Towards a new position-controlled electrospinning setup

T.A. Baede

DCT 2009.052

Master’s 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

Table of contents Summary......................................................................................................................................................... 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

1

Summary Electrospinning is a powerful and versatile technique for the fabrication of very thin fibers from polymer solutions. In contrast to mechanical drawing processes, spinning with an electric field provides the ability to produce fibers with much thinner diameters, typically in the micro- and nanometer range. In a conventional electrospinning process, a polymer solution is fed to a metallic nozzle so that a drop appears at the tip of this nozzle. A high voltage is then applied to the nozzle, while at some distance below it a grounded plate electrode is placed. The potential difference between both points generates an electric field. The drop becomes charged and a thin jet is ejected from the nozzle. Due to electrostatic forces, the jet is pulled towards the electrode. By placing a thin sheet of collecting material between nozzle and electrode, the fiber is deposited on this so-called collector. Due to instabilities in the process, the material is deposited randomly, forming a so-called nonwoven mesh with a chaotic structure. For future electrospinning applications in the fields of filtration, tissue engineering and nano-electronics, it is necessary to make the fiber deposition controllable. A technique developed at Eindhoven University of Technology consisting of a moving collector and a thin, positionable grounded needle electrode makes this possible and was implemented in an electrospinning setup. However to improve fiber deposition control, more knowledge of the electrospinning process is required. The goal of this research study is to acquire this knowledge through experiments. In a first experiment, the deposition mechanisms for the grounded plate and needle electrodes were studied. It was learned that although the deposition mechanisms are different, the deposition looks similar. Secondly, PEO and PCL fibers were “written” on a moving collector consisting of a mylar sheet in a straight line. Interestingly, it was discovered that both fibers had a rectangular 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 does indeed play a role and that reducing this thickness leads to more pronounced deposition. This experimental result corresponds with electrostatic analysis, which suggests that although a dielectric collector does not change the overall shape of the electric field, it does locally reduce the field strength. This leads to a decrease in electrostatic pulling force and thus the amount of deposition. In a fourth experiment, the goal was to find out whether a small airgap between electrode and collector prevents, hinders or otherwise alters the deposition. No apparent visible difference was detected in deposition between both types of samples. The airgap did not prevent or hinder deposition. Although great strides have been made in the control of the fiber deposition point, there is still a difference between desired and realized deposition points. To further improve fiber deposition control, 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 investigate this. Therefore, a new setup must be designed. Design requirements for such a new electrospinning setup have been set. Two designs that meet these 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 consists of 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 reproducible results. Furthermore, a motion control scheme was implemented.

2

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 the

rotation axis. Both problems were analyzed. It was discovered that the electrostatic properties of the design are sound, 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 reduced wall thickness is unpractical due to issues with ease-of-handling, safety and durability. Through experiments it was determined that a thin-walled PET cylinder offers significantly better focusing without any of the before-mentioned complications and could therefore be a viable alternative to the installed glass cylinder. To address the collector speed problem, it is recommended to use a polymer that requires a lower collecting speed. Alternatively, the rotation axis could be redesigned.

3

Introduction Electrospinning is a powerful and versatile technique for the fabrication of very thin fibers from polymer solutions or melts. The resulting fibers are continuous and have a uniform diameter. This diameter can vary from tens of micrometers down to a few nanometers, depending on the process parameters. Furthermore, fibers with both solid and hollow interiors can be achieved. It is also possible to give the fibers special properties by adding metallic, ceramic or even biological compounds such as proteins or DNA to the source polymer material. In literature, the technique is often compared to the traditional fiber spinning process where a polymer 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. The advantage of this contactless drawing is that fibers with significantly smaller diameters can be produced. As mentioned before, both polymer solutions and melts can be used as a source material, but in this 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 pump to generate a constant flow of fluid through the needle. This needle is connected to a metallic capillary, the so-called nozzle, with a transport tube. The polymer solution is pumped through the tube and a small liquid drop appears at the tip of the nozzle. A high voltage is then applied to the nozzle, usually between 5 and 50 kV, while at some distance below the nozzle a grounded electrode is placed. The potential difference generates an electric field between nozzle and electrode.

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 surface tension alone to a conical shape called a Taylor cone [1]. When the electric field is sufficiently high and charge buildup in the cone reaches a critical level, the electrostatic forces will overcome the surface tension of the polymer solution and a thin, viscoelastic jet is ejected from the capillary nozzle. Under the action of the electric field, the fibers are forced to travel towards the grounded electrode. While the charged jet is travelling downwards, the polymer is continuously stretched by the electrostatic forces. At the same time, the solvent evaporates. The combination of these two effects results in a significant reduction of the jet’s diameter.

4

The jet does not follow a straight path to the electrode, however. Instead, shortly after exiting the nozzle, 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 of collecting material (also called collector) on the electrode so that the electrospun material is deposited 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 predict and 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 are oriented. 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-size of a filter is of prime importance as it determines the flow resistance and selectivity [14]. Making the fiber spacing in the mesh constant and reproducible would be very desirable. Such control would also allow researchers in the biomedical field to carefully optimize scaffold structures for tissue engineering. Finally, when one wants to create highly efficient sensors and electronics based on nanofibers, well-aligned and highly ordered architectures are required. Control of the orientation of nanofibers is therefore very desirable and numerous methods with various levels of success are described in literature. Worldwide, many research groups have experimented with methods to control the orientation of fibers during electrospinning with various levels 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 of the fibers. When this is achieved solely through the shape of the setup, we define this as electrostatic. Finally, if both means are important for the end result, we classify the method as such. The results from literature will be compared on two qualitative fronts: alignment and controllable spacing.

