Investigation of an embedded closed-loop stimulation current control principle based on the use of nonlinear ceramic capacitors

Over the years, a constant progress in the development of implantable medical devices (IMD's)can be observed. On one hand, the advancedimplantable electronics enable the implementation of numerous smart functionalities, on the other hand, the varietyof electronic componentsincluding sensors and a bulkybatteryseverely restrict theirdegree of miniaturizationand reliability. To overcome this limitation,our approach is to realize smart functionalities in leadless and battery-free IMD's emerging from frugal innovation by exploiting the intrinsic nonlinear properties of the components to be used anyway.The aim of this work is to deepen the understanding of the dynamic behavior of circuit topologies of nonlinear ferroelectric ceramic capacitors and to investigate their potential use for an embedded closed-loop control of the stimulation current. We characterized aselection of 40 commercial ceramic capacitors by measurement and simulation. The degree of nonlinearity resulting from a circuit topology consisting of one, two series and two parallel connected nonlinear capacitors wasmodeled and evaluated in Mathcad.We present a model with parameterized nonlinear capacitors to simulate the dynamic behaviorof an inductively coupled implantable system.The stabilization and amplitude of the stimulation current is controlled by two features.These features arein turn controlled by the circuit topology and the degree of nonlinearity of the capacitors.We found that ahigh degree of nonlinearity allows the stimulation current to be stabilized within a reasonable range, but it makes the system more prone to instability.However, our model needs to include the dynamic behavior of ferroelectric materials used as dielectric in ceramic capacitors to extend the current investigationsand to deepen the understanding of the physics behind the nonlinear properties of ferroelectric capacitors.