GUIDEWIRE COUPLED HELICAL MICROROBOT SYSTEM FOR MECHANICAL THROMBECTOMY

Abstract

The present invention relates to a medical robot system capable of effectively removing a calcified thrombus in a blood vessel. The present invention proposes a new guide-wired helical microrobot for mechanical thrombectomy applied to a calcified thrombus. Also, the present invention proposes an electromagnetic navigation system (ENS) which uses a high frequency operation that is based on a resonant effect in order to enhance the boring force of a microrobot. The microrobot system of the present invention can precisely tunnel through a blood vessel blockage site by means of the electromagnetic navigation system without damaging blood vessel walls. The microrobot system of the present invention has a wide range of applications including not only for thrombosis, but also thromboangiitis obliterans caused by vasoocclusion, cerebral infarction, strokes, angina or myocardial infarction, peripheral artery occlusive disease, or atherosclerosis, etc.

Claims

1. An electromagnetic actuation system comprising: (a) 3-axis Helmholtz coils comprising one pair of circular Helmholtz coils disposed in an x-axial space, one pair of square Helmholtz coils disposed in a y-axial space, and one pair of square Helmholtz coils disposed in a z-axial space; and (b) a variable capacitor connected in series to each of the Helmholtz coils.

2. The electromagnetic actuation system of claim 1, further comprising a switch selectively activating the variable capacitor.

3. The electromagnetic actuation system of claim 1, wherein in the 3-axis Helmholtz coils, one pair of square Helmholtz coils are placed inside one pair of circular Helmholtz coils and another pair of Helmholtz coils are placed inside the pair of square Helmholtz coils.

4. The electromagnetic actuation system of claim 2, wherein the electromagnetic actuation system cancels the reactance of an electromagnetic coil by the selectively activated variable capacitor at the resonant frequency (fr) and delivers the maximum current at the resonant frequency (fr): $f_r=\frac{1}{2.Math.\pi .Math.\sqrt{{\mathrm{LC}}_v}}$ wherein L is coil inductance, and Cv is variable capacitance.

5. The electromagnetic actuation system of claim 2, further comprising a current amplifier supplying a current to each electromagnetic coil in response to a control command of the switch.

6. A method for controlling a microrobot, the method comprising: (a) placing a microrobot inside the electromagnetic actuation system of claim 1; (b) applying a current to the electromagnetic actuation system to generate an electromagnetic field by 3-axis Helmholtz coils; and (c) controlling the microrobot by adjusting the intensity and direction of a current applied to a microrobot magnetized by the electromagnetic field or a microrobot containing a ferromagnetic substance.

7. The method of claim 6, wherein the electromagnetic actuation system is remote-controlled by a control system disposed in a separated space.

8. A medical microrobot system comprising: (a) a microrobot main body comprising a helical head, a rotatable spherical joint, and a magnetic substance 130; (b) an electromagnetic navigation system (ENS); and (c) a three-dimensional imaging system.

9. The medical microrobot system of claim 8, wherein the rotatable spherical joint is installed in the tip of the microrobot main body and the spherical joint is connected to a guidewire.

10. The medical microrobot system of claim 8, wherein the electromagnetic navigation system comprises an electromagnetic actuation (EMA) system for controlling a microrobot.

11. The medical microrobot system of claim 10, wherein the electromagnetic actuation system comprises 3-axis Helmholtz coils and variable capacitors.

12. The medical microrobot system of claim 11, wherein the 3-axis Helmholtz coils comprise one pair of circular Helmholtz coils disposed in an x-axial space, one pair of square Helmholtz coils disposed in a y-axial space, and one pair of square Helmholtz coils disposed in a z-axial space.

13. The medical microrobot system of claim 11, wherein the variable capacitors are connected in series to the 3-axis Helmholtz coils, respectively, to form a resonance control circuit.

14. The medical microrobot system of claim 8, wherein the imaging system comprises X-ray fluoroscopy, computed tomography (CT), positron emission tomography (PET), positron emission tomography-computed tomography (PET/CT), radioisotope imaging (RI), or ultrasonography.

15. The medical microrobot system of claim 14, wherein the X-ray fluoroscopy is mono-plane X-ray fluoroscopy, bi-plane X-ray fluoroscopy, or multi-plane X-ray fluoroscopy.

16. The medical microrobot system of claim 8, wherein the magnetic substance is a permanent magnet or an electromagnet.

17. The medical microrobot system of claim 8, wherein the guidewire is a magnetically steering guidewire.

18. The medical microrobot system of claim 8, further comprising a suction device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0124] FIG. 1 shows the entire structure of a guide-wired helical microrobot system of the present invention.

