Development of a method for calculating the parameters of shape memory polymer orthosis

The object of consideration is a new design of a finger orthosis. It is manufactured using tubular blanks made of cross-linked polyethylene (CLPE). CLPE products exhibit hyperelastic and viscoelastic properties, including the shape memory effect (SME), but the scope of application of heat-shrinkable tubes (HST) is mainly limited to connecting structures. A study was conducted on the use of HST as an orthosis that fixes the position of a damaged joint or limb of a person. A hollow heat-shrinkable tube is considered, which is deposited on a bandage wound on a finger. The advantages of the new orthosis include its insensitivity to the non-cylindrical shape of the real object of study (finger). The obtained formulas include averaged parameters that automatically take into account variations in the geometry and compliance of the finger along its length. It is possible, as the soft tissue edema resolves, to intermediately settle the primary fixator from HST by means of its additional heating and cooling. The design is lightweight and has minimal dimensions. An incorrect description of the thermomechanical behavior of a tube using a combined viscoelastic Prony model and a hyperelastic Mooney – Rivlin model in a range of deformations exceeding 70 % is shown. A refinement of the previously described hyperviscoelastic physical model using Ogden relations is performed. The criteria for selecting the geometric parameters of the orthosis blanks were obtained, including a formula for estimating the pressure on the patient's finger, adjusted taking into account numerical calculations in ANSYS. An experimental method for determining the properties of low-modulus materials based on screw tape winding was developed and implemented. The obtained properties of living soft tissues of the finger and a multilayer gauze bandage were implemented in ANSYS as a multi-elastic model. A spatial finite element model of the finger-bandage-orthosis system was constructed for an accurate assessment of the stress-strain state of the system. Additional numerical experiments were conducted to assess the rigidity, in which the initial finite element mesh was the deformed model after shrinkage. Based on the obtained pressure and rigidity values, a general criterion for assessing the orthosis configuration was constructed.