We because of nonconductive fixed collector and overcoming the

 

We have introduced a new
method to produce spiral shapes at micro and nanoscale by changing effective
parameters on the electrospinning. Microneedle is used to make thin and bead
free nanofibers. The better nanofibers and Taylor cone are achieved by small inner
and outer radius of the microneedle hole and tall length of the microneedle that these results
are got by COMSOL simulation based on enhancement of the electric field on the
microneedle surface. The inner and outer radius of the microneedle and the length
of the microneedle are chosen 20, 50 and 100µm, respectively based on
simulation results by considering the limitation of the microneedle fabrication
and distance between each one is
??µm.  the microfabricated microneedle
array is fabricated using the combination of dry and wet etching of silicon
wafer. The experimental setup is applied to adjust the solution flow rate,
voltage and distance between the microneedle array and the collector. The
distance and applied voltage are decreased to have a controlled nanofiber. The
spiral templates at the micro and nanoscale of single nanofiber from PVP
solution with the concentration of 20Wt% are formed because of nonconductive fixed
collector and overcoming the elastic force and expulsion force of charges on nanofibers.
a thinner spiral shape nanofiber is obtained by microneedle in comparison of
insulin syringe under same conditions. The spiral shape of single nanofiber is lost by enhancing
the voltage at the fixed distance. Sinuate shapes of nanofiber is
generated by moving the collector and speed of motion is effected on the dispensation region.

Conclusion

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In the electrospinning,
the nanofibers were bent and split until getting to the collector. So, coils of
fibers were generated. We can use microneedle with 20µm hole radius that was
applied instead of insulin syringe to create a small droplet. the best
characteristics of microneedle were examined in order to achieve the best
results by the simulation. So, microneedle array was fabricated based on these
specifications. Then the microneedle array was used for producing special
shapes by electrospinning. The polymer solution was conducted with 0/1ml/hr
flow rate toward microneedle tip. There were two sections up to arrive at the
collector for the nanofibers. The first part was related to the straight liquid
jet with micrometer diameter and the second one had whirling thinner
nanofibers. The first section was happened at the smaller distance and it was
suitable for deposition of the controlled single nanofiber. Shortening voltage
and the distance between microneedle and collector was made the ordered single
nanofiber instead of buckling nanofibers based on near filed electrospinning
(NFES) method. Unlike the NFES method, the microneedle array was used instead
of the probe for electrospinning. So, the thinner fiber without beads in the
low distance was resulted by the microneedle because of the little droplet on
the small outer radius of microneedle. Therefore the single thin regular nanofiber
was achieved. The single nanofiber can be electrospun from PVP solution with
the concentration of 20Wt%, the voltage of 1/5?2 kV, the distance between microneedle tip and lam glass of
2mm and solution flow rate of 0/1 mL/hr. Microneedle array and lam glass were
mounted on the aluminum sheet as the positive voltage and copper sheet as the negative
pole ,respectively. Voltage was applied between aluminum sheet and copper sheet
to electrospun nanofibers. Taylor cone was formed when the voltage was overcome
the surface tension and viscous force. The base of Taylor cone was related to
the microneedle hole radius. Therefore diameter, required force for the
formation of Taylor cone, softness and uniformity of nanofibers were changed by
using microneedle. thus the growth of the voltage was caused the reduction of
the stretch time. speed of jet was grown by elevating the voltage between
microneedle and collector. Voltage and distance between microneedle array and
collector were the effective parameters on the electric field, diameter and
shape of the nanofibers. Diameter and deviation of fibers were impressed by the
electric field. So, change in diameter and produced shapes were observed by
varying voltage at a fixed distance and the inner and outer radius of the microneedle. as the hole radius of
microneedle was decreased, the acceleration of jet and average fiber diameter
were decreased and the surface tension was increased. When Surface tension was
mounted, more time was required for stretching the polymer solution. So, min
voltage for drawing polymer solution was enhanced by the microneedle in comparision
of the insulin syringe. The electrical field was altered after putting the lam glass
on the copper
sheet as the collector because of it?s insulation therefore the behavior of electrospinning was
the same as abnormal ground on this collector. distance between nanofibers on
the isolator collector was more than the conductive collector. Expulsion force
of charges and elongation of surface tension were effected on the
electrospinning process and the created patterns by nanofibers. the width of
the distribution area was raised because of the expulsion force of charges on
nanofibers and the elastic force. So, nanofibers didn?t want to compress
together. If the collector wasn?t conductive, charges couldn?t deliver rapidly
and the expulsion force overcome the other forces thus The nanofibers were
collected on the collector with spirality pattern. Also, charged nanofibers were
dragged more because of the repulsion force on the dielectric collector. So,
the diameter of nanofibers was diminished on the lam glass. Spiral shape single
nanofiber was formed when nanofibers were placed away from nanofibers spread on
lam glass because of the insulation collector. the first point of this pattern
was the junction between microneedle tip axis and lam glass substrate and then
the continuous single nanofiber was moved environment the initial point with
the shape of spirality which was made with the slow speed. The area between
close nanofibers along the spirality pattern almost was ???? µm.// Nanofibers are expanded on the
wider surface on the lam glass based on SEM pictures of…. Different shape and diameter of
nanofibers are shown in figure14 by raising voltage discontinuously at the
fixed distance of 2mm. As the voltage was enhanced, the instability of nanofibers
was lifted. Min voltage was achieved 1/5kv from the microneedle to have Taylor
cone at the fixed distance of 2mm. The spiral shape of nanofibers by two close
microneedles is exhibited in figure15. The different shapes and distribution area
were made by moving the substrate. When collector moved slowly, nanofibers were
laid on the collector into wave-shape in direction of moving. The speed of the collector was adjusted to
create different shape and diameter of nanofibers. If the velocity of the collector was enhanced,
the distribution area was decreased. The moving collector was caused
that the spirality pattern wasn?t produced And waved nanofiber was specified in
the direction of transfering. As
the speed of collector increases the area distribution decreases. According to the SEM
pictures, the nanofiber width on the collector is in the range of ?????????nm, that electrospun from 20Wt% PVP under voltage of ??? v as shown in figure??.

