Fabrication of poly(ester-urethane)urea elastomer/gelatin electrospun nanofibrous membranes for potential applications in skin tissue engineering

Abstract

In this study, PEUU elastomer was synthesized and nanofibers with five different PEUU/gelatin ratios (PEUU/gelatin = 0 : 100, 25 : 75, 50 : 50, 75 : 25, 100 : 0, which were called PU0, PU25, PU50, PU75, PU100, respectively) were fabricated via electrospinning. All samples were treated in glutaraldehyde (GA treating) for 24 h. The SEM image results show that there is a uniform distribution for all the nanofibers and the mean fiber diameters increase with the increase in the percentage of PEUU. The FTIR spectra indicate that PEUU was successfully synthesized. The XRD results show that the GA treating process decreased the crystallinity of PEUU to some extent. It can be concluded from the mechanical results that the mechanical properties of PU100 and PU75 nanofibers are superior to other groups. Only a small amount of gelatin significantly improved the hydrophilic properties of the nanofibers. The results of cell proliferation and cell morphology indicate that the PEUU/gelatin nanofibers, particularly the PU75 group, could better contribute to the cell proliferation than the PU0 and PU100 groups, supporting the application of PU75 in skin tissue engineering. Two different types of electrospun nanoyarns were also fabricated, which showed better comprehensive properties than nanofiber, thus paving way for further research in the tissue engineering field.

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1. Introduction

Polyurethane is one of the most popular synthetic polymers applied in various fields, and in the tissue engineering field in particular.^1–4 PEUU (poly(ester-urethane)urea) is one of the most important members of the polyurethane family, and a lot of results have been reported for PEUU for application in tissue engineering.^5–15 PEUU could be used as optimal scaffold material because of its multiple properties, such as good mechanical strength, tunable properties, versatile processability, biocompatibility and biodegradability, while the hydrophilicity and biocompatibility of PEUU could be improved for application in skin tissue engineering.

Gelatin is a natural polymer that can be considered to be a cheap alternative to soluble collagen. It should be noted that gelatin possesses almost all the properties of collagen, such as good biocompatibility, excellent biodegradability, tiptop hydrophilicity, etc. Gelatin is used extensively in the tissue engineering field^16–18 and can be prepared in multiple forms, such as sponges, hydrogels and electrospun membranes.^19–33

Electrospinning is still an exclusive processing method for the continuous preparation of nanofibers.^34–36 The nanofiber membranes prepared by electrospinning can mimic the natural tissue of the extracellular matrix, thus paving way for the substitution for broken tissue. A soaring increase in related results on electrospinning biomimetic scaffolds has identified it as a simple and effective processing method for producing biomaterials and tissue engineering scaffolds.^37–41

A multitude of results has been reported for the electrospinning of synthetic PEUU and other polymers for tissue engineering application. Todd Courtney et al.⁴² synthesized PEUU by applying polycaprolactone diol, 1,4-diisocyanatobutane and putrescine as monomer, and then electrospun nanofiber membranes were fabricated for the soft tissue engineering. John J. Stankus et al.⁴³ prepared blood vessel constructs via special electrospinning equipment from PEUU. Ryotaro Hashizume et al.⁵ prepared a wet electrospun PEUU by a combination of electrospinning and electrospraying, and the scaffold was then transplanted into a rat's abdominal wall for further study. Nicholas J. Amoroso et al.⁴⁴ studied the PEUU/PCL hybrid scaffold in the heart valve tissue engineering field. Considering the high elasticity of PEUU, which has almost the same mechanical strength as natural skin tissue, it could be possible for PEUU to be prepared as a substitute for human skin. To improve the hydrophilicity and biocompatibility of PEUU, it could be blended with gelatin.

In this study, PEUU consisting of polycaprolactone diol, hexamethylene diisocyanate and butanediamine was synthesized. Electrospun membranes were prepared by blending gelatin and PEUU together. The micro structure of PEUU/gelatin is depicted in Fig. 1, and the soft and hard segments of PEUU are shown. The chemical structure, fiber morphology, mechanical strength, surface wettability, pore size, porosity, thermal properties, cell viability and morphology of the hybrid nanofiber membranes were studied.

Fig. 1 Schematic of the electrospinning equipment, and the micro structure of the PEUU/gelatin nanofibers on the molecular level.

