A novel non-woven fabric from bamboo fiber in medical lifestyle products

Abstract

The present study aims to investigate the appropriate size of bamboo fibers derived from waste bamboo and determine the optimal duration for soaking in bio-fermented water to facilitate fabric molding. Additionally, we seek to explore the properties of non-woven fabric products manufactured from bamboo fibers. The study factors encompass three grades of bamboo fibers, designated A, B, and C, as well as five levels of fermentation time: 2, 4, 6, 8, and 10 days. A Completely Randomized Design (CRD) experiment is planned to assess both physical and microbial properties. Furthermore, we will analyze the inhibition of Staphylococcus aureus (S. aureus) and Escherichia coli (E.coli) bacteria in accordance with AATCC test standards and determine the optimal ratio for non-woven fabric molding. Regarding the ratio of bamboo fiber to polyester fiber, we will investigate three levels: 50:50, 70:30, and 100:0, with the objective of designing medical lifestyle products.Preliminary findings indicate that grade B bamboo fibers exhibit a light brown color and a relatively thin structure, with lengths ranging from 1 to 3 cm, rendering them suitable for most fabric production processes. Notably, unfermented bamboo fibers demonstrate higher inhibition of S. aureus and E.coli bacterial growth compared to their fermented counterparts. Additionally, our results suggest that the ingredient ratio significantly impacts the molding process, with a 99.94% effect. Remarkably, a blend of bamboo fiber and polyester fiber in a 70:30 ratio can be mechanically processed to create needle-pressed fabric, utilizing a method that involves pressing and tight weaving. This innovative approach facilitates the production of antibacterial non-woven fabric products suitable for elderly individuals, ready for practical use in community settings as a best practice solution.

1. Introduction

Bamboo, a versatile and sustainable plant, has garnered significant attention in recent years due to its diverse applications and ecological benefits. Thailand boasts a rich diversity of bamboo species, with 13 genera and 60 species covering approximately 5,062,500 rai (5.5% of the total forested area). Among these, Dendrocalamus asper Backer, commonly known as Sweet Bamboo or Tong bamboo, is widely cultivated for its economic value [1,2]. Tong bamboo is renowned for its hardiness, fast growth, and ease of propagation, making it a valuable resource for various industries [3]. Its shoots, stems, leaves, flowers, and fruits provide both direct and indirect benefits, contributing significantly to the livelihoods of bamboo growers [4].

In Thailand, Tong bamboo plays a crucial role in the basketry handicraft industry, generating substantial income for local farmers. The stems of Tong bamboo typically measure 10–20 cm in diameter, with internodes spanning 20–50 cm and flesh thickness ranging from 1 to 3.5 cm. The unique properties of Tong bamboo, such as its strength, durability, and flexibility, make it an ideal material for construction, furniture, and handicrafts. However, the sexual propagation of Tong bamboo is a lengthy process, with some species requiring 30–60 years to bloom and produce seeds. This highlights the need for alternative propagation methods and the efficient utilization of bamboo waste [5,6].

Bamboo waste, generated from various processing stages, has the potential to be transformed into value-added products. Bamboo fiber, derived from bamboo waste, has emerged as a promising material for textile and other industries due to its unique structure and functional properties [7]. Bamboo fibers exhibit excellent mechanical properties, such as high tensile strength and elasticity, making them suitable for a wide range of applications [8]. Moreover, bamboo fibers possess inherent antimicrobial properties, which can be attributed to the presence of bioactive compounds like flavonoids, phenols, and lignin [9]. The extraction of bamboo fibers from waste bamboo has been explored using various methods, including mechanical, chemical, and enzymatic processes. However, these methods often involve the use of harsh chemicals or energy-intensive processes, which can have negative environmental impacts [10]. Therefore, there is a growing interest in developing eco-friendly and sustainable methods for bamboo fiber extraction and treatment.