Results

In this study, microneedle
array was examined. figure13 shows the microneedle array experiment setup. It
was combined of a lam glass that mounted on a copper sheet as a collector, a
0/5ml plastic syringe, a microneedle array (microneedle with inner diameter 45µm
and outer diameter 105µm), a stepper motor, a micrometer head, a distance
controller and a voltage generator. The whole setup was placed perpendicular to
the collector. Microneedle array was deposited by gold. Aluminum sheet as an
electrode to be connected to the voltage generator was attached to the
microneedle array. A lam glass mounted on the copper plane was used to collect
fibers. Aluminum sheet was straddled on a insulin syringe as a polymer solution
source. the polymer solution was forced by micrometer head which was attached
to the stepper motor. The flow rate was controlled by stepper motor rotation.
Applied force was caused the formation of drops of the polymer solution at the
microneedles tip. Taylor cone was formed by applying a voltage to the Aluminum
sheet and the copper surface. A jet of polymer solution toward collector was
made by overcoming the electric field over the surface tension of the solution.
At last, fiber samples were deposited with gold to decrease the charging
effect. The gathered fibers were characterized by using a scanning electron
microscope(???). The distance between
microneedle array and collector and applied voltage were the important
parameters affecting on electric field, uniformity and diameter of fibers. So,
these factors were examined to
show their effects on the fibers.

 Experimental setup:

Polyvinylpyrrolidone
(PVP) (MW=1,300,000, Merck) solution was provided by dissolving PVP powder in
methanol to prepare a solution with a 20 Wt% concentration pursued by stirring
for 5 min at about 30?. Since this solution was passed through the microneedles,
it?s concentration should be sufficient. The good concentration was occurred
when the microneedles weren?t clogged and electrospinning  was happened.

Preparation of
materials:

An innovative manner related to the
electrospinning was followed by microneedle that different patterns such as
spiral templates at micro and nanoscale were made with low distance and the
applied voltage by changing effective parameters on electrospinning for various
usages like cell patterning. A concentration of 20Wt % solution of
polyvinylpyrrolidone (PVP) was got as optimum concentration, so this
concentration was conducted to make nanofibers.

