Microbotics

thumb|Jasmine minirobots each smaller than in width

Microbotics (or microrobotics) is the field of miniature robotics, in particular mobile robots with characteristic dimensions less than 1 mm. The term can also be used for robots capable of handling micrometer size components.

History

Microbots were born thanks to the appearance of the microcontroller in the last decade of the 20th century, and the appearance of microelectromechanical systems (MEMS) on silicon, although many microbots do not use silicon for mechanical components other than sensors. The earliest research and conceptual design of such small robots was conducted in the early 1970s in (then) classified research for U.S. intelligence agencies. Applications envisioned at that time included prisoner of war rescue assistance and electronic intercept missions. The underlying miniaturization support technologies were not fully developed at that time, so that progress in prototype development was not immediately forthcoming from this early set of calculations and concept design. As of 2008, the smallest microrobots use a scratch drive actuator.

The development of wireless connections, especially Wi-Fi (i.e. in household networks) has greatly increased the communication capacity of microbots, and consequently their ability to coordinate with other microbots to carry out more complex tasks. Indeed, much recent research has focused on microbot communication, including a 1,024 robot swarm at Harvard University that assembles itself into various shapes; and manufacturing microbots at SRI International for DARPA's "MicroFactory for Macro Products" program that can build lightweight, high-strength structures.

Microbots called xenobots have also been built using biological tissues instead of metal and electronics. Xenobots avoid some of the technological and environmental complications of traditional microbots as they are self-powered, biodegradable, and biocompatible.

Definitions

While the "micro" prefix has been used subjectively to mean "small", standardizing on length scales avoids confusion. Thus a nanorobot would have characteristic dimensions at or below 1 micrometer, or manipulate components on the 1 to 1000 nm size range. A microrobot would have characteristic dimensions less than 1 millimeter, a millirobot would have dimensions less than a cm, a mini-robot would have dimensions less than , and a small robot would have dimensions less than .

Many sources also describe robots larger than 1 millimeter as microbots or robots larger than 1 micrometer as nanobots.

Design considerations

The way microrobots move around is a function of their purpose and necessary size. At submicron sizes, the physical world demands rather bizarre ways of getting around. The Reynolds number for airborne robots is less than unity; the viscous forces dominate the inertial forces, so "flying" could use the viscosity of air, rather than Bernoulli's principle of lift. Robots moving through fluids may require rotating flagella like the motile form of E. coli. Hopping is stealthy and energy-efficient; it allows the robot to negotiate the surfaces of a variety of terrains. Pioneering calculations (Solem 1994) examined possible behaviors based on physical realities.

One of the major challenges in developing a microrobot is to achieve motion using a very limited power supply. The microrobots can use a small lightweight battery source like a coin cell or can scavenge power from the surrounding environment in the form of vibration or light energy. Microrobots are also now using biological motors as power sources, such as flagellated Serratia marcescens, to draw chemical power from the surrounding fluid to actuate the robotic device. These biorobots can be directly controlled by stimuli such as chemotaxis or galvanotaxis with several control schemes available. A popular alternative to an onboard battery is to power the robots using externally induced power. Examples include the use of electromagnetic fields, ultrasound and light to activate and control micro robots.

The 2022 study focused on a photo-biocatalytic approach for the "design of light-driven microrobots with applications in microbiology and biomedicine".

Locomotion of microrobots

Microrobots employ various locomotion methods to navigate through different environments, from solid surfaces to fluids. These methods are often inspired by biological systems and are designed to be effective at the micro-scale. Several factors need to be maximized (precision, speed, stability), and others have to be minimized (energy consumption, energy loss) in the design and operation of microrobot locomotion in order to guarantee accurate, effective, and efficient movement.

When describing the locomotion of microrobots, several key parameters are used to characterize and evaluate their movement, including stride length and transportation costs. A stride refers to a complete cycle of movement that includes all the steps or phases necessary for an organism or robot to move forward by repeating a specific sequence of actions. Stride length (𝞴s) is the distance covered by a microrobot in one complete cycle of its locomotion mechanism. Cost of transport (CoT) defines the work required to move a unit of mass of a microrobot a unit of distance

Other options for actuating microrobots' surface locomotion include magnetic, electromagnetic, piezoelectric, electrostatic, and optical actuation.

Swimming locomotion

Swimming microrobots are designed to operate in 3D through fluid environments, like biological fluids or water. To achieve effective movements, locomotion strategies are adopted from small aquatic animals or microorganisms, such as flagellar propulsion, pulling, chemical propulsion, jet propulsion, and tail undulation. Swimming microrobots, in order to move forward, must drive water backward.

At Air-Fluid Interface locomotion

In the specific instance when microrobots are at the air-fluid interface, they can take advantage of surface tension and forces provided by capillary motion. At the point where air and a liquid, most often water, come together, it is possible to establish an interface capable of supporting the weight of the microrobots through the work of surface tension. Cohesion between molecules of a liquid creates surface tension, which otherwise creates 'skin' over the water's surface, letting the microrobots float instead of sinking. Through such concepts, microrobots could perform specific locomotion functions, including climbing, walking, levitating, floating, and or even jumping, by exploring the characteristics of the air-fluid interface.

Due to the surface tension, σ, the buoyancy force, Fb, and the curvature force, Fc, play the most important roles, particularly in deciding whether the microrobot will float or sink on the surface of the liquid. This can be expressed as

<math>\sigma=F_b+F_c</math>

Fb is obtained by integrating the hydrostatic pressure over the area of the body in contact with the water. In contrast, Fc is obtained by integrating the curvature pressure over this area or, alternatively, the vertical component of the surface tension, <math>\sigma\sin\theta</math>, along the contact perimeter.

One example of a climbing, walking microrobot that utilizes air-fluid locomotion is the Harvard Ambulatory MicroRobot with Electroadhesion (HAMR-E). The control system of HAMR-E is developed to allow the robot to function in a flexible and maneuverable manner in a challenging environment. Its features include its ability to move on horizontal, vertical, and inverted planes, which is facilitated by the electro-adhesion system. This uses electric fields to create electrostatic attraction, causing the robot to stick and move on different surfaces. With four compliant and electro-adhesion footpads, HAMR-E can safely grasp and slide over various substrate types, including glass, wood, and metal.

To calculate the necessary aerodynamic power for maintaining a hover with flapping wings, the primary physical equation is expressed as

<math>mg=2*\rho*l^2*\phi*\upsilon_i^2</math>

where m is the body mass, L is the wing length, Φ represents the wing flapping amplitude in radians, ρ indicates the air density, and Vi corresponds to the induced air speed surrounding the body, a consequence of the wings' flapping and rotation movements. This equation illustrates that a small insect or robotic device must impart sufficient momentum to the surrounding air to counterbalance its own weight.

One example of a flying microrobot that utilizes flying locomotion is the RoboBee and DelFly Nimble, which, regarding flight dynamics, emulate bees and fruit flies, respectively. Harvard University invented the RoboBee, a miniature robot that mimics a bee fly, takes off and lands like one, and moves around confined spaces. It can be used in self-driving pollination and search operations for missing people and things. The DelFly Nimble, developed by the Delft University of Technology, is one of the most agile micro aerial vehicles that can mimic the maneuverability of a fruit fly by doing different tricks due to its minimal weight and advanced control mechanisms. These microscopic robots are designed to navigate the human body with a degree of precision previously unimaginable. and magnetically guided engineered bacterial microbots for 'precision targeting' for fighting cancer that all have been tested with mice.