About

Pore Density

Track-etched membranes are made differently from other filter types. They start with a transparent polymeric film which is integral – nothing passes through it – which then goes through a series of radiation and chemical treatments that open up holes straight through the material. What becomes important in terms of filter characteristics of a track-edged membrane is the number of pores per unit surface area.

Consider a piece of material with 100 pores per cm². This material would have twice the filtration capacity of a piece with only 50 pores per cm². Pore density is measured by placing a membrane sample under a scanning electron microscope (SEM) and physically counting the number of pores per unit surface area. Track-etched membranes of the same pore size can exhibit a gradient of different filtration rates dependent upon the pore density.

Wettability Characterization

For liquid filtration, a membrane must be wettable with the fluid being filtered. The wettability of a membrane is tied to the chemical properties of the membrane surface. Most polymers used to manufacture microporous membranes are naturally hydrophobic, meaning they will not wet out with water.

Some exceptions are nylon and cellulose which are naturally hydrophilic and will wet out with water. The distinction between hydrophobic and hydrophilic relates to the surface energy of the polymer. If the surface energy is >70 dynes/cm, the polymer is hydrophilic. Below 70 dynes/cm, the polymer is hydrophobic.

Hydrophobic membranes are wettable with alcohols. If the polymer is compatible with alcohols, it can be wet first with the alcohol and then equilibrated in water prior to filtering the aqueous fluid. For many applications, this is impractical; the membrane must be directly wettable with the aqueous fluid.

To overcome the hydrophobicity of the polymer, the membrane can be treated with a secondary chemistry that coats the base polymer. The secondary chemistry becomes primary in determining wettability. It is important to recognize that the base polymer remains hydrophobic unless the secondary chemistry is a covalent modification of the polymer.

In venting applications, the filter is used as a porous barrier that allows escape of gas bubbles from a liquid stream or exchange of gases between a liquid stream and the external atmosphere. To ensure that the filter does not wet out under any circumstances, it can be treated with a secondary chemistry that renders it superhydrophobic or oleophobic. This reduces the surface energy to <20 dynes/cm. The membrane cannot be wet with water or alcohols.

Flow Rate

Flow refers to the time it takes for a particular flow stream to pass through the filter. The flow rate of a filter is important in determining how rapidly filtration can be completed. Although flow rate generally decreases with pore size among membranes of a single type, membranes with the same pore rating, but made from different materials or by different methods, can have very different flow rates. Flow rate differences can be caused by differences in thickness, porosity, and pore architecture.

After a microporous membrane is produced, flow rate or flow time is measured using an ideal liquid or gas. Hydrophilic filters are usually tested with water. Hydrophobic membranes are usually tested with an alcohol. Membranes to be used for air filtration can be tested with dry air or dry nitrogen. By using ideal liquids and gases, the flow properties of the filter can be assessed independently of particulates or other contaminants that could clog the pores. If there is nothing in the sample stream to clog the pores, the flow rate should remain constant. For ultrafiltration, there are special considerations on flow rate.

During ultrafiltration, it is important to balance flow rate with retention to obtain optimal performance. With ultrafiltration membranes the term more commonly used is flux. A membrane’s flux is defined as flow divided by the membrane area. The reason that flux is used with ultrafiltration membranes is the need for scalability. Ultrafiltration membranes are commonly used in the purification of expensive biomolecules. Separations are investigated on a small scale in the laboratory before being scaled up to larger volumes in a production setting. Characterizing separations on the basis of flux makes is easier to convert a lab scale investigation to a production scale process.

Using membranes with higher NMWL ratings will increase the flow rate, but at the same time lower the retention. A membrane should be selected for the required retention, coupled with the desired flow rate. This is determined by:

·         Surface area

·         Macrosolute type

·         Solubility

·         Concentration and diffusivity

·         Membrane type

·         Temperature effects on viscosity

·         Pressure

When concentration polarization is rate-controlling, flux is affected by solute concentration, fluid velocity, flow channel dimensions, and temperature.

Since sterility is a common requirement of vent membranes, pore rating is an important consideration. Please note that the mechanism of bacterial retention by hydrophobic membranes in a gas stream differs from that for hydrophilic membranes. Bacteria and other pathogens float in air attached to particles (aerosol or dust). Consequently, in air filtration, pathogens can be rejected by membranes with pore sizes larger than the pathogen alone. Membranes with pore sizes up to 5.0 µm are claimed to exhibit >99.99% bacterial retention efficiency by some suppliers. Similar claims exist for viral retention on 0.2 µm membranes. Therefore, membranes with larger pore sizes are used in less critical applications, yielding the benefits of higher flow rate.

When comparing the air flow rates of different membranes, it is important to note the units in which flow rate is reported and any differences in the conditions under which testing was done. Small changes in pressure and temperature can dramatically affect reported air flow rates.

Liquid flow is measured by placing the filter into an appropriate holder, adding a defined volume of liquid to the holder, and then pulling the liquid through the filter with a constant vacuum. Flow thus depends on the nature of the liquid, the surface area of the membrane, and the vacuum level. To compare different membranes, the same liquid and vacuum level should be used.

While water and alcohols can be used to test flow rate in large scale testing, they may not provide enough discrimination in predicting membrane performance when a more complex solution such as serum or cell culture medium is to be processed. For specific applications, it is appropriate to use other test solutions. Since complex solutions, such as cell culture media, are considerably more expensive than water or alcohols, sampling plans and test protocols should balance the amount of extra data required against the additional cost.

Types of Filtration

Filtration is widely used in biotechnology for separating substances based on relative particle size. Types of filtration most commonly used include:

·         Microfiltration (MF)

·         Ultrafiltration (UF)

·         Reverse Osmosis and nanofiltration (RO/NF)

Microfiltration (MF) is the physical retention of particles behind a filter medium while the liquid they were suspended in passes through the filter. Particles are retained because they are larger than the pores in the filter. Other factors affecting retention are fluid viscosity and chemical interactions between the membrane and the particles in the solution. Microfiltration removes particles with a pore size of .05 and 5.0 µm.

Ultrafiltration (UF) works basically that same way as microfiltration, except that the pore sizes are considerably smaller. Solutes are retained behind the filter on the basis of molecular size while the bulk of the liquid and dissolved salts pass through. A pressure gradient across the membrane, known as transmembrane pressure, drives the filtration process. Ultrafiltration membranes are designed for the concentration and separation of complex protein mixtures.

Reverse osmosis (RO) and nanofiltration (NF) are the processes of separating very low molecular weight molecules (typically <1500 Daltons) from solvents, most often water. The primary basis for separation is rejection of solutes by the membrane on the basis of size and charge. Unlike UF membranes, RO and NF membranes retain most salts, as well as uncharged solutes. NF membranes are a class of RO membranes which allow passage of monovalent salts but retain polyvalent salts and uncharged solutes > ~400 Daltons. Reverse osmosis membranes (RO) have very small pore sizes and are designed to separate ions from each other.

For most high volume filtration applications, the properties of membrane and depth filters are clearly complementary. Depth filtration allows the removal of a bulk of particles economically. Composite filtration combines relatively high dirt-holding capacity with clearly defined retention characteristics.

Membrane filtration permits complete removal of particles and microorganisms above a certain size as qualified by pre-established specifications and testing regimens.