Regenerated Cellulose Membrane

Q. What is a membrane filter?

A. A membrane filter is a matrix with channels which act as a screen and retain particles larger than the filter on the surface of the membrane.  Membrane filters allow the retention of sub-micron particles and organisms. 

Note that pores have not been observed in RO or NF membranes using a microscope. 

Q. What is the maximum temperature for the different filter membranes?
A. 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) - 180°C
• Sterlitech Polypropylene - 82°C
• Sterlitech Cellulose Acetate - 135°C
• Sterlitech PTFE (Laminated) - 130°C
• Sterlitech PTFE (Unlaminated) - 260°C
• Sterlitech PVC Data not available

Q. What is the difference between hydrophilic vs hydrophobic membranes?  

A. Hydrophilic filters posses an affinity for water and can be wetted with almost any liquid. Sterlitech carries a wide variety of hydrophilic membranes, including:

• Silver Metal
• Polyethersulfone (PES)
• Glass Fiber
• Polycarbonate Track Etch (PCTE)
• Polyester (PETE)
• Mixed Cellulose Esters (MCE)
• Nylon
• Cellulose Acetate

Hydrophobic filters lack an affinity for water and are best suited for venting applications. Example of hydrophobic filters are Sterlitech polypropylene membranes and Sterlitech PTFE (Teflon®), either laminated or unlaminated.

Q. How is the performance of a filter measured?

A. Design and material selection determines the performance of a filter. Three important measures of filter performance are flow rate, throughput and bubblepoint, defined as follows:

Flow Rate: Determines the volume of liquid or air that will flow through the filter at a fixed pressure and temperature. This is usually displayed as ml/minute/cm^2.

Throughput: Describes the dirt handling capacity of a filter. Namely, how long the liquid will continue to flow through the membrane before the membrane clogs. The lower the flow rate and throughput, the longer it takes the researcher to complete the analysis.

Bubble point: A test to determine the integrity and pore size of a filter. The differential pressure at which a steady stream of gas bubbles is emitted from a wetted filter under specific test conditions. The bubble point test measures the largest pore. Bubble point is generally determined using water or an alcohol (methanol or isopropynol) and is displayed as PSI.

Q. What variables affect the performance of a filter?

A. Viscosity: The viscosity of a liquid determines its resistance to flow; the higher the viscosity, the lower the flow rate and the higher the differential pressure required to achieve a given flow rate.

Porosity: The flow rate of a membrane is directly proportional to the porosity of a membrane, eg. the more pores, the higher the flow rate.

Filter Area: The larger the filter area, the faster the flow rate at a given pressure differential and the larger the expected filter throughput volume prior to "clogging for a given solution."

Q. How are pore sizes rated?

A. A pore size rating is determined by the diameter of the particle that it can be expected to retain with a defined, high degree of efficiency. The rating is stated in nominal or absolute terms.

Q. How is pore size determined?

A. The pore size of a filter, normally stated in micrometers (µm), is determined by the diameter of a particle that is retained by the filter. This is determined using a challenge organism and/or bubble point testing.

Q. What is the maximum operating pressure for the Capsule filters?

A. The maximum operating pressure for the Capsule filters varies with liquids and gases.

Liquids - 80 psi
Gases - 55 psi

Q. What is the difference between nominal and absolute pore size ratings?

A. Nominal pore size rating describes the ability of the filter to retain the majority of the particles at the rated pore size and larger (60-9%). Glass fiber filters and screen filters are a good example of nominally rated filtration.

Absolute size rating describes the pore size at which a challenge organism of a particular size will be retained with 99.9% efficiency under strictly defined test conditions. Most membrane filters are rated as absolute terms.

Q. What organisms are used to determine pore size?

A.

Q. What is a bubble point test?

A. A test to determine the integrity and pore size of a filter. The differential pressure at which a steady stream of gas bubbles is emitted from a wetted filter under specific test conditions. The bubble point test measures the largest pore.

Q. How many times can the Capsule filters be autoclaved?

A. The Capsule filters can be autoclaved at 121°C for up to five 30 min cycles.

Q. What is a KD (kiloDalton)?

A. KD, or kD is the abbreviation for kiloDalton and is equal to 1,000 Molecular Weight Cut-Off (MWCO).  A solution having a molecular weight of 1,000,000 would be equivalent to 1,000 KD.  The table listed below gives a general relationship between kiloDalton to Microns (micrometers), Nanometers, and Angstroms:

As a general rule, choose a membrane with a pore size (MWCO) that is less than half of the compound of interest.

