ABSTRACT Electrospinning is a promising fabrication method for three dimensional tissue engineeri... more ABSTRACT Electrospinning is a promising fabrication method for three dimensional tissue engineering scaffolds due to its ability to produce a nano-/micro-sized non-woven fibrous structure which resembles the natural extracellular matrix. We investigated the mechanical behavior of two different electrospun microstructures. Polycaprolactone (PCL) fibers with or without "point-bonding" exhibited different deformation behaviors having significant biomedical consequences. While fibers with point-bonded structure failed due to the generation of voids by the fracture of fiber interconnections under strain, fibers without point-bonds produced a 'bamboo' structure with fiber joining visible at higher levels of strain. In addition, gelatin and PCL were electrospun and the residual solvent contents were systematically investigated. A simple and effective means of reducing residual solvent content was developed. The interaction between these electrospun matrices and chondrocytic cells were compared to other topographies having the same chemistry. Electrospun polycaprolactone fibers supported better proliferation and extracellular matrix production than the corresponding semi-porous and dense surfaces and even, at some time points, glass surfaces. The intrinsic capability of electrospinning to produce high porosity appears to offset the relative hydrophobicity of polycaprolactone resulting in a more uniform cell seeding. Electrospun fibers induced a higher level of glycosaminoglycans (GAG) production by providing a 'dynamic scaffold' in which chondrocytes are able to maintain a morphology associated with the appropriate phenotype. Finally, based on this study, a method producing macro-pores within an electrospun scaffold was developed. With this method, not only can cellular infiltration into a thick electrospun scaffold be facilitated, but scaffolds having designed, anisotropic structures can be produced that better approximate the final tissue.
Background: During normal physical activities cartilage experiences dynamic compressive forces th... more Background: During normal physical activities cartilage experiences dynamic compressive forces that are essential to maintain cartilage integrity. However, at non-physiologic levels these signals can induce inflammation and initiate cartilage destruction. Here, by examining the pro-inflammatory signaling networks, we developed a mathematical model to show the magnitude-dependent regulation of chondrocytic responses by compressive forces.
Meshes of electrospun ͑ES͒ polycaprolactone ͑PCL͒ and polyethylene terephthalate nanofiber meshes... more Meshes of electrospun ͑ES͒ polycaprolactone ͑PCL͒ and polyethylene terephthalate nanofiber meshes were structured by ablation of linear grooves with a scanned femtosecond laser. Focus spot size, pulse energy, and scanning speed were varied to determine their affects on groove size and the characteristics of the electrospun fiber at the edges of these grooves. The femtosecond laser was seen to be an effective means for flexibly structuring the surface of ES PCL scaffolds. Femtosecond ablation resulted in much more uniformly ablated patterns compared to Q-switched nanosecond pulse laser ablation. Also, the width of the ablated grooves was well controlled by laser energy and focus spot size, although the grooves were significantly larger than the spot size. Also, some melting of fibers was observed at the edges of grooves. These affects were attributed to optical radiation from laser-induced plasma at higher pulse energies and melting of fibers at laser fluences lower than the ablation threshold. The ablation threshold for the PCL mesh was estimated to be significantly larger than that of bulk ͑solid͒ PCL, a result attributed to multiple scattering of the laser energy within the volume of the nanofiber mesh.
In this work, α-Fe2O3 nanofibers were synthesized via electrospinning and characterized to observ... more In this work, α-Fe2O3 nanofibers were synthesized via electrospinning and characterized to observe optimal morphological and dimensional properties towards chromate removal. The Fe2O3 nanofiber samples were tested in aqueous solutions containing chromate [Formula: see text] to analyze their adsorption capabilities and compare them with commercially-available Fe2O3 nanoparticles. Synthesized Fe2O3 nanofibers were observed with a variety of different average diameters, ranging from 23 to 63 nm, while having a constant average grain size at 34 nm, point zero charge at pH 7.1, and band gap at 2.2 eV. BET analysis showed an increase in specific surface area with decreasing average diameter, from 7.2 to 59.2 m(2)/g, due to the increased surface area-to-volume ratio with decreasing nanofiber size. Based on [Formula: see text] adsorption isotherms at pH 6, adsorption capacity of the Fe2O3 nanofibers increased with decreasing diameter, with the 23 nm sized nanofibers having an adsorption cap...
