**2.2 Micro machining**

The microfluidic chip fabricated by micro machining, such as femtosecond laser or endmill, has relatively high accuracy and can be used repeatedly [20]. Combing with computer numerical control (CNC), micro machining can rapidly and accurately construct devices at several microns scale [21]. However, microchannels obtained from micro milling are too rough to use as a microfluidic chip, especially for the machining of polymer materials [22]. However, compared with micro milling, laser micro machining with improved accuracy can perform the direct writing ablation of polymers such as PMMA, PC, etc. Direct-write laser machining with UV and CO2 assistance can realize microstructuring in polymer materials.

#### **Figure 2.**

*PDMS casted microfluidic chip: PDMS substrate with microchannels (a); enclosing of microchannel (b, e, f); PDMS molding(g); PDMS curing (c, h); substrate integration to fluid pump (d); SWB was applied for prototyping PDMS (I; forces in reversible bonded microdevice (j), mold characterization and cross-section (k) [30].*

**Figure 3.** *Direct-write laser machining for microfluidic chip [31].*

For laser micro machining, when the laser beam is moving above the workpiece space, the heated spots will gather to form various patterns. Through the reflection in the light path and the focusing effect of the focusing lens, the laser is finally focused on the workpiece, where the temperature of the focal point increases rapidly. After the polymer material melts and decomposes, scratches will be left on the surface of the polymer, where the microchannels are formed [24]. The scanning speed is programmed and controlled by a computer. **Figure 3** shows the specific process methods of laser micro machining and machined glass microfluidic chip.
