wöhlk wissen Plasma surface technology and contact lenses - when and why? EDITION # 3 Basic plasma technology

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1 wöhlk wissen DATE 16. Mai 2010 // PAGE _ 1 EDITION # 3 Plasma surface technology and contact lenses - when and why? Nowadays modern contact lenses are manufactured in a wide range of materials and parameters and are individually fitted by the optician or ophthalmologist for many different applications. The present article provides information on the background and variety of plasma technology surface treatments for contact lenses, as well as on the characteristics of products treated in this way. contact lenses. At Wöhlk plasma technology has been used successfully for several decades in the production of classic hard contact lenses (e.g. CAB) and the early silicone products. Since it also has innumerable applications in research and production (automotive, semiconductors, medical devices etc.), it has undergone considerable technological development, which has also resulted in a formidable diversity of technological processes. The optimization of wearing comfort and spontaneous comfort of contact lenses, which have become lighter and thinner in the past 10 years, requires an increasing multiplicity of characteristic potential in the materials and the products made from them, including the product surfaces which are particularly relevant to quality and comfort. Modern polymers for contact lenses are made from very complex monomer compounds and, in the case of some rigid polymers and some soft silicone hydogels, special surface treatment ensures improved surface wettability and spontaneous wearing comfort. Basic plasma technology Plasma processing has become an established feature in optometry, not only through surface treatment for silicone hydogel This variety and the great wealth of experience gained from basic research is also being used increasingly in modern contact lens production to improve quality and comfort. The first generation silicone hydogels without plasma treatment were not adequately functional and compatible, whereas plasma treatment of materials for RGP products can be applied to improve spontaneous comfort, and is therefore an extra option which is not essential for good functioning. What is plasma? Plasma is a reactive gas or gaseous mixture consisting of free, energy-rich electrons, ions and neutral particles. Plasma (Greek: shapeable) is described as the 4th aggregate state of matter because its behaviour differs from that of solid matter, liquids and gases. In a plasma the atoms or molecules, ions and electrons move around freely and interact with each other.

2 DATE 16. Mai 2010 // PAGE _ 2 www. woehlk. com Fig. 1 Schematic diagram of plasma generation Fig. 2 AP plasma flame There are two types of plasma: Low and high temperature plasmas. In the case of contact lenses only low temperature processes are applicable and it is here that surface modification of plastics is most efficient. These low temperature plasmas can be created partly at atmospheric pressure (AP plasma) or at lower pressure (LP plasma). To date AP plasma is of little significance in the production of contact lenses, so that the technically more complex and very efficient LP plasma processes dominate. In every case only the surface of the product is modified and the basic properties of the material are not altered. plasma treatment is largely free of hazardous decomposition products or solvents. The type of gas activating high frequency waves for generating ions, electrons and radicals is important for the quality of the surface structure and its durability, in addition to the type of plasma, the chemical composition of the process gases, the process time, a possible pulse frequency and other peripheral parameters,. Systems are available with a 40 KHz, 13,56 MHz or 2,45 GHz generator, for instance. Practical trials and contact lens durability tests will ultimately assist in making an optimal decision regarding investment in an appropriate system. Contact lenses are geometric high-precision plastics, and are not thermically compromised if they are appropriately processed. Plasmas can be pulsed, allowing for less stressful application. High-energy photons are generated during the process, whereby a biocide effect, amongst others, is achieved. Special LP plasmas follow the contours well and are predestined for the modification of the smallest structures. In contrast to wet chemical processes, Application to contact lenses The surface of the contact lens can be modified by plasma in a wide variety of ways, so that even extremely hydrophobic products can acquire hydrophilic properties. For many years now the silicone hydrogels, which contain a very high proportion of hydrophobic siloxanes, have been a particularly good example

3 wöhlk wissen DATE 16. Mai 2010 // PAGE _ 3 EDITION # 3 for the need for a durable hydrophilic coating. Here the durability of the hydrophilic surfaces is a core criterion since these materials are extremely hydrophobic. b) Plasma activation / functionalization It is a different matter with RGP contact lenses, which have been used successfully for many years without additional surface processing. However, the increasing number of plasma-treated RGP products can make it easier to start wearing these excellent vision aids which may be somewhat less comfortable and wettable at the beginning. Plasma treatment will make this high quality material group more attractive. The different plasma processes vary considerably and have different primary objectives and requirements. The following summary lists the main processes which may find application in the production of modern contact lenses: Fig. 3b Plasma activation/functionalization a) Plasma cleaning c) Plasma polymerisation Fig. 3a Plasma cleaning Fig. 3c Plasma polymerisation Organic particle impurities are removed physically through oxidation, and also chemically, if oxygen is used. After this procedure the products are thoroughly degreased and clean. Processes b) and c) go a stage further than the thorough cleaning of the contact lenses, and produce a nanoscale surface which remains more or less stable long-term and is conducive to longterm wetting stability. The reason for this is the character of the chemical-structural new surface with altered cross linking density, surface finish and surface energy.

