IEEE Solid-States Circuits Magazine - Fall 2021 - 92

attracted to the metal charges and
form a double layer. This layer of charged
particles, whose average distance from
the metal lies on the Helmholtz plane,
essentially forms a capacitance (CDL)
between the electrode and tissue.
Alternatively, a constant phase element,
which captures the imperfections
of the double layer capacitance,
can provide a more accurate model of
the impedance.
There is also a parallel charge transfer
resistance RCT in the model that
accounts for reactions and exchange
currents that result at the interface.
Both CDL and RCT are electrode material
dependent and scale with the effective
surface area of the electrode, with the
capacitance increasing and the resistance
decreasing. Exemplary values
for various materials can be found in
[33]. The best electrode metals will be
inert, low impedance, and biocompatible
and have high charge delivery
capacity; they include platinum, platinum
alloys, and iridium oxide [34].
Here, RS, which is in series with the
other elements,
the tissue itself and dependent only
on electrode geometric dimensions
and the resistivity of the tissue. A
round, flat electrode with radius a has
a spread resistance of
R =
S 4a
t
,
where t is the tissue resistivity. Any
interconnect between the stimulator
and the electrode may be modeled
in series with RS.
Figure 6 illustrates some common
monophasic [Figure 6(a)] and biphasic
[Figure 6(b)] stimulation waveforms.
While voltages can be used, current
provides a more controlled charge
delivery since it is agnostic to electrode
and tissue impedance fluctuations.
Monophasic cathodic pulses are highly
efficacious but leave residual charge
at the electrode interface, potentially
inducing corrosion and neural tissue
damage. A passive recharge phase
CDL
Body
RS
RCT
CPEDL
Body
Electrode Edge
d Helmholtz Plane
(a)
RS
RCT
(b)
FIGURE 5: Circuit models of (a) the electrode-tissue interface using (b) the capacitance (CDL) and a constant phase element (CPEDL).
Circuit
ZCAP = RS +
CPE =
1 + (jω)nQ
RCT
1 + (jω)nQRCT
1
(0 < n <1)
Circuit
ZCAP = RS +
RCT
1 + jωCDLRCT
is the resistance of
may be added after each pulse to short
the stimulation electrodes and clear
built-up charge; however, this can
induce large current spikes. Biphasic
stimulation, consisting of a cathodic
pulse followed by an anodic pulse
in the opposite direction, is used to
limit the peak reverse current and
promote balance in the total amount
of charge delivered. The current
amplitude, pulsewidth, period, and
interphase gap (which improves
efficacy) are tuned on a patient-specific
basis.
When stimulating nervous tissue,
the most important consideration is
patient safety. The Shannon criteria
[35] constitute an empirical rule for
evaluating the possibility of damage
to nervous tissue and relate the
maximum charge density per phase
(Q/A) to the maximum charge per
phase (Q) with a dimensionless constant
k:
loglog().
cm=-k
Q
A
Q
Period
Anodic
Current
Cathodic
Current
Interphase Gap
(a)
(b)
FIGURE 6: (a) A monophasic stimulation pulse train. (b) A biphasic stimulation pulse train with an interphase gap.
92
FALL 2021
IEEE SOLID-STATE CIRCUITS MAGAZINE
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