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Textile fabricated biodegradable composite stents with core-shell structure
Journal Pre-proof Textile fabricated biodegradable composite stents with core-shell structure Mei-Chen Lin, Jia-Horng Lin, Chih-Yang Huang, Yueh-Sheng Chen
PII:
S0142-9418(19)31491-6
DOI:
https://doi.org/10.1016/j.polymertesting.2019.106166
Reference:
POTE 106166
To appear in:
Polymer Testing
Received Date: 24 August 2019 Revised Date:
4 October 2019
Accepted Date: 15 October 2019
Please cite this article as: M.-C. Lin, J.-H. Lin, C.-Y. Huang, Y.-S. Chen, Textile fabricated biodegradable composite stents with core-shell structure, Polymer Testing (2019), doi: https://doi.org/10.1016/ j.polymertesting.2019.106166. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Textile Fabricated Biodegradable Composite Stents with Core-Shell Structure Mei-Chen Lina, Jia-Horng Linb,c,d,e,f,g,h, Chih-Yang Huangi,j,k,l and Yueh-Sheng Chenm, * a
Graduate Institute of Biomedical Sciences, China Medical University, Taiwan Fujian Key Laboratory of Novel Functional Textile Fibers and Materials, Minjiang University, Fuzhou 350108, China.
b
c
Innovation Platform of Intelligent and Energy-Saving Textiles, School of Textiles, Tianjin Polytechnic University, Tianjin 300387, China. d College of Textile and Clothing, Qingdao University, Shangdong 266071, China e
Department of Fashion Design, Asia University, Taichung 41354, Taiwan; School of Chinese Medicine, China Medical University, Taichung 40402, Taiwan;
f
g
Tianjin and Ministry of Education Key Laboratory for Advanced Textile Composite Materials, Tianjin Polytechnic University, Tianjin 300387, China; h
Laboratory of Fiber Application and Manufacturing, Department of Fiber and Composite Materials, Feng Chia University, Taichung City 40724, Taiwan. i Department of Medical Research, China Medical University Hospital, China Medical University, Taichung, Taiwan j
Department of Biotechnology, Asia University, Taichung, Taiwan Holistic Education Center, Tzu Chi University of Science and Technology, Hualien, Taiwan l Cardiovascular and Mitochondria Related Diseases Research Center, k
Hualien Tzu Chi Hospital, Buddhist Tzu Chi Medical Foundation, Hualien, Taiwan m School of Chinese Medicine, China Medical University, Taiwan *
Corresponding
Author.
E-mail
address:
(Yueh-Sheng Chen).
1
[email protected]
Abstract This study proposes composite stents with core–shell structure. Biodegradable polyvinyl alcohol (PVA) yarns are twisted and then coated with polycaprolactone/polyethylene glycol (PEG) blends. The coated yarns are weft knitted into braids and then thermally treated to form composite stents with core–shell structure. The morphological, mechanical, and biological characteristics of the formed composite stents are evaluated to determine the effects of PEG concentration. Results show that composite stents acquire the flexibility of PVA yarns and elasticity of weft knits. The presence of PEG positively influences composite stent performance. When the PEG concentration is 30 wt%, composite stents exhibit a compressive strength of 6.15 N and cell viability of 97.32 % after a 24 h of culture. The selected materials are biodegradable, and the novel structure meets the requirements of bioresorbable vascular stent, which suggests that the proposed composite stents have good potential for advancement. Keywords: Polycaprolactone (PCL), Polyethylene glycol (PEG), Polyvinyl alcohol (PVA), Biodegradable stent 1. Introduction Generally, biomedical materials can help or replace the functions of the tissues and organs of organisms and have a direct contact with the cells, blood, tissues, or proteins of organisms. Hence, biomedical materials are commonly used inside or outside the human body as stents, catheters, artificial organs, and wound dressings. To contact with body fluid, blood, or tissues, biomedical materials need to be nontoxic, noninflammatory, biologically active, and degradable. Bioresorbable vascular stent (BRS) has been commonly studied. Grafted BRS can be decomposed 6–12 months after the impaired part is healed [1, 2]. Biodegradable metallic materials have limited usages and development because of their short degradation, high corrosion, and low extensibility [3]. Biodegradable polymer stents compensate for these setbacks and became the first BRS clinically. However, the first generation of BRS also has disadvantages, such as difficulty of grafting and a high thrombus occurrence rate. Subsequently, the development of new BRS has been the topic of an increasing number of studies [4-8]. In our previous study, we built different BRS models made of polyvinyl alcohol (PVA) fibers and successfully yielded good mechanical and biological 2
characteristics [9]. Similarly, in a previous study, researchers built stents out of PVA yarns via electrospinning and found that a spiral structure is beneficial for cell growth and activity, as well as responsiveness against shear stress [10]. Meanwhile, stents made from polycaprolactone (PCL) and gelatin are highly hydrophilic and have a porous structure, which are required by BRS [11]. Composite stents composed of polylactide (PLA) and PCL demonstrate excellent anticollapse ability and good recovery from deformation [12]. Overall, BRS is not generally used as a specified morphology material but in demands in vivo and clinical uses. BRS made from polymer and fibers by using different manufacturing parameters exhibit flexible morphology and essential properties. In the present study, composite stents with a core–shell structure are built. PVA yarns are made into twisted yarns (i.e., the core layer), which are coated with PCL/polyethylene glycol (PEG) blends (i.e., the shell) and then weft knitted. The coated braids are thermally treated to stabilize the structure, thereby forming composite stents. The PEG concentration changes during the manufacture, after which its effects on the PCL/PEG blends are examined. Afterward, the morphology of the stents is observed, and the thermal behavior and mechanical and biological properties of the stents are evaluated. 2. Experimental 2.1 Materials PVA yarns (Asiatic Fiber Corporation, Taiwan) with the specifications of 75D/25f and diameter of 0.05 mm were used. PCL pellets (Capa™ 6800, Solvay, Ltd., US) with the molecular weight of 80,000 and melting point of 60 °C were also utilized. The PEG powders (Sanyo Chemical Industries, Ltd., Japan) with an average molecular weight of 4,000, pH in the range of 5–7, and a melting point of 55 °C were used. 2.2 Composite Stent Preparation Three barrels of PVA yarns were fitted in the machine. Then, PVA yarns were twisted, coated, and weft knitted. The PCL/PEG ratios were 100/0, 90/10, 80/20, 70/30, 60/40, and 50/50 wt%. PCL and PEG were melted and blended five times at the processing temperatures of 70 °C, 80 °C, 90 °C, 100 °C, and 90 °C. The output and collecting rates of the extruder were 100 and 50 rpm, respectively. Next, the PVA twisted yarns were coated with PCL/PEG mixtures by using a coating machine. Then, the coated yarns were weft knitted, thereby exhibiting a core–shell structure and a diameter of 3 mm. Finally, these coated 3
braids underwent heat treatment at 60 °C for 15 min to form the composite stents (Figure 1).
