07 Bioprocessing and Bioreactors

Bioreactors and vessel design

A bioreactor provides a controlled environment for growing cells or microorganisms and often producing a desired product. In upstream production, cells are prepared and expanded, cultivated under defined conditions, and then harvested for downstream processing. Design and operation aim to provide a suitable, reproducible environment and support reliable transfer from laboratory to manufacturing scale.

Vessel design and functions

A common production system is the stirred-tank bioreactor. Its closed, cleanable vessel holds the culture medium and cells, and the vessel and its connections must support aseptic operation. An impeller and motor agitate the culture to mix nutrients, distribute heat, and help disperse gas; the impeller type and speed affect mixing and the forces experienced by cells.

A sparger introduces air or other gases as bubbles. Oxygen transfer is especially important in aerobic cultures, although excessive aeration or agitation can harm shear-sensitive cells. Baffles reduce swirling and improve mixing in many stirred tanks, while a jacket or other heat-transfer surface helps maintain target temperature. Sensors, ports, and controls allow measurement, sampling, feeding, and adjustment of operating conditions.

Rocking-motion bags and other single-use vessels achieve mixing and gas transfer differently. The appropriate design depends on the organism or cell type, production objective, scale, and process requirements.

Upstream production

Upstream production commonly starts with a preserved cell bank or microbial stock. A small culture is revived and expanded through a seed train—a sequence of progressively larger cultures—before inoculation into the production bioreactor.

The process usually includes vessel preparation, sterilization or other validated contamination-control steps, inoculation, cultivation, and harvest. Process controls support consistent cell growth and product formation. Monitoring may include growth, viability, and productivity, alongside attention to bioburden and contamination risks.

Cultivation modes

Production cultures can be operated in different modes, depending on how nutrients and culture broth are handled.

• Batch: The main nutrients are supplied at the start, and the culture proceeds until harvest.

• Fed-batch: Additional nutrients are fed during cultivation. Feed rate can help manage growth and product formation.

• Continuous: Fresh medium enters while culture broth is removed, maintaining ongoing operation under defined conditions.

Monitoring and oxygen transfer

Operators monitor variables that influence cell growth and product quality. Common measurements include temperature, pH, dissolved oxygen (DO), agitation, gas flow, pressure, foam, and liquid level. Cell density, viability, nutrient use, and product concentration may be assessed using in-line or at-line instruments, or by analyzing samples.

For an aerobic culture, oxygen transfer is described by:

Here, $\text{OTR}$ is the oxygen transfer rate, $k_La$ is the volumetric oxygen-transfer coefficient, $C^*$ is the oxygen concentration at saturation, and $C_L$ is the dissolved oxygen concentration in the culture. The relationship shows that oxygen supply depends on both the reactor’s transfer capacity and the difference between saturation concentration and the current dissolved oxygen concentration. DO monitoring is therefore important when oxygen demand is high.

Process control

A control system can respond to low DO by increasing agitation, increasing airflow, or enriching the inlet gas with oxygen, subject to process limits. Operators may adjust pH through controlled addition of acid, base, or gas.

Operating variables interact. For example, increasing agitation may improve oxygen transfer but also increase energy input and cell stress. Monitoring and control strategies should therefore be set for the particular culture and product.

Scale-up and process representation

Scale-up transfers a process from a smaller vessel to a larger one while aiming to preserve the conditions that matter to cells and products. Simply enlarging vessel dimensions does not guarantee equivalent performance: large vessels can mix more slowly and develop gradients in oxygen, nutrients, pH, or temperature.

Engineers select one or more criteria to guide scale-up, such as:

• Maintaining constant oxygen-transfer capacity, $k_La$.

• Maintaining constant power input per unit volume, $P/V$.

• Keeping mixing time or impeller tip speed similar.

• Maintaining an adequate DO level.

These criteria generally cannot all be held constant at once, and no single criterion works for every process. The choice depends on the culture’s oxygen demand, sensitivity to shear, mixing needs, and other process characteristics. Scale-down models can help test how cells respond to conditions or gradients that may occur in larger equipment.

• oxygen transfer rate (OTR)