Technical Field

The invention relates to a method for producing individual solid polymer fibers from a precursor liquid comprising polymerizable and/or cross-linkable polymer precursors by microfluidicbased wet spinning. Furthermore, the invention is directed to a microfluidic-based wet spinning device for producing an individual solid polymer fiber.

Background Art

Synthetic fibers are used in the manufacture of materials in many fields of technology ranging e.g. from optics over textiles to mechanical engineering. Usually, synthetic fibers are produced by spinning. Thereby, depending on the material of the synthetic fiber, continuous filaments or fiber mats are produced by different spinning techniques. For example, melt spinning allows to produce synthetic fibers from molten thermoplastic materials. Another approach, called solution spinning uses spinning solutions comprising precursor materials in a solvent whereby the precursor materials of the spinning solutions are solidified to form the target fibers. Solution spinning can be carried out quite differently. Well-known methods are inter alia dry spinning, wet spinning, dry-jet wet spinning, and electrospinning.

WO 2014/ 143866 A1 (Arsenal Medical, Inc.) discloses for example a method for obtaining multicomponent fibers by coaxial electrospinning. Thereby, fibers are provided, which comprise (a) a polymeric core that comprises a core-forming polymer and (b) a polymeric sheath that comprises a sheath-forming polymer that is different than the core-forming polymer. Examples of core-forming polymers include, for instance, crosslinked polysiloxanes and thermoplastic polymers, among others. Examples of sheath-forming polymers include, for instance, solventsoluble polymers, degradable polymers and hydrogel-forming polymers, among others. However, this method produces fiber meshes formed by a collection of rather thin fibers interlaced to form a three-dimensional network but is not able to directly produce individual fibers in a targeted manner. KR 10169598 B 1 (Dong-A Univ. Res. Found for Industry-Academy Coop.) relates to a method for manufacturing a polymer fiber using a microfluidic device and, particularly, to a method for manufacturing a polymer fiber manufactured by injecting a precursor solution and a support crosslinking solution to a microfluidic device. The method comprises the steps of: (1) preparing (i) a precursor solution containing a monomer, a crosslinking agent, an alginate, and a photoinitiator, and (ii) an alignate crosslinking solution containing a metal cation; (2) injecting the precursor solution and the alginate crosslinking solution into the microfluidic device to manufacture a metal-alginate support structure; (3) photopolymerizing the monomer by irradiation with a light source in the microfluidic device, in order to manufacture a cross-linked polymer/alginate support composite fiber; and (4) reacting the manufactured cross-linked polymer/alginate support composite fiber with a chelating agent to remove the alginate support from polymer/alginate support composite fiber so as to obtain a cross-linked polymer fiber without support. However, fibers produced in this manner comprise a considerable amount of imperfections, making them for example unsuitable for optical applications. Furthermore, this method is practical only for producing hydrophilic fibers, of which the precursors are miscible with the alginate aqueous solution. It is however not possible to produce hydrophobic fibers from other precursors that are not miscible with alginate aqueous solutions, such as e.g. polydimethylsiloxane (PDMS) fibers.

There is thus a need to develop new and improved methods for producing polymer fibers, which at least partly overcome the aforementioned drawbacks.

Summary of the invention

It is the object of the present invention to provide new and improved solutions for producing polymer fibers. Especially, the method should allow to produce individual polymer fibers consisting of different materials and having various thicknesses and lengths in a targeted manner. Further preferred, the solution should make it possible to produce polymer fibers as uniform as possible and in particular having a quality suitable for optical applications.

Surprisingly it was found that these objects can be achieved with a method according to claim 1.

Thus, according to a first aspect, the invention is concerned with a method for producing an individual solid polymer fiber from a precursor liquid comprising aggregatable, polymerizable and/or cross-linkable polymer precursors, especially from a precursor liquid comprising polymerizable and/or cross-linkable polymer precursors, by microfluidic-based wet spinning, the method comprising the steps of: a) Introducing the precursor liquid through a first inlet into a first capillary tube, whereby at least a downstream end of the first capillary tube coaxially protrudes into a second capillary tube, and injecting the precursor liquid into the second capillary tube, to obtain a core flow of the precursor liquid in the second capillary tube; b) Producing a liquid fiber with core-shell structure in the second capillary tube by simultaneously introducing a shell liquid into the second capillary tube through a second inlet such that the second liquid forms a tubular and coaxial shell flow around the core flow of the precursor liquid; c) Optionally, injecting the liquid fiber with core-shell structure produced in step b) into a third capillary tube to obtain a core-shell flow in the third capillary tube, whereby at least a downstream end of the second capillary tube coaxially protrudes into the third capillary tube, and simultaneously introducing a third liquid into the third capillary tube through a third inlet, such that the third liquid forms a tubular and concentric sheath flow around the core-shell flow in the third capillary tube whereby a liquid fiber with core-shell-sheath structure is produced; d) Guiding the liquid fiber with core-shell structure, optionally having a sheath, as produced in step b) or in step c) through a stationary liquid phase; e) Curing the shell liquid of the fiber with core-shell structure at least in one of steps c) and d), especially by using a curing agent for the shell liquid as the third liquid in step c) and/or as the stationary liquid phase in step d), to provide a fiber with core-shell structure having a liquid core embedded in a cured shell; f) Collecting the fiber from the stationary liquid phase in the form of the fiber having a liquid core embedded in a cured shell; g) After step f), solidifying the liquid core of the fiber having a liquid core embedded in a cured shell by aggregation, polymerization and/or crosslinking, in particular by polymerization and/or crosslinking, to obtain a fiber with a solid core within the cured shell; h) Removing the cured shell from the solid core to obtain the solid polymer fiber.

