1. Introduction
Intestinal-related diseases, even intestinal cancer, all around the world are increasing nowadays, and are gradually become stubborn diseases threatening human health and lives [
1,
2]. Researchers have paid more attention to diagnostic methods of intestinal diseases for decades. As is known, the intestine is slippery, viscoelastic, and tubular [
3]. As such, an efficient diagnostic device suitable for the intestinal environment is urgently needed.
Since the first traditional endoscope, built in 1983 by American company Welch Allyn, it has become a very mature technology. Though endoscopes with high resolution can observe the fine structure of the intestinal mucosa and find tiny lesions, it brings pain to patients and even complications, which are potentially harmful [
4]. More importantly, for the insertion endoscope, most parts of the small intestine are still blind spots [
5].
In 2000, researchers Iddan et al. proposed the first non-invasive wireless capsule endoscope, with a size of Φ 11 mm × 26 mm [
6]. The wireless capsule endoscope moves forward by intestinal peristalsis like a capsule moving in the intestine, without discomfort for patients, and could compensate for the blind spots of traditional endoscopic techniques mentioned previously. However, for this type of movement, it is impossible to conduct fixed-point and long-term observation of a specific part in the intestine, let alone repeated inspection. Moreover, lacking the ability to expand the intestinal folds, lesions in folds cannot be examined, leading to a certain rate of missed diagnosis [
7].
In order to overcome the limitation of passive motion of the wireless capsule endoscope, one kind of capsule endoscope with active motion, called a magnetic drive capsule endoscope, has been developed by researchers in the past few years [
8,
9,
10,
11]. Based on the traditional capsule endoscope structure, a permanent magnet is embedded in the capsule. Outside the body, a magnetic field is constructed, which is used to produce magnetic interaction with the magnet in the body. By controlling the direction and intensity of the external magnetic field, the capsule endoscope is actuated inside the body. The magnetically-actuated capsule endoscope could explore the intestine to some extent by active motion, However, it may cause some damage to the internal surface of the intestine, due to its fast speed [
12]. In addition, the small magnetic driven force, insufficient positioning accuracy, and inability of expanding intestinal folds limit the range of the inspection area.
Nowadays, with the development of technology, more and more attention has been paid to micro-robots, which possess actuators on board. Researchers Dario et al. designed a series of bio-inspired leg-based robots [
13,
14,
15,
16], with four- and eight legs. In spite of exhibiting bidirectional motion and stable anchorage, legged robots may cause damage to the intestine due to sharp tips of legs, and high power consumption relying on a tethered power supply makes it inconvenient for intestine inspection. Imitating paddling, a kind of paddling-based micro-robot has been developed [
17,
18]. By moving expanded legs from the front to the back of the robot body, the micro-robot travels forward. The in vivo experiments demonstrated a highly efficient motion in the intestine. However, the robot is only able to achieve one-way motion and could not anchor at specific points, which cannot be ignored for accurate inspection. A tracked micro-robot that moves by using micro-patterned treads has also been studied [
19,
20]. It has a large contact surface with the intestine, which improves the safety of the leg-intestine interaction. However, because of its large size, it is only suitable for the examination of the colon and rectum, and not suitable for the small intestine. In addition, as a tethered micro-robot, it cannot go deep into the intestine for inspection. The capsule robot proposed in [
21] is designed considering movement safety and efficiency. However, the variable diameter ratio of the capsule robot is too small (only 2.1) to explore the intestine with a large diameter.
To sum up, an ideal non-invasive diagnostic device should be an untethered micro-robot with a swallowable size and possess effective bidirectional motion, stable anchorage, and sufficient visualization of the lumen without insufflation (that is to say, the ability to expand intestinal folds).
In this article, a novel inchworm-like intestinal micro-robot (IIMR) with a swallowable size (14 mm in diameter) is proposed, which is powered by wireless power transmission. The IIMR system consists of two main mechanisms, and expanding mechanism and a telescoping mechanism, to help it move forward, backward, and anchor stably. Meanwhile, an expanding mechanism makes it possible to easily obtain an adequate visualization.
This paper is organized as follows: The overview of the system and the mechanisms design are introduced and details about the kinematics and dynamics analysis of the IIMR system are provided in
Section 2; then experiments are conducted for validation in
Section 3. Finally, in
Section 4, conclusions are drawn.
4. Discussion and Conclusions
This research has presented the design, analysis and experimental validation of a novel inchworm-like intestinal micro-robot (IIMR), which provides a non-invasive way in exploring intestinal diseases. The proposed mechanisms are not only minimized in size but also provide active locomotion. The micro-robot with the novel mechanisms has the following advantages: First, the diameter of the micro-robot can be as small as 14 mm and the designed mechanisms can save more space for surgical tools to be embedded. The small size makes the robot easier to swallow, and allow the micro-robot could move through the pylorus to monitor the small intestine. Second, the expanding mechanism has a larger variable diameter ratio of 3.43. The larger expanding diameter makes it possible to anchor itself at a specific suspicious lesion points in the large intestinal lumen. Furthermore, the expanding mechanism could easily distend folds for an adequate visualization without the need for insufflation. Third, the designed micro-robot possesses bidirectional motion and stable anchorage, so the inspection efficiency and accuracy of intestinal diseases can be improved. In addition, the lengthened parts at the tips of the expanding legs increase the contact surface with the intestine, which could help avoid damage to the intestine. The telescoping mechanism possesses a self-locking lead screw nut system, and could drive the IIMR system forward and backward smoothly.
After careful modelling and analysis, a prototype of the IIMR system has been fabricated. In experiments, the expanding force of the expanding mechanism could reach 4.359 N, and the expansion time is 1.352 s. The axial thrust from the telescoping mechanism was tested at 6.7048 N and the axial speed it provides is 3.894 mm/s. All of these above characteristic meet the requirements of the kinetics for an intestinal micro-robot. The in vitro experiments, whether in pipes or in porcine intestine, show an efficient motion. Especially in the in vitro intestine experiment, it has shown good performance in terms of safety and reliability when the micro-robot moves in the intestine.
For this research solely, there are still some aspects that need to be improved. As can be seen in the in vitro intestine experiment, there is a large difference in speed when the prototype moves upward and downward in the intestine. This step loss may be caused by the malleability of the intestine or external forces, such as gravity, acting as a key factor during the uphill and downhill stages. Further analysis including these factors should be considered.
Though the micro-robot is small to a certain degree, it will still be desired to make the IIMR system as compact as possible, saving space for advanced functions, such as drug delivery and tissue extraction, in future research.