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Fig. 5. Concept of acoustic actuation (a) before actuation (b) D. Hybrid Actuation
after actuation The benefits of several actuation techniques can be combined
through hybrid actuation, which can also enhance the perfor-
tions and thus the movement of microrobots. In actuality, mance and functionality of microrobots in complex environ-
acoustic fields are commonly used in two main ways: stand- ments [66]. The most commonly used actuation techniques
ing waves and traveling waves [58]. Ultrasound is a rela- in micro-robotics are magnetic and optical. They can both be
tively new method that has been applied to drive micro and used to actuate a set of microrobots quickly, precisely, and
nanorobots [59]. Acoustic field actuation was dependent on over a longer distance. Microrobots with magnetic actuation
sound waves that radiated from the microrobot. The sound may penetrate nontransparent tissues at great depths for med-
propagation direction is the same as the movement direction. ical applications, but microrobots with optical actuation are
Examples of sound waves include surface sound, pulsed ul- better suited for biotechnology [67].
trasound, ultrasonic standing waves, and ultrasonic [60, 61].
Acoustic field actuation has several benefits including great The potential for combining the benefits of both approaches
tissue permeation, biocompatibility, and high flexibility [59]. in new robot designs is very important. Although synthetic mi-
Acoustic levitation appears to be the best option for micro- cro/nanomotors are effective at delivering medications, using
assembly in most cases. Acoustic actuation is a biocompatible them inside the human body presents a number of challenges.
kind of actuation that can be utilized in combination with Due to their improved biocompatibility, biodegradability, and
magnetic fields to increase the swimmers’ propulsion and functional connections with physiological tissue, biological
steering [62]. The energy of an ultrasonic beam is attenuated hybrid systems have recently become an alternative [68]. Tar-
as it passes through tissue due to absorption and scattering. get medication delivery techniques use electromagnetic actu-
Heating occurs as a result of tissue energy absorption [63, 64]. ation to move the microrobot and acoustic bubble actuation
The main benefit of acoustic processing is that it can manip- to manipulate the drug [36]. A microrobot in the shape of
ulate any material by acoustic forces. When given enough a bullet is equipped with an air bubble that is acoustically
sound power, a stable equilibrium position always exists. Sur- trapped inside its internal body cavity. Due to the net fluidic
face features, such as roughness, were influenced by fluid flow brought on by the bubble oscillation, the microrobot is
movement and the acoustic field. Acoustic stream and force driven laterally at extremely high speeds while also creating
field non-uniformity are the main disadvantages of acoustic an attractive force toward the wall. The direction of the mi-
levitation [65]. Fig. 5 illustrates the concept of acoustic actua- crorobots’ movements is controlled by directing them into a
tion of the microrobot. The acoustic field’s vibrations, which uniform magnetic field [69].
are the basis for bubble propulsion, are strong when acous-
tic frequencies reach the bubble’s resonance frequency. Fig. III. MOTION CONTROL OF MICROROBOT
5(a) shows the inactivity of the liquid surface by ultrasonic
waves before acoustic actuation, due to the acoustic field’s The primary goal of the control scheme is to get the micro-
insufficient selectivity to the object being manipulated and robot to the desired position using the Manipulation Control
Fig. 5(b) shows the effect of the liquid surface after directing Algorithm. In addition, any actuation method needs auxiliary
ultrasounic waves towards it after acoustic actuation, when the actuation, control, and imaging equipment to drive and control
sound field’s frequency and the bubble’s resonance frequency mobile microrobots remotely [70, 71]. Planning robot trajec-
are equal, the bubble vibrate effectively. tories is a challenging task that is essential and plays a crucial
role in the design process [72]. Therefore, the development of
model-based control algorithms requires a robust microrobot
motion model. The movement of these small robots is af-
fected by surface tension, friction, and viscous forces, which
make it difficult to obtain a good estimate of these forces [73].
At low fluid flow velocities, the drag force was less signifi-
cant than the friction force. The friction force contributed a
lot to microrobot control and drag force measurements [74].
Cylindrical microrobots have the same diameter but different
lengths. The frontal area and not the length were the only
variables influencing the drag forces [75].
A closed-loop control technique based on location infor-
mation feedback was developed [41, 76]. Time-delay estima-
tion (TDE) is used to model and implement a closed-loop
control scheme for a magnet-actuated microrobot [29]. In the
