- Letter
- Open Access

# Improving guidewire-mediated steerability of a magnetically actuated flexible microrobot

- Sungwoong Jeon†
^{1, 2}, - Ali Kafash Hoshiar†
^{2}, - Sangwon Kim
^{3}, - Seungmin Lee
^{1, 2}, - Eunhee Kim
^{1, 2}, - Sunkey Lee
^{1, 2}, - Kangho Kim
^{1, 2}, - Jeonghun Lee
^{1, 2}, - Jin-young Kim
^{1, 2}and - Hongsoo Choi
^{1, 2}Email authorView ORCID ID profile

**Received:**24 October 2018**Accepted:**10 December 2018**Published:**13 December 2018

## Abstract

Here, we develop a flexible microrobot enhancing the steerability of a conventional guidewire. To improve steerability, a microrobot is attached to the tip of the guidewire and guided using an external magnetic field generated by an electromagnetic coil system. The flexible microrobot is fabricated via replica-molding and features a body made of polydimethylsiloxane (PDMS) and a single permanent magnet. As the robot is made of a deformable material, it can be steered using a low-intensity external magnetic field; the robot can potentially be guided into the coronary artery. To study steering performance, we employed mathematical modeling and a finite element model (FEM), and performed experiments under various magnetic fields. We found that a mathematical model using the Euler–Bernoulli beam could not precisely calculate the deformation angles. The FEM more accurately estimated those angles. The deformation angle can be controlled from 0 to 80° at a magnetic field intensity of 15 mT. The trackability at angles of 45 and 80° of the guidewire-based microrobot was confirmed in vitro using a two-dimensional blood vessel phantom.

## Keywords

- Flexible microrobot (robot)
- Guidewire
- PCI surgery
- Steerability
- Trackability
- Magnetic actuation
- Angioplasty

## Background

Biomedical microrobots (here termed simply “robots”) will potentially revolutionize medicine. Many research groups have studied the biomedical applications of robots, including targeted drug delivery, biopsy-taking, hyperthermia control, radioactive therapy, scaffolding applications, in vivo ablation, stenting, sensing, and marking [1–7]. Most robots are controlled and operated in low Reynolds number fluids [8–10]. Therefore, the robots must be appropriately designed and fabricated; size, geometry, and material properties must be considered if they are to operate within the body [8, 11, 12]. More recently, tethered robots have been developed for diverse biomedical applications.

Especially, robots may play roles in minimally invasive vascular surgery. One treatment for chronic total occlusion (CTO), i.e., complete blockage of a coronary artery, is percutaneous coronary intervention (PCI) [13–17] employing guidewires and catheters. Some guidewires penetrate blood clots while others guide catheters. The guidewires are manually controlled by pushing them back-and-forth or rotating them about their axes. However, this is time-consuming and highly dependent on operator skill. Thus, to reduce operative time and increase success rates, many groups are seeking robotic solutions.

Several groups have performed distal shaping of catheters and guidewires to improve steerability and controllability [18–26]. Krings et al. and Lalande et al. developed magnetic microguidewires fitted with permanent magnets at the tips; an external magnetic field was used for steering [20, 21, 24]. However, a high-intensity magnetic field was required because the microguidewires were not very flexible and the steering system was composed of permanent magnets, which limits the real-time steering for a catheter.

Clogenson et al. developed a steerable guidewire compatible with magnetic resonance imaging (MRI) [27]. Settecase et al. designed a steering catheter for use with MRI systems. A mathematical model was derived by exploiting the equilibrium between the magnetic and mechanical restorative torque; the equation was expressed in a linear form to simplify modeling. Thus, the model has certain limitations when used to estimate large nonlinear deformations [25]. Thus, we here develop a novel magnetic robot attached to a guidewire that exhibited high-level steerability through multi-angled branches of a blood vessel phantom under the influence of a low-level magnetic field, improving real-time delivery performance. In this study, an electromagnetic steering system was used to improve the steering performance. Moreover, the proposed guidewire-based microrobot with flexible material is easier to control using external magnetic field compared with the previously reported magnetic catheter in Ref. [21].

Our robot was fabricated from polydimethylsiloxane (PDMS), which has a low elastic modulus and a high Poisson ratio. Because it is highly deformable, the robot can be readily steered through various vessel branches using a low-level magnetic field. A permanent magnet placed at the end of the PDMS beam is used for steering. To verify robot deformation and effective steering, we used a mathematical model based on the Euler–Bernoulli beam and a finite element model (FEM) to predict the deformation angles of the microrobot tip. We compared the results of the mathematical model, the FEM simulation, and the experimental values. To demonstrate its practical utility, we performed steering and tracking using a complex two-dimensional phantom. To demonstrate the potential of the robot, we performed biocompatibility testing using human colorectal cancer (HCT116) cells.

## Methods

### Robot design

### The mathematical model and the FEM

*m*is the extent of magnetization of the permanent magnet,

*B*is the magnitude of the external magnetic field,

*γ*is the field direction,

*θ*is the deformation angle of the robot tip,

*L*is the robot length,

*E*is the Young’s modulus of PDMS, and

*I*is the second moment of inertia of the beam as shown in Fig. 2. Although this linear model can be used to create a deformation curve in the presence of an external magnetic field, the large nonlinear deformations of the robot compromise mathematical accuracy. The assumption that the microrobot length (L) is constant reduces the accuracy of the mathematical model. At the higher strain angles, therefore, the difference between the mathematical model and the experimental results diverges. However, the COMSOL model takes into account the structural deformation, leading to a better match between the experimental and simulated results.

