Fluid-like Soft Machines with Liquid Metal
2021; Elsevier BV; Volume: 4; Issue: 2 Linguagem: Inglês
10.1016/j.matt.2021.01.009
ISSN2590-2393
Autores Tópico(s)Innovative Microfluidic and Catalytic Techniques Innovation
ResumoLiquid metal is emerging as a key ingredient for soft machines that mimic natural tissue and organs. These systems have the potential for transformative impact as actuators, pumps, and transport devices for the next generation of soft and biologically inspired robots. Liquid metal is emerging as a key ingredient for soft machines that mimic natural tissue and organs. These systems have the potential for transformative impact as actuators, pumps, and transport devices for the next generation of soft and biologically inspired robots. Karel Čapek first introduced the term "robot" in a 1921 play that described artificial humans that were manufactured in a factory from organic matter.1Čapek K. RUR (Rossum's universal robots). Penguin, 2004Google Scholar Unlike our modern notions of robots, these human-like machines were not motorized or mechanical. Instead, they were composed of artificial tissue and organs produced from liquid vats. It wasn't until Čapek's vision was put into practice that robots became defined as motorized machines composed of rigid materials and mechanical parts. Because of their mechanical stiffness and load-bearing properties, such machinery allows for power, strength, and precision—features that are important in applications of robotics for industrial automation and manufacturing. However, the mechanical nature of robots is also an artifact of the rigid and bulky actuator technologies that are currently available. Such dependence on rigid hardware significantly interferes with the ability to engineer machines that can match the mechanics of natural organisms or be intrinsically safe and compatible for physical human interaction. To truly achieve the original vision of robots composed of artificial tissue and organs, we need a new generation of soft machines (Figure 1) that do not rely on rigid components and are instead engineered from fluids, elastomers, and other forms of condensed soft matter. Soft machines represent an emerging and potentially transformative class of new technologies that extends well beyond robotics. Efforts dating back to the early 1990s focused on soft microfluidic devices composed of lithographically patterned elastomer that are capable of manipulating biological assays, liquid analytes, and reactants.2Squires T.M. Quake S.R. Microfluidics: Fluid physics at the nanoliter scale.Rev. Mod. Phys. 2005; 77: 977Crossref Scopus (3163) Google Scholar These lab-on-a-chip technologies use pneumatics, hydraulics, thermal actuation, and other alternatives to electrical motors to drive fluid flow. With subsequent progress in tissue engineering, such work has led to more recent advancements in organ-on-a-chip devices as well as "biohybrid" actuators3Ricotti L. Trimmer B. Feinberg A.W. Raman R. Parker K.K. Bashir R. Sitti M. Martel S. Dario P. Menciassi A. Biohybrid actuators for robotics: A review of devices actuated by living cells.Sci. Robot. 2017; 2: eaaq0495Crossref PubMed Scopus (174) Google Scholar composed of micropatterned elastomers integrated with natural muscle tissue. In contrast to other alternatives to the electrical motor, biohybrid actuators are advantageous because they can be stimulated with low electrical voltage and can thus be operated using highly miniaturized and portable electronics. However, under in vitro conditions, they lack the supporting environment required to sustain long-term functionality needed for practical applications. Despite tremendous progress in artificial muscle technologies, there still remain opportunities for new actuators that can power soft machines and microfluidic systems. Liquid metal (LM) overcomes key limitations of existing soft actuator materials and, in this way, represents a highly promising material for next generation soft machines. Among LMs, eutectic gallium-indium (EGaIn) is especially promising since it is easily deformable (2 mPas viscosity), structurally stable (forms an oxide skin to hold its shape), and electrically responsive.4Dickey M.D. Chiechi R.C. Larsen R.J. Weiss E.A. Weitz D.A. Whitesides G.M. Eutectic gallium-indium (EGaIn): a liquid metal alloy for the formation of stable structures in microchannels at room temperature.Adv. Funct. Mater. 