The inventive method makes use of a removable shell as tubular mold for shaping and trapping the curable polymer precursor in a core channel. This without need of mixing the curable polymer precursor and the shell liquid beforehand. This allows for producing a wider range of individual polymer fibers and by adjusting the dimensions of the capillary tubes, polymer fibers with various thicknesses up to several millimetres and lengths of more than one meter can be produced in a highly targeted manner. Furthermore, the polymer fibers obtainable with the inventive method are highly uniform and, depending on the materials used, even suitable for optical applications.

Furthermore, during the inventive method, a fiber having a liquid core embedded in a cured shell is produced. This allows for decoupling the curing of the liquid curable polymer precursor from the wet spinning process.

In contrast, in existing microfluidic spinning methods, such as for example described in KR 10169598 B1 , the solidification of the polymer fibers has to be induced during the spinning process, in particular when the flow of the liquids in the wet spinning apparatus is well maintained. Furthermore, such an approach requires rapid curing of the precursor fluid during the rather short spinning process, thus limiting the materials suitable for fiber production. Also, because specific polymer materials require different solidification methods, e.g. photo-initiated polymerization, chemical crosslinking and/or solvent exchange, the microfluidic wet spinning devices have to be designed and fabricated with specific considerations, thus making polymer fiber production rather complicated for users.

All these drawbacks can be circumvented with the inventive method, whereby curing of the liquid curable polymer precursors can be decoupled in time and place from the wet spinning process and in particular of the curing of the shell. Therefore, for example, curing of the liquid curable polymer precursors can be effected outside the wet spinning apparatus and during any long time, if desired.

Therefore, in a preferred implementation, steps g) and/or h) take place outside the stationary liquid phase and/or in time after steps a) to e). In particular, unlike processes such as common melt spinning or thermal drawing, the inventive method allows to produce high quality continuous polymer optical fibers (POF) even with sensitive functional molecules, e.g. perovskite nanocrystals.

Furthermore, with the inventive method, polymer opticalfibers (POF) having a low modulus of elasticity of < 3 MPa or even < 1 MPa are available. Such POF feature an increased sensitivity for small pressures and cannot be produced by common moulding techniques because the fibers would break during demoulding due to the too low modulus of elasticity.

In particular, the polymerizable and/or cross-linkable polymer precursors are selected from polymerizable and/or crosslinkable monomers, oligomers and/or polymers.

For example, the precursor liquid comprises: polyols as well as diisocyanates, triisocyanates and/or polyurethane prepolymers, capable of forming polyurethane polymers epoxy resins, optionally with hardeners crosslinkable siloxane polymers, especially polydimethylsiloxane polymers ethylenically unsaturated monomers, especially alkenes, vinyl monomers, alkenyl monomers and/or (meth)acrylates, monomers having amine, carboxyl and/or acyl groups, e.g. amino acids, capable of forming polyamides, and/or monomers and/or oligomers containing active groups for click-chemistry and capable of forming polymers, e.g. monomers and/or oligomers containing groups that react in azidealkyne cycloaddition reactions, thiol-ene reactions, and/or amino-yne reactions, monomers selected from a-hydroxy acids, especially lactic acid or its cyclic di-ester lactide, glycolic acid, mandelic acid, and/or citric acid; aggregatable polymers, especially polylactic acid (PLA) and/or polycaprolacton (PCL).

According to an especially preferred implementation, the precursor liquid comprises (meth)acrylates and/or crosslinkable polydimethylsiloxane polymers. Examples of aggregatable polymers are polylactic acid (PLA) and/or polycaprolacton (PCL). Such polymers can be solidified via solvent-exchanged polymer aggregation. Thus, in particular, aggregatable polymers are dissolved in one or more solvent(s) as the precursor liquid.

However, depending on the desired solid polymer fiber, other polymerizable and/or crosslinkable polymer precursors can be used as well.

Further preferred polymerizable precursors are lactic acid or its cyclic di-ester lactide.

Other highly preferred polymerizable precursors are polyols as well as diisocyanates, triisocyanates and/or polyurethane prepolymers, capable of forming polyurethane polymers.

Further preferred polymerizable precursors are mixtures of crosslinkable siloxane polymers or alkene polymers and/or macromers, e.g. dodecyldimethacrylateand, and (metha)acrylate precursors, especially capable of forming amphiphilic polymer co-networks.

Especially, the precursor liquid furthermore comprises for example a solvent, a crosslinking agent, a thermal polymerization initiator, a photopolymerization initiator, a chain transfer agent, a functional molecule and/or a molecular weight regulator. In particular, these substances are chosen depending on the specific polymerizable and/or cross-linkable polymer precursors and are in particular added in order to enable and control the solidifying of the liquid core in step g).

Functional molecules can e.g. be selected from fluorophores, chromophores and/or nanoparticles. Such molecules can for example be incorporated in the fibers to adapt the fibers to specific applications. Especially, functional molecules may comprise nanocrystals, e.g. perovskite nanocrystals.

The shell liquid in particular is a solution of a solvent, especially water, and a solvent-soluble non-crosslinked polymer.

