Relative Efficiencies of Double Filters or Tighter Filters ...

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iven the lack of standards for pore-size ratings, a performance measurement — namely, the retention of 1 107 Brevundimonas diminuta organisms per.
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Relative Efficiencies of Double Filters or Tighter Filters for Small-Organism Removals Theodore H. Meltzer, Maik W. Jornitz, and Peter R. Johnston*

Theodore H. Meltzer, PhD, is a consultant with Capitola Consult-

ing Co., Bethesda, MD. He is also a member of Pharmaceutical Technology’s Editorial Advisory Board. Maik W. Jornitz is director of product management with Sartorius AG, Göttingen, Germany. Peter R. Johnston, PE, is a consultant with Johnston & Associates, 302 Morningside Drive, Carrboro, NC 27510, tel./fax (919) 942-9092, e-mail ([email protected]). *To whom all correspondence should be addressed.

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PHOTOGEAR, INC.

There is concern in the area of pharmaceutical filtration regarding the dependable filtrative removal of nanobacteria and L-form bacteria from liquid suspensions by 0.2-m rated membranes qualified to retain Brevundimonas diminuta. Whether filter membranes with reduced pore diameters or those of double construction are more suited to the retention of organisms smaller than B. diminuta is a matter of relative filter efficiencies. This article discusses the several factors governing filter efficiency. Given the paucity of relevant experimental information, the authors rely upon theoretical issues. Filters of narrower flow-averaged pore size offer a more likely retention of smaller organisms. However, comparisons cannot be made among filters of different polymeric compositions because their propensities for adsorptive sequestrations are not known. The need for confirmation by experimental validation is clear.

iven the lack of standards for pore-size ratings, a performance measurement — namely, the retention of 1  107 Brevundimonas diminuta organisms per cm2 of membrane surface — serves to define a sterilizing filter. This definition would serve well if the organism retention mechanism were solely sieve-type captures, which is often conveniently, if erroneously, assumed. However, the influences of the various filtration conditions and the characteristics of the particle-suspending liquids governing the adsorptive sequestration of organisms complicate the picture. Consequently, a sterilizing filter, as qualified by a filter manufacturer in a particular (but nonstandard) filtration mode, may not perform as such in another mode. Absolute particle capture is achieved only when the smallest particle of a particle-size distribution is larger than the smallest pore of the pore-size distribution. Normally, absent this condition, filters do not act as perfect sieves. Relatively larger particles are retained by sieve arrests at the metering pores. Particles smaller than the pores are arrested by adsorption to the walls of the pores they penetrate (1,2). These surfaces are extensive. For example, if the thickness of the medium is 150 m and the flow-averaged diameter is 0.2 m, the ratio of the pore length to diameter is greater than 150  0.2  750. Moreover the pore passageways are tortuous, leading to even longer travel paths for penetrating particles. The increased pore length is promotive of adsorption effects. The situation is particularly aggravating in contexts wherein the capture of organisms smaller than B. diminuta is involved. Among the smaller organisms, the filtrative removal of which is currently of interest, are L-form bacteria, mycoplasmas, and nanobacteria (3–5). Such smaller organisms may not be sterile-filtered using 0.2-m rated membranes. Indeed, the L-forms found in sera, where the nanobacteria are also found, are not adequately removed by 0.1-m rated membranes, or even by 0.04-m rated filters

............................................................................ (6). Concerns about viable but nonculturable organisms are also increasing (7). In these circumstances, some researchers argue for the use of tighter filters, namely 0.1-m rated membranes, if only as an act of prudence for filtrative sterilizations involving organisms smaller than B. diminuta (8,9). The impracticality of the consequent flow rates — and the need for system revalidations when 0.1-m rated membranes are substituted for their 0.2-m rated counterparts — has been discussed in the literature (10,11). Other investigators opt for the use of double filters — two 0.2-m rated entities in series. (Here, reference is not made to filter constructions composed of two layers of microporous membrane that are necessitated because single layers cannot meet the desired retention values, as indicated by integrity testing.) The purpose of this article is to explore the relative filter efficiencies of both these filtration possibilities and to attempt to derive some guidance for the filtrative removal of organisms smaller than B. diminuta from pharmaceutical preparations. FILTER EFFICIENCY

