title: Methods for specific electrode resistance measurement during transcranial direct current stimulation.
abstract: BACKGROUND
  Monitoring of electrode resistance during tDCS is considered important for tolerability and safety. Conventional resistance measurement methods do not isolate individual electrode resistance and for HD-tDCS devices, cross talk across electrodes makes concurrent resistance monitoring unreliable.
  
  
  OBJECTIVE
  We propose a novel method to monitor individual electrode resistance during tDCS, using a super-position of direct current with a test-signal (low intensity and low frequency sinusoids with electrode-specific frequencies) and a sentinel electrode (not used for DC).
  
  
  METHODS
  We developed and solved lumped-parameter models of tDCS electrodes with or without a sentinel electrode to validate this methodology. Assumptions were tested and parameterized in participants using forearm stimulation combining tDCS (2 mA) and test-signals (38 and 76 μA pk-pk at 1 Hz, 10 Hz, & 100 Hz) and an in vitro test (creating electrode failure modes). DC and AC component voltages across the electrodes were compared and participants were asked to rate subjective pain.
  
  
  RESULTS
  A sentinel electrode is required to isolate electrode resistance in a two-electrode tDCS system. Cross talk aggravated with electrode proximity and resistance mismatch in multi-electrode resistance tracking could be corrected using proposed approaches. Average voltage and pain scores were not significantly different across test current intensities and frequencies.
  
  
  CONCLUSIONS
  Using our developed method, a test signal can predict DC electrode resistance. Since unique test frequencies can be used at each tDCS electrode, specific electrode resistance can be resolved for any number of stimulating channels - a process made still more robust by the use of a sentinel electrode.
authors.affiliation: Department of Biomedical Engineering, The City College of New York, CUNY, 160 Convent Ave., New York 10031, USA. Electronic address: nironzan@gmail.com.
authors.forename: Niranjan
authors.initials: N
authors.lastname: Khadka
authors.affiliation: Department of Biomedical Engineering, The City College of New York, CUNY, 160 Convent Ave., New York 10031, USA.
authors.forename: Asif
authors.initials: A
authors.lastname: Rahman
authors.affiliation: Department of Biomedical Engineering, The City College of New York, CUNY, 160 Convent Ave., New York 10031, USA.
authors.forename: Chris
authors.initials: C
authors.lastname: Sarantos
authors.affiliation: Department of Biomedical Engineering, The City College of New York, CUNY, 160 Convent Ave., New York 10031, USA.
authors.forename: Dennis Q
authors.initials: DQ
authors.lastname: Truong
authors.affiliation: Department of Biomedical Engineering, The City College of New York, CUNY, 160 Convent Ave., New York 10031, USA.
authors.forename: Marom
authors.initials: M
authors.lastname: Bikson
mesh_terms: D000328:Adult; D017097:Electric Impedance; D004567:Electrodes, Implanted / Q000706:statistics & numerical data*; D006801:Humans; D008297:Male; D008954:Models, Biological; D010147:Pain Measurement; D065908:Transcranial Direct Current Stimulation / Q000379:methods*; D055815:Young Adult
keywords: Brain stimulation; Electrode impedance; Electrode resistance; Neuromodulation; Tissue resistance; tDCS