One on the MNPs and constitute an excellent barrier

One of the major
criteria dominating the choice of materials used for coating magnetic
nanoparticles is that they should enable formation of conjugates with any given
biological entity (Barakat, 2009). The role of
coating polymers is to inhibit opsonisation, thereby permitting longer
circulation time. Since they are used for biomedical purposes the coating
materials ought to be safe, less toxic, stable and biocompatible. Cationic
excipients for coating are favoured for entry of magnetic nanoparticles into
negatively charged cells. When polymeric materials are employed as stabilizers,
the adsorption of polymers onto the MNPs confer protective steric repulsion and
act as a barrier against the interaction between the particles, thereby
stabilizing the particles and preventing them from agglomeration. Such
protection is most e?cient when using amphiphilic copolymers bearing a
hydrophilic segment with a tendency to spread into the aqueous medium and a
hydrophobic segment anchoring onto the particle surface. If the polymer chains
are charged, an additional electrostatic repulsion may occur, thus conferring a
combined ionic and steric (electrosteric) stabilization e?ect.  In other words, polymeric coatings can be designed
with surface properties (chemical functionality and surface charge) on the MNPs
and constitute an excellent barrier for preventing aggregation, leading to physicochemically
stable magnetic nanoformulations. Regarding, the vascular administration
intravenous (i.v.) or intra-arterial of MNPs, the gravitational settling in
the blood ?ow can be considered as negligible, thanks to these polymer coatings
that generally decrease the mean density of the magnetic cores. After
particles are injected into the blood stream they are rapidly coated by
components such as plasma proteins, by a mechanism known as Opsonization and
this formation will dictate the circumstances of injected particles. However
the macrophages cells of RES will uptake these opsonised particles. Hence to
avoid this, surface coatings with biodegradable polymers or non biodegradable
organic and inorganic coatings are used to retard the uptake by macrophages
cells of liver, spleen etc to name few.  Key
characteristics of the polymer such as length and molecular weight, chemical
structure (biodegradability and hydrophobic/hydrophilic character), conformation,
degree of surface coverage, and attachment mechanism to the particle surface
(covalent, hydrophobic, or ionic binding) must be taken into consideration.
Common coatings used on magnetic nanoparticles are derivatives of dextran,
Polyethylene glycol, polyethyleneime, poloxamers, polyoxamines                   (Barakat,
Co-polymerization with polymeric acidic amino acids such as poly (L-glutamate)
or poly (L-aspartate) provide additional functional groups to interact and form
conjugates with the biological entity. Two most commonly used coating techniques
are discussed below:

Emulsion polymerization method: In this
technique polymer coating is carried at a high surfactant to monomer ratio in a
given microemulsion system. Magnetite nanoparticles are prepared by co-precipitation
and further coated these nanoparticles using emulsion polymerization method.
Here methyl methacrylate
(MMA) and acrylic acid are used as comonomers with a weight ratio of 9/1 with
high surfactant to monomer ratio conducted in microemulsion system. The sodium dodecyl sulphate acts
as surfactant and potassium persulphate as initiator (Sayar, Güven, & Pi?kin, 2006; Businova,
Chomoucka,  Prasek, Hrdy, Drbohlavova,
Sedlacek & Hubalek, 2011).

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Method: In this method,
polyethylene glycol (PEG) is most commonly used. PEG, a linear neutral
polyether acts at the surface by providing a stealth shielding effect, delaying
the action of RES from protein resistant character of PEGylated surfaces. This
contributes to lower the interfacial energy in water and steric stabilization
effect (Meng, Engbers & Feijen, 2004).  However, hydrophobic surfaces
on the particles are effectively coated with plasma components and rapidly
excreted, whereas particles that are more hydrophilic can resist this coating
process and are cleared slowly (Gaur, Sahoo, De Tapas. Ghosh, Maitra & Ghosh, 2000).