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

**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 melts into a liquid because of the elevated temperature.

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 ratio of SU-8 to Blu-Tack is 5:3 by volume. The mixture is then baked at 110°C for 4 hours to evaporate off the majority of the solvent (i.e. GBL).

Before exposure to any UV light, it is essential to ensure that the SU-8/impurity composite can be reshaped upon heating and retain its chemical properties after cooling down. However, when this SU-8/impurity composite is exposed to a UV light, as shown in **Figure 1a**, only the surface exposed on which the ultraviolet light has shone to can be polymerized into in a thin layer. The thickness of this layer can be changed by changing the duration of the UV exposure. In general, the longer the UV exposure, the thicker this layer will be.

Since the UV light cannot reach the SU-8/impurity composite underneath the UV exposed surface, the SU-8/impurity composite underneath the UV-exposed surface will remain uncured. The heat then applied to this experimental setup will melt this uncured SU-8/ impurity composite into a liquid, as illustrated in **Figure 1b**. Since the removed SU-8/impurity composite removed remains photo-curable, it can be reused to fabricate other hollow parts.
