Biomedical subjects

F Volke

Publications and source records attributed to F Volke.

10 recordsLinked to original sources

PubMed2005

Towards a medically approved technology for alginate-based microcapsules allowing long-term immunoisolated transplantation.

The concept of encapsulated-cell therapy is very appealing, but in practice a great deal of technology and know-how is needed for the production of long-term functional transplants. Alginate is one of the most promising biomaterials for immunoisolation of allogeneic and xenogeneic cells and tissues (such as Langerhans islets). Although great advances in alginate-based cell encapsulation have been reported, several improvements need to be made before routine clinical applications can be considered. Among these is the production of purified alginates with consistently high transplantation-grade quality. This depends to a great extent on the purity of the input algal source as well as on the development of alginate extraction and purification processes that can be validated. A key engineering challenge in designing immunoisolating alginate-based microcapsules is that of maintaining unimpeded exchange of nutrients, oxygen and therapeutic factors (released by the encapsulated cells), while simultaneously avoiding swelling and subsequent rupture of the microcapsules. This requires the development of efficient, validated and well-documented technology for cross-linking alginates with divalent cations. Clinical applications also require validated technology for long-term cryopreservation of encapsulated cells to maintaining a product inventory in order to meet end-user demands. As shown here these demands could be met by the development of novel, validated technologies for production of transplantation-grade alginate and microcapsule engineering and storage. The advances in alginate-based therapy are demonstrated by transplantation of encapsulated rat and human islet grafts that functioned properly for about 1 year in diabetic mice.

Alginates↗

PubMed2005

[Experimental testing of a new coil design for endoluminal MRI applied to the pig stomach].

PURPOSE: Experimental feasibility study of a new MR-Coil concept for enhanced visualization of the gastric wall. MATERIAL AND METHODS: The newly developed single-loop receiver coil for endoluminal imaging (Fraunhofer Institute, St. Ingbert, Germany) was evaluated in 4 explanted pig stomachs in a 1.5T MR unit (Siemens Symphony, Erlangen, Germany) with T1 w and T2 w MR sequences in three planes. The new coil consists of a foldable and self-expanding single loop coil (receiver coil) of a shape memory metal (nitinol). It was covered with a biocompatible material (silicone) to prevent direct contact of the wire with stomach tissue. The coil assumes a circular configuration with a diameter of 8 cm because of its memory metal properties. The flexible characteristics of the material used allow the passage through the instrument channel (13 mm diameter) of a specially designed MR-compatible endoscope. The purpose of our study was to assess feasibility of the coil design as a first step in developing a new endoluminal MRI-concept. Additionally the number and signal intensity of visible gastric wall layers were evaluated and findings were correlated with histopathological results of a pig stomach. RESULTS: The new coil concept was a feasible system in all 4 cases and showed good image quality for analysis. On T1 w images, 3 layers were visible in all cases, and on T2 w images 4 different gastric wall layers were seen in 2 cases. Due to histopathological correlation, the different gastric wall layers were identified as follows: mucosa, submucosa and muscularis propria if three layers were depicted; in cases of 4 visible wall layers, serosa and subserosa could be detected additionally. For each gastric wall layer, a distinct signal intensity was found. CONCLUSION: The new MR coil concept for endoluminal imaging proved to be a feasible technique. Good differentiation of gastric wall layers in the pig stomach could be demonstrated. We have shown that endoscopic MR-imaging with our new coil concept is a valuable technique for the visualization of gastric wall layers. Due to this fact, follow-up studies including assessing safety aspects are necessary to finally conduct an experimental-clinical study on in-vivo human gastric specimens to detect tumor growth and morphology within the gastric wall. Endoscopic MRI may have the potential in the future to overcome today's limitations of diagnostic imaging in gastric cancer.

Animals↗

PubMed2003

Cross-linking properties of alginate gels determined by using advanced NMR imaging and Cu(2+) as contrast agent.

