Prototyping of microfluidic systems with integrated ... - DTU Nanotech

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22 Polymer materials – mainly Topas the cost. Also, even though all the processes for silicon processing have been used in large-scale production, the risk of damaging the structure increases for each process step the substrate goes through. Furthermore, many of the processes involve harmful and potentially dangerous wet chemistry like HF-etching. Using polymer as a substrate material, the fabrication cost and complexity can be significantly decreased using processes like injection moulding or hot embossing. A third limitation to the traditional processing methods are the geometrical constrictions which are imposed by the structuring methods available in standard cleanroom processing. Using isotropic chemical etching, semicircular channels can be manufactured. Dry etching is anisotropic, and structures with roughly vertical sidewalls can be created. However, the exact dimensions are often difficult to control, and especially structures with several different levels require many process steps, making fabrication tedious and difficult. The fourth limitation cited concerns the surface chemistry, where biomolecules often bond to the substrate. This is not desired in e.g. continuous flow systems, and can be avoided by surface coatings. This, however, implies yet another process step. In review articles, Becker and Gärtner [17], Soper et al. [18] and de Mello [19] list a number of properties for the ideal substrate material for microfluidic and analytical systems, and conclude that although no single polymer offer all the ideal properties, it is almost always possible to find a suitable polymer for a given application. The properties listed include: • Low cost of the material. This issue is even more important in microfluidics than in e.g. microelectronics, since physical restrictions in the former make downscaling impossible without performance loss. A 2 cm long channel used for electrophoresis cannot be fitted onto a chip shorter than 2 cm on one side without bending the channel, and thereby decreasing the performance and the resolution of the system. Also compatibility with e.g. pipetting robots when working with microtiter plates could be an issue. • Appropriate electrical and thermal properties for the system. This is important especially in systems where electrophoresis is applied. The material must be able to withstand large electrical fields and to dissipate the Joule heat generated in the system.
3.2 Polymer types 23 • Compatibility with the biological chemical substances introduced in the system, so that the system is not affected by the substances or vice versa. • Good machinability by methods such as laser ablation (infrared, ultraviolet or X-ray) or micro milling, and applicable to mass replication methods. • Good optical properties, including transparency in the visible, infrared and ultraviolet parts of the electromagnetic spectrum, so that the system can be monitored from the outside. Also, the autofluorescence of the polymer should be limited, so that the polymer does not interfere with the light used in measurements. • Surface modification options, so that the chemical and physical properties of the polymer can be tailored to match the needs of specific processes and systems. The polymers used in microfluidic today all posses several of the properties listed above, but differ in their characteristics, such as absorption spectrum and chemical resistance. A few of the most commonly used polymers are compared in section 3.4.2. Development of polymer types with specific properties, which are tailored for specific applications in microfluidics is not common. Contrary to industrial scale production of macroscopic objects, the polymer quantities needed for making even millions of microfluidic structures is in the order of a few tons. This is by no means enough for a manufacturer to cover development costs, so polymers with specific features and qualities are typically still initially produced with other applications in mind, and then at a later point introduced in µTAS and microfluidics. 3.2 Polymer types It is beyond the scope of this thesis to make a general introduction to polymers and to go into details with subjects such as polymerisation and polymer chemistry. The reader is referred to e.g. [20]. However, a few remarks on key concepts relevant for microfluidic systems are presented in this section.
Page 1 and 2: Prototyping of Microfluidic Systems
Page 3 and 4: Preface This thesis is one of the r
Page 5 and 6: joining the substrate, optical laye
Page 7 and 8: Disse metoder inkluderer: • Mikro
Page 9 and 10: Many thanks to the collaborators in
Page 11 and 12: microfluidic systems. In Klavs Jens
Page 13 and 14: Contents Preface i Abstract ii Resu
Page 15 and 16: CONTENTS xiii 5 Optical waveguides
Page 17 and 18: Chapter 1 Introduction In the last
Page 19 and 20: 1.1 Downscaling 3 Figure 1.1: Scali
Page 21: 1.2 Prototyping of microfluidic sys
Page 24 and 25: 8 Theoretical aspects - microfluidi
Page 26 and 27: 10 Theoretical aspects - microfluid
Page 40 and 41: 24 Polymer materials - mainly Topas
Page 52 and 53: 36 Fabrication methods results are
Page 54 and 55: 38 Fabrication methods An impressiv
Page 56 and 57: 40 Fabrication methods Average roug
Page 58 and 59: 42 Fabrication methods a b c d e 1m
Page 60 and 61: 44 Fabrication methods Generally, t
Page 62 and 63: 46 Fabrication methods branch. Sec-
Page 64 and 65: 48 Fabrication methods and the heig
Page 66 and 67: 50 Fabrication methods Topas on Top
Page 68 and 69: 52 Fabrication methods Table 4.2: C
Page 70 and 71: 54 Fabrication methods Figure 4.8:
Page 72 and 73: 56 Fabrication methods that the str
Page 74 and 75: 58 Fabrication methods the excimer
Page 76 and 77: 60 Fabrication methods 500 µm 500
Page 78 and 79: 62 Fabrication methods wedge tests
Page 81 and 82: Chapter 5 Optical waveguides in Top
Page 83 and 84: 5.1 Waveguide fabrication 67 It sho
Page 85 and 86: 5.2 Propagation loss measurements 6
Page 87 and 88: 5.3 Thermal treatment of the surfac
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5.3 Thermal treatment of the surfac
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5.3 Thermal treatment of the surfac
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5.4 Optical cuvette absorbance meas
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5.4 Optical cuvette absorbance meas
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5.5 A refractive index sensor 81 fr
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5.5 A refractive index sensor 83 a
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5.6 Summary 85 5.5.3 Improvements I
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Until further notice, Chapter 6 is
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96 Discussion and outlook transpare
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References [1] A. Manz, N. Graber,
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REFERENCES 101 [20] David Walton an
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REFERENCES 103 [41] Kiha Lee and Da
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REFERENCES 105 prototyping of polym
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REFERENCES 107 [81] M. Grumann, J.
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