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Converting a Dilute Slurry of Hollow Tube-like Papermaking Fibers into Phase-reversible, Self-healable, and Stretchable Hydrogels

  • Commercially, assembly-directed packing of hollow tube-like papermaking fibers with diameters of roughly 10-50 μm into sustainable micro-fibrous bio-assemblies (i.e., cellulosic paper) starts with a dilute fiber slurry. In this process, a huge amount of water is required to disperse and transport fibers, which also facilitates colloidal interactions and formation of inter-fiber bonds. To form bio-assemblies in their dry states, unit operations associated with dewatering and drying are routine practices and treatment of the generated wastewater is a necessity. We herein present a facile, scalable concept of converting fiber slurry into dynamic hydrogels by using chemical additives (similar to papermaking wet-end additives), but without water removal. We used a typical group of additives as an example in an attempt to demonstrate the applicability of the concept. With boron-based dynamic chemistry as a key theoretical foundation, the combination of crosslinking and hydrogen bonding led to the formation of phase-reversible, self-healable, and stretchable hydrogels. Essentially, the characteristics of hydrogels can be facilely tunable, and process parameters such as polymer dosage are rather critical. It is worth noting that fibers can act as a structural skeleton or mechanical support for tailorable design of hydrogels. The concept demonstrated in this study would provide useful insights into value-added utilization of mass-producible biopolymeric fibers in accordance with existing mill facilities. Fiber-based hydrogels would find use in diversified applications:toys, 3D/4D printing materials, soft robots, drug delivery systems, among others.
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    Han J Q, Lei T Z, Wu Q L, 2014. High-water-content mouldable polyvinyl alcohol-borax hydrogels reinforced by well-dispersed cellulose nanoparticles:dynamic rheological properties and hydrogel formation mechanism. Carbohydrate Polymers, 102:306-316. DOI:10.1016/j.carbpol.2013.11.045.
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    Ke H, Yang L P, Xie M, et al., 2019. Shear-induced assembly of a transient yet highly stretchable hydrogel based on pseudopolyrotaxanes. Nature Chemistry, 11(5):470-477.DOI:10.1038/s41557-019-0235-8.
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    沈阳化工大学材料科学与工程学院 沈阳 110142

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Converting a Dilute Slurry of Hollow Tube-like Papermaking Fibers into Phase-reversible, Self-healable, and Stretchable Hydrogels

Abstract: Commercially, assembly-directed packing of hollow tube-like papermaking fibers with diameters of roughly 10-50 μm into sustainable micro-fibrous bio-assemblies (i.e., cellulosic paper) starts with a dilute fiber slurry. In this process, a huge amount of water is required to disperse and transport fibers, which also facilitates colloidal interactions and formation of inter-fiber bonds. To form bio-assemblies in their dry states, unit operations associated with dewatering and drying are routine practices and treatment of the generated wastewater is a necessity. We herein present a facile, scalable concept of converting fiber slurry into dynamic hydrogels by using chemical additives (similar to papermaking wet-end additives), but without water removal. We used a typical group of additives as an example in an attempt to demonstrate the applicability of the concept. With boron-based dynamic chemistry as a key theoretical foundation, the combination of crosslinking and hydrogen bonding led to the formation of phase-reversible, self-healable, and stretchable hydrogels. Essentially, the characteristics of hydrogels can be facilely tunable, and process parameters such as polymer dosage are rather critical. It is worth noting that fibers can act as a structural skeleton or mechanical support for tailorable design of hydrogels. The concept demonstrated in this study would provide useful insights into value-added utilization of mass-producible biopolymeric fibers in accordance with existing mill facilities. Fiber-based hydrogels would find use in diversified applications:toys, 3D/4D printing materials, soft robots, drug delivery systems, among others.

1.   Introduction
  • Sustainable conversion of renewable biopolymeric feedstocks into environmentally benign products for diversified, tailorable applications fits well into a "greener" economy. Chemical or mechanical liberation of lignocellulosic fibers naturally confined in lignocellulosic matrices while largely maintaining their plant cell structure, which is now a widely commercialized process known as pulping, significantly unlocks the potential of green renewables. Typically, these hollow fibers have a length of 1–4 mm, a width of 10–50 µm, and a cell wall thickness of 1–10 µm. With colloidal chemistry and supramolecular assembly as key process basics, assembly-guided packing of fibers to form network- structured commodities is an industrial paradigm of using fibers for mass production of biobased materials (Wu et al., 2018).

