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Selection Criteria For Bioprocess Addition Vessels

Mon, 07/14/2008 - 11:44am

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Untitled Document
Key considerations for your biopharmaceutical facility
By Karin Koller
Bioengineering AGddition vessels are used as components in fermentation or cell culture plants for dosing, storage, transfer or harvest of liquids. Inadequate vessels can lead to the contamination and loss of product samples and as such, deserve proper attention and consideration.

When contemplating the purchase of a biopharmaceutical processing plant, the user has many options to consider. One such consideration is the type and functionality of the vessels that will be employed, as well as the measurements, controls and aeration systems of those vessels as central components of the plant. Careful selection of these components is critical for smooth operation, but with the many stresses of such an acquisition, is there ample time to devote to these seemingly more insignificant components? Are they merely storage vessels, or are they truly vital to the successful operation of the plant?

Addition vessels are necessary, even on the most simple fermenters or cell culture reactors. They provide storage for acids and bases involved in pH-control processes as well as antifoam agents for foam control. Other dosing agents in more complex plants, such as precursor, media, vitamins and minerals, or nitrogen sources can determine the success or failure of the cultivation process. If no pre-culture reactors are present, the inoculation is carried out with addition vessels. Such vessels are not only used for dosing liquids, but for harvesting the product, transfer of intermediates and active ingredients, and storage of culture broth before down-stream processing.
Connection to the reactor

Each application involving these vessels requires transport to or from the reactors. As such, the transfer has to be sterile and in most cases must comply with strict regulations (e.g. cGMP). The addition and removal of liquids represent, by far, the greatest contamination risk to a properly sterilized fermenter or cell culture reactor. The addition vessels, along with the liquids stored in them, must be properly and reproducibly sterilized to ensure the sterility of the whole system. The weak link in maintaining a sterile process chain is often the connection linking the reactor and the addition vessel. Every plant operator must decide if the techniques they use to establish these connections are safe enough to ensure sterile operation are in compliance with the regulations, and perhaps most importantly, the highest quality product.

Since the design of the addition vessel ultimately determines the method of the connection, careful selection of these vessels becomes significantly more important than it may have seemed at first glance. Liquid transfer must be precise and reliable in order to deliver exact dosages, properly control culture conditions, provide worry-free operation of the fed-batch and process the culture broth as efficiently as possible. If the reactor becomes contaminated, its contents must be discarded and the process has to be aborted. This leads to significant losses of both time and money.
Process suitability

Several other selection criteria are directly related to liquid transfer and process application. It is of utmost importance to define how the addition vessel is to be sterilized during the process. For example, if the dosed liquids also have to be heat sterilized, they should be autoclaved directly in the addition vessel. If they are sterile filtered, the addition vessel should be steam sterilized to save time and work. To prevent the formation of vacuums, addition vessels should be vented with sterile air. Depending on the plant's control system, the dosing is achieved with pumps or valves. With valve control the liquid is transported with compressed air. The addition vessel must therefore be pressure resistant.

Additionally, in order to handle the required tasks, the vessel must be equipped with a sufficient number of ports and connectors. For safe dosing with compressed air, a pressure gauge is required to monitor the internal pressure of the vessel. A sterile filter is needed as a vent if the transfer is done by pump. Liquids can't be refilled if all the available ports are occupied. Foresight into such details can help to avoid compromising the entire process.

Not only must the design be considered, but also the materials from which the vessels are made. Glass vessels must be pressure and heat resistant while steel vessels must be manufactured from stainless steel (e. g. 316L). Just as with all other plant components that come into contact with the product, surface roughness must not exceed the specified value. Seals must be hygienic and made from approved material for the application since unsuitable materials could lead to corrosion or dissolution of the substances being flushed into the product.

These materials and the design of the vessel must be suitable in order for it to be completely and efficiently cleaned. Any crevices, roughness, dead-legs or unevenness will be difficult to clean and sterilize. When substances remain in these places, microorganisms can proliferate uninhibited, and greatly increase the risk of contamination.

Otherwise healthy plants can watch as the operation comes to a standstill because of unvalidated and inadequate addition vessels. They can cause contamination of the product as well as excessive labor input if not carefully chosen, and as such, their selection should not be taken lightly.
One vessel for all purposes

Addition and storage vessels are also needed for a multitude of tasks involved in unsterile operation. Most, if not all of those vessels are unsuitable for aseptic applications, and not all vessels suitable for sterile operation can be used freely for all steps of the process.

To ensure operational safety and applicational suitability, some addition and storage vessels such as those built by Bioengineering have been designed for sterile use in the biotech and pharmaceutical industries. These vessels are made of stainless steel with a surface roughness of Ra 0.8µm in areas in contact with the product, and they come in volumes ranging from 5 to 60 liters. Alternatively, vessels made of borosilicate glass reinforced with epoxy resin offer solutions in smaller plants. Both the glass and steel vessels can be autoclavable and steam sterilizable so liquids can be sterilized directly in the vessels or separately according to the techniques described above. Steel vessels come equipped with three DN19 ports fitted with standard sterile connections, sterile connections with an immersion tube, hypodermic needles with a cup and diaphragm, blind plugs, sterile filters, and pressure gauges. Everything from the number of ports, the multitude of accessories, and the combination of options offer limitless applications for sterile liquid transfer with pumps or compressed air, including sterile refill and aseptic replacement of connectors.

In the case of harvest or transfer from the vessels, the connection is made using the hypodermic needle and diaphragm. The diaphragms are secured with a blind plug and the connection is established when the needle is removed from the cup, piercing through the diaphragm, and screwed tightly.

Vessels such as these are designed to be simple and intuitive to make cleaning and sterilization regulation compliance as easy as possible, yet because of their modular components, they are also ideal for numerous aseptic liquid transfer applications. The vessels can be fully integrated into a plant for dosing, storage, harvest and transport of sterile liquids. Above all, it is important to remember that only through the selection of suitable, high-grade components, can the smooth and sterile operation of the entire plant be ensured.
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