Opening remarks: The smart textile market is gradually becoming a new engine of global economic growth. Market research company ReportLinker predicts that the global smart textile market is expected to grow from $2.52 billion in 2021 to $9.3 billion in 2026, with a compound annual growth rate of 28.7%. In the next decade, in the era of the Internet of Things, smart textiles are likely to change human life together with artificial intelligence, human-machine interfaces, and cloud technology. This magazine has a special column on new developments in smart textiles, aiming to provide readers with a deeper understanding of smart textiles through a comprehensive introduction.
Textile actuators can act on or interact with the human body through mechanical, thermal, magnetic, chemical, electrical, or other stimuli, and can even perceive external conditions (stimuli) and respond in a predetermined manner. According to common actuator driving principles, the main classifications of textile actuators are shown in Figure 1. Interactivity is the core of most smart textiles, but in the past, most research has focused on the stimulation/input aspects of smart textiles, and recently more scholars have paid attention to their response/output parts. The second generation of intelligent textiles is characterized by execution and mechanical output, which is significantly different from the first generation of intelligent textiles dedicated to perception. At present, most of the reported textile actuators mainly use one of three types: electric field, ion, and thermal drive, and integrate actuators into textiles using traditional textile manufacturing methods such as weaving, knitting, and weaving. By designing the materials and their geometric shapes, the required characteristics are achieved, such as high impact resistance, thermal regulation, or conductivity. However, the vast majority of smart textiles only focus on sensing and information collection functions, but fabrics with adjustable mechanical properties can provide mechanical feedback to the human body and perform functions such as joint assistance, support, and tactile sensation, which have broad market prospects and application promotion value.
Figure 1 Classification of typical intelligent textile actuators
Textile actuators can help patients obtain more consistent therapeutic dose, and play an important role in drug delivery, pain management, asthma management, rehabilitation and orthopedic treatment of various chronic diseases (such as chronic pain, bedsore and ulcer, asthma, diabetes, etc.). Usually, these medical processes require minimal static forces (or other stimuli) to be applied locally to the human body, and many types of textile actuators (such as electroactive polymers) have relatively small strains, which perfectly meet the needs of therapeutic applications. For example, when the resistance is heated to 130 ℃, a flexible actuator based on spandex filament twisting and curling can shrink by 45%, and the shrinkage generated by the actuator can reach 7% at around 30 ℃. There have also been studies combining two types of natural fiber fabrics to develop an electrodynamic ion driver, which provides electrical properties with high surface area viscose fiber based activated carbon cloth, achieves lateral mechanical properties with silk fabric, and prevents contact with conductive textiles. By injecting electronic charges into the activated carbon cloth electrode, electrolyte ions migrate in the porous network between the electrodes, causing uneven expansion of the layers and ultimately resulting in bending of the layers, displaying a strain difference of about 0.8%.
Image source:《High-performance electroionic artificial muscles boosted by superior ion transport with Ti3C2Tx MXene/Cellulose nanocomposites for advanced 3D-motion actuation》
Figure 2: Flexible actuator based on two-dimensional material MXene/cellulose ionic liquid
Artificial muscles are one of the main application areas of textile actuators and are also the focus of many scholars. Scholars have combined cellulose textile processing technology with metal free deposition method to coat electroactive polymers, and manufactured wearable flexible artificial muscles with adjustable strength and tension through machine weaving and knitting. They have found that the magnitude of their contraction force is linearly related to the number of yarns in the fabric. Flexible exoskeletons are another achievable goal for textile actuators. At present, only pneumatic exoskeleton sets have appeared in the market, but they are not integrated into textiles. Fluid actuation is the foundation of soft robot exoskeleton technology. Due to the current lack of active fluid pumps that can be easily integrated into textiles, experts from the Swiss Federal Institute of Technology in Lausanne have reported a stretchable fibrous fluid pump that integrates a pressure source directly into textiles, achieving wireless wearable fluid control and potentially achieving textile integration of active fluid pumps. These studies confirm that textile structures increase the flexibility and mechanical stability of actuators, and textile processing can achieve mass production of artificial muscles and other actuators, making new methods for designing auxiliary equipment possible.
Image source: Network
Figure 3 Lower limb exoskeleton robot Fourier X1
The designers of textile actuators have introduced standards that define unit mass output power, unit volume output power, and actuator efficiency as the three basic characteristics of actuators. The formulation of standards helps to promote the standardized development of such textiles. However, currently textile actuators still face a series of technical challenges, including the need to overcome the high power consumption of thermal actuators, develop materials and soft robot structures to maximize strain and other execution effects, while minimizing energy consumption; Developing flexible fiber yarns and fabric structures to maximize the tension of electric actuators; Pneumatic drive has high force, but it cannot be well integrated into textiles. It is necessary to improve integration, reduce size, and design execution mechanisms and clothing to effectively combine force with the wearer.
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