Influence of Annealing and Biaxial Expansion on the Properties of Poly(l-Lactic Acid) Medical Tubing

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.

Direct Preparation of High Thermal Stable PLA‐Based Nanocomposite via Extra‐Low Loading of In Situ Exfoliated Ultrathin MWW Zeolite Nanosheets

“…Although all samples exhibited the broad scattering reflections of amorphous phases (Figure 1D), a broad scattering reflection around ≈33° appeared in those of the MCM‐22(P)‐filled samples which are different from the typical XRD pattern of pure PLA. [ 11,30 ] The similar phenomenon was also found in the research on PLA samples with annealing or CO 2 treatments, [ 31–33 ] which is assigned to the (018) crystallographic planes of PLA. According to the general principle of XRD, it might imply the generation of the local ordered tiny clusters or nucleus of PLA molecular chain after adding MCM‐22(P) and raise the VST of the nanocomposites.…”

Interaction of Poly L-Lactide and Tungsten Disulfide Nanotubes Studied by In Situ X-ray Scattering during Expansion of PLLA/WS2NT Nanocomposite Tubes

“…In this and other applications, the limited strength and brittleness of PLLA in comparison to permanent metal stents also limit its use. To overcome these issues, previous research has investigated the use of alternative copolymers [ 29 , 30 ], the addition of nanoparticles [ 31 , 32 ], and process-induced microstructural modification [ 33 , 34 ].…”

Modelling Stretch Blow Moulding of Poly (l-lactic acid) for the Manufacture of Bioresorbable Vascular Scaffold

“…The process simulation had its practicality as there had been the operation of stretch blow moulding within this temperature window [ 13 , 16 , 56 ]. Since the processing condition covering higher temperature has been suggested [ 12 , 16 , 28 ], the applicability of the FEA modelling needs to be further addressed by the experiment at elevated forming temperature. As the forming process is a load-controlled deformation, the magnitude of the pressure influences the deformation behaviour significantly, which is not covered in the current study.…”

“…PLLA tubes usually have non-organised state of material morphology prepared by extrusion or solution casting [ 9 , 10 ]. By performing stretch blow moulding (SBM), the PLLA tubes are heated above the glass transition temperature ( T g ) then biaxially deformed inside a mould [ 11 , 12 , 13 ]. The PLLA material experiencing SBM gains the orientation and crystallisation of the morphology, and the stiffness, strength and ductility are significantly improved, which can be further machined by a femtosecond laser [ 11 , 13 , 14 ].…”

“…The operation sequence can affect the material morphology by applying a constant-width (CW) and equal biaxial (EB) stretch on PLLA sheets [ 23 , 24 , 25 ]. At biaxial deformation with high deformation rate (of over 1 s −1 ) similar to SBM [ 12 , 26 ], PLLA material showed strong dependence on strain rate and deformation mode at low-temperature level (of between 70 and 80 °C) by evident yielding stress and early strain hardening [ 27 ]. PLLA sheets or films were widely used in the previous study to investigate the mechanical behaviour, whilst the PLLA tubes have to be used in stretch blow moulding.…”