EP2045375A1

EP2045375A1 - Apparatus and method for electrospinning 2D- or 3D-structures of micro- or nano-fibrous materials

Info

Publication number: EP2045375A1
Application number: EP07117780A
Authority: EP
Inventors: Yousef Mohammadi; Akbar Gazme; Masoud Soleimani; Vahid Polymer Engineering Department Haddadi-Asl
Original assignee: Stem Cell Technology Co
Current assignee: Stem Cell Technology Co
Priority date: 2007-10-02
Filing date: 2007-10-02
Publication date: 2009-04-08
Anticipated expiration: 2027-10-02
Also published as: DE602007013237D1; ATE502140T1; EP2045375B1

Abstract

The invention relates to an apparatus for electrospinning 2D- or 3D-structures of micro- or nano-fibrous materials, comprising at least one electrospinning spinneret (3) being electrically charged at a first potential; electrode means (4A, 4B) disposed adjacent said at least one electrospinning spinneret (3) and being electrically charged at a second potential different from said first potential to thereby generate an electric field between said at least one electrospinning spinneret (3) and said associated electrode means for aligning micro- or nano-fibers injected by said at least one electrospinning spinneret (3); and collecting means (9) for collecting micro- or nano-fibers injected by a respective spinneret and aligned by said associated electrode means.

In order to enable a precise alignment of the fibers and a precise control over the properties and characteristics of the hybrid material deposited onto the surface of the collecting means, according to the invention the electrode means comprises at least two conductive electrodes (4A, 4B), which are disposed in parallel with each other and rotate in the same rotating direction, wherein the collecting means (9) is movably disposed between said at least two conductive electrodes (4A, 4B) for collecting said aligned micro- or nano-fibers.

Description

FIELD OF THE INVENTION

The present invention relates in general to electrospinning of fiber forming materials, in particular polymer materials, and relates in particular to an apparatus and method for electrospinning 2D- or 3D-structures of micro- or nano-fibrous materials

