EP4689247A1

EP4689247A1 - Cholesteric liquid crystal elastomer based fibres, a production method thereof and their uses

Info

Publication number: EP4689247A1
Application number: EP24710465.6A
Authority: EP
Inventors: Yong GENG; Jan LAGERWALL
Original assignee: Universite du Luxembourg
Current assignee: Universite du Luxembourg
Priority date: 2023-03-29
Filing date: 2024-03-14
Publication date: 2026-02-11
Also published as: LU503770B1; WO2024200037A1

Abstract

An aspect of the present invention relates to cholesteric liquid crystal elastomer (CLCE)-based fibres, having a cylindrically symmetric shape and internal order. A further aspect of the present invention relates to methods for producing a CLCE-based fibre, comprising a step of filling a CLCE-precursor composition into a tube, annealing, polymerising and optionally cross-linking the CLCE-precursor composition in the tube and removing the tube by dissolving in a solvent. A further aspect of the present invention relates to the use of the CLCE-based fibres of the invention, as well as items employing such fibres.

Description

CHOLESTERIC LIQUID CRYSTAL ELASTOMER BASED FIBRES, A PRODUCTION METHOD THEREOF AND THEIR USES

Field of the Invention

[0001] The invention generally relates to cholesteric liquid crystal elastomer (CLCE) based fibres and methods for producing CLCE-based fibres. The invention further relates to the use of the CLCE-based fibres according to the invention and items which make use of the CLCE-based fibres according to the invention, such as sensors, wearable items and soft actuators.

Acknowledgment

[0002] The project leading to this application has received funding from the European Research Council under the Horizon Europe Framework Programme/ERC Grant Agreement no. 101069416 (Proof of Concept project REVEAL).

Background of the Invention

[0003] Liquid crystal-forming materials can exist in different phases. For example, the liquid crystal may be in a nematic phase, in which the molecules are preferentially aligned along a common direction. If the mesogens (liquid crystal molecules) are chiral, or if the system is chirally doped, the nematic phase becomes a chiral nematic phase, also called a cholesteric liquid crystal (CLC) phase. Here, the preferential alignment direction rotates continuously, such that the molecular order forms a helical structure, the axis of which is always perpendicular to the preferential molecule alignment direction.

[0004] In CLCs, the helical modulation of the refractive index, due to the preferential molecular alignment direction rotating in the helical structure, gives rise to selective (Bragg) reflection of light in a narrow wavelength band, the central wavelength of which is determined by the pitch of the helix and the angle of incidence of the light. The reflected light is circularly polarized with the same handedness as the cholesteric helix [1, 2, 3, 4]. CLCs maybe brought into a defined shape by confinement in a container, thereby forcing them into a specific shape. However, upon removal of the container, the same CLCs will collapse into droplets (Plateau- Rayleigh instability). Many efforts have been made to polymerize cholesteric liquid crystals in order to make the helical structure permanent and robust, thereby significantly widening the application area [5, 6, 7, 8, 9, 10].

[0005] Mechanochromic and/or thermochromic elastomeric fibres, changing color in response to strain and temperature change, respectively, can be very useful for strain and temperature sensing in, e.g., health care, sports or safety gear, where the light-weight soft and flexible form factor and independence of power sources gives advantages with respect to alternative technologies [11, 12, 13, 14, 15]. They can also enable entirely new possibilities in fashion and artistic design [16, 17].

[0006] Cholesteric liquid crystal elastomer fibres (CLCE fibres) are known in the art [18], formed by deposition of an extruded oligomeric cholesteric liquid crystal elastomer (CLCE) precursor solution on a rotating mandrel, and photo-crosslinking after solvent removal and annealing, where high viscosity was used to prevent the surface modulation or breaking up of the oligomer filament before UV exposure. The cross-section of the fibres obtained in this process is belt-like. Also, the various production parameters (viscosity of the precursor, mandrel rotation speed, extrusion rate) require a delicate balance of simultaneous processes taking place prior to the crosslinking step. This renders the procedure sensitive to deviation from optimum processing conditions. Upscaling the mandrel-based process to industrial scale is difficult, given the slow extrusion rates and the fact that the mandrel size limits the maximum fibre length.

