Low surface tension

PharmTech: Low surface tension of some adjuvant solutions can reduce the efficiency of filters' bacterial retention. How can this problem be mitigated?

Bromm (Sartorius Stedim): It is advisable and required by regulators to carry out a comprehensive filter validation study, including bacteria-retention testing, simulating worst-case process parameters with actual product formulation using process related (i.e., pleated) scale-down filter devices. The design of the filtration system should consider reducing filtration time and differential pressure because these two parameters, among others, may increase the risk for bacterial breakthrough. During a filter evaluation study, the impact of different inlet pressure filtration conditions should be assessed, including constant flow or constant pressure conditions. Constant flow conditions may increase the risk of bacterial breakthrough, because of the increased differential pressure required to keep the flow constant during the filtration process and increased filter blocking.

Martin (Pall): Statistical and empirical studies at Pall Corporation have identified low surface tension of some adjuvant solutions as a risk factor for reduced bacterial retention efficiency of most sterilizing grade 0.2 micron rated membrane filters. The mechanism by which bacterial retention is reduced under lower surface tension in these fluids is not yet fully elucidated. Some mitigating factors appear to be membrane structure and layering of multilayer media, operating conditions, as well as reduction of bacterial bioburden or challenge levels and reducing challenge duration. Fluid surface tension affects the interactions between the bacteria and the membrane flow-path surfaces, but detailed mechanisms are not well known and specific surface tension thresholds cannot be determined.

Membrane surface chemistry is also an element that may mitigate the negative impact of fluid surface tension. Determining how and to what extent membrane-surface chemistry can enhance retention requires extensive studies. Filters with positive zeta potential, which provide enhanced adsorptive removal properties for bacteria in aqueous ionic solutions, have been used in the past for such purposes. This was also one of the capability advantages of asbestos-containing filters, although these are no longer used because of asbestos safety concerns.

Koklitis (3M): Such reduced filter efficiency can be related to the mechanisms involved in bacterial retention, which can be based not only on sieving but also on entrapment and electrostatic attraction. The adsorption of bacteria to the membrane polymer surface can be caused by any combination of forces, including hydrogen bonding, charge-induced, and Van der Waals interactions. The presence of liposomes, oils, or surfactants in a process stream can disrupt these adsorptive interactions and consequently reduce retention of bacteria within the membrane structure.

When there may be a high risk of bacterial penetration, it should be identified and considered in the planning of a filter validation study. The required minimum bacteria challenge (1 × 10⁷ colony forming units of Brevundimonas diminuta per cm² effective filter area) must apply, although an upper challenge level can be considered and restricted to one log higher. In a full-scale production process, the bacterial challenge to the final filter membrane may be controlled by introducing a prefiltration stage that has been demonstrated to be effective for bioburden reduction. The careful management and control of the operating conditions during process filtration will also help mitigate the risk of bacterial penetration, with attention to flow rate and filter area sizing to avoid high pressure drop.

Powell (Asahi): This issue is typically not applicable to Asahi products, but with some filters, the lower surface tension can change the effective porosity rating of the membrane, allowing larger particles to slip through the membrane's holes. These low viscosity adjuvants effect the thickness of the boundary layer (where flow velocities at the membrane surface are at or close to zero) which, in turn, alters the effective pore size under those conditions. It can also affect how the API and contaminants build up around the membrane's pores hence altering the effective pore size. One can screen different membrane types, porosities, and brands of filters, and work closely with the membrane filter supplier to choose the best filter for the application.