Poly-ε-Caprolactone-Polytetrahydrofurane Copolymers as Alternative for the Treatment of Bone Fractures - METTLER TOLEDO

Poly-ε-Caprolactone-Polytetrahydrofurane Copolymers as Alternative for the Treatment of Bone Fractures

Introduction

Up until now, bone fractures have been treated by fixing them in position while they mend by encasing them in bandages impregnated with plaster of Paris. Such plaster casts are however relatively heavy and lose their rigidity if they become wet. In recent years, people have been on the lookout for alternatives to plaster of Paris. A very promising material is the copolymer of poly-ε-caprolactone and polytetrahydrofurane (PCL-PTHF), which is synthesized by polycondensation of mixtures of PLC and PTHF diols in appropriate ratios with diisocyanatohexane (HDI).   Thermoanalytical methods have supplied valuable information in the development of this material.

For example, the influence of the composition of the copolymer on the processing temperature can be investigated by differential scanning calorimetry (DSC) and the mechanical properties of different copolymers can be compared using dynamic mechanical analysis (DMA).

 

Experimental 

The PCL homopolymer is semicrystalline and melts at about 60 °C. The material supercools on cooling and the crystallization temperature is just over 30 °C. This has the important advantage that the clinical application can be performed at temperatures that are acceptable and pleasant for patients.

In this study, the main point of interest was to determine the influence of PTHF content on the crystallization temperature and the storage modulus in the crystalline state. 

 

Evaluation 

Figure 1 shows the melting/crystallization behavior of the PCL homopolymer. The diagram displays the shear modulus (storage part, G′, and loss part, G″) of a DMA measurement. During melting, the shear modulus (G′) decreases by about 5 decades from an initial value of 7⋅107 Pa. In the range in which G″ > G′, the sample is in the liquid state. This is the reason for the slight noise level observed on the curve - it is due to the low applied and measured forces in this phase. It is particularly worth noting that the DMA/SDTA861e can measure meaningful modulus values even when the sample is in the low viscosity, liquid state. This is possible due to the high dynamic range of the DMA/SDTA861e. On cooling, the viscosity increases, which in turn causes a linear increase in G′. The storage modulus of 70 MPa in the crystalline state is relatively low; one has the impression that the sample is not sufficiently stiff for the application. The stiffness is however at just the right level. If it were too high, the material would be too brittle - there would be a risk of material breakage under the influence of mechanical shock such as a blow on the immobilized limb.

 

Figure 2 displays the DSC and DMA curves of heating and cooling experiments for one of the PCL-PTHF copolymers (9:1). During heating (shown on the right-hand side of the DSC curve) one observes two endothermic effects, first the melting of PTHF and then the melting of PCL. On cooling (displayed on the left-hand side), the endothermic effect is observed as a two-step crystallization process of PCL followed by crystallization of PTHF. The mechanical properties, shown here as the shear modulus curve, G′, are in good agreement with the measured calorimetric effects. The shear modulus, which has a value of 55 MPa in the initial semicrystalline state, decreases during melting to that of the PTHF; the material is already soft at about 40 °C. If the material is cooled down slowly from 80 °C, the PCL content crystallizes in two steps as in the DSC measurement. A further step can be seen in the shear modulus of PTHF. During this phase transition, the modulus returns to the original level. Here, calorimetrically and mechanically determined phase transition temperatures are in good agreement.

Conclusions

DSC and DMA are excellent complimentary methods for the investigation of the melting and crystallization behavior of PCL-PTHF copolymers. The phase transition temperatures determined by both methods are in good agreement. With increasing PTHF content, both the supercooling effect of crystallization and the time required for complete crystallization increase with increasing PTHF content. This allows the processing temperature to be optimized by adjusting the composition of the material. The PTHF content also affects the modulus value of the crystalline material. This means that the mechanical properties can be adapted to the medical application. The challenge is to find the right composition for an optimal balance between the processing temperature and stiffness of the material. Finally, regarding the medical application, one might point out that polymer systems are already today used to immobilize fractures. They are however more expensive in comparison with plaster of Paris and demand more skill and greater experience in use.

Poly-ε-Caprolactone-Polytetrahydrofurane Copolymers as Alternative for the Treatment of Bone Fractures | Thermal Analysis Application No. UC 174 | Application published in METTLER TOLEDO Thermal Analysis UserCom 17