[0007] Non-chiral nematic liquid crystal elastomer fibres have been synthesized using injection of a precursor into a PTFE-tube [19, 20]. Since the PTFE-tubes employed could not be dissolved in a solvent, the fibres had to be removed mechanically. This mechanical removal is cumbersome and unsuitable for scale-up.

Technical Problem

[0008] It is an object of the present invention to provide elastomeric fibres which overcome the problems of the cited prior art.

[0009] It is a further object of the present invention to provide methods for making such elastomeric fibres. General Description

[00010] As used herein, a “liquid crystal” is a state of matter which has properties between those of conventional liquids and those of solid crystals. In other words, a liquid crystal can flow like a liquid but has some degree of long-range ordering in the arrangement of its molecules.

[00011] As used herein, a “cholesteric liquid crystal” (also called chiral nematic liquid crystal) is a liquid crystal including chiral molecules that exhibits a twisting of its molecules along an axis perpendicular to the preferred orientation of the molecules.

[00012] As used herein, a “cholesteric liquid crystal elastomer” is (at least) a loosely cross-linked cholesteric liquid crystal. It has an anisotropic structure as of a traditional CLC while it cannot flow.

[00013] A cylindrically symmetric shape may be a shape that is invariant under a (continuous) rotation between [o, 2JI[ about an axis.

[00014] A “rotationally symmetric cross-section” may be a cross-section that is invariant under a (continuous) rotation between [o, 2JI[.

[00015] As used herein, a fibre is an elongated object that is significantly longer than it is wide, e.g. at least 5 time longer than it is wide.

[00016] As used herein, an "elastomer” may be a polymer exhibiting both viscous and elastic characteristics when undergoing deformation. In particular, in the context of the present document, an “elastomer fibre” may be a fibre that is able to retain its form even in the absence of a container (for example a tube), and it is insoluble. This is, for example, in stark contrast to a fluid which is not able to retain the same form after removal of the container, when compared to its form before the removal of the container, and which can be dissolved in a suitable solvent. An elastomer may be swelled by a suitable solvent, but the network topology prevents dissolution.

[00017] According to a first aspect of the invention, a CLCE-based fibre is provided, wherein the fibre has a cylindrically symmetric shape and wherein the fibre has a rotationally symmetric cross-section, and optionally with a cylindrically symmetric internal cholesteric order (i.e. the axis along which the orientational order is modulated helically is oriented radially). According to this aspect, CLCE-based fibres are available which may be uniform in thickness, may exhibit high light selective- reflectance and maybe chemically stable. They maybe suitable for advanced wearable technology and mechanical and thermal sensing applications. In a preferred embodiment, the CLCE-based fibre consists of CLCE. In other words, the CLCE-based fibre may be a CLCE fibre.

[00018] In an embodiment, the CLCE-based fibre comprises CLCE.

[00019] According to one embodiment of the invention, the CLCE-based fibres may display a mechanocromic response. As used herein, a “mechanocromic response” is a color change in response to mechanical strain.

[00020] According to one embodiment of the invention, the CLCE-based fibres may display a thermocromic response. As used herein, a “thermocromic response” is a color change in response to temperature change.

[00021] According to one embodiment of the invention, the CLCE-based fibres may display both mechanocromic and thermochromic responses. According to a further embodiment of the invention, the mechanocromic and thermochromic responses may be reversible.

[00022] In an embodiment, the CLCE-based fibres may display a thermomechanical response, which may be reversible. Any combination of the above- mentioned response is contemplated.

[00023] In an embodiment, the CLCE-based fibre, in its ground state at room temperature (e.g. 20°C), reflects light in a wavelength band having a central wavelength in the near IR-region (e.g. in the range from 750 nm to 1000 nm, or in the visible light region (e.g. in the range from 400 nm to 750 nm), or in the near-UV region (e.g. in the range from 380 nm to 400 nm).

[00024] In an embodiment, the CLCE-based fibre comprises at least one of: a chiral dopant, a photo-initiator, a catalyst and a radical scavenger.

[00025] According to a second aspect of the present invention, a method for producing CLCE-based fibres is provided, which comprises the steps of (i) providing a tube made of a soluble material as a template for fibre production, (ii) providing a CLCE-precursor material including a chiral dopant, (hi) filling the provided CLCE- precursor composition into the provided tube, (iv) annealing the CLCE-precursor composition inside the provided tube, (v) polymerising and optionally cross-linking the annealed CLCE-precursor composition inside the provided tube to form a CLCE- based fibre, and (vi) dissolving the tube in a (suitable) solvent so as to remove the tube from the CLCE-based fibre. By suitable solvent, it is meant a solvent that is capable of dissolving the soluble tube (at some temperature).

