Soft robotic devices have many desirable features for minimally invasive surgery (MIS) applications, but many interdisciplinary challenges remain unresolved. To understand the current state of the technologies, a keyword search was performed using the Web of Science and Scopus databases. From the resulting list of articles, a selection of articles was made based on inclusion and exclusion criteria, which were used to compare various features of soft robotic devices for MIS. There was a low diversity in device designs and a high level of detail in technical capabilities. We propose a standardized comparison methodology to characterize soft robotics for different MIS applications to help designers in the production of the next generation of devices.
1 Introduction
Minimally invasive surgery (MIS) uses long, rigid or flexible surgical instruments that are inserted into the body through small incisions or through natural openings. In contrast, open surgery involves making large incisions in the body in order to reach the desired site directly. The aim of MIS is to perform a surgical procedure as safely and as quickly as possible while minimizing damage to the surrounding tissue. MIS is increasingly being used as an alternative to open surgery, as it can bring improvements in patient safety, cosmetics, recovery time, length of hospital stay, number of postoperative complications and less pain [1]. This review is a detailed summary of current literature on novel soft robotic devices for MIS.
At the center of MIS is the field of endoscopy, the process of observing the inside of the body by directly inserting an optical device into the area of interest. The optical device is called an endoscope. There are different types of endoscopes. Today, an endoscope generally refers to a long flexible tube of about 1.5-2 m in length, equipped with a high-resolution camera and a light source at the tip of the endoscope. The head can be actively controlled by means of two thumb-operated rotary knobs at the operating end. Typically, there are working channels along the length of the endoscope for the supply of air and water and those through which small instruments can be flexibly inserted to perform simple therapeutic procedures. Flexible endoscopes of this type are used to visualize the upper gastrointestinal (GI) tract (gastroscope) and the lower GI tract (colonoscope). However, rigid endoscopes of different lengths and diameters are also used in many applications, for example to visualize the abdomen (laparoscope), the brain (neuroendoscope), the joints (arthroscope) and the esophagus (esophagoscope). There are a variety of endoscopic procedures that enable either diagnosis or treatment of different parts of the body. Flexible and rigid endoscopes can vary in diameter and length depending on the application and patient.
Surgical instruments allow the surgeon to grasp, dissect, remove and suture tissue inside the body [2]. A common example of MIS is endoscopic surgery for abdominal procedures, in which a laparoscope and two or three long, rigid surgical instruments with a typical diameter of around 5 mm are inserted into the abdomen through several individual small incisions. Various approaches have been developed to make MIS even less invasive and to enable new procedures that are not possible with traditional open surgery. One of these improved approaches is single-incision laparoscopic surgery, in which not only a laparoscope but also two rigid instruments are inserted through a single larger incision in the abdomen, preferably at the umbilicus, reducing the number of incisions but increasing the difficulty of the procedure. Natural orifice transluminal endoscopic surgery (NOTES) is a technique in which the abdomen is accessed via a long flexible endoscope inserted through the mouth, anus or vagina, with the advantage of completely avoiding abdominal incisions [3]. Instead of incisions, NOTES is performed through internal incisions that allow the endoscope to pass between tubular structures in the body, called the lumen, to neighboring cavities. MIS can also be performed on the brain by removing a portion of the skull and placing a port through which a neuroendoscope and surgical instruments are passed to gain access to the target tissue deep within the brain [4].
MIS is associated with small, easily deformable, dynamically changing and unstructured working spaces, poor visibility with few visual markers for orientation and the use of long, narrow instruments. Long, rigid instruments used in some forms of MIS make the instruments difficult to use due to the so-called fulcrum effect, which is caused by the insertion point of the instrument into the body. The rigid instrument rotates around this point, reversing the surgeon's movements and increasing hand tremor [5]. In current robotic MIS approaches, a surgeon controls a rigid robotic device, which in turn controls the movement of the modified surgical instruments. The forces exerted at the tip of manually operated laparoscopic instruments can range from 0.1 to 10 N [6]. Designers of robotic systems strive for similar performance. In addition, robotic systems offer precision, stability, motion scaling, and other advantages, but due to their immobility and in some cases their size, they cannot navigate tortuous paths, which means they cannot access the entire target anatomy. If the surgical site cannot be performed with rigid devices, flexible endoscopes and instruments are used. However, if flexible devices cannot be used, open surgery may be the only option. In robotic systems with multiple instruments, the possibility of instrument collisions must also be considered [7], which makes manipulation of the instruments more complex due to their overlapping working areas. When using some robotic systems, difficulties also arise when changing instruments during a procedure.