Figure 1. Image of composite stent. 2.3 Measurements 2.3.1 Observation of Composite Stent Structure A scanning electron microscope (SEM; S4800, HITACHI, Japan) was used to analyze the morphology of the composite stents. The samples were affixed to a platform with a carbon conductive tape, after which they were coated with a thin layer of Au by using a sputter coater. The difference in the surfaces of the PCL/PEG blends was worth observing because two incompatible polymers may affect the properties of the composite stents. The SEM images were evaluated in terms of porosity and fiber diameter. According to the specified error bars of each SEM image, the fiber coverage area and diameter were computed using Images-Pro Plus (Media Cybernetics, USA), and then the average was calculated to yield the porosity of the composite stents by using Equation (1), as follows:
Porosity
(1),
100%
where A and Af were the composite stent (including voids) area, and Af was the fiber coverage of composite stent. 2.3.2 Thermal Behavior The composite stents were cut into pieces with the size of 9–10 mg and then sealed in an Al plate for differential scanning calorimetry (DSC) measurement (Q20, TA Instruments, US). The temperature of DSC was first set. The initial temperature was −20 °C, and the samples were heated to 200 °C and cooled to −20 °C, with the heating rate of 10 °C/min. The melting and crystal temperatures of the composite stents were recorded. The degree of crystallinity (Xc) of the PCL in the composite stents was determined using Eq. 4
(2) as follows. ∆ ∆
(
)
(2),
100%
where Xc was the degree of crystallinity, ∆Hm was the enthalpy of fusion for composite stent, ∆Hom was the enthalpy of fusion for complete crystallization, and f was the weight ratio of PEG. The ∆Hom values of PCL and PEG were 139.5 [13] and 196.8 J/g, respectively [14]. 2.3.3 Compressive Strength The feasibility of the compressive strength of composite stents was evaluated using a computer servo control material testing machine (HT-2402, Hung Ta Instrument, Taiwan). Composite stents were mounted on the compressive strength testing plate and compressed by another plate at a compression rate of 1 mm/min with a specified displacement of 2 mm. Ten samples for each specification were used. 2.3.4 Biological Characteristics As specified in ISO 10993-5, the measurement was conducted on a horizontal laminar flow table, and the samples and instruments were processed through UV sterilization. 3-[4,5-Dimethylthiazol-2– yl]-2,5-diphenyltetrazolium bromide (MTT) assay was performed to compute cell viability. The sample extract and L929 fibroblast suspension were poured into a 96-well plate for a 24 h culture at 37 °C in an incubator with 5% CO2. Afterward, minimum essential medium was replaced with a MTT agent, and the plate was kept in the dark for 4 h, after which MTT agent was replaced with dimethyl sulfoxide solvent. An ELISA reader was used to measure the optical density (OD) of the sample extract at 570 nm, and the cell viability was computed using OD. The cellular viability (%) of the composite stent was determined using Eq. (3) as follows. Cellular viability (%)
"#$% &'( "#) *+, '
100%.
(3).
2.3.5 Statistical Analysis The quantitative data was presented with standard deviations (SD). When SD was covered in a range of test data, one-way ANOVA statistical analysis (SPSS Statistics 17) was used to examine whether there was significant difference at * p<0.05 and ** p<0.01.
5
3. Results and discussion 3.1 Observation of Composite Stent Structure Figure 2 shows the structure diagrams and SEM images of composite stents. Figure 2 (a) demonstrates the weft-knitted and coating structures of the composite stents. The weft knits are composed of interconnected loops, which induce the stents high deformability. When composite stents are exerted with force, the loops in the proximity deform in order, thereby improving their mechanical properties. However, the acquired mechanical strength remained insufficient; hence, the composite stents need further reinforcement [12, 15, 16]. In the present study, PVA yarns are first twisted to form twisted yarns (Figure 2 [b]), which are then coated with PCL/PEG blends and weft knitted. The core–shell structure reinforces structure and properties. PVA yarns are composed of multifilaments that are arranged in parallel to one another at a loose state. During the twisting process, with one end of PVA yarns being fixed, PVA yarns are twisted from the other end, thereby mechanically reinforcing the twisted yarns. This process increases the friction among yarns and decreases the possibility for PVA yarns to slip and unwrap. Figure 2 (c) shows the cross section of the coated yarns, thereby exemplifying that PVA twisted yarns are cohered firmly and enwrapped with a PCL/PEG shell. Figure 2 (d) shows that weft knitting provides composite stents with a tubular structure. The SEM image shows that the inner thickness of the shell is 3 mm. The interlacing points of weft knits are shown in Figure 2 (e), where the coating layer is not damaged by the weft-knitting process. This result indicates that soft thin shells not only protect the yarns but also demonstrate the flexibility of filaments. The purpose of the finishing is to reduce the water solubility of PVA yarns because they have considerable potential as biodegradable stents but have limited application in the biomedical field. Considering the presence of its –COOR groups, PCL can reduce the contact area between PVA twisted yarns and water, thereby diminishing the water solubility of the composite stents. Given its biodegradability and nontoxicity as a polymer with high hydrophobicity, PCL is commonly used as a shape-memory and anti-stick material [17, 18]. PCL coating can also decrease its volume ratio in composite stents. The combination of PCL coating and PVA twisted yarns is beneficial for the in vivo retention time of composite stents, thereby meeting the clinical demands.
6
Figure 2. a) Scheme, d) overlook image, and e) structural diagram of composite stent. Images of b) twisted yarns and c) coating structure. Figure 3 shows the interlacing points of composite stents. Weft knitting is characterized by interconnected loops, which make the interlacing points the part that primarily supports the whole frame. Efficient heat treatment is frequently used to reinforce the basic properties of materials [19-21] and used in the present study to secure the interlacing points and stabilize the structure of composite stents. The composite stents undergo a heat treatment at 60 °C for 15 min to melt the coating layer of PCL/PEG blends and then are cooled. Figure 3 (a) shows the interlacing points of nonthermally treated composite stent. The inset shows the magnified image that indicated that PVA coated yarns do not generate cracks when bent and are damage-free when weft knitted into loops. This observation indicates that the stent with the weft-knitted structure possesses flexible with a wide diameter range. The tubular braids are porous and do not require other finishing processes. Therefore, composite stents have a diversity of applications and are evaluated with different measurements accordingly. Figure 3 (b)–(g) show the interlacing points of thermally treated composite 7
stents relative to the different PEG contents. Regardless of whether they are thermally treated, composite stents preserve the original weft-knitted structure. The PCL coating layer is melted, and it subsequently covers the interlacing points as one-piece forming, which frees the weft-knitting structure from the ease of dismantling. Generally, when the loops of weft knits fall off, all the loops along the perpendicular direction also fall off. The crimped edge of weft knits can also be flattened due to the presence of interlacing points. Overall, the application of heat does not harm the structures of composite stents but stabilizes them. Figure 3 (b)–(g) show that the PCL/PEG coating layer of the PVA yarns melts to form a smooth surface, which in turn protects the vascular vessels when composite stents are grafted in vivo, and reduces waste accumulation in the lower part of the human body.