The term filter efficiency is a characterization of the degree of completeness with which a given filter will remove a specific type of particle (organism) from its population under stipulated conditions. Filter efficiency is not an invariant property of a filter. It reflects the complex interplay of filtration conditions, filter characteristics, suspending-fluid qualities, and particle chemistry and size. The efficiency of a given filter may be different under different conditions, and small particles that adsorb onto filter surfaces may be captured with different degrees of efficiency in other circumstances. When determining filter efficiency, the influences making for adsorptive sequestrations, as well as for sieve retentions, are operative. Among these influences are the following. Filter composition. The polymeric composition of the filter plays a role in the exercise of its zeta potential, van der Waals forces, hydrogen bonding, and other charge-related allurements. Hydrophobic attractions without charge considerations are also possible. Membrane thickness is an important influence on filter efficiency (12–14). By adding to the length of the pore passageways, filter thickness slows the flow characteristics and affects adsorption. Some investigators hold that increased filter thickness, to a point, increases the bubble-point values (13). This has implications of enhanced sieve retentions. Others disagree (15,16). Pore size is not the only determinant of filter action. The extent of internal pore surface area encountered by a unit volume of feed stream is also important. Thus, a coarse, thick medium can be as efficient as a fine, thin one. Indeed, this is reflected in considering the choice between double filters and those of tighter dimensions for the capture of smaller organisms. Porosity. The various porosity considerations — as expressed by total porosity, pore-size average, and pore-size distribution — merit consideration, as do their possible alterations caused by subtle incompatibilities with the liquid preparation being filtered. Pore-size distribution is removed as a variable if the filtration media are composed of a random array of solids. In that case, the pore-size distribution is the same from one medium to another (16).

Temperature is important because it can influence organism proliferation and viability and because it affects the Brownian motion of the suspended organisms, increasing the likelihood of their adsorptive contact with pore walls. In this connection, the reciprocal relationship of temperature and viscosity bears notice. Brownian motion of colloids and visible particles (organisms) is the result of the vectors imparted to them by collisions with liquid (suspending) molecules, which themselves have movements in direct response to temperatures above absolute zero, along with the vibrational and rotational motions of their constituting atoms. These collisions may randomly impel a particle from within the stream to adsorptive contact with a pore surface. The smaller the particle or organism, the greater the Brownian-motion effect upon it. Viscosity has an influence on filter efficiency, independent of temperature, because of its limitations upon Brownian motion and inertial impactions imposed by higher viscosity values. To be sure, liquid flow through a filter medium is laminar, as shown by Rosenstein et al. (14). These investigators illustrated that a single fiber positioned in a liquid creates eddy currents, a manifestation of turbulent flow. No such currents develop within fibrous filter media. Filtrations are performed in the fluid velocity range — called viscous, Hagen-Poiseuille, or laminar flow — wherein the ratio of velocity to driving force is constant. Nevertheless, particles that are too small to be sieve-retained but that are positioned within liquid streams may depart the laminar flow pattern as a result of their inertial moment, particularly when the liquid flow direction alters. They may impact a pore surface to become adsorptively fixed. This effect is magnified in gas (air) filtrations wherein the lower viscosity of the fluids exerts a lesser drag on the particle. In addition, because inertia is the product of mass and the square of velocity, it favors the trapping of the larger (heavier) particles. Liquid composition. Composition of the suspending liquid is a consideration in filter efficiency because it may alter particleadsorbing forces. It may also change pore dimensions through polymer plasticization. Also, it may disturb the organism-size/ pore-size relationship by mutating the organism size, e.g., by high ionic strengths (17,18). Flow rate and the differential pressure that engenders it are factors that govern organism velocities during their passage through a pore system. These factors define the residence time of the particle in the pore passageways and affect the possibilities of adsorptive arrests. Mittelman et al. discuss these and certain other pore-size/ organism-size relationships that bear on the retention of organisms by microporous membranes (18).

