Tissue engineering, which means the generation of artificial three-dimensional (3D) tissues, is intended as a powerful tool for regenerative medicine and for drug screening [1-5]. 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 [2]. Cell-based therapy concepts include: (1) the direct transplantation ofisolated cells; (2) the implantation of a bioactive scaffold for the stimulation of cell growth within the original tissue; and (3) the implantation of a 3D biohybrid structure of a scaffold and cultured cells or tissue. Furthermore, nonimplantable tissue structures can be applied as external support devices (e.g., an extracorporal liver support when a compatible donor organ is not readily available [6, 7]), or engineered tissues can be used as in-vitro physiological models for studying disease pathogenesis and developing new molecular therapeutics (e.g., in-vitro assays for drug screening [7-9]).

For drug screening based on cell models, 2D cellular assays are mostly applied, using often well-described cell line models [10, 11]. These assays can be performed quite efficiently in high-throughput screening (HTS) systems, but to date their value for predicting the clinical response of new agents, especially with respect to cancer therapy, is limited. This lack of predictability of 2D cellular assays is attributed to the fact that such systems do not mimic the response of the 3D microenvironment present in a tissue, or tumor, in vivo [10, 12-14]; therefore, 3D-cellular assays are required. Compared to 3D-tissue cultures intended for medical applications, here the size of the tissue construct is not a major problem, as smaller tissue constructs are also appropriate. The main challenge is the efficient, reproducible handling of a large quantity of tissue constructs in parallel.

The increasing market in highly specific biological pharmaceuticals such as antibodies, cytokines, growth factors or cells and tissue products highlighted the need for specific human relevant test systems on immunofunctions. In-vitro assays and transgenic animal models can be used in this situation. The development and use of immunotests in vitro, as well as transgenic animal models, must consider the remarkable specificity ofthe immune systems ofdifferent mammalian species. The investigation of effects on human patients at the research level of drug screening, in addition to tests on potency in samples of process development and production and an adapted preclinical risk assessment, requires human in-vitro immunotests to be conducted.

Established in-vitro tests based on acute lymphocyte reactions for induced cytokine release or cell proliferation of freshly prepared blood samples, such as mixed lymphocyte reaction, are inadequate for investigations into complex or long-term effects such as induced hypersensitivity and allergy.

The effects of immunogenicity or immunotoxicity must have been monitored in long-term culture under physiological and histological equivalency of secondary lymphatic organs using primary cells, tissue preparations, or immunocompetent cell lines. Recently, the use of transgenic animals mimicking isolated functions of the human immunosystem has been inadequate. In the future, immune cell-based in-vitro test systems may have to compete with transgenic animal models having a reconstituted human immune system.

The in-vitro generation of 3D tissue constructs requires not only a biological model (e.g., an adequate source of proliferative cells with appropriate biological functions; a protocol for proliferating cells while maintaining the tissue-specific phenotype) but also the further development of new culture strategies, including bioreactor concepts [7, 15, 16]. Bioreactors established for the cultivation of microbes or mammalian cells under monitored and controlled environmental and operational conditions (e.g., pH, temperature, oxygen tension, nutrient supply) are mostly inapplicable to 3D tissue constructs. Furthermore, each type of tissue

2.2 Important Aspects for Bioreactor Design | 55

(e.g., skin, bone, blood vessels, cartilage) will likely require an individualized bioreactor design [15]. Therefore, tissue-specific bioreactors should be designed on the basis of a comprehensive understanding of biological and engineering aspects. Additionally, typical engineering aspects such as reliability, reproducibility, scalability and safety should be addressed [7, 16]. In the following sections, the 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 [6, 7, 15-24], and also reviewed [25]; therefore, they will be discussed only briefly at this point. Particular focus will be given to the interaction between biological and engineering aspects and the special demands of using these reactor systems for drug screening. Using an artificial immunosystem 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.

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