The Experts below are selected from a list of 336 Experts worldwide ranked by ideXlab platform
Shaun Eshraghi - One of the best experts on this subject based on the ideXlab platform.
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mechanical and microstructural properties of polycaprolactone scaffolds with one dimensional two dimensional and three dimensional orthogonally oriented porous architectures produced by selective laser sintering
Acta Biomaterialia, 2010Co-Authors: Shaun Eshraghi, Suman DasAbstract:In the US, approximately 1.5 million surgical procedures are conducted annually to repair damaged or fractured bone [1]. Conventional treatments involve the use of autogenous bone-grafts or allogenic bone. However, the amount of donor tissue available and complications at the donor site are limiting factors for autogenous bone grafting. In the case of allogenic bone-grafts, cell-mediated immune responses and pathogen transfer can be problematic [2–4]. Tissue engineering has the potential to resolve these areas of concern. It holds great promise for providing improved patient care and decreased health care costs by reducing the number of surgical procedures and recovery time associated with current medical practices. Tissue engineering focuses on the use of cells and engineered materials to restore tissue as opposed to making repairs using autografts, allografts, and prosthetics [5, 6]. It has been observed that isolated cells are unable to form mechanically and physiologically suitable neotissues if growth is left unassisted [7]. Consequently, tissue engineering typically involves the use of porous, bioresorbable scaffolds to serve as temporary, three-dimensional scaffolds to guide cell attachment, differentiation, proliferation, and subsequent tissue regeneration. Recent research strongly suggests that the choice of scaffold material and its internal porous architecture significantly affect regenerate tissue type, structure, and function [8, 9]. The effects of mean pore size has been extensively studied [8, 10–16], and Chang et al showed that the direction of bone ingrowth was along the long axis of the porous channels [10]. In addition to possessing the appropriate material composition and internal pore architecture for regenerating a specific target tissue, scaffolds must also have mechanical properties appropriate to support the newly formed tissue [17, 18]. In the case of bone, failure to provide adequate mechanical load-bearing function will cause a loss of function as the scaffold defines the ultimate shape of the new bone [18]. Conventional methods for scaffold fabrication rely on a variety of techniques involving the use of woven and non-woven fabrics, solvent casting and particulate leaching, solution casting and gel casting with porogens, pressurized gas foaming, forging, injection molding, cold or hot pressing, and electrospinning. However, these methods provide inadequate control over the porous architecture and require a separate mold for each exterior geometry [19]. An alternate approach known as direct digital Manufacturing (DDM) combines the use of computer-aided design (CAD) and finite element analysis (FEA) techniques to design scaffolds with interior porous architectures that achieve the desired effective mechanical properties, and the use of solid freeform fabrication (SFF) methods to construct scaffolds that are reproducible, cost-effective, and consistent with these designs. SFF methods build three-dimensional objects layer-by-layer, depositing or consolidating materials in selected regions thereby enabling the rapid investigation of a wide range of scaffold geometries with a high degree of dimensional control and with fewer limitations on the scaffold exterior shape or the porous architecture. Consequently, scaffold fabrication through SFF has received worldwide attention. Extensive reviews on the emerging use of solid freeform fabrication and computer-aided design methods in tissue engineering are available in the literature [7, 18–29]. Selective Laser Sintering (SLS) is a laser-based SFF technique in which an object is built layer-by-layer using powdered materials, radiant heaters, and a computer controlled laser [30]. In SLS, the digital representation of an object is mathematically sliced into a number of thin layers. The object is then created by scanning a laser beam and selectively fusing (melting or sintering) patterns into sequentially deposited layers of a powder. Each patterned layer of powder is also fused to its underlying layer and corresponds to a cross-section of the object as determined from the mathematical slicing operation. This Layered Manufacturing method allows the fabrication of scaffolds with a high degree of geometric complexity and enables the direct conversion of a scaffold’s computer model into its physical realization—allowing patient-specific and tissue-specific reconstruction strategies to be easily developed [22, 31–37]. In order to investigate the capabilities of a scaffold fabrication technique such as selective laser sintering in making functional scaffolds for load-bearing tissues, it is crucially important to assess the attainable mechanical properties both in bulk and in porous specimens, and to compare them against published data. SLS of non-resorbable materials has been investigated by several groups worldwide for biomedical applications such as tissue