2. Experimental

2.1. Materials

Polycaprolactone diol (PCL, M_n = 2000), hexamethylene diisocyanate (HDI), stannous octoate (Sn(Oct)₂), butanediamine and gelatin were provided by Sigma-Aldrich and used as received unless specified. 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP) was purchased from Shanghai Darui Chemical Ltd., dimethyl sulfoxide (DMSO) was obtained from J&K Chemical, L929 (rat fibroblasts) cells were acquired from the Institute of Biochemistry and Cell Biology (Chinese Academy of Sciences, China). All culture media and related reagents were supplied by Gibco Life Technologies Co. (USA), except where mentioned otherwise.

2.2. Synthesis of PEUU elastomer

PEUU was synthesized by a two-step solution polymerization, which is one of the most basic methods in the synthesis of the polyurethane family. To start with, PCL2000 (polycaprolactone diol, M_n = 2000) was dried for 12 h in a vacuum oven at 60 °C to remove residual water before synthesis and was then added to a three-necked flask with stannous octoate. HDI dissolved in DMSO was then added to the system in the nitrogen atmosphere and the reaction was carried out for 3 hours at 80 °C. The chain extension reaction was then put into effect with the dropwise addition of butanediamine. The molar ratio of PCL2000, HDI and butanediamine was 1 : 2 : 1. The polymerization process was carried out for 18 hours and the polymer solution was then extruded into the deionized water for precipitation. Finally, the polymer precipitated in the deionized water was transferred to ethanol for further purification over one day, and dried in a lyophilizer for two days. The yield of PEUU was over 90%.

2.3. Nanofiber fabrication

The nanofibers were prepared by the electrospinning process. Briefly, PEUU and gelatin with different mass ratios (PEUU/gelatin = 0 : 100, 25 : 75, 50 : 50, 75 : 25, 100 : 0, which were called PU0, PU25, PU50, PU75, PU100, respectively) were dissolved in HFIP at the concentration of 10% (w/v). The polymer solution was then made into a randomly oriented electrospun membrane, via traditional electrospinning equipment (Fig. 1) consisting of a syringe pump, a high voltage supplier and a flat collector. To keep the electrospinning process steady, the parameters were set as follows: a voltage of 12.5 kV was applied to the solution while the flow rate and the distance between the needle tip and the collector were set as 1.5 mL h⁻¹ and 15 cm respectively. The membranes containing gelatin were placed in a sealed desiccator with glutaraldehyde vapor (evaporating from 10 mL of 25% glutaraldehyde aqueous solution) at room temperature and were treated for around 24 hours. Finally, the samples were placed in a vacuum oven for over 7 days.

2.4. Nanoyarn fabrication

2.4.1. Dynamic liquid electrospun nanoyarn fabrication. Dynamic liquid electrospinning equipment was utilized for the fabrication of the nanoyarn tissue engineering scaffold.⁴⁵ It was also applied in this study for the preparation of nanoyarn. Briefly, the PU75 nanofibers were electrospun on a water surface in a basin with a hole on the bottom. When water flowed down through the hole, a vortex was formed, which twisted the nanofiber into yarns. The yarns flowing down out of the hole were then collected by a rotating mandrel to give nanoyarns, and called PU75-DLY.

2.4.2. Conjugated electrospun nanoyarn fabrication. The conjugated electrospinning equipment (Fig. 2) was designed on the basis of traditional electrospinning equipment and is first reported in this study. In Fig. 2, PU75 nanofibers from two opposite spinning nozzles generated at positive high voltage and negative high voltage, respectively, were banded together into yarns, which were then collected by a rotating mandrel to give PU75 nanoyarn (PU75-CY). The morphology of PU75-CY is illustrated in Fig. 2.

Fig. 2 Illustration of the conjugated electrospinning equipment and morphology of the PU75 nanoyarn fabricated by using the conjugated electrospinning equipment (PU75-CY).

2.5. Nanofiber morphology

The morphology of the PEUU/gelatin nanofibers was characterized by scanning electron microscopy (SEM, TM-100; Hitachi, Tokyo, Japan). The nanofiber membranes were sputter-coated with gold for 20 s at 4 mA and the acceleration voltage was 10 kV.

