Donaldson, N

Jiang, D

Lancashire, H

Niederhoffer, T

Vanhoestenberghe, A

English

UCL Discovery

72 pt1 in25.4 mm54 pt0.75 in19.1 mm54 pt0.75 in19.1 mm54 pt0.75 in19.1 mmMargin requirements for first pagePaper size this page US LetterAbstract— Advances in electrodes hold the potential to improve the stimulation safety for implantable electrical stimulators, but electrode developers should understand some electronics design to optimise overall device safety.I. INTRODUCTIONInnovative  research  on  electrode  materials  and manufacture  promises  ever  more  flexible  electrode  arrays, some with higher electrode density, or able to conform to the three dimensional shape of the target (CNS or PNS). As electrode dimensions decrease, the surface density of charges on the electrodes increases, and with it, the risks of electrode corrosion and cell damage. Lilly’s 1955 paper first suggested  that  balancing  the  charges  delivered  during stimulation  was  key  to  preventing  damage  [1].  In  1992, Shannon proposed  an  empirical  safety  criterion of  charge-balanced waveforms, expressed as a relation between charge density and charge per phase [2].  Much work has sought to further  our  understanding  of  the  mechanisms that  mediate cell  damage  and  electrode  corrosion  in  chronic  use,  for various  electrode  dimensions  and  materials,  and  types  of balanced waveforms; that may enable safe stimulation “to the right of Shannon’s line” [3, 4].Inspired by efforts to share knowledge across the many disciplines  of  our  fields,  such  as  the  interpretation  of electrochemical  electrode  characterisation,  to  improve stimulation safety [5, 6], we wish to discuss the impact, on overall  safety,  of  compromises,  made  when  designing  the electronics  of  the  stimulator,  to  comply  with  various sometimes conflicting, demands. In this presentation, we will consider  the  relationships  between  waveform,  safety  and stimulation efficacy from an electronics design point of view. II.WAVEFORM, SAFETY AND STIMULATION EFFICACYIt  is  now  generally  accepted  that  the  benefit  of “balancing” a stimulation pulse is not to reverse all reactions like  for  like,  but  to  maintain  the  electrode-electrolyte potential within a safe range, whilst maximising the charges that can be delivered safely during the cathodic phase.  An imbalance  between  phases,  charging,  or  discharging,  the double  layer  capacitor,  may  allow  the  potential  to  “slide back” to a more favourable level [7, 8].In the context  of this paper,  the stimulation efficacy  is related to the ability of a cathodic pulse to elicit  a desired response.  Whilst  electrodes  are  characterised  in  terms  of charge capacity, the strength-duration curve reminds us that amplitude and duration must both be above a threshold for efficacious  stimulation.  For  the  electronics  engineer designing the stimulator,  the waveform should be described not only in terms of charge per phase,  but  also acceptable range of amplitude and duration. Owing to the potential for cathode-break and anodal excitation, as well as the inhibitory effect  of  depolarising  pulses,  this  requirement  applies  not only to the cathodic phase, but also to the reversed pulse. III. ELECTRONICS SAFETY, COMPLEXITY AND BLOCKING CAPACITORCircuits that produce current waveforms with controlled charges per phase are more complex (and larger) than those that  deliver  monophasic  voltage  pulses.  Increasing  design complexity,  and  the  desire  for  smaller  implants,  increases likelihood  of  failure,  because  with  increasing  component density on a substrate, every connection is at higher risk of failure. Application Specific Integrated Circuits (ASICs) may be used to integrate more functions on a single component, the component density is more favourable,  but routing and interconnections are not, and a single fault failure can lead to overload and rapid propagation of faults throughout.Electronics  engineers  have long relied on an additional “blocking” capacitor in series with the electrodes, to block, or considerably limit, the passage of direct current through the electrodes. These enable simpler stimulators to achieve well balanced  passive  discharge,  but  increase  the circuit’s  size. They may also provide safety under single fault conditions, but only for some output stage designs [9]. IV. CONCLUSIONThanks  to  remarkable  advances  in  the  field,  it  is becoming  possible  to  define  electrode-specific  safe stimulation parameters, unconditional on exact knowledge of the  electrode-electrolyte  interface  (which  is  difficult  to monitor chronically in vivo). As progress improves electrode and stimulation safety, an understanding of their impact on the electronics design will be needed to reach compromises between “ideal” electrical stimulation waveforms and implant circuit complexity, to optimise overall device safety.REFERENCES[1] J.C.  Lilly  et  al,  “Brief  non-injurious  electric  waveforms  for stimulation of the brain”, Science, 121, 1955, pp 468-69[2] R.V.  Shannon  "A  model  of  safe  levels  for  electrical  stimulation." IEEE Transactions on biomedical engineering 39.4 1992 pp 424-26.[3] D.R. Merrill et al Electrical stimulation: design of efficacious and safe protocols. J Neuroscience Methods 141 2005 pp 171–98[4] S.F.  Cogan  et  al.  "Tissue  damage  thresholds  during  therapeutic electrical stimulation."J of neural engineering13.2, 2016, 021001.[5] A.  Ghazavi, & S.F. Cogan “Electrochemical characterization of high frequency  stimulation  electrodes:  Role  of  electrode  material  and stimulation  parameters  on  electrode  polarization.”  J.  Neural Engineering, 15(3), 2018, 036023.[6] J.T. Mortimer & N. Bhadra. Fundamentals of electrical stimulation. In Neuromodulation, 2018 pp. 71-82. Academic Press.[7] N  Donaldson  and  P.E.K.  Donaldson.  "When are  actively  balanced biphasic  (‘Lilly’)  stimulating  pulses  necessary  in  a  neurological prosthesis?" Medical & Biological Eng & Comp, 24, 1986, pp 41-49.[8] D.W. Kumsa et al “Electron transfer processes occurring on platinum neural stimulating electrodes: pulsing experiments for cathodic-first, charge-imbalanced,  biphasic  pulses  for  0.566  k  2.3  in  rat⩽ ⩽  subcutaneous tissues”. J Neural Engineering,16(2), 2018, 026018.[9] A. Nonclercq et al “Safety of multi-channel stimulation implants: a single blocking capacitor per channel is not sufficient after single-fault failure”. Medical & Biological Eng & Comp, 50(4), 2012, pp 403-10.Charge balancing strategies: electronics design impact on safety and electrode stability Anne Vanhoestenberghe, Dai Jiang, Senior Member, IEEE, Henry Lancashire, Thomas Niederhoffer, Student Member, IEEE and Nick Donaldson, 

Charge Balancing Strategies: Electronics Design Impact on Safety and Electrode Stability

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Charge Balancing Strategies: Electronics Design Impact on Safety and Electrode Stability

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