Methylcellulose-Cellulose Nanocrystal Composites for Opto- mechanically Tunable Hydrogels and Fibers

Chemical modification of cellulose offers routes for structurally and functionally diverse biopolymer derivatives for numerous industrial applications. Among cellulose derivatives, cellulose ethers have found extensive use, such as emulsifiers, in food industries and biotechnology. Methylcellulose, one of the simplest cellulose derivatives, has been utilized for biomedical, construction materials and cell culture applications. Its improved water solubility, thermoresponsive gelation, and the ability to act as a matrix for various dopants also offer routes for cellulose-based functional materials. There has been a renewed interest in understanding the structural, mechanical, and optical properties of methylcellulose and its composites. This review focuses on the recent development in optically and mechanically tunable hydrogels derived from methylcellulose and methylcellulose-cellulose nanocrystal composites. We further discuss the application of the gels for preparing highly ductile and strong fibers. Finally, the emerging application of methylcellulosebased fibers as optical fibers and their application potentials are discussed.


Introduction
Cellulose, in general, is a renewable and nearly inexhaustibly available natural polymer that can be processed into various forms ranging from pulp to microfibers to nanoparticles and down to chemically derivatized polymer chains. [1] Each cellulose derivative shows its own characteristics and functionalities depending on its processing history, for instance, high surface charge, water solubility, lyotropic liquid crystallinity, thermoresponsivity, and extreme mechanical strength. Consequently, a range of fully cellulose-derived advanced and functional materials and (nano)composites can be prepared by selecting the correct raw materials and their combinations. [2][3][4][5][6] Furthermore, the inherent relative ease of modifiability makes cellulose applicable in a wide range of conditions and enables synergies with other materials, such as carbon nanotubes, graphene, other biomolecules and metallic nanoparticles. [7][8][9][10][11][12] Potential applications for advanced cellulosic materials include, for example, biomedicine, tissue engineering, photonics, smart materials, emulsions, foaming, and sensors. [13][14][15][16][17] The rich history and applications of cellulose and various cellulose-based materials and nanocomposites are evident from several excellent reviews. [4,5,[18][19][20][21][22][23][24] Yet, there are numerous properties and potential applications of celluloses, nanocelluloses, and their composites that remain unexplored. Especially the trend towards sustainably produced optical and optoelectronic materials and devices has gained attraction recently, and therein cellulose derivatives and their composites appear as key components. [20] Here, this review focuses on the recent findings on the optomechanical properties of methylcellulose (MC), a commonly used cellulose derivative and its composites with cellulose nanocrystals (CNCs). We first provide a brief introduction to methylcellulose and cellulose nanocrystals and their properties. Then we discuss how the inverse biocompatibility, and relative structural simplicity. Its applications range, among others, from general viscosity modifying agents to food technology and biotechnology. [25] MC provides a naturally functional and physically cross-linked water-based matrix or scaffolds within nanocomposites, whose stiffness is feasibly tunable from viscous fluid to stiff self-standing gels by simply adjusting the MC concentration or the surrounding temperature. [32,33] Charge-neutral and amphiphilic nature of the MC also ensures good compatibility and homogeneous mixing with a wide range of polymers, dopants and modifying agents without sacrificing the characteristic inverse thermoreversible behavior. [31,[34][35][36] Accordingly, the versatility of the MC hydrogels is highlighted and essential also for the MC-CNC nanocomposite hydrogels and fibers.
CNCs, on the other hand, represent a completely another type of cellulosic material that is particularly known for its high mechanical strength, rigidness, and colloidal rodlike appearance. [37][38][39] Hence, CNCs have often been considered as potential strengthening agents and reinforcing nanofillers, which also applies to the MC-CNC nanocomposites. [40] Importantly, the surface functionality and thereby the properties of CNCs can be modified depending on the preparation methods, their source and post-chemical modification. [41][42][43] Additionally, the CNCs self-assemble into left-handedly twisted chiral nematic, i.e., cholesteric, liquid crystalline structures above a critical concentration (c*), which may serve as a scaffold for hierarchical structural materials, to increase mechanical order and strength in composites, and to adjust optical properties of a material, among others. [44][45][46][47] Both the mechanical and liquid crystalline properties are relevant for the MC-CNC nanocomposites.
