Poly(tetrafluoroethylene) is ubiquitous in our daily lives. Applications range from non-stick cookware to artificial arteries and from wet weather apparel to high-altitude mountaineering ropes. Its serendipitous discovery is a story worth telling

Chandler-Temple, Adrienne

Grondahl, Lisbeth

Wentrup-Byrne, Edeline

Chandler-Temple, Adrienne F.

Name not available

Expanded poly(tetrafluoroethylene): from conception to biomedical device

Queensland University of Technology ePrints Archive

               QUT Digital Repository:  http://eprints.qut.edu.au/ Chandler-Temple, Adrienne F. and Wentrup-Byrne, Edeline and Grondahl, Lisbeth (2008) Expanded poly(tetrafluoroethylene): from conception to biomedical device. Chemistry in Australia 75(8):pp. 3-6.            © Copyright 2008 The Royal Australian Chemical Institute  Expanded Poly(tetrafluoroethylene): From Conception to Biomedical Device  Adrienne F. Chandler-Temple1*†, Edeline Wentrup-Byrne2, Lisbeth Grøndahl1  1School of Molecular and Microbial Sciences, The University of Queensland, St Lucia Q 4072 2Tissue BioRegeneration and Integration Program & School of Physical and Chemical Sciences, Queensland University of Technology, 2 George St., GPO Box 2434, Brisbane Q 4001                            * Corresponding Author † Current Address: Centre for Materials and Surface Science, La Trobe University, Bundoora Vic 3086, email: a.chandler-temple@latrobe.edu.au Initial Discovery Isaac Asimov said that the most exciting phrase in science is not ‘Eureka!’, but ‘That’s funny...’ This phrase epitomises Dr Roy Plunkett’s serendipitous discovery in 1938 of poly(tetrafluoroethylene) (PTFE, Figure 1) whilst working on new forms of the DuPont refrigerant Freon®. PTFE stood out as a highly unusual tough thermoplastic with unique physical properties stemming from the strength of the C-F bond, the high electronegativity of fluorine, and the fact PTFE forms helical rods rather than mobile zig-zag chains.1    Hence, PTFE is extremely hydrophobic, chemical and heat resistant as well as being prone to cold flow. These properties initially made processing the new polymer a challenge.  As Plunkett himself delightfully admitted: “While I had recognised that PTFE was an unusual material, I was somewhat at a loss as to what to do with it.”2. However, it was not long before processing methods were formulated and a full-scale plant was built in 1950.3  Figure 1 near here.  PTFE Manufacture and Development Currently PTFE is manufactured and sold in three forms: granular, as a fine powder or as an aqueous dispersion4.  Further processing is by compression, automatic and isostactic  ? molding, ram extrusion, and dispersion coating.4-6 PTFE was initially used in small quantities as tape, sheets, rods and tubes, gaskets, insulated wire for handling hot corrosive liquids, lubricant powder, and as an electrical insulator. Its potential was quickly acknowledged and large-scale production led to its use in the space industry, nuclear power industry, in electronics, and telecommunication.2  The key process that made it possible to transform this “wonder material” into the highly porous or expanded version (ePTFE), used today in many in vivo biomedical and dental applications such as surgical sutures, vascular grafts, cardiovascular and soft tissue patches, facial implants, and endovascular prostheses, was initially discovered by Wilbert and Robert Gore. Its use in biomedical applications stemmed from yet another serendipitous encounter with Wilbert Gore’s friend, surgeon Dr Ben Eiseman. Whilst on a Colorado skiing trip, Gore showed Eiseman a piece of tubing and mentioned that he had no idea of what it could be used for. Two weeks later, Eiseman successfully trialed the tubing as a vascular graft in a pig.7 He subsequently became the first person to use Gore-Tex® in humans replacing a portal vessel in a cancer patient.8 Thus, a biomedical device was born. An examination of selected Gore patents (Table 1) sheds an interesting perspective on the historical development of this versatile material.  The Initial Patents Wilbert Gore worked as a research chemist at DuPont (Wilmington). He had tried for some time to interest the company in developing further PTFE applications without success. He left DuPont and started his own family business at his home in Newark in 1958. He began by using Du Pont PTFE as an insulator in electrical applications. The processing which involved forming sheets by compressing PTFE moulding (fine dispersion) powder to eliminate voids proved a challenge from the start. These sheets were heated and stretched through rollers which improved elongation but it also promoted splintering, hence, common two-way stretching techniques could not be used. He experimented with extrusion techniques and