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Towards a new position-controlled electrospinning setup

T.A. Baede

DCT 2009.052

Masters thesis

Coach(es): dr. ir. M.J.G. van de Molengraftdr. ir. G.W.M. Peters

Supervisor: prof. dr. ir. M. Steinbuch

Eindhoven University of TechnologyDepartment of Mechanical EngineeringDynamics and Control Technology Group

Eindhoven, June, 2009


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Table of contentsSummary......................................................................................................................................................... 1

Introduction ..................................................................................................................................................... 3

Chapter 1: Electrostatics................................................................................................................................. 8 1.1: Electric field without dielectric ............................................................................................................. 8 1.2: Refinements ...................................................................................................................................... 10

Chapter 2: Process parameters, materials and cabin ................................................................................... 13 2.1: Process parameters .......................................................................................................................... 13 2.2: Materials............................................................................................................................................ 14 2.3: Electrospinning cabin ........................................................................................................................ 14

Chapter 3: Pre-design experiments .............................................................................................................. 16 3.1: Introduction ....................................................................................................................................... 16 3.2: Experiments and results.................................................................................................................... 16

3.2.1: Fiber deposition: experiments.................................................................................................... 16 3.2.2: Fiber deposition: results ............................................................................................................ 17 3.2.3: Writing: experiments.................................................................................................................. 19 3.2.4: Writing: results........................................................................................................................... 20 3.2.5: Influence of collector thickness: experiments ............................................................................ 22 3.2.6: Influence of collector thickness: results ..................................................................................... 22 3.2.7: Influence of collector-electrode airgap: experiments ................................................................. 24 3.2.8: Influence of collector-electrode airgap: results .......................................................................... 24

3.3: Design implications ........................................................................................................................... 25

Chapter 4: Design of a new electrospinning setup........................................................................................ 26 4.1: Introduction ....................................................................................................................................... 26 4.2: Design requirements ......................................................................................................................... 26 4.3: Configurations ................................................................................................................................... 27

4.4: The cylinder spinner.......................................................................................................................... 29 4.5: Material selection .............................................................................................................................. 30 4.6: Rotation axis ..................................................................................................................................... 30 4.7: Translation axis ................................................................................................................................. 30 4.8: Realisation ........................................................................................................................................ 30 4.9: End stops, homing strategy, endpoint detection................................................................................ 34 4.9: Control design................................................................................................................................... 35

4.9.1: System identification.................................................................................................................. 35 4.9.2: Inter-axis disturbances .............................................................................................................. 36 4.9.3: GUI and real-time control .......................................................................................................... 36 4.9.4: Feedback control ....................................................................................................................... 37

4.10: Spacing error................................................................................................................................... 40

Chapter 5: Post-design experiments............................................................................................................. 42 5.1: Stationary deposition experiments on glass cylinder......................................................................... 42

5.1.1: Experiments............................................................................................................................... 42 5.1.2: Results ...................................................................................................................................... 42

5.2: Testing with motion ........................................................................................................................... 42 5.3: Troubleshooting ................................................................................................................................ 43 5.4: Stationary deposition experiments on PET ....................................................................................... 46

5.4.1: Experiments............................................................................................................................... 46 5.4.2: Results ...................................................................................................................................... 46

Conclusion and recommendations................................................................................................................ 48

References.................................................................................................................................................... 49


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SummaryElectrospinning is a powerful and versatile technique for the fabrication of very thin fibers frompolymer solutions. In contrast to mechanical drawing processes, spinning with an electric fieldprovides the ability to produce fibers with much thinner diameters, typically in the micro- andnanometer range.

In a conventional electrospinning process, a polymer solution is fed to a metallic nozzle so that adrop appears at the tip of this nozzle. A high voltage is then applied to the nozzle, while at somedistance below it a grounded plate electrode is placed. The potential difference between bothpoints generates an electric field. The drop becomes charged and a thin jet is ejected from thenozzle. Due to electrostatic forces, the jet is pulled towards the electrode. By placing a thin sheetof collecting material between nozzle and electrode, the fiber is deposited on this so-calledcollector .

Due to instabilities in the process, the material is deposited randomly, forming a so-callednonwoven mesh with a chaotic structure. For future electrospinning applications in the fields offiltration, tissue engineering and nano-electronics, it is necessary to make the fiber depositioncontrollable. A technique developed at Eindhoven University of Technology consisting of amoving collector and a thin, positionable grounded needle electrode makes this possible and wasimplemented in an electrospinning setup. However to improve fiber deposition control, moreknowledge of the electrospinning process is required. The goal of this research study is to acquirethis knowledge through experiments.

In a first experiment, the deposition mechanisms for the grounded plate and needle electrodeswere studied. It was learned that although the deposition mechanisms are different, thedeposition looks similar. Secondly, PEO and PCL fibers were written on a moving collectorconsisting of a mylar sheet in a straight line. Interestingly, it was discovered that both fibers had arectangular cross section. In a third experiment, it was investigated whether the collector

thickness influences the amount of deposition. It was discovered that the collector thickness doesindeed play a role and that reducing this thickness leads to more pronounced deposition. Thisexperimental result corresponds with electrostatic analysis, which suggests that although adielectric collector does not change the overall shape of the electric field, it does locally reducethe field strength. This leads to a decrease in electrostatic pulling force and thus the amount ofdeposition. In a fourth experiment, the goal was to find out whether a small airgap betweenelectrode and collector prevents, hinders or otherwise alters the deposition. No apparent visibledifference was detected in deposition between both types of samples. The airgap did not preventor hinder deposition.

Although great strides have been made in the control of the fiber deposition point, there is still adifference between desired and realized deposition points. To further improve fiber depositioncontrol, it is imperative to find out which process parameters influence the fiber deposition error.The current electrospinning setup lacks the required position accuracy to successfully investigatethis. Therefore, a new setup must be designed.

Design requirements for such a new electrospinning setup have been set. Two designs that meetthese requirements were generated, one with a planar and another with a cylindrical configuration.A cylindrical configuration was determined to be superior and was further developed. It consistsof a rotating, thin-walled glass, cylindrical collector that can also translate along its center axis.This cylinder spinner was built and equipped with a homing mechanism to enable reproducibleresults. Furthermore, a motion control scheme was implemented.


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During testing of the new design two problems were encountered:

1. Focusing did not occur on the glass cylindrical collector.

2. The desired collector speed could not be reached due to mechanical problems in therotation axis.

Both problems were analyzed. It was discovered that the electrostatic properties of the design aresound, but that the relatively large wall thickness of the glass cylinder prevented focusing.Changing the spinning parameters had no effect on focusing. A new glass cylinder with a reducedwall thickness is unpractical due to issues with ease-of-handling, safety and durability. Throughexperiments it was determined that a thin-walled PET cylinder offers significantly better focusingwithout any of the before-mentioned complications and could therefore be a viable alternative tothe installed glass cylinder. To address the collector speed problem, it is recommended to use apolymer that requires a lower collecting speed. Alternatively, the rotation axis could beredesigned.


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Introduction

Electrospinning is a powerful and versatile technique for the fabrication of very thin fibers frompolymer solutions or melts. The resulting fibers are continuous and have a uniform diameter. Thisdiameter can vary from tens of micrometers down to a few nanometers, depending on theprocess parameters. Furthermore, fibers with both solid and hollow interiors can be achieved. It isalso possible to give the fibers special properties by adding metallic, ceramic or even biologicalcompounds such as proteins or DNA to the source polymer material.

In literature, the technique is often compared to the traditional fiber spinning process where apolymer melt or solution is extruded through a die with a small hole. In electrospinning, however,instead of using mechanical forces to form the fibers, electrostatic forces are employed. Theadvantage of this contactless drawing is that fibers with significantly smaller diameters can beproduced.

As mentioned before, both polymer solutions and melts can be used as a source material, but inthis report we will focus on electrospinning using polymer solutions.

In a typical procedure (see Fig. 1), a syringe with polymer solution is placed in an infusion pumpto generate a constant flow of fluid through the needle. This needle is connected to a metalliccapillary, the so-called nozzle , with a transport tube. The polymer solution is pumped through thetube and a small liquid drop appears at the tip of the nozzle. A high voltage is then applied to thenozzle, usually between 5 and 50 kV, while at some distance below the nozzle a groundedelectrode is placed. The potential difference generates an electric field between nozzle andelectrode.

