A study on the mechanical and antimicrobial properties of biomimetic shark skin fabrics with different denticle size via 3D printing technology

Although 3D-printed shark skin fabrics for functional textiles have been of great interest recently in both academic and industrial research, there is still a lack of understanding in the effect of denticle size on fabric performance. Up to date, the smallest size of 3D printed denticles on biomimetic shark skin fabrics previously reported was still larger than that on shark skin found in nature (100–500 μm). In this study, smaller denticles ranging from 0.65 mm to 1.30 mm were fabricated to decorate on a smooth film using a Form3 3D printer, resulting in biomimetic shark skin fabrics. The effect of denticle size on mechanical properties (stiffness, Young’s modulus, tensile strength, and breaking elongation) and antimicrobial properties of the biomimetic shark skin fabrics were evaluated to assess the applications in functional clothing. The results suggested that when the size of the denticles was decreased, the stiffness of the fabrics was increased. The tensile strength and the breaking elongation of the fabric with 1.04 mm denticles were larger than those of fabric materials used in commercial swimwear. In antimicrobial testing, the shark skin fabrics with 0.65 mm and 1.04 mm denticles were found to be less susceptible to bacterial attachment, suggesting great potential for functional clothing applications.


Introduction
Biomimicry is to create innovative solutions for human challenges by emulating nature's time-tested strategies [1]. It has been recently recognized as an innovative strategy for developing functional clothing and textiles. One biomimetic invention of great interest for the apparel industry is biomimetic shark skin fabrics that demonstrate exceptional characteristics such as water drag reduction, mechanical and antimicrobial properties [2][3][4]. A shark in the natural environment is covered with scales that are called dermal denticles [5]. Each denticle has a 3-prong skin structure with specific size and spacing between adjacent denticles, parallel to the direction where the shark swims [6]. The 3 prongs structure has been found to prevent vortexes from forming as well as staying on the shark skin surface and hence can reduce the dragging force when sharks move in the water [7]. Biomimetic shark skin fabrics have been previously reported using 3D printing technology [2,5,8]. Domel et al [5] and Wen et al [6,8] completed the analysis of the hydrodynamic properties of a biomimetic shark skin fabric covered with denticles under static and dynamic conditions and demonstrated that it was more effective in reducing drag than smooth control surfaces. The previous studies have suggested that the integration of shark skin characteristics into fabrics could be significantly beneficial to functional clothing such as swimming wear [9].
Bacteria spread quickly in swimming pools. Although chlorine is commonly used to prevent bacteria from spreading widely in the pool, water contamination from bacteria such as cryptosporidium, E. coli, and legionella is Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
still not in great control, resulting in swimming-related illness such diarrhea, vomiting, skin rash, cough, and congestion [10]. It is known that shark skin is antimicrobial because the 3 prongs structure on the surface of shark skin has the characteristics of preventing external objects from being attached, resulting in prevention of bacterial contamination and breeding on shark skin [4]. The principle of antimicrobial property on a riblet structure is that it has a thin air layer hidden in the surface of shark skin due to the microstructure. The air layer can act as a physical barrier to prevent the adhesion of bacteria [11]. The antibacterial properties of shark skin are very attractive in the application of functional clothing such as swimming wear. Rostami et al. (2021) prepared a solution using chitosan solution, lactic acid, ampicillin sodium salt, and caffeic acid phenethyl ester and used the solution to create a biomimetic shark skin surface via a solution casting method. The biomimic shark skin surface was found to reduce the growth of bacterial biofilm by 11.1% to 19%. Purandare (2020) reported that 3D printed sharkskin fabrics made by polyurethane photopolymer resin demonstrated more than 14% bacterial reduction compared with copper foil that is a common antimicrobial agent used in health-care industry. Because the antimicrobial effect of the biomimic shark skin fabric as a physical barrier is non-toxic and harmless to the environment due to no additional chemicals added on, the application of biomimic shark skin fabrics in functional clothing offers sustainable solutions to textile and apparel innovation.
Wen et al [6] 3D printed shark skin fabric with a denticle size of about 1.87 mm with different spacing arrangements of denticles and suggested that denticles arranged as the staggered-overlapped pattern was the most efficient in reducing drag forces. The size of the denticle found in shark skin in nature is between 100 μm and 500 μm [5]. Domel et al [5] demonstrated the 2 mm denticles promoted the most drag reduction. The smallest denticle size created by 3D printing for biomimetic fabrics to date was 2 mm that is approximately tentime larger than that in nature. Therefore, exploring the performance of biomimetic shark skin fabrics with denticles that are close to natural size can help maximize the benefits of biomimicry for functional clothing applications. In addition, the use in functional clothing requires flexibility and good hand in textile fabrics, suggesting the smaller size of denticle offers the better hand and flexibility in the biomimetic shark skin fabrics.
The primary objective of this study was to provide implication of the effects of denticle size on the fabric performance characteristics of stiffness, mechanical, and antimicrobial properties. In this study, biomimetic shark skin fabrics were created using a commercial resin (Flexible80A, Formlabs, Somerville, MA) that contains 75%-95% methacrylate monomer, 3%-6% urethane dimethacrylate, and <1.5% photoinitiator and Form3 SLA 3D printer (Somerville, MA). The fabrics were made using a thin film substrate decorated with small denticles simultaneously printed on the film via 3D printing. The denticles were designed in sizes smaller than 2 mm. The thickness of the thin films was set at 0.3 mm that was found in the range of fabric thickness of commercial swimsuits. The printed fabric swatches were characterized for three textile performance properties, including stiffness performance, mechanical properties (specifically, Young's modulus, tensile strength, and breaking elongation), and antimicrobial properties. First, the optical images of the samples were captured and used to analyze the structure of denticles in three different sizes. Secondly, the stiffness of the samples was tested in a customized method. The results indicated that the stiffness of the sample increased as the denticle size of the sample decreased. In addition, mechanical tests showed that the samples with medium-sized denticles had higher tensile strength and elongation at break than commercial swimsuit fabrics such as nylon, representing their potential for swimwear applications. Finally, in the antimicrobial test, the samples with medium-and small-sized denticles exhibited antimicrobial properties compared to control samples (without denticles).