BACKGROUND OF THE INVENTION

Electrospinning, or electrostatic field-assisted fiber deposition, has been extensively explored as a quick and facile technique for preparing ultrathin fibers with diameters ranging from tens of nanometers to micrometers [D. Li and Y. N. Xia, Adv. Mater., 19, 1151-70, 2004 and Y. Dzenis, Science, 304, 1917-9, 2004]. It is based on electrostatic surface charging of a melt or solution droplet, drawing a jet moving at a high speed toward a grounded stationary or rotating surface. The highly extensional flow results in ultrahigh draw ratios, which lead to the formation of a continuous nanofiber. Most nanofibers obtained so far are in non-woven form, which are suggested for applications in energy storage, healthcare, biotechnology, environmental engineering, and defense and security [S. Ramakrishna, K. Fujihara, W. E. Teo, T. Yong, Z. Ma and R. Ramaseshan, Materialstoday, 9(3), 40-50, 2006].
However, as it could be understood from traditional fiber and textile industry, only when continuous single nanofibers or uniaxial fiber bundles are obtained, can their applications be expanded into unlimited fields. Nevertheless, this is a very tough target to be achieved for electrospun nanofibers, because the polymer jet trajectory is in a very complicated three-dimensional "whipping" way caused by bending instability rather than in a straight line. Efforts are being made in various research groups all over the world. Up to date, however, there is no continuous long nanofiber yarn obtained and the publications related to aligned nanofibers are very limited. The following techniques are some possible means, which have been attempted to align electrospun nanofibers.
The most basic form of getting aligned nanofibers rather than random mesh is through the use of a rotating drum [J. A. Matthews, G. E. Wnek, D. G. Simpson and G. L. Bowlin, Biomacromolecules, 3, 232-238, 2002 and E. D. Boland, G. E. Wnek, D. G. Simpson, K. J. Palowski and G. L. Bowlin, J. Macromol. Sci. Pur Appl. Chem., A38, 12, 1231-1243, 2001]. This is a simple, mechanical method of aligning the fibers along the circumference of the drum. As the nanofibers are formed from the electrospinning jet, the drum that is used to collect the nanofibers is rotated at a very high speed up to thousands of revolutions per minute (rpm). Although there is obvious alignment of the nanofibers, the degree of alignment is not very good as there are still a substantial number of misaligned nanofibers collected.
To improve on the alignment of the nanofibers, the introduction of a sharp pin with a negative potential applied in the rotating drum can be used to create an electric field that leads from the tip of the needle and converges at the sharp pin in the rotating drum [B. Sundaray, V. Subramanian, T. S. Natarajan, R. Z. Xiang, C. C. Chang and W. S. Fann, Appl. Phys. Lett., 84(7), 1222-1224, 2004]. The pin in the drum is mounted vertically and it lies directly below the positively charged needle. However, in this method, the distance between the two electrodes is highly limited to maintain the electric field from the needle to the sharp pin. Thus this method of collection may not be suitable when a less volatile solvent which is used to dissolve the polymer.
Katta et. al. used a rotating wire drum to collect aligned electrospun nanofibers [P. Katta, M. Alessandro, R. D. Ramsier and G. G. Chase, Nano. Lett., 4, 2215-2218, 2004]. The rotation speed of the drum has been reported to be 1 revolution per minute (rpm). This is a much slower rotating speed compared to other nanofiber alignment methods using rotating drum. However, although there is significant alignment of the nanofibers along the circumference of the axis for the first 15 min, the fiber alignment starts to go astray after that. The loss in alignment is attributed to accumulation of charge on the deposited nanofibers on the drum. Since the drum is rotated at such a low speed, the aligning of the nanofibers may not be due to the mechanical winding of the nanofibers around its circumference. The electric field profile created by the parallel wires with gaps between them may play a part in aligning the nanofibers.
Zussman and co-workers have modified the design of a drum and used a tapered, wheel-like disk as the collector [A. Theron, E. Zussman and A. L. Yarin, Nanoteclnology, 12, 384-390, 2001]. It was found that most of the nanofibers could be collected on the sharp edge. The collected fibers were oriented parallel to each other along the edge. These authors have also simulated the electrostatic field of this configuration and revealed that the field strength increased dramatically near the edge of the disk. Because of the strong electrostatic attraction, the charged nanofibers were continuously wound on the edge when the disk was rotating at a relatively high speed. They further demonstrated that nanofiber crossbars could be readily fabricated using this collector.
With the use of a similar setup, Xu et. al. have also fabricated well-aligned nanofibers [C. Y. Xu, R. Inai, M. Kotaki, S. Ramakrishna, Biomaterials, 25, 877-894, 2004]. In addition to drums, metal or wooden frames have been explored by Vaia, Wendorff, and their co-workers to collect electrospun nanofibers as more or less aligned arrays [H. Fong, W. D. Liu, C. S. Wang and R. A. Vaia, Polymer, 43, 775-780, 2002 and R. Dersch, T. Liu, A. K. Schaper, A. Greiner and J. H. Wendorff, J. Polym. Sci. Part A: Polym. Chem., 41, 545-553, 2003].
US 4,689,186 (A. Bomat ) discloses a method to fabricate tubular products for blood vessel prosthesis and urinary and bile duct applications. According to this approach, deposited fibers can be circumferentially oriented substantially by employing an auxiliary electrical field. The auxiliary electrode made of parallel conducting strips was given a negative voltage such that the electric field extends from the positively charged needle to the auxiliary electrodes. The parallel conducting strips has the effect of concentrating the electric field along the orientation of the parallel strips. Thus the electrospinning jet has a greater tendency to spin in the direction of the orientation of the strips. When a nonconducting rotating mandrel is placed between the needle tip and the auxiliary electrode, the fibers can be easily picked up as it accelerates towards the electrode. In this way, aligned nanofibers along the circumference of the nanofibers can be obtained at a lower rotation speed as compared to using a rotating mandrel alone. To alter the electric field such that more electric field lines converge towards the auxiliary electrode, parallel knife-edged bars can be used instead of strips [W. E. Teo, M. Kotaki, X. M. Mo and S. Ramakrishna, Nanotechnology, 16, 918-924, 2005].
Xia and Li demonstrated that the geometrical configuration of a conductive collector had a profound effect on the orientation of electrospun nanofibers [D. Li, Y. Wang and Y. Xia, Nano Lett., 3(8), 1167-1171, 2003]. By using a collector consisting of two conductive strips separated by a void gap with variable width, electrospun nanofibers could be uniaxially aligned over long length scales during the spinning process. Xia and Li disclose that it is convenient to transfer the aligned nanofibers onto other solid substrates for further processing steps and applications. Moreover, making use of the behavior of the electrospinning jet across the gap between electrodes, modification to the setup can be used to achieve more complicated arrangement of the electrospun nanofibers. Finally, it should be mentioned that repulsion caused by the charges on the deposited nanofibers means that the collection time cannot be more than a few minutes thus this method may not be suitable for collection of a large patterned mesh or thick patterned mesh.
Recently, Pan et. al. have reported a novel method to prepare ordered fibers by refitting the setup of conventional electrospinning [H. Wu, D. Lin, R. Zhang and W. Pan, J. Am. Ceram. Soc., 90(2), 633-634, 2007]. According to this approach, a large electric field and a relatively short distance are used, and essentially different from conventional electrospinning process, the nanofibers are collected between the electrospinning tip and the collector. Furthermore, Li et. al. have utilized a simple and versatile technique to manufacture aligned polymer nanofibers with infinite length and over large collector area [H. Pan, L. Li, L. Hu and X. Cui, Polymer, 47, 4901-4904, 2006]. Unlike the conventional technique, in this method, two needles with opposite voltages spray simultaneously. The electrospun fibers with opposite charges attract each other, stick together and form a yarn. Then the yarn, which is neutral as a whole is easily collected.
US 6,713,011 B2 (Chu et al. ) discloses an apparatus and a method for electrospinning polymer fibers and membranes according to the preamble of claim 1. The apparatus comprises an array of electrospinning spinnerets electrically charged at a first potential. A ground plate is positioned below the array such that an electric field is created between the charged spinnerets and the ground plate, which causes tiny jets of a conducting, fiber-forming fluid to be ejected from the spinnerets and spray towards the ground plate, forming sub-micron diameter filaments or fibers. A moving support membrane is positioned between the charged spinnerets and the ground plate to collect the fibers and to form an interconnected web of the fibers. The support membrane is substantially flat and moves passed the spinnerets by being unwinded from an upstream unwind roll and winded onto a downstream rewind roll.