Critical Reviews™ in Eukaryotic Gene Expression, 2008
Cartilage is a mechanosensitive tissue, which means that it can perceive and respond to biomechan... more Cartilage is a mechanosensitive tissue, which means that it can perceive and respond to biomechanical signals. Despite the known importance of biomechanical signals in the etiopathogenesis of arthritic diseases and their effectiveness in joint restoration, little is understood about their actions at the cellular level. Recent molecular approaches have revealed that specific biomechanical stimuli and cell interactions generate intracellular signals that are powerful inducers or suppressors of proinflammatory and reparative genes in chondrocytes. Biomechanical signals are perceived by cartilage in magnitude-, frequency-, and time-dependent manners. Static and dynamic biomechanical forces of high magnitudes induce proinflammatory genes and inhibit matrix synthesis. Contrarily, dynamic biomechanical signals of low/physiologic magnitudes are potent antiinflammatory signals that inhibit interleukin-1beta (IL-1beta)-induced proinflammatory gene transcription and abrogate IL-1beta/tumor necrosis factor-alpha-induced inhibition of matrix synthesis. Recent studies have identified nuclear factor-kB (NF-kB) transcription factors as key regulators of biomechanical signal-mediated proinflammatory and antiinflammatory actions. These signals intercept multiple steps in the NF-kappaB signaling cascade to regulate cytokine gene expression. Taken together, these findings provide insight into how biomechanical signals regulate inflammatory and reparative gene transcription, underscoring their potential in enhancing the ability of chondrocytes to curb inflammation in diseased joints.
Label-free chemiresistive sensors based on a polypyrrole (PPy) nanoribbon (width: 500 nm, thickne... more Label-free chemiresistive sensors based on a polypyrrole (PPy) nanoribbon (width: 500 nm, thickness: 25-100 nm) were batch-fabricated by a lithographically patterned nanowire electrodeposition (LPNE) technique. A plant pathogen specific antibody was covalently conjugated on the surface of the
Journal of the mechanical behavior of biomedical materials, 2016
Mechanical factors among many physiochemical properties of scaffolds for stem cell-based tissue e... more Mechanical factors among many physiochemical properties of scaffolds for stem cell-based tissue engineering significantly affect tissue morphogenesis by controlling stem cell behaviors including proliferation and phenotype-specific differentiation. Core-shell electrospinning provides a unique opportunity to control mechanical properties of scaffolds independent of surface chemistry, rendering a greater freedom to tailor design for specific applications. In this study, we synthesized electrospun core-shell scaffolds having different core composition and/or core-to-shell dimensional ratios. Two independent biocompatible polymer systems, polyetherketoneketone (PEKK) and gelatin as the core materials while maintaining the shell polymer with polycaprolactone (PCL), were utilized. The mechanics of such scaffolds was analyzed at the microscale and macroscales to determine the potential implications it may hold for cell-material and tissue-material interactions. The mechanical properties of...
Transactions of the Korean Society of Mechanical Engineers B
Water management has been recognized as a crucial factor for achieving better performance and sta... more Water management has been recognized as a crucial factor for achieving better performance and stability in polymer electrolyte membrane fuel cells (PEMFCs). Proper water management should provide favorable water conditions, including the local humidity, membrane water content, and liquid water saturation in PEMFCs, thereby leading to more uniform electrochemical reaction and current generation. In this study, computational fluid dynamics (CFD) simulation was conducted to investigate the effects of the cathode relative humidity (RH) on the performance of a 3 by 3 cm 2 PEMFC with serpentine flow fields. The CFD results showed that the best performance of the PEMFC was obtained for the cathode RH of 80%, but the performance variation was small for the cathode RH range of 60~100%. However, the loss of the PEMFC performance was significant when the cathode RH was reduced below 40%. The reason for such performance variation was investigated through the detailed inspection of ohmic loss, a...
