Measurement of protein diffusion through poly(D,L-lactide-co-glycolide)

A novel method was developed for studying the diffusion of proteins through poly(D,L-lactide-co-glycolide) (PLG), using a diffusion cell. To develop improved formulations for the controlled release of encapsulated drugs it is important to understand the underlying release mechanisms. When using low-molecular-weight PLG as the release-controlling polymer, diffusion through the pores is often proposed as the main release mechanism. The experimental set-up and method of determining the diffusion coefficient were thoroughly evaluated with regard to the reliability and the influence of the stirring rate. A procedure for spraying thin films of PLG onto a filter, which could be placed in the diffusion cell, was optimized. The method was then applied to the determination of the diffusion coefficient of human growth hormone (hGH) through a PLG film. The results show that the method enables measurements of the diffusion coefficient through the polymer film. Neither the stirring rate nor the concentration of hGH influenced the diffusion coefficient. The diffusion coefficient of hGH through degraded PLG films was 5.0.10(-13) m(2)/s, which is in the range that could be expected, i.e., several orders of magnitude smaller than its the diffusivity in pure water. The reproducibility was good, considering the dynamic properties of PLG, i.e., the difference in diffusion coefficients, at, for example, different stages of degradation and for different compositions of PLG, is expected to be much higher. The variation is probably also present in PLG films used for controlled-release formulations. Although the PLG film contains a large amount of water, a considerable time elapsed before pores of sufficient size formed and diffusion through the film started. In two-component diffusion experiments, the difference in diffusion rate did not correspond to the difference in molecular weight of the solutes, indicating a size exclusion effect. This method can be used to study the effect of changes in the formulation specification. By studying the change in the diffusion coefficient through the degradation process of PLG, or similar polymers, a better understanding of diffusion and, thus, also release mechanisms can be obtained

Molecular characterization of native and processed waxy maize starch in relation to the recrystallization behavior of model systems and starch microspheres

The recrystallization behavior of five starch materials, native waxy maize starch, and starches modified by acid hydrolysis and mechanical treatments, were investigated by DSC (differential scanning calorimetry) and electron microscopy (SEM). The starches were known to vary significantly in molecular weights but not in the degree of branching. Microspheres manufactured from one of these starches were also studied by both methods. The acid hydrolysis did not produce major breakdown of the granular structure of starch as evidenced by SEK or the amount of crystalline amylopectin, as measured by DSC, but broadened the melting temperature range compared to native starch. No crystallinity could be detected in the material after the mechanical treatment, which also seemed to destroy the granular structure. The crystallinity was regained after the material had been dispersed in 70 % water during heating followed by 20 hour storage at 6 degreesC, called as one-step temperature treatment, while within the set of samples stored at 20 degreesC only one of the starches showed presence of regained crystallinity. No significant differences could be observed with regard to endothermic transition temperature or temperature range between the samples of native starch and both the acid hydrolysed and the mechanically treated samples, as measured by DSC. Apparently from the enthalpy values and presence of endothermic transitions, amylopectin molecular weight and modification method seemed to affect the amount of starch crystallites formed and/or the recrystallization rates, with acid hydrolysed samples showing higher enthalpy values compared to mechanically treated samples. The same conclusion, as for one-step temperature treatment of starch materials, was done for a two-steps temperature treatment of the starch materials, regarding transitions temperatures within the set of experiment. The two-steps treatment with nucleation at a lower temperature and crystal growth and perfection at a higher temperature resulted in an increase in the melting temperatures and narrower melting intervals. This treatment is also similar to the actual microsphere manufacture process. DSC thermogram of placebo starch microspheres revealed that the crystallites, obtained in production process of the microspheres, melted at temperatures similar to melting temperatures of the starch dispersion after 14 days/6 degreesC- storage but showed higher enthalpy value and smaller melting interval

Recrystallisation behaviour of native and processed waxy maize starch in relation to the molecular characteristics

Molecular characteristics were determined for native waxy maize starch and maize starch modified in different way (by mechanical treatment or/and acid hydrolysis). Recrystallisation behaviour was studied. Methods used in this study were MALLS, HPAEC-PAD, NMR, DSC, SEM, light microscopy. Five starch materials were subjected to storage under the same conditions in the presence of water (70 w/w%). Molecular weight, radius of gyration, initial crystallinity, and degree of polymerisation, degree of branching, chain length distribution profiles, were related to nucleation rate during the recrystallisation process, rate of recrystallisation, thermal stability and amount of obtained crystallinity. This allowed the following connections between the molecular characteristics and kinetic of recrystallisation to be proposed: Amylopectin molecular weight appeared to affect the number of starch crystallites formed and amount of crystallinity but not the stability of the rebuilt crystallites. The stability of rebuilt crystallites can be controlled by degree of polymerization, degree of branching and unit chain length distribution, characteristics which were similar for the starches. A mixture of two starches, with and without crystalline structure in initial state but with molecular weight in same range, were stored and scanned in order to understand possible cocrystallisation effects. (C) 2004 Elsevier Ltd. All rights reserved