RELATIVE FILTER EFFICIENCIES

It should be noted in particular how sparse are the data on the factors governing filter efficiencies. Yet the design of a filter or filter scheme to remove organisms smaller than B. diminuta will derive from a consideration of relative filter efficiencies. One can attempt to compare the relative filter efficiencies of two similar filters of different designs, provided that the pore characteristics and polymeric compositions are the same. A filter with smaller pores may be expected to be more retentive of smaller particles. However, the filter with more generously proportioned

............................................................................ pores may have the advantage of surface qualities that are more conducive to organism removal by adsorption — positivecharge membranes, for instance. Therefore, comparisons of filters of different compositions, like those of different porosity characteristics (usually unknown and, in any case, nonstandard), are difficult, if not impossible, to address. Given an organism challenge level, the ratio of the challenge density to the number of microbes penetrating the filter is represented as R. It turns out that working arithmetically with log R is easier than calculating by way of R. This is often referred to as the log reduction value (LRV). It is a description of the efficiency of a filter. Consider a challenge of 100 organisms, of which 2 penetrate. The filter retained 98% of the confronting organisms. The filter efficiency — the log of the ratio, R — is 100  2  50, and log R is 1.7. Doubling the filter thickness would increase the LRV by a factor of 2, to 3.4. If the number of challenging organisms were 1000, and 2 escaped capture, then R would be 1000  2  500, and log R would be 2.7. Using such calculations requires the analytical availability of the challenging and penetrating organism populations. With the concerns regarding L-form organisms, nanobacteria, and viable but nonculturable microbes, such quantitation is not available. A reasonable rule of thumb is that doubling a filter will increase its LRV (its organism removal efficiency) by a factor of 2.

log R2 Z2 2   2 log R1 Z1 1 If a membrane with narrower pores and a flow-averaged pore diameter were to be confronted with the same challenges, then 2

log R2 d 1   1  4 4 log R1 d 22 0.5 2 1 The increase in LRV caused by halving the flow-averaged pore diameter is fourfold. Specifically, given a membrane with d1  0.2-m rating, the d2 required for the same efficiency, double log R, would be

log R2 2 0.2 2   2 log R1 1 d2 0.2 d2   0.14 m 2 The 0.14-m rated membrane would have the same capture efficiency as a double 0.2-m rated filter. The 0.1-m rated filter would offer a greater efficiency than would its double 0.2m rated counterparts (19). To equal the fourfold increase in LRV occasioned by the tighter membrane, four layers of media would be needed. It bears repetition, however, that this analysis is not based upon the parameters of any actual filtration, nor does it offer any description of the separations involved. From this mathematical exercise it can be concluded that the use of a double filter would not offer as much advantage to the retention of smaller organisms as would the use of a filter of tighter construction. However, given the inherent simplifying assumptions involved, definitive conclusions might not be prudent short of actual trial measurements. This is particularly true

when competitive filters are being evaluated. In such cases, it is likely that porosity characteristics are not the same. Additionally, almost no data exist relating to pore-size distributions, extents of pore surfaces, adsorption tendencies, etc. And poresize ratings are not standardized in terms of interpreting comparative pore dimensions. CONCLUSIONS