[00026] The soluble material of the tube may be polymeric, preferably noncrosslinked. Of course, the tube may be solid.

[00027] It has been found that with the method of producing CLCE-based fibres according to the second aspect of the invention, a variety of parameters of the obtained fibres may be tuned and adapted according to specific requirements. This may be achieved by varying e.g. the size and shape of the tubes, the concentration and handedness of the chiral dopant, the nature of the CLCE-precursor composition, or the degree of polymerization and/or cross-linking.

[00028] According to one embodiment, the soluble material forming the tube may be selected from low-density polyethylene (LDPE) and polyvinylalcohol (PVA). LDPE- tubes may be dissolved using e.g. toluene, whereas PVA tubes may be removed by dissolving in water.

[00029] According to the method of the second aspect of the invention, the CLCE fibre formed will conform to the inner shape of the tube provided. Any shapes deemed useful may be considered. According to one embodiment, the tube provided may have a cylindrical, preferably uniformly cylindrical, inner shape, leading to the formation of cylindrical fibres with a rotationally symmetric cross-section. The cross-section is defined in the plane perpendicular to the longitudinal axis of the fibre. The fibres are not necessarily along a straight line but maybe bent, thereby running along a curve, so that the longitudinal axis maybe seen as a longitudinal curve. Nevertheless, this curve is locally straight so that a plane locally perpendicular to the (local) longitudinal axis may be defined.

[00030] The inner diameter of the tube may be chosen according to the requirements, for example the inner diameter of the tube maybe 1 mm or less, such as for example 800 pm or less, or 600 pm or less, or 500 pm or less, or even 400 pm or less. For example, the inner diameter of the tube maybe between 50 pm and 1 mm, or between 100 pm and 800 pm, or between 200 pm and 600 pm, or between 300 pm and 500 pm. The thickness of the tube may be comprised in the range from 0.1 to 0.3 mm, but values outside this range are also contemplated. [00031] It is worth noting that the diameter of the CLCE fibre may correspond (is equal) to the inner diameter of the tube. This remains true even after removal (step vi) of the tube since the fibre is an elastomer.

[00032] The diameter of the CLCE fibre may be 1 mm or less, such as for example 800 pm or less, or 600 pm or less, or 500 pm or less, or even 400 pm or less. For example, the diameter of the CLCE fibre maybe between 50 pm and 1 mm, or between 100 pm and 800 pm, or between 200 pm and 600 pm, or between 300 pm and 500 pm.

[00033] According to one embodiment, the CLCE-precursor composition may comprise liquid crystal monomers, a chain extender and a chiral dopant. The liquid crystal monomers and the chain extender may react to form liquid crystal oligomers. Alternatively, the CLCE-precursor composition may comprise liquid crystal oligomers and a chiral dopant. In an embodiment, the CLCE-precursor composition may comprise a melt containing liquid crystal monomers A combination of the above is also contemplated. A photo-initiator and/or a catalyst may be included. In addition, a solvent may be present. The use and amount of solvent may help tune the viscosity of the precursor composition, thereby aiding the injecting of the composition into the tube, and it may also help increase the precision in concentration of certain components, such as a catalyst added to stimulate the reaction of monomers into oligomers. In one embodiment, the solvent may be selected such that it may diffuse through the soluble material of the tube during the annealing step. A radical scavenger may also be incorporated.

[00034] During the annealing step, which may be carried out by subjecting the tube comprising the CLCE-precursor composition to elevated temperature, the mixture of the oligomer (or the liquid crystal monomers and chain extenders) and the chiral dopant transforms into an equilibrium cholesteric liquid crystal state with the desired radial helical structure. In the annealing step, the tube filled with the CLCE- precursor composition maybe heated to a temperature of 40 °C or higher, such as for example between 40°C and 50°C, such as for example to about 45°C. Since the development of the helical structure requires some time, depending on the viscosity and the relative concentrations of the components in the CLCE-precursor composition, the duration of the annealing step may be 12 hours or more, such as for example 24 hours or more, such as for example between 24 hours and 48 hours, or even 72 hours. If any solvent is present in the CLCE-precursor composition, it may leave during the annealing step through the soluble tube.