engineering and drug delivery [38–46]. More recently, the SLS has been used to fabricate tissue engineering scaffolds from bioresorbable polymeric biomaterials and their composites [47–58]. While these efforts have shown promise by documenting the feasibility of fusing powder particles together by laser sintering of such materials to form scaffold, they have not reported on the range of attainable mechanical properties when the designed solid regions, i.e. the struts of the scaffold, are fully or near-fully dense. In a previous article [57], we demonstrated a bone tissue engineering approach using polycaprolactone scaffolds fabricated by SLS. The porosity of these scaffolds was between 37 and 55%, the compressive modulus of such scaffolds was in the 52–68MPa range, and the ultimate compressive strength was in the 2.0–3.2MPa range. Such scaffolds were shown to have ompressive mechanical properties within the reported lower range of properties for human trabecular bone. However, in that work, the scaffolds were shown to be incompletely dense in the designed solid regions resulting in approximately 20% porosity where none was intended. This porosity, termed Manufacturing-induced porosity, results from the incomplete coalescence of polymer particles during sintering, melting and resolidification. Furthermore, the designed porosity composed of orthogonal porous channels, was not faithfully reproduced according to design due to excess powder being sintered and bonded to the pore channel interior surfaces, resulting in actual total porosity of the scaffold being less than the design porosity. Both of these effects were attributed to the use of sub-optimal SLS processing parameters, including the laser power, the scan speed, and the powder bed preheat temperature. Subsequently, we conducted a thorough study to identify optimal SLS processing parameters (Table 1) based on a design of experiments approach [58]. This study resulted in the development of processing parameters that achieved densities in excess of 95% relative density in the designed solid regions of the scaffold while faithfully constructing the overall scaffold and the pore geometry to within 3–8% of designed dimensions. Table 1 Optimal SLS parameter settings for processing PCL as determined by Partee et al [58]. This article reports on the tensile and compressive mechanical property testing, finite element analysis and microtomographic microstructure assessments carried out on PCL scaffolds produced by SLS using the improved processing parameters. To the best of our knowledge, this article represents the first effort to report both tensile and compressive mechanical properties of SLS-processed, solid PCL and porous PCL specimens using the same Manufacturing technique, under identical conditions. The capability of producing anatomic shaped scaffolds with complex designed porous architecture and the potential of SLS as a versatile technique for fabricating functional bone tissue engineering scaffolds for load bearing applications can then be considered a direct extension of this work [57, 58].
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mechanical and microstructural properties of polycaprolactone scaffolds with one dimensional two dimensional and three dimensional orthogonally oriented porous architectures produced by selective laser sintering
Acta Biomaterialia, 2010Co-Authors: Shaun EshraghiAbstract:Abstract This article reports on the experimental determination and finite element modeling of tensile and compressive mechanical properties of solid polycaprolactone (PCL) and of porous PCL scaffolds with one-dimensional, two-dimensional and three-dimensional orthogonal, periodic porous architectures produced by selective laser sintering (SLS). PCL scaffolds were built using optimum processing parameters, ensuring scaffolds with nearly full density (>95%) in the designed solid regions and with excellent geometric and dimensional control (within 3–8% of design). The tensile strength of bulk PCL ranged from 10.5 to 16.1 MPa, its modulus ranged from 343.9 to 364.3 MPa, and the tensile yield strength ranged from 8.2 to 10.1 MPa. These values are consistent with reported literature values for PCL processed through various Manufacturing methods. Across porosity ranged from 56.87% to 83.3%, the tensile strength ranged from 4.5 to 1.1 MPa, the tensile modulus ranged from 140.5 to 35.5 MPa, and the yield strength ranged from 3.2 to 0.76 MPa. The compressive strength of bulk PCL was 38.7 MPa, the compressive modulus ranged from 297.8 to 317.1 MPa, and the compressive yield strength ranged from 10.3 to 12.5 MPa. Across porosity ranged from 51.1% to 80.9%, the compressive strength ranged from 10.0 to 0.6 MPa, the compressive modulus ranged from 14.9 to 12.1 MPa, and the compressive yield strength ranged from 4.25 to 0.42 MPa. These values, while being in the lower range of reported values for trabecular bone, are the highest reported for PCL scaffolds produced by SLS and are among the highest reported for similar PCL scaffolds produced through other Layered Manufacturing techniques. Finite element analysis showed good agreement between experimental and computed effective tensile and compressive moduli. Thus, the construction of bone tissue engineering scaffolds endowed with oriented porous architectures and with predictable mechanical properties through SLS is demonstrated.
