C - DTU Nanotech - Danmarks Tekniske Universitet

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18 3 Theoretical Considerations of Polymers for Bio-Microsystems CH 2 CH3 C C O OCH 3 n Figure 3.1: Structure of PMMA [33] provided by a laser. Infrared laser machining evaporates substrate material directly by applying heat with the laser beam [35]. If UV radiation is used, the irradiated polymer decomposes, presumably by a mixture of two mechanisms: thermal and direct bond breaking [11]. As with infrared radiation, thermal bond breaking is induced by heat. In direct bond breaking, polymer molecules directly absorb ultraviolet photons so that the chemical bonds within the polymer chains will break if they absorb enough energy, which is often the case. The resulting smaller polymer chains are volatile or melt at much lower temperatures than the bulk polymer, thereby leaving a void in the material [11]. The smallest feature size attainable with laser machining depends strongly on the quality of the optical system, the laser wavelength and also the material properties of the polymer. As shown in equation 3.1 for small feature sizes, short–wavelength radiation together with a high relative aperture of the optical system should be used. d = πλf 4D d = Focal spot diameter λ = Wavelength f = Focal length D = Pre–focus diameter (3.1) From this the advantage of UV over infrared radiation can be seen: In practical terms the spot size that can be obtained with a UV system can be as small as 1.5 µm, whereas the feature size for an infrared laser is typically around 200 µm [2]. An advantage of the infrared laser is that fast prototyping is possible and it is available at moderate prices [35]. A disad- vantage of the infrared laser is, that the machined channels have Gaussian shape like shown in Fig. 3.2. For this research project only an infrared laser (CO2–laser, 1060 nm, Fig. 3.3)
100µm 3.2 Topas R○ and the CNC micromilling machine 19 200µm Figure 3.2: Profile of a channel made by an infrared laser system (the picture has been made by D. Snakenborg (MIC) using an optical microscope) and no UV laser is available. When PMMA is heated up, it remains in the solid glassy state until it reaches its glass transition temperature, at higher temperatures PMMA becomes easily rubbery and mouldable [35]. The thermal decomposition begins if more energy is added and results in the breaking of long polymer chains into smaller ones. For PMMA this process is known as depropagation, it is initiated by random chain breaking, which finally leads to the development of volatile methyl methacrylate and end–chain scission [1, 8, 16]. The main part of the depropagation occurs at temperatures between 350 − 380 ◦ C [35]. 3.2 Topas R○ and the CNC micromilling machine Topas R○ is a thermoplastic cyclo–olefin copolymer (COC). The family of the cyclo–olefin copolymers are amorphous, transparent copolymers based on cyclo–olefins and linear olefins (see Fig. 3.4) and the property profile can be varied over a wide range during polymerization [38]. The COC Topas R○ consists of the two monomers ethylene and norbornene [27]. By controlling the norbornene content in the polymer, the glass transition temperature Tg can be engineered (Tg = 80, 130, 150, 170 ◦ C) [19]. One reason for using Topas R○ as material for microsystems is its transparency to light with wavelengths above 300 nm [38]. Furthermore, Topas R○ is chemically resistent to soap solutions, hydrolysis, acids and organic polar solvents,
Page 1: Danmarks Tekniske Universitet Insti
Page 5: Zusammenfassung Diese Arbeit besch
Page 8 and 9: 8 Contents 6 Development of a Bio-M
Page 10 and 11: 10 Nomenclature COC Cyclo-olefin co
Page 12 and 13: 12 1 Introduction sampling system w
Page 14 and 15: 14 2 Overview of Small-Scale Cultiv
Page 17: 3 Theoretical Considerations of Pol
Page 21 and 22: 3.2 Topas R○ and the CNC micromil
Page 23 and 24: 4 Basic Principles of a Peltier Ele
Page 25: p-semiconductor p-semiconductor 4 B
Page 28 and 29: 28 5 Freezing Different Samples usi
Page 42 and 43: 42 6 Development of a Bio-Microreac
Page 64 and 65: 64 7 Concluding Remarks Reactor cha
Page 67 and 68: Appendix Freezing experiments with
Page 69:
Experiments with 20 µl PBS-buffer
Page 72 and 73:
72 List of Figures 6.8 Heat wire de
Page 75 and 76:
Bibliography [1] Arisawa, H.; Brill
Page 77 and 78:
Bibliography 77 [26] Mark, H. F.: E
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Bibliography 79 [50] Zimmermann, H.
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C - DTU Nanotech - Danmarks Tekniske Universitet

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