Page 122 - IJB-7-3

Systematic Thermal Analysis for Accurately Predicting the Extrusion Printability
of the fibers printed at the controlled syringe temperatures extrusion velocity contributes to a secondary swell. For
of 26°C, 27°C, 28°C, and 29°C using the 23-G nozzle are example, the printing condition was set to a pressure of
plotted in Figures S1F-I. An almost linear relationship 600 kPa for the 32-G nozzle and 130 kPa for the 23-G
was also found between the value of c and the temperature, nozzle at a temperature of 27°C. The effect of nozzle
which is presented in Figure 6(B) (R = 0.96). This was velocity on linewidth is presented in Figures S1J-K. The
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also the case for the relationship between the shape faster the velocity, the thinner the linewidth. There was a
coefficient, b, and the temperature (R = 0.96), as shown gradual decline in the rate of change, as shown by Eq. (16).
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in Figure 6D. The derived shear coefficient, c, and the shape efficient, b,
The differences in coefficients c and b for different were substituted into the equation, and an approximation
nozzles could be attributed to the nozzle diameter. The was found between the theoretical and experimental
contact angle could be affected when the experimental linewidths (R = 0.9807 and 0.9421, respectively, for the
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linewidth was widened by the collapse of more 32-G and 23-G nozzles).
materials extruded by the larger nozzle. A negative The above discussion results can be summarized as
correlation was found between the shear coefficient, c, follows: (a) The formation of filaments is a synergistic
and the temperature. This could be attributed to the high consequence of various factors, including pressure,
temperature decreasing the viscosity of the material, velocity, and extrudate temperature. (b) The predicted
thereby weakening the die-swell phenomenon. The extrudate temperature (T ), which was derived from the
E
increase in temperature decreased the material-slide syringe temperature (T ) and the AT (T ), was close to the
C
A
contact angle, which increased the shape coefficient, b. real extrudate temperature. (c) The modified model with
In Figure S1, the linewidths, predicted using consideration of the die-swell phenomenon was used to
the conventional model (Eq. [12]) at the extrudate predict the printed linewidth. The results were superior
temperature, do not match the experimental results to those of the ordinary model. (d) A reasonably accurate
(R = 0); Chen et al. reported a similar conclusion . open-loop control could be established based on the
[34]
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However, when using k and n’s values at the predicted proposed physical model.
extrudate temperature and the proposed physical model,
good agreement is found between the predicted and 3.4. Linewidth printing steps
experimental linewidths (R > 0.8 for each experimental The first step to printing 3D scaffolds is to use an
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set). Those results reveal that the extrudate temperature appropriate linewidth, which can be achieved using
predicted by the syringe temperature and the AT, rather the established physical model (Eq. [16]). The model
than by the syringe temperature alone, was similar to describes the relationship between the extrudate’s
the actual printing temperature. And the physical model temperature, pressure, moving velocity, and linewidth.
considering the die-swell phenomenon could describe the A series of lines with a stepped linewidth were printed
extrusion process more exactly. This could explain the using this model. The shape coefficient (b) and shear
difference between the predicted and experimental results coefficient (c) were derived from Figure 6, and the AT
reported in Chen et al.’s study . was set to 25°C.
[34]
When comparing the linewidths predicted by the A series of experiments was conducted to modify
proposed and conventional models, a deviation between the physical model’s different parameters. Lines were
two lines is evident for the 32-G nozzle but not for the printed using the 32-G and 23-G nozzles separately with
23-G nozzle. This could be attributed to differences a 50-μm linewidth step. Figures 7A and B show the
in shear stress and flow duration. A greater pressure is fabricated lines, and a comparison between the printed
required to force materials through the smaller diameter and predicted linewidths is given in Figures 7C-H. The
nozzle, which increases the wall shear stress and causes white dotted line denotes the outline of the printed lines.
the extrudate to swell. For the larger nozzle (23-G), the With the 32-G nozzle, the initial linewidth was set
deviation is decreased by the fast flow rate and the low to be equal to its nozzle diameter of 110 μm. The printed
shear stress. lines were expected to have linewidths of 110, 160, 210,
The influence of the AT between 15°C and 40°C 260, and 310 μm. In the first set of experiments, the
(data not shown) was also investigated. The significant printing pressure and velocity were maintained at 200
difference between the predicted and printed linewidths kPa and 8 mm/s, respectively. The extrudate temperature
suggests that the AT could result in different contact was controlled to 26.1°C, 27.3°C, 28.1°C, 28.7°C, and
angles and collapse of the printed line, and it was 29.3°C separately by changing the nozzle temperature,
demonstrated that the contact angle influences the the values of which were determined by the thermal
measured linewidth [32,52] . model and the simulation (Section 3.2). Then, the AT of
During the third stage of the extrusion process, the 25°C and the thermal parameters (as given in Table 1)
mismatch between the nozzle’s moving velocity and the were substituted. In the second set of experiments, the

118 International Journal of Bioprinting (2021)–Volume 7, Issue 3