Inflating single-use bioreactor bags using dual-valve pressure controllers

Inflating single-use bioreactor bags using dual-valve pressure controllers

Background: installing & inflating single-use vessels

Single-use bioreactors (SUBs) offer key advantages including increased sterility and subsequently simpler and faster changeover between batches or campaigns. After the product is removed from the bioreactor, the single-use bioreactor bag can simply be removed from its holding container and disposed of (generally via incineration). The new vessel can then be installed into the same container, with no need for expensive and time-consuming cleaning or sterilization skids and processes.

The installation specifics will vary by bioreactor manufacturer, but all will include an inflation step after the bag is placed into the container. At this point, the various gas lines and sensing ports are all sealed shut, and the bag is inflated with only an exhaust valve open to prevent overpressurization.

Once the bag is fully inflated, the sensing, sampling, and other ports can be used. Following final installation steps, the culture medium and cell strain will be added and the bioprocess can proceed.

Challenges: safety, automation, and process integrity

Schematic showing an Alicat PCD used to inflate a closed volume, representative of a single-use bioreactor bag.

A typical Alicat PCD (dual-valve pressure controller) setup. When inflating a SUB vessel, the closed volume shown here represents the bioreactor bag. The PCD initially inflates the bag to the chosen pressure setpoint, then actively relieves the excess air pressure as the bag is filled with liquid at a constant volumetric flow rate.

Bioreactor bags are not pressure vessels, and their integrity (and subsequently the integrity of the overall bioprocess) will be compromised should they be over-pressurized during either inflation or operation – even if it doesn’t burst, leak, or otherwise fail. The appropriate pressurization must therefore take into account gases which will be introduced, consumed, and generated during the growth period, as well as the influences of culture medium, antifoaming agents, and other fluids introduced into the broth.

It is also necessary for the vessel to be sufficiently inflated during installation. This ensures that the bag can support the full installation and setup process, including the introduction of the various fluids.

The inflation process may be fully or partially automated, with controls built into the overall bioreactor controller system. In some facilities, inflation may be a completely manual affair, requiring an operator to start the process while making sure the system doesn’t over-pressurize. In either case, inflation may be completed using only a pressure controller. However, SUB manufacturers also recommend using a pressure sensor and transducer for best monitoring of the inflation process.

Various safeties work to ensure that the bioreactor bag is not damaged during setup, ranging from alarms built into controllers or pressure transducers to interlocks that prevent inflation when the bag is improperly installed. The inflation process generally takes less than 30 minutes, but is ultimately dependent on  the volume of the vessel and the fill rate of the gas.

Example: dual-valve pressure controllers to inflate single-use bioreactor bags

Single-use bioreactors range in scale from milliliters to about 5,000 L, with several bag sizes at each order of magnitude. For example, a manufacturer may make bags at 50, 100, 250, 1,000 and 2,000 L, all of which will need to be inflated as part of installation.

These bags will come with instructions to fill them with air to a specific pressure setpoint. Additional pressure may be introduced (often about 1 PSIG) to remove any creases or folds in the bag based upon visual inspection. The dual-valve pressure controller is used to ensure that the air in the bioreactor does not exceed the manufacturer’s designed fill pressure, maintaining the integrity of the vessel at all times.

Following inflation, the bag is filled with liquid for the bioreaction to proceed. At this point, the air pressure in the bag can be exhausted at a rate matching the incoming liquid flow. As the pressure controller was designed with valves to accommodate specific inlet and outlet airflow rates, one Alicat dual-valve pressure controller can be used to manage singe-use bioreactor bag pressures at all phases of the reaction process.

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