Results and discussion

In this study, Hollow
microneedles were fabricated using the combination of Wet Etching and Deep
Reactive Ion Etching (DRIE) of the silicon oriented, 510µm thick
silicon wafer. the Fabrication process of microneedle array is shown in
figure10. A solution of (NH4OH: H2O2: H2O   
1:1:5)  at 75? temperature for 10
minutes was acted for cleaning of silicon wafers. Plasma Enhanced Chemical
Vapor Deposition (PECVD) was applied to deposit a silicon nitride layer on to
both sides of silicon with a thickness of 225nm to protect silicon against Wet
Etching. Chromium was deposited on to the both sides of wafer to a thickness of
160nm by electron beam evaporation system to keep silicon nitride against
Reactive Ion Etching (RIE) and Deep Reactive Ion Etching (DRIE). then it was
lithographically patterned (using Shipley photoresist) to get a 4×4 array of
squares each gaging with 658µm in width, where the
center to center spacing between squares in each row and column was 1/5mm. The
photoresist was spin coated onto the other side of the wafer to protect
chromium. Next, the exposed chromium layer and silicon nitride were removed by
etchant chromium and Reactive Ion Etching respectively. The acetone was used to
remove photoresist. Pyramid shaped was fabricated to the wafer using KOH (30Wt%)
heated to 80?. 54/7?tapered walls were built using wet etching along crystal
plane. When depth removal was achieved 470µm, the wet etching process was
stopped. SEM images after Wet etch are shown in figure11. The photoresist was spin
coated onto the other side of the wafer. A second mask was aligned with silicon
membrane to expose the photoresist to UV light in the same 4×4 array of microneedles
each measuring 45µm and 60µm in inner diameter and wall thickness, respectively
and center to center spacing was 1/5mm. the photoresist was developed with KOH(??). The spacing between neighbor
microneedles must be enough and didn?t effect together. The height of microneedles ??. when exposed chromium
layer was removed, silicon nitride and silicon were etched directly by Deep
Reactive Ion Etching until silicon membrane was disappeared. So, hollow
microneedles with good surfaces are found out as shown in figure12.

Microneedle Fabrication

Figure7
displays the electric field along the z-axis in the central line for the four
systems with various microneedle hole radius, outer radius and length that set
as 10, 20, 30 and 40µm for microneedle hole radius and 50,100,150 and 200µm for
microneedle outer radius and 25, 50, 75 and 100µm for microneedle length. it showed that with the increase distance from
microneedle, the field intensity decreased slowly except for the
position beside the microneedle apex. At the end fixed and low electric field
was received on the collector. It was clear that the microneedle with the smaller hole and outer radius and larger length made
the larger electric field in the region close the microneedle and displayed a better
Taylor cone in the spinning path. Also, It caused the long jet direction and
thinner nanofibers. So it was better to use microneedle with the lower hole and
outer radius and bigger length.  According
to the limitations of the fabrication process, a microneedle with 20µm hole
radius, 50µm outer radius and 100µm length was used. From the simulation
results are shown in figure8(a),
we can observe that the microneedle with smaller outer radius makes a stronger
electric field strength along the microneedle surface at the different
microneedle outer radius. So, Taylor cone and nanofibers were improved and this
result proved the outcome of figure7(b). So, effective parameters such as
voltage, the distance between microneedle and collector, length of the
microneedle and inner hole and outer radius of the microneedle effected on the
electric field and electric potential. At the end, the suitable conditions for
fabricating microneedle was followed to achieve the best results.

Electric field and
electric potential simulation for design microneedle:

The electric potential
(V) was applied to an aluminum sheet that microneedle array was mounted on it. The
surface that microneedle was extruded from it and microneedle tip were coated
with a 30nm gold layer. Potential of zero was put on the copper sheet that the
lam glass plate as the fiber collector was straddled on the copper surface. the
relationship between electric field and electric voltage with altering distance between
microneedle surface to horizontal line graph with z=250, 500, 750 and 1000µm
(on the collector) are shown in figure4. As the distance microneedle to
detachment horizontal line  increases,
the electric potential decreases As shown in figure5(a). it was indicated that
max and min electric potential related to the microneedle tip and collector,
respectively. Taylor cone was made due to the
variation of electric potential. As a result, nanofibers was been formed. analyze
of the electric field is done at different horizontal line z=0, 100, 300, 500,
700, 900 and 1000µm in the XY plane as shown in figure5(b). the max electric
field was at the edges of the microneedle and collector and the min amount was
at the hole of microneedle along z=0. Microneedle with edge was caused the max
field intensity around it. So polymer solution was stretched because of
concentrate charges. As the distance microneedle to horizontal line enhances
the electric field decreases slowly as long as it reaches to the collector with
constant and low electric field. Extension of the field was happened by
increasing the distance from the microneedle surface. So distance caused
different field strength. Also vertical lines with y=-500, 0, 500µm in the YZ plane is displayed various electric
potential along increase
distance from microneedle upto collector as shown in figure6(a). the equal distribution of electric potential on
the microneedle was caused the fine nanofibers.()().