3D Models development and 3D printing
Previously, an Environmental Scanning Electron Microscope (ESEM) was used to photograph the mid-trunk position morphology and dimensions of freshly dead mako shark (Isurus oxyrinchus) denticles [8]. In this study, Wen's model was adopted to create 3D models of sharkskin fabrics with denticles. First, Autodesk Fusion360 (Autodesk, Inc.) was used to develop a 3D model of a single denticle with precise parameters. Second, the single denticle model was arranged into the staggered-overlapped pattern [6] on a 50 mm × 30 mm × 0.3 mm (length ×width × height) film that creates the base layer of shark skin fabric. Biomimetic shark skin fabric swatches with the same staggered-overlapped pattern, but different denticle sizes were developed via Autodesk Fusion360. Figure 1 and table 1 show the dimensions of large, medium, and small denticles and spacing dimensions between denticles. The spacing between two adjacent denticles was decreased with a decrease in the denticle size was decreased. This was because the density of denticles (/mm 2 ) was kept as consistent as a previous study of denticle distribution on real sharks by Kanagusuku et al's [12]. The denticle sizes and the spacing between two adjacent denticles were proportionally determined as shown in table 1.
In the printing step, a Form3 3D printer (Formlabs, Somerville, MA) was used to fabricate shark skin fabric swatches. Because the surface of shark skin fabric is covered with a complex microstructure, a photopolymerization method is utilized in the Form3 printer to fabricate the fabrics. The Form3 printer has a resolution of 25 microns on the X-axis and Y-axis, which can ensure that the details of the shark denticle are printed to the greatest extent. In printing shark skin fabrics, the film and the denticles were simultaneously printed with a Flexible 80A resin. Flexible 80A is an acrylate-based resin, which is commonly used and already is compatible with most 3D printers using a photopolymerization mechanism on the market. After printing was completed and the bae supports were removed, the printed part was washed with tripropylene glycol monomethyl ether (TPM) for 10 min to remove waxiness, and then cured for 10 min at 60°C in the Form Cure (Formlabs, Somerville, MA).

Morphological analysis
Investigation of biomimetic shark skin fabric swatch morphology was carried out using an optical microscope (Motic Digital Microscope DMB3-223). The motif image plus 2.0 software (Motic Instruments Inc., Canada) was used to capture images for morphological analysis. Denticle size and spacing were measured using Image J software.

