Filtration fundamentals

BQ+ would like to discuss some fundamental filtering principles with our clients. The following section discusses some critical features of filtering technology as it relates to certain applications. Our Sales Engineers are always available to provide further information. When choosing the appropriate filter medium for your application, the following qualities should be examined.


Filtration via a mesh ensures that particles bigger than the mesh size rating are trapped by the screen. BQ+ uses medical-grade meshes that meet stringent international standards for hygiene. Monofilament is used to construct the displays. Polyamide (PA6.6) or polyester are used as standard materials (PE). Mechanical filtration is filtering via a mesh. Except in exceptional circumstances, mesh does not have the power to block air. Mesh is defined by its mesh size, which is merely one of numerous critical properties.

ISO1135 specifies that blood transfusion filters shall have a mesh size of 200 microns and an efficiency of more than 80%.

170 micron mesh is used in certain markets for this purpose. ISO8536 specifies that disc filters for IV drip chambers should have a mesh size of 15 micron and an efficiency of more than 80%.

Nota bene: Certain of our filters are constructed using a different medium than mesh. We use a variety of hydrophobic and hydrophilic membranes that are typically reserved for very specialized applications. These are available in our catalog.

Mesh properties

BQ+ manufactures the mesh for our medical filter products with a consistent weave and an exact open-mesh structure, ensuring the lowest possible flow restriction. Typically, the mesh’s “windows” are square in form.

Polyamide (PA 6.6) or polyester are utilized as raw materials for the monofilament (PE). Other basic materials are available, although they are not widely used.

– Mesh opening (micron): this is the measurement of the size of a window or aperture. Throughout the manufacturing process, electronic analysis imaging systems are used to inspect the apertures.

– Open area: this is a percentage (%) of the total mesh area that is “open” to allow for flow.

It is critical to have a high open area percentage in order to minimize flow limitation. This is also verified throughout manufacturing using computerized image analysis devices.

– Mesh count: the number of threads per centimeter or inch (n/cm or n/in).

– Thread diameter (microns): this is the thread diameter. During manufacture, it is also subjected to electronic analysis imaging systems.

– Mesh weight (g / m2) or (oz /yd2): this is critical for determining the mesh’s quality.

– Mesh thickness. The thickness is stated in microns (m), and its stability is critical for effective mesh management throughout manufacturing.

  • The meshes employed by BQ+ are always more efficient than those required by international standards.

Stability of Sterilization

This property enables proper operation at high temperatures. Generally, the mesh used in BQ+ filters is compatible with all existing sterilization methods: EtO, Gamma or e-beam radiation, or steam sterilization without causing any harmful effects.


These tests are carried out in accordance with ISO-10993 and USP Class VI standards.

Cytotoxicity, Sensitization, Irritation, or Intracutaneous Reactivity, Systemic Toxicity (Acute), and Hemocompatibility tests are undertaken (Hemolysis)


Pyrogens are chemical or biological substances that may be present on the filter mesh or other components; when they enter the human body, they mostly induce a temperature increase. Pyrogens may also be associated with bacterial breakdown or death. Filters that are pyrogenic may contribute to the pyrogenicity of solutions. They are not eliminated after sterilization, which is why it is essential to utilize non-pyrogenic filter media and components in the manufacture of medical filter devices. The LAL test is used to detect pyrogenicity (Limulus Amebocyte Lysate test).


Extractables are pollutants (usually chemicals) that elute from filters and may impair the effluent’s quality. The primary source of undesirable extractables is wetting agents (surfactants) or manufacturing or sterilizing residuals. The following applications exhibit typical difficulties created by extractables:

– High-performance liquid chromatography (strange result)

– Culture of cells (cytotoxicity)

– Microbiological examination (affects the microorganism)

– Environmental impact assessment (contaminants)

Prior to usage, flushing the line may help decrease Extractables and their negative consequences.

The following rules specify the maximum quantity of extractables permitted in mesh filters:

21CFR177.1500 (PA) (PE)

Filter Efficiency

This is the percentage of particle retained in comparison to the total amount of particulate challenged by the filter. It is given as a percentage and relates to a certain particle size.

Area of Effective Filtration

This is the real filtration region of a filtration-sensitive device. For example, in a tubular filter, the plastic frame (socket, two ribs, and top cover) should be excluded from the EFA calculations. In mesh filters, just the seal area should be eliminated.


