Biomedical subjects

V Normand

Publications and source records attributed to V Normand.

3 recordsLinked to original sources

PubMed2001

Dynamic experimentation on the confocal laser scanning microscope: application to soft-solid, composite food materials.

Confocal laser scanning microscopy (CLSM) is used to follow the dynamic structural evolution of several phase-separated mixed biopolymer gel composites. Two protein/polysaccharide mixed gel systems were examined: gelatin/maltodextrin and gelatin/agarose. These materials exhibit 'emulsion-like' structures, with included spherical particles of one phase (i.e. polymer A) within a continuous matrix of the second (i.e. polymer B). Compositional control of these materials allows the phase order to be inverted (i.e. polymer B included and polymer A continuous), giving four basic variants for the present composites. Tension and compression mechanical tests were conducted dynamically on the CLSM, with crack/microstructure interactions investigated using a notched compact tension geometry. Gelatin/maltodextrin composites exhibit a 'pseudo-yielding' stress/strain response in both tension and compression, when the gelatin-rich phase is continuous, which was attributed to debonding of the particle/matrix interface. This behaviour is significantly less apparent for both the gelatin/agarose composites, and the maltodextrin continuous gelatin/maltodextrin composites, with these materials responding in a nominally linear elastic manner. Values of the interfacial fracture energy for selected compositions of the two biopolymer systems were determined by 90 degrees peel testing, where a gelatin layer was peeled from either a maltodextrin or agarose substrate. For biopolymer layers 'cast' together, a value of 0.2 +/- 0.2 J m-2 was obtained for the fracture energy of a gelatin/maltodextrin interface, while a significantly higher value of 6.5 +/- 0.2 J m-2 was determined for a gelatin/agarose interface. The interfacial fracture energy of the two mixed systems was also determined following an indirect elastomer composite debonding model. An interfacial fracture energy of approximately 0.25 J m-2 was determined using this approach for the gelatin continuous gelatin/maltodextrin composite, which compares favourably with the value calculated directly by peel testing (i.e. approximately 0.2 J m-2). A somewhat higher value was estimated for the gelatin continuous gelatin/agarose system (1.0-2.0 J m-2), using this model, although there are severe limitations to this approach for this mixed gel system. In the present case, it is believed that the differing mechanical response of the two mixed biopolymer systems, when the gelatin phase is continuous, arises from the order of magnitude difference in interfacial fracture energy. It is postulated that polymer interdiffusion may occur across the interface for the gelatin/agarose system, to a significantly greater extent than for interfaces between gelatin and maltodextrin, resulting in a higher interfacial fracture energy.

Biopolymers↗

PubMed2000

Interpenetrating network formation in gellan--agarose gel composites.

Thermal, mechanical, turbidity, and microscope evidence is provided which strongly suggests molecular interpenetrating network (IPN) formation by mixtures of the bacterial and seaweed polysaccharides gellan and agarose. There is no evidence for synergistic coupling of the networks, and simple phase separation (demixing) can definitely be ruled out. Some changes in the gellan gelling behavior are suggested, however, by the increased gellan effective concentrations implicit in cure curve data. The dependence of this effect on the agarose nominal concentration seems consistent with a previous model that focused on gelling parameters, and changes in these rather than real concentration effects. In large deformation mechanical tests, the influence of agarose added to gellan is to re-enforce the network (higher compression and shear moduli, higher stresses-to-break) without significantly changing the strain to break, or the gellan brittle failure mechanism.

Algorithms↗

PubMed2000

New insight into agarose gel mechanical properties.

The current study focuses on the effects of the molecular weight on the mechanical behavior of agarose gels. The small strain rheology and large strain deformation/failure behavior of three different molecular weight agarose gels have been examined, with the results expressed in term of molar concentration. For small deformation strains, the gelation temperature at low concentrations and the critical concentration for gel formation are strongly affected by the molecular weight. In addition, the elasticity of the network is also very sensitive to this parameter. It has been demonstrated that the experimental gelation cure curves can be superimposed on a universal gelation master curve, independent of the cure time. This would indicate self-similarity of the network at different scales, irrespective of concentration. A relationship between the elastic modulus and the molecular weight has been extracted from these results, where the molecular weight dependence exhibits a power law exponent of 2.42. For large deformation strains, the Poisson ratio has been estimated to be 0.5 for each of the agarose types examined, which indicates that these gels are incompressible. The strain at failure is largely dependent on the molecular weight, and is essentially independent of the biopolymer concentration. This result highlights the fact that the strain at failure is sensitive to the connectivity distances in the gel network. However, the failure stress and Young's modulus of agarose gels show a dependence on both concentration and molecular weight. The observations regarding Young's modulus are in good agreement with those found for small deformation strain rheology for the shear modulus. One of the primary advantages of using the lowest molecular weight agarose is that higher molar concentrations can be reached (more molecules per unit volume). However, the mechanical response of agarose gels is very sensitive to the molecular weight at fixed molar concentration, and if the present results are extrapolated to very low molecular weight, it can be suggested that below a limiting molecular weight a percolating network will not be formed, as suggested by the Cascade model (Carbohydr. Polym. 1994, 23, 247-251). This speculation is based on the influence of the "connectivity" at long distances, which influences the strain at failure (when the strain at failure is zero, the system is not connective).

Algorithms↗