GVS would like to share some basic concept about filtration with our customers. The following clarifies some key aspects of filtration technology applied to the specific applications. Our Sales Engineers are always at your disposal for any further explanation. The following properties should be considered at the time of selecting the proper filter media on your application.
Filtration through a mesh means that the fabrics will stop particles larger than the mesh size rating.
Medical meshes adopted by GVS are medical grade and comply with the very strict international requirements for cleanliness.
The fabrics are composed of monofilaments. Standard material is polyamide (PA6.6) or polyester (PE).
Filtration through a mesh is mechanical filtration. Mesh does not have the ability to stop air, except in special situations. Mesh is specified by its mesh size, which is just one of the several key characteristics.
According to ISO1135, blood transfusion filters should have a mesh size of 200 micron and efficiency higher than 80%. Specific markets are using 170 micron mesh for this application.
According to ISO8536, the IV drip chamber disc filters should have mesh size of 15 micron, and efficiency higher than 80%.
Note: Some of our filters are manufactured with different media instead of mesh. We use many hydrophobic and hydrophilic membranes, they are normally dedicated to very special applications. You can find these in our catalog.
The mesh used by GVS for our medical filter products is manufactured with uniform weave and accurate open-mesh structure which guarantees the lowest possible flow restriction. The "windows" of the mesh normally have a square shape.
- Raw materials used for the monofilament are polyamide (PA6.6) or polyester (PE). Other raw materials are available but not very popular.
- Mesh opening (micron): this is the size of any window or opening. The openings are tested by electronic analysis image systems during production.
- Open area: this is a percentage (%) of the total mesh area which is "open" to let the flow go through. It's important to have high open area percentage to reduce flow restriction. This is also tested by electronic analysis image systems during production.
- Mesh count: this is the quantity of threads per cm or per inch (n/cm) or (n/in).
- Thread diameter (micron) this is the diameter of the filament. It is also tested by electronic analysis image systems during production.
- Weight of the mesh (g/m2) or (oz/yd2), important to qualify the quality of the mesh.
- Thickness of the mesh. This is expressed in microns (µm) and its stability is very important to achieve the proper handling of the mesh during production.
The meshes used by GVS have efficiency which is always higher than that of international standards.
Sterilization Stability
This characteristic allows proper performance at elevated temperatures. The mesh used in GVS filters is usually compatible with any of the current sterilization methods: EtO, Gamma, Beta or e-beam radiation, or steam sterilization with no adverse affects.
Biosafety
These tests are conducted in compliance with ISO-10993 and USP Class VI.
Tests conducted are:
Cytotoxicity
Sensitization
Irritation or Intracutanous Reactivity
Systemic Toxicity (Acute)
Hemocompatibility (Hemolysis)
Pyrogenicity
Filters that are pyrogenic can make solutions pyrogenic. Pyrogens introduced to patients can elevate the patient's fever, and can cause complications - even death. Therefore, filters must be non-pyrogenic. The test to determine the pyrogenicity is the LAL test (Limulus Amebocyte Lysate test).
Extractables
Extractables are contaminants (typically chemicals) that elute from the filter which might affect quality of the effluent. Wetting agents (surfactants), manufacturing or sterilization residuals are the main cause of undesired extractables. Typical problems caused by extractables are found in the following applications:
- HPLC analysis (strange result)
- Cell culture (cytotoxicity)
- Microbiological analysis (affects the microorganism)
- Environmental analysis (contaminants)
Flushing of the line prior to use can reduce Extractables and their adverse effects.
The amount of extractables allowed for mesh filters are described in following regulations:
21CFR177.1500 (PA)
21CFR177.1630 (PE)
Filter Efficiency (FE)
This is the quantity of particulate retained compared to the total quantity of particulate to which the filter is challenged. It is expressed in % and refers to a specific size of particles.
Effective Filtration Area (EFA)
This is the actual filtration area in a device that is subject to filtration. For instance, in a tubular filter, the frame (socket, two ribs and top cover) made by plastics should be eliminated from the calculations of the device EFA. In mesh filters you should only eliminate the seal area.
Hydrophilic - Hydrophobic Membranes
- Hydrophilic membranes have permeability of aqueous solutions and once wetted, they stop gasses. This means that aqueous solutions pass through hydrophilic membranes but gas is stopped when the membrane is wet until the applied pressure exceeds the "bubble point", at which time the air will evacuate the pore, the liquid is expelled, and the gas will go through. Dry hydrophilic membrane allows gas to pass through. Our HI-FLO PES membranes are hydrophilic membranes.
- Hydrophobic membranes have permeability to the gas, but they stop aqueous solutions. In other words, they do the opposite job when compared to hydrophilic membranes. This means that gas will pass through these membranes, but aqueous solutions will be stopped. If air or gas can reach the hydrophobic membrane, it will go through, but if the contact with the hydrophobic membrane is not possible, then the gas will not pass through. The pressure at which aqueous solutions will pass through a hydrophobic membrane is called the water breakthrough (WBT) or water intrusion pressure (WIP).