Figure 3. Images of a) nonthermally and thermally treated composite stents containing b) 0, c) 10, d) 20, e) 30, f) 40, and g) 50 wt% of PEG. The porosity feature of vascular stents is indispensable because the interconnected pore structure allows the metabolism and exchange of materials inside the vascular vessels. The fibrous composite stent process stability and have an evenly composed structure, that is, the pore size of weft knits is uniform and evenly distributed. Figure 4 (a) shows the porosity and mesh size of the composite stents. The fiber coverage and void area ratio are computed based on the SEM images. The test results show that an increase in the PEG content affects the porous structure of composite stents, and the mesh size is inversely proportional to PEG content. Composite stents composed of PCL/PEG ratios of 100:0 and 90:10 wt% have relatively higher porosities of 24.93% and 26.50%, respectively. Increasing the PEG contents to 20 and 30 wt% decreases the porosity of the composite stents to 23.39% and 23.29%, respectively. The higher the PEG content is, the lower the porosity of composite stents will be. When the PEG contents are 40 and 50 wt%, the porosity decreases to 21.53% and 19.09%, respectively. The test results indicate that an increase in the PEG content has a negative influence on the 8
porosity of composite stents, which is similar to the case for mesh area. A small mesh area causes a low porosity, which may be ascribed to the swelling phenomenon of the hydrophilic PVA yarns when the composite stents are cooled in a water bath. In addition, over the surface of composite stents, PEG that possesses -OH group enables the constitutional PVA yarns of core layer to swell to a greater extent, which subsequently affects the porosity. As a result, a rise in the content of PEG adversely affects the porosity and mesh size (Figure 4 (a)). Figure 4 (b) shows that, in contrast to the cases for porosity and mesh area, the fiber diameter of composite stents increases with the increase in PEG content. When the PEG content increases from 10 wt% to 50 wt%, the fiber diameter increases by 11.54% from 0.23 mm to 0.26 mm. The fiber measurement precisely indicates that the swelling phenomenon of PVA yarns is attributed to the fact that PVA core layers are in contact with water and subsequently swell. When the PCL/PEG ratio changes from 100:0 wt% to 90:10 wt%, the fiber diameter decreases from 0.24 mm to 0.23 mm. This exception may be attributed to the high-mobility PEG that permeates PVA yarns and then decreases the fiber diameter. PEG is a common medical material that is used to improve the biological characteristics and drug loading [22, 23]. During composite stent preparation, PEG as the coating layer contributes to good hydrophilicity and dominates the morphology of the composite stents. Variations in PEG significantly influence the structure and pore size of composite stents. Hence, these variations should be investigated further.
Figure 4. a) Composite stent porosity, mesh area, and b) fiber diameter. 3.2 Thermal Behavior of Composite Stents DSC measurement is conducted mainly to investigate PCL/PEG blends. 9
Polymer compounds may exhibit changes in crystallization behavior, on which the mechanical properties is dependent upon. Table 1 and Figure 5 show the DSC results of composite stents, where the melting points of PCL are close to that of PEG and do not change when they are synthesized. Similar and low processing temperatures melt both materials. However, the crystallization temperature of the composite stents slightly increased with the increase in PEG content. Table 1 shows that the crystallization temperature of composite stents is between the crystallization temperatures of PCL and PEG, thereby suggesting that the synthesis of the two materials is beneficial for crystallization. PEG serves as the core and effectively improves the crystallization behavior of PCL. An increase in PEG from 0 wt% to 50 wt% also increases the crystallization degree of composite stents from 25.22% to 47.96%, respectively. This result is attributed to the presence of PEG, which has high crystallization degree and heat restoration property. The presence of a small amount of PEG significantly changes the heat property of composite stents, and the crystallization degree also proves that the incorporation of PEG affects the thermal behavior of composite stents [24, 25]. By contrast, the crystallization degree of PEG-free composite stents reduces by 5.88%, which is ascribed to the interface between the PCL coating layer and PVA yarns. Hence, the presence of PEG positively influences the crystallization degree of composite stents.
10
Figure 5. Thermal behavior of composite stents with various PEG contents. Table 1. DSC results of composite stents PEG Content (wt%)
∆Hm (J/g)
Tm (oC)
Tc (oC)
XcPCL(%)
0
35.19
54.14
20.51
25.22
10 20 30 40 50
33.50 32.82 35.63 40.14 45.37
53.82 54.02 53.70 54.39 55.17
19.83 22.00 21.35 21.74 21.66
26.68 29.40 36.49 47.96 65.05
PCL(pellets) PEG(pellets)
43.38 158.7
53.45 59.14
16.99 27.24
31.10 80.64
3.3 Mechanical Properties of Composite Stents The compressive properties of composite stents are highly related to the supportive force by which the stents provide the vascular vessels. A sufficient anticompression performance can expand the narrow space inside vascular vessels and ensure fluent blood flow. Figure 6 (a) and (b) show the compressive strength test and composite stent results. Figure 6 (a) shows that the samples are compressed between the two platforms. Composite stents 11
counteract the exerted force with a displacement of 2 mm, and the anticompression performance displayed is recorded as compressive strength. Following an increase in the PEG content (0–50 wt%), the compressive strength of composite stents first increases and then decreases, that is, 1.97 N for 0 wt%, 6.15 N for 30 wt%, and 4.5 N for 50 wt%. Comparing the crystallization degree and compressive strength of composite stents shows that thermal treatment exerts a lesser influence on mechanical properties than on thermal behavior. The blends improve mechanical properties because the fillers may strengthen the crystallization level and support the morphology and structure. The difference in the composite stent structure reinforces mechanical properties. Low porosity indicates high density of fibrous structures over the surface. Thus, additional fibers generate high compressive strength (cf. Figure 4). However, excessive PEG content facilitates the mobility of segments in the blends, which easily render failure to PCL and provide compressive strength. In addition to PCL that generates reinforcement, PEG is also necessary due to its plasticizing performance and biological activity [26-28]. This topic will be discussed in the next section.