As mentioned above, several techniques have been developed to align electrospun nanofibers and some breakthroughs have been obtained. The results are promising, but these methods need to be further improved for practical applications. In the technique of using a rotating drum as the collector, only partial fiber alignments have been achieved. Other techniques can produce well-aligned nanofibers, but only of limited length, area, and/or thickness. Thus, it is still challenging to fabricate precisely oriented nanofibers, given the chaotic nature of the spun jet motion, build-up of surface charges on the collector, and electrical conductivity of the collector influencing the electrostatic force.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide an improved apparatus and method to further increase the control over aligning the micro- or nano-fibers produced from different materials during the process of electrospinning. A closely-related object of the present invention is to control the density and other physical, morphological, mechanical, biological, and chemical properties of the micro- or nano-fibrous materials, produced through the electrospinning process. It is a further closely-related object of the present invention, to enable a reliable design and production of 2D and/or 3D structures composed of aligned micro- or nano-fibers of the same or different materials.
The above and further objects are solved by an apparatus according to claim 1 and by a method according to claim 8. Further advantageous embodiments are the subject-matter of the dependent claims.
According to a first aspect of the present invention there is provided an apparatus for electrospinning 2D- or 3D-structures of micro- or nano-fibrous materials, in particular for electrospinning 2D- or 3D-structures consisting of or comprising a plurality of precisely aligned and densely packed micro- or nano-fibrous materials, comprising at least one electrospinning spinneret being electrically charged at a first potential; electrode means disposed adjacent said at least one electrospinning spinneret and being electrically charged at a second potential different from said first potential to thereby generate an electric field between said at least one electro spinning spinneret and said associated electrode means for aligning micro- or nano-fibers injected by said at least one electrospinning spinneret; and collecting means for collecting micro- or nano-fibers injected by a respective spinneret and aligned by said associated electrode means. According to the present invention the electrode means comprises at least two conductive electrodes, which are disposed in parallel with each other and rotate in the same rotating direction, wherein the collecting means is movably disposed between said at least two conductive electrodes for collecting said aligned micro- or nano-fibers.
The two electrodes which are disposed spaced apart from each other and in parallel with each other serve to generate an electric field of a precisely specifiable strength and electric field distribution that is virtually not effected by external stray fields and other sources. According to the invention a droplet of a fiber-forming material, in particular melt or solution, is ejected by the associated spinneret(s) into the space between the electrodes to thereby form stretched micro- or nano-fibers that are immediately aligned with each other in a precise positional relationship with respect to each other due to the electric field generated by the electrodes. Preferably the two opposite ends of these fibers adhere to the circumference or surface of the electrodes due to electrostatic and/or adhesive forces. After being aligned the fibers are transported away from the location of injection to a collecting means disposed downstream in the rotating direction of the rotating electrodes. Thus, further droplets can be injected substantially unhindered and unaffected by any previously generated fibers to thereby form fresh micro- or nano-fibers. Thus, a substantially continuous or batch-wise feed of precisely aligned micro- or nano-fibers can be generated that adhere to the circumference or surface of the rotating electrodes. This feed of precisely micro- or nano-fibers is continuously transported to the downstream collecting means to be deposited onto the surface thereof.
Thus, according to the invention the alignment and further parameters of the feed, such as density, thickness and further properties of the fibers can be precisely controlled, which enables the production of hybrid materials consisting of or comprising a plurality of micro- or nano-fibers that are arranged in a unique densely packed arrangement in precise alignment with each other and precisely controllable geometrical configuration. Thus enables the production of new hybrid materials offering unique and unprecedented features and characteristics.
The electrodes can have a virtually arbitrary geometric configuration, e.g. can comprise curved or bulged surfaces that are disposed in parallel with each other. More preferably the surfaces of the electrodes are flat enabling the generation of a predetermined electric field according to the well-known parallel-plate-capacitor-principle. Even more preferably the circumference of the electrodes is circumventing, in particular elliptical or circular shaped. Adherence of the micro- or nano-fibers at the outer circumference of circular disc-shaped electrodes enables a steady and precisely controllable feed of fibers to the downstream collecting means.
In order to avoid stretching and/or twisting of the fibers aligned between the electrodes while being rotated to the downstream collecting means, according to another embodiment the oppositely disposed electrodes are rotating synchronously with each other about a common rotating axis, which preferably extends perpendicular to a surface of the electrodes and through a center thereof.
According to another embodiment the collecting means comprises at least one flat conductive plate or at least one cylindrical mandrel extending perpendicular to a surface of said associated electrodes. The plate or mandrel may bridge substantially the entire gap between the oppositely disposed electrodes although according to other embodiments there may exist a certain gap between the periphery of the plate or mandrel and the surface of the electrodes. Thus, according to the present invention the fibers transported with a given velocity towards the surface of the collecting means by the rotating electrodes finally impinge onto the surface of the collecting means and are smoothly peeled off the surface of the electrodes to be deposited on the surface of the collecting means. In this step of depositing the fibers, the orientation and alignment of the fibers remains substantially unchanged as the deposition of the fibers is not affected by the generation of new fibers at the upstream injecting location due to the geometrical separation between the respective spinneret and the associated collecting means.
According to another embodiment, deposition of the fibers on the surface of the collecting means is even less effected if the collecting means is grounded or precisely controlled to a predetermined electrostatic potential, which most preferably is identical to that of the associated electrodes between which the collecting means is disposed.
According to another embodiment of the present invention the collecting means can be moved with an e.g. constant or variable linear speed between said associated electrodes and/or can be rotated at an e.g. constant or variable rotating speed about a given or variable rotation axis in the three-dimensional space between said associated electrodes. Thus, the conditions during deposition of the aligned fibers, which are transported towards the surface of the collecting means by the rotating electrodes, can be varied easily and in a reliable manner, by controlling the speed and orientation of the rotation axis of the collecting means. This enables to vary many different characteristics of the hybrid material deposited on the surface of the collecting means, such as density of the fibers, morphology, homogeneity of the alignment and density of the hybrid material, porosity etc.
According to another embodiment of the present invention a relative speed between the associated electrodes and the collecting means can be varied for varying the density of micro- or nano-fibers collected by said collecting means. The relative speed may be varied gradually, stepwise, ramp-like manner or in any other manner, as will become apparent to a person skilled in the art.
According to another embodiment of the present invention each of said electrospinning spinnerets is configured for ejecting different materials simultaneously or subsequently or in accordance with any desired time-sequence, in order to enable the deposition of a predetermined mixture of precisely aligned micro- or nano-fibers on the surface of the collecting means, said mixture being determined by the time sequence of actuating the electrospinning spinnerets.
According to another embodiment of the present invention the apparatus comprises a plurality of electrospinning spinnerets disposed at different angular positions with respect to a rotating direction of said conductive electrodes. These angular position can be fixed or may be varied continuously, step-wise or in accordance with any desired motion-sequence in order to even further vary the properties, composition and geometrical configuration of the hybrid material deposited onto the surface of the collecting means.
As will become apparent to a person skilled in the art, further related aspects of the present invention are directed to method for electrospinning, in correspondence with the afore-mentioned apparatus, and to a hybrid material produced by performing such an electrospinning process.
In such a process the fiber-forming material ejected by the respective spinneret(s) for generating the micro- or nano-fibers may consist of dissolved and/or molten polymers, ceramics, metals and mixtures of at least two of the afore-mentioned materials and/or different precursors or mixtures thereof.
In such a process the molten mixtures and/or solutions may contain nanoparticles.
Preferably such nanoparticles are nanoparticles of metals, metal oxides and/or ceramics.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following preferred embodiments of the present invention will be described in exemplary manner and with reference to the drawings, from which further objects, features and advantages may be derived by the person skilled in the art and wherein:

