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.

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 linkage in the polymer's backbone and is influenced by the polymer's initial molecular weight and degree of crystallinity. To evaluate its degradation PLLA pellets were processed by compression moulding into tensile test specimens and by extrusion into 2 mm diameter lengths of rod, prior to being sterilized by ethylene oxide gas (EtO) and degraded in both in vitro and in vivo environments. On retrieval at predetermined time intervals, procedures were used to evaluate the material's molecular weight, crystallinity, mechanical strength, and thermal properties. Additionally, the in vivo host tissue's biological response was analysed. The results from this study suggest that in both the in vitro and in vivo environments, degradation proceeded at the same rate and followed the general sequence of aliphatic polyester degradation, ruling out enzymes contributing and accelerating the degradation rate in vivo. Additionally, the absence of cells marking an inflammatory response suggests that the PLLA rods investigated in vivo were biocompatible throughout the 44 weeks duration of the study, before any mass loss was observed.

No abstract

No abstract

No abstract

No abstract

Weapon size has been shown to be a better predictor of competitive success than body size (carapace width) in ®ghts between shore crabs, Carcinus maenas. However, when the weapon size disparity is small between two opponents, it is still dif®cult to predict the victor. The role of weapon strength in pairwise ®ghts between male shore crabs was investigated, to determine if relative force in¯uences contest content, duration and outcome. Weapon strength was ascertained using a force transducer on live crabs, then ®ghts between crabs were staged between size matched males. Winning crabs had major (crusher) claws and minor (cutter) claws that exerted a signi®cantly greater force than losing crabs even when claw length was the same. Winners and losers were matched for carapace width, claw length and dactyl length but not claw height or claw length minus the dactyl. Winners had greater claw height and claw length to the dactyl in the major claw giving them a higher mechanical advantage when closing the claw and thus exerting a greater force. The forces exerted by the major and minor claws were analysed for any relationship between force and morphological measurements. Winning crabs appear to be ®tter in having a better claw structure which exerts a greater force and they are more successful in agonistic interactions.

No abstract