E= -?V    (1)

The 3D electric field
and electric potential of the electrospinning system was examined using the
finite element method (FEM).The display of the electric field on Arrow and the
electric potential on contour were exhibited in figure1(b). So the distribution
of electric field was mentioned upgrade and electrospinning will be done with
the best results. The figure2 shows the distribution of the electric potential
on surface.The electric potential on surface at the microneedle tip with height
is displayed in interpolation of figure3(b) that a concentrated point of
voltage is observed. Max electric potential at microneedle surface is placed on
the microneedle because of microneedle?s length as shown in figure3(b).The
center region of the collector has smaller field intensity contrasted to the
corners of the collector as shown in figure3(a). The electric potential at the
collector surface with height in the microneedle system exhibits in the
interpolation of figure3(a). The electric field (E) was computed by the
gradient of electric potential (V), as shown in equation 1.

The system was simulated
using Comsol ® Ver 5.2 add-on AC/DC module under Windows 10 operating system
that microneedle electrospinning system was modeled for forming of spiral shape
single nanofiber and a spherical- shaped air environment was been modeled. At
first, the physical geometries of the setup, such as microneedle, collector and
the electrode were determined based on their experimental dimensions, positions
and substance properties. The configuration of a microneedle spinneret was
shown in figure1(a). The processing parameters were summarized in table 1.

Electric field and
electric potential simulation:

Simulation

Electrospinning is a
beneficial process to fabricate nanofibers of which the diameter ranging from
tens to hundreds of nanometers from polymer solutions at circumstance
temperature and atmospheric pressure1. nanofibers have great
characteristics such as smaller diameter, high surface-to-volume ratio and long
length that use in various industrial and scientific fields2Such as filtration
membranes 3, optical tools 4, bio-scaffolds 5 and MEMS device 6. conventional
electrospinning process acts at high voltages above the tens of kilo-volts and
provides randomly coiled fibers because of the turning instability of fiber jet
7. many parameters such
as polymer concentration, solution viscosity and conductivity, the applied
voltage, needle to collector distance, solution flow rate, needle diameter and
temperature and humidity of environment 2 effect on the electrospinning
process. The smaller diameter of fibers is created by the minor diameter
capillaries 8. Also, a lesser needle
diameter is conduced to have the bead free fibers and the beads are produced
due to  instability of the jet initiation
9. So needle with the
small diameter is beneficial to electrospun nanofibers. Micro-electromechanical
systems (MEMS) technology has been applied to many types of research. Microneedle
array has been fabricated by using this technology. Microneedles have found
some applications in cancer treatment 10, transfer of DNA
solution to the core 11, blood extraction 12, three-dimensional
measurement 13 and stimulate the
nervous signals 14. Electrospinning
process is done by micromachining spinneret to form the nanofibers 15. different methods are
applied to fabricate hollow needles such as the combination of deep etching of
silicon substrates and rotating small angle deposition 16. the width of deposite nanofibers
distribution area is changed by varying the collector material and this area of
the silicon surface is smaller than that on SiO2 surface 17. Near field
electrospinning (NFES) has been used to achieve a nanofiber instead of randomly
nanofibers by decreasing spinneret to collector distance in electrospinning 18. spiral patterns are detected
widely in nature and art. Benefits of this type of shape such as microarrays with spiral channels for numerous
hybridization rates 19, spiral molds to
specify the action of polymers injected on fabrics 20, spiral-like electrodes
for energy harvesting 21 and spiral turning capabilities
in soft robots 22 have been caused to try to have this kind of shape. There is a new method
of electrospinning that it is known as micro-pyro-electrospinning(µ-PES) to
have fibers into spiral patterns which have various applications, containing
cell patterning. µ-PES offers a mask free and single step process without waste
time and expensive lithography methods or direct printing to build spiral types
for the first time  23. In this study, we show
a new method by using microneedle, collector material and electrospinning by
decreasing microneedle to collector distance and voltage to have a microscale
spiral shape controlled nanofiber. Also, the haracteristics of microneedle  are examined to have thin and free bead
nanofibers. c

Introduction

In this study a new
manner present to construct micro and nanoscale spiral shape nanofiber.
Multiple conditions were observed to achieve this type of nanofiber by electrospinning.
 Microneedle is assisted instead of
needle to reduce the diameter of nanofibers which electrospun from PVP polymer
solution with a concentration level of 20Wt%. The best specifications of
microneedle are defined by simulation to have fine and free bead nanofibers.
The microneedle array are fabricated by utilization of MEMS technology and
combination of Wet and Dry Etching. Single nanofiber is obtained by small
distance and the applied voltage between microneedle and collector by electrospinning.
The nonconductive collector is caused the formation of spiral figure type of
single nanofiber because of overcoming the elastic force of nanofibers and
expulsion force of charged nanofibers. Finally, the effects of the applied
voltage and spinneret characteristics have been examined on the nanofibers
patterns.

Abstract

 

 

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