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Principles of Within Electrodes Current Steering (WECS)
Niranjan Khadka*, Dennis Q.Truong, Marom Bikson
Department of Biomedical Engineering, The City College of New York, CUNY
Neural Engineering
Group
http://neuralengr.com/
Introduction
Results
Electrode Design
A
Figure 1: Computational
head model showing
streamline current distribution
inside the active electrode
assembly and to the returning
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A modified tDCS saline-saturated sponge: 7x5x3 cm (σ = 1.4S/m)
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Top face perforated with cylindrical Ag/Agcl rivets (dout= 1.5cm, din= 0.61cm,
extrusionouter =1cm, and extrusioninner = 0.50cm σ = 5.99E7S/m)
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Electrodes align with the top surface and protrude through half the sponge thickness
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Exposed on all surfaces and connect the lead via male receptacles at the top
Figure 5: FEM computational
analysis using a previously
developed tDCS workflow to
validate the underlying
assumption of within
electrode current steering. (A)
Current density observed at
the scalp electrode interface.
(B) Presents an electric field
distribution found in the brain
target.
electrode.
A1
B
Metal Rivets
Active electrode assembly
•
•
•
•
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Within Electrode Current Steering (WECS) as a novel
method to enhance the reliability and tolerability of transcranial
direct stimulation (tDCS).
Distinct from (across electrode) current steering, as developed
for implanted devices such as Deep Brain Stimulation (DBS),
where current is steered between electrodes that are each in
contact with tissue.
WECS adjusts current between electrodes not in contact with
tissue but rather embedded in an electrolyte on the body
surface.
Goal here is not to alter brain current flow, but rather
compensate for non-ideal conditions at the surface.
This technology leverages our technique for independently
isolating electrode impedance and over-potential during multichannel stimulation.
A
B
B1
Figure 3:Schematics diagram of an active electrode assembly. Top perforated face of the
saline soaked sponge (A) has four cylindrical metal (e.g. Ag/Agcl) rivets (B) around the
edges (for this exemplary example) that align with the top surface of the sponge and
protrude through half the sponge thickness. Changing the diameter and distance between
the metal rivets, the distance between the electrodes and the skin, or electrolyte conductivity
will discriminate how current from the electrode reaches the skin.
Current Steering
Dermis
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At the electrode-assembly interface with the skin, the current density distribution varied
only incrementally across conditions (e.g. less than would be expected with even minor
changes in electrode assembly or skin properties; with no significant difference in peak
current density (~2 A/m2; typically predicted around edges)
Electric field at the brain target under all three current split conditions was essentially
identical ( 0.35 V/m)
A
Conclusion
Methods
PRINCIPLES:
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WECS applies to non-invasive electrical stimulation with two or
more electrodes (metal-rivets) embedded in an electrolyte (saline or
gel) on the skin. Each electrode is independently powered by a
current source. Success in implementation of WECS depends on
geometry and material of each component of the assembly and an
algorithm for current steering between electrodes.
•
B
Return electrode
Figure 4: 2D view of
current distribution inside
the electrode assembly
(A) under three current
split conditions (even,
partially uneven, and fully
uneven) and streamline
plots (B) of within sponge
current flow under each
condition from electrodes
to the skin surface.
Active electrode assembly
•
Figure 2: A FEM model of a head with realistic electrode assembly to
achieve the principles of within electrode current steering ( current is
steered between electrodes that are in contact with tissue but without
altering brain current flow). The active electrode is placed on the scalp over
the motor region (M1) while the return electrode is placed at over the
contra-lateral orbit (not of concern here).
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•
•
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Electrode assembly receives a fixed total current of 1 mA (with -1 mA collected by the
return electrode)
Current is actively divided across the electrodes (rivets) within the assembly
“Even” current split: 0.25 mA is delivered to each electrode.
“Partially Uneven” current split: 0.5, 0.25, 0.25, and 0 mA is delivered
“Fully Uneven” current split: 1.0 mA is delivered to one electrode and 0 mA to the
remaining
•
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We illustrated how current flow in the brain can remain unaltered even as current is steered
between electrodes inside the electrode-assembly.
WECS can be generalized to other noninvasive electrical stimulation technique and
potentially to invasive techniques where an artificial or natural electrolyte barrier exists
between the electrode and the tissue.
For invasive techniques, WECS may complement traditional current steering but be used
to protect electrode and tissue from injury.
Success of this approach depends on the appropriate design of the electrode assembly
and the algorithm used to steer current between electrodes – topics to be considered in
future design efforts
References
•
Nitsche, MA., Liebetanz D., Lang N., Antal A., Tergau F., Paulus W., 2003, “Safety criteria
for transcranial direct current stimulation”, Clin Neurophysiol., 114(11): 2220-2., doi:
10.1016/j.clinph.2009.03.018
•
Kronberg, G., Bikson, M., 2012, “Electrode assembly design for transcranial Direct
Current Stimulation: A FEM modeling study”, Conf Proc IEEE Eng Med Biol Soc.
•
Khadka, N., Rahman, A., Sarantos, C., Truong, DQ., Bikson, M., 2015, “Methods for
Specific Electrode Resistance Measurement during Transcranial Direct Current
Stimulation”, Brain Stimul., 8: 150-159. doi:10.1016/j.brs.2014.10.004
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