Preparation and Performance Study of Bacterial Nanocellulose Yarns

Abstract

This study investigated the preparation of bacterial nanocellulose yarn, a high-strength and high-modulus cellulose-based textile material. Compared with the previously used wet spinning and electrospinning methods, the film-cutting, drawing and twisting treatment method in this paper retains the natural structure of BNC. This can greatly transfer the high performance of BNC nanofibers to BNC yarns, making the mechanical properties of the prepared yarn much higher than those of the BNC yarns prepared by the above two methods. It was produced through a film-cutting and twisting process utilizing bacterial nanocellulose as the primary component. The effects of drafting and twisting on the characteristics and properties of the yarn were systematically examined. Comparative analyses were conducted between the bacterial nanocellulose yarn and conventional cotton yarn of equivalent fineness and twist in terms of appearance, tensile properties, frictional behavior, and bending resistance. Optimal tensile mechanical properties of the bacterial nanocellulose yarn were achieved at 1% elongation and a twist number of 160 r/20 cm, resulting in a breaking strength of 751.56 MPa and an elongation at break of 11.56%, surpassing those of cotton yarn of similar specifications. The spinnability assessment revealed a smooth surface for the bacterial nanocellulose yarn, characterized by low friction coefficient, robust bending resistance with a bending modulus of 718.76 GPa. These findings offer valuable empirical data and theoretical insights to guide the subsequent textile processing and utilization of bacterial nanocellulose yarn.

1. Introduction

With the ongoing depletion of petroleum-based materials, the advancement of sustainable and renewable resources is gaining increasing attention. Cellulose is the most abundant and widely distributed environmentally friendly, renewable biomass resource in nature. Its diverse sources and complete biodegradability make it an ideal candidate for the development of sustainable polymer materials. Additionally, the high density of hydroxyl groups in cellulose imparts excellent moisture absorption and wearing comfort, making it one of the most extensively used fibers in textile applications. Despite their widespread use, the physical and mechanical performance of cellulose fiber yarns prepared via conventional spinning has remained largely stagnant, and such yarns are generally classified as medium- to low-strength. The Chinese national standard, for example, specifies a minimum breaking strength of 19.8 cN/tex (about 158 MPa) for cotton yarn [1], whereas that of linen bundle fibers reaches 45 cN/tex (about 405 MPa) [2]. As a result, traditional cellulose yarns often fall short of the high performance requirements demanded by specialized applications. For instance, the tensile strength of dental floss should not be less than approximately 566 MPa [3], and the tensile strength of medical sutures should not be less than 300 MPa, with a knotting strength of no less than 200 MPa. This intrinsic strength deficit limits the broader utilization of traditional cellulose yarns.

Theoretically, cellulose fibers have the potential to exhibit exceptional physical and mechanical properties. For instance, individual cellulose nanofibers can achieve tensile strengths ranging from 1.6 to 3.0 GPa [4], while cellulose nanocrystals can reach tensile strengths up to 10 GPa and exhibit elastic moduli as high as 150 GPa [5]. However, cellulose exists in various forms due to differences in its structural organization. Currently, commonly used cellulose fibers are classified into plant-based cellulose fibers (e.g., cotton and linen) and regenerated cellulose fibers (e.g., viscose and Lyocell) [6]. These fibers are linear in morphology, typically featuring micrometer-scale diameters and millimeter-scale lengths. Their physical and mechanical properties are comparatively limited due to their relatively loose structure and the presence of amorphous regions. For example, cotton fibers exhibit tensile strengths ranging from 287 to 597 MPa, while flax fibers range from 343 to 1035 MPa, and hemp fibers from 310 to 900 MPa [7]. Similarly, viscose fibers exhibit tensile strengths ranging from 8.8 to 24 cN/tex (approximately 128–370 MPa), while Lyocell fibers range from 30 to 45 cN/tex (approximately 450–675 MPa) [8]. The breaking strength of yarns spun from these fibers is notably lower than that of the individual fibers. As a result, yarns produced from the aforementioned fibers exhibit a relatively limited upper bound in their physical and mechanical performance.

