Study Of Shape Memory Polyurethane Orthodontic Archwires Based On Viscoelasticity

With the development of social economy and the improvement of people’s living standard, people’s aesthetic standard have been improved, they not only demand for the improvement of the aesthetic after orthodontic treatment, at the same time also hope that the fixed orthodontic application can be more beautiful during the orthodontic treatment. Metals wires are the most reliable from the view points of rigidity, flexibility, fatigue, durability and operability. Although there are many efficient points in metals, it is inevitable to cause the unnatural effect in the visual impression of the patients and impossible to possess esthetic, and the potential biologic hazards from Ni-Ti archwires is still a possible concern. In order to improve the security and esthetic of the treatment, scholars have put forward to use the shape memory polymers(SMP), The SMP of polyurethane series has good biocompatibility and recovery properties, and the material is transparent which is potential of an ideal material for orthodontic archwires. Currently, existing research focused on the metal archwires materials, the research of archwire materials made of SMPU in the field of orthodontics is few. this paper will conduct research work on the SMPU materials including the following aspects:(1) Assuming the shape memory polymers is an isotropic material, by the application of solid mechanics and thermal viscoelasticity theory, a three dimensional constitutive equation of SMP is developed based on Tobushi’s one dimensional constitutive equation.(2) A UMAT subroutine is programmed with Fortran based on ABAQUS secondary development platform, then the SMP thermo-mechanical behavior is numerically simulated by ABAQUS. Numerical simulation results coincide with Tobushi’s experimental results, show that the thermo-mechanical behavior of SMP and it’s shape memory effect can be described by the three dimensional constitutive model of SMP.(3) In order to confirm the viscoelastic behavior of SMPU in the vicinity of the glass transition, The basic thermomechanical properties of SMPU are examined by unixial tensile tests and relaxation tests under constant temperature, laid the foundation for the evolution of the properties of SMPU wires where the viscoelastic character of SMP is the important factors, and also provide the necessary experimental date and material parameter for subsequent simulations.(4) The orthodontic finite element model was built in ABAQUS which was excellent in geometric and biomechanical resemblance with clinical condition, the maxillary lateral incisors, canine was chosen as research object, the arch-wire assembly process was equivalent to moving the brackets by applying displacement in brackets to control the tooth movement, the process of SMPU archwires fixed into bracket of orthodontics patient’s teeth have the direction of the lip and cheek or lingual dislocation was simulated, the initial orthodontic force was obtained from the finite element analysis which can predict the orthodontic force acting on brackets in the course of the treatment, orthodontic finite element simulation results show that the SMPU arch-wire can provide recovery force for tooth movement, but the force is tend to be small, need to further improve its mechanical properties.