Laser-assisted modification of polystyrene surfaces for cell culture applications
Introduction
While silicon based micro-devices such as sensors or actuators are well established commercial products in microsystem technology (MST), non-silicon, in particular polymer based, micro-fluidic systems have very recently been introduced to the market [1]. For this purpose techniques of rapid prototyping [2], rapid tooling and rapid manufacturing are increasingly used in MST. Large volume industrial manufacturing can be provided by replication of a micromachined master tool [3]. Typical replication methods are hot embossing [4] and injection molding [5]. They provide low-cost mass production of microstructured components with large aspect ratios, structural details in the sub-micron range and a precision better than 2 μm in the final polymer product [6], [7].
In MST UV-laser-assisted processes are of particular interest for applications in microfluidics, bio-analytics, bioreactors and micro-optics [8], [9], [10], [11], [12]. The current state of the art of UV-laser micro-processing of polymer materials with respect to laser ablation, micro-patterning and Laser-LIGA has been described elsewhere [13]. For the packaging of micro-structured polymers, laser transmission welding was successfully developed [14], and in current research even for channel structures with a width of 20 μm. UV-photon-induced surface modification of polymers for a functionalization of polymer-based micro-devices is a relatively new research field. For this purpose, laser radiation sources or UV-lamp systems may be applied [15], [16], [17], [18]. The main advantage of laser-based technology is its high process flexibility. Three-dimensional structures may be modified and a variety of processing conditions (e.g. processing gases, liquids) can be applied. Excimer laser processing enables high local resolution via direct writing or direct optical imaging of complex structures or motorized masks. The process is in general initiated by direct bond breaking (e.g. separation of side chains or homolytic fissions) which leads to the formation of new bonds or radicals. As a consequence of this the formation or grafting of functional groups, such as amino-groups or carboxyl-groups is possible, which in turn leads, e.g. to a change of biocompatibility. This type of functionalization was studied in detail for polystyrene (PS) with respect to wettability and adhesion of animal cells.
Section snippets
Laser modification
Laser-induced modifications based on excimer laser radiation were performed with the following different laser micromachining systems: (1) Exitech PS2000 operates with a Lambda LPX 210i as radiation source at 193 nm (pulse length 20 ns); (2) Promaster (Optec s.a.) operates with an ATLEX-500-SI at 248 nm (pulse length 4–6 ns). It is expected that short laser pulses (∼ns range) reduce significantly thermal contributions to a laser process. A high beam homogeneity or “flat top” profile with intensity
Laser processing parameters
UV-laser assisted ablation and modification processes are mainly influenced by the laser fluence ɛ. In principle three different process regimes with respect to the laser fluences are of interest: Firstly, below the ablation threshold ɛt, secondly in the range of ɛt and thirdly, significantly above ɛt. Above the ablation threshold ɛt three-dimensional shapes with a very small surface roughness of Ra = 50 nm can be realized [3], [13]. In the range of ɛt the surface roughness significantly increases
Summary
Laser processing at short wavelengths is an appropriate tool for a selective patterning of polymer surfaces. Here, patterning stands for chemical or topographic modification of surfaces on nanometer and micrometer scale. But patterning also encompasses ablation for the generation of two- and three-dimensional shapes in polymer surfaces. It was demonstrated that these kinds of “patternings” could be combined with high lateral resolution, e.g. ablation and modification of polymer surfaces with
Acknowledgments
We are grateful to our colleague M. Beiser for his technical assistance in SEM. We are indebted to Mrs. M. Marin for her excellent contributions to this research project. We also thank Mrs. V. Trouillet for her support in XPS and H. Besser for laser material processing. We gratefully acknowledge the financial support by the program NANOMIKRO of the Helmholtz association and the EU within the Sixth Framework Programme (“Network of Excellence in Multi-Material Micro Manufacture (4M)” and “Network
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