Biomedical Engineering Reference
In-Depth Information
that are well characterized and have been investigated for biomedical application
are all
Magnetospirillum
, and all of these form cubo-octahedral magnetosomes,
with no shape variation. Equally, although the biomineralization process results
in a pure and consistent magnetite with rigid magnetic characteristics, it provides
no fl exibility by which the magnetic properties of the particles can be altered and
thus enhanced, for various purposes.
Importantly, both of these disadvantages are being addressed via research inves-
tigations. In recent years, the growth and yields of magnetic bacteria and magne-
tosomes have increased vastly, due to the optimization of large- scale fermenter
growth [46, 111] , where magnetosome yields of 6.3 mg l
− 1
of bacterial medium per
day were achieved [46]. Growth has also been increased and optimized for geneti-
cally engineered magnetic bacteria harboring fusion genes for magnetosome func-
tionalization [112]. More recent optimizations have shown dramatic increases in
cell and magnetosome yields, with cell culture densities reaching OD
565
= 7.24,
and cell yields of 2.17 g l
− 1
giving impressive magnetosome yields of 16.7 mg l
− 1
per
day [93]. Although it must be recognized that syntheses using fermenters may be
costly, these recent levels of yield make magnetosome considerably more com-
mercially viable.
Recent developments have also been made to combat the lack of fl exibility in
the magnetic characteristics of magnetosomes. Magnetite is magnetically isotropic
and soft, but becomes harder if it becomes anisotropic, which in turn endows the
material with a preferential bias so as to increase the coercivity. Whilst this can
be achieved by increasing the length/width ratio of the particles, such a change in
shape may have wider implications on their behavior and toxicity. An increased
coercivity can also be achieved by the addition a small amount of cobalt; this adds
atomic anisotropy and increases magnetic hardness, without affecting the particle
shape. Recently, the cobalt doping of magnetosomes was achieved
in vivo
for three
strains of
Magnetospirillum
, simply by growing the bacterial cells in a medium
containing specifi c concentrations of iron and cobalt [63]. These cobalt-doped
magnetosomes showed an increased magnetic coercivity, which in turn should
increase the magnetic heating power of the magnetosomes, making them a very
attractive proposition as a practical material for hyperthermic cancer treatments.
11.6
The Future for Biomedical Magnetosomes
Over the past few years, investigations into biomedical nanoscience have increased
dramatically, such that research activity in the area is now intense and pushing
forward towards the development of therapies. This, in time, should result in
additional clinical trials and the production of commercial, magnetically targeted
therapies. Although magnetosomes show superior characteristics for biomedical
applications, they have in the past been overlooked due to their low yields and
costly specialized syntheses. Yet, recent advances in growth optimization have led
to magnetosome yields being dramatically improved. Increasing magnetosome
