Frequently Asked Questions

We have a Chemical Compatibility Chart that you can use for reference. 

cellQART® cell culture inserts feature a polyester (polyethylene terephthalate, PET) track-etched membrane rather than a polycarbonate membrane. Differences in membrane chemistry, surface energy, and pore architecture can impact cell attachment, proliferation, morphology, and differentiation. When transitioning from polycarbonate to PET membranes, variations in extracellular matrix adsorption and cell–substrate interactions may alter growth behavior.

To ensure optimal performance in cell culture, transwell assays, and in vitro permeability or migration studies, Sterlitech recommends performing seeding density optimization by testing both increased and decreased cell concentrations relative to your current protocol. This approach allows users to identify conditions that support improved confluence, barrier integrity, and reproducibility when using Sterlitech cellQART® membrane inserts.

Yes. Membrane transparency can directly influence imaging quality and experimental monitoring in cell culture applications. Sterlitech’s transparent cellQART® track-etched membranes are optimized for phase contrast microscopy, live-cell imaging, and real-time monitoring of cell morphology and confluence. Improved optical clarity enables clearer visualization of cells without membrane interference, supporting more accurate assessment during transwell assays, co-culture systems, and permeability studies.

Both transparent and translucent cell culture inserts are suitable for fluorescence microscopy and fluorescent imaging applications. Each membrane type allows sufficient light transmission for fluorescent analysis, making them compatible with common fluorescence microscopes.

Transparent membranes are ideal when maximum optical clarity is desired, such as for high-resolution imaging, cell visualization, or detailed morphology studies.

Translucent membranes are also fully compatible with fluorescence microscopy and are frequently used in routine fluorescent assays, permeability studies, and transport experiments.

Ultimately, the choice between transparent or translucent inserts depends on your imaging resolution requirements, experimental design, and personal preference, rather than limitations in fluorescence compatibility.

Vivaflow® crossflow devices offer a fast, cost-effective way to concentrate 0.1–5 L samples without large-scale systems. With PES or Hydrosart® membranes and single-use or reusable options, they can achieve up to 50× concentration—1 L in ~30 min or 5 L in <75 min—with near-total recovery after a single buffer rinse.

To reduce protein degradation, optimize buffer composition and minimize shear stress—e.g., use lower centrifuge speeds for linear proteins and devices with membrane areas suited to your starting volume. Vivaflow® crossflow devices minimize shear compared to centrifugal or stirred cell systems, and diafiltration tools like the Vivaflow® reservoir help maintain optimal buffer conditions during concentration.

For virus concentration, use a MWCO ~⅓ of the target’s molecular weight (or match pore size to diameter). For Lentivirus (~90 nm), a 300 kDa Hydrosart® membrane offers the best balance of speed and recovery, with 100 kDa PES as a slower alternative. Always validate device, membrane, and MWCO for your specific virus.

The maximum operating temperatures for Sterlitech filter membranes are listed below.

• Sterlitech Silver Metal - 427°C
• Sterlitech Ceramic - 350°C
• Sterlitech Polycarbonate Track Etch - 140°C
• Sterlitech Polyester - 140°C
• Sterlitech Nitrocellulose (MCE) - 130°C
• Sterlitech Nylon - 180°C
• Sterlitech Polyethersulfone (PES) - *130°C
• Sterlitech Polypropylene - 82°C
• Sterlitech Cellulose Acetate - 135°C
• Sterlitech PTFE (Laminated) - 130°C
• Sterlitech PTFE (Unlaminated) - 260°C

*5.0um and 8.0um - max temp is 180°C

The pores of microporous membrane filters act as small capillaries.  When hydrophilic membranes come into contact with water, capillary action associated with surface tension forces causes the water to spontaneously enter and fill the pores.  In this manner, the membranes are easily wetted and allow the bulk flow of water through the pores.  Once wetted, hydrophilic membranes will not allow the bulk flow of air or other gasses, unless they are applied at pressures greater than the membrane’s bubble point.

Hydrophilic membrane filters are typically used with water and aqueous solutions.  They can also be used with compatible non-aqueous fluids.  Hydrophilic membrane filters are typically not used for air, gas or vent filtration since the filters would block flow if inadvertently wetted, by condensation for example.