    Forming a dilute, well-dispersed biopolymeric fiber slurry is a prerequisite for commercial production of microfibrous bioassemblies (paper-based products). The low solids concentration of the slurry contributes significantly to sufficient mixing and tunable interactions. Various chemical additives, such as hydrosoluble polymers, nanosized mineral particles, dyes, and sizing emulsions, are widely used in combination with fibers to endow bioassemblies with unique functionalities, or to improve dewatering and machine runnability. In most cases, these additives interact with fibers in an aqueous medium via intermolecular forces, facilitating tailorable conversion of the dilute fiber slurry into diversified products.

    Evidently, for the generation of microfibrous bioassemblies, the removal of a huge amount of water is an energy-consuming process, and the effluent needs to be sufficiently post-treated or reused. Inspired by well- established commercial practices for the production of microfibrous bioassemblies (cellulosic paper), we herein present a concept of converting a dilute slurry of hollow, microsized biopolymeric fibers into biobased hydrogels with dynamic characteristics. Once dynamic bonding interactions (involving the formation of reversible bonds) occur in a colloidal medium containing biopolymeric fibers, some key basics of state-of-the-art commercial fiber assembling processes (involving the use of chemical additives) can be employed to facilely generate hydrogel- based products without the need for water removal. With industrial colloidal (wet-end) chemistry and dynamic chemistry as key theoretical foundations, this concept would facilitate direct mass production of valuable, sustainable hydrogels in the conventional paper industry.

2.   Materials and Methods
  • Softwood-derived paper-grade bleached chemical pulp (hollow tube-like fibers) produced in Russia was purchased from Liaoning Jiali Trade Co. Ltd., China. This pulp was mechanically refined and disintegrated prior to use. Polyvinyl alcohol with a weight-average molecular weight of 75 000–85 000 g/mol was purchased from Tianjin Basifu Chemicals Co. Ltd., China. Sodium tetraborate decahydrate (Na2B4O7·10H2O), commonly known as borax, was supplied by Tianjin Tianda Chemical Reagents Factory, China. Both polyvinyl alcohol and sodium tetraborate decahydrate were used as received.

  • A facile strategy was used to convert a dilute slurry of biopolymeric fibers into dynamic hydrogels. Initially, a 3% clear aqueous solution of polyvinyl alcohol was prepared by heating at 90 ℃ for 1 h. Biopolymeric fibers were then added to polyvinyl alcohol solution, followed by mixing for 1 h (90 ℃), allowing for sufficient fiber-polymer interaction. This interaction can induce the adsorption of some polymer chains to fibers. Afterwards, sodium tetraborate decahydrate was added, and the resulting slurry was mixed for 20 min to induce crosslinking and hydrogen bonding interactions. Cooling down to room temperature finally resulted in the formation of fiber-based hydrogels. It is noteworthy that, the weight ratio of sodium tetraborate decahydrate to polyvinyl alcohol during hydrogel formation was 1꞉2. Varying amounts of fibers were used in combination with these two additives. For comparison purposes, the annotation of a given hydrogel sample is based on the polymer dosage (i.e., the amount of polyvinyl alcohol relative to the dry mass of fibers, g/g).

  • Freeze-dried samples were fractured in liquid nitrogen and sputter-coated with gold (conducting metal coating). Scanning electron microscopic (SEM) imaging was then performed with a JSM-7500F scanning electron microscope (Japan) to identify morphological features. After powdering of freeze-dried samples, Fourier transform infrared (FT-IR) spectra were recorded on a Thermo Scientific Nicolet iS10 spectrophotometer (USA) to identify chemical features. Freeze-dried samples were also characterized by thermogravimetry (TG) and derivative thermogravimetry (DTG), with a 6000-SQ8 simultaneous thermal analyzer (STA) (USA). For this analysis, samples were heated at 10 ℃/min from 30 ℃ to 700 ℃ under nitrogen atmosphere (purged at 20 mL/min).

