Biomedical Engineering Reference
In-Depth Information
magnetite formation time [93] coinciding with a slower iron uptake. Conversely,
standard media and available iron show mineralization to occur within 30 min [53,
91], with rapid iron uptake occurring within this time frame [53, 91]. Interestingly,
it seems that magnetosome formation is dependent on, and occurs concomitantly
with, iron uptake, and that a faster iron uptake occurs when the cells have been
starved of iron. However, a full explanation of the factors that affect the speed of
iron uptake and magnetite formation has not yet been provided.
11.5
Progress and Applications of Novel Biomedical Magnetosome Materials
When considering the types and specifi cations of nanomagnets used for biomedi-
cal applications, it is clear that magnetosomes represent ideal starting materials
for biomedical nanomagnets. These magnetosomes composed of magnetite that
is ferrimagnetic up to high temperatures, is nontoxic, and also stable. The particles
are ideally sized (30-100 nm) for many applications, with highly regulated size and
morphology providing a uniform, consistent, and well-defi ned magnetic signal.
Moreover - and most signifi cantly - magnetosomes have an integral 4 nm - thick
lipid coating that makes them ideal for biomedical functionalization and ensures
that they will not aggregate. As these materials are synthesized biologically, active
biosubstrates can be attached and integrated into the lipid membrane by elegantly
using genetic manipulations of the magnetosomes. All of these advantages show
huge potential for magnetosomes in biomedical applications, and several signifi -
cant systems to modify magnetosomes
in vitro
are currently under development,
mainly by Matsunaga and coworkers [3]. Magnetosome-based systems are being
developed by attaching proteins (e.g., luminescent proteins and antibodies) and
DNA, for applications such as immunoassays, cell separation, labelling (e.g., bio-
marker detection) and drug screening, by utilizing several different functionaliza-
tion strategies to attach bioactive substrates. One method of attaching proteins to
magnetosomes is via a crosslinker reaction, where the lipid's surface amine groups
bind aldehydes or esters that in turn link to a range of proteins. This has been
successful for the display of antibodies and the immobilization of streptavidin, by
using biotin- modifi ed magnetosomes [94].
Another more elegant method of protein attachment uses genetic engineering
to express fusion proteins on the magnetosomes
in vivo
; the fusion proteins consist
of a native magnetosome membrane protein as an anchor, attached to a functional
protein. For this method, the choice of anchor protein is very important for the
stability and coverage of the active protein. MagA was the original choice of anchor
protein, but was found to be too large and hydrophobic to accommodate other
bulky membrane fusion proteins. Thus, the smaller Mms16 protein was used and
found to show a greater expression on the magnetosome, especially for large
transmembrane receptors. However, the stability and density of attachment was
seen to be far more effective when utilizing Mms13 as the anchor protein [95].
Mms13 is not only very small, providing next to no steric hindrance and increased
