Artificial Muscle Devices: Innovations and Prospects for Fecal Incontinence Treatment

Abstract

Fecal incontinence describes the involuntary loss of bowel content, which is responsible for stigmatization and social exclusion. It affects about 45% of retirement home residents and overall more than 12% of the adult population. Severe fecal incontinence can be treated by the implantation of an artificial sphincter. Currently available implants, however, are not part of everyday surgery due to long-term re-operation rates of 95% and definitive explantation rates of 40%. Such figures suggest that the implants fail to reproduce the capabilities of the natural sphincter. This article reviews the artificial sphincters on the market and under development, presents their physical principles of operation and critically analyzes their performance. We highlight the geometrical and mechanical parameters crucial for the design of an artificial fecal sphincter and propose more advanced mechanisms of action for a biomimetic device with sensory feedback. Dielectric electro-active polymer actuators are especially attractive because of their versatility, response time, reaction forces, and energy consumption. The availability of such technology will enable fast pressure adaption comparable to the natural feedback mechanism, so that tissue atrophy and erosion can be avoided while maintaining continence during daily activities.

Keywords: Biomimetic design; Electro-active polymer actuator; Fecal sphincter.

Figure 1

The flow chart shows the wide variety of the more or less complex FI treatments. If the conservative therapies fail, i.e., dietetics, medication, and pelvic floor training or pelvic floor incidents are present, a surgical approach is often advisable, which includes sphincter plastics, graciloplasty, and sacral nerve stimulation. In severe cases of FI, the artificial bowel sphincter systems can be applied. The ultima ratio is the creation of a stoma.

Figure 2

Opening in natural and fluid-cuff-based artificial sphincters. (a) Closeness depends on the intact continence organ consisting of an internal and external anal sphincter (IAS, EAS), hemorrhoid cushion, and puborectal muscle (m. puborectalis). (b) The puborectal muscle loops around the rectum and pulls it towards the os pubis, a ventral bone of the pelvis. (c) When the puborectal muscle is activated, the rectum is closed, and feces cannot descent from the rectal ampulla to the anal canal. For defecation, the puborectal muscle is relaxed, the rectum straightens, and faces decent. The relaxation changes the anorectal angle indicated by green color. The IAS, which is an involuntary muscle, relaxes as well by the reflex triggered by the distension of the rectal ampulla. If the EAS is voluntary relaxed, defecation is possible. (d) Currently, the artificial sphincters are generally based on fluid-filled cuffs. (e) Activating the pump to defecate, the fluid from the cuff is transported to the reservoir. To restore continence, the fluid is pushed back in the cuff as shown in (d). The majority of the FI systems are implanted around the EAS. The simple fluid-filled cuff systems constantly act on the underlying tissues. Because continence can only be ensured for rather high pressures, the tissues are compromised, which usually results in atrophy and erosion.

Figure 3

Scheme of the PAS in (a) close and (b) open states. As the other fluid-filled cuff systems, the PAS works through mechanical obstruction. Similar to the puborectal muscle, cf. Fig. 2(c), the PAS increases the angle of the anorectal junction to gain continence. The PAS also contains a gel-filled cushion, given in yellow, which deforms, when the pressure in fluid-filled cuff is changing. The PAS is placed above the sphincter around the rectum close to the anorectal junction implying many advantages. First, there is much more space and less vulnerable structures than close to the anus. Second, the minimally invasive surgery for the PAS placement is comparably easy and safe, which results in a relatively low infection rate and the absence of extended scar tissue.

Figure 4

Schematic representation of the actuation principles currently used for commercially available artificial sphincters and systems under development. (a) An inflatable membrane of a fluid-filled cuff squeezes the tissues according to the induced pressure. (b) Two plates made from the shape memory alloy Ni–Ti deform as the result of a temperature increase to 55 °C. Thermal isolation is required to avoid the heating of the surrounding tissues above 43 °C. (c) An electromagnet switches the hinged clamp mechanism. The silicone rubber housing has to be carefully designed to ensure secured gripping. (d) The elastic scaling cuff contains a circularly stretchable mechanism with an integrated steel wire rope. The gearbox connected to a micro-motor rolls up the wire resulting in contraction. The springs are encapsulated in silicone.

Figure 5

Roadmap of approaches toward breakthrough in non-biological artificial sphincter development. Active sphincter devices to replace the function of the continence organ include electromagnetic, shape memory alloy-based, and, in particular, fluid-filled cuff systems. The diagram shows from top to bottom the currently commercially available implants, the prototypes currently in animal studies, and the systems under development for future artificial sphincters. The columns list the features of the individual systems, the related physical principles of operation and the system’s denomination. The Acticon™ Neosphincter is the only device approved by the Food and Drug Administration (FDA) so far. The Soft Anal Band is clinically available as investigational device. The Prosthetic Anal System (PAS) has also been implanted in humans. For the German Artificial Sphincter System (GASS) and the shape memory alloy sphincter reports on in vitro and animal studies are available. A Swiss consortium, the SmartSphincter team of nano-tera.ch, develops a low-voltage, dielectric actuator sphincter with sensory feedback. AMS: American Medical Systems, A.M.I.: Agency for Medical Innovations, AASS: Artificial Anal Sphincter System.