Charged nozzle

Collecting material

Grounded electrode

High voltage supply

Jet

Taylor cone

Infusion pump

Syringe

Figure 1: A characteristic electrospinning setup

Due to the electric field, the shape of the drop starts to deform from the shape caused by surfacetension alone to a conical shape called a Taylor cone [1]. When the electric field is sufficientlyhigh and charge buildup in the cone reaches a critical level, the electrostatic forces will overcomethe surface tension of the polymer solution and a thin, viscoelastic jet is ejected from the capillarynozzle. Under the action of the electric field, the fibers are forced to travel towards the groundedelectrode. While the charged jet is travelling downwards, the polymer is continuously stretched bythe electrostatic forces. At the same time, the solvent evaporates. The combination of these twoeffects results in a significant reduction of the jets diameter.


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The jet does not follow a straight path to the electrode, however. Instead, shortly after exiting thenozzle, a chaotic oscillation occurs, caused by charge repulsion between material elements of the

jet and an aerodynamically driven bending instability. The oscillation is known as the whipping

phenomenon in literature [2]. Because of this phenomenon, the material is randomly deposited,creating a so-called nonwoven mesh (see Fig. 2). It is also possible to place a thin sheet ofcollecting material (also called collector ) on the electrode so that the electrospun material isdeposited on this sheet instead of directly on the electrode.

Figure 2: An electrospun nonwoven mesh [32]

Since the structure of the nonwoven mesh is essentially chaotic, its properties are hard to predictand characterize. Although a random structure is sufficient for some electrospinning applications,for others one would prefer or even require a better control over how the micro- or nanofibers areoriented.

Examples of application fields where orientation control could play a key role are: filtration [3, 4],sensors [5-8], electronics [9, 10] and tissue engineering [11-13]. In the filtration field, the pore-sizeof a filter is of prime importance as it determines the flow resistance and selectivity [14]. Makingthe fiber spacing in the mesh constant and reproducible would be very desirable. Such controlwould also allow researchers in the biomedical field to carefully optimize scaffold structures fortissue engineering. Finally, when one wants to create highly efficient sensors and electronicsbased on nanofibers, well-aligned and highly ordered architectures are required.

Control of the orientation of nanofibers is therefore very desirable and numerous methods withvarious levels of success are described in literature. Worldwide, many research groups haveexperimented with methods to control the orientation of fibers during electrospinning with variouslevels of success. An overview of these methods can be found in Table 1.

The methods can be divided into three groups based on how the orientation is achieved:mechanically, through electrostatic means or both. We define mechanical as using movement (i.e.rotation and/or translation) inside the setup to accomplish alignment, focusing and positioning ofthe fibers. When this is achieved solely through the shape of the setup, we define this aselectrostatic . Finally, if both means are important for the end result, we classify the method assuch.

The results from literature will be compared on two qualitative fronts: alignment and controllablespacing.


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T a

b l e 1 :

O r i e n

t a t i o n c o n

t r o

l m e

t h o

d s

i n l i t e r a

t u r e

M e

t h o

d

A l i g n m e n

t

C o n

t r o

l l a

b l e s p a c

i n g

A u

t h o r

R e

f .

M e c

h a n

i c a

l :

R o

t a t i n g w o o

d e n o r a

l u m

i n u m

f r a m e a s c o

l l e c

t o r

V

X

H u a n g

[ 1 5 ]

R o

t a t i n g

d i s k c o

l l e c

t o r

V

X

S u

b r a m a n

i a n

[ 1 2 ]

A l u m

i n i u m

f o

i l o n r o

t a t i n g c

i l i n

d e r

X

X

B h a

t t a r a

i

[ 1 3 ]

L i q u

i d b a

t h - a s - c o

l l e c

t o r

V

X

S m

i t

[ 1 6 ]

E l e c t r o s t a t

i c :

R i n g a u x

i l i a r y e

l e c

t r o

d e

X

X

J a e g e r

[ 1 7 ]

E l e c

t r o s

t a t i c l e n s a s

f o c u s

i n g e

l e m e n

t

X

X

D e

i t z e

l

[ 1 8 ]

C y l

i n d r i c a

l a u x i

l i a r y e

l e c

t r o

d e

X

X

K i m

[ 1 9 ]

A u x

i l i a r y e

l e c

t r i c f i e l d

V

X

H u a n g

[ 1 5 ]

E l e c

t r o

d e - g a p s p

i n n

i n g

V

X

L i / X i a

[ 2 0 ]

M e

t a l f r a m e e

l e c

t r o

d e

V

X

D e r s c

h

[ 2 1 ]

K n

i f e - e

d g e e

l e c

t r o

d e

i n n e e

d l e l e s s s p

i n n i n g

X

X

Y a r i n

[ 2 2 ]

M e

t a l g r i

d c o

l l e c

t o r

V

X

G i b s o n

[ 2 3 ]

B o t

h :

C o p p e r w

i r e

d r u m

c o

l l e c

t o r

V

X

K a

t t a

[ 2 4 ]

B o

b b i n c o

l l e c

t o r

V

X

T h e r o n

/ Z u s s m a n

[ 2 5 ]

C y l

i n d e r s p

i n n e r

V

X

S u n

d a r a y

[ 2 6 ]

X , Y , Z

- p o s

i t i o n e

d c o

l l e c

t o r

V

V

M i t c h e

l l

[ 2 7 ]

S c a n n

i n g

t i p s p

i n n

i n g

V

X

K a m e o

k a

[ 2 8 ]

N e a r -

f i e l d e

l e c

t r o s p

i n n

i n g

V

V

S u n

[ 2 9 ]

P a r a

l l e l g r i

d o

f a

l u m

i n i u m

s t r i p s a s e

l e c t r o

d e

V

X

T e o

[ 3 0 ]

K n

i f e - e

d g e

d b l a d e e

l e c

t r o

d e s

V

X

T e o

[ 3 0 ]

A l u m

i n i u m

f o

i l o n r o

t a t i n g c

i l i n

d e r

/ w c o p p e r e

l e c

t r o

d e s

V

X

B h a

t t a r a

i

[ 3 1 ]


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Alignment is defined as whether there is control over how parallel all fibers are positioned to eachother.

Controllable spacing is defined as whether or not it is possible to deposit two fibers next to eachother with a controllable spacing between them.

From the results in Table 1, it follows that achieving alignment of nanofibers along a single axis iscurrently well-understood. The controllable spacing, however, is a different issue. Only twogroups, [27] and [29], report that they can carefully control the distance between two fibers.

At Eindhoven University of Technology, a new technique has been devised and patented tocarefully control the position of the fiber deposition point. Instead of using a grounded plate aselectrode, a grounded needle is used which can be actuated in the plane (see Fig. 3). Thistechnique can potentially improve the alignment and degree of spacing control significantly.

The feasibility of the technique was demonstrated by Solberg [32]. He also implemented thetechnique in an experimental setup and showed that it is possible to align and deposit a single,continuous fiber on the collecting material by moving the collector with respect to the electrode.Fig. 4 shows a glass sample with neatly aligned electrospun fibers deposited on it using thissetup.

The experimental setup by Solberg has provided much insight into the electrospinning process.Due to its components, however, the attainable position accuracy of the electrode and collectingmaterial is limited and this prevents the precise fiber deposition that is required for advancedoriented fiber meshes necessary for many future applications. Therefore, a new electrospinningsetup is required.

Figure 3: Overview of new electrospinning techniqueusing an actuated grounded needle electrode

In this project, the following goals have been set:

Acquire more knowledge of the fiber deposition process for future improvements oforientation control.

Design, build and test a new electrospinning setup with higher position accuracy than thecurrent one which is capable of depositing straight fibers with an adjustable spacing in areproducible manner.


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Figure 4: Electrospun fiber spirals on a glass disc with a spacing of 1 mm [32]

This work will start with a description in Chapter 1 of the electrostatic mechanisms that play a rolein electrospinning. Then, a short overview is given of important process parameters, studiedpolymers and equipment in Chapter 2.

A number of experiments were performed to obtain more knowledge of the fiber depositionprocess. These will be discussed along with results in Chapter 3. The design of the newelectrospinning setup and its real-time motion control is addressed in Chapter 4. In Chapter 5,tests performed with the new setup are described. Finally, conclusions are drawn andrecommendations for future work given.


