Formation and relaxation kinetics of starch-particle complexes.

The formation and relaxation kinetics of starch-particle complexes were investigated in this study. The combination of cationic nanoparticles in suspension and anionic starch in solution gave rise to aggregate formation which was studied by dynamic light scattering, revealing the initial adsorption of the starch molecules on the particle surface. By examining the stability ratio, W, it was found that even in the most destabilized state, i.e. at charge neutralization, the starch chains had induced steric stabilization to the system. At higher particle and starch concentrations relaxation of the aggregates could be seen, as monitored by a decrease in turbidity with time. This relaxation was evaluated by fitting the data to the Kohlrausch-Williams-Watts function. It was found that irrespective of the starch to particle charge ratio the relaxation time was similar. Moreover, a molecular weight dependence on the relaxation time was found, as well as a more pronounced initial aggregated state for the higher molecular weight starch. This initial aggregate state could be due to bridging flocculation. With time, as the starch chains have relaxed into a final conformation on the particle surface, bridging will be less important and is gradually replaced by patches that will cause patchwise flocculation. After an equilibration time no molecular weight dependence on aggregation could be seen, which confirms the patchwise flocculation mechanism.

Competitive adsorption of amylopectin and amylose on cationic nanoparticles: a study on the aggregation mechanism.

In this study we investigate the interactions between cationic nanoparticles and anionic starch, where the starch was composed of 20 wt% of amylose, a linear polymer, and 80 wt% of amylopectin, a branched polymer. The mechanism of aggregation was investigated by scattering techniques. It was found that the cationic particles formed large aggregates with the starch as a result of selective adsorption of the amylopectin. Amylose did not participate significantly in the aggregate formation even when the charge ratio of starch to particles was <1. For starch to particle ratio >1 stabilization was recovered mostly due to the large hindrance brought about by the highly branched amylopectin. This results in a shift of the stabilization mechanism from electrostatic to electrosteric. The internal structure of the aggregates was composed of primary particles with starch coils adsorbed on the surface. This information supports the proposed aggregation mechanism, which is based on adsorption of the negatively charged starch in patches on the positively charged nanoparticles causing attractive interaction between the particles.

Polymer leaching from film coating: effects on the coating transport properties.

The release mechanism of metoprolol succinate pellets coated with a blend of a water-insoluble polymer, ethyl cellulose (EC), and a water-soluble polymer, hydroxypropyl cellulose (HPC), is mechanistically explained. The kinetics of drug release and HPC leaching were followed for drug doses. The coating was initially not permeable to the drug, and release started only after a critical amount of the HPC had been leached out. Drug release occurred mainly through pores created in the coating by the HPC dissolution. Single-pellet release experiments were also performed. The coating thickness and size of each pellet were measured. In order to quantitatively characterize the transport properties of the coating of the individual pellets, and to determine the effective diffusion coefficient (D(e)) of the drug in the coating, a mechanistic model was used to fit the single-pellet release data. It was found that D(e) increased with time due to an increase in the amount of HPC leached. It was also found that D(e) was dependent on the coating thickness, and increased more slowly with a thicker coating. This agreed well with the finding that the HPC leaching rate decreased with increasing film thickness.

Influence of different polymer types on the overall release mechanism in hydrophilic matrix tablets.

The effect of three different types of polymer chain structures on the polymer release from hydrophilic matrix tablets was investigated by comparing a synthetic semi-crystalline linear polymer (PEO), a branched amorphous polysaccharide (dextran) and an amorphous substituted cellulose derivative (HPMC). The polymer release rates for tablets containing mixtures of high and low molecular weight grades in different ratios were determined by using a modified USP II method and a SEC-RI chromatography system. The results showed that independent of polymer type: (i) plots of the release versus time had similar shapes, (ii) the release of long and short polymer chains was equal and no fractionation occurred during the release and (iii) the release rate could be related to the average intrinsic viscosity of the polymer mixtures. This confirms the hypothesis that the release rate can be related to a constant viscosity on the surface of the hydrophilic matrix tablet and that it is valid for all the investigated polymers.

Coated formulations: new insights into the release mechanism and changes in the film properties with a novel release cell.

The effect of the blend ratio of water-insoluble ethyl cellulose (EC) and water-soluble hydroxypropyl cellulose (HPC-LF), on the properties of sprayed films and on the drug release mechanism of formulations coated with the material was investigated. When the original HPC-LF content exceeded 22%, both the amount of HPC-LF leached out and the water permeability of the films increased drastically when they were immersed in a phosphate buffer solution. The release mechanism of potassium nitrate through EC/HPC-LF films containing 20, 24 and 30% HPC-LF was elucidated in a new release cell equipped with a manometer to measure the pressure build-up inside the cell. A lag phase in the release accompanied by a pressure build-up was observable in all the experiments showing that all the films were initially semi-permeable to KNO3. However, pressure data revealed that films with 30% HPC-LF became permeable to KNO3 during the release process due to HPC-LF leaching. Importantly, the blend ratio influenced not only the release rate (which increased as the amount of HPC-LF increased), and the lag time (which increased as the amount of HPC-LF decreased), but also the release mechanism, which changed from osmotic pumping to diffusion as the amount of HPC-LF increased.