Fig. 1: is a schematic representation of an apparatus a method according to a first embodiment of the present invention;
Fig. 2a and 2b: show in a schematic side view and front view the positional relationship between the conductive electrodes for aligning the fibers injected by an electrospinning spinneret and a collector of the apparatus according to the embodiment shown in Fig. 1;
Fig. 2c: shows the electric field in the embodiment of Fig. 1;
Fig. 3a and 3b: show an apparatus for the production of different patterns of totally aligned micro/nano-fibers according to another embodiment of the present invention together with the structure of a layered material produced in this manner;
Fig. 4a and 4b: show an apparatus for the production of totally aligned micro/nano-fibers with desired and tunable distances from one another according to another embodiment of the present invention together with the structure of a layered material produced in this manner;
Fig. 5: shows an apparatus and method for changing the patterns of the totally aligned micro/nano-fibers in different layers according to the first embodiment in two different operational conditions;
Fig. 6a and 6b: show an apparatus and method for the production of composites of different aligned micro/nano-fibrous materials according to another embodiment of the present invention with the collector in two different angular positions; and
Fig. 7a and 7b: show in a schematic side view and front view an apparatus and method for the deposition of totally aligned micro/nano-fibers over a cylindrical collector according to another embodiment of the present invention.

Throughout the drawings the same reference numerals relate to the same or technically equivalent features or elements.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE PRESENT INVENTION