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Chapter 1: ElectrostaticsIn order to control the location of material deposition and improve deposition accuracy, goodknowledge of the underlying electrospinning process is required. Key to this control is the electricfield. First, the overall shape of the field is discussed based on electrostatics. Thereafter, anumber of effects that also influence the shape will be described to refine the situation description.

1.1: Electric field without dielectric

From an electrical standpoint, the electrospinning setup is composed of two elements: thecharged nozzle at the top and the grounded needle electrode at the bottom. This situation can bemodeled as an electric dipole. It consists of two point sources of charge, one at the tip of eachelement, with a certain separation distance h . A positive charge is applied to the nozzle; hencenegative charge accumulates on the electrode to maintain equilibrium. The potential difference V between both elements is kept constant.

h

Figure 5: (a) Electric field for two point sources. Blue lines represent the field lines withthe arrows indicating the direction of the electrostatic force. Red contour lines representthe equipotential lines. (b) The same plot, but incorporating a dielectric collector.

In Fig. 5a, the shape of the electric field is shown. The red dashed contour lines represent theequipotential lines. As the name suggests, all points on such a line are at the same electricpotential. The nozzle is at the maximum potential, while the electrode is at zero potential due togrounding. Hence, the potential decreases while moving downwards from one equipotential lineto the next. The blue solid lines represent the electric field lines and are always perpendicular tothe equipotential lines. By convention, the direction of the lines is from positive to negative charge.


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The closer the field lines are to each other, the stronger the field in that region. The field is thusstrongest near nozzle and electrode.

The relationship between potential difference and electric field strength is given by:

ld E V b

a

rr= (1.1)

Here V is the potential difference between the nozzle at location a and the electrode at location b

in [V]. E r

is the electric field strength in [V/m] and ld r

is an infinitesimal increment ofdisplacement on any line or curve between both points [m].

When we assume the electric field is uniform over the separation distance, like between twocharged parallel plates where fringing is ignored, the magnitude of the electric field strength canbe estimated using:

h

V E = (1.2)

with h in [m].

From this equation it can be concluded that the electric field strength is highest on the verticalfield line since the field line is shortest. In order to investigate what this means for theelectrospinning process, assume that a positive test charge is inserted in the field. The force thatthis test charge will experience can be calculated from:

E QF rr

= (1.3)

with the force F r

in [N], the test charge Q in [C] and the strength of the electric field E r

in [V/m].

The electrostatic force acts in the direction of the electric field and therefore the pulling force onthe vertical field line will be highest. All field lines converge towards the electrode and thus thepoint of the needle there will exert the largest pulling force.

Although in reality the electric field is clearly not uniform over the distance, the order of the fieldstrength can be estimated with Eq. 1.2. When we enter a characteristic V = 12 kV andh = 0.1 m, we arrive at a field strength of E = 1.210 5 V/m. This is one order of magnitude belowthe dielectric strength of air, E breakdown = 3.0 10

6 V/m, which is the field strength at which airbecomes electrically conductive [33]. The breakdown phenomenon puts a limit on how far thefield strength can be increased to accomplish focusing.

In the above text, we started from point charges. However, in reality both electrical elements havefinite dimensions and their shape influences how the electric field looks like. In Fig. 6, the electricfield around real needle or nozzle configurations is shown. The equipotential lines wrap aroundthe contour of the nozzles.

Figure 6: Electric field around two nozzle configurations:(a) An isolated nozzle. (b) A nozzle attached to a horizontal plate.


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1.2: Refinements

Field with dielectric:

In the electrospinning setup, we use a dielectric, which we have called collector to capture thespun polymer fibers. When a dielectric is inserted in an external electric field, the shape of theoverall electric field is not influenced, see Fig. 5b. Only within the dielectric itself, the electric fieldwill change. This effect is called polarization. The externally applied electric field will induce someseparation of charge in the dielectric molecules. Negative charge will orient itself towards theupper boundary of the dielectric; positive charge orients itself towards the lower boundary of thedielectric. Therefore, the net effect is as if there is a negative charge on the upper surface and apositive charge on the lower surface of the dielectric. A close-up of the dielectric, demonstratingthe polarization effect, can be seen in the following figure:

Figure 7: Molecular view of polarization within a dielectric

The field in the dielectric d E r

is a vector sum of the externally applied field ext E r

and the field

ind E r

due to the induced charge on the surfaces of the dielectric:

ind ext d E E E rrr

+= (1.4)

The strength of the induced field is given by a function:

),( 0 P f E ind rr

= (1.5)

The exact definition depends on the geometry of the dielectric. In this equation, 0 is the dielectric

constant of vacuum [C 2 /Nm 2] and Pr

is the polarization density vector in [C/m 2].

The level of polarization that occurs can be calculated from the following relation:

d e E Prr

0= (1.6)

where e is the electric susceptibility of the dielectric [-]. Note that the polarization depends on thetotal electric field inside the dielectric.

Since in the electrospinning setup the induced field and external field are in opposite directions,the field in the dielectric E d is weaker than the external field E ext .


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In the discussion above, we assumed that the dielectric is one layer of a single material. When

the collector consists of two or more layers of different materials, then ind E r

in those layers willdiffer from each other. However, as for one layer, the shape of the electric field outside of thedielectric will not be influenced.

The same holds for a collector that is not positioned exactly halfway between nozzle andelectrode, but closer to one of the two. Although the field within the collector is no longer uniform,the electric field outside the dielectric will keep its shape.

Introduction of the polymer:

During the electrospinning process, a drop of polymer solution is elongated and guided towardsthe grounded electrode by a combination of gravity and electrostatic force. Since the polymersolution is charged, in essence it can be seen as an extension of the nozzle. When the materialmoves towards the dielectric, this is equivalent to the dielectric moving towards the nozzle.The distance between the upper needle and dielectric decreases and thus the electric fieldstrength and consequently the electrostatic force increases. This leads to a distortion of theelectric field (see Fig. 8a). However, since the accumulated charge is hard to measure, theamount of field distortion is currently unknown.

Dielectric

Nozzle with polymer

(a)

Dielectric

Nozzle with polymer

(b) Figure 8: (a) Distortion of the electric field due to presence of polymer. (b) Contactsituation.

While the material travels downwards in the form of a jet, the solvent evaporates. Consequently,the conductive properties of the jet change, as the material changes from a solution to a solidifiedpolymer.

Contact situation:

When the polymer fiber comes into contact with the dielectric, a new situation occurs. A currentwill start flowing through the conductive polymer fiber.

E Qv I += (1.7)

where Q is the charge [C], v is the polymer flow velocity [m/s], the conductivity [m/ ] and E theelectric field strength [V/m].

The first term represents transport of charge due to movement of the polymer. The second termis conduction following Ohms Law. Whether this latter term plays a role depends on theconductive properties of the polymer when it comes into contact with the dielectric.


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Since the collector is fabricated from an electric isolator, the charge deposited by the polymercannot flow away (see Fig. 8b). Therefore, the collector will become locally charged because ofthe presence of charged fiber segments. These deposited segments will repel newly spun

material that arrives at the collector. On a flat collector, this leads to a radially expanding drop ofsolidified polymer as this distributes charge in the most optimal way [32]. The repulsion effect isexpected to be localized; however experiments are necessary to confirm this hypothesis.

The dielectric will not stay charged forever, however, as charge will diffuse into the surroundingatmosphere. Therefore, deposited segments will gradually lose their charge.


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Chapter 2: Process parameters, materials and cabinPosition-controlled electrospinning is a complex process and requires knowledge ofelectrohydrodynamics, chemistry, rheology and motion control to operate and fully address itspotential.

2.1: Process parameters

Many parameters play a role in the electrospinning process. These can be broken down inmaterial properties and operational parameters:

Material properties of the polymer:

Molecular weight Molecular weight distribution Architecture, such as linear, branched, etc.

Material properties of the solution:

Viscosity Electrical conductivity Surface tension

Operational parameters:

Electric field strength E Polymer solution feed rate f Distance between nozzle and collector d Distance between collector and electrode (airgap) a Velocities of the nozzle, collector and electrode Ambient parameters (temperature, humidity, cabin air velocity)

The electric field strength is determined by the applied voltage V and the layout of the setup.The deposition time period t plays a role in the characteristics of the produced mesh. A numberof operational parameters are shown in Fig. 9.

