Vaccines are very much in the public eye now because of the Covid 19 pandemic. The use of vacuum is integral not only to their manufacture but also for some of the vaccines such as the Pfizer Biontech which requires a temperature of -60oC for its storage and transport.
The use of vacuum for freeze drying in the pharmaceutical industry is well known but there are several steps within the synthesis where vacuum is crucial. Purification within the manufacturing process is paramount and an ultra-high-speed centrifuge is employed to facilitate this step. The different settling coefficients or buoyancy density of the components within the mixture allows the purification process to be achieved. High rotational speeds of more than 30,000 RPM are required to produce complete separation of the active species and unwanted contaminants. Such large rotational speeds will cause air friction within the mixture and result in heat generation which could damage the active components. Use of a high vacuum pump system incorporating a turbomolecular pump (TMP), and ideally a dry vacuum pump, allows heat to be extracted from the mixture as illustrated below.
The key components of vaccines are active microorganisms and enzymes are which are live and must remain so to be effective.
The finished live vaccine is mixed with a water-based stabiliser to form a suspension and then the material is frozen. A vacuum is then applied with a little heat, such that the ice changes from solid to vapour or sublimes. Because of the low temperature of the sublimation process, the components of the vaccine remain active and undamaged.
The phase diagram for water shown below illustrates how at low pressure the solid ice changes directly to a vapour, with no intermediate liquid phase involved.
Vapour pressure curve/saturated vapor pressure curve
Phase diagram for water
The freeze-dried vaccine can be sealed and stored under vacuum; this has the advantages of offering a long shelf life, rapid dissolution with diluent during use, and unchanged recovery characteristics. It is currently the most common method of preservation of live vaccines.
Glass Vial production
Prior to transportation and dispensing, the vaccine is dispensed into glass vials. The correct choice of glass is crucial to maintain the efficacy of the vaccine, and only low borosilicate glass has the required high chemical stability needed to achieve long term stability of the vaccine. In addition, it has excellent stability to thermal expansion and contraction, important for long term storage at below-ambient temperatures.
Vacuum is required in two stages of the production of borosilicate vials:
- The melting process to remove air trapped in the glass, typically operating at around 50 mbar pressure. Glass dust and high temperatures are issues which must be addressed, and traditionally liquid ring pumps have been used. But increasingly, to reduce running costs, both oil rotary vane and screw pumps are employed. Dry screw pumps offer an oil free alternative.
- The moulding process requires vacuum levels of around 100mbar. Short pumpdown times and continuous operation are key, and again oil rotary vane and screw pumps, as well as dry screw pumps, are increasingly used.
Transportation and Storage prior to use.
As mentioned, the first approved vaccine, Pfizer Biontech, requires storage at -60o C. Maintaining this temperature presents a significant challenge. The use of Vacuum Insulation Panel (VIP) technology offers a method to maintain these temperatures in an energy efficient way. VIP offers very low thermal conductivity of 0.004W (m.K), with a typical container wall thickness of 25 -60mm. By comparison, conventional mineral wool of 150mm thickness would have a value of 0.04W (m.K).
This means not only greater efficiency but also more storage within the refrigeration unit.
This technology has wider applications for insulating older buildings without significant loss of internal space, whilst greatly reducing the carbon footprint.
The structure of VIP comprises three parts: insulating material, a gas adsorption material (Getter) and a closed insulating film (barrier). This closed insulating barrier is pumped to a high vacuum level before sealing, thus offering exceptional insulation properties.
A typical vacuum system is shown below:
The high speed forevacuum pumping train minimises the time before the diffusion pump kicks in, to give rapid turnaround of the panels.
- Use of a high vacuum pumping system in conjunction with an Ultra-High-Speed centrifuge allows vaccine purification whilst minimising any detrimental effect of heat on the product.
- Vacuum freeze drying offers long term bulk storage prior to transfer to dispensing vials.
- Production of vials is dependent on vacuum for air removal in the melting process, and additionally vacuum is key to uniform moulding of the vials.
- Vacuum Insulating Panel technology gives reliable and energy efficient low temperatures crucial to the long-term stability of some vaccines.