Bio-fermented water, obtained by composting plant or animal residues in an airless environment, has been explored as a potential treatment for bamboo fibers [11]. The fermentation process involves microorganisms that decompose organic waste into a nutrient-rich slurry [12]. Bio-fermented water contains beneficial microorganisms, such as lactic acid bacteria, yeasts, and photosynthetic bacteria, which can enhance the properties of treated materials [13]. Previous studies have investigated the effects of bio-fermented water on various crops and materials, demonstrating its potential to improve growth, yield, and quality [14,15]. The application of bio-fermented water in the treatment of bamboo fibers remains largely unexplored. However, the potential benefits of using bio-fermented water in this context are manifold. First, the fermentation process can help break down the lignin and hemicellulose components of bamboo fibers, leading to improved flexibility and softness [16]. Second, the beneficial microorganisms present in bio-fermented water can impart antimicrobial properties to the treated fibers, enhancing their resistance to bacterial and fungal growth. Third, the use of bio-fermented water can reduce the reliance on harsh chemicals and energy-intensive processes, making it an eco-friendly and sustainable approach to bamboo fiber treatment [17].

The present study aims to address several key objectives related to the utilization of bamboo waste and the development of value-added products from bamboo fibers. First, it seeks to investigate the appropriate size of bamboo fibers derived from waste bamboo for fabric production. This is important because the size of the fibers can significantly influence the physical and mechanical properties of the resulting fabrics [18]. Second, the study aims to determine the optimal duration of soaking bamboo fibers in bio-fermented water to enhance their properties. The soaking time can affect the extent of fermentation and the degree of fiber modification, which in turn can impact the quality of the treated fibers. Third, the study aims to identify the optimal ratio of bamboo fibers to polyester fibers for non-woven fabric formation. Non-woven fabrics are widely used in various applications, including medical and hygiene products, due to their excellent absorbency, breathability, and barrier properties [[19], [20], [21]]. The incorporation of bamboo fibers into non-woven fabrics can impart additional benefits, such as antimicrobial properties and improved sustainability [22]. Fourth, the study seeks to assess the quality of non-woven fabric products manufactured from bamboo fibers in terms of physical and microbial properties. This is important to ensure that the developed fabrics meet the required standards and can be effectively used in the intended applications. Finally, the study aims to design and develop medical lifestyle products utilizing bamboo fiber fabrics. Medical lifestyle products, such as wound dressings, face masks, and bedding materials, require materials that are biocompatible, hygienic, and comfortable [23]. Bamboo fiber fabrics, with their inherent antimicrobial properties and breathability, have the potential to meet these requirements and provide enhanced benefits to users.

Recent research underscores the innovative use of natural materials for sustainable product development. A study highlighted the design and satisfaction of banana rope woven baskets, reflecting a community-driven approach to eco-friendly manufacturing [24]. Another research focused on improving daily life utility items by incorporating natural fibers, enhancing both functionality and environmental friendliness [25]. Additionally, efforts in using locally sourced materials to craft fashion bags exemplify the blend of traditional techniques with contemporary needs [26]. These efforts demonstrate a significant shift towards sustainable practices in product design, driven by both environmental concerns and consumer satisfaction.

By addressing these objectives, this research seeks to contribute to the efficient utilization of bamboo waste, the development of sustainable textile materials, and the creation of value-added products. The findings of this study have the potential to benefit farmers, emerging entrepreneurs, and the wider community by promoting the use of bamboo fibers in functional textiles and medical lifestyle products. Moreover, the use of bio-fermented water in the treatment of bamboo fibers represents an eco-friendly and sustainable approach, aligning with the growing demand for green and clean technologies [27].

This study aims to explore the potential of bamboo waste as a source of valuable fibers for textile and medical applications. By investigating the optimal size of bamboo fibers, the duration of bio-fermented water treatment, and the ratio of bamboo to polyester fibers in non-woven fabrics, this research seeks to develop high-quality, sustainable, and functional materials. The design and development of medical lifestyle products utilizing bamboo fiber fabrics further demonstrate the practical applications and benefits of this innovative approach. Overall, this study contributes to the advancement of sustainable textile materials and the promotion of bamboo as a versatile and eco-friendly resource.

2. Materials and methods

2.1. Raw materials

• -Bamboo waste (Dendrocalamus asper Backer) was sourced from Pimtha Company Limited, located in Non Hom Subdistrict, Muang District, Prachinburi Province, Thailand. The bamboo waste consisted of discarded bamboo culms, branches, and leaves from the company's bamboo processing operations.
• -EM Extra Bio-fermented Water, a commercially available product containing beneficial microorganisms, was used for the treatment of bamboo fibers.

