Biomedical subjects

W Borchard

Publications and source records attributed to W Borchard.

7 recordsLinked to original sources

PubMed2005

Phase diagram of the system sodium alginate/water: a model for biofilms.

Sodium alginate is a polyelectrolyte consisting of the monomer units beta-D-mannuronate and alpha-L-guluronate. Mainly based on the theory of Khokhlow et al., the state diagram of the binary system alginate/water has been calculated using different sets of parameters like degree of ionization, degree of polymerization and interaction function. The calculations comprise miscibility gaps, liquidus curves, eutectic points and the behaviour at temperatures below the melting point of water. Also gel and swelling curves have been treated, where gels are physically crosslinked. The DSC diagram of a 0.5 by wt.% polymer sol shows a double melting peak, which is explained by a heterogeneity above 0 degrees C. The crystallization of water seems to concentrate the gelled system irreversibly.

Alginates↗

PubMed2003

Phase separation in calcium alginate gels.

Alginates are polysaccharides consisting of beta-D-mannuronate and alpha-L-guluronate units. In the presence of bivalent cations like calcium the guluronate blocks form physically cross-linked gels. The gelation properties of alginates play an important role in the stability of extracellular polymer substances and in the food industry. When stock solutions of Ca2+ ions and alginate are mixed, the gelation starts before the Ca2+ ions are evenly distributed, which leads to non-uniform gels. In this contribution, Ca alginate gels were prepared by in situ gelation using glucono-delta-lactone and CaCO3. In this way, uniform gels could be prepared directly in the measuring cell. Below a critical concentration, highly viscous solutions were obtained, which were below the critical point of gel formation. In these solutions at low rotational speeds a Schlieren peak arose, which became smaller and steeper with increasing time until a new meniscus could be detected. This behaviour is in contrast to the peak broadening due to diffusion after a synthetic boundary was formed. Evaluation of the data leads to negative diffusion coefficients. It has been shown by others that the mutual diffusion coefficient must be negative in the spinodal region. This phenomena is known as uphill diffusion and leads to phase separation of a binary system. The formation of the gel phase in this case is therefore discussed as uphill diffusion.

Alginates↗

PubMed2003

The thermodynamic water retention capacity of solutions and gels.

The thermodynamic water retention capacity (WRC) has been defined and applied to different heterogeneous phase equilibria. This definition includes others known from the literature for testing heterogeneous systems. For the type of a real solution it is shown that at constant values of temperature and pressure the WRC is related to the difference of the chemical potential of water between the original state and the state after having applied a constraint. The dependence of WRC on concentration of a solute is predicted to be described by an e-function which has been experimentally confirmed in the literature.

Models, Theoretical↗

PubMed2001

Uniaxial compression measurement device for investigation of the mechanical stability of biofilms.

The mechanical stability of biofilms is important for biotechnology, as sloughing of the biomass due to mechanical failure of the biofilm matrix can lead to severe interferences with biofilm processes. In cases of biofouling, biofilms have to be removed, in which case their mechanical stability must be overcome. The apparent modulus of elasticity and the yield strength as obtained from uniaxial compression experiments can be taken as parameters indicative for the mechanical stability of a biofilm. A film rheometer is presented which allows for the determination of these quantities, using model biofilms of Pseudomonas aeruginosa grown on membrane filters. The compressive stress-strain behaviour up to the point of failure is recorded at a compression speed of 1 microm s(-1). In accordance with the stress-strain curve, the investigated biofilm can be described as viscoelastic material, which demonstrates plastic flow properties. The extracellular polymeric substances (EPS), which keep biofilms together, form a temporary network of fluctuating junction points. Above the yield point, the gel structure fails and the system behaves as a highly viscous fluid. The apparent modulus of elasticity and the yield point are considered to be useful parameters for characterizing the mechanical properties of biofilms.

Biofilms↗

PubMed2001

Influence of calcium ions on the mechanical properties of a model biofilm of mucoid Pseudomonas aeruginosa.

The mechanical properties of biofilms and in particular their mechanical strength is of great importance for both biofilm reactors and for the removal of undesired biofilms as in cases of biofouling and biocorrosion. By uniaxial compression measurements, it is possible to determine the apparent elastic or Young's modulus and the yield stress as parameters for mechanical stability. This was performed with a recently developed device, using model biofilms of mucoid strain Pseudomonas aeruginosa SG81. The biofilms were grown on membrane filters placed on nutrient agar medium with different concentrations of calcium ions. The compressive stress-strain behaviour up to failure was recorded at a compression speed of 1 micron s-1. The apparent Young's modulus, representing the stiffness of the biofilm, and the yield stress obtained from the stress--strain diagram were used for the description of mechanical properties of biofilms. A certain critical concentration of calcium ions was found where the Young's modulus of the P. aeruginosa biofilms increases strongly and subsequently remains constant for higher calcium concentrations. This behaviour is explained by the presence of calcium ions crosslinking alginate, which is the major component of the extracellular polymeric substances produced by the mucoid P. aeruginosa strain used in this investigation.

Biofilms↗

PubMed1999

The role of intermolecular interactions: studies on model systems for bacterial biofilms.

The mechanical stability of biofilms and other microbial aggregates is of great importance for both the maintenance of biofilm processes and the removal of undesired biofilms. The binding forces are weak interactions such as London dispersion forces, electrostatic interactions and hydrogen bonds. In a first attempt to rank their contribution, the viscosity of solutions of extracellular polymeric substances (EPS) from a mucoid strain of Pseudomonas aeruginosa is measured. In order to distinguish the binding forces, substances are chosen which individually address the different types of bonds. Polyacrylic acid is identified as a suitable model system for EPS when molecular interactions are studied. Electrostatic interactions and hydrogen bonds are found to be the dominating forces among macromolecules within the biofilm.

Acrylic Resins↗

PubMed1994

A modified experimental setup for sedimentation equilibrium experiments with gels. Part 2: Technical developments.

This part of the paper trilogy describes technical developments for an efficient experimental setup to investigate gels with equilibrium analytical ultracentrifugation. New 10-channel centerpieces for the Schlieren optics, a new programmable multiplexer, a modified Schlieren optical system, and a photo pickup with impulse transformer are introduced as major developments. Also, some new centerpieces suitable for equilibrium experiments with solutions using the Rayleigh interference and the uv-absorption optics are presented. These centerpieces allow the investigation of 10, 12, or even 26 samples per centerpiece. The problem to find suitable materials for cell centerpieces and windows in the case of adhering samples is discussed for the system gelatin/water. A phase volume calculation for circular sample channels as a correction for the case of broadened menisci is presented. The method described allows an accurate measurement of up to 70 samples simultaneously in an equilibrium experiment if the 8-hole rotor presented in part 1 of the trilogy is used. The number of samples is sufficient to characterize a gel/solvent system in the experimentally accessible range under identical conditions, which is not possible by means of any of the methods known before. All parts described are also applicable for the investigation of solutions.

Gelatin↗