Cellulose fibers at the nanoscale exist in three primary forms: cellulose nanocrystals (CNC), cellulose nanofibrils (CNF), and bacterial nanocellulose (BNC). CNCs are typically produced by hydrolyzing plant-based materials, such as cotton, hemp, wood pulp, agricultural waste and others. CNC can be prepared by various methods such as acid hydrolysis, enzymatic hydrolysis, oxidative hydrolysis, and mechanical methods. Among them, acid hydrolysis is the most commonly method. Under the action of acid, the amorphous region of cellulose materials is preferentially hydrolyzed, while the crystalline region remains intact due to its high resistance to acid, achieving the purpose of separating and preparing high-crystallinity CNC [9]. They possess diameters ranging from 2 to 20 nm and lengths of several hundred nanometers, characterized by high crystallinity and excellent dispersibility [10,11,12,13]. CNFs are obtained by mechanically disintegrating cellulose into individual fibrils using methods such as high-pressure homogenization, ultrasonication, or ball milling. CNFs generally exhibit diameters between 1 and 100 nm and lengths extending to several micrometers, featuring high aspect ratios and abundant surface hydroxyl groups [14,15,16]. However, due to their relatively short lengths, both CNCs and CNFs are not well suited for the direct fabrication of cellulose yarns [17]. BNC is a high-molecular-weight polymer biosynthesized by specific microorganisms under controlled conditions. Compared to plant-derived cellulose, BNC exhibits several advantageous properties due to its unique biosynthetic process and nanofibrous network structure. These include high purity, elevated moisture content, a high degree of polymerization, high crystallinity, and outstanding mechanical performance [18,19,20]. More importantly, BNC is a film with self-growing nanocellulose network structure. Its microfibrils combine to form nanofibers, with diameters ranging from 25 to 100 nm and lengths of several micrometers [21]. Leveraging this intrinsic structure for self-twisting presents a theoretically promising approach for fabricating high-strength cellulose-based materials.

In the film-cutting and twisting process for yarn production, the polymer or staple fiber is first formed into a sheet-like film, which is then cut into narrow strips of defined width and subsequently twisted into yarn [22]. During the twisting process, torque is introduced into the fiber or yarn, inducing a helical arrangement of the polymer chains and resulting in a more compact fiber structure. This structural reorganization leads to changes in the mechanical properties of the yarn [23,24,25,26]. Wet drafting is a promising technique for the fabrication of BNC yarn, as it not only compensates for the random orientation of BNC nanofibers and enhances their alignment, but also facilitates moisture removal. This process reduces inter-fiber gaps in the resulting dry yarn, thereby improving its mechanical properties [27,28,29,30]. Given that BNC is inherently a nanofibrous network film, cutting it into strips followed by drawing and twisting is theoretically expected to yield yarns with enhanced physical and mechanical properties compared to conventional cellulose yarns. Hu [29] et al. first produced super-hard BNC yarn with a diameter of about 300 µm in 2017. The team used stretch-twist methods to produce high-strength yarns that reached record strengths but was relatively large in diameter. In 2020, Wu [31] et al. prepared BNC yarns with a diameter of about 200 µm by peeling the BNC film from top to bottom and further twisting. The team reduced the yarn diameter further while maintaining yarn strength. However, the BNC yarns reported in these two studies exhibit relatively large diameters, which may hinder their suitability for downstream applications. In recent years, research on BNC yarns has continued, Takagi [32] et al., inspired by the Japanese “kami-ito” technique in 2025, prepared BNC yarns that can replace cotton and industrial man-made cellulose fibers. The method of film cutting and twisting is adopted, but its diameter is large and its strength is relatively low, and its tensile strength is around 200 MPa. Further research is desired to develop high-strength, high-modulus BNC yarns with reduced diameters.

This study employs a film-cutting and twisting technique to address these challenges. BNC yarns with reduced diameters, high strength, and high modulus were fabricated through a three-step process comprising wet stretching, wet twisting, and drying. The morphology and structure of the resulting yarns were characterized using scanning electron microscopy and optical microscopy. Their mechanical and textile performance properties were evaluated using a universal testing machine, friction coefficient tester, and single-fiber bending tester. By optimizing key process parameters, BNC yarns with favorable mechanical properties and wearing performance were successfully produced. This straightforward preparation method offers a practical approach and provides valuable knowledge for the development of high-performance, high-strength cellulose-based yarns.

2. Materials and Methods

2.1. Materials

BNC hydrogel films with dimensions of 35 cm × 25 cm × 0.3 cm (length × width × thickness) were purchased from Hainan Yide Food Co., Ltd. (Haikou, Hainan, China). Sodium hydroxide (NaOH, AR) was purchased from Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China).

