Biomimetic Color Changing Anisotropic Soft Actuators with Integrated Metal Nanowire Percolation Network Transparent Heaters for Soft Robotics

Abstract

To add more functionalities and overcome the limitation in conventional soft robots, highly anisotropic soft actuators with color shifting function during actuation is demonstrated for the first time. The electrothermally operating soft actuators with installed transparent metal nanowire percolation network heater allow easy programming of their actuation direction and instantaneous visualization of temperature changes through color change. Due to the unique direction dependent coefficient of thermal expansion mismatch, the suggested actuator demonstrates a highly anisotropic and reversible behavior with very large bending curvature (2.5 cm −1 ) at considerably low temperature (≈40 °C) compared to the previously reported electrothermal soft actuators. The mild operating heat condition required for the maximum curvature enables the superior long-term stability during more than 10 000 operating cycles. Also, the optical transparency of the polymer bilayer and metal nanowire percolation network heater allow the incorporation of the thermochromic pigments to fabricate color-shifting actuators. As a proof-of-concept, various color-shifting biomimetic soft robots such as color-shifting blooming flower, fluttering butterfly, and color-shifting twining tendril are demonstrated. The developed color-shifting anisotropic soft actuator is expected to open new application fields and functionalities overcoming the limitation of current soft robots.Unlike the conventional rigid actuators, the soft actuators are composed of elastic and lightweight materials with simple operating systems. [1] Due to their unique soft features, the soft actuators have been utilized in bioapplications such as artificial muscles, [2,3] soft manipulators, [4,5] biomimicking robots, [6][7][8][9] prosthesis, [10] and so on. The soft actuators operates by various physical, chemical, and optical stimulus such as electricity, [11][12][13][14][15] heat, [16][17][18] light, [7,8,19] magnetism, [20] pressure, [21] and humidity. [9,18,[22][23][24] Among them, considering the practical uses, the electrical signal has been the most popular input signal due to its easy and intuitive control of actuators. Typically the approaches to establish electrically operated soft actuators can be divided into two categories which are using electroactive polymeric (EAP) materials [4,5,12,15,25,26] and thermal expansive materials. [16][17][18]24,[27][28][29] However, since the EAP-based actuators need high operating voltage [4,5] and electrolyte environment, [12,25,26] their application in various fields is limited. On the other hand, the electrothermal actuator (ETA) which basically operates by different thermal expansive volume changes between the polymers composing a bilayer requires much lower operating voltage and Color-Changing Soft ActuatorsThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.

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