Chronic inflammation is one of the major causes of cartilage destruction in osteoarthritis. Here,... more Chronic inflammation is one of the major causes of cartilage destruction in osteoarthritis. Here, we systematically analyzed the changes in gene expression associated with the progression of cartilage destruction in monoiodoacetate-induced arthritis (MIA) of the rat knee. Sprague Dawley female rats were given intra-articular injection of monoiodoacetate in the knee. The progression of MIA was monitored macroscopically, microscopically and by micro-computed tomography. Grade 1 damage was observed by day 5 post-monoiodoacetate injection, progressively increasing to Grade 2 by day 9, and to Grade 3-3.5 by day 21. Affymetrix GeneChip was utilized to analyze the transcriptome-wide changes in gene expression, and the expression of salient genes was confirmed by real-time-PCR. Functional networks generated by Ingenuity Pathways Analysis (IPA) from the microarray data correlated the macroscopic/histologic findings with molecular interactions of genes/ gene products. Temporal changes in gene expression during the progression of MIA were categorized into five major gene clusters. IPA revealed that Grade 1 damage was associated with upregulation of acute/innate inflammatory responsive genes (Cluster I) and suppression of genes associated with musculoskeletal development and function (Cluster IV). Grade 2 damage was associated with upregulation of chronic inflammatory and immune trafficking genes (Cluster II) and downregulation of genes associated with musculoskeletal disorders (Cluster IV). The Grade 3 to 3.5 cartilage damage was associated with chronic inflammatory and immune adaptation genes (Cluster III). These findings suggest that temporal regulation of discrete gene clusters involving inflammatory mediators, receptors, and proteases may control the progression of cartilage destruction. In this process, IL-1b, TNF-a, IL-15, IL-12, chemokines, and NF-kB act as central nodes of the inflammatory networks, regulating catabolic processes. Simultaneously, upregulation of asporin, and downregulation of TGF-b complex, SOX-9, IGF and CTGF may be central to suppress matrix synthesis and chondrocytic anabolic activities, collectively contributing to the progression of cartilage destruction in MIA.
Exercise is vital for maintaining bone strength and architecture. Follistatin like 3 (FSTL3), a m... more Exercise is vital for maintaining bone strength and architecture. Follistatin like 3 (FSTL3), a member of Follistatin family, is a mechanosensitive protein upregulated in response to exercise and is involved in regulating musculoskeletal health, we investigated the potential role of FSTL3 in exercise-driven bone remodeling. Exercise-dependent regulation of bone structure and functions was compared in mice with global Fstl3 gene deletion (Fstl3-/-) and their age-matched Fstl3+/+ littermates. Mice were exercised by low-intensity treadmill walking. The mechanical properties and mineralization were determined by μCT, three-point bending test and sequential incorporation of calcein and alizarin complexone. ELISA, Western-blot analysis and qRT-PCR were used to analyze the regulation of FSTL3 and associated molecules in the serum specimens and tissues. Daily exercise significantly increased circulating FSTL3 levels in mice, rats and humans. Compared to age-matched littermates, Fstl3-/- mic...
Electrospinning provides a means to synthesize nanofibrous structures with very high surface area... more Electrospinning provides a means to synthesize nanofibrous structures with very high surface area-tovolume ratio, which enhance the sensitivity of conductive polymer (CP)-based gas sensors in a cost effective manner. To enhance processability, insulating host polymers are often used with the CP for electrospinning. Unlike CPs, however, the contribution of insulating polymers on overall sensing performance of composites has not been systemically investigated. In this study, we examined the effects of insulating polymers in electrospun composite nanofibers on the sensitivity to various analytes. Different composition ratios of polyaniline (PANI)/poly(-caprolactone) (PCL) nanofibers were produced by electrospinning, and their structure and chemistry were characterized. The PANI/PCL electrospun composite nanofibers were configured in a chemiresistor and subjected to different analytes, including H 2 O vapor, NH 3 , and NO 2 . H 2 O vapor and NO 2 showed a polarity change in sensitivity, having a compositional threshold of PANI-to-PCL ratio. To investigate this polarity change, the temperature dependence of electrical conductivity was examined. When H 2 O vapor was exposed to the composite with the highest PANI content at 20 wt%, there was a decrease in hopping distance; on the other hand, an increase in hopping distance was observed when H 2 O vapor was exposed to the composite with the lowest PANI content at 9 wt%. These results show an existence of competition between the conductive polymer, PANI, and the insulating host polymer, PCL, for analyte interaction, both of which integratively determine the overall sensitivity. The work demonstrates that the host polymer plays an important role in structural swelling as well as chemical interaction with analytes, which critically modulate sensing behavior. (J. Nam). analytes by modifying functional side groups or blending with different polymers as a composite .