The entrapment of enzymes, drugs, cells or tissue fragments in alginates cross-linked with Ca(2+) or Ba(2+) has great potential in basic research, biotechnology and medicine. The swelling properties and, in turn, the mechanical stability are key factors in designing an optimally cross-linked hydrogel matrix. These parameters depend critically on the cross-linking process and seemingly minor modifications in manufacture have a large impact. Thus, sensitive and non-invasive tools are required to determine the spatial homogeneity and efficacy of the cross-linking process. Here, we show for alginate microcapsules (between 400 microm and 600 microm in diameter) that advanced (1)H NMR imaging, along with paramagnetic Cu(2+) as contrast agent, can be used to validate the cross-linking process. Two- and three-dimensional images and maps of the spin-lattice relaxation time T(1) of Ba(2+) cross-linked microcapsules exposed to external Cu(2+) yielded qualitative as well as quantitative information about the accumulation of Cu(2+) within and removal from microcapsules upon washing with Cu(2+) free saline solution. The use of Cu(2+) (having a slightly higher affinity constant to alginate than Ba(2+)) for gelling gave a complementary insight into the spatial homogeneity of the cross-linking process together with information about the mechanical stability of the microcapsules. The potential of this technique was demonstrated for alginates extracted from two different algal sources and cross-linked either externally by the conventional air-jet dropping method or internally by the "crystal gun" method.

Alginates↗

PubMed2003

Fabrication of homogeneously cross-linked, functional alginate microcapsules validated by NMR-, CLSM- and AFM-imaging.

Cross-linked alginate microcapsules of sufficient mechanical strength can immunoisolate cells for the long-term treatment of hormone and other deficiency diseases in human beings. However, gelation of alginate by external Ba(2+) (or other divalent cations) produces non-homogeneous cross-linking of the polymeric mannuronic (M) and guluronic (G) acid chains. The stability of such microcapsules is rather limited. Here, we show that homogeneous cross-linking can be achieved by injecting BaCl(2) crystals into alginate droplets before they come into contact with external BaCl(2). The high effectiveness of this crystal gun method is demonstrated by confocal laser scanning microscopy and by advanced nuclear magnetic resonance imaging. Both techniques gave clear-cut evidence that homogeneous cross-linkage throughout the microcapsule is only obtained with simultaneous internal and external gelation. Atomic force microscopy showed a very smooth surface topography for microcapsules made by the crystal gun method, provided that excess Ba(2+) ions were removed immediately after gelation. In vitro experiments showed greatly suppressed swelling for crystal gun microcapsules. Even alginate extracted from Lessonia nigrescens (highly biocompatible) yielded microcapsules with long-term mechanical stability not hitherto possible. Encapsulation of rat islets, human monoclonal antibodies secreting hybridoma cells and murine mesenchymal stem cells transfected with cDNA encoding for bone morphogenetic protein (BMP-4) revealed that injection of BaCl(2) crystals has no adverse side effects on cell viability and function. However, the release of low-molecular weight factors (such as insulin) may be delayed when using alginate concentrations in the usual range.

Alginates↗

PubMed1997

Characterisation of antibiotic moenomycin A interaction with phospholipid model membranes.

Using a combination of physico-chemical techniques (MAS NMR, DSC, freeze-fracture electron microscopy, molecular modelling) the antibiotic moenomycin A was found to be anchored by its hydrophobic chain into multilamellar POPC membranes. The lamellar phase structure of the modified membrane is retained, while moenomycin A in water at different concentrations does not form any other but isotropic phase structures. The mobility of POPC molecule segments is reduced with increasing moenomycin A concentrations. Freeze-fracture electron microscopy images show ripple like structures for low moenomycin A concentrations, which are rare for high concentrations. A sugar-group network of the antibiotic seems to cover the whole membrane surface for molar ratios moenomycin A/POPC of 1:2, which is supported by 13C-MAS (Magic Angle Spinning) 31P-NMR, and molecular modelling.

Bambermycins↗

PubMed1995

Computer simulation of the temperature- and hydration-dependent lateral diffusion of phosphatidylcholine in lipid bilayers.

A lattice model of a lipid bilayer near the so-called main phase transition between the liquid crystalline (L alpha) and the gel (L beta) phase is presented. It is based on a two-state model. Jump dynamics are defined for the lattice molecules to simulate lateral diffusion. The temperature and hydration dependence of the lateral diffusion coefficients of the model are calculated for the L alpha phase using Monte Carlo simulation techniques. The results obtained allow the estimation of the hydration dependent part of the lateral diffusion activation energy by thermodynamical quantities. We compare these results with measured activation energies of dipalmitoylphosphatidylcholine (DPPC) and propose a model to describe the total lateral diffusion activation energy of such systems.

Chemical Phenomena↗

PubMed1995

Membrane hydration and structure on a subnanometer scale as seen by high resolution solid state nuclear magnetic resonance: POPC and POPC/C12EO4 model membranes.