developed a method to form biaxially expanded fibrillated sheets. The specialty extruder head is the subject of his 1967 U.S. Patent. Control of fibrillation was made possible by his discovery that using organic lubricants reduced the sheer stress of the particles. The nature of the fibrils was dependent on the nature of the lubricant used, initially naphtha gasoline, kerosene, glycol: basically any organic liquid that was miscible with wet PTFE. Fillers, such as mica, graphite, glass fibre, bronze powder, and potassium titanate fibres, were used to improve hardness, rigidity, wear resistance, creep resistance, and heat conductivity. These fillers and lubricants were added prior to fibrillation and subsequently removed either by heat processing or chemical dissolution.  Table 1 near here.  In the 1972 Patent the term ‘expanded’ was first used. It referred to a material that had a lower specific gravity after processing. The microstructure was described as ‘fibrillar’. It is clear from this patent that Gore’s original aim was to produce an easy to use joint-sealing material with the excellent qualities of PTFE but using less starting material. It outlines how the extruder head described in his first patent was used to produce ribbons of PTFE that were subsequently converted into what is now commonly referred to as plumber’s tape.  Addressing Porosity In 1969, Robert Gore discovered that PTFE could be made porous while retaining its strength, maintaining uniformity and cross-sectional shape, and without causing any significant change in its thickness. He found that high temperatures and fast stretching techniques produced an optimal product. In his 1976 patents, Gore mentions the demand for improved porous products, particularly with reference to reducing the need for fillers. Removing fillers from the process not only made the process more economical but eliminated the risk of impurities.  He discovered that the expansion process was greatly improved if the starting polymer was highly crystalline (>98%). The 1976 patents stress that although uniaxial expansion increased the porosity, it also increased the strength, particularly with high-rate expansion. The process of stretching the material at temperatures above the lowest crystalline melting point of PTFE caused an increase in disorder with a corresponding increase in amorphous content. This locked the fibrils and crystallites: thereby increasing creep resistance. The result was improved material strength.  The expanded material could now be used as a membrane to separate wetting and non-wetting liquids as well as solids from liquids including those corrosive to other materials. It could bond to itself and other materials more readily than unexpanded PTFE. It also demonstrated improved thermal resistance. As a consequence of the lower density the expanded material had a lower dielectric constant than the unexpanded, 1.2 – 1.8ε compared to 2.2ε. Hence, it could also be used for finer wires and cables. In addition, an extreme increase in final length, up to 1760 times the original length, was attainable without diminishing the thickness of the product. The new materials produced were described as having a ‘microstructure characterised by nodes interconnected by fibrils’. This is clearly visible in Figure 2A. The size of the nodes and the character of the fibrillar structures depended on the conditions used in the expansion process. Both 1976 patents describe methods for producing sheets, films, tubes, rods, and continuous filaments by methods as diverse as uniaxial expansion, biaxial expansion, pressure differential, and a combination of pressure differential and moulding. The articles produced from these patents are the subject of a 1980 U.S. patent with 77 claims in total.  Figure 2 near here.  Medical Applications The advances made thus far regarding physical properties such as strength and porosity, as well as the minimisation of impurities such as the fillers, now meant that the material was ready for long term in vivo applications. By the time of the 1984 patent, the ePTFE tubes described in the three previous patents as “having internodal distances of 5 – 1000 µm” were being used as vascular prostheses. The internodal spaces meant that in many cases tissue ingrowth occurred after implantation.9, 10 Some applications such as soft tissue replacement required thicker materials. Gore and his associates achieved this by constructing consecutive layers of unsintered lubricated PTFE. As described in the November 1984 patent once the desired thickness was reached the lubricant was removed and the material was expanded in the desired manner: uni-, bi-, or multi-axially below the crystalline melt temperature and at a selected rate.  