IEEE Solid-States Circuits Magazine - Fall 2020 - 37

Electrochemical Transcutaneous
Electrodes
Transcutaneous sensors have been a
mainstay in the medical field since
the 1970s. Severinghaus invented the
first stable electrode for measuring
CO2 in the blood in 1953, improving on Richard Stow's electrode [27].
Lelend Clark introduced the platinum (Pt)-based O2-sensing electrode
in 1956 [14]. It has been used with
good success, especially with neonate
patients and in wound treatment.
Severinghaus would later improve
on that design and combine it with
his CO2 sensors. In 1993, Larsen and
Linnet published findings on a solidstate CO2 electrode that was more stable and reliable than previous designs
[40]. Electrochemical electrodes are
still the standard for transcutaneous measurements today. However,
newer optical methods may begin to
replace the 60-year-old technology
[17], [41]. An example cross section
of an electrochemical transcutaneous sensor is illustrated in Figure 3.
The basic concept for the electrochemical transcutaneous CO2 electrode is as follows. The CO2 diffuses
from the tissue through a CO2-permeable membrane to an electrolyte-covered glass electrode. The CO2 reacts
with the water in the solution, forming
carbonic acid and changing the electrolyte solution's pH. The change in
the pH of the solution is related to the
concentration of CO2, which causes
a potential difference to develop
between the glass electrode and the

reference silver (Ag)/Ag chloride electrode. The potential difference caused
by the pH can be related to the CO2
concentration with the Henderson-
Hasselbalch equation [42].
Historically, transcutaneous O2
has been measured using Clark electrodes. During the 1950s, concerns
grew over O2-related blindness in
premature infants. If premature infants
receive excessive O2, the underdeveloped blood vessels in their eyes
could be damaged, and the resulting scar tissue can cause vision

loss. This is known as retinopathy
of prematurity. Transcutaneous O2
electrodes, such as those developed
by Clark [14] and later improved
by Severinghaus [15], enable the
accurate measurement of O2 without having to take an arterial blood
sample from infants.
The Clark-style O2 electrode works
on the reduction reaction on a platinum cathode coupled with oxidation on an AG anode. The reduction
reaction generates a current that is
proportional to the concentration of

100
2.3 DPG
Temperature
PtcCO2
pH
FHbF
FCOHb
FMetHb

90
% Oxygen Saturation (SO2)

surrogate measurement. It should be
noted that oxygenation is not ventilation. In many cases, patients have a
reasonable O2 saturation level but still
have a breathing problem due to high
CO2 levels. These high CO2 levels can
be caused by underlying conditions
that prevent the patient from exhaling
correctly. Therefore, in current practice, the invasive blood gas measurement has not been entirely replaced.
Considering this need, we claim that
noninvasive multimode blood gas monitoring solutions should be explored
[22], [38], [39].

80
70
60
50
40

2.3 DPG
Temperature
PtcCO2
pH

30
20
10
0

10 20 30 40 50 60 70 80 90 100
Partial Pressure of Oxygen (PtcO2) (mmHg)

FIGURE 2: The relationship between blood saturation and blood partial pressure. Parameters that can shift this relationship are highlighted. Adapted from [37]. mmHG: millimeters
of mercury; DPG: disphosphoglycerate; HbF: fetal hemoglobin; COHb: carboxyhemoglobin;
MetHb: methemoglobin.

Ag/AgCl Reference Electrode
Electronics

Pt Electrode (O2) or
pH Electrode (CO2)

Housing
Heater

Adhesive

Membrane
42-45° C
Blood Gas Diffusion

Epidermis
Derma

Subcutaneous Tissue
FIGURE 3: An electrochemical transcutaneous gas sensor. Ag/AgCl: silver/silver chloride; Pt: platinum.

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IEEE Solid-States Circuits Magazine - Fall 2020

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