One of the majorcriteria dominating the choice of materials used for coating magneticnanoparticles is that they should enable formation of conjugates with any givenbiological entity (Barakat, 2009). The role ofcoating polymers is to inhibit opsonisation, thereby permitting longercirculation time. Since they are used for biomedical purposes the coatingmaterials ought to be safe, less toxic, stable and biocompatible. Cationicexcipients for coating are favoured for entry of magnetic nanoparticles intonegatively charged cells. When polymeric materials are employed as stabilizers,the adsorption of polymers onto the MNPs confer protective steric repulsion andact as a barrier against the interaction between the particles, therebystabilizing the particles and preventing them from agglomeration. Suchprotection is most e?cient when using amphiphilic copolymers bearing ahydrophilic segment with a tendency to spread into the aqueous medium and ahydrophobic segment anchoring onto the particle surface. If the polymer chainsare charged, an additional electrostatic repulsion may occur, thus conferring acombined ionic and steric (electrosteric) stabilization e?ect.
In other words, polymeric coatings can be designedwith surface properties (chemical functionality and surface charge) on the MNPsand constitute an excellent barrier for preventing aggregation, leading to physicochemicallystable magnetic nanoformulations. Regarding, the vascular administrationintravenous (i.v.) or intra-arterial of MNPs, the gravitational settling inthe blood ?ow can be considered as negligible, thanks to these polymer coatingsthat generally decrease the mean density of the magnetic cores. Afterparticles are injected into the blood stream they are rapidly coated bycomponents such as plasma proteins, by a mechanism known as Opsonization andthis formation will dictate the circumstances of injected particles.
Howeverthe macrophages cells of RES will uptake these opsonised particles. Hence toavoid this, surface coatings with biodegradable polymers or non biodegradableorganic and inorganic coatings are used to retard the uptake by macrophagescells of liver, spleen etc to name few. Keycharacteristics of the polymer such as length and molecular weight, chemicalstructure (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,2009).Co-polymerization with polymeric acidic amino acids such as poly (L-glutamate)or poly (L-aspartate) provide additional functional groups to interact and formconjugates with the biological entity.
Two most commonly used coating techniquesare discussed below:Emulsion polymerization method: In thistechnique polymer coating is carried at a high surfactant to monomer ratio in agiven microemulsion system. Magnetite nanoparticles are prepared by co-precipitationand 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 withhigh surfactant to monomer ratio conducted in microemulsion system. The sodium dodecyl sulphate actsas surfactant and potassium persulphate as initiator (Sayar, Güven, & Pi?kin, 2006; Businova,Chomoucka, Prasek, Hrdy, Drbohlavova,Sedlacek & Hubalek, 2011).Co-precipitationMethod: In this method,polyethylene glycol (PEG) is most commonly used. PEG, a linear neutralpolyether acts at the surface by providing a stealth shielding effect, delayingthe action of RES from protein resistant character of PEGylated surfaces. Thiscontributes to lower the interfacial energy in water and steric stabilizationeffect (Meng, Engbers & Feijen, 2004).
However, hydrophobic surfaceson the particles are effectively coated with plasma components and rapidlyexcreted, whereas particles that are more hydrophilic can resist this coatingprocess and are cleared slowly (Gaur, Sahoo, De Tapas. Ghosh, Maitra & Ghosh, 2000).