In particular, the shell liquid is a solution of water and a homopolymer and/or a copolymer formed for example from one or more of the following monomers: ethylene oxide, vinyl pyrrolidone, vinyl alcohol, vinyl acetate, vinyl pyridine, methyl vinyl ether, acrylic acid and salts thereof, methacrylic acid and salts thereof, hydroxyethyl methacrylate, acrylamide, N,N- dimethyl acrylamide, N-hydroxymethyl acrylamide, alkyl oxazolines, saccharide monomers, polysaccharides, dextran, alginate, amino acids, hydrophilic polypeptides, proteins and/or gelatin. Especially preferred, the shell liquid is an aqueous hydrogel precursor solution, in particular an aqueous polysaccharide solution. Particular preferred, the shell liquid is an aqueous alginate solution, in particular an aqueous alkaline metal alginate solution, e.g. a sodium alginate solution. These kind of solutions turned out to produce highly stable shells and are compatible with various precursor liquids. Additionally, these solutions can easily be removed later on.

In particular, with respect to the total weight of the shell liquid, a concentration of the homopolymer and/or a copolymer, especially the hydrogel precursor, in particular an alginate, is from 0.05 - 10 wt%, especially 0.1 - 5 wt%, in particular 1 - 3 wt%.

Especially, the precursor liquid is essentially immiscible with the shell liquid. Thereby, in step b) a well-defined phase interface between the core and the shell in the liquid fiber can be obtained. Additionally, diffusion of precursor liquid into the cured shell is reduced.

In step d) the liquid fiber with core-shell structure, optionally having a sheath, as produced in step b) or in step c) preferably is introduced into the stationary liquid phase below the liquid surface of the stationary liquid phase. This reduces turbulences and improves the overall quality of the fibers obtainable.

Thus, preferably, the second capillary tube or the third capillary tube, if the latter is present, is submerged in the stationary liquid phase.

Especially, the third liquid in step c) and/or the stationary liquid phase in step d) comprises or consists of a polar solvent, especially methanol, ethanol and/or water, in particular water.

Particularly, the stationary liquid phase can have a temperature between 0 - 95°C, in particular between 15 - 90°C, especially between room temperature and 80°C.

According to a first preferred embodiment, the temperature of the stationary liquid phase is between 15 - 30°C, especially room temperature.

In a second preferred embodiment, the temperature of the stationary liquid phase is an elevated temperature, especially above room temperature, in particular above 30°C. This allows performing a pre-curing step and/or a partial curing of the liquid core in stationary liquid phase.

Especially, the temperature of the stationary liquid phase is between 45 - 85°C, especially between 55 - 75°C, particularly between 60 - 70°C, e.g. 65°C. These temperatures are especially beneficial when using crosslinkable siloxane polymers, especially polydimethylsiloxane polymers, as precursor liquid and/or when producing PDMS fibers.

In particular, the third liquid in step c) and/or the stationary liquid phase in step d) is a curing agent for the shell liquid and is selected from an aqueous solution of a salt of a divalent metal cation, especially selected from of Ca ²⁺ , Mg ²⁺ , Zn ²⁺ , Fe ²⁺ , Cu ²⁺ , and/or Ba ²⁺ . Especially preferred is an aqueous solution Ca ²⁺ salt, e.g. an aqueous solution of CaCI ₂ .

In particular, with respect to the total weight of the aqueous solution, a concentration of the salt of the divalent metal cation, especially CaCI ₂ , in the aqueous solution is from 0.05 - 10 wt%, especially 0.1 - 5 wt%, in particular 0.5 - 2 wt%.

According to a particularly preferred embodiment, optional step c) is performed and in step c) a curing agent for the shell liquid selected from an aqueous solution of a salt of divalent metal cation is used as the third liquid. In this case, the shell of the liquid fiber with core-shell structure is directly cured through the concentric sheath flow of the third liquid within the third capillary tube. Thereby, the third liquid forms another tubular mold for shaping and trapping the liquid fiber with core-shell structure. This may further improve uniformity of the liquid fiber. In this case, the stationary liquid phase in step d) allows the fibers to be spun-out of the device in a smooth and stable manner. In particular, the stationary liquid phase in step d) is an aqueous solution, e.g. water.

According to another highly preferred embodiment, step c) is not performed and in step d) a curing agent for the shell liquid selected from an aqueous solution of a salt of divalent metal cation is used as the stationary liquid phase. In this case, the shell of the liquid fiber with coreshell structure is cured within the stationary liquid phase, which in addition allows the fibers to be spun-out of the device in a smooth and stable manner.

According to a preferred embodiment, in the liquid fiber with core-shell structure produced in the second capillary tube, the shell liquid is in direct contact with the precursor liquid at least in a downstream section of the second capillary tube.

Thereby, preferably, the downstream end of the first capillary tube is located within the second capillary tube, whereby the downstream end of the first capillary tube, in flow direction of the liquid fiber, in particular is located inside the first half, especially within the first quarter, of the second capillary tube. In this case, if a third capillary tube is present, the downstream end of the second capillary tube in particular is located inside the third capillary tube.

In particular, with respect to an upstream end, at least an outer diameter, especially an inner and the outer diameter, of the first capillary tube tapers, especially step-like, towards the downstream end, in particular to form a first capillary nozzle. Likewise, in a preferred embodiment, with respect to an upstream end, at least an outer diameter, especially an inner and the outer diameter, of the second capillary tube tapers, especially step-like, towards the downstream end, in particular to form a second capillary nozzle.

This allows for focussing the flow of the precursor liquid when injecting it into the second capillary tube and for focussing the liquid fiber with core-shell structure when leaving the second capillary tube. However, other configurations might be suitable as well.

In particular, a length of the tapered section of the first capillary tube is 25 - 75%, especially 40 - 60%, of the whole length of the first capillary tube; and, especially, a length of the tapered section of the second capillary tube is 25 - 75%, especially 40 - 60%, of the whole length of the second capillary tube.

