Use of Expanded PTFE Membranes in Medical Filtration
PTFE and ePTFE offer medical device manufacturers a range of benefits for their finished products. Using the material as a filtration media, it is ideally suited to several application areas, such as in drug delivery, urine collection, and laparoscopic surgery. This article reviews the properties of the material and the specific advantages of using it in filter applications.
Eric C. Wigner was formerly a market specialist and Keith Fritsky is a product specialist with W. L. Gore & Associates Inc. Fritsky can be reached at 410-506-8845 or firstname.lastname@example.org.
Medical device designers seek to identify safe and effective filtration solutions for critical healthcare applications. The use of expanded polytetrafluoroethylene (ePTFE) membranes is a desirable option for designers and engineers looking to optimize flow and retention characteristics.
PTFE and ePTFE—A Quick Primer
Polytetrafluoroethylene (PTFE) is a polymer consisting of a carbon backbone covalently bonded to a uniform sheath of fluorine atoms. It can be manipulated and engineered into a variety of forms, many possible as a result of an expansion process (the tenets of which are well documented).1 This expanded form is referred to as expanded PTFE or ePTFE. ePTFE takes many forms, including tapes, membranes, films, tubes, fibers, sheets, and rods.
The molecular structure of PTFE produces properties that are beneficial when considering its use in medical applications. Of these properties, one of the most significant is chemical inertness. With that, PTFE is known to be biocompatible, meaning that it will not cause adverse reactions when implanted or placed in contact with the body. In addition, it will resist wetting
by biological liquids and is not chemically changed or degraded by medical fluids. Furthermore, the mechanical properties of ePTFE enable biostability, meaning that the material performance is maintained over time. The vascular graft is a classic example of where PTFE/ePTFE is used in medical devices; such devices are well covered in literature.2 Unlike non-expanded forms of PTFE, ePTFE exhibits high porosity and tensile strength, and enhanced creep and cold flow resistance. Finally, PTFE/ePTFE is extremely hydrophobic (Figure 1).
The expanded membrane form of PTFE is relevant for use in external medical devices that require separation functionality. Typically, the membrane acts as a vent, blocking liquids while allowing gas to flow through the membrane. It can also act as an air/gas filter, removing particles and aerosols that may contain microbial
contaminants.3 More can be learned about the membrane's filtration capability by understanding the membrane's structure at a microscopic level. Figure 2 shows a scanning electron microscope image of the surface of a microporous, ePTFE membrane. The microstructure is a tangled web or "forest" of nodes and fibrils. The fibrils are thin connections between the nodes and are submicron in size. Thin fibrils are used to create more tortuosity and surface area in a membrane, impacting the filtration efficiency. The three-dimensional structure is non-shedding and continuous with no loose ends. These characteristics of the microstructure provide vital attributes in medical filtration as one looks to maximize flow and retention and minimize pressure drop. Figure 3 provides examples of various membrane microstructures.
Lamination and surface modifications further extend the versatility of ePTFE membranes. Lamination is a set of processes where single or multiple layers are joined together with the membrane. The additional layer(s)—such as woven, non-woven, and extruded materials—act as a mechanical support and enhance handling and processing of the membrane in a manual or automated device manufacturing environment. Mechanical support layers are designed and selected to enable easy integration through heat, ultrasonic, or radio frequency welding, adhesive bonding, or insert molding. The additional layer(s) can also provide additional functionality to the vent system. The addition of a coarse, pre-filter media would be an example of creating a multi-functional filtration/venting laminate. This laminate is ideal for venting applications with high particle loadings such as surgical smoke evacuation.
Surface modification refers to a process for changing the surface energy of ePTFE and manipulating the properties of the membrane from hydrophobic to oleophobic or hydrophilic (Figure 4). As a result of modifying the surface energy of ePTFE, membranes can operate with a variety of fluids over a wide range of surface tensions. Such fluids include water, bodily fluids, nutritional fluids, and diagnostic reagents. By employing proven surface treatments, the natural hydrophobic properties of the ePTFE membrane can be enhanced, significantly increasing the number of applications for this material.
ePTFE membrane, as compared to other porous materials, shows superior performance as a barrier to liquids, particles, and aerosols that may contain viable bacteria and viruses. While protecting against contaminants is critical, the membrane must also provide other required functionalities such as pressure equalization, air/gas elimination, and air/gas delivery.
Figure 5 demonstrates the venting performance of ePTFE membrane in terms of airflow and resistance to liquid breakthrough. Microporous membranes made of different polymers and processes (e.g., phase separation) are also included for reference. Airflow and the resistance to liquid breakthrough are opposing material properties. The optimization and superior combination of other material characteristics enable innovative and effective medical devices.
In urine collection devices, oleophobic ePTFE membranes are used as vents, equalizing pressure as liquid fills the device (Figure 6A). Air residing in the device is displaced, and the membrane blocks liquid and airborne contaminants.
During the drug delivery process, it is common to use in-line intravenous filters to remove fungi, bacteria, and particulate from parenteral solutions (Figure 6B). Air bubbles can collect within these filter devices, which can negatively affect the performance of the medical device, drug delivery regimen, and ultimately patient safety. ePTFE membranes provide an effective and reliable solution by safely eliminating entrained air bubbles while preventing leakage of parenteral solution from the filter device.
Insufflation is used in laparoscopic (or ‘key hole') surgery to inflate the abdomen with CO2 gas, allowing space for surgical and visualization instruments (Figure 6C). Insufflation filters are used to protect the patient from bacteria and particulate that may be present in the CO2 tanks and delivered to the patient via the insufflator device. The filter also protects the insufflator from potential biohazardous contamination from the patient. ePTFE membrane is commonly used in insufflation filters due to desirable combinations of membrane gas flow, particle retention, and hydrophobicity.
ePTFE membranes are an important building block for designing safe and effective solutions to challenging medical filtration and separation problems. PTFE and ePTFE are inherently inert, biocompatible, biostable, and hydrophobic. Through a thorough understanding and innovative manipulation of the microstructure, ePTFE membranes can be designed to provide the desired combination of air or gas flow and the retention of potentially hazardous liquids, particles, and aerosols. By joining ePTFE with other functional layers, ePTFE integrates easily into various types of medical devices. Functionalization of the ePTFE membrane extends the versatility of the membrane across a broad range of fluids and application requirements. By leveraging the inherent strengths of the PTFE polymer and engineering unique microstructures and functionality, ePTFE is an effective solution for medical filtration and separation problems.
1 Robert W. Gore, "Process for producing porous products," US Patent 3,953,566.
2 Chiesa R, et al. Extensible expanded polytetrafluoroethylene vascular grafts for aortoiliac and aortofemoral reconstruction. Cardiovascular Surgery 2000; 8(7): 538-544.
3 Wendy Goldberg, Mary Tilley, and Jim Rudolph, Design Solutions Using Microporous Hydrophobic Membranes. Reprinted from Medical Plastics and Biomaterials; March/April 1999.