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 HuangPresenter: Hou-Yu ChouDate: 102/12/11

Outline

1. Introduction2. Materials and Methods3. Results4. Discussion5. Conclusion

Introduction

ECG:– Advantages:

better signals– Disadvantages: 1. skin irritations2. specific preparation before

measurements 3. limitations related to the

connecting cables

http://romatranslations.com/emergency-acls-practice-ecg-strips/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.

http://physics.appstate.edu/laboratory/quick-guides/electrostatics-0 102/11/14

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.

ConnectedThe 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 Methods1. 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

Materials and Methods

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) Δ is the Laplace operatoris the electric potential fieldρ is the charge density ε is the permittivity

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 usedThe derivation can then be approximated by

Results

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.