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Ortega Z, Douglas P, Hanna PR, Garrett G, Clarke A, Cunningham E, Suárez L. Characterization of PLA Sheets Prepared by Stretching under Different Conditions: Influence of Reprocessing and Establishing Optimal Conditions. MATERIALS (BASEL, SWITZERLAND) 2023; 16:5114. [PMID: 37512389 PMCID: PMC10386445 DOI: 10.3390/ma16145114] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/04/2023] [Revised: 07/13/2023] [Accepted: 07/18/2023] [Indexed: 07/30/2023]
Abstract
Polylactide (PLA) is one of the most commonly used biomaterials nowadays, with many recognized benefits, particularly in the packaging and single-use products industries. However, little research has been conducted on its stretching behavior. This work investigates the optimal conditions of biaxial stretching of injection-molded PLA samples produced under different processing conditions (pressure, drying, and pre-processing by extrusion, to simulate a recycling step). The injection-molded samples were characterized to determine their mechanical, thermal and thermo-mechanical behavior, water absorption, thermal behavior, and crystallization kinetics. The extruded samples showed reduced thermal stability, lower viscosity, decreased mechanical properties, and higher crystallization rates due to thermal degradation. However, the stretched samples provided similar properties regardless of the materials pre-processing. Regarding the assessment of the biaxial stretching process, processing at lower temperatures provides the films with a higher yield and breaking strength, while the time and strain rates have little influence on such properties. It was then determined that 82 °C is the optimal temperature for stretching the PLA samples. An increase in the stretch ratio provided a higher elastic modulus and higher values of opacity due to an increased crystallinity induced by stress during the process. Films as thin as 50 μm can be obtained by biaxially stretching injection-molded preforms, producing a deformation over 150% and acquiring good mechanical properties: about 90 MPa for the yield and a breaking strength and elastic modulus of 4000 MPa.
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Affiliation(s)
- Zaida Ortega
- Departamento de Ingeniería de Procesos, Universidad de Las Palmas de Gran Canaria, 35017 Las Palmas de Gran Canaria, Spain
| | - Paula Douglas
- Polymer Processing Research Centre, School of Mechanical and Aerospace Engineering, Ashby Building, Queen's University of Belfast, Belfast BT9 5AH, Northern Ireland, UK
| | - Paul R Hanna
- Polymer Processing Research Centre, School of Mechanical and Aerospace Engineering, Ashby Building, Queen's University of Belfast, Belfast BT9 5AH, Northern Ireland, UK
| | - Graham Garrett
- Polymer Processing Research Centre, School of Mechanical and Aerospace Engineering, Ashby Building, Queen's University of Belfast, Belfast BT9 5AH, Northern Ireland, UK
| | - Alan Clarke
- Polymer Processing Research Centre, School of Mechanical and Aerospace Engineering, Ashby Building, Queen's University of Belfast, Belfast BT9 5AH, Northern Ireland, UK
| | - Eoin Cunningham
- School of Mechanical and Aerospace Engineering, Ashby Building, Queen's University Belfast, Belfast BT9 5AH, Northern Ireland, UK
| | - Luis Suárez
- Departamento de Ingeniería Mecánica, Universidad de Las Palmas de Gran Canaria, 35017 Las Palmas de Gran Canaria, Spain
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Interaction of Poly L-Lactide and Tungsten Disulfide Nanotubes Studied by in Situ X-ray Scattering during Expansion of PLLA/WS 2NT Nanocomposite Tubes. Polymers (Basel) 2021; 13:polym13111764. [PMID: 34072208 PMCID: PMC8198810 DOI: 10.3390/polym13111764] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/21/2021] [Revised: 05/19/2021] [Accepted: 05/24/2021] [Indexed: 11/27/2022] Open
Abstract
