**7. Fabrication of an embossing stamp**

**Micro-stereolithography**

**Table 1.** Comparison between different 3D micro-fabrication technologies.

**6. SU-8 combined with a UV-blocking impurity**

**Micro-injection molding [11–13]**

No Yes Yes Yes

No Yes Yes Yes

**Resolution** Micron range Millimeter range Micron range Micron range

**Figure 1.** (a) A cross-sectional view showing that SU-8 mixed with a UV-opaque impurity is exposed to ultraviolet light. The gray region represents the UV-exposed area where cross-linking of SU-8 monomers occurs and (b) a crosssectional view showing that the UV-exposed structure from **Figure 1a** is subjected to heat. The uncured SU-8 resin

Standard SU-8 fabrication processes normally require a UV-transparent SU-8 [14–17] to proceed. To start fabricating a micro-structure with shells or suspended layers, however, we need a custom-made SU-8 monomer. This SU-8/impurity composite essentially contains a mixture of an SU-8 monomer and a UV-opaque impurity. The UV-opaque impurity is an organic chemical compound satisfying the following criteria: i) it must be opaque to UV lights, ii) it must be highly soluble in SU-8, and iii) it must be substantially non-adhesive. Our experimental results suggest store-bought plasticines or synthetic rubber Blu-TackTM can be used as an example of the UV-opaque impurities in the proposed process. In this study, Blu-Tack plasticine is the UV-opaque impurity added to the SU-8 to increase its UV opacity. The

**Grayscale lithography [9,10]** **This technology**

**[5–7]**

**[1,2,8]**

6 Lab-on-a-Chip Fabrication and Application

melts into a liquid because of the elevated temperature.

**Suitability for fabricating large**

**Suitability for mass-**

**objects**

**production**

Another key element for mass production of micro-structures is an embossing stamp, as shown in **Figure 2e**. The embossing stamp serves as a patterned template for casting the desired 3D patterns on the surface of the SU-8/impurity composite. A variety of methods can be used to fabricate this embossing stamp. If the 3D features to be formed have many complicated contours, then gray tone lithography or micro-stereolithography [1–4] should be used. If the 3D patterns to be formed are just a combination of several arbitrarily profiled sidewalls, then the method illustrated in [5,7,18] may be adopted.
