PREPARATION OF PATTERNED SURFACES

Micropatterning of small molecules, macromolecules and cells on matrix surfaces has a wide range of potential applications in molecular electronics, biosensing, diagnostics, tissue engineering and micromachining. To achieve this micropatterning several methods are in use such as photolitography, ion implantation, electron beam and ion beam irradiation, electrochemical methods and patterning with the tip of the probe of atomíc force microscope.

Our objective was to achieve domain-separated surface by radiation grafting, using an electron accelerator. In the work reported here, a thermoresponsive monomer, N-isopropylacrylamide; a pH-sensitive monomer, acrylic acid, and a functional monomer, glycydilmethacrylate were grafted on low density polyethylene.

9. MATERIALS AND METHODS

N-isopropylacrylamide (NIPAAm), acrylic acid (AAc), and glycidylmethacrylate (GMA) were used after standard purification methods. All solvents were reagent grade and used as received. The base polymer was low density polyethylene that was cleaned ultrasonically with methanol three times before use.

A series of irradiations were carried out with the 4 MeV linear accelerator in both single pulse and scanned beam regime combined with a conveyor. In the scanned beam regime the conveyor speed and sample distance from the beam window was adjusted to obtain the specified dose, that varied between 50 and 200 kGy, while in single beam operation mode a stationary sample position and a pulse length of 2.6 µs was used.

The absorbed dose was determined by ethanol-monochlorbenzene (ECB) dosimeter.

The conductivity of the irradiated ECB solutions was measured by the K-302/2 type oscillotitrator (Radelkis Electrochemical Instruments, Budapest, Hungary)12

To achieve a domain-separated surface, aluminum and copper masks were prepared to partially cover the surface of the samples.

Before the irradiation of the samples, a radiochromic dye film dosimeter (PVB/pararosaniline) was placed under the mask for dose mapping. After irradiation, the dosimeter film develops color with a maximum adsorption at 554 nm. The relationship between the absorbed dose and optical density is linear up to 100 kGy ¹³.

After the irradiation, the surfaces were thoroughly washed to remove the physically adsorbed but not grafted monomer and homopolymer, and dried. The grafted polymer samples were then evaluated by FTIR spectroscopy, (ATI Mattson Model RS-1 equipped with ATR and PAS detectors), and the chemical composition was also determined by taking XPS survey and high resolution C 1 s and O 1 s spectra. The modified surfaces were also visualized by scanning electron microscopy (JEOL Model 5600 L V).

10. RESULTS AND DISCUSSION

Polyethylene films were irradiated by various doses in the range of 1-200 kGy, under the masks and in the presence of a monomer. (As reference, the following samples were compared: irradiated PE without a monomer, monomer adsorbed on PE but not irradiated, monomer adsorbed on PE and irradiated without mask.)

On Fig. 10. a representative FTIR spectra of base PE and GMA-grafted PE are shown.

(The grafting was obtained by irradiation with absorbed dose of 35 kGy.) On the lower spectrum, that shows the GMA-g-PE, the grafting could be clearly identified from the new peaks that appear at 1151 cm-1 and 1730 cm-1 and belong to ester C-O and C=O groups, respectively, as well as the peak at 904–910 nm assigned to the epoxy groups.

The results of the XPS measurements are shown on Figs 11 and 12. The survey spectra of PE shows only one peak that corresponds to C 1 s but that of the GMA-g-PE has also a huge O 1 s peak. (The grafting shown here corresponds to the absorbed dose of 70 kGy.)

The peak fitting of the C 1s showrs only the existence of CH-type bonds in the base polymer (278 eV binding energy in our case — not corrected). When the C 1s peak of the GMA-g-PE is fitted, the shifts to the higher binding energies of about 2 eV from the hydrocarbon binding energy belongs to ether type C-O bonds. The C=C type bond usually shows another shift to still higher binding energies of about another 2 e V.

FIG. 10. Scheme for the preparation of domain-separated surfaces.

FIG. 11. FTIR spectra of virgin PE (upper spectrum) and GMA-grafted PE (lower spectrum).

FIG. 12. XPS spectra of virgin and GMA-grafted PE.

FIG. 13. XPS survey spectra of virgin and GMA-grafted PE.

Similar results were obtained witn other monomers, AAc and NIPAAm too. Besides the guided cell growth, grafting with NIPAAm had also another objective. Since this polymer shows typical soluble-ínsoluble changes in response to temperature changes across a lower critical solution temperature (LCST) at around 32–34 oC in aqueous solution, the polyNIPAAm-grafted surfaces will be hydrophilic below, and hydrophobic above this temperature. Cells generally adhere and grow on hydrophobic surfaces but not on hydrophilic ones, thus cells grown on hydrophobic polyNIPAAm surfaces at 37 oC could be detached and harwested without chemicals by lowering the temperature of the surface below the LCST.

The grafted samples were used for cell culture experiments, but unfortnately, the cells could not be grown satisfactortly neither on reference nor on the grafted surfaces. We are now planning to do alI cell culturing experiments in collaboration witn The Netherlands, as this laboratory has long experience in this field.

ACKNOWLEDGEMENT

The work reported here includes contributions from Mariano Grasselli and Eduardo Smolko {Argentina) and André Deelder (Netherlands). I would like to thank them for their interest and enthusiastic and diligent work. I am also indebted to Katalin Gonter and Peter Hargittai (Hungary) for their excellent assistance. The partial financial support from the International Atomic Energy Agency as well as from the Hungarian Research Foundation {OTKA) and Hungarian-Argentine Bilateral Foundation is gratefully aknowledged.

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AFFINITY PATTERNING OF BIOMATERIALS USING