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. Key words: tissue engineering, bioreactor, design considerations, cartilage. The loss and damage of tissues cause serious health problems . In the US, almost one-half of the costs for medical treatments are spent on implant devices annually. Worldwide, 350 billion USD are expended for substitute of organs.
The substitution of tissues (such as bone or cartilage) or joints with allograft materials includes the risk of infections by viruses (such as HIV, hepatitis C) or a graft rejection. Artificial implants such as those used in knee or hip replacement, have limitations due to their limited lifespan, insufficient bonding to the bone, and allergic reactions caused by material abrasion. New therapy concepts for practical medical applications are required. To this end, tissue engineered substitutes generated in vitro could open new strategies for the restoration of damaged tissues.
The goal of tissue engineering can be defined as the development of cell-based substitutes to restore, maintain or improve tissue function. These substitutes should have organ-specific properties with respect to biochemical activity, microstructure, mechanical integrity and biostability.
The generation of 3D tissue substitutes in vitro requires not only a biological model (e. g. , an adequate source for proliferable cells with appropriate biological functions, a protocol for proliferating cells while maintaining the tissuespecific phenotype), but also the further development of new culture strategies including bioreactor concepts.
Bioreactors are well established for the cultivation of microbes or mammalian cells under monitored and controlled environmental and operational conditions (e. g. pH, temperature, oxygen tension, and nutrient supply) up to an industrial scale. However, as individual cells are mostly applied, these concepts are inapplicable to 3D tissue constructs. Furthermore, each type of tissue construct (e. g. , skin, bone, blood vessel, and cartilage) will likely require an individualized bioreactor design
The question is, however, whether bioreactors can indeed fulfil these expectations. In this review, key technical challenges are identified and an overview of existing culture systems and bioreactors used for tissue engineering is provided. These topics have been addressed to some extent by several authors. Therefore, they will be discussed only briefly.
Particular focus will be given to the interaction between biological and engineering aspects. Using cartilage tissue formation as an example, it will be shown how an increased fundamental understanding of biological, biochemical, and engineering aspects can significantly improve the properties of 3D tissue constructs.
Bioreactors for the generation of 3D tissue constructs can provide a better process control by taking into account different demands of cells during cultivation. Furthermore, they can provide the technical means to perform controlled studies aimed at understanding specific biological, chemical or physical effects. Moreover, bioreactors enable a safe and reproducible production of tissue constructs.
An overall comparison of different culture methods shows clearly the advantages of bioreactor culture. Not only can the properties of cultivated 3D tissue constructs be improved, aspects such as safety of operation argue for the use of bioreactor systems. Furthermore, the bioreactor can be used to study effects such as shear flow and/or hydrostatic pressure on the generation of tissues. For future 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 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. On one hand, devices are required with a well-described microenvironment of cells for fundamental studies.
On the other hand, a ransition from a laboratory scale to an industrial scale will require a high adaptability of specialized bioreactors in a standardized production process. These advances will aid in ensuring that tissue engineering fulfil the expectations for revolutionizing medical care.We would like to thank Drs. Klaus Baumbach, Frank Feyerabend, Jan-Philipp Petersen and Jens Schroder for scientific input and help in animal trials, Sven Cammerer and Katja Schmid for modelling the cartilage reactor as well as Katharina Braun, Ditte Siemesgeluss and Richard Getto for technical support.
The financial support of Biomet Deutschland GmbH, Berlin under the BMBF grant no. 03N4012 and the city of Hamburg within the “Qualtitatsoffensive Tissue Engineering” is gratefully acknowledged. FIG. 5. Images of immunohistological staining for collagen type II of implanted cartilage-carrier construct (mini-pig) after explanation and decalcification of carrier. Cartilage-carrier constructs were cultivated prior to implantation under loading with intermittent hydrostatic pressure in a bioreactor aerated with 21% O2 (a, b) and 10% O2 (c, d). Time of observation for the animals: 8 weeks.
The theoretical simulations indicate that even under ideal conditions (no mass transfer limitation in the fluid phase) a severe oxygen limitation within the engineered tissue should be expected. If oxygen supply would be the limiting factor during cartilage formation, a bioreactor system (flow chamber) with an improved oxygen supply should lead to a better quality of the engineered cartilage. On the other hand, lower oxygen concentrations in the gas phase seem to improve some matrix properties.
From the results discussed above, these discrepancies can be solved only to some extent. With respect to important biochemical properties, particularly the content of GAG, the constructs from the flow chamber bioreactor showed significantly lower values than those from the 12-well plates, probably due to a higher, detrimental oxygen concentration in the matrix. On the other hand, other matrix properties, particularly the attachment between the cartilage and carrier was better for constructs from the flow chamber than for those from 12well plates.
This can be due to a better oxygen supply within the matrix close to the surface of the carrier. The best results were obtained from constructs cultivated under intermittent hydrostatic pressure and a decreased oxygen concentration in the gas phase. Initially, this phenomenon is difficult to understand. As the partial pressure of oxygen in the gas phase depends on total pressure, a higher pressure should even increase oxygen concentration significantly, leading to even worse matrix properties.
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