2.2. Preparation of BNC Hydrogel Strip

A 0.1 M NaOH solution was prepared by dissolving 2 g of NaOH in 500 mL of distilled water at room temperature. The BNC film was first rinsed with distilled water and then immersed in the 0.1 M NaOH solution in a 90 °C water bath for 1 h to remove residual biomass. After treatment, the film was thoroughly washed with distilled water until the pH reached 7. It was then cut into hydrogel strips with dimensions of 23 cm × 0.2 cm × 0.3 cm for subsequent use.

2.3. Fabrication of BNC Yarn

The hydrogel strips were twisted using a manual twist tester with a gauge length of 20 cm. To investigate the effect of twist on yarn properties, twist numbers were set at 60, 100, 120, 140, 160, 180, and 200 turns per 20 cm (r/20 cm), and the corresponding samples were labeled BNC-60, BNC-100, BNC-120, BNC-140, BNC-160, BNC-180, and BNC-200, respectively.

To examine the effect of elongation, the hydrogel strips were fixed on the twist tester (gauge length: 20 cm), and the draft length was adjusted to achieve elongations of 0.5%, 1%, and 1.5%, respectively. Each strip was then twisted at the optimal twist number identified earlier. The resulting samples were designated as 0.5%-BNC, 1%-BNC, and 1.5%-BNC.

Following twisting, each yarn sample was held on the twist tester for 10 min to allow partial water evaporation under tension, helping to roughly fix the yarn structure. The processed yarns were then transferred to an oven and dried at 105 °C for 2 h to fully set the structure and enhance the stability and durability of the yarn. The finalized BNC yarns were used for subsequent experiments and characterization. The preparation process is shown in Figure 1.

Cotton sliver was spun into yarn using a spinning frame to match the fineness and twist number of the BNC yarn for performance comparison. The conversion formula for fineness is shown below:

$${\mathrm{N}}_{\mathrm{e}}={\displaystyle \frac{1+{\mathrm{W}}_{\mathrm{k}}}{1+{\mathrm{W}}_{\mathrm{e}}}}·{\displaystyle \frac{590.5}{{\mathrm{N}}_{\mathrm{t}}}}={\displaystyle \frac{\mathrm{C}}{{\mathrm{N}}_{\mathrm{t}}}}$$

(1)

where N_e represents the imperial count of the yarn, s; W_k is the standard moisture regain of the yarn, %; W_e represents the standard moisture regain rate in the imperial system, %; N_t is the linear density of the yarn, tex; C is the conversion constant. The C value of pure-spun cotton yarn is 583.1. Control cotton yarns of 18s (32 tex) and 22s (26 tex) were prepared, respectively, and labelled as 18s-CY and 22s-CY.

2.4. Characterization

Moisture absorption-desorption equilibrium: The dried yarn samples were first weighed and then placed under ambient conditions at room temperature (temperature ≈ 24 ℃, humidity ≈ 50%). As the yarn gradually absorbed moisture from the air, its mass increased over time. The yarn was weighed at 1 h intervals until a constant mass was achieved. The moisture regain at room temperature was calculated using Equation (2) as follows:

$$\mathrm{W}={\displaystyle \frac{\mathrm{G}-{\mathrm{G}}_0}{{\mathrm{G}}_0}}\times 100\%$$

(2)

where W represents the moisture regain of the yarn, %; G represents the wet weight of the yarn, g; G₀ represents the dry weight of the yarn, g.

Morphology: The surface morphology of the BNC film was examined using a Regulus 8100 field emission scanning electron microscope (FE-SEM) operated at an acceleration voltage of 3 kV. Prior to imaging, the samples were coated with a thin layer of gold. The nanostructure of BNC nanoyarns with varying twist levels was observed using a JSM-6390LV SEM at an acceleration voltage of 15 kV, also following gold sputter coating. The diameter distribution of the yarn samples was analyzed using a DM2700M RL polarizing microscope in conjunction with Nano Measure 1.2.5 software. Additionally, the surface and cross-sectional morphology of the 1%-160rBNC yarn sample were characterized using a Sigma 500 FE-SEM operated at 5 kV, with gold coating applied prior to observation.