Stiffness testing
According to AATCC TM66 (Test Method for Wrinkle Recovery of Woven Fabrics: Recovery Angle) and ASTM standard D1388-08 (Standard Test Method for Stiffness of Fabrics), a customized method was developed to test stiffness of the biomimetic shark skin fabric swatches using a wrinkle recovery tester (Momsanto Chemicals,  Leverkusen). In sample preparation, fabrics were cut into a rectangle shape (15 mm×40 mm). The three-prong of the denticle was parallel to the long side of the sample. Most of the fabric samples were not flat, so a 4-pound weight was used to flatten fabric samples for 24 h. Figure 2 shows a fabric sample being tested using the wrinkle recovery tester. In the stiffness testing, first, a sample was squarely aligned in a 15 mm wide. Second, as shown in figure 2, the short straight line of the circular dial was oriented to the 90°indicator on the dial to ensure the clamp was completely perpendicular to gravity's force. When the sample on the clamp fell due to gravity, a mark was made on the circular dial where the sample hung before the clamp was removed. Then, a straight line was drawn to connect the mark with the center point of the dial and extended to the edge of the dial. The dial was rotated 180• to enable the short straight line to eclipse the original straight line below the tester; the position of the clamp remained horizontal. The angle indicated by the numbers on the dial was recorded to measure the stiffness of the fabrics. The smaller the angle of bending, the stiffer the sample was. Each result was the average mean of five specimens (N = 5).

Mechanical properties analysis
In this study, the tensile strength, elongation at break, and Young's modulus of biomimetic shark skin fabrics were tested using Instron 4442 Mechanical Tester (Norwood, MA). The testing was conducted by following ASTM standard D412-16 (2021) (Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers -Tension). The samples were cut into dumbbell shapes and placed on the grips. The grip separation speed was 500 ± 50 mm min −1 . The stress-strain curve and load-extension curve were recorded by Bluehill 2.0 software. As shown in the figure 3, the fabrics were tested in both lengthwise (3 prongs of the denticle parallel to vertical direction) and widthwise (3 prongs of the denticle parallel to horizontal direction) directions. In figure 3(A), the denticles are all aligned in a straight line in the lengthwise direction, while the denticles are arranged in a zigzag pattern in the widthwise direction. Each result was the average of five specimens (N = 5). All 30 tests (3 denticle sizes × 2 directions × 5 repetitions) were conducted at room temperature.

Antimicrobial properties analysis
The antimicrobial properties of the biomimetic shark skin fabrics with different denticle sizes were evaluated using an immersion inoculation assay method described by Mann et al [13] and Purandare [3]. The prevention of bacterial growth was measured by the area occupied by bacteria after incubation. Statistical analysis was performed using ANOVA tests. 3D printed fabrics with smooth surfaces (no denticles) were used as control samples in the tests. All 3D printed fabric samples were cut into 1 cm×1 cm squares. Five replicates (N = 5) for each sample were used for statistical analysis. First, a LB culture media solution was used to inoculate and to dilute E. coli ATCC25922. The LB solution was made of 950 ml of DI water, 9.5 g of tryptone, 4.75 g of NaCl, and 4.75 g of yeast extract. The solution was autoclaved at 273°F and 28 psi for 1 h. Two premade E. coli ATCC25922 pellets (1.0 × 104 cfu/pellet) were put in a hydration liquid and mixed by shaking and, then hydrated for 30 min at 37°C. The hydrated E. coli was inoculated in a 250 ml LB media solution. The mixture was incubated at 37°C for 24 h in an incubator (Fisher, Waltham, MA). The incubated E. coli bacteria were diluted at 1:100 with a 700 ml LB media solution. The diluted solution was sub-cultured in the incubator at 37°C for 4 h.
A soft agar solution was made by mixing 700 ml of DI water, 7 g of tryptone, 3.5 g of NaCl, 3.5 g of yeast extract, and 6.3 g of agar powder. The solution was autoclaved at 273°F and 28 psi for 1 h before the solution was added into 20 sterile petri dishes, covered and kept at room temperature for 2 h. The soft agar plates were stored in the refrigerator at 4°C for the following experiments.
1 L of 1 x phosphate buffered saline (PBS) was made using 800 ml of DI water, 8 g of NaCl, 0.2 g of KCl, 1.44 g of Na2HPO4, and 0.245 g of KH2PO4 mixed well. The solution was adjusted to pH value 7.4. More DI water was added to increase the volume to be 1 L.
Before E. coli was introduced to the shark skin fabric samples, each sample was firmly adhered to the bottom of a petri dish with the side with denticles facing up. The sample fixed in the petri dish was sterilized with 95% ethanol for 10 m and then was rinsed 3 times with DI water before it was allowed to air dry for 1 h. According to the immersion inoculation assay method [13], first, a sub-cultured E. coli solution was poured into petri dishes, and samples were submerged and stored at room temperate for 1 h. Second, the E. coli that were not attached to the samples had to be removed by rinsing with 1 × PBS for 10 s while rotating 3 times. Third, the petri dishes with samples were allowed to air dry at room temperature for 1 h. Each sample was taken from the petri dish and pressed onto a soft agar plate for 5 s to minimize air bubbles between the sample surface and the agar. The soft agar plates, then, were incubated at 37°C in the incubator for 24 h. After the incubation, each soft agar plate was photographed. The area covered by the bacteria were measured using Image J software (Purandare, 2020).

