S OFT LITHOGRAPHY AND STANDARD PDMS DEVICES

Figure 4-1: Chemical structure of PDMS: oligomer (left), crosslinked (right).

4.1. Manufacturing microfluidic devices

The utility of microfluidic devices to manipulate fluids is of widespread interest in many scientific and industrial contexts. Their design often requires unusual geometries and the interplay of multiple physical effects such as pressure gradients, surface tensions, and capillarity. The earliest microfluidic systems were fabricated in silicon by using technology originally developed for the microchip industry.^109-111 For biological research, silicon is often not the ideal material for microfluidic applications due to optical opacity, difficulty in component integration, and unsuitable surface characteristics. Furthermore, faster and less expensive device fabrication is needed. In order to satisfy these demands, soft lithographical techniques to create microfluidic devices have been developed over the past two decades.^112-114

4.1.1. Soft lithography and standard PDMS devices

Soft lithography involves the replication of a master structure into a soft elastomer, such as Sylgard 184 poly-(dimethylsiloxane) (PDMS, Dow Corning GmbH, Wiesbaden, Germany). Although several polymers (e.g. polyurethane) have been used for soft lithography, PDMS has a useful combination of properties. It is nontoxic, permeable for water and many gases, readily available commercially, and optically transparent down to a wavelength of approximately λ ≈ 300nm. It has a Young’s modulus of about 1MPa (depending on cross-linking density) making it a moderately stiff elastomer.¹¹² Consisting of repeating units of -O-Si(CH3)2-groups (Figure 4-1), it is intrinsically hydrophobic (advancing contact angle of water: θa(H2O) ≈ 110°).^{112, 115} Typically, standard photolithography is used for manufacturing masters. Figure 4-2 shows the principle steps of master fabrication. A silicon wafer (Si-Mat, Landsberg/Lech, Germany) is cleaned with isopropanol and dried on a hotplate for 5

4. Microfluidic Devices

Figure 4-2: Schematic representation of principle steps of the master fabrication.

(1) Clean silicon wafer as a substrate. (2) Spin coating the substrate with photo resist. (3) Exposure to UV light through a high resolution lithography mask. (4) Master containing the developed structure.

minutes at 200°C. A layer of negative photo resist (SU-8, Micro Resist Technology GmbH, Berlin, Germany) is spin coated onto the wafer. Depending on the type of photoresist and the spin coater (SCS, Indianapolis, USA) parameters, a thickness range of 50-200µm can be achieved. To polymerize selected regions of the photo resist, the wafer is exposed to UV light (wavelength λ = 365nm) through a lithography mask using a Karl Süss MJB3 mask aligner (Süss Microtech AG, Garching, Germany).

Lithography masks are printed transparencies (JD-Photo-Tools Ltd., Oldham, UK) or chrome masks (ML&C, Jena-Maue, Germany), with microstructures drawn using AutoCAD 2005 (Autodesk, München, Germany). For resist layers thicker than a few micrometers, an iterative exposure is necessary. In order to completely crosslink the exposed photo resist areas, the wafer is post-exposure baked and developed. After dissolving the unpolymerized photoresist, a positive relief of the channel structure is left on the wafer and cleaned with isopropanol. This structure acts as a master for casting PDMS channels.

To replicate the 3D microstructure in PDMS (Figure 4-3), the pre-polymer is mixed with the cross-linker (ratio 10:1 (w/w)), degassed thoroughly using an exsicator and poured onto the master. The liquid PDMS pre-polymer conforms to the shape of the master and replicates features of the master with high fidelity (10’s of nm).¹¹⁶ After curing the pre-polymer at 65°C for approximately 3h, the PDMS replica is peeled off of the master. To prevent irreversible bonding between silicon and PDMS, the surface of the wafer is treated with heptafluoropropyl-trimethylsilane (97%, Sigma-Aldrich, Germany) by absorption from the gas phase. Dimensional limits in channel design are in the range of 1 to 300 micrometers. The width of the channels is measured with optical bright field microscopy resulting in an accuracy of ±0.2µm.

To introduce fluids into the microchannels, Teflon tubing (NovoDirect, Kehl, Germany) with an inner diameter of 500µm and an outer diameter of 1000µm is used.

4. Microfluidic Devices

Figure 4-3: Schematic representation of the manufacturing process of a standard microfluidic device from an existing master. (1) Photolithographic fabrication of a master with relief structure. (2) Casting of PDMS against the master. (3) Peeled off PDMS replica after cross linking the PDMS at 65°C. (4) Sealing the channel structure with a cover slide.

To connect the tubing, holes slightly smaller than the outer tube diameter are punched in the PDMS replica. When the tubing is inserted, it exerts pressure on the PDMS and provides a waterproof seal. To improve tightness, the tubes are additionally glued into the PDMS by using Loctite^® 406/770 glue (Henkel Loctite, München, Germany).

The PDMS replica can be sealed irreversibly to PDMS, glass, silicon, polystyrene, polyethylene, or silicon nitride by exposing both the surface of PDMS and the surface of the substrate for 10s to an air plasma using a plasma cleaner (Harrick Scientific Corporation, Ossining, USA). Oxidization using plasma produces silanol (Si-OH) groups on the PDMS surface, converting it to a hydrophilic form (θa(H2O) ≈ 10°),^{112, 115} and -OH-containing functional groups on the other materials. These two types of polar groups form covalent -O-Si-O-bonds when the surfaces are brought into contact.¹¹¹ For standard microfluidic PDMS devices, microchannels in the cross-linked PDMS stamp are sealed with a glass slide.

Elastomeric polymer molding enables the generation of topologically sophisticated microfluidic structures according to experimental demands. A significant advantage of this technique is the rapid turn-around time from conception of the experiment to fabrication.¹¹¹ One master can be used several times to fabricate replicas, resulting in an extreme cost and time efficency. In addition, the list of advantages of microfluidic devices manufactured using PDMS includes robustness, reduced size of operating systems, flexibility in design, reduced use of reagents, increased speed of analyses, and easy integration with outside components because the polymer conforms to most materials.

When studying complex fluids in a microchannel device, the utilization of a multitude of methods of analysis is desirable. Asides from different optical techniques, X-ray microdiffraction in microflow in particular provides new opportunities to study complex fluids.^{59-62, 108} Due to its optical properties, PDMS is compatible with many

4. Microfluidic Devices

Figure 4-4: Schematic representation of the manufacturing process of KSK devices:

(1) Stainless steel plate as starting point. (2) Spark erosion of the channel structure. (3) Covering with a thin self-adhesive Kapton foil on one side. (4) Closing the channel structure by covering the other side with a second Kapton foil. Four holes are punched into this Kapton foil, which fit the inlet positions of the microfluidic device. (5) Thin double side sticking tape with cavities at the inlet position and the measuring area is used for mounting the microfluidic device on a PMMA slab assisting the connections to the fluid pumping system (6).

optical detection methods. However, X-ray diffraction measurements directly on a microfluidic chip cannot be performed using standard PDMS devices. The major obstacle here for is the X-ray absorption and scattering properties of PDMS. In the following sections, two straightforward and scalable methods are presented for fabricating long lifetime X-ray microdiffraction compatible microfluidic devices with thin polyimide (Kapton) windows.