Patent Publication Number: US-11384452-B2

Title: Electrospinning device and method

Description:
FIELD OF THE INVENTION 
     The invention relates to an electrospinning device for producing a fibrous structure. The invention also relates to method of electrospinning. 
     BACKGROUND ART 
     U.S. patent publication US2005/224999 discloses an electrospinning device for producing fibrous materials. The device has an extrusion element configured to electrospin a substance using an electric field extraction of the substance from a tip of the extrusion element, a collector, and a chamber enclosing the collector and extrusion element. An ion generator is present to generate ions for injection into a Rayleigh instability zone in the chamber during operation of the device. 
     US patent publication US2007/042069 discloses a fiber spinning apparatus for charging a polymer-containing liquid stream using a point-electrode positioned adjacent the intended path of the liquid stream during operation. E.g. an ion flow is generated by a corona discharge to impart electrical charge to the polymer-containing liquid stream. 
     US patent publication US2005/104258 discloses an electrospinning device allowing to direct a polymer from a source electrode into an electric field that drives the formation of electrospun fibers that are deposited onto a collecting surface (being a counter electrode or a collecting surface between the source electrode and a counter electrode). Multiple electrically charged areas underneath the counter electrode allow to produce a pattern of areas where fibers are collected. 
     Electrospinning is a method to produce continuous fibers with a diameter ranging from a few tens of nanometers to a few tens of micrometers. To electrospin fibers, a suitable liquefied material may be fed through a small, electrically conductive nozzle. The liquefied material may be electrically charged by applying a high voltage between the nozzle and a counter electrode. The generated electric field causes a cone-shape deformation of the droplet at the nozzle tip. Once the surface tension of this droplet is overcome by the electrical force, a jet is formed out of the droplet and a fiber forms that moves towards the counter electrode. During the flight towards the counter electrode the fiber is continuously stretched and elongated by the different forces acting on it, reducing its diameter and allowing it to solidify by evaporation of the solvent or cooling of the material such that a solid fiber is deposited on the collector which is placed before the counter electrode or the counter electrode is used as collector directly. 
     Electrospinning uses an electric field, generated by a high voltage potential between nozzle and collector, to produce a fiber from a droplet at the nozzle tip. In alternative configurations fibers are drawn e.g. from a liquid bath, liquid covered ball, liquid filled opening or liquid covered wire. After stretching, the fiber is deposited on the collector surface. However, even with conductive collector surfaces, residual electric charges might remain inside the deposited fiber. These residual charges have an adverse effect on the process since they act as a repulsive force on the subsequent section of the fiber arriving at the collector. These residual charges are not always easy to remove efficiently, even with conductive collectors. Eventually, fibers are not in direct contact with the collector anymore but with underlying, poorly conducting fibers. 
     Several methods are proposed to improve the removal/neutralization of residual charges at the deposited fibrous structure. However, these methods rely on either reducing charge on the fiber in mid-air, or bombarding the collector surface with ions to alter the charge on the fibrous structure, see e.g. patent publication WO2016/147951. In WO2016/147951 a nanofiber manufacturing apparatus is described equipped with a collecting unit, a discharging unit, a power source unit, and an electricity-removing unit. The collecting unit dispenses a deposit-receiving material from one end and collects same at the other end. The discharging unit discharges a feedstock liquid and deposits nanofibers on a collecting surface. The power source unit generates a potential difference between the discharging unit and the collecting surface. The electricity-removing unit removes the charge with which the deposited nanofibers are charged. Rotatable bodies cause the collecting surface to face the discharging unit and the electricity-removing unit alternately. The electricity-removing unit extends across the whole width of the collecting surface. 
     SUMMARY OF THE INVENTION 
     It is on object of the present invention to provide an improved electrospinning device. 
