IEEE Circuits and Systems Magazine - Q3 2022 - 26

The SenTiva uses changes in heart rate that associated
with seizures to detect epileptic seizures. However,
heart-rate change based detection could lead to a misidentification
of a seizure. The ANDS-300 extracts biosignals
from the vagus nerve because it is the simplest
way to record and build a closed-loop VNS. In fact, it is
difficult to distinguish a seizure signal from the weak
CAP signal from the vagus nerve located at the neck
because the vagus nerve is far from the seizure foci in
the brain. The RNS System entails high-risk surgery because
its leads need to be implanted into brain, to which
both patients and their guardians are hesitant. Thus,
these closed-loop neurostimulators have obvious shortcomings.
However, there is a demand for a closed-loop
neurostimulator that can extract brain signals with noninvasive
leads, automatically detect an epileptic seizure,
and deliver stimuli in a timely manner in response to
stimulation effects. With these points in mind, we believe
that a HybridVNS that can sense the scalp EEG via the
WER, and suppress the epileptic seizure via the IVNS is
feasible for treatment of RE.
III. Architecture of the HybridVNS
The HybridVNS comprises a WER, an IVNS, a wireless
power transmitter, and a controller (Fig. 2). The WER
records and monitors brain activity, the IVNS stimulates
the vagus nerve, the wireless power transmitter charges
the IVNS wirelessly, and the controller controls both
the WER and the IVNS.
The WER consists of an EEG recorder and a headset.
The EEG recorder includes a cabled charge management
unit, a data telemetry module, a low-power microcontroller
unit (MCU), and a 4-channel preamplifier. The cabled
charge management unit consists of a battery charger and
a rechargeable battery. The recorded multichannel EEG
signals are firstly amplified by the preamplifier and
then are fed forward to a multichannel ADC (which is
available in the MCU). Next, the digitized signals are input
to a thresholding epileptic seizure detector. Once a seizure
onset is detected, the MCU immediately sends a message
to the IVNS so that it begins to suppress the seizure. The
headset is able to accommodate two reference electrodes
(each on an ear), four recording electrodes (i.e., dry comb
electrodes with extended prongs to accommodate longer
hair, or wet gold cup electrodes with conductive paste such
as Ten20), and the WER. One example of such headset is
the Ultracortex (OpenBCI, Brooklyn, NY, USA), which is capable
of sampling up to 16 EEG channels from up to 35 different
10-20 locations [20].
The IVNS comprises a pulse generator, an electrodetissue
interface (ETI) impedance extractor, an ultra-lowpower
MCU, a temperature sensor, two stimulating electrodes,
a wireless power receiver, and a data telemetry
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IEEE CIRCUITS AND SYSTEMS MAGAZINE
module. The ETI impedance extractor is used to gives an
indication of whether the electrodes are well in contact
with the target tissue, and the electrodes and leads are
not broken. The temperature sensor monitors the tissue
temperature and alerts when there is an increment above
2 °C during either stimulation or wireless power charging.
The controller can be an application (App) on a mobile
phone or Tablet (Pad). Fig. 3 shows the design of an example
App based on the Android operating system. This App
contains four functional zones. The first zone presents the
status of the wireless (Bluetooth) connection, which indicates
the current connections among the controller, the
WER, and the IVNS. The Bluetooth connection between
two components can be switched manually or automatically.
The second zone exhibits the operating status of
the IVNS (Stimulator), such as information of the battery
residual lifetime, system standby, battery charging,
temperature, bio-impedance, and stimuli. The frequency,
width, and amplitude of the stimulating pulse can be adjusted
manually or automatically. In the IVNS, to prevent
the repeated battery denial of service (BDOS) attacks
on the pulse generator, firstly, an AES-128 algorithm (as
part of the Bluetooth communication protocol) is used to
encrypt the data. Secondly, a data verification method is
employed when two components are connected. The primary
Bluetooth link does not send data until it receives a
return message from the secondary Bluetooth link. Thirdly,
the battery charger in the wireless power receiver is
passive. It starts to charge as soon as the wireless power
transmitter works, and there are no instructions involved.
The third zone in the App demonstrates the operating
status of the WER (Recorder) and includes information
on battery residual lifetime, standby, recording, temperature,
seizure onset parameters, and seizure onset status
(Normal, Epileptic Pre-seizure, and Epileptic Seizure). The
fourth zone shows a guardian alert that sends a text message
to the patient's guardian when the onset of an epileptic
seizure is detected. The message includes both seizure
information and the current location of the patient.
IV. Hardware Implementation of the HybridVNS
With Discrete Components
The WER (as shown in Fig. 4) of the HybridVNS can be implemented
with two discrete component boards: a scalp EEG
recording board and a cabled charge management and data
telemetry board. The scalp EEG recording board comprises
a low-power MCU (i.e., MSP430FR5969, TI, which
has up to 16 channels ADC and each channel has a resolution
of 12 bit), a DC impedance extractor, and a 4-channel
preamplifier. The cabled charge management board
consists of a battery charger or a power management IC
(i.e., BQ24232, TI), a 3.3 V low dropout (LDO) regulator
(i.e., NCP583, ON Semiconductor), and a data telemetry
THIRD QUARTER 2022
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