Material properties of flexible microrobots used in mathematical modeling and establishment of the FEM

Material | Polydimethylsiloxane (PDMS) | Neodymium magnet (NdFeB, N52) |
---|---|---|

Density (kg/m | 0.97 | 7.500 |

Young’s modulus (Pa) | 750 × 103 | 160 × 109 |

Poisson’s ratio | 0.49 | 0.24 |

Relative permeability | 1 | 1.05 |

Remanence flux density (T) | 1.43 |

Geometric properties of flexible microrobots with guidewires PDMS, polydimethylsiloxane

Material | Diameter (μm) | Length (μm) |
---|---|---|

PDMS | 500 | 3000 |

Neodymium magnet (NdFeB, N52) | 400 | 800 |

Brass pipe | O.D 500, I.D 300 | 2000 |

Micro-spring | 500 | 2000 |

Guidewire | 340 (1Fr) |

### Robot fabrication

*H*,1

*H*,2

*H*,2

*H*-perfluorooctyl) silane (PFOTS) (Sigma-Aldrich, USA) to decrease the surface energy and render robot detachment easier. A PFOTS thin film was deposited on the PDMS mold in a vacuum chamber for 2 h [28]. As shown in Fig. 4a, the robot components were first aligned on the mold. The distance between the microspring and the permanent magnet was 3 mm. A brass pipe was placed in front of the magnet to keep it steady during molding. As shown in Fig. 4b, the mold was filled with Sylgard 184 silicone elastomer mixture (PDMS; Dow Corning Corp., USA) at a PDMS:curing agent weight ratio of 10:1; trypan blue was used to visualize the mixture within the mold. Oven-curing for 24 h followed and the robot was detached from the mold (Fig. 4c). The brass pipe was removed and the final structure connected to a conventional guidewire (Fig. 4d).

### Experimental setup

## Results and discussion

### Robot steering

Experimental and simulated conditions used to evaluate steering of the flexible microrobot

Parameter | Value |
---|---|

Length of flexible microrobot (μm) | 3800 |

Direction of magnetic field (°) | 0–170 |

Intensity of magnetic field (mT) | 5, 10, 15 |

### Tracking experiments

### Biocompatibility

_{2}in a humid (95% relative humidity) incubator. The cell culture processes were carried out as follows [34]:

- 1.
The microrobot without guidewire was sterilized twice with 70% ethanol and phosphate buffered saline (PBS, Welgene, Gyeongsan, South Korea) for sterilization and then dried for 20 min.

- 2.
The microrobot was placed in each well of a 24-well plate except for the control groups.

- 3.
One mL cell culture medium with HCT 116 cells (5 × 10

^{5}cells/mL) was dropped into each well plate. - 4.Well plates were incubated for 3 days in an incubator (Fig. 10c) and then the cell viability test was performed according to the manufacturer’s instructions.

Cell viability was assessed using a LIVE/DEAD Cell Imaging Kit (excitation wavelength 488 nm, emission wavelength 570 nm; Molecular Probes, Life Technologies Corp., USA) to stain live (green fluorescent signal) and dead (red fluorescent signal) cells. Cells cultured without robots (control cells) are shown in Fig. 10a, and cells cultured with microrobots in Fig. 10b, c. Figure 10d shows the cell viabilities, which were similar between the two groups. Thus, the robot is non-toxic.

## Conclusion

We designed and fabricated, via replica-molding, a guidewire-based robot for use during PCI. A low-level magnetic field affords actuation; the robot is highly flexible, smaller than a coronary artery, and connected to a conventional guidewire. The robot is steered and controlled using a low-intensity, external magnetic fields (5–15 mT). Robot deformation is controlled using geometric parameters (length and diameter), magnetic steering (extent and direction), and the material properties. We used Euler–Bernoulli theory and an FEM to predict robot deformation. The experimental results were in good agreement with the simulated values, but not the mathematical data. The robot moved within a two-dimensional blood phantom featuring many branches. The robot was guided to desired locations and steered into branches at angles of 45 and 80º. Furthermore, the robot was not toxic to human cells, and can thus be used in vivo.

## Notes

## Declarations

### Authors’ contributions

SJ and AKH carried out the simulation, the experiment and wrote the manuscript. SK and SL guided analysis data and reviewed the manuscript. EK cultured HCT116 cells and conducted biocompatibility test of microrobot. SL and KK designed and fabricated microrobots. JL conducted the experiment. JK and HC conceived of the study and wrote the manuscript. All authors read and approved the final manuscript.

### Acknowledgements

The authors are grateful to all members of DGIST-ETH Microrobot Research Center and Bio-Micro Robotics Lab. for their sincere help and comments.

### Competing interests

The authors declare that they have no competing interests.

### Funding

Funding for this research was provided by the Korea Evaluation Institute of Industrial Technology (KEIT) funded by the Ministry of Trade, Industry & Energy (No. 10052980) and the Global Research Laboratory from the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (No. NRF 2017K1A1A2013237).

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## Authors’ Affiliations

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