  • At room temperature, freeze-dried hydrogels were soaked in phosphate-buffered saline (pH 7.4). The resulting swollen samples were taken out of the solution at different time intervals, which were then wiped with paper tissue to remove excess surface water. Samples before and after swelling were weighed, and degree of swelling (DS) (Mansur et al., 2008) was subsequently calculated, according to the following equation:

    where Ws is the weight of a swollen sample, and Wd is the dry weight of a hydrogel sample.

    To evaluate acid-sensitive phase reversibility, a selected hydrogel (1 g) was transferred to a bottle, and 0.5 mL of hydrogen chloride solution (1 mol/L) was incorporated and mixed to test the possibility of gel-sol transformation. Post-addition of 0.5 mL of sodium hydroxide solution (1 mol/L) followed by mixing was conducted to identify its role in gelling of the sol. Photographs were taken to assess these pH-tunable phase changes.

    To roughly assess the stretchability of a typical hydrogel, it was manually stretched, and a ruler was used to show response to tensile forces. Photographs were taken to visualize the change of the hydrogel. Prior to a typical self-healing analysis, samples were shaped into rectangular cuboids. Each cuboid was roughly cut in half, although two halves were not-so-regularly-shaped due to high stretchability. One half of the cuboid was colored by immersing in methylene blue solution (0.01%) for 5 min. These two halves with different colors were brought into close contact with each other, allowing for recombination due to self-healing. In addition, photographs were taken to assess self-healing characteristics.

    An AR2000ex rotational rheometer (TA Instruments, USA) equipped with a parallel plate (25 mm in diameter) was used to assess viscoelastic and self-healing properties of hydrogels. Samples were shaped into disks of about 20 mm diameter and 2 mm thickness. To ensure all measurements were performed in linear viscoelastic region, oscillatory strain sweeps were carried out at 25 ℃ on a typical sample, with a strain of 0.01%–100% and an angular frequency of 1 Hz. Frequency sweeps (0.1– 100 rad/s) were then conducted at 25 ℃, with a strain of 1%. Time sweeps were implemented in 20 min, while strain and angular frequency were held constant at 1% and 1 Hz, respectively. Storage modulus and loss modulus as a function of time, with an oscillatory strain of 1% or 20%, was tested at 1 Hz frequency. Self-healing properties of hydrogels in response to shear forces were also examined with the dynamic rheometer (Ding et al., 2018), and the strain was alternatively varied: 1% (180 s)—20% (120 s)—1% (180 s)—20% (120 s) —1% (180 s).

3.   Results and Discussion
  • Flow-related packing of biopolymeric fibers to generate microfibrous bioassemblies (paper-based products) is a sustainable commercial process (Tayeb et al., 2017), where intermolecular forces play a central role in determining process efficiency and product functionalities. It would be interesting to identify new possibilities of utilizing existing facilities and/or process basics of the well-established green paper industry to produce value-added products. Essentially, the facile strategy of assembling biopolymeric fibers, i.e., papermaking process, would be extended to any liquid systems containing liquid-dispersible particles (e.g., graphene oxide) (Fukahori et al., 2003; Dikin et al., 2007). The use of mass-producible paper-based bioassemblies as substrates for diversified functional materials would also be very promising (Sadri et al., 2018). Indeed, there is an ongoing need to identify new strategies of product design in light of inspirations from the paper industry. In this context, we herein propose a process concept of converting a dilute papermaking fiber slurry into dynamic hydrogels (Fig. 1).

    Figure 1.  Concept of using crosslinking additives to convert a dilute slurry of bio-derived, micro-sized fibers into dynamic hydrogels for diversified, value-added applications

    Hypothesized features of the process concept are: (1) production of valuable dynamic hydrogels is readily implementable by utilizing existing facilities of commercial paper mills; (2) production of hydrogels instead of paper- based commodities leads to the unnecessity of dewatering and whitewater treatment; (3) commercial practices of wet-end chemistry, such as those related to the use of chemical additives, provide insights into hydrogel formation; (4) paper-grade pulp fibers function as skeletons for hydrogel formation, while still contributing to intermolecular bonding due to accessible functional groups (hydroxyls in particular); and (5) manipulation of dynamic interactions in fiber slurry by using fiber-compatible additives can lead to tailorable formation of dynamic hydrogels with diversified functionalities.