When combined into nanocomposite hydrogels and fibers, MC and CNCs express multiple complementary properties and synergistic effects: importantly, mechanical adjustability, thermally tunable stiffness and structural stability, feasible spinnability, tunable transparency, and controllable structural degradability, which are all achievable through relatively straightforward and gentle processing. [31,48,49] Thus, versatile and extraordinary materials from freely flowing liquids to stiff and solid nanocomposites can be prepared from relatively simple cellulose-derived components. This also emphasizes the potential of cellulosic materials in general. Next, the special characteristics of MC and CNCs are presented in more detail, followed by the discussion on the corresponding nanocomposite hydrogels, manufacturing of such hydrogels into highly ductile fibers and, lastly, their tuning into and potential as cellulose-derived optical fibers

Preparation and general properties of MC hydrogels
Hydrogels are viscoelastic materials consisting of either chemically or physically cross-linked three-dimensional (3D) networks encapsulating a large number of water molecules. Gels, in general, are state of matter intermediate between solids and liquids containing at least two components viz., a continuous phase (i.e., a solid network) and dispersion media (typically a liquid or solvent). [50] Therein, solutes and water molecules may rather freely flow within the gel matrix, while a stable structure and shape are, nonetheless, maintained. Despite containing a large amount of liquid, the gels display solidlike mechanical properties under oscillatory rheological experiments. Typically, strong gels show an order of magnitude higher storage modulus (G`) compared to loss modulus (G´´). [51] The gel's rigidity and mechanical properties can be adjusted and enhanced via the addition of rigid supportive nanofillers, such as high-aspect ratio CNCs. [40,48] When MC is dissolved in water, the polymer chains readily hydrate, resulting in a transparent viscous fluid (Figure 1b). However, proper measures are encouraged to ensure homogeneous MC dissolution and to avoid undesired aggregation during the preparation phase. Due to the LCST behavior, dry MC powder is advisably first added into hot water (>80 °C) to avoid untimely and uneven hydration. [25,49] Only after the MC powder is homogeneously distributed and wetted by the hot solvent, the hydration and closer interactions with water molecules are triggered and driven by cooling down the system to ensure an even dissolution. Upon the cooling induced MC hydration, the solution turns from cloudy to transparent. [28] When preparing nanocomposite hydrogels, water-soluble nanoparticles may be feasibly mixed with the MC powder in hot water to facilitate homogeneous distribution.
Dilute solutions of MC typically show liquid-like behavior at low temperatures (below the LCST temperature) and flow readily. [25] Higher viscosities and more gel-like properties can be reached by increasing the concentration or molecular weight of the MC, or via the addition of rigidifying (nano)components. [27,33,48,52] Shear thinning, which facilitates fibers spinning methods involving narrow spinnerets or capillaries, is characteristically observed under steady shear experiments at low temperatures, while transformation to shear thickening may be observed close to or above the gelation temperature. [53] The behavior can be further controlled by substituting the MC polymer chains with chemically cross-linkable side groups, such as allyls, which allow solid, stable materials regardless of the temperature. [54] At elevated temperatures above the LCST gelation point, the storage and loss moduli of MC solutions increase considerably, and turbid hydrogels (G' >> G'') are achieved (Figures 1b-d and 2). [33] However, in addition to the gel composition, the exact gelation temperature is affected by the heating rate. [28] A significant change in volume or syneresis, i.e., the excretion of network-bound water, does not occur upon the MC gelation, unless heating is continued for substantially extended periods of time, such as days or weeks. [55] However, considerable hysteresis is observed upon cooling back to the solution state, even though the thermally induced gelation is otherwise fully reversible. [28,33,56] The hysteresis has been suggested to be induced by the slow unraveling and low re-hydration rate of the fibrillar MC bundles present in the gel stage. [28] As indirect evidence, the addition of negatively charged highly hydrophilic CNCs significantly reduces the observable hysteresis by facilitating the gel re-hydration upon cooling ( Figure 2). [48]  1% MC hydrogel freeze-dried from room temperature and then fractured. d) SEM of a 1% MC hydrogel freeze-dried and fractured from 75 °C shows enhanced presence of fibrillar structures. a-b) adapted with permission from [57]. Copyright 2013 American Chemical Society. c-d) adapted with permission from [31]. Copyright 2018 American Chemical Society.
When aqueous MC is heated, the MC polymer chains assemble into a percolative network consisting of fibrillar structures with a high water content of ca. 60 % and a rather constant and persistent diameter of ~15 nm, which has been confirmed and visualized both by experimentally (cryo-TEM imaging, and small angle neutron and X-ray scattering) and via computational simulations ( Figure 2). [57][58][59][60][61][62][63] Additionally, fibril formation may be induced by the addition of salt. [64,65] Rather recently, the tightly-packed core region of the fibers has been suggested to comprise semicrystalline twisted MC chain domains aligned along the fibril long axis and surrounded by amorphous and less-ordered regions. [66] The percolation of the semi-stiff fibril network contributes to the increased macroscopic gel stiffness and turbidity at elevated temperatures. The current knowledge over such thermally induced MC fibrillar aggregates and their significance on the MC gelation process have been comprehensively summarized in the recent reviews by Morozova [59] and Coughlin et al. [67,68] Furthermore, the MC fibrils may grant the strain stiffening property to the hydrogels, which is relatively rarely encountered in carbohydrate-based hydrogels. [69] Instead, strain stiffening is a characteristic feature of various extracellular matrix components and cytoskeleton, such as collagen, fibrin, actin, and vimentin. [70,71] In strain-stiffening systems, the material's stiffness increasingly grows as a response to increasing deformation, to resist over-extension and to prevent catastrophic mechanical failure. [70] In MC hydrogels at elevated temperatures, strain stiffening has been reported in samples consisting of relatively high-molecular weight (300 000) MC, and in MC-CNC nanocomposite hydrogels containing lower molecular weight MC (88 000) chains. In the MC-CNC composites, the strain stiffening was rendered detectable by the addition of CNCs, suggesting an intriguing synergistic interplay between the MC network and the nanodopants. [31,53] Finally, aqueous solutions of high-molecular weight (≥ 380 000) MC have been shown to exhibit concentration, molecular weight, and temperature-dependent liquid crystallinity. [72] Thus, in addition to ordered fibrillar structures upon gelation, MC appears to have an inherent tendency for collectively structured assemblies. By adding suitable nanodopants favoring liquid crystalline packing, such as CNCs, the order can be further effectively enhanced. [73] Overall, MC has been shown to readily interact with CNCs both in aqueous conditions and as stabilizers for oil-water emulsions and double-morphology latexes. [73][74][75] Furthermore, MC's ability to efficiently interact at the interfaces between hydrophilic and hydrophobic substances can allow surprisingly rigid and controllable structures as demonstrated with the stabilization of oil-in-water emulsions. [76] There the droplet coalescence, and the stabilization ability and structure of the surrounding MC layer were significantly adjusted by simply tuning the oil/solid fat composition of the oil droplets.