2008; 18: 1097-1104Crossref Scopus (898) Google Scholar The electrically responsive properties of EGaIn arise from the ability to control its effective surface tension through electrowetting and voltage-controlled reduction/oxidation reactions.5Khan M.R. Eaker C.B. Bowden E.F. Dickey M.D. Giant and switchable surface activity of liquid metal via surface oxidation.Proc. Natl. Acad. Sci. USA. 2014; 111: 14047-14051Crossref PubMed Scopus (216) Google Scholar As shown in several recent studies, such changes in surface tension can be harnessed for linear actuation.6Russell L. Wissman J. Majidi C. Liquid metal actuator driven by electrochemical manipulation of surface tension.Appl. Phys. Lett. 2017; 111: 254101Crossref Scopus (20) Google Scholar,7Liao J. Majidi C. Soft actuators by electrochemical oxidation of liquid metal surfaces.Soft Matter. 2020; https://doi.org/10.1039/D0SM01851ACrossref Google Scholar This is enabled through an electrochemical reaction that controls the thickness of the surface oxide, thereby tuning the surface tension that an EGaIn droplet exerts on an elastic spring or mechanical load. Unlike piezoelectric materials or dielectric elastomer actuators, such EGaIn-based actuators can be stimulated with low voltages on the order of ∼1 V and therefore have the potential to be operated using highly miniaturizable electronics. Beyond actuation, the electrically responsive nature of EGaIn has been utilized in soft machines capable of relatively complex functionality. This includes a fluidic pump for an integrated liquid cooling system8Zhu J.Y. Tang S.Y. Khoshmanesh K. Ghorbani K. An integrated liquid cooling system based on galinstan liquid metal droplets.ACS Appl. Mater. Interfaces. 2016; 8: 2173-2180Crossref PubMed Scopus (78) Google Scholar and electrical rotor that is driven by an LM-brush motor, as is described in the January 22, 2021 issue of iScience.9Wang E. Shu J. Jin H. Tao Z. Xie J. Tang S.-Y. Li X. Li W. Dickey M.D. Zhang S. Liquid metal motor.iScience. 2021; 24: 101911Abstract Full Text Full Text PDF PubMed Scopus (11) Google Scholar In both of these implementations, a droplet of EGaIn drives fluid flow through a "Marangoni effect" in which continuous electrowetting induces an interfacial tension gradient that pulls liquid along the surface of the droplet. In recent years, there has also been exciting progress in the development of LM-based "soft robots" that are capable of locomotion through rolling, crawling, and propulsion through fluid.10Wang X. Guo R. Liu J. Liquid metal based soft robotics: materials, designs, and applications.Adv. Mater. Technol. 2019; 4: 1800549Crossref Google Scholar As with the LM actuators and pumps, these mobile systems are typically powered through surface tension or surface tension gradients that are controlled by mechanisms such as electrochemical redox and continuous electrowetting. In some cases, they can be self-powered through chemical reactions and sustain motion without the need for electrical stimulation. These systems are not robots in the conventional sense since they are not capable of sensing, on-board control, and decision making. However, such LM-based soft machines could serve as key building blocks for future soft robots that are capable of more autonomous functionality. While there has been impressive progress in the development of soft machines with LM, this field is still in its nascent stages. Researchers are still learning about the underlying mechanics of EGaIn droplets and exploring novel ways in which surface interactions can be controlled. In the forthcoming years, there is tremendous opportunity to build on existing work and explore new methods of actuation, locomotion, and organ-like functionality. This includes designs for novel structures that harness fluid-structure interactions for directed or reciprocating motion, architectures for integrated control electronics, and methods to control the electrical and chemical response of EGaIn droplets by tailoring their surface composition. Such efforts will help accelerate progress in the creation of actuators and artificial organs that leverage the unique chemical, electrical, and electrochemically responsive properties of EGaIn. In this way, it may be possible to eventually realize the truly life-like machines that were originally conceived of by Čapek and other robotics visionaries. Liquid metal motorWang et al.iScienceDecember 8, 2020In BriefMechanical Design; Metals; Electrical Materials Full-Text PDF Open Access
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