Degradation of poly-L-lactide. Part 2: Increased temperature accelerated degradation

Abstract

Poly-L-lactide (PLLA) is one of the most significant members of a group of polymers regarded as bioresorbable. The degradation of PLLA proceeds through hydrolysis of the ester linkages in the polymer's backbone; however, the time for the complete resorption of orthopaedic devices manufactured from PLLA is known to be in excess of five years in a normal physiological environment. To evaluate the degradation of PLLA in an accelerated time period, PLLA pellets were processed by compression moulding into tensile test specimens, prior to being sterilized by ethylene oxide gas (EtO) and degraded in a phosphate-buffered solution (PBS) at both 50 degrees C and 70 degrees C. On retrieval, at predetermined time intervals, procedures were used to evaluate the material's molecular weight, crystallinity, mechanical strength, and thermal properties. The results from this study suggest that at both 50 degrees C and 70 degrees C, degradation proceeds by a very similar mechanism to that observed at 37 degrees C in vitro and in vivo. The degradation models developed also confirmed the dependence of mass loss, melting temperature, and glass transition temperature (Tg) on the polymer's molecular weight throughout degradation. Although increased temperature appears to be a suitable method for accelerating the degradation of PLLA, relative to its physiological degradation rate, concerns still remain over the validity of testing above the polymer's Tg and the significance of autocatalysis at increased temperatures.

Irradiation of bioresorbable biomaterials for controlled surface degradation

“…Weir et al [Weir et al, 2004] conducted hydrolytic degradation experiments on PLLA at an elevated temperature of 50 o C and 70 o C. These studies showed that by elevating the temperature the rate of hydrolytic degradation increased exponentially, with no change in mechanism above the polymer's glass transition temperature. Samples that were hydrolytically degraded at 70 o C took only 23 days to degrade, whereas samples degraded at 50 o C took over 115 day to degrade to the same extent.…”

In vitro degradation and mechanical properties of PLA-PCL copolymer unit cell scaffolds generated by two-photon polymerization

“…At each time point every scaffold was weighed and mechanically tested allowing the degradation pathway of each individual scaffold to be followed with time. An Arrhenius equation was utilised to extrapolate results at elevated temperatures to physiological temperature [49]. …”

“…The degradation activation energies obtained were 87.9, 82.7 and 94.9 kJ mol −1 for LC16 : 4, LC18 : 2 and LC9 : 1 respectively. Arrhenius relation has been conducted for the change in several parameters of biodegradable polymers such as molecular weight, breaking strength, viscosity and mass loss [34,38,49,71,72]. The majority of previous publications used change in molecular weight over degradation time at different temperatures.…”

“…Therefore, the change in rate of mass loss at different temperatures was used in this study to calculate the degradation activation energy. Weir et al [49] and Agrawal et al [71] determined activation energies for 50%PLA-50%PGA copolymer and PLLA (113.9 and 100.5 kJ mol −1 ) through the change in molecular weight against degradation temperature. Degradation activation energy was also determined for 10%PLLA-90%PGA through change in tensile breaking strength during degradation at different temperatures [40].…”

Degradation mechanisms of bioresorbable polyesters. Part 2. Effects of initial molecular weight and residual monomer

“…It is often the case that polymer samples show zero mass loss for a long period at the start of degradation [15][16][17][18]. An indication of the likely duration of this delay before mass loss onset is characterised here by the time taken for water-soluble monomers and oligomers to account for 10% of the total polymer weight.…”