Motion artefacts reduction in capacitive ECG recordings using an injection current

Capacitive electrodes are an interesting alternative to conventional adhesive gel electrodes in the sense that they allow biopotential recordings through different layers of cloths. Capacitive electrodes can also be invisibly embedded in everyday objects like chairs and beds. These electrodes are however very sensitive to motion artefacts which are caused by distance variations between the electrode and the body surface. These variations induce a change in the capacitive coupling of the electrode, which affects the transfer function of the recording system. The dynamic and unknown variations of the transfer function corrupt the biopotential recordings, making them sometimes hardly usable. The goal in this study is to dynamically track the variations of the transfer function due to motion and correct the biopotential recordings accordingly. As a first step, a model for the body/electrode interface [1] is adapted to our specific measurement setup, yielding a model with four parameters (i.e. coupling capacitance, coupling resistance, input capacitance and bias resistance). As a second step, a known sum of two sinusoids is injected through the system and recorded jointly with the biopotentials. The real and imaginary parts of these injected sinusoids are used to dynamically estimate the four model parameters. Once the behaviour of the transfer function is known over time, the biopotential recordings can be corrected for motion artefacts. A MATLAB Simulink model was developed to simulate the capacitive sensor and the body/electrode interface in presence of motion artefacts. An electrocardiogram recorded with capacitive electrodes was used as an input to the system. Two sinusoids of respectively 110 and 120 Hz were injected to the system to estimate the four parameters of the transfer function. Since our model-based correction for motion artefacts shows promising results in this simulated environment, the method is currently tested in a lab setup. Motion artefacts are not the only interferences corrupting capacitive biopotential recordings. However, a proper cancellation of motion artefacts is a first step towards a robust capacitive system enabling unobtrusive ECG or EEG recordings on dressed patients or in ubiquitous systems [2].