Overcoming the Force Limitations of Magnetic Robotic Surgery: Magnetic Pulse Actuated Collisions for Tissue-Penetrating-Needle for Tetherless Interventions

Abstract

The field of magnetic robotics aims to obviate physical connections between the actuators and end-effectors. Such tetherless control may enable new ultra-minimally invasive surgical manipulations in clinical settings. While wireless actuation offers advantages in medical applications, the challenge of providing sufficient force to magnetic needles for tissue penetration remains a barrier to practical application. Applying sufficient force for tissue penetration is required for tasks such as biopsy, suturing, cutting, drug delivery, and accessing deep seated regions of complex structures in organs such as the eye. To expand the force landscape for such magnetic surgical tools, an impact-force based suture needle capable of penetrating in vitro and ex vivo samples with 3-DOF planar motion is proposed. Using custom-built 14G and 25G needles, we demonstrate generation of 410 mN penetration force, a 22.7-fold force increase with more than 20 times smaller volume compared to similar magnetically guided needles. With the MPACT-Needle, in vitro suturing of a gauze mesh onto an agar gel is demonstrated. In addition, we have reduced the tip size to 25G, which is a typical needle size for interventions in the eye, to demonstrate ex vivo penetration in a rabbit eye, mimicking procedures such as corneal injections and transscleral drug delivery.

Keywords: magnetic systems; medical robotics; miniature robotics; surgical robots; tissue penetration.

Figure 1:

Magnetic suturing and tissue penetration with the benefit of impact forces. a.i) A typical procedure for hernia repair is to use a mesh and a suturing needle to close a defect. A magnetic needle controlled by external magnetic fields could accomplish this procedure remotely by revolutionizing the surgery with an ultra minimally invasive, magnetically controlled suture needle approach. a.ii) Ttissue access, i.e. cornea access to eye, requires strong penetration forces from a needle. The proposed magnetic device could be able to provide transcorneal or transscleral access for various medical procedures. b) To overcome the force limitation for miniature magnetic robots, we designed and manufactured MPACT-Needle which utilizes a moving magnetic piston’s momentum to realize momentarily high force outputs for tissue penetration. c) The running suture path to stitch a mesh into an agar gel and eye penetration are accomplished by the four electromagnetic coil system.

Figure 2:

Impact-based needle design, impact mechanism with magnetic actuation sequence, and magnet length optimization. a) The impact-based magnetic needle consists of five main components: the needle tip, impact plate, tubular body, permanent magnet, and a cap. b) These components are assembled via cyanoacrylate-based adhesives or press-fit inside the tubular structure. The permanent magnet is slightly smaller than the diameter of the tubular structure and is sized so as to allow it to freely move back and forth within the tube. c) Increasing the length of the magnet increases the applied magnetic force but reduces the possible travelling distance within a limited tubular body. To maximize the impact force, the optimum size of the magnet is found to be 0.66 times of the overall tubular body length. d) The motile magnet is being pulled back and forth in the tubular body to create repetitive impact forces. The magnetic field created keeps the alignment of the magnet along the penetration direction. e) High speed recording snapshots of the collision moment and the overall magnet stroke.

Figure 3:

Characterization experiments and scaling analysis. a) The forces are measured using a load cell under the workspace of the electromagnetic coil system. b) For a selected well-performing period duration, T = 150 ms, the impact force measurements with respect to time are shown. The duty ratio, D, values ranging from 0.2 to 0.8 is swept with 0.1 increments. Having D in the range of 0.4 to 0.6 yields more than 400 mN momentarily forces in the needle. c) Selection of impractical D and T values results in degradation of the force performance. The optimal value for D and T is found to be 0.5 and 0.15 s for this study. d) Compared to the DC pulling force, the impact-based mechanism provides 22.7 times higher forces to allow penetration into tissue. e) Scaling of the needle dimensions are analyzed by manufacturing and experimentally measuring the impact force of each size of needle. The diameter and needle length are the two important factors in scaling. Larger the needle is, the more impact force we can acquire. f) Higher electrical power levels on the electromagnetic coils allow exerting stronger impact forces. MagnetoSuture^™ System results in more than 400 mN forces for an optimized miniature needle dimensions at 14G.

Figure 4:

Suturing on agarose gel with a gauze mesh. a) The experimental setup for demonstrating the suturing capacity on an agar gel: we prepared a clamp mechanism that holds the sample and allows mobility for the suturing needle. b) The agar gel is 0.6% and 2 mm in thickness. A gauze mesh is covered around the agar gel to represent the meshes being used in hernia repair. c-h) The needle is being steered by a handheld remote controller in an open-loop fashion. An overhead camera is being used to provide real-time monitoring of the workspace. The needle has demonstrated three penetrations in less than 3 minutes. j-m) The suture thread used for suturing the mesh and the agar gel is shown after the completion of the suturing task. The suture thread used for the experiments is 50 μm thick.

Figure 5:

Eye penetration experiments. a) The holder structure used to stabilize the eye, mount the needle inside the coil system. b) The 25G needle being used with the impact force mechanism. c) The experimental snapshots of the penetration video provided in the Supplementary Video 2. The MPACT-Needle is able to puncture the eye in less than 30.1 seconds. d) Images of the rabbit eye before penetration and after penetration by the MPACT-Needle. Due to the puncturing deep in the eye, a passage from the cornea to the deeper tissue layers is opened. This passage allows for a fluid exchange and delivering fluid drugs. We observe the leakage of the fluid from the inside of the eye towards the inside of the tube that holds the needle. A closer look reveals the location of penetration on the surface of the cornea.