IEEE Solid-States Circuits Magazine - Spring 2020 - 39

VDD

el1

Vload

el2

Vload

GND

Time

Istim
RloadIstim
VSS
FIGURE 6: The biphasic-constant current stimulator, along with the equivalent model of
the electrode-tissue interface and the stimulation current and resulting voltage across the
electrode-tissue interface.

Constant
Current

Vbat

Anodic

Cathodic

dc-dc
Converter

UHF
Stimulation

A1
Time

Time

Rload Cload

1 µs

Current

Istim

Optical Stimulation Methods
Alternative methods for actuating
the nervous system are becoming
available: we highlight two in this
tutorial. The first is optogenetic
stimulation, which is a cell-typespecific stimulation method that
combines photonics and molecular
genetics [26]. This method allows
for the excitation and inhibition of

Light can be delivered to the neural
tissue by using custom-engineered
devices that combine light emission
and electrical recording, the so-called
optrodes. The construction of precise, smart, and seamlessly integ rated optrodes is an engineering
challenge that involves multidisciplinary endeavors.
Photolithography is the method of
choice for fabricating optrodes based
on its reproducibility, ease of patterning precise structures with a high
aspect ratio at submicrometer resolution, and CMOS compatibility. However, truly seamless integration of
light sources on the optrode remains
one of the most difficult challenges.
This is mainly caused by the intrinsic need to use substrates in which
direct bandgap materials (e.g., gallium

neural networks in deep brain structures at a very high spatiotemporal
resolution. Optogenetic neurostimulation fundamentally relies on emitting light at a specific wavelength
and intensity to electrically activate or inhibit genetically modified
light-gated ion channel proteins in
the neurons, the so-called opsins.
There are many families of opsins
but Channelrhodorpsin-2 typically is
the one used for the activation of the
neural cells when exposed to a band
of blue light (440-500 nm) and an
intensity threshold below 1 mW/mm2
while Halorhodopsin inhibits the
neural activity when exposed to a
band of yellow light (540-600 nm). A
more detailed review of how optogenetics can be used for neural interaction can be found in [27].

1 µs

In such a UHF pulse-based current-stimulator circuit, each stimulation phase is produced from a
sequence of current pulses injected
into the tissue at a high rate. The
amplitude of the pulses, indicated
as A in Figure 7(a), can be regulated
and sets the stimulation intensity. A
buck-boost dc-dc converter without
the output filtering capacitance is
used to generate the pulses. When
more channels are operated concurrently, the pulses generated by the
only inductor are sent to all of the
activated channels in a time-interleaved fashion [see Figure 7(b)]. This
technique avoids the use of a highvoltage supply, thereby maximizing
the power efficiency while allowing
the architecture to be scaled up to
potentially several hundreds of electrodes. Moreover, the use of a buckboost converter topology renders
the stimulator circuit single-fault tolerant and thus no longer necessitates
the use of (bulky medical-grade) coupling capacitors.

2 µs

Example of Two
Time-Interleaved,
Channels

el1
Rload1

el3

Cload1
el2

Cload2
el4

Rload2

A2
2 µs

1 µs
(a)

(b)

FIGURE 7: (a) A sketch of a biphasic pulse produced by a constant current stimulator [12] (top) and by UHF stimulation (bottom). (b) The highlevel architecture of a UHF neural stimulator with its current waveforms during multichannel operation.

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