Q. Are Sterlitech Syringe Filters certified pyrogen free?

A. No, we do not perform pyrogen tests on our filters.

Q. Can I filter aqueous solutions through a polytetrafluoroethylene (PTFE) hydrophobic membrane?

A. Yes, but the membrane requires pre-wetting with alcohol (like Isopropanol or Methanol) to establish flow with reasonable pressure differentials.  We often use a 60/40 solution (60 Isopropanol/ 40 water) to prewet the membrane.

Q. I am interested in gridded, sterile membranes, but I am concerned about the high costs of shipping Nitrocellulose internationally. Do you have other types of membranes that can be used?

A. We offer two alternatives to gridded Nitrocellulose membranes, which incur no Hazardous materials charges, Nylon and Polyethersulfone (PES).

Q. Can my Nylon membranes be treated exactly like Nitrocellulose during colony hybridization?

A. One of our customers was wondering if drying E. coli on Nylon transfer membranes prior to denaturation effects the colony hybridization.  Further, they were wondering if after drying the membranes, would treating with SDS before denaturation effect the hybridization?  The answer is...NO, this will work!  The hybridization will not be adversely affected.  This is the procedure they used.

Procedure

1. performed colony lifts
2. air dried nylon membranes with E. coli
3. incubated 3 minutes on 3MM paper saturated with 10% SDS.
4. incubated 5 minutes on 3MM with 0.5M NaOH, 1.5 M NaCI.
5. incubated 5 minutes on 3MM with 1.5 M NaCI, 0.5M Tris pH 7.5.
6. incubated 5 minutes on 3MM with 2X SSC.
7. UV crosslinked without drying.
8. prehybridized and hybridized as usual.

Q. Which membrane is recommended for size characterization analysis?

A. The polycarbonate track-etch (PCTE) membrane is recommended for size characterization analyses.  The pores of this membrane are exceptionally uniform and are offered as low as 0.01um in size.

Q. We would like to filter fluids and wonder which membrane to use for cytology?

A. Cytology:

Cells are removed from the body, then stained and examined under a microscopy.  The trained physician or cytotechnologist is able to detect the presence of malignancy.

Use of either the 5.0 um or 8.0 um polycarbonate membrane.

• Pore Structure and Porosity

Minimum clogging by red blood cells and protein.  Well preserved cellular morphology.  High recovery rate.  Rapid filtration with low pressure.  Surface capture.  No cover slip edge sealing.

• Smooth, Flat Surface

High cell visibility.  Improved morphologic resolution.  Surface capture.

• "Thinness"

Easy mounting.  Immediate microscopic examination.

• Low absorption and adsorption

Improved contrast.  Greater cell isolation.  Easy mounting.

• Non-staining

Improved contrast.  Simpler microscopic analysis routine.

• Transparency

Simpler microscopic analysis routine.

• Chemical Resistance

Unaffected by conventional cytologic fixatives and stains.

• Strength

Less critical handling techniques needed.

Equipment needed
25mm or 47mm filter holder
Stainless steel forceps
5.0um - 25mm or 47mm PCTE membranes
General Procedure - procedure may vary

Most body fluids contain blood in various amounts.
Collect them in a container with an anticoagulant.
One recommended using polycarbonate membranes is EDTA (0.1 molar)
For each 5-ml specimen generally 1 ml of EDTA solution is sufficient; for specimens of higher blood content, 1 ml of EDTA for each 20-ml specimen is recommended.
Fixatives or preservatives recommended are:
30 to 50% alchol
10% buffered formalin
IMucolexx

Diagnostic procedures

For optimum results employ a limited amount of fluid when preparing specimens that exhibit high cell concentrations.  If too much fluid is used with cellular specimens, such as endometrial washing, ascites, gastrics, pleurals, sputums, and urines, the cells crowd and clog the filter.  A 5-ml specimen or smaller volume provides a good sample if it is properly mixed before filtration.

The 8um pore size is recommended for filtering most body fluids. 

For samples of higher probability of small cancer cells or fewer cancer cells, such as CSF or baby urine, the 5um pore size is recommended.

um = micrometer = micron

Q. Which membranes are recommended for gravimetric analysis?

A. We have several membranes to recommend for gravimetric analysis.

• Mixed Cellulose Esters (MCE) Membrane Filters, Plain: In gravimetric analysis using ashing techniques, (MCE) Nitrocellulose filters yield a residue of less than 0.045% of their initial weight.  They are hydrophilic with a non-cytotoxic wetting agent extractable level of less than 4% of their weight.