Morphological analysis
The 3D model of the denticle is a 3-prong structure that consists of a centrally-located long prong paralleled by two short prongs, one on either side, extending from the same main body creating a trident shape. Figure 4 shows optical images of shark skin fabrics with different size denticles. Table 2 shows denticle size and spacing of the fabrics as printed. It was found that the large denticles of 3D printed shark skin fabric (figure 4(A)) had same morphology as the 3D model that contains the completed 3-prong structure. However, the medium denticles as shown in figure 4(B) were not printed as precisely as large ones according to the 3D models. Although the 3-prong structure was still presented, the two laterally-located short prongs were not fully printed, resulting in shorter prongs than what were shown in the 3D model. The denticle was proportionally imbalance, which resulted in a longer centrally-located prong that is highlighted. It was most likely due to the printer resolution limitations. When the denticle size continued to decrease, the 3D printed shark skin fabrics started to lose the unique 3-prong structures. Figure 4(C) shows the morphology of small denticles, which is quite different from the 3D model. The resolution of the Form3 3D printer limited the presentation of such a small 3-prong structure. Only the most obvious feature, the central long prong, was presented. The prongs on both sides of the small-sized denticle merged to the prong in the middle, and the printed denticle no longer resembled the shape of the designed 3-prong structure. Figure 5 shows bending angles of shark skin fabrics pressed, which therefore illustrated the effect of denticle size on stiffness of shark skin fabric. Data was analyzed by ANOVA at a 5% significance level. There was statistical significance for the main factor denticle size (p < 0.05) and denticle density (P < 0.05), for stiffness. The fabric with large denticles had the largest bending angle, suggesting that it was the softest fabric. The fabric with small denticles showed the smallest bending angle, suggesting it was the stiffest fabric. The results may be greatly associated with different denticle density due to different denticle size. Denticle density is defined by the number of denticles in one cm 2 of the fabric as shown in figure 6. It was found that the fabrics with small denticles had a high denticle density and small space between denticles. Therefore, the overall thickness of the fabrics with small denticles may be larger than the other two fabrics, which made it difficult to bend, resulting in the small bending angle in the fabrics with small denticles. On the other hand, medium-and large-sized-denticle samples had less  denticle density and more space between denticles, which lent to the softness of the sample. In a summary, the large denticles that had large spacing between adjacent denticles promoted the stiffness of the shark skin fabrics.