5

Ta

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tati

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[12]

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Yarin

[22]

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ibson

[23]

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V

M

itchell

[27]

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[28]

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6

Alignment is defined as whether there is control over how parallel all fibers are positioned to each other. Controllable spacing is defined as whether or not it is possible to deposit two fibers next to each other with a controllable spacing between them. From the results in Table 1, it follows that achieving alignment of nanofibers along a single axis is currently well-understood. The controllable spacing, however, is a different issue. Only two groups, [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 to carefully control the position of the fiber deposition point. Instead of using a grounded plate as electrode, a grounded needle is used which can be actuated in the plane (see Fig. 3). This technique can potentially improve the alignment and degree of spacing control significantly. The feasibility of the technique was demonstrated by Solberg [32]. He also implemented the technique 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 this setup. 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 collecting material is limited and this prevents the precise fiber deposition that is required for advanced oriented fiber meshes necessary for many future applications. Therefore, a new electrospinning setup is required.

Figure 3: Overview of new electrospinning technique using 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 of orientation control.

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

7

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 role in electrospinning. Then, a short overview is given of important process parameters, studied polymers and equipment in Chapter 2. A number of experiments were performed to obtain more knowledge of the fiber deposition process. These will be discussed along with results in Chapter 3. The design of the new electrospinning 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 and recommendations for future work given.

8

Chapter 1: Electrostatics In order to control the location of material deposition and improve deposition accuracy, good knowledge of the underlying electrospinning process is required. Key to this control is the electric field. First, the overall shape of the field is discussed based on electrostatics. Thereafter, a number 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: the charged nozzle at the top and the grounded needle electrode at the bottom. This situation can be modeled as an electric dipole. It consists of two point sources of charge, one at the tip of each element, with a certain separation distance h. A positive charge is applied to the nozzle; hence negative 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 with the arrows indicating the direction of the electrostatic force. Red contour lines represent the 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 the equipotential lines. As the name suggests, all points on such a line are at the same electric potential. The nozzle is at the maximum potential, while the electrode is at zero potential due to grounding. Hence, the potential decreases while moving downwards from one equipotential line to the next. The blue solid lines represent the electric field lines and are always perpendicular to the equipotential lines. By convention, the direction of the lines is from positive to negative charge.

9

The closer the field lines are to each other, the stronger the field in that region. The field is thus strongest near nozzle and electrode. The relationship between potential difference and electric field strength is given by:

ldEV

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]. Er

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

is an infinitesimal increment of

displacement on any line or curve between both points [m]. When we assume the electric field is uniform over the separation distance, like between two charged parallel plates where fringing is ignored, the magnitude of the electric field strength can be estimated using:

h

VE = (1.2)

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

EQFrr

= (1.3)

with the force Fr

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

in [V/m]. The electrostatic force acts in the direction of the electric field and therefore the pulling force on the vertical field line will be highest. All field lines converge towards the electrode and thus the point 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 field strength can be estimated with Eq. 1.2. When we enter a characteristic V = 12 kV and h = 0.1 m, we arrive at a field strength of E = 1.2·10

5 V/m. This is one order of magnitude below

the dielectric strength of air, Ebreakdown = 3.0 ·106 V/m, which is the field strength at which air

becomes electrically conductive [33]. The breakdown phenomenon puts a limit on how far the field strength can be increased to accomplish focusing. In the above text, we started from point charges. However, in reality both electrical elements have finite dimensions and their shape influences how the electric field looks like. In Fig. 6, the electric field around real needle or nozzle configurations is shown. The equipotential lines wrap around the contour of the nozzles.

Figure 6: Electric field around two nozzle configurations:

(a) An isolated nozzle. (b) A nozzle attached to a horizontal plate.

10

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

Figure 7: Molecular view of polarization within a dielectric

The field in the dielectricdEr

is a vector sum of the externally applied fieldextEr

and the field

indEr

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

indextd EEErrr

+= (1.4)

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

),(0

PfEind

rrε= (1.5)

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

ε is the dielectric

constant of vacuum [C2/Nm

2] and P

r is the polarization density vector in [C/m

2].

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

de EPrr

χε0

= (1.6)

where eχ is the electric susceptibility of the dielectric [-]. Note that the polarization depends on the

total 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 Ed is weaker than the external field Eext.

11

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 indEr

in those layers will

differ from each other. However, as for one layer, the shape of the electric field outside of the dielectric will not be influenced. The same holds for a collector that is not positioned exactly halfway between nozzle and electrode, 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 towards the grounded electrode by a combination of gravity and electrostatic force. Since the polymer solution is charged, in essence it can be seen as an extension of the nozzle. When the material moves 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 field strength and consequently the electrostatic force increases. This leads to a distortion of the electric field (see Fig. 8a). However, since the accumulated charge is hard to measure, the amount 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) Contact situation.

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 solidified polymer. Contact situation: When the polymer fiber comes into contact with the dielectric, a new situation occurs. A current will start flowing through the conductive polymer fiber.

EQvI σ+= (1.7)

where Q is the charge [C], v is the polymer flow velocity [m/s], σ the conductivity [m/Ω] and E the electric field strength [V/m]. The first term represents transport of charge due to movement of the polymer. The second term is conduction following Ohm’s Law. Whether this latter term plays a role depends on the conductive properties of the polymer when it comes into contact with the dielectric.

12

Since the collector is fabricated from an electric isolator, the charge deposited by the polymer cannot flow away (see Fig. 8b). Therefore, the collector will become locally charged because of the 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 of solidified polymer as this distributes charge in the most optimal way [32]. The repulsion effect is expected 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 surrounding atmosphere. Therefore, deposited segments will gradually lose their charge.