The earlier materials had a microstructure characterised by small nodes and extended fibrils that have been drawn directionally, with a strength concentration in the direction of orientation. Further development aimed at creating products with a ‘coarse’ microstructure: larger nodes and larger pore sizes. However, although a coarse microstructure resulted when the PTFE was stretched, the material had decreased tensile strength. U.S. Patents 4,482,516 and 4,598,011 used a densification die, calendering, or a press, to densify the paste-extruded unsintered PTFE extrudate. This resulted in a uni- and bi-axially stretched product of coarse microstructure with increased tensile strength. Stretching could be performed continuously either post- or pre-densification. Alternatively a desired shape could be obtained mechanically using lubricant-loaded extrudate. The 1986 patent with 23 claims in total covers articles produced by this approach. One of the important outcomes of this process is that the product had increased crush resistance; hence porosity could be maintained under limited pressure. This advance was critical to the manufacture of materials suitable for applications such as cranio-maxillo-facial augmentation that require permanent aesthetic integrity for positive patient image outcomes.  Today expanded ePTFE is the in vivo material of choice for many sutures, vascular grafts, and as an augmentation material for soft and hard tissue replacement. The nature of the processing techniques determines the target properties of the device such as strength and chemical resistance to withstand the harsh in vivo environment. Although ePTFE is used successfully as a biomedical device in these situations, for some applications an optimum degree of material-tissue response is desirable. In order to improve this bioactivity researchers have in recent years been applying surface modification techniques. The incorporation of heparin improves the anti-clotting behaviour of the vascular grafts while phosphate groups improve the bone-bonding ability of materials.  Thanks to Wilbert Gore’s foresight and persistence, we now have a material that continues to make significant improvements in people’s lives. There is a salutary lesson in the PTFE story: it demonstrates that advances are made with persistence and that the development of new biomedical capabilities can result from the observation “that’s interesting...”.  Acknowledgement: The authors would like to acknowledge Dr Richard Lewandowski of “Operation Smile” for his time and suggestions for projects from which this story stem. (http://operationsmile.com.au)  Figure 1: Chemical structure of poly(tetrafluoroethylene).    Figure 2: Scanning electron micrographs of Gore-Tex® ePTFE (A) and Sumitomo® ePTFE (B) illustrating the gross structure (nodes interconnected by fibrils) formed by different expansion methods.      A B Table 1: An examination of the relevant Gore patents sheds an interesting perspective on the historical development of this versatile material.  Year U.S. Patent Number Invention 1967 3,315,020 Extrusion die 1972 3,664,915 Expansion process, plumber’s tape 1976 3,953,566 Expansion process variety of expansion parameters, for uni-, bi-, multi-axial expansion 1976 3,962,153 Expansion parameters for increasing expansion length 1980 4,187,390 The products produced by the two 1976 patents 1984 4,482,516 Increasing the thickness by layering 1984 4,478,665 Densification of unsintered PTFE after lubricant removal and prior to stretching 1986 4,598,011 Products from densification process  References   1. Schiers, J., Modern Fluoropolymers. 1st ed.; John Wiley & Sons: Chichester, 1997; p 637. 2. Kauffman, G. B. Education in Chemistry 1988, November, 173 - 175. 3. Sperati, C. A., High Performance Polymers: Their Origin and Development, Elsevier Science Publishing Co., Inc.: 1986; pp 267 - 278. 4. Nuyken, O.; et al, Handbook of Polymer Synthesis, Marchel Dekker: New York, c2005; pp 151 - 239. 5. Drobny, J. G., Technology of Fluoropolymers. 1st ed.; CRC Press: Florida, 2000. 6. Hintzer, K.; Löhr, G., Modern Fluoropolymers, John Wiley & sons Ltd: Chichester, 1997; pp 239 - 255. 7. Shipper, F.; Manz, C. C., Cases in Strategic Management, 1998; pp 492 - 513. 8. Beger, H. G. Langenbecks Archives of Surgery 2004, 389, 311 - 312. 9. Cumming, R. D.; et al. Transactions - American Society for Artificial Internal Organs 1983, 29, 163 - 168. 10. Hydorn, W. H.; et al. Annals of Thoracic Surgery 1977, 23, (6), 539 - 544.    

Expanded poly (tetrafluoroethylene): From conception to biomedical device

http://eprints.qut.edu.au/14948/1/14948.pdf

Expanded poly (tetrafluoroethylene): From conception to biomedical device

Abstract

Similar works

Full text

Available Versions

Name not available

Queensland University of Technology ePrints Archive