PTFE membranes are hydrophobic membranes. PES membranes are hydrophilic membranes.
Pore size
Pore size is determined by the size of the particle that is expected to be retained with a high degree of efficiency. Pore size is typically stated in micrometers or microns (µm), and should clearly be designated as either nominal or absolute.
Nominal pore size is the ability to retain a majority (60% - 98%) of particles having a specific dimension.
Retention efficiency is also depending on such process conditions as concentration, operating pressure etc.
Rating parameters can vary among manufacturers. When the pore size, or retention, is "nominal", it should be stated at a particle size and a percent, i.e., 99.97% retention of ≥ 0.3 µm particles.
Absolute pore size is the ability to retain the 100% of particles of a specific dimension under defined test conditions (particle size, challenge pressure, concentration, detection method)
Chemical compatibility
This is the ability of the membrane to resist to chemicals without mechanical or chemical damage from chemical exposure. Information about the liquid used with a specific filter material should be outlined before application to determine compatibility, GVS can assist customers in choosing the proper filter (and housing) materials.
Extractables
Extractables are contaminants (typically chemicals) that elute from the filter which might affect quality of the effluent. Wetting agents (surfactants), manufacturing or sterilization residuals are the main cause of undesired extractables. Typical problems caused by extractables are found in the following applications:
- HPLC analysis (strange result)
- Cell culture (cytotoxicity)
- Microbiological analysis (affects the microorganism)
- Environmental analysis (contaminants)
Flushing of the line prior to use can reduce Extractables and their adverse effects.
Binding
This is the property of substances to be filtered having affinity with membranes.
This could be a positive effect in some circumstances, but most of the time it can create adverse effects. Particularly it could lead to loss of active components of the liquid to be filtered reducing its beneficial effect.
Our PES HI-FLO membrane is low protein binding.
Thermal Stability
This characteristic allows unchanged performance at elevated temperatures. Some membranes can only be sterilized by EtO. Others can be gamma, beta or e-beam sterilized, as well as EtO. Others can be also steam sterilized with no adverse affects. Membrane performance is sometimes reduced at temperature higher than 25°C, and high temperatures can also reduce chemical stability. PTFE membrane is widely stable (any type of sterilization) if the product is designed properly. PES membrane is suggested for EtO and irradiation (it could be also steam sterilized).
Biosafety
These tests are conducted in compliance with ISO-10993 and USP class VI, see specifications
Tests that are conducted are:
- Cytotoxicity
- Sensitization
- Irritation intracutaneous reactivity
- Systemic toxicity (acute)
- Hemocompatibility (Hemolysis)
Pyrogenicity
Filters that are pyrogenic can make solutions pyrogenic. Pyrogens that are introduced to patients can increase the patient's fever, and can cause complications - even death. Therefore, filters must be non-pyrogenic. The test to determine the pyrogenicity is the LAL test (Limulus Amebocyte Lysate test). The LAL testing components are derived from the horseshoe crab, which can be found offshore around the USA and other countries. Fortunately, the nice thing about these components is that the extraction of the components does not hurt or destroy the horseshoe crab.
Bubble Point (BP)
Typically this test that is performed on hydrophilic membranes. The BP pressure is the pressure to force air through a wetted hydrophilic membrane. These tests are typically performed with water; however, this test can be conducted on hydrophobic membranes using liquids other than water that will wet the membrane. The BP is an indication of the membrane pore size, as related to actual bacterial retention. This test can also be performed on hydrophobic membranes if the correct solvent (instead of aqueous solution) is used, and is compatible with the entire product.
Water Breakthrough (WBT)
This is the test performed on hydrophobic membranes, and it is also related to the pore size of the membrane. The WBT pressure (sometimes referred to as water intrusion pressure) is the pressure it takes to force an aqueous solution through a hydrophobic membrane.
Water Flow Rate (WFR)
This is a test performed on hydrophilic or hydrophobic membranes. It is the amount of liquid that passes through a fixed surface of membrane with a specific applied pressure. It is usually referred to as ml/min (milliliters per minute), cc/min (cubic centimeters per minute) or LPM (liters per minute) at a given pressure. Of course, with hydrophobic membranes, the solution cannot be plain water. Or, the flow rate can be performed with water, if first the hydrophobic membrane is wetted with a compatible solvent that mixes with water.
Air Flow (AF)
This is a flow rate typically related to hydrophobic membranes. It is the amount of air that passes through a fixed surface of membrane with a specific applied pressure.
Filter Efficiency (FE)
Quantity of particulate retained compared to the total quantity of particulate to which the filter is challenged. It is expressed in % and referred to a specific size of particles.
Effective Filtration Area (EFA)
This is the actual filtration area in a device that is subject to filtration. For instance, whereas a 25mm device may start out with a disc of filter media that is cut to 25mm, the sealing surfaces should be eliminated from the calculations of the device EFA.