Characteristics of cellulose nanocrystals
Cellulose nanocrystals (CNCs) are highly crystalline rod-like high-aspect ratio nanoparticles that are obtained via top-down deconstruction from cellulosic raw materials ( Figure 3). [18] Depending on the source and processing conditions, the length of the CNCs is typically some hundreds of nanometers and their aspect ratio ranges from ~5 to over 100. [77] For instance, cotton based CNCs typically display 70-300 nm in length and around 7-10 nm in lateral dimension ( Figure 3). [77] Often sulfuric acid hydrolysis is preferred since it produces CNCs with high negative surface charge emerging from sulfate half ester groups, high hydrophilicity, and excellent colloidal stability. [78,79] During the processing, the more loosely packed and amorphous regions become degraded, while the organized crystallites remain as the CNC particles. The benefits and drawbacks of various preparation methods have been recently thoroughly reviewed by Vanderfleet and Cranston. [18] Currently, industrial scale production of CNCs is readily available, making the material suitable for large scale manufacturing and applications. [80] Figure 3. Cellulose nanocrystals. a) A photograph of 1% aqueous solution of cotton-based CNCs produced using sulfuric acid hydrolysis -i.e., below the critical liquid crystalline concentrationappears grayish due to scattering and behaves like water. b) TEM image of cotton-based CNCs. c) Polarized optical microscopy image showing cholesteric tactoids in an 8.0% cotton-based CNC solution, i.e., above the critical liquid crystalline concentration. The inset shows a schematic illustration of a left-handed cholesteric CNC assembly.
CNCs are structurally very strong and stiff, which makes them excellent reinforcing units for nanocomposites. As such, the mechanical properties of individual CNCs supersede, for example, steel or glass fibers, and are comparable to Kevlar. [81,82] However, due to the anisotropic rod-like crystalline packing, the mechanical performance of CNCs is also direction-dependent, and the longitudinal elastic modulus of the CNCs (~150 GPa) far exceeds the transverse modulus (~8-57 GPa). [37,83] In order to maximally transfer the beneficial properties of CNCs to nanocomposites, it is important to ensure homogeneous distribution, alignment and interactions of the nanorods and to prevent particle aggregation. CNC surface typically contains a multitude of hydroxyl (-OH) groups that allow hydrogen bonding and efficient coupling to the surrounding matrix to provide reinforcement. [41] These groups, along with electrostatic interactions, may also be used to chemically adjust the CNC functionalities or to drive certain interactions and self-assembled structures. [7,12,[84][85][86] However, at certain conditions, CNCs may self-aggregate, which results in the loss of available surface groups and surface area, and in the potential formation of structural defects, weak points, and local inhomogeneities, which potentially compromise the mechanical benefits of the nanocomposites. [87] Thus, ensuring proper distribution and stability during material processing is essential.
CNCs show lyotropic liquid crystalline behavior, that is, they spontaneously form liquid crystalline assemblies when dispersed in water at sufficiently high concentration. [88,89] For example, the typical critical concentration (c*) to trigger the onset of phase separation of aqueous cotton-based CNCs into ordered liquid crystalline and disordered phases is approximately 3-4%. [88] While individual CNCs are structurally right-handedly twisted, collectively they tend to form left-handed chiral nematic, i.e., cholesteric, liquid crystalline assemblies (Figure 3c). [7,44,90] The chirality can be retained upon drying, and used for templating of hierarchical solids. [47,91] In nanocomposite hydrogels and solids, the organized assemblies may serve as an additional source of structural stability and as a template for anisotropic mechanical and optical properties, such as oriented swelling, birefringence, and structural colors. [14,46,77,[92][93][94][95] Even though achieving the c* and feasible CNC packing while being embedded inside a polymer matrix or a hydrogel may pose some challenges. However, the polymer network, on the other hand, may also facilitate the packing via particle confinement or co-alignment. [31] Additionally, in polymer nanocomposites the matrix polymer has been shown to form a semi-solid interface on top of the nanodopants, thus, contributing their effective size and volume fraction, and simultaneously lowering the actual c* and potential percolation threshold. [96,97]

Mechanical tunability of MC-CNC composite hydrogels
While pure aqueous MC solutions appear transparent at room temperature, both the mechanical and optical properties can be significantly adjusted by adding nanodopants. In general, water-soluble nanoparticles are feasibly mixed within the MC matrix during the gel preparation process. [31,36,48,52] First, the addition of high-aspect ratio stiff nanorods, such as CNCs or chitin nanocrystals (ChNC), effectively induces the sol-gel transition and enables self-standing MC-based hydrogels also at low temperatures. [31,48,52] It has been shown that already an addition of ~1.0% of sulfuric acid hydrolyzed cotton-based CNCs (length ~200 nm) into 1.0% aqueous MC (MW 88 000), that is, total solid content of 2.0% results in gelation with storage modulus (G') > loss modulus (G''), and G' > 10 Pa at room temperature (20 °C). [31,48] Thus, a proper gel is achieved, even though the nanocomposite structure comprises 98.0% of water. Furthermore, within the identical 1.0% MC solutions, G' has been observed to exceed G'' already by adding as low as 0.5% of cottonbased CNCs. However, there the composite structure remains very delicate and weak having the absolute G' of only ca. 5-8 Pa. [31,48] For comparison, pure aqueous MC solution of identical molecular weight with a higher total solid content of 4.0%, still remains as a flowing liquid having G'' above the G', regardless of its significantly higher absolute G' of ca. 170 Pa in the linear viscoelastic range. [36] The nanocomposite gel's stiffness can be increased further by increasing the concentration of CNCs within the MC matrix ( Figure 4). However, the mechanical enhancement does not proceed linearly as a function of the CNC amount, but shows a rapid initial increase followed by a more plateau-like region suggesting a percolation phenomenon. [31] Generally, the effect of CNC content has been studied in MC-CNC nanocomposite gels, where the MC content