# Endoscopic Optical Imaging Technologies and Devices for Medical Purposes: State of the Art
## 5 Actuation Methods
### Electromagnetic Actuators  
An electromagnetic actuator works based on the conversion of electric energy into mechanical energy by means of a magnetic field. It comprises a stationary part called a stator, and a moving part called a rotor. The conversion of energy takes place in the area separating the two components (also called an air gap). The interaction between the stator and rotor produces a high vibration of the rotor component. These actuators have two different configurations depending on the moving component. In one configuration, the moving part is represented by a bulk magnet or a thin magnetic film deposited on a device component, while the stationary part is represented by an electric coil. In the other configuration, an electric coil is the moving part, and the static magnetic field is provided by the stationary magnets placed around the coil [81].  
[PERSON] and [PERSON] designed an OCT probe where the current passing through the outer coil slides the tube containing the magnet at its distal end and the fiber inside the tube. The developed OCT probe is shown in Figure 18[113]. [PERSON] et al. placed a magnet around an optical fiber, which was vibrated at resonance using the electromagnetic force generated by a tilted coil placed on the tube surface via lithography [114], and 3-D printing technology [115]. Later, the authors used a pair of driving coils embedded in polyimide film and wrapped it around the tube surface to drive an optical fiber at a second resonance mode to get a higher scanning angle [66].  
Many MEMS micromirror scanners are actuated using a moving coil configuration of electromagnetic actuators. [PERSON] et al. developed an optical scanner by making the mechanical structure using bulk silicon micromachining, and the electric circuit coil was fabricated using deep UV lithography and gold electroplating [116]. Using this technique, [PERSON] et al. developed a similar 1-D MEMS scanner for the  
Figure 17: Schematic diagram of scanning fiber endoscope (SFE), and the schematic of the actuator used (taken with the permission of [3]).  
Figure 18: Electromagnetically driven OCT probe: (**A**) schematic; (**B**) developed probe; (**C**) test of probe through a 25-gauge port (taken with the permission of [113] © The Optical Society).  
confocal microscope, guaranteeing the reliability and stability of the scan angle. A macroscale scanner is used together with an electromagnetic scanner to achieve a 2-D scan [117; 118]. [PERSON] et al. designed a 2-D scanner driven using this kind of electromagnetic actuation method for OCT imaging. In this scanner, two different sets of coils are placed on a moveable plane to actuate the plane in the X or Y direction inside a permanent magnet. Two coils exhibit the X direction motion of the central mirror, and four coils placed at intermediate frames permit the Y axis motion [119].  
Electromagnetic actuation also finds use in capsule endoscopy to achieve the movement of the device [120]. Recently, [PERSON] et al. proposed a new biopsy capsule endoscope, which uses an external rotating magnetic field to linearly actuate the permanent magnet connected to the biopsy needle. The linear motion of the needle permits the collection of the biopsy sample and brings it back in the capsule using the movement in the reverse direction. Such devices define a new field for less invasive diagnosis procedures [121].  
Magnetic materials are characterized by a special property, which allows them to change their dimensions in the presence of a magnetic field. This effect is called magnetostriction. The material can undergo a change in dimension until it reaches the value of saturation magnetostriction, which depends on the magnetization and thus on the applied magnetic field [122]. [PERSON] et al. developed a 2-D optical scanner based on the magnetostrictive effect where a silicon cantilever is coated with a magnetostrictive film [123].