The Operation of Bioreactors

Bioreactors allow for different process strategies including batch, fed-batch, or continuous cultivation (Figs. 2.2 and 2.3).

Continuous perfusion, in particular, enables cultivation to be carried out under constant and controlled environmental conditions [44, 51, 68, 69, 73, 74]. Martin et al. [7] summarized some of the effects of direct perfusion on tissue-specific properties such as growth, differentiation and mineralized matrix deposition by bone cells, the proliferation of human oral keratinocytes, rates of albumin synthesis by hepatocytes, the expression of cardiac-specific markers by cardiomyocytes, and glucosaminoglycan (GAG) synthesis and matrix formation by chondrocytes (Fig. 2.4) [69].

On the other hand, a bioreactor system becomes more complex when additional features such as feeding pumps, vessels for fresh and spent medium, and control strategies are required, particularly in the case of mechanical stimulation. With regard to the formation of an implantable tissue, the bioreactor system must be integrated into the entire cultivation scheme, including biopsy, proliferation (cell expansion, usually in T-flasks), cell seeding of the bioreactor, tissue formation, and delivery to the site of application (e.g., the hospital). This is particularly important with regard to the manufacture of engineered tissue constructs for clinical applications, when good manufacturing practice (GMP) requirements must also be met [15, 16].

Fig. 2.2 The operation of tissue culture systems.

(a) Fresh medium is pumped continuously through the culture unit in a plug-flow. The required flow rate is determined by the substrate demands (especially oxygen) of the cells (1 = culture unit; 2 = circulation pump;

(b) Medium is pumped continuously from a medium vessel through the cultivation unit and back. The medium in the medium vessel can be changed at intervals (1 = culture unit; 2 = circulation pump; 3 = medium vessel).

(c) The culture unit consists of two culture chambers separated by a semipermeable membrane. Each culture chamber is supplied with fresh medium from an individual medium vessel. This technique is intended for the co-cultivation of different types of cells (1 = culture unit; 2 = circulation pumps; 3 = medium vessels).

Fig. 2.2 The operation of tissue culture systems.

(a) Fresh medium is pumped continuously through the culture unit in a plug-flow. The required flow rate is determined by the substrate demands (especially oxygen) of the cells (1 = culture unit; 2 = circulation pump;

(b) Medium is pumped continuously from a medium vessel through the cultivation unit and back. The medium in the medium vessel can be changed at intervals (1 = culture unit; 2 = circulation pump; 3 = medium vessel).

(c) The culture unit consists of two culture chambers separated by a semipermeable membrane. Each culture chamber is supplied with fresh medium from an individual medium vessel. This technique is intended for the co-cultivation of different types of cells (1 = culture unit; 2 = circulation pumps; 3 = medium vessels).

Fig. 2.3 The set-up of a flow-chamber bioreactor system to cultivate cartilage-carrier-constructs. The system consists of: (a) a flow chamber with inserts for tissue constructs; (b) peristaltic pumps for medium circulation; (c) a medium vessel; (d) a medium exchange bottle; (e) a humidifier; and (f) a flask to trap the exhaust gas. This set-up refers to a culture unit indicated in Fig. 2.1c (flow chamber with special inserts) operated according to the operation mode shown in Fig. 2.2b.

Fig. 2.3 The set-up of a flow-chamber bioreactor system to cultivate cartilage-carrier-constructs. The system consists of: (a) a flow chamber with inserts for tissue constructs; (b) peristaltic pumps for medium circulation; (c) a medium vessel; (d) a medium exchange bottle; (e) a humidifier; and (f) a flask to trap the exhaust gas. This set-up refers to a culture unit indicated in Fig. 2.1c (flow chamber with special inserts) operated according to the operation mode shown in Fig. 2.2b.

Fig. 2.4 Photograph (side view) of a cartilage-carrier-construct after cultivation in a flow chamber bioreactor on scale paper (scale 1 mm) (for comparison, see [69]).
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