Tom A. P. Engels

Time-dependent mechanical strength of 70/30 poly(L,DL-lactide): shedding light on the premature failure of degradable spinal cages

Study Design. In vitro studies on the mechanical strength of 70/30 poly(l,dl-lactic acid) (70/30 PLDLLA) cages. Objective. To evaluate the effect of loading rate, humidity, temperature, and continuous static loading on the strength of 70/30 PLDLLA, to elucidate the mechanism of premature failure of degradable spinal cages observed in earlier studies. Summary of Background Data. Degradable 70/30 PLDLLA cages have been designed to withstand mechanical loads in a goat lumbar spine for at least 6 months. Yet mechanical failure was observed after only 3 months in vivo. We hypothesize that this observation can be related to the time-dependent nature of the polymer. Methods. Degradable 70/30 PLDLLA cages were loaded to failure at loading rates between 10−3 and 10−1 mm/s under standard loading conditions (in air at room temperature: ±23°C). The experiments were also done at body temperature (37°C) and under wet conditions. Furthermore, we determined the time-to-failure for 70/30 PLDLLA cages subjected to loads well below their instantaneous mechanical strength. Results. The mechanical strength of 70/30 PLDLLA cages was lower for lower loading rates, higher temperature, and higher humidity. The cages already failed within less than 5 minutes when statically loaded at 75% of their strength, and within 1 day when loaded at about 50% of their strength. Extrapolation predicts cage failure at 3 months when loaded at 25% of their strength. Conclusion. Premature failure of 70/30 PLDLLA cages, as observed in vivo in earlier studies, is owing to mechanical loading and the time-dependent mechanical properties of the material. The standards for mechanical testing of implants made of strongly time-dependent materials like polylactide should be reconsidered.

Time-dependent failure of amorphous polylactides in static loading conditions

Polylactides are commonly praised for their excellent mechanical properties (e.g. a high modulus and yield strength). In combination with their bioresorbability and biocompatibility, they are considered prime candidates for application in load-bearing biomedical implants. Unfortunately, however, their long-term performance under static load is far from impressive. In a previous in vivo study on degradable polylactide spinal cages in a goat model it was observed that, although short-term mechanical and real-time degradation experiments predicted otherwise, the implants failed prematurely under the specified loads. In this study we demonstrate that this premature failure is attributed to the time-dependent character of the material used. The phenomenon is common to all polymers, and finds its origin in stress-activated segmental molecular mobility leading to a steady rate of plastic flow. The stress-dependence of this flow-rate is well captured by Eyring’s theory of absolute rates, as demonstrated on three amorphous polylactides of different stereoregularity. We show that the kinetics of the three materials are comparable and can be well described using the proposed modeling framework. The main conclusion is that knowledge of the instantaneous strength of a polymeric material is insufficient to predict its long-term performance.

Quantitative Prediction of Mechanical Performance of Polymer Products directly from Processing Conditions

A method is presented that enables the prediction of mechanical properties of an amorphous polymer based on the thermal history during processing. It is demonstrated that the method quantitatively predicts both short‐ and long‐term failure of polymer products, thus making it a powerful tool for true product optimization.

Programmable helical twisting in oriented humidity-responsive bilayer films generated by spray-coating of a chiral nematic liquid crystal

A facile method is presented that generates humidity-driven helical twisting in thin, oriented polyamide 6 (PA6) films by spray-coating of a chiral nematic liquid crystal followed by photopolymerization into a network. The liquid crystal network (LCN) is self-aligned by the PA6 substrate, and the internal twist angle of the LCN provides the direction and extent of twisting for the expanding PA6 substrates. The handedness and extent of helical twisting in the bilayer film can be finely programmed by the degree of twist in the thin topping LCN layer. This straightforward spray-coating process and self-alignment of the liquid crystals (LCs) is compatible with high-throughput industrial manufacturing processes and is applicable to other stretched commodity polymers.