• Polycarbonate Track-Etch Membranes (PCTE) - 25mm:  Polycarbonate Track-Etch or our Polyester Track-Etch (PETE) membranes are two membranes that offer exceptionally low tare weights, are non-hygroscopic, and exhibit extremely low absorption and adsorption losses.

Since these membranes are non-hygroscopic, they are particularly well suited for gravimetric analysis.  They do not require drying when used directly out of the package.  If they are wet, they can be dried rapidly and will not pick up moisture from the air during weighing.

• Analytical Filter Funnels:  Funnels are available complete with low hold-up polypropylene support pads, gravimetric analysis is one of the ideal applications for this product.  Each package of 12 units comes with an extender that adapts the unit for side-arm flask filtrations.  Optional No. 8 rubber stoppers may be ordered for filtering with flask.


• Glass Fiber Filters & Prefilters:  Glass fiber filters without binders are recommended for analytical and gravimetric determinations.

Q. How does Epifluorescent microscopy work?

A. Epifluorescent microscopy uses a UV-visible light source and specific filters to excite fluorescent stains added to microbiological specimens to aid in identifications and enumeration.

Sterlitech Black Polycarbonate membranes are ideal for epifluorescent microscopy, since they retain bacteria on the surface of the membrane and provide a non-distracting background to view fluorescence against.

Q. What membrane has been used to prevent water vapor from passing, but would allow regular air to pass?

A. The common membranes used for gas/air filtration are our hydrophobic Polypropylene and PTFE Membranes.  They both inhibit the flow of water vapors (hydrophilic) while allowing regular air molecules (such as oxygen) to pass.

For the PTFE membrane the water intrusion pressure (which is inversely related to pore size) is greatest with the smaller sizes:

Q. What is the shelf life of your membranes?

A. Cellulose acetate (CA) and nitrocellulose (MCE) membranes will last 2 1/2 to 3 years before reverting to their natural hydrophobic states.  Nylon, on the other hand, is naturally hydrophilic so it does not have a shelf life.

An easy test for older membranes is to perform a "wetting out" test.  Dipping a small portion of the membrane into water (hydrophilic membranes) or an alcohol (hydrophobic membranes) works well.  If the membrane absorbs the material, or "wets out", it has not gone past the shelf life.

Q. Is your Cellullose Acetate (CA) membrane made of mono-, di-, or triacetate?

A. Sterlitech CA membranes are made from cellulose diacetate.

Q. What is the difference between cellulose and "regenerated" cellulose?

A. Cellulose Acetate is a specific type of polymer used for low protein binding applications.

Regenerated cellulose, on the other hand, is a pure cellulose which has been treated in a chemical bath for better chemical resistance.  It has a lower molecular weight and the structure is not as orderly as it is for a cellulose.  An example of regenerated cellulose is "cellophane" used in a dialysis application for artificial kidneys.

Q. What is the amount of Lignin, if any, in the Cellulose Acetate membranes?

A. According to the MSDS, the product is 100% cellulose acetate (CA). Some Lignin may still be found within the membrane material due to the fact that the Lignin comes from the cellulose cell walls.

CA is available as a membrane roll, or flat sheet, or syringe filter.

Q. What is the function of a binder in the glass fiber filter?

A. The binders purpose is to increase strength and dirt-loading capacity while decreasing fiber slough.  They are used for filtration of long duration under pressure.  Binders are Acrylic material added to the borosilicate glass fibers.

Our TCLP glass fiber filters have a pore size of 0.7um and are available.  Glass fiber filters without binders are autoclavable and have a maximum temperature of 500°C.

Q. What is the thickness of the polypropylene membranes?

A. The thicknesses of our polypropylene membranes are:

• 0.1um polypropylene: 75-110um
• 0.2um polypropylene: 140-180um
• 0.45um polypropylene: 140-180um

Q. What is a Polycarbonate or Polyester Track Etch filter membrane?

A. These types of filter membranes are precise, two-dimensional micro porous screens with straight through, cylindrical pores.

As in the case of other screen-type filters, particle capture takes place only on the surface, therefore there is more accurate separation cut-off. The precision cylindrical pores of Track Etch membranes have the most accurate size cut-off of any membrane. In depth filters, particles get caught throughout the torturous paths within the matrix as well as on the surface of the membrane.

Track Etch filters are also very thin (between 6 - 15 microns thick) but very durable (can withstand over 3,000 psi when properly supported).   They range in color from opaque to almost transparent and black.