has been kept constant (1.0%) while varying the amount of CNCs. [31,36,48] Hence, the gel's total solid content and the relative MC-to-CNC ratio are modified simultaneously. A similar approach with fixed MC composition accompanied and a tunable nanodopant composition has been used when exploring MC-ChNC nanocomposite hydrogels. [52] Experimentally, the addition of cotton-based CNCs up to ~1.5% has been reported to enhance the gel stiffness, above which the reinforcement efficacy decreases. [31] It has been suggested that above the 1.5% CNC loading, the inter-CNC interactions begin to outweigh the particle-matrix interactions. This results in less effective entanglement and reduced reinforcement capacity. [31] In total, by tuning the CNC content from 0 to 3.5% while keeping MC constant at 1.0%, approximately two orders of magnitude increase in G' from ~1 Pa to ~100 Pa is achievable at RT (Figure 4). [31,48] Simultaneously with the onset of the plateau-like region around 1.5% of added CNC, birefringence observable with a polarized optical microscope appears, which is discussed below, implying a percolation threshold. [31]  In the literature, only the use of cotton-based CNCs have been reported with MC gels, but the required CNC content to enable initial gelation and significant reinforcement is expected to clearly diminish when utilizing CNCs with higher lengths and aspect ratios, such as those extracted from tunicates (length ~1000 nm). [98] Higher aspect ratio particles are known to display better reinforcement potential and to lower the percolation threshold in nanocomposites in general. [96,99] Likewise, MC network consisting of higher MW polymer chains would provide better entanglement and increased gel stiffness. On the other hand, small metal nanoclusters mixed into MC hydrogel have been observed to promote more liquid-like behavior and soften the gel network. [36] This has been suggested to be caused by the tiny size (d~3 nm), spherical shape and surface structure of the metal clusters that do not provide sufficient additional rigidity or stiff entanglement points for the MC polymer chains like the CNCs. Thus, the MC-metal cluster interactions are unable to provide reinforcement and rather tend to loosen the structure. [36] The observation well agrees with the nanocomposite literature, where, in general, fine nano objects with high aspect ratio and prominent surface-to-volume ratio are known to facilitate both favorable particle-matrix and particle-particle interactions, and to provide superior reinforcement compared to simpler spherical nanodopants. [100] Nonetheless, the possibility to use of tiny nanodopants to tune the MC rheological properties towards more liquid-like state may also open interesting opportunities for processing and material properties in the future. Additionally, nanodopants may simultaneously provide additional benefits and properties, for instance, sensing and photoluminescence, to the nanocomposites.
The thermoreversible response characteristic of pure MC is retained in the MC-CNC nanocomposite hydrogels regardless of the CNC addition, which allows reversible stiffening of the structure upon heating (Figure 4). That is, the baseline stiffness and sol-gel characteristics of an MC-CNC, or an MC gel modified with any analogous nanoparticles, at low temperatures can be adjusted by controlling the amount of nanodopants and gel composition. Furthermore, fully reversible additional stiffening of approximately one order of magnitude may be triggered on demand by increasing the temperature (Figure 4bc). Hence, a two-step and partially fully reversibly method to adjust MC-CNC or any analogous MC-nanodopant hydrogel exists. Moreover, the typical hysteresis of MC hydrogels upon cooling can be reduced by the inclusion hydrophilic CNCs or nanodopants, which facilitates the re-hydration of the entire nanocomposite network. [48]

Optical birefringence in MC-CNC composite hydrogels
Emergence of birefringence in MC-CNC hydrogels has been observed at certain CNC loadings, specifically at and above ~1.5 % of cotton-based CNCs when keeping the amount of MC (MW 88000) constant at 1% ( Figure 5). [31] The appearance of birefringence matches with the onset of diminishing mechanical returns from the addition of CNCs as mentioned above and suggests percolation. Birefringence indicates two important features viz, (i) aligned structural order exists and can be maintained within the composite hydrogel, and (ii) the polarization and direction of the incoming light dictates how it will refract through the sample material. Birefringence can be observed with polarized optical microscopy (POM), which is typically used to visualize and identify liquid crystalline phases and materials. [101] Thus, structural order and stability can be imparted onto MC gels by adding CNCs. However, it should be noted that the total observed intensity of birefringence may be enhanced by the local shear induced alignment emerging from the contact between the material and the used imaging chamber walls. [31] Nonetheless, the structure is able to also maintain the possible shear-determined orientations. Thus, opening possibilities for the manufacturing of nematically oriented composite materials. For instance, anisotropic acrylamide hydrogel-bound CNC films have already shown prospects as ionic strength and pressure sensors. [102] The addition of CNCs also affects the macroscopic light scattering and gel transparency turning by the general appearance from clear (with pure MC) towards increasingly grayish and cloudy with the increased CNC content. [31,36,48] As a rule of thumb, nanoparticles exceeding the size limit of ca. 50 nm can be expected to significantly increase the light scattering which is then observed as enhanced opaqueness. [103] Figure 5. Birefringence in MC-CNC composite hydrogels. a) MC-CNC 1%-2.5% hydrogel imaged through crossed polarizers at 20 °C. A full-wave plate is used for enhanced colors. b) SEM micrographs of a MC-CNC hydrogel freeze-dried and fractured from 70 °C at two different magnifications show CNCs dispersed in the MC medium. c) The birefringence of MC-CNC gels intensifies upon heating to 70 °C and recovers upon cooling down to 20 °C. d) Increase of birefringence intensity as a function of the hydrogels CNC content when MC is kept fixed at 1.0%. Note the logarithmic scale to highlight the differences also at the lower CNC loadings. Birefringence is visible above the intensity threshold of ~1. Adapted with permission from [31] (https://pubs.acs.org/doi/full/10.1021/acs.biomac.8b00392). Copyright 2018 American Chemical Society.