[00035] The annealed CLCE-precursor composition may be polymerized (and optionally cross-linked) by exposing it to UV light, which passes (at least partially) through the tube. A polymerized and optionally cross-linked CLCE is formed at this stage.

[00036] Finally, according to one embodiment, the soluble tube may be removed by dissolving in a (suitable) solvent. For example, if the tube is an LDPE-tube, it may be removed by dissolving in a toluene bath, at elevated temperature (e.g. at a temperature comprised between 8o°C and ioo°C, preferably 9O°C). If the tube is a PVA-tube, it may be by dissolving in a water bath either at room temperature or at elevated temperature. According to one embodiment, in a further method step, the fibres left after removal of the polymer tubes may be further washed several times in one or more solvents (may be different from the solvent used in the precursor composition), in order to remove any remaining polymer and solvent. Of course, a combination of solvents may be used.

[00037] It has been found that the CLCE-based fibres obtained using the method according to the second aspect of the present invention are stable, robust, uniformly shaped optionally with cylindrically symmetric internal order, and they may display reversible mechanocromic, thermochromic and/or thermomechanical properties.

[00038] According to a third aspect of the present invention, the CLCE-based fibres of the invention, and the CLCE-based fibres according to the present invention and obtained according to the method of the invention may be used in the production of garments, such as for example wearable garments such as smart clothing, sports or safety gear.

[00039] According to a fourth aspect of the present invention, the CLCE-based fibres of the invention, and the CLCE-based fibres obtained according to the method of the invention may be used in the production of mechanical or thermal sensors.

[00040] In the present document, the verb “to comprise” and the expression “to be comprised of’ are used as open transitional phrases meaning “to include” or “to consist at least of’. Unless otherwise implied by context, the use of singular word form is intended to encompass the plural, except when the cardinal number “one” is used: “one” herein means “exactly one”. Ordinal numbers (“first”, “second”, etc.) are used herein to differentiate between different instances of a generic object; no particular order, importance or hierarchy is intended to be implied by the use of these expressions. Furthermore, when plural instances of an object are referred to by ordinal numbers, this does not necessarily mean that no other instances of that object are present (unless this follows clearly from context). When reference is made to “an embodiment”, “one embodiment”, “embodiments”, etc., this means that these embodiments may be combined with one another. Furthermore, the features of those embodiments can be used in the combination explicitly presented but also that the features can be combined across embodiments without departing from the invention, unless it follows from context that features cannot be combined.

Brief Description of the Drawings

[00041] The accompanying drawings illustrate several aspects of the present invention and, together with the detailed description, serve to explain the principles thereof. In the drawings:

Fig. 1. is a macroscopic image of a random mixture of CLCE fibres according to the present invention, with red, green and blue ground state retroreflection colour, respectively;

Fig. 2. is a reflection mode microscopy image of a knotted red reflecting CLCE fibre according to the present invention;

Fig. 3. is an SEM image of the same fibre as shown in Fig. 2;

Fig. 4. shows the chemical representations of a reactive liquid crystal monomer RM257 and a spacer EDDET, which can react to form a liquid crystal oligomer, and a chiral dopant LC756 providing chirality in the obtained mixture;

Fig. 5. is a schematic representation of a liquid crystal state precursor mixture, including chiral dopant, injected into a templating tube;

Fig. 6. is a schematic drawing of the director distribution of the liquid crystalline state precursor mixture after helical structure development;

Fig. 7. is a schematic drawing of the director distribution of the crosslinked cholesteric liquid crystal elastomer fibre after removal of the tube;

Fig. 8. is a transmission mode optical microscopy image of a 5 pm thick microtome section of a CLCE fibre embedded in light-curable glue (Norland Optical Adhesive); Fig. 9. is a series of 9 photographs (a to i) of observed colour of a CLCE fibre according to the present invention as a function of axial strain e_zz;

Fig. 10. is a graphical representation of the reflection spectra of a CLCE fibre according to the present invention upon which different axial strains have been applied;

Fig. 11. is a graphical representation of the colour shifts detected on CLCE fibres according to the present invention as a function of applied axial strain e_zz, on the examples of fibres having IR, red and yellow ground state retroreflection wavelengths, respectively; and

Fig. 12. is a graphical representation of the colour shifts detected on CLCE fibres according to the present invention as a function of temperature during heating and cooling on the example of a fibre having a green ground state (room temperature) retroreflection wavelength.

