Triboelectricity in Capacitive Biopotential Measurements IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 58, NO. 5, MAY 2011 Tobias Wartzek*, Thomas Lammersen, Benjamin Eilebrecht, Marian Walter, Member, IEEE,and Steffen Leonhardt, Senior Member, IEEE Adviser: Ji-Jer Huang Presenter: Hou-Yu Chou Date: 102/12/11

Outline 1.Introduction 2.Materials and Methods 3.Results 4.Discussion 5.Conclusion

Introduction ECG: – Advantages: better signals – Disadvantages: 1.skin irritations 2.specific preparation before measurements 3.limitations related to the connecting cables 102/12/9

Introduction Capacitive ECG : Advantages: – does not need contact – make continuous – an office chair, a bed, a toilet seat,automotive Disadvantages: – still suffer from strong artifacts

Introduction If two objects contact each other, interaction between the atoms will appear at the boundary layer /11/14

Introduction 1.Isolator–Isolator: 2.Isolator–Metal : 3.Metal–Metal:

Materials and Methods Driven right leg Disconnected  If only one foot is lifted from the ground, voltage changes up to several volts are measured. Connected  The electrostatic voltmeter does not measure any voltage change.  Global triboelectric effects cannot be the dominating reason for the severe artifacts in capacitive biopotential measurements.

Materials and Methods Simplified equivalent circuit The definition of the current i = dQ/dt

Materials and Methods 1.The capacitance is only slightly changed, but very strong artifacts occur. 2.Arbitrary high voltages can be generated by movement, even though the capacitor is discharged.

Materials and Methods Equivalent circuit

Materials and Methods

An appropriate electrode design: 1.The surface of the electrode should be matched to the patient. 2.The generation of charge due to rubbing should be avoided. 3.Charges on the surface should quickly discharge.

Materials and Methods 1.Reduce the accumulated charge. 2.Decrease the discharge time.

Materials and Methods The signal of interest was simulated by a fixed potential located 1 mm above the electrode. (to simulate the ECG)

Materials and Methods An actively driven grid makes the simulation difficult, since it leads to an algebraic loop in the simulation.

Materials and Methods The grid is kept on the same potential as the electrode by the op amp

Materials and Methods The Newton method is used The potential of the grid ugrid is adapted until the condition  f(ugrid) − ugrid = uelectrode − ugrid ≈ 0

Materials and Methods Broyden's method is used  The derivation can then be approximated by

Results

This small current together with a conservative estimated resistance of Rc = 100MΩ. Produces a voltage artifact as high as 2000 V.

Results Using a very small current of dQ/dt = 1nA, – the model still predicts an artifact in the order of magnitude of the ECG signal.  This result shows how sensitively a capacitive ECG measurement system reacts to small currents generated by contact electrification.

Results The charge separation of the artifact voltage uout mainly depends on Ri, Rc, and Cin. The discharge time of the artifact voltage uout mainly depends on Rin and Cin.

Results High humidity will mainly reduce Rg and Rc. A reduced discharge time constant τ of the coupling capacitor Cc (t). – τ = Cc · [Rc(Rin + Ri + Rg )].

Results A possible explanation is that charges are separated on the surface of the material, where the grid can drain them off.

Results At the beginning, the op amp is saturated at 1 V due to large electrostatic input voltage the discharge behavior of the electrode after being rubbed against cotton as secondary material

Results In case of an actively driven grid, the potential on the electrode is much less influenced.

Results As expected, the larger the grid,the less the signal voltage on the electrode.

Results The upper graph, the grid is switched off, The lower graph, the grid is active

Results In order to verify the simulations : A signal generator with a frequency far above the cut off frequency – The grounded grid the received voltage was 33% – Actively driven grid The received voltage was 97%

Discussion In case of the analyzed electrodes, one could argue that the electrode with no isolation rather than a capacitive electrode. However, when clothes are between the body and the electrode, such an electrode still acts like a capacitive electrode.

Conclusion The simulation results show that the grid has only a slight influence on the signal of interest. An adaptive filtering of those artifacts will be difficult due to the random nature of triboelectricity.