Suman Das - One of the best experts on this subject based on the ideXlab platform.
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mechanical and microstructural properties of polycaprolactone scaffolds with one dimensional two dimensional and three dimensional orthogonally oriented porous architectures produced by selective laser sintering
Acta Biomaterialia, 2010Co-Authors: Shaun Eshraghi, Suman DasAbstract:In the US, approximately 1.5 million surgical procedures are conducted annually to repair damaged or fractured bone [1]. Conventional treatments involve the use of autogenous bone-grafts or allogenic bone. However, the amount of donor tissue available and complications at the donor site are limiting factors for autogenous bone grafting. In the case of allogenic bone-grafts, cell-mediated immune responses and pathogen transfer can be problematic [2–4]. Tissue engineering has the potential to resolve these areas of concern. It holds great promise for providing improved patient care and decreased health care costs by reducing the number of surgical procedures and recovery time associated with current medical practices. Tissue engineering focuses on the use of cells and engineered materials to restore tissue as opposed to making repairs using autografts, allografts, and prosthetics [5, 6]. It has been observed that isolated cells are unable to form mechanically and physiologically suitable neotissues if growth is left unassisted [7]. Consequently, tissue engineering typically involves the use of porous, bioresorbable scaffolds to serve as temporary, three-dimensional scaffolds to guide cell attachment, differentiation, proliferation, and subsequent tissue regeneration. Recent research strongly suggests that the choice of scaffold material and its internal porous architecture significantly affect regenerate tissue type, structure, and function [8, 9]. The effects of mean pore size has been extensively studied [8, 10–16], and Chang et al showed that the direction of bone ingrowth was along the long axis of the porous channels [10]. In addition to possessing the appropriate material composition and internal pore architecture for regenerating a specific target tissue, scaffolds must also have mechanical properties appropriate to support the newly formed tissue [17, 18]. In the case of bone, failure to provide adequate mechanical load-bearing function will cause a loss of function as the scaffold defines the ultimate shape of the new bone [18]. Conventional methods for scaffold fabrication rely on a variety of techniques involving the use of woven and non-woven fabrics, solvent casting and particulate leaching, solution casting and gel casting with porogens, pressurized gas foaming, forging, injection molding, cold or hot pressing, and electrospinning. However, these methods provide inadequate control over the porous architecture and require a separate mold for each exterior geometry [19]. An alternate approach known as direct digital Manufacturing (DDM) combines the use of computer-aided design (CAD) and finite element analysis (FEA) techniques to design scaffolds with interior porous architectures that achieve the desired effective mechanical properties, and the use of solid freeform fabrication (SFF) methods to construct scaffolds that are reproducible, cost-effective, and consistent with these designs. SFF methods build three-dimensional objects layer-by-layer, depositing or consolidating materials in selected regions thereby enabling the rapid investigation of a wide range of scaffold geometries with a high degree of dimensional control and with fewer limitations on the scaffold exterior shape or the porous architecture. Consequently, scaffold fabrication through SFF has received worldwide attention. Extensive reviews on the emerging use of solid freeform fabrication and computer-aided design methods in tissue engineering are available in the literature [7, 18–29]. Selective Laser Sintering (SLS) is a laser-based SFF technique in which an object is built layer-by-layer using powdered materials, radiant heaters, and a computer controlled laser [30]. In SLS, the digital representation of an object is mathematically sliced into a number of thin layers. The object is then created by scanning a laser beam and selectively fusing (melting or sintering) patterns into sequentially deposited layers of a powder. Each patterned layer of powder is also fused to its underlying layer and corresponds to a cross-section of the object as determined from the mathematical slicing operation. This Layered Manufacturing method allows the fabrication of scaffolds with a high degree of geometric complexity and enables the direct conversion of a scaffold’s computer model into its physical realization—allowing patient-specific and tissue-specific reconstruction strategies to be easily developed [22, 31–37]. In order to investigate the capabilities of a scaffold fabrication technique such as selective laser sintering in making functional scaffolds for load-bearing tissues, it is crucially