2.2. Bamboo fiber preparation and grading

The bamboo waste was manually sorted to remove any non-bamboo materials and contaminants. The sorted bamboo waste was then mechanically processed using a fiber extraction machine (model: FX-100, Fiber Tech Co., Ltd., Thailand) to obtain bamboo fibers of varying sizes. The extracted fibers were sieved through a 60-mesh sieve (aperture size: 250 μm) to separate them into different size ranges. The fibers were then classified into three grades based on their length and thickness.

• -Grade A: Fine, powdery fibers with lengths less than 1 cm and diameters less than 50 μm.
• -Grade B: Thin fibers with lengths ranging from 1 to 3 cm and diameters between 50 and 150 μm.
• -Grade C: Thick, hard fibers with lengths ranging from 4 to 6 cm and diameters greater than 150 μm.

The length and diameter of the fibers were measured using a digital caliper (model: CD-6″ ASX, Mitutoyo Corporation, Japan) with a resolution of 0.01 mm. Fifty fibers from each grade were randomly selected and measured to obtain average values. Grade B fibers were chosen for further experiments due to their optimal length and thickness, which make them suitable for fabric production.

2.3. Bio-fermented water treatment

Grade B bamboo fibers were soaked in bio-fermented water at a ratio of 100:1 (weight of fermented substance to weight of bamboo fibers). The bio-fermented water was prepared by mixing EM Extra Bio-fermented Water with distilled water at a ratio of 1:1000 (v/v). The fiber-water mixtures were placed in sealed glass containers and allowed to ferment at room temperature (25 ± 2 °C) for periods of 2, 4, 6, 8, and 10 days. After the designated fermentation times, the treated fibers were removed from the bio-fermented water, rinsed thoroughly with distilled water, and dried in an oven (model: UF55, Memmert GmbH + Co. KG, Germany) at 60 °C until a constant weight was achieved.

2.4. Non-woven fabric production

The treated bamboo fibers were mixed with polyester fibers (1.5 denier, 38 mm length; Nanya Plastic Co., Ltd., Taiwan) in ratios of 50:50, 70:30, and 100:0 (weight of bamboo fibers to weight of polyester fibers). The fiber mixtures were then processed using a carding machine (model: TC-600, Textima Export Import GmbH, Germany) to create a uniform fiber web. The carded webs were then needle-punched using a needle-punching machine (model: NP-200, Dilo Group, Germany) with a needle density of 200 needles/cm² and a punching depth of 10 mm to form non-woven fabrics.

2.5. Characterization of bamboo fibers and non-woven fabrics

2.5.1. Scanning electron microscopy (SEM)

The surface morphology and cross-sectional structure of the treated bamboo fibers were examined using a scanning electron microscope (model: JSM-6610LV, JEOL Ltd., Japan). Fiber samples were sputter-coated with a thin layer of gold (thickness: ∼10 nm) using a sputter coater (model: SC7620, Quorum Technologies Ltd., UK) to improve their conductivity. SEM images were acquired at accelerating voltages of 10–20 kV and magnifications ranging from 100X to 5000X.

2.5.2. Antibacterial properties

The antibacterial properties of the treated bamboo fibers and non-woven fabrics were evaluated against Staphylococcus aureus (S. aureus) (ATCC 6538) and Escherichia coli (E.coli) (ATCC 8739) using the AATCC 100–2012 test method (Antibacterial Finishes on Textile Materials: Assessment of). Fiber and fabric samples (0.5 g each) were placed in sterile containers containing 50 mL of bacterial suspension (concentration: ∼10^5 CFU/mL) and incubated at 37 °C for 24 h. After incubation, the bacterial concentrations in the suspensions were determined by serial dilution and plating on nutrient agar. The antibacterial activity was expressed as the percentage reduction in bacterial concentration compared to a control sample (untreated bamboo fibers or non-woven fabric without bamboo fibers).

2.5.3. Physical properties of non-woven fabrics

The physical properties of the bamboo fiber non-woven fabrics were assessed using the following methods.

• -Weight: ASTM D3776/D3776M-09a (Standard Test Methods for Mass Per Unit Area (Weight) of Fabric)
• -Thickness: ASTM D1777-96(2019) (Standard Test Method for Thickness of Textile Materials)
• -Tensile strength: ASTM D5035-11(2019) (Standard Test Method for Breaking Force and Elongation of Textile Fabrics (Strip Method))

For each physical property, five specimens were tested, and the average values were reported.