Figure 1 shows an apparatus for electrospinning according to a first embodiment of the present invention, capable of precisely controlling both the degree and pattern of the alignment of the micro- or nano-fibers. The apparatus has overall a structure comparable to that of conventional systems and comprises a spinneret for ejecting droplets of fiber-forming material, which is fed by a pump 1 to deliver the electrospinning melt or solution 1. The output of a high-voltage power supply 10, more specifically an AC power supply, is applied to the spinneret 3 in order to generate a predetermined potential difference between the spinneret 3 and the grounded electrodes 4A, 4B, for alignment of the produced micro- or nano-fibers 5 (only schematically shown). Overall the principle of generating the micro- or nano-fibers from the melt or solution using the spinneret 3 is comparable to that according to prior art systems well known to the person skilled in the art, so that a detailed description can be omitted. However, both the alignment mechanism and collection method of the produced micro- or nano-fibers are different, which will be described in detail below.
According to the embodiment shown in Fig. 1, the micro- or nano-fibers 5 are aligned between the two parallel disc-shaped rotating electrodes 4A and 4B of a conductive material. In this embodiment the disc-shaped electrodes 4A, 4B are grounded via the rotating shaft 7 of electrically conductive material. Thus, the electrospun fibers 5 are aligned in parallel with each other between the two rotating electrodes 4A and 4B. As shown in Fig. 1, the fibers 5 accumulate substantially at the periphery of the disc-shaped electrodes 4A, 4B. These are driven by the motor 6 and via the rotating shaft 7 at a constant angular speed, which of course can also be varied as wished according to further embodiments of the present invention. In the embodiment shown in Fig. 1 the two parallel electrodes 4A, 4B are rotated synchronously with each other so that the aligned fibers 5 between the electrodes 4A, 4B are not stretched while being transported by the rotating electrodes.
As shown in Fig. 1, at a side of the electrodes 4A, 4B substantially diametrical opposite with regard to the spinneret 3, a collector 9 in the form of a flat conductive plate is disposed symmetrically between the two electrodes 4A, 4B. The collector 9 extends substantially perpendicular to the surfaces of the electrodes 4A, 4B and is disposed at a radius smaller than the radius D/2 of the two electrodes 4A, 4B. Thus, the fibers 5, which are aligned by the electrodes 4A, 4B and are transported along a circular path by the rotating electrodes 4A, 4B, finally come into contact with the collector 9, where they are deposited in a configuration, which is substantially determined by the alignment caused by the rotating electrodes. Thus, in this embodiment, the collected fibers 8 are deposited on the collector plate 9 substantially in parallel with each other.
As shown in Fig. 1, the collector plate 9 is grounded, although according to further embodiments the collector plate 9 may also be charged to a predetermined electrostatic potential relative to the spinneret 3 and electrodes 4A, 4B, as desired. Thus, the fibers 8 experience no electrostatic counter-forces when being deposited on the collector plate so that their alignment remains substantially unaffected white being deposited. As shown in Fig. 1, the width of the collector plate 9 in x-direction is substantially identical to the distance between the electrodes 4A, 4B, although according to further embodiments this condition may not necessarily be satisfied.
As shown in Fig. 1, the collector plate 9 is coupled with a drive 11 to be moved in radial outward direction of the electrodes and/or rotated between the electrodes 4A, 4B. If one assumes as an example that the collector plate 9 is moved at constant speed in radial outward direction, the fibers 8 will be deposited on the surface of the collector plate 9 in parallel with each other and at a constant density, which is given inter alia by the amount of fibers 5 collected and transported by the rotating electrodes 4A, 4B and the moving speed of the collecting plate 9. Thus, overall the alignment, density and other physical, morphological, mechanical, biological, and chemical properties of the micro- or nano-fibrous materials, produced through the electrospinning process can be controlled precisely and in a reliable manner.
Figure 2 shows a schematic diagram of the different parts of the collector of the present invention.
Fig. 2c shows the electric field lines between the two parallel disc-shaped electrodes 4A, 4B. As can be seen in Fig. 2c, the electric field lines between the two parallel disc-shaped rotating electrodes 4A, 4B are divided into two parts, in a way that the direction of these field lines is toward the circumference of the discs 4A, 4B. As is well-known to the person skilled in the art, during the electrospinning processes, the direction of the electrospinning jet between the spinneret 3 and target (collector) is highly dependent on the electric field lines. Thus according to the present embodiment, the electrospinning melt or solution or in other words the micro- or nano-fibers to be produced, are alternatively aligned in parallel to one another between the two parallel disc-shaped rotating electrodes 4A, 4B. In the cross-sectional view according to Fig. 2b the aligned fibers are shown as to extend in parallel with each other between the rotating electrodes 4A, 4B to thereby substantially follow the electric field lines. From the location of injection into the space between the electrodes 4A, 4B the fibers are then transported towards the collector plate 9, due to the rotary movement of the disc-shaped electrodes 4A, 4B. As can be derived from Fig. 2a and 2b, the fibers are then moved into immediate vicinity of the collector plate, from where they finally hit the collector plate to be separated from the electrodes 4A, 4B and be deposited onto the surface of the collector plate 9. If the collector plate 9 is not moved the fibers will accumulate on the same location in the form of a stripe-shaped staple of parallel micro- or nano-fibers. In order to avoid an excessive accumulation of fibers on the same location on the collector plate and to efficiently control parameters, such as density and thickness of the layer of micro- or nano-fibers to be deposited onto the collector plate, the collector plate 9 is moved radially outward or outward, as indicated by the double-arrow V in Fig. 2a. Thus, a quasi homogeneous layer consisting of micro- or nano-fibers and having a predetermined density is deposited onto the surface of the collector plate, the thickness being determined by parameters of the apparatus itself, such as rotating speed of the electrodes 4A, 4B, the ejection rate of the spinneret 3 and the velocity of the collector plate 9, as will become apparent to a person skilled in the art when studying this description. Thus, the properties of the layer to be deposited on the surface of the collector plate 9 can be controlled efficiently and precisely.
As it is shown Fig. 2a, the space between the two disc-shaped rotating electrodes 4A, 4B is virtually divided into two half space I and II. Half space I is a segment of the space between the two disc-shaped rotating electrodes that is between point N and the collector 9, where the micro- or nano-fibers are most loaded over the collector, whereas region II is the remaining half space between the two parallel disc-shaped electrodes 4A, 4B, where literally no micro- or nano-fibers are deposited on the collector 9. As will be apparent to a person skilled in the art, the collector 9 can be positioned in the space between the two parallel disc-shaped rotating electrodes 4A, 4B at any point, as desired and/or can be moved with a suitable speed.
According to a preferred embodiment of the present invention, it is preferred that the collector 9 enters the space between the two parallel disc-shaped rotating electrodes 4A, 4B at a proper speed and point, to thereby adjust the density of the layer of fibers deposited on the collector 9. According to a most preferred embodiment of the present invention, it is preferred that the collector 9 enters the space between the two parallel disc-shaped rotating electrodes 4A, 4B at an appropriate distance from point N.
As shown in Fig. 1, the collector 9 can be driven by the motor 11 to be moved precisely between the two parallel disc-shaped rotating electrodes 4A, 4B at any direction, preferably in a direction perpendicular to the circumference of the disc-shaped rotating electrodes 4A, 4B. This allows preventing the build-up of the micro- or nano-fibers in one section of the collector 9 and also can be used for depositing different number of layers over one another.
According to another embodiment of the present invention, the collector can be moved stepwise at one or several movements in a direction perpendicular to the circumference of the disc-shaped electrodes 4A, 4B, which can help create more layers in one or several segments of the layer on the collector 9 while keeping the other segments in the vicinity as single layers or layers of different numbers.
According to another embodiment of the present invention, the collector 9 can also be designed to whirl completely or to some extent with respect to the two parallel disc-shaped rotating electrodes 4A, 4B.
According to another embodiment of the present invention, the movement of the collector 9 in any of the mentioned directions and with any desired speed(s) can lead to the formation of two or three-dimensional structures of micro- or nano-fibers each layer of which can have same or different materials and/or alignments.
According to another embodiment of the present invention, and opposite to the conventional rotating cylinder method, high angular speeds are not required for the two parallel disc-shaped rotating electrodes 4A, 4B. According to another embodiment of the present invention, the angular speed of the two parallel disc-shaped rotating electrodes 4A, 4B can vary from a minimum value ((ω_min>0) up to any desired maximum value.
It is noteworthy that the higher the speed of the rotation of the two parallel disc-shaped rotating electrodes is, the lower can become the extent of the alignment of the micro- or nano-fibers. This is because one of the forces affecting the micro- or nano-fibers before they are positioned on the collector is the resistance of the atmosphere between the two parallel disc-shaped rotating electrodes. The faster the rotation is, the higher becomes this resistance force, and the harder it becomes for the micro- or nano-fibers to maintain the implied alignment, which decreases the extent of alignment.
According to another embodiment of the present invention, the distance between the two parallel disc-shaped rotating electrodes, being a determining factor affecting the length of the aligned micro/nano-fibrous structure, can be designed to be fixed or to vary from a minimum optimum distance H_min to a maximum optimum one H_max. According to another embodiment of the present invention the distances H_min and H_max can vary further depending on the materials used for electrospinning.
It is noteworthy that these minimum and maximum values are the minimum or maximum gap length between the two parallel disc-shaped rotating electrodes respectively which let the formation of aligned micro/nano-fibers.
The two parallel disc-shaped rotating electrodes used in the present invention can be of any size and dimensions and they can be either disc-shaped, straps, and/or hallow rings. The discs may also be of the same or of different sizes. It is noteworthy that the smaller is the size of the two parallel disc-shaped rotating electrodes, the harder it is to handle the process. According to a preferred embodiment of the present invention, the diameter of the two parallel disc-shaped rotating electrodes is preferred to be the same and their thickness can be arbitrary.
According to another embodiment of the present invention, different number of parallel disc-shaped electrodes as well as collectors can be used to build an electrospinning device according to the present invention, however at least two parallel disc-shaped rotating electrodes and one collector are necessary. The number of electrospinning jets and collectors, according to another embodiment of the present invention can vary. However, at least one electrospinning jet and one collector are used.
The shape of the collector can be chosen based on the aim of the design of system. However, according to the preferred embodiment of the present invention it is preferred to be round, square, or rectangular. According to the most preferred embodiment of the present invention the shape of the collector is preferred to be rectangular or square.