ABSTRACT Electrospinning is a promising fabrication method for three dimensional tissue engineeri... more ABSTRACT Electrospinning is a promising fabrication method for three dimensional tissue engineering scaffolds due to its ability to produce a nano-/micro-sized non-woven fibrous structure which resembles the natural extracellular matrix. We investigated the mechanical behavior of two different electrospun microstructures. Polycaprolactone (PCL) fibers with or without "point-bonding" exhibited different deformation behaviors having significant biomedical consequences. While fibers with point-bonded structure failed due to the generation of voids by the fracture of fiber interconnections under strain, fibers without point-bonds produced a 'bamboo' structure with fiber joining visible at higher levels of strain. In addition, gelatin and PCL were electrospun and the residual solvent contents were systematically investigated. A simple and effective means of reducing residual solvent content was developed. The interaction between these electrospun matrices and chondrocytic cells were compared to other topographies having the same chemistry. Electrospun polycaprolactone fibers supported better proliferation and extracellular matrix production than the corresponding semi-porous and dense surfaces and even, at some time points, glass surfaces. The intrinsic capability of electrospinning to produce high porosity appears to offset the relative hydrophobicity of polycaprolactone resulting in a more uniform cell seeding. Electrospun fibers induced a higher level of glycosaminoglycans (GAG) production by providing a 'dynamic scaffold' in which chondrocytes are able to maintain a morphology associated with the appropriate phenotype. Finally, based on this study, a method producing macro-pores within an electrospun scaffold was developed. With this method, not only can cellular infiltration into a thick electrospun scaffold be facilitated, but scaffolds having designed, anisotropic structures can be produced that better approximate the final tissue.
Background: During normal physical activities cartilage experiences dynamic compressive forces th... more Background: During normal physical activities cartilage experiences dynamic compressive forces that are essential to maintain cartilage integrity. However, at non-physiologic levels these signals can induce inflammation and initiate cartilage destruction. Here, by examining the pro-inflammatory signaling networks, we developed a mathematical model to show the magnitude-dependent regulation of chondrocytic responses by compressive forces.
Meshes of electrospun ͑ES͒ polycaprolactone ͑PCL͒ and polyethylene terephthalate nanofiber meshes... more Meshes of electrospun ͑ES͒ polycaprolactone ͑PCL͒ and polyethylene terephthalate nanofiber meshes were structured by ablation of linear grooves with a scanned femtosecond laser. Focus spot size, pulse energy, and scanning speed were varied to determine their affects on groove size and the characteristics of the electrospun fiber at the edges of these grooves. The femtosecond laser was seen to be an effective means for flexibly structuring the surface of ES PCL scaffolds. Femtosecond ablation resulted in much more uniformly ablated patterns compared to Q-switched nanosecond pulse laser ablation. Also, the width of the ablated grooves was well controlled by laser energy and focus spot size, although the grooves were significantly larger than the spot size. Also, some melting of fibers was observed at the edges of grooves. These affects were attributed to optical radiation from laser-induced plasma at higher pulse energies and melting of fibers at laser fluences lower than the ablation threshold. The ablation threshold for the PCL mesh was estimated to be significantly larger than that of bulk ͑solid͒ PCL, a result attributed to multiple scattering of the laser energy within the volume of the nanofiber mesh.
In this work, α-Fe2O3 nanofibers were synthesized via electrospinning and characterized to observ... more In this work, α-Fe2O3 nanofibers were synthesized via electrospinning and characterized to observe optimal morphological and dimensional properties towards chromate removal. The Fe2O3 nanofiber samples were tested in aqueous solutions containing chromate [Formula: see text] to analyze their adsorption capabilities and compare them with commercially-available Fe2O3 nanoparticles. Synthesized Fe2O3 nanofibers were observed with a variety of different average diameters, ranging from 23 to 63 nm, while having a constant average grain size at 34 nm, point zero charge at pH 7.1, and band gap at 2.2 eV. BET analysis showed an increase in specific surface area with decreasing average diameter, from 7.2 to 59.2 m(2)/g, due to the increased surface area-to-volume ratio with decreasing nanofiber size. Based on [Formula: see text] adsorption isotherms at pH 6, adsorption capacity of the Fe2O3 nanofibers increased with decreasing diameter, with the 23 nm sized nanofibers having an adsorption cap...
