Biomedical Engineering Reference
In-Depth Information
Electrodes
Tissue culture medium
(electrolyte)
Cells
Capillary
Small gold
electrode
Counter-
electrode
R
Figure 4.1-24 The measuring capillary cell of a Coulter counter.
4.1.15 Skin instrumentation
Lock-in
amplifier
4.1.15.1 Fingerprint detection
PC
Data acquisition
and processing
Electronic fingerprint systems will in the near future
eliminate the need for keys, pin-codes and access cards in
a number of everyday products. While fingerprint recog-
nition traditionally has been used only in high security
applications, it is now gaining acceptance in mainstream
consumer applications worldwide. One such large-scale
application will be the need for securemobile transactions
when paying for the groceries with your mobile phone at
the local supermarket. Several bioimpedance based fin-
gerprint sensors have been developed, such as the elec-
trode array based sensor from the company Idex ASA
(
www.idex.no
). An array of electrodes is scanned as the
fingertip is swept over the sensor stripe, giving a 500 dpi
resolution impedance image of the fingerprint.
The fingerprint sensor market also demands systems
for detecting fake fingers on the sensor, and a bioimpe-
dance based solution for spoof detection was described
by Martinsen et al. (2007). Their system is based on the
simultaneous measurement of skin impedance at differ-
ent depths, and the use of multivariate models to classify
the fingers as living or fake.
Figure 4.1-22 Impedance motion sensing with cells on a small
gold electrode.
is a commercially available version of this system
(Applied BioPhysics Inc., Troy, New York, USA).
4.1.14.4 Coulter counter
The principle is based on letting cells in suspension pass
a narrow orifice. If a cell has different electrical proper-
ties than the liquid, the impedance of the pore will
change at each cell passage. Cell counting is possible, and
it is also possible to have information about each cells
size, form or electrical properties.
Figure 4.1-24
shows
the basic set-up of the two-electrode conductance mea-
suring cell. Typical dimensions (diameter, length) for
a capillary is 50 and 60
m
m (erythrocytes); 100 and
75
m
m (leukocytes).
4.1.15.2 Stratum corneum hydration
10
Stratum corneum hydration is essential for proper
function and appearance of the skin. The moisture con-
tent can be measured in vitro by means of gravimetry or
electron microscopy, or by magnetic resonance tech-
niques in vivo. The resolution of the latter technique is,
however, currently not sufficiently high to enable isolated
measurements on the stratum corneum. Compared to
these techniques, assessment of stratum corneum hy-
dration by means of electrical measurements would
represent a tremendous reduction in instrumental cost
and complexity.
A prerequisite for using electrical measurements in
this way is of course a detailed knowledge of how the
different parts of the skin influence on the electrical
8
Cell motion
6
4
Cell attachment and spreading
2
0
0
2
4
6
8
Time (h)
Figure 4.1-23 Changes in electrical resistance reflecting
attachment and motion of cells on a small gold electrode.
