Melt blending of poly(lactic acid) with biomedically relevant polyurethanes to improve mechanical performance

Abstract

Minimally invasive surgery procedures are of utmost relevance in clinical practice. However, the associated mechanical stress on the material poses a challenge for new implant developments. In particular PLLA, one of the most widely used polymeric biomaterials, is limited in its application due to its high brittleness and low elasticity. In this context, blending is a conventional method of improving the performance of polymer materials. However, in implant applications and development, material selection is usually limited to the use of medical grade polymers. The focus of this work was to investigate the extent to which blending poly-l-lactide (PLLA) with low contents of a selection of five commercially available medical grade polyurethanes leads to enhanced material properties. The materials obtained by melt blending were characterized in terms of their morphology and thermal properties, and the mechanical performance of the blends was evaluated taking into account physiological conditions. From these data, we found that mixing PLLA with Pellethane 80A is a promising approach to improve the material's performance, particularly for stent applications. It was found that PLLA/Pellethane blend with 10% polyurethane exhibits considerable plastic deformation before fracture, while pure PLLA fractures with almost no deformation. Furthermore, the addition of Pellethane only leads to a moderate reduction in elongation at yield and yield stress. In addition, dynamic mechanical analysis for three different PLLA/Pellethane ratios was performed to investigate thermally induced shape retention and shape recovery of the blends.

Keywords: PLLA; PLLA polyurethane blends; Polymer stent; Shape memory polymers.

Graphical abstract

Fig. 1

DSC thermograms of PLLA and PLLA/PU-blends as obtained by injection molding and after thermal annealing.

Fig. 2

Macrophotographical images of PLLA and PLLA/PU-blend dog bone specimens for mechanical testing as obtained by Laser cutting and after thermal annealing.

Fig. 3

Cross-sectional SEM images of cryo-fractured PLLA/PU blends after etching for 72 h with 1 M NaOH.

Fig. 4

ATR-FTIR spectra of PLLA and PLLA/PU blends before and after thermal annealing. As reference, spectra of pure polymer granules were added. Characteristic bands of PLLA and PU are indicated with dotted lines. To enhance visibility, the spectra were stretched in y-axis.

Fig. 5

Stress strain curves of PLLA and PLLA/PU blends after injection molding and thermal annealing measured in dry state.

Fig. 6

Stress strain curves (averaged over 5 measurements) of PLLA and PLLA/PU blends after injection molding and thermal annealing measured after incubation in physiological saline solution for 7 d.

Fig. 7

Yield stress versus yield strain of PLLA/PU blends with 10 wt% PU after injection molding and thermal annealing measured in dry state and after incubation in physiological saline solution for 7 days, calculated from stress-strain curves by 0.2% offset method. Dashed line indicate PLLA values, measured in dry state as reference.

Fig. 8

Stress strain curves of PLLA/Pellethane blends with a ratio of 80:20 and 70:30 after injection molding and thermal annealing measured in dry state, and yield stress versus yield strain of PLLA/Pellethane 80:20 and 70:30 measured in dry state and after incubation in physiological saline solution for 7 days, calculated from stress-strain curves by 0.2% offset method. Dashed line indicate PLLA values measured in dry state as reference.

Fig. 9

I: Representative diagram of displacement, force and temperature as function of time for pure PLLA for the first complete DMA cycle for SM characterization and schematic of the mechanical load on the sample. Characteristic values D_s, D_r and D_R are indicated. 2nd and 3rd cycles were similar, except the initial thermal equilibration step (i) was skipped. DMA protocol was the same for all measured samples. II: Representative 3D plots of DMA measurements for PLLA and PLLA/Pellethane blends (90:10, 80:20 and 70:30) after three cycles. Colors are indicative for cycle 1 (blue), cycle 2 (red) and cycle 3 (green). Thermal equilibration step is not shown in 3D plots.

Fig. 10

Photographic images of PLLA and PLLA/Pellethane after controlled displacement using DMA device. The test chamber was heated to 70 °C and the sample was equilibrated for 30 min before a force of 8.0 N was applied at a rate of 0.8 N/min. The samples were held at 8.0 N for 30 min before temperature was lowered to 15 °C. Samples were kept at 15 °C for 30 min, and then the force was released. The bottom line shows photographic images of the same samples after 1 min immersion in deionized water heated to 70 °C.

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