[0125] FIG. 2A shows an ENS system used in the present invention.

[0126] FIG. 2B shows an ENS platform of the present invention using bi-plane X-ray fluoroscopy.

[0127] FIG. 2C shows a tracking image of the helical microrobot of the present invention in the blood vessel.

[0128] FIG. 3A shows a structural design of a microrobot.

[0129] FIG. 3B shows a 3D-printed helical robot. The F-directional arrow represents the X-directional coordinate system of the microrobot. The arrow at the tip of the microrobot represents the Y-directional coordinate system.

[0130] R: Radius of microrobot body

[0131] 2r: Helical thickness

[0132] λ: Helical pitch

[0133] T: Rotation period, w: angular speed

[0134] F: propulsion force, u: speed

[0135] FIG. 3C shows the locomotion coordinate of a helical microrobot having a predetermined rotating electromagnetic field.

[0136] α: angle between x-y plane and magnet, θ: angle between x-axis and magnet

[0137] u: vector of projection direction (heading direction) of microrobot, n: vector of rotaional direction

[0138] FIG. 4A shows a conceptual design of an electromagnetic actuator circuit applying a resonance control system. Rp: Internal resistance

[0139] FIG. 4B shows one embodiment of an electromagnetic coil contained in an electromagnetic actuator circuit of the present invention. CHCx: x-directional circular Helmholtz coil, SHCy: y-directional square Helmholtz coil, SHCz: z-directional square Helmholtz coil.

[0140] FIG. 5 shows the maximum magnetic flux density and available operating frequency range of the ENS system of the present invention with a series resonance control system.

[0141] FIG. 6A shows in-vitro experimental results of a guide-wired microrobot, depicting the locomotion and drilling motions in a 5-branch phantom (4 mm in diameter).

[0142] FIG. 6B shows in-vitro experimental results of a guide-wired microrobot, depicting the actuation and drilling motion in a flow tube.

[0143] FIG. 7 schematically shows a guide-wired microrobot and a suction device of the present invention, which are inserted through a catheter.

DETAILED DESCRIPTION

[0144] Hereinafter, the present invention will be described in more detail with reference to examples. These examples are only for illustrating the present disclosure more specifically, and it would be apparent to those skilled in the art that the scope of the present disclosure is not limited by these examples according to the gist of the present disclosure.

EXAMPLES

[0145] I. Present Inventive System Overview

[0146] From the point of view of a practical fully controllable helical microrobot, the system of the present invention is at least composed of a helical microrobot, an electromagnetic navigation system (ENS), and an imaging system (see FIG. 1).

[0147] The microrobot is externally controlled by an electromagnetic actuation (EMA) system in the ENS. This microrobot is controlled outside an operating room so that an operator can avoid X-ray exposure. The key components of the medical microrobot system of the present invention are as follows.

[0148] A. Helical Microrobot for Intravascular Drilling

[0149] For the effective locomotion and drilling of the microrobot in the blood vessel, the microrobot needs to be helical or spiral. The size of the robot needs to be minimized for the insertion into the blood vessel and the improvement of locomotion.

[0150] In order to externally actuate the microrobot by using the electromagnetic navigation system (ENS) that provides sufficient force to implement external actuation and drilling motion, a small-sized permanent magnet is placed inside the robot main body.

[0151] The magnetization direction of the microrobot is also important capable of designing control strategies of ENS.

[0152] B. Electromagnetic Navigation System (ENS) for External Actuation

[0153] ENS plays an important role to generate a magnetic field necessary for producing the torque to align the microrobot and the force to control and actuate the microrobot in a three-dimensional space by utilizing a uniform magnet field and a gradient magnet field.

[0154] In previous studies, the present inventors successfully demonstrated several configurations and sizes of ENS based on air-core type electromagnets that enables the 3D locomotion of the microrobot.

[0155] C. Imaging System for Microrobot Tracking

[0156] Tracking and recognizing the direction and location of the robot in the human body is an important issue in clinical applications. This is associated with targeting accuracy and intravascular intervention safety and further a remote control application of a microrobot during the mechanical thrombectomy procedure. Since X-ray fluoroscopy is currently available in the operating room, an X-ray imaging system was used in the present invention.

[0157] As an alternative, an MRI system can be used to simultaneously manipulate and recognize an object. However, the control sequencing for monitoring conflicts with the control, and thus a relatively small actuation force has been generated in actuation of the microrobot in the blood vessel.

[0158] Hence, the imaging system of the present invention has been developed to have capability of grasping and detecting the location of the microrobot by utilizing bi-plane X-ray fluoroscopy through three dimensional object reconstructions (see FIGS. 2B and 2C).