When hydrophobic membranes come into contact with water, surface tension forces act to repel the water from the pores.  Water will not enter the pores and the membranes will act as a barrier to water flow, unless the water is applied at pressures greater than the membrane’s water entry pressure.  Low surface tension fluids, such as alcohols, can spontaneously enter and fill the pores of hydrophobic membranes.  Once all the air in the pores is displaced, there are no longer any surface tension forces and water can easily enter the pores, displace the low surface tension fluid, and pass through the membrane.  The membrane will then allow bulk flow of water for as long as the pore remain water filled.  If the membrane is allowed to dry (i.e. air enters the pores), then it must be pre-wet with a low surface tension fluid again prior to use with water.

Hydrophobic membrane filters are typically used with compatible non-aqueous fluids.  They are also commonly used as air, gas, or vent filters.  Hydrophobic membrane filters are sometimes used with water or aqueous solutions; and, in these applications, they must first be prewet with a low surface tension, water miscible fluid prior to use.

Nominal pore size ratings provide a general indication of filter retention efficiency, meaning some particles equal to or larger than the stated pore size may pass through the filter. Nominal ratings can vary by manufacturer, so filters with the same nominal pore size may not offer equivalent filtration performance.

Absolute pore size ratings are determined through controlled particle or microbial retention testing and represent the smallest particles that are consistently retained by the membrane. These ratings are often correlated with bubble point specifications and are generally more comparable across manufacturers.

Important: Actual filtration performance depends on application conditions, even when using filters with absolute pore size ratings.

Silver membrane filters do not have a predetermined shelf life when stored properly. They should be kept sealed in their original packaging until use to minimize environmental exposure.

Over time, surface discoloration or silver compound formation may occur. This is largely cosmetic and does not affect pore structure, filtration efficiency, or membrane performance.

Slight discoloration of a silver membrane filter is normal and typically cosmetic. Although silver metal membranes are made from 99.97% pure silver, they can form surface compounds over time due to environmental exposure, even when not in use.

Common silver compounds that may cause discoloration include silver sulfide (Ag₂S), silver chloride (AgCl), and other silver salts. These surface compounds do not affect pore structure, filtration efficiency, or membrane performance and are usually not a cause for concern.

To minimize discoloration, silver membrane filters should be stored in sealed packaging. In some cases, silver chloride can be removed with a brief rinse or soak in an ammonia solution, while other surface compounds may be reduced using alcohols such as methanol or ethanol.

Do not confuse discoloration with the membrane’s natural grayish-white appearance, which results from its microporous structure. Slight color differences between the two sides of the membrane are normal and are most noticeable in 3 µm and 5 µm pore sizes.

Aluminum oxide (anodic aluminum oxide, AAO) membrane filters are free of organic extractables and leachables, making them ideal for high-purity filtration. They exhibit very low nonspecific adsorption, helping preserve sample integrity and maximize analyte recovery. Their inorganic structure provides excellent chemical and thermal stability, along with uniform pore size and high porosity, supporting reliable, efficient filtration in analytical and research applications.

Unfortunately, we are unable to supply the aluminum oxide membrane filters with custom diameters. Please contact us at [email protected] to inquire about alternatives.

Yes. Sterlitech aluminum oxide (AAO) membrane filters can be used as alternatives to Whatman Anodisc filters. Please note that Sterlitech alumina membrane filters do not have perimeter support rings, so additional care is recommended during handling to prevent damage.

Sterlitech Cellulose Acetate (CA) membranes are made from cellulose diacetate. These membrane filters also have an integral nonwoven polyester (polyethylene terephthalate) support layer. When evaluating application compatibility, both materials should be considered.

Excluding the polyester support layer, Cellulose Acetate (CA) membrane filters are composed entirely of cellulose acetate polymer. There may be a very small amount of residual lignin present as a trace component. This minimal lignin content does not affect the performance, purity, or reliability of CA membrane filters in typical laboratory, analytical, and sample filtration applications.

