The minimum coating layer thickness required to guarantee efficient particle
coating depends on a number of parameters including the potential absorbable proteins and the nanocarrier size . Studies have shown that a minimum effective hydrodynamic layer thickness is about 5% of the particle diameter . Moghimi et al. demonstrated that efficient protection Inhibitors,research,lifescience,medical of 60–200nm polystyrene particles from complement activation and protein adsorption can be obtained with 4kDa PEG that provides for a coating thickness of 5nm . The thickness of the polymer coating depends on the polymer chemical composition. In aqueous medium, PEG can provide for a maximum thickness corresponding to its full chain length. For copolymer such as poloxamers and poloxamines instead the
thickness is linearly related to the number of EO monomers since only this function of the polymer can extend outward from Inhibitors,research,lifescience,medical the nanocarrier Pifithrin-�� order surface . A hydrophilic polymer can provide for a surface coating thickness of hc = aN(a/D)1/v, where N is the degree of polymerization, a is the size of the monomer, and D is the mean distance between grafting points . For a good solvent the exponent is 3/5. In general, proper particle stabilization is achieved when A(b/hc) < T where T = temperature, A = Hamaker constant, and b = Inhibitors,research,lifescience,medical particle radius. As A/T is typically in the order of 1/10, a coating with a thickness corresponding to 10% of the particle diameter is conventionally considered adequate to provide for efficient steric stability . 2.3.6. Polymer Flexibility Studies have Inhibitors,research,lifescience,medical demonstrated that polymer chain mobility is required for repelling proteins from polymer chains on particle surface yielding stealth nanocarrier . Accordingly, the lower complement activation of PEG as compared to dextran can be explained on the basis of polymer chain flexibility. In a CH50 assay, an in vitro haemolytic complement consumption assay, 10% complement activation was obtained with 20cm2of 5kDa dextran coated and 120cm2 5kDa PEG-coated polycaprolactone Inhibitors,research,lifescience,medical nanoparticles . The results
normalized by the particle surface area show that the PEG those coated particle surface induces a lower complement activation as compared to the dextran-coated surface. This is due to continuous change of the well-hydrated PEG chain conformation that reduces the exposure of fixation sites for complement proteins. The rapid movement of the flexible chains allows for the polymer to occupy a high number of possible conformations and leads to a temporary squeezing out of water molecules, making the surface impermeable for other solutes such as plasma proteins . Therefore, the water cloud surrounding the PEG chains confers an interfacial free energy on the particle surface that protects the nanocarriers from opsonisation and recognition by macrophages. 2.3.7.