    In an attempt to demonstrate the proposed concept shown in Fig. 1, we used polyvinyl alcohol and sodium tetraborate decahydrate as chemical additives to construct dynamic bonding/networking interactions in a dilute slurry of paper-grade pulp fibers. In the paper industry, polyvinyl alcohol has found widespread commercial use as a chemical additive for paper surface treatment. It is also used in the formulation of pigmented coating colors for paper. Cationic modification of this hydrosoluble synthetic polymer can facilitate its efficient use in papermaking wet-end applications (Fatehi et al., 2011). According to recently published works (Spoljaric et al., 2014; Lu et al., 2017), a combined use of nanoscale cellulosic particles, polyvinyl alcohol, and borax can generate dynamic hydrogels, and these components are likely to interact synergistically with each other. However, there remains a need to assess the possibility of using regular paper-grade pulp fibers for hydrogel formation.

    The fibers used in the current study exhibited typical microsized characteristics (Fig. 2A). They are also widely known as hollow tube-like structures (Tejado et al., 2010). The combination of these fibers with polyvinyl alcohol resulted in the formation of bridged structures (Fig. 2B–C). Noticeably, the addition of sodium tetraborate decahydrate generated interlocked composite networks with significantly increased bonding sites (Fig. 2D–E). It is noted that, sodium tetraborate decahydrate can form dynamic crosslinks with both cellulose and polyvinyl alcohol, and hydrogen bonding also contributes to hydrogel formation (Han et al., 2014). The networks of the hydrogel containing no fibers, as shown in Fig. 2E–F, were somehow "finer" in comparison to fiber-based hydrogels. Nevertheless, fibers were largely embedded in matrices of hydrogels, and polyvinyl alcohol networks were efficiently anchored onto fibers.

    Figure 2.  Morphologies, FTIR spectra, TG-DTG profiles, and a colored hydrogel*s photograph. (A) Freeze-dried fiber slurry containing no additives. (B) Freeze-dried fiber slurry containing polyvinyl alcohol (polymer dosage: 1 g/g). (C) Freeze-dried fiber slurry containing polyvinyl alcohol (polymer dosage: 0.67 g/g). (D) Freeze-dried, fiber-based hydrogel (polymer dosage: 1 g/g). (E) Freeze-dried, fiber-based hydrogel (polymer dosage: 0.67 g/g). (F) Freeze-dried polyvinyl alcohol hydrogel containing no fibers. (G-H) TG-DTG diagrams of freeze-dried fiber slurry containing no additives (F), freeze-dried fiber slurry containing polyvinyl alcohol (polymer dosage: 0.67 g/g) (F-H), and freeze-dried polyvinyl alcohol hydrogel containing no fibers (polymer content: 5%) (P-H). (I) FT-IR spectra of freeze-dried fiber slurry containing no additives (F), freeze-dried fiber slurry containing polyvinyl alcohol (polymer dosage: 3.33 g/g) (F-H), and freeze-dried polyvinyl alcohol hydrogel containing no fibers (polymer content: 3.33%) (P-H). (J) Photograph of a fiber-based hydrogel (polymer dosage: 0.67 g/g)

    Structural bonding plays a governing role in thermal decomposition. TG-DTG profiles (Fig. 2G-H) show notable differences among freeze-dried samples (biopolymeric fibers and hydrogels) as regards pyrolysis characteristics. Evaporation of water molecules is responsible for initial weight losses (e.g., at a temperature of less than 100 ℃). For the tested samples, fast removal of water molecules was achieved at around 70 ℃. Clearly, as indicated from TG-DTG profiles, the water content of freeze-dried polyvinyl alcohol hydrogel was much higher than that of freeze-dried, fiber-based hydrogel or freeze-dried fibers, largely due to high hydrophilicity of polyvinyl alcohol. The use of fibers in hydrogel formation shifted the peak of weight loss rate to a higher temperature, indicating their role as a skeleton in hurdling thermal decomposition. On the other hand, fiber-induced increase of the yield of final residue (after the end of heating) can be linkable to the networking and structural bonding role of fibers in hydrogel formation.

