Reconfigurable Liquid‐Bodied Miniature Machines: Magnetic Control and Microrobotic Applications

Abstract

Soft miniature machines demonstrate multimodal actuation and morphology change capabilities in narrow spaces smaller than their dimension. The wirelessly controlled soft‐bodied features make them promising candidates for microrobotic manipulation and targeted operation in a noninvasive manner. Liquid‐bodied machine offers an ultrasoft body with extreme deformability owing to its fluid nature, enabling adaptive navigation with smooth contact with objects and environmental restrictions. Over the last decade of development, significant research progress has been achieved in wirelessly controlling liquid‐bodied machines for diverse manipulation applications. Herein, an overview of the recent research results in magnetic control methods and diverse microrobotic applications of liquid‐bodied machines is provided. Considering the control mechanisms and application challenges, ferrofluid‐based, liquid metal‐based, and liquid marble‐based machines are mainly discussed with a brief discussion on droplet‐based machines. The connection between control methods and applications is highlighted with a detailed analysis of machine–object and machine–environment interactions. The current challenges and research opportunities on liquid‐bodied miniature machines are outlined, aiming at designing intelligent liquid‐bodied machine‐based microrobotic systems and promoting the development of small‐scale robotics.

Switchable Wettability States of a Ferrofluid Droplet atop a Hydrophobic Interface

“…Soft machines, holding the capabilities of programmable alterations in their wetting states, offer exciting possibilities in a plethora of applications ranging from biomedical engineering to chemical processing . While such “smart” machines can be actuated by different stimuli, magnetic fields appear to be particularly attractive for their precise controllability and efficacy in realizing a wide variety of shapes and functionalities. , The most common variant of such a micromachine can simply be a liquid drop made of a suspension of magnetic nanoparticles commonly known as ferrofluids. Despite being elusively simple to synthesize, the ferrofluids offer great promise in a wide variety of automated applications ranging from liquid cargo delivery, − cell and tissue engineering, , water purification to on-chip mixing of liquid samples .…”

“…While such “smart” machines can be actuated by different stimuli, magnetic fields appear to be particularly attractive for their precise controllability and efficacy in realizing a wide variety of shapes and functionalities. , The most common variant of such a micromachine can simply be a liquid drop made of a suspension of magnetic nanoparticles commonly known as ferrofluids. Despite being elusively simple to synthesize, the ferrofluids offer great promise in a wide variety of automated applications ranging from liquid cargo delivery, − cell and tissue engineering, , water purification to on-chip mixing of liquid samples . Although their different shape modulations, splitting, and merging were successfully demonstrated in previous works, the proposition of tuning their wettability at will, preferably in a programmable manner, continues to offer an outstanding challenge when the applied field strength exceeds the saturation magnetization limit.…”

Untethered Small-Scale Machines for Microrobotic Manipulation: From Individual and Multiple to Collective Machines

“…Besides, direct contact manipulation limits the application scenarios when a noncontact operation scheme is required. As for wireless manipulation, an essential requisite for the manipulation strategies is to obtain controlled motion state of the small objects, which is usually conducted by applying external fields. − The targeted objects should exhibit a relatively fast response to the field input, and they are able to show distinctive behaviors from the environment for precise on-demand manipulation. This brings limitations to the intrinsic features.…”

Programmable Matter with Free and High‐Resolution Transfiguration and Locomotion

“…Mechanisms for deforming transfigurable objects include -in order of increasing potential for autonomous operation: external contact forces on the object boundary; [3][4][5][6][7][8][9] contactless interaction with a passive energy landscape (self-organization); [10] contactless interaction with an actively controlled external force field (magnetic, [11][12][13] dielectrophoretic, [14] acoustic, [15] photonic), [16,17] and; integrated actuation mechanisms (folding/origami, [18][19][20][21][22][23][24] shape memory, [25,26] piezoelectric, [27] soft actuators, [28,29] series actuator chains, [30] tendon-based, [31] gyroscopic, [32] and electromagnetic). [33] Integrated actuation mechanisms are essential to creating programmable matters with full autonomy, while contactless external force fields allow for contactless operation within the range of the field.…”

“…Transfigurable material systems can be divided into three categories: continuous (non-modular) objects, [11,[18][19][20][21][22][23][24][25][26][27][28][29][30][31] modular objects, [3][4][5][6]10,12,[14][15][16][17]32,33] and objects allowing reflow of their boundary. [7][8][9]13,34] Continuous objects can be transfigured by elastic deformation or internal mechanical systems but are limited in shape freedom by either the yield strength of the material or the number and position of deformation zones machined in the object. To transform objects into free shapes necessitates rearranging the objects' constituent parts.…”