According to another preferred embodiment, in the liquid fiber with core-shell structure produced in the second capillary tube, the shell liquid and the precursor liquid in the liquid fiber with core-shell structure are separated by the first capillary tube when flowing through the second capillary tube. Put differently, in this case, the shell liquid and the precursor liquid are not in direct contact in the second capillary tube.

Thereby, preferably, the first capillary tube extends completely through the second capillary tube, and, preferably, the downstream end of the first capillary tube, in flow direction of the liquid fiber, is located further downstream the downstream end of the second capillary tube. If a third capillary tube is present, the downstream ends of the first and the second capillary tubes are located inside the third capillary tube, especially within the first half, in particular within the first quarter, of the third capillary tube.

Thereby, especially, the sheath fluid first is contacted with the shell fluid in the third capillary tube in order to pre-cure the shell fluid before the precursor fluid is introduced through the first capillary tube further downstream into the hollow central section of the pre-cured shell fluid. The pre-cured shell fluid thereby still is fluid but forms a more stable interface with the precursor fluid what further reduces mixing of the fluids at the interface.

Thus, the expression "whereby at least a downstream end of the first capillary tube coaxially protrudes into a second capillary tube" in particular is to be interpreted as to include the first capillary tube extending completely through the second capillary tube, and, preferably, the downstream end of the first capillary tube, in flow direction of the liquid fiber, beeing located further downstream the downstream end of the second capillary tube.

In the embodiment with the first capillary tube extending completely through the second capillary tube, the first and the second capillary tubes, and optionally the third capillary tube, in particular each have a constant inner and/or outer diameter.

The embodiment where the shell liquid and the precursor liquid are not in direct contact in the second capillary tube and the embodiment with the first capillary tube extending completely through the second capillary tube allows the pre-curing of the shell liquid before it gets in contact with the precursor liquid. This is due to the extended first capillary tube or core channel, respectively. This improves the stability of the liquid fiber with core-shell structure by having an at least partly gelled shell at the onset when forming the core-shell interface. Fluctuations of the core-shell interface are thus reduced due to the less fluid or non-liquid nature of the shell liquid (hydrogel flow).

As such: these embodiments in particular provide polymer fibers with better structural consistency and/or allow fiber spinning at lower spinning rate (hence easier post-spinning online treatment such as UV irradiation and automatic collection).

Independently of the specific embodiment, in particular, an inner diameter of the first capillary tube at the downstream end equals 40 - 60%, especially 45 - 55%, of the inner diameter of the second capillary tube at the upstream end; and, especially, an inner diameter of the second capillary tube at the downstream end equals 40 - 60% especially 45 - 55%, of the inner diameter of the third capillary tube at the upstream end. This allows for producing a liquid fiber with core-shell structure in the second capillary tube with a preferred ratio of core thickness to shell thickness.

Especially, an inner diameter of the second capillary tube at the upstream end equals 1 10 - 150%, especially 120 - 140%, of the outer diameter of the first capillary tube at the downstream end; and, especially, an inner diameter of the third capillary tube at the upstream end equals 1 10 - 150%, especially 120 - 140%, of the outer diameter of the second capillary tube at the downstream end. These configurations allow for introducing the first capillary tube into the second capillary tube and leaving an annular opening for introducing a liquid at the upstream end of the second capillary tube. This is also true for the second and the third capillary tube.

For example, an inner diameter of the first capillary tube at the downstream end is 0.3 - 5.0 mm and/or an inner diameter of the second capillary tube at the upstream end is from 0.7 - 6.0 mm; and, especially, an inner diameter of the second capillary tube at the downstream end is 0.3 - 5.0 mm, and/or an inner diameter of the third capillary tube at the upstream end is from 0.7 - 6.0 mm. However, inner and outer diameters preferably are choses depending on desired thicknesses of the fibers to be produced.

Especially, the shell liquid is introduced into the second capillary tube at the upstream end face of the second capillary tube, especially through an annular opening formed by the first capillary tube and/or the downstream end of the first capillary tube coaxially protruding into or extending through the second capillary tube; and, especially, the third liquid is introduced into the third capillary tube at the upstream end face of the third capillary tube, especially through an annular opening formed by the downstream end of the second capillary tube coaxially protruding into the third capillary tube.

In particular, the shell liquid is guided along the outer surface of the first capillary tube, especially the tapered section, before introducing it into the second capillary tube or the third capillary tube; and, especially, the third liquid is guided along the outer surface of the second capillary tube, especially along the tapered section, before introducing it into the third capillary tube.

This allows for producing highly laminar flows of the respective liquids in the capillary tubes.

Preferably, a ratio of the flow rates of core flow : shell flow is from 1 : 0.1 - 10, particularly 1 : 0.5 - 1 .5, in particular 1 : 0.8 - 1.2; and, especially, a ratio of the flow rates of core flow : shell flow: sheat flow is from 1 : (0.1 - 10) : (1 - 50), particularly 1 : 0.5 - 1.5 : 2 - 15, in particular 1 : 0.5 - 0. 9 : 5 - 12. With such ratios, highly laminar flows are achievable and high quality fibers can be produced. The flow rate is meant to be the volumetric flow rate or the volume of liquid that passes per unit time.

In particular, the flow of the precursor liquid in the second capillary tube, the liquid fiber with core-shell structure in the second capillary tube, the core-shell flow in the third capillary tube and/or the liquid fiberwith core-shell-sheath structure are controlled to flow laminarly. This can be achieved by appropriate dimensions of the capillaries and flow rates of the liquids.

