A molecular view of melting in anhydrous phospholipidic membranes.

A high-flux backscattering spectrometer and a time-of-flight disk chopper spectrometer are used to probe the molecular mobility of model freeze-dried phospholipid liposomes at a range of temperatures surrounding the main melting transition. Using specific deuteration, quasielastic neutron scattering provides evidence that, in contrast to the hydrocarbon chains, the headgroups of the phospholipid molecules do not exhibit a sharp melting transition. The onset of motion in the tails is located at temperatures far below the calorimetric transition. Long-range motion is achieved through the onset of whole-lipid translation at the melting temperature. Atomistic simulations are performed on a multibilayer model at conditions corresponding to the scattering experiments. The model provides a good description of the dynamics of the system, with predictions of the scattering functions that agree with experimental results. The analysis of both experimental data and results of simulations supports a picture of a gradual melting of the heterogeneous hydrophobic domain, with part of the chains spanning increasingly larger volumes and part of them remaining effectively immobile until the thermodynamic phase transition occurs.

Potential of mean force between two nanometer-scale particles in a polymer solution.

Expanded ensemble density-of-states simulations and a connectivity altering algorithm are used to investigate the effective interactions that arise between nanoparticles suspended in polymer solutions. Our calculations with systems of long polymeric chains reveal oscillations in the effective polymer-induced interactions between the particles, even at low concentrations. The range of these interactions is considerably longer than originally anticipated, and their origin is traced back to the chain-end effects and density fluctuations that were absent in previous treatments of these systems.

Polymer-particle mixtures: depletion and packing effects.

The structure of polymers in the vicinity of spherical colloids is investigated by Monte Carlo simulations and integral equation theory. Polymers are represented by a simple bead-spring model; only repulsive Lennard-Jones interactions are taken into account. Using advanced trial moves that alter chain connectivity, depletion and packing effects are analyzed as a function of chain length and density, both at the bond and the chain level. Chain ends segregate to the colloidal surface and polymer bonds orient parallel to it. In the dilute regime, the polymer chain length governs the range of depletion and has a negligible influence on monomer packing in dense polymer melts. Polymers adopt an ellipsoidal shape, with the larger axis parallel to the surface of the particle, as they approach larger colloids. The dimensions are perturbed within the range of the depletion layer.

Controlled release of salicylic acid from poly(D,L-Lactide).

Poly(D,L-Lactide) of high molecular weight (Mv) was prepared by ring-opening bulk polymerization of D,L-Lactide and characterized in terms of Mv, melting point and swelling behavior in buffer solution. Samples of the polymers with low and high Mv (2000 and 22 000 respectively) loaded with various amounts of salicylic acid (SA) were immersed in a buffer solution and the release of SA was recorded. The results obtained showed that swelling of the poly(D,L-Lactide) samples obeyed Fick's law, especially for those with high molecular weight, where biodegradation proceeds slowly. The release of SA seemed to follow a simplified relationship which is linear with time, at least for the early stages of delivery. The extent of linearity is dependent on the content of the acidic SA, which probably accelerates decomposition of the high molecular weight products.

Controlled release systems based on poly(lactic acid). An in vitro and in vivo study.

A new biodegradable delivery system based on poly(lactic acid) has been formulated, with potential applications in sustained antibiotic release against bone infection. The in vitro release of a new quinolone (pefloxacin) from low molecular weight poly(D,L-lactic acid) Mw = 2 x 10(3) lasted for 56 d whereas the in vivo delivery lasted 33 d. In both cases, the release rate is controlled by the drug diffusion and the polymer degradation, which seems to be the predominant factor. For the release experiments, discs were prepared from poly (D,L-lactide) Mw = 2 x 10(4) with drug loadings of 2% and 10% w/w. It was concluded that pefloxacin concentration remains higher than the Minimum Inhibitory Concentration (MIC) against the major causative bacteria of bone infection. The results indicate that the two different types of poly(lactic acid) can be used effectively in an implantable antibiotic release system.

Synthesis and properties of poly(lactic acid).

Poly(D,L lactic acid) was prepared by bulk polymerization of D,L lactide, both under atmospheric pressure and in vacuum. The obtained polymeric products were characterized in terms of molecular weight, Mw, melting point, calorimetric response and swelling behaviour. All products were amorphous. Their molecular weights were determined by viscosimetry and ranged from 2x10(3) to 9x10(4). Similarly, the melting points ranged from 90 to 210 degrees C. Swelling experiments, with specimens immersed in buffer solutions, showed that hydrolytic degradation started in a few days for the low Mw material, whereas for the higher molecular weight products it took much longer and probably followed a two-stage mechanism. This study suggests that the high molecular weight material could be an interesting carrier for the preparation of controlled release products, in cases where prolonged delivery is necessary.