4 DATE 16. Mai 2010 // PAGE _ 4 www. woehlk. com Plasma treatment of RGP contact lenses Modern RGP contact lenses which have undergone plasma treatment are also available. These products are not coated but are rendered hydrophilic through plasma cleaning and plasma surface activation, principally with oxygen plasma. The advantages and effectiveness of this additional processing have been frequently debated and are considered below. Plasma oxidation of RGP contact lenses chemically generates a variety of oxygen-functional groups, whereby the lens surfaces loose their relatively water repellent material properties without undergoing any geometric or optical changes or showing any other deficits. The new functional groups can be detected with sensitive analysis techniques (REM-EDX, XPS etc.). The hydroxyl (OH) and carbonyl (C=O) groups are particularly worthy of mention, whereby many stable and less stable groups occur. Greater surface energy of the products is demonstrable, but this is not permanent. The greater surface energy is evident in the improved wettability which can be seen in contact angle measurements, and is noticeable for the lens wearer in improved wearing comfort, which makes the adaptation period for new fittings easier. Plasma treatment covers a range of different qualities and goals, which can make a comparison meaningful and, in individual cases, makes the modified product successful. Improved wettability, harder surface and slightly reduced adhesion of deposits have frequently been reported, in conjunction with less irritation to the eye tissue. For many years now the experts at Wöhlk have been combining observation of the performance of the first plasma-treated RGP contact lenses with our own extensive experience in the field of plasma technology, and have optimized process technology for the production of plasma modified RGP contact lenses. Our comprehensive experience with different plasma frequencies was employed to develop a particularly effective and gentle activating process for contact lenses. Long-term observation and nanoscopic surface analysis in our own research department were the basis for these high-quality products, in which time-tested traditional plastics are combined with new plasma surfaces. Surface structures after plasma treatment Changes in plasma-treated RGP contact lenses are in the nanometer range, in ranges which can be examined only with the most modern methods of analysis. Changes to the surface structure can be determined very exactly with Atomic Force Acoustic Microscopy (AFM) in these extremely small orders of magnitude. Nanoscale material abrasion occurs during plasma cleaning and plasma oxidation, which is, however, not inevitably also linked to changes in the structure which would result in a new surface topography. In comparative analyses products were examined which experienced material abrasion but no change to topography and rms-roughness. However, it can not be deduced from this that more or less surface finish necessarily correlates with better or worse wettability or better spontaneous comfort. The nature of plasma physics (the type of electrodes, the high frequency, the setting etc.) and the type of process gases used produce specific results in different contact lenses and materials. Fig. 4 (a and b) shows an AFM view of a Wöhlk RGP contact lens (CFA) and its topography with and without plasma treatment. During the plasma process the roughness of modern Wöhlk RGP contact lenses decreases (e.g. from 430 angstrom to 140 angstrom), resulting in an altered height-depth profile. The topography of these lenses therefore also changes to a somewhat smoother surface. The named chemical changes/ functionalities in the topmost molecule layer, which reach more polarity through a higher degree of ionization of the plasma process than was the case with older and lower energy plasma technologies, also have an effect. In extensive trials with different materials Wöhlk has ascertained that an adequate microwave stimulation carries less risk of material defects (micro tears etc.) or of pre-existing damage than was sometimes detected with radio frequency. Thus slightly cross-linked RGPs, soft plastics and comparatively hydrophobic plastics are rendered hydrophile more effectively with microwave frequency in short process sequences than can be achieved with other plasma frequencies.