Especially, the diameter of the first, the second and optionally the third capillary tube and/or the flow rates of the core flow, the shell flow and optionally the sheath flow are selected such that the solid polymer fiber obtainable in step h) has a diameter of 1 - 5'000 gm, especially 10 - 2'500 |im, e.g. 100 - 1 '500 gm.

Preferably, the capillary tubes are embedded within a solid material, especially such that the capillary tubes pass though the solid material along a straight line. Thereby, preferably, around each tapered section of the capillary tubes and/or around a section of the first and the second capillary tubes, an annular cavity is formed for guiding a liquid between the outer surface of the tapered section and/or outer surface of the respective capillary tube and the solid material, and whereby each cavity is accessible from the outside through a free passage in the solid material forming the second inlet, and optionally the third inlet. The solid material for example is an inorganic material and/or a plastic material. An inorganic material is for example glass. A plastic material is for example a photopolymerized resin, polydimethylsiloxane and/or poly(methyl methacrylate) (PMMA).

Especially, the capillaries are made from glass. Since glass is highly inert to various chemicals, the inventive method can be implemented with essentially any kind of substances. However, capillaries made from plastic and/or metallic materials might be suitable as well.

In particular, solidifying the core of the core-shell fiber in step g) is effected by irradiation with electromagnetic radiation and/or heating. This allows for curing and/or polymerizing the precursor liquid in the core of the fibers. Appropriate radiation and/or heat sources are known to the skilled person.

Typically, for photopolymerization, ultraviolet radiation sources are used. Thus, preferably, solidifying the core of the core-shell fiber in step g) is effected by electromagnetic radiation having a wavelength of 280 - 380 nm. Heating can be effected at temperatures in the range of for example 40 - 200°C, e.g.

60 - 100°C, in particular for a duration of 10 min to 10 hours, especially 0.5 to 5 hours.

Aggregation in step g) can for example be effected by solvent-exchanged polymer aggregation. In this case, the fiber with a solid core comprises aggregated polymers. This is meant to be non- covalently bonded polmyers. E.g. bonding between the polymers in the fibers is for example effected by weak intermolecular forces, such as van der Waal forces.

However, depending on the specific composition of the precursor liquid, other measures might be suitable as well for solidifying.

Removing the cured shell from the solid core is in particular affected by dissolving the cured shell in a solvent, e.g. an aqueous solution. In particular the solvent is present as an aqueous alkaline metal salt solution, e.g. an NaCI solution, and/or as an aqueous solution comprising a chelating agent, e.g. ethylenediaminetetraacetic acid, capable of binding divalent metal cations, especially Ca ²⁺ , Mg ²⁺ , Zn ²⁺ , Fe ²⁺ , Cu ²⁺ , and/or Ba ²⁺ . This turned out to be highly efficient and reliable methods for shell removal. Nevertheless, other techniques for removing the cured shell might be used as well.

Especially, during or after step f), the fiber having a core embedded in a cured shell, is taken up on a storage unit, especially on a winder and/or a reel. Thereby, in particular, the fiber having a core embedded in a cured shell is guided with one or more godet unit(s).

Especially, the individual solid polymer fiber is produced without application of electrical fields and/or the inventive method does not comprise a step of applying an electrical field to produce the fibers.

Another aspect of the present invention is directed to a liquid fiber having a liquid core embedded in a liquid shell, whereby the liquid core is made of a precursor liquid comprising polymerizable and/or cross-linkable polymer precursors and the liquid shell is an aqueous hydrogel precursor solution.

Another aspect of the present invention is directed to a fiber having a liquid core embedded in a cured shell, whereby the liquid core is made of a precursor liquid comprising polymerizable and/or cross-linkable polymer precursors and the cured shell is a hydrogel. Furthermore, the present invention is directed to a solid fiber having a cured core embedded in a cured shell, whereby the core is made of a cured precursor liquid comprising polymerizable and/or cross-linkable polymer precursors and the cured shell is a hydrogel.

Also, the present invention is directed to a solid fiber obtained or obtainable by a method as described above.

With the fibers described above, the liquid core, the cured core, the shell liquid and the cured shell are composed, configured and/or obtainable as described above in connection with the inventive method. Preferably, the diameter of the liquid core or the cured core is from 1 - 5'000 |im, especially 10 - 2'500 gm, e.g. 100 - 1 '500 ,m. A length of the fibers preferably is at least 10 cm, especially at least 25 cm, in particular at least 50 cm or at least 100 cm. According to a further advantageous embodiment, the length of the fibers is at least 10 m, especially at least 100 m, in particular at least 1’000 m.

A further aspect is directed to a microfluidic-based wet spinning device for producing an individual solid polymer fiber from a precursor liquid comprising polymerizable and/or crosslinkable polymer precursors, whereby the device comprises: a) a first inlet for introducing a precursor liquid into a first capillary tube, whereby at least a downstream end of the first capillary tube coaxially protrudes into a second capillary tube, for injecting the precursor liquid into the second capillary tube, to obtain a core flow of the precursor liquid in the second capillary tube; b) a second inlet configured for introducing a shell liquid into the second capillary tube such that the second liquid forms a tubular and coaxial shell flow around the core flow of the precursor liquid whereby a liquid fiber with core-shell structure is producible in the second capillary tube; c) optionally, a third capillary tube, whereby at least a downstream end of the second capillary tube coaxially protrudes into the third capillary tube, such that the liquid fiber with coreshell structure producible in the second capillary tube can be injected in the third capillary tube, to obtain a core-shell flow in the third capillary tube, whereby the third capillary tube has a third inlet for introducing a third liquid into the third capillary tube, such that the third liquid forms a tubular and concentric sheath flow around the core-shell flow in the third capillary tube, whereby a liquid fiber with core-shell-sheath structure is producible; d) A receptacle for providing a stationary liquid phase for guiding through the liquid fiber with core-shell structure, optionally having a sheath, as producible in the second or the third capillary tube; e) A device for collecting the fiber from the stationary liquid phase in the form of a fiber having a core embedded in a cured shell, in particular a storage unit, especially a winder and/or a reel; f) A device for solidifying the core of the core-shell fiber by polymerization and/or crosslinking to obtain a fiber with a solid core within the cured shell, in particular a device for emitting electromagnetic radiation and/or a heating device; g) A device for removing the cured shell from the solid core to obtain the solid polymer fiber; especially a further receptacle for immersing and/or guiding through the fiber having a solid core embedded in the cured shell.

