Research Of Constant Force Structure Based On SMA Superelasticity And Its Application In Artificial Sphincter

The biomechanical mismatch between the artificial organs and their surrounding tissues can lead to tissue atrophy,ischemic necrosis or implant failure.This is the key problem need to be solved to maintain the long term performance of the artificial organs that achieving the functional replacement by the mechanical interaction.The artificial sphincter treating fecal incontinence is the research object of this dissertation.The technical principle,optimization method and evaluation criterion of stable constant force clamping in large deformation range were studied which provided a theoretical basis in designing the artificial sphincter with simple and compact structure while providing constant force compression.The main idea of the dissertation is making the artificial sphincter device achieve constant force clamping within large deformation range under the compression load by using the superelasticity of the shape memory alloy(SMA)in stretching,structural design and parameters optimization.The artificial sphincter embedding constant force component was designed for the stomal incontinence.The contact stress between the artificial sphincter and the intestinal tissue was studied,and the adaptability of constant force range of the artificial sphincter in the long term thickening the intestinal tissue was studied.The effectiveness and safety of the artificial sphincter based on SMA constant force component were verified by experiments.The main contents of this dissertation includes the following aspects:(1)In order to solve the biomechanical compatibility of artificial sphincter and its clamping intestinal tissue,the biomechanical design and evaluation criteria of artificial sphincter were put forward.Firstly,the function of effective clamping the intestine can be achieved.Secondly,injuries of the intestinal tissue caused by the clamping overpressure can be avoided.In addition,constant force clamping of artificial sphincter is still able to be ensured when the intestinal tissue is thickened under long term mechanical stimulation.Mechanics evaluation criteria of artificial sphincter was established: a constant clamping load with a small fluctuation range in the larger deformation interval can be provided by the device.At the same time,the surface contact stress between the device and the intestinal wall need to be lower than the mesenteric blood vessels driving pressure threshold.(2)A structure design method was proposed to achieve constant force compression function within large deformation using SMA tensile superelasticity.Since SMA has constant force characteristics in the stretching process,as well as wire has application limitations in medical equipment.The stress state of the SMA sheet or round bar at different positions of the functional element was changed by structure design,and the constant force deformation function of the functional element under the compressive force was realized.The force and deformation characteristics of typical structural elements such as i-shaped structure,heart-shaped structure,M-shaped structure,V-shaped structure,O-shaped structure and C-shaped structure were studied.C-shaped sheet and rod bar were selected as the basic functional components of the artificial sphincter under pressure based on the integrated requirements of the artificial sphincter with constant deformation,simple structure and compact size.This breaks the design limitations of the constant force structure using external auxiliary equipment such as additional drives,complex sensing systems etc.,and relying on the spring or flexible energy storage components in previous researches.(3)The parameter optimization method of C-shaped constant force element design was proposed.Firstly,the influence of the structural parameters of C-shaped round bar on constant force characteristics was studied.The results showed that the working range and fluctuation amplitude of constant force characteristics can be changed by modifying the chord height,chord length and diameter of C-shaped circular bar.Secondly,the structural parameters of constant pressure compression function component with rectangular cross section were optimized by the combination of genetic algorithm and finite element method.The structural analysis function of finite element method was applied for SMA,and the nonlinear analysis ability and the global Search ability of genetic algorithm were applied for SMA.The results showed that the constant deformation characteristics of the C-shaped structural functional elements can be changed by modifying the profile parameters and cross section parameters of C-shaped sheet.To meet the small design space needs in medical field,it is more effective to obtain the large constant deformation range by changing cross section parameters of the sheet.(4)A miniature artificial sphincter device with integrated constant force loading element was designed.The circular four bar mechanism that was similar to the shape of the cross section was designed.It was then used as the displacement change actuator of the device and the support body of the constant force loading element.The constant force loading element adopted the C-shaped configuration,and both ends of C-shaped sheet were respectively connected with the peripheral annular four-bar mechanism and the clamping element which was in contact with the intestinal wall through the hinge.The load of the constant force loading element was determined according to the resting pressure in the intestine;the size of the clamping element was determined according to the contact stress threshold of the intestinal wall.The biomechanical property experiment of the intestine was designed.The resting pressure of the artificial intestines and the pig intestines with three kinds of thickness were measured.The results showed that the contact stresses of intestines were lower than the mesenteric blood vessels driving pressure threshold.And the fluctuation error of the constant clamping load was lower than 4% within the range of the three layers thickness of artificial sphincter.The effectiveness of the proposed design method for long term biomechanical effects of sphincter was verified.In view of the above,aiming at solving the biomechanical compatibility problem of artificial sphincter,the constant force clamping component with large stroke compression deformation was designed through the combination of SMA superelasticity and structural optimization,and was optimized by combining genetic algorithm with finite element method.A comprehensive method for designing constant force component using SMA was proposed.The artificial sphincter device was designed with four-bar mechanism to provide contract displacement,C-shaped components to achieve constant force clamping,and clamping components to realize uniform pressure.