Guaranteeing aseptic liquid processing

Aseptic processing operations place great demands on conventional technology and none more so than in the mixing process, which inherently agitates materials and often places them under pressure.

Although advances have been made in sealing techniques, utilising methods such as cartridge and gas seals can only control leakage, not guarantee total zero ingress or egress – of particular concern where potent materials are being used – over the lifetime of the product. For a number of applications, therefore, the solution lies in seal-free magnetic mixing technology where the requirement for a mechanical connection between the mixer drive and the in-tank impeller is dispensed with, thus ensuring total integrity of the tank. Bottom-entry magnetically-driven mixers operate by transmitting torque with a magnetic field. Depending on the torque requirement, the same number of magnets – each a pole – are fitted to the impeller, which has a female bearing, and to a drive unit. The male bearing is attached to a tank plate and the drive unit connects to the exterior face of the tank plate. A magnetic field is transmitted through the non-magnetic tank wall between the impeller and the bearing and as there are the same number of poles in each, magnetic coupling is synchronous. The impeller will only operate slower than the drive unit if the maximum torque the mixer has been designed for is exceeded. This capability has been available for some time and is particularly relevant to the blending of liquid products in applications such as injectables, infusion solutions, plasma fractions and bacteria and cell cultures. Not surprisingly, therefore, it is the "coming of age" of biotechnology and the resultant increase in biopharma processing which has thrown the spotlight on seal-free technology. The key advantage for the biopharma sector and aseptic processing in general is the ability to CIP/SIP the tank contents – including the impeller – while keeping the contents hermetically sealed to the outside environment. However, it is vital this process is carried out as effectively as possible, as the products which benefit most from this capability are also those which demand the highest levels of purity. Consequently, NovAseptic set out to substantiate the "cleanability" of its NA-magnetic mixer and commissioned the independent research institute, the Biotechnological Institute¹ in Kolding, Denmark, to conduct the tests. Before testing, the test machine and the reference pipe (0.5mm Ra) were dismantled – the main working components of the mixer are the mixing head, male bearing and tank plate – thoroughly cleaned, degreased by hand and autoclaved at 120°C for 30 minutes. Both were then reassembled and soiled under pressure with a soured milk solution containing spores of the test strain Bacillus stearothermophilus var. calidolactis (NIZO C953). The test machine was also drained and dried in the vessel by flushing it with dry filtered air. Both the machine and the reference pipe were then cleaned in place by: • Rinsing with cold water for one minute • Circulating a 1% (w/v) detergent solution at 63°C ± 2°C for 10 minutes • Rinsing with cold water for one minute After cleaning, the inner surface of the reference pipe and the surface of the mixing head and the male bearing were covered with molten agar. Once this had solidified, the machine and reference pipe were placed in an incubator at 58°C for 24 hours. Following incubation, they were examined for the presence of yellow discolouration in the agar. The test was carried out five times, with the mixing head not covered with detergent solution in tests one to three and covered with 10cm of the solution in tests four and five. As Table 1 shows, using just a spray ball, the NA mixer is at least as cleanable in place as the reference pipe.

Particulate concerns The nature of biopharma process products – injectables, infusions, fractions and cultures – also makes it imperative that, apart from being biologically untainted, they must also be particulate-free, or at least as particulate-free as the USP standards demand. Thus, having verified the cleanability of its mixer, NovAseptic next set out to determine its performance against this standard. Particles are generated by the bearings, and the number and size of these particles will depend on both the material used and the design of the male and female bearings. With this in mind, both of these bearings on the NA-mixer are made from silicone carbide and are mounted using a heat shrinking method, therefore eliminating O-Rings or glue in their construction. This time the Kolding Institute's objective was to determine the size and quantity of the particulates in 0.22µm of filtered water after mixing at 75% of maximum speed for 30 and 60 minutes and 12, 24 and 48 hours. Two test methods were used – measurement of the particulate matter by weight and by the Light Obscuration Particle Count Test. Following a thorough cleaning of the impeller and silicon carbide male bearing which were to be the objects of the weight testing, control samples were taken of their weights and of the water. The tests were then carried out and even after 48 hours of mixing, no weight difference was observed for the impeller and just 0.0005g of weight loss was observed for the spindle. Not surprisingly, therefore, the total particulate count was acceptable at all times of testing and well within the USP specification of particulate matter for large volume parenterals. When translated into the working life of an NA mixer, these results show that just 1g of silicone carbide would be lost from the male bearing over a minimum of a five-year period if the machine worked five days week, eight hours a day. The tests show the advantages magnetic mixing can provide in the form of product integrity and quality, which are obviously of primary concern to manufacturers. However, there are also a number of other areas in which magnetic mixing technology has advantages. For example, improvements in productivity can be achieved as the CIP/SIP capabilities reduce downtime, while the seal-free technology simplifies maintenance routines and reduces both the time taken and the number of annual inspections required. Minimal particulate contamination can also shorten batch times by eliminating the requirement for post-mix filtration. The bottom-entry nature of magnetic mixing also confers certain advantages. For example, there is no requirement for a mechanical lifting device to raise the impeller mechanism and the machines have the ability to agitate even when virtually empty. Cleanable in place, ensured product integrity and quality, high productivity, small footprint and the ability to process minimal batches on any size machine – to date, magnetic mixers have only been ubiquitous in the laboratory, but the time for scale-up, especially in the biopharma sector, may well be upon us.