micromachines Perspective Liquid-Metal Enabled Droplet Circuits Yi Ren 1 and Jing Liu 1,2,3,* ID 1 Department of Biomedical Engineering, Tsinghua University, Beijing 100084, China; [email protected] 2 Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China 3 School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049, China * Correspondence: [email protected]; Tel.: +86-10-6279-4896 Received: 13 April 2018; Accepted: 2 May 2018; Published: 5 May 2018 Abstract: Conventional electrical circuits are generally rigid in their components and working styles, which are not flexible and stretchable. As an alternative, liquid-metal-based soft electronics offer important opportunities for innovation in modern bioelectronics and electrical engineering. However, their operation in wet environments such as aqueous solution, biological tissue or allied subjects still encounters many technical challenges. Here, we propose a new conceptual electrical circuit, termed as droplet circuit, to fulfill the special needs described above. Such unconventional circuits are immersed in a solution and composed of liquid metal droplets, conductive ions or wires, such as carbon nanotubes. With specifically-designed topological or directional structures/patterns, the liquid-metal droplets composing the circuit can be discrete and disconnected from each other, while achieving the function of electron transport through conductive routes or the quantum tunneling effect. The conductive wires serve as electron transfer stations when the distance between two separate liquid-metal droplets is far beyond that which quantum tunneling effects can support. The unique advantage of the current droplet circuit lies in the fact that it allows parallel electron transport, high flexibility, self-healing, regulation and multi-point connectivity without needing to worry about the circuit break. This would extend the category of classical electrical circuits into newly emerging areas like realizing room temperature quantum computing, making brain-like intelligence or nerve–machine interface electronics, etc. The mechanisms and potential scientific issues of the droplet circuits are interpreted and future prospects in this direction are outlined. Keywords: droplet circuits; liquid metal; quantum tunneling effect; solution electronics; electron transport; ionic conduction; quantum computing; brain-like intelligence 1. Introduction Since the origin of electricity, it has become a necessity in daily life. Generally speaking, electrical circuits are rigid and continuous in their structures and components. Print circuit boards (PCB) have been commonly used in various situations (Figure1A). However, classic rigid circuits cannot easily adapt to the human body due to poor flexibility and biocompatibility, limiting their value in the biomedical and health care fields. The increasing advancement of wearable devices and implantable systems has led to significant growth in flexible electronics (Figure1B) [ 1]. Polymer nanomaterials, silk fibroin and liquid metal are being gradually adopted in soft electronics. Another main development trend in artificial circuits is molecular electronics. This was first proposed in 1974 by Aviram and Ratner [2], and refers to a field that seeks to fabricate electrical devices and circuits with single molecules and molecular monolayers [3–6]. The fabrication of molecular electrical devices includes single-molecule break junctions and molecular monolayer devices [3]. Tailored by chemical design and synthesis, the function of molecular components can be rather diverse. Deoxyribonucleic acid Micromachines 2018, 9, 218; doi:10.3390/mi9050218 www.mdpi.com/journal/micromachines Micromachines 2018, 9, 218 2 of 12 Micromachines(DNA) has 2018 potential, 9, x for molecular devices for its unique structure [5]. Figure1C presents the logic2 of 11 ‘AND’ and ‘OR’ gate fabricated by DNA. Until now, molecular components including diodes, switches, switches,memory andmemory transistors and transistors have been have intensively been intensively researched [researched6]. Those components [6]. Those components can be combined can be to combinedconstruct molecular-scaleto construct molecular-scale electronic computers electronic [7 ].computers Based on the[7]. electricalBased on propertiesthe electrical of theproperties molecular of thediode molecular switches, diode quantum switches, mechanical quantum calculations mechanical can calculations also possibly can be also performed. possibly be performed. FigureFigure 1. 