important to assess the attainable mechanical properties both in bulk and in porous specimens, and to compare them against published data. SLS of non-resorbable materials has been investigated by several groups worldwide for biomedical applications such as tissue engineering and drug delivery [38–46]. More recently, the SLS has been used to fabricate tissue engineering scaffolds from bioresorbable polymeric biomaterials and their composites [47–58]. While these efforts have shown promise by documenting the feasibility of fusing powder particles together by laser sintering of such materials to form scaffold, they have not reported on the range of attainable mechanical properties when the designed solid regions, i.e. the struts of the scaffold, are fully or near-fully dense. In a previous article [57], we demonstrated a bone tissue engineering approach using polycaprolactone scaffolds fabricated by SLS. The porosity of these scaffolds was between 37 and 55%, the compressive modulus of such scaffolds was in the 52–68MPa range, and the ultimate compressive strength was in the 2.0–3.2MPa range. Such scaffolds were shown to have ompressive mechanical properties within the reported lower range of properties for human trabecular bone. However, in that work, the scaffolds were shown to be incompletely dense in the designed solid regions resulting in approximately 20% porosity where none was intended. This porosity, termed Manufacturing-induced porosity, results from the incomplete coalescence of polymer particles during sintering, melting and resolidification. Furthermore, the designed porosity composed of orthogonal porous channels, was not faithfully reproduced according to design due to excess powder being sintered and bonded to the pore channel interior surfaces, resulting in actual total porosity of the scaffold being less than the design porosity. Both of these effects were attributed to the use of sub-optimal SLS processing parameters, including the laser power, the scan speed, and the powder bed preheat temperature. Subsequently, we conducted a thorough study to identify optimal SLS processing parameters (Table 1) based on a design of experiments approach [58]. This study resulted in the development of processing parameters that achieved densities in excess of 95% relative density in the designed solid regions of the scaffold while faithfully constructing the overall scaffold and the pore geometry to within 3–8% of designed dimensions. Table 1 Optimal SLS parameter settings for processing PCL as determined by Partee et al [58]. This article reports on the tensile and compressive mechanical property testing, finite element analysis and microtomographic microstructure assessments carried out on PCL scaffolds produced by SLS using the improved processing parameters. To the best of our knowledge, this article represents the first effort to report both tensile and compressive mechanical properties of SLS-processed, solid PCL and porous PCL specimens using the same Manufacturing technique, under identical conditions. The capability of producing anatomic shaped scaffolds with complex designed porous architecture and the potential of SLS as a versatile technique for fabricating functional bone tissue engineering scaffolds for load bearing applications can then be considered a direct extension of this work [57, 58].
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processing and characterization of a carbon black filled electrically conductive nylon 12 nanocomposite produced by selective laser sintering
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2010Co-Authors: Siddharth Ram Athreya, Kyriaki Kalaitzidou, Suman DasAbstract:Abstract Selective laser sintering (SLS), a Layered Manufacturing technique was explored to process an electrically conductive polymer nanocomposite made of Nylon-12 reinforced with 4 wt% of carbon black. SLS process parameters were optimized in order to maximize the flexural modulus. The porosity and morphology were studied using optical microscopy and scanning electron microscopy (SEM). The crystalline state was characterized using differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The electrical conductivity was determined using the four probe technique. Results indicate that carbon black-filled Nylon-12 nanocomposites can be successfully made by SLS. Maximum flexural modulus values of 1750 MPa and 1450 MPa were achieved for the neat polymer and the nanocomposite, respectively. A reduction in the flexural modulus of the nanocomposite is likely due to the formation of a segregated structure in the nanocomposite and a weak polymer–filler interface. The optimized neat polymer and the nanocomposites had average densities of around 97% and 96% relative to full density, respectively. The electrical conductivity of the nanocomposite was approximately 1 × 10−4 S/cm, which is five orders of magnitude higher than that of the neat polymer processed by SLS, and indicates that the onset of percolation behavior occurs below the 4 wt% loading of carbon black.
