Wireless Energy and Signal Transmission System for Micro Implantable Medical System

Xiuhan Li, Jinbin Hu Haixia Zhang*

Xiuhan Li, Jinbin Hu School of Electronics and Information Engineering

Beijing Jiaotong University Beijing, China

E-mail: lixiuhan@bjtu.edu.cn

Haixia Zhang National Key Laboratory of Nano/Micro Fabrication

Technology, Institute of Microelectronics Peking University, Beijing, China E-mail: zhanghx@ime.pku.edu.cn

Abstract—In this paper, we present the wireless energy and signal transmission system and circuit design for implantable medical system to achieve power and data transmission and signal processing. This paper gives a detailed design of the power and the ASK demodulation circuit and their improved circuits. The first design has been fabricated in a 0.5um 2P3M standard CMOS process, the test results have demonstrated that the comparator detects the digital data precisely. The improved circuits greatly reduce the chip area and get more stable and precise 1-v dc voltage and restore the digital data precisely. They are implemented in 0.13-um 1P8M standard CMOS process.

Keywords—Implantable system, Wireless transmission, CMOS, MEMS

I. INTRODUCTION

T present, the implantable wireless energy and signal transmission system is being actively carried out in the

global and micro sensors and signal processing chips are still being studied[1].In order to achieve power and data transmission and processing for implantable medical system,integrate a MEMS device and the corresponding CMOS interface circuits [2]and inductively coupled energy and signal transmission[3], not only solve the problem of the limited battery life of the implantable medical devices and suffering connect percutaneous for patients, but also significantly reduce the size of implanted chips and energy consumption for the miniaturization and high-volume of the technology, which lay a solid technology base for implementing the low-cost, small size and low power consumption implantable system by using wireless energy transmission. In this paper, the realization of the wireless energy and signal transmission system applicable to cardiac pacemakers, artificial retinal implants and a variety of medical systems, which has vast market prospects and value.

II. SYSTEM DESCRIPTION

The overall block diagram of the wireless energy and signal transmission system is shown in Fig.1. Briefly, the system

consists of an external part and an internal part. The digital data are encoded, which are sent to the class-E driver with the high efficiency RF transmitter and are inductively coupled to the implantable module [4].

The internal part consists of a power regulator, a signal processing circuit and an actuator. The power regulator includes a full wave rectifier and a voltage regulator,and itextracts power and data and regulates the voltage to a stable and precise dc voltage source for the whole implantable chip [5]. The signal processing circuit consists of a ASK demodulator, a controller and a current driver. The digital signal carried by the external transmitting RF signal is restored by the ASK demodulator and role in the organism through the current drive in order to restore or replace the function of certain organs [6].

III. ELECTRONIC PARTS DESIGN

A. Power and ASK demodulator design

Power supply circuit is used to provide DC voltage for micro implantable medical system, mainly consisting of two parts of a full-wave rectifier and a voltage regulator. It receives RF power signals from the external system by inductively coupled wireless transmission,then rectifier the signals, reduce voltage and regulator,eventually the input AC voltage is transformed into a stable required DC voltage. The

A VoltageRegulator

Power Regulator

signal processing

Figure 1. Overall block diagram of the wireless power and data transmission system

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Proceedings of the 2010 5th IEEE International Conference on Nano/Micro Engineered and Molecular Systems January 20-23, Xiamen, China

978-1-4244-6545-3/10/$26.00 ©2010 IEEE

linear regulator is the important part of the power module, due to the RF power signals received through inductive coupling will usually fluctuate within a certain range,and then they can not achieve a very stable reference DC voltage, which can not be used directly by the circuits of the implantable system.

Fig. 2 depicts a full-wave rectifier, which receives an ASK modulated 2-MHz signal through inductor coupling, and then the 2-MHz AC voltage rectified through the double voltage to convert the DC voltage contains a pulsating component. In addition, the constant of the capacitor’s discharge time is greater, the discharge process is slower and the output voltage is higher, meanwhile pulse components fewer, filter better.

Fig. 3 shows a linear regulator including three parts of a self-bias circuit, an operational amplifier and negative feedback. In order to provide an accurate reference voltage to the operational amplifier,the self-bias circuit structure is used in this design, which is improved from the Wilson current source. An additional start circuit is added to this bias circuit to make sure to avoid the wrong balance of the circuit. The output voltage of the regulator can be changed within a certain range through the negative feedback, allowing flexibility supply the power voltage for the various circuit blocks of the implantable systems. Input 2-MHz AC voltage after cascading the rectifier and the linear regulator, the power’s output is a stable and precise 3-v dc voltage.

In order to achieve low-power implantable system on a chip,

an ASK demodulation circuit is used,which structure is simple and easy to implement.It controls the amplitude of carrier with digital modulated signal.Fig. 4 depicts the ASK demodulator composed of an inverting amplifier, a low-pass filter, and a comparator. The simulation results of the demodulator output

is shown in Fig. 5.

After the pre-simulation results of the circuit meet the

Performance indicators, the layout of the circuit is designed. In order to reduce the parasitic capacitance and parasitic resistance and save the area of the layout, some layout design techniques such as a total of centroid symmetrical, interdigital structure, merge the source and drain of the adjacent MOS are used in this layout.In addition, protection rings are also increased in this layout, so that not only avoid the latch-up effect of the CMOS but also reduce the grounding resistance, the interaction effect of the different potentials of devices and reduce noise. After the DRC and LVS checks, the layout parasitic parameters are extracted for post-simulation.The circuit can be fabricated in foundry after the post-simulation results of the circuit meet the Performance indicators.