Figure 6. a) Image of compressive measurement and b) compressive strength of composite stents. 3.4 Biological Characteristics of Composite Stents The biological activity of composite stents is measured through cytotoxicity test, and MMT assay is performed to record cell viability in 24 h (Figure 7). The test results indicate that the cell viability of composite stents increases from 36.76% to 97.32% when the PEG content increases from 0 wt% to 30 wt% and then decreases to 44.07% when the PEG content is 50 wt%. Attachment-type 12
L929 fibroblasts are used in cell biological characteristics measurement. The fibroblasts execute the autophagy mechanism in the medium, during which they need to adhere to matrices for growth. Therefore, the matrices must have strong affinity to fibroblasts; otherwise, the fibroblasts would suspend or die in the medium [29]. The biological characteristics corroborate that the presence of PEG improves the cell viability of composite stents. Hydrophilic PEG is beneficial for cell growth because it provides additional areas for cells to grow. PEG also has considerable distribution because of the formula of PCL/PEG blends and makes up for hydrophobic PCL. Meanwhile, PCL is composed of ester bonds that demonstrate hydrolysis and is degradable, which makes PCL a considerable candidate for BRS. Nonetheless, PCL is restricted by its degradation duration and low biological characteristics. Hence, the formula of PCL/PEG blends is beneficial for these setbacks. The cell viability result is consistent with the mechanical strength of composite stents, both of which show the same trend. The higher the compressive strength is, the better the cell viability will be. Given that attachment-type fibroblasts grow and depend on the mechanical properties of matrices, the matrices should possess sufficiently high mechanical strength. This result conforms with the findings in a previous study [30].The cells require complex environment; a relatively a hard matrix enables cell expansion to a greater area. Thus, the cells can migrate efficiently, as demonstrated by considerable cell adhesion and proliferation. By contrast, a soft matrix restricts cell growth, and the cells exhibit spherical shape without proliferation and migration.
Figure 7. Cell viability of composite stents. (* p<0.05, ** p<0.01) 13
To evaluate the correlation between matrices and determine whether cells are important for composite stents, we observe the microscopic morphology of composite stents in this study. Blend microstructure highly affects the cell attachment. Previous studies showed that the cell functions are primarily dependent upon the chemical properties of the matrices’ surface [31, 32]. The compatibility of cells is also positively correlated with matrix wettability, that is, the hydrophilic matrix improves cell adhesion and proliferation. A flat matrix also limits protein transmission, thereby restricting and decreasing cell adhesion and proliferation. Figure 8 shows the scheme of different matrices, where the cells are oval on a flat matrix and equivalently display self-protective state. By contrast, a rough matrix facilitates cell attachment because of the additional space and polar groups (–OH) provided by the matrix. An excessively rough matrix also generates a crowded environment, and low nutrition also adversely affects cell growth. Hence, this study evaluates the microstructure of composite stents (Figure 9), where PCL/PEG blends exhibit a rough surface when the PEG content increases from 0 wt% to 50 wt%. With a PEG content exceeding 30 wt%, composite stents exhibit a densely wavy surface, which is beneficial to cell viability, as shown in Figure 7. This result proves that the microstructure of matrix and cell growth are correlated. According to the biological characteristics data, matrices with slightly rough surface demonstrate improved cell attachment, but those with excessively rough surface exhibit low cell attachment and proliferation.
Figure 8. Scheme of cell growth relative to different matrices.
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Figure 9. Microstructures of composite stents relative to the PEG content of a) 0, b) 10, c) 20, d) 30, e) 40, and f) 50 wt%. 4. Conclusion In this study, we successfully combine fibers and polymers to create composite stents with a core–shell structure. First, PVA yarns are twisted as the core layer to provide the composite stents with soft filament while preserving the elasticity of weft knits. The application of heat treatment melts and stabilizes the interlacing points of composite stents, which improves size stability. The porosity of composite stents is correlated with PEG content; the greater the PEG content is, the more hydrophilic the composite stents are. Subsequently, the PVA core yarns swell, which in turn increases the diameter of fibers and decreases porosity. The addition of PEG reinforces the thermal behavior and mechanical properties of composite stents, as demonstrated by the differences in their crystallization level and shape. The presence of PEG also significantly improves the cell viability and biological activity of PCL, thereby resulting in blends with rough surface and providing cells with a friendly environment to attach and grow. Acknowledgements This research project was financially supported by the Ministry of Science and Technology of Taiwan under contract MOST 107-2622-E-039 -002 -CC3. References [1] E. Mostaed, M. Sikora-Jasinska, J.W. Drelich, M. Vedani, Zinc-based alloys for degradable vascular stent applications, Acta Biomater. 71 (2018) 1-23. 15
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Appl. Therm. Eng. 118 (2017) 398-407. [26] A.J. Guerra, J. San, J. Ciurana, Fabrication of PCL/PLA Composite Tube for Stent Manufacturing, Procedia CIRP, 65 (2017) 231-235. [27] M. Sladkova, M. Palmer, C. Öhman, R.J. Alhaddad, A. Esmael, H. Engqvist, G.M. de Peppo, Fabrication of macroporous cement scaffolds using PEG particles: In vitro evaluation with induced pluripotent stem cell-derived mesenchymal progenitors, Mater. Sci. Eng. C 69 (2016) 640-652. [28] T. Serra, M. Ortiz-Hernandez, E. Engel, J.A. Planell, M. Navarro, Relevance of PEG in PLA-based blends for tissue engineering 3D-printed scaffolds, Mater. Sci. Eng. C 38 (2014) 55-62. [29] B.D. James, J.B. Allen, Vascular Endothelial Cell Behavior in Complex Mechanical Microenvironments, ACS Biomater. Sci. Eng. 4 (2018) 3818-3842. [30] R.W. Tilghman, E.M. Blais, C.R. Cowan, N.E. Sherman, P.R. Grigera, E.D. Jeffery, J.W. Fox, B.R. Blackman, D.J. Tschumperlin, J.A. Papin, J.T. Parsons, Matrix Rigidity Regulates Cancer Cell Growth by Modulating Cellular Metabolism and Protein Synthesis, PLOS ONE, 7 (2012) e37231. [31] H. Yu, Y. Jia, C. Yao, Y. Lu, PCL/PEG core/sheath fibers with controlled drug release rate fabricated on the basis of a novel combined technique, J. Pharm. 469 (2014) 17-22. [32] R.A. D’Sa, J. Raj, P.J. Dickinson, F. McCabe, B.J. Meenan, Human Fetal Osteoblast Response on Poly(Methyl Methacrylate)/Polystyrene Demixed Thin Film Blends: Surface Chemistry Vs Topography Effects, ACS Appl. Mater. Interfaces 8 (2016) 14920-14931.
18
Figure Captions Figure 1. Image of composite stent. Figure 2. a) Scheme, d) overlook image, and e) structural diagram of composite stent. Images of b) twisted yarns and c) coating structure. Figure 3. Images of a) nonthermally and thermally treated composite stents containing b) 0, c) 10, d) 20, e) 30, f) 40, and g) 50 wt% of PEG. Figure 4. a) Composite stent porosity, mesh area, and b) fiber diameter. Figure 5. Thermal behavior of composite stents with various PEG contents. Figure 6. a) Image of compressive measurement and b) compressive strength of composite stents. Figure 7. Cell viability of composite stents. (* p<0.05, ** p<0.01) Figure 8. Scheme of cell growth relative to different matrices. Figure 9. Microstructures of composite stents relative to the PEG content of a) 0, b) 10, c) 20, d) 30, e) 40, and f) 50 wt%.
