# Low-Power Wireless Data Transfer System for Stimulation in an Intracritical Visual Prosthesis
## 1 Introduction  
An implanted intracortical visual prosthesis is perhaps the only possible technological option for most cases of blindness. Stimulating the visual cortex can be the last resort when the visual pathway is damaged or inaccessible. Approximately 216 million people worldwide are visually impaired, 36 million are blind [1], and most would need an intracortical visual prosthesis to regain some (rudimentary) form of vision. An intracortical visual prosthesis system generally comprises an external camera, a pocket processor, implanted electrode arrays, and a feedback loop [2]. A very high-count integrated wired system (65,536 electrodes) for neural recording and stimulation was reported in [3]. A desirable aspect for the visual prosthesis is that the powering and communication to and from the implanted electrodes be done wirelessly, to avoid infections and to enable free movement [4]. Wireless enablement is part of the goal of the NESTOR project which has the aim to implant a high count of electrodes (~1000) [5], using 16 Blackrock Microsystems' Utah arrays of 64 electrodes each [6]. The wireless-enabled visual prosthesis will need a communication link to send stimulation information to the implant (downlink), another link for retrieving recorded neural activity from the brain (uplink), and wireless powering of the implant. The uplink is needed for adaptation, monitoring, and calibration over time. This work focuses on the downlink part of the system. The wireless power transfer system is designed separately in another sub-project. There are a few possibilities in laying out this wireless system, which may directly impact the communication technology used for transmitting stimulation information. Figure 1 shows three possible layouts. The first option (Figure 1a) is to place the implanted transceiver embedded into each electrode array [7]. This form requires individual antennas/receive coils for each electrode array, which may face variable signal reception as the number of electrode array increases spatially. Another form (Figure 1b) is to use a central transceiver placed beneath the skull, but this space may not be convenient for the implanted transceiver if its components are bulky. The proposed form in this work (Figure 1c) is to place the implanted transceiver beneath the skin where more space is available, and a shorter distance to the external front end is present [8]. The challenge of this option is the possible micro-motion of the implanted connecting wires, which can be partly alleviated with better packaging and implantation. Figure 1: Various system layouts for the wireless enabled visual prosthesis. (**a**) place the implanted transceiver embedded into each electrode array; (**b**) use a central transceiver placed beneath the skull; (**c**) place the implanted transceiver beneath the skin. Several general implanted medical links have been proposed for communication through the body [9; 10; 11; 12; 13; 14]. Frequency shift keying (FSK) was investigated in [9]. Differential phase shift keying (DPSK) was used in [10]. In [11; 12], binary phase shift keying (BPSK) was developed. Pulse delay modulation was implemented in [13]. A non-coherent analog BPSK demodulator was attempted in [14]. However, for the application to a visual implant, in which the overall system has to do simultaneous communication and powering and in which power consumption is an issue, the earlier works mentioned above are not suited. Instead, here we take an approach to arrive at a low-power system. Therefore, the requirements of developing the downlink for stimulation include low-power consumption \(<\)10 mW, data rates between 200 kbps and 4 Mbps, robustness to interference from the power carrier, and link security. These requirements led to take a low-power system approach which involves using low frequency \(<\)100 MHz, an inductive link for communication, phase shift keying (PSK), a nearly digital receiver, and non-coherent demodulation, as introduced in [15; 16].