*2.2. Digital Light Processing Method*

Digital light processing (DLP) is a rapid additive fabrication technology with superior sensitivity. The processing basis of DLP technology is explained in [51,52] and can be briefly abstracted as follows. A commercial 3D printer, MiiCraft 125 (Rapid City, Canada), was utilized in this paper, as shown in Figure 2b.

As given in Figure 2a, the 3D pattern of the matter is first sliced into layers horizontally (in the *x*-axis). Thin layers are then transformed into 2D mask images. A light projection device is utilized to harden the photopolymer resin. This device employs a digital masking method to reflect a dynamically described mask image on the resin plane. With respect to [51], a bottom-up projection system has many benefits compared to a top-bottom system. In this system, the mask image is reflected on the bottom of a resin tank with cured resin at the bottom of the tank. This process continues until the desired design is created.

We established a bottom-up DLP system, as shown in Figure 2b. The DLP projector was equipped to provide a 400 nm full HD ultraviolet light source. The contrast ratio of the DLP system projector is 900:1. The XY resolution of the 3D printing device is 65 µm and the maximum building size 125 × 70 × 120 mm. With respect to [52,53], the masking method primarily occurs as three types: liquid crystal display (LCD), digital micro-mirror device (DMD), and liquid crystal on silicon (LcoS). In our improvement, the projector uses the DMD method. An optical reflector is employed to set the direction of the UV light. A position adjusting device is employed to set the position and behavior of the reflector. The adjustable angle range is ± 15◦ , and the accuracy is 0.003◦ . A flexible compressing device is used to press the resin vat. This device allows the vat to be lifted to a specific height.

For the DLP method, IP-S resin, which is a photopolymer, was used as the material. This resin was designed to tend the double function of index-matching the dip fluid for final focusing of the object and photo-polymerizable, thus enabling the highest resolution at a given magnification. The elemental composition and fundamental features of the resin are given in Table 2 [54]. The composition was decided upon by using the procedure discussed in [55] and is significant for this study since it decided the x-ray absorption characteristics of the foam [56]. The foam structure extracted from the glass substrate was qualified using optical microscopy and scanning electron microscope (SEM). SEM sample preparation included a sputter coating of 30 nm thick gold to allow electrical charge conductivity while imaging.

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(**a**)

(**b**) (**c**)

**Figure 2.** (**a**) Schematic of a digital light processing (DLP) system [53]; (**b**) 3D printing process in progress; (**c**) MiiCraft 3D printer. **Figure 2.** (**a**) Schematic of a digital light processing (DLP) system [53]; (**b**) 3D printing process in progress; (**c**) MiiCraft 3D printer.


