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Lee, et al.
A

B C

Figure 1. An overview of the formulation of coconut cream inks added with coconut oil, and results from oil separation test. (A) Coconut
cream base was initially formulated by mixing coconut cream powder with different water concentrations (20, 25, and 33%) and fixed
concentration of pandan extract at 0.2%. Next, different weight concentrations of coconut oil (% w/w, with respect to the weight of the
coconut cream base) were added to the coconut cream base to observe the occurrence of the oil separation. (B) A plot showing oil separation
ratio as a function of oil concentration. (C) A diagram showing inks that exhibited or did not exhibit oil separation at different oil and water
content.

expected, the oil separation ratio increased as the amount 4.2. Rheological characterization of coconut
of oil separated increased (Figure 1B). Oil separation cream ink
occurred in inks containing 20% (w/w) water content with
additional oil content >5% (w/w), inks containing 25% Next, we studied the rheological properties of the inks.
(w/w) water content with the additional oil content >10% Rheological properties such as yield stress and storage
(w/w), and inks containing 33% (w/w) water content with modulus (G’) are important to determine the printability
[42]
the additional oil content >15% (w/w) (Figure 1B). As of ink . The yield stress of the ink is the minimum
the water content increased from 20% to 33% (w/w), the shear stress needed to initiate flow in DIW 3D printing.
tendency for oil separation to occur decreased. The oil G’ is a measure of mechanical strength at rest condition
separation ratio increased from 0.10 (10% w/w oil) to which determines the structural integrity of the printed
0.80 (30% w/w oil) for inks containing 20% water, 0.01 material after deposition. The rheological properties were
(12.5% w/w oil) to 0.50 (30% w/w oil) for inks containing determined using the same method described in previous
[23]
25% water, and 0.01 (20% w/w oil) to 0.35 (30% w/w work . All inks displayed shear thinning property,
0
3
oil) for inks containing 33% water. As expected, the oil where the viscosity decreased from an order of 10 to 10
−1
separation became prominent at high concentrations of Pa·s with increasing shear rates between 0.01 and 100 s
the oil. With the increase in water content, the amount (Figure 2A). Shear-thinning properties are desirable for
of dispersed water also increased which allowed the the extrusion of ink from the nozzle on applied pressure.
[41]
emulsifier to achieve continuous interface between water The increase in water content from 25 to 33% (w/w)
and oil without phase separation. However, the increased resulted in a reduced viscosity due to the weakened
water content decreased the viscosity of the overall ink, colloidal network caused by lower ratios of emulsifier.
which would affect the 3D printability of inks. Overall, Similarly, the increase in the oil content from 10 to 12.5%
we selected three samples (Ink A contains 25% water (w/w) led to a decreased viscosity. We also observed mild
with 10% (w/w) oil, Ink B contains 25% water with oil separation in Ink B presumably due to the weakened
12.5% (w/w) oil, and Ink C contains 33% water with colloidal network within the ink.
10% oil (w/w)) for the characterization of the rheological The previous studies have suggested that the inks
properties to ensure the 3D printability by cold extrusion. with the yield stress of 106–330 Pa and the storage

International Journal of Bioprinting (2021)–Volume 7, Issue 2 117