The position on a subnanometer scale and the dynamics of structurally important water in model membranes was determined using a combination of proton magic-angle spinning NMR (MAS) with two-dimensional NOESY NMR techniques. Here, we report studies on phosphocholine lipid bilayers that were then modified by the addition of a nonionic surfactant that is shown to dehydrate the lipid. These studies are supplemented by 13C magic-angle spinning NMR investigations to get information on the dynamics of segmental motions of the membrane molecules. It can be shown that the hydrophilic chain of the surfactant is positioned at least partially within the hydrophobic core of the lipid bilayer. With the above NMR approach, we are able to establish molecular contacts between water and the lipid headgroup as well as with certain groups of the hydrocarbon chains and the glycerol backbone. This is possible because high resolution proton and 13C-NMR spectra of multilamellar bilayer membranes are obtained using MAS. A phase-sensitive NOESY must also be applied to distinguish positive and negative cross-peaks in the two-dimensional plot. These studies have high potential to investigate membrane proteins hydration and structural organization in a natural lipid bilayer surrounding.

Carbon Isotopes↗

PubMed1994

Dynamic properties of water at phosphatidylcholine lipid-bilayer surfaces as seen by deuterium and pulsed field gradient proton NMR.

The dynamic properties of water in phosphatidylcholine lipid/water dispersions have been studied, applying a combination of 2H-NMR techniques (quadrupole splitting and spin-lattice relaxation time) and self-diffusion measurements using pulsed filed gradient (PFG) 1H-NMR. The hydration properties of POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine) were compared with those of DOPC (1,2-dioleoyl-sn-glycero-3-phosphatidylcholine) and EYL (egg yolk phosphatidylcholine (lecithin)). A model is presented that assumes an exponentially decaying influence of the bilayer surface on water dynamics as well as on water orientation with increasing hydration. This assumption is based on an exponentially decaying hydration potential which results from direct lipid-water and water-water interactions. The model describes successfully the experimental data for a large water concentration range, especially at low hydration, where other models failed. With the exception of a small fraction of water which is significantly influenced by the surface in slowing down the mobility, the interbilayer water has isotropic, free water characteristics in terms of correlation times and molecular order. Hydration properties of POPC are comparable with those of EYL but differ from DOPC. At very low water content the correlation times of headgroup segmental reorientation and water are similar, indicating a strong coupling of this water to the lipid lattice. The hydration properties of the three lipids studied are explained in terms of slightly different headgroup conformations due to different lateral packing of the molecules by their fatty acid chain composition.

Chemical Phenomena↗

PubMed1994

Hydration force parameters of phosphatidylcholine lipid bilayers as determined from 2H-NMR studies of deuterated water.

The continuous decrease of the quadrupolar splitting of deuterated water interacting with phosphocholine lipid bilayers with growing water concentration is analyzed as a function of the water activity. From the apparent linear dependence on water activity a measure for hydration forces is obtained. The forces calculated are in the range of published data using sorption isotherms and osmotic stress technique in combination with SAXS. A simple interaction potential which includes orientational order of water adsorbed on surfaces gives a physical base for these findings. Therefore, deuterium NMR may become a powerful tool for hydration force analysis complementing well-known methods.

Biophysical Phenomena↗

PubMed1990

Use of chemical fractionation and proton nuclear magnetic resonance to probe the physical structure of the primary plant cell wall.

Proton magnetic resonance has been used to monitor the microscopic physical properties of etiolated hypocotyl cell walls from Phaseolus vulgaris L. at all stages in a series of chemical fractionations with ammonium oxalate and potassium hydroxide. Solid echo measurements indicate that 75% of the polymers in the intact cell wall, including the cellulose and most of the hemicelluloses, are arranged such that there is almost complete restraint of molecular motion. The chemical fractionations generally altered the physical structures of the remaining cell wall components. Digestion with 0.25% ammonium oxalate/oxalic acid solubilized the pectin and increased the mobility of the hemicellulose I component. Extraction with 4% potassium hydroxide removed the hemicellulose I component and loosened the hemicellulose II. Further extraction with 24% potassium hydroxide removed the hemicellulose II and loosened some of the cellulose. The cellulose crystallinity, as monitored by Jeener echo measurements decreased from 83% to 63% during these fractionations. We conclude that, while hemicellulose I is firmly attached to hemicellulose II, it is not in a closely packed structure. Hemicellulose II is strongly bound to cellulose and has a much more closely packed structure.

Journal Article↗