13

Chapter 2: Process parameters, materials and cabin Position-controlled electrospinning is a complex process and requires knowledge of electrohydrodynamics, chemistry, rheology and motion control to operate and fully address its potential. 2.1: Process parameters

Many parameters play a role in the electrospinning process. These can be broken down in material 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 number of operational parameters are shown in Fig. 9.

Figure 9: Operational parameters

14

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

Polymer 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 is relatively simple to prepare and use. Therefore, a lot is known about this material, its behaviour and 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 this material is that it is commonly dissolved in chloroform, which evaporates during electrospinning and 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 outfitted with 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]

15

One risk is the high voltage (up to 25 kV). When a user accidently comes into contact with an electrified element of the setup, the voltage enables a current to flow through the body which is harmful 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 with a 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 during electrospinning. Therefore, the cabin is connected to a forced ventilation system that captures and leads any emitted gases away from the setup.

16

Chapter 3: Pre-design experiments In this Chapter, an overview will be presented of all experiments that were performed to obtain more knowledge of the electrospinning process and to assist in the design of a new electrospinning setup. Additionally, the results of these experiments will be analyzed and discussed. 3.1: Introduction In the work of Solberg [32], the feasibility of focusing the fiber deposition by using a thin grounded needle 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 a velocity difference between collector, nozzle and electrode. These experiments will be taken as starting 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 of experiments, described in Paragraph 3.2.1, is to compare the mechanism of fiber deposition on a grounded plate electrode with that on a grounded needle electrode. In Paragraph 3.2.3, the deposition on a moving collector is investigated. The goal of this experiment is to determine whether fibers can be “written” with both PEO and PCL and if so what the properties are of these fibers. Then, in Paragraph 3.2.5, a set of experiments is described to determine whether the collector 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 and electrode 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 of fiber 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 is positioned 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 the electrospinning cabin’s high voltage supply. Directly below the nozzle, a circular, copper plate electrode with diameter of 5 cm is placed. The plate electrode is connected to the minus terminal of the cabin. A Harvard PHD 2000 infusion pump is used to feed PEO solution (see previous Chapter 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 in direct contact with the grounded plate electrode. In Fig. 11a, the setup used for this test is depicted. As preparation for the experiment, the infusion pump is switched on so that a constant feed of polymer solution begins to flow through the tubing. During experimentation, the pump is never turned off as the polymers under study are viscous and we want to minimize transient behaviour of the fluid. Once the solution is ejected in a steady-state manner from the nozzle, experiments can take place. Paper covering that keeps the electrode clean is removed and the collecting material is placed on the electrode. The experiment starts when an uniform drop has formed and the high voltage is applied. After spinning for a set duration, the high voltage is switched off, the collecting material removed and the cabin and equipment cleaned with ethanol to remove any remaining charged polymer strands that could disturb subsequent experiments. All experiments are performed at ambient cabin temperature (22.4°C) unless noted otherwise.

17

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

In the second test, the plate electrode is exchanged for a grounded, conical-shaped needle electrode with an outer diameter of 0.9 mm (see Fig. 11b). All other elements of the setup and procedure are kept the same. For these tests, the spinning parameters are chosen such that a stable polymer jet is established and deposition is clearly visible. Note that the main focus of the present tests is not to compare deposition 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, where V is the applied voltage in kV, f is the polymer solution feed rate in [µl/min], ∆t is the time period in which deposition occurs in [s] and d is the vertical distance between nozzle tip and collecting material in [cm]. Table 3: Spinning parameters for fiber deposition experiments

Experiment Electrode type Collecting material V [kV] f [µl/min] ∆t [s] d [cm]

1 plate mylar 17 18 120 13

2 plate glass 17 18 120 13

3 needle mylar 17 18 60 12

4 needle glass 17 8 120 12

These tests are different from those described by Solberg [32] as a higher V and different values for 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 is ejected 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 of the incoming polymer has no preference for a specific point of the plate electrode and is thus deposited randomly on the collecting material. Upon deposition, the segment retains a residual charge that exerts a repulsive force on subsequent segments and sweeps them to another location. As a result, the collecting material is covered with randomly deposited fibers that

18

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 using optimal 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 are occasionally kept up with black foam tubing or a combination of thin white and red strips and pins to facilitate photography. These aids are not important and should be ignored. The bright, little drops in the center of the photographs in this Chapter are caused by the initial polymer drop when the voltage is applied or the last drop after the voltage has been removed. They are not part of the 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 (see Figs. 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. This can be explained by the fact that the fiber, which has been deposited on the sheet, retains an amount of residual charge which exerts a repellant force. After the first fiber segment has been collected, the continuous deposition leads to pile up of new fiber material at the initial deposition

19

point which ultimately buckles. The already deposited fiber segment’s charge pushes the new fiber away to another location. As more fiber is collected, the repellant forces will direct new material 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 be concluded that a needle electrode focuses fiber deposition. After a limited time, material is no longer deposited directly above the electrode because of repulsive forces. A solution to this problem is to move already deposited fiber segments away from the deposition point and put untouched collecting material in the path, so that incoming fiber segments stay attracted to the tip of the needle electrode and focusing is maintained. All photographs show similar circular deposition although the deposition mechanisms of a plate electrode 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 a single straight fiber and create oriented fiber meshes, however, it is necessary to create a velocity difference between nozzle and electrode on one hand and the collector on the other hand. In this simple test, the grounded needle electrode setup introduced earlier is reused. PEO and PCL will be selected as polymer solutions. The same experimental procedure is followed with the exception that a long strip of mylar sheet is used which protrudes from the electrospinning cabin through a slit on one end. During electrospinning, the strip is manually pulled further out of the cabin with a velocity of several cm/s. In this way, the mylar translates over the needle electrode while it keeps in direct contact with it (see Fig. 13). Deposited fiber is thus transported away from the 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 that yield the most pronounced polymer “tracks” on mylar for both polymers. After selection, the writing test is performed three times for PEO and one time for PCL with the following parameters: Table 4: Spinning parameters for writing experiments

Polymer solution V [kV] f [µl/min] d [cm]

PEO 20 18 13

PCL 22 30 7

20

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 the parallel polymer tracks written on the sheet. The left track is curved over the bottom half of its length because it was not pulled in a entirely straight, continuous motion. The right track however is significantly better due to better steering of the sheet. It is observed that the polymer fiber is deposited in an entwined, thread-like fashion as the horizontal velocity of the sheet is too low to collect 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.