Ryan B Wicker - One of the best experts on this subject based on the ideXlab platform.
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reticulated mesh arrays and dissimilar array monoliths by additive Layered Manufacturing using electron and laser beam melting
2010Co-Authors: Frank Medina, Ryan B Wicker, Lawrence E Murr, Sara M. GaytanAbstract:Compositions and methods for making a three dimensional structure comprising: designing a three-dimensional structure; melting the three-dimensional structure from two or more layers of a metal powder with a high energy electron or laser beam is described herein. The position where the metal is melted into the structure is formed along a layer of metal powder, wherein the location and intensity of the beam that strikes the metal layer is based on the three-dimensional structure and is controlled and directed by a processor. The instant invention comprises a novel dry state sonication step for removing metal powder that is not melted from the three dimensional structure.
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microstructure and mechanical behavior of ti 6al 4v produced by rapid layer Manufacturing for biomedical applications
Journal of The Mechanical Behavior of Biomedical Materials, 2009Co-Authors: L E Murr, Sara M. Gaytan, Stella Quinones, M I Lopez, A Rodela, E Martinez, D H Hernandez, F Medina, Ryan B WickerAbstract:Abstract The microstructure and mechanical behavior of simple product geometries produced by Layered Manufacturing using the electron beam melting (EBM) process and the selective laser melting (SLM) process are compared with those characteristic of conventional wrought and cast products of Ti–6Al–4V. Microstructures are characterized utilizing optical metallography (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and included α (hcp), β (bcc) and α ′ (hcp) martensite phase regimes which give rise to hardness variations ranging from HRC 37 to 57 and tensile strengths ranging from 0.9 to 1.45 GPa. The advantages and disadvantages of Layered Manufacturing utilizing initial powders in custom building of biomedical components by EBM and SLM in contrast to conventional Manufacturing from Ti–6Al–4V wrought bar stock are discussed.
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microstructure and mechanical behavior of ti 6al 4v produced by rapid layer Manufacturing for biomedical applications
Journal of The Mechanical Behavior of Biomedical Materials, 2009Co-Authors: L E Murr, Sara M. Gaytan, Stella Quinones, M I Lopez, A Rodela, E Martinez, D H Hernandez, F Medina, Ryan B WickerAbstract:The microstructure and mechanical behavior of simple product geometries produced by Layered Manufacturing using the electron beam melting (EBM) process and the selective laser melting (SLM) process are compared with those characteristic of conventional wrought and cast products of Ti-6Al-4V. Microstructures are characterized utilizing optical metallography (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and included alpha (hcp), beta (bcc) and alpha(') (hcp) martensite phase regimes which give rise to hardness variations ranging from HRC 37 to 57 and tensile strengths ranging from 0.9 to 1.45 GPa. The advantages and disadvantages of Layered Manufacturing utilizing initial powders in custom building of biomedical components by EBM and SLM in contrast to conventional Manufacturing from Ti-6Al-4V wrought bar stock are discussed.
E Martinez - One of the best experts on this subject based on the ideXlab platform.