Filtration across a membrane ensures that particles bigger than the pore size rating are trapped by the filter material. This permits the determination of the absolute pore size of membranes that are clearly categorized. Bacterial retention claims might be made in relation to the membrane’s pore size.

Membranes Hydrophilic – Hydrophobic

Hydrophilic membranes have the permeability of aqueous solutions and, when wet, they prevent gasses from passing through. This implies that although aqueous solutions flow through hydrophilic membranes, gas is blocked until the applied pressure surpasses the “bubble point,” at which point the air evacuates the pore, the liquid is ejected, and the gas passes through. Gas may travel through a dry hydrophilic membrane. Hydrophilic membranes are what our HI-FLO PES membranes are.

– Hydrophobic membranes are permeable to gases but are impermeable to aqueous solutions. In other words, they perform the inverse function of hydrophilic membranes.

This implies that although gases will flow through these membranes, aqueous solutions will not. If air or gas can make touch with the hydrophobic membrane, it will pass through; however, if this contact is not feasible, the gas will not flow through. Water breakthrough (WBT) or water incursion pressure refers to the pressure at which aqueous solutions flow through a hydrophobic membrane (WIP).

Hydrophobic membranes are PTFE membranes. Hydrophilic membranes are PES membranes.

Pore diameter

The size of the pores is decided by the particle size that is anticipated to be maintained with a high degree of efficiency. The size of the pores is commonly expressed in micrometers or microns (m), and should be explicitly labeled as nominal or absolute.

The nominal pore size is defined as the capacity of a pore to retain a majority (60–98 percent) of particles of a given dimension.

Additionally, retention efficiency is dependent on process variables like as concentration, operating pressure, and so on.

Manufacturers’ rating criteria may vary. When the pore size or retention is “nominal,” it should be expressed in terms of particle size and percentage, e.g., 99.97% retention of 0.3 m particles.

The capacity of a pore to retain 100% of particles of a certain dimension under specified test circumstances (particle size, challenge pressure, concentration, and detection technique) is referred to as the absolute pore size.

Compatibility with chemicals

This refers to the membrane’s capacity to withstand chemical exposure without suffering mechanical or chemical damage. Prior to application, information regarding the liquid being used with the filter material should be detailed to ensure compatibility; BQ+ can help clients in selecting the appropriate filter (and housing) materials.


Extractables are pollutants (usually chemicals) that elute from filters and may impair the effluent’s quality. Unwanted extractables are mostly caused by wetting agents (surfactants), manufacturing or sterilizing residuals.

The following applications exhibit typical difficulties created by extractables:

– High-performance liquid chromatography (strange result)

– Culture of cells (cytotoxicity)

– Microbiological examination (affects the microorganism)

– Environmental impact assessment (contaminants)

Prior to usage, flushing the line may help decrease Extractables and their negative consequences.


This is a trait of chemicals that are to be filtered that they have an affinity for membranes. This may have a beneficial impact in certain instances, but it often has a detrimental effect. It is possible that this will result in the loss of active components in the liquid to be filtered, so diminishing its positive impact. Our PES HI-FLO membrane has a low affinity for proteins.

Stability Under Thermal Conditions

This property enables performance to be maintained at increased temperatures.

Certain membranes can be sterilized exclusively with EtO. Others, such as EtO, may be sterilized using gamma, beta, or e-beam radiation. Others may also be steam sterilized without causing any harm. Membrane performance is sometimes diminished above 25°C, and high temperatures may also affect chemical stability. If the product is developed appropriately, PTFE membranes are quite stable (under any form of sterilization).

For EtO and irradiation, a PES membrane is recommended (no steam sterilization).


These tests are done in accordance with ISO-10993 and USP class VI requirements, as specified in the specifications. Cytotoxicity – Sensitization – Irritation intracutaneous reactivity – Systemic toxicity tests are undertaken (acute) – Hematological compatibility (Hemolysis)


Pyrogens are compounds that accumulate on filter media and other components as a result of dead bacteria.

When administered to a patient, they may cause the patient’s temperature to rise, resulting in difficulties – and even death.

Filters that are pyrogenic may contribute to the pyrogenicity of solutions.

They are not eliminated after sterilization, which is why it is essential to utilize non-pyrogenic filter media and components in the manufacture of medical filter devices.

