INTRODUCTION TO ePTFE AND ITS USE IN BIOMATERIALS APPLICATIONS
Poly(tetrafluoroethylene) (PTFE), a fully fluorinated polymer, is polymerized from tetrafluoroethylene (CF2CF2). It is a linear thermoplastic polymer, which was discovered in 1938 by Plunkett while working for Du Pont. The strong CC and CF bonds[2, 3] together with the helical structure of the polymer chain caused by the relatively larger size of fluorine atoms compared to carbon atoms[2, 4] give PTFE high thermal and chemical stability. Furthermore, since the carbon atoms in the chain are enclosed within a sheath of electronegative fluorine atoms the polymer chains are very stiff and this results in a highly crystalline material. PTFE exhibits good electrical insulating properties, has high toughness, is non-adhesive, has anti-frictional properties and is extremely hydrophobic.[5, 6]
The expanded form of PTFE (ePTFE) was developed by Wilbert Gore with the first patent for the process granted in 1976 followed by several subsequent inventions over the following years.[7, 8] The stretching process was conducted at temperatures above the lowest crystalline melting point of PTFE and resulted in an increase in amorphous content of ePTFE compared to the starting material. The ePTFE materials can be described as having a microstructure composed of nodes interconnected by fibrils. The size of the morphological features as well as the crystallinity can be tailored by the conditions used in the expansion process. This is illustrated in Figure 1 which shows the SEM images of ePTFE membranes from three different suppliers. The percent crystallinity was determined as previously reported, and is indicated in the figure text.
The first reported biomaterials application of PTFE was as an artificial heart valve. Shortly after, a woven textile graft of PTFE found application as a vascular graft material, however, it was found not to be ideal as it unraveled post implantation. In contrast, ePTFE has proven more favorable as a biomaterial due to its antithrombotic surface and porosity which allow tissue in-growth (e.g., fibrovascular and dermis). Furthermore, it displays enhanced mechanical integrity. PTFE is one of few (if not the only) synthetic polymer which is truly biostable and an in vivo study of ePTFE showed that it is stable for up to 6.5 years (length of study) after implantation. Because of its overall good performance in the human body PTFE and in particular ePTFE have found numerous biomaterials applications, some of which are listed in Table 1. While W. L. Gore and Associates continues to manufacture ePTFE materials and has an extensive product range in the medical implant market, other companies included in Table 1 likewise provide PTFE materials for biomaterials applications.
|Vascular graft||Gore-Tex®||W. L. Gore and Associates|
|AV Access Graft||BARD®|
|Advanta™ Graft||Atrium Medical Corporation|
|Bypass graft||Gore® Hybrid Vascular Graft||W. L. Gore and Associates|
|Tissue space-filler in soft tissue reconstruction||Gore® DUALMESH®||W. L. Gore and Associates|
|GORE-TEX® Soft Tissue Patch||W. L. Gore and Associates|
|PTFE knitted mesh||SurgicalMesh™|
|Guided bone regeneration||Cytoplast™ micro textured high density membrane||Osteogenics Biomedical|
|TefGen regenerative Membrands™||Lifecore Biomedical|
|Hernia membrane||Gore® DUALMESH®||W. L. Gore and Associates|
|PTFE knitted mesh||SurgicalMesh™|
|Sutures||Gore-Tex®||W. L. Gore and Associates|
|Suture support||Cytoplast® monofilament||Osteogenics Biomedical|
|Pledget||Santec medicalprodukte gmbp|
It has long been established that ePTFE is suitable in vascular graft applications in general, however, it cannot be used for small diameter grafts due to occlusion caused by thrombosis and intimal hyperplasia. This issue has been addressed in a modified ePTFE material manufactured by W.L. Gore under the trade name Propaten® and more detail of this product will be given in the Vascular section. Another application where it has been identified that the material itself is not ideal but requires modification of its surface in order to provide an implant with optimal performance is in its use as soft tissue fillers interfacing with bone tissue. To the best of our knowledge these are the two applications where the largest research efforts have been conducted in order to improve the medical performance of ePTFE and this article therefore seeks to provide a comprehensive review of these two applications. The focus will be on research done in the period from 2004 to July 2013.