Q. What are the benefits of using Sterlitech Polycarbonate or Polyester filter membranes?

A. Sterlitech Polycarbonate Track Etch (PCTE) and Polyester Track Etch (PETE) filters offer the lowest, non-specific binding of any filter membrane. The capture of samples occurs on a flat, glass-like smooth surface with an even distribution of particles captured on a single plane, simplifying microscopic and SEM examination of samples captured on the surface of the membrane.

• Sterlitech Track Etch filter membranes are manufactured and produced under class 100 condition during critical manufacturing steps. Therefore, the membrane is free of contaminants and pyrogens.
• Sterlitech PCTE and PETE membranes offer very low extractables. Both PCTE and PETE membranes are integral, plastic films, therefore, there is no sloughing or particle shedding.
• Sterlitech PCTE and PETE membranes are biologically inert. 
• They offer superior strength, with pressure tolerances in excess of 3,000 psi (when placed in an appropriate filter holder).
• Both filter membranes offer excellent chemical resistance and thermal stability, with PETE offering a higher chemical resistance.

Q. Will Sterlitech Track Etch filter membranes keep liquid behind the filter and let gases pass through?

A. The need for a gas vapor barrier requires the use of a hydrophobic filter with a water contact angle greater than 120°. If the contact angle is less than 120°, the filter will "bleed" through. The best filter material to use for a vapor barrier is made of PTFE (Teflon™). Most hydrophilic membranes generally have a contact angle that is less than 50°, and an hydrophobic membrane generally has a water contact angle greater than 120°. Raw polycarbonate film has a contact angle of approximately 70°, which is neither hydrophilic nor hydrophobic. It is for this reason that on most Sterlitech PCTE filter membranes we apply the wetting agent PVP. As a result they are considerably more hydrophobic than standard membranes. They also have a typical water contact angles that range from 70° to 90°.

Sterlitech PETE filter membranes are naturally hydrophilic with a contact angle of approximately 40°, therefore, no wetting agent is added in the manufacturing process.

Q. Do Sterlitech Track Etch filter membranes have any type of wetting agent on them?

A. Sterlitech Polcarbonate Track Etch filter membranes use a very mild wetting agent; PVP (Polyvinylpyrrolidone) to make them more hydrophilic and more easily wettable. PVP is a very useful and safe chemical, originally used as a blood plasma extender in surgeries. PVP does have some effects in chemotaxis where the cells seem to migrate faster with PVP on the membrane that, in certain chemotaxis experiments, is not desirable. However, in most cases, PVP is negligible and does not affect any analysis beign performed with the Sterlitech Track Etch membrane.

Sterlitech Polcarbonate Track Etch filter membrane is available with or without PVP. Sterlitech Polyester Track Etch membrane is natrually hydrophilic, therefore, it is made without PVP.

Q. What materials can I use to dissolve Polycarbonate or Polyester membranes?

A. 

Polycabonate membranes can be dissolved with:

• N-mehyl-2pyrrolidone
• Methylene chloride
• Chloroform

Polyester membranes can be dissolved with:

• m-Cresol
• o-Chlorophenol
• Hexafluoroisopropanol

Q. Can Polycarbonate membrane be bonded with adhesives?

A. Polycarbonate membranes can be adhesive bonded to each other or to other plastics, metals, glass or wood using commercially available one component, two component and pressure adhesives.

Q. How clear are the PCTE membrane filters? I am interested in looking at them under an optical microscope.

A. As a rule, PCTE and PETE filters range from opaque in the lower pore density ranges and translucent in the higher pore density ranges. Because most depth filters are fairly thick, they are usually opaque and cannot be made transparent.

When using with a microscope, note that the polycarbonate and polyester have two (2) refractive indices (Birefringent at 1.584 and 1.625). Under the microscope, the pores display outlines (shadows). The 2 indicates polarize transmitted light into two sets of rays at right angles to one another. Using a mounting medium of 1.584 in a combination with polarized light (matching one refractive index) and focusing in polarized light makes the pore outline invisible.

One can also use the Cyto-Clear Slides, which virtually eliminate distracting pore outlines in polycarbonate filters, the shadows from the pores, and ensures clear observation of the specimen. There is no need to dissolve the filter with chloroform, to use pore-clearing solutions, nor to transfer the cells to a slide. All the valuable benefits of the polycarbonate filter are preserved.

The procedure is simpler and requires less time than present protocols. There are only three steps: filtering the specimen, fixing and staining the cells, and mounting the membrane and sample between glass for examination and preservation.