Figure 9: Operational parameters


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2.2: Materials

As mentioned in the first Chapter, the research described in this report focuses on using polymersolutions for electrospinning. A solution is made by dissolving polymer powder in a suitablesolvent. Huang [3] lists over forty different polymers that have been electrospun in solution formusing a wide range of solvents. In this research project, two polymer solutions were usedextensively. These are listed in Table 2.

Table 2: Used polymer solutionsPolymer Molecular weight Solvent Concentration

PEO (Polyethylene oxide) 400,000 Distilled water / ethanol(2:3) 8 wt%

PCL (Polycaprolactone) 200,000 Chloroform 18 wt%

Polyethylene oxide solution is by far the most used material in electrospinning studies as it isrelatively simple to prepare and use. Therefore, a lot is known about this material, its behaviourand properties. For these reasons, PEO was selected as main component of this research.Polycaprolactone is a biodegradable polymer used extensively in tissue engineering research,which is one of the proposed fields of application for oriented fiber meshes. A drawback of thismaterial is that it is commonly dissolved in chloroform, which evaporates during electrospinningand leads to harmful gas formation. As a result, the use of PCL solution was limited in this project.

2.3: Electrospinning cabin

All experiments have been performed in a dedicated cabin (see Fig. 10). This cabin is outfittedwith a high voltage supply that can be controlled from an operating panel. It also protects the user

from a number of risks.

Figure 10: Electrospinning cabin [32]


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One risk is the high voltage (up to 25 kV). When a user accidently comes into contact with anelectrified element of the setup, the voltage enables a current to flow through the body which isharmful and could potentially lead to death. Therefore, the cabin, which is equipped with a door

that provides access to the electrodes and other parts of the setup placed inside, is outfitted witha safety mechanism that shuts down the voltage when the door is opened.

In the previous Paragraph it was mentioned that harmful gases may be produced duringelectrospinning. Therefore, the cabin is connected to a forced ventilation system that capturesand leads any emitted gases away from the setup.


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Chapter 3: Pre-design experimentsIn this Chapter, an overview will be presented of all experiments that were performed to obtainmore knowledge of the electrospinning process and to assist in the design of a newelectrospinning setup. Additionally, the results of these experiments will be analyzed anddiscussed.

3.1: Introduction

In the work of Solberg [32], the feasibility of focusing the fiber deposition by using a thin groundedneedle electrode instead of a grounded plate electrode was shown through experiments.Furthermore, it was demonstrated that a single straight fiber could be produced by applying avelocity difference between collector, nozzle and electrode. These experiments will be taken asstarting point of this investigation to gain more insight into the electrospinning process.

3.2: Experiments and results

In the following sections, four sets of experiments will be performed. The goal of the first set ofexperiments, described in Paragraph 3.2.1, is to compare the mechanism of fiber deposition on agrounded plate electrode with that on a grounded needle electrode. In Paragraph 3.2.3, thedeposition on a moving collector is investigated. The goal of this experiment is to determinewhether fibers can be written with both PEO and PCL and if so what the properties are of thesefibers. Then, in Paragraph 3.2.5, a set of experiments is described to determine whether thecollector thickness influences the amount of deposition. The goal of the last set of experiments,described in Paragraph 3.2.7, is to find out whether a small airgap between collector andelectrode prevents, hinders or otherwise alters the deposition.

3.2.1: Fiber deposition: experiments

As mentioned previously, the goal of the first set of experiments is to compare the mechanism offiber deposition on a grounded plate electrode with that on a grounded needle electrode.

In the first test, a thin, blunt 23G Terumo injection needle with an inner diameter of 0.337 mm ispositioned in a brass holder. The combined needle and holder will be called nozzle henceforth.This nozzle is placed in a stand with a variable height and connected to the plus terminal of theelectrospinning cabins high voltage supply. Directly below the nozzle, a circular, copper plateelectrode with diameter of 5 cm is placed. The plate electrode is connected to the minus terminalof the cabin. A Harvard PHD 2000 infusion pump is used to feed PEO solution (see previousChapter for properties) to the nozzle via a Teflon tube. Thin mylar sheets (thickness of 0.01 mm)and thin glass samples (thickness of 0.55 mm) are used as collecting material and placed indirect contact with the grounded plate electrode. In Fig. 11a, the setup used for this test isdepicted.

As preparation for the experiment, the infusion pump is switched on so that a constant feed ofpolymer solution begins to flow through the tubing. During experimentation, the pump is neverturned off as the polymers under study are viscous and we want to minimize transient behaviourof the fluid. Once the solution is ejected in a steady-state manner from the nozzle, experimentscan take place. Paper covering that keeps the electrode clean is removed and the collectingmaterial is placed on the electrode. The experiment starts when an uniform drop has formed andthe high voltage is applied. After spinning for a set duration, the high voltage is switched off, thecollecting material removed and the cabin and equipment cleaned with ethanol to remove anyremaining charged polymer strands that could disturb subsequent experiments. All experimentsare performed at ambient cabin temperature (22.4C) unless noted otherwise.


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Figure 11: Fiber deposition setup. (a) With grounded plate electrode. (b) With groundedneedle electrode.

In the second test, the plate electrode is exchanged for a grounded, conical-shaped needleelectrode with an outer diameter of 0.9 mm (see Fig. 11b). All other elements of the setup andprocedure are kept the same.

For these tests, the spinning parameters are chosen such that a stable polymer jet is establishedand deposition is clearly visible. Note that the main focus of the present tests is not to comparedeposition for different parameters, but rather to assess the deposition mechanisms that occur.In the following table, the spinning parameters for both tests are summarized for reference, whereV is the applied voltage in kV, f is the polymer solution feed rate in [ l/min], t is the time period inwhich deposition occurs in [s] and d is the vertical distance between nozzle tip and collectingmaterial in [cm].

Table 3: Spinning parameters for fiber deposition experimentsExperiment Electrode type Collecting material V [kV] f [l/min] t [s] d [cm]

1 plate mylar 17 18 120 132 plate glass 17 18 120 133 needle mylar 17 18 60 124 needle glass 17 8 120 12

These tests are different from those described by Solberg [32] as a higher V and different valuesfor f have been used. Furthermore, in this research both mylar and glass are studied, while [32]

solely studied mylar.3.2.2: Fiber deposition: results

The first results that will be discussed are those of the fiber deposition experiments.

In experiments 1 and 2, where a plate electrode is used, it is observed that the polymer jet isejected straight down from the Taylor cone. Several centimeters above the collecting material, the

jet starts to diverge from its vertical path and an oscillating haze is observed. The first segment ofthe incoming polymer has no preference for a specific point of the plate electrode and is thusdeposited randomly on the collecting material. Upon deposition, the segment retains a residualcharge that exerts a repulsive force on subsequent segments and sweeps them to anotherlocation. As a result, the collecting material is covered with randomly deposited fibers that


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ultimately form a nonwoven fabric. Images of these results are shown in Figs. 12.1 and 12.2,respectively. Note that special care was taken to visualize the delicate fiber deposition usingoptimal camera angles and with optimal lighting conditions and these might differ from sample to

sample. The mylar sheets and glass samples shown in the images of this Chapter areoccasionally kept up with black foam tubing or a combination of thin white and red strips and pinsto facilitate photography. These aids are not important and should be ignored. The bright, littledrops in the center of the photographs in this Chapter are caused by the initial polymer drop whenthe voltage is applied or the last drop after the voltage has been removed. They are not part ofthe main deposition process we want to study and should therefore be ignored.

Figure 12: Photographs of collecting materials after deposition experiments

Different behaviour is observed in experiments 3 and 4 where a needle electrode is used (seeFigs. 12.3 and 12.4, respectively). The whipping phenomenon is significantly reduced and,although some haze is observed, the fiber is first collected very close to or, under ideal conditions,exactly above the needle electrode. The circular deposition spot grows in diameter with time. Thiscan be explained by the fact that the fiber, which has been deposited on the sheet, retains anamount of residual charge which exerts a repellant force. After the first fiber segment has beencollected, the continuous deposition leads to pile up of new fiber material at the initial deposition


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point which ultimately buckles. The already deposited fiber segments charge pushes the newfiber away to another location. As more fiber is collected, the repellant forces will direct newmaterial further out of the center of the deposition spot. The spot remains circular, however, since

the needle electrode is still attracting charged fiber segments. From these experiments, it can beconcluded that a needle electrode focuses fiber deposition. After a limited time, material is nolonger deposited directly above the electrode because of repulsive forces. A solution to thisproblem is to move already deposited fiber segments away from the deposition point and putuntouched collecting material in the path, so that incoming fiber segments stay attracted to the tipof the needle electrode and focusing is maintained.

