Plasma-induced graft polymerization of acrylic acid onto poly(ethylene terephthalate) films: characterization and human smooth muscle cell growth on grafted films
Introduction
Polymers have generated considerable interest as biomaterials in the field of tissue engineering, such as tissue replacement, tissue reinforcement and organ transplant [1]. The polymeric materials provide support surfaces for the immobilization of biologically active molecules and living cells. Scaffolds have also been made from extracellular protein matrix components such as collagen, laminin or fibronectin. These materials show excellent cell adhesion, biodegradability and biocompatibility but suffer from the disadvantage that they cannot be freely or reproducibly processed into stable objects with three-dimensional shapes of good mechanical strength. Scaffolds made out of thermoplastic polymers, on the other hand, have excellent strength and ductility and can readily be made into various shapes, but their surfaces do not favorably interact with cells. It is the physico-chemical and morphological nature of polymers that governs the cell interaction at the interface [2], [3], [4], [5]. Synthetic polymers therefore often require selective modification to introduce specific functional groups to the surface for the binding of biomolecules [6], [7].
Plasma-induced graft polymerization is an attractive way of modifying the surface chemistry and morphology of polymeric materials [8], [9], [10], [11], [12]. A desired monomer may be polymerized onto the surface of a plasma-activated polymer resulting in the formation of a grafted brush layer on top of the surface. The grafted surfaces may then provide active sites for the binding of protein molecules. This method is highly surface selective, where the modification is confined to a depth of a few nanometers without modification of the bulk properties. Plasma-induced graft polymerization has consequently proven highly successful as a means to develop functional interfaces for the immobilization of biomolecules and apt for culture [13], [14], [15], [16].
Considerable effort has been made to modify poly(ethylene terephthalate) (PET) surfaces for the immobilization of biomolecules [17], [18], [19], [20], [21], [22], [23]. Piglowski et al. [18] reported that PET can be suitably modified by plasma exposure under argon (which makes the surface hydrophilic) or perfluorohexane (leading to a more hydrophobic surface). Both types of surfaces were found to show excellent biocompatibility during in vitro and in vivo evaluation. Surface modification of PET films, fibers and fabrics has been carried out using plasma, UV, ozone and radiation induced graft polymerization of various monomers [19], [20], [21], [22], [23], [24], [25]. The grafting of such a hydrophilic monomer leads to a surface with suitable chemical functionality for biomolecule interaction at the interface. However, the photosensitizers used in UV grafting have to be thoroughly removed from the polymer surface as a pre-condition for any biomedical application. In a recent report [26], PET films were grafted with acrylic acid using oxygen plasma in order to immobilize insulin and heparin on the surface.
Very little is currently available in the literature on the physico-morphological changes that take place as a result of the grafting at PET surfaces. In particular, the surface structure and dynamics of the plasma grafted PET have not been properly addressed in the context of polymer surface-cell interactions. Therefore, it is extremely interesting to monitor eventual alterations in the surface morphology, as they are likely to affect cell growth considerably. With this in mind, we describe here the preparation of PET surfaces with various densities of poly(acrylic acid) (PAA) grafts by plasma processing, so that the modified surfaces may be used for the coupling of tailored amounts of collagen promoting smooth muscle cell growth in vitro.
The surface modification of PET films by plasma treatment has already been described in detail in an earlier report [27]. The modified surfaces contain oxygen functionality as demonstrated by X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance (ATR) measurements. However, these surfaces are relatively unstable undergoing rapid reorganization which results in a loss of surface hydrophilicity. In order to maintain a surface with high level of hydrophilicity and functionality at the surface, subsequent modification by grafting of acrylic acid onto plasma treated PET films was carried out. The influence of the plasma treatment and reaction conditions on the graft density has been discussed elsewhere [28]. In the present investigation, we report on the grafting-induced structural changes on PET surfaces and the immobilization of collagen and seeding of human smooth muscle cells.
Section snippets
Materials
Biaxially oriented PET films of 23 μm thickness used in this study were supplied by Goodfellow, England. Acrylic acid, Mohr's salt, acetic acid, sulfuric acid and sodium hydroxide were supplied by Fluka. Toluidine Blue O was supplied by Aldrich. Deionized water was used throughout and the acrylic acid was purified by distillation under reduced pressure.
Vitrogen (a mixture of collagen type I and III) (Cohesion, Palo Alto, CA, US) was used. PBS, trypsin, and Hams F10 nutrient mixture were supplied
Variation in contact angle
Grafting led to a considerable decrease in the contact angle of the PET films as shown in Fig. 1. The virgin film had contact angle of 73° which decreased to 33° for the film plasma treated for 60 s. This value is selected for zero graft density in Fig. 1, giving an unexpected increase in contact angle for the surface grafted with 1 μg/cm2 polyacrylic acid.3
Conclusion
The observations in this investigation suggest that plasma processing is an extremely useful technique to design PET films for the immobilization of proteins and cell growth on the modified surfaces. The surface structure changes at each stage of the plasma treatment as well as during the grafting of acrylic acid. The grafted surface is unstable and undergoes rapid reorganization similar to that observed in the plasma treated films. However, the rearrangement was limited at higher graft
Acknowledgements
Authors acknowledge the assistance of Mr. N. Xanthopoulos, Laboratory of chemical metallurgy, EPFL for carrying out XPS measurements on samples. Authors are grateful to Swiss Federal Institute of Technology, Lausanne, Switzerland for financial support to carry out this work.
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