Explain what a bioreactor controls, the batch, fed-batch and perfusion modes, and what changes when you scale up.
Why it matters
A shake flask is fine for a few hundred millilitres. Producing a vaccine or a monoclonal antibody for thousands of patients needs bioreactors of hundreds to thousands of litres, run identically batch after batch. Technicians in upstream manufacturing set up, monitor and sample bioreactors, and must understand what each control loop is doing and why a parameter drifting out of range matters.
The stirred-tank bioreactor
The most common design is a stirred tank: a vessel (stainless steel, or a single-use plastic bag inside a support) with:
- an impeller that mixes the culture;
- a sparger that bubbles air, oxygen, nitrogen or CO2 into the liquid;
- probes for temperature, pH and dissolved oxygen (DO), plus often online cell density, and off-gas analysis;
- ports for adding base, feeds and antifoam, and for taking samples aseptically;
- a jacket or heating blanket for temperature control.
A control system reads the probes and adjusts the equipment to hold each parameter at its set point.
The main control parameters
| Parameter | Typical mammalian set point | How it is controlled |
|---|---|---|
| Temperature | about 36 to 37 °C (sometimes shifted lower later in the run) | Jacket heating and cooling |
| pH | about 6.9 to 7.2 | CO2 sparging lowers pH; base addition raises it |
| Dissolved oxygen | often 30 to 50 percent of air saturation | Sparging air or O2, agitation speed |
| Agitation | set to mix well without damaging cells | Impeller speed |
| Feeds, glucose | kept in a target range | Scheduled or controlled feed additions |
| Foam | kept low | Antifoam addition |
Microbial fermentations run hotter and much faster, and need far more oxygen, so their agitation and aeration are more intense. Oxygen transfer is often the limit, described by the kLa (volumetric mass transfer coefficient).
Operating modes
- Batch: everything is added at the start; cells grow until a nutrient runs out. Simple but low yield.
- Fed-batch: concentrated nutrient feeds are added during the run, extending growth and production. This is the standard for most monoclonal antibody production in CHO (Chinese hamster ovary) cells, typically over about two weeks.
- Perfusion: fresh medium flows in while spent medium (and often product) is removed, with cells retained by a filter. High cell densities and continuous production, but more complex.
Scale-up
Production starts from a vial of the working cell bank, which is expanded through a seed train of progressively larger flasks and bioreactors until there are enough cells to inoculate the production bioreactor.
Scaling up is hard because not everything scales together. In a bigger tank:
- Mixing time gets longer, so added base or feed takes longer to disperse, and local pH or nutrient gradients form.
- Oxygen transfer and CO2 removal change: dissolved CO2 can build up in large tanks.
- Shear from the impeller tip and bursting bubbles can damage sensitive cells.
- The surface-to-volume ratio falls, so heat removal and aeration behave differently.
Engineers therefore choose which criteria to keep constant, commonly power input per volume (P/V), kLa, impeller tip speed or gas flow per volume (vvm), and accept changes in others. Scale-down models (small bioreactors that mimic the large one) are used to study problems without running full-size batches.
Worked example. A 2 L bench bioreactor runs at 0.1 vvm air (0.1 volume of gas per volume of liquid per minute), which is 0.2 L/min. Keeping vvm constant at 2,000 L means 200 L/min. Keeping superficial gas velocity constant instead would give a very different number, which is why scale-up needs a deliberate choice and verification runs.
Process data and GMP
In GMP manufacturing, critical process parameters and their ranges are defined during development and validation. Excursions outside the ranges are deviations that must be investigated. Batch data (set points, alarms, samples, operator entries) form part of the batch record you will review in module 3. Real bioreactor operation is learned on the job, under supervision and the employer's training programme.
Common mistakes
- Treating a DO or pH probe reading as truth without regular calibration and offline checks.
- Ignoring dissolved CO2 build-up at large scale.
- Changing agitation to fix DO without considering shear.
- Breaking sterility at a sampling port.
- Assuming a process that works at 2 L will behave the same at 2,000 L.
Quick recap
- Stirred-tank bioreactors control temperature, pH, DO, agitation, feeds and foam.
- Batch, fed-batch (most common for mAbs) and perfusion modes.
- Seed train from the working cell bank to the production bioreactor.
- Scale-up changes mixing, oxygen and CO2 transfer, shear and heat; pick scale-up criteria deliberately.
Try it
The Do tab has a quiz on control parameters and scale-up.