21

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, it should 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 confocal microscopy and interferometry using a Sensorfar PLu 2300 optical imaging profiler. Several locations along the fiber were selected and the diameter was measured. Interestingly, the measurements showed that the fibers did not have circular cross sections, but rather a

22

rectangular shape. The PEO fibers had a height of 380 nm and a width of 8 µm, thus a h:w ratio of 1: 21. The PCL fibers had a height of 14 µm and a width of 27 µm, which is a h:w ratio of approximately 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 at Eindhoven University of Technology is whether the collector thickness influences the amount of deposition. To investigate this question, the grounded plate electrode setup will be used and strips of glass and polycarbonate with varying thickness are used as collector material and placed directly on the electrode, while all other spinning parameters are kept constant. The amount of deposition on the samples will then be compared qualitatively. The plate electrode is chosen over the needle electrode as the former gives a more pronounced deposition effect in the same time frame. Different collector thicknesses lead to different nozzle-collector distances when the vertical distance between these two points is not adjusted. It is assumed for these experiments that this difference of a few millimeters is negligible when considering a vertical distance of 5-15 cm and will not lead to different electrospinning behaviour. Four experiments will be performed, as two polymer solutions (PEO and PCL) and two collector materials (glass and polycarbonate) will be investigated. For each experiment, either two or three collector thicknesses are compared. In the following table, the spinning parameters are summarized. Here t1, t2 and t3 are the thickness of the collectors that are compared in [mm]. Table 5: Spinning parameters for collector thickness experiments

V [kV] f [µl/min] d [cm] t1 [mm] t2 [mm] t3 [mm] ∆t [s]

Experiment 1: PEO with glass as collector material:

17 18 13 0.55 1.1 - 120

Experiment 2: PEO with polycarbonate as collector material:

17 18 13 1.0 3.0 - 120

Experiment 3: PCL with glass as collector material:

12 17 7.8 0.2 1.4 - 60

Experiment 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 collector thickness of 0.55 mm, while the sample on the right has a collector thickness of 1.1 mm. In the photograph, 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 black circles. This experiment was performed using polycarbonate as collector and it was observed that it 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 the left) is covered with the most polymer deposition. The results for PCL are more pronounced than those for PEO. In Fig. 20, two circular glass samples are shown with a thickness of 0.2 mm on the left and 1.4 mm on the right. The density of polymer on the left sample is significantly higher and the deposition is more focused than on the right sample where it appears more spread out, even though the plate electrode is used for both.

23

Figure 18: Collector thickness - experiment 1 Left: t = 0.55 mm. Right: t = 1.1 mm.

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

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

24

Fig. 21 shows the results of the last experiment where polycarbonate is used as collector in three thicknesses. The deposition is marked with black circles. Again we observe that the thinnest sample 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 role and reducing this thickness leads to a more pronounced deposition. This can be understood by taking into account that the collector partially shields off the electrode and thus locally reduces the field strength. It acts as a sort of resistor in the field. This reduces the electrostatic pulling force and therefore the amount of deposition. The shape of the overall field is not changed, however.

Figure 21: Collector thickness - experiment 4 Left: 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 electrode prevents, hinders or otherwise alters the deposition. To address this question, the grounded needle electrode setup is used. PEO solution is selected as polymer source and mylar as collecting material. We will compare deposition on samples in direct contact with the electrode to deposition on samples with a small collector-electrode airgap a of 1 or 2 mm. The applied voltage V is set to 17 kV, the polymer solution feed rate f to 18 µl/min and the deposition period ∆t was kept constant at 60 s. Four experiments will be performed. In each experiment, deposition on a sample in direct contact with 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 experiments

Experiment d [cm] a [mm]

1 12 1

2 13.5 1

3 13.5 1

4 13.5 2

3.2.8: Influence of collector-electrode airgap: results

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

25

Both pictures show a similar circular deposition with some fibers that are deposited radially outward 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 immediately above the needle electrode is covered with deposition. The electrode is essentially shielded off and thus the polymer jet follows the electrostatic field lines which now curve from nozzle towards electrode 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 there was direct contact and where an airgap between mylar collector and needle electrode existed.

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

3.3: Design implications

From the experiments in this Chapter we have learned several things that are important when designing 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.

26

Chapter 4: Design of a new electrospinning setup 4.1: Introduction

To achieve the precise fiber deposition necessary for advanced oriented meshes, a new electrospinning setup is required with a significantly higher position accuracy of the deposition point than currently available. In this Chapter, the design requirements of such a setup are presented. Subsequently, a number of potential configurations are generated and evaluated. Then, one design is chosen and elaborated. Finally, motion controllers are designed and a graphical 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 depositing straight fibers with adjustable spacing in a reproducible manner. The straightness of fibers depends on the velocity difference between the nozzle and electrode on one hand and the collector on the other hand. If this velocity difference is too low, an entwined thread is produced instead of a single, straight, continuous fiber. For PEO, the polymer solution we would like to study, Solberg [32] showed that this speed difference must be at least 8 m/s for the spinning parameters he used. In order to be certain that straight fibers are producible, the new electrospinner should be able to reach this collecting speed.