     A first aspect of the invention provides an electrospinning device comprising:
         a container for holding a liquid comprising a polymer melt or a polymer solution;   a nozzle arranged to outlet a stream of the liquid from the container;   a collecting surface for collecting electro spun material coming from the nozzle during an electrospinning process so as to form a fibrous structure on the collecting surface;   a voltage supply system arranged to create a voltage difference between the nozzle and the collecting surface,   one or more electrostatic emitters arranged to locally distribute positive and/or negative ions onto the fibrous structure and collector surface, and   one or more rotatable bodies arranged to cause the collecting surface to face the nozzle and the electrostatic emitters in turn.       

     The present invention deploys the known technique of using ions to alter the charge on the deposited fibrous structure in a local manner. To obtain this, the one or more electrostatic emitters may be relatively small and positioned close to the surface of the collecting surface/fibrous structure, and have e.g. an effective area around the emitters with a radius of only 5-10 mm. This new technique offers precise control over the attractiveness/repulsiveness of certain areas of the collector/fibrous structure for subsequent fiber deposition. This enables a local built up of fibers, which enables patterning of the fibrous structure. So what was regarded previously as a problem (i.e. built up of charge in the fibrous structure during manufacturing) is now used by the inventors to its advantage. 
     Optionally, the device comprises a rotatable cylindrical body, the surface of which forms the collecting surface. 
     Optionally, the device comprises at least two rotatable bodies, and a looped conveyer belt arranged around the two rotatable bodies, wherein the surface of the belt forms the collecting surface. 
     Optionally, the collecting surface is arranged between the nozzle and the one or more electrostatic emitters. This allows to have the collecting surface, in combination with the rotatable bodies to face in turn (i.e. subsequently) the nozzle and the one or more electrostatic emitters. Furthermore, as in this embodiment, the electrostatic emitters are located at the opposite side from the collecting surface when viewed from the nozzle, the electrostatic emitters will have less influence on the area in the electrospinning device where the fibers are formed from the jet exiting the nozzles (the Rayleigh instability area). 
     Optionally, the electrostatic emitters are arranged in a row. 
     Optionally, the electrostatic emitters are arranged in an array. 
     Optionally, the electrostatic emitters are movable in a direction parallel to a rotation axis of the rotatable body or bodies. 
     Optionally, the electrostatic emitters comprise ion generators. 
     Optionally, the device comprises a control unit arranged to control the electrostatic emitters so as to create a pattern into the fibrous structure. 
     According to a further aspect there is provided a method of electrospinning comprising:
         holding a liquid comprising a polymer melt or a polymer solution in a container;   letting out a stream of the liquid from the container through at least one nozzle;   creating a voltage difference between the nozzle and a collecting surface;   collecting electro spun material coming from the nozzle so as to form a fibrous structure on the collecting surface;   distributing positive and/or negative ions onto the fibrous structure by way of one or more electrostatic emitters;   rotating the collecting surface by means of one or more rotatable bodies causing the collecting surface to face the nozzle and the one or more electrostatic emitters in turn.       

     Optionally, the method further comprising the step of controlling the electrostatic emitters so as to form a pattern in the fibrous structure. 
    
    
     
       SHORT DESCRIPTION OF DRAWINGS 
       These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawings, 
         FIG. 1  schematically shows an embodiment of an electrospinning device; 
         FIG. 2  schematically shows an electrospinning device according to an embodiment of the invention; 
         FIG. 3  schematically shows an electrospinning device according to a further embodiment of the invention; 
         FIG. 4  shows a flow chart of a method of electrospinning according to a further aspect of the invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  schematically shows an embodiment of an electrospinning device  1 . The electrospinning device  1  may be arranged inside an enclosure (not shown in  FIG. 1 ) for quality or security reasons. The electrospinning device  1  may comprises a container  2  for holding a liquid comprising a polymer melt or a polymer solution, and a nozzle  3  arranged to outlet a stream of the liquid from the container  2 . The electrospinning device  1  further comprises a collecting surface  4  for collecting electro spun material coming from the nozzle  3  during an electrospinning process. A voltage supply system  5  may be arranged to create a voltage difference between the nozzle and the collector. The voltage supply system  5  may comprise at least one AC or DC voltage supply to create the voltage difference or it may comprise two voltage supplies, one creating a voltage difference between the collecting surface  4  and ground and one creating a difference between the nozzle  3  and ground. Due to the applied voltage(s), an electro spun fiber is created that flies from the nozzle  3  to the collecting surface  4  on which it is collected to form an electro spun fibrous structure  8 . 