In situ synchrotron X-ray scattering was used to reveal the transient microstructure of poly(L-lactide) (PLLA)/tungsten disulfide inorganic nanotubes (WS2NTs) nanocomposites. This microstructure is formed during the blow molding process (“tube expansion”) of an extruded polymer tube, an important step in the manufacturing of PLLA-based bioresorbable vascular scaffolds (BVS). A fundamental understanding of how such a microstructure develops during processing is relevant to two unmet needs in PLLA-based BVS: increasing strength to enable thinner devices and improving radiopacity to enable imaging during implantation. Here, we focus on how the flow generated during tube expansion affects the orientation of the WS2NTs and the formation of polymer crystals by comparing neat PLLA and nanocomposite tubes under different expansion conditions. Surprisingly, the WS2NTs remain oriented along the extrusion direction despite significant strain in the transverse direction while the PLLA crystals (c-axis) form along the circumferential direction of the tube. Although WS2NTs promote the nucleation of PLLA crystals in nanocomposite tubes, crystallization proceeds with largely the same orientation as in neat PLLA tubes. We suggest that the reason for the unusual independence of the orientations of the nanotubes and polymer crystals stems from the favorable interaction between PLLA and WS2NTs. This favorable interaction leads WS2NTs to disperse well in PLLA and strongly orient along the axis of the PLLA tube during extrusion. As a consequence, the nanotubes are aligned orthogonally to the circumferential stretching direction, which appears to decouple the orientations of PLLA crystals and WS2NTs.
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Optimisation on Thermoforming of Biodegradable Poly (Lactic Acid) (PLA) by Numerical Modelling. Polymers (Basel) 2021; 13:polym13040654. [PMID: 33671769 PMCID: PMC7926767 DOI: 10.3390/polym13040654] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/31/2021] [Revised: 02/12/2021] [Accepted: 02/17/2021] [Indexed: 11/16/2022] Open
Abstract
Poly (lactic acid) (PLA) has a broad perspective for manufacturing green thermoplastic products by thermoforming for its biodegradable properties. The mechanical behaviour of PLA has been demonstrated by its strong dependence on temperature and strain rate at biaxial deformation. A nonlinear viscoelastic model by the previous study was employed in a thermoforming process used for food packaging. An optimisation approach was developed by achieving the optimal temperature profile of specimens by defining multiple heating zones based on numerical modelling with finite element analysis (FEA). The forming process of a PLA product was illustrated by modelling results on shape evolution and biaxial strain history. The optimal temperature profile was suggested in scalloped zones to achieve more even thickness distribution. The sensitivity of the optimal results was addressed by checking the robustness under perturbation.
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Hua R, Tian Y, Cheng J, Wu G, Jiang W, Ni Z, Zhao G. The effect of intrinsic characteristics on mechanical properties of poly(l-lactic acid) bioresorbable vascular stents. Med Eng Phys 2020; 81:118-124. [PMID: 32482508 DOI: 10.1016/j.medengphy.2020.04.006] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/31/2019] [Revised: 03/23/2020] [Accepted: 04/05/2020] [Indexed: 12/31/2022]
Abstract
Poly(L-lactic acid) (PLLA) is currently the bioresorbable polymer of choice for vascular stents with its superior biocompatibility and mechanical properties. However, it is still difficult to enhance the radial supporting capacity of PLLA stents without increasing the strut thickness. In this study, the performance of laser-cut thin-strut stents from two groups of PLLA tubes are investigated. We considered two groups of PLLA tubes. Group 1 indicates the longitudinally stretched from original 150-μm-thick tubes, and Group 2 indicates the directly thinned from original 150-μm-thick tubes. Three stages of mechanical tests were conducted in this study, which are defined as tensile tests of dog-bone specimens, radial loading tests of tubes and radial loading tests of stents. The results suggest that Group 2 has higher radial supporting capacity than Group 1 with the same wall thickness. This work serves as a basis for manufacturing thin-strut stents with sufficient radial supporting capacity.
