The degradation properties of co-continuous calcium phosphate polyester composites: insights with synchrotron micro-computer tomography

Abstract

This study investigates the in vitro degradation properties of composites consisting of a porous tricalcium phosphate (TCP) foam filled with degradable poly(dl-lactic acid) (PDLLA) via either in situ polymerization or infiltration. The motivation was to develop a material for bone repair that would be initially mechanically strong and would develop porosity during degradation of one of the components. A thorough analysis of the physical in vitro degradation properties has been conducted and reported by the same authors elsewhere. Synchrotron micro-computer tomography analysis (conducted at ID19, ESRF, Grenoble, France) allowed detailed insights to be gained into the process of the composites' degradation, which was discovered to be strongly influenced by the manufacturing method. The polymer phase of in situ-polymerized TCP-PDLLA degraded as a bulk sample, with faster degradation in the centre of the sample as a whole. In contrast, the polymer phase of infiltrated TCP-PDLLA degraded as individual polymer spheres with faster degradation in the centre of each sphere.

Figure 1.

Schematic of possible ways of polymer degradation within the composite: (a) bulk degradation; (b) degradation of individual spheres. Black, ceramic; grey, polymer; white, degrated polymer.

Figure 2.

(a,c) Comparison of in situ-polymerized and (b,d) infiltrated TCP–PDLLA composites as made (top row) and after 1.25 days of degradation in PBS at 42°C (bottom row). Circles in the image of as-made infiltrated composites (b) indicate cracks in the composite that were present in the ceramic matrix initially. (a–d) Bar, 200 µm.

Figure 3.

(a) Water absorption and mass loss of infiltrated and (b) in situ-polymerized TCP–PDLLA composites. Crosses, water absorption (deg. study); open diamonds, mass loss (deg. study); filled triangles, water absorption (ESRF); filled diamonds, mass loss (ESRF). Samples were investigated during a long-term degradation study (triplicates) and with synchrotron micro-CT at the ESRF (one sample per time point). Molecular weight changes for both in situ-polymerized and infiltrated composites are shown in (c). Open squares, infiltrated TCP–PDLLA (ESRF); filled squares, infiltrated TCP–PDLLA (deg. study); grey diamonds, in situ TCP–PDLLA (ESRF); black diamonds, in situ TCP–PDLLA (deg. study).

Figure 4.

Micro-CT images of in situ-polymerized composites after degradation. (a) Five days (magnified section: b), (c) 14 days* (magnified section: d), (e) 25 days* (central slice, magnified sections: f and g; h: slice close to the edge). (a,c,e,h) Bar, 200 µm (not given for the magnifications). In samples highlighted with asterisks (*), the greyscale range was adjusted to increase the contrast between polymer and PBS.

Figure 5.

Micro-CT images of infiltrated TCP–PDLLA composites after degradation for (a) 10 days, (b) 35 days* (magnified section: c), (d) 50 days* (magnified sections: e,f). (a,b,d) Bar, 200 µm (not given for the magnifications). In samples highlighted with asterisks (*), the greyscale range was adjusted to increase the contrast between polymer and PBS.

Figure 6.

Ratio of molecular weight (Mn_edge/Mn_centre) for TCP composites degraded in PBS at 42°C for various times. Samples were from the main degradation study and the additional degradation for synchrotron measurements. Values less than 1 indicate a faster degradation in the edge, whereas values greater than 1 indicate a faster degradation in the centre. Filled diamonds, in situ TCP–PDLLA (deg. study); filled squares, infiltrated TCP–PDLLA (deg. study); open diamonds, in situ TCP–PDLLA (ESRF); open squares, infiltrated TCP–PDLLA (ESRF).

Figure 7.

Polydispersity index (PDI = Mw/Mn) for TCP–PDLLA samples after degradation. The open symbol represents the edge of the sample; the closed symbol the centre of the sample. Lighter colours refer to samples investigated at the ESRF; dark symbols refer to samples from the degradation study. Grey filled triangle, in situ as made; grey open diamond, in situ, ESRF, edge; black open diamond, in situ, deg. study, edge; grey open square, infiltrated, ESRF, edge; black open square, infiltrated, deg. study, edge; black filled triangle, infiltrated as made; grey filled diamond, in situ, ESRF, centre; black filled diamond, in situ, deg. study, centre; grey filled square, infiltrated, ESRF, centre; black filled square, infiltrated, deg. study, centre.