Q. Do cells adhere to the surface of the Sterlitech track etch membranes?

A. Depending on the cell line, most exfoliated human cells adhere with some tenacity. Most epithelial cells will adhere if the membrane has a negative charge applied by gas plasma or has a suitable attractant applied to the surface of the membrane. Endothelial cells will generally not attach to the surface of Track Etch membranes.

Q. Should I use the shiny or dull side of the membrane?

A. The shiny side and dull side only appear on polycarbonate (PCTE) membranes, because polyester (PETE) membranes are shiny on both sides. With PCTE membranes, most users opt for the shiny side up, because it is the smoother surface.

This is particularly important in microscopy for optimum sample observation and for avoiding the necessity of continuously refocusing the microscope. However, in applications where you are trying to attach or adhere cells, it is better to use the dull or matte side.

It is fairly easy to determine the shiny side by simply holding the membrane up to the light and observing which side is more reflective. All filters disks are packaged with the same orientation—shiny side up. A blue paper is used to separate the membranes.

Q. I am looking for a black polycarbonate membrane pore size that is not listed. Can I dye the standard polycarbonate membrane? How do I do this?

A. Yes, but we usually recommend the PVP-free polycarbonate membranes. Note that black polycarbonate membranes are used for counting particulates and performing fluorescent microscopy.

If you need to create a black membrane that is not listed on this site, the standard method for rendering polycarbonate membranes black in a lab follows:

Dissolve 2 gm of Irgalan black (Chemical Index, acid black 107) in 1 liter of 2% acetic acid.
Soak membranes in Irgalan black solution for 24 hours.
Rinse with water, air dry.
To obtain a darker black, dry at 180ºF (82ºC) for 15 minutes.

**Note: There have been instances of Irgalan Black "bleeding" off of the membrane.   This is not common, but has occurred.

Q. Do you have a method for coating the PVP-Free Polycarbonate membranes for Chemotaxis?

A. General Chemotaxis Notes:

Membranes are used in the study of white blood cell reactions to toxins (chemotaxis), to determine the natural immunity in whole blood. Since immunity is transferable, this could lead to the development of vaccines for treatment of cancer and other diseases.

Membranes may be polycarbonate, with or without polyvinylpyrrolidone (PVP). It depends on the type of cells used. Apparently, Neutrophils have a tendency to round up and fall off the membrane when around PVP - not so with some of the other types of cells. Refer to each procedure.

Characteristics of PCTE Membrane and Advantages in Chemotaxis Studies

Uniform Properties yield reliable, consistent, and reproducible results. Less cell distortion for improved morphology.
Zero leaching of offensive surfactants yields reliable, consistent, and reproducible results.
Smooth, flat surface contributes to high cell visibility on proximal and distal surfaces. Improved morphologic resolution. Easy removal of excess cells from proximal surface.
Thinness of the membrane allows shorter incubation time. More assays made faster and simpler. Chamber sterilization not needed for most studies. Facilitates study of slower moving cells (e.g., monocytes and macrophages).
Low adsorption and absorption of cells causes higher yields. Enhanced radiometric analysis.
Non-staining results in improved contrast. Simplifies optical microscopic analysis routine.
Transparency simplifies optical microscopic analysis routine.
Heat sealable characteristics aids in experimenting with disposable chambers.
Strength of membrane means that less critical handling techniques needed.
Coating Membranes with Gelatin

Wash membrane with 0.5% Acetic Acid.
Make a stock gelatin solution with 50mg/10mL water.
Working solution is 1-mL stock to 1-liter water for a light coating.
Then the membranes are placed in a rolling boil of this solution in a beaker for 1 hour.
The membranes are individually pulled out and placed on filter paper to air dry. If an oven is used, use very low temperature for 20 minutes. Air-drying is best in a covered dish left over night. Keep clean.
When the membranes are dry, place them back in their original box.
NOTE: For Chemotaxis, the most important issue is to keep everything at 37 degrees C., the water bath, buffers, glassware, etc. for 1 hour.

Studies in cold produce cells that "round up", curl up and fall off.

Q. Why offer both polyester and polycarbonate track-etch filter membranes?

A. The greatest advantage of using polyester is that it has better solvent resistance and it is prone to fewer wrinkles in the manufacturing process. One of the main reasons polycarbonate was originally used and continues to be used by most people is that for over 30 years the end-users have known mainly polycarbonate.

Polycarbonate works better with live or fixed cells than does polyester. However, both polycarbonate  and polyester offer very high quality end product that is suitable for a large variety of applications.