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Affiliation(s)
- Rixin Hua
- School of Mechanical Engineering, Jiangsu Key Laboratory for Design and Manufacture of Micro-Nano Biomedical Instruments, Southeast University, Nanjing, 211189, China
| | - Yuan Tian
- School of Mechanical Engineering, Jiangsu Key Laboratory for Design and Manufacture of Micro-Nano Biomedical Instruments, Southeast University, Nanjing, 211189, China
| | - Jie Cheng
- School of Mechanical Engineering, Jiangsu Key Laboratory for Design and Manufacture of Micro-Nano Biomedical Instruments, Southeast University, Nanjing, 211189, China
| | - Gensheng Wu
- School of Mechanical and Electronic Engineering, Nanjing Forestry University, Nanjing, 210037, China
| | - Wei Jiang
- State Key Laboratory of Pollution Control and Resources Reuse, School of the Environment, Nanjing University, Nanjing, 210023, China
| | - Zhonghua Ni
- School of Mechanical Engineering, Jiangsu Key Laboratory for Design and Manufacture of Micro-Nano Biomedical Instruments, Southeast University, Nanjing, 211189, China.
| | - Gutian Zhao
- School of Mechanical Engineering, Jiangsu Key Laboratory for Design and Manufacture of Micro-Nano Biomedical Instruments, Southeast University, Nanjing, 211189, China.
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Blair R, Dunne N, Lennon A, Menary G. Characterisation and constitutive modelling of biaxially stretched poly(L-lactic acid) sheet for application in coronary stents. J Mech Behav Biomed Mater 2019; 97:346-354. [DOI: 10.1016/j.jmbbm.2019.05.039] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/05/2018] [Revised: 05/22/2019] [Accepted: 05/23/2019] [Indexed: 10/26/2022]
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Blair RW, Dunne NJ, Lennon AB, Menary GH. Multi-objective optimisation of material properties and strut geometry for poly(L-lactic acid) coronary stents using response surface methodology. PLoS One 2019; 14:e0218768. [PMID: 31449528 PMCID: PMC6709949 DOI: 10.1371/journal.pone.0218768] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/08/2019] [Accepted: 08/10/2019] [Indexed: 11/29/2022] Open
Abstract
Coronary stents for treating atherosclerosis are traditionally manufactured from metallic alloys. However, metal stents permanently reside in the body and may trigger undesirable immunological responses. Bioresorbable polymer stents can provide a temporary scaffold that resorbs once the artery heals but are mechanically inferior, requiring thicker struts for equivalent radial support, which may increase thrombosis risk. This study addresses the challenge of designing mechanically effective but sufficiently thin poly(L-lactic acid) stents through a computational approach that optimises material properties and stent geometry. Forty parametric stent designs were generated: cross-sectional area (post-dilation), foreshortening, stent-to-artery ratio and radial collapse pressure were evaluated computationally using finite element analysis. Response surface methodology was used to identify performance trade-offs by formulating relationships between design parameters and response variables. Multi-objective optimisation was used to identify suitable stent designs from approximated Pareto fronts and an optimal design is proposed that offers comparable performance to designs in clinical practice. In summary, a computational framework has been developed that has potential application in the design of high stiffness, thin strut polymeric stents.