[0159] II. Improved Helical Microrobot System

[0160] Hereinafter, a mechanical thrombectomy device of the present invention will be described. More specifically, the structure of a guide-wired helical microrobot and a power- and frequency-range improved ENS will be described.

[0161] A. Guide-Wired Helical Microrobot

[0162] FIG. 3A shows a structure of a helical microrobot, and FIG. 3B shows a prototype of the helical microrobot, made by a 3D printer (VeroClear-RGD810 from Stratasys).

[0163] The device of the present invention is equipped with a rotatable spherical joint in the tip of the helical microrobot, and the spherical joint is connected to a guidewire.

[0164] The guidewire connection is performed for the purpose of supporting the microrobot against the blood flow in the blood vessel.

[0165] Moreover, the spherical joint enables the supporting guidewire to maintain the drilling motion without twisting. The geometric parameters of the helical-head guidewire are summarized in Table 1.

TABLE-US-00001 TABLE 1 Parameter Explanation Value R Body radius [mm] 1 λ Helix pitch [mm] 5.5 2r Spiral thickness [μm] 120 — Spiral height [mm] 1.2 — Head length [mm] 12 — Permanent magnet [mm] 1 (diameter) × 5 (length)

[0166] The main body size of the microrobot was minimized so that the microrobot was inserted into a commercially available catheter for artery therapeutics. A permanent magnet is placed inside the main body in order to interact with the electromagnetic field generated by ENS to control the motion. The magnetization orientation of the tip is orthogonal to the axis of the main body. The ball joint is designed such that a 0.011″ super-elastic guidewire is connected to the microrobot main body. The length and forward and backward motions of the guidewire is controlled by a guidewire feeder.

[0167] B. Power- and Frequency-Range Improved ENS

[0168] Three orthogonal pairs of air-core type electromagnetic coils are used in ENS:

[0169] circular Helmholtz coils (CHCx); and

[0170] y- and z-directional square Helmholtz coils, SHCy and SHCz, respectively.

[0171] Two pairs of square Helmholtz coils were designed inside the circular Helmholtz coils to maximize the region of interest (ROI) (see FIG. 4B). The frame of this system was designed to adopt a nonconductive material (Bakelite) to avoid eddy current effects and heat emission.

[0172] The detailed specifications of ENS are shown in Table 2.

TABLE-US-00002 TABLE 2 Specification CHCx SHCy SHCz Coil radius/length (mm) 162 168 120 Number of turns 472 380 300 Resistance (Ohm) 6.75 9.27 5.42 Inductance (mH) 258 92 37 Magnetic field intensity (A/m) 1982.4 2679.1 2913.3

[0173] Since the electromagnetic coils are made by copper coil winding, the magnetic flux density reduction and phase delay are unavoidable due to an inductance effect. In the three pairs of coils, the magnetic flux density dramatically decreased and the phase delay increased with respect to the operating frequency. In experiments, the magnetic flux density showed a reduction of 90% and the phase delay showed an increase of 90° with respect to a frequency variation of approximately 30-300 Hz.

[0174] Therefore, to enhance the actuation force and widen the operating frequency range of the ENS system, the inductance effect needs to be reduced. An inductance reduction circuit and a pseudo-continuous switching algorithm were designed using a variable capacitor circuit composed of various capacitors, relays, and switching circuits, in each coil. Assuming that each coil is a simple RL circuit, the variable capacitor circuit to each coil in series forms an RLC equivalent circuit as shown in FIG. 4A. Then, the output current (I(s)) of the RLC circuit is calculated as followings

[00002]$\begin{array}{cc}I\left(s\right)=\frac{{\mathrm{sC}}_v.Math.V_i\left(s\right)}{{\mathrm{LC}}_v.Math.s^2+{\mathrm{RC}}_v.Math.s+1}& \left(1\right)\end{array}$

[0175] s: Laplace constant

[0176] Cv: Variable capacitance value

[0177] Vi: Input voltage

[0178] L: Coil inductance

[0179] R: Resistance value in RLC circuit

[0180] The phase of the RLC circuit is as follows:

[00003]$\begin{array}{cc}\phi ={\tan }^{-1}\left(\frac{2.Math.\pi .Math..Math.\mathrm{fL}}{R}-\frac{1}{2.Math.\pi .Math..Math.{\mathrm{fRC}}_v}\right)& \left(2\right)\end{array}$

[0181] where Cv was designed as a variable capacitor that can match the resonant frequency of the coil system. For the given desired frequency, the capacitor is automatically switched to match the coil impedance and then cancel the inductor reactance by changing the capacitance.