2.6. Fourier transform infrared spectroscopy

Chemical analysis of the electrospun membranes were performed using a Fourier transform infrared spectrometer (FTIR, Thermo Nicolet) equipped with attenuated total reflectance (ATR) accessories, over a range of 4000–800 cm⁻¹.

2.7. X-ray diffraction

To identify the crystallinity of the hybrid nanofiber membranes before and after treating, a D/max-2550 PC-XRD instrument (Rigaku, Japan) with Cu K_α radiation (wavelength λ = 0.154 nm) was used. The voltage and current were set as 40 kV and 40 mA, respectively.

2.8. Mechanical properties

The mechanical properties of the nanofibers were characterized by a universal materials testing machine (H5K-S, Hounsfield, UK) with the ambient temperature of 20 °C and humidity of 65%. All specimens (50 mm × 10 mm, n = 5) were tested with a cross-head speed of 10 mm min⁻¹ until breakage.

2.9. Surface wettability

To visualize the surface wettability of the nanofiber membranes, the contact angle measurement equipment (OCA40, Dataphysics, Germany) was applied with a dosing volume of 0.02 mL of deionized water and 5 different parts of the samples were averaged.

2.10. Pore size and porosity

The pore size of the electrospun nanofiber membranes was measured using Image J software on the basis of the SEM images. One hundred pore diameters on different parts of the SEM images were chosen randomly, thus measuring the pore size distribution. The porosity of the membranes was tested via the ethanol infiltration method.^46–48 A slice of the membrane was immersed in the ethanol; the volume of ethanol in the measuring cylinder before and after the nanofiber membrane immersion was set as V₁ and V₂, respectively. After 15 minutes, the membrane was removed from the ethanol, and the remaining volume was marked as V₃; the porosity of the tested membranes was calculated according to eqn (1):

2.11. Cell proliferation and morphology

For each electrospun membrane, 3 parallel samples were chosen for the cell proliferation test. All the membranes with stainless rings were put into the 24-well plates individually, and were sterilized by putting them in a sealed desiccator with 75% ethanol for 24 hours. The membranes were then washed with PBS (phosphate-buffered saline) solution more than 3 times and were washed with culture medium one more time before seeding cells. L929 cells were cultured in DMEM (Dulbecco's Modified Eagle's Medium) with 10% FBS (fetal bovine serum) and 1% antibiotic–antimycotic in an atmosphere of 5% CO₂ at 37 °C, L929 cells were seeded with a density of 1.0 × 10⁴ cells per well and the culture medium was replaced every two days. The MTT (methyl thiazolyl tetrazolium) test was performed at three time points of 1 d, 4 d and 7 d. At each time point, the samples were taken out from the CO₂ (carbon dioxide) incubator, and were washed with PBS after the removal of the culture medium. Then, 400 μL of MTT and culture medium with the volume ratio of 1 : 9 were added to the well and the samples were again incubated for 4 hours in the CO₂ incubator. Four hours later, the precipitates were dissolved in DMSO and the absorbance, which represents the cell viability, was tested using a microplate reader at the wavelength of 490 nm.^15,17,18,36

L929 cells cultured for about 4 days were dehydrated with ethanol of different concentrations (10%, 30%, 50%, 60%, 70%, 80%, 90%, 95% and 100%). The membranes were washed with PBS following the dehydration process. After that, the membranes were lyophilized for about 3 days and were sputter-coated with gold so that the cell morphology could be illustrated from the SEM images.

To identify the cell morphology, L929 cells were stained by rhodamine-conjugated phalloidin (Invitrogen, USA) and 4,6-diamidino-2-phenylindole hydrochloride (DAPI, USA). After culturing for 3 days, the cells were fixed in 4% paraformaldehyde for about 1 hour. The samples were then washed with PBS more than 3 times and immersed into 0.1% TritonX-100 (Sigma, USA) solution for around 5 minutes. After that, the samples were washed again, and then DAPI and phalloidin were added for the staining of the nuclei and cytoplasm of the cells, respectively.

2.12. Statistical analysis

Origin 8.0 (Origin Lab Inc., USA) was applied for statistical analysis. All the values were expressed as means ± standard deviation (SD). Statistical differences, determined by the one way analysis of variance (ANOVA), were considered significant at p < 0.05.