All photographs show similar circular deposition although the deposition mechanisms of a plateelectrode and a needle electrode, as described earlier, are clearly different.

3.2.3: Writing: experiments

In the previous section, focusing on a stationary collecting material was investigated. To obtain asingle straight fiber and create oriented fiber meshes, however, it is necessary to create a velocitydifference between nozzle and electrode on one hand and the collector on the other hand. In thissimple test, the grounded needle electrode setup introduced earlier is reused. PEO and PCL willbe selected as polymer solutions. The same experimental procedure is followed with theexception that a long strip of mylar sheet is used which protrudes from the electrospinning cabinthrough a slit on one end. During electrospinning, the strip is manually pulled further out of thecabin with a velocity of several cm/s. In this way, the mylar translates over the needle electrodewhile it keeps in direct contact with it (see Fig. 13). Deposited fiber is thus transported away fromthe deposition point above the electrode and we expect that focusing is maintained.

Figure 13: Writing experiment

The writing experiment is prepared by varying the spinning parameters and selecting those thatyield the most pronounced polymer tracks on mylar for both polymers. After selection, thewriting test is performed three times for PEO and one time for PCL with the following parameters:

Table 4: Spinning parameters for writing experimentsPolymer solution V [kV] f [l/min] d [cm]

PEO 20 18 13PCL 22 30 7


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3.2.4: Writing: results

In Fig. 14, a photograph of the mylar sheet used in the PEO writing test is shown. In this image,

two nearly vertical white lines can be seen, marked with two white arrows. These lines are theparallel polymer tracks written on the sheet. The left track is curved over the bottom half of itslength because it was not pulled in a entirely straight, continuous motion. The right track howeveris significantly better due to better steering of the sheet. It is observed that the polymer fiber isdeposited in an entwined, thread-like fashion as the horizontal velocity of the sheet is too low tocollect the fiber as a straight, single fiber.

The result of the PCL writing test is presented in Fig. 15. Here, only one polymer track was spun,again marked with white arrows.

Figure 14: Writing test result for PEO. Height of image = 10 cm.

Figure 15: Writing test result for PCL. Length of image = 12 cm.


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From these experiments, it is learned that it is possible to write polymer tracks on mylar sheets.These results correspond with those of Solberg [32]. By increasing the collection velocity, itshould therefore be possible to write a single, straight fiber on a collector.

Figure 16: Topography of PEO sample

Figure 17: Topography of PCL sample

To learn more about the properties of these fibers, the samples were analyzed with confocalmicroscopy and interferometry using a Sensorfar PLu 2300 optical imaging profiler. Severallocations along the fiber were selected and the diameter was measured. Interestingly, themeasurements showed that the fibers did not have circular cross sections, but rather a


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rectangular shape. The PEO fibers had a height of 380 nm and a width of 8 m, thus a h:w ratioof 1: 21. The PCL fibers had a height of 14 m and a width of 27 m, which is a h:w ratio ofapproximately 1:2. Results are shown in Figs. 16 and 17 for PEO and PCL, respectively.

3.2.5: Influence of collector thickness: experiments

One research question that has not been properly addressed in electrospinning research atEindhoven University of Technology is whether the collector thickness influences the amount ofdeposition. To investigate this question, the grounded plate electrode setup will be used andstrips of glass and polycarbonate with varying thickness are used as collector material and placeddirectly on the electrode, while all other spinning parameters are kept constant. The amount ofdeposition on the samples will then be compared qualitatively. The plate electrode is chosen overthe needle electrode as the former gives a more pronounced deposition effect in the same timeframe.

Different collector thicknesses lead to different nozzle-collector distances when the verticaldistance between these two points is not adjusted. It is assumed for these experiments that thisdifference of a few millimeters is negligible when considering a vertical distance of 5-15 cm andwill not lead to different electrospinning behaviour.

Four experiments will be performed, as two polymer solutions (PEO and PCL) and two collectormaterials (glass and polycarbonate) will be investigated. For each experiment, either two or threecollector thicknesses are compared. In the following table, the spinning parameters aresummarized. Here t 1, t 2 and t 3 are the thickness of the collectors that are compared in [mm].

Table 5: Spinning parameters for collector thickness experimentsV [kV] f [l/min] d [cm] t 1 [mm] t 2 [mm] t 3 [mm] t [s]

Experiment 1: PEO with glass as collector material:17 18 13 0.55 1.1 - 120Experiment 2: PEO with polycarbonate as collector material: 17 18 13 1.0 3.0 - 120Experiment 3: PCL with glass as collector material: 12 17 7.8 0.2 1.4 - 60Experiment 4: PCL with polycarbonate as collector material: 12 17 8.4 1.0 2.0 3.0 60

3.2.6: Influence of collector thickness: results

In Fig. 18, an image of the first experiment is shown. The glass sample on the left has a collectorthickness of 0.55 mm, while the sample on the right has a collector thickness of 1.1 mm. In thephotograph, it can be observed that the PEO deposition on the left sample is denser and optically

whiter than on the right sample.The results of the second experiment are shown in Fig. 19. The deposition is marked with blackcircles. This experiment was performed using polycarbonate as collector and it was observed thatit is a lot harder to spin on polycarbonate than glass as it seems to repulse incoming polymer jets.When we compare deposition on both samples, the thinnest strip of polycarbonate (shown on theleft) is covered with the most polymer deposition.

The results for PCL are more pronounced than those for PEO. In Fig. 20, two circular glasssamples are shown with a thickness of 0.2 mm on the left and 1.4 mm on the right. The density ofpolymer on the left sample is significantly higher and the deposition is more focused than on theright sample where it appears more spread out, even though the plate electrode is used for both.


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Figure 18: Collector thickness - experiment 1

Left: t = 0.55 mm. Right: t = 1.1 mm.

Figure 19: Collector thickness - experiment 2Left: t = 1.0 mm. Right: t = 3.0 mm.

Figure 20: Collector thickness - experiment 3Left: t = 0.2 mm. Right: t = 1.4 mm.


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Fig. 21 shows the results of the last experiment where polycarbonate is used as collector in threethicknesses. The deposition is marked with black circles. Again we observe that the thinnestsample is covered with the most polymer fibers and this amount of deposition diminishes nicely

with increasing collector thickness.From these experiments, it can be concluded that the collector thickness does indeed play a roleand reducing this thickness leads to a more pronounced deposition. This can be understood bytaking into account that the collector partially shields off the electrode and thus locally reduces thefield strength. It acts as a sort of resistor in the field. This reduces the electrostatic pulling forceand therefore the amount of deposition. The shape of the overall field is not changed, however.

Figure 21: Collector thickness - experiment 4Left: t = 1.0 mm. Middle: t = 2.0 mm. Right: t = 3.0 mm.

3.2.7: Influence of collector-electrode airgap: experiments

Another interesting question is whether a small airgap between collecting material and electrodeprevents, hinders or otherwise alters the deposition. To address this question, the groundedneedle electrode setup is used. PEO solution is selected as polymer source and mylar ascollecting material. We will compare deposition on samples in direct contact with the electrode todeposition on samples with a small collector-electrode airgap a of 1 or 2 mm. The applied voltageV is set to 17 kV, the polymer solution feed rate f to 18 l/min and the deposition period t waskept constant at 60 s.

Four experiments will be performed. In each experiment, deposition on a sample in direct contactwith the electrode is compared to one with an airgap, while all other conditions are kept the same.The parameters are summarized in the following table:

Table 6: Spinning parameters for airgap experimentsExperiment d [cm] a [mm]

1 12 12 13.5 13 13.5 14 13.5 2

3.2.8: Influence of collector-electrode airgap: results

All four experiments show similar results. However, to conserve space only one representativeresult will be presented. Photographs of the first experiment are presented below. In Fig. 22a, theresult for direct contact is shown, with the result for a 1mm airgap in Fig. 22b.