Cellulose Acetate (CA) membrane filters are among the lowest protein-binding membranes available, making them ideal for applications requiring maximum protein recovery. They provide high throughput with protein-rich solutions and are commonly used for filtering proteins, enzymes, tissue culture media, serums, and biological fluids.

CA membranes feature an integral nonwoven polyester (PET) support layer, offering strength, dimensional stability, and easy handling, with resistance to tearing and curling. They are naturally hydrophilic, wet easily with aqueous solutions, offer good chemical resistance, are compatible with low molecular weight alcohols, and can withstand steam sterilization up to 135 °C.

Ceramic membrane filters are manufactured from a robust inorganic matrix of zirconium oxide and titanium dioxide, providing exceptional mechanical strength, chemical resistance, and thermal stability. These inert ceramic filtration membranes can operate at temperatures up to 350 °C, far exceeding the limits of conventional polymer membrane filters.

Thanks to their superior resistance to aggressive chemicals, solvents, and extreme pH conditions, ceramic membranes are ideally suited for demanding filtration applications. Their durability allows for repeated regeneration, backflushing, and high-temperature or chemical cleaning without loss of performance.

These advantages make ceramic membrane filters well suited for industrial filtration, pharmaceutical processing, biotechnology, food and beverage applications, and other processes requiring long service life, consistent performance, and high-temperature operation.

The ceramic membrane filters are only available with 47mm and 90mm diameters. You may want to consider other inorganic membranes, such as alumina oxide or silver, for applications requiring different diameters.

The ceramic membrane disk filters are considerably thicker than conventional membrane disk filters and will not fit in conventional disk filter holders.  The ceramic membrane disk filters must be used with the specially designed holders offered here.

Glass fiber filters offer excellent thermal stability and can operate at high temperatures, making them suitable for demanding laboratory and industrial applications. They are particularly economical and effective as pre-filters, where they help extend the life of final membrane filters by capturing larger particles and high particulate loads.

Due to their high dirt-holding capacity and fast flow rates, glass fiber filters are commonly used in air and liquid filtration, sample clarification, and analytical applications. Their combination of high-temperature resistance, cost-effectiveness, and reliable pre-filtration performance makes glass fiber filter media a versatile choice across many filtration processes.

The acrylic (PMA) resin binder used in glass fiber filters significantly enhances their wet strength, improving durability during liquid filtration. This resin binder helps hold the glass fibers together, making resin-bonded glass fiber filters easier to handle and more resistant to fiber shedding.

As a result, glass fiber filters with an acrylic (PMA) binder provide more consistent performance and improved integrity in both laboratory and industrial filtration applications. When assessing chemical compatibility and application suitability, it is important to consider the presence of the acrylic (PMA) resin binder.

DOP is an abbreviation for dioctyl phthalate, a compound historically used to generate monodisperse aerosol particles for air filter testing and efficiency characterization. DOP aerosols produce particles with a highly uniform size of approximately 0.3 µm, which corresponds to the most penetrating particle size (MPPS) for many air filtration media.

Because of this consistency, DOP aerosol testing has been widely used to evaluate air filter retention and performance, including in standards such as ASTM D2986, Standard Practice for Evaluation of Air Assay Media by the Monodisperse DOP (Dioctyl Phthalate) Smoke Test.

Note: Due to health and safety considerations, DOP has largely been replaced in modern testing by alternative aerosols (such as PAO), but the term “DOP test” is still commonly used in the filtration industry.

MCE membrane filters offer fast flow rates, high protein-binding capacity, and strong thermal stability, making them a common choice for environmental and biological laboratory filtration. They’re available as presterilized, individually wrapped membranes, and gridded MCE filters can be used for easy microbial colony counting and quantification.

Unfortunately, in most instances, we are unable to supply the MCE membrane filters with custom diameters. Please inquire with your Sterlitech sales representative about alternatives.

Sterile mixed cellulose esters (MCE) membrane filters are used in vast quantities for microbiological studies across many industries and are manufactured in very high volumes to accommodate this demand.  Economies of scale and process automation allow the sterile MCE membrane filters to be offered at lower pack costs compared to non-sterile MCE membrane filters.  Non-sterile MCE filters are used much less frequently, necessitating less efficient, smaller volume manufacturing runs and packaging methods.  Consequently, the non-sterile MCE membrane filters have intrinsically greater manufacturing costs and must be offered at higher prices.