[00042] The reader’s attention is drawn to the fact that the drawings are not necessarily to scale. Furthermore, for the sake of clarity, proportions between height, length and/ or width may not have been represented correctly.

Detailed Description of Preferred Embodiments of the Invention

[00043] A mixture of CLCE fibres according to the first aspect of the present invention are represented in Fig. 1. The CLCE fibres may be any ground state colour as desired. Fig. 1 shows a macroscopic image of a random mixture of CLCE fibres with red, green and blue ground state colour, respectively. Due to the molecular alignment direction rotating in a helical structure, a selective (Bragg) reflection of light in a narrow wavelength band occurs. The central wavelength of this band is determined by the pitch of the helix and the angle of incidence of the light. Therefore, the ground state colour of the CLCE fibres according to the first aspect of the present invention maybe tuned in line with requirements, by tuning the helix pitch during the formation of the CLCE fibres. As also apparent from Fig. 1, the CLCE fibre may be flexible in the sense that they may bend and form e.g. a knot. Multiple fibres may also intertwine and/ or form a knot.

[00044] As is apparent from Figs. 2 and 3, the fibres have a cylindrical shape with uniform diameter and a smooth surface. [00045] The method of production of the CLCE fibres, preferably according to the first aspect of the invention represents a second aspect of the invention. The method allows for the formation of CLCE fibres having a size and shape defined by the morphology of the inner surface of templating tubes made of a soluble material, which may be polymeric and preferably non-crosslinked. The tube is preferably solid. In particular, the method according to this aspect of the invention allows to produce CLCE fibres of a predefined shape and morphology, wherein the formation of the CLCE from a CLCE precursor occurs within the templating tube, and the templating tube may be removed by simple dissolution in a suitable solvent. In this way, the process of harvesting the CLCE fibre is simple and does not risk deterioration of the fibre structure. The general principle of formation of a CLCE from a CLCE-precursor composition comprising a liquid crystal oligomer (obtained by linking different numbers of liquid crystal monomers using a chain extender) and a chiral dopant for forming droplets and shells is known in the art. The application of this principle for forming CLCE fibres using a dissolvable templating tube has not been previously described.

[00046] The dissolvable tube template is a key for the CLCE fibre production in this aspect of the invention. As shown in Figs. 4 and 5, a solution of chiral dopant and liquid crystalline oligomer (linear polymer obtained by linking different numbers of liquid crystal monomers using a chain extender), or a melt preferably of low molar mass components, was filled into a templating tube. As shown in the figures, RM257 (i,4-bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene) and EDDET (2,2- (ethylenedioxy) diethanethiol) may be used as liquid crystal monomer and chain extender, respectively, and LC756 (3R,3aS,6aS)-hexahydrofuro[3,2-b]furan-3,6-diyl bis(4-(4-((4-(acryloyloxy)butoxy) carbonyloxy) benzoyloxy )benzoate) may be used as a chiral dopant. The skilled person in the art will be aware of different liquid crystal monomers, chain extenders and chiral dopants which may be equally suitable for use in the method according to this aspect of the invention. A catalyst (e.g. dipropylamine), a photo-initiator (e.g. Irgacure 2022 (Sigma Aldrich)) and/or a radical scavenger (e.g. butylated hydroxytoluene) may be added.

[00047] Different degrees of crosslinking, tuned by using oligomers of varying lengths or varying the ratio of liquid crystal monomer to chain extender, are expected to yield different mechanical properties of the final fibres, as well as a different thermochromic response.

[00048] When starting with an oligomeric CLCE precursor, a solvent may be used to lower the viscosity in order to have good fluidity of the oligomer and chiral dopant mixture during the tube filling and to have a faster annealing time as well. The choice of solvent may well depend on the choice of material for the templating tube and the choice of components of the CLCE-precursor composition. For example, in case LDPE templating tubes are employed, dichloromethane is a suitable choice of solvent, since this may diffuse through the templating tube during the annealing step of the method according to this aspect of the invention.