Critical Reviews™ in Eukaryotic Gene Expression, 2008
Cartilage is a mechanosensitive tissue, which means that it can perceive and respond to biomechan... more Cartilage is a mechanosensitive tissue, which means that it can perceive and respond to biomechanical signals. Despite the known importance of biomechanical signals in the etiopathogenesis of arthritic diseases and their effectiveness in joint restoration, little is understood about their actions at the cellular level. Recent molecular approaches have revealed that specific biomechanical stimuli and cell interactions generate intracellular signals that are powerful inducers or suppressors of proinflammatory and reparative genes in chondrocytes. Biomechanical signals are perceived by cartilage in magnitude-, frequency-, and time-dependent manners. Static and dynamic biomechanical forces of high magnitudes induce proinflammatory genes and inhibit matrix synthesis. Contrarily, dynamic biomechanical signals of low/physiologic magnitudes are potent antiinflammatory signals that inhibit interleukin-1beta (IL-1beta)-induced proinflammatory gene transcription and abrogate IL-1beta/tumor necrosis factor-alpha-induced inhibition of matrix synthesis. Recent studies have identified nuclear factor-kB (NF-kB) transcription factors as key regulators of biomechanical signal-mediated proinflammatory and antiinflammatory actions. These signals intercept multiple steps in the NF-kappaB signaling cascade to regulate cytokine gene expression. Taken together, these findings provide insight into how biomechanical signals regulate inflammatory and reparative gene transcription, underscoring their potential in enhancing the ability of chondrocytes to curb inflammation in diseased joints.
Label-free chemiresistive sensors based on a polypyrrole (PPy) nanoribbon (width: 500 nm, thickne... more Label-free chemiresistive sensors based on a polypyrrole (PPy) nanoribbon (width: 500 nm, thickness: 25-100 nm) were batch-fabricated by a lithographically patterned nanowire electrodeposition (LPNE) technique. A plant pathogen specific antibody was covalently conjugated on the surface of the
Journal of the mechanical behavior of biomedical materials, 2016
Mechanical factors among many physiochemical properties of scaffolds for stem cell-based tissue e... more Mechanical factors among many physiochemical properties of scaffolds for stem cell-based tissue engineering significantly affect tissue morphogenesis by controlling stem cell behaviors including proliferation and phenotype-specific differentiation. Core-shell electrospinning provides a unique opportunity to control mechanical properties of scaffolds independent of surface chemistry, rendering a greater freedom to tailor design for specific applications. In this study, we synthesized electrospun core-shell scaffolds having different core composition and/or core-to-shell dimensional ratios. Two independent biocompatible polymer systems, polyetherketoneketone (PEKK) and gelatin as the core materials while maintaining the shell polymer with polycaprolactone (PCL), were utilized. The mechanics of such scaffolds was analyzed at the microscale and macroscales to determine the potential implications it may hold for cell-material and tissue-material interactions. The mechanical properties of...
Transactions of the Korean Society of Mechanical Engineers B
Water management has been recognized as a crucial factor for achieving better performance and sta... more Water management has been recognized as a crucial factor for achieving better performance and stability in polymer electrolyte membrane fuel cells (PEMFCs). Proper water management should provide favorable water conditions, including the local humidity, membrane water content, and liquid water saturation in PEMFCs, thereby leading to more uniform electrochemical reaction and current generation. In this study, computational fluid dynamics (CFD) simulation was conducted to investigate the effects of the cathode relative humidity (RH) on the performance of a 3 by 3 cm 2 PEMFC with serpentine flow fields. The CFD results showed that the best performance of the PEMFC was obtained for the cathode RH of 80%, but the performance variation was small for the cathode RH range of 60~100%. However, the loss of the PEMFC performance was significant when the cathode RH was reduced below 40%. The reason for such performance variation was investigated through the detailed inspection of ohmic loss, a...
