**1. Introduction**

On the basis of channels with dimensions of tens to hundreds of micrometers, microfluidics principally deals with the processing and manipulation of tiny amounts (normally from 10−9 to 10−18 liters) of fluids [1]. Although early microfluidic devices relied mainly on silicon and glass, the use of polymer materials has become increasingly common, which is largely attributed to its relatively low cost, admirable replication accuracy, optical transparency, biocompatibility, chemical stability, and good electrical insulation. Integrating with other apparatuses such as detectors and purifiers, polymeric microfluidics contains micro components, which principally includes microchannels [2, 3], microvalves [4–6], micropumps [7], micromixers [3, 8]. In fact, the elementary surface structures, which generally include channels, wells, chambers, and protruding arrays in microscale and submicro scale, are crucial for the functionalities of the entire microfluidic systems. The cross-sectional shape is normally square, and the aspect ratio is commonly less than two with a surface roughness being smaller than 50 nm.

For mass production of a microstructured plastic part for microfluidic applications, an appropriate tooling technology with the capability of enabling the

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 be particularly efficient for mass production.

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 bonding, the interfaces and bulk material have homogenous properties.

This chapter will overview these prototyping and mass production technologies and process chains for manufacturing of polymer microfluidic chips.