5 wöhlk wissen DATE 16. Mai 2010 // PAGE _ 5 EDITION # 3 Fig. 4a RGP-Wöhlk contact lens without plasma (AFM photo) Fig. 4b RGP-Wöhlk Contact lens with plasma treatment

6 DATE 16. Mai 2010 // PAGE _ 6 www. woehlk. com Fig. 5a Re-hydrophobisation after plasma oxidation (trend) Durability of plasma-induced product characteristics In the literature concerning plasma physics and experience with plasma processes on plastic materials and their many specialties, a relative instability of surface energies and the chemical surface functionalities depending on external influences on the treated products have been described. For example, the quality of the hydrophilization of different contact lenses is affected by a temporaly very product-specific re-hydrophobization. Plasma-treated Wöhlk RGP products also exhibit a slow and unavoidable loss of surface energy during storage as well as through continuous care and cleaning of the contact lenses during use. To start with the products have excellent wetting characteristics which, depending on when the contact angle is measured, on the type of measuring method used and the pretreatment of the samples, can be as much as 20 to 30 (drop method). After 24 hours the contact lenses loose a small part of these causative hydrophilic chemically functional groups and loads through saturation from the hydrophobic environment/ air and through restitution and rotation into a more stable, lower energy state. After about 1 week contact angles are about 50, which is still an excellent result, although it is important to use the same measuring method when comparing like with like.

7 wöhlk wissen DATE 16. Mai 2010 // PAGE _ 7 EDITION # 3 Fig. 5b Graphic representation of fundamental influences on extended durability of hydrophilic properties of RGP CL surface There are known favorable factors which contribute to a much longer-lasting good wettability and which are taken into account. Thus the Wöhlk products are not stored unnecessarily dry and are generally dispatched in suitable viscous storage solutions. The customer can use all the usual and recommended hygiene and care methods. The long-term instability of the hydrophilic properties is not directly hastened by the care solution and cleaning, although this can lead to a more pronounced re-hydrophobization than when the plasma-treated contact lenses are only stored in care solution. This is why the perceptibly better comfort decreases somewhat over a period of a few weeks to the normal level. The formation of deposits due to the lipophile components in the tears is also reduced long-term however because the surface topography, which results from material abrasion and slight cross linking in the plasma process, remains unchanged.

8 DATE 16. Mai 2010 // PAGE _ 8 www. woehlk. com Conclusion Literature Plasma technology or plasma surface-treatment are being increasingly applied to modify the relatively hydrophobic material properties of rigid gas permeable contact lenses (RGPs), and to optimize the surface quality of these products so that the wearer experiences better spontaneous compatibility and wearing comfort. The hydrophilization of the material involved in the process affects the surface finish and the adhesion characteristics of the product. However, there are additional chemical groups and carriers which do not remain permanently hydrophilically active. The improved wetting behaviour is therefore only noticeable for the user at the beginning, although the reduced roughness or the somewhat smoother topography does mean that hydrophobic deposits (lipids etc.) will be reduced long-term. Since plasma treatment is not a coating process, there is no risk that the normal recommended care and hygiene measures, including the use of abrasive cleaners, will cause any damage. Scratch resistance is even slightly better in plasma-treated lenses, so this procedure is a further measure for increasing biocompatibility and product tolerance. Roland Fromme, Dipl. Ing. (FH) Chemie, Head Chemical Engineering wöhlk contactlinsen 2010 Berke A: Wechselwirkungen zwischen Bakterien und Kontaktlinsenoberflächen, die Kontaktlinse 9/2009, Gonzalez-Meijome MJ, Lopez-Alemany A, Almeida BJ, Parafita AM: Surface AFM Microscopy of Unworn and Worn Samples of Silicone Hydrogel Contact Lenses, J Biomedical Mat Research Part B, 2008, p1-8 Plasma Technology: Firmenschriften Künzel P: Die Behandlung des kontaktlinsenbedingten Trockenen Auges; die Kontaktlinse 10/2008, 4-9 Plasma Plus: homepage CONIL AG, No7 Contact Lenses Deutschland e.k.: Firmenschriften Plasma Behandlung Brown K: Plasma Treatment for gas Permeable Contact Lenses, Polymer Technology, Boston update, Global edition Volume 19, July 2007 King-Smith PE, Nichols JJ, Nichols KK, Fink BA, Braun RJ: Die Auswirkung von Tränenfilmverdunstung und anderen Mechanismen auf die Verdünnung und das Aufreißen des Tränenfilms; die Kontaktlinse 1-2/2009 S12-19 Medical Device Technology: Novel Technology for the surface Modification of Biomedical Polymers; Whitehouse, Al-Lamee K; 2009 Thai LC, Tomlinson A, Doane MG: Effect of Contact Lens Materials on Tear Physiology, Tapper TT: Plasmaoberflächenbehandlung; DOZ ; Wöhlk-Contact-Linsen GmbH Postfach 4520, Kiel Bürgermeister - Schade - Str Schönkirchen Contact: Roland Fromme, roland.fromme@woehlk.com Telefon + 49 (0) Everaert EP et al: Hydrophobic recovery of repeatedly plasma treated silicone rubber; J. Adhesion Sci Technol // ART 1726_CK // 04.10