3. Results and discussion

3.1. Morphology and structure

The SEM images of nanofibers with different mass ratios are shown in Fig. 3, from which it can be seen that there is no significant difference in the morphology of the different nanofibers, which indicates that all of these random nanofiber membranes could mimic the extracellular matrix, without considering the components. It can be concluded from the picture of the fiber diameter distribution that there is a uniform distribution for all the nanofiber membranes and the mean fiber diameters increased with the increasing percentage of PEUU.

Fig. 3 SEM images (a–e) and fiber diameter distribution (a′–e′) of PEUU/gelatin nanofibrous membranes with different ratios. PEUU/gelatin (a and a′) 0 : 100; (b and b′) 25 : 75; (c and c′) 50 : 50; (d and d′) 75 : 25; (e and e′) 100 : 0. The scale bar for all these SEM images is 50 μm.

3.2. Fourier transform infrared spectroscopy

FTIR spectroscopic studies of the PEUU/gelatin series nanofibers after GA treatment were carried out via ATR. The main chain of PEUU contains masses of CH₂ parts, including both the hard and the soft segments. As is shown in Fig. 4, the structure of the CH₂ parts could be identified by the peaks at 2937.13 cm⁻¹ and 2862.93 cm⁻¹, which represent the stretching vibrations of H–C–H. The strong peak at 1729.89 cm⁻¹ represents the stretching vibration of C=O, and the peak at 1571.71 cm⁻¹ represents the N–H deformation vibration, the broad peaks at 1161.09 cm⁻¹ and 1236.32 cm⁻¹ represent the stretching vibration of C–O–C; all these peaks indicate the PEUU structure formation. The broad peak at 3335.88 cm⁻¹ represents the stretching vibration of O–H, probably due to the ethanol remaining from the post precipitate treatment process during the synthesis of PEUU. With the increasing gelatin content, as shown in Fig. 4, the intensities of the peaks at 2937.13 cm⁻¹, 2862.93 cm⁻¹, 1729.89 cm⁻¹, 1161.09 cm⁻¹ and 1236.32 cm⁻¹, which represent the structure of PEUU, decrease while the intensities of the peaks at 3500–3300 cm⁻¹ and 1700–1300 cm⁻¹, which represent the structure of gelatin, increase gradually.

Fig. 4 Fourier transform infrared spectra of PEUU/gelatin nanofibrous membranes with different ratios.

3.3. X-ray diffraction analysis

The XRD (X-ray diffraction) of the series of nanofiber membranes was tested. A sharp peak that may represent the crystalline structure of the soft segment in PEUU appeared at around 21° (2θ). The crystalline peak of PU100 (Fig. 5d) shows no significant difference before and after GA treating, which indicates that there is no need for the pure PEUU to have a treatment. Meanwhile, the nanofiber membranes containing gelatin (Fig. 5c, set PU75 as an example) show great difference before and after GA treating, which demonstrates that the treating process decreases the crystallinity of PEUU to some extent. The reason might be that the appearance of new bonds during the GA treating process of gelatin disturbed the regularity of the soft segment in PEUU.

Fig. 5 XRD profiles of PEUU/gelatin nanofibrous membranes with different ratios before (a) and after (b) GA treating. XRD profiles of PU75 (c) and PU100 (d) before and after treating with glutaraldehyde.

3.4. Mechanical properties

The mechanical properties of the nanofiber membranes with different PEUU and gelatin mass ratios after GA treatment in dry and wet conditions (immersed in water for 3 hours) were tested separately. It can be seen from Fig. 6 that the mechanical properties of the nanofiber membranes are proportional to the PEUU percentage. In other words, the mechanical properties of PU100 and PU75 nanofiber membranes are superior to the other membranes. Meanwhile, it can be concluded from Fig. 6 that the mechanical strength of all the nanofiber membranes decreases after immersion in water for 3 hours.

Fig. 6 Stress–strain curves of PEUU/gelatin nanofibrous membranes with different ratios, after treating in dry conditions (a) and in wet conditions (b) (immersed in water for 3 hours). The data for PU0 was abandoned because it was too brittle.

