# Analysis and Comparison of Electromagnetic Microrobotic Platforms for Biomedical Applications
## 2 Theoretical Background
### Discussions  
Various arrangements of electromagnetic coils can generate various magnetic field distributions. The EMA setup should be properly defined with respect to the envisioned biomedical application. To do so, the main characteristics should be specified, such as: the environment of the workspace, the type of microrobot and the various magnetic tasks. The required number of electromagnets for different motions control has been studied in past works [45; 81; 83]. On this basis, the relations between the specifications and the number of coils to design an EMA system can be proposed, and are depicted in Figure 3.  
Specifically, for the choice of a proper EMA system, five main characteristics of an application are required:  
1. The dimensions of workspace;
2. The media of the environment;
3. The type of microrobots;
4. The medical tasks;
5. The required motion control.  
First of all, the dimension of workspace is determined by the desired biomedical application that can be either 2D or 3D. The media of workspace could be divided into easy-to-operate and non-easy-to-operate for the placed microrobot. Commonly, the media with high viscosity or non-Newtonian fluid and the flowing status are difficult conditions to manipulate microrobot. In contrast, low viscosity and static environment are easy for the operation of microrobot.  
Moreover, the type of applied microrobot and its locomotion must be specified. Especially, the helical microrobot or microswimmer could be selected to move in a flowing environment and/or high viscosity media since these microrobots can perform drilling motion by the rotating magnetic field (see also Figure 2). Combining with magnetic force produced by magnetic gradient, the helical microrobot or microswimmer can be also actuated by a strong propulsion force. Besides, the cylindrical, ellipsoidal, spheroidal and irregularly shaped microrobot are chosen to the suitable environments. In addition, the type of locomotion of the microrobot should be determined with respect to the given application. As presented in Figure 3, six main types of biomedical applications are here considered. However, a distinction is made according to a 2D or 3D workspace that is considered. For a 2D workspace, as illustrated in Figure 3a, four main types of biomedical applications are considered for a 2D workspace: (i) surface treatment, (ii) marking/sensing, (iii) in vitro micromanipulation, and (iv) controllable structure. Whereas for a 3D biomedical operations, the main types of tasks are: (i) material removal, (ii) marking/sensing, (iii) targeted therapy, and (iv) controllable structure, as depicted in Figure 3b. For instance, almost all types of microrobot could be used for targeted drug delivery. However, the spheroidal microrobot rather than the helical microrobot is suitable for marking/sensing application. If a helical microrobot is applied for targeted drug delivery, the possible motion of microrobot is required as translation and rotation. If a spheroidal microrobot is used for targeted drug delivery, the possible motion could be translation, rotation and punching.  
Finally, the number of electromagnetic coils is determined by the specific motions of the selected microrobot. For instance, the translational locomotion can be achieved by the magnetic force on the spheroidal microrobot, and it can also be reached by the magnetic torque generated by rotating magnetic field on the helical microrobot. Considering a 3D workspace as an example, the reliable locomotion of spheroidal microrobot requires \(n=5\) numbers of electromagnets, while the locomotion of a helical microrobot only demands \(n=3\) numbers of electromagnets. Similarly, other necessary information for designing EMA system can also be obtained in the same way following Figure 3. Therefore, from the above five requirements, the specifications of desired EMA system are obtained.