Selection of our books indexed in the Book Citation Index in Web of Science™ Core Collection (bkci)

Class of sensor
Biomedical sensor
Physical sensors
Geometric
Mechanical
Thermal
Hydraulic
Electric
Optical
Chemical sensors
Gas
Electrochemical
Photometric
Other physical chemical methods
Biopotential electrodes
Body surface biopotential electrode
Metal plate
Intracavitary and intratissue electrode
Microelectrode
Bioanalytic (or biosensor)
Enzyme
Protein
Antigen
Antibody
Ligand
Cell
DNA
Table 1. Classifications of biomedical sensor
Advances in Bioengineering
184

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Although bioanalytic sensors are essentially chemical sensors, they are often classified as a
separate major sensor category. These devices incorporate biologic recognition reaction such
as enzyme-substrate to identify complex biochemical molecules. The use of biologic reactions
gives bioanalytic sensors high sensitivity and specificity in identifying and quantifying
biochemical substances.
2.2. Oxygen and carbon dioxide sensor for blood
Measurements of arterial blood gas (pO
2
and pCO
2
) and pH are frequently performed by on
critical patients in both the operating rooms and intensive care unit. They are selected and used
by the physician to adjust mechanical ventilation or administer pharmacological agents. Such
measurement could provide information about the respiratory and metabolic imbalance in the
body and reflect the change of blood oxygen increment and carbon dioxide(CO
2
) elimination.
Noninvasive sensors for measuring O
2
and CO
2
in arterial blood are based on the discovery
that gases such as O
2
and CO
2
can easily diffuse from body skin. Diffusion occurs due to a
partial pressure difference between the blood in the superficial layers of the skin and the
outermost surface of the skin. Such idea has been used to develop two types of noninvasive
electrochemical sensors pO
2
and pCO
2.
The discovery that blood changes its color depending
on the percent of oxygen has led to the development of several optical methods to measure
the oxygen saturation in blood.
2.2.1. Oxygen sensor for blood
The method for measuring blood oxygenation is very great important in assessing the
circulatory and respiratory condition of a patient. The blood from the lungs to the tissues in
two distinct states transports oxygen. Under normal physiological conditions, approximately
2% of the total amount of oxygen carried by the blood is dissolved in the plasma. This amount
is proportional to the blood pO
2.
. The 98% remain is carried inside the erythrocytes in a loose
reversible chemical combination with hemoglobin (Hb) as oxyhemoglobin (HbO
2
). Thus, there
are two methods for measuring the blood oxygenation: either using polarographic pO
2
sensor
or measuring oxygen saturation (the relative amount of hemoglobin dioxide HbO
2
in the
blood) by means of an optical oximeter.
A pO
2
sensor, also widely known as a Clark electrode, is used to measure the partial pressure
of O
2
gas in a sample of air or blood. This sensor is categorized as an amperometric sensor and
requires an external polarization bias source. The measurement is based on the principle of
polarography as illustrated in figure 4. The electrode utilizes the ability of oxygen O
2
molecules
to react chemically with H
2
O in the presence of electrons to produce hydroxyl (OH
-
) ions. This
electrochemical reaction, called an oxidation/reduction or redox reaction, generates a small
current and requires an externally applied constant polarizing voltage source of about 0.6V.
Oxygen is reduced (consumed) at the surface of a noble metal (such as platinum or gold)
cathode (this electrode is connected to the negative side of voltage source) according to the
following the chemical reaction:
Biomedical Sensor, Device and Measurement Systems
http://dx.doi.org/10.5772/59941
185

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O
2
+ 2H
2
O + 4e ↔4OH
−
In this reduction reaction, one O
2
molecule takes four electrons and reacts with two water
molecules, generating four hydroxyl ions. The resulting OH
-
ions migrate and react with a
reference Ag/AgCl anode (this electrode is connected to the positive side of voltage source),
causing a two-step oxidation reaction as follows:
Ag ↔ Ag
+
+ e
Ag
+
+ Cl
−
↔ AgCl
↓