Electrospinning device and method

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, andone 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.

DESCRIPTION OF EMBODIMENTS

FIG. 1schematically shows an embodiment of an electrospinning device1. The electrospinning device1may be arranged inside an enclosure (not shown inFIG. 1) for quality or security reasons. The electrospinning device1may comprises a container2for holding a liquid comprising a polymer melt or a polymer solution, and a nozzle3arranged to outlet a stream of the liquid from the container2. The electrospinning device1further comprises a collecting surface4for collecting electro spun material coming from the nozzle3during an electrospinning process. A voltage supply system5may be arranged to create a voltage difference between the nozzle and the collector. The voltage supply system5may 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 surface4and ground and one creating a difference between the nozzle3and ground. Due to the applied voltage(s), an electro spun fiber is created that flies from the nozzle3to the collecting surface4on which it is collected to form an electro spun fibrous structure8.

In the embodiment shown inFIG. 1the device1also comprises one or more electrostatic emitters10arranged to locally distribute positive and/or negative ions onto the fibrous structure and/or collector, thereby locally changing the charge of the fibrous structure8, and so attract or repel the incoming ‘flying’ fibers. The electrostatic emitters10are, for example, electrostatic emitters (ionizers) such as ion generators. Furthermore, the device1comprises a rotatable body6arranged to cause the collecting surface to face the nozzle3and the static emitters10in turn (or alternately in position, and hence during operation also alternately in time). In this embodiment, the rotatable body is a rotatable cylindrical body6, the surface of which forms the collecting surface4. The rotatable cylindrical body6is arranged on a shaft7which is driven by a motor (not shown).

In the embodiment shown inFIG. 1the static emitters10are arranged in a row. In the example ofFIG. 1, the static emitters10are arranged in an array with equidistant space between two consecutive static emitters10. Each static emitter10is arranged to distribute positive and/or negative ions on the fibrous structure8over a distance Di. This distance is smaller than the width W of the rotatable cylindrical body6, and thus smaller than the width of the collecting surface4. It is noted that in other embodiments, the static emitters10can be arranged having arbitrary intermediate spaces, i.e. non-equidistant spaces. The electrostatic emitters10may be relatively small, and positioned close to the surface of the collecting surface4(and thus fibrous structure8). The electrostatic emitters are e.g. pin or spike formed, and may have an effective area around the emitters10with a radius of only 5-10 mm.

The device1may also comprise a control unit15arranged to control the static emitters10so as to create a pattern into the fibrous structure8, as will be explained below.

For example, when the tip portion of the nozzle3is positively charged, the fibrous structure8deposited on the collecting surface4has a positive charge. In such a case, since the positively charged fibers8repel each other, it is difficult to deposit the fibers consecutively. By locally distributing negative ions on the fiber structure8, 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 nozzle3again, 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. 2schematically shows an electrospinning device1according to an embodiment of the invention. The device1is similar to the device shown inFIG. 1, except that inFIG. 2the static emitters10are movable in a direction parallel to a rotation axis of the rotatable body6, i.e. parallel to the longitudinal direction of the shaft7. This is indicated by arrows. In this example some static emitters11are stationary, some are movable individually, see12, and some are movable jointly, see13. 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 toFIG. 1, by providing positive or negative ions onto the fibrous structure respectively collecting surface, the fibrous structure is locally charged or discharged. The control unit15may be arranged to control the static emitters10so as to create a pattern into the fibrous structure8. For example, in a first stage, the control unit15may equally activate all of the static emitters10, which may cover the whole of the width W of the collecting surface4. This will result in a substantially flat layer of fibers on the collecting surface4. In a second stage, the control unit15may activate two of the static emitters10remote 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. 3schematically shows an electrospinning device30according to a further embodiment of the invention. The device30comprises two rotatable bodies34,35, and a looped conveyer belt36arranged around the two rotatable bodies34,35, wherein the surface of the belt36forms the collecting surface/carrier for the fibrous mesh4. In the embodiment ofFIG. 3, the device30also comprises a counter electrode31. By applying a voltage difference between the nozzle3and the counter electrode31, an electrical field is created between the nozzle3and the collecting surface4when situated between the nozzle3and the electrode31. The electrode31may have all sorts of configurations such as for example beam shaped or plate shaped. In an embodiment the belt36will be made of a polymer and thus exhibit electrically insulating properties. The belt36should 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 electrode31. It is noted that more than two rotatable bodies may be used to guide the belt36along the collecting location, the charge or discharge location, and possible some other locations for additional processing of the fibrous structure.

FIG. 3shows an electrostatic emitter38, which represents a whole row of a number of electrostatic emitters38arranged along the surface of the belt in a direction parallel to a rotation axis of the rotatable bodies34,35. Although a number of electrostatic emitters38are preferred, only a single electrostatic emitter38will already produce a pattern in the fibrous structure. The same accounts for the number of electrostatic emitters10ofFIGS. 1 and 2.

In the embodiments shown inFIG. 1-3the collecting surface4is arranged between the nozzle3and the one or more electrostatic emitters10,38. This allows to have the collecting surface4, in combination with the rotatable bodies6, to face, in turn (i.e. subsequently), the nozzle3and the one or more electrostatic emitters10,38. Furthermore, as the electrostatic emitters10,38are located at the opposite side from the collecting surface4when viewed from the nozzle3, the electrostatic emitters10,38will have less influence on the area in the electrospinning device where the fibers are formed from the jet exiting the nozzles3, i.e. the Rayleigh instability area. Alternative arrangements are conceivable, as long as the positioning of the electrostatic emitters10,38is such that the formed fibrous structure8on the collecting surface4is facing the nozzle3and the electrostatic emitters10,38in turn (i.e. subsequently during operation).

FIG. 4shows a flow chart of a method of electrospinning40according to a further aspect of the invention. The method40comprise holding, see step41, a liquid comprising a polymer melt or a polymer solution in a container. Furthermore the method comprises letting out, see step42, a stream of the liquid from the container through a nozzle3. Furthermore the method comprises creating43a voltage difference between the nozzle and a collecting surface. Furthermore the method comprises collecting, see step44, 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 step45, 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 step46, causing the collecting surface to face the nozzle and the one or more static emitters10alternately.

The method shown inFIG. 4may also comprise the step of controlling the static emitters so as to form a pattern in the fibrous structure.