[00049] After filling, the templating tubes containing CLCE precursor are placed in an incubator with a temperature setting of between 4O°C and 5O°C for the oligomer to anneal. The helical structure development may take between 12 and 72 hours, but with other CLCE precursor compositions the time may be lower, and the temperature may be higher or lower. When a solvent is present, this may leave the core through the templating tube during the annealing, eventually evaporating when it reaches the exterior.

[00050] During the annealing process after filling into the tube, the CLCE precursor transforms into a well-aligned equilibrium cholesteric liquid crystal state with radial helix. Because the CLCE precursor is liquid, the core at this stage would break up into droplets due to the Rayleigh-Plateau instability, if it were not for the templating tube confining the core. When solvent is used, capillary forces between the liquid core CLCE-precursor and the inside of the tube (which may swell and become somewhat softened, yet not liquid, as a result of solvent leaving the core and diffusing into the tube) ensure that there is never any gap between core and tube, thus preventing void formation or the Rayleigh- Plateau instability to develop at any point. The use of the templating tubes allows for CLCE precursors with much lower viscosity to be used compared to the state of the art.

[00051] As schematically shown in Fig. 6, a well aligned helical structure with the axis in the radial direction is obtained upon annealing (the same orientation scale is used for Fig. 6 and Fig. 7).

[00052] After annealing, the core is photopolymerized by exposing it to UV light (at least partly passing through the templating tube). The templating tube is then dissolved by immersion into a solvent bath. The CLCE fibres may then be collected and washed repeatedly to remove any remaining tube material or solvent. The CLCE fibres may be dried in a vacuum oven at elevated temperature to remove all remaining washing solvent. A representation of the structure after removal of the templating tube is shown in Fig. 7. The orientation scale indicates the angle between the fibre director and the fibre axis.

[00053] In case of templating tubes made of LDPE, toluene may be used as a solvent for removing the tubes. In case the templating tubes are made of PVA, water may be employed to dissolve and remove the tubes.

[00054] Fibres produced in this way adopt the morphology of the inner surface of the templating tube and with uniform thickness, as shown in Figs. 1 to 3.

[00055] The radial helix arrangement of the obtained CLCE fibres may be confirmed by analysis of a microtomed cross-sectional slice of the obtained CLCE fibre. Fig. 8 shows a 5 pm microtome section of a CLCE fibre according to the present invention. The appearance in polarizing optical microscopy confirms the radial helix arrangement.

[00056] By adjusting the concentration of the chiral dopant in the oligomer solution, the ground state pitch of the cholesteric liquid crystal elastomer can be tuned such that it reflects wavelength bands ranging from IR to blue. Accordingly, a higher amount of chiral dopant present in the CLCE-precursor composition generally leads to a shorter helical pitch and therefore a shorter relaxed state retroreflection wavelength X*_o of the Bragg reflection of the CLCE. In other words, increasing the amount of chiral dopant in the CLCE-precursor composition leads to a blue shift of the ground state colour of the obtained CLCE fibre.

[00057] The CLCE fibres according to the present invention may display mechanochromic responses. When an axial strain is applied to the CLCE fibres, the helical structure of the CLCE is deformed, but the periodicity remains and gets smaller, hence the reflected wavelength becomes shorter. This is shown qualitatively in Fig. 9, in which varying degrees of axial strain have been applied to a CLCE fibre which is red in its ground state. The axial strains applied are indicated in the figure. More quantitatively, Fig. 10 shows the reflectance and wavelengths of the reflected light of a CLCE fibre at various applied axial strains, indicating that a higher axial strain leads to a shorter reflected central wavelength. Fig. 11 shows the central wavelength shifts occasioned by the application of axial strain to CLCE fibres with IR, red and yellow X*_o (including best-fit lines). This shows that the application axial strain to the CLCE fibres according to the present invention leads to a blue shift in the fibre colour. This is due to the reduction of the helical pitch upon application of axial strain to the CLCE-based fibres. The experiments also showed that the mechanocromic response is reversible- once the applied axial strain is released, the fibre returns to its original (ground state) colour.

[00058] The CLCE fibres according to the present invention may also display thermochromic responses. As illustrated by Fig. 12, the central wavelength of the reflected light increases upon heating of a CLCE fibre according to the present invention, which is green in its ground state at room temperature. Upon cooling, the central wavelength of the reflected light reduces again, albeit with some delay. In other words, heating of the CLCE fibres of the present invention leads to a red shift of their colour, and their cooling leads to a blue shift.