2.6. Development of medical lifestyle products

Based on the results of the characterization tests, the bamboo fiber non-woven fabric with the optimal properties (i.e., highest antibacterial activity, suitable mechanical strength, and good thermal insulation) was selected for the development of medical lifestyle products. As a proof-of-concept, an antibacterial bed mat for the elderly was designed and fabricated using the selected non-woven fabric. The bed mat was constructed by layering the non-woven fabric with a breathable, waterproof backing material (e.g., polyurethane-coated polyester) and edge-sealing the layers using a hot-melt adhesive. The dimensions and design features of the bed mat were determined based on ergonomic considerations and input from healthcare professionals.

3. Result and discussion

3.1. Bamboo fiber grading and selection

The bamboo waste obtained from Pimtha Company Limited was processed and sieved to obtain fibers of different sizes. The fibers were classified into three grades based on their length and thickness: Grade A (fine, powdery fibers; <1 cm), Grade B (thin fibers; 1–3 cm), and Grade C (thick, hard fibers; 4–6 cm). Fig. 1 presents representative images of the fibers from each grade.

Fig. 1.

Representative images of bamboo fibers: (a) Grade A, (b) Grade B, and (c) Grade C.

After sifting the bamboo waste through a 60-mesh sieve, the resulting fibers were classified into three distinct grades: Grade A, Grade B, and Grade C shown in Fig. 1(a), Fig. 1(b), and Fig. 1(c), respectively. Grade A fibers were characterized by their fine texture and small size, rendering them unsuitable for compaction into powder form. As reported by Ref. [2], natural fibers used in fabric production typically have a length of approximately 2.54 cm (1 inch). The fine texture and small size of Grade A fibers make them more appropriate for the manufacture of decorative products or furniture [28].

Grade B fibers were distinguished by their light brown color and relatively thin structure, with lengths ranging from 1 to 3 cm. Similarly, Grade C fibers also exhibited a light brown color but were notably thicker and harder than Grade B fibers, with lengths spanning from 4 to 6 cm. The substantial size and thickness of Grade C fibers made them unsuitable for fabric production. A comprehensive assessment of the morphological characteristics of all three fiber grades indicated that Grade B fibers were the most appropriate for fabric production and further development, as their size and length were optimal for molding into fabrics composed of natural fibers.

To ensure optimal results, it is recommended that the fiber length be maintained at approximately 2.54 cm (1 inch). This length prevents the fibers from spiraling into yarn during the processing stage. Excessively long fibers may become brittle and susceptible to breakage during the threading process.

3.2. Effects of bio-fermented water treatment on bamboo fiber morphology

The SEM images presented in Fig. 2 reveal the impact of bio-fermented water treatment on the surface morphology and cross-sectional structure of bamboo fibers, showing the longitudinal view of the fibers at a magnification of 350X.

Fig. 2.

Characteristics of bamboo fibers that have been fermented for different periods of time (magnification 350X) of (a) Control is not fermented, (b) Marinate for 2 days, (c) Marinate for 4 days, (d) Marinate for 6 days, (e) Marinate for 8 days and (f) Marinate for 10 days.

Comparing the untreated control fibers with the treated fibers (Fig. 2(a)), it is evident that the bio-fermented water treatment induces significant changes in the fiber morphology. As the treatment duration increases from 2 (Fig. 2(b)),4 (Fig. 2(c)), 6 (Fig. 2(d)),8 (Fig. 2(e)), and10 days (Fig. 2(f)), the fibers exhibit progressively rougher surfaces, with more pronounced longitudinal striations and an increased number of micropores. This increase in surface roughness and porosity can be attributed to the degradation of lignin and hemicellulose components in the fibers by the microorganisms present in the bio-fermented water.

The cross-sectional views of the fibers (Fig. 2) further highlight the structural changes induced by the bio-fermented water treatment. The untreated control fibers display a relatively smooth and compact structure, with closely packed microfibrils. In contrast, the treated fibers show a more loosely arranged microfibrillar structure, with increased spacing between the microfibrils. This loosening of the fiber structure is particularly evident in the fibers treated for 8 and 10 days, which exhibit a more open and porous cross-section.

