A versatile output stage for implantable neural stimulators

Abstract

Neural stimulators have the potential of becoming very important devices for the treatment of a wide variety of diseases. One of the major problems with existing stimulators is the limited waveform adjustability. This precludes the use of sophisticated stimulation programs and thereby affects the efficacy of the therapy applied. Another issue is the limitted implantability of the device, resulting in long subcutaneous wires. Because of these two reasons a new type of stimulator is required. Electrodes implanted in neural tissue are modeled using a highly non linear model with a capacitive nature. It is however shown that the response of the electrode tissue interface can be modeled accurately enough using a linear capacitive model. The physical process associated with the stimulation of neural tissue essentially comprises lifting up the tissue potential above (or below) a certain threshold value. This means that stimulation essentially is the injection of a particular amount of charge into the tissue in order to lift the potential of the tissue. Furthermore the injected charge needs to be canceled precisely in order to prevent tissue damage. First a system level design of a complete stimulator system is presented. This design includes the possibility of feedback: based on the brain activity recorded by electrodes a certain stimulation pattern is applied. After the system definition the design of the output stage, responsible for injecting the stimulation pattern into the tissue, is treated. Most existing stimulators use a current based architecture in which the charge is controlled by enabling the stimulator for a certain amount of time. Voltage based stimulation however is shown to have a higher power efficiency. A novel type of voltage based architecture is proposed using indirect current feedback of the tissue current. Using a current integrator with a very high dynamic range the injected charge can be controlled very precisely, while any arbitrary voltage waveform can be used for stimulation. Circuit simulations prove the feasibility of the approach and show a charge mismatch in the order of 0.1% is possible, paving the way to full charge balancing. Furthermore, they predict correct functionality over all process corners, including mismatch. The system only uses a singleended supply and the quiescent power consumption of the system is less than 17 ?W. Therefore it can be concluded that the novel approach for the output stage design proposed in this thesis allows the use of a very versatile stimulator; any arbritrary waveform can be injected while assuring charge balancing. Furthermore power consumption is minimized in order to relax the requirements for the battery and thus improve the implantability of the system.