Thereby, preferably, the capillaries and the inlets are configured and arranged as described above in connection with the inventive method.

Additionally, the device preferably comprises at least two, especially at least three, independently controllable pump devices for introducing the precursor liquid into the first capillary tube, the shell liquid into the second capillary tube, and optionally the third liquid into the third capillary tube.

In a still further aspect, the invention is directed to the use of a device as described above for producing an individual solid polymer fiber from a precursor liquid comprising polymerizable and/or cross-linkable polymer precursors.

Other advantageous embodiments and combinations of features come out from the detailed description below and the entirety of the claims.

Brief description of the drawings

The drawings used to explain the embodiments show: Fig. 1 A schematic cross-section of a first microfluidic wet spinning device for producing an individual solid polymer fiber from a precursor liquid comprising polymerizable and/or cross-linkable polymer precursors;

Fig. 2 A cross-section of a liquid fiber with core-shell-sheath structure flowing through the last capillary tube at section S of Fig. 1 .

Fig. 3 A schematic illustration of the transformation from a fiber with a liquid core embedded within a cured shell (A) to a fiber with a cured core embedded within the cured shell (B) to the solid target fiber after removal of the shell (C);

Fig. 4 Thin polydimethylsiloxane (PDMS) fibers (A) and thick PDMS fibers (B) produced with the inventive method; A digital micrograph (C) shows light guiding behavior of the thick PDMS fiber;

Fig. 5 Polyacrylate fibers produced with the inventive method: (A) thick polyacrylate fibers; (B) thin polyacylate fibers and (C) a digital micrograph showing light guiding behavior of the polyacrylate fibers;

Fig. 6 A schematic cross-section of a second microfluidic wet spinning device for producing an individual solid polymer fiber from a precursor liquid comprising polymerizable and/or cross-linkable polymer precursors;

Fig. 7 Polylactic acid (PLA) fibers produced with the inventive method;

Fig. 8 Amphiphilic polymer optical fibers (polysiloxane/polyacrylate amphiphilic polymer co-network) produced with the inventive method;

Fig. 9 Polyurethane fibers produced with the inventive method;

Fig. 10 A diagram showing the mechanical behavior of soft POF produced with the inventive method under tensile load;

Fig. 1 1 A diagram showing the decrease in light intensity in polymer optical fibers (POF) as a function of applied pressure.

In the figures, the same components are given the same reference symbols. Examples

First wet spinning device and method of operation

Fig. 1 shows a schematic cross-section of a first inventive microfluidic wet spinning device 10 for producing an individual solid polymer fiber with the inventive method. At the upper end, the device 10 comprises a first capillary tube 1 1 protruding coaxially into an upper end 12a of a second capillary tube 12. The lower ends 1 1 b, 12b of both, the first and the second capillary tubes 1 1 , 12 taper, such that glass nozzles are formed. The tapered lower end 12b of the second capillary tube 12 protrudes coaxially into a third capillary tube 13 in the form a straight glass capillary. The three capillary tubes 1 1 , 12, 13 are embedded within a solid block 14 of a synthetic material, i.e. PDMS elastomer, and each has a length of for example 40 mm. The two upper capillary tubes 1 1, 12 at their upper ends 1 1 a, 12a have for example an outer diameter of 1 mm and an inner diameter of 0.772 mm. At their lower ends 1 1 b, 12b, the two upper capillary tubes 1 1 , 12 have for example an outer diameter of 0.6 mm and an inner diameter of 0.4 mm. The lowest capillary tube 13 for example has an outer diameter of 1 mm and an inner diameter of 0.772 mm over its entire length.

The upper end 1 1a, i.e. the upstream end, of the topmost capillary tube 1 1 is open and forms a first inlet 11 for introducing a first liquid L1, e.g. a precursor liquid.

At the upper or upstream ends faces of the two lower capillary tubes 12, 13, annular openings are formed, which allow to introduce a second liquid L2, e.g. a shell liquid, and a third liquid L3, e.g. a sheath liquid, into the respective capillary tubes 12, 13. The annular openings communicate with annular cavities 14a, 14b formed around each tapered section of the two upper capillary tubes 1 1, 12. The annular cavities 14a, 14b are configured for guiding the liquids L2, L3 between the outer surface of the tapered section of the respective capillary tube 1 1 , 12 and the solid material 14. Each annular cavity 14a, 14b is accessible from the outside through a free passage in the solid material forming a second inlet I2 for the second liquid L2 and a third inlet I3 for the third liquid L3.