The LAL test is used to detect pyrogenicity (Limulus Amebocyte Lysate test).

Point de bubble

This test is often done on hydrophilic membranes and is used to determine the integrity of the membrane filter. Typically, this test is done using water; however, it may be performed on hydrophilic membranes with any liquid that can wet the membrane. The BP value indicates the pore size of the membrane in relation to real bacterial retention. This test may also be done on hydrophobic membranes if the appropriate solvent is used (rather than aqueous solution) and the whole product is suitable.

Water Revolution

This is the test used to determine the hydrophobicity of membranes, and it is also connected to the membrane’s pore size. The WBT pressure (also known as water incursion pressure) is the amount of force required to drive an aqueous solution through a hydrophobic membrane.

Rate of Water Flow

This test is often conducted on hydrophilic membranes.

The WFR is used to determine the flow rate of a liquid across a wetted hydrophilic membrane at a certain pressure and time. Typically, this test is conducted using water; however, other solutions may be used as long as the filter medium is compatible with the liquid.

Flow of Air

This is a typical flow rate for hydrophobic membranes. It is the quantity of air that travels through a given membrane surface when a certain pressure is applied.

Filter Efficiency

The amount of particle or bacteria retained in comparison to the total amount of particulate or bacteria exposed to the filter. It is given as a percentage and relates to a certain particle size.

Area of Effective Filtration

This is the real filtration region of a filtration-sensitive device. For example, although a 25 mm device may begin with a disc of filter media that has been cut to 25 mm in diameter, the sealing surfaces should be excluded from the device EFA calculations.

How are hydrofobic and hydrofilic membranes different?

A membrane must be wettable with the fluid being filtered in order to be used for liquid filtration. The wettability of a membrane is determined by its chemical characteristics. The majority of polymers used to fabricate microporous membranes are inherently hydrophobic, which means they do not wet out when exposed to water.

Nylon and cellulose are exceptions since they are inherently hydrophilic and will wet out when exposed to water. The contrast between hydrophobic and hydrophilic membranes is based on their surface tension. If the surface tension of the polymer is more than 70 dynes/cm, it is hydrophilic. The polymer is hydrophobic below 70 dynes/cm.

The Wetting Angle is critical in defining and comprehending the physical difference between the two types of membranes; if it is less than 90 degrees, we are dealing with a hydrophilic membrane; if it is more than 90 degrees, we are dealing with a hydrophobic membrane.


The two fundamental classes of plastic materials are thermoplastics and thermosets. Because thermoplastic resins may be melted and solidified again by heating and cooling, any scrap created during production can potentially be reused. Generally, no chemical changes occur during formation. Typically, thermoplastic polymers are delivered in pellet form, which may include additives to aid in processing or to impart desired qualities to the end product (e.g., color, conductivity, etc.). The temperature range across which thermoplastics may be used is restricted by their physical strength loss and ultimate melting at extreme temperatures.

Thermoplastic resins that are often utilized in medical applications

Elastomers Thermoplastic

TPEs are a class of polymers that can be stretched repeatedly without irreversibly deforming the part’s shape. They do not need curing or vulcanization, in contrast to rubber-like elastomers, since they are real thermoplastics. TPEs may be processed using standard thermoplastic processes such as injection molding, extrusion, or blow molding. In many applications, thermoplastic elastomers have replaced rubber. Styrenic block copolymers, polyolefin blends (TPOs), elastomeric alloys, thermoplastic polyurethanes (TPUs), thermoplastic copolyesters, and thermoplastic polyamides are the six major thermoplastic elastomer categories encountered commercially.

Butadiene Acrylonitrile Styrene

ABS is a multi-polymer based on acrylic that is impact-modified and utilized for injection molding and extrusion of medical equipment, medical packaging, as well as food packaging, toys, and appliance components.

When cost is a consideration, ABS provides the optimum balance of features.

It has excellent chemical and stress resistance, as well as a blend of toughness, stiffness, and creep resistance. It is resistant to water, aqueous salt solutions, dilute acids and alkalis, saturated hydrocarbons, and a broad range of vegetable and animal fats and oils chemically.

Polymer composed of methacrylate, acrylonitrile, butadiene, and styrene

Is a translucent, amorphous thermoplastic based on the MABS polymer (alternatively referred to as “transparent ABS”). Although these grades are mainly intended for injection molding, they may also be extruded.