Q. I am having a problem when I force liquids under pressure through a membrane with a very small pore size. Little or no liquid is seen to pass through the membrane, what could be the problem?

A. There could be a number of things going on to restrict the flow through the membrane.

First you want to take the original flow rate into consideration, for example a 25mm polycarbonate membrane with a 0.05 micron pore size and a flow rate of 0.4ml/min/cm² is only going to get about 1.2 mil of liquid through a minute. This amount decreases with smaller pore sizes.

Another thing to consider is if the liquid is being properly prefiltered. With such small pore sizes, 0.05-0.01micron, you need to filter in a step down manner so as not to plug the pores. Step down means a series of filters with decreasing pore size.

This should eliminate most problems with flow through the membrane; other issues need to be evaluated on an individual basis.

Q. I just received a box of Sterlitech Track-Etch filter membranes and I am trying to use them, but there is no liquid passing through. What is wrong?

A. Believe it or not, in most cases end-users tend to use the blue separator paper instead of the membrane because the separator paper has the physical qualities that make it look more like a typical depth filter than the Track-Etch filter membrane.

Always remember that the filter membrane is usually opaque or translucent and is generally white or white with a yellow or green tint to it. The separator paper is usually blue or imprinted and should not be used for filtration.

Another reason for lack of filtration is due to particle loading on the membrane that would cause the filter to plug. Because Sterlitech Track-Etch filter membranes are made of plastic with cylindrical pores, they capture 100% of all particles larger than the pore size. Therefore, they have a higher incidence of plugging due to their high level of capturability. It is highly recommended that unless the end-user is capturing particles on the surface for analysis that a glass fiber prefilter is used on top of the track-etch membrane. The glass fiber prefilter will capture most of the larger particulate, thereby keeping it from loading up on the surface of the membrane and plugging it.

The use of a mesh spacer (drain disks) is also helpful for maximizing flow. A mesh spacer is set under the membrane and on top of the filter holder. The mesh spacer lifts the membrane off the mostly solid filter holder surface, allowing increased filtration by allowing tangential flow to occur. This is particularly applicable with stainless steel supports.

Q. We would like a higher density Polycarbonate membrane (PCTE). Is it available?

A. Yes, custom density PCTE is available.   Please contact our sales team for a quote (sales@sterlitech.com)

Q. Are the polycarbonate and polyester track-etch membranes biocompatible?

A. Yes. Polycarbonate and polyester membranes are neither cytotoxic nor bactericidal. Both cells and bacteria will grow on the membranes when proper nutrients are supplied.

Q. What membrane works best for cell studies?

A. Sterlitech Polycarbonate (PCTE) and Polyester (PETE) membranes are ideal for cell studies. They are neither cytotoxic nor bactericidal. Cells will grow on the membranes if provided with a nutrient.

The membranes can be repeatedly autoclaved at 121°C (250°F), and no damage has been observed in membranes with a sustained exposure to temperatures of 140°C (284°F) in air or steam.

PCTE membranes are neutral biologically, being neither cytotoxic nor bactericidal. Cells and bacteria will grow on PCTE membranes when proper nutrients are supplied. PCTE membranes pass all USP Class VI tests for bio-compatibility and are completely safe to use in implant studies.

Randomly selected samples of polycarbonate and polyester track-etched membranes were tested by an independent laboratory with MEM Extract/L929 Mouse Fibroblast Cells. No evidence of cytotoxic response was noted during a seventy-two (72) hour exposure period, and the materials were judged to be negatively cytotoxic.

Q. Is there any way I can get rid of the pores on Sterlitech's Track-Etch filter membranes?

A. If you want to completely get rid of the pores, the easiest way to do so is to dissolve the filter away using chloroform or touch prep.

Q. How do I remove the PVP hydrophilic coating from the surfaces of a polycarbonate membrane filter?

A.  Sterlitech’s polycarbonate membranes are naturally hydrophobic but are coated with PVP to produce a hydrophilic version. While the full range of pores sizes (0.01um-30.0um) are available as hydrophilic filters, we are only able to offer a limited number of pore sizes (0.1um-10.0um) as hydrophobic (PVP-Free).

Fortunately, the PVP can be removed without much hassle.  Boil the membrane(s) for at least 1 hour in deionized ultra-pure water, remove, then air dry the membrane(s).  Once the membrane dries it will be hydrophobic and PVP-Free.

Q. Why are black polycarbonate membranes better than other membranes for counting bacteria?

A. Black polycarbonate membranes are better than cellulose membranes for the direct counting of bacteria, because they have a uniform pore size and a flat surface that retains all of the bacteria on top of the filter. Although cellulose filters also retain all of the bacteria, many become trapped inside the filter where they cannot be counted.

