Surface Treatment

Many of the varied species within a plasma (ions, electrons, radicals and photons) can alter the surface of a polymer (Figure 1.20). The extent at which these species affect the polymer surface depends on many things. These include the energy, density and chemistry of the species, the wavelength of the photons, the distance they need to travel and the chemistry of the polymer surface. When the species reach a polymer surface they can change the structure provided they have enough energy. Ions are known to cause etching of a surface (typically from molecular gases e.g. O2, N2…), UV photons can break surface bonds or cause cross-linking, radicals and ions can be incorporated into the surface [162, 169, 197, 198].

Figure 1.20 Cartoon of the species involved in plasma treatment including UV photons

(hv), ions, electrons and radicals.

When a surface is treated with a plasma created from “inert” gases such as helium, argon and neon it often results in the addition of oxygen, even if the surface is treated in vacuum. This phenomenon is due to radical implantation on the sample surface. These surface radicals act as intermediates in the changing surface chemistry. Their free electrons quickly combine with other atoms or molecules in thermodynamically favourable reactions. Most often the surface radicals react with oxygen in the air to create hydroxyl, carboxyl and other oxygen containing functional groups. Radicals created on the surface of materials by inert gas plasmas in vacuum can combine with oxygen species (such as molecular oxygen or water vapour) when they are removed from the vacuum into air (Figure 1.21). This post- processing reaction has often been observed, even when gases such as nitrogen have been used, or following plasma polymerisation [169, 197-200]. When plasmas are generated in air or with the addition of oxygen, it is difficult to distinguish whether the oxygen present on the surface following treatment was incorporated directly during treatment, or was due to radicals created on the surface, however it is likely to be a combination of these effects. The incorporation of polar oxygen functional groups onto polymer surfaces is typically characterised by a reduction in contact angle and the observation of these species by x-ray photoelectron spectroscopy (XPS).

Figure 1.21 Cartoon of post-treatment functionalisation by air creating hydroxl and carboxyl functional groups.

A study by Yuen et al. (2006) investigated the plasma treatment (low pressure) of common IOL materials: PMMA, silicone and hydrophobic acrylic (Alcon’s Acrysof® lenses). PMMA was treated with a nitrogen plasma, whereas silicone and Acrysof® were treated with air plasmas. All samples were placed in water following treatment to initiate oxygen functionalisation via interaction with water and surface radicals as outlined in previous work [201]. Plasma treatment decreased the contact angle of all materials and also significantly increased the number of primary bovine LECs observed on treated materials at 16 and 24 days post-seeding. Yuen and co-authors also comment that the morphology of LECs on treated PMMA and silicone were more epithelial than those on untreated materials [64].

Matsushima et al. (2006) treated acrylic IOLs (Hoya’s VA-60 BB lenses) with either a UV/ozone system or an argon plasma (pressure not stated). The authors demonstrated the presence of hydroxyl (-OH) and carboxyl (COOH) groups, not present on untreated surfaces, following either treatment. The treatment of lenses increased the adhesion of fibronectin and primary rabbit LECs. Untreated and treated lenses were also implanted in rabbits following phacoemulsification. After 2 weeks the animals were sacrificed and the thickness of the LEC growth between the posterior capsule and lenses was examined. There was a significantly thinner layer of LECs on the central portion of the capsule/lens when treated lenses were implanted compared to untreated lenses [202].

D’Sa et al. (2010) demonstrated that atmospheric pressure DBD plasma treatment in air of polystyrene (PS) and PMMA increased the surface oxygen of these polymers. This corresponded to the adsorption of albumin in a different conformation on treated surfaces compared to untreated surfaces. In a competitive protein adsorption study the authors noted that serum proteins replaced some of the albumin which had adhered on the surface, but only on treated surfaces [5]. The authors also demonstrated that the difference in the protein adsorption on treated and pristine PS and PMMA resulted in a greater number of adhered B3 LECs on the treated surfaces. The LECs on treated surfaces also had a more spread morphology compared to LECs on untreated surfaces [5, 18]. In other work D’Sa and co-authors demonstrate that the hydroxyl groups bound to the surface following plasma treatment can be used in wet chemical reactions, to bind both poly(ethylene glycol) methyl ether methacrylate [203] and hyaluronic acid [155]. In this latter experiment the authors demonstrated that plasma treatment enabled the chemisorption of 3- aminopropyltrimethoxysilane (APTMS) which created an amine surface functionality. Hyaluronic acid could then be immobilised to the amine functionality. Whereas APTMS encouraged LEC adhesion (as amine functionalised surfaces are known to increase cell adhesion) the immobilised hyaluronic acid inhibited LEC adhesion to the PS [155]. If the amine or plasma treated surfaces could be used to encourage an epithelial monolayer which retained it phenotype, then any of these surfaces (plasma treated, amine and hyaluronic acid) could be used to combat PCO.

