Ming Liu

Power harvesting using PZT ceramics embedded in orthopedic implants

Battery lifetime has been the stumbling block for many power-critical or maintenance-free real-time embedded applications, such as wireless sensors and orthopedic implants. Thus a piezoelectric material that could convert human motion into electrical energy provides a very attractive solution for clinical implants. In this work, we analyze the power generation characteristics of stiff lead zirconate titanate (PZT) ceramics and the equivalent circuit through extensive experiments. Our experimental framework allows us to explore many important design considerations of such a PZT-based power generator. Overall we can achieve a PZT element volume of 0.5 times 0.5 times 1.8 cm, which is considerably smaller than the results reported so far. Finally, we outline the application of our PZT elements in a total knee replacement (TKR) implant.

Low-Power Circuits for the Bidirectional Wireless Monitoring System of the Orthopedic Implants

This paper proposes an architecture of the wireless monitoring system for the real-time monitoring of the orthopedic implants, which monitors the implant duty cycle, detects abnormal asymmetry, high amounts of force, and other conditions of the orthopedic implants. Data for diagnosis are communicated wirelessly by the radio-frequency (RF) signal between the embedded chip and the remote circuit. In different working modes, the system can be powered by the RF signal or stiff lead zirconate-titanate (PZT) ceramics which are able to convert mechanical energy inside the orthopedic implant into electrical energy. The power circuits with a variable ratio switched-capacitor (SC) dc-dc converter have been taped out with 0.35-mum complementary metal-oxide semiconductor (CMOS) technology. The test results show that the SC converter can transfer the input voltage that ranges from 5 V to 14 V from the PZT ceramics into the voltage ranging from 2 V to 2.5 V which will be dealt with by a low drop-out circuit in the future work. The total efficiency of the SC converter is from 28% to 42% at full-time working mode. The analog-to-digital converter (ADC) circuits have been fabricated in a 0.18-mum 1P6M CMOS process. The test results show that the ADC chip consumes only 12.5 muW in working mode and 150 nW in the sleep mode. The circuits, including RF circuits, ADC, and the microcontrol unit, have been implemented in a 0.18-mu m CMOS process. Future work includes some clinical experiments test in the application where PZT elements are used for power generation in total knee-replacement implants.This paper proposes an architecture of the wireless monitoring system for the real-time monitoring of the orthopedic implants, which monitors the implant duty cycle, detects abnormal asymmetry, high amounts of force, and other conditions of the orthopedic implants. Data for diagnosis are communicated wirelessly by the radio-frequency (RF) signal between the embedded chip and the remote circuit. In different working modes, the system can be powered by the RF signal or stiff lead zirconate-titanate (PZT) ceramics which are able to convert mechanical energy inside the orthopedic implant into electrical energy. The power circuits with a variable ratio switched-capacitor (SC) dc-dc converter have been taped out with 0.35-mum complementary metal-oxide semiconductor (CMOS) technology. The test results show that the SC converter can transfer the input voltage that ranges from 5 V to 14 V from the PZT ceramics into the voltage ranging from 2 V to 2.5 V which will be dealt with by a low drop-out circuit in the future work. The total efficiency of the SC converter is from 28% to 42% at full-time working mode. The analog-to-digital converter (ADC) circuits have been fabricated in a 0.18-mum 1P6M CMOS process. The test results show that the ADC chip consumes only 12.5 muW in working mode and 150 nW in the sleep mode. The circuits, including RF circuits, ADC, and the microcontrol unit, have been implemented in a 0.18-mu m CMOS process. Future work includes some clinical experiments test in the application where PZT elements are used for power generation in total knee-replacement implants.

Low Power IC Design of the Wireless Monitoring System of the Orthopedic Implants

This paper proposes an architecture of the wireless monitoring system for the real-time monitoring of the orthopedic implants. The system monitors the implant duty cycle, detects abnormal high amounts of force, and other conditions of the orthopedic implants. Data for diagnosis is communicated wirelessly by radio frequency (RF) signal between the embedded chip (inside body) and the remote circuit (outside body). In different working modes the system can be powered by the RF signal or stiff lead zirconate titanate (PZT) ceramics which are able to convert mechanical energy inside the orthopedic implant into electrical energy. The radio frequency (RF) circuits with the working frequency of 2.4 GHz have been taped out with 0.18 mum CMOS technology with 50 muW. It can supply 400 muW power over a distance of 20 cm between the two transceivers. The power circuits have been taped out with 0.35 mum CMOS technology. The circuits including RF circuits, analog digital converter (ADC), and micro control unit (MCU) have been implemented in 0.18 mum CMOS process.