     In the embodiment shown in  FIG. 1  the device  1  also comprises one or more electrostatic emitters  10  arranged to locally distribute positive and/or negative ions onto the fibrous structure and/or collector, thereby locally changing the charge of the fibrous structure  8 , and so attract or repel the incoming ‘flying’ fibers. The electrostatic emitters  10  are, for example, electrostatic emitters (ionizers) such as ion generators. Furthermore, the device  1  comprises a rotatable body  6  arranged to cause the collecting surface to face the nozzle  3  and the static emitters  10  in turn (or alternately in position, and hence during operation also alternately in time). In this embodiment, the rotatable body is a rotatable cylindrical body  6 , the surface of which forms the collecting surface  4 . The rotatable cylindrical body  6  is arranged on a shaft  7  which is driven by a motor (not shown). 
     In the embodiment shown in  FIG. 1  the static emitters  10  are arranged in a row. In the example of  FIG. 1 , the static emitters  10  are arranged in an array with equidistant space between two consecutive static emitters  10 . Each static emitter  10  is arranged to distribute positive and/or negative ions on the fibrous structure  8  over a distance Di. This distance is smaller than the width W of the rotatable cylindrical body  6 , and thus smaller than the width of the collecting surface  4 . It is noted that in other embodiments, the static emitters  10  can be arranged having arbitrary intermediate spaces, i.e. non-equidistant spaces. The electrostatic emitters  10  may be relatively small, and positioned close to the surface of the collecting surface  4  (and thus fibrous structure  8 ). The electrostatic emitters are e.g. pin or spike formed, and may have an effective area around the emitters  10  with a radius of only 5-10 mm. 
     The device  1  may also comprise a control unit  15  arranged to control the static emitters  10  so as to create a pattern into the fibrous structure  8 , as will be explained below. 
     For example, when the tip portion of the nozzle  3  is positively charged, the fibrous structure  8  deposited on the collecting surface  4  has a positive charge. In such a case, since the positively charged fibers  8  repel each other, it is difficult to deposit the fibers consecutively. By locally distributing negative ions on the fiber structure  8 , using the negative ion generator, the positive charges of the already deposited fibers can be locally neutralized. As a consequence, once the collecting surface faces the nozzle  3  again, at these locally neutralized locations, the fibers will be attracted, while at the still positively charged locations, the new fibers will be repelled. In this way a pattern can be created into the fiber structure. It is noted that instead of neutralizing certain locations of the fibrous structure, they can be charged negative, giving the same or sometimes even better results. 
       FIG. 2  schematically shows an electrospinning device  1  according to an embodiment of the invention. The device  1  is similar to the device shown in  FIG. 1 , except that in  FIG. 2  the static emitters  10  are movable in a direction parallel to a rotation axis of the rotatable body  6 , i.e. parallel to the longitudinal direction of the shaft  7 . This is indicated by arrows. In this example some static emitters  11  are stationary, some are movable individually, see  12 , and some are movable jointly, see  13 . It will be clear to the skilled reader that many combinations of movable and non-movable (static) static emitters are conceivable. 
     As mentioned above with reference to  FIG. 1 , by providing positive or negative ions onto the fibrous structure respectively collecting surface, the fibrous structure is locally charged or discharged. The control unit  15  may be arranged to control the static emitters  10  so as to create a pattern into the fibrous structure  8 . For example, in a first stage, the control unit  15  may equally activate all of the static emitters  10 , which may cover the whole of the width W of the collecting surface  4 . This will result in a substantially flat layer of fibers on the collecting surface  4 . In a second stage, the control unit  15  may activate two of the static emitters  10  remote from each other with a distance L, and having an effective discharge area of Di and Dj. Once the second stage progresses, the fibrous structure will contain a bottom layer with two rims on it having a real valued distance K of about K=L−Di/2−Dj/2. In this way all kind of pattern with rings can be manufactured. By switching between the individual emitters during the rotation of the collector other projected patterns (not limited by) like squares, stripes and circles are possible. 