According to another embodiment of the present invention the material used for building the collector can be any proper material of different chemical and/or physical properties. It is also noteworthy that it is preferred that the collector be made of conducting materials. This is due to the fact that, in the case of the application of an insulating material in the construction of the collector, the static charges that have been built on the body of the micro/nano-fibers cannot be discharged to the collector. The layers that have been built first will repel the new micro- or nano-fibers that are being deposited on the 2D/3D produced array of micro- or nano-fibers. Conductive materials lack this disadvantage as a result of their ability to discharge the deposited micro- or nano-fibers.
As a result, it is preferred that the collector used in the system of the present invention be made of a conductive material, according to another embodiment of the present invention, such aluminum, steel, copper, gold, silver, platinum. According to the most preferred embodiment of the present invention, it is preferred that the collector used in the system be made of aluminum.
Also, one must note that, in case the discharge of the collector and hence the layers of the micro/nano-fibers produced on it, is facilitated in some way the alignment of each and all layers and their disposition on one another can be modified leading to better 2D/3D structures of micro/nano-fibers.
According to another embodiment of the present invention, the collector that is made of a conductive material is connected to the earth or a pole of opposite charge to help the discharge happen with a higher efficiency.
The speed of the movement of the collector between the two parallel disc-shaped rotating electrodes, V(t), is another important factor that affects the efficiency of the present method. The radial movement (i.e., movement perpendicular to the circumference of the rotating discs) of the collector in the space between the two parallel disc-shaped rotating electrodes helps avoid the build-up of the produced micro- or nano-fibers in one point of the collector, and rather distribute them evenly all over its surface. It is ineluctable to mention that the higher this speed is the higher the space between the deposited micro- or nano-fibers will be and on the contrary the slower it moves the more dense will become the produced micro- or nano-fibrous structure.
According to another embodiment of the present invention, the speed of the movement of the collector between the two parallel disc-shaped rotating electrodes can be designed to vary according to any functions of desire or be completely constant over the whole path.
According to another embodiment of the present invention, the overall (i.e. over the whole layer(s) on the collector) or local (i.e. over one spot of each layer over the collector) density of the aligned micro- or nano-fibers can be determined by determining the speed of the movement of the collector.
According to another embodiment of the present invention, even the number of the layers in one location of the layer over the collector can also be determined by either stopping the collector at desired intervals or moving it in a perpendicular path in addition to the radial one, or even returning it over a short path in the middle of the overall path of the collector for one to several times.
According to another embodiment of the present invention, in case a uniform distribution of the micro/nano-fibers is of desire, the speed of the movement of the collector is kept constant.
According to another embodiment of the present invention, the density of the distribution of the micro/nano-fibers can be increased by means of decreasing the constant speed of the movement of the collector, while the density can be decreased by increasing this speed.
According to another embodiment of the present invention, the minimum and maximum speeds of the movement of the collector are between zero and any positive desired maximum value.
Fig. 3a and 3b show an apparatus for the production of different patterns of totally aligned micro/nano-fibers according to another embodiment of the present invention together with the structure of a layered material produced in this manner. According to Fig. 3a the apparatus comprises a plurality of n separate electrospinning spinnerets 3 that are referred to with indices 1 to n. All or some of the jets produced by the spinnerets 3_1 to 3_n can be used to inject the same material or different ones in order to produce different hybrid patterns, e.g. patterns containing different micro/nano-fibrous materials of desire.
As shown in Fig. 3a the spinnerets 3_1 to 3_n are disposed at different angular positions along the circumference of the rotating disc-shaped electrodes 4A and 4B (not shown), spaced apart to each other. These angular positions can either be fixed over the time or can be shifted, e.g. by angular movement of each or individual ones of the spinnerets 3_1 to 3_n. In the later case a driving motor would be associated to each, several or all of the spinnerets 3_1 to 3_n.
In order to enable the production of variable hybrid patterns the spinnerets 3_1 to 3_n can be actuated all at the same time or individually in arbitrary actuation patterns, as desired. Further, the spinnerets 3_1 to 3_n can be used to inject the same or different melts or solutions in order to obtain such hybrid patterns, i.e. patterns containing different micro/nano-fibrous materials, of desire.
Thus, the use of a plurality of fixed or shiftable (movable) electrospinning spinnerets enables the production of rather complicated patterns.
Referring in particular to Fig. 3b and Fig. 4b, according to another embodiment of the present invention, the width W_i of the i-th layered section (with i ranging from 1 to n) of the micro- or nano-fibrous material deposited on the collector 9 and/or the density of each of these sections, composed of different or the same micro/nano-fibrous materials, can be varied by adjusting the constant or variable speed(s) of the collector 9, the speed of the jets, and other effective parameters, irrespective of whether they are superimposed on another layer of another or the same micro/nano-fibrous material or as a single layer
According to another embodiment of the present invention, the position, angle, and injection rate of the jets and also the position, angle, and speed of the collector can be adjusted so that more than one jet can perform its role over one point of the collector at the same time with the other jet(s).
Referring to Fig. 4a and Fig. 4b, according to another embodiment of the present invention, in the case of the application of one jet of a given material, by adjusting the relative speed of the collector 9 with respect to the jet and the angular speed of the two parallel disc-shaped rotating electrodes 4A and 4B (not shown), different physical, mechanical and morphological properties of the product can be adjusted.
According to another embodiment of the present invention, in the case of the application of a plurality of jets (of the same or several different materials for electrospinning), by adjusting the relative speed of the collector with respect to the jet and angular speed of the two parallel disc-shaped rotating electrodes, not only different physical, mechanical and morphological properties of the product can be obtained, but also new chemical, biological and other properties.
These above capabilities of the system of the present invention are of very great importance especially in the case of designing hybrid scaffolds for the regeneration of complicated living tissues using tissue engineering strategies.
According to the embodiment of Fig. 5, the collector 9 can be rotated horizontally with respect to an axis perpendicular to the surface of the collector 9, i.e. about an axis that is perpendicular to the axis of rotation of the rotating disc-shaped electrodes 4A and 4B. By moving the collector plate 9 radially inward (as shown) and/or radially outward with a constant speed v during deposition individual layers will be deposited onto the surface of the collector plate, each layer consisting of a fibers aligned in parallel with each other and with a given unique rotational orientation with respect to the side edges of the collector plate 9. As an example, while the fibers in the left-hand part of Fig. 5 are aligned in parallel with the bottom and top edge of the collector plate 9, the fibers in the right-hand part of Fig. 5, where the collector plate has been rotated further by an angle α counterclockwise, are aligned under the angle α with respect to the bottom and top edge of the collector plate 9. In this way new patterns can be knitted over the collector plate 9. In this way also different 2D/3D aligned micro/nano-fibrous structures of the same or different materials can be produced. The degree of the horizontal rotation of the collector with respect to an axis perpendicular to its plane is preferred to be 0 < α < 90.
As will become apparent to a person skilled in the art the feature of rotating the collector plate about an axis perpendicular to its surface can be combined with any of the afore-mentioned embodiments in order to obtain even more variable hybrid materials.
According to another embodiment of the present invention, in case the different aligned micro/nano-fibrous layers are desired to be composed of different materials (hybrid structures), either one jet (injecting different materials at desired time intervals) or a plurality of jets (each injecting any desired materials) can be designed in the structure of the electrospinning system of the present invention (Fig. 6a and Fig. 6b).
As will become apparent to a person skilled in the art, the collector can have any shapes, sizes, and can be either smooth or with any desired pattern or surface.
According to another embodiment of the present invention, the collector should have dimensions that let it fit between the two parallel disc-shaped rotating electrodes, and rotate freely in any desired directions.
According to another preferred embodiment of the present invention, the collector can have other 3D spherical, cubic, or cylindrical structures.
As an example for another most preferred embodiment of the present invention and as shown in Fig. 7a and Fig. 7b, the collector is formed as a cylindrical mandrel 9 that extends in a direction in parallel with the axis of rotation of the disc-shaped rotating electrodes 4A, 4B and can be rotated about this axis with the same or a different rotational speed as the rotating electrodes 4A, 4B, as indicated by the index ω'.
Such a cylindrical collector 9 can have any length and any diameter provided it does not prevent the formation of the aligned micro/nano-fibers. Furthermore, the cylindrical collector can rotate at any desired speed.
As will become apparent to a person skilled in the art, all dissolved and/or molten polymers, ceramics, metals, and mixtures of one or some of these materials and also different precursors or mixtures thereof capable of forming micro- or nano-fibers by electrospinning can be used in the system according to the present invention. The spinnerets for producing the jets of such materials can be arranged in any desired geometrical alignment.
According to another embodiment of the present invention, such molten mixtures and/or solutions that additionally contain nanoparticles can be converted to highly aligned micro/nano-fibers of desired patterns using the system of the present invention.
According to a more preferred embodiment of the present invention, such molten mixtures and/or solutions can contain nanoparticles of metals, metal oxides, ceramics, and polymers to be converted to highly aligned micro/nano-fibers of desired patterns using the system of the present invention.
According to the most preferred embodiment of the present invention, 2D and/or 3D matrices composed of highly aligned carbon micro/nano-fibers can be prepared using appropriate precursors and applying the system of the present invention.
According to another embodiment of the present invention, highly aligned micro/nano-fibers of different morphologies including porous, flatted or ribbon-like, core-sheath, helical and/or hollow micro/nano-fibers that have already been produced can also be produced using the system of the present invention.
In the following further advantageous features will be summarized briefly that can be combined with any of the above embodiments to obtain further embodiments according to the present invention:

