Micro-UFO (Untethered Floating Object): A Highly Accurate Microrobot Manipulation Technique

Abstract

A new microrobot manipulation technique with high precision (nano level) positional accuracy to move in a liquid environment with diamagnetic levitation is presented. Untethered manipulation of microrobots by means of externally applied magnetic forces has been emerging as a promising field of research, particularly due to its potential for medical and biological applications. The purpose of the presented method is to eliminate friction force between the surface of the substrate and microrobot. In an effort to achieve high accuracy motion, required magnetic force for the levitation of the microrobot was determined by finite element method (FEM) simulations in COMSOL (version 5.3, COMSOL Inc., Stockholm, Sweden) and verified by experimental results. According to position of the lifter magnet, the levitation height of the microrobot in the liquid was found analytically, and compared with the experimental results head-to-head. The stable working range of the microrobot is between 30 µm to 330 µm, and it was confirmed in both simulations and experimental results. It can follow the given trajectory with high accuracy (<1 µm error avg.) at varied speeds and levitation heights. Due to the nano-level positioning accuracy, desired locomotion can be achieved in pre-specified trajectories (sinusoidal or circular). During its locomotion, phase difference between lifter magnet and carrier magnet has been observed, and relation with drag force effect has been discussed. Without using strong electromagnets or bulky permanent magnets, our manipulation approach can move the microrobot in three dimensions in a liquid environment.

Stabilization of Microrobot Motion Characteristics in Liquid Media

“…While the resistance of air is negligible, the drag force acting on the microrobot in the fluid cannot be omitted. For this reason, while the lifter-magnet follows a step function like a motion profile with sharp edges, and the microrobot has a parabolic motion profile with smoother edges [ 32 ]. Figure 1 shows the schematic diagram of the head-tilting reaction and phase difference for the microrobot moving in the x axis.…”

“…Since the pyrolytic graphite is a diamagnetic material with a magnetic permeability of , it encloses the microrobot within the boundaries of magnetic field lines of the lifter-magnet. Because of this, we can achieve more stable levitation characteristics inside the liquid [ 32 , 33 ].…”

“…As shown in Figure 1 , the microrobot moving in the x direction is not parallel to the surface during its motion due to the undesired torque acting on it. For a microrobot, for which the moment of inertia is taken as (μg mm 2 ), a relation between the angular acceleration and undesired torque can be determined [ 32 , 33 , 34 ]. Accordingly, Equation (1) will be used to calculate the angle value at which the lifter-magnet should be held in order to avoid the generation of undesired torque values ( Figure 2 ).…”

“…The acceleration-dependent mathematical model is expressed in Equations (3)–(5), in which the robot mass is m r [ 32 , 33 , 34 ]. The forces exerted on the microrobot are shown in Figure 4 B.…”

“…For a robot with a mass of 2.92 μg, robot acceleration can be determined for known values of lifting force (12.788 μN), speed-dependent friction force [ 33 ], and gravitational force (28.741 μN). The relationship between phase difference and the microrobot acceleration has been investigated in a different study [ 32 ]. …”