*Review* **Wearable Actuators: An Overview**

#### **Yu Chen <sup>1</sup> , Yiduo Yang <sup>1</sup> , Mengjiao Li <sup>2</sup> , Erdong Chen <sup>2</sup> , Weilei Mu <sup>2</sup> , Rosie Fisher <sup>1</sup> and Rong Yin 1,\***


**Abstract:** The booming wearable market and recent advances in material science has led to the rapid development of the various wearable sensors, actuators, and devices that can be worn, embedded in fabric, accessorized, or tattooed directly onto the skin. Wearable actuators, a subcategory of wearable technology, have attracted enormous interest from researchers in various disciplines and many wearable actuators and devices have been developed in the past few decades to assist and improve people's everyday lives. In this paper, we review the actuation mechanisms, structures, applications, and limitations of recently developed wearable actuators including pneumatic and hydraulic actuators, shape memory alloys and polymers, thermal and hygroscopic materials, dielectric elastomers, ionic and conducting polymers, piezoelectric actuators, electromagnetic actuators, liquid crystal elastomers, etc. Examples of recent applications such as wearable soft robots, haptic devices, and personal thermal regulation textiles are highlighted. Finally, we point out the current bottleneck and suggest the prospective future research directions for wearable actuators.

**Keywords:** smart textiles; wearable; fiber actuators; soft exoskeleton; haptic action

**1. Introduction**

The global wearable market showed a dramatic swell in the past decade with a market size valued at USD 28 billion in 2020 and is expected to expand continually in the next decade. Wearable technology, an emerging trend, integrates sensors, actuators, and electronics that can be worn, embedded in fabric or accessories, or tattooed directly onto the skin to assist daily activities and address changing lifestyles. Wearable actuators, a subcategory of wearable technology, require a compatible modulus to the human body, a huge scope of motion with high precision and velocity, great strain energy density to generate a high force level, a low fatigue rate for a long lifetime, and good reliability.

Conventional actuators including rotary or linear electrical motors, pneumatic and hydraulic actuators [1] provide high power, fast response time, and have been applied in industries for centuries; they are, however, stiff, heavy, noisy, and nonbiological, which limit their applications in wearables. People expect the wearable actuators to be lightweight, inconspicuous, lifelike, and versatile when on the human body, while still achieving their purpose outstandingly. These requirements impelled the development of soft actuation technologies and have attracted enormous interest from researchers in various disciplines [2]. Unlike those traditional actuators, these actuators are small and light, and are not limited to the electrical–mechanical force conversion method. These soft actuators can respond to multiple stimuli such as heat, light, electricity and moisture, exerting force or producing shape changes [2].

Applications of wearable actuators mainly include wearable robotics, haptic devices, and smart textiles. Wearable robotics have been proven valuable in rehabilitation, body assistance, and/or virtual reality [3]. These applications cover systems of various sizes, from millimeter-scale biorobots to large deployable structures. Haptic devices allow the intent

**Citation:** Chen, Y.; Yang, Y.; Li, M.; Chen, E.; Mu, W.; Fisher, R.; Yin, R. Wearable Actuators: An Overview. *Textiles* **2021**, *1*, 283–321. https:// doi.org/10.3390/textiles1020015

Academic Editor: Ivo Grabchev

Received: 1 June 2021 Accepted: 20 August 2021 Published: 24 August 2021

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recognition and information transmission through the interface link between the device and the skin. They contain flexible tactile actuators that can transfer the signal through vibration or morphological change [4]. Smart textiles are textiles that can interact with the environment or respond to stimuli [5,6]. Examples of smart textile applications include electrocardiography-T-shirts/wristbands, electroencephalography caps and photovoltaic curtains [1].

There are several review articles introducing the material and working principles of various soft actuators, but few of them are focused on the scope of wearable actuators [2,5,7,8]. In this paper, we review the actuation mechanisms, structures, applications, and limitations of recently developed wearable actuators including pneumatic and hydraulic actuators, shape memory alloys and polymers, thermal and hygroscopic materials, dielectric elastomers, ionic and conducting polymers, piezoelectric actuators, electromagnetic actuators, liquid crystal elastomers, etc. Examples of the recent applications such as wearable soft robots, haptic devices, and personal thermal regulation textiles are highlighted. Finally, we point out the current bottleneck and suggest the prospective future research directions for wearable actuators.
