Construction Of In Vitro Bone Defect Model And Detection Device For Shape Memory Recovery Force

During the repair of bone defects by using artificial bones, delayed bone repair or even bone non-union may take place because of the formed gap between the artificial bone and native bone and the produced stress shielding by internal or external fixation. Shape memory polymer(SMP), when heated to its recovery temperature, will change its shape and produce a recovery force. By utilizing the shape change and recovery force, the SMP-based artificial bone(SMP bone) is able to fill the gap and apply compressive stress stimuli to the native bone, which may potentially promote bone repair and even prevent bone non-union. Obviously, the magnitude of recovery force is crucial for the performance of SMP bones, which is dependent on the recovery ratio of SMP. Therefore, a quantitative detection of the recovery force and recovery ratio is of great importance. Unfortunately, there is no such a detection device. In addition, critical size defect(CSD) animal models are often employed to evaluate the in vivo performance of artificial bones. These animal models are featured with high cost, time-consuming and complex operations. The aim of this study is to construct a cantilever-based device, acting as both a bone defect model and a detector of recovery force. The obtained device should be able to: ① simulate various sizes of bone defects and various elastic coefficients of native bones(so as to represent the bone status related to the age, gender or heath); ② quantitatively detect the recovery force and the relationship between recovery force and recovery ratio; ③ detect the deformation of various native bones. By using this device, the temporary and permanent shapes of SMP bone may optimized in vitro, so that the in vivo animal tests might be minimized. The main work and results are listed as follows:(1) The principle for construction of cantilever-based bone defect model was deduced through theoretical calculation. A relationship was obtained between the elastic coefficient of native bone(K), cantilever’s parameters including the elastic modulus(E), beam width(b), beam thickness(h), and the length(a) between the acting point of SMP bone and the fixation end of cantilever: =?3 This relationship suggests that various native bones(K) could be represented by regulating cantilever’s parameter E, b, h and a. Especially, regulation of a is the most convenient.(2) The principle for construction of cantilever-based recovery force detector was deduced through theoretical calculation. A relationship was obtained between the recovery force F and cantilever’s parameters(E, b, h, a), the deflections of cantilever beam at the free end(δ0) and at point a(δa): 0=22 This relationship suggests that detection of δ0 could give F and F-induced deformation of SMP bone(δa) and of native bone w.(3) According to the above principles, the cantilever-based device integrating a bone defect model and a detector of recovery force was constructed, consisting of a base, a upper/lower sample support, a heating chamber, a deflection detection system by using a LVDT-based displacement transducer(LVDT: linear variable differential transformer), and a control software programmed by using Labview. The procedure to operate the device was established and the device was calibrated. δ0 values corresponding to a series of known F values were detected. The experimental Ke value was comparable to the theoretical one Kt, confirming that the constructed device could reasonably simulate the bone defects and the above-mentioned principle for bone defect model is correct as well.(4) Shape memory polyurethane(PUU-PPZ) was synthesized and characterized. An improved bench-scale method was developed to fabricate cylindrical, laminar or coiled samples.(5) The effect of recovery ratio on recovery force was investigated by using the compressed cylindrical samples. A porous composite scaffold of polylactide and tricalcium phosphate(PLA-TCP) was employed as a control since it has no shape memory effects. The results indicate that PUU-PZZ samples demonstrated obviously higher recovery force compared to PLA-TCP. Moreover, the recovery force of PUU-PPZ decreased with increasing recovery ratios.(6) The recovery force of the bent PUU-PPZ laminates was further detected. The bent laminate exhibited obviously higher recovery force(1.10N) than its compressed cylindrical counterpart(0.86N), suggesting that SMP bone comprising bent or coiled structures might produce higher recovery force and present better recovery effects. A theoretical calculation was employed to explain this difference.