19
Table Captions Table 1. DSC results of composite stents Table 1. DSC results of composite stents PEG Content (wt%)
∆Hm (J/g)
Tm (oC)
Tc (oC)
XcPCL(%)
0 10
35.19 33.50
54.14 53.82
20.51 19.83
25.22 26.68
20 30 40 50
32.82 35.63 40.14 45.37
54.02 53.70 54.39 55.17
22.00 21.35 21.74 21.66
29.40 36.49 47.96 65.05
PCL(pellets)
43.38
53.45
16.99
31.10
PEG(pellets)
158.7
59.14
27.24
80.64
20
Highlights 1. The stent is composed of core–shell structure includes fiber and polymers. 2. The stent uses the twisted PVA fiber as the core layer. 3. The stents acquire the flexibility of PVA yarns and elasticity of weft knits. 4. The biological activity test results showed no toxicity. 5. The components of the stent are all degradable materials.
The authors of submission entitled “Textile Fabricated Biodegradable Composite Stents with Core-Shell Structure”do not have any conflict of interest to others.
PII:
S0142-9418(19)31491-6
DOI:
https://doi.org/10.1016/j.polymertesting.2019.106166
Reference:
POTE 106166
To appear in:
Polymer Testing
Received Date: 24 August 2019 Revised Date:
4 October 2019
Accepted Date: 15 October 2019
Please cite this article as: M.-C. Lin, J.-H. Lin, C.-Y. Huang, Y.-S. Chen, Textile fabricated biodegradable composite stents with core-shell structure, Polymer Testing (2019), doi: https://doi.org/10.1016/ j.polymertesting.2019.106166. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Textile Fabricated Biodegradable Composite Stents with Core-Shell Structure Mei-Chen Lina, Jia-Horng Linb,c,d,e,f,g,h, Chih-Yang Huangi,j,k,l and Yueh-Sheng Chenm, * a
Graduate Institute of Biomedical Sciences, China Medical University, Taiwan Fujian Key Laboratory of Novel Functional Textile Fibers and Materials, Minjiang University, Fuzhou 350108, China.
b
c
Innovation Platform of Intelligent and Energy-Saving Textiles, School of Textiles, Tianjin Polytechnic University, Tianjin 300387, China. d College of Textile and Clothing, Qingdao University, Shangdong 266071, China e
Department of Fashion Design, Asia University, Taichung 41354, Taiwan; School of Chinese Medicine, China Medical University, Taichung 40402, Taiwan;
f
g
Tianjin and Ministry of Education Key Laboratory for Advanced Textile Composite Materials, Tianjin Polytechnic University, Tianjin 300387, China; h
Laboratory of Fiber Application and Manufacturing, Department of Fiber and Composite Materials, Feng Chia University, Taichung City 40724, Taiwan. i Department of Medical Research, China Medical University Hospital, China Medical University, Taichung, Taiwan j
Department of Biotechnology, Asia University, Taichung, Taiwan Holistic Education Center, Tzu Chi University of Science and Technology, Hualien, Taiwan l Cardiovascular and Mitochondria Related Diseases Research Center, k
Hualien Tzu Chi Hospital, Buddhist Tzu Chi Medical Foundation, Hualien, Taiwan m School of Chinese Medicine, China Medical University, Taiwan *
Corresponding
Author.
address:
(Yueh-Sheng Chen).
1
[email protected]
Abstract This study proposes composite stents with core–shell structure. Biodegradable polyvinyl alcohol (PVA) yarns are twisted and then coated with polycaprolactone/polyethylene glycol (PEG) blends. The coated yarns are weft knitted into braids and then thermally treated to form composite stents with core–shell structure. The morphological, mechanical, and biological characteristics of the formed composite stents are evaluated to determine the effects of PEG concentration. Results show that composite stents acquire the flexibility of PVA yarns and elasticity of weft knits. The presence of PEG positively influences composite stent performance. When the PEG concentration is 30 wt%, composite stents exhibit a compressive strength of 6.15 N and cell viability of 97.32 % after a 24 h of culture. The selected materials are biodegradable, and the novel structure meets the requirements of bioresorbable vascular stent, which suggests that the proposed composite stents have good potential for advancement. Keywords: Polycaprolactone (PCL), Polyethylene glycol (PEG), Polyvinyl alcohol (PVA), Biodegradable stent 1. Introduction Generally, biomedical materials can help or replace the functions of the tissues and organs of organisms and have a direct contact with the cells, blood, tissues, or proteins of organisms. Hence, biomedical materials are commonly used inside or outside the human body as stents, catheters, artificial organs, and wound dressings. To contact with body fluid, blood, or tissues, biomedical materials need to be nontoxic, noninflammatory, biologically active, and degradable. Bioresorbable vascular stent (BRS) has been commonly studied. Grafted BRS can be decomposed 6–12 months after the impaired part is healed [1, 2]. Biodegradable metallic materials have limited usages and development because of their short degradation, high corrosion, and low extensibility [3]. Biodegradable polymer stents compensate for these setbacks and became the first BRS clinically. However, the first generation of BRS also has disadvantages, such as difficulty of grafting and a high thrombus occurrence rate. Subsequently, the development of new BRS has been the topic of an increasing number of studies [4-8]. In our previous study, we built different BRS models made of polyvinyl alcohol (PVA) fibers and successfully yielded good mechanical and biological 2
characteristics [9]. Similarly, in a previous study, researchers built stents out of PVA yarns via electrospinning and found that a spiral structure is beneficial for cell growth and activity, as well as responsiveness against shear stress [10]. Meanwhile, stents made from polycaprolactone (PCL) and gelatin are highly hydrophilic and have a porous structure, which are required by BRS [11]. Composite stents composed of polylactide (PLA) and PCL demonstrate excellent anticollapse ability and good recovery from deformation [12]. Overall, BRS is not generally used as a specified morphology material but in demands in vivo and clinical uses. BRS made from polymer and fibers by using different manufacturing parameters exhibit flexible morphology and essential properties. In the present study, composite stents with a core–shell structure are built. PVA yarns are made into twisted yarns (i.e., the core layer), which are coated with PCL/polyethylene glycol (PEG) blends (i.e., the shell) and then weft knitted. The coated braids are thermally treated to stabilize the structure, thereby forming composite stents. The PEG concentration changes during the manufacture, after which its effects on the PCL/PEG blends are examined. Afterward, the morphology of the stents is observed, and the thermal behavior and mechanical and biological properties of the stents are evaluated. 2. Experimental 2.1 Materials PVA yarns (Asiatic Fiber Corporation, Taiwan) with the specifications of 75D/25f and diameter of 0.05 mm were used. PCL pellets (Capa™ 6800, Solvay, Ltd., US) with the molecular weight of 80,000 and melting point of 60 °C were also utilized. The PEG powders (Sanyo Chemical Industries, Ltd., Japan) with an average molecular weight of 4,000, pH in the range of 5–7, and a melting point of 55 °C were used. 2.2 Composite Stent Preparation Three barrels of PVA yarns were fitted in the machine. Then, PVA yarns were twisted, coated, and weft knitted. The PCL/PEG ratios were 100/0, 90/10, 80/20, 70/30, 60/40, and 50/50 wt%. PCL and PEG were melted and blended five times at the processing temperatures of 70 °C, 80 °C, 90 °C, 100 °C, and 90 °C. The output and collecting rates of the extruder were 100 and 50 rpm, respectively. Next, the PVA twisted yarns were coated with PCL/PEG mixtures by using a coating machine. Then, the coated yarns were weft knitted, thereby exhibiting a core–shell structure and a diameter of 3 mm. Finally, these coated 3
braids underwent heat treatment at 60 °C for 15 min to form the composite stents (Figure 1).