The use of instruments that are difficult to use leads to lengthy procedures and carries a high risk of causing unwanted harm to the patient [8]. In addition, sometimes years of training are necessary to become expert in their use. Patient pain is often caused by the instruments deforming or perforating the surrounding tissue, which can be caused by using endoscopic instruments that are too stiff [9]. Damage or pain to the patient can also be caused by the use of flexible devices. An example of this is the looping of the colon during colonoscopy. [10]. In addition, there are still problems with positioning, dexterity, force transmission and visualization when using flexible instruments and endoscopes [11]. Research in the field of soft robotics aims to bring together the controllability of rigid robotics, the ability of flexible instruments to reach hard-to-reach areas, and the safety of soft materials.
The field of soft robotics focuses on the use of soft, compliant materials to construct robotic devices. Due to their manufacturing materials, soft robots are ideal for unstructured environments and for human use as they can deform according to their environment [12]. This differs from the traditional robotic design approach where rigid materials are used for both the robotic joints and articulations. Soft robotics is excellent for medical applications where avoiding trauma and pain to the patient is of primary importance [13]. Due to the challenges faced by MIS, compliance with regulations, the ability to adjust variable stiffness and patient safety are some of the most important design criteria [14]. Soft robotics fulfill these requirements to a good degree. In the authors' experience, soft materials achieve high patient acceptance compared to robotic devices made of metallic or other rigid materials. A group representing colorectal cancer patients found that a non-intimidating appearance is more important than the cross-section of the device. Clinicians specializing in GI tract applications and familiar with its delicate mechanical properties also expressed a preference for soft devices that were less likely to cause pain and trauma to patients.
Unfortunately, when using soft robotic devices, many compromises must be made in exchange for increased patient safety. These include lower force exertion, poorer controllability and lack of sensing capabilities, as discussed, for example, in Hughes et al [15]. Simulation of soft robots is difficult and computationally expensive because compliant materials respond nonlinearly to loading and soft devices have many degrees of freedom, making predictable design of soft robots difficult [16]. Soft robots can achieve large changes in volume, shape and stiffness that are impossible for conventional robots, giving them a unique advantage. Harnessing these capabilities and overcoming the problems of low force exertion and controllability will provide the next generation tools for MIS.
One advantage of soft materials is that they are often more economical, readily available and easy to handle; the most commonly cited advantage is the wide range of elastomers used in much of the current research. By using low-cost soft materials and new manufacturing techniques, it is also possible to design robots that are designed to be disposable for use in MIS, that are patient-specific, that are low-cost, or that can be manufactured using rapid prototyping. Because soft robots are less expensive than traditional robotic systems, they have the potential to be widely available, making them a candidate for frugal design approaches with high utility value. Advances in materials and manufacturing will increase the potential for patient-specific soft robotics in the future. However, there may be additional regulatory requirements for customizable medical devices in the future. On a case-by-case basis, manufacturers must ensure that each customized device meets the appropriate quality and safety requirements; therefore, a reliable manufacturing process is extremely important.
The aim of this review is to provide an overview of soft robotic devices being developed in the field of MIS, so that designers can more easily find new ways to overcome the many challenges they currently face.
The next section reports on the methodology used to conduct the literature review, followed by a description of the comparison process and the results of the literature review. The areas of application of the devices include endoscopic procedures for both diagnostic and therapeutic purposes. In the remainder of this review, the working principles and materials found in the selected soft robotic devices are described in more detail. Attention is drawn to the challenges in many disciplines that need to be considered and the lack of a standard comparative method for soft robotic devices in MIS.
-continued-
