Study On Volume Conduction Energy Transfer For Implantable Devices

The medical implantable microelectronic devices are developing very fast, and become used widely, such as pacemakers and artificial cochlear. Power supply for the implantable devices is one of the most critical techniques. Although there are a variety of possible energy supply ways for implantable devices, only battery and magnetic induction technologies are widely applied in clinical.The largest disadvantage of battery technology is lifetime constraint of battery. When the battery power of the implantable devices runs out, it has to be replaced through the surgery. On the other side, battery size is also a key factor affecting the miniaturization of implantable devices. Magnetic induction technology can transfer energy in vitro to implantable devices in vivo. However, energy transfer efficiency of this technology is very low, and magnetic induction coil is larger. In addition, magnetic induction technology works in the high-frequency environment, and that would produce RF interference to medical equipments nearby.In order to avoid the major drawback of magnetic inductive coupling method and expand the service life of implantable devices, scholars at abroad presented an energy delivery process based on volume conduction across skin and connective tissue. Through the electrodes close to the skin in vitro, this technology is making use of biological organization ionic fluid as a carrier to transfer the energy power in vitro to the electrodes in vivo, and then supply power for implantable electronic devices in vivo or charge the rechargeable battery. So far, scholars at home and abroad have done a preliminary study on this technology, initially proved the feasibility of volume conduction energy transfer, and proposed a variety of specific programs and evaluation index for it.According to the preliminary study of our laboratory, efficiency of volume conduction energy transfer is higher than that of magnetic induction technology, and the key factor affects volume conduction energy transfer efficiency is the size of shorting current, which is affected by separation between electrodes. However, previous work did not do detailed research. The topic focused on the impact of separation of electrodes on volume conduction energy transfer.Additionally, previous studies focused on charging rechargeable battery of implantable devices in vivo under circumstances that it did not cause tissue injury. This just considered about that current which flew through the skin did not cause physical damage to the skin, but not took the effect of frequency and amplitude of the signal source on nervous system into account. The author believed that this is very important to avoid these situations that cause patients feel tingling, burning or other inappropriate feelings. On the basis of considering biological reactions, the topic focused on the impact of waveform, amplitude and frequency of signal source in vitro on volume conduction energy transfer.An Electromagnetic field model and circuit model of volume conduction energy transfer were established to study the effect of separation of electrodes, waveform, amplitude and frequency of signal source in vitro to volume conduction energy transfer, and then verified the view by the agar experiments and swine skin experiments.A general model was established according to the simulation and experimental results. Under the circumstances that current size does not lead to tissue injury and current frequency does not cause neurological reactions, the optimal separation between the electrodes is 3 cm, the best waveform of signal source is sine wave, and the optimal frequency is 200KHz. At this time, the current transfer efficiency are 28.13% and 20.65%, the energy transfer efficiency are 9.86% and 6.90% in the agar experiments and swine skin experiments, respectively.Theoretical analyses and experimental results have proved that electrical energy in vitro can be transformed efficiently into implantable devices in vivo through volume conduction. We can obtain the optimal efficiency of energy transfer by setting separation between the electrodes, waveform, amplitude and frequency of the signal source appropriately.In order to take advantage for surgical implantation and to get higher energy tran-sfer efficiency, our laboratory has designed a symmetrical electrode, which can preve-nt shorting current between electrodes significantly. The electrode skin model is in the optimization now, and agar and swine skin experiments will be taken recently.