Abstract
Fecal incontinence caused by sphincter dysfunction is an unresolved problem which has a serious effect on patients, both physically and psychologically. For patients with severe symptoms, treatment with an artificial anal sphincter could be a potential option to restore continence. Acticon Neosphincter still has safety and efficacy problems, which is the only device certified by the US Food and Drug Administration. The biomechanical compatibility is the key problem of artificial anal sphincter in the clinical application. This paper describes an artificial anal sphincter with superelastic shape memory alloys and its mechanical properties assessment in artificial intestine models. The new design was developed as a low invasive and constant force or pressure prosthesis with a simple structure to solve the problem of severe fecal incontinence in patients.
Introduction
Fecal incontinence caused by sphincter dysfunction is a difficult medical problem which has not been fully resolved. Fecal incontinence is the inability to control the passage of feces through the anus [1–3]. From a medical perspective the treatment methods of fecal incontinence include nonsurgical treatment and surgical treatment However, these treatment methods may be insufficient for patients having severe fecal incontinence. From an engineering perspective, the artificial anal sphincter is a potential treatment method for severe patients [4–6].
In 1999, the Acticon Neosphincter (AMS, Minnetonka, MN, USA) received a humanitarian device exemption of US Food and Drug Administration (FDA) and was formally FDA-approved in 2001 [7]. The device has been implanted in many patients. However, the patients suffered from a wide range of complications, such as infection, erosion, tissue ischemia, and mechanical failure. So the only device certified FDA still has safety and efficacy problems [8,9]. The crucial problem of this study was to keeping long-term biomechanical compatibility between the artificial anal sphincter and the surrounding tissue.
Excessive pressure can cause intestinal tissue injury, low pressure can cause intestine tissue slippage, and the intestinal tissue will be thickened under long-term pressure, which will lead to mechanical mismatch between the artificial anal sphincter and the intestinal tissue. Clamping the intestinal tissue of human body with constant force or pressure in a safe range is considered as an effective solution for this issue. Constant force can be realized using the superelastic mechanical properties of shape memory alloys (SMA), which are functional materials with the unique characteristics during its stress-induced transformation [10,11].
This paper proposed a novel artificial anal sphincter with a simple structure and constant force, which using C-shaped shape memory alloys sheet. The purpose of this work was to reduce the number of parts of the device, to develop a compact design for a less invasive prosthesis, and improve the long-term biomechanical compatibility between the artificial anal sphincter and the surrounding tissue.
Materials and method
Design of constant force component
A schematic drawing of the proposed constant force component is shown in Fig. 1a. It can be seen that the model consists of a C-shaped superelastic SMA sheet, two sleeves and two pins. The pins and sleeves were designed to simulate loading on ends of the sheet. The sleeves were rigid-connected to the superelastic SMA sheet. The mechanical property of the structure was simulated using finite element method. The material and geometric parameters and boundary conditions of the simulation were as follows:
The model of C-shaped SMA sheet. (a) C shape SMA sheet, (b) The dimension parameters and the constrained surfaces of model, (c) The boundary conditions of model, (d) The deformation of model.
(1) The material of the sheet was defined as SMA. A Ti-55.82at%Ni SMA was used as the material of the sheet with thickness 0.2 mm. The stresses of start and finish points of phase transformation from austenite to martensite were 313 MPa and 389 MPa, respectively. The stresses of start and finish points of phase transformation from martensite to austenite were 157 MPa and 79 MPa, respectively. The modulus of elasticity was 26,957 Mpa and the Poisson’ratio was 0.3. The response difference of material tension-compression was 0.12, and the residual strain was 0.07. The material of the pin and sleeve was defined as rigid body with elastic modulus of 150,000 MPa and Poisson’s ratio of 0.3.
(2) The contact pair was established using element target170 and contact174 between pin and sleeve, and the contact parameter fcn was 0.1.
(3) The equation for the contour curve of C-shape sheet was
(4) In simulation a 1/4 model was applied due to the symmetry. Symmetry constraints were applied on section B,C, D and E, the degree of freedom in Y-direction of area A was constrained and the displacement in X-direction of area A was applied, as shown in Fig. 1b–c. The reaction force in X-direction of area A was obtained during the numerical simulation. The force-displacement values during all applying displacement were analysed. The deformation of model is shown in Fig. 1d. The structure optimization for superelastic SMA component to obtain most constant force was done by combining finite element analysis in ANSYS with genetic algorithm in MATLAB. The result of optimization design was validated by performing the experiment of solid model.
The designed artificial anal sphincter was composed of two upper claw, two lower claw, two clamping elements, two superelastic SMA sheets and two ropes with position limit. The upper claw connected to lower claw by hinges, and the rope with position limit was a link between clamping element connected and upper claw. The geometric dimensions of superelastic SMA sheet were determined by the optimization in Section 2.1. The device in its open (left) and closing (right) state is shown in Fig. 2.
Schematic drawing of the proposed artificial anal sphincter in its open (left) and closing (right) state.
