**Figure 1.**

*Advances in Microfluidics and Nanofluids*

be particularly efficient for mass production.

fabrication of multi-scale features and controllable surface quality is required, because the feature size and quality of microinjection molded microfluidic chip is highly related to the characteristics and quality of corresponding micro mold insert [9–12]. The essence of replication is the reproduction of microfluidic structures from the master to the substrate materials. In terms of the replication of surface structures using polymer materials, techniques such as injection molding [13, 14] (including microinjection molding, variotherm-assisted injection molding, and injection compression molding), embossing [15, 16] (hot embossing, UV embossing, roll-to-roll embossing), nanoimprinting [17] and 3D printing [18] are commonly applied. Among all the replication processes, injection molding and hot embossing are more viable for industrial production, due to their advantages of relatively low cost and suitability for different materials and product design. Injection molding can

After microinjection molding of microfluidic devices, sealing is required to achieve enclosed microchannels [19]. Some important parameters should be taken into consideration before selecting bonding methods. Bond strength should be one of the most important parameters. In some applications, the interfacial bond energy is expected to be as high as the cohesive strength of the substrate, while in other applications the weak and reversible bonds between the cover and the substrate are required. Another parameter should be the solubility of thermoplastic and solvent. The interfaces used for bonding should be compatible with some solvent so that they can be dissolved and bonded together, meanwhile, the microchannels should not be subjected to the deformation in the bonding process [19]. Other parameters include the surface roughness, optical properties as well as material compatibility. In general, bonding methods can be either indirect or direct. In the indirect bonding process, an intermediate adhesive layer is used to bond two substrates together [20]. The interfaces applied with adhesive will have different properties than the bulk substrate. In terms of direct bonding, no other material is added between the interfaces, and the surface of the substrates can be mated directly [21]. After bond-

ing, the interfaces and bulk material have homogenous properties.

**2. Rapid prototyping of microfluidic chips**

and process chains for manufacturing of polymer microfluidic chips.

This chapter will overview these prototyping and mass production technologies

PDMS casting is also named soft lithography. The polydimethylsiloxane (PDMS)

is a kind of silicon-based organic polymer material that has been widely used in microfluidic devices by rapid prototyping [26]. PDMS casting fabrication process is typically divided into the following steps (see **Figure 1**): molds developing, PDMS casting, curing and releasing, bonding, and integration [14]. A master mold needs to be prepared firstly, where SU-8 epoxy resin usually is applied as the mold material [27]. The standard PDMS compositions are composed of silicon elastomer base and the curing reagent in a ratio of 10:1. The uncured PDMS is poured into the mold, followed by curing at 70–80 °C for an hour. After releasing PDMS from the mold, PDMS microfluidic chips can be obtained [28]. The post-process of PDMS

In recent years, microfluidic devices find many advanced applications in chemical analysis [22], polymerase chain reaction (PCR) [23], biological analysis [24], and chemical synthesis [25]. Main rapid prototyping methods of fabricating microfluidic chips include PDMS casting, micromachining, and 3D printing, etc.

**34**

**2.1 PDMS casting**

*Process steps of PDMS casting.*

casting usually includes bonding and integration. Bonding can reduce the hydrophobicity of the PDMS chip to encapsulate its microchannel. To enhance the bonding strength of chips with other materials, adjusting bonding process parameters and surface modification by using oxygen plasma to form O-Si-O covalent bonds at the interface of PDMS microfluidic channels are commonly used [29]. Finally, some microsensors, microheaters and microfluidic pumps are integrated onto microfluidic chip for diversified performance [18]. **Figure 2** shows the practical microfluidic chip fabrication approaches by the PDMS casting process.

Although PDMS casting is a rapid prototyping process for disposable microfluidic chip fabrication, it is a complex process with many drawbacks. The microfluidic chip fabricated by PDMS casting has insufficient mechanical strength, non-conductivity, and non-magneticity. Also, under high temperature, high voltage, and pressure conditions, it is not easy to integrate other precision components on the microfluidic chip [19].