Stiffness performance analysis
To compare the shark skin fabrics with currently used fabrics in swimwear, a nylon spandex blend was also measured using the same testing method and the bending angle was 53.8 ± 3.03 degrees. Therefore, the 3D printed fabric in this study was much stiffer than the nylon spandex blend. The stiffness of shark skin fabric may reduce the comfort level of the functional clothing if it was used, which is still a challenge of using 3D printed shark skin fabric in functional clothing. Figure 7 shows the stress-strain curves of biomimetic shark skin fabrics with different denticle sizes in lengthwise and widthwise directions. Table 3 shows three mechanical properties including tensile strength, young's modulus, and breaking elongation, which are critical to textile performance testing. First, in a comparison of lengthwise and widthwise directions, all of the fabrics in the widthwise direction had higher tensile strength, Young's modulus, and breaking strength, than those in the lengthwise direction, suggesting mechanical anisotropy in the 3D printed fabrics. The mechanical anisotropy of the biomimetic shark skin fabrics with different denticle sizes was about 31%-56%. Mechanical anisotropy was previously reported in polymeric parts fabricated using additive manufacturing techniques, such as DLP [14]. Zohdi and Yang [14] have reported that the mechanical anisotropy of a part made by DLP was around 5%. The higher percentage was given because denticles were arranged in the staggered-overlapped pattern, which included the denticles aligned in a straight line in the lengthwise direction and the denticles arranged in a zigzag pattern in the widthwise  . Denticle density of biomimetic shark skin fabric with large-sized denticles (A. 49 denticles/cm2), fabric with medium-sized denticles (B. 72 denticles/cm2), and fabric with small-sized denticles (C. 154 denticles/cm2). direction. The denticles in a zigzag pattern in widthwise direction might be more favorable in mechanical testing than the denticles in a straight line in the lengthwise direction, resulting in better mechanical properties in the widthwise direction than in the lengthwise direction. Additionally, the mechanical anisotropy might be due to the uneven thickness of the fabric base layer produced during 3D printing. Figure 8 shows (a) tensile strength, (b) Young's modulus, and (c) breaking elongation of the shark skin fabrics with different denticle sizes. The tensile strength of the fabric with the small denticles in the widthwise direction was found greatest as shown in figure 8(a). When the denticle size was increased by 50% from small denticle to large denticle, the tensile strength was decreased by 29.67% from 8.73 MPa to 6.14 MPa. When the denticle size was increased by 20% from medium denticle to large denticle, the tensile strength was decreased by 18.35% from 7.52 MPa to 6.14 MPa. Therefore, the tensile strength increased with a decrease in the denticle size. On the other hand, in the lengthwise direction, the fabric with medium denticles had the highest tensile strength and breaking elongation. The shark skin fabric with medium size denticles had the most elongation at the point of structural failure in the lengthwise direction. In the lengthwise direction, the elongation at break increased 7.17% for the medium denticle from that for the small denticle, and decreased 19.07% for the large denticle from that for the medium denticle. In the widthwise direction, the fabric with small denticles had highest elongation at break. The small denticle size could help increase the breaking elongation of the fabrics in the widthwise direction more than the other sized denticle fabrics. Therefore, there may be a certain denticle size that capitalizes on the opposing properties of smaller and larger denticles' tensile strength. However, this critical point was not easily recognizable within the measured size range in the widthwise direction due to its mechanical anisotropy. The shark skin fabric with medium size denticles had the highest Young's modulus as shown in figure 8(c), suggesting high tensile stiffness in lengthwise and widthwise directions [15]. In the lengthwise direction, the Young's modulus of the fabric with medium denticles was 10.26% and 11.79% higher than that of  small and of large denticle fabrics, respectively. On the other hand, in the widthwise direction, the Young's modulus of medium denticle fabric was 17.00% and 20.88% higher than that of small and large denticle fabrics, respectively. The Young's modulus of the shark skin fabric with the same size denticles was different in lengthwise and widthwise directions primarily due to mechanical anisotropy of the 3D-printed fabrics.

Mechanical properties analysis
The results of mechanical testing of biomimetic shark skin fabric suggested that the 3D printed shark skin fabrics had the potential to be integrated into competitive and commercial swimwear designs. Typically, fabrics are mechanically anisotropic due to the weaving structure and fiber content (Klevaityte · & Masteikaite · , 2007). Therefore, mechanical anisotropy of 3D printed shark skin fabrics would not be an obstacle to potential applications in functional clothing. According to Manshahia and Das [16], nylon or polyester fibers that are often used in swimwear have a maximum tensile strength around 5.4 Mpa. The minimum tensile strength of the medium-sized-denticle shark skin fabric was 5.74 Mpa. The biomimetic shark skin fabric elongation was between 154.12% and 200.13% while the maximum elongation of commonly used nylon-spandex blends is only 40.49% [15]. Therefore, the 3D printed biomimetic shark skin fabrics had higher tensile strength and elongation than the fabrics currently used in the swimwear market, indicating a great potential as an innovative fabric for sportswear applications.

Statistical analysis
The testing results were subjected to 2-way ANOVA tests and hence assessed to determine the effects of denticle size and mechanical anisotropy on the properties of biomimetic shark skin fabric. It was found that there were significant differences in tensile strength, Young's modulus, and elongation at break showed by the fabrics with different denticle sizes at 0.05 level (p < 0.05). However, the differences between lengthwise and widthwise directions of the fabrics were not statistically significant (p0.05). In addition, the denticle size and the direction did not have combined effect on the tensile strength, Young's modulus, and elongation at break (p0.05).

