Abstract
Smart wearables equipped with integrated flexible actuators possess the ability to autonomously respond and adapt to changes in the environment. Fibrous textiles have been recognised as promising platforms for integrating flexible actuators and wearables owing to their superior body compliance, lightweight nature, and programmable architectures. Various studies related to textile actuators in smart wearables have been recently reported. However, the review focusing on the advanced design of these textile actuator technologies for smart wearables is lacking. Herein, a timely and thorough review of the progress achieved in this field over the past five years is presented. This review focuses on the advanced design concepts for textile actuators in smart wearables, covering functional materials, innovative architecture configurations, external stimuli, and their applications in smart wearables. The primary aspects focus on actuating materials, formation techniques of textile architecture, actuating behaviour and performance metrics of textile actuators, various applications in smart wearables, and the design challenges for next-generation smart wearables. Ultimately, conclusive perspectives are highlighted.
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Introduction
Over the past decade, there has been growing interest and demand for wearable technology [1]. Most commercial wearables, such as glasses, watches, and wristbands, are composed entirely or partially of rigid microelectronics that detect or monitor the wearer’s environmental or human physiological data [2,3,4]. However, the rapid development of wearables has promoted the emergence of smart wearables, which can sense external stimuli, react, and adapt to their surroundings by incorporating actuators. Actuators are known for their capacity to reversibly translate a physical domain into mechanical motion in response to an external stimulus, which can be categorised into different categories depending on various features. According to external stimuli, actuators are primarily grouped into electrical, thermal, humidity, light, bladder, and magnetic actuators [5,6,7,8]. Based on their actuating motion characteristics, actuators are categorised into linear, bending, and rotating actuators; in accordance with their flexibility, actuators are classified into two categories, namely, hard and soft actuators [9, 10]. Hard actuators built using rigid materials can be divided into several well-established domains, including electric motors, springs, hydraulics, and pneumatics. Compared with hard actuators, soft actuators composed of soft materials exhibit superior characteristics, including but not limited to softness, lightness, flexibility, compliance, air permeability, and close fitting, which enables the wearer to feel more comfortable, demonstrating their great potential for smart wearable applications.
As two representative soft materials, polymer films and textiles show great flexibility, good biocompatibility with human skin, lightweight, and high mechanical strength, making them favourable platforms for soft actuators [11, 12]. Compared with film-based actuators, textile actuators exhibit better breathability owing to their richer porous structures [13, 14]. These pore features enable the transportation and evaporation of steam or heat from human skin, thereby improving the comfort of wearables. Furthermore, film-based actuators often need to be bonded with other fibrous materials to form multilayered wearables. In contrast, textile actuators can be assembled with other fibrous materials through preprogrammed technology to form smart wearables with unique patterns and structures, thanks to their intrinsic hierarchical structure. With these benefits, the adoption of textiles as actuating carriers for soft actuators is a good choice for smart wearables. Conventional textiles are closely related to our daily lives in the form of garments, bedding, bags, shoes, or interior furnishings. With the advancement of interdisciplinary technology integration, many actuating materials go hand in hand with textile-forming technologies to develop flexible textile actuators. As demonstrated in Fig. 1, through traditional textile-forming techniques as well as bonding methods, actuating materials can be structurally expanded into textile actuators [15,16,17,18,19]. These flexible textile actuators demonstrate great potential for meeting the intelligent requirements for smart wearables, which are able to automatically sense and respond to environmental changes.
Overview of flexible textile actuators in smart wearables. Mobility-assistive exosuit; Reproduced with permission from ref [15], Copyright 2022, Amer Assoc Advancement Science. Healthcare wearable; Reproduced with permission from ref [16], Copyright 2023, Wiley–VCH Verlag Gmbh. Aerospace garment; Reproduced with permission from Ref. [17], Copyright 2022, Wiley. Comfort-adapting clothing; Reproduced with permission from Ref. [18], Copyright 2023, Springer Nature. Human–computer interaction; Reproduced with permission from Ref. [19], Copyright 2022, Wiley–VