25th anniversary article: A soft future: from robots and sensor skin to energy harvesters

Abstract

Scientists are exploring elastic and soft forms of robots, electronic skin and energy harvesters, dreaming to mimic nature and to enable novel applications in wide fields, from consumer and mobile appliances to biomedical systems, sports and healthcare. All conceivable classes of materials with a wide range of mechanical, physical and chemical properties are employed, from liquids and gels to organic and inorganic solids. Functionalities never seen before are achieved. In this review we discuss soft robots which allow actuation with several degrees of freedom. We show that different actuation mechanisms lead to similar actuators, capable of complex and smooth movements in 3d space. We introduce latest research examples in sensor skin development and discuss ultraflexible electronic circuits, light emitting diodes and solar cells as examples. Additional functionalities of sensor skin, such as visual sensors inspired by animal eyes, camouflage, self-cleaning and healing and on-skin energy storage and generation are briefly reviewed. Finally, we discuss a paradigm change in energy harvesting, away from hard energy generators to soft ones based on dielectric elastomers. Such systems are shown to work with high energy of conversion, making them potentially interesting for harvesting mechanical energy from human gait, winds and ocean waves.

Keywords: artificial eyes; camouflage; self-healing and -cleaning; soft robots and energy generators; ultrathin electronics.

Figure 1

Two extremes: Hard and soft robots. Hard robots are rigid-linked, with actuators for every joint. Soft robots on the other hand have actuators integrated and distributed throughout the robot. Robots are (a) dexterous, (b) able to monitor and control position, (c) able to manipulate and (d) to load objects. Reproduced with permission. Copyright 2008, Taylor & Francis.

Figure 2

Grippers based on dielectric elastomers and PneuNets. (a) Dielectric elastomer actuator gripping a small Teflon cylinder. Reprinted with permission. Copyright 2007, American Institute of Physics. (b) Finite element modelling of the dielectric elastomer actuator in (a). Reprinted with permission. Copyright 2008, American Institute of Physics. (c) PneuNet actuator gripping a raw egg. Reprinted with permission. Copyright 2011, Wiley-VCH.

Figure 3

Examples of ultrathin and flexible electronic circuits. (a) Organic transistor circuits on plastic films enabling bending down to a radius of 100 μm. Reprinted with permission. Copyright 2010, Macmillan Publishing Ltd. (b) Epidermal electronic systems with releasable connectors directly on skin Reprinted with permission. Copyright 2013, Wiley-VCH. (c) Ultrathin silicon circuits on plastic films. Reprinted with permission. Copyright 2013 American Chemical Society. (d) Imperceptible tactile sensor film that can be crumpled like a piece of paper. Reprinted with permission. Copyright 2013, Macmillan Publishing Ltd.

Figure 4

Stretchable light emitting diodes and displays and stretchable organic solar cells. (a) Waterproof ultrathin inorganic AlInGaP light emitting diodes on stretchable substrates. Reprinted with permission. Copyright Macmillan Publishing Ltd. (b) Intrinsically stretchable polymer based light emitting electrochemical cells. Reprinted with permission. Copyright 2011, Wiley-VCH. (c) Ultrathin organic polymer light emitting diodes withstanding crumpling like a piece of paper. Reprinted with permission. Copyright 2013, Macmillan Publishing Ltd. (d) Stretchable GaAs solar cells with high areal coverage (left image) (Reprinted with permission. Copyright 2011, Wiley-VCH); stretchable organic solar cell (middle image). (Reprinted with permission. Copyright, Wiley-VCH), and ultrathin and lightweight organic solar cell (right image) (Reprinted with permission. Copyright 2013, IEEE).

Figure 5

(a) Mechanically tunable colours in a hydrogel with micro­domains of bilayers periodically stacked in the polymer host. Reprinted with permission. Copyright 2011, American Chemical Society). (b) Tunable structural color by combining polymer opals and dielectric elastomer actuators. Under an applied voltage the color changes from green to blue. Reprinted with permission. Copyright 2012, American Institute of Physics. (c) Electrically triggered surface wrinkling in elastomeric coatings changes the appearance of the coating from transparent to translucent. Reprinted with permission. Copyright 2013, Wiley-VCH. (d) Camouflage and display of soft machines by pumping coloured liquids into networks of microfluidic channels. Reprinted with permission. Copyright 2012, AAAS.

Figure 6

Self-cleaning and healing. (a) Voltage controlled dynamic topography surfaces based on dielectric elastomers for detaching bacterial biofilms. The electric field causes deformation of the elastomer surface (left image), which detaches over 95 % of the biofilm (middle and right image before and after deformation). Reprinted with permission. Copyright 2013, Wiley-VCH. (b) Rubber composite healing at room temperature. The sequence of images shows cutting, joining, mending and stretching the rubber after healing. Reprinted with permission. Copyright 2008, Macmillan Publishing Ltd. (c) and (d) show two examples of electrically and mechanically self-healable polymer composites, in (c) the composite contains nickel microparticles Reprinted with permission. Copyright 2012 Macmillan Publishing Ltd.) in (d) silver nanowires to achieve electrical conductivity. Reprinted with permission. Copyright 2013, Wiley-VCH.

Figure 7

Artificial eye demonstrators. (a) Fluid filled elastomeric lens with an annular dielectric elastomer actuator working as an artificial muscle. Reprinted with permission. Copyright 2011, Wiley-VCH. (b) Electrode-free configuration of a fluid filled elastomeric lens with electrically tunable focal length. Reprinted with permission. Copyright 2010, National Academy of Sciences USA. The focal length is tuned by spraying charges on the elastomer surfaces. (c,d) Hemispherical eye (Reprinted with permission. Copyright 2011, National Academy of Sciences USA), and insect-like eye demonstrators (Reprinted with permission. Copyright 2013, Macmillan Publishing Ltd). with inorganic photodetector arrays on elastomeric surfaces deformable in hemispherical shapes.

Figure 8

Stretchable batteries. (a) Zinc carbon chemistry based battery stretched biaxially. Reprinted with permission. Copyright 2010 Wiley-VCH. (b) Zinc carbon based battery with stretchable textile meshes as current collectors. Reprinted with permission. Copyright 2012 Wiley-VCH. (c) Ultra-stretchable and rechargeable Li-ion battery with serpentine interconnects. Reprinted with permission. Copyright 2013 Macmillan Publishing Ltd. (d) Alkaline manganese rechargeable battery twisted and stretched during charging. Reprinted with permission. Copyright 2013 Royal Society of Chemistry.

Figure 9

Hard and soft energy generators. (a) Rigid versus soft ocean wave energy converters (WEC). In the rigid generator steel and concrete dominate as used materials, whereas for the soft generator one may deploy rubber tubes into the ocean to be deformed by waves. (b) Experimental rubber tube system under water, showing the deformation by caused by waves Reprinted with permission. Copyright 2012, SPIE. The varying pressure level is indicated with a Ruben’s tube. (c) Soft energy harvester employing only soft materials, rubber, plastics and stretchable carbon black/oil composite conductors. Reprinted with permission. Copyright 2011, American Institute of Physics.