3.5. Surface wettability and porosity

The surface wettability of the PEUU/gelatin nanofiber membranes was measured. The pure PEUU electrospun membranes tend to be hydrophobic, with a steady contact angle of 120.2° (as is shown in Table 1), while after the addition of gelatin, the nanofiber membranes became much too different in wettability properties. The water disappeared soon after coming in contact with the nanofiber membranes containing gelatin. The contact angle of different membranes at the 0.5 second time point during the testing process was captured. It can be seen from Table 1 that the contact angles of the hybrid nanofiber membranes increase with the decrease of the proportion of gelatin, and only a small amount of gelatin can significantly improve the hydrophilic properties of electrospun membranes. This could be the reason that the gelatin disperses evenly into the PEUU/gelatin electrospun system, and the hydrophilic nature of gelatin contributes to this phenomenon. It can also be concluded from Table 1 that the contact angle of the nanofiber membranes at 0.5 second become smaller after GA treatment, compared with the untreated groups. It goes without saying that the diameter of the nanofibers increased after submerging in glutaraldehyde, thus expanding the pore diameter (Table 1). The water could flow down to the lower side along the larger pore tunnel, thus producing the decrease in contact angle. The pore size and porosity results are shown in Table 1. It can be concluded that the mean pore diameter of the treated nanofiber membranes is larger than that of untreated groups. Meanwhile, the porosity of these nanofiber membranes increases with the increase in the proportion of PEUU.

Table 1 The pore size, porosity and contact angle of PEUU/gelatin nanofiber membranes with different mass ratios

Name	Mean pore diameter ± SD (μm)		Porosity (%)		Contact angle/° (0.5 s)
	Before treating	After treating	Before treating	After treating	Before treating	After treating
PU0	2.6 ± 0.7	4.9 ± 1.4	20.0% ± 3.0%	45.0% ± 2.0%	36.1 ± 0.5	17.5 ± 0.5
PU25	2.8 ± 0.8	3.4 ± 0.9	28.6% ± 2.5%	50.0% ± 1.5%	38.4 ± 0.8	31.8 ± 0.3
PU50	2.8 ± 0.8	3.5 ± 1.1	35.0% ± 2.0%	57.1% ± 3.0%	46.1 ± 0.9	42.7 ± 0.9
PU75	2.0 ± 0.5	3.0 ± 0.9	50.0% ± 1.5%	71.4% ± 5.0%	49.3 ± 0.6	44.6 ± 0.9
PU100	2.4 ± 0.9	3.0 ± 0.8	64.3% ± 4.0%	85.7% ± 4.5%	120.2 ± 0.9	119.1 ± 0.5

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3.6. Cell proliferation

The cell viability of L929 cells was tested. The result of cell proliferation (Fig. 7) indicates that the L929 cells appear to grow adaptably on the PU0 nanofiber membrane, compared to the other groups on the first day. After culturing for 4 days, the cell viability on PU0 and PU25 was not as good as other groups that had a relatively larger amount of PEUU. It can be concluded from the 7 day result that the PU75 group shows superior cell proliferation properties compared to the other groups. The cell viability on the TCP (tissue culture plate) is obviously better than all groups as a control. It can be concluded from these results that the PEUU/gelatin hybrid nanofiber membranes, especially the PU75 group can contribute to the cell proliferation better than the PU100 group. This might be the reason that the addition of gelatin increases the hydrophilicity and surface cell-recognition sites of PEUU.^1,49

Fig. 7 Cell proliferation results for PEUU/gelatin nanofibrous membranes with different ratios after treatment. * represents a significant difference between these groups.

3.7. Cell morphology

The cell morphology results are shown in Fig. 8 and 9. It can be seen from the cell SEM results (Fig. 8) that the cell morphology of PU75, PU50 and PU25 are better than PU0. The cell shapes of these three groups tend to be multi-directional, rather than a round shape. The cells on these nanofiber membranes grew along the fiber and showed a relatively good attachment on the materials. Compared to the PU100 group, cells on PU75 seemed to be more compatible on the nanofiber membrane. The same can be concluded from Fig. 9. Cells on PU75 tend to be more stretchable and the amount of fusiform cells on the PU75 group was markedly greater than the other groups. The cell morphology results were consistent with the cell proliferation results, supporting the application of PU75 in skin tissue engineering.

Fig. 8 SEM images of L929 cells on the electrospun nanofiber membrane of PU0 (a and a′), PU25 (b and b′), PU50 (c and c′), PU75 (d and d′), PU100 (e and e′). The scale bar of (a–e) represents 50 μm, while the scale bar of (a′–e′) represents 30 μm.