the single electrospinning jet or each electrospinning jet is either fixed in an optimum position or mounted on a device that is capable of moving it to any desired position, at any desired speed along the circumference of the rotating electrodes;
the collector can move at a constant or variable linear speed of V(t), in any desired direction, between the parallel rotating electrodes;
the collector can also be configured to rotate around the three Cartesian axes of x, y, and z at any constant or variable speeds;
the number of the afore-mentioned parallel disc-shaped rotating electrodes is preferred to be 2;
the speed of rotation of the afore-mentioned two parallel disc-shaped electrodes is preferred to be between 0 and any positive desired maximum value;
the total number of the electrospinning jets varies based on the structure of desire and also on the number and type of materials to be used in the electrospinning process;
each of the electrospinning jets can be used to inject the same or several different materials, or mixtures thereof, at the same time or at different intervals;
in case different materials are used during the electrospinning process, different numbers of electrospinning jets can be used for the injection of each material, depending the structure of desire;
in case different materials are used during the electrospinning process, different electrospinning jets are preferred to be used for the injection of each material;
the electrospinning jets can have fixed positions and angle with respect to the rotating electrodes and the collector;
the collector can be configured to be moved in any desired direction between the two parallel disc-shaped rotating electrodes;
the collector can be configured to be moved at different constant and/or variable speeds in each of the afore-mentioned directions of desire;
the collector can be configured to be moved perpendicular to the circumference of the two parallel rotating electrodes;
the speed of the collector over the diameter of the rotating electrodes is preferred to be between 0 and any positive desired maximum value;
the speed of the collector can be either constant or variable at the mentioned direction;
the relative speed between the parallel rotating electrodes and the collector can be used to increase or decrease the density of the micro/nano-fibers in at least a part of the final product;
the total number of layers of the aligned micro/nano-fibers of desire can be increased in at least a part of the produced structure, by decreasing the relative speed between the two parallel rotating electrodes and collector to zero, at desired intervals;
the collector can be configured to be rotated about any of the Cartesian axes of x, y, and z or other axes at any constant or variable speeds;
different 2D/3D structures of the produced aligned micro/nano-fibers can be obtained by applying different rotation angles;
the rotation angle, α, of the collector can be chosen to vary between 0 and 90°;
all dissolved and/or molten polymers, ceramics, metals, and mixtures of one or some of these materials and also different precursors or mixtures thereof can be used to yield the micro/nano-fibers of desired alignment using the system of the present invention;
molten mixtures and/or solutions that contain nanoparticles can also be converted to highly aligned micro/nano-fibers of desired patterns using the system of the present invention;
more preferably molten mixtures and/or solutions that contain nanoparticles of metals, metal oxides, ceramics, and polymers can also be converted to highly aligned micro/nano-fibers of desired patterns using the system of the present invention;
in the case of using different materials different 2D/3D hybrid aligned micro/nano-fibrous structures of different materials, with different alignment can be produced;
micro/nano-fibers of different morphologies including porous, flatted or ribbon-like, core-sheath, helical and/or hollow micro/nano-fibers that have already been produced can be produced using the system of the present invention.