Antimicrobial properties analysis
The immersion assay method was used to evaluate antimicrobial properties of the 3D printed shark skin fabrics. Figure 9 shows the results of a typical antimicrobial testing. The photographs show E. coli growth area after 24 h for the fabrics with different size denticles as well as the control fabric sample (without denticle). The control fabric sample's E. coli growth area was larger than the fabrics with medium and small denticles, but smaller than the large-sized denticle fabric. The E. coli growth area for the fabric with large denticles was found to be large as shown in figure 9(B), while the fabrics with medium and small denticles had the small E. coli growth area. Figure 10 shows the average of bacterial growth area on each sample. The area of bacteria growth was reduced from 6.01 cm 2 in control groups to 2.41 cm 2 in samples with medium denticles and to 2.25 cm 2 in samples with small denticles, respectively. The reduction in comparison with the control sample was 60% (p < 0.05) with medium denticles and 63% (p < 0.05) with small denticles, respectively. The results were statistically significantly different, suggesting good antibacterial properties demonstrated by the small and medium denticles. On the other hand, the bacterial growth area was increased by 42% (p<0.05) on the 3D printed shark skin fabric with large denticles compared to smooth controls. However, the increase in bacterial growth was not statistically significant, suggesting that the antimicrobial behaviors of the control group and the sample with large denticles were not significantly different. In addition, the statistical comparison between the medium denticles and the small denticles was not significantly different (p>0.05) either. It was probably because the 3-prong structure of the small-sized denticle was not completely and accurately printed due to the resolution of the printer as shown in figure 10. Another hypothesis to explain the differences is associated with actual surface area of the 3D printed shark skin fabrics. When the denticle size of the fabrics was changed, the surface area of the fabric was also changed. The surface area for each fabric shown in figure 10 was theoretically estimated according to the 3D model with different denticle size. Overall, the biomimetic shark skin fabrics had higher surface areas than the control sample without denticles (1 cm2/1 cm2). Although the fabric with large denticles had the largest surface area, no statistically significant changes in antimicrobial properties were found in the fabric with large denticles in comparison with the control smooth sample. On the other hand, when the size of the denticle was changed from large to medium and then small, the surface areas of the fabrics with medium and small denticles were slightly smaller than that of the fabrics with large denticles, but still larger than that of the control sample. The spacing between adjacent denticles was reduced when the denticle size was decreased, resulting in small spaces between denticles for the fabrics with small denticles. When bacteria were introduced on the fabrics, the attachment of the bacteria to the fabric surface was likely reduced due to the steric hindrance, which was able to improve antimicrobial properties of the fabrics [11]. It is in a good agreement to the antimicrobial property results. In a comparison of the fabrics with medium denticles and small denticles, however, the differences in the surface area were trivial, which might explain why no statistical differences were found in their antibacterial testing. The results suggested that steric hindrance is essential for improving antimicrobial performance of the 3D printed shark skin fabrics. The steric hindrance was determined by the denticle size as well as the spacing between adjacent denticles. Future work could include a study of steric hindrance on antimicrobial performance of the shark skin fabrics.
In summary, samples with large-sized denticles did not demonstrate antimicrobial properties, while samples with medium-and small-sized denticles exhibited significant antimicrobial properties.

Conclusion
This study investigated the fabric performance properties of 3D printed biomimetic shark skin fabrics decorated with different size denticles (small, medium, large), including (1) stiffness performance, (2) mechanical properties, and (3) antimicrobial properties of 3D printed biomimetic shark skin fabrics. When the size of the denticle was decreased, the stiffness of the biomimetic shark skin fabric was increased. All the tested fabrics exhibited mechanical anisotropy to different degrees. The biomimetic shark skin fabric in the widthwise direction (3 prongs of the denticle parallel to horizontal direction) had higher tensile strength, Young's modulus, and breaking strength than those in the lengthwise direction (3 prongs of the denticle parallel to vertical direction). Additionally, the fabric with the small denticles had the largest tensile strength and breaking elongation. The fabric with medium-and small-sized denticles showed good antimicrobial properties, while fabrics with large denticles did not demonstrate antimicrobial properties. In a summary, the biomimetic shark skin fabrics with small denticles showed great potential for integration into swimwear production in terms of mechanical properties and antimicrobial properties, The results provided guidance in continuous development of 3D printed shark skin fabrics for functional clothing and textile innovation. Future work may include (1) fabrication strategies of creating biomimic shark skin fabrics with nanoscale denticles to further enhance the true biomimicry of textiles; (2) a study of hydrodynamic properties of the fabrics with small denticles; (3) a testing of fabric comfort using hand value measurement because fabric comfort is critical to functional clothing applications.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files). Figure 10. Antimicrobial testing results and surface area, average (n = 5) of E. coli growth area in smooth control groups (Control), 3D printed shark skin fabrics with large size denticles (Large), medium size denticles (Medium), and small size denticles (Small).