Tensile mechanical property test: The tensile properties of the BNC yarn and reference yarn (both are 32 tex and 26 tex, respectively) were evaluated using an Instron 5300 universal testing machine equipped with a 1 kN load cell. The tests were conducted at a speed of 20 mm/min with a clamping distance of 10 mm.

Friction coefficient test: The test was conducted in accordance with Chinese standard GB/T 45179-2024 [33]. The prepared yarn was cut into 5 cm segments, and a fixed tension of f₁ = 2 cN was applied to both ends. A total of 20 specimens were tested for each group to ensure statistical accuracy.

Bending resistance test: Bending resistance of the yarn samples was measured using a single-fiber tensile and compressive bending tester. The test was performed at a bending speed of 0.1 mm/s, with a yarn length of 10 mm. Twenty specimens were tested for each sample group to reduce random errors.

Detailed information of the above-mentioned test equipment is provided in the Supplementary Materials.

3. Results and Discussion

3.1. Analysis of Moisture Absorption-Desorption Equilibrium

Since the experiments were conducted under ambient conditions, the dried BNC yarn was subject to environmental humidity. As the rate of moisture uptake exceeded that of moisture release, the yarn absorbed moisture, leading to a gradual increase in mass. A moisture absorption-desorption equilibrium test was performed to determine the yarn’s moisture regain under ambient conditions.

Figure 2 illustrates the moisture regain of BNC yarn over time. The yarn’s mass increased steadily during the first 8 h, after which it plateaued, indicating that moisture absorption-desorption equilibrium had been reached. This behavior reflects a dynamic balance between the absorption of atmospheric moisture and the concurrent release of residual moisture and adsorbed gases from the yarn. After 8 h, the moisture regain stabilized at around 3.8%, which is lower than the typical 6.5–8.5% reported for cotton yarns. This difference is attributed to BNC’s higher crystallinity, which results in fewer amorphous regions and thus a lower moisture absorption capacity. The relatively low moisture regain also suggests that BNC yarns have a compact structure with limited internal porosity. It is obvious that the moisture content kept stable from 8 h to 15 h, suggesting the BNC should be conducted moisture adjustment longer than 8 h in the following characterization, which would be helpful to obtain the stable results.

3.2. Surface Morphology Analysis

Figure 3 presents a representative FE-SEM image of the surface morphology of the BNC film (the full image is provided in Figure S1, Supplementary Materials). The film consists of a dense network of filamentous nanofibers with diameters ranging from 10 to 100 nm. These nanofibers are intricately and randomly distributed, interwoven to form a highly entangled network structure. Numerous pores are present within the film, with pore diameters of approximately 100 nm, which may result in good air permeability and water permeability.

After cutting and twisting the BNC film into yarns, SEM was used to examine the morphology of yarns with varying twist levels, including the yarn with the optimal tensile properties (1%-160rBNC). Representative SEM images are presented in Figure 4 (original images are available in Figure S2, Supplementary Materials). The yarns were immersed in liquid nitrogen and fractured using tweezers to observe the cross-sectional morphology.

As shown in the images, at a low twist level of 100 r/20 cm (Figure 4a,d), the orientation of individual fibers is more visible, and the yarn surface appears relatively rough. When the twist level increased to 160 r/20 cm (Figure 4b,e), the yarn became more compact and exhibited a smoother surface. Further increasing the twist to 180 r/20 cm (Figure 4c,f) resulted in an even tighter structure; however, excessive twisting caused the surface fibers to tilt and form grooves, which disrupted the yarn structure. This structural damage contributes to the observed decline in tensile mechanical properties, as discussed in later sections. Therefore, 160 r/20 cm was identified as the optimal twist level.

At 1% elongation, the surface of the 1%-160rBNC yarn is relatively smooth and uniform, with consistent dryness and no visible defects (Figure 4g,h). Compared to the unstretched yarn, its diameter was significantly reduced to just over 100 µm, which is notably smaller than the diameters of BNC yarns reported in previous studies [29,31]. The cross-section reveals elongated fiber bundles arranged in a distinct counterclockwise spiral pattern, indicative of the twisting process. The internal structure is densely packed, with fibers tightly bound together due to the combined effects of stretching and twisting.