Birefringence is characteristic for cholesteric liquid crystalline assemblies, and typically such behavior emerges in pure high-concentration CNC (above 3 -4%) dispersions. It is atypical to encounter liquid crystalline assemblies at such a low CNC content of ~1.5% (i.e., total hydrogel solid content of 2.5%) used in the MC-CNC composites. Importantly, that is below the usual c* of 3-4% for the cotton-based CNCs. [31,88] Therefore, the MC matrix appears to contribute to the CNC alignment and confinement. Furthermore, even though liquid crystallinity has been reported in some pure high-molecular weight MC solutions, MC solutions with lower MW (88 000) have showed no birefringence at similar conditions. However, it could be transformed birefringent by the addition of CNCs. [31,72] Hence, MC-induced CNC confinement and mutual alignment of MC chains in the MC-CNC nanocomposite hydrogels have been suggested to result in the increased structural order above the percolation threshold. Consequently, it manifests as birefringence and is significantly tunable by the CNC content. [31] Potentially, interactions between the carbohydrate backbones of MC and CNCs provide additional support and rigidity to the composite hydrogel similar to mixtures of cellulose nanofibers (CNFs) and chitosan in composite fibers. [104] When the CNC concentration exceeds the percolation threshold, the particle-particle interactions eventually begin to outweigh the particle-MC matrix interactions. Consequently, gel rigidness increases, and local CNC-driven more organized regions emerge which is, then, observed as birefringence. [31] A similar concentration-dependent aggregation has been observed in solid fiber nanocomposites, as will be discussed more below. It is possible that gel preparation conditions could affect the orientation and possible interactions of the CNCs. For example, fast cooling and gelation may lock the particles more randomly, while slow and gentle cooling could allow time for more controlled CNC rod assemblies within the emerging gel matrix. By carefully controlling the processing conditions it is visioned that the liquid crystalline capabilities of the CNCs within the MC nanocomposites could be better utilized to create, for example, internally hierarchical, moldcast nanocomposites.
Like the mechanical response, birefringence in MC-CNC gels also responds to changes in temperature: Above the gel transition temperature, the birefringence reversibly increases ( Figure 5). While the magnitude is relatively modest compared to G', the alterations, nonetheless, suggest tunable packing and structural order. [31] It has been hypothesized that upon heating, the fibrils formed by MC mutually co-align and pack themselves with the CNCs which leads to the observed reversible increase of birefringence. [31] MC fibrils may also increase the local confinement of the CNCs and reduce the freedom of movement. Upon cooling, the MC chains relax back towards random coil formation, which loosens CNC packing and allows more mobility, resulting in dimming the birefringence and loss of structure. [31] In summary, the baseline mechanical and optical properties of the MC-based nanocomposite hydrogels are readily tunable. Importantly, the effect of the concentration of MC and the composition of reinforcing nanoparticles, especially the cotton-based CNCs, have been described here in more detail. [31,48] In general, high-aspect ratio particles enable good mechanical reinforcement and birefringent structures at surprisingly low concentration. Interaction between the gel matrix and the nanoparticles is deemed important in determining the structural strength, packing, and birefringent properties, which is highlighted especially at the elevated temperature, where the thermosensitivity of MC chains facilitates both mechanical and optical response enhancement. Furthermore, the characteristics of the added nanocomponents contribute to the gel clarity, color, and potential additional functionalities and thus, also broaden the application potential. For instance, CNC-derived birefringent patterns have been suggested as safe and forgery-proof security markers. [105]

MC-CNC composite fibers
Methylcellulose solutions and MC-based nanocomposite gels at sufficient compositions can be extruded into fibers via wet-spinning, a process that has previously been applied to CNF hydrogels. [49,106] The solution is pumped through a thin capillary into a coagulation bath, where the extruded fiber solidifies ( Figure 6). [106] The process is performed at ambient temperature without the need for chemical crosslinkers. Typically, ethanol that is mildly polar and water-miscible has been used as the coagulation solvent. Finally, the fibers can be lifted from the coagulation bath, and after drying solid macroscopic fibers are obtained. [36,49,106] Like in hydrogels, the addition of dopants or reinforcing agents can be used to tune the processing and fiber properties. In general, highaspect ratio CNCs improve the wet-strength, rigidity, and facilitate handling of the nascent fibers, while, for instance, tiny gold nanoclusters have been observed to soften the spinning dope and increase fiber softness similar to hydrogels. [36,49]  When spinning pure MC, high molecular weight and sufficient concentration facilitate polymer chain entanglement and fiber strength and, thus, determine the optimal spinning conditions. Instead, low molecular weight and low concentrations of MCs fail to produce fibers and freely diffuse into the coagulation bath.
