Adaptive Signal Acquisition and Wireless Power Transfer for Implantable Medical Devices

April 2, 2009
Interschool Lab, Room 750 CEPSR
Speaker: Stephen O’Driscoll, Stanford University


Implantable medical devices (IMDs) are a rapidly growing area of technology. In-vivo monitoring and treatment of key biological parameters can greatly assist in managing health and preventing disease. Excess analog power consumption and insufficient power supply prohibit the widespread deployment of IMDs for many applications. Using implanted neural sensors as an application driver, this talk presents technologies to solve both of those challenges.

An analog-to-digital converter (ADC) array which digitizes the neural signals sensed by an implanted microelectrode array is described. The resolution of each ADC cell is varied according to the neural data content of the signal from the corresponding electrode. The base ADC cell resolution can be varied from 3 to 8-bits with corresponding power consumption of 0.23uW to 0.90uW at 100kS/s in 0.13um CMOS. Resolution adaptation reduces power consumption by a factor of 2.3 whilst maintaining an effective 8-bit resolution across all channels, achieving a figure of merit of 15fJ per conversion step.

A new approach to wireless power transfer is developed which achieves the same efficiency as previous work using 100 times smaller antennae. This requires three steps: firstly the optimal frequency for wireless power transmission through tissue to area constrained receivers is derived. Second, new matching techniques to achieve the theoretical link gain at this optimal frequency are introduced and an adaptive matching scheme which increases the robustness of the link gain to inevitable dielectric, range and alignment variations associated with an IMD is presented. Thirdly, a low voltage rectifier is presented which reduces the voltage drop per stage to considerably less than a threshold voltage, achieving 67% efficiency for 0.75V input. The power receiver including adaptive matching, rectifier, and regulator have been implemented in 0.13um CMOS, delivering 120uW at 1.2V DC to the implanted circuits from a 2mm x 2mm on-board receive antenna, through 15mm of tissue whilst meeting all safety constraints.

Both the ADC and power transfer work demonstrate that performance requirements of analog circuits for IMDs vary as a function of patient physique, health, device placement, patient activity etc, and thus cannot be known accurately prior to deployment. I will outline some of the new IMDs I intend to develop by applying adaptive analog circuits and mm-sized implantable power receivers. I will also discuss extending the capabilities of these new wireless power links by adding beamforming to increase link efficiency, bi-directional data communication, and transmit-side receiver localization to enable addressing of multiple receivers.


Stephen received the Bachelor of Engineering degree in Electrical Engineering from University College Cork, Ireland in 2001. He received the SCI, Motorola scholarships and IEE fellowship while at UCC. He spent the summers of 1999 and 2000 at Farran Technology, Ireland where he designed a 77GHz radar front end. From 2001 to 2003 he was at Cypress Semiconductor, San Jose, California where he designed clock and data recovery PLLs for gigabit ethernet transceivers. He joined Stanford University in 2003 as the Nicky Lu Stanford Graduate Fellow. He received the MS in 2005 and will receive his PhD in June of this year, both in Electrical Engineering. He has been developing implantable biomedical ICs with Prof. Teresa Meng since 2003. His research interests are in the area of analog and mixed-signal circuits, wireless power transfer, and adaptive circuit and system design especially for medical applications.

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