The diameter distributions of two types of BNC yarns (BNC-160 and 1%-160rBNC) and two reference cotton yarns (18s-CY and 22s-CY) were compared. For each yarn type, 150 randomly selected samples were measured using an optical microscope and analyzed with Nano Measure 1.2.5 software. The average diameters and coefficient of variation (CV) of the four yarn types are illustrated in Figure 5. The CV was calculated using the following formula:

$$\mathrm{C}\mathrm{V}={\displaystyle \frac{\mathsf{\sigma }}{\overline{\mathrm{x}}}}\times 100\%$$

(3)

where CV is the coefficient of variation, %; $\mathsf{\sigma }$ is the standard deviation; $\overline{\mathrm{x}}$ is the mean diameter.

The average diameter of BNC-160 was 152.64 μm with a CV of 9.99%, while 18s-CY had an average diameter of 269.61 μm and a CV of 18.78%. For the drawn yarns, 1%-160rBNC had an average diameter of 134.67 μm with a CV of 11.29%, and 22s-CY showed an average diameter of 253.26 μm with a CV of 13.78%. The drawn yarns (1%-160rBNC and 22s-CY) exhibited smaller diameters than their undrawn counterparts (BNC-160 and 18s-CY), indicating a finer yarn structure.

The CV value reflects the degree of diameter variation and is an important parameter for evaluating the uniformity of yarn dryness. Lower CV values indicate better uniformity. Compared with cotton yarns, the BNC yarns exhibited lower CV values, suggesting superior uniformity. In addition, the BNC yarns showed no visible defects or fuzz, resulting in a smoother and more consistent appearance.

3.3. Analysis of Tensile Properties

Drawing and twisting are critical processes for enhancing the mechanical performance of BNC yarns. Stretching (drawing) modifies the alignment and uniformity of fibers within the BNC hydrogel strips, while twisting consolidates the fiber structure into a cohesive yarn with improved form and performance. By adjusting the twist number and applying controlled wet tensile strain during processing, the degree of fiber orientation can be regulated, thereby influencing the tensile properties of the resulting yarns.

The tensile behavior of BNC yarns with varying twist numbers is shown in Figure 6a. As the twist number increased, the breaking strength of the yarn initially improved, reaching a maximum tensile stress of 400.75 MPa and an elongation at break of 9.23% at 160 r/20 cm. At this point, the average diameter of the BNC yarn is 152.64 μm, and the fineness is 32 tex. Beyond this point, further increases in twist number resulted in a decline in tensile performance. This reduction is attributed to the increasing inclination angle of fibers relative to the yarn axis, which diminishes the load-bearing efficiency in the axial direction and disrupts the internal orientation of the yarn structure. A comparison of mechanical properties between BNC-160 and 18s-CY cotton yarn is presented in Figure 6b. BNC-160 exhibited significantly higher tensile strength, with a breaking stress 6.41 times greater than that of the cotton yarn of equivalent fineness 18 s (32tex). However, its elongation at break was considerably lower, amounting to only 14.72% of that of 18s-CY, indicating limited ductility.

Yarns with different wet tensile strains were evaluated at the optimal twist level (160 r/20 cm) to further explore the effects of drawing. Under the condition that yarn integrity was maintained during both drawing and twisting, three groups of BNC yarns were prepared with wet elongation rates of 0.5%, 1%, and 1.5%. Their tensile properties are shown in Figure 6c. All wet-drawn yarns exhibited higher breaking strength and elongation at break compared to the undrawn BNC-160 yarn. The yarn drawn at 1% elongation demonstrated the best performance, with a breaking strength of 751.56 MPa, 1.88 times that of BNC-160, and an elongation at break of 11.56%, 1.25 times that of the undrawn counterpart. At this point, the average diameter of the BNC yarn is 134.67 μm, and the fineness is 26 tex. These results highlight the significant role of drawing in improving the tensile properties of BNC yarns by enhancing fiber alignment and compaction.

The tensile performance of the optimized yarn (1%-160rBNC) was further compared with that of 22s-CY cotton yarn (26 tex). As shown in Figure 6d, the BNC yarn displayed a breaking stress 15.63 times greater than that of the 22s cotton yarn, though its elongation at break accounted for only 21.46% of the cotton yarn’s. This trade-off between tensile strength and extensibility is inherent to the internal structural characteristics and deformation mechanisms of the yarn. Therefore, optimizing process parameters based on specific application requirements is essential to achieve an appropriate balance between strength and ductility.

