1. M. Ward, A.-L. Brisse, G.Y.H. Choong, A.J. Parsons, D.M. Grant and D.S.A. De Focatiis
Faculty of Engineering, University of Nottingham
Mechanical response and structure evolution of degrading
medical polylactides for resorbable medical implants
www.nottingham.ac.uk
Introduction
Current structural implants used for the support of healing of broken bones utilise the
strength of metallics. These implants greatly limit the full healing of the bone due to
stress shielding effects [1].
Poly(lactic acid) (PLA) is a bioresorbable polymer; it will therefore break down in the
body, to eventually be metabolised. The properties of PLA alone are not enough to
withstand the forces required for structural applications. Novel nanoparticles produced
at the University of Nottingham could be the answer [2]. The incorporation of
reinforcing nanoparticles has the potential to significantly improve the mechanical
properties of these polymer implants, whilst still allowing for melt processing and in-
theatre shaping.
Research methods and results
After drying under vacuum at (50 ± 1) °C for over 16 h rectangular bars of PLA were
produced by compression moulding at 170 °C for 23 min. The molecular weight (Mw) of
the polymer prior to degradation was 445.9 kDa, as measured in methylene chloride at
25 °C using a MALLS detector. After moulding, the bars were fully submerged in
phosphate buffer saline (pH 7.42 ± 0.10) at a volume ratio of polymer to saline <1:30
in an accelerated environment of (50 ± 1) °C for a range of times up to 67 d.
Acknowledgements
The authors wish to acknowledge funding from the
EPSRC (EP/J017272/1), the contribution of Ms Beata
Wiktorska and Ms Maja Stępień from the Polish
Academy of Sciences in carrying out the GPC
measurements of the PLA materials, to Ms Alessia
Canciani for her assistance with data analysis, and
tof the members of the BENcH project
(www.nottingham.ac.uk/bench) for helpful
discussions.
References
[1]- Pilliar, R. M.; Binnington, A. G.; Szivek J.; Macnab I., Journal of Biomedical Materials Research, 1979.
13(5): p. 799-810.
[2]- Gimeno-Fabra M.; Tang S. V.; Lester E. H.; Grant D. M. 'Continuous hydrothermal synthesis of
functional-hydroxyapatite rods and plates for biomedical applications' ICCE-22, July 13-19, Malta, 2014
[3]- DVR Anatomic Narrow Short Right Plate. Available at http://tinyurl.com/lvlkoa7 Sourced January 2015
[4]- Choong, G. Y. H.; Parsons, A. J.; Grant, D. M.; De Focatiis, D. S. A., "Rheological Techniques for
Determining Degradation of Polylactic Acid in Bioresorbable Medical Polymer Systems," in PPS 30,
Cleveland, Ohio, 2014.
[5]- Lyu, S. P.; Schley, J.; Loy, B.; Lind, D.; Hobot, C.; Sparer, R.; Untereker, D., Biomacromolecules, 2007,
8, 2301-2310.
Conclusions
The polymer lost some of its initial mechanical properties due to water ingress over the 1st day of
accelerated degradation. Little change in the mechanical properties or mass were then seen between 1
and 10 d. After 10 d, white regions appeared, possibly due to autocatalytic degradation. Further
degradation led to a more dramatic fall in the mechanical properties, with the molecular weight and Tg
following soon after. The reduction in chain length finally allowed the formation of α-crystals and gross
mass loss of the specimens.
1 d 10 d 21 d
Stage 1:
0-1 d
• Diffusion of
water into
the polymer
Stage 2:
1-10 d
• Constant rate
controlled by the
water
concentration
• Little change in
the mechanical
properties and
mass
Stage 3:
10-21 d
• Autocatalysis as
a result of slow
diffusion of the
acidic chain ends
• Whitening of
selected regions,
followed by an
increase in mass
and decline in
mechanical
properties
Stage 4:
>21 d
• Mass loss
due to
polymer
dissolving
into the
solution
• Specimens
eventually
disintegrate
after 29 d
log degradation time (s)
Molarmass(kDa)
Masschange(%)logCreepcompliance(Pa)Yieldstress(MPa)
Creeprecovery(%)Crystallinity(%)
Tg(oC)
60mm
5 mm
• Differential scanning calorimetry (DSC) using a TA Instruments Q10 was
employed to obtain measurements of the glass transition (Tg), and
crystallinity on dry specimens. Specimens were subjected to two
heating cycles at a heating rate of 10 °C min-1 under a nitrogen
atmosphere, but were cooled at 13 °C min-1 to provide a cooling rate
representative of the compression moulding process. The degree of
crystallinity was determined by integration of the crystallisation and melt
peak during the 1st heating, whilst the Tg results were obtained on the 2nd
heating from the peak in the derivative of heat flow.
• Selected X-ray diffraction (XRD) patterns were obtained using a
Siemens D500 Kristalloflex 810, CuK radiation (λ= 0.154 nm), for
10°<2θ<40° in 0.05° wide scan steps at 0.25 Hz.
Results shown on the right demonstrate that the degradation process
appears to progress through 4 stages [5].
This work focuses on a medical
grade 70:30 poly(L-lactide-co-D,L-
lactide) (PLDLA), RESOMER®
LR706S (intended to be amorphous
as it degrades through its functional
life).
Example of a metallic fracture plate [3]
PLDLA specimen
Physical properties
• Optical micrographs were taken using a Fuji Finepix S2000HD digital camera under consistent
lighting conditions.
• Water uptake was recorded through regular monitoring of the mass of the specimens after drying
their surface with a paper towel.
• Molar mass was not measured directly using chromatographic methods since these would not transfer
easily to nanocomposites. Instead, an estimate of molar mass was obtained from interpretation of
rheological measurements based on Tuminello’s approach [4]. This technique identified the molecular
weight prior to degradation to be 552.1 kDa.
Mechanical properties
determined under 3-pt. bending in water at body temperature (37 ± 1) °C:
• Creep compliance was determined through linear viscoelastic measurements at small strains
(<0.5%).
• After a 1 h creep experiment, creep recovery at zero load was measured for a further 30 min.
• Yield stress was measured in large strain flexural experiments using a BOSE ElectroForce 3330 series
II at a constant strain rate of 0.1 s-1. The yield stress was determined as the peak in the stress-strain
curve.
Structural properties