Fecal incontinence is a common clinical symptom, which defined as the voluntary loss of the body's ability to control stool or gas for more than three months [1], [2]. Studies have shown that the prevalence of fecal incontinence in the population is approximately 2-3%, which women twice as high as men. And among adults over the age of 45, women are 8 times more prevalent than men [3]. The most common cause of fecal incontinence for the woman is sphincter damage during pregnancy or giving birth [4]. In addition, the incidence of fecal incontinence also increases with age. According to the survey, about 7% to 18% of healthy people have experienced fecal incontinence, but due to personal privacy and other issues, most adults who have experienced the disease have not been included in the statistical incidence of fecal incontinence [5]. Due to the complex etiology of fecal incontinence and the large individual differences in patients, it is difficult to uniformly treat all cases.

The treatment options for fecal incontinence include conservative treatment and surgical treatment. Doctors often offer different treatment options based on the cause and severity of fecal incontinence after clinical evaluation. Conservative treatment mainly includes diet management, medication-assisted therapy, biofeedback and non-surgical electrical stimulation [6], [7], [8]. For patients with mild symptoms, conservative treatment with dietary modification and supplemental medication is usually used as the initial treatment plan. When conservative treatment fails, surgical treatment is required. At present, there are many drawbacks in surgical treatment. Different treatment plans are required for different patients, which mainly due to the complex etiology of fecal incontinence. In addition, the incidence of postoperative complications is also high. Surgical treatment includes sacral nerve stimulation, enterostomy and artificial anal sphincter [9], [10]. Sacral nerve stimulation is an effective method to treat fecal incontinence in patients with sphincter dysfunction and intact sphincter. However, for patients with severe defects in sphincter morphology, enterostomy or artificial anal sphincter implantation is often required. There are many clinical problems associated with enterostomy, such as bowel prolapse, hernia, infection and bleeding. Therefore, artificial anal sphincter is an ideal surgical treatment option for patients with severe anal incontinence with congenital, neurogenic and colorectal cancer.

The artificial anal sphincter is designed to imitate the function of normal human intestinal muscles and help patients achieve free bowel control. At present, the artificial anal sphincter has common complications such as infections and mechanical failures. After long-term implantation, it is prone to local intestinal tissue ischemia and necrosis. The main reason for these problems is the biomechanical compatibility between the implanted artificial anal sphincter and the intestine.

Biomechanical compatibility refers to the property and ability of artificial implants to match their elastic shape when they produce mechanical effects on the biological tissue at the site. Based on the biomechanical properties of intestinal tissue, long-term use of artificial anal sphincter can lead to tissue thickening. When too much pressure is applied, it can cause rectal tissue to shrink and damage, and when too little pressure is applied, it can cause the device to fail.

In this paper, a mechanically matched means of constant force clamping is provided by the device to control the proper working pressure of the artificial anal sphincter on the intestine. The Constant force clamping is that a constant or minimally variable output force over a wide range of input motion displacement. The constant force clamping is achieved by superelasticity of shaped shape memory alloy (SMA). The superelasticity of SMA is that SMA can be greatly deformed by applying a certain stress under a certain ambient temperature, and the SMA can still return to the initial shape before deformation after the stress is released. It is optimized and verified initially that the novel artificial anal sphincter can achieve constant force clamping of the intestine within a large deformation range by finite element analysis.