In textile applications, there is a significant trade-off between yarn strength and elongation, which is crucial to the performance of the final product. This trade-off usually means that increasing the strength of the yarn may lead to a decrease in its elongation, and vice versa. The selection of yarn strength and elongation often depends on the application direction of the final product. The yarn prepared in this paper is intended for the production of dental floss, high-strength surgical suturing, industrial ropes and other products. Therefore, the high strength of the yarn is pursued while the elongation requirement is reduced. Moreover, through the stretching treatment of BNC strips in this paper, the orientation degree of the fibers in the yarn is improved. It has significantly enhanced the strength of the yarn and also moderately increased its elongation, achieving a balance between the strength and elongation of the yarn and meeting the individualized demands for high-strength and high-modulus yarns.

In summary, compared with traditional cellulose yarns such as cotton, BNC yarns exhibit significantly higher tensile strength, better resistance to breakage, and lower susceptibility to deformation. These features make BNC yarns particularly suitable for demanding applications in industrial, medical, and high-performance textile fields.

3.4. Analysis of Friction Performance

The coefficient of friction is a critical parameter in evaluating yarn performance in textile applications. It reflects the interaction between the yarn surface and other materials, such as machine components, other yarns, or human skin. Yarn-yarn friction affects fabric characteristics such as anti-pilling and abrasion resistance, while yarn-skin friction influences tactile comfort in garments. Depending on the application, yarns may require either low or high friction. For instance, surgical sutures require minimal friction to reduce tissue damage during insertion, whereas safety belts and high-performance ropes benefit from increased friction to enhance fiber cohesion and tensile strength. Thus, the frictional performance of yarns is a key factor in determining their suitability for practical applications.

To assess the potential applications of BNC yarns, the static (μ_S) and dynamic (μ_D) coefficients of friction for 1%-160rBNC yarn and 22s-CY cotton yarn were measured, and the results are summarized in

Table 1. Dynamic, static and composite friction coefficients of BNC yarn and cotton yarn.

	μS	CV (%)	μD	CV (%)	Comprehensive Coefficient of Friction
22s-CY	0.1817	46.7	0.1587	49.8	0.1656
1%-160rBNC	0.1651	41.7	0.1229	39.3	0.1356

. The static coefficient of friction (μ_S) represents the maximum ratio of frictional to normal force before movement begins, while the dynamic coefficient (μ_D) represents this ratio during continuous sliding. A comprehensive coefficient of friction was calculated using a weighted average method, assigning a weight of 30% to μ_S and 70% to μ_D. As shown in Table 1, the μ_S and μ_D values for BNC yarn were 0.1651 and 0.1229, respectively, both lower than those of cotton yarn (0.1817 and 0.1587, respectively). The calculated comprehensive coefficient of friction for BNC yarn was 0.1356, compared to 0.1656 for cotton yarn. This reduction in friction can be attributed to the smoother surface of the BNC yarn, which exhibits fewer surface defects or fuzz. Lower friction implies that less force is needed to initiate and maintain yarn movement, resulting in reduced resistance during operation or use. Additionally, the coefficient of variation (CV) for all measured friction parameters was lower in BNC yarn than in cotton yarn, indicating better uniformity in surface dryness and structural consistency of the BNC yarn.

Compared with other cellulose-based yarns, the BNC yarn also exhibits lower friction: for example, the friction coefficient between moor yarn and a metal roller is 0.272 ± 0.001, and for Lyocell yarn, 0.235 ± 0.002 [34]. In contrast, the friction coefficient of BNC yarn is significantly lower. These properties suggest that BNC yarn, with its combination of high tensile strength and low friction, holds strong potential for use in biomedical applications such as surgical sutures, where smooth passage through tissue is essential to minimize trauma and reduce the risk of thread jamming.

3.5. Analysis of Bending Resistance

The bending resistance of yarn is another critical factor influencing both its processability and the tactile quality of the final textile product. Yarns with high bending resistance tend to be stiffer and better retain their shape, while those with lower bending resistance are softer, more flexible, and drape more smoothly.

