*Prototyping and Production of Polymeric Microfluidic Chip DOI: http://dx.doi.org/10.5772/intechopen.96355*

*Advances in Microfluidics and Nanofluids*

optimization is introduced.

the stainless steel mold insert machined by μEDM. With the assistance of other technologies, the surface finishing issues can be solved, such as ultrasonic-assisted μEDM. Overall, μEDM is a competitive technology, which has non-contact mechanical force, heat, and stress generation for machining the mold insert for microfluidic applications.

Micro injection molding is an important process to transfer patterns from mold to the surface of microfluidic patterns. Thanks to fast solidification and relatively high viscosity, filling of polymer melts into tiny micro patterned mold is challenging. Upon micro pattern is formed, separation of patterns from mold is problematic due to the distortion or damage of plastic micro structure from friction and adhesion between polymers. Here a brief introduction of these issues is presented by using a microfluidic chip as an example and a simulation strategy for microfluidic structure

**Figure 10(a)** illustrates a fast prototyping mold for the injection molding of a flow cytometer chip, which has an area of 26.12 mm × 26.12 mm and a thickness of 1.05 mm [50]. **Figure 10(b)** shows surface structures that are used to fabricate inverted channels, all of which are 250 μm wide and 150 μm deep for the parts that had rectangular cross-sectional areas. The horizontal channel works as a fluidic channel, in which biological particles, such as cells, are focused by sheath flow from side inlets. Meanwhile, an optical fiber is integrated into the microfluidic chip using tilted channels. One channel is used for a fiber laser excitation beam and the other two are allocated for detection of forwarding scattering and side/fluorescence scattering by fibers. In order to minimize light intensity reduction of the excitation

*Flow cytometer chip: (a) cassette mold, (b) three-dimensional model of surface structures on the chip,* 

*(c) actual chip assembly, (d) connection with tubing [55].*

**3.2 Micro injection molding of microfluidic structures**

*3.2.1 Structures filling mechanism for micro injection molding*

**42**

**Figure 10.**

laser and to decrease potential scattering when the laser beam passes through plastic material, the distance between the fluidic channel and optical fiber channels has to be as small as possible. This requirement necessitates surface structures as shown in the upper image in **Figure 10(b)**. The protruding structures demonstrate a gradual aspect ratio, and the highest ones are at the places between the edge and the fluid channel, which have an aspect ratio of 3:1 nominally; it is these that were mainly investigated. **Figure 10(c)** shows the actual microfluidic chip (after bonding), which is assembled into a designed chip holder and connected with samples and PBS (Phosphate-buffered saline) sheath flow using PTFE (Polytetrafluoroethylene) tubing (**Figure 10(d)**).

In theory, the surface structures should fill more easily under such a layout because the gate was parallel to the feature of interest and the centre fluid channel [51]. However, replication of micro/nanoscale surface structures is challenging due to their high surface to volume ratios, especially for high aspect ratio features. Fast heat transfer rates mean that the polymer melts are inclined to solidify before the cavities are fully filled. For example, experimental results show that the cross-sectional area of the surface structure emphasized in **Figure 10(b)** only reaches up to 70.67% of the criterion. Combined with microscopy, process monitoring, and morphology, A combined melt flow and creep deformation model is proposed to explain the complex filling behavior of the surface structure. As shown in **Figure 11**, the overall replication is ordinarily composed of two parts: melt flow during the injection stage and creep deformation during the packing stage. Melt flow is related to injection velocity and pressure, while the extent of creep deformation is linked to mold temperature, packing pressure, packing time, etc. Based on this, increasing shot size to improve the replication quality of surface structures is proposed, and the replication is significantly improved and sufficient to satisfy the practical requirements of a microfluidic flow cytometer chip.
