Implantable hemodynamic controlling system with a highly miniaturized two axis acceleration sensor

This paper presents an overall system for hemodynamic controlling consisting of a transmitter/reader, which operates outside the human body, and an implantable sensor unit. The implantable sensor module detects pressure, acceleration, temperature, voltage, and impedance. In order to achieve the required miniaturization, the acceleration sensor was developed in a MEMS (micro electro mechanical system) technology. Except the MEMS pressure sensor and accelerometer the other sensory functions are integrated in an ASIC (application-specific integrated circuit). The three individual components (pressure sensor, acceleration sensor, ASIC) are mounted on a ceramic interposer module. While the acceleration sensor detects the module orientation within the earth gravitational field as well as sudden motion events the pressure sensor measures the blood pressure inside a blood vessel. This system is developed within a joined project by the Fraunhofer Institutes IMS in Duisburg and ENAS in Chemnitz, both in Germany. The design of the acceleration sensor, the fabrication of the lithographic masks and the sensor manufacturing on 6-inch wafer level as well as the system integration are done by Fraunhofer ENAS. Fraunhofer IMS developed the electronics for control and wireless read-out of the microsystem, evaluated the measurements and provided the ASIC and the pressure sensor. The requirements for the MEMS accelerometer are to detect sudden motions and shock events in the range of ±5 g and inclination in two directions in the range from -180° to +180° with an accuracy of ±5°. The sensing device is a two axis micromechanical acceleration sensor with capacitive detection. The MEMS accelerometer is fabricated by using the Deep Reactive Ion Etching (BDRIE) technology. The core die (sensor area) has a dimension of 1 × 1 mm2. The overall die size is 1.2 × 1.5 mm2. The maximum height of the sensor chip is 0.65 mm. Each axis is measured with two differentially arranged comb-shaped electrodes to extract the acceleration data. The sensor is highly miniaturized and tiny enough for implantation in the human body. The characterization tests showed that the system is working properly and the results are very promising. Further work will be focused on the integration and packaging technologies of all the components including biocompatible thin film encapsulation [1].