       FIG. 3  schematically shows an electrospinning device  30  according to a further embodiment of the invention. The device  30  comprises two rotatable bodies  34 ,  35 , and a looped conveyer belt  36  arranged around the two rotatable bodies  34 ,  35 , wherein the surface of the belt  36  forms the collecting surface/carrier for the fibrous mesh  4 . In the embodiment of  FIG. 3 , the device  30  also comprises a counter electrode  31 . By applying a voltage difference between the nozzle  3  and the counter electrode  31 , an electrical field is created between the nozzle  3  and the collecting surface  4  when situated between the nozzle  3  and the electrode  31 . The electrode  31  may have all sorts of configurations such as for example beam shaped or plate shaped. In an embodiment the belt  36  will be made of a polymer and thus exhibit electrically insulating properties. The belt  36  should be thin enough for the electric field to pas-trough, but intrinsically will limit the release of charge from the fibrous structure to the counter electrode  31 . It is noted that more than two rotatable bodies may be used to guide the belt  36  along the collecting location, the charge or discharge location, and possible some other locations for additional processing of the fibrous structure. 
       FIG. 3  shows an electrostatic emitter  38 , which represents a whole row of a number of electrostatic emitters  38  arranged along the surface of the belt in a direction parallel to a rotation axis of the rotatable bodies  34 ,  35 . Although a number of electrostatic emitters  38  are preferred, only a single electrostatic emitter  38  will already produce a pattern in the fibrous structure. The same accounts for the number of electrostatic emitters  10  of  FIGS. 1 and 2 . 
     In the embodiments shown in  FIG. 1-3  the collecting surface  4  is arranged between the nozzle  3  and the one or more electrostatic emitters  10 ,  38 . This allows to have the collecting surface  4 , in combination with the rotatable bodies  6 , to face, in turn (i.e. subsequently), the nozzle  3  and the one or more electrostatic emitters  10 ,  38 . Furthermore, as the electrostatic emitters  10 ,  38  are located at the opposite side from the collecting surface  4  when viewed from the nozzle  3 , the electrostatic emitters  10 ,  38  will have less influence on the area in the electrospinning device where the fibers are formed from the jet exiting the nozzles  3 , i.e. the Rayleigh instability area. Alternative arrangements are conceivable, as long as the positioning of the electrostatic emitters  10 ,  38  is such that the formed fibrous structure  8  on the collecting surface  4  is facing the nozzle  3  and the electrostatic emitters  10 ,  38  in turn (i.e. subsequently during operation). 
       FIG. 4  shows a flow chart of a method of electrospinning  40  according to a further aspect of the invention. The method  40  comprise holding, see step  41 , a liquid comprising a polymer melt or a polymer solution in a container. Furthermore the method comprises letting out, see step  42 , a stream of the liquid from the container through a nozzle  3 . Furthermore the method comprises creating  43  a voltage difference between the nozzle and a collecting surface. Furthermore the method comprises collecting, see step  44 , electro spun material coming from the nozzle so as to form a fibrous structure on the collecting surface. Furthermore the method comprises locally distributing, see step  45 , positive and/or negative ions onto the fibrous layer by way of one or more static emitters. 
     Finally, the method comprises rotating the collecting surface by means of one or more rotatable bodies, see step  46 , causing the collecting surface to face the nozzle and the one or more static emitters  10  alternately. 
     The method shown in  FIG. 4  may also comprise the step of controlling the static emitters so as to form a pattern in the fibrous structure. 
     It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb “comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. In the device claims several means are enumerated. These means may be embodied by one and the same item of hardware or software. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.