Similarly, Zhang et al. (2009) bonded poly ethylene glycol (PEG) and/or heparin to PMMA IOL surfaces following low pressure argon plasma treatment. The authors suggest the plasma treatment with argon created surface radicals which formed oxygen functionalities when the samples were removed from vacuum to air. Following this samples were soaked in PEG or heparin solutions then treated with the plasma again. This is known as plasma- induced polymerisation. The resultant PEG and/or heparin surfaces were demonstrated to reduce the adhesion of platelets [154]. The attachment of LECs was not investigated.

An atmospheric pressure argon plasma was used by Wang et al. (2009) to modify the surface of hydrophobic acrylic lenses (66 Vision Tech Co.’s FV-60A lenses). Again, the authors believe exposure to air following treatment introduced the increased concentration

of oxygen and nitrogen (nitrogen increased by only 0.3%), and subsequent reduction in contact angle from 92° to 70-77° (depending on storage time). The authors demonstrate that the plasma treatment of acrylic IOLs significantly decreased the number of platelets and macrophages, when examined 24hrs after the cells were seeded. When B3 LECs were seeded onto untreated and treated lenses there were significantly fewer LECs on the lenses treated for the longest times (180 and 360s treatment times) compared to untreated at 24hrs; however there was no difference between the number of LECs on untreated lenses and those plasma treated for shorter durations (10 and 60s treatment times). At 72hrs post-seeding there was no significant difference between the numbers of LECs on treated or untreated surfaces. The LEC morphologies were also similar between materials [153].

1.6.1.1 Spatial Resolution

The use of atmospheric pressure microplasma jets for functionalising surfaces is a research topic of growing interest. Yet there are not many current publications which investigate the spatial confinement of the treat areas. Even fewer papers have reported the spatially resolved cellular attachment onto surfaces treated with microplasma jets.

It has been demonstrated that microplasma jets can be used to encourage spatially defined cell growth on polymeric materials [19, 170]. In both studies plasma jets, with nozzles diameters of 100-150µm, were scanned in a line across a polymer surfaces. The authors demonstrated that cells (human aortic endothelial and HeLa cells) adhered to tracks with widths of a few hundred micrometers. Yet these reports have vague methodology and possibly lack of repetition. The spatial modification of the surface also lacks detailed characterisation [19, 170].

In 2007 Soba et al. reported surface functionalisation by a helium/ammonia with an area of ~500µm by XPS imaging. In this study however the pressure was “near atmosphere” (720- 730Torr; 760Torr = 1atm), and a reaction chamber with various pumps were used. Although operating in the same pressure range this system still required a plasma reactor chamber and pumping mechanisms [204]. The smallest spatial modifications induced by atmospheric pressure plasma jets reported, to the best of the authors knowledge, has taken place in Shizuoka University, Hamamatsu, Japan. Kakei et al. (2010) reported etching of acrylic resin

resist films in tracks of 500-700nm, using a jet with a 500nm nozzle. The track widths were measured by AFM. The change in the surface chemistry was not studied and could possibly be wider than these etched tracks [181]. Motrescu et al. (2012) et al. used a 2-step process to create amine functionalised dots as small as 20µm in diameter. Polymer surfaces (polyethylene and polyurethane) are first pre-treated with a helium plasma for 0.01-0.1s and subsequent treated with helium + 3% ammonia plasmas for 3s. The size of the resultant amine functionalisation was determined using a fluorescent microscope and a fluorescently tagged sulfodicholorphenol ester which binds to amines. The authors were able to achieve dots of 20µm with a 1µm ID nozzle [205]. The size of the oxygen functionalisation was not investigated and would be of interest from a biomaterial perspective. These results may be promising for biomaterial applications yet no work in this field has yet been published.