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Both pictures show a similar circular deposition with some fibers that are deposited radiallyoutward from the deposition point on these 6 x 6 cm mylar sheets. One possible explanation of

the deposition of this radial arrangement is that the fibers are created when the area immediatelyabove the needle electrode is covered with deposition. The electrode is essentially shielded offand thus the polymer jet follows the electrostatic field lines which now curve from nozzle towardselectrode over the edge of the mylar sheet. The fibers dry before reaching their ultimate target.

Figure 22: Characteristic result of airgap experiment

Two conclusions can be drawn from these results:

1. No apparent visible difference was detected between deposition on mylar where therewas direct contact and where an airgap between mylar collector and needle electrodeexisted.

2. The airgaps used in this section do not prevent focusing of polymer nor hinder depositionon the mylar collector.

3.3: Design implications

From the experiments in this Chapter we have learned several things that are important whendesigning a new electrospinning setup:

The collector should be as thin as possible to maximize focusing power. A small airgap between collector and electrode of 1 or 2 mm does not hamper focusing.


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Chapter 4: Design of a new electrospinning setup4.1: Introduction

To achieve the precise fiber deposition necessary for advanced oriented meshes, a newelectrospinning setup is required with a significantly higher position accuracy of the depositionpoint than currently available. In this Chapter, the design requirements of such a setup arepresented. Subsequently, a number of potential configurations are generated and evaluated.Then, one design is chosen and elaborated. Finally, motion controllers are designed and agraphical user interface is constructed that controls the spinner in real-time.

4.2: Design requirements

The new design should meet the following requirements:

1. Collecting speed 8 m/s

The goal of this project is to create an electrospinning setup capable of depositingstraight fibers with adjustable spacing in a reproducible manner. The straightness offibers depends on the velocity difference between the nozzle and electrode on one handand the collector on the other hand. If this velocity difference is too low, an entwinedthread is produced instead of a single, straight, continuous fiber. For PEO, the polymersolution we would like to study, Solberg [32] showed that this speed difference must be atleast 8 m/s for the spinning parameters he used. In order to be certain that straight fibersare producible, the new electrospinner should be able to reach this collecting speed.

2. Nozzle and electrode placed in-lineIn the electrospinning process, a charged polymer fiber flows from the nozzle to theelectrode. The fiber needs to be deposited on the collector exactly above the electrode.When the nozzle and electrode are not in-line (see Fig. 23), a deposition error occurs.

nozzle

electrode

collector

desired deposition point

actual deposition point

Figure 23: Deposition error

This is because, first of all, sideways bending of the fiber is limited and breakup occursfor excessive misalignment of nozzle and electrode. Secondly, the charged materialfollows the electrostatic streamlines which do not flow through the desired depositionpoint. To make the deposition process as simple as possible, it is desirable to have thecharged fiber travel directly downwards from the nozzle toward the electrode. Tominimize the deposition error, the nozzle and electrode should therefore be in-line andmechanically coupled, for instance by a shared support structure.

3. Minimize influence on electric field As mentioned before, the electric field determines where a nanofiber is deposited. Hence,to ensure a proper working of the setup, the disruption of the electric field should beminimized. Therefore, all design components should either be made of electric isolators


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or, where this is not possible, these conducting elements should be shielded off.

4. Compatible with existing infrastructure

Electrospinning experiments are always conducted in a dedicated cabin which protectsthe user from exposure to high voltages and is outfitted with a dedicated ventilationsystem. The new electrospinner should fit within this existing cabin. The maximumdimensions of the new design are therefore: length < 0.48 m, width < 0.66 m, height V ,then vertical polymer lines will appear. When 0=V and 0> x& , horizontal lines will appear.When V and x&are chosen equal, a 45line is deposited (see also Fig. 26). Hence, for an arbitraryangle , we can write:

xV &

=)tan( (4.3)

From this same figure, it can be concluded that the deposition angle can also be expressed inthe spacing:

s R

2

)tan( = (4.4)

Combining Eqs. 4.3 and 4.4 leads to:

V x R

s & 2= (4.5)

With Eq. 4.5 it is possible to calculate the spacing s for a certain translation velocity of thecylinder. This equation can also be rewritten to obtain the translation velocity as a function ofspacing:

RVs

x 2

=& (4.6)

4.4: The cylinder spinner

After comparing both configurations, the cylinder configuration is selected. Now, the newelectrospinner can be designed in detail. A concept drawing is shown in Fig. 27. The stationarypart consists of a tower from which a nozzle arm and electrode arm protrude. The translating partconsisting of the cylinder and its drivetrain will slide over the electrode with a small airgap of1 mm between cylinder interior and electrode. Several actuator mechanisms were consideredand a ballscrew drive was selected. The translating part of the spinner will be mounted on thisdrive.


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Figure 27: Cylinder spinner concept drawing (translation mechanism not shown)

4.5: Material selection

The rotating cylinder will be made from glass with a wall thickness of 2.5 mm. The material is anelectric isolator, readily available, easy to machine and resistant to aggressive solvents which areoccasionally used in electrospinning.

The frame of the cylinder spinner is built from thick polymer sheets. Polyvinylchloride (PVC), withits good electric isolation properties, was selected for the majority of frame. Polyetheretherketone(PEEK) was chosen for the electrode arm and polycarbonate (PC) for the nozzle arm because ofthe shape retention and moisture resistance properties of these materials.

4.6: Rotation axis

At one of the endpoints of the glass cylinder, a PVC plug is inserted and connected to a MaxonRE25 (type 118746) electromotor. A WM Berg bellow coupling is used to prevent alignmentproblems. The electromotor rotates the cylinder up to 3820 rpm, which corresponds with 8 m/s.The cylinder is supported by two metal groove contact bearings. A Maxon HEDL5540 encoderwith 500 counts/turn is mounted on the motor to control the angular velocity of the cylinder. Thisoptical encoder is preferred over a tachometer because we want to be able to control thedeposition position, not just the angular velocity. For data acquisition and interfacing with themotor and encoder, a TU/e Microgiant is used.

4.7: Translation axis

The translating part in the cylinder spinner design will be mounted on a NSK Monocarrier ball

screw actuator (type MCM05-020-H05K) with a stroke of 200 mm, ball screw lead of 5 mm,repeatability of 10 m and backlash < 20 m. The ball screw is connected to a Maxon RE30 (type268219) electromotor with ceramic gear ( Maxon , GP32C) via another WM Berg bellow coupling.A Maxon HEDL5540 encoder with 500 counts/turn is used as sensor. For data acquisition andinterfacing with the motor and encoder, the same TU/e Microgiant as for rotation is used. Assoftware we employ Matlab Simulink .

4.8: Realisation

The cylinder spinner design was built in Mechanical Engineerings departmental workshop.Images of the realized design are presented below. In Fig. 28, the stationary part is shown withthe main frame in black PVC. The beige PEEK beam that is mounted to the frame is the electrodearm. The electrode is made from brass and has a diameter of 0.6 mm and length of 9.5 mm. The


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matte, glass-like PC beam above it is the nozzle arm. The brass nozzle itself is on the outer edgeon the right. The Teflon polymer transport tube is visible, coming out of the nozzle. The verticalposition of the nozzle arm can be adjusted to change the nozzle-collector and nozzle-electrode

distances. The spindle, carriage, motor and encoder for the spindle axis are also visible in thisphoto, but these belong to the translating part which is depicted in Fig. 29.

Figure 28: Stationary part

The translating part consists of an U-shaped frame on which the glass cylinder is mounted. Thecylinder is held up by a large bearing on the left side, which is mounted in a vertical stand, andthe black PVC plug on the right side. The plug is connected to the rotation motor and encoderwhich are housed in the black box on the right side of the picture.

The assembled spinner, where the translating part has been mounted on the carriage, can beseen in Fig. 30. A back-side view of the spinner is shown in Fig. 31. The three wired protrusionswith orange tags at the bottom of the image are the homing switch on the left (marked with zero)and the two end stops (marked with one and two). More information on the elements can befound in the next section. In Fig. 32, the spinner is placed in the electrospinning cabin. Theinfusion pump is located left of the cabin while the amplifiers and control hardware are locatedright of the cabin.