Nylon membrane filters offer high protein binding, making them suitable for applications that benefit from strong biomolecule retention. They also provide excellent solvent resistance and broad chemical compatibility, supporting filtration of aqueous solutions and many organic solvents. In addition, nylon membranes deliver strong dimensional stability and durability, often reinforced by an inert polyester support layer, which improves handling and helps the membrane maintain its structure during filtration.

Sterlitech nylon membrane filters are made from nylon 66 (PA66) polymer, which is inherently hydrophilic, non-toxic, and resistant to many organic solvents. These nylon 66 membrane filters also include an integral nonwoven polyester (PET) support layer for added strength and dimensional stability. When evaluating chemical compatibility and filtration performance, both the nylon 66 membrane and the PET support layer should be considered.

PAN Membranes combine excellent selectivity, high flow rates and low pressure requirements for use. 

The polyacrylonitrile (PAN) membrane filters are absolute rated at 0.2µm and are bacterially retentive with typical 6 log reduction value (LRV).  This level of retention can be expected to meet EPA standards for safe drinking water with respect to microorganisms.  It is important to realize that the integrity of the combined filter holder and disk filter assembly must be considered in critical applications.

No, the PAN membrane filters cannot withstand autoclave sterilization. The fiters can be sanitized with hot water at 90C water for 30min or by soak in ethanol.

Polycarbonate (PC) and Polyester (PET) track-etch membrane filters are precision, two-dimensional microporous “screen” membranes with straight-through, cylindrical pores created by the track-etching process. Because the pore structure is uniform and non-tortuous, particles are captured primarily on the membrane surface, providing a highly accurate and reproducible separation cut-off compared to depth filter media.

Track-etch membranes are known for having some of the most precise pore size distributions of any membrane filter, making them ideal for applications that require exact particle sizing and surface capture, such as microscopy, particle analysis, microbial enumeration, and sample preparation.

These membranes are also very thin (typically ~6–15 µm) yet surprisingly durable, and can withstand high differential pressures (over 3,000 psi when properly supported). They are available in a range of appearances, from opaque to nearly transparent, including black options for enhanced contrast in imaging and microscopy.

Sterlitech Polycarbonate (PCTE) and Polyester (PETE) track-etched filter membranes offer ultra-low non-specific binding and a smooth, flat surface that captures particles on a single plane—ideal for microscopy, SEM, and particle analysis. Manufactured under Class 100 cleanroom conditions, they are contaminant- and pyrogen-free, with very low extractables and no fiber shedding. Both membranes are biologically inert, provide precise, uniform pore sizes, and deliver excellent chemical and thermal stability, with PETE offering higher solvent resistance.

Sterlitech Track-Etched Polycarbonate membranes are not recommended for venting applications. PVP-free polycarbonate membranes have a water contact angle of ~90° and can wet out under low differential pressure, allowing liquid to pass through. As a result, they do not effectively retain liquids while venting gases. For vent filter applications where gas permeability and liquid blocking are required, Sterlitech recommends hydrophobic PTFE, hydrophobic polyethylene, or oleophobic polyester membranes, which provide higher water entry pressure and allow gases and water vapor to pass while preventing liquid breakthrough.

PES (Polyethersulfone) membrane filters offer very low protein binding and high flow rates, making them ideal for sterile filtration of tissue culture media, buffers, and other life science and microbiology fluids. PES membranes provide excellent throughput, broad chemical compatibility, and reliable performance for cell culture media sterilization and routine laboratory filtration applications.

Sterlitech PES (polyethersulfone) membrane filters have an asymmetric pore structure, meaning one side has larger pores and the other has smaller pores. To identify the correct side, view the membrane under reflected light at a low angle—the larger-pore side appears dull/matte, while the smaller-pore side looks shinier/smoother.

For maximum flow rate and throughput, orient the filter with the dull/matte (larger-pore) side facing upstream. For microscopy or particle capture, you may place the shinier (smaller-pore) side upstream to keep particles closer to the surface (with reduced throughput).

