current driver. The relaxation oscillator generates clock signal. The two control signals, PUSH and PUSHB, are generated by the non-overlapping clock generator. The biphasic driver generates the two biphasic stimulation currents, i CH and i REF. The two output currents are injected to the retina cells. The output currents are amplitude modulated (AM) signals with the envelope of the light intensity. International Journal of Advancements in Computing Technology(IJACT) Volume 5, Number 12, August

2 3. Circuit Description Figure 1. Top level architecture of proposed stimulator IC The photocurrent amplifier is shown is Figure 2. The photocurrent generated by the photodiode is converted to the voltage output and amplified by the transimpedance amplifier (TIA). The output voltage of the TIA is converted to the output current by the voltage-to-current converter (V-I). The output current of the V-I converter, i light, is modulated with the two non-overlapping clock signals, PUSH and PUSHB. Figure 2. Photocurrent amplifier Figure 3. shows the biphasic current driver circuit. When STIM_EN is H, the biphasic driver generates the biphasic currents, i CH and i REF using i light. The PUSH signal determines the direction of the stimulating current. When the PUSH is H, the anodic current, i CH, flows from the channel electrode to the reference electrode as shown in Figure 4(a). When the PUSHB is H, the cathodic current, i REF, flows from reference electrode to channel electrode as shown in Figure 4(b). Figure 3. Biphasic current driver circuit 165

3 Figure 4. Biphasic current flow The relaxation oscillator circuit is shown in Figure 5. The cross-coupled pair, M5 and M6, forms a positive feedback loop. Assuming M5 is turn-on and M6 is turn-off, the current flows from M5 to M8, while charging C3. When C3 is fully charged, the source voltage of M5 is increased and the voltage difference between gate and source of M5 is decreased. Thus, M5 becomes turn-off. This result M6 to be turn-on, and C3 is charged again. The clock signal is generated by repeating this process. Figure 5. Relaxation oscillator circuit The generated clock is converted to the non-overlapping clock generator, as shown in Figure 6. The non-overlapping clock signals, OUT and OUTB are shown in Figure 7. When the CLK signal is L, the OUT becomes L and the OUTB is changed to H after propagation delay time of four inverters 166

4 and a NOR gate. When the CLK signal change to H, the OUTB becomes L and the OUT is changed to H after same delay time as before. The non-overlapping clocks can prevent the unwanted short-circuit current in biphasic current driver. Figure 6. Non-overlapping clock generator Figure 7. Non-overlapping output signal Figure 8. shows the stimulation currents of biphasic current driver according to sinusoidal input current. The output stimulation currents are modulated by the input light amplitude. 167

5 4. Conclusion Figure 8. Light amplitude modulated stimulation current The neural stimulator IC with photodiode for visual prostheses is presented. The stimulator provides the light amplitude modulated stimulation current using photodiode. The fully implantable and smaller stimulation system can be implemented with the presented scheme without external camera system. The size of the unit pixel with photodiode scheme is 20% compared to the conventional neural stimulator IC because the DAC can be removed. Furthermore, assuming the number of stimulator pixel is increased for higher resolution, the proposed neural stimulator IC with photodiode will be more effective to reduce the size of the pixel array. 5. Acknowledgement This research was supported by the Happy tech. program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology ( ). This work was also supported by IC Design Education Center (IDEC). 6. References [1] M. Ortmanns, A. Rocke, M. Gehrke, and H. Tiedtke, A 232-Channel Epiretinal Stimulator ASIC, Solid-State Circuits, IEEE Journal of, Vol. 42, No. 12, pp , December [2] K. Chen, Z. Yang, L. Hoang, J. Weiland, M. Humayun, and W. Liu, An Integrated 256-Channel Epiretinal Prosthesis, Solid-State Circuits, IEEE Journal of, Vol. 45, No. 9, pp , September [3] E. Noorsal, K. Sooksood, H. Xu, R. Hornig, J. Becker, and M. Ortmanns, A Neural Stimulator Frontend With High-Voltage Compliance and Programmable Pulse Shape for Epiretinal Implants, Solid-State Circuits, IEEE Journal of, Vol. 47, No. 1, pp , January [4] L. Theogarajan, A Low-Power Fully Implantable 15-Channel Retinal Stimulator Chip, Solid- State Circuits, IEEE Journal of, Vol. 43, No. 10, pp , Nov [5] T. Tokuda, K. Hiyama, S. Sawamura, K. Sasagawa, Y. Terasawa, K. Nishida, Y. Kitaguchi, T. Fujikado, Y. Tano, and J. Ohta, CMOS-Based Multichip Networked Flexible Retinal Stimulator Designed for Image-Based Retinal Prosthesis, Electron Devices, IEEE Transactions on, Vol. 56, No. 11, pp , Nov