[49] Naturally, also the operating limits of the extrusion setup need to be taken into consideration. The polymer alignment and fiber mechanical properties can be controlled by adjusting the extrusion capillary diameter, capillary length, and flow speed. [106] In general, high flow speed and shear stress tend to improve fiber stiffness and internal alignment when spinning cellulosic fibers. However, the ductility and ultimate strain often become sacrificed in the process. [106][107][108] As a broadly applicable post-modification, stretching may be used to further enhance fiber alignment and mechanical strength. [107] The addition of CNCs generally reinforces the solid composite fiber structure similar to hydrogels. However, optimal properties have been typically achieved at lower-end CNC loadings. For example, 1% or less CNC loadings in the MC-based hydrogel spinning dopes, where the CNCs provide entanglement points and structural rigidity. At the same time allow the MC chains to flow and reorganize under stress agreeing with the typical CNC-reinforced polymer nanocomposite behavior.[49, 109,110] Examples of the undesired effects of high CNC dopant concentration and their aggregation, and consequential macroscopic structural changes have been reported, among others, in MC-CNC, alginate-CNC, and in poly(vinyl alcohol) (PVA)-CNC fibers. [36,[110][111][112] Typically, after exceeding the optimal CNC concentration, the particles tend to aggregate and significantly stiffen the fiber. This results in overall sub-optimal mechanical performance, including high brittleness, and inhomogeneous structure. At such conditions rearrangement of the CNCs towards cholesteric or twisting assemblies -that are characteristic for pure CNCs -may emerge. Therefore, microphase separation of liquid crystalline regions or larger scale structural twisting and buckling of the fiber structure are observed ( Figure  6). [36,[110][111][112] On the other hand, if homogeneous CNC distribution is successfully maintained, the structural reinforcement effect analogous to the ordered -sheet units within the natural silk fibers may be achieved. [113] Specifically, pure MC, and MC-CNC fibers with low CNC loading coagulated in high concentration ethanol show good transparency and relatively smooth morphology (Figure 6).
[49] However, due to gravitational pull and competing inward-and outwardbound solvent diffusion processes during the coagulation phase, fibers with fully spherical cross-sections are challenging to obtain ( Figure 6). [114] Nonetheless, aligned homogeneous arrays of rod-like building nanoscale blocks that conform to the fiber shape can be observed within fractured fiber cross-sections ( Figure 6). [36,49] The so far reported MCbased fibers also have shown high ductility up to 30-50% depending on the composition, but relatively low Young's modulus (< 8 GPa) and ultimate strength (< 200 MPa) ( Figure  7). [36,49] Compared to the literature, the magnitude of ductility of the MC-based fibers has rarely been achieved with other fully cellulose-derived fibers. However, significantly higher moduli and maximum stress have been reported, especially in highly aligned CNFbased fibers. [49,115] The strength and stiffness could likely be further enhanced to an extent by optimizing the spinning conditions. However, so far, the research has mainly focused on exploring the effects and characteristic features of different MC-CNC fiber compositions and concentrations rather than the optimization of technical processing parameters. [36,49] Nonetheless, the optimal composition of the MC-CNC fibers for mechanical performance -that is, minimizing the CNC content -also produces the highest transparency and relatively smooth and homogeneous morphology. [36] These properties are beneficial for optical fiber capabilities and potential applications that are discussed in the next section.

Figure 7.
Mechanical properties of the MC-based fibers. a) Stress-strain curves of pure MC fibers wet-spun from 2%, 3%, and 4% MC hydrogels (MW 88000). Increased gel total solid content appears to stiffen the resulting fibers. With low solid content, exceptional ductility above 50% is achieved. b) Stress-strain curves of MC-CNC fibers with different total solid contents but with identical MC-to-CNC ratios (4:1). There the difference in total solid content of composite hydrogel is reflected mostly in the fiber diameter, whereas significant alterations in stiffness or ductility are not observed. Adapted with permission from [36]. Copyright 2021 John Wiley and Sons.