Sterlitech PES (polyethersulfone) membranes are asymmetric, meaning the largest pores are on one side and the smallest pores are on the opposite side. You can identify the sides by viewing the membrane under reflected light at a low angle—the larger-pore side looks dull/matte, while the smaller-pore side appears shinier/smoother.

The membrane can be installed either way without changing retention, but for best results:

Dull/matte side upstream = higher flow and throughput
Shiny side upstream = better surface capture for microscopy/particle analysis

Yes—you can estimate the pore size of Sterlitech PETE (polyester track-etched) membrane filters using SEM imaging, and SEM is commonly used to characterize track-etched pore diameters during manufacturing. However, pore size measurements can vary between instruments due to SEM calibration, magnification accuracy, image resolution, sample preparation, coating thickness, and measurement method. Because of these variables, user-measured pore diameters may not exactly match Sterlitech’s manufacturing pore size specifications. For best accuracy, use a calibrated scale standard, measure multiple pores across several fields of view, and report results as an average with a distribution.

Sterlitech Polyester Track-Etched (PETE) membrane filters are made from polyethylene terephthalate (PET)—a durable polyester film known for its strength, dimensional stability, and chemical resistance, making it ideal for track-etched membrane filtration and microscopy/particle analysis applications.

No—Sterlitech PCTE and PETE track-etched membranes are not ideal vent filters unless they are hydrophobic and have sufficient water entry pressure. Standard polycarbonate (PCTE) membranes, even when hydrophobic, typically have low water entry pressure and can allow liquid water to pass under relatively low differential pressure.

For applications that must retain liquid while allowing gases or water vapor to pass, Sterlitech recommends hydrophobic PTFE or hydrophobic/oleophobic PETE membranes, which provide much higher water entry pressure and are commonly used as vent filters in laboratory and industrial systems.

The thickness of Sterlitech polypropylene membrane filters varies by pore size:

0.1 µm polypropylene: 75–110 µm
0.2 µm polypropylene: 140–180 µm
0.45 µm polypropylene: 140–180 µm

Polypropylene (PP) membrane filters are naturally hydrophobic and provide reliable performance for venting and gas filtration applications where aqueous liquids should be repelled. PP membranes are also a cost-effective alternative to PTFE for users who don’t require PTFE’s higher chemical resistance, while still offering good durability and consistent filtration performance.

PTFE (polytetrafluoroethylene) membrane filters are extremely hydrophobic and offer exceptional chemical compatibility, making them ideal for aggressive solvents, acids, and bases. PTFE membranes are widely used for venting, gas filtration, and solvent filtration applications where liquid repellency, high purity, and chemical resistance are critical.

Advantec unlaminated hydrophilic PTFE membrane filters are not permanently hydrophilic. Once the membrane is wetted, it can revert to hydrophobic behavior if it is allowed to dry. In addition, the membrane may become hydrophobic after autoclave sterilization or other exposure to temperatures above 100 °C.

The Smooth PTFE side should face towards the feed solution or liquid ingress.

Sterlitech syringe filters are not certified pyrogen-free. Pyrogen testing is not performed on these filters. Users requiring pyrogen-tested or endotoxin-certified filtration products should select filters specifically labeled and validated for those applications.

The maximum recommended filtration volumes for Sterlitech syringe filters are:

17 mm syringe filter: up to 12 mL
17 mm with glass fiber prefilter: up to 20 mL
30 mm syringe filter: up to 120 mL
30 mm with glass fiber prefilter: up to 180 mL

Sterlitech 17 mm and 30 mm syringe filters feature a polypropylene housing, allowing them to withstand higher temperatures than acrylic designs. The maximum operating temperature is 180 °C, and the filters are autoclavable for sterilization.

Sterlitech crossflow test cells (Sepa® CF, CF042, and CF016) operate in true crossflow filtration mode, meaning the feed flows tangentially across the membrane and produces both a permeate stream and a concentrate (retentate) stream. These systems allow continuous operation, with user-controlled pressure and crossflow rate, and enable ongoing sampling from both streams during testing.