2. Nozzle and electrode placed in-line In the electrospinning process, a charged polymer fiber flows from the nozzle to the electrode. 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 occurs for excessive misalignment of nozzle and electrode. Secondly, the charged material follows the electrostatic streamlines which do not flow through the desired deposition point. To make the deposition process as simple as possible, it is desirable to have the charged fiber travel directly downwards from the nozzle toward the electrode. To minimize the deposition error, the nozzle and electrode should therefore be in-line and mechanically 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 be minimized. Therefore, all design components should either be made of electric isolators

27

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 protects the user from exposure to high voltages and is outfitted with a dedicated ventilation system. The new electrospinner should fit within this existing cabin. The maximum dimensions of the new design are therefore: length < 0.48 m, width < 0.66 m, height < 0.56 m.

5. Spinnable area at least 0.12 x 0.12 m A spinnable area of at least 0.12 x 0.12 m is desired, based on application considerations. The fibers are to be positioned as parallel lines with an adjustable spacing s in[m].

4.3: Configurations With the before-mentioned requirements as constraints, a number of designs were generated. In order to fulfill the first and second requirements simultaneously, there are two practical options:

• A stationary collector and a moving pair of nozzle and electrode.

• A moving collector and a stationary pair of nozzle and electrode. The nozzle and electrode need to be mechanically coupled, for instance by a U-shaped frame. When we select the first option, the frame will have to move. To study the deposition behaviour, this option would require a moving camera which is not practical. Therefore, the second option is selected. Now that the moving and stationary parts are known, a collector configuration can be selected. Essentially, there are two configurations that fit the requirements mentioned above: planar and cylindrical (see Figs. 24a and 24b, respectively).

ω

x&x&

y&

Figure 24: Collector configurations

The planar configuration consists of a flat rectangular collector which is actuated in the x,y-plane just above a needle electrode. A mylar sheet of the same size is to be placed on the collector to capture the fiber deposition. Another solution is to use a configuration where a thin-walled, hollow cylinder with one open end and radius R is used as collector, which can rotate and translate along its center axis. A needle electrode is mounted on an arm that sticks into the cylinder. Now, a mylar sheet of the same size as the collector will be wrapped around it. After electrospinning, the sheet will be unwrapped.

28

A truly two-dimensional mesh with spacing s can only be woven with a planar configuration (see Fig. 25). Note that the fiber will only be straight if it is deposited at the required collecting speed. Therefore, in the planar case, there is an effective spinning area where the collecting speed is reached and an area outside where it is not. A major disadvantage of this configuration is that a collector speed of 8 m/s in the plane is hard to achieve. Especially within the limited confines of the electrospinning cabin which places rather extreme requirements on the acceleration of the collector when we want to be able to spin with a constant speed for at least 120 mm. This can be seen by calculating the available acceleration length:

2

eff

acc

lll

−= (4.1)

here l is the width of the electrospinning cabin in [m], leff is the length over which spinning should occur with a constant velocity in [m] and lacc is the length available for acceleration in [m]. Entering the corresponding values in Eq. 4.1 yields: lacc = 0.27 m.

s

effl

Figure 25: Schematic of a mesh produced with a linear configuration

The required acceleration follows from:

acc

reql

Vx

2

2

=&& (4.2)

with reqx&& the required acceleration in [m/s2] and V the collecting speed in [m/s]. This gives

reqx&& = 119 m/s2. This required acceleration puts very high demands on the actuators.

If we only want parallel fibers, it is possible to use a fast axis to deposit the fiber along the length of the collector and a short axis to move this axis sideways over a distance that corresponds to the spacing. However, one fast actuator is still required.

α

Rb π2=

s Figure 26: Schematic of a mesh produced with a cylindrical configuration (folded out)

Using a cylindrical configuration, the collector is rotated with a surface velocity of 8 m/s. Translation of the collector is necessary to achieve the spacing and fibers will appear as spirals

29

on the collector. In Fig. 26, a schematic of a mesh produced with the cylindrical configuration is shown. Note that in this configuration, the cabin dimensions are no longer a constraint when it comes to the collecting speed. As can be seen in the figure, the width of the mesh b in [m] is equal to the circumference of the cylinder and thus depends on the radius R in [m]. The required

angular velocity reqω in [rad/s] to achieve the collecting speed is also dependant on the radius.

Taking all design requirements into account, we select R = 0.02 m. This results in

reqω = 400 rad/s = 3820 rpm. The required angular acceleration can be set freely as it depends

on how long the spinner is allowed to spin up to the required angular velocity. Now that the rotation of the cylinder has been investigated, attention can be focused on the translation. It can be gathered from Fig. 24b that the interfiber spacing s that is realized depends

on the collecting speed V and the selected translation velocity x& . Suppose that 0=x& and 0>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 45° line is deposited (see also Fig. 26). Hence, for an arbitrary

angle α, we can write:

x

V

&=)tan(α (4.3)

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

s

Rπα

2)tan( = (4.4)

Combining Eqs. 4.3 and 4.4 leads to:

V

xRs

&π2= (4.5)

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

R

Vsx

π2=& (4.6)

4.4: The cylinder spinner

After comparing both configurations, the cylinder configuration is selected. Now, the new electrospinner can be designed in detail. A concept drawing is shown in Fig. 27. The stationary part consists of a tower from which a nozzle arm and electrode arm protrude. The translating part consisting of the cylinder and its drivetrain will slide over the electrode with a small airgap of 1 mm between cylinder interior and electrode. Several actuator mechanisms were considered and a ballscrew drive was selected. The translating part of the spinner will be mounted on this drive.

