IEEE Solid-States Circuits Magazine - Fall 2020 - 35

In conclusion, remote and continuous monitoring of blood gas content
with a miniaturized, noninvasive, and
comfortable device is essential, not
only for known respiratory diseases,
including COPD and sleep apnea in
adults and babies, but for uprising
diseases such as COVID-19. Moreover,
offering personalized medicine for
patients experiencing the same disease
in various conditions would be possible with remote and continuous monitoring of blood gas content. In this
article, we cover emerging blood gas
monitoring devices and techniques to
create the sensors and IC structures to
transform bulky benchtop instruments
into miniaturized, next-generation biomedical devices. We also explore currently available technologies, with a
short glance back to the evolution of
blood gas sensing systems.

State-of-the-Art Blood Gas
Monitoring Sensors
Today, many smartwatches and fitness trackers come with sensors to
measure physiological parameters and
activity, such as heart rate and movement, and support software to track
exercise and diet [18], [23]. Hospitals
are starting to use wearables that wirelessly transmit data to a base station
in a ward to monitor patients' postsurgery conditions [24], [25]. Researchers are looking for ways to leverage
machine learning and artificial intelligence to make these tools " smarter. "
These advancements have demonstrated exceptional promise in providing an infrastructure for performing
medicine in a futuristic way, where
health-care providers and patients are
better connected and informed.
However, amid this transformation, there has been little progress
in miniaturizing respiration devices
to a suitable form factor for longterm wear and comfort. Research and
development of wearables has heavily
focused on pulse oximeters and electrocardiograms (ECGs) [20], [26]. The
respiration rate and blood O2 saturation, well-known respiration parameters, are only a subset of respiration
parameters. Beyond that, there are

A miniaturized blood gas monitor attached
to a patient's body can help with monitoring
individuals in their home.
more parameters, such as blood O2
and CO2 partial pressures, which
have medical significance and the
potential for being measured noninvasively from the body.
Currently, in the hospital setting,
respiration parameters are observed
using bedside instruments with
wired probes attached to the patient.
The invention of these instruments
follows a pretty impressive history
that begins just before World War
II and includes rapid technological
development after 1970. Figure 1
offers a snapshot of the evolution of
respiration monitoring technologies
from 1956 to 2020.
Before the late 1950s, blood gas
analysis was performed using blood
samples drawn from an arterial or a
venous line. Arterial blood gas monitoring remains the gold standard for
respiration assessment. During the
late 1950s, inventors John W. Severinghaus, A. Freeman Bradley, and Leland
Clark developed electrodes that would
enable the revolutionary electrochemical transcutaneous sensors still used
today [27]. During the early 1970s,
Takou Aoyagi and Michio Kishi created the first practical pulse oximeter,
under the Nihon Kohden Company.
They devised a mathematical formula
that distinguishes O2 absorption data
from pulse information. This way, pulsatile noise is removed [16]. Through
the next two decades, noninvasive res--
piration monitors became staples in
hospital wards around the world. In
particular, pulse oximetry began to
dominate through its relative simplicity and ease of use.
Pulse oximeters are some of the
most widely used medical sensors and
are becoming commonplace in wearable fitness trackers [18], [23]. During
the 2010s, companies including FitBit
and Apple released high-quality health
and fitness trackers to the consumer
market. These products generated a

desire for smart and connected medical tools and for giving individuals more control over their well-being.
During the past couple of years, universities such as the University of California, Berkley, Worcester Polytechnic
Institute, and Northwestern University
developed new and exciting respiration monitoring devices [20]-[22].
This research has been driven by
concerns about effective remote and
continuous monitoring and patient
comfort. Improving the quality of
remote care has increased demand
for hospital-grade wearable medical devices. These new instruments
would enable medical professionals
to safely monitor patients from individuals' homes, reducing long and
expensive hospital stays and potentially speeding up recovery. In this
section, we briefly discuss the various types of state-of-the-art respiration probes and electrodes that are
in use in the health-care industry.

Pulse Oximeters:
Photoplethysmography
Photoplethysmography (PPG) is an optical method to measure the change
in blood volume in a capillary bed. A
pulse oximeter is an instrument that
illuminates the skin through LEDs,
typically placed on a fingertip or an
ear lobe, and it captures the transmitted and reflected light with a photodetector (PD). It measures changes in
light absorption by the tissue. The
difference in light intensity is related
to the change in the blood volume to
the capillaries in the dermis and subcutaneous tissue. Two wavelengths
of light, typically red and infrared,
are harnessed due to the different
absorption spectra for oxygenated
and deoxygenated hemoglobin. Some
pulse oximeters use green or blue
light instead of infrared [28]. The
ratio of these two waveforms is used
to infer the percentage of hemoglobin

IEEE SOLID-STATE CIRCUITS MAGAZINE

FA L L 2 0 2 0

IEEE Solid-States Circuits Magazine - Fall 2020

Table of Contents for the Digital Edition of IEEE Solid-States Circuits Magazine - Fall 2020