IEEE Solid-States Circuits Magazine - Fall 2021 - 93

Intuitively, both the charge density
and charge per phase should play
important roles in safety. Charge
density captures the notion that
passing the same amount of current
through a larger-surface-area
electrode should be safer and that
the total amount of charge that is
delivered should also be limited,
independent of the electrode size.
Shannon observed that experiments
where k exceeded 1.85 resulted in
tissue damage, leading to the use of
this value as an absolute maximum.
Clinical deep brain stimulators
observe a maximum charge density
of 30 µC/cm2, a limit derived
from a more conservative k value
of 1.75. While the Shannon criteria
are widely used for stimulation in
the central and peripheral nervous
systems, a more comprehensive
set of safety metrics would capture
the effects of frequency, the duty
cycle, and the duration of exposure
and would be extended to smallarea
microelectrodes (<30 µm in
diameter) [34].
Figure 7(a) reveals the boundary
between safe and unsafe stimulation
at k 185. The relationship between
=
.
the charge and charge density at the
limit is defined by the electrode area,
with an exemplary 1-mm diameter
electrode having a limit of less than
1 µC/phase. This safe stimulation
boundary can be translated into a
relationship between the stimulation
current amplitude and pulsewidth,
whose product defines the charge in
each phase [Figure 7(b)]. Figure 7(c)
illustrates one commonly used stimulator
topology, the push-pull stimulator,
connected to the electrode model.
It provides a negative current in one
104
5
Safe
Stimulation
4
103
Tissue Damage
3
102
2
Vc = 5 V
101
1
Vc = 1 V
10-2
10-1
100
Charge (µC/Phase)
(a)
φC
Pin
+VDDH
{DC/AC/Radio
Frequency}
to DC
-VDDL
Clock
Generator
Control Logic
and Memory
φC
φshort
φA
1 mm Diameter Platinum Electrode
-VDDL
CDL
RCT
RS
(c)
427 nF
38.5 MΩ
1.75 kΩ
-IC(RS + Tstim/CDL)
(d)
FIGURE 7: (a) A safety curve relating the maximum charge per phase and charge density. (b) Exemplary maximum safe stimulation parameters
for a 1-mm diameter platinum electrode. (c) An electrode model connected to push-pull stimulator, with exemplary model values.
(d) Waveforms of the stimulation current and voltages for biphasic pulses.
IEEE SOLID-STATE CIRCUITS MAGAZINE
FALL 2021
93
IC
φC
Istim
IA
φA
Pstim Vstim
CDL
+Voffset-
Vstim
Tstim
IARS
Time
VDDH
Electrode Model
RCT
RS
Istim
QA
QC
101
102
100
200
Pulse Width (µS)
300
(b)
φA
φshort
Time
400
500
600
Tissue Damage
Charge Density (µC/cm2/Phase)
φshort
Stimulus Current (mA)
Safety Curve
Safe Stimulation
k = 1.85
1-mm Diameter
Electrode
Vc = 15 V
Vc = 10 V

IEEE Solid-States Circuits Magazine - Fall 2021

Table of Contents for the Digital Edition of IEEE Solid-States Circuits Magazine - Fall 2021

Contents
IEEE Solid-States Circuits Magazine - Fall 2021 - Cover1
IEEE Solid-States Circuits Magazine - Fall 2021 - Cover2
IEEE Solid-States Circuits Magazine - Fall 2021 - Contents
IEEE Solid-States Circuits Magazine - Fall 2021 - 2
IEEE Solid-States Circuits Magazine - Fall 2021 - 3
IEEE Solid-States Circuits Magazine - Fall 2021 - 4
IEEE Solid-States Circuits Magazine - Fall 2021 - 5
IEEE Solid-States Circuits Magazine - Fall 2021 - 6
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IEEE Solid-States Circuits Magazine - Fall 2021 - Cover3
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