Since the MC-based fibers are prepared through simple coagulation treatment without covalent chemical cross-linkers, they also show interesting degradability in wet conditions that is tunable by the surrounding temperature due to the thermosensitivity of the underlying MC matrix. [36] The MC matrix allows the rapid intake of water and swelling of the fiber, which is potentially beneficial for sensing applications, but it also results in full fiber degradation in approximately 4 hours if immersed in pure water at room temperature. [36] This could be considered as a drawback considering, for example, biological applications involving tissues and cell culture conditions with high humidity. However, due to the LCST behavior of MC, MC fibers have been shown to stabilize already at a biologically relevant temperature of 37 °C and survive for extended periods of time (at least 6 hours without loss of structure) in extreme humidity in a water bath. Therefore, the fiber stability and the lifespan of the MC-based fibers are feasibly extended by relatively moderate heating. [36]

MC-based biopolymeric optical fibers
Optical fibers are an essential part of modern data and communications networks, and they hold several benefits over traditional copper wire-based electronics. Optical fibers, in general, have substantial information capacity, lower signal loss and more affordable material costs, generate less heat, and are lightweight, insensitive to electromagnetic disturbances and energy spikes. [116] The current commercial optical fibers are typically made from silica glass or plastics, such as poly(methyl methacrylate) (PMMA). [116] The glass optical fibers (GOFs) are optimal for long-range communication networks having very low signal attenuation of approximately 0.2 dB/km. [116] That is, the signal can be transmitted across tens of kilometers without irreparable signal degradation or the need for amplification. Plastic optical fibers (POFs), on the other hand typically express approximately an order of magnitude higher attenuation coefficients and thicker multimode profile, which makes them more suitable for more local networks and applications. [116] However, both GOFs and POFs possess certain limitations or sub-optimal features for specific occasions that could be potentially overcome or reduced by utilizing cellulosic, or more generally, biopolymeric raw materials. [117] For instance, while GOFs are superior in long-range communication, they are stiff, potentially brittle, nondegradable, may be challenging to modify or dope, and require extreme temperatures to produce and frequent special handling skills in order to preserve the performance across connections and necessary junctions. POFs partially fix some of the problems as, for example, they can be feasibly produced via extrusion, are thicker, softer, lightweight, modifiable and cost-effective. [116] However, also the standard POFs cannot avoid issues in degradability and sustainability, even though they tend to react to the environment and may become more easily damaged by temperature and humidity than the GOFs. [116] Thus, preparing optical fibers from biopolymeric materials, particularly from cellulose, with high environmental sensitivity, ease of modifiability, and degradability is envisioned to complement and support the traditional POF technologies in cases where the current properties are not optimal. [36,118] That is, the main intention of biopolymeric and cellulose-derived optical fibers is not to directly compete with the GOFs and POFs, but to complement them where appropriate properties and functionalities are needed. One significant reason for this is that the current biopolymeric optical fibers are effectively usable only on distances on a sub-meter scale, which, however, is useful, for instance, biological or biomedical devices and sensors. [119][120][121][122][123] Nonetheless, cellulose, in general, has the potential to allow novel, feasibly modifiable, and multifunctional optical fibers for shortrange applications and devices from renewable and nearly endless raw materials. The preparation processes of cellulosic fibers are benign and mild compared, for example, to the traditional GOFs whose preparation requires temperatures up to 2000 °C. [124] So far, a limited number of attempts to prepare fully cellulose-derived optical fibers -including the MC-based optical fibers -have been reported. [36,118,125] Nonetheless, the potential of such fibers beyond simple light transmission has been already envisioned since they have demonstrated prospects for, e.g., drug delivery and sensory applications, controlled degradation, and feasibly modifiable functional fiber optic nanocomposites. [36,118,125] On the other hand, a range of cellulosic components have been rather frequently coupled with traditional optical fibers and into photonic devices to utilize, for instance, their sensitivity to humidity or the structural colors and chirality of liquid crystalline CNCs which broadens the application potential and cooperation with the traditional light transmission methods. [13,[126][127][128] Overall, work still remains, especially in finding and optimizing the best cellulose derivatives for optical fiber preparation for different purposes and standardizing such fibers' quality and processing conditions before any larger scale production becomes topical. Up to date, most of the research focusing on cellulose fiber spinning, in general, has strongly focused on the optimization of the mechanical properties. [129] While it is a valuable approach in itself and potentially largely translatable also into the manufacturing of the cellulose-derived optical fibers, it is unfortunately still rather limited and neglects aspects, such as transparency, that are essential for optical fibers and in need of more active research.
At the moment, biopolymeric optical fibers -either solid or in gel form -have been reported from, among others, silk, agarose, and alginate. [119][120][121][122][123][130][131][132] In addition, even collection of a bacterial organism have been successfully utilized to modulate light propagation analogous to fiber optics even though in significantly shorter length scales. [133,134] Nonetheless, such bacterial experiments may turn out critical in providing inspiration and insights into how to more efficiently control and tune light propagation inside biopolymeric materials and composites. The state-of-the-art biopolymeric optical fibers can reach signal attenuation in the range of 0.2 -1.0 dB/cm, which also holds to the reported cellulose-derived optical fibers. [119] Out of the explored biomaterials, spider silk is logically of high interest because of its inherently excellent mechanical properties and high transparency. [123,135,136] However, the availability of spider silk may turn out as the limiting factor when considering large-scale production, even though synthetic biology potentially holds the keys to solve the issue. [137] In comparison, wood-derived optical fibers from a near endless renewable raw material, and with long research history aiming towards good mechanical performance, appear as a tempting alternative. Moreover, cellulose-based hydrogels are widely established and generally non-toxic, and thus, could also allow cellulose-derived hydrogel fibers. [138] Hydrogel fibers are interesting especially for tissue and biomedical applications where mechanical tunability -to match the properties of living tissue -, sufficient porosity, cell encapsulation potential, diffusion of nutrients, and biocompatibility are essential. [120,139] Pure methylcellulose-derived optical fibers, specifically, have reached the attenuation coefficient () of 1.47 dB/cm at best measured with cutback methods, which is on par, for example, with various reported silk-based optical fibers (Figure 8). [36,119,122,123,132] In addition, it is similar to the other two up-to-date reported cellulose-derived optical fibers consisting of cellulose acetate coated regenerated cellulose, and hydroxypropyl-filled cellulose butyrate tubes whose attenuations remain in the range of ~1.0 -5.9 dB/cm. [118,125] Furthermore, it is good to notice that the reported MC-based fibers did not have any cladding, and in the future, by adding a suitable coating to the structure the scattering can be potentially lowered further. [36] As a drawback, though, the antisolventdependent coagulation and processing of the MC-fibers may not be optimal for upscaling due to its extended coagulation period compared to, for example, the highly efficient fiber spinning by ionic interactions. [36,49] In the future, combinations and use of different cellulose derivatives have the potential to tune both the mechanical and optical capabilities of the fibers, since various cellulose derivatives allow a multitude of potential combinations and fabrication routes for optical fibers.