Bending modulus refers to the intrinsic ability of the yarn material to resist bending deformation, reflecting the hardness of the material itself. In contrast, bending stiffness refers to the overall resistance of the yarn structure to bending, determined by both material properties and geometric factors. A higher bending stiffness indicates that greater force is required to bend the yarn. Understanding the bending resistance of yarns provides valuable insight into their potential applications. In this study, a single-fiber tensile and compressive bending tester was used to assess the bending behavior of yarns. The bending process of the yarn is shown in Figure 7. Yarn samples were cut into 10 mm segments, with the lower end fixed in a metal clamp and the upper end extended freely (Figure 7a,c). Due to the high length-to-diameter ratio of the yarn, eccentric loading during compression caused it to bend (Figure 7b,d). The upper end of the device includes a micro-hinged sensor, which measures the bending resistance. As also seen in the images, the BNC yarn exhibits more uniform surface dryness and is free of fuzz.

The bending modulus and bending stiffness of 1%-160rBNC and 22s-CY cotton yarn were measured, and the results are presented in Figure 8 and

Table 2. Bending stiffness and bending modulus of BNC yarn and cotton yarn.

Sample	Modulus (Gpa)	CV (%)	Stiffness (cN·cm2)	CV (%)
22s-CY	0.2014	58.2	0.0102	26.9
1%-160rBNC	718.7564	35.7	1.7365	31.8

. The BNC yarn demonstrated significantly higher values in both parameters. The bending modulus of 1%-160rBNC reached 718.76 GPa, which is 3569 times greater than that of the control cotton yarn. This remarkable difference is attributed to fundamental differences in composition and nanostructure. BNC is biosynthesized by microorganisms and consists of nearly 100% pure cellulose with exceptionally high crystallinity. Its nanofibrous network is densely packed with hydrogen bonds, forming a rigid internal framework that limits molecular chain mobility and elastic deformation. In contrast, cellulose fibers such as cotton and linen naturally contain impurities such as lignin and pectin, exhibit lower crystallinity, and possess more amorphous regions. These structural features disrupt molecular chain alignment and result in a reduced modulus.

The bending stiffness of 1%-160rBNC was measured at 1.7365 cN·cm², which is 170 times higher than that of 22s-CY. This substantial difference is not only due to the inherently higher bending modulus of BNC but also to the geometric advantages of the yarn structure. The BNC yarn features a dense, axially aligned structure. During the drawing and twisting processes, the nanofibers are highly oriented and packed tightly, enabling efficient load sharing among fibers. In contrast, cotton yarn contains internal voids between fiber bundles and a looser overall structure, reducing the effective load-bearing area and thus lowering stiffness.

The bending resistance of BNC yarns exceeds that of many other natural cellulose-based yarns. The high modulus and rigidity of BNC yarns make them well-suited for applications requiring strong dimensional stability and resistance to external compressive forces. These characteristics allow BNC yarns to maintain their shape under load, reduce deformation-related damage, and provide excellent structural support in functional textiles and high-performance materials.

4. Conclusions

In this study, high-strength, high-modulus, and stretchable yarn was successfully fabricated using BNC as the raw material via a film-cutting and twisting method, incorporating both stretching and twisting processes. The tensile, bending, and frictional properties of the resulting yarns were comprehensively investigated. The main conclusions are as follows: 1. Morphological Analysis: SEM images revealed that the BNC film consists of a dense three-dimensional nanofiber network formed by irregularly arranged nanofibers interconnected through abundant hydrogen bonds. Yarn produced at the optimal twist level exhibited the smoothest surface and compact structure without grooves, while yarn produced at the optimal elongation rate had a significantly reduced diameter. The resulting BNC yarn demonstrated high fiber orientation, uniform dryness, and an absence of surface defects or fuzz. 2. Tensile Properties: The 1%-160rBNC yarn exhibited the best tensile performance, with a breaking strength of 751.56 MPa, which is 15.63 times higher than that of the reference cotton yarn. Its elongation at break was 11.56%, equivalent to 21.46% of the control, indicating high tensile strength and moderate extensibility. 3. Friction Properties: The static and dynamic friction coefficients of the 1%-160rBNC yarn were lower than those of cotton yarn, resulting in reduced friction with contact surfaces and a smoother tactile feel. 4. Bending Resistance: The 1%-160rBNC yarn exhibited exceptional bending resistance, with a bending modulus of 718.76 GPa and bending stiffness of 1.7365 cN·cm², 170 times greater than that of the control cotton yarn. This reflects a strong capacity to resist bending deformation and maintain structural integrity. These findings highlight the potential of BNC yarns for use in high-performance textile applications where strength, stability, and low surface friction are required, such as in biomedical materials, industrial textiles, and technical fabrics.