Rheological behaviour of solid lipid nanoparticles

Purpose. To characterize the rheological behaviour of solid lipid nanoparticles (SLN) prepared with different lipid volume fraction. Methods. Cetylpalmitate was heated above its melting point (65°C) and added to a hot aqueous solution of polysorbate 80 (2% w/v) under stirring (8000 rpm, Ultra Turrax T25, IKA, Germany) to form the pre-emulsion. The pre-emulsion was homogenized at a pressure of 150000 KPa by using an EmulsiFlex-C5 (Avestin, Ottawa, Canada) thermostated at 65°C. The homogenized hot dispersion was cooled in an ice bath to form SLN. Dimensional distribution analyses were performed using a Nicomp 380 autocorrelator (PSS Inc., Santa Barbara, USA). Lipid fractions (φ) ranged from 0.009 to 0.255. Rheology was performed using a Stresstech HR (Rheologia Instruments AB, Milan, Italy) equipped with both plate/plate and cone/plate geometry. Results. SLN mean hydrodynamic diameter (MHD) and Gaussian distribution width were seen to be susceptible to lipid fraction with MHD ranging from about 170 to 290 nm. SLN with volume fractions in the range 0.009-0.161 behave as Newtonian fluids while higher fractions deviate from this behaviour, showing a viscous (Newtonian) region followed by a thickening. As expected, viscosity increased as function of the volume fraction and ranged from 1.33 ± 0.06 cP (φ, 0.009) to 16.3 ±0.48 cP (φ, 0.255). Viscosity increase trend deviated from theoretical values obtained with both Einstein and Krieger-Dougherty equations. Even though viscosity increased with the raise of plate/plate gap (compatibles with common syringe needle internal radius), values were low enough to permit an easy needle extrusion. Conclusion. SLN rheological characterization demonstrated good flow properties up to a lipid volume fraction of 0.161 with no syringability issues.