Lower‐Voltage‐Driven Deformable Soft Electrothermal Actuators with Embedded Liquid Metal EGaIn

Abstract

An interactive human-machine interface (iHMI), a platform for the two-way exchange of information between humans and computers for a variety of symbols and actions, has been the subject of extensive attention. [1,2] Traditionally, rigid sensors and actuators have been used for sensing in iHMI. However, rigid components are often poorly compatible with humans and uncomfortable to wear. Therefore, flexible, stretchable, and deformable actuators have been a focal point in recent studies. Soft actuators play an important role in a variety of fields such as haptic systems, [3,4] soft robots, [5,6] and rehabilitation devices [7,8] due to their low cost, easy deformation, high concealment, and good environmental adaptation. [9,10] Soft actuators can respond to a wide range of stimuli, including electrical [11] and magnetic fields, [12] light [13] and hydraulic pressure, [14] and so on. Based on the types of stimuli, soft actuators can be classified into electrothermal actuators (ETAs), magnetic response actuators, light thermal actuators, electrochemical actuator temperature response actuators, humidity response actuators, and so on. [15][16][17][18] Among them, the ETAs have attracted more interest due to their simple construction and ease of control. As a response to the input electrical signal, the ETAs can generate deformation because of the mismatch in the thermal expansion coefficient (TEC) between different materials/ layers of ETAs when heated. Compared with other electrically responsive actuators, the ETAs can respond to lower voltages rapidly and accurately, indicating these ETAs are more suitable for iHMI. [19] In addition, the highest level of national safety voltage is 6 V, which will not cause harm to human body. Thus, more studies have been carried out focusing on ETAs driven by lower voltage. The key to achieving fast low-voltage-driven ETAs is the acquisition of large, fast temperature increments, which depends on the Joule heat produced by the heating layer/conductive circuit. Thus, designing and fabricating suitable materials for the heating layer/conductive circuit is of significance for lower-voltage-driven ETAs.The conductive composite materials can be obtained by mixing conductive materials (such as carbon nanotubes, [20] silver nanowires, [21] and graphene [22] ) and flexible materials (such as polydimethylsiloxane (PDMS), [23] silicone rubber, and liquid crystal elastomer (LCE) [24] ). Ahn et al. proposed a new deformable ETA utilizing nonhomogeneous electrical conductivity. In the new ETA, carbon nanotubes/silver nanowires/PDMS composites are used as the heating layer, by which the temperature can be increased to up to 120 °C at a driving voltage of 10 V.

Actuation for flexible and stretchable microdevices

“…231,232 The actuation should provide the intended dexterity to achieve motions such as extension, contraction, twisting, and bending. 232 Actuation mechanisms such as fluid-driven, 68,233,234 thermal, 72,88 electrical, 225,235,236 and magnetic 113,237,238 have been widely used for soft robotics. In addition, chemical actuation 239 was also applied in tactile soft robotics.…”

“…6E(i), or tethered Fig. 6E(ii) 72 manners. Tethered actuation results from the deformations of the actuator material by joule heating.…”

“…Overall, the low compressibility of liquids compared to gases 36 (C) A non-metallic bi-morph actuator embedded with an eutectic GaIn heater. 72 (D) An anisotropic transparent shape morphing actuator consists of transparent silver nanowire (AgNW) percolation network as the heating element. 73 (E) A color-changing electro-thermal actuator consists of thermochromic pigments coated on the passive layer.…”

“…Since the two layers are bonded, the bimorph structure bends toward the layer with the lowest CTE, 82 resulting in actuation stroke and force. Bi-morph actuators are classified into in-plane, out-of-plane, 72,83,84 twodimensional (2D) spiral, 77 and three-dimensional (3D) spiral 85 according to their spatial motion. The most recent advancements of bi-morph actuators are the transparent electro-thermal, 86 Fig.…”

“…51 (B) A micropump works by expansion and contraction of a tubular structure and bi-directional fluid flow 36. (C) A non-metallic bi-morph actuator embedded with an eutectic GaIn heater 72. (D) An anisotropic transparent shape morphing actuator consists of transparent silver nanowire (AgNW) percolation network as the heating element 73.…”