By R. L. Rama Chandra
Director, Porous PTFE Products
EDITED BY STEPHEN J. MRAZ
When it comes to specifying filter materials for medical and laboratory equipment, "one size fits all" and "best fit" are seldom both applicable. Almost every new filtration system presents a unique combination of working specifications for variables such as pressure, flow rate, and working temperature. Some lab applications, such as the clarifi-cation of cryogenic fluids, for instance, need filters that operate reliably at extremely low temperatures. On the other hand, filters in some sensors must work in continuously high temperatures.
Today, no standard material can meet every specialized and demanding medical environment. Instead, engineers are using more and more customized materials developed for specific applications. And one of the materials they commonly choose to customize is expanded polytetrafluoroethylene, or ePTFE.
BENEFITS OF EPTFE
Expanded PTFE has excellent filter characteristics such as extremely high tensile strength, durability, and chemical resistance, and it can withstand continuous high temperatures. It is also a pure sub-stance and chemically inert. Expert material suppliers can customize ePTFE to suit a wide variety of filtration applications. They can closely control the longitudinal and biaxial porosity for uniform filtration in applications that include gas and blood filtration and liquid clarification.
Engineers specify customized ePTFE for the same reason mechanical engineers create an entirely new part: for a best fit. Customized materials can also help simplify existing products. For example, by employing ePTFE or an ePTFE laminate, a medical designer may reduce the number of materials and parts in an assembly. A single, smaller component might do the job previously handled by a multilayered, multimaterial design.
Medical designers have recently begun using ePTFE in filtration products more often. They are more comfortable choosing it for several reasons:
- It complies with FDA standards.
- Standard ePTFE and many customized varieties have passed the strict testing and quality-assurance procedures common in the medical industry.
- The special properties of ePTFE make it an ideal replacement for some traditional materials. For example, nitrocellu-lose has long been the filter material of choice in many lab devices. More recently, nitrocellu-lose filters have been widespread in the blotting apparatus used in DNA research. Nitrocellulose is inexpensive, useful, and reliable. However, filters made of it cannot handle shock, friction, or prolonged exposure to heat. And it decomposes in contact with strong alkaline and acidic materials, amines, or oxidizing agents. Expanded PTFE, on the other hand, provides a more durable alternative to nitro-cellulose in harsh environments or those involving chemical exposure. Also, ePTFE doesn't decompose in the presence of fluorine analogues.
As customized ePTFE continues to spread through the medical product market, more designers are seeing the advantages of customizing it for medical applications. Here are a few tips for simplifying the customization process:
Contact a supplier. Suppose an engineer is developing a filter set for an intravenous device. The filter must block bacteria and unwanted particulate matter while allowing unimpeded liquid flow. In the initial call to a supplier, the designer would describe the filter as envisioned and explain its function and the environment it must endure.
The filter's function determines the filter material's flow rate and permeability. In the case of this particular filter, pore sizes must range from 0.2 to 0.22 microns for sterile liquid filtration (also a common size for use with antibiotic solutions). Pore size might be larger if it were supposed to filter lipid-containing liquids. Ideal pore sizes for filters change with the solutions used and the size and nature of particulates in the solution.
The solution being filtered determines other material characteristics. If low drug-binding properties are required, for example, ePTFE's chemical resistance makes it a good fit for such a filter.
Solution pressure drives the flow through filters, so designers should specify a required tensile strength for the material, otherwise the supplier will determine it. Strength is rarely an issue for ePTFE, which has a longitudinal tensile strength of 2,000 psi.
Develop a sample. If the designer's specifications are close to a customized material that already exists, the supplier sends the designer a sample for examination and testing.
If the material requirements are new, the supplier tries to create a sample based on those requirements. In addition to adjusting pore size, customizing could involve altering the oleophilic nature of ePTFE to make it more oil repellent. If the filter will be die cut for manufacture, or must be pleatable, the supplier may bond ePTFE to a laminate of porous ultra-high-molecular-weight poly-ethylene (UHMW-PE). Its larger pore size does not change the filtration properties of ePTFE, and the laminate is easier to manipulate physically than unlaminated ePTFE. Sample preparation takes two to four weeks, depending on the number and complexity of the customizing processes.
Test the sample. The supplier should test samples thoroughly before sending them to designers. World-class suppliers have labs extensively equipped for sample analysis. Materials are tested for pore size and uniformity, porosity, moisture vapor permeability, water-entry pressure, airflow rate, and tensile and dielectric properties, among others. Each lot is tested individually, and test results should be available on request.
Sometimes designers require special tests involving equipment neither the supplier nor the customer has. Examples include scanning electron microscopy, moisture-vapor-transmission rate, or Mullens water entry pressure test. Suppliers can arrange for certified outside labs to perform these tests.
Refine performance. Even after testing, designers often have the opportunity to fine-tune the material specifications. Testing the filter with actual solutions may suggest a different porosity, for example. This is when designers realize world-class suppliers can help them take greater advantage of customization to improve the final product.
Sometimes at this late design stage, a product design can be changed to exploit a customized material. For example, if the previous multilayer IV filter needed to be a certain length to do its job, it may be possible to reduce that length and thereby reduce filter cartridge costs using a single-layer filter.
Test in the field. Finally, the filter material is field tested in the finished device. OEMs normally conduct field tests to see if the device meets stringent in-plant quality programs. Although it is still possible to customize ePTFE at this point, by now the material is usually a best fit for the application. The entire customizing process, from the first phone call to release for production, generally takes one to three months.
Develop a solid partnership with your supplier. The successful release of custom filter material is often the beginning of a partnership between the OEM and the material supplier. The success of a company's initial product can inspire a family of related products, each building on previous design experiences and drawing on the capabilities of customized ePTFE. The partnership can also give designers another source of expert information on new applications, new products, and even entirely new markets for a customized material.
For example, a company that develops flexible ribbon cable for data transfer had used ePTFE as an insulator because of its low coefficient of friction. Cables that bend or flex, such as those common in monitoring applications, generate heat by friction in their insulation, shortening the life of the insulator. Wrapping wire in ePTFE before adding an additional insulation reduces friction, thereby lengthening its effective life. The same company also used ePTFE with customized dielectric properties for highly specialized signal cables. As the example shows, designers who use customized ePTFE will create next-generation, high-precision filters using best-fit materials and learn more about a material that may come in handy in another application.
Why some engineers refuse to change
Some engineers never seriously consider customized materials for filters. Instead, they favor existing materials they know from other applications. This decision is understandable and is based on several underlying reasons:
- The FDA maintains a list of materials that comply with standards for medical use. Designers often rely solely on this list when looking for filter materials.
- Medical and laboratory products are subject to tight quality control and testing. Designers are unlikely to select materials that have not already been through these processes.
- Some materials, such as nitrocellulose and polyolefins, may not exactly fit into all applications, but are suitable for a great many.
- Newer materials, even those already used in medical and laboratory applications, are not widely known to designers.
Underlying these reasons, and perhaps more important than all of them, is the assurance of specifying familiar materials that other designers have used in similar applications.
While these are all seemingly valid reasons, engineers who make the time to check out the option of customizing material usually end up with an improved, more reliable, and better-designed product.