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Influence of electrohydrodynamic jetting parameters on the morphology of PCL scaffolds

patibility has stimulated extensive research into its (>250 mm/s), while C=60% is easy to trigger the
potential application in the biomedical field. The so- coiled/wave structure, C=70% yields fibres that are
lubility of PCL is strongly dependent on the MW of not thinner enough to form the coiled/wave structure
polymer. Hence, the viscosity of the polymer solution due to the higher solution concentration. Hence, there
is proportional to the MW of PCL, for the same con- is no transition from straight to coiled/wave structure
centration of solution. As the PCL has a low melting is seen.
point, the viscosity of the polymer solution decreases (3) Effects of Stage Speed on the Coiled Scaffold
rapidly with the increase of temperature, thus lowering To investigate the influence of stage speed on the
the viscoelastic force acting on the Taylor cone during coiled scaffold pattern, the process parameters were
EHD-jetting. Moreover, during the printing process, set as following: nozzle-to-substrate distance of 3 mm,
the polymer chain tends to orient along forced direc- solution concentration of 60%, solution feed rate of
tion and solidifies on substrate upon evaporation of 1.5 μL/min, ambient temperature of 25 °C, and the
the solvent in which the temperature and humidity stage speed was varied from 100 mm/s to 250 mm/s at
show significant influence on the solvent evaporation increments of 50 mm/s. Table 4 shows the optical
rate [29–30] . Therefore, the temperature variation directly microscope images of fabricated single layer scaffolds.
affects the EHD jetting process and the morphology of In this experiment, the temperature was 25 °C,
the scaffold. which is 5 °C higher than the previous single layer grid
(2) Effects of Process Parameters on the Layout of
Scaffold structure fabrication (Table 2). With the increase of
In this experiment, process parameters were temperature, the solution viscosity decreased, aiding
C=60% and 70%, FR=1 µL/min, D=3 mm, V=3 kV, the traveling liquid jet stream to flow easily, being
and T=25°C. A transition from coils to waves and fi- subject to a variety of forces. And then, the EHD jet-
nally to straight fibres was observed, with the varia- ting process undergoes an instable phase, which di-
tion of the stage speed, as shown in Table 3. Two crit- rectly results in the formation of the coiled structure
ical speeds were observed to aid in this transition be- scaffold. The stage speed plays a s econdary role in
tween two shapes: straight, and coiled/waved struc- this experiment by just guiding the scaffold pattern.
tures, namely 100 mm/s and 250 mm/s, when C=60%. Therefore, all the scaffold patterns are coiled and the
When the stage speed was relatively slow, the elec- stage speed variation resulted in slight differences in
trical force, pressure from the pump, and the other the coiled structure morphology. During the EHD-
forces take charge of the formation of the fibres. Since jetting process, the liquid jet stream exiting from the
the stage moves very slowly, there is enough time for nozzle is subjected to a variety of forces with different
the fibres to fold and form coiled/wave structures. effects. If the forces are unbalanced, instability of the
However, when the speed increases, the mechanical jet occurs. The increased surface tension at the Taylor
drawing force of the stage play a main role, and the cone and the electrical force causes the whole EHD-
fibres align in a straight line. With the increase of the jetting process to be unstable during this period and
stage speed, the fibre diameter becomes thinner and forms coiled structures. However, by tuning the para-
thinner. At a certain point, when the fibre diameter is meters, consistent coiled patterns were obtained. For
too thin, the mechanical drawing force generated by instance, at a s tage speed of 100 mm/s, the coiled
the robotic stage will lose control due to the small fibre structure was more uniform when the feed rate was
diameter and the fibres become coiled or wave-shaped increased from 1.5 to 2 µL/min.
again. Splaying might also play a role in the formation From Table 4, when the stage speed is low, there
of coiled / wave structure but further research is req- were more coiled loops in a single fibre. Apart from
uired to study how significant the effect is. This ex- this, the low stage also resulted in thicker fibre di-
plains the two critical transition speeds of 100 mm/s and ameter. Comparing the fibres printed at a stage speed
250 mm/s. At C=70%, the lower bound of the transi- of 100 mm/s with that of 150 mm/s, the fibre diameter
tion speed holds good at 100 mm/s while straight to is smaller and the loop diameter is larger. Both the low
coiled/wave structure transition is not seen at 250 mm/s. stage speed and high stage speed have less likelihood
As mentioned above in the case for C=60%, at lower to fabricate a uniform coiled pattern. For both the
stage speed, there is enough time for the fibres to fold concentration values of 60% and 70%, the coils were
and form coiled/wave structures. But at higher spHHGV unstable and non-uniform at the extreme ends of stage

78 International Journal of Bioprinting (2017)–Volume 3, Issue 1