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Figure 31: Electrospinner (back view)

Figure 32: Electrospinner placed in cabin


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4.9: End stops, homing strategy, endpoint detection

As described before, the cylinder spinner consists of two parts: a rotating cylinder and a

translating slide. The cylinder is mounted on top of the carriage of the slide. A long, u-shapedcover is also mounted on the carriage of the ball screw actuator to shield off the slides ball screwbelow and prevent it from collecting polymer strands.

The stationary support of the cylinder is outfitted with two Crouzet microswitches which areplaced at each extremity of the slide. These limit points of the slides travel are marked as A andB in Fig. 33.

A pin protrudes from the cover and can come into contact with a switch. The switches are wired insuch a way that they cut the current to the translation electromotor when one comes into contactwith the pin. As such, they act as end stops and provide mechanical safety.

Figure 33: End stops

It is practical to have a homing procedure to make sure that electrospinning always starts from awell-defined position and yields a reproducible result. Also, we want to maximize the distanceover which we can electrospin.

There are several methods to home the cylinder spinner. Lets assume we want to home toposition A, and then do spinning while the slide moves from A to B. Since we know the directionof movement from the encoder data, one way of homing is to move towards and touch end stop 1.There is a problem with this approach however since once the slide runs into an end stop, anymotor current is interrupted and thus the user is required to manually remove the slide from itslocked position.

Another strategy is to add an additional microswitch as reference point. The next step is then tochoose the location of this reference point. It is possible to place the switch in the middle betweenthe endpoints of the slide. The problem with this, however, is that it is possible the spinner is in aposition just past the midpoint on the right side when we want to spin from position B.

The best solution is to move the spinner in one set direction upon initialization, i.e. towards A,and place the reference point just in front of the associated end stop. With this set up, just asingle microswitch is required for homing. This solution was implemented in the electrospinnerdesign (see Figs. 31 and 34).

Figure 34: Bottom view of endstops (left) and homing switch


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The current spindle position of the spinner is logged by the motion control and this software shutsthe movement down once the endpoint of the slide is reached.

4.9: Control design

4.9.1: System identification

To enable cylinder motion with high position accuracy, controllers will need to be developed.Frequency response function measurements were done for both axes to get a good view ofsystem dynamics. The FRF of the translation axis ( spindle ) is shown in Fig. 35. A constantvelocity of 1.5 rad/s was supplied as reference. This corresponds to a constant movement of thespindle of 1.2 mm/s (see Paragraph 4.10). A chirp signal with a decreasing frequency from 2000Hz to 1 Hz was used as noise signal. The chirp signal resulted in better coherence than astandard Gaussian noise signal over a larger range of frequencies. The response was measuredfor 100 s with a sampling frequency of 4000 Hz.

100

101

102

103

104

-200

-100

0

100

M a g n

i t u

d e

[ d B ]

100

101

102

103

104

-200

0

200

P h a s e

[ d e g

]

100

101

102

103

104

0

0.5

1

C o

h e r e n c e

[ - ]

Frequency [Hz]

Figure 35: Plant FRF for spindle (fs = 4000 Hz)

The magnitude plot of the FRF shows a slope of -2, which corresponds with a simple movingmass. In the phase plot we see that delay is present in the system. This delay was modeled withthe following transfer function:

d T je j H =)( (4.7)

where T d is an aggregate of all effects that contribute to delay, such as discretization andcomputation time. Fitting this model on the phase loss yields a delay of 0.65 ms.


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100

101

102

103

104

-100

-50

0

50

M a g n

i t u

d e

[ d B ]

100

101

102

103

104

-200

0

200

P h a s e

[ d e g

]

100

101

102

103

104

0

0.5

1

C o

h e r e n c e

[ - ]

Frequency [Hz]

Figure 36: Plant FRF for cylinder (fs = 4000 Hz)

The FRF of the rotation axis ( cylinder ) is shown in Fig. 36. Going from low frequency to highfrequency, we see a -2 slope, an anti-resonance at 55 Hz, a resonance at 600 Hz and finallyagain a -2 slope. A sine wave with frequency of 0.1 Hz and amplitude of 12 rad was suppliedas reference, while a white noise signal with variance of 0.5 was used as noise signal. Theresponse was measured for 60 s with a sampling frequency of 4000 Hz. A similar fittingprocedure as before yields a delay of 0.80 ms.

4.9.2: Inter-axis disturbances

To see whether movement of one axis registers on the encoders of the other axis and vice versa,a quick experiment was performed. The same reference signals as in the previous section weresupplied. First, a reference was supplied to the spindle and the cylinder encoders were checked.Thereafter, a reference was supplied to the cylinder and the spindle encoders were checked. Nointer-axis disturbances were measured. This shows that both axes can operate independently.

4.9.3: GUI and real-time control

A real-time motion controller has been developed with Matlab Simulink. This controller consists ofa graphical user interface (GUI) and real-time code which is generated from a Simulink model.The GUI is shown in Fig. 37. With this GUI, the user can independently start the translating androtating axis, perform a homing procedure and stop the spinner.


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Figure 37: GUI

4.9.4: Feedback control

Since we want to be able to control where on the cylinder a fiber is deposited, position control willbe implemented for both axes.

First, a feedback controller will be designed for the spindle. As a starting point, a desiredbandwidth of 30 Hz was chosen. By analyzing the open-loop, we notice that the system needsphase. Therefore, a lead-filter is required ( K = 24, z = 30/3 Hz, p = 90 Hz). Furthermore, toremove steady-state errors, a weak integrator ( z = 6 Hz) needs to be implemented. Finally, a low-pass filter ( p = 300 Hz) is necessary to reduce the influence of measurement noise.Secondly, a feedback controller will be designed for the cylinder. As a starting point, a desiredbandwidth of 10 Hz was chosen. By analyzing the open-loop, we notice that the system needsphase. Therefore, a lead-filter is required ( K = 1.2, z = 10/3 Hz, p = 30 Hz). From the FRF, it isdetermined that it is desirable to also include a notch to combat the effects of the resonance at600 Hz and a low-pass filter to reduce measurement noise influences. However, stablecontrollers turned out to be unstable after implementation. Therefore, as controller, the beforementioned lead-filter was used.

An overview of performance is shown in the following table:

Table 7: Performance for both axesAxis Bandwidth [Hz] Modulus margin [dB] Phase margin [] Gain margin [dB]

Spindle 49.7 6.0 36.5 9.9Cylinder 3.7 3.1 95.7 11.3

Since the spindle will be operated with real-time control that specifies and changes a second-degree setpoint on-the-fly, feedforward control will not be implemented. The complete controlscheme is shown in Fig. 38.

To test the system, suitable references need to be supplied. A spacing of 100 m was selectedas target. This target can be achieved for 4108 = x& m/s and V = 1 m/s ( = 50 rad/s). In Fig.39, reference signals r 1 and r 2 are shown for the spindle and cylinder axis respectively which arenecessary to achieve this target. The homing procedure takes up the first 50 s of the reference.


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Then, the spinning procedure starts. For r 1, a constant velocity of4108 = x& m/s is supplied. For

r 2 , a smooth third order reference is supplied that accelerates for 100 s until it reaches V = 1 m/s.

Figure 38: Simulink control scheme

The associated error signals in time domain are shown in Fig. 40. The position error of thespindle e 1 is roughly 5 m in the spinning interval. The position error of the cylinder e 2 reaches asteady state value of 1 rad at 150 s. This value is unsatisfactory, but cannot be improved at timeof writing.

0 50 100 150-0.1

-0.05

0

0.05

0.1

r 1 [ m ]

0 50 100 1500

500

1000

1500

2000

2500

r 2 [ r a

d ]

t [s]

Figure 39: Reference trajectory for s = 100 m

u18

e17

r16

y15

u24

e23

r22

y21

stop2

stop2

stop1

stop1

SystemIO

u1 [V]

u2 [V]

y1 [rad]

y2 [rad]

homing [-]Subsystem1

startaccvel

posRef3

StartRef3

r

Spindle Ref

r1

Spindle Ctrl

e1 u1

Cylinder Ctrl

e2 u2


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50 60 70 80 90 100 110 120 130 140 1502

4

6

8

10x 10

-6

e 1

[ m ]

50 60 70 80 90 100 110 120 130 140 1500

0.5

1

1.5

e 2

[ r a

d ]

t [s]

Figure 40: Measured error signals (time domain)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-120

-110

-100

-90

-80

-70

-60

-50

-40

Frequency (kHz)

P o w e r

/ f r e q u e n c y

( d B / H z

)

PSD of e 1

Figure 41: Measured error signal e1 (frequency domain)


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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

Frequency (kHz)

P

o w e r /

f r e q u e n c y

( d B / H z

)

PSD of e 2

Figure 42: Measured error signal e2 (frequency domain)

To learn more about the error signals, a frequency analysis was performed. The results areshown in Figs. 41 and 42. The results can be explained by taking the reference trajectories intoaccount. The references have no high-frequency content and will therefore not excite the systemat high frequencies. Therefore, only errors with low frequencies are expected. This is also whatfollows from the figures.

