Live demonstration: Inductive power and telemetry for micro-implant

A prototype for a wireless implantable sensor system is demonstrated intended for a pill sized micro-implant for blood sugar monitoring. Power is sent and telemetry data received over a near field inductive link. The implant system is almost completely realized on a single CMOS ASIC, excepting three surface mounted capacitors, the coil antenna and the sensor. For this demonstration, the ASIC is packaged and mounted with the 4 other components on a PCB, and the glucose sensor is substituted with either a potentiometer or an atmospheric pressure sensor. In the targeted highly miniaturized implantable micro system all components (including antenna and sensor) will be integrated into a pill sized LTCC (low temperature cofired ceramics) package. The total implant power budget with the minimal supply voltage of 2.5V is at 76μW. This number includes the power for various test structures that will be unnecessary for an industrial prototype, which is expected to operate below 20μW. A simple demonstrator of the reader is implemented on another PCB with a two digit hexadecimal LED display to show the telemetry data. This demo is associated with the BioCAS and the Sensory Systems track and is based on the publication [1] I. DEMONSTRATION SETUP The demonstrator consists of two PCBs: a prototype for the reader and a prototype for the implant as depicted in figure 1. The upper board is the reader prototype, powered by a 9V battery. It sends power on a 13.56MHz carrier to the implant prototype and receives telemetry data on the same link. The main component is a Xilinx CoolRunner II CPLD. The sensor value is displayed on a two digit hexadecimal LED display. The lower board is the implant prototype. Either a potentiometer or a air pressure sensor can be connected, in lieu of the targeted glucose sensor that is still under development. The implant prototype depicted contains a number of configuration switches which will be removed and the configuration will be hardwired for the demonstrator that will be brought to ISCAS 2010. This prototype’s approximate dimension will either be 2.5cm x 5cm, an area which is required by the ASIC package and the coil antenna, or merely 2cm x 2cm if the ASIC is wire bonded directly onto the PCB. II. VISITOR INTERACTION AND EXPERIENCE A. Asynchronous Pulse Density Modulation The implant does not perform a normal analog to digital conversion (ADC), but an analog to analog conversion, where the analog sensor value is translated into an analog/asynchronous inter pulse interval [1]. These pulses are conveyed to the reader over the inductive link by load modulation. The real ADC then simply consists of counting the pulses over a given interval. The decoded pulses can be observed on a portable oscilloscope and the correspondence of pulseintervals, the LED display and the ’sensor’ can be observed while the visitor tweaks the ’sensor’-potentiometer. B. Tuning the Transmitted Power If the reader is not close enough to the implant antenna, the implant is neither powered nor can it send data on the inductive link: the LED display shows 00. Only if the implant is moved close enough to the reader, will pulses be conveyed and a number be displayed. However, at first the power will not be sufficient for the ASIC’s internal regulator to operate properly. As a consequence, the pulse frequency will not truly reflect the sensor value. Moving closer, the power will become sufficient, but at some point the internally rectified voltage on the ASIC will become even too high and threaten to damage the ASIC. This is prevented by an internal reversible fuse (turning on an infinite load) which also sends pulses at a high frequency back to the reader. The reader display will indicate this by flashing digit points. A more advanced prototype reader shall achieve an optimal power transmission by gradually increaseing the carrier power until an over-voltage is signalled and then slightly reduce the power before each reading (once every 5 minutes).