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

Brian Dillon,Andrew V Healy,John G Lyons,Noel M Gately,Ian Major,Evert Fuenmayor,Patrick Doran

doi:10.3390/polym11071172

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.

Highlights

Poly-l-lactic acid (PLLA) is an aliphatic polyester derived from renewable resources
Thermal analysis using differential scanning calorimetry (DSC) was carried out on Annealed and Expanded tubing, to determine the effect different processing conditions had on the thermal properties and Xc of the material
There was a very subtle drop in modulus between 21 ◦ C, 37 ◦ C and 50 ◦ C, which are analogous with the results reported in a previous study [49] and highlight the temperature dependence of mechanical properties below the glass transition temperature in PLLA tubing

Summary

Introduction

Poly-l-lactic acid (PLLA) is an aliphatic polyester derived from renewable resources. Despite having reasonably good mechanical strength compared to other bioabsorbable polymers, with a tensile strength of 60–70 MPa and elastic modulus of 2–4 GPa, it is limited in its use due to its brittle nature with less than 5% strain and poor fracture toughness [3]. It has inferior mechanical properties compared to many of the competing metal-based permanent and bioabsorbable materials used in similar load-bearing medical applications, such as scaffolding, orthopaedics and fixation devices

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Conclusion