Biomedical Engineering Reference
In-Depth Information
order of 0.1 to 10
m. The larger end of the glass tube (the stem) is then filled typically with
a 3M KCl electrolyte solution. A short piece of Ag/AgCl wire is inserted through the stem
to provide an electrical contact with the electrolyte solution. When the tip of the microelec-
trode is inserted into an electrolyte solution, such as the intracellular cytoplasm of a
biological cell, ionic current can flow through the fluid junction at the tip of the micro-
electrode. This establishes a closed electrical circuit between the Ag/AgCl wire inside the
microelectrode and the biological cell.
A different kind of microelectrode made from a small-diameter strong metal wire (e.g.,
tungsten or stainless steel) is illustrated in Figure 10.8b. The tip of this microelectrode is
usually sharpened down to a diameter of a few micrometers by an electrochemical etching
process. The wire is then insulated up to its tip.
Solid-state microfabrication techniques commonly used in the production of integrated
circuits can be used to produce microprobes for multichannel recordings of biopotentials
or for electrical stimulation of neurons in the brain or spinal cord. An example of such a
microsensor is shown in Figure 10.8c. The probe consists of a precisely micromachined silicon
substrate with four exposed recording sites. One of the major advantages of this fabrication
technique is the ability to mass-produce very small and highly sophisticated microsensors
with highly reproducible electrical and physical properties.
m
10.3 PHYSICAL MEASUREMENTS
10.3.1 Displacement Transducers
Displacement transducers are typically used to measure physical changes in the position of
an object or medium. They are commonly employed in detecting changes in length, pressure,
or force. Variations in these parameters can be used to quantify and diagnose abnormal phys-
iological functions. In this section, we will describe inductive types of displacement transdu-
cers that can be used to measure blood pressure, electromagnetic transducers to measure
blood flow, potentiometer transducers to measure linear or angular changes in position,
and other types of elastic, strain gauge, capacitive, and piezoelectric type transducers.
Inductive Displacement Transducers
Inductive displacement transducers are based on the inductance
L
of a coil given by
2
L
¼
m
n
l
A
ð
10
:
1
Þ
where m is the permeability of the magnetically susceptible medium inside the coil (in henry
per meter),
n
is the number of coil turns (in turns per meter),
l
is the coil length (in meters),
and
is the cross-sectional area of the coil (in square meters). These types of transducers
measure displacement by changing either the self-inductance of a single coil or the mutual
inductance coupling between two or more stationary coils, typically by the displacement of
a ferrite or iron core in the bore of the coil assembly. A widely used inductive displacement
transducer is the linear variable differential transformer (LVDT) shown in Figure 10.9.
This device is essentially a three-coil mutual inductance transducer that is composed of a
primary coil (P) and two secondary coils (S
1
and S
2
) connected in series but opposite in
A
