Biomedical subjects

Sean P Rigby

Publications and source records attributed to Sean P Rigby.

7 recordsLinked to original sources

PubMed2006

Simulation of nonwetting phase entrapment within porous media using magnetic resonance imaging.

Models representing the pore structures of amorphous, mesoporous silica pellets have been constructed using magnetic resonance images of the materials. Using magnetic resonance imaging (MRI), maps of the macroscopic (approximately 0.01-1 mm) spatial distribution of porosity and pore size were obtained. The nature and key parameters of the physical mechanism for mercury retraction, during porosimetry experiments on the silica materials, were determined using integrated gas sorption experiments. Subsequent simulations of mercury porosimetry within the structural models derived from MRI have been used to successfully predict, a priori, the point of the onset of structural hysteresis and the final levels of mercury entrapment for the silicas. Hence, a firm understanding of the physical processes of mercury retraction and entrapment in these amorphous silica materials has been established.

Computer Simulation↗

PubMed2005

NMR and confocal microscopy studies of the mechanisms of burst drug release from PLGA microspheres.

Pulsed-field gradient (PFG) NMR and confocal microscopy techniques have been used to study the structural evolution and drug release profile of poly(d,l-lactide-co-glycolide) (PLGA) microspheres over time during immersion in an aqueous phase. Variation of the drying process used in the synthesis of the PLGA microspheres has been found to significantly influence the degree of permeability of the spheres to water. PFG NMR has been used to study the change in the cavity sizes within the pore structure of the microspheres over time following initial immersion. In these studies, the temperature of the secondary emulsion, used in the sphere synthesis, has been found to significantly change the temporal evolution of the pore structure. Confocal microscopy studies of the release of a model drug from within the microspheres suggest that the rate-limiting step in drug release is the swelling rate of the polymer matrix, and that the mechanism may be a percolation process. These studies also showed that the local rate of drug release is heterogeneously distributed across a microsphere, and thus, strictly, cannot be modelled as purely a simple diffusive release process from a sphere.

Algorithms↗

PubMed2005

Modeling the fractal growth of templated, mesoporous silica films.

Brewster angle micrographs have been obtained of the development of a silica film grown at an air-water interface with use of the surfactant template cetylpyridinium chloride. The micrograph images showed that the growing silica film exhibited complex, fractal-like patterns. These images have been analyzed to determine the values of the fractal dimension and lacunarity, and the forms of the autocorrelation function and Euclidean crossover behavior of the silica clusters have been observed. These statistical descriptors have been compared with the equivalent properties of simulated images of model structures generated by computer using a particular variant of the cluster-cluster aggregation (CCA) algorithm. Good agreement was found between the characteristic properties of typical experimental images and the simulated images. It was, therefore, suggested that the CCA process is a good model for the growth of the silica films.

Journal Article↗

PubMed2005

An experimental study of gas adsorption on fractal surfaces.

The validity of the fractal versions of the FHH and BET theories for describing the adsorption of butane and nitrogen on a variety of partially dehydroxylated silica surfaces has been tested. The fractal dimensions obtained from adsorption data have been compared with those obtained completely independently using SAXS. It was found that the fractal dimensions obtained from butane adsorption isotherms, using both the fractal FHH and fractal BET theories, agreed well with the corresponding values obtained from SAXS over overlapping length scales. However, in general, a systematic deviation between the fractal dimension obtained from nitrogen adsorption and that obtained from SAXS was observed. The fractal dimensions obtained from nitrogen adsorption were consistently larger than those obtained from SAXS, which is the opposite of what has often been found in the literature. It has been suggested that the differences in the suitability of the adsorption theories tested to describe butane and nitrogen adsorption is due to the significant difference between the interaction strengths of these two different molecules with silica surfaces. A modified theory that can account for the discrepancy between the fractal dimensions obtained from nitrogen adsorption and SAXS has been proposed. The implications of the new theory for the accuracy of nitrogen adsorption BET surface areas for silicas are discussed.

Journal Article↗

PubMed2004

Determining drug spatial distribution within controlled delivery tablets using MFX imaging.

In this study, the potential of micro-focus X-ray (MFX) imaging as a tool in the design and quality control of drug delivery systems has been demonstrated. MFX imaging has been used to map the spatial distribution of a drug down to length-scales of approximately 10-100 microm and has shown that the spatial distribution of the drug amiodarone within a hydroxypropylmethylcellulose (HPMC) controlled release tablet (diameter 10 mm) is not homogeneous. MFX imaging allows an assessment to be made of the degree of mixing of the original powders. Despite vigorous mixing during the manufacture of the tablets, the drug particles, originally of size 180-420 microm, were found to be concentrated within isolated but spatially extended patches of sizes up to 1 mm that were scattered across the tablet.

Chemistry, Pharmaceutical↗

PubMed2002

A statistical model for the heterogeneous structure of porous catalyst pellets.

The complex structures of the void space of porous media are often characterised by parameters such as pore network connectivity and lattice size. This paper presents a comparison of the estimates of these parameters obtained from two previous methods based on nitrogen sorption and mercury porosimetry, and also from a new, completely independent approach based on pulsed-gradient spin-echo nuclear magnetic resonance (PGSE NMR). It was found that the new PGSE NMR technique obtains estimates of connectivity and lattice size in agreement with nitrogen sorption but different to mercury porosimetry. This difference was attributed to the various physical processes involved actually probing different aspects of the pore space geometry. It was further suggested that the representation of the pore structure derived from either nitrogen sorption or PGSE NMR is really a mapping of the real pore space onto an equivalent abstract, random pore bond network. However, it has been shown that this mapping does capture some of the characteristic properties of the pore space that control transport over mesoscopic ( < 10 microm) length scales. For materials which additionally possessed macroscopic (> 10 microm) structural heterogeneity, it was found that the model could also be adapted to predict the macroscopic transport properties of the porous medium.

Journal Article↗

PubMed2002

The influence of mercury contact angle, surface tension, and retraction mechanism on the interpretation of mercury porosimetry data.

The use of a semi-empirical alternative to the standard Washburn equation for the interpretation of raw mercury porosimetry data has been advocated. The alternative expression takes account of variations in both mercury contact angle and surface tension with pore size, for both advancing and retreating mercury meniscii. The semi-empirical equation presented was ultimately derived from electron microscopy data, obtained for controlled pore glasses by previous workers. It has been found that this equation is also suitable for the interpretation of raw data for sol-gel silica spheres. Interpretation of mercury porosimetry data using the alternative to the standard Washburn equation was found to give rise to pore sizes similar to those obtained from corresponding SAXS data. The interpretation of porosimetry data, for both whole and finely powdered silica spheres, using the alternative expression has demonstrated that the hysteresis and mercury entrapment observed for whole samples does not occur for fragmented samples. Therefore, for these materials, the structural hysteresis and overall level of mercury entrapment is caused by the macroscopic (> approximately 30 microm), and not the microscopic (< approximately 30 microm), properties of the porous medium. This finding suggested that mercury porosimetry may be used to obtain a statistical characterization of sample macroscopic structure similar to that obtained using MRI. In addition, from a comparison of the pore size distribution from porosimetry with that obtained using complementary nitrogen sorption data, it was found that, even in the absence of hysteresis and mercury entrapment, pore shielding effects were still present. This observation suggested that the mercury extrusion process does not occur by a piston-type retraction mechanism and, therefore, the usual method for the application of percolation concepts to mercury retraction is flawed.

Journal Article↗