The observed changes in fiber morphology have important implications for the properties and potential applications of the treated bamboo fibers. The increased surface roughness and porosity can enhance the mechanical interlocking and adhesion between fibers, leading to improved strength and durability in composite materials. Moreover, the more open and accessible structure of the treated fibers may facilitate the penetration and retention of moisture, as well as the incorporation of functional additives, which can be beneficial for applications such as wound dressings or drug delivery systems.

3.3. Microbial quality analysis results

The antibacterial properties of bamboo fibers treated with bio-fermented water for 2 and 10 days were evaluated against S. aureus and E.coli. The results are presented in Table 1, Table 2.

Table 1.

Antibacterial activity of bamboo fibers against S. aureus.

Sample	Microbial population count (CFU/g)		Reduction in bacterial count (%)
	0 h	24 h
Control	(1.85 ± 0.02) x 105	<100	99.95 ± 0.03a
2-days treatment	(1.85 ± 0.02) x 105	(1.50 ± 0.02) x 105	18.92b ± 0.02b
10-days treatment	(1.85 ± 0.02) x 105	(1.55 ± 0.02) x 105	16.22c±0.02c

Note: CFU/g = colony-forming units per gram. Control = untreated bamboo fibers. Values are presented as mean ± standard deviation (n = 3). Different superscript letters indicate statistically significant differences (p < 0.05) based on one-way ANOVA followed by Tukey's post hoc test.

Table 2.

Antibacterial activity of bamboo fibers against E.coli.

Sample	Microbial population count (CFU/g)		Reduction in bacterial count (%)
	0 h	24 h
Control	(1.61 ± 0.02) x 105	<100	99.94
2-days treatment	(1.61 ± 0.02) x 105	(3.00 ± 0.05) x 106	−1762.73
10-days treatment	(1.61 ± 0.02) x 105	(3.00 ± 0.05) x 106	−1762.73

Note: CFU/g = colony-forming units per gram. Control = untreated bamboo fibers. Values are presented as mean ± standard deviation (n = 3). Negative values indicate an increase in bacterial count.

Table 1 of the analysis of the antibacterial activity of S. aureus showed that bamboo fibers were not fermented. Bamboo fiber fermented for 2 days and bamboo fiber fermented for 10 days contain bacteria at the beginning of both strains equal to 1.85 x 10⁵ CFU/g when incubated at 38 °C. For 24 h, it was found that unfermented bamboo fibers have the highest antibacterial effect. There was a statistical difference (P < 0.05) due to saponins, flavonoids, and tannins. Bamboo has antibacterial effects on some types and can help relieve inflammation and reduce the risk of bacterial infections [4]. S. aureus accounted for 18.92 % and 16.22 % of the antibacterial activity, respectively.

Non-woven fabrics were formed by mixing bamboo fiber and polyester fiber, with the bonding adhesive comprised of a 95:5 mixture ratio. Table 2 presents the antibacterial activity of bamboo fibers against E. coli. The untreated control fibers showed a 99.94 % reduction in bacterial count after 24 h. However, the fibers treated with bio-fermented water for 2 and 10 days exhibited a substantial increase in bacterial count, with a 1762.73 % increase compared to the initial count. These results suggest that the bio-fermented water treatment may have negatively affected the inherent antibacterial properties of the bamboo fibers against E. coli.

The contrasting effects of the bio-fermented water treatment on the antibacterial activity of bamboo fibers against S. aureus and E. coli warrant further investigation. The decreased antibacterial activity against S. aureus and the increased bacterial growth of E. coli in the treated fibers could be attributed to changes in the chemical composition, surface properties, or the introduction of nutrients from the bio-fermented water. Additional studies are necessary to elucidate the underlying mechanisms and to develop strategies for maintaining or enhancing the antibacterial properties of bamboo fibers during the bio-fermented water treatment process.

3.4. Study of the optimal ratio for molding fabric from bamboo fiber

Form non-woven fabrics at a mixture ratio between bamboo fiber and polyester iber..

As illustrated in Fig. 3, the molding process for non-woven fabrics comprised four primary steps.

