A highly miniaturized two-axis acceleration sensor for implantable hemody-namic controlling system
The paper shows the development of a micro machined sensor for implantation and monitoring the blood pressure of patients quite accurately. A functional system for hemodynamic controlling was developed and consists at least of an externaltransceiver and an implantable multi sensor unit. The implantable sensor elements are used to measure pressure, acceleration, temperature, voltage and impedance. In order to achieve the required miniaturization the acceleration sensor is based on MEMS technology. Other sensor elements are integrated in an application-specific integrated circuit. The three individual components as pressure sensor, acceleration sensor, and circuitry will be mounted on a ceramic interposer module. The acceleration sensor detects the module's orientation in the earth gravitational field as well as strong impulsive events. The design of the acceleration sensor, the fabrication of lithography masks and the sensor manufacturing on 6-inch wafers were done by Fraunhofer ENAS. Fraunhofer IMS developed the pressure sensor, the electronics for controlling the microsystem, the evaluation of the measurement as well as the integrated circuit. The measurement range of the acceleration sensor is from -180 ° to + 180 ° for two directions with an accuracy of +/- 5 ° especially for orientation monitoring. Furthermore the sensor enables the recognition of "impulsive events" with an extended measurement range of ±5 g. Such events will influence the pressure measurement and should be eliminated for evaluation. The sensing device is a two-axis micromechanical acceleration sensor with differential capacitive detection. With a dimension of 1 x 1 mm² of the MEMS core and a complete die size of 1.2 x 1.5 mm² the sensor is highly miniaturized and small enough for implantation. First characterization tests showed the system is working and are quite promising. Further work is focussed on the integration and packaging technologies of all the components including biocompatible thin film encapsulation.