In an ex-vivo test, the bilayer hydrogel microrobot revealed potential for intraocular mobility by applying a magnetic field in bovine vitreous (Figure 6).
Figure 6a summarises the results of the bilayer hydrogel microrobot's ex vivo test in bovine vitreous. First, a minimally invasive technique was performed by rolling up the bilayer hydrogel microrobot in a 21G syringe needle. After that, a syringe needle was used to introduce the bilayer hydrogel microrobot in a minimally invasive manner into the bovine vitreous. Finally, an externally produced magnetic field was used to propel the bilayer hydrogel microrobot into the bovine vitreous, demonstrating its potential for intraocular mobility.
The bilayer hydrogel microrobot was made of a thin sheet that could be folded or crumpled into a 21G syringe needle while having a huge surface area (Figure 6b). This tiny robot could be forced into the needle of a syringe without any restrictions or could be dragged into the needle while submerged in a solution like saline. Within 2-4 days of the eye's extraction, the bovine vitreous to be injected with the bilayer hydrogel microrobot was removed and deposited in gel-like shape on a dish (Figure 6c).
When a magnetic field from an external permanent magnet was supplied to the bilayer hydrogel microrobot injected into the bovine vitreous, it displayed multiple motions (Figure 6d-i). The bilayer hydrogel microrobot could travel back and forth to a certain spot, as seen in Figure 6d. Additionally, it could follow angles in geometric shapes like triangles (Figure 6e) and squares (Figure 6f). Additionally, as illustrated in Figures 6g and 6h, I the bilayer hydrogel microrobot may travel to the z-axis in a flat or upright position and change orientation.
It could migrate easily up to 5 mm from the injection site, but not across many centimetres. Additionally, it was unable to go along the curved path of the bilayer hydrogel microrobot or penetrate the area with the highest density.
Untethered mobile microrobots offer a variety of uses, but how can we control them- In both controlled and uncontrolled contexts, a variety of strategies have been created. The utilization of magnetic and electric fields for manipulating tiny actuators from a distance is one of the most frequent manipulation techniques.
The following are a few common approaches :
Magnetic control : Because magnetic fields can pass through most materials, they are the best choice for remote actuation of microscale components in difficult-to-reach areas. Micro manipulations are conceivable due to the ease with which magnetic field spatial gradients can be created.. Magnetic coils or permanent magnets are used to generate these forces beyond the micro robot's working sphere. Multiple magnetic actuation techniques have been created by researchers, allowing bespoke work spheres and many degrees of freedom (DOF) with a variety of control methods in both open loop and closed loop control systems. Ferromagnetism and paramagnetism are the two mechanism that are very dominant in all materials that are used for micro robots. The magnetic flux through the electromagnetic coils is manipulated by adjusting the currents passing through the coils [54].
Electric Field control : Electrical field manipulation is a common alternative to magnetic field manipulation and is described in some of the applications. To activate the bot, the general concept is to establish attracting and repulsive forces on it. The electrodes can drive the microrobot directly through capacitive coupling [55].
Experiments have been carried out with an array of electrodes on the substrate on which the microrobot motion is to take place. All of these electrodes may be controlled separately, allowing many microrobots to be commanded from any position on the substrate. The motion is controlled with the help of four or more electrode chambers filled with an ionic solution, surrounding the actuation work-area [56].
Light Propulsion : When light is focused on a specific substrate, it can be utilised to convey energy or momentum to that substrate, allowing the microrobot to be controlled from afar. By inducing thermal expansion, a laser was used to operate the bimorph limbs of a microrobot .The step size was varied by varying the light intensity. Light also exerts a pressure on the surface on which it falls, which is insignificant at the macro scale but cannot be overlooked at the micro size. Sailboats were inflated to a pressure of roughly 0.6 Pa, resulting in speeds of up to 10 m/s [57].
Chemical Propulsion : Chemical reactions are the primary actuating mechanism in chemical propulsion. The components are propelled by a micron-scale jet, which uses oxygen bubbles created inside the jet tube as a result of chemical reactions with the liquid medium [58].