Fabrication and Characterization of Nylon 6 Fiber via Wet Fabrication and Characterization of Nylon 6 Fiber via Wet Spinning Method for Application as a Reinforcing Material for a Spinning Method for Application as a Reinforcing Material for a Direct Dental Bridge Direct Dental Bridge

Nylon is a biocompatible thermoplastic polymer that is well known for its excellent fracture resistance, making it suitable for fabricating fiber-reinforced composite (FRC)-based dental bridges. FRC is widely used in dentistry. This study aimed to investigate the structural and morphological characteristics of the nylon 6 fiber prepared using five different weights of nylon 6 dissolved in formic acid. The nylon 6 fiber was successfully fabricated via a simple wet spinning method using water as a coagulant. The fiber was then characterized using Fourier transform infrared (FTIR) spectroscopy, optical microscopy


Introduction
Losing teeth can lead to issues related to esthetics, speech, masticatory function, and socialization, leading to problems related to an individual's psychological and social well-being.These problems can be minimized with prompt tooth replacement treatments.The fiber-reinforced composite bridge is a decent option for replacing missing teeth, realizing minimum invasive therapy and high esthetics [1].Fiber-reinforced composite (FRC) is a material comprising a fiber as the core or framework and a resin matrix as an external surface [2,3].Fiber is applied to the abutment teeth before being coated with a composite and molded to fit the tooth anatomy [2].However, failure in FRC treatment is not uncommon [1].The physical and mechanical qualities of the prosthesis material determine the strength of the dental bridge [4].Hence, the fiber used to fabricate the dental bridge structure must be strong enough to withstand intraoral forces [5].
Polymer-based biomaterials have undergone substantial advancements over the years, making them one of the most rapidly evolving dental bridge materials currently available [6].Clinicians prefer dental bridges made of various fibers (such as polyethylene, carbon, and glass fiber) over traditional materials like metal or ceramic in specific cases due to their distinct advantages [7].However, each type also has inherent limitations that must be considered before use in treatment procedures.Despite its relatively high success rate as a restorative material for dental bridges compared to other types of dental bridges, such as metal-ceramic bridges, FRC bridges has been reported to fail [8,9].Therefore, researchers and practitioners explored the factors behind such failures.Using an expanded research approach to study these materials, we hope to design highly effective treatment methodologies with great longevity and low scope of failure.
This study employs a thermoplastic polymer-based material, Nylon 6, which is a common type of nylon used in fiber and engineering applications, to fabricate dental bridges.The important characteristics of nylon, such as resistance to fracture, can be advantageous in dental applications [10].Nylon is widely used as a denture base material in dentistry and has several other advantages, including outstanding aesthetic value, biocompatibility, lack of residual monomers, high flexibility, low solubility, and high temperature and abrasion resistance.However, nylon has some disadvantages, such as tendency for discoloration and high propensity for water absorption [11].However, the fiber in fiber-reinforced composites used in fabricating direct dental bridges is coated with a composite [1].It appears the disadvantage of nylon properties can be minimized; Thus, the fiber does not come in contact with water and the discoloration of the fiber is minimized.
In this study, microsized nylon 6 fibers with different concentrations (10%, 15%, 20%, 25%, and 30%) were produced via wet spinning.Wet spinning is a fiber fabrication method involving a polymer solution, a spinneret, and a coagulation bath [12].Wet spinning can used to fabricate high-molecular-weight polymers; molecular weight has a substantial influence on the tensile strength of the fibers [13].The diameter of the fabricated fiber was then characterized using an optical microscope.The strength of a fiber depends on its diameter [14].When the concentration of the solution increases, large-diameter fibers may develop [15].Furthermore, the fabricated fiber was characterized via Fourier transform infrared (FTIR) spectroscopy to determine the functional groups that exhibit the properties of nylon 6 and via scanning electron microscopy (SEM) to examine its surface morphology [16,17].
This study systematically investigated the structural and morphological characteristics of the nylon 6 fiber.
Moreover, this study aimed to investigate the fabrication of nylon 6 fiber based on a reproducible, simple, and cost-effective method and its potential as an alternative reinforcing material for fabricating direct dental bridges.

