Magnetic microrobot and its application in a microfluidic system
© Wang et al.; licensee Springer. 2014
Received: 16 July 2014
Accepted: 16 October 2014
Published: 22 December 2014
This paper researches the design and control method of a microrobot in a microfluidic system by electromagnetic field. The microrobot can move along the microchannel to a required position, and by changing the magnetic torque, the microrobot can also rotate in the microfluidic chip. As an application of the microrobot, it is used as a mobile micromixer to mix two solutions in the microfluidic chip, and the experimental results verify its effectiveness.
Microrobot is a hot research field in robotics. Using microrobots as a tool to surgery in vivo has the advantages of small trauma, safety, reliability, and significant reduction of medical costs and rehabilitation duration, so entering the liquid environment of the human body and making in vivo detection and treatment – are an important developing direction of microrobots. Recently, many researches on microrobots have carried out a lot of various actuation methods for microrobots, such as electromagnetic actuators ,, chemical bubble actuators ,, swimming tail actuators –, bacterial actuators –, and so on. Especially, the electromagnetic driving method has many advantages. First, electromagnetic field can be used to drive the microrobot with high controllability through changing the currents of the electromagnetic coils. Second, magnetic field will not cause harm or side effects to humans.
Among the researches of magnetic controlled microrobots, Bradley J. Nelson's group, inspired by the natural design of bacterial flagella, proposed an artificial bacterial flagella microrobot  that could swim in a controllable fashion using weak applied magnetic fields. The flagella microrobot consisted of a helical tail and a small soft magnetic material as a head on one end. The flagella microrobot's helical propulsion could be precisely controlled by electromagnetic coils. They also proposed porous microniches using 3D laser lithography as a transporting microrobot for targeted cell delivery . Metin Sitti proposed a microrobot driven by magnetic torque. It could be controlled by two pairs of Helmholtz coils and a pair of clamping coils with a stick-slip motion . The horizontal movement was controlled by the Helmholtz coils, and the clamping coils made the driving signals for the stick-slip motion. Sukho Park proposed a kind of microrobot controlled by magnetic force. The driving system consisted of two pairs of Helmholtz coils and two pairs of Maxwell coils . The magnetic force was provided by the Maxwell coils, and the Helmholtz coils were used to generate the torque for the rotation of the microrobot. However, in manipulating microrobots to explore the human body, there are still a lot of works that need to be done.
In order to control the microrobot to fulfill a specific function in the internal liquid environment of the human body, it is necessary to firstly control the microrobot’s movement in the microfluidic environment simulating the vessels. Since a microfluidic system has an effective functional zone of microscale as well as precise control of flow rate, it can well simulate the vessel liquid environment .
This paper researches the design and fabrication of a microrobot controlled by outside electromagnetic field and proposes a control method for the motion of the microrobot in the microfluidic system. This microrobot can move along the microchannel to a required position. By changing the magnetic torque, the microrobot can also rotate in the microfluidic chip. As an application of the microrobot, it can be used as a mobile micromixer in the microfluidic chip.
The counter-diagonal of ∇B can be considered to be 0, and M x and M y are constants in the magnetic field simultaneously.
where I is the current through the coil, L is the integral path, r is the full displacement vector from the wire element to the point where the field needs to be calculated, dl is a vector of the differential element of the current through the wire, and μ0 is the magnetic permeability (μ0 = 4π × 10−7 H/m).
To manipulate the microrobot accurately, the magnetic field produced by the electromagnetic coils must be controlled nicely. The microrobot is placed in the controlled area that is surrounded by four electromagnetic coils, and each electromagnetic coil can be controlled accurately according to the current outputting by the PC. Four coils can be controlled separately or together. COMSOL multiphysics is used to simulate the magnetic field and the force generated by the electromagnetic coils.
Microrobot and system configuration
Fabrication of the microrobot
Carve the microrobot's mold out of the acrylic plate using an engraving machine (Benchtop Engravers EGX-600/400, Roland DGA Corporation, Irvine, CA, USA) which is equipped with a milling cutter with a diameter of 200 μm.
Mix the polydimethylsiloxane and its curing agent at a mass ratio of 10:1 and then stir for 30 min.
Put NdFeB magnetic microparticles into mixed PDMS and then put them into the microrobot's mold.
Degas for 30 min in a vacuum box to remove bubbles.
Put the microrobot into an oven to heat for 3 h at 60°C.
Remove the microrobot from the mold with tweezers after magnetic PDMS is cured.
Setup of the magnetic driving system
Parameters of coils
Insider radius/outsider radius (mm)
Diameter of copper wires (mm)
Results and discussion
Motion control of the microrobot
The microrobot starts to move in the channel due to the magnetic force. In the initial case, the magnetic force is greater than the liquid viscous force, with the increase of microrobot speed; fluid viscous force is gradually increased until the magnetic force is equal to the liquid viscous force. The microrobot reaches equilibrium.
The microrobot works as a mobile micromixer
The microrobot can move to any position in the microfluidic chip and can be controlled to produce rotary motion, so the microrobot can be used as a mobile micromixer in microfluidic chips.
The rotation made by the microrobot can generate significant tensile deformation and folding effect on the fluid streamline, so it increases the contact area between different liquids effectively. The rotational frequency of the microrobot can be increased by increasing the frequency of the sinusoidal signals provided in electromagnetic coils. To generate a rotating magnetic field, the sinusoidal signal is provided in each of the four coils, whose phase difference is 90°. When the rotational frequency increases, the microrobot can stir the liquid more quickly. Therefore, the microrobot can enhance the mixing efficiency with a larger rotational frequency. The microrobot can also move in the mixing chamber, and the motion can accelerate the mixing speed and enhance the mixing efficiency.
This paper researches the design and control method of a microrobot in microfluidics. A theoretical analysis is performed for the microrobot and magnetic driving system. The microrobot is made of magnetic PDMS. With four electromagnetic coils and other attachments, the magnetic driving system is constructed. By controlling the coils, the microrobot can move and rotate in a microfluidic chip. The microrobot can be used as a mobile micromixer, and a mixing experiment is accomplished inside a microfluidic mixing chip, which verifies the microrobot's effectiveness. Future studies will focus on the motion control of the microrobot in 3D and its interaction with live cells in a vessel-like microfluidic system.
This paper is supported by the National Natural Science Foundation of China (Grant Nos. 61106109 and 61304251) and the CAS FEA International Partnership Program for Creative Research Teams.
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