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Edgar Y. S. Tan and Wai Yee Yeong

between the layers should not occur. The wall thick- rence pixel for the perimeter of the cavity, P.
ness, w, determines the minimum resolution of ma- ( ) 2
P
terial deposited onto the platform. The spreading ef- R = (2)
π
fect, e, was calculated as an angle from the platform. 4.A
Spreading displayed the effect of viscosity on the The ideal roundness of a tubular construct is 1.0,
physical dimensions of the construct. The opaque which implies that the area and perimeter are equal. In
layer thickness (OW) shows the degree of calcium this experiment, we showed that the pre-designed
diffusion into the material. As the calcium diffuses roundness was achieved with 2% XG (see Figure 5),
into the material, the ions cross-link within the algi- indicating that an apparent optimal viscosity threshold
exist for a desired roundness value. The printed tubu-
nate-XG solution. This reaction also caused the struc- lar structure became increasingly out-of-roundness
ture to turn white, allowing it to be quantifiable when the viscosity was too low at 1% XG or too high
through the thickness of the diffusion zone.
at 3% XG. It was estimated that the movement of the
3.4 Wall Thickness printing platform may have affected the shape fidelity
as the extrusion pressure and speed were kept constant
The wall thickness was measured to develop a pres- for all viscosity materials. When the viscosity is too
entation of how the viscosity affects the resolution. high, the volume of hydrogel that was dispensed may
From the results in Figure 4, it can be seen that at 2% have been insufficient, causing the material to be un-
XG concentration, the width variation is minimal; in- stable and become out-of-round shape.
dicating that printing these material is most stable at
that viscosity range. It can also be established that at
the concentration of 2%, less material has spread as
compared to at the 1% XG concentration. In this study,
as pressure was fixed at a specific value for the print,
the thickness of the wall was allowed to be a variable
factor. Thus, the optimal surface area can be deter-
mined to be sufficient to support the weight of the gel
deposited on it.

Figure 5. Circularity at different concentrations of xanthan gum.

3.6 Spreading
Better shape fidelity can be derived from the spread-
ing angle of the construct. Mirroring the concept of
contact angle for droplet spreadability, the right and
left angles of the base layer were measured and aver-
°
aged. The optimal spread angle would be 90 , indicat-
ing no spreading has occurred and thus representing
Figure 4. Wall thickness versus concentrations of xanthan gum.

3.5 Roundness of Tubular Construct

Generally, there are multiple methods for measuring
the roundness of an object. In this case, roundness of
the construct was determined from the ratio between
the square of the perimeter and the area of the cavity
(see equation (2)). To calculate A and P in equation (2),
ImageJ was utilized to analyzed the cavity. Segmenta-
tion was performed on the image and the result was
calculated based on the number of pixels for area of Figure 6. Spread angle at the different concentrations of xan-
the cavity, A, and the length of the overall circumfe- than gum.

International Journal of Bioprinting (2015)–Volume 1, Issue 1 53