The lower end 13b of the third capillary tube 13 protrudes into a receptacle 15 comprising a stationary liquid phase SP. Additionally, there are three godets 16 arranged to direct a fiber produced from the receptacle 15 towards a storage unit 17 in the form of a winder for taking up the fiber. The device can for example be operated as follows: The first liquid L1 , e.g. a precursor liquid comprising polymerizable and/or cross-linkable polymer precursors, is introduced at the first inlet 11 into the topmost capillary tube 1 1 . The first liquid L1 then is injected into the second capillary tube 12, to obtain a core flow of the first liquid in the second capillary tube 12.

In the second capillary tube 12 a liquid fiber with core-shell structure is produced by simultaneously introducing the second liquid L2, e.g. a hydrogel precursor, into the second capillary tube 12 through the second inlet I2 and the annular cavity 14a, such that the second liquid L2 forms a tubular and coaxial shell flow around the core flow of the first liquid L1 .

The liquid fiber with core-shell structure produced in the second capillary tube 12 then is injected into the third capillary tube 13 to obtain a core-shell flow in the third capillary tube 13. Thereby, a third liquid L3 or a sheath liquid, e.g. a curing agent for the second liquid, is introduced into the third capillary tube 13 through the third inlet I3 and the annular cavity 1 b, such that the third liquid L3 forms a tubular and concentric sheath flow around the core-shell flow in the third capillary tube 13, whereby a liquid fiber with core-shell-sheat structure is produced.

The so produced liquid fiber with core-shell-sheat structure then is guided through the stationary liquid phase SP, and taken up on the storage unit 17, i.e. the winder.

Fig. 2 shows a cross section of the liquid fiber with core-shell-sheat structure at section S of Fig. 1. Specifically, the liquid precursor liquid L1 forms a core of the liquid fiber, whereby the core is encapsulated in the sheet liquid L2. Thus, the sheet liquid L2 forms a tubular coaxial shell flow around the core flow of the precursor liquid L1. Likewise, sheath liquid L3 forms a tubular coaxial shell flow around the shell flow L2. Thereby, due to the contact with the sheath liquid L3, the shell liquid L2, e.g. a hydrogel precursor, is cured, whereby a fiber with liquid core 22 of the precursor liquid L1 embedded within the cured shell 21 as shown in Fig. 3-A is produced.

However, in an alternative implementation, no liquid is introduced into the first inlet and the first liquid, i.e. the precursor liquid comprising polymerizable and/or cross-linkable polymer precursors is introduced into the second inlet I2. The second liquid, e.g. a hydrogel precursor, then is introduced into the third inlet I3. In this case no third liquid is used. Instead of it, the stationary liquid phase SP comprises the curing agent for the second liquid. So to say, in this alternative, the second inlet I2 acts as the first inlet and the third inlet I3 acts as the second inlet.

Irrespective of the previous procedure, after taking the fiber from the stationary liquid phase SP, the fiber is treated as illustrated in Fig. 3. Specifically, the fiber with the liquid core 22 within the cured shell 21 is subjected to a solidification treatment ST. e.g. by irradiating the fiber with UV light and/or by providing thermal energy, in order to cure the precursor liquid in the liquid fiber core 22. This results in a fiber as shown in Fig. 3-B having a solid core 22' within the cured shell 21.

Subsequently, the cured shell 21 is removed from the fiber in a removal step RS, e.g. by immersing the fiber in a solvent, to obtain the solid target fiber without shell as shown in Fig. 3- C.

Production of PDMS optical fibers

As an example, PDMS optical fibers were fabricated with the above described device 10 according to the above described alternative implementation without using inlet 11. In this example, liquid polymer precursor in the form of Sylgard™ 184 (ratio of base : crosslinker = 1 ; available from Dow) were introduced into inlet I2 (fow rate: 50 ,L/min), and an aqueous alginate solution (2 wt% in H ₂ O) was introduced through inlet I3 (flow rate: 50 ,L/min). An aqueous solution of CaCI ₂ (0.7 wt% in H ₂ O) was used as the stationary liquid phase SP in the receptacle 15.

PDMS fibers with different diameters were produced as follows: For producing thin fibers, a glass capillary with an outer diameter of 1 mm and an inner diameter of 0.722 mm (tip dimension: outer diameter = 0.6 mm, inner diameter = 0.4 mm) was used as the second capillary tube 12 and a capillary tube with an outer diameter of 1 mm and an inner diameter of 0.722 mm as the third capillary tube. For thick fibers, capillary tubes with larger diameters (outer diameter of 1 .5 mm and an inner diameter of 1 mm) have been used.

In these examples, the liquid core of Sylgard 184 polymer precursors were cured by heat (80 °C for 2 hours) and the shell was removed in a sodium chloride solution. Fig. 4 shows photographs of a two types of fibers obtained in this manner: The left side (A) shows a fiber having a diameter of 700 pm and the right side (B) a fiber with a diameter of 1 '304 pm. As evident from the digital micrograph (C), the fibers are suitable for guiding visible light.

Production of fibers

As another example, polyacrylate polymer optical fibers were produced with the above described device 10. Thereby, acrylate monomer precursors (Norland Optical Adhesive 83H, i.e. NOA83H) were used as precursor liquid and introduced into inlet 11. An aqueous alginate solution (2 wt% in H ₂ O) was used as the shell liquid and introduced into inlet I2 whereas an aqueous solution of CaCI ₂ (0.7 wt% in H ₂ O) was used as the third liquid or sheath flow, respectively, and introduced into inlet I3. Polyacrylate fibers with different diameters were achieved by controlling the core and shell flow rates. Thick fibers were produced with the following flow rates: 500 pL/min for the precursor liquid, 300 pL/min for the sheet liquid and 5000 pL/min for the sheath liquid. Thin fibers were produced with the following flow rates: 50 pL/min for the precursor liquid, 40 pL/min for the sheet liquid and 300 pL/min for the sheath liquid.