1. RespectiveRespective kinds kinds of of electrical electrical circuits. circuits. ( (AA)) Traditional Traditional rigid rigid print print circuit circuit boards boards (PCB) (PCB) circuits circuits (https://upload.wikimedia.org/wikipedia/commons/0/0f/MOS6581_chtaube061229.jpg);(https://upload.wikimedia.org/wikipedia/commons/0/0f/MOS6581_chtaube061229.jpg (B)); (B)Soft Soft electronics.electronics. Reproduced Reproduced with with permission permission from from [8]; [8]; (C) (C) Logic Logic AND AND Gate Gate and and OR OR Gate Gate based based on on DNA. DNA. ReproducedReproduced with permission from [6]; [6]; (D) (D) Several Several typical typical types types of of neural neural circuits; circuits; (E) (E) Brain–computer Brain–computer interface.interface. Reproduced Reproduced with with permission permission from from [9]. [ 9Note:]. Note: TMS: TMS: transcranial transcranial magnetic magnetic stimulation; stimulation; ERD: event-relatedERD: event-related desynchronization; desynchronization; MEP: motor-evoked MEP: motor-evoked potential; potential; EEG: electroencephalogram; EEG: electroencephalogram; EMG: electromyogram.EMG: electromyogram. BesidesBesides these these artificial artificial circuits, circuits, electrical circuits in fact intrinsically intrinsically exist exist throughout throughout the the human human body,body, as shown in Figure 11D.D. ForFor instance,instance, DejeanDejean etet al.al. recentlyrecently studiedstudied thethe neuralneural circuitscircuits andand cellcell typestypes that that mediate mediate conditioned conditioned fear fear expression expression and and recovery recovery [8]. [8]. Moreover, Moreover, voltage-gated channels areare important switches switches for for signal signal transportation transportation in in the the central central and and peripheral peripheral neural neural systems systems [10]. [10]. KeepingKeeping the aboveabove factsfacts inin mind mind and and without without losing losing generality, generality, we we can can divide divide electrical electrical circuits circuits into into two twomain main categories: categories: biologically-inspired biologically-inspired natural natural circuits circuits and human-madeand human-made artificial artificial circuits. circuits. One moreOne moretrend, trend, as indicated as indicated in Figure in 1FigureE, is that 1E, a is new that category a new iscategory emerging is emerging to combine to naturally combine occurringnaturally occurringneural circuits neural and circuits artificial and artificial circuits tocircuits carry to out carry complex out complex functions. functions. The core The ofcore such of such circuits circuits can cangenerally generally be called be called brain–computer brain–computer interfaces interfaces (BCIs) (BCIs) [11–14 [11–14].]. Clearly, Clearly, BCIs are BCIs significant are significant for patients for patientswith serious with disabilitiesserious disabilities such as tetraplegiasuch as tetraplegia and stroke. and Theystroke. especially They especially mean a mean lot for a future lot for humanfuture humanneeds in needs extending in extending the limits the of limits biological of biological capability. capability. At this At stage, this BCIsstage, based BCIs onbased rapid on serialrapid visualserial visualpresentation presentation have already have already been used been to detectused to and detect recognize and objects,recognize providing objects, aproviding viable approach a viable to approachprompt human–machine to prompt human–machine systems [14]. systems [14]. DespiteDespite thethe widespread widespread application application of currentlyof currently available available circuits, circuits, they all unavoidablythey all unavoidably encounter encounterthe possibility the possibility of circuit break,of circuit which break, severely which affects severely the affects normal the operation normal operation of devices. of As devices. a remedy, As awe remedy, here propose we here an unconventionalpropose an unconventional concept for electrical concept circuits for electrical to tackle circuits the above to challenges,tackle the above which challenges,we call droplet which circuits. we call Such droplet an electrical circuits. circuit Such isan enabled electrical from circuit liquid-metal is enabled droplets from liquid-metal (LMDs) and dropletsis conductive (LMDs) in and discontinuous is conductive form. in discontinuous The structure form. and workingThe structure style ofand this working kind of style circuit of this are kindhighly of analogouscircuit are andhighly similar analogous to that and of the similar neuro-network to that of the system. neuro-network Therefore it system. is expected Therefore to be veryit is expecteduseful in innovatingto be very useful newly in emerging innovating areas newly such emerging as room temperature areas such as quantum room temperature computing, brain-likequantum computing,intelligence, brain-like and brain–machine intelligence,