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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 an electric isolator, readily available, easy to machine and resistant to aggressive solvents which are occasionally used in electrospinning. The frame of the cylinder spinner is built from thick polymer sheets. Polyvinylchloride (PVC), with its 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 of the 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 Maxon RE25 (type 118746) electromotor. A WM Berg bellow coupling is used to prevent alignment problems. 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 encoder with 500 counts/turn is mounted on the motor to control the angular velocity of the cylinder. This optical encoder is preferred over a tachometer because we want to be able to control the deposition position, not just the angular velocity. For data acquisition and interfacing with the motor 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 µm and backlash < 20 µm. The ball screw is connected to a Maxon RE30 (type 268219) 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 and interfacing with the motor and encoder, the same TU/e Microgiant as for rotation is used. As software we employ Matlab Simulink. 4.8: Realisation

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

31

matte, glass-like PC beam above it is the nozzle arm. The brass nozzle itself is on the outer edge on the right. The Teflon polymer transport tube is visible, coming out of the nozzle. The vertical position 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 this photo, 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. The cylinder is held up by a large bearing on the left side, which is mounted in a vertical stand, and the black PVC plug on the right side. The plug is connected to the rotation motor and encoder which 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 be seen in Fig. 30. A back-side view of the spinner is shown in Fig. 31. The three wired protrusions with 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 be found in the next section. In Fig. 32, the spinner is placed in the electrospinning cabin. The infusion pump is located left of the cabin while the amplifiers and control hardware are located right of the cabin.

32

Figure 29: Translating part

Figure 30: Electrospinner (front view)

33

Figure 31: Electrospinner (back view)

Figure 32: Electrospinner placed in cabin

34

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-shaped cover is also mounted on the carriage of the ball screw actuator to shield off the slide’s ball screw below and prevent it from collecting polymer strands. The stationary support of the cylinder is outfitted with two Crouzet microswitches which are placed at each extremity of the slide. These limit points of the slide’s travel are marked as A and B in Fig. 33. A pin protrudes from the cover and can come into contact with a switch. The switches are wired in such a way that they cut the current to the translation electromotor when one comes into contact with 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 a well-defined position and yields a reproducible result. Also, we want to maximize the distance over which we can electrospin. There are several methods to home the cylinder spinner. Let’s assume we want to home to position A, and then do spinning while the slide moves from A to B. Since we know the direction of 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, any motor current is interrupted and thus the user is required to manually remove the slide from its locked position. Another strategy is to add an additional microswitch as reference point. The next step is then to choose the location of this reference point. It is possible to place the switch in the middle between the endpoints of the slide. The problem with this, however, is that it is possible the spinner is in a position 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 a single microswitch is required for homing. This solution was implemented in the electrospinner design (see Figs. 31 and 34).

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

35

The current spindle position of the spinner is logged by the motion control and this software shuts the 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 of system dynamics. The FRF of the translation axis (spindle) is shown in Fig. 35. A constant velocity of 1.5 rad/s was supplied as reference. This corresponds to a constant movement of the spindle of 1.2 mm/s (see Paragraph 4.10). A chirp signal with a decreasing frequency from 2000 Hz to 1 Hz was used as noise signal. The chirp signal resulted in better coherence than a standard Gaussian noise signal over a larger range of frequencies. The response was measured for 100 s with a sampling frequency of 4000 Hz.

100

101

102

103

104

-200

-100

0

100

Magnitude [

dB

]

100

101

102

103

104

-200

0

200

Phase [

deg]

100

101

102

103

104

0

0.5

1

Cohere

nce [

-]

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 moving mass. In the phase plot we see that delay is present in the system. This delay was modeled with the following transfer function:

dTjejH

ωω −=)( (4.7)

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

36

100

101

102

103

104

-100

-50

0

50

Magnitude [

dB

]

100

101

102

103

104

-200

0

200

Phase [

deg]

100

101

102

103

104

0

0.5

1

Cohere

nce [

-]

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 high frequency, we see a -2 slope, an anti-resonance at 55 Hz, a resonance at 600 Hz and finally

again a -2 slope. A sine wave with frequency of 0.1 Hz and amplitude of π12 rad was supplied

as reference, while a white noise signal with variance of 0.5 was used as noise signal. The response was measured for 60 s with a sampling frequency of 4000 Hz. A similar fitting procedure 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 were supplied. 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. No inter-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 of a 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 and rotating axis, perform a homing procedure and stop the spinner.

37

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 will be implemented for both axes. First, a feedback controller will be designed for the spindle. As a starting point, a desired bandwidth of 30 Hz was chosen. By analyzing the open-loop, we notice that the system needs phase. Therefore, a lead-filter is required (K = 24, z = 30/3 Hz, p = 90 Hz). Furthermore, to remove 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 desired bandwidth of 10 Hz was chosen. By analyzing the open-loop, we notice that the system needs phase. Therefore, a lead-filter is required (K = 1.2, z = 10/3 Hz, p = 30 Hz). From the FRF, it is determined that it is desirable to also include a notch to combat the effects of the resonance at 600 Hz and a low-pass filter to reduce measurement noise influences. However, stable controllers turned out to be unstable after implementation. Therefore, as controller, the before mentioned lead-filter was used. An overview of performance is shown in the following table: Table 7: Performance for both axes

Axis Bandwidth [Hz] Modulus margin [dB] Phase margin [°] Gain margin [dB]

Spindle 49.7 6.0 36.5 9.9

Cylinder 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 control scheme is shown in Fig. 38. To test the system, suitable references need to be supplied. A spacing of 100 µm was selected

as target. This target can be achieved for 4

108−⋅=x& m/s and V = 1 m/s (ω = 50 rad/s). In Fig.

