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Sun J, et al.
A B

Figure 3. Effect of the applied voltage on centroid and diameter under varied distance (65 wt/v% polycaprolactone, feed rate = 0.7 μl/min,
stage speed = 150 mm/s). (A) Effect of voltage and distance on centroid. (B) Effect of voltage and distance on diameter

cone is inversely proportional to the applied voltage. It is we can manipulate the solidified electrospun fiber flying
found that a voltage around 2.6 kV can generate a huge in a stabilized helical manner. Thus, versatile serpentine
size of Taylor cone, and this size reduces with the increase structures can be direct written on a moving substrate
of the voltage. The cone shape becomes excessively with diverse amplitude and cycles.
tiny when the voltage is around 3.4 kV. In other words,
increasing the applied voltage above a threshold will 3.3.2 Effect of the SS
reduce the stability of the jet . Similar phenomena are We conduct several tests to examine the effect of the SS
[10]
also observed for the nozzle-substrate distance at 3.5 mm (100–300 mm/s) on Taylor cone and jet. The mechanical
and 4 mm. Similarly, we also investigate the size and drawing force acting on the jetting increases with the
shape of Taylor cones using 70 wt/v% PCL and draw the SS. This leads to an increasing degree of slant jet and
same conclusion. its elongation rate and eventually generates diverse
About the effect of the applied voltage on the cone-jet trajectory. When the SS is greater than the downward
diameter in Figure 3B, increasing the applied voltage speed of the jet, the fiber length deposited on the substrate
leads to slightly decrease of the jet diameter when the is larger than the jet length fallen on substrate per unit
nozzle-substrate distance is 3 mm. The jet diameter time .
[13]
decreases more significantly when the nozzle-substrate As shown in Figure 5, the EHDP jet falls down almost
distance increases to 3.5 and 4 mm. As the electrostatic vertically and buckles in some degree near the substrate
force becomes weaker under a larger distance, EHDP at the SS range of 50–150 mm/s. The jet evolves into a
process generates a thicker jet which can be further compressed “heel” shape (Figure 5A and B) and becomes
stretched with the increase of the applied voltage . unstable with periodically meandering. Thus, a lower SS
[10]
The captured cone images at 3 kV under different nozzle- may lead to a series of bifurcations, such as alternating
substrate distances and their corresponding deposited loops, and translated coiling on the substrates as shown in
fiber patterns are shown in Figure 3B. Compared with the Figure 2. This jet buckling effect is less obvious when the
Taylor cone shape at 3 mm, the cone deforms significantly SS is above 200 mm/s, and straight fibers can be observed
at 4 mm. To achieve stable printing, the ratio of the on the substrate.
applied voltage to the nozzle-substrate distance should be
kept in a reasonable range. At 3 mm, EHDP could avoid 4. CNN in EHDP Cone Modes’ Classification
the whipping instability and realize the direct writing
of straight micro-/nano-fibers. With the increase of this Conventional machine learning algorithms such as
distance to 3.5 mm and above, the serpentine structures support vector (v) machines have limited capability
are observed on the substrate. to process natural data (such as the pixel values of
At 3 mm, the deposited fiber patterns on the substrate an image). Constructing a pattern recognition system
may vary with the applied voltage as shown in Figure 4. requires considerable domain expertise to design a
When the applied voltage is about 2.6–3 kV, the straight feature extractor which transforms these data into
fiber can be collected on the substrate. When the applied suitable features for these learning algorithms. Applying
voltage reaches to 3.2 kV, the fiber patterns change to these algorithms in EHDP cone mode classification need
serpentine structure. The amplitude of the serpentine to go through both feature extraction and classification
structure increases significantly at 3.4 kV applied processes in a separated way, which inevitably lead to
voltage. Through adjusting the key process parameters, information loss. Furthermore, a feature selection process

International Journal of Bioprinting (2019)–Volume 5, Issue 1 5