Q.  What are the maximum filtration volumes for the 17mm and 30mm syringe filters?

A.  The 17mm and 30mm syringe filters offer greater filtration volume than smaller filters.

• 17 - 12ml
• 17 with glass fiber prefilter - 20ml
• 30 - 120ml
• 30 with glass fiber prefilter - 180ml

Q. What is the maximum operating temperature for the 17mm and 30mm syringe filters?

A.  The 17mm and 30mm syringe filters have a polypropylene housing, which allows them to withstand higher temperatures than an acrylic housing.  The 17mm and 30mm's have a maximum operating temperature of 180°C.  They are also able to be autoclaved.

A. A membrane filter is a matrix with channels which act as a screen and retain particles larger than the filter on the surface of the membrane.  Membrane filters allow the retention of sub-micron particles and organisms.

Note that pores have not been observed in RO or NF membranes using a microscope. 

A. Hydrophilic filters posses an affinity for water and can be wetted with almost any liquid. Hydrophilic membranes inlcude:

• Silver Metal
• Polyethersulfone (PES)
• Glass Fiber
• Polycarbonate Track Etch (PCTE)
• Polyester (PETE)
• Mixed Cellulose Esters (MCE)
• Nylon
• Cellulose Acetate

Hydrophobic filters lack an affinity for water and are best suited for venting applications. Example of hydrophobic filters are polypropylene membranes and PTFE (Teflon®), either laminated or unlaminated.

A. Design and material selection determines the performance of a filter. Three important measures of filter performance are flow rate, throughput and bubblepoint, defined as follows:

Flow Rate: Determines the volume of liquid or air that will flow through the filter at a fixed pressure and temperature. This is usually displayed as ml/minute/cm^2.

Throughput: Describes the dirt handling capacity of a filter. Namely, how long the liquid will continue to flow through the membrane before the membrane clogs. The lower the flow rate and throughput, the longer it takes the researcher to complete the analysis.

Bubble point: A test to determine the integrity and pore size of a filter. The differential pressure at which a steady stream of gas bubbles is emitted from a wetted filter under specific test conditions. The bubble point test measures the largest pore. Bubble point is generally determined using water or an alcohol (methanol or isopropynol) and is displayed as PSI.

A. Viscosity: The viscosity of a liquid determines its resistance to flow; the higher the viscosity, the lower the flow rate and the higher the differential pressure required to achieve a given flow rate.

Porosity: The flow rate of a membrane is directly proportional to the porosity of a membrane, eg. the more pores, the higher the flow rate.

Filter Area: The larger the filter area, the faster the flow rate at a given pressure differential and the larger the expected filter throughput volume prior to "clogging for a given solution."

A. A pore size rating is determined by the diameter of the particle that it can be expected to retain with a defined, high degree of efficiency. The rating is stated in nominal or absolute terms.

A. The pore size of a filter, normally stated in micrometers (µm), is determined by the diameter of a particle that is retained by the filter. This is determined using a challenge organism and/or bubble point testing.

Q. What is the difference between nominal and absolute pore size ratings?

A. Nominal pore size rating describes the ability of the filter to retain the majority of the particles at the rated pore size and larger (60-9%). Glass fiber filters and screen filters are a good example of nominally rated filtration.

Absolute size rating describes the pore size at which a challenge organism of a particular size will be retained with 99.9% efficiency under strictly defined test conditions. Most membrane filters are rated as absolute terms.

A.

A. A test to determine the integrity and pore size of a filter. The differential pressure at which a steady stream of gas bubbles is emitted from a wetted filter under specific test conditions. The bubble point test measures the largest pore.

A. KD, or kD is the abbreviation for kiloDalton and is equal to 1,000 Molecular Weight Cut-Off (MWCO).  A solution having a molecular weight of 1,000,000 would be equivalent to 1,000 KD.  The table listed below gives a general relationship between kiloDalton to Microns (micrometers), Nanometers, and Angstroms:

As a general rule, choose a membrane with a pore size (MWCO) that is less than half of the compound of interest.

A. Yes, but the membrane requires pre-wetting with alcohol (like Isopropanol or Methanol) to establish flow with reasonable pressure differentials.  We often use a 60/40 solution (60 Isopropanol/ 40 water) to prewet the membrane.

A. The polycarbonate track-etch (PCTE) membrane is recommended for size characterization analyses. The pores of this membrane are exceptionally uniform and are offered as low as 0.01um in size.

