Time-dependent failure in load-bearing polymers: a potential hazard in structural applications of polylactides

With their excellent biocompatibility and relatively high mechanical strength, polylactides are attractive candidates for application in load-bearing, resorbable implants. Pre-clinical studies provided a proof of principle for polylactide cages as temporary constructs to facilitate spinal fusion, and several cages already made it to the market. However, also failures have been reported: clinical studies reported considerable amounts of subsidence with lumbar spinal fusion cages, and in an in vivo goat study, polylactide spinal cages failed after only three months of implantation, although mechanical testing had predicted sufficient strength for at least eight months. The failures appear to be related to the long-term performance of polylactides under static loading conditions, a phenomenon which is common to all glassy polymers and finds its origin in stress-activated molecular mobility leading to plastic flow. This paper reviews the mechanical properties and deformation kinetics of amorphous polylactides. Compression tests were performed with various strain rates, and static stress experiments were done to determine time-to failure. Pure PLLA appeared to have a higher yield strength than its co-polymers with d-lactide, but the kinetic behaviour of the polymers was the same: an excellent short-term strength at higher loading rates, but lifetime under static stress is rather poor. As spinal implants need to maintain mechanical integrity for a period of at least six months, this has serious implications for the clinical application of amorphous polylactides in load bearing situations. It is recommended that standards for mechanical testing of implants made of polymers be revised in order to consider this typical time-dependent behaviour

Time-dependent failure in load-bearing polymers: a potential hazard in structural applications of polylactides

With their excellent biocompatibility and relatively high mechanical strength, polylactides are attractive candidates for application in load-bearing, resorbable implants. Pre-clinical studies provided a proof of principle for polylactide cages as temporary constructs to facilitate spinal fusion, and several cages already made it to the market. However, also failures have been reported: clinical studies reported considerable amounts of subsidence with lumbar spinal fusion cages, and in an in vivo goat study, polylactide spinal cages failed after only three months of implantation, although mechanical testing had predicted sufficient strength for at least eight months. The failures appear to be related to the long-term performance of polylactides under static loading conditions, a phenomenon which is common to all glassy polymers and finds its origin in stress-activated molecular mobility leading to plastic flow. This paper reviews the mechanical properties and deformation kinetics of amorphous polylactides. Compression tests were performed with various strain rates, and static stress experiments were done to determine time-to failure. Pure PLLA appeared to have a higher yield strength than its co-polymers with d-lactide, but the kinetic behaviour of the polymers was the same: an excellent short-term strength at higher loading rates, but lifetime under static stress is rather poor. As spinal implants need to maintain mechanical integrity for a period of at least six months, this has serious implications for the clinical application of amorphous polylactides in load bearing situations. It is recommended that standards for mechanical testing of implants made of polymers be revised in order to consider this typical time-dependent behaviou

Inverse Detection and Heteronuclear Editing in 1H-15N Correlation and 1H-1H Double Quantum NMR Spectroscopy in the Solid State under Fast MAS

Signal enhancement in heteronuclear correlation spectra as well as signal selection in 1H experiments can be achieved through inverse, i.e., 1H, detection in the solid state under fast MAS conditions. Using recoupled polarization transfer (REPT), a heteronuclear 1H–15N single-quantum correlation (HSQC) experiment is presented whose symmetrical design allows the frequency dimensions to be easily interchanged. By observing the 15N dimension indirectly and detecting on 1H, the sensitivity is experimentally found to be increased by factors between 5 and 10 relative to conventional 15N detection. In addition, the inverse 1H–15N REPT-HSQC scheme can be readily used as a filter for the 1H signal. As an example, we present the combination of such a heteronuclear filter with a subsequent 1H–1H DQ experiment, yielding two-dimensional 15N-edited 1H–1H DQ MAS spectra. In this way, specific selection or suppression of 1H resonances is possible in solid-state MAS experiments, by use of which the resolution can be improved and information can be unravelled in 1H spectra

Quadruple hydrogen bonds of ureido-pyrimidinone moieties investigated in the solid state by H-1 double-quantum MAS NMR spectroscopy

The structure of the quadruple hydrogen bond formed by ureido- pyrimidinone moieties is investigated in dimerised model compounds, as well as in a supramolecular polymer, by solid- state H-1 double-quantum (DQ) NMR spectroscopy under fast magic-angle spinning (MAS). This NMR method combines the sensitivity of H-1 NMR chemical shifts to the strengths of hydrogen bonds with quantitative information about dipole dipole couplings between pairs of protons. Thus, two- dimensional H-1 DQ MAS spectra provide particularly detailed insight into the arrangement of hydrogen bonds and allow proton proton distances to be measured. For the supramolecular polymer, a thermally induced irreversible tautomeric rearrangement of the hydrogen-bonded moieties is elucidated in the bulk material. This process is associated with an Arrhenius activation energy of (145 +/- 15) kJ mol(-1), which can be rationalised in terms of hydrogen-bond dissociation and the reorientation of the supramolecular polymer chain