[0182] The circuit will resonate at the following frequency:

[00004]$\begin{array}{cc}{f}_r=\frac{1}{2.Math.\pi .Math.\sqrt{{\mathrm{LC}}_v}}.Math..Math.V_L=2.Math.\pi .Math..Math.{\mathrm{fLI}}_0,V_C=I_0/\left(2.Math.\pi .Math..Math.{\mathrm{fC}}_v\right),\mathrm{and}.Math..Math.C_v=1/{\left(2.Math.\pi .Math..Math.f\right)}^2.Math.L& \left(3\right)\end{array}$

[0183] were set. Therefore, the voltages at both ends of the coil and the capacitor were equal (V.sub.L=V.sub.C), and the net voltage across the coil and the resonance control circuit became 0 V by the Kirchhoff's voltage law. Since the remaining parasitic resistance was relatively small, the applied voltage to maintain the parasitic resistance can deliver the maximum current with a zero phase delay through the ENS at the given input frequency.

[0184] III. Results

[0185] FIG. 5 shows the wide operating frequency range of the ENS of the present invention. The controllable lowest to highest resonant frequency ranges of CHCx, SHCy, and SHCz were 16.58-100 Hz, 26.27-131.2 Hz, and 41.42-370 Hz, respectively. The CHCx coil had a lowest maximum operating frequency of 100 Hz. The coil system requires a high voltage to maintain a high coil current, and the reason is that the coil system shows the largest inductance value, resulting therefrom.

[0186] To examine the resonance control circuit suggested in the present invention, the step-out frequency experiment was performed. The step-out frequency is directly related to the actuation force of the microrobot. The rotating magnetic field frequency was continuously increased until the microrobot speed dropped down, and the input frequency was regarded as the limit frequency of the microrobot motion. As a result, the step-out frequency of a robot without a resonance control system was around 50 Hz, but the step-out frequency of the ENS of the present invention showed 200 Hz, which was much higher than that of the previous system. Since the step-out frequency could be improved by 400%, the locomotion speed was improved by approximately 388% (As the input frequency increases, the varying speed of the magnetic field, and thus the rotating speed of the robot increases, resulting in an increase in locomotion speed of the microrobot).

[0187] To show the feasibility of the guide-wired helical microrobot system of the present invention, two in-vitro experiments were conducted: direction control into different branches (FIG. 6A) and driving force and drilling force in the water flowing tube (FIG. 6B).

[0188] As shown in FIG. 6A, the three-dimensional control force and the obstacle removal motion experiments of the helical microrobot were conducted in the phantom with five branches.

[0189] First, the guide-wired microrobot was controlled to move into different directions of the phantom branches of 15°, 0°, and −15°.

[0190] Second, after successful guidewire was inserted into a desired branch, the drilling motion and propulsion were performed simultaneously for the effective removal of an obstacle. For the application of mechanical thrombectomy, a 3D printed phantom was made to mimic the blood vessel environment. To mimic an obstacle in the human blood vessel, an agar-block region (0.5% agarose which was 7 times harder than a previous one [F. Carpi et al., IEEE Trans. Biomed. Eng., vol. 58, no. 2, pp. 231-234, 2011]) was made at the end of the desired portion.

[0191] The system of the present invention could effectively drill through the obstacle within 20 seconds, indicating a significant increase in drilling force (see FIG. 6A).

[0192] The effect of the guidewire method against flow was evaluated in the water flow tube as shown in FIG. 6B. The guide-wired microrobot could maintain its position during drilling motion and move forward and backward freely without being influenced by the water flow. Furthermore, the guide-wired microrobot could move forward and backward while simultaneously performing drilling motion.

[0193] IV. Conclusion

[0194] The present invention verified the feasibility of the novel mechanical thrombectomy device using the guide-wired helical microrobot and the improved ENS system. A prototype of the helical microrobot having a spherical joint connected to the guidewire was constructed, and the drilling motion and steering locomotion of the microrobot were verified through in-vitro experiments. Furthermore, a resonance control system generating a high frequency and an electromagnetic field was developed to enhance drilling force of the helical microrobot system of the present invention.

[0195] It was experimentally proved that the ENS of the present invention could generate a wide range of resonant frequency with a maximum magnetic field and little phase delay. Furthermore, the motional controllability and drilling forces could be obtained to remove a thrombus in the vascular system.

INDUSTRIAL APPLICABILITY

[0196] The present invention relates to a guide-wired microrobot system for mechanical thrombectomy. More specifically, the present invention relates to a medical robot system composed of a helical microrobot equipped with a guidewire at the tip thereof, an electromagnetic actuation system for rapidly rotating the helical microrobot, and a real-time imaging system.