Consistent with the previous studies in our laboratory on the uncrosslinked copolymer PPF-co
], the copolymer characterizations including GPC, FT-IR, NMR and DSC, showed a successful synthesis from PPF and PCL precursors. FT-IR confirmed the presence of PCL precursors in the copolymer composition by the appearance of PCL characteristic peaks. 1
H NMR showed a good agreement of initial PCL feed ratio with final PCL composition in the copolymer. This allows us to use the ratio as its composition in later discussions for simplicity without affecting the conclusion [19
]. All the copolymers had only a single glass transition in DSC curves indicating that copolymerization of semicrystalline PCL with amorphous PPF efficiently decreased the crystallinity of PCL fragment. Moreover, the glass transition for all the copolymers was narrower (less than 15 °C) than PPF precursors. This confirmed that the copolymerization was quite successful since the single glass transition for miscible polymer blends is generally much broader than those of pure polymers. Tg
of the uncrosslinked copolymer dramatically decreased with increasing PCL feed ratio. It possibly resulted from an increasing flexibility of copolymer backbone with increasing PCL feed ratio [22
The heat produced from polymerization of injectable biomaterials is one of the major concerns in the clinical application. The maximum heat produced from PMMA bone cement exceeds 100 °C. This could pose a risk of tissue damage especially when it is injected into the fractured vertebral body where the vulnerable spinal cord is close by [23
]. In our study, both PPF and PPF-co
-PCL copolymer showed a much lower heat produced during the crosslinking (around 38~50 °C) than PMMA. These results agree with some previous reports on the PPF-based composites [24
]. The crosslinking temperature of the copolymer increased slightly with increasing PCL precursor's molecular weight. A higher molecular weight of PCL fragment would introduce more carbon-carbon single bonds into the backbone of copolymer, hence increase the mobility of copolymer chain and the accessibility of double bonds during crosslinking reactions, and lead to a higher speed of crosslinking reaction and heat release.
The gelation time of all the copolymers decreased approximately to half the time of their PPF precursors. This interesting finding further illustrated the role of PCL on the mobility of the copolymer chain. As a result, a more fully crosslinked copolymer might be achieved and the scaffold's biocompatibility would be increased by reducing the amount of unreacted monomers [14
]. Our preliminary in vitro
data on the cytotoxicity comparison between crosslinked PPF and PPF-co
-PCL did show a more favorable biocompatibility of the copolymer in consequence of a higher double-bond conversion. (Detailed data will be shown in a subsequent paper from our lab.) So far, the in vitro
cytotoxicity of parts of our PPF-co
-PCL formulations together with PPF and PMMA were already investigated in our lab using MTS Assay. Three time points (1 day, 3 days, and 7 days) were chosen. No cytotoxic response was demonstrated from the different formulations of PPF-co-PCL and PMMA bone cement. Only on the 7 days time point, there was a slight decrease of cell viability in PPF samples compared with the control group. However, no statistical difference was shown. Further studies are needed to exactly assess the crosslinked copolymer's biocompatibility in vitro
and in vivo
The gelation time of the crosslinked copolymer using a specific amount of accelerator fell between 4.2±0.2 and 8.5±0.7 min. Moreover, our study showed that it could be easily adjusted by changing the amount of accelerator for different clinical use. An increase in DMT concentration led to an accelerated production of free radicals which increased the rate of crosslinking and resulted in shorter gelation time. Additionally, we also investigated the mechanical properties changes of the crosslinked scaffold using different amount of accelerator. The results showed that the final compressive modulus of the crosslinked scaffold began to drop when accelerator's amount was decreased to a lower level (0.125 wt % of DMT/copolymer). This was due to a decrease in crosslinking levels that rendered a reduced mechanical property.
Our data revealed that compressive modulus of the crosslinked copolymer increased with a higher PPF precursor molecular weight and lower PCL feed ratio. Both increasing PPF Mn and decreasing PCL ratio led to an increase of fumarate double bonds density in the copolymer. More reacting double bonds in the crosslinked copolymer would yield a stronger material. Furthermore, our preliminary in vitro biocompatibility study showed that the degradation rate of the copolymer increased with increasing PCL feed ratio. The weight loss of copolymer with the highest PCL feed ratio was almost twice as much as that of the copolymer with the lowest PCL feed ratio from the same PPF and PCL precursors. No significant difference of weight loss was found from different formulations varying in PPF or PCL precursor's molecular weights (detailed data not shown). This means that PPF-co-PCL copolymer can be tailored to vary from a higher mechanical property for some non-biological reconstructions where a slow degradability is preferred (like some load-bearing metastatic bone lesions) to a relatively lower mechanical property for cases where bone healing is the goal and a more rapid degradability is preferred (like some non load-bearing upper limb benign bone lesions or defects). This surely offers the copolymer a wider and more individualized application in clinics.
The amount of energy/volume a material can absorb before failure defines the intrinsic toughness of the material. It is calculated as the magnitude of the force times the deformation produced and is equal to the area under stress-strain curve. This concept is very useful in clinics because many injuries may impart a specific energy to the body. For example, a fall from a height may turn the potential energy of the body weight at the original height into the energy of deformation especially in the osteoporotic spine, causing fracture [27
]. Wolfe et al reported that the fracture toughness of crosslinked PPF scaffold is much lower than that of human cortical bone. This makes PPF unsuitable for use especially in load-bearing bone defect area [28
]. In our study, copolymer 13 (PPF2000
) had a much higher compressive toughness than its PPF precursor (PPF2000
) even though their compressive moduli were similar (). This indicated that, in this specific formulation of PPF-co
-PCL copolymer, the unsaturated PPF part provided enough mechanical stiffness while the flexible PCL part offered a higher toughness than its PPF precursor. This interesting property makes the crosslinked copolymer a more favorable substitute for clinical use than its brittle PPF precursor, especially in loading required bone defect treatment such as vertebroplasty for compressive spinal fractures.
Finally, the changes in the compressive modulus over time were investigated. The compressive moduli of all the tested copolymer scaffolds gradually increased over time. The greatest increase occurred in the first two weeks after setting (). PPF also had the phenomenon of mechanical property enhancement due to the continuous crosslinking reaction over time [29
]. However, unlike PPF-co
-PCL that showed an increase in mechanical properties up to two weeks, the compressive modulus of PPF kept increasing up to the sixth week even in a degradation environment. This difference between PPF-co
-PCL and PPF illustrated that the PCL fragment in the copolymer chain could help to produce a much faster and complete crosslinking process.