Biomedical subjects

Ralf Pörtner

Publications and source records attributed to Ralf Pörtner.

9 recordsLinked to original sources

PubMed2006

Redifferentiation of chondrocytes and cartilage formation under intermittent hydrostatic pressure.

Since articular cartilage is subjected to varying loads in vivo and undergoes cyclic hydrostatic pressure during periods of loading, it is hypothesized that mimicking these in vivo conditions can enhance synthesis of important matrix components during cultivation in vitro. Thus, the influence of intermittent loading during redifferentiation of chondrocytes in alginate beads, and during cartilage formation was investigated. A statistically significant increased synthesis of glycosaminoglycan and collagen type II during redifferentiation of chondrocytes embedded in alginate beads, as well as an increase in glycosaminoglycan content of tissue-engineered cartilage, was found compared to control without load. Immunohistological staining indicated qualitatively a high expression of collagen type II for both cases.

Alginates↗

PubMed2005

Experimental and modelling study of different process modes for retroviral production in a fixed bed reactor.

Pseudotype vectors are promising for gene transfer in many gene therapy approaches, however, low-vector concentration in batch cultures and high temperature-dependent decay do limit sufficiently large-scale production. To overcome these obstacles, the kinetic relations of cell growth and vector formation in different culture modes need to be understood. Effective optimisation of process modes is needed to achieve sufficient yields. Experimental and modelling studies were carried out in order to analyse the impact of different process modes such as perfusion, perfused fed-batch or repeated-batch on vector titer and productivity. Retroviral pseudotype vector, derived from the murine leukaemia virus carrying the HIV-1 envelop protein MLV (HIV-1) were produced using a 200 ml fixed bed reactor for high cell density cultivation on macroporous carriers. After starting the cultivation in batch mode, the reactor was either run in perfusion, perfused fed-batch or repeated-batch. A mathematical model of the bioreaction was developed on the basis of experimental data measured in culture dishes. The ability of the model to describe all different process modes of fixed-bed cultivation without additional fitting of the parameters was proven by three long-term cultivations for more than 400 h. The results of optimisation with the aid of the model, leads to the conclusion that perfusion with optimised harvest cycles and fed flows, result in a higher yield in comparison to batch or fed batch culture.

Animals↗

PubMed2005

Cultivation of three-dimensional cartilage-carrier-constructs under reduced oxygen tension.

Three-dimensional cartilage-carrier-constructs were produced according to a standard protocol from chondrocytes of an adult mini-pig. Experiments with different oxygen concentrations (21, 10 and 5%, v/v O(2)) were performed and the constructs were compared qualitatively and quantitatively. The appearance of the cartilage obtained under reduced oxygen tension seemed to be closer to native cartilage with respect to shape of the cells, distribution of the cells within the matrix, smoothness of the surface, etc. The thickness of the cartilage formed by free swelling was always in the same range as for native cartilage (approximately 1mm). Qualitatively the most stable attachment of the cartilage on top of the carrier was found for 10% O(2) (v/v). Especially at 5% O(2) (v/v) the attachment between cartilage and carrier was not sufficient. The constructs generated at lower oxygen tensions had a significantly higher amount of glycosaminoglycan per DNA, but still significantly less when compared to native cartilage. Furthermore, the cultivated cartilage contained a large amount of collagen type II. The experiments proved the applied concept for generation of cartilage-carrier-constructs and the usefulness of cultivation under reduced oxygen tension.

Animals↗

PubMed2005

Bioreactor cultivation of three-dimensional cartilage-carrier-constructs.

A flow-chamber bioreactor was designed for generation of three-dimensional cartilage-carrier-constructs. A specific attribute of the flow-chamber is a very thin medium layer for improved oxygen supply and a counter current flow of medium and gas. Three-dimensional cartilage-carrier-constructs were produced according to a standard protocol from chondrocytes of an adult mini-pig. The final step of this protocol was performed either in the bioreactor or in 12-well plates. The bioreactor experiments showed a significantly higher matrix thickness but a lower ratio of glycosaminoglycan to DNA. For both culture methods the constructs contained a high amount of collagen II. Appearance of the cartilage obtained in the bioreactor seemed to be closer to native cartilage with respect to distribution of the cells within the matrix, smoothness of the surface etc. All results considered the flow-chamber bioreactor is a very useful tool for generation of three dimensional cartilage-carrier constructs.

Bioreactors↗

PubMed2005

Relationship between physical, biochemical and biomechanical properties of tissue-engineered cartilage-carrier-constructs.

Three-dimensional cartilage-carrier-constructs were produced according to a standard protocol from chondrocytes of an adult mini-pig. Physical parameters (height and weight) correlated very well with total DNA content (r2 = 0.86, re. 0.94). The relation between DNA content and glycosaminoglycan content was less but still significant. No significant relationship was found between the elasticity module and the DNA content, even if the elasticity module increased slightly at higher DNA content. With respect to later implantation, selection of a construct for implantation based on the weight, which can be determined non-invasive and under sterile conditions, seems to be justifiable.

Animals↗

PubMed2005

Bioreactor design for tissue engineering.

Bioreactor systems play an important role in tissue engineering, as they enable reproducible and controlled changes in specific environmental factors. They can provide technical means to perform controlled studies aimed at understanding specific biological, chemical or physical effects. Furthermore, bioreactors allow for a safe and reproducible production of tissue constructs. For later clinical applications, the bioreactor system should be an advantageous method in terms of low contamination risk, ease of handling and scalability. To date the goals and expectations of bioreactor development have been fulfilled only to some extent, as bioreactor design in tissue engineering is very complex and still at an early stage of development. In this review we summarize important aspects for bioreactor design and provide an overview on existing concepts. The generation of three dimensional cartilage-carrier constructs is described to demonstrate how the properties of engineered tissues can be improved significantly by combining biological and engineering knowledge. In the future, a very intimate collaboration between engineers and biologists will lead to an increased fundamental understanding of complex issues that can have an impact on tissue formation in bioreactors.

Animals↗

PubMed2004

Evaluation of selected control strategies for fed-batch cultures of a hybridoma cell line.

While fed-batch suspension culture of animal cells continues to be of industrial importance for the large-scale production of pharmaceutical products, existing control concepts are still insufficient. The present paper illustrates the advantages and disadvantages of different fed-batch strategies, including fixed-feed trajectories, control via OUR (oxygen uptake rate) (stoichiometric feeding), a priori determination of feed trajectories based on a kinetic model and the model-based adaptive OLFO (open-loop-feedback-optimal) control strategy. A recommendation as to which control strategy should be used for a specific process has to consider the respective process. For an established process with a well characterized and stable production cell line, probably the application of a fixed feed trajectory should be recommended. An adaptive, model-based control strategy could be the method of choice during cell-line development or for rapid production of small amounts of product for clinical trials, owing to its universal character and because it does not require intensive process development.

Algorithms↗

PubMed2002

Determination of dissolved CO(2) concentration and CO(2) production rate of mammalian cell suspension culture based on off-gas measurement.

The determination of dissolved CO(2) and HCO(3)(-) concentrations as well as the carbon dioxide production rate in mammalian cell suspension culture is attracting more and more attention since the effects on major cell properties, such as cell growth rate, product quality/production rate, intracellular pH and apoptosis, have been revealed. But the determination of these parameters by gas analysis is complicated by the solution/dissolution of carbon dioxide in the culture medium. This means that the carbon dioxide transfer rate (CTR; which can easily be calculated from off-gas measurement) is not necessarily equal to carbon dioxide production rate (CPR). In this paper, a mathematical method to utilize off-gas measurement and culture pH for cell suspension culture is presented. The method takes pH changes, buffer and medium characteristics that effect CO(2) mass transfer into account. These calculations, based on a profound set of equations, allow the determination of the respiratory activity of the cells, as well as the determination of dissolved CO(2), HCO(3)(-) and total dissolved carbonate. The method is illustrated by application to experimental data. The calculated dissolved CO(2) concentrations are compared with measurements from an electrochemical CO(2) probe.

Animals↗

PubMedDate not supplied

Adaptive, model-based control by the Open-Loop-Feedback-Optimal (OLFO) controller for the effective fed-batch cultivation of hybridoma cells.

Although fed-batch suspension culture of animal cells continues to be of industrial importance for the large scale production of pharmaceutical products, existing control concepts are still insufficient. Changes in cell metabolism during cultivation and between similar cultivations, the complexity of the cell metabolism, and the lack of on-line state variables restrict the transfer of available control strategies established in bioprocess engineering. A process control strategy designed to achieve optimized process control must account for all these difficulties and fit sophisticated requirements toward adaptability and flexibility. The combination of a fed-batch process and an Open-Loop-Feedback-Optimal (OLFO) control provides a new approach for cell culture process control that couples an efficient cultivation concept to a capable process control strategy. The application of an adaptive, model-based OLFO controller to a hybridoma cultivation and experimental results are presented.

Algorithms↗