Nanoscopic friction behavior of pharmaceutical materials

Evaluation of the lubricating effect of magnesium stearate and glyceryl behenate solid lipid nanoparticles in a direct compression process

The aim of this study was to develop solid lipid nanoparticles (SLN) and introduce them into a direct compression process to evaluate their lubricant properties. The study consisted of preparing glyceryl behenate SLN (Compritol® 888 ATO) by hot dispersion, and magnesium stearate SLN by a novel nanoprecipitation/ion exchange method. The ejection force was measured for nanosystems and raw materials in a formulation typically used for direct compression. The smallest particle sizes obtained were 456 nm for Compritol® 888 ATO and 330 nm for magnesium stearate. Results show that the NPs used as lubricants in a direct compression model formulation provided efficient lubrication by maintaining the lubricating properties of the system, thereby decreasing the amount of lubricant used compared to the raw material. The lubricating effect showed an increase of 15-30% for magnesium stearate and Compritol® 888 ATO, compared to the raw material at concentrations above 2%.

Effect of pendent chains on the interfacial properties of thin polydimethylsiloxane (PDMS) networks

The interfacial properties of end-linked polydimethylsiloxane (PDMS) films on silicon are examined. Thin cross-linked PDMS films (∼10 μm thick) were synthesized over a self-assembled monolayer supported on a silicon wafer. By systematically varying the concentration of monofunctional PDMS in a mixture with telechelic precursor molecules, structures ranging from near-ideal elastic networks to poorly cross-linked networks composed of a preponderance of dangling/pendent chains were synthesized. Lateral force microscopy (LFM) employing bead probes was used to quantify the effect of network structure on the interfacial friction coefficient and residual force. Indentation measurements employing an AFM in force mode were used to characterize the elastic modulus and the pull-off force for the films as a function of pendent chain content. These measurements were complemented with conventional mechanical rheometry measurements on similar thick network films to determine their bulk rheological properties. All networks studied manifested interfacial friction coefficients substantially lower than that of bare silicon. PDMS networks with the lowest pendent chain content displayed friction coefficients close to 1 order of magnitude lower than that of bare silicon, whereas networks with the highest pendent chain content manifested friction coefficients about 3 times lower than that of bare silicon. At intermediate sliding velocities, a crossover in the interfacial friction coefficient was observed, wherein cross-linked PDMS films with the least amount of pendent chains exhibit the highest friction coefficient. These observations are discussed in terms of the structure of the films and relaxation dynamics of elastic strands and dangling chains in tethered network films.