Biomedical Engineering Reference
In-Depth Information
ANSWER:
B
. Bisphosphonates do not add bone mass, but instead encourage osteo-
clasts, which destroy bone, to undergo cell death and thereby preventing
bone turnover.
electrical properties of tissues
In addition to mechanical properties, tissues possess thermal and
electrical properties that arise from their composition and structural
arrangement. Thermal properties are not of great interest in orthopaedic
applications since, as yet, prosthetic devices intended for implantation
by orthopaedic surgeons are not sources of heat. The exception is the
role of heat release by polymethyl methacrylate-type bone cements dur-
ing curing. However, electrical properties have become of great interest,
both in the process of understanding naturally occurring processes of
growth, remodeling, and repair and in the application of exogenous sig-
nals to aid these processes. Thus, a brief discussion of electrical behavior
of tissues is in order.
Passive electrical
properties
Electricity, or the flow of current, is simply the transfer of charge from
one point to another. Solids, liquids, and gases conduct electricity
according to Ohm's law:
E
=
I
∙
R
where
E
is the electrical potential difference (in volts),
I
is the current (in
amperes), and
R
is the resistance (in ohms). This is the relationship used
in electrical circuits with direct current (net current flowing in one direc-
tion without reversal). It may be converted to an intrinsic expression,
suitable for consideration of tissue properties, and written thus:
Δ
V
=
i
∙ ρ
where Δ
V
is the voltage gradient at a point (in V ∙ m),
i
is the current den-
sity (in A/m
2
), and ρ is the resistivity (in ohm ∙ m). Thus, ρ is the intrinsic
electrical conduction parameter of the material, describing the density of
current that will pass a point in response to an applied voltage gradient.
Current flows in tissue primarily by the movement of ions. If two
electrodes are inserted in a portion of tissue and a voltage is placed
across them, positive ions (such as H
+
, Na
+
, K
+
, and Ca
2+
) will move
toward the cathode and be reduced, whereas negative ions (such as OH
−
and Cl
−
) will move toward the anode and be oxidized. The net effect, in
the external circuit, is that a current of electrons obtained by oxidation at
the anode will flow to the cathode. This current will be equal in magni-
tude to the current, produced by these ion movements, flowing between
the electrodes in the tissue.
Search WWH ::
Custom Search