Of the reported MC optical fibers in general, the ones with the thinnest cross-sections have also showed the lowest signal attenuation. [36] Since the fiber thickness is partially affected by the initial hydrogel spinning dope concentration, its tunability is closely related to the mechanical properties: low concentrations typically allow softer, more ductile, and thinner fibers with lower attenuation. The higher concentrations produce thicker and stiffer fibers with relatively higher signal loss. [36,49] This is logical considering the optical fibers in general, where thicker fibers allow transportation of a higher number of simultaneous propagation modes, resulting in overall increased signal dispersion. [116] The mechanical strength can also be increased via the addition of, for instance, CNCs. However, nanoadditives often tend to contribute to higher signal loss due to increased light absorption and scattering because of, for instance, their highly absorbing chemical groups, particle sizes matching to visible light wavelength, and increased amorphouscrystalline boundary regions within the material. [36,103] Thus, the feasibility of adding nanodopants to the MC fibers to achieve complementary functions often needs to be balanced against the loss of transparency. Figure 8. MC-based multifunctional optical fibers. a) The attenuation of MC-based fibers has been studied with cutback method, where the transmitted light intensity has been measured, the fiber shortened, and the measurement cycle repeated for several rounds. The results reveal signal attenuation per length unit. b) Photograph of an attenuation measurement system with an MC fiber coupled to light source. c) Representative attenuation measurement data (black circles) and mathematical fitting to the data (blue line). d) MC fiber doped with gold nanoclusters (AuNC) is photoluminescent and shines red light upon UV irradiation. e) AuNC-modified MC fiber as Hg 2+ sensor. Under UV light, the absorption of Hg 2+ ions (10 mM solution) into the MC-AuNC fiber quenches the luminescence providing a visual cue of the presence of Hg2+. The luminescence of a dry MC-AuNC fiber and a similar control fiber immersed in pure water remains unaltered. Adapted with permission from [36]. Copyright 2021 John Wiley and Sons.
The effective operating range due to the relatively high signal attenuation of > ~1.5 dB/cm of the MC-based optical fibers is limited to centimeter scale even without any modifications and, thus, is only usable for short-distance applications and sensors. [36] Therefore, as it is acknowledged from the start that the aim of the MC-based optical fibers is not to pursue superior long-distance performance and usurp the GOFs, the additional loss caused by the dopants may often be acceptable and not compromise the envisioned applications. Moreover, the dopants may allow multifunctional optical fibers, for example, by coupling the material with sensory or luminescent properties, or higher tunable mechanical strength as has been demonstrated via the addition of gold nanoclusters (AuNCs) (Figure 8d-e). [36,[140][141][142] While such properties and analogous dopants can be typically added to GOFs mainly via their modified cladding layers, in MC fibers, and other biopolymeric fibers, it is possible to make the actual fiber core active and responsive to its environment while still allowing further finetuning of the properties via suitable coatings. [36,118,120,126,130] The combination of functional cores and claddings appears as a fruitful ground for further research in the future. For instance, double-core cellulose-derived optical fibers from cellulose butyrate and hydroxypropyl cellulose have already been envisioned as a candidate, for example, for drug delivery and sensing purposes in addition to the typical signal transmission. [125]

Conclusions
Methylcellulose is one of the extensively utilized cellulose derivatives. Despite its interesting phase changing behavior and thermoreversibility several properties have been identified only recently. More importantly, the structural transition and self-assembly that drives the inverse thermoresponsive behavior was only understood in recent findings using advanced cryo-TEM imaging analysis. The compatibility of methylcellulose with structurally and functionally diverse nanoparticles allow access to various composite systems. As discussed in this review, the addition of one-dimensional cellulose nanocrystals reinforces the gels and solid composites. The shear thinning properties of the gels offer a facile and environmentally benign route for wet spinning without the need for chemical cross-linking. Rather surprisingly, the MC and MC-CNC composite fibers display high ductility and strength that is in par with other biopolymeric fibers prepared using highly sophisticated methods and technologies. More importantly, the high optical transparency of MC-based fibers offers a new opportunity to explore their application in waveguiding. The MC fibers have shown to act as short-distance optical fibers with a low attenuation coefficient encouraging more active research towards cellulosic optical fibers. Furthermore, several nanoadditives can be incorporated in the MC matrix to enhance their optical properties and functionalities, allowing, for example, luminescent fibers or sensing of toxic metal ions. [36] Author