PDMS-Based Molding

Some representative properties of PDMS material have been presented in Table 10.10. Due to the special properties of PDMS, the following observations can be made on its role in microfluidics  

Its Young s modulus is a function of ratio between the PDMS and reticulating agent concentration. The value given in Table 10.10 is a higher end value. 

The elastomeric quality of PDMS facilitates the watertightness of microfluidic connections. The elasticity of the material facilitates the fabrication of valves and pumps using membranes. 

Untreated PDMS is hydrophobic in nature. It becomes hydrophilic after exposure to the oxygen plasma or after immersion in strong base. Oxidated PDMS adheres by itself to glass and silicon as long as the other surfaces are also exposed to oxygen plasma. 

The permeability of PDMS to gas facilitates filling of the channel. However, its permeability to other nonpolar organic solvents makes it unsuitable for other applications. 

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PDMS-based molding technique plays a very important role in microfluidics. PDMS is a family of polymers containing silicon. Its formula is (Si(CH3)20) , where n is the number of repeating monomer. Figure 10.26 shows the semistructural representation of PDMS. 

The other methods used for fabrication of PDMS-based MEMS device are (1) casting and (2) injection molding. During casting, the mold is pressed into the heated deformable material. After cooling and separation, the negative structure of the mold is obtained. In microinjection, the heated PDMS material is injected in a liquid state into the mold. After separation, the negative structure of the mold is obtained. 

A mammalian cell-based biochip for allergen detection was also proposed (Matsubara et al, 2004). They cultured a basophilic mast cell line (RBL-2H3) that releases histamine upon activation by an allergen, in a microfluidic biochip molded with PDMS (poly-dimethylsiloxane). The mast cells were pretreated with a fluorescent dye, quinacrine, that binds to histamine in acidic compartments of the cell granule by mass action. Upon exposure to the allergen, the IgE-activated cells release histamine along with the fluorescent dye, which is measured by a photomultiplier tube attached to a microscope. 

Poly(ethyleneglycol) (PEG) hydrogel-based micropatches were first molded within two small PDMS channels (see Figure 10.19). Glucose oxidase and HRP were physically entrapped during the polymerization of micropatches for the... 

Fluorosilicone compounds can be processed by the same methods used for silicone elastomers based on PDMS. They can be milled, calendered, extruded, and molded. A large proportion of fluorosilicone compounds is used in compression molding. Molded parts produced in large series are made by injection molding, and parts with complex shapes are produced by transfer molding. Calendering is used to produce thin sheets and for coating of textiles and other substrates. 

A third way to build up pFCs based on MEMS-polymers such as poly-dimethylsiloxane (PDMS) or polymethyl methacrylate (PMMA) or PCB-materials such as polyimid (PI) or FR4. These polymers can be micro-machined by molding or by laser ablation. Shah et al. [22,23] have developed a complete PEMFC system consisting of a PDMS substrate with micro-flow channels upon which the MEA was vertically stacked. PDMS micro-reactors were fabricated by employing micro-molding with a dry etched silicon master. The PDMS spin coated on micro-machined Si was then cured and peeled off from the master. The MEA employed consisted in a Nafion - 12 membrane where they have sputtered Pt through a Mylar mask. Despite an interesting method, this FC gave poor results, a power density of 0.8 mW cm was achieved. 

The concept of polymer-based 3D microstructure utilizes replica molding. Therefore, the key fabrication processes include 3D mold fabrication and pattern transfer. The material property is a very important factor when choosing the polymer and mold materials for special applications. For easy handling and accurate shape transfer, the mold requires smooth surface and relatively low thermal expansion coefficient since the curing of the polymer often involves heat. Nowadays, several polymer materials such as PDMS, PMMA, and SU-8 are popular choices for microfluidic applications. 

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