[00059] According to a third aspect of the present invention, the CLCE fibres of the first aspect of the invention, or obtained according to the second aspect of the invention, may be employed in the production of wearable garments, such as sports gear, safety gear or clothes to be worn for novelty purposes, artistic performances or for demonstration purposes.

[00060] According to a fourth aspect of the present invention, the CLCE fibres of the first aspect of the invention, or obtained according to the second aspect of the invention, may be employed in the design and production of sensing applications, for example in sensors for use as soft actuators in medical devices or for biological or medical research.

[00061] While specific embodiments have been described herein in detail, those skilled in the art will appreciate that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims and any and all equivalents thereof. References

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Claims

1. A cholesteric liquid crystal elastomer (CLCE) -based fibre, wherein the fibre has a cylindrically symmetric shape and wherein the fibre has a rotationally symmetric cross-section.

2. The CLCE-based fibre according to claim 1, which has a mechanochromic and/ or a thermochromic response.

3. The CLCE-based fibre according to claim 2, wherein the said mechanochromic and/or thermochromic responses are reversible.

4. The CLCE-based fibre according to any one of the previous claims which in its ground state at room temperature reflects light in a wavelength band having a central wavelength in the near IR-region, or in the visible light region, or in the near-UV region.

5. The CLCE-based fibre according to any one of the previous claims, wherein the CLCE-based fibre comprises at least one of: a chiral dopant, a photo-initiator, a catalyst and a radical scavenger.

6. A method for producing a CLCE-based fibre, comprising the steps of o providing a tube made of a soluble material as a template for fibre production; o providing a CLCE-precursor material including a chiral dopant; o filling the provided CLCE-precursor composition into the provided tube; o annealing of the CLCE-precursor composition inside the provided tube; o polymerising and optionally cross-linking the annealed CLCE-precursor composition inside the provided tube to form a CLCE-based fibre; and o dissolving the tube by immersion in a solvent so as to remove the tube from the CLCE-based fibre.

7. The method according to claim 6, wherein the soluble material is polymeric, preferably non-crosslinked, the soluble material being preferably selected from low-density polyethylene (LDPE) and polyvinylalcohol (PVA).

8. The method according to any one of claims 6 to 7, wherein the said tube has an inner shape which is cylindrical, preferably uniformly cylindrical, preferably with a diameter of 1 mm or less.

9. The method according to any one of claims 6 to 8, wherein the said CLCE- precursor composition comprises liquid crystal monomers and a chain extender.

10. The method according to any one of claims 6 to 9, wherein the said CLCE- precursor composition comprises liquid crystal oligomers.

11. The method according to any one of claims 6 to 10, wherein the said annealing step is carried out by subjecting the CLCE-precursor composition inside the provided tube to elevated temperature for a specified period of time.

12. The method according to any one of claims 6 to 11, wherein the said polymerisation and/or crosslinking steps are carried out using UV radiation.

13. The method according to any one of claims 6 to 12, wherein the said removal of the tube is carried out by immersion into a solvent, which may be optionally heated.

14. The method according to any one of claims 6 to 13, comprising the further step of washing the obtained CLCE-based fibre.

15. The use of a CLCE-based fibre of any one of claims 1 to 5 or of the CLCE-based fibre formed according to the method of any one of claims 6 to 14 for the production of wearable garments.

16. The use of a CLCE-based fibre of any one of claims 1 to 4 or of the CLCE-based fibre formed according to the method of any one of claims 6 to 14 in mechanical and/or thermal sensors.

17. A sensor comprising a CLCE-based fibre of any one of claims 1 to 5 or of the CLCE-based fibre formed according to the method of any one of claims 6 to 14.

18. A wearable item comprising a CLCE-based fibre of any one of claims 1 to 5 or of the CLCE-based fibre formed according to the method of any one of claims 6 to 14.

19. A soft actuator comprising a CLCE-based fibre of any one of claims 1 to 5 or of the CLCE-based fibre formed according to the method of any one of claims 6 to 14.

20. A method for measuring strain and/or temperature in a biological system or in an enclosed environment comprising the use of a sensor including a CLCE-based fibre of any one of claims 1 to 5 or of the CLCE-based fibre formed according to the method of any one of claims 6 to 14.