39, reference signals r1 and r2 are shown for the spindle and cylinder axis respectively which are necessary to achieve this target. The homing procedure takes up the first 50 s of the reference.

38

Then, the spinning procedure starts. For r1, a constant velocity of 4

108−⋅=x& m/s is supplied. For

r2, 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 the spindle e1 is roughly 5 µm in the spinning interval. The position error of the cylinder e2 reaches a steady state value of 1 rad at 150 s. This value is unsatisfactory, but cannot be improved at time of 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 [

rad]

t [s]

Figure 39: Reference trajectory for s = 100 µm

u18

e17

r16

y15

u24

e23

r22

y21

stop2

stop2

stop1

stop1

SystemIO

u1 [V]

u2 [V]

y1 [rad]

y2 [rad]

homing [-]Subsystem1

start

acc

vel

posRef3

StartRef3

r

Spindle Ref

r1

Spindle Ctrl

e1 u1

Cylinder Ctrl

e2 u2

39

50 60 70 80 90 100 110 120 130 140 1502

4

6

8

10x 10

-6e

1 [

m]

50 60 70 80 90 100 110 120 130 140 1500

0.5

1

1.5

e2 [

rad]

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)

Pow

er/

frequency (

dB

/Hz)

PSD of e1

Figure 41: Measured error signal e1 (frequency domain)

40

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)

Pow

er/

frequency (

dB

/Hz)

PSD of e2

Figure 42: Measured error signal e2 (frequency domain)

To learn more about the error signals, a frequency analysis was performed. The results are shown in Figs. 41 and 42. The results can be explained by taking the reference trajectories into account. The references have no high-frequency content and will therefore not excite the system at high frequencies. Therefore, only errors with low frequencies are expected. This is also what follows from the figures. 4.10: Spacing error

Quadrature encoders are used as position sensors. Encoders for both axes have a resolution of 500 counts/revolution. The encoder of the spindle is connected to the motor via a gearbox with a reduction of 23/4:1. The spindle has a lead of 5 mm, thus the translating part moves 5 mm for every revolution. One count of the spindle encoder then corresponds with:

rad4

1046.54/234500

2 −⋅=

⋅⋅

π (4.8)

mrevm 7

1035.44/234500

/005.0 −⋅=

⋅⋅ (4.9)

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

rad0031.04500

2=

⋅

π (4.10)

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

41

We define the spacing error ∆s as the difference between the desired spacing s and the realized

spacing s :

sss ˆ−=∆ (4.11)

The desired spacing can be calculated from the desired translation velocity x& and the realized

collecting speed V :

V

xRs

&π2= (4.12)

The realized spacing can be calculated from the realized translation velocity x& and the realized

collecting speed V :

V

xRs

ˆ

ˆ2ˆ

&π= (4.13)

As the derivatives of the positions of the translation and rotation axes were very noisy, it was not possible to determine the realized spacing error at the time of writing.

42

Chapter 5: Post-design experiments Now that the cylinder spinner design has been realized, experiments can be performed to review its performance. In Paragraph 5.1, deposition experiments are performed on the stationary cylinder to determine whether focused deposition occurs testing a wide range of spinning parameters. Thereafter, in Paragraph 5.2, it is checked to see if the electric field disturbs the sensors and electromotors. A number of problems with the setup manifest themselves during testing. In Paragraphs 5.3 and 5.4, the causes of these problems are analyzed and some solutions 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. The goal of these experiments is to determine whether focused deposition occurs for a wide range of spinning parameters. These initial experiments will be performed with a stationary cylinder. PEO is 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 µl/min and d the vertical distance between nozzle tip and collecting material 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 indicate the location of the needle electrode. The electrode was placed at the interception point of the black 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 a gap in the deposition at the bottom of the image, because this was the location of a piece of tape to stitch both edges of the mylar sheet together. From tuning V and keeping other parameters constant we learn that the deposition for low V is less concentrated and less dense than for high V. The changes are marginal, though, and all results are very similar to Fig. 43. An interesting observation was that for V > 22 kV the jet splits in 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 what is observed. Clearly, this is a significant problem. Since spinning parameters were varied over a wide range, it is highly likely that the lack of sharp focusing is due to the construction rather than operating conditions. In Paragraph 5.3, a number of experiments will be described to determine the cause of this problem. 5.2: Testing with motion

As a first motion test, the cylinder spinner was supplied with standard position references for the rotation 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 same trajectories were supplied. No significant differences in the position errors of both axes were observed. The test was repeated for 15, 20 and 25 kV with identical results. It was therefore concluded that the electric field has no influence on the encoders and motors and that the measures employed to shield off the electric components are sound.

43

During further testing it was discovered that the current hardware of the cylinder spinner is not capable of reaching the required collection speed of 8 m/s. The upper boundary is currently 1 m/s. This limitation is due to mechanical problems in the rotational axis of the setup. Causes are expected to be: the out of roundness of the cylinder (measured: 0.3 mm), misalignment between cylinder axis and rotation motor axis, and significant friction in the bearing.

Figure 43: Representative result of spinning on mylar wrapped around glass cylinder

5.3: Troubleshooting To locate the cause of the focusing problem, as systematic approach will be undertaken using the grounded needle electrode setup as reference. Elements of the setup are exchanged with those of the cylinder spinner and the following combinations are investigated:

1. Grounded needle electrode setup, described in Chapter 3, modified so that the nozzle is just an injection needle protruding from a charged horizontal copper plate.

2. Same as 1, but with the cylinder spinner’s nozzle arm. 3. Same as 1, but with the cylinder spinner’s electrode arm. 4. Cylinder spinner setup with mylar sheet as collector. 5. Cylinder spinner setup with glass cylinder (t = 2.5 mm) as collector.

