Shun Bai

Wireless technologies for closed-loop retinal prostheses

In this paper, we discuss various technologies needed to develop retinal prostheses with wireless power and data telemetry operation. In addition to the need to communicate with the implanted device, supply of power to the retinal prosthesis is especially difficult. This is because, in the implanted state, the device is not fixed in position due to constant motion of the eye. Furthermore, a retinal prosthesis incorporating a high density electrode array of more than 1000 electrodes is expected to consume approximately 45 mW of power and require 300 kbps of image and stimulation data. The front end of the wireless power and data transmission, the antenna, needs to be small compared to the size of the eye. Also, the wireless module is expected to operate in the reactive near-field region due to small separation between the transmit and receive antennas compared to their size and corresponding operating wavelength. An inductive link is studied as a means to transfer power and for data telemetry between the implant and external unit. In this work, the use of integrated circuit and microfabrication technologies for implementing inductive links is discussed. A closed-loop approach is taken to improve performance and reach optimum operation condition. Design and simulation data are presented as the basis for development of viable wireless module prototypes.

A Complete 256-Electrode Retinal Prosthesis Chip

This paper presents a complete 256-electrode retinal prosthesis chip, which is small and ready for packaging and implantation. It contains 256 separate programmable drivers dedicated to 256 electrodes for flexible stimulation. A 4-wire interface is employed for power and data transmission between the chip and a driving unit. Power and forward data are recovered from a 600 kHz differential signal, while backward data are sent at 100 kbps rate simultaneously. The stimulator possesses many stimulation features, supporting various stimulation strategies. Many safety features are included such as real-time monitoring of voltage compliance and temperature, electrode self-locking in the event of out-of-compliance, and ESD protection circuit at every electrode. The chip is fabricated in a 65 nm CMOS process. The electrode driver pitch is 150 μm, and total chip area is 8 mm 2 . The chip has been extensively tested and all the requirements have been successfully verified. The measured DC current error for single driver stimulation without electrode shorting is 20 nA. The average power consumption per electrode with typical stimulus pulse parameters and full-scale output current is 129 μW, inclusive of all standby power. The chip overall power efficiency is 70% with 23 mW of power delivered to load.

A fully flexible stimulator using 65 nm cmos process for 1024-electrode epi-retinal prosthesis

This paper presents a fully flexible stimulator using 65 nm CMOS process for a 1024-electrode epi-retinal prosthesis. The stimulator can select any number of electrodes at any time and also supports both mono-polar and multi-polar stimulation. Furthermore, the stimulator supports a wide range of stimulus parameters. A novel feature is that the electrode driver operates in an alternately pull-push manner, which helps reduce headroom voltage while guaranteeing charge balance at the active electrode. The use of positive supplies instead of both positive and negative supplies simplifies CMOS circuit design. The current distribution between two nearby simultaneously active electrode groups was investigated and measurement result showed a maximum current crosstalk of 8%.

Wireless power delivery for retinal prostheses

Delivering power to an implanted device located deep inside the body is not trivial. This problem is made more challenging if the implanted device is in constant motion. This paper describes two methods of transferring power wirelessly by means of magnetic induction coupling. In the first method, a pair of transmit and receive coils is used for power transfer over a large distance (compared to their diameter). In the second method, an intermediate pair of coils is inserted in between transmit and receive coils. Comparison between the power transfer efficiency with and without the intermediate coils shows power transfer efficiency to be 11.5 % and 8.8 %, respectively. The latter method is especially suitable for powering implanted devices in the eye due to immunity to movements of the eye and ease of surgery. Using this method, we have demonstrated wireless power delivery into an animal eye.

