Biomedical Engineering Reference
In-Depth Information
2.
The equation of the linear regression line describing the input-output calibration data can be
written as
Reading
¼
a
ð
Reference temperature
Þþ
b
where,
a
¼
is the slope and
b
is the y-intercept of the regression line. Accordingly, the offset
) are equal to 10.87 mV and 0.84 mV/
C, respectively.
3.
From the equation of the linear regression line, the average sensitivity for readings between
70
C to 100
C is 0.17 mV/
C.
(
b
) and sensitivity (
a
10.2 BIOPOTENTIAL MEASUREMENTS
Biopotential measurements are made using different kinds of specialized electrodes. The
function of these recording electrodes is to couple the ionic potentials generated inside the
body to an electronic instrument. Biopotential electrodes are classified either as noninvasive
(skin surface) or invasive (e.g., microelectrodes or wire electrodes).
Biopotential measurements must be carried out using high-quality electrodes to minimize
motion artifacts and ensure that the measured signal is accurate, stable, and undistorted.
Body fluids are very corrosive to metals, so not all metals are acceptable for biopotential
sensing. Furthermore, some materials are toxic to living tissues. For implantable applications,
we typically use relatively strong metal electrodes made, for example, from stainless steel or
noble materials such as gold, or from various alloys such as platinum-tungsten, platinum-
iridium, titanium-nitride, or iridium-oxide. These electrodes do not react chemically with
tissue electrolytes and therefore minimize tissue toxicity. Unfortunately, they give rise to
large interface impedances and unstable potentials. External monitoring electrodes can use
nonnoble materials such as silver with lesser concerns of biocompatibility, but they must
address the large skin interface impedance and the unstable biopotential. Other considera-
tions in the design and selection of biopotential electrodes are cost, shelf life, and mechanical
characteristics.
10.2.1 The Electrolyte/Metal Electrode Interface
When a metal is placed in an electrolyte (i.e., an ionizable) solution, a charge distribution
is created next to the metal/electrolyte interface, as illustrated in Figure 10.5. This localized
charge distribution causes an electric potential, called a half-cell potential, to be developed
across the interface between the metal and the electrolyte solution.
The half-cell potentials of several important metals are listed in Table 10.2. Note that the
hydrogen electrode is considered to be the standard electrode against which the half-cell
potentials of other metal electrodes are measured.