In this example, the liquid core of NOA83H monomer precursors was cured by UV irradiation (for 20 min at 10 mW/cm ² at a wavelength of 365 nm).

Fig. 5 shows photographs of the thicker (A) and the thinner fibers (B) having diameters of 372 pm or 325 pm, respectively. As evident from the digital micrograph (C), the fibers are suitable for guiding visible light.

Second wet spinning device and method of operation

Fig. 6 shows a schematic cross-section of a second inventive microfluidic wet spinning device 10’ for producing an individual solid polymer fiber with the inventive method. The second device 10’ at the upper end comprises a first capillary tube 1 1’ extending coaxially through a second capillary tube 12’ and ending in an upper end 13a’ of the third capillary tube 13’. The lower end 12b’ of the second capillary tube 13’ coaxially protrudes into the upper end 13a’ of the third capillary tube 13’ too. Thereby, the lower end 1 1 b’ of the first capillary tube 1 1’ protrudes out of the lower end 12b’ of the second capillary tube 12’. In this embodiment, all of the capillary tubes 1 1’, 12’, 13’ have constant inner and outer diameters.

The upper end 1 1a’, i.e. the upstream end, of the topmost capillary tube 1 1’ is open and forms a first inlet 11’ for introducing a first liquid L1 , e.g. a precursor liquid. At the upper or upstream ends faces of the two lower capillary tubes 12’, 13’, annular openings are formed, which allow to introduce a second liquid L2, e.g. a shell liquid, and a third liquid L3, e.g. a sheath liquid, through inlets 12’, 13’ into the respective capillary tubes 12’, 13’.

The lower end 13b’ of the third capillary tube 13’ protrudes into a receptacle 15’ comprising a stationary liquid phase, e.g. water. Additionally, there are godets and storage units (not shown in Fig. 6) similar to the ones shown in Fig. 1 .

The device 10’ can for example be operated as follows: The first liquid L1 , e.g. a precursor liquid comprising polymerizable and/or cross-linkable polymer precursors, is introduced at the first inlet 11’ into the topmost capillary tube 1 1’. The first liquid L1 then is guided into the second capillary tube 12’, to obtain a core flow of the first liquid in the second capillary tube 12’. Thereby, the core flow is confined within the first capillary tube 1 1’ extending through the second capillary tube 12’.

In the second capillary tube 12’ a liquid fiber with core-shell structure is produced by simultaneously introducing the second liquid L2, e.g. a hydrogel precursor, into the second capillary tube 12’ through the second inlet I2’ such that the second liquid L2 forms a tubular and coaxial shell flow around the first capillary tube 1 1’ and the core flow of the first liquid L1 flowing therein. Thereby, inside the second capillary tube 12’, liquids L1 and L2 are separated by the first capillary tube 1 1’. Nevertheless, they form a liquid fiber with core-shell structure.

The liquid fiber with core-shell structure produced in the second capillary tube 12’ then is injected into the third capillary tube 13’ whereby the third liquid L3 or a sheath liquid, e.g. a curing agent for the second liquid, is introduced into the third capillary tube 13’ through the third inlet I3’, such that the third liquid L3 forms a tubular and concentric sheath flow around the core-shell flow in the third capillary tube 13’. Thereby the third liquid L3 is first brought in contact with the second liquid L2 in the pre-curing zone PZ whereby the sheet liquid L2 is precured by the sheath liquid L3. Subsequently, the precursor fluid L1 is introduced through the first capillary tube 1 1’ further downstream into the hollow central section within the pre-cured shell fluid L2, whereby a liquid fiber with core-shell-sheat structure is produced.

Compared to the embodiment shown in Fig. 1 , in the embodiment of Fig. 6 the pre-cured shell fluid L2 forms a more stable interface with the precursor fluid L1 what further reduces mixing of the fluids.

The so produced liquid fiber with core-shell-sheat structure then is guided through the stationary liquid phase and taken up on a storage as explained with Fig.1.

It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed examples are therefore considered in all respects to be illustrative and not restricting.

Especially, it is for example possible to configure device 10 with only two capillary tubes if production of the fibers always is effected in line with the alternative implementation.

For producing fibers with different diameters, capillary tubes 1 1 , 12, 13 with other dimensions can be foreseen.

Fig. 7 shows a photograph of polylactic acid (PLA) fibers produced with the inventive method. Thereby a non-toxic solvent, i.e. Cyrene® (Dihydroglucosenon; polar solvent), was used instead of toxic solvents (e.g. dichloromethane) usually used with known production processes.

Fig. 8 shows a photograph of amphiphilic polymer optical fibers composed of polysiloxane/polyacrylate amphiphilic polymer co-network produced with the inventive method. The fibers have a low modulus of elasticity of E ~ 2 MPa. Such fibers can be produced from precursors with very low viscosities what so far was not possible with any other method.

Fig. 9 shows polyurethane fibers produced with the inventive method from polyol and isocyanate precursors.

Fig. 10 shows a strain (e) versus stress (a) diagram obtained by tensile testing of a soft polymer optic fiber (POF) produced according to the inventive method. The data shows a modulus of elasticity (E) of only around 0.35 MPa. Fig. 1 1 shows the decrease in light intensity in polymer optical fibers (POF) produced according to the inventive method with low modulus of elasticity (E ~ 0.35 MPa and 1 MPa) as a function of applied pressure, in comparison with an ordinary fiber (E ~ 5 MPa).