• 1.Determining the optimal mixture ratio between bamboo fiber and polyester fiber, which was found to be 70:30 respectively shown in Fig. 3(a–b).
• 2.Carding: The two fiber types were introduced into a fiber mixer to form a carded lap. This involved passing the bamboo-polyester blend through a carding machine equipped with 100 small teeth (Fig. 3(c)). The resulting fiber sheet exhibited a transverse or parallel fiber arrangement. An air-laid process employing air currents was then utilized to produce fiber webs by depositing the fibers onto a grid. The fibers were fed into the machine and subjected to beating by a rotating roller at 100 °C for 10 min to ensure uniform strength properties in all directions, a process termed web formation. The bamboo fiber and polyester fiber sheets demonstrated lightweight characteristics akin to cotton pads.
• 3.Creation of adhesive forces: Mechanical bonding, a process in which fibers are compressed and consolidated, was employed. Needle punching entailed embroidering the fibers within the sheet, resulting in disordered entanglement (Fig. 3(d)). This imparted enhanced stability and strength to the fiber sheet while maintaining dimensional integrity and preventing elongation. The resulting fabric exhibited properties resembling that of a felt-pressed fabric [29].

Fig. 3.

Fabric molding from bamboo fiber of (a) Ratio of mixture between bamboo and polyester fiber, (b) Mix bamboo fiber, (c) Bamboo fiber carding, and (d) Forming bamboo fiber sheet.

3.5. Characterization of bamboo fiber non-woven fabric products

A prototype product, a bamboo fiber scrubbing pad, was developed utilizing bamboo fibers with lengths ranging from 1 to 5 cm shown in Fig. 4. The manufacturing process consisted of five primary steps: fiber weighing, lengthwise cutting, fiber softening through beating, sheet formation, and the sewing of polishing pads. This approach represents a simple, convenient, and energy-efficient method for producing scrubbing pads. The pads were constructed by sewing together bamboo fibers and raw yarn, with the front surface comprising bamboo fibers and the back composed of raw yarn to prevent fiber detachment shown in Fig. 4(a). Squares measuring 1 × 1 cm were sewn onto the pads Fig. 4(b).

Fig. 4.

Bamboo fiber scrubbing pads of (a) front face, and (b) rear aspect.

The bamboo fiber scrubbing pad prototype is a product that can be manufactured in regions with abundant bamboo fiber sources. Furthermore, it can be distributed through community stores, catering to the interests of cosmetic consumers seeking natural, sustainable products.

4. Conclusions

Unfermented bamboo fibers exhibit remarkable antibacterial properties, effectively inhibiting the growth of S. aureus and E.coli bacteria. The fibers are classified into three distinct grades: A, B, and C. Grade B fibers, characterized by their delicate thinness and lengths ranging from 1 to 3 cm, are ideally suited for fabric production. Furthermore, these fibers possess the potential for enhancement through fermentation with bio-fermented water specifically tailored for bamboo fibers. Notably, unfermented bamboo fibers demonstrate exceptional antibacterial efficacy, surpassing that of their fermented counterparts by an impressive 99.94 %. To capitalize on the exceptional antibacterial qualities of bamboo fibers, a ratio of 70:30 bamboo to polyester fibers is employed in the formation of non-woven fabrics. This innovative process involves the creation of a diffused fiber sheet, yielding a non-directional fiber matrix that ensures consistent strength properties in all directions. This breakthrough in textile technology holds significant promise for a wide range of applications, particularly in the healthcare sector, where the demand for high-performance antibacterial fabrics is paramount. The unique properties of these bamboo fiber-based non-woven fabrics position them as a potential solution for various medical and hygiene-related applications.

Data availability statement

Data will be made available on request.

Additional information

No additional information is available for this paper.

CRediT authorship contribution statement

Sakorn Chonsakorn: Writing – review & editing, Writing – original draft, Funding acquisition, Formal analysis, Data curation, Conceptualization. Rath Chombhuphan: Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. Kasidit Rattanaporn: Methodology, Formal analysis, Data curation, Conceptualization. Supanicha Srivorradatphisan: Formal analysis, Data curation, Conceptualization. Chanakarn Ruangnarong: Investigation, Formal analysis, Data curation, Conceptualization. Sujira Khojitmate: Writing – review & editing, Writing – original draft, Visualization, Validation, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.