The results of experiments with these combinations will be compared for the same spinning parameters. Note that all results have been repeated several times and the images of representative results are presented below.

44

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 hamper focusing. On the contrary, deposition seems to be more aggressively focused and resembles “writing”.

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) is shown, while in Fig. 45b combination 3 (electrode-arm + nozzle out of plate + mylar) is represented.

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

45

Both figures show a random deposition although this deposition is denser in Fig. 45a. In the right figure, two big white drops are present. The one in the center of the image was caused by a drop deposited after the electric field was switched off and should therefore be ignored. It is concluded that using the cylinder spinner’s electrode arm gives similar results, although less concentrated. Comparing results of 2 with 4: In Fig. 46a, the result with combination 2 (grounded needle + nozzle-arm + mylar) is shown, while in Fig. 46b combination 4 (electrode-arm + nozzle-arm + mylar) is represented. From this figure it can be observed that both depositions consist of a “writing”-like deposition with some thick fibers produced halfway through the experiment.

Figure 46: Comparing deposition of: (a) Combination 2. (b) Combination 4.

It is logical to conclude that writing is enabled by the nozzle-arm. Comparing results of 4 with 5:

Figure 47: Comparing deposition with electrospinning setup on:

(a) Combination 4. (b) Combination 5.

46

In Fig. 47a, the result with combination 4 (electrode-arm + nozzle-arm + mylar) is shown, while in Fig. 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 from focused to random. From the experiments in this Paragraph it can be concluded that the electrostatic properties of the nozzle and electrode arms are satisfactory. The glass cylinder is the element that prevents proper focusing. 5.4: Stationary deposition experiments on PET Through experiments it was discovered that the glass cylinder with a thickness of 2.5 mm prevents proper focusing of polymer fibers. While one can produce a glass cylinder with a smaller wall thickness, one has to take the operating conditions into account. The rotation axis of the cylinder spinner operates at high angular velocities and therefore a thinner glass tube might be problematic. Aspects like ease-of-handling, safety and durability could become issues. In electrospinning experiments we wrap a thin sheet of collecting material around the cylinder on which the deposition takes place. A thin glass tube breaks easily and special care must be taken when mounting the collecting material. As mentioned before, high angular velocities are used and designing a setup that does not vibrate during motion and does not lead to failure of the glass is nontrivial. Rather than installing a new cylinder with smaller wall thickness, it was deemed more prudent to investigate 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 the same 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 of 0.25 mm were obtained from the departmental workshop. For each experiment, a new PET sample will be mounted in the cylinder spinner setup to replace the glass cylinder. An airgap of 10 mm between PET collector and electrode is established. PEO solution will be electrospun on the samples using the following spinning parameters: Table 8: Spinning parameters for PET deposition experiments

V [kV] f [µl/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 in the previous section. Even with an airgap of 10 mm, which is significantly larger than the airgaps used in this research project for mylar and glass, focusing is very good. All three experiments show 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 variety we are accustomed to from other experiments where the cylinder spinner’s nozzle arm is used. This deposition can be recognized as the hazy spot of fibers in the centre of the image. After this time has lapsed, the deposition becomes less focused. The polymer jet seems to lash from the centre of the collector outwards. During this movement, the majority of the polymer seems to slide along the already established or solidified jet while the deposition point traverses radially away

47

from the deposition point. This is not unlike other experiments where the same phenomenon has been observed. The associated deposition can be seen in the center-left part of the image and consists of thicker fibers.

Figure 48: Representative result of spinning on a PET cylinder

It is postulated that this different deposition is due to the fact that the needle electrode has been shielded off by the writing-like deposition that has happened earlier, but further tests are necessary to confirm this hypothesis. When a rotating cylinder is used instead of a stationary one, the deposited charge is moved away from the deposition point and shielding is no longer an issue. PET offers significantly better focusing and is therefore a viable alternative for glass as cylinder material.

48

Conclusion and recommendations In this report, a new position-controlled electrospinner was designed, constructed and tested. The design was based on electrostatics and the results of a set of pre-design experiments with PEO and PCL. A number of conclusions can be drawn from these experiments:

• Although deposition mechanisms for the grounded plate and needle electrodes on mylar and glass are different, the deposition looks similar.

• Mylar writing samples were created with PEO and PCL. Fibers of both polymers have a rectangular cross section. The PEO fibers had a height of 380 nm and a width of 8 µm, while the PCL fibers had a height of 14 µm and a width of 27 µm.

• It was qualitatively determined for glass and polycarbonate that the influence of collector thickness does indeed play a role in the amount of deposition that takes place.

• No apparent visible difference was detected between deposition on mylar in direct contact with a needle electrode and with an airgap in between. The airgap did not prevent or hinder deposition.

Two problems popped up:

1. Focusing did not occur when using a glass cylinder with thickness of 2.5 mm. 2. The desired collecting speed of 8 m/s could not be reached.

Through a number of post-design experiments, it was found that:

• The lack of focusing was caused by the relatively high thickness of the glass cylinder. Changing parameters V, f, d had no effect.

• The nozzle arm of the cylinder spinner enables better focusing than a nozzle protruding from a horizontal charged plate.

• A PET cylinder is a good alternative to the glass cylinder currently installed in the setup. There are a few recommendations for further research:

• It is suggested to redesign the rotation axis so that the required collecting speed can be attained.

• Another solution is to use a different polymer that requires a lower collecting speed than PEO.

• Better position control might be possible by implementing feedforward control.

• The glass cylinder can be replaced with a PET cylinder for better focusing.

• The current electronics that are used to operate the cylinder spinner are rather bulky. It might be practical to rearrange them in a more compact form.

49

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