Materials and Methods
Herein, a wet-spun fiber was fabricated using nylon 6 pellets (25038-54-4) (Sigma-Aldrich).Formic acid with 98%-100% purity (64-18-6) was utilized as a solvent (Merck), and distilled water was employed as a coagulant.The nylon 6 fiber was fabricated using various concentrations of polymer solutions based on the wet spinning method reported in a previous study by Ko et al.'s study (2010) [18].First, nylon 6 pellets were dissolved in 10 mL of 98%-100% formic acid under magnetic stirring at a speed of 350 rpm and room temperature (25 ℃) for 1 h.Five samples of nylon 6 with different weights (g) (1.0, 1.5, 2.0, 2.5, and 3.0 g) were used to prepare five solutions having different concentrations (10%, 15%, 20%, 25%, and 30%), respectively.Each of these homogeneous solutions was then transformed into a fiber via wet spinning and transferred to a 1 mL syringe fitted with a spinneret needle no.23.After the wet spinning speed was set to 250 mL/h, the machine was connected to a syringe containing a nylon 6 solution and switched on to pump the solution into a distilled watercontaining coagulation bath.Fibers were then extracted from the coagulation bath using tweezers and washed using demineralized aqua.After drying, the fibers were rolled.The same procedure was used to produce the five different fiber samples.

Figure 1. Schematic Workflow of the Fabrication and Characterization of the Nylon 6 Fiber
The morphology and structure of the fabricated fibers were examined.First, sample images were taken using a digital optical microscope Dino-Lite Premier AM3111/ 3113, and their diameter was measured using the Dino Capture 2.0 application.Chemical structural analysis using Bruker-Tensor II Instrument for Fourier transform infrared (FTIR; Bruker-Tensor II Instrument, Billerica MA, United States) spectroscopy was performed to demonstrate the functional groups on nylon 6 fiber.The morphology of nylon 6 fiber was examined using a Scanning Electron Microscope (SEM; Hitachi SU3500, Tokyo, Japan).The workflow is shown in Figure 1.
Characterization data in the form of digital images and graphics are crucial in understanding and analyzing the properties of materials.The results for the collected sample variants were then described.By using advanced imaging techniques and analytical tools, the researchers further examined the microstructural details of the samples.These visual representations captured the structural and morphological features of the materials and revealed valuable information about their physical properties.A comprehensive procedure was carried out in February 2022 at the BRIN Advanced Materials Research Center (Pusat Riset Material Maju BRIN) located in Tangerang, West Java, Indonesia.