The HP4750 Stirred Cell, by comparison, is a sealed batch filtration device (up to 300 mL feed volume) typically pressurized with compressed gas. It runs in normal-flow (dead-end) mode and does not have a concentrate stream. A stir bar helps reduce concentration polarization and simulates crossflow-like mixing at the membrane surface, but it is not true crossflow.

Crossflow velocity limits for commercially available spiral-wound membrane elements depend on several factors, including element construction, maximum allowable pressure drop, and feed stream characteristics (such as viscosity and solids content). Recommended operating ranges are typically provided by the element manufacturer. For application-specific guidance, please contact Sterlitech for assistance in selecting appropriate crossflow velocities and operating conditions.

In most cases, a foulant spacer imprint on the membrane is normal and not a cause for concern. Light impressions can occur due to normal compression during operation and do not necessarily indicate damage.

However, you should verify spacer thickness to ensure the foulant spacer (or spacer + shim combination) is not thicker than the feed channel. If the spacer is too thick, it can over-compress the element and risk damaging the membrane, reducing performance and element life.

The Reynolds number is a dimensionless number that is related to the ratio of inertial forces to viscous forces experienced by a fluid for given flow conditions. The Reynolds number can be used to predict whether flow conditions result in a laminar or turbulent flow.

In theory, the cross section area of the test cell feed channel can be used to calculate the Reynolds number for the feed flow. In practice, it is very difficult to calculate the Reynolds number because of the complex geometry of the foulant spacer occupying the feed channel. There are empirical methods to estimate the Reynolds number by characterizing the relationship between feed flow and differential pressure.

Please contact us at [email protected] if you need assistance.

You can usually tell the low foulant (34 mL) and high foulant (68 mL) feed spacers apart by their cell pattern, stiffness, and surface texture:

Low foulant (34 mL) feed spacer:
Has smaller square openings
Feels lighter and more flexible (bends easier in your hands)
Not as stiff as the medium/high foulant styles

High foulant (68 mL) feed spacer:
Has a corrugated, ridged texture (similar to cardboard)
Typically has no punched holes
Feels stiffer and thicker overall

Tip: If you have both on hand, compare how easily each piece flexes—the low foulant spacer will bend noticeably easier, while the high foulant spacer holds its shape due to the corrugation.

Used flat sheet crossflow membranes must be stored wet at all times. If a membrane is allowed to dry out, it can irreversibly lose water permeability and performance.

For storage, place the membrane fully submerged in one of the following solutions:
0.5% formaldehyde solution (helps prevent microbial growth)
1.0% sodium metabisulfite (SMBS) solution -- Replace the SMBS solution monthly to maintain effectiveness

UPDI water (ultrapure deionized water)--Replace the water weekly to reduce contamination and biofouling

Sterlitech flat sheet crossflow membranes should be stored sealed in the original packaging in a climate-controlled environment, protected from direct light, heat, and temperature extremes. While we recommend using membranes as soon as practical after receipt, most flat sheet membranes can typically be stored for up to one (1) year without affecting performance when stored properly.

Yes—flat sheet membranes can be cut to fit your stirred cell. For best results, use the stirred cell support disk as a template to trace the correct size, then cut the membrane carefully to match.

Any standard magnetic stir plate will work with Sterlitech stirred cells. Sterlitech recommends the Jeiotech TS-18QG Hotplate & Magnetic Stirrer (Digital, 180 mm) because the large 180 mm plate provides a stable base for stirred cells, and the digital speed control ensures precise, repeatable stirring performance during membrane filtration testing.

Yes. cellQART® Cell Culture Inserts are designed to be fully compatible with standard 6-well, 12-well, and 24-well plates. Their patented hanging design ensures accurate positioning within the well while maintaining excellent pipetting access for routine cell culture handling.

No. Rocker vacuum pumps should not be moved while operating. Before moving the pump, turn it off and open the inlet to atmosphere to release vacuum. Moving a running Rocker 300 or 400 series pump can cause the prote® protection device inside the inlet filter to close, which may restrict airflow and affect pump performance.

No. Rocker vacuum pumps must be operated on a level, flat surface. Running Rocker 300 or 400 series pumps when they are tilted or not level can cause the prote® protection device inside the inlet filter to close, which may restrict airflow and reduce pump performance.