MABS’s impact strength is derived from a rubber phase composed of polybutadiene that is submicroscopically embedded in a matrix composed of styrene, acrylonitrile, and methyl methacrylate. Due to the correct balance of these fundamental building blocks, MABS exhibits exceptional transparency despite its high impact strength and stiffness, distinguishing it from the majority of impact-modified thermoplastics. MABS combines the characteristics of ABS, such as a balanced stiffness/toughness ratio, with the excellent transparency associated with PMMA moulding components. This unique mix of characteristics distinguishes MABS from other transparent thermoplastics.


PMMA (polymethyl-methacrylate) is a thermoplastic polymer that is amorphous and has excellent optical characteristics. PMMA is a tough, rigid, and medium-strength material that is easy to scratch and notch sensitive, but also simple to polish. Exceptional outdoor performance, including resistance to weather and sunshine, without sacrificing optical or mechanical qualities.

Polyvinyl Chloride

It is structurally identical to polyethylene, except that each unit includes a chlorine atom. While the chlorine atom makes it susceptible to some solvents, it also makes it more resistant in a wide variety of applications (PVC has extremely good resistance to oils and very low permeability to most gases). Polyvinyl chloride is a clear material with a little blue tint. When PVC is combined with phthalate esters plasticizers, it becomes soft and malleable, allowing for the manufacture of tubing of any diameter. PVC is the most extensively used vinyl family member. Chemical processing tanks, valves, fittings, and piping systems are all common uses.

PVC Sheets, Rods, and Tubes exhibit exceptional resistance to corrosion and the elements.

It has a high strength-to-weight ratio and is an excellent conductor of electricity and heat. Additionally, PVC is self-extinguishing according to UL flammability testing. PVC is suitable for temperatures up to 140°F (60°C).


It is identical to polyethylene, except that each chain unit has a methyl group. It is transparent, self-contained, and does not contain any known solvent at room temperature. Due of the many branches (methyl groups in this instance), it is significantly more vulnerable to strong oxidizing agents than regular polyethylene. Polypropylene is well-known for its resilience to chemicals in harsh situations. This polymer is very easy to weld and process. In the chemical and semiconductor industries, homopolymer and copolymer grades, as well as a popular heat-stabilized formulation, are employed in a variety of applications.


This is a family of linear polymers having repetitive amide connections running through them. These are formed when diamines are amidated with dibasic acids or when amino acids are polymerized. Nylon is a strong and durable material. It is abrasion, fatigue, and impact resistant. When employed with organic solvents, nylon exhibits superior chemical resistance with minimal absorption rates. It is, however, susceptible to severe mineral acids, oxidizing agents, and some salts.


This is a unique form of polyester in which dihydric phenols are linked together by carbonate bonds (O-CO-O). These connections are susceptible to chemical reactivity with bases, strong acids, and other organic solvents, rendering PC soluble in a wide variety of organic solvents. PC is transparent, very robust, and stiff. It is autoclavable, non-toxic, and one of the hardest thermoplastics available. PC retains its impact resistance across a broad temperature range and even in very harsh weather conditions.

It is resistant to both low and high temperatures ranging from -50°C to +130°C and has exceptional optical qualities in addition to a strong resilience to sunlight exposure (UV radiation).


It is formed when formaldehyde is polymerized. At extreme temperatures, acetal preserves its dimensions and other characteristics. Its tolerance to strong acids and bases is exceptional. Acetal (POM) copolymer is a naturally opaque material that combines high strength and stiffness with improved dimensional stability and ease of machining. Acetal, being a semi-crystalline substance, also has a low coefficient of friction and excellent wear characteristics, particularly in moist settings.

Acetal is utilized as a weight-saving metal alternative because to its excellent strength, modulus, and resistance to impact and fatigue.

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A Shanghai-Based, Export-oriented Medical Device Manufacturer

With 3 facilities, 400+ employees, ISO Certified, Lean Production, High-Quality

BQ+ Medical is a manufacturer dedicated in Infusion therapy products.  

For you, BQ+ customizes sets of Infusion, Transfusion, Oncology & Enteral feeding with in-house Components molding, Assembling, and EO Sterilization.

BQ+ Medical not only produces components for manufacturers worldwide but also provides private-label manufacturing for leading brand distributors in different countries.

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