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Lee, et al.
microstructure with thicker films between each bubble. small- and medium-sized bubbles than Ink 4 as shown
It also explains why the EW inks were denser than the in the red box in column IV. This could be attributed to
HPMC inks. the higher density of Ink 5, which made it harder for
Comparing the EW only foam to Inks 1 and 2, the foam formation and reduced its ability to trap air.
baked EW foam was flakier and had more disconnected
sections as shown in the blue boxes in column I for A, B, 3.6. Texture profile of foam inks
and C. The honeydew extract, which contained a large From Figure 5A, all the inks have relatively low hardness
portion of small polysaccharide , seemed to increase and gumminess ranging from 0 to 2 N. The adhesiveness
[38]
the adhesiveness of the liquid phase of the foams. Even of all inks is below 5 mJ. Adhesiveness helps increase the
after the foams were baked, most bubbles retained their ability of the foam to self-support. A good combination
shapes for Inks 1 and 2. In Inks 1 and 2, thin films of slightly higher hardness and good adhesiveness of Ink
covered most of the bubbles. More bubbles were intact 2 as well as Inks 4 and 5 resulted in better printability
in Ink 2 than Ink 1 (red boxes of B and C in column II);
suggesting that the XG kept the foams from collapsing, than inks without XG, such as Inks 1 and 3. While Ink 1
especially at elevated temperature during baking, and had a higher hardness, it had poor adhesiveness. Hence,
the gas bubbles tend to grow larger. Furthermore, in while it could be extruded, it was unable to maintain a
Ink 2, the films over the bubbles were more wrinkled, good shape, resulting in poor printability. Ink 5 had a
suggesting that the liquid phase with XG was stretched lower adhesiveness than Inks 2 and 4 but higher than Ink
when hydrated to prevent the bubbles from bursting and 1. This explains the better printability of Ink 5 than Ink 1
the foam from collapsing. as it can retain shape better than Ink 1.
Comparing Control 2 to Inks 4 and 5, the pores in While Inks 1, 2, 4, and 5 were all printable and
Control 2 were larger and exhibited a thinner layered retained their shapes after baking, Ink 5 could not maintain
interface between pores pointed out in blue arrows the height of 20 mm after baking and collapsed. Hence,
in column IV. Most of the bubbles in Control 2 were it was not included in the texture profile analysis in the
torn or burst after being dehydrated by baking. The baked form (B is added to the ink names to differentiate
honeydew extract increased the adhesion between each baked samples). After baking, all the printed inks
bubble and thickened the interfaces. In Ink 4, the film became meringue-like after dehydration by baking. This
over the larger bubbles was intact, whereas the film over increased the hardness as reflected in Figure 5C. The
smaller bubbles was torn slightly as seen in column chewiness of the printed inks increased too. However,
IV (Figure 4E). In comparison, Ink 5 had more burst the springiness and adhesiveness reduced. The other
or torn bubbles than Ink 4. Ink 5 also showed more parameters remained similar. The lack of water made the

A D

E
B

C F

Figure 4. Microstructure of foam inks after baking for 1 h at 70°C. (A) Control 1: Egg white only foam. (B) Ink 1. (C) Ink 2. (D) Control
2: Foam Magic only foam. (E) Ink 4. (F) Ink 5.

International Journal of Bioprinting (2021)–Volume 7, Issue 4 163