Fig. 6 shows the layout photograph of the processing circuit chip, with silicon area of 2 0.05mm (including I/O pads). The chip has been fabricated in a 0.5 um 2P3M standard CMOS process. The comparator detects the digital data precisely as shown in Fig. 7.

Fig. 6. Layout photograph and floor plan of the Processing circuit

Fig. 5. Output singnal of the ASK demodulator

Digitalsignal

RFsignal

Fig. 4. Schematic of the ASK demodulator

Figure 3. Schematic circuit structure for the regulator

Figure 2. Schematic circuit structure for the rectifier

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B. Improved Power and ASK demodulator design

In order to achieve low power and small size wireless energy and signal transmission system used in micro implantable medical system,our new design (Figs. 8 and 9) has done a relatively large improvement. The difference is the rectifier has been integrated in the chip and a band-gap reference voltage source is used in the regulator so that the output dc voltage is more stable and precise and little affected by temperature. In addition, ASK demodulator uses non-synchronous envelope detector circuit which avoids the use of large capacitors and resistors and greatly reduces the chip area.

Fig. 8 shows the improved power, which mainly consists of a bridge rectifier, an off-chip filter capacitor and a low-dropout linear regulator. Based on two considerations of the chip performance and area, the single-phase diode bridge rectifier circuit is used, which transforms AC signals into pulsating DC voltage with a good performance. As the implanted chip requires very low supply voltage sensitivity, means that within a certain range of the supply voltage fluctuation, the output reference voltage should maintain stable, therefore, the band-gap reference voltage source is used, which is not sensitive to the change of temperature, technology and supply voltage. Through two operational amplifiers, integrated reference voltage source and transmission pipes produce the depth of negative feedback, 1V DC voltage is generated.

After extracting the layout parasitic parameters, post-simulation starts with a input sine signal , which frequency is 0-3MHZ and DC component is 2-3V.The simulation results of the power output is shown in Fig. 9. The output dc voltage is

1±1%V which basically stable in the 1V, and the power is 0.5mW

A non-synchronous detection circuit is used in our new

design.It uses envelope detection method to demodulate ASK modulated signals. The envelope circuit without any capacitors replaces a large area of low-pass filter, and it only has a small resistor to achieve the effect of low-pass filter and realize the envelope detect. Fig. 10 shows the improved ASK demodulator, in which the bias circuit that nothing to do with the power change constitutes the envelope detector and the schmitt trigger constitutes the voltage comparator.

In the post simulation of the ASK demodulator, the carrier

frequency of the input modulated signal is 2MHZ, the rate of data transfer is 20KHZ, the low potential and the high potential of modulated signal is 0.5V and 1V.The simulation results of the ASK demodulator output are shown in Fig. 11. The modulated signal has been restored by the ASK demodulator. The output demodulated signal is square wave,which frequency is 20KHZ ,the duty cycle is about 52%,the low potential and the high potential is 0V and 1V.The improved circuits will be fabricated in a 0.13-um 1P8M standard CMOS process through SMIC.

Figure 10. Schematic of the improved ASK demodulator

Figure 9. Output of the improved power

Figure 8. Schematic of the improved power circuit

Figure 7. Output of the comparator

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IV. CONCLUSIONS

In this paper, the power and ASK demodulator have been implemented for an implantable wireless Energy and Signal transmission system, which have been fabricated in a 0.5 um 2P3M standard CMOS process and completed the chip-level test in the human body temperature range. Meanwhile, the detailed design of the improved power and ASK demodulator with SMIC 0.13-um 1P8M standard CMOS process are presented,which greatly reduce the chip power and area, and greatly improve the performance of the circuit

ACKNOWLEDGMENT

This paper was supported by National Science Foundation

of China (60706031), The National High Technology Research and Development Program of China (2006AA04Z359), Project supported by the Beijing Jiaotong University, Project supported by the “Talents Project” of Beijing Jiaotong University and China Postdoctoral Science Foundation (200902004).

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[3] S. Lee and S. Lee, “An Implantable Wireless Bidirectional Communication Microstimulator for Neuromuscular Stimulation”, IEEE Trans. Circuits and Systems-I, Vol. 52, No. 12, pp.2526-2538, Dec. 2005.

[4] M.Ghovanloo and K.Najafi,“A Wireless Implantable Multichannel Microstimulating System-on-a-Chip With Modular Architecture,”IEEE Trans.Neural Systems and Rehabilitation Eng., Vol. 15, No. 3, pp. 449-457, Nov. 2007.

[5] G.W.Wang,W.t. Liu and M. Sivaprakasam,“High Efficiency Wireless Power Transmission with Digitally Configurable Stimulation Voltage for Retinal Prosthesis,” in Proc. EMBS 2005, pp.543-546.

[6] C.S.A.Gong,M.T.Shiue,K.W.Yao,T.Y.Chen,Y.Chang and C.H.Su,“A Truly Low-Cost High-Efficiency ASK Demodulator Based on Self-Sampling Scheme for Bioimplantable Applications ,”IEEE Trans.Circuits and Systems-I, Vol. 55, No. 6, pp. 1464~1477, 2008.

Figure 11. Output of the improved ASK demodulator

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