4.10: Spacing error

Quadrature encoders are used as position sensors. Encoders for both axes have a resolution of500 counts/revolution. The encoder of the spindle is connected to the motor via a gearbox with areduction of 23/4:1. The spindle has a lead of 5 mm, thus the translating part moves 5 mm forevery revolution. One count of the spindle encoder then corresponds with:

rad 41046.54 / 234500

2 =

(4.8)

mrevm 71035.4

4 / 234500

/ 005.0 =

(4.9)

The cylinder encoder is directly coupled to the encoder and one count thus corresponds with:

rad 0031.04500

2=

(4.10)

With this information, a measured constant position error e in [rad] can be related to the erroralong the associated axis. The error in the spacing can then also be calculated.


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We define the spacing error s as the difference between the desired spacing s and the realizedspacing s :

sss = (4.11)

The desired spacing can be calculated from the desired translation velocity x&and the realizedcollecting speed V :

V x R

s & 2= (4.12)

The realized spacing can be calculated from the realized translation velocity x&and the realizedcollecting speed V :

V

x Rs

2

& = (4.13)

As the derivatives of the positions of the translation and rotation axes were very noisy, it was notpossible to determine the realized spacing error at the time of writing.


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Chapter 5: Post-design experimentsNow that the cylinder spinner design has been realized, experiments can be performed to reviewits performance. In Paragraph 5.1, deposition experiments are performed on the stationarycylinder to determine whether focused deposition occurs testing a wide range of spinningparameters. Thereafter, in Paragraph 5.2, it is checked to see if the electric field disturbs thesensors and electromotors. A number of problems with the setup manifest themselves duringtesting. In Paragraphs 5.3 and 5.4, the causes of these problems are analyzed and somesolutions are presented.

5.1: Stationary deposition experiments on glass cylinder

5.1.1: Experiments

Now that the cylinder spinner is realized, deposition experiments can be performed with it. Thegoal of these experiments is to determine whether focused deposition occurs for a wide range ofspinning parameters. These initial experiments will be performed with a stationary cylinder. PEOis used as polymer solution. A mylar sheet is wrapped around the cylinder to collect the fiber.

Fourteen experiments will be executed, varying: the applied voltage V in a range from 12 to 25 kV,the feed rate f from 10 to 18 l/min and d the vertical distance between nozzle tip and collectingmaterial from 8 to 13 cm.

5.1.2: Results

All mylar films showed similar random depositions. In Fig. 43, a representative result is shown.Two black lines at the top and right edges of the cylinder were added to the sample to indicatethe location of the needle electrode. The electrode was placed at the interception point of theblack lines in the center of the image. The deposition resembles that of a plate electrode, but it

was checked that there was no contact between the cylinder and electrode. Note that there is agap in the deposition at the bottom of the image, because this was the location of a piece of tapeto stitch both edges of the mylar sheet together.

From tuning V and keeping other parameters constant we learn that the deposition for low V isless concentrated and less dense than for high V . The changes are marginal, though, and allresults are very similar to Fig. 43. An interesting observation was that for V > 22 kV the jet splitsin multiple sub-jets immediately after leaving the nozzle tip.

Tuning f and d did not enhance the focus of the deposition.

It had been expected that a well-defined spot of deposition would be formed, but this is not whatis observed. Clearly, this is a significant problem. Since spinning parameters were varied over awide range, it is highly likely that the lack of sharp focusing is due to the construction rather thanoperating conditions. In Paragraph 5.3, a number of experiments will be described to determinethe cause of this problem.

5.2: Testing with motion

As a first motion test, the cylinder spinner was supplied with standard position references for therotation and translation axes. Simultaneously, the realized positions and errors were recorded.Then, the electric field was powered up with an applied voltage of 10 kV and the sametrajectories were supplied. No significant differences in the position errors of both axes wereobserved. The test was repeated for 15, 20 and 25 kV with identical results. It was thereforeconcluded that the electric field has no influence on the encoders and motors and that themeasures employed to shield off the electric components are sound.


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Comparing results of 1 to 2:

In Fig. 44a, the result with combination 1 (grounded needle + nozzle out of plate + mylar) is

shown, while in Fig. 44b combination 2 (grounded needle + nozzle-arm + mylar) is represented.From the results it is learned that the nozzle arm of the cylinder spinner does not hamperfocusing. On the contrary, deposition seems to be more aggressively focused and resembleswriting.

Figure 44: Comparing deposition of:(a) Combination 1. (b) Combination 2.

Comparing results of 1 to 3:

In Fig. 45a, the result with combination 1 (grounded needle + nozzle out of plate + mylar) isshown, while in Fig. 45b combination 3 (electrode-arm + nozzle out of plate + mylar) isrepresented.

Figure 45: Comparing deposition of:(a) Combination 1. (b) Combination 3.


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In Fig. 47a, the result with combination 4 (electrode-arm + nozzle-arm + mylar) is shown, while inFig. 47b combination 5 (electrode-arm + nozzle-arm + glass) is represented. When switching

from the thin mylar to the relatively thick glass as collector, the deposition type changes fromfocused to random.

From the experiments in this Paragraph it can be concluded that the electrostatic properties of thenozzle and electrode arms are satisfactory. The glass cylinder is the element that prevents properfocusing.

5.4: Stationary deposition experiments on PET

Through experiments it was discovered that the glass cylinder with a thickness of 2.5 mmprevents proper focusing of polymer fibers. While one can produce a glass cylinder with a smallerwall thickness, one has to take the operating conditions into account. The rotation axis of thecylinder spinner operates at high angular velocities and therefore a thinner glass tube might beproblematic. Aspects like ease-of-handling, safety and durability could become issues. Inelectrospinning experiments we wrap a thin sheet of collecting material around the cylinder onwhich the deposition takes place. A thin glass tube breaks easily and special care must be takenwhen mounting the collecting material. As mentioned before, high angular velocities are used anddesigning a setup that does not vibrate during motion and does not lead to failure of the glass isnontrivial.

Rather than installing a new cylinder with smaller wall thickness, it was deemed more prudent toinvestigate the possibility of using a different cylinder material. Polyethylene terephthalate (PET)was selected because it cannot break, is easy to process, readily available and essentially thesame material as mylar.

5.4.1: Experiments

Cylindrical PET samples with a diameter of 8.5 cm, length of 9.5 cm and wall thickness of0.25 mm were obtained from the departmental workshop. For each experiment, a new PETsample will be mounted in the cylinder spinner setup to replace the glass cylinder. An airgap of 10mm between PET collector and electrode is established. PEO solution will be electrospun on thesamples using the following spinning parameters:

Table 8: Spinning parameters for PET deposition experimentsV [kV] f [ l/min] t [s] D [cm] a [mm]

17 18 60 11 10

5.4.2: Results

It was discovered that it is relatively easy to electrospin on PET cylinders using the parameters inthe previous section. Even with an airgap of 10 mm, which is significantly larger than the airgapsused in this research project for mylar and glass, focusing is very good. All three experimentsshow similar results. Here, one representative result will be presented (see Fig. 48).

Again, black lines were added to the sample to indicate the location of the needle electrode.In the first 20 seconds of the experiment, the deposition process resembles the writing-like varietywe are accustomed to from other experiments where the cylinder spinners nozzle arm is used.This deposition can be recognized as the hazy spot of fibers in the centre of the image. After thistime has lapsed, the deposition becomes less focused. The polymer jet seems to lash from thecentre of the collector outwards. During this movement, the majority of the polymer seems to slidealong the already established or solidified jet while the deposition point traverses radially away


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from the deposition point. This is not unlike other experiments

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