Results and Discussion
The choice of material to be used in a dental treatment is related to the ability of the material to resemble the physical, mechanical, and aesthetic properties of the natural tooth structure [19].In dentistry, FRC has been used for various clinical applications, especially in prosthodontics to restore missing teeth [3].The chemical structure as well as the chemical and physical characteristics of a fiber and the chemical composition of a matrix have an influence on the strength of the bond between the fiber and matrix [14].Moreover, the morphology of a fiber substantially affects its physical characteristics, particularly in polymer-based fibers [20].
The wet spinning method was used to fabricate fibers using five different concentrations of nylon 6 polymer solutions.Digital images of the nylon 6 fibers obtained using an optical microscope demonstrate that the fiber diameters vary.The surface of the fiber in sample 1 (Figure 2A) appears white and has a reasonably uniform diameter of 271 ± 5.57 µm, although the fiber surface does not appear as smooth as the surface of the fibers obtained using other concentrations.Meanwhile, Figure 2B illustrates the large variation in fiber diameters in the microscope data from sample 2, and the diameter is 284 ± 89.03 µm.The diameter of sample 3 (Figure 2C) is 279 ± 37.27 µm, which is similar to that of sample 2. In contrast to the other samples, the fiber surface of sample 4 (Figure 2D) seems whiter, more homogeneous, and smoother.The diameter of sample 4 is approximately 330 ± 22.14 µm.Sample 5 (Figure 2E) resembles the other samples with respect to fiber appearance, and its diameter is 268 ± 56.00 µm.The diameters of each fiber are different at three different points; however, the diameter of each fiber sample is nearly uniform.The fiber irregular shape of the fiber accounts for the slight difference in diameter [15].The average diameter measurements related to the five samples of the nylon 6 fiber (Figure 3) were based on the measurements taken at three different parts of the fibers.Figure 3 shows a slight increase in fiber diameter.The diameter of the fiber increases from fiber sample 1 to fiber sample 4; however, the diameter of fiber sample 5 decreases.This finding is consistent with the reports in a study by Chowdhury, M [15].on a nylon 6 fiber fabricated via electrospinning, using formic acid concentrations ranging from 15% to 25%.Chowdhury, M stated that increasing the concentration of the solution can lead to the production of fine fibers with larger diameters.Furthermore, the average diameter size in this study (Figure 3) spans from 268 to 330 µm.According to Causin, V. [21], the typical diameter of nylon fiber used for reinforcement (FRC) is between 3 and 500 µm.According to Jin et al., average diameters of the fibers fabricated via wet spinning and electrospinning are 248.5 and 0.22 µm, respectively [22].
FTIR spectroscopy was used to interpret wave crest to ensure that the produced fiber was made of nylon.Infrared spectroscopy is an essential tool for analyzing molecule structures and their interactions [23].Figure 4 shows the FTIR spectra of the five samples at various concentrations.The infrared spectra are identical for all five nylon 6 fiber samples, indicating similar wavenumber positions for each chemical bonding.The presence of amide groups in nylon 6 is reflected by a strong band at about 1635  −1 and a broad band at approximately 3300  −1 [24].Figure 4 presents the characteristics of the polyamide group in nylon 6.The absorption peak that emerged 3295  −1 in the five samples is ascribed to the N-H stretching vibration.The peaks at 1635  −1 in samples 1, 2, 4, and 5 and the peak at 1636  −1 in sample 3 are attributed to C=O stretching.The peak at 3295  −1 indicate the vibration of the N-H group, and those at 1635 and 1636  −1 indicate the presence of a C=O group [25].This finding is in line with the study of Liu et al. [26] on nylon 6 fiber, which revealed an absorption peak at 3300  −1 due to the stretching of the N-H group.Zhang et al. [23] supported this theory by reporting the formation of peaks at 3294 and 1635  −1 for pure nylon 6 fiber as measured by FTIR spectra.
The peaks at 2934  −1 for samples 1, 4, and 5 and at 2935 and 2864  −1 for samples 2 and 3, respectively, are characteristic of C-H stretching.Vedamurthy et al. [27] analyzed pure nylon 6 samples and revealed peaks at 2860 and 2930  −1 generated by the vibration of the September 2023  Vol.27  No. 3 C-H group.The peak at 1538  −1 shows N-H deformation and C-N stretching.This result is consistent with the study of Liu et al. [26] on pure nylon 6 fiber samples demonstrating N-H deformation and C-N stretching at the peak of 1540  −1 .The characteristic peaks in the FTIR spectra of the five samples confirm the presence of nylon 6. SEM images show in-depth details of the nylon 6 fiber's morphological appearance.The cross-sectional view of the fiber surface is represented by an SEM micrograph with a magnification of 50 µm.A pore size of 1-2 µm was measured from the transverse direction of the sample (Figure 5).SEM was applied to explore the morphology of nylon 6 prepared by wet spinning.Figure 5 illustrates the fiber's cross-section, which exhibits an irregular and porous fiber form.Ko et al. [18] explained that in manufacturing a mixture of SF/nylon 6 fibers, the higher the nylon 6 ratio, the more irregular the fiber shape and the less smooth the surface.Furthermore, Gupta et al. [28] reported the porosity of electrospun nylon 6 fiber as seen in its cross-section SEM image.In their study, the calcium concentration increased with the number and size of pores [28].The type of fiber studied by Sana et al. [29] has a porous and rough surface that is beneficial for good adhesive bonding and affects friction between the fiber and matrix.However, this result contradicts the findings of Pan et al. [30], who found that natural fiber, which is porous, has the potential to absorb and store water, causing fiber degradation and a decrease in mechanical qualities.Hence, the presence of porosity is considered unfavorable.The fracture resistance feature of fiber is crucial in its usage in dentistry [31].The binding strength between the fibers and the matrix significantly influences the success of dental restorations and the support for fractureresistant qualities [32].A flexural strength test must be evaluated to determine the material's ability to withstand deflection when subjected to a load [33].Therefore, further investigation of the fiber's flexural strength test is needed to evaluate the fiber strength and the effectiveness of the alternative material for direct dental bridge treatment.In future research, researchers must conduct strength tests, such as flexural strength test, for fiber strength assessment to support its efficacy in direct dental bridge treatments.

Conclusion
The nylon 6 fiber was successfully fabricated via wet spinning using water as the coagulant and polymer solutions with five different concentrations (10%-30%).FTIR spectroscopy confirmed that the prepared fiber possesses nylon 6 characteristics.Characterization based on optical and SEM analyses showed that the nylon 6 fiber has a nearly uniform diameter and morphological shape, suggesting its potential to be used as an alternative reinforcement material for fabricating direct dental bridges.