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Affiliation(s)
- Ross W. Blair
- School of Mechanical and Aerospace Engineering, Queen’s University, Belfast, United Kingdom
- * E-mail:
| | - Nicholas J. Dunne
- School of Mechanical and Manufacturing Engineering, Dublin City University, Dublin, Ireland
- Centre for Medical Engineering Research, School of Mechanical and Manufacturing Engineering, Dublin City University, Dublin, Ireland
- School of Pharmacy, Queen’s University, Belfast, United Kingdom
- Trinity Centre for Bioengineering, Trinity College Dublin, Dublin, Ireland
- Advanced Materials and Bioengineering Research Centre (AMBER), Royal College of Surgeons in Ireland and Trinity College Dublin, Dublin, Ireland
| | - Alex B. Lennon
- School of Mechanical and Aerospace Engineering, Queen’s University, Belfast, United Kingdom
| | - Gary H. Menary
- School of Mechanical and Aerospace Engineering, Queen’s University, Belfast, United Kingdom
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Sweeney J, Spencer P, Nair K, Coates P. Modelling the Mechanical and Strain Recovery Behaviour of Partially Crystalline PLA. Polymers (Basel) 2019; 11:polym11081342. [PMID: 31412588 PMCID: PMC6723906 DOI: 10.3390/polym11081342] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/04/2019] [Revised: 08/07/2019] [Accepted: 08/09/2019] [Indexed: 11/29/2022] Open
Abstract
This is a study of the modelling and prediction of strain recovery in a polylactide. Strain recovery near the glass transition temperature is the underlying mechanism for the shape memory in an amorphous polymer. The investigation is aimed at modelling such shape memory behaviour. A PLA-based copolymer is subjected to stress–strain, stress relaxation and strain recovery experiments at large strain at 60 °C just below its glass transition temperature. The material is 13% crystalline. Using published data on the mechanical properties of the crystals, finite element modelling was used to determine the effect of the crystal phase on the overall mechanical behaviour of the material, which was found to be significant. The finite element models were also used to relate the stress–strain results to the yield stress of the amorphous phase. This yield stress was found to possess strain rate dependence consistent with an Eyring process. Stress relaxation experiments were also interpreted in terms of the Eyring process, and a two-process Eyring-based model was defined that was capable of modelling strain recovery behaviour. This was essentially a model of the amorphous phase. It was shown to be capable of useful predictions of strain recovery.
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Affiliation(s)
- John Sweeney
- IRC in Polymer Science and Technology, Mechanical and Energy Systems Engineering, Faculty of Engineering and Informatics, University of Bradford, Bradford BD7 1DP, UK.
| | - Paul Spencer
- IRC in Polymer Science and Technology, Mechanical and Energy Systems Engineering, Faculty of Engineering and Informatics, University of Bradford, Bradford BD7 1DP, UK
| | - Karthik Nair
- IRC in Polymer Science and Technology, Mechanical and Energy Systems Engineering, Faculty of Engineering and Informatics, University of Bradford, Bradford BD7 1DP, UK
| | - Phil Coates
- IRC in Polymer Science and Technology, Mechanical and Energy Systems Engineering, Faculty of Engineering and Informatics, University of Bradford, Bradford BD7 1DP, UK
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Influence of Annealing and Biaxial Expansion on the Properties of Poly(l-Lactic Acid) Medical Tubing. Polymers (Basel) 2019; 11:polym11071172. [PMID: 31373323 PMCID: PMC6680782 DOI: 10.3390/polym11071172] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/11/2019] [Revised: 07/04/2019] [Accepted: 07/05/2019] [Indexed: 01/30/2023] Open
Abstract
Poly-l-lactic acid (PLLA) is one of the most common bioabsorbable materials in the medical device field. However, its use in load-bearing applications is limited due to its inferior mechanical properties when compared to many of the competing metal-based permanent and bioabsorbable materials. The objective of this study was to directly compare the influence of both annealing and biaxial expansion processes to improve the material properties of PLLA. Results showed that both annealing and biaxial expansion led to an overall increase in crystallinity and that the crystallites formed during both processes were in the α’ and α forms. 2D-WAXS patterns showed that the preferred orientation of crystallites formed during annealing was parallel to the circumferential direction. While biaxial expansion resulted in orientation in both axial and circumferential directions, with relatively equal sized crystals in both directions, Da (112 Å) and Dc (97 Å). The expansion process had the most profound effect on mechanical performance, with a 65% increase in Young’s modulus, a 45% increase in maximum tensile stress and an 18-fold increase in strain at maximum load. These results indicate that biaxially expanding PLLA at a temperature above Tcc is possible, due to the high strain rates associated with stretch blow moulding.
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Mechanical and chemical characterisation of bioresorbable polymeric stent over two-year in vitro degradation. J Biomater Appl 2019; 34:61-73. [DOI: 10.1177/0885328219839591] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
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