Study On Energy Delivery Rationale And Method For Implantable Devices Through Volume Conduction

The power supply is a common concern for implantable devices. There are several possible approaches to power implants, however, only battery and magnetic inductive coupling are practical. The service life of an implantable device powered by a battery is short because of the limited battery capacity. For example, the real service life for a cardiac pacemaker is 2-5 years. On the other hand external rich energy can be transformed into implantable devices by magnetic inductive coupling. But its conversion efficiency in power delivery is generally poor due to the energy loss in conductive biological tissue . In order to maintain a sufficient amount of power transfer, a large RF power must be transmitted. This causes RF interference to nearby medical instruments.In order to avoid the major drawback of magnetic inductive coupling method and expand the service life of implantable devices this paper presents the energy delivery process based on volume conduction across skin and connective tissue. An x-type equivalent circuit model for the energy delivery process has been constructed and a circuit analysis method based on the model has been proposed. The circuit analysis method is useful and can be applied for whole volume conduction energy delivery system. Theoretical analyses and experiments had been conducted to evaluate the characteristics of energy delivery based on volume conduction, such as, AC current transmission efficiency, implantable battery recharging condition, recharging current transmission efficiency, energy transmission efficiency. According to circuit analysis three important parameters were defined to evaluate performance of power delivery. These parameters are Input Loop Impedance Ratio (ILIR), Output Loop Impedance Ratio (OLIR) and Voltage Ratio (VR) for determining recharging current transmission efficiency. Two methods for enhancing recharging current transmission efficiency have been proposed: those are impedance method and voltage method. According to the implantable battery recharging condition derived from circuit analysis, it is concluded that square wave is the best one for exciting external electrodes in energy delivery based on volume conduction.According to the circuit analysis results, a circuit prototype based on volume conduction had been developed. In our preliminary experiment, it had delivered a 5 kHz current of up to 2.8 mA across freshly harvested pigskin with a current transmission efficiency of 27%.Theoretical analyses and experiments have proved that electrical energy can be transformed efficiently from an external body into implantable devices in the body through volume conduction.