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Biomedical Optics & Medical Imaging

Irradiating polymers improves biocompatibility

Exposing polymers to UV could ease fabrication of cell-coated medical implants and cell micro-arrays for high-throughput screening.
9 October 2006, SPIE Newsroom. DOI: 10.1117/2.1200608.0345

The interaction of cells with synthetic polymers is important for many potential applications in medicine and biotechnology.1 Several groups have attempted to make the inner surface of cardiovascular prostheses—made from polytetrafluoroethylene (PTFE) or polyethyleneterephthalate (PET)—more compatible with cells.2,3 Others have worked to produce microscopic arrays of cell spots on a polymer surface for high-throughput screening applications.4

The physical and chemical surface properties of polymeric supports strongly influence cell function, adhesion, and proliferation. Radiation treatments of polymers have been extensively investigated as a way to improve biocompatibility. The aim of these treatments is mainly to produce new chemical groups at the polymer surface that enhance cell attachment and adhesion through their polar or chemical character. The substrate roughness also strongly influences cell growth and adhesion. The optimal micro-roughness depends on the type of cell and, for many cell types, nano-scale surface roughness is also reported to promote cell adhesion and proliferation.5

In our recent work,6–8 we demonstrated highly-effective photochemical surface modification. We improved the compatibility of mammalian cells to PTFE, PET, and polyvinyl alcohol (PVA) by irradiating the materials with wavelengths shorter than 200nm in a reactive ammonia (NH3), acetylene (C2H2) or oxygen (O2) atmosphere. In addition, we found that photodissociated fragments of these gases can react with the polymers or be deposited on their surfaces.

The polymer samples were placed inside a reaction chamber that was connected to the UV source by a window. The newly-introduced chemical groups at the polymer surface can lead to an increased surface wettability. So, if we use a contact mask, we can localize treatment of the surface, thus allowing us to control where on the surface cells are most likjely to adhere (see Figure 1). This kind of surface would be useful in the production of micro-arrays of cells for genomic high-throughput screening.

Figure 1. Human embryonic kidney cells cluster on irradiated spots on a polymer surface. The PTFE surface was exposed to 172nm xenon-lamp radiation for 30 minutes in 5mbar ammonia. The image was obtained by phase-contrast microscopy after the cells had grown for one day in culture. This image was adapted from Reference 4.

UV irradiation in an acetylene atmosphere leads to the deposition of amorphous hydrogenated-carbon films on the polymer samples. As is indicated in Figure 2, deposition for 20 minutes produced a surface that shows a higher cell-proliferation rate than surfaces resulting from longer or shorter irradiation times. We believe that changes in the surface nano-roughness are responsible for this effect. Another way to nanostructure the surface is to use a laser to induce ripples.9 We produced 10–20nm-deep ripples with 130nm periodicity at the surface of PET foils by irradiating them with 157nm light from a fluorine laser, as shown in Figure 3.

Figure 2. Treated samples of PTFE quickly show higher populations of human umbilical vein endothelial cells than the untreated material after 1, 3, and 7 days of growth. The three treated materials were exposed to 172nm-light at intensities of 20mW/cm2, in a 5mbar atmosphere of acetylene for either 10, 20, or 30 minutes. PS: Standard tissue-culture polystyrene Petri dish. This figure was adapted from Reference 1.

Figure 3. Ripples result from irradiating a PET foil with a p-polarized fluorine laser beam. The beam had a fluence of 4.7mJ/cm2/pulse. The image was obtained by atomic force microscopy, the scan area is 5 × 5μm, and the height scale is 10nm/division.

Our study demonstrates simple methods that increase the biocompatibility of polymers for tissue engineering and biotechnology applications. We modify both the chemical properties and the nano-roughness of the polymer surfaces by exposing them to UV radiation. In further work, we will focus on forming ripples with a laser as a promising method of structuring surfaces for biomedical applications.

This work was supported by grant N0102-NAN (NSI-NBPF) from the Austrian NANO Initiative, by GA CR Grant 204/06/0225 and GAAS CR Grant A 5011301.

Johannes Heitz
Institute of Applied Physics, Johannes Kepler University
Linz, Austria

Dr. Johannes Heitz is a professor at the Institute of Applied Physics of the Johannes Kepler University. In recent years in Linz and during a fellowship at the National Institute of Materials Science and Chemistry in Tsukuba, Japan, he has worked mainly on laser-matter interaction with a special emphasis on polymeric materials. In addition, he has given invited talks on these topics at several SPIE conferences.

9. D. Bäuerle,
Laser Processing and Chemistry,
Springer, Berlin Heidelberg New York, 2000.