A. Cytology:

Cells are removed from the body, then stained and examined under a microscopy.  The trained physician or cytotechnologist is able to detect the presence of malignancy.

Use of either the 5.0 um or 8.0 um polycarbonate membrane.

• Pore Structure and Porosity

Minimum clogging by red blood cells and protein.  Well preserved cellular morphology.  High recovery rate.  Rapid filtration with low pressure.  Surface capture.  No cover slip edge sealing.

• Smooth, Flat Surface

High cell visibility.  Improved morphologic resolution.  Surface capture.

• "Thinness"

Easy mounting.  Immediate microscopic examination.

• Low absorption and adsorption

Improved contrast.  Greater cell isolation.  Easy mounting.

• Non-staining

Improved contrast.  Simpler microscopic analysis routine.

• Transparency

Simpler microscopic analysis routine.

• Chemical Resistance

Unaffected by conventional cytologic fixatives and stains.

• Strength

Less critical handling techniques needed.

Equipment needed
25mm or 47mm filter holder
Stainless steel forceps
5.0um - 25mm or 47mm PCTE membranes
General Procedure - procedure may vary

Most body fluids contain blood in various amounts.
Collect them in a container with an anticoagulant.
One recommended using polycarbonate membranes is EDTA (0.1 molar)
For each 5-ml specimen generally 1 ml of EDTA solution is sufficient; for specimens of higher blood content, 1 ml of EDTA for each 20-ml specimen is recommended.
Fixatives or preservatives recommended are:
30 to 50% alchol
10% buffered formalin
IMucolexx

Diagnostic procedures

For optimum results employ a limited amount of fluid when preparing specimens that exhibit high cell concentrations.  If too much fluid is used with cellular specimens, such as endometrial washing, ascites, gastrics, pleurals, sputums, and urines, the cells crowd and clog the filter.  A 5-ml specimen or smaller volume provides a good sample if it is properly mixed before filtration.

The 8um pore size is recommended for filtering most body fluids. 

For samples of higher probability of small cancer cells or fewer cancer cells, such as CSF or baby urine, the 5um pore size is recommended.

um = micrometer = micron

A. We have several membranes to recommend for gravimetric analysis.

• Mixed Cellulose Esters (MCE) Membrane Filters, Plain: In gravimetric analysis using ashing techniques, (MCE) Nitrocellulose filters yield a residue of less than 0.045% of their initial weight.  They are hydrophilic with a non-cytotoxic wetting agent extractable level of less than 4% of their weight.

• Polycarbonate Track-Etch Membranes (PCTE) - 25mm:  Polycarbonate Track-Etch or our Polyester Track-Etch (PETE) membranes are two membranes that offer exceptionally low tare weights, are non-hygroscopic, and exhibit extremely low absorption and adsorption losses.

Since these membranes are non-hygroscopic, they are particularly well suited for gravimetric analysis.  They do not require drying when used directly out of the package.  If they are wet, they can be dried rapidly and will not pick up moisture from the air during weighing.

• Analytical Filter Funnels:  Funnels are available complete with low hold-up polypropylene support pads, gravimetric analysis is one of the ideal applications for this product.  Each package of 12 units comes with an extender that adapts the unit for side-arm flask filtrations.  Optional No. 8 rubber stoppers may be ordered for filtering with flask.


• Glass Fiber Filters & Prefilters:  Glass fiber filters without binders are recommended for analytical and gravimetric determinations.

Q. How does Epifluorescent microscopy work?

A. Epifluorescent microscopy uses a UV-visible light source and specific filters to excite fluorescent stains added to microbiological specimens to aid in identifications and enumeration.

Black Polycarbonate membranes are ideal for epifluorescent microscopy, since they retain bacteria on the surface of the membrane and provide a non-distracting background to view fluorescence against.

A. The common membranes used for gas/air filtration are hydrophobic Polypropylene and PTFE Membranes. They both inhibit the flow of water vapors (hydrophilic) while allowing regular air molecules (such as oxygen) to pass.

For the PTFE membrane the water intrusion pressure (which is inversely related to pore size) is greatest with the smaller sizes:

A. The binder's purpose is to increase strength and dirt-loading capacity while decreasing fiber slough.  They are used for filtration of long duration under pressure.  Binders are acrylic material added to the borosilicate glass fibers.

Our TCLP glass fiber filters have a pore size of 0.7um and are available. Glass fiber filters without binders are autoclavable and have a maximum temperature of 500°C.