KhadkaBiksonWECS - Neural Engineering Group
Transcription
KhadkaBiksonWECS - Neural Engineering Group
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 • A modified tDCS saline-saturated sponge: 7x5x3 cm (σ = 1.4S/m) • Top face perforated with cylindrical Ag/Agcl rivets (dout= 1.5cm, din= 0.61cm, extrusionouter =1cm, and extrusioninner = 0.50cm σ = 5.99E7S/m) • Electrodes align with the top surface and protrude through half the sponge thickness • 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 • • • • • 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 • • 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: • 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). • • • • 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 • • 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 [email protected]