Fig. 9 Cell morphologies of PU25 (a), PU50 (b), PU75 (c) and PU100 (d), viewed on an inverted fluorescence microscope; the scale bar is 100 μm. The PU0 image was too hard to identify because the cytoplasm and the material tend to be stained with the same color, so there was no data.

3.8. Electrospun nanoyarn

As is shown in Fig. 9, SEM images of PU75-DLY (Fig. 10-a) and PU75-CY (Fig. 10-b) show that these two nanoyarns have a bunchy and aligned morphology. It can also be concluded that L929 cells could spread more easily along the aligned PU75-DLY (Fig. 10-c) and PU75-CY (Fig. 10-d). It should be noted that the nanoyarn played a guiding role for the cell proliferation and cells could grow more easily along the yarn bundle, compared to the nanofiber membrane. Masses of study have shown that nanoyarn contributes more easily to the proliferation of cells, compared to nanofiber membranes.

Fig. 10 SEM images of the fiber (a and b) and cell (c and d) of the dynamic liquid electrospun nanoyarn (a and c) and conjugated electrospun nanoyarn (b and d) of PU75. The scale bar of (a–c) is 50 μm and (d) is 25 μm.

Both PU75-DLY and PU75-CY had high porosities, with that of PU75-DLY being larger (85.7%) than that of PU75-CY (80%) (Fig. 11-a). The porosity of nanoyarn is higher than that of nanofiber. The mechanical properties were also tested. It can be seen from Fig. 11-b that on the whole, the mechanical properties of PU75-CY were superior to PU75-DLY, especially in the direction parallel to the aligned direction of the nanoyarn. It is obvious that the PU75-DLY and PU75-CY have better potential applications in the tissue engineering field; the specific properties will be studied further in future research.

Fig. 11 Properties of PU75-DLY and PU75-CY. (a) Porosity of the two electrospun nanoyarns; (b) stress–strain curves of two different directions (⊥ represents the direction perpendicular to the aligned direction of the nanoyarn, while ∥ represents the direction parallel to the aligned direction of the nanoyarn) of the two electrospun nanoyarns. PU75-DLY and PU75-CY represent the dynamic liquid electrospun nanoyarn and conjugated electrospun nanoyarn of PU75, respectively.

4. Conclusions

PEUU elastomer was synthesized by using PCL2000, HDI and butanediamine as monomers and stannous octoate as the catalyst. A series of PEUU/gelatin nanofiber membranes were then fabricated and the properties were studied. SEM images showed that all of these nanofiber membranes could mimic the extracellular matrix without considering the components. The FTIR spectra of the PEUU/gelatin series membranes after treatment indicated that PEUU was successfully synthesized. The XRD results showed that the treatment process decreased the crystallinity of PEUU to some extent. It can be concluded that the mechanical properties of PU100 and PU75 nanofiber membranes are superior to other groups. Only a small amount of gelatin significantly improved the hydrophilic properties of the electrospun nanofiber membranes. Meanwhile, the porosity of the nanofiber membranes increased with the increase in the proportion of PEUU. The results of cell proliferation indicated that PEUU/gelatin nanofiber membranes, especially the PU75 group, could contribute to the cell proliferation better than the groups of PU0 and PU100; the cell morphology results were consistent with the cell proliferation results, supporting the application of PU75 in skin tissue engineering. Consequently, PU75 could be a good alternative for regenerative nanofiber membranes. PU75-DLY and PU75-CY, with higher porosity compared to nanofiber membranes, were also fabricated. Nanoyarn played the guiding role in the cell proliferation and the cells grew more easily along the yarn bundle, compared to the nanofiber membrane. Further studies are required for potential application in the field of tissue engineering.

Acknowledgements

The study was supported by National Nature Science Foundation of China (31470941, 31271035), Science and Technology Commission of Shanghai Municipality (14JC1492100), Science and Technology Commission of Shanghai Municipality (15JC1490100, 15441905100), Yantai Double Hundred Talent Plan and Ph.D. Programs Foundation of Ministry of Education of China (20130075110005) and light of textile project (J201404). The authors extend their appreciation to the International Scientific Partnership Program ISPP at King Saud University for its funding research through (ISPP-0000).

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