As will become apparent to a person skilled in the art the term 'microfiber or microfibers' as used herein relates to fibers with strands less than one denier. As will become apparent to a person skilled in the art the term 'nanofiber or nanofibers' as used herein relates to fibers with diameters less than 100 nanometers. Special properties of micro- or nanofibers make them suitable for a wide range of applications like medical, consumer products, industrial, high-tech applications for aerospace, capacitors, transistors, drug delivery systems, battery separators, energy storage, fuel cells, information technology, and filtration and it is expressly noted that the present application is to cover any of the afore-mentioned applications as well as any related application.
While throughout the above description of the invention it was referred to the case of two parallel disc-shaped electrodes, it should be noted that all the description can also be applied in the case of two parallel rings, straps or any other conductive structures for use in a system according to the present invention.

LIST OF REFERENCE NUMERALS

1: Melt or solution to be electrospun
2: Pump to deliver the electrospinning melt or solution
3: Spinneret to form the droplet
4A, 4B: Parallel disc-shaped rotating electrodes
5: Aligned micro/nano-fibers between the parallel disc-shaped rotating electrodes
6: Motor to rotate the parallel disc-shaped rotating electrodes
7: Rotating shaft
8: Collected micro/nano-fibers on the surface of the collector
9: Collector to collect the aligned micro/nano-fibers produced between the parallel disc-shaped rotating electrodes
10: High-voltage power supply
11: Motor to move or rotate the collector between the parallel disc-shaped rotating electrodes
D: Diameter of the parallel disc-shaped rotating electrodes
H: Gap length between the parallel disc-shaped rotating electrodes
V(t): Linear speed of the collector
ω(t): Angular speed of the parallel disc-shaped rotating electrodes
Wi:: width of the ith aligned micro/nano-fibrous section produced from ith material
α:: degree of the horizontal rotation of the collector with respect to an axis perpendicular to its plane
d:: diameter of the cylindrical collector
ω':: angular speed of the cylindrical collector

Claims

An apparatus for electrospinning 2D- or 3D-structures of micro- or nano-fibrous materials, comprising:
at least one electrospinning spinneret (3) being electrically charged at a first potential;

electrode means (4A, 4B) disposed adjacent said at least one electrospinning spinneret (3) and being electrically charged at a second potential different from said first potential to thereby generate an electric field between said at least one electrospinning spinneret (3) and said associated electrode means for aligning micro- or nano-fibers injected by said at least one electrospinning spinneret (3); and

collecting means (9) for collecting micro- or nano-fibers injected by a respective spinneret and aligned by said associated electrode means;

characterized in that

said electrode means comprises at least two conductive electrodes (4A, 4B), which are disposed in parallel with each other and rotate in the same rotating direction and

said collecting means (9) is movably disposed between said at least two conductive electrodes (4A, 4B) for collecting said aligned micro- or nano-fibers.
The apparatus according to claim 1, wherein said electrodes are flat circular plates rotating synchronously with each other about a rotating axis.
The apparatus according to any of the preceding claims, wherein said collecting means comprises at least one flat conductive plate (9) or at least one cylindrical mandrel extending perpendicular to a surface of said associated electrodes.
The apparatus according to any of the preceding claims, wherein said collecting means (9) is configured to move with a constant or variable linear speed between said associated electrodes (4A, 4B) and/or to rotate at a constant or variable speed about a given or variable rotation axis in the three-dimensional space between said associated electrodes.
The apparatus according to any of the preceding claims, wherein a relative speed between said associated electrodes and said collecting means can be varied for varying the density of micro- or nano-fibers collected by said collecting means.
The apparatus according to any of the preceding claims, wherein each of said electrospinning spinnerets is configured for injecting different materials simultaneously or subsequently.
The apparatus according to any of the preceding claims, said apparatus comprising a plurality of electrospinning spinnerets disposed at different angular positions with respect to a rotating direction of said conductive electrodes.
A method for electrospinning 2D- or 3D-structures of micro- or nano-fibrous materials, comprising:
injecting at least one jet of a liquid of a fiber-forming material via at least one electrospinning spinneret (3) being electrically charged at a first potential;

aligning a respective jet by means of an electric field generated between electrode means (4A, 4B) disposed adjacent said at least one electrospinning spinneret (3) and being electrically charged at a second potential different from said first potential; and

collecting a respective aligned jet on a collecting means (9) for forming said 2D-or 3D-structures; wherein:

said electric field is generated by at least two conductive electrodes (4A, 4B), which are disposed in parallel with each other and rotate in the same rotating direction and

said collecting means (9) is moved between said at least two conductive electrodes (4A, 4B) while collecting said aligned micro- or nano-fibers.
The method according to claim 8, wherein said electric field is generated by at least two flat circular plates rotating synchronously with each other about a rotating axis.
The method according to claim 8 or 9, wherein said micro- or nano-fibers are collected by at least one flat conductive plate (9) or at least one cylindrical mandrel extending perpendicular to a surface of said associated electrodes.
The method according to any of claims 8 to 10, wherein said collecting means (9) is moved with a constant or variable linear speed between said associated electrodes (4A, 4B) and/or rotates at a constant or variable speed about a given or variable rotation axis in the three-dimensional space between said associated electrodes.
The method according to any of claims 8 to 11, wherein the density of micro- or nano-fibers collected by said collecting means is varied by varying a relative speed between said associated electrodes and said collecting means.
The method according to any of claims 8 to 12, wherein each of said electrospinning spinnerets optionally injects different materials simultaneously or subsequently.
The method according to any of claims 8 to 13, wherein a plurality of electrospinning spinnerets are disposed at different angular positions with respect to a rotating direction of said conductive electrodes for injecting the same or different fiber-forming materials.
The method according to any of claims 8 to 14, wherein said fiber-forming material consists of dissolved and/or molten polymers, ceramics, metals and mixtures of at least two of the afore-mentioned materials and/or different precursors or mixtures thereof.
The method according to claim 15, wherein said molten mixtures and/or solutions contain nanoparticles.
The method according to claim 16, wherein said nanoparticles are nanoparticles of metals, metal oxides and/or ceramics.
A hybrid material consisting of or comprising a plurality of densely packed micro- or nano-fibers which are aligned with each other in a predetermined geometrical configuration, said hybrid material being produced by performing the process steps according to any of claims 8 to 17.