IEEE Solid-States Circuits Magazine - Fall 2020 - 38

PtcO2 (mmHg)

150

100

50

50

100
PaO2 (mmHg)

150

200

FIGURE 4:  The relationship between the PtcO2 and PaO2. Adapted from [43].

PD
LED

LED
Luminescent Sensing Film

Epidermis
Blood Gas Diffusion
Derma

Subcutaneous Tissue
(a)

0.1

More O2
450
650
Wavelength (nm)
(b)

2

cO
Pt
g

0.06

in

Less O2

ea
s

Emission
Spectrum

0.08

0.04

In
cr

Exciting
Spectrum

Intensity (a.u.)

Intensity

0.12

0

40 mmHg
120 mmHg

0.02
0

10

20
30
Time (µs)

40

50

(c)

FIGURE 5:  (a) A cross section of a luminescent O2 sensor. (b) A Stokes shift of luminescent
O2 sensing dye. (c) The luminescent lifetime of the sensor versus the O2 concentration.
a.u.: arbitrary unit.

38	

FA L L 2 0 2 0	

IEEE SOLID-STATE CIRCUITS MAGAZINE	

O2. Clark electrodes measure the
transcutaneous partial pressure of O2
(PtcO2). The PtcO2 is linearly related
to the arterial partial pressure of O2
(PaO2) at an increased temperature
of ~44° C, typically with a small dc
offset due to O2 consumed by the tissue and other metabolic processes
[43], as exhibited in Figure 4. Other
factors can impact the relationship
between the PtcO2 and PaO2, such as
the temperature to which the skin is
heated. A higher temperature brings
the partial pressure of O2 in the tissue
closer to the arterial tension; however,
it also increases O2 consumption by
cells in the tissue.
Electrochemical sensors have a lag
time between a change in the arterial
tension and the value measured at
the sensor. The lag is approximately
15-30 s, with transcutaneous CO2
diffusing faster than transcutaneous O2 [44]. Electrochemical sensors
also suffer long stabilization periods
(minutes) and drift; therefore, they
require regular recalibration [8], [41],
[44]. The accuracy and measurement
times of electrochemical transcutaneous monitors can be im--
proved
through local heating with temperatures between 42 and 45° C. However, this heating creates local hot
spots, which can potentially damage a
patient's skin.

Photophysical O2 Probes

Luminescent O2 sensors for transcutaneous sensing have been explored
since the 1980s. However, they fell
out of fashion with the popularity and success of pulse oximetry.
Recently, luminescent O2 sensors
have begun reemerging as a compact
way to monitor transcutaneous O2,
which will help overcome some of
the limitations of using pulse oximetry alone [22], [38], [39], [41]. Luminescent O2 sensors are also finding
use in wound treatment to assess tissue oxygenation [21].
Photophysical O2 sensors rely on
luminescence to detect O2. The sensor consists of luminescent dyes suspended in a polymer matrix and is
excited with an LED; the response is



IEEE Solid-States Circuits Magazine - Fall 2020

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

Contents
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