Biomedical Engineering Reference
In-Depth Information
Because both ferromagnetic and ferrimagnetic materials depend on their
domain structure in order to remain magnetic in the absence of an applied fi eld,
their properties undergo an important change when their dimensions are decreased
to less than domain size. Particles of this size are said to be superparamagnetic
because, although their dipoles line up parallel to an applied magnetic fi eld, the
ambient thermal energy is suffi cient to spontaneously disorganize the direction
of their magnetization when the fi eld is no longer applied. This is important for
biotechnology applications, because it would not be possible to resuspend mag-
netic particles in solution following a magnetic separation due to mutual attraction
if they remained magnetic. The force (magnetic moment) experienced by a particle
depends on the strength of the applied fi eld and the size and composition of the
particle. Because of their small sizes, individual superparamagnetic nanoparticles
(Figure 2.2a) respond relatively slowly to an applied magnetic fi eld. The makers
of large magnetic microspheres overcome this problem by dispersing large
numbers of superparamagnetic nanoparticles in an organic or silica matrix, as
shown in Figure 2.2b. The core of the particle shown is packed with 17% (by
weight) of
7.5 nm diameter magnetite nanoparticles. Because the nanoparticles
are present in such large numbers, the microspheres respond rapidly to an applied
fi eld, but because each individual nanoparticle is so small the ambient thermal
energy is suffi cient to disorganize their dipoles as soon as the applied fi eld is
removed. Magnetic microspheres are available from companies such as Dynal
(now part of Invitrogen), Bangs Laboratories, Micromod, Seradyne, Polysciences
and Estapor, in sizes ranging from 0.3 to 25
∼
m, and with a wide range of surface
chemistries for easy conjugation to biological and other molecules. Magnetic
nanoparticles are available from Micromod and Microspheres- Nanospheres;
however, the development of new particles and surface chemistries is at present
a highly active area of research [8].
μ
(a)
(b)
10 nm
1
m
m
Figure 2.2
(a) Transmission electron
microscopy (TEM) image of water- soluble
superparamagnetic colbalt nanoparticles.
Reproduced from Ref. [7] (http://dx.doi.
org/10.1039/b713528a ), with permission from
The Royal Society of Chemistry; (b) TEM
image of a thin section through a magnetic
microsphere; the lighter areas correspond to
the polymer matrix, and the dark specks are
superparamagnetic iron oxide nanoparticles.
