Biomedical Engineering Reference
In-Depth Information
2.1.2
Better Targeting of MNPs
The separation of tumor cells from healthy cells is a vital problem in tumor
hyperthermia. One must protect healthy cells from hurt to the greatest extent during
raising temperature of the tumor cells. MNPs show a feasibility to fulfill these
needs through combination with tumor cells. Firstly, they have controllable sizes
which are smaller than or comparable to a cell, so they can enter the human body
and get close to the area of interest. Nanoparticles must pass capillary endothelium
before reaching targeted location. The majority of nanoparticles with size bigger
than 7 mm are obstructed by pulmonary capillary. The liver and spleen uptake
remaining particles with diameters above 100 nm size, while those smaller than
100 nm are swallowed by osteoclast. These features allow nanoparticles to reach
corresponding organ by adjusting their size. This is termed by passive targeting.
Secondly and importantly, they can be coated with biological molecules, such as
dextran, PVA and phospholipids, to make them to bind to or interact with the tumor
cells so that providing a controllable means of tagging them. Besides, the MNPs
can be controlled by the external magnetic field gradient to gather at the tumor
which can result in interaction with target tissues as soon as possible.
2.1.3
Heat Generation Mechanism
In this part, we will discuss how the MNPs convert other forms of energy into thermal
energy.
Electromagnetic fields E and H can affect heating of tissues. The electrocaloric
effect is caused by electric field component E, based on the Maxwell equations and
thermodynamic relations. The magnetocaloric effect is caused by variations in H.
These effects are mainly determined by the tissue dielectric and magnetic properties
of substances, respectively. Because magnetism of biological objects is negligibly
small, biologically compatible nontoxic MNPs (based on magnetite and so on) are
used to strengthen the influence of an external magnetic field.
Hysteresis, eddy current, Neel paramagnetic switching, and friction from
Brownian rotation are the four possible mechanisms to generate heat for magnetic
materials exposed to an alternating magnetic field (Rosensweig
2002
; Hergt et al.
2006
). It is quite possible that all four mechanisms may contribute to the total heat
generated by a particular magnetic sample in an alternating magnetic field. But it is
expected that only one or two of the mechanisms will dominate. This is determined
by the properties of the magnetic material, its environment and the magnetic field.
Considering a spherical particle of diameter
D
and resistivity
ρ
Ω
in the presence
of a uniform periodic magnetic field of strength
=
BB w
, where
B
is the field
amplitude and
w
is the frequency of the field (in radians per second), the power of
eddy current heating is
sin
p
2
(
p
fB D
)
P
0
(3)
P
=
E
20
r
W
where,
º /2
f wp
(in cycles per second).
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