Figure 1. Image of composite stent. 2.3 Measurements 2.3.1 Observation of Composite Stent Structure A scanning electron microscope (SEM; S4800, HITACHI, Japan) was used to analyze the morphology of the composite stents. The samples were affixed to a platform with a carbon conductive tape, after which they were coated with a thin layer of Au by using a sputter coater. The difference in the surfaces of the PCL/PEG blends was worth observing because two incompatible polymers may affect the properties of the composite stents. The SEM images were evaluated in terms of porosity and fiber diameter. According to the specified error bars of each SEM image, the fiber coverage area and diameter were computed using Images-Pro Plus (Media Cybernetics, USA), and then the average was calculated to yield the porosity of the composite stents by using Equation (1), as follows:
Porosity
(1),
100%
where A and Af were the composite stent (including voids) area, and Af was the fiber coverage of composite stent. 2.3.2 Thermal Behavior The composite stents were cut into pieces with the size of 9–10 mg and then sealed in an Al plate for differential scanning calorimetry (DSC) measurement (Q20, TA Instruments, US). The temperature of DSC was first set. The initial temperature was −20 °C, and the samples were heated to 200 °C and cooled to −20 °C, with the heating rate of 10 °C/min. The melting and crystal temperatures of the composite stents were recorded. The degree of crystallinity (Xc) of the PCL in the composite stents was determined using Eq. 4
(2) as follows. ∆ ∆
(
)
(2),
100%
where Xc was the degree of crystallinity, ∆Hm was the enthalpy of fusion for composite stent, ∆Hom was the enthalpy of fusion for complete crystallization, and f was the weight ratio of PEG. The ∆Hom values of PCL and PEG were 139.5 [13] and 196.8 J/g, respectively [14]. 2.3.3 Compressive Strength The feasibility of the compressive strength of composite stents was evaluated using a computer servo control material testing machine (HT-2402, Hung Ta Instrument, Taiwan). Composite stents were mounted on the compressive strength testing plate and compressed by another plate at a compression rate of 1 mm/min with a specified displacement of 2 mm. Ten samples for each specification were used. 2.3.4 Biological Characteristics As specified in ISO 10993-5, the measurement was conducted on a horizontal laminar flow table, and the samples and instruments were processed through UV sterilization. 3-[4,5-Dimethylthiazol-2– yl]-2,5-diphenyltetrazolium bromide (MTT) assay was performed to compute cell viability. The sample extract and L929 fibroblast suspension were poured into a 96-well plate for a 24 h culture at 37 °C in an incubator with 5% CO2. Afterward, minimum essential medium was replaced with a MTT agent, and the plate was kept in the dark for 4 h, after which MTT agent was replaced with dimethyl sulfoxide solvent. An ELISA reader was used to measure the optical density (OD) of the sample extract at 570 nm, and the cell viability was computed using OD. The cellular viability (%) of the composite stent was determined using Eq. (3) as follows. Cellular viability (%)
"#$% &'( "#) *+, '
100%.
(3).
2.3.5 Statistical Analysis The quantitative data was presented with standard deviations (SD). When SD was covered in a range of test data, one-way ANOVA statistical analysis (SPSS Statistics 17) was used to examine whether there was significant difference at * p<0.05 and ** p<0.01.
5
3. Results and discussion 3.1 Observation of Composite Stent Structure Figure 2 shows the structure diagrams and SEM images of composite stents. Figure 2 (a) demonstrates the weft-knitted and coating structures of the composite stents. The weft knits are composed of interconnected loops, which induce the stents high deformability. When composite stents are exerted with force, the loops in the proximity deform in order, thereby improving their mechanical properties. However, the acquired mechanical strength remained insufficient; hence, the composite stents need further reinforcement [12, 15, 16]. In the present study, PVA yarns are first twisted to form twisted yarns (Figure 2 [b]), which are then coated with PCL/PEG blends and weft knitted. The core–shell structure reinforces structure and properties. PVA yarns are composed of multifilaments that are arranged in parallel to one another at a loose state. During the twisting process, with one end of PVA yarns being fixed, PVA yarns are twisted from the other end, thereby mechanically reinforcing the twisted yarns. This process increases the friction among yarns and decreases the possibility for PVA yarns to slip and unwrap. Figure 2 (c) shows the cross section of the coated yarns, thereby exemplifying that PVA twisted yarns are cohered firmly and enwrapped with a PCL/PEG shell. Figure 2 (d) shows that weft knitting provides composite stents with a tubular structure. The SEM image shows that the inner thickness of the shell is 3 mm. The interlacing points of weft knits are shown in Figure 2 (e), where the coating layer is not damaged by the weft-knitting process. This result indicates that soft thin shells not only protect the yarns but also demonstrate the flexibility of filaments. The purpose of the finishing is to reduce the water solubility of PVA yarns because they have considerable potential as biodegradable stents but have limited application in the biomedical field. Considering the presence of its –COOR groups, PCL can reduce the contact area between PVA twisted yarns and water, thereby diminishing the water solubility of the composite stents. Given its biodegradability and nontoxicity as a polymer with high hydrophobicity, PCL is commonly used as a shape-memory and anti-stick material [17, 18]. PCL coating can also decrease its volume ratio in composite stents. The combination of PCL coating and PVA twisted yarns is beneficial for the in vivo retention time of composite stents, thereby meeting the clinical demands.
