**3. Outlook**

Recent applications of wearable actuators are mainly concentrated in three categories: wearable robots, haptic devices, and personal thermal regulation textiles. Current orthosis robots are dominated by electric motors [18], which are high-output, stable, multifunctional, but also bulky, stiff, and inconvenient. In the future, these rigid electric motors will be gradually replaced by high-performance soft and smart materials, although they have a long way to go. For pneumatic and hydraulic actuators, research on the structure and functional design enables this type of actuator to achieve complex motions with simple strategies. Further, the efficiency of these actuators has been improved through the integration of motors, pumps and valves [170]. Recently, the combination of pneumatic and hydraulic actuators and textiles allows us to see new possibilities [22] which make the pneumatic/hydraulic wearable robots not limited to medical treatment and rehabilitation. With the emerging material science and technology, new smart materials enable the possibility of wearable robots, getting rid of bulky, rigid, and heavy equipment. Thermally driven materials such as shape memory alloys have problems such as high thermal hysteresis, high temperature, and high cost [29]. The actuation force of IPMCs, CNTs and CPs is too small, and it is difficult to make large-scale devices [18]. Among these new materials, DEAs are the most promising, due to fast response speed, high electromechanical performances, and high efficiency [18]. The research direction of reducing the operating voltage, improving the stability, and structure design of DEAs will be the mainstream. Wearable orthosis robots require high output force and precise control, especially for lower limb rehabilitation, which needs to be addressed in the future. Moreover, ideal combination of textile structure and smart materials for wearable robots is still a major challenge and we look forward to seeing more wearable assistive robots based on textile structures in the future.

Virtual and augmented reality (VR/AR) technology is a system that generates perceptual information through a computer and interacts with the real or virtual world. As the VR/AR technology evolves, the market demands matching or even better performance equipment. Haptic technology is an indispensable part of the VR/AR system. Wearable haptic devices mainly include three different methods: skin-attachable haptic interfaces, wearable haptic interfaces, and touch-based haptic interfaces [171]. Traditional haptic feedback is usually achieved by using the vibration motor such as the eccentric rotating mass (ERM) and linear resonant actuator (LRA) [171]. These actuators have the problems of poor portability and low resolution. Fine tactile feedback can be achieved through fine texture and tiny shapes, which makes soft actuators a potential alternative [171]. Soft actuators such as dielectric, electromagnetic, and piezoelectric actuators have been widely applied in this field. Future research is likely to take the advantage of soft materials to achieve multi-point stimulations and multi-shape generation [171]. Moreover, reducing the driving voltage and increasing the reliability of materials are also main research directions. Integration of haptic actuators and textiles into smart textiles to replace traditional tactile feedback devices is another potential for breakthrough of haptic technologies.

Thermal regulation textiles, as the first generation of smart textile, have been used in commerce and the market for decades [1]. These textiles are typically made of materials (nylon, wool, silk etc.) that can undergo expansion by moisture or heat. Through structural design such as twisting or chemical modification, these materials can passively respond to environmental stimuli. Future research may lie on the improvement of material performance and the design of the active regulation system.

Due to the advantages of programmability and comfort, textiles are very likely to be the main implementation method for wearable actuators. The new soft material has good compatibility with the textile structure. CNTs, shape memory alloys, and LCEs can be directly used as yarns for textile design. As an indispensable element of human life, textiles can also serve as a carrier of actuators. How to achieve actuation without affecting the original function of the textile is a problem that needs to be overcome.

#### **4. Conclusions**

In this review, we summarized the materials and structures that can be applied to 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. We have cited examples of wearable applications recently reported. These actuator applications are mainly concentrated in orthosis robots, haptic actuators, and smart textiles. There is no doubt that the application prospects of smart wearable actuators are wide, and the research of new soft actuator materials and structures is still in progress. The prospective future research directions for wearable actuators include: (1) smart materials with high performance, precise control, durable, and low cost; (2) integration of smart materials into textile structures to achieve required functions; (3) exploration of more applications for wearable actuators. We expect to see the application of wearable actuators that are comparable to biological structures in all aspects in the future.

**Author Contributions:** Writing—original draft preparation, visualization, analysis, Y.C.; writing original draft preparation, visualization, analysis, Y.Y.; writing—original draft preparation, visualization, analysis, M.L.; writing—original draft preparation, visualization, analysis, E.C.; writing original draft preparation, visualization, analysis, Y.Y.; review and editing, W.M.; review and editing, R.F.; conceptualization, supervision, writing—original draft preparation, review and editing, R.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the start-up at North Carolina State University.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available within the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

