A new method for measuring porous microbial barriers: Part I
The work behind ASTM International Standard Test Method F2638  

By Paul F. Herman & Curtis Larsen, CPP, Fellow

The introduction in 1997 of the European Committee for Standardization (CEN) and International
Organization for Standardization (ISO) standards for packaging and materials for terminally
sterilized medical devices highlighted the need for a universally recognized microbial barrier test
for such materials. Both the CEN and ISO working groups decided, in the absence of a widely
accepted procedure for evaluating the integrity of the sterile package {sterile barrier system
[SBS]def. ISO11607-01:2006(1)}, that the best way forward was to carry out separate tests for seal
integrity, integrity of the package materials, films and the microbial barrier of the porous
materials.

Although there were a number of recognized tests for evaluating seal integrity, this was not the
case for determining the microbial barrier of permeable or porous materials. Tests such as ASTM
International F-1608(2), DIN 58953 Part 6 sub-clauses 2.14 and 2.15, and BS 6256 Appendix C
(Methylene Blue) were all used, but not universally recognized. In addition, all of these
referenced tests take a long time to perform, with results not available for as long as three to four
days in some cases.

Filtration Theory

Challenges to sterile barrier systems used to protect medical devices from bacteria and viruses
come in the form of aerosols of suspended particles. Microbial spores can exist as individual
entities or clusters, or they can be attached to inert particles such as dust particles. Thus, the size
of a particulate challenge can range from 0.002 microns for the smallest virus up to 100 microns,
which is the size of the largest dust particle that can remain suspended in air for a significant
length of time.

Filtration theory predicts that materials which are permeable to air and gases employ three
mechanisms to remove particles from the air stream:

Interception. This occurs when a filter fiber splits the air stream that a particle is following.
The particle continues on its original path and collides with the fiber. Interception is therefore
a constant particle removal mechanism that is a function of the material’s fiber structure. It is
independent of both the particle’s mass and its velocity.

Inertial Impaction. This occurs when a particle, as a result of its mass, deviates from the air
stream flowing around a fiber and collides with it. The effectiveness of this method of capture
is directly related to the mass of the particle and the speed of the air stream. The higher the
velocity and the mass of the particle, the greater the chance of it colliding with a fiber.

Diffusion. This is the interception of a particle with a fiber as a result of random particle
movement (Brownian motion) and, for some materials, electrostatic attraction. The
effectiveness of this capture mechanism is inversely related to the mass of the particle and the
velocity of the air stream. The lighter the particle and the slower its velocity, the greater the
chance of capture.

All three of these mechanisms are in operation at all flow rates and for all particle sizes.
However, larger particles moving at higher flow rates are more likely to be trapped by inertial
impaction, whereas lighter ones moving at slower speeds are more likely to be caught by
diffusion.

The BTC Project

In the absence of an internationally recognized test to evaluate the microbial barrier of permeable
or porous materials, a group of seven companies within the Sterile Barrier Association (SBA),
previously known as ESPA (European Sterile Packaging Association), formed the Barrier Test
Consortium Ltd. (BTC) in 1998 to develop a rapid, easy-to-use, microbial barrier test for porous
medical packaging materials. The BTC consisted of the following companies:

Amcor Flexibles (formerly Rexam Medical Packaging)
Billerud (formerly Henry Cooke)
DuPont Medical Packaging
Kimberly-Clark
Oliver Medical (formerly Oliver Products)
Perfecseal, a Division of Bemis Corp.
Westfield Medical Packaging

Members of the BTC agreed that the new test had to meet certain key requirements. Specifically,
the test had to be:

- based upon established, scientifically sound filtration principles
- applicable to the range and variety of commercially available medical packaging materials
(including coated and uncoated papers, nonwovens and cellulose/synthetic mixed papers)
- reliable, reproducible and rapid
- able to be correlated with a recognized microbiological barrier test method (i.e. ASTM
International F1608)
- fully definable and describable and, as such, be able to support the requirement for microbial
barrier performance as part of the CEN and ISO Standards for medical packaging materials
Following a call for research proposals, the project was awarded to Air Dispersions Ltd. (ADL),
based in Manchester, UK. The project tasks included:
- defining the range of microbiological barrier performances of commercially available porous
packaging materials
- selecting and adapting commercially available particulate filter test equipment and optimizing
test conditions
- establishing physical versus microbiological barrier correlations using materials of defined
construction
- confirming that the correlation applies to commercially available materials
- specifying the particulate test method for defining microbiological barrier performance
To ensure erroneous results were not obtained due to variations in commercially available
materials, the initial work was to prove a correlation existed using research-quality papers
designed and produced by ADL. These papers had a high degree of uniformity and a wide range
of porosity.