Integrated power management circuit for piezoelectronic generator in wireless monitoring system of orthopaedic implants

Piezoelectric (PZT) materials are capable of converting the mechanical energy of compression into electrical energy. With the recent advent of extremely low-power electrical devices, PZT generators have become attractive in many kinds of applications, especially for biomedical applications. Piezoelectronic generators are used in a wireless monitoring system of orthopaedic implants. Due to their poor source characteristics, the efficiency of PZT generator is low. A hybrid direct current (DC)-DC, comprising a switched capacitor (SC) DC-DC converter and a low dropout (LDO) linear voltage regulator, is presented to improve conversion efficiency. A bandgap reference (BGR) circuit which works in sub-threshold region is also presented. Because SC DC-DC converter works in the highest voltage region in this system, small power supply current, including supply current through BGR and other auxiliary modules, means low power consumption. BGR-s power supply voltage can be varied from 3 to 16 V. Its supply current is only 3.2 muA at 125degC and its temperature coefficient is 46 ppm. Stacked switches technique is proposed to reduce leakage current in switching process of SC converter. Simulation results show that the efficiency of SC-s converter can reach 88%, that of LDO can reach 80% and that of the overall system can reach 66%, including power consumption of all auxiliary components, which is far higher than previous work.

A low-power IC design for the wireless monitoring system of the orthopedic implants

A low power IC of the wireless monitoring system of the orthopedic implants (WMSoOI) is put forth. The analog-digital mix-mode system monitors the implant duty cycle, detects abnormal asymmetry, wear, and high amounts of force, and other conditions of the orthopedic implants. Data for diagnosis is communicated wirelessly between the embedded chip (inside body) and the remote circuit (outside body). The radio frequency (RF) circuits have been taped out and tested. The power circuit is presented and verified by simulation. Future work includes the test of circuits of analog digital converter (ADC) and micro control unit (MCU) and the integrating of the entire system.

A wireless force measurement system for Total Knee Arthroplasty

A wireless force measurement system is presented in this paper. The system, which is used during the operation of Total Knee Arthroplasty(TKA), is designed to assist surgeons to determine the proper position of the knee implants and consequently enhance the success ratio of the treatment. It consists of three parts: a device to measure and transmit force data, a receiver and a terminal to display the force data in real time. The transmitter communicates with the receiver by 2.4GHz Radio Frequency (RF) signal. The system consumes no more than 17mA current with 3V voltage supply typically. So it can work with a button battery cell. Experimental results show that the performance of the system meets the requirements.

A wide dynamic range and fast update rate integrated interface for capacitive sensors array

A low-power CMOS capacitance-to-digital converter for capacitive sensors array is presented. It consists of a 16-channel MUX, a front-end with wide dynamic range for capacitance-voltage conversion and a novel voltage-pulses convertor. This novel circuit charges the measured capacitor indirectly and converts it into a proportional voltage, and then produces a 16-bits pulses-formed single-line output with multi-steps quantization method. The circuit can operate under 1.2V to 3.6V voltage supply for single battery application. The maximum input is 350pF with 0.75ms/ch update rate and 90μA power consumption when the operating clock is 100KHz. The chip has been design and fabricated in 0.18-μm 1P6M CMOS process with active area of 640×630μm2.

Low-power circuits design for the Wireless Force Measurement system of the Total Knee Arthroplasty

This paper proposes a novel wireless force measurement system for the Total Knee Arthroplasty (TKA) to improve the ligament balancing procedure during TKA. The force measurement system is comprised of a Wireless Force Measurement Spacer (WFMS) and the display part. They communicate with each other by the Radio Frequency (RF) signal. The WFMS is designed to measure the force between the WFMS and the femoral component of the artificial implants and to transmit the force data wirelessly by a low power transceiver. The display part demonstrates the force data in 3D images in real time. The WFMS composes of a sensors array, a Universal Transducer Interfaces (UTIs) array, a low-power sub-threshold microprocessor and a transceiver. The sub-threshold 8-bit microprocessor is taped out with 0.18µm CMOS technology. The testing results of the microprocessor show that the leakage power of 46nW and the dynamic power of 385nW@165kHz are achieved with the operating voltage of 350mV. The test results of the system are given and the errors of the system are analyzed. The results verified the reliability of the system. The future work is to design the microprocessor and a lower power transceiver within a single chip.

Low-power IC design for a wireless BCI system

Integrated circuit (IC) design for a wireless BCI system is put forward in this paper. The system is composed of an electrode, a stimulator, antennas, and an integrated circuit including a preamplifier, an analog to digital converter (ADC), a current mode digital to analog converter (DAC), a transceiver and a micro control unit (MCU). The system is closed loop, which can detect and record electroencephalogram (EEG) signals, send the signals to computer wirelessly, and generate stimulating signals according to analysis results from the computer. A 16-bit ADC is designed and some non-idealities parameters (e.g. kT/C noise, 1/f Noise and so on) are given. Sub-threshold circuit technology is used to realize ultra low-power MCU. Both of the ADC and the MUC are verified by simulation results. Next work will include the design of the preamplifier, IDAC, and a single chip design integrating all the circuits.

A fast computable delay model for subthreshold circuit

The subthreshold circuit is a promising ultra-low-power solution for such applications that dont require high speed but are extremely power-stringent. The characteristic of the current under subthreshold voltage is different from the normal domain. A delay model is essential to predict the performance, analyze the variation impacts and optimize the design. This paper proposes a fast computable delay model for subthreshold circuit. The model requires a few parameters and is easy to calculate quickly. A 32-stage inverter-chain designed by 180nm process is used to verify the model and the calculated results indicate that the model error is less than 10% compared with HSPICE.