Under present circumstances, except the above analysis, there can be little meaningful guidance, whether from regulatory bodies or filtration engineers, directing filtration activities to the use of double filters or tighter filters in the quest for more reliable retentions. Until research investigations generate the required missing data, reliance can be acquired only by trial and error. This poses a problem especially for regulatory bodies, whose obligation to protect the public outstrips the available data essential for technical certainty. Be that as it may, at present there is no way, except for trial and error, to determine which among competitive filters (either double filters or tighter filters) would be more retentive of smaller organisms. For this purpose, the bioburden facing the filter and the propensities for adsorption would have to be known. With regard to the choice of double filters or tighter filters, fluids with higher viscosities may profit from the use of double filters. These would limit flow rates to a lesser degree (a consequence of viscosity) and would benefit from the dependence of flow upon the fourth powers of the pore radius in accordance with the Hagen-Poiseuille teaching. Otherwise, use of tighter filters rather than double filters is indicated. In the absence of reliable guidance based on experimental data, dependence upon hypotheses may be necessary. It would be best, however, that such recommendations be justified by validation exercises. From such endeavors alone can assurances of filter efficiencies and filter appropriateness be made. REFERENCES 1. G.B. Tanny, D.K. Strong, W.G. Presswood, and T.H. Meltzer, “The Adsorptive Removal of Pseudomonas diminuta by Membrane Filters,” J. Parenteral Drug Assoc. 33 (1), 40–51 (1979). 2. U. Osumi, N. Yamada, and U. Toya, “Bacterial Retention Mechanisms of Membrane Filters,” PDA J. Pharm. Sci. Technol. 50 (1), 30–34 (1996). 3. K.K. Akerman, I. Kuronen, and E.O. Kajander, “Scanning Electron Microscopy of Nanobacteria — Novel Biofilm Producing Organisms in Blood,” Scanning 15, Suppl. III (1993). 4. E.O. Kajander, K. Kuronen, and N. Ciftcioglu, “Fetal Bovine Serum: Discovery of Nanobacteria,” Mol. Biol. Cell 7 (Suppl.), 517a (1996). 5. P. Hargreaves, “Paul Hargreaves Speaks Out on Pharmaceutical Manufacturing,” PDA Letter 29, 10–13 (1993). 6. J.W. Haycock et al., “Bacterial L-Forms Are Omnipresent in ‘Sterile’ Culture Sera,” unpublished communication. 7. R.R. Colwell and A. Huq, “Viable but Non-Culturable Bacteria and Their Implications for Water Purification,” Ultrapure Water 122 (3), 67–74 (1995). 8. P. Blosse, E. Boulter, and S. Sundaram, “Diminutive Bacteria — Implications for Sterile Filtration,” Amer. Biotechnol. Lab. 16 (12), 38–40 (1998). 9. S. Sundaram et al., “Application of Membrane Filtration for Removal of Diminutive Bioburden Organisms in Pharmaceutical Products and Processes,” PDA J. Pharm. Sci. Technol., in press. 10. T.H. Meltzer, M.W. Jornitz, and A.M. Trotter, “Application-

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11. 12.

13.

14.

Directed Selection of 0.1-m or 0.2-m Rated Sterilizing Filters,” Pharm. Technol. 22 (9), 116–122 (1998). M.W. Jornitz and T.H. Meltzer, “Sterile Double Filtration,” Pharm. Technol. 22 (10), 92–100 (1998). H.W. Piekaar and L.A. Klarenburg, “Aerosol Filters: Pore-Size Distribution in Fibrous Filters,” Chem. Eng. Sci. 22, 1399–1408 (1967). D.B. Pall and E.A. Kirnbauer, “Bacteria Removal Prediction in Membrane Filters,” presented at 52nd Colloids and Surfaces Symposium, University of Tennessee, Knoxville, 1978. N.D. Rosenstein, A. Dybbs, and R.V. Edwards, “Nonlinear Flow in a Porous Medium,” publication FTAS/TR (Cleveland: Case Western Reserve University, Dept. of Mechanical and Aerospace Engineering, 1980).

15. P.R. Johnston, Fluid Sterilization by Filtration (Interpharm Press, 2nd ed., Buffalo Grove, IL, 1997). 16. P.R. Johnston, Fundamentals of Fluid Filtration: A Technical Primer (Interpharm Press, 2nd ed., Buffalo Grove, IL, 1998). 17. R.V. Levy, “The Effect of pH, Viscosity, and Additives on the Bacterial Retention Capabilities of Membrane Filters Challenged with Pseudomonas diminuta,” ASTM Special Publ. 975, Vol. II, 80–89 (1986). 18. M.W. Mittelman, M.W. Jornitz, and T.H. Meltzer, “Bacterial Cell Size and Surface Charge Characteristics Relevant to Filter Validation Studies,” PDA J. Pharm. Sci. Technol. 50 (1), 37–42 (1998). 19. T.H. Meltzer, “Filtrative Particle Removal from Liquids,” in Filtration in the Pharmaceutical Industry (Marcel Dekker, New York, 1986), pp. 343–346.

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