    FTIR spectra of selected samples are shown in Fig. 2I. Freeze-dried fiber slurry without any additive shows characteristic peaks for cellulose, particularly 1030 cm–1 and 1053 cm–1 associated with C—OH bonds (primary and secondary alcohols) (Bian et al., 2018). For this sample, O—H stretching vibrational bonds at 3332 cm–1 can be attributed to hydrogen bonding in cellulose I (Liu et al., 2011). The peak at 2895 cm–1 pertains to C—H stretching vibration (Yang et al., 2007). Conversion of the fiber slurry into a hydrogel (polymer dosage, 3.33 g/g) resulted in noticeable change of FT-IR spectrum, and the characteristic peaks at 1417 cm–1 and 1338 cm–1 are indicative of the formation of boron-based dynamic crosslinks (Spoljaric et al., 2014). These crosslinks can bind polyvinyl alcohol to fibers, forming 3D networks. In such a reaction medium, hydrogen bonds can also be generated, contributing to complex network formation. As seen in Fig. 2I, polyvinyl alcohol hydrogel and fiber-based hydrogel had similar FT-IR spectra, presumably due to the efficient embedment of fibers in complex networks. Thus, these FT-IR results provide useful evidence for the role of fibers as structural skeleton in hydrogel formation. The photograph shown in Fig. 2J further confirms that a dilute slurry of hollow, micro-sized fibers can be facilely converted into hydrogel-based networks with the aid of chemical additives.

    Degree of swelling is a critical parameter for the design of hydrogels with tailorable functionalities (Rauner et al., 2017; Nojoomi et al., 2018). For hydrogels generated from a dilute slurry of hollow tube-like, micro-sized fibers, degree of swelling was governed by polymer dosage (Fig. 3A–C). Very interestingly, after about 3 d, a degree of swelling of as high as about 800 was achieved at a polymer dosage of 3.33%. In other words, 1 g of freeze-dried sample can absorb about 800 g of water. Reducing polymer dosage led to lowered degree of swelling. It is noteworthy that swelling behavior has a strong correlation with the nature of crosslinkers. A high degree of swelling translates to the possibility of using only a small amount of starting materials to produce hydrogels. These products are usable as a replacement for commercially available petroleum-based plastics, and in such circumstances huge environmental benefits can be expected (Wang et al., 2010).

    Figure 3.  Degree of swelling (DS), pH-directed phase-reversibility, and stretchability. (A–C) Swelling of fiber-based hydrogels with polymer dosages of 3.33, 1.00, and 0.67 g/g, respectively. (D) pH-responsiveness of a representative fiber-based hydrogel with polymer dosage of 3.33 g/g. Addition of hydrochloric acid solution (1 mol/L) is responsible for gel-sol transformation, and post-addition of sodium hydroxide solution (1 mol/L) can lead to sol-gel transformation. (E) Stretchability of a fiber-based hydrogel (polymer dosage, 3.33 g/g)

    Hydrogels that can easily undergo phase transition when subjected to stimuli would have interesting smart applications (Piest et al., 2011; Li et al., 2018). As shown in Fig. 3D, the selected fiber-based hydrogel was pH-sensitive, and the addition of hydrogen chloride solution (1 mol/L) resulted in gel-sol transition. Upon further addition of sodium hydroxide solution (1 mol/L), the sol can be converted into a gel. This pH-induced phase-reversibility agrees well with previous studies involving the use of boron-based crosslinking additives for hydrogel formation (Piest et al., 2011; Lu et al., 2017; Hong et al., 2018). Such a phase-reversible nature is due to the reversible crosslinking interactions.

    Despite the fact that hydrogel-related research is booming, most hydrogels are brittle (i.e., not easily stretchable), which limits their scope of applications (Sun et al., 2012). Delivering stretchability to hydrogels involves the design of functional structures such as those related to capability of "frictional" sliding of molecular tubes (Ke et al., 2019). As shown in Fig. 3E, a dilute slurry of papermaking fibers can be converted into stretchable structures (note that the hydrogels can be easily stretched to two folds of original length), favorable for the design of functional products like stretchable electronics and devices.