The next phase, a blind study, utilized 16 of 30 samples submitted by BTC members. The
samples represented the range of porous packaging materials commercially available at the time.
The range of properties for these materials is shown in Table I.


Table I. Range of properties for 16 sample materials* used in tests conducted by ADL
Property
Unit
Range of Value
Basis weight
g/m2
53.6-129.0
Thickness
Mm
69-541
Air permeability (Bendtsen)
mL/min
29-43,717
Mean maximum spore penetration
1.46 x 10-3 – 5.37 x 101 (0.00146 – 53.7%)
*Four of the 16 materials tested were adhesive coated.


The specialized microbiological test (3) used for the project was developed by ADL several years
before work began on the BTC project. This test challenges the sample with standardized
dispersions of microorganisms at a range of pressure differentials corresponding to overall flow
rates of 0.1 to 100.0 cm3/min/cm2. This range encompasses typical conditions experienced by
medical packages during normal conditions of handling, distribution and storage.

The equipment used for this test was designed as a research instrument. It was highly specialized
to allow for a wide operating range of flow rates. It also featured very sensitive control
mechanisms for flow control, particle control, particle size and enumeration.

Porous materials are basically filters or filtration systems. To test the effectiveness of any filter, a
concentration of challenge media is forced through a sample of material. The extent of microbial
penetration is determined at each flow rate by measuring the concentration of air-dispersed micro
organisms both upstream and downstream of the material under test.

This test measures the extent of microbial penetration at each flow rate by measuring the
concentration of air-dispersed micro organisms both before and after passing through the
material. For a given test organism, the extent of penetration through the test sample is strongly
dependent on the flow rate. This results in a maximum penetration value being obtained at a
specific flow rate for a particular organism and packaging material. Endospores of bacillus
subtilis var. niger are typically used. The maximum penetration value is a measure of the
microbial barrier performance of the packaging material.

It is important to note that although this test gives an accurate result, it is a microbiological test
and takes several days to complete. In addition, this test requires a high level of technical
expertise and a dedicated microbiological laboratory. Although ADL considered the test to be a
good research tool, it didn’t meet the criteria requirement of being rapid.

To reduce equipment cost, the particle generator, size classifier and neutralizer were replaced by
an atomizer and an aerosol of polystyrene beads measuring 1.0 µm in diameter. This
modification simplified the process, allowing for variable airflow capacity and a single size,
synthetic particle for the test medium.

The commercially available equipment selected for the BTC project was known to fulfil these
criteria very well, enabling relatively inexperienced operators to obtain reproducible results for
the physical particulate barrier performance of filter media in a matter of minutes. As with the
microbiological test, a maximum penetration value is obtained.

To test medical packaging materials as opposed to filter media, it was necessary to design a
special sample holder to use in conjunction with the equipment. Once the sample holder
mechanism was available, it was then possible to identify percentage penetration of the most
penetrating particle size (maximum particle penetration) for any material using synthetic
substances as the test particle.


Basis for New ASTM Method


The BTC project was successfully completed in 2000 and formed the basis of the new ASTM
international standard test method F2638-07 (4). ADL was able to demonstrate correlation
between results obtained using commercially available equipment for testing the physical
particulate barrier performance of fibrous filter media and those generated using ADL’s
specialized microbiological test. (3) Furthermore, the relationship extends over a wide range of
commercially available packaging materials including coated and uncoated paper, and coated and
uncoated nonwovens (DuPont™ Tyvek® for sterile packaging). This was not unexpected because
both tests measure the effectiveness of materials to act as filter media.

Part 2 of this series will discuss the development of ASTM F2638-07, as well as provide a
comparison of this new microbial barrier test method vs. ASTM F1608, which is commonly
called the log reduction value (LRV) test. An in-depth look at the aerosol filtration equipment
required to perform the new test method will also be presented.

References
1. ISO 11607-1:2006 Packaging for terminally sterilized medical devices Part 1: Materials,
sterile barrier systems and packaging systems.
2. ASTM International F1608-00(2004) Standard Test Method for Microbial Ranking of Porous
Packaging Materials (Exposure Chamber Method).
3. Microbiological Barrier Testing of Porous Medical Packaging Materials, Alan Tallentire and
Colin Sinclair at the Meeting of the Society of Plastics Engineers (Scandinavian Section),
May 1994.
4. ASTM International F2638-07 Standard Test Method for Using Aerosol Filtration for
measuring the Performance of Porous Packaging Materials as a Surrogate Microbial Barrier

Paul F. Herman is a Nonwovens Application Consultant for DuPont Medical and Industrial
Packaging in Richmond, VA. Curtis L. Larsen is a Packaging Consultant for DuPont Medical
Packaging and Spartan Design Group, LLC located in Tonka Bay, MN.