Novel non rotational mixing

New type of up and down agitation has several additional advantages:

The up and down stirring produces no vortex (eddy) and therefore eliminates the need for baffles. This spares costs, simplifies setting up and cleaning of the vessel and allows the creation new types of vessels (because the large vessel opening otherwise necessary for baffle insertion is no more needed).

The very expensive head plates which are heavy, complicated and long to set up and clean are no longer used. Because no special head plates are required anymore, the cost of passing to vessels of different volumes is much lower. For the same reason, it is no more necessary to buy shorter or longer probes and all other parts*). In the MINIFOR system they are all reused and only few, low cost parts need to be changed (see Optimized vessel design).

*) This important costs lead many users to rather buy fermenters with the largest possible vessel volume and to work under more expensive and high volume conditions, even though this was not required by their experiments. With the LAMBDA MINIFOR fermenter-bioreactor system this is now a thing of the past! Clever users always try to work with the lowest possible medium volume as this offers so many benefits.

Disadvantages of traditional rotational mixing systems

Almost all laboratory fermenters-bioreactors use circular rotation to agitate the culture medium.

The major technical problem is that the axis of the stirrer (and the motors axis) rotates while the vessel is fixed. Thus, it is a physical necessity that a free space must exist between both, the moving axis and the immobile vessel, otherwise the rotation of the axis would not be possible. This free space allows viruses and microorganisms to get into the vessel. To limit the probability of contamination three ways are used:Almost all laboratory fermenters-bioreactors use circular rotation to agitate the culture medium.

• 1. The cheapest and less efficient solution is the use of so called lip-seals, which consist of elastic material with a central opening smaller than the axis diameter. This lip pushes onto the axis surface and should make the system tight. At the beginning, the closure can be satisfactory, but with time and especially at high rotation speed the lip is used up and the seal is no longer tight. Contaminating microorganisms can penetrate into the vessel. Therefore, such a system is not recommended for long time cultures or continuous cultures. By the way, even one major provider gives the following advice: "A magnetic stirrer assembly is available by special order if contamination-free work is critical."....

• 2. The second solution is the so-called mechanical seal or axial face seal. In this mechanically more advanced joint the stirrer axis is connected to the head plate by two discs, which glide on each other under a given pressure. The problem of this solution is that the system is mechanically stable only for certain time and if medium salts dry out between these discs their destruction is fast and contamination inevitable. Hence, they must be changed even though they are quite expensive. Much larger seals of this type are used in large, industrial scale fermentors. However, because of the knowledge of the mentioned problems, they are used in double sets with sterile water in between to protect the culture if the packing breaks during a run.

• 3. Today's best solution with respect to contamination problems connected with the rotational stirring is the magnetic coupling. The stirrer axis is completely separated from the motor axis and from the outside environment of the vessel and the driving force is transmitted by two sets of magnets. Since the magnetic force diminishes strongly with distance between poles, the slot separating the rotating cup and the stationary one is very narrow. Frequently, medium deposits and dries out in this space which leads to problems.

Because of the length of the axis and high transmitted force the magnetic coupling is technically quite complex and very expensive. For this reason, it is never proposed as standard equipment for laboratory fermenters. The client can sometimes buy it as an expensive option. In this way, the initial prices of many laboratory fermenters are kept lower despite of expensive consequences for the client at a later stage, when he is basically forced to buy the magnetic coupling option from the same producer.

Conclusion

LAMBDA has found a very simple, innovative solution for this mixing problem by selecting a non-rotational vertical up and down mixing solution. A simple elastic membrane allows the movement of the stirring axis and serves at the same time as a quality seal between the vessel and its central threaded cap. The membrane separates completely the interior of the vessel from the outside environment and this at low cost for the user.