A prototype 64-electrode stimulator in 65 nm CMOS process towards a high density epi-retinal prosthesis

This paper presents a highly flexible 64-electrode stimulator using 65 nm CMOS process fabricated as a stage towards a 1024-electrode epi-retinal prosthesis, which aims to restore partial vision in patients suffering from eye diseases such as retinitis pigmentosa (RP) and age-related macular degradation (AMD). The stimulator drives 64 electrodes with many flexible features, which are necessary before making a complete 1024-electrode implant chip. Each electrode driver can provide a bi-phasic stimulus current with fully programmable parameters such as amplitude, pulse duration, inter-phase gap, and stimulation rate. The electrode driver operates in an alternately pull-push manner with only one current source working at a time, which helps reduce headroom voltage while controlling charge balance at the active electrode. The stimulator varies both stimulus current amplitude and stimulation rate to represent phosphene brightness. The stimulus current amplitude starts from the tissue depolarization threshold with 64 different levels. The selection of active and return electrodes is arbitrary, any electrodes and any number of them can be selected at any time. The power consumption of the stimulator is 400 μW excluding the stimulus power. Measurement results verify correct operation. The stimulator is easily scaled up to drive 1024 electrodes.

High-Q flexible spiral inductive coils

A limitation on the optimal design of inductive coils for wireless power transfer is its physical size. We investigated the effect of varying width and spacing of conductive trace of spiral inductive coils in order to improve its quality factor and hence power transfer efficiency between two coils. These spiral coils have inner and outer diameter of 23 mm and 36.5 mm, respectively. We found that for the same number of turns, quality factor Q increases with an increase in spacing. This is attributed to proximity effects in adjacent conductive tracks of the coil. An increase of Q at 6.78 MHz by 121% from the minimum value was achieved by systematically varying the different topologies. We conclude that an optimal topology of choice for a spiral coil is larger spacing and smaller number of turns.

A super low power MICS band receiver in 65 nm CMOS for high resolution epi-retinal prosthesis

We report a super low power MICS band receiver for a high resolution epi-retinal prosthesis (BionicEye). The FSK receiver consumes less than 1.5 mW power with 1 V supply. It can achieve a maximum data rate of 400 kb/s. In this paper, we present the research work carried out on designing a fully-integrated sub-threshold receiver fabricated on a 65nm CMOS chip. In order to achieve super low power consumption, more than 90% of the transistors in all analog building blocks are operated in sub-threshold region. System level issues, such as required receiver architecture and specifications are also addressed1.

Closed-loop inductive link for wireless powering of a high density electrode array retinal prosthesis

A retinal prosthesis intended for rehabilitation of vision impaired patients will require continuous power supply in order to achieve real-time moving images. In this work, we explore the use of inductive coils fabricated using flexible circuit technologies for inductive powering of the implanted prosthetic device. We found that manufacturing technologies dictate the optimum operating frequency of the coil. For a minimum track width and spacing of 4 mils, the optimum frequency was found to be 2.9 MHz. We also looked at the distribution of electric and magnetic fields generated by the inductive coils in and surrounding the eye. These simulation results show that there are electric field concentrations around the conductive coils. Apart from the coils, we need to design an efficient circuit to drive the transmit coil and recover the transmitted power. In order to maintain optimal operation of the link, a closed-loop load modulation feedback operation is proposed. Adaptive control using back-telemetry of the induced voltage on the secondary side can close the power supply loop and result in optimum power transfer by boosting the supply voltage on the primary side when load is high and reducing this voltage when load is small.

A simple voltage reference with ultra supply independency

An ultra wide dynamic range voltage reference operating with power consumption of pico-watt has been designed for a retinal prosthesis device using IBM 65nm CMOS process. The temperature coefficient is zeroed at 37 °C while achieving 2.5 ppm/°C over the range from 25 °C to 75 °C. A perfect suppression of the supply voltage dependence is realised to provide line sensitivity of 0.04%/V with ultra low input voltage of 0.5 V up to 3.5 V. The power supply noise attenuation is simulated to maintain at -100 dB across whole frequency band up to 100GHz. The occupied chip area is maintained as low as 0.003 mm2 with size of 35μm by 75μm.

A super low power MICS band receiver front-end down converter on 65 nm CMOS

This paper presents a super low power MICS band receiver front-end down converter on 65 nm CMOS for implantable biomedical devices. This down converter, including a LNA and a quadrature mixer, only consumes 500 µA DC current under 1 V supply. With a small LO swing of 300 mV, it provides a voltage conversion gain of 35 dB and a noise figure of 7.4 dB, while a −20 dBm IIP3 is obtained. In order to achieve super low power, current-reuse structure is adopted and all transistors are operated in deep sub-threshold region. Circuits level issues and techniques are also discussed.