Biomedical Engineering Reference
In-Depth Information
size, magnetic and coating characteristics. It is therefore crucial that the synthetic
method chosen can achieve a uniform formation of the required particles, hope-
fully at low cost.
11.2.2.2
Magnetic Properties
The required magnetic properties of particles depend on the intended application.
Some applications require a weak magnetic moment, whereas others need an
increased magnetic moment. Many applications will require a material that is
magnetically “soft” (i.e., the magnetization is easily reversed, and lost when a fi eld
is reversed or removed), while some applications require a hard magnet (i.e., the
magnetization is retained when the fi eld is reversed or removed). For example,
MRI enhancers typically need to be soft magnets, whereas potential hysteric hyper-
thermic cancer treatments require hard magnetic properties, and magnetic- fl uid
hyperthermic treatments utilize smaller super-paramagnetic particles. The mag-
netic properties are defi ned by the composition and type of material used. All
magnetic materials are super-paramagnetic below a certain size (i.e., depending
on the material and shape, typically between 5 and 40 nm), and above this size will
obtain a bulk magnetism, becoming either ferro/ferri or antiferromagnetic. Above
a critical size, the magnetism of the material will compartmentalize into “multiple
domains” in order to stabilize the material. This is again dependent on the mate-
rial and its shape: elongated, needle-shaped particles will remain single domain
over much larger sizes and have a much larger magnetic anisotropy associated
with them than will more spherical particles. Typically, the multiple domain transi-
tion is approximately 100 nm, and so “ single - domain ” magnets can usually be
found with sizes in range of tens of nanometers.
11.2.2.3
Particle Size and Shape
The size and shape of the particles must also be considered; both factors will
infl uence a particle's magnetic characteristics, such as whether they are super-
paramagnetic or single domain. But size and shape are signifi cant for their other
effects also. For example, micrometer-sized particles are limited to traveling in the
blood vessels, while nanosized particles can penetrate the tissues and cells. Particle
shape must also be considered, as different-shaped particles interact differently
with the various biological components. For example, needle- shaped particles
may often be toxic, although a “spearing” effect can occasionally have a positive
functionality.
11.2.2.4
Particle Coatings
One of the most important design features of magnetic nanoparticles for biomedi-
cal applications is the nanomagnet's coating and medical functionalization. This
not only provides the active medicine, such as the drug, protein, selection site or
genetic material, but can also act as a barrier between the magnet and the body,
helping to preventing degradation of the particle and the possible leaching out of
any toxic particle components. Essentially, the coating must be nontoxic and bio-
compatible, as well as having the synthetic means to attach any functional biomol-
