PSI - Issue 15

Raasti Naseem et al. / Procedia Structural Integrity 15 (2019) 55–59 Naseem et al. / Structural Integrity Procedia 00 (2019) 000–000

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3. Results The results of our experimental study demonstrated a decline in modulus with increasing indentation depth for both continuous and singular indentations, as shown in Figure 1. This could be a result of indentation size effect (ISE) or variation of properties through the thickness of the sample (associated with polymer chain orientations during manufacturing process). Both continuous and singular indentations gave consistent modulus values.

Fig. 1. Elastic modulus as a function of maximum indentation depth obtained with continuous and individual indentation.

The stress-strain curves for the tubing sample were obtained using the Field and Swain analysis of the spherical indentation data (Figure 2). The material yielded at about 3% strain, with a slight variation among the four indents. This variation might be caused by the nonuniformity of local phases and microstructures, associated with the semi crystalline nature of the sample as well as the mixing process. After yielding, a decline in stress with increasing strain was observed, which could be indicative of material softening. The results for the co-polymer tubing are comparable with those obtained for PLLA, in terms of trend and behaviour, but with a much lower stress response. This indicates the loss of strength for the co-polymer. Figure 3 demonstrates the elastic modulus of the material, obtained from spherical nanoindentation, at 0, 6 and 10 days of degradation. The results agree with those observed from the stress-strain response, showing a decline with time. It is noted that the significant decrease in modulus was accompanied by a simultaneous increase of displacement. At day 10, there was almost a 5-fold increase of the displacement at 15 mN when compared to the virgin state sample. In comparison to the PLLA, the degradation rate of the material is significantly higher, and the co-polymer material would not be able to maintain the integrity over a sufficient period of time after stent implantation. 4. Conclusions Mechanical properties of the PLA-PCL-PEG polymer blend do change as a result of manufacturing processes imposed on the material, which must be considered in design and development of the material for stent application. The results obtained for degradation indicate that this material was not yet adequately optimised for use in cardiovascular implants, and further development is required to achieve a desirable degradation rate as well as suitable mechanical properties.