Chronic inflammation is one of the major causes of cartilage destruction in osteoarthritis. Here,... more Chronic inflammation is one of the major causes of cartilage destruction in osteoarthritis. Here, we systematically analyzed the changes in gene expression associated with the progression of cartilage destruction in monoiodoacetate-induced arthritis (MIA) of the rat knee. Sprague Dawley female rats were given intra-articular injection of monoiodoacetate in the knee. The progression of MIA was monitored macroscopically, microscopically and by micro-computed tomography. Grade 1 damage was observed by day 5 post-monoiodoacetate injection, progressively increasing to Grade 2 by day 9, and to Grade 3-3.5 by day 21. Affymetrix GeneChip was utilized to analyze the transcriptome-wide changes in gene expression, and the expression of salient genes was confirmed by real-time-PCR. Functional networks generated by Ingenuity Pathways Analysis (IPA) from the microarray data correlated the macroscopic/histologic findings with molecular interactions of genes/ gene products. Temporal changes in gene expression during the progression of MIA were categorized into five major gene clusters. IPA revealed that Grade 1 damage was associated with upregulation of acute/innate inflammatory responsive genes (Cluster I) and suppression of genes associated with musculoskeletal development and function (Cluster IV). Grade 2 damage was associated with upregulation of chronic inflammatory and immune trafficking genes (Cluster II) and downregulation of genes associated with musculoskeletal disorders (Cluster IV). The Grade 3 to 3.5 cartilage damage was associated with chronic inflammatory and immune adaptation genes (Cluster III). These findings suggest that temporal regulation of discrete gene clusters involving inflammatory mediators, receptors, and proteases may control the progression of cartilage destruction. In this process, IL-1b, TNF-a, IL-15, IL-12, chemokines, and NF-kB act as central nodes of the inflammatory networks, regulating catabolic processes. Simultaneously, upregulation of asporin, and downregulation of TGF-b complex, SOX-9, IGF and CTGF may be central to suppress matrix synthesis and chondrocytic anabolic activities, collectively contributing to the progression of cartilage destruction in MIA.
Exercise is vital for maintaining bone strength and architecture. Follistatin like 3 (FSTL3), a m... more Exercise is vital for maintaining bone strength and architecture. Follistatin like 3 (FSTL3), a member of Follistatin family, is a mechanosensitive protein upregulated in response to exercise and is involved in regulating musculoskeletal health, we investigated the potential role of FSTL3 in exercise-driven bone remodeling. Exercise-dependent regulation of bone structure and functions was compared in mice with global Fstl3 gene deletion (Fstl3-/-) and their age-matched Fstl3+/+ littermates. Mice were exercised by low-intensity treadmill walking. The mechanical properties and mineralization were determined by μCT, three-point bending test and sequential incorporation of calcein and alizarin complexone. ELISA, Western-blot analysis and qRT-PCR were used to analyze the regulation of FSTL3 and associated molecules in the serum specimens and tissues. Daily exercise significantly increased circulating FSTL3 levels in mice, rats and humans. Compared to age-matched littermates, Fstl3-/- mic...
Electrospinning provides a means to synthesize nanofibrous structures with very high surface area... more Electrospinning provides a means to synthesize nanofibrous structures with very high surface area-tovolume ratio, which enhance the sensitivity of conductive polymer (CP)-based gas sensors in a cost effective manner. To enhance processability, insulating host polymers are often used with the CP for electrospinning. Unlike CPs, however, the contribution of insulating polymers on overall sensing performance of composites has not been systemically investigated. In this study, we examined the effects of insulating polymers in electrospun composite nanofibers on the sensitivity to various analytes. Different composition ratios of polyaniline (PANI)/poly(-caprolactone) (PCL) nanofibers were produced by electrospinning, and their structure and chemistry were characterized. The PANI/PCL electrospun composite nanofibers were configured in a chemiresistor and subjected to different analytes, including H 2 O vapor, NH 3 , and NO 2 . H 2 O vapor and NO 2 showed a polarity change in sensitivity, having a compositional threshold of PANI-to-PCL ratio. To investigate this polarity change, the temperature dependence of electrical conductivity was examined. When H 2 O vapor was exposed to the composite with the highest PANI content at 20 wt%, there was a decrease in hopping distance; on the other hand, an increase in hopping distance was observed when H 2 O vapor was exposed to the composite with the lowest PANI content at 9 wt%. These results show an existence of competition between the conductive polymer, PANI, and the insulating host polymer, PCL, for analyte interaction, both of which integratively determine the overall sensitivity. The work demonstrates that the host polymer plays an important role in structural swelling as well as chemical interaction with analytes, which critically modulate sensing behavior. (J. Nam). analytes by modifying functional side groups or blending with different polymers as a composite .
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