# Crawling Magnetic Robot to Perform a Biopsy in Tubular Environments by Controlling a Magnetic Field
## 1 Introduction  
A biopsy is a medical test to examine the presence or extent of a disease using tools such as a scalpel, a needle, or forceps to extract sample tissues from a patient's body. While incisional or needle aspiration biopsies take a tissue sample close to human skin, endoscopic or cardiac biopsies collect sample tissues located deep inside the human body by inserting a long tube or catheter along the tubular structure of the human body. Conventional wired biopsies have low steering ability and controllability and mostly depend on the experience of physicians. Furthermore, there is a possibility of causing bacteremia due to the penetration of external bacteria into the patient's body through the repeated insertion of the wires [1, 2]. This may cause sepsis in patients who have low levels of immunity and could lead to death [3, 4]. To overcome these problems, many researchers have studied wireless biopsy robots. Previous researchers have mainly explored biopsy robots for capsule endoscopes. However, such robots do not have their own propulsive force and are driven by the passive peristaltic motion of the intestine or by a micromotor [5, 6, 7]. One was designed to contain a microspike, which moves in and out of the robot using a slider-crank mechanism to perform safe biopsies [5]. Other researchers have proposed a magnetic robot that contains permanent magnets inside and can be controlled by using an external magnetic field to generate propulsive force instead of a micromotor and battery [8, 9, 10]. These magnetic robots include swimming robots with helical structures or biomechanical wings and inchworm and earthworm robots that locomote using wormlike motions [11, 12, 13, 14, 15, 16, 17, 18, 19]. These robots are useful for navigation, but it is difficult to maintain their positions, especially in the fluidic pulsatile environment of human blood vessels, where external forces are continually applied. Toaddress this problem, several researchers have proposed a crawling robot operated in the tubular environments with a diameter of 9 to 21 mm [20; 21; 22]. Magnetic torque generates asymmetric frictional force between the legs of the crawling robot and the external wall, and this allows the crawling robot to navigate forward and backward. This is useful in performing other tasks because the crawling robot always maintains its position compared with other swimming robots, even without the application of an external magnetic field [20]. However, it is still difficult to generate other mechanical motions of the crawling robot without affecting the crawling motion itself, because the source of motion is the external magnetic field that is applied to the entire body of the crawling robot. [PERSON] et al. proposed a crawling robot capable of selectively or simultaneously performed drilling motion in addition to its crawling motion by controlling the external precessional magnetic fields [21]. Recently, [PERSON] et al. succeeded in achieving selective motion control of a crawling magnetic robot for wireless self-expandable stent delivery in narrowed tubular environments, in addition to crawling and drilling motions [22]. However, further research is still required to generate independent medical actions of the crawling robots such as drug deliveries or biopsies. In this paper, we propose a crawling magnetic robot (CMR) that can wirelessly crawl and perform biopsies in tubular environments. The proposed CMR performs various mechanical actions under the control of an external magnetic field, such as a crawling motion with the needle covered, an uncovering motion to expose the needle, the insertion of the needle into the target area, and a covering motion of the needle after the collection of sample tissues. First, we explain the structure of the CMR and the magnetic manipulation method.