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microstructure and mechanical behavior of ti 6al 4v produced by rapid layer Manufacturing for biomedical applications
Journal of The Mechanical Behavior of Biomedical Materials, 2009Co-Authors: L E Murr, Sara M. Gaytan, Stella Quinones, M I Lopez, A Rodela, E Martinez, D H Hernandez, F Medina, Ryan B WickerAbstract:Abstract The microstructure and mechanical behavior of simple product geometries produced by Layered Manufacturing using the electron beam melting (EBM) process and the selective laser melting (SLM) process are compared with those characteristic of conventional wrought and cast products of Ti–6Al–4V. Microstructures are characterized utilizing optical metallography (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and included α (hcp), β (bcc) and α ′ (hcp) martensite phase regimes which give rise to hardness variations ranging from HRC 37 to 57 and tensile strengths ranging from 0.9 to 1.45 GPa. The advantages and disadvantages of Layered Manufacturing utilizing initial powders in custom building of biomedical components by EBM and SLM in contrast to conventional Manufacturing from Ti–6Al–4V wrought bar stock are discussed.
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microstructure and mechanical behavior of ti 6al 4v produced by rapid layer Manufacturing for biomedical applications
Journal of The Mechanical Behavior of Biomedical Materials, 2009Co-Authors: L E Murr, Sara M. Gaytan, Stella Quinones, M I Lopez, A Rodela, E Martinez, D H Hernandez, F Medina, Ryan B WickerAbstract:The microstructure and mechanical behavior of simple product geometries produced by Layered Manufacturing using the electron beam melting (EBM) process and the selective laser melting (SLM) process are compared with those characteristic of conventional wrought and cast products of Ti-6Al-4V. Microstructures are characterized utilizing optical metallography (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and included alpha (hcp), beta (bcc) and alpha(') (hcp) martensite phase regimes which give rise to hardness variations ranging from HRC 37 to 57 and tensile strengths ranging from 0.9 to 1.45 GPa. The advantages and disadvantages of Layered Manufacturing utilizing initial powders in custom building of biomedical components by EBM and SLM in contrast to conventional Manufacturing from Ti-6Al-4V wrought bar stock are discussed.
L E Murr - One of the best experts on this subject based on the ideXlab platform.
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microstructure and mechanical behavior of ti 6al 4v produced by rapid layer Manufacturing for biomedical applications
Journal of The Mechanical Behavior of Biomedical Materials, 2009Co-Authors: L E Murr, Sara M. Gaytan, Stella Quinones, M I Lopez, A Rodela, E Martinez, D H Hernandez, F Medina, Ryan B WickerAbstract:Abstract The microstructure and mechanical behavior of simple product geometries produced by Layered Manufacturing using the electron beam melting (EBM) process and the selective laser melting (SLM) process are compared with those characteristic of conventional wrought and cast products of Ti–6Al–4V. Microstructures are characterized utilizing optical metallography (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and included α (hcp), β (bcc) and α ′ (hcp) martensite phase regimes which give rise to hardness variations ranging from HRC 37 to 57 and tensile strengths ranging from 0.9 to 1.45 GPa. The advantages and disadvantages of Layered Manufacturing utilizing initial powders in custom building of biomedical components by EBM and SLM in contrast to conventional Manufacturing from Ti–6Al–4V wrought bar stock are discussed.
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microstructure and mechanical behavior of ti 6al 4v produced by rapid layer Manufacturing for biomedical applications
Journal of The Mechanical Behavior of Biomedical Materials, 2009Co-Authors: L E Murr, Sara M. Gaytan, Stella Quinones, M I Lopez, A Rodela, E Martinez, D H Hernandez, F Medina, Ryan B WickerAbstract:The microstructure and mechanical behavior of simple product geometries produced by Layered Manufacturing using the electron beam melting (EBM) process and the selective laser melting (SLM) process are compared with those characteristic of conventional wrought and cast products of Ti-6Al-4V. Microstructures are characterized utilizing optical metallography (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and included alpha (hcp), beta (bcc) and alpha(') (hcp) martensite phase regimes which give rise to hardness variations ranging from HRC 37 to 57 and tensile strengths ranging from 0.9 to 1.45 GPa. The advantages and disadvantages of Layered Manufacturing utilizing initial powders in custom building of biomedical components by EBM and SLM in contrast to conventional Manufacturing from Ti-6Al-4V wrought bar stock are discussed.