    Endowing hydrogels or other categories of materials with self-healing characteristics has been an active research area (Taylor et al., 2016; Azevedo et al., 2017; Chakraborty et al., 2019). As shown in Fig. 4A, fiber-based hydrogels with polymer dosages of 3.33, 1.00, and 0.67 g/g, respectively, had noticeable self-healing characteristics. Self-healing was easily achievable, and the healed sites were basically intact when hydrogels were turned upside down. Fig. 4B shows that the healed sites were strong enough to support two stainless steel weights (totally 200 g). The change of storage modulus and loss modulus due to alternatively varied strains indicates the self-healability of hydrogels (Fig. 4C). Under mechanical forces with a strain of 1%, storage modulus was higher than loss modulus, indicating solid-like nature of hydrogels. When the stain switched from 1% to 20%, both moduli were lowered, and storage modulus was lower than loss modulus, indicating liquid- like nature of hydrogels. It is interesting to note that both moduli were easily recoverable in combined treatments of alternatively varied strains. Self-healing characteristics combined with phase-reversibility would facilitate the use of fiber-based hydrogels in diversified applications, e.g., toys, soft robots, 3D/4D printing materials, drug delivery systems, and medical devices.

    Figure 4.  Self-healing characteristics. (A) Self-healing characteristics of fiber-based hydrogels with polymer dosages of 3.33, 1.00, and 0.67 g/g (top, middle, and bottom), respectively. For all hydrogels, self-healing was easily achievable after 15 seconds of contact. Self-healing sites remained intact under the influence of hydrogels' gravities. (B) A self-healed hydrogel (polymer dosage, 0.67 g/g) easily supported two stainless steel weights (totally 200 g). (C-D) Storage modulus (G') and loss modulus (G") as a function of alternatively varied strains [1% (180 s)—20%(120 s)—1% (180 s)—20%(120 s)—1% (180 s)]. C and D represent a fiber-based hydrogel (polymer dosage, 0.67 g/g) and a polyvinyl alcohol hydrogel (polymer dosage, 3.33 g/g), respectively

    To further elucidate the dynamically crosslinked structures of hydrogels, viscoelastic characteristics were examined by strain sweeps, frequency sweeps, and time sweeps. The linear viscoelastic region was determined by strain sweeps (0.01%–10%) (Fig. 5A-B). In this region, storage modulus was constantly higher than loss modulus, indicating "solid-like" behaviors of polyvinyl alcohol hydrogels and fiber-based hydrogels. On the other hand, at a high strain (e.g., around 50%), noticeable reduction of storage modulus and loss modulus were identified, which is due to significant disruption of internal bonding interactions; in such circumstances, storage modulus was lower than loss modulus, representing "liquid-like" behaviors. For polyvinyl alcohol hydrogels, an increase of polymer content during hydrogel formation resulted in the increase of both storage modulus and loss modulus (Fig. 5C) due to enhanced crosslinking. As shown in Fig. 5C–E, the "solid-like" nature of all hydrogels was enhanced with the increase of angular frequency. Time sweep profiles (Fig. 5F–5G) show good structural stabilities of tested hydrogels.

    Figure 5.  Viscoelastic characteristics. (A–B) Dynamic strain sweeps of a polyvinyl alcohol hydrogel (polymer content: 5%) (left) and a fiber-based hydrogel (polymer dosage: 0.67 g/g) (right). (C–E) Storage modulus (G'), loss modulus (G"), and loss tangent (tan δ) as a function of angular frequency. (F–G) Dynamic time sweeps (time ramps) of different hydrogels. P-H-a and P-H-b represent polyvinyl alcohol hydrogels with polymer contents of 3.33% and 5%, respectively. F-H-a and F-H-b represent fiber-based hydrogels with polymer dosages of 1 g/g and 0.67 g/g, respectively

4.   Conclusion
  • A facile concept of using chemical additives to produce dynamic hydrogels from a dilute slurry of papermaking fibers was demonstrated. This concept is essentially built upon well-established commercial practices of converting microsized biopolymeric fibers into paper-based microfibrous bioassemblies. Hydrogels with phase- reversibility and self-healing characteristics were fabricated with boron-based dynamic chemistry as a key theoretical foundation. Polymer dosage was identified as a critical factor governing the nature of hydrogels. Our proposed strategy of hydrogel formation would shed light on value-added applications of paper-grade pulp using existing industrial facilities, opening the door to new scopes of applications.

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