6
Figure 2. a) Scheme, d) overlook image, and e) structural diagram of composite stent. Images of b) twisted yarns and c) coating structure. Figure 3 shows the interlacing points of composite stents. Weft knitting is characterized by interconnected loops, which make the interlacing points the part that primarily supports the whole frame. Efficient heat treatment is frequently used to reinforce the basic properties of materials [19-21] and used in the present study to secure the interlacing points and stabilize the structure of composite stents. The composite stents undergo a heat treatment at 60 °C for 15 min to melt the coating layer of PCL/PEG blends and then are cooled. Figure 3 (a) shows the interlacing points of nonthermally treated composite stent. The inset shows the magnified image that indicated that PVA coated yarns do not generate cracks when bent and are damage-free when weft knitted into loops. This observation indicates that the stent with the weft-knitted structure possesses flexible with a wide diameter range. The tubular braids are porous and do not require other finishing processes. Therefore, composite stents have a diversity of applications and are evaluated with different measurements accordingly. Figure 3 (b)–(g) show the interlacing points of thermally treated composite 7
stents relative to the different PEG contents. Regardless of whether they are thermally treated, composite stents preserve the original weft-knitted structure. The PCL coating layer is melted, and it subsequently covers the interlacing points as one-piece forming, which frees the weft-knitting structure from the ease of dismantling. Generally, when the loops of weft knits fall off, all the loops along the perpendicular direction also fall off. The crimped edge of weft knits can also be flattened due to the presence of interlacing points. Overall, the application of heat does not harm the structures of composite stents but stabilizes them. Figure 3 (b)–(g) show that the PCL/PEG coating layer of the PVA yarns melts to form a smooth surface, which in turn protects the vascular vessels when composite stents are grafted in vivo, and reduces waste accumulation in the lower part of the human body.
Figure 3. Images of a) nonthermally and thermally treated composite stents containing b) 0, c) 10, d) 20, e) 30, f) 40, and g) 50 wt% of PEG. The porosity feature of vascular stents is indispensable because the interconnected pore structure allows the metabolism and exchange of materials inside the vascular vessels. The fibrous composite stent process stability and have an evenly composed structure, that is, the pore size of weft knits is uniform and evenly distributed. Figure 4 (a) shows the porosity and mesh size of the composite stents. The fiber coverage and void area ratio are computed based on the SEM images. The test results show that an increase in the PEG content affects the porous structure of composite stents, and the mesh size is inversely proportional to PEG content. Composite stents composed of PCL/PEG ratios of 100:0 and 90:10 wt% have relatively higher porosities of 24.93% and 26.50%, respectively. Increasing the PEG contents to 20 and 30 wt% decreases the porosity of the composite stents to 23.39% and 23.29%, respectively. The higher the PEG content is, the lower the porosity of composite stents will be. When the PEG contents are 40 and 50 wt%, the porosity decreases to 21.53% and 19.09%, respectively. The test results indicate that an increase in the PEG content has a negative influence on the 8
porosity of composite stents, which is similar to the case for mesh area. A small mesh area causes a low porosity, which may be ascribed to the swelling phenomenon of the hydrophilic PVA yarns when the composite stents are cooled in a water bath. In addition, over the surface of composite stents, PEG that possesses -OH group enables the constitutional PVA yarns of core layer to swell to a greater extent, which subsequently affects the porosity. As a result, a rise in the content of PEG adversely affects the porosity and mesh size (Figure 4 (a)). Figure 4 (b) shows that, in contrast to the cases for porosity and mesh area, the fiber diameter of composite stents increases with the increase in PEG content. When the PEG content increases from 10 wt% to 50 wt%, the fiber diameter increases by 11.54% from 0.23 mm to 0.26 mm. The fiber measurement precisely indicates that the swelling phenomenon of PVA yarns is attributed to the fact that PVA core layers are in contact with water and subsequently swell. When the PCL/PEG ratio changes from 100:0 wt% to 90:10 wt%, the fiber diameter decreases from 0.24 mm to 0.23 mm. This exception may be attributed to the high-mobility PEG that permeates PVA yarns and then decreases the fiber diameter. PEG is a common medical material that is used to improve the biological characteristics and drug loading [22, 23]. During composite stent preparation, PEG as the coating layer contributes to good hydrophilicity and dominates the morphology of the composite stents. Variations in PEG significantly influence the structure and pore size of composite stents. Hence, these variations should be investigated further.
Figure 4. a) Composite stent porosity, mesh area, and b) fiber diameter. 3.2 Thermal Behavior of Composite Stents DSC measurement is conducted mainly to investigate PCL/PEG blends. 9
Polymer compounds may exhibit changes in crystallization behavior, on which the mechanical properties is dependent upon. Table 1 and Figure 5 show the DSC results of composite stents, where the melting points of PCL are close to that of PEG and do not change when they are synthesized. Similar and low processing temperatures melt both materials. However, the crystallization temperature of the composite stents slightly increased with the increase in PEG content. Table 1 shows that the crystallization temperature of composite stents is between the crystallization temperatures of PCL and PEG, thereby suggesting that the synthesis of the two materials is beneficial for crystallization. PEG serves as the core and effectively improves the crystallization behavior of PCL. An increase in PEG from 0 wt% to 50 wt% also increases the crystallization degree of composite stents from 25.22% to 47.96%, respectively. This result is attributed to the presence of PEG, which has high crystallization degree and heat restoration property. The presence of a small amount of PEG significantly changes the heat property of composite stents, and the crystallization degree also proves that the incorporation of PEG affects the thermal behavior of composite stents [24, 25]. By contrast, the crystallization degree of PEG-free composite stents reduces by 5.88%, which is ascribed to the interface between the PCL coating layer and PVA yarns. Hence, the presence of PEG positively influences the crystallization degree of composite stents.
10
Figure 5. Thermal behavior of composite stents with various PEG contents. Table 1. DSC results of composite stents PEG Content (wt%)
∆Hm (J/g)
Tm (oC)
Tc (oC)
XcPCL(%)
0
35.19
54.14
20.51
25.22
10 20 30 40 50
33.50 32.82 35.63 40.14 45.37
53.82 54.02 53.70 54.39 55.17
19.83 22.00 21.35 21.74 21.66
26.68 29.40 36.49 47.96 65.05
PCL(pellets) PEG(pellets)
43.38 158.7
53.45 59.14
16.99 27.24
31.10 80.64
3.3 Mechanical Properties of Composite Stents The compressive properties of composite stents are highly related to the supportive force by which the stents provide the vascular vessels. A sufficient anticompression performance can expand the narrow space inside vascular vessels and ensure fluent blood flow. Figure 6 (a) and (b) show the compressive strength test and composite stent results. Figure 6 (a) shows that the samples are compressed between the two platforms. Composite stents 11
counteract the exerted force with a displacement of 2 mm, and the anticompression performance displayed is recorded as compressive strength. Following an increase in the PEG content (0–50 wt%), the compressive strength of composite stents first increases and then decreases, that is, 1.97 N for 0 wt%, 6.15 N for 30 wt%, and 4.5 N for 50 wt%. Comparing the crystallization degree and compressive strength of composite stents shows that thermal treatment exerts a lesser influence on mechanical properties than on thermal behavior. The blends improve mechanical properties because the fillers may strengthen the crystallization level and support the morphology and structure. The difference in the composite stent structure reinforces mechanical properties. Low porosity indicates high density of fibrous structures over the surface. Thus, additional fibers generate high compressive strength (cf. Figure 4). However, excessive PEG content facilitates the mobility of segments in the blends, which easily render failure to PCL and provide compressive strength. In addition to PCL that generates reinforcement, PEG is also necessary due to its plasticizing performance and biological activity [26-28]. This topic will be discussed in the next section.
Figure 6. a) Image of compressive measurement and b) compressive strength of composite stents. 3.4 Biological Characteristics of Composite Stents The biological activity of composite stents is measured through cytotoxicity test, and MMT assay is performed to record cell viability in 24 h (Figure 7). The test results indicate that the cell viability of composite stents increases from 36.76% to 97.32% when the PEG content increases from 0 wt% to 30 wt% and then decreases to 44.07% when the PEG content is 50 wt%. Attachment-type 12
L929 fibroblasts are used in cell biological characteristics measurement. The fibroblasts execute the autophagy mechanism in the medium, during which they need to adhere to matrices for growth. Therefore, the matrices must have strong affinity to fibroblasts; otherwise, the fibroblasts would suspend or die in the medium [29]. The biological characteristics corroborate that the presence of PEG improves the cell viability of composite stents. Hydrophilic PEG is beneficial for cell growth because it provides additional areas for cells to grow. PEG also has considerable distribution because of the formula of PCL/PEG blends and makes up for hydrophobic PCL. Meanwhile, PCL is composed of ester bonds that demonstrate hydrolysis and is degradable, which makes PCL a considerable candidate for BRS. Nonetheless, PCL is restricted by its degradation duration and low biological characteristics. Hence, the formula of PCL/PEG blends is beneficial for these setbacks. The cell viability result is consistent with the mechanical strength of composite stents, both of which show the same trend. The higher the compressive strength is, the better the cell viability will be. Given that attachment-type fibroblasts grow and depend on the mechanical properties of matrices, the matrices should possess sufficiently high mechanical strength. This result conforms with the findings in a previous study [30].The cells require complex environment; a relatively a hard matrix enables cell expansion to a greater area. Thus, the cells can migrate efficiently, as demonstrated by considerable cell adhesion and proliferation. By contrast, a soft matrix restricts cell growth, and the cells exhibit spherical shape without proliferation and migration.
Figure 7. Cell viability of composite stents. (* p<0.05, ** p<0.01) 13
To evaluate the correlation between matrices and determine whether cells are important for composite stents, we observe the microscopic morphology of composite stents in this study. Blend microstructure highly affects the cell attachment. Previous studies showed that the cell functions are primarily dependent upon the chemical properties of the matrices’ surface [31, 32]. The compatibility of cells is also positively correlated with matrix wettability, that is, the hydrophilic matrix improves cell adhesion and proliferation. A flat matrix also limits protein transmission, thereby restricting and decreasing cell adhesion and proliferation. Figure 8 shows the scheme of different matrices, where the cells are oval on a flat matrix and equivalently display self-protective state. By contrast, a rough matrix facilitates cell attachment because of the additional space and polar groups (–OH) provided by the matrix. An excessively rough matrix also generates a crowded environment, and low nutrition also adversely affects cell growth. Hence, this study evaluates the microstructure of composite stents (Figure 9), where PCL/PEG blends exhibit a rough surface when the PEG content increases from 0 wt% to 50 wt%. With a PEG content exceeding 30 wt%, composite stents exhibit a densely wavy surface, which is beneficial to cell viability, as shown in Figure 7. This result proves that the microstructure of matrix and cell growth are correlated. According to the biological characteristics data, matrices with slightly rough surface demonstrate improved cell attachment, but those with excessively rough surface exhibit low cell attachment and proliferation.
Figure 8. Scheme of cell growth relative to different matrices.
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Figure 9. Microstructures of composite stents relative to the PEG content of a) 0, b) 10, c) 20, d) 30, e) 40, and f) 50 wt%. 4. Conclusion In this study, we successfully combine fibers and polymers to create composite stents with a core–shell structure. First, PVA yarns are twisted as the core layer to provide the composite stents with soft filament while preserving the elasticity of weft knits. The application of heat treatment melts and stabilizes the interlacing points of composite stents, which improves size stability. The porosity of composite stents is correlated with PEG content; the greater the PEG content is, the more hydrophilic the composite stents are. Subsequently, the PVA core yarns swell, which in turn increases the diameter of fibers and decreases porosity. The addition of PEG reinforces the thermal behavior and mechanical properties of composite stents, as demonstrated by the differences in their crystallization level and shape. The presence of PEG also significantly improves the cell viability and biological activity of PCL, thereby resulting in blends with rough surface and providing cells with a friendly environment to attach and grow. Acknowledgements This research project was financially supported by the Ministry of Science and Technology of Taiwan under contract MOST 107-2622-E-039 -002 -CC3. References [1] E. Mostaed, M. Sikora-Jasinska, J.W. Drelich, M. Vedani, Zinc-based alloys for degradable vascular stent applications, Acta Biomater. 71 (2018) 1-23. 15
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Figure Captions Figure 1. Image of composite stent. Figure 2. a) Scheme, d) overlook image, and e) structural diagram of composite stent. Images of b) twisted yarns and c) coating structure. Figure 3. Images of a) nonthermally and thermally treated composite stents containing b) 0, c) 10, d) 20, e) 30, f) 40, and g) 50 wt% of PEG. Figure 4. a) Composite stent porosity, mesh area, and b) fiber diameter. Figure 5. Thermal behavior of composite stents with various PEG contents. Figure 6. a) Image of compressive measurement and b) compressive strength of composite stents. Figure 7. Cell viability of composite stents. (* p<0.05, ** p<0.01) Figure 8. Scheme of cell growth relative to different matrices. Figure 9. Microstructures of composite stents relative to the PEG content of a) 0, b) 10, c) 20, d) 30, e) 40, and f) 50 wt%.
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Table Captions Table 1. DSC results of composite stents Table 1. DSC results of composite stents PEG Content (wt%)
∆Hm (J/g)
Tm (oC)
Tc (oC)
XcPCL(%)
0 10
35.19 33.50
54.14 53.82
20.51 19.83
25.22 26.68
20 30 40 50
32.82 35.63 40.14 45.37
54.02 53.70 54.39 55.17
22.00 21.35 21.74 21.66
29.40 36.49 47.96 65.05
PCL(pellets)
43.38
53.45
16.99
31.10
PEG(pellets)
158.7
59.14
27.24
80.64
20
Highlights 1. The stent is composed of core–shell structure includes fiber and polymers. 2. The stent uses the twisted PVA fiber as the core layer. 3. The stents acquire the flexibility of PVA yarns and elasticity of weft knits. 4. The biological activity test results showed no toxicity. 5. The components of the stent are all degradable materials.
The authors of submission entitled “Textile Fabricated Biodegradable Composite Stents with Core-Shell Structure”do not have any conflict of interest to others.