Citation: Lee, S.; Kim, N.; Kwon, J.; Jang, G. Identification of the Position of a Tethered Delivery Catheter to Retrieve an Untethered Magnetic Robot in a Vascular Environment. Micromachines 2023, 14, 724. https:// doi.org/10.3390/mi14040724 Academic Editor: Junyang Li Received: 17 February 2023 Revised: 20 March 2023 Accepted: 21 March 2023 Published: 24 March 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). micromachines Article Identification of the Position of a Tethered Delivery Catheter to Retrieve an Untethered Magnetic Robot in a Vascular Environment Serim Lee, Nahyun Kim, Junhyoung Kwon and Gunhee Jang * Department of Mechanical Convergence Engineering, Hanyang University, Seoul 04763, Republic of Korea; tpfla8243@hanyang.ac.kr (S.L.); shqpffhtm@hanyang.ac.kr (J.K.) * Correspondence: ghjang@hanyang.ac.kr Abstract: In this paper, we propose a method of identifying the position of a tethered delivery catheter in a vascular environment, recombining an untethered magnetic robot (UMR) to the tethered delivery catheter, and safely retrieving them from the vascular environment in an endovascular intervention by utilizing a separable and recombinable magnetic robot (SRMR) and a magnetic navigation system (MNS). From images of a blood vessel and a tethered delivery catheter taken from two different angles, we developed a method of extracting the position of the delivery catheter in the blood vessel by introducing dimensionless cross-sectional coordinates. Then, we propose a retrieval method for the UMR by using the magnetic force considering the delivery catheter’s position, suction force, and rotating magnetic field. We used thane MNS and feeding robot to simultaneously apply magnetic force and suction force to the UMR. In this process, we determined a current solution for generating magnetic force by using a linear optimization method. Finally, we conducted in vitro and in vivo experiments to verify the proposed method. In the in vitro experiment, which was in a glass tube environment, by using an RGB camera, we confirmed that the location of the delivery catheter in the glass tube could be recognized within an average error of 0.05 mm in each of the X- and Z-coordinates and that the retrieval success rate was greatly improved in comparison with that in the case without the use of magnetic force. In an in vivo experiment, we successfully retrieved the UMR in the femoral arteries of pigs. Keywords: position identification; retrieval of an untethered magnetic robot; vascular environment 1. Introduction Occlusive vascular diseases (OVDs), such as stroke in the brain, myocardial infarction in the heart, and peripheral artery disease in the leg, occur when a blood vessel becomes narrowed or blocked due to accumulated blood clots and lipids. Such diseases are expected to become even more important health issues globally, since the incidence rate of OVDs is certain to increase due to bad eating habits and an aging society [1]. OVDs are treated through endovascular interventions, in which medical doctors first secure a path through existing blood vessels to blocked lesions by using catheters and guidewires, and then insert a balloon or a stent through the secured path to widen the lesion [2,3]. During the procedure, medical doctors monitor the blocked lesion, guidewires, and catheters through X-ray imaging devices. This procedure has become popular because this treatment does not entail the opening of the body. However, since medical doctors can manipulate only the proximal part of a conventional tethered device, which is located outside of the patient’s body, it is almost impossible to precisely control the magnitude and direction of the force and the torque at the distal end of the device [4–6]. Furthermore, medical doctors are continually exposed to hazardous X-ray radiation [7–9]. As an alternative for overcoming these limitations, a magnetic robot system has been proposed [10]. The magnetic robot system comprises a magnetic navigation system (MNS) that generates an external magnetic field and a magnetic robot to which magnetic materials Micromachines 2023, 14, 724. https://doi.org/10.3390/mi14040724 https://www.mdpi.com/journal/micromachines