The intestinal tissue can be thickened under long-term pressure of artificial anal sphincter. To measure the constant force characteristics of the artificial intestine with various thicknesses, single layer artificial intestine, two layers artificial intestine and three layers artificial intestine with soft rubber were prepared, as shown in Fig. 3. To verify the safety of the artificial anal sphincter, the surface contact stress of intestinal tissue under the clamping of artificial anal sphincter was tested. The schematic diagram of the experimental device for measuring the contact stress of intestinal tissue is shown in the Fig. 4. The artificial intestine with soft rubber was installed into the clamping component to test the surface contact stress to the bowel provided by the device. The surface contact stress was measured with contact sensor with high sensitivity (KYOWA), and the sampling frequency is 20 Hz. Four sensors are attached to the clamping element of the artificial anal sphincter, and the intestine is installed into the device. The surface contact stress of the artificial intestine was monitored and collected simultaneously by data acquisition unit (KYOWA, PCD-300A) during the closing state of the device.
The artificial intestine models with three kinds of thicknesses for experiment.
Schematic diagram of experimental setup for measuring contact stress at the intestinal surface.
Based on the design method, the obtained the design solution is l = 18 mm. The solid model of l = 18 mm, 0.2 mm in thickness, 2 mm in width was fabricated, and its actual force-displacement curve was measured. Figure 5 shows the experimental curve of the C-shaped sheet with l = 18 mm It can be seen that the curve shows good flatness between displacement of 5 and 16 mm, and the force of experimental curve is roughly 1.11 N in the constant force region. The constant force region was approximately 68.75% of the entire input displacement.
The corresponding experiment curve of the optimized solution.
The force for closing the intestine is decided by occlusion pressure of artificial anal sphincter and the contact area of the intestine with the clamping element. An artificial anal sphincter pressure of 50 to 60 mmHg is thought be appropriate based on clinical experience. The contact area of the intestine with a clamping element is established as 96 mm2. So the clamping force to close the intestine canal is 0.64 N to 0.77 N. The clamping force to close the intestine can be obtained by adjusting the width of C-shaped superelastic SMA sheet, this is the reason that the force of SMA sheet is proportional to its width. The force of constant force region was roughly 1.11 N for the superelastic SMA with l = 18 mm, 0.2 mm in thickness, and 2 mm in width. Therefore, for the superelastic SMA with l = 18 mm, 0.2 mm in thickness, the width of constant force element can be 1.15 mm to 1.39 mm.
Figure 6 shows the prototype of the developed artificial anal sphincter with the constant force element. The device has the dimension of 60 mm in length, 60 mm in width, 6 mm in thickness for its open state, and the dimension of 70 mm in length, 40 mm in width, 6 mm in thickness for its close state. The superelastic SMA with l = 18 mm, 0.2 mm in thickness, 1.28 mm in width is as the constant force element of the artificial anal sphincter.
The dimensions of the artificial anal sphincter in its open (left) and closing (right) state.
The experimental device for measuring the contact stress of intestinal tissue is shown in the Fig. 7. The experimental comparison for surface contact stresses of artificial intestines with three kinds of thicknesses is shown in Fig. 8. The average contact stress of artificial intestines with three kinds of thicknesses at point 1 is 7.534 Kpa, the maximum difference is 0.11 Kpa, and the fluctuation error is about 1.5%. The average contact stress of artificial intestines with three kinds of thicknesses at point 2 is 7.546 Kpa, the maximum difference is 0.118 Kpa, and the fluctuation error is about 1.6%. The average contact stress of artificial intestines with three kinds of thicknesses at point 3 is 7.545 Kpa, the maximum difference is 0.15 Kpa, and the fluctuation error is about 2%. The average contact stresses of artificial intestines with three kinds of thicknesses at point 4 is 7.536 Kpa, the maximum difference is 0.127 Kpa, and the fluctuation error is about 17%. The fluctuation error of constant clamping load at four test points of artificial intestine with three kinds of thicknesses is less than 2%. The results show that artificial anal sphincter had minimal change in the surface contact stress, even if the thickness of the artificial intestine is changed. This could prevent ischemic necrosis of intestinal tissues caused by overpressure. The study reported that the driving pressure threshold of mesenteric vessels was 10 Kpa, and the measured contact stress values were lower than 10 Kpa, which could avoid the clamping injury of intestinal tissues.
The experiment for contact stress at the artificial intestinal surface.
The contact stress at the artificial intestinal surface with three kinds of thicknesses.
It was demonstrated that the proposed artificial anal sphincter with superelastic SMA sheet had minimal change in surface contact stress, even if the thickness of the artificial intestine was changed. This could prevent ischemic necrosis of soft tissues caused by excessive pressure. The measured contact stress values were lower than the driving pressure threshold of mesenteric vessels, which could avoid the clamping injury of intestinal tissues. It was possible that this device would be helpful for patients with fecal incontinence and could have long term effective. To further verify the constant force property and effectiveness of the device, experiments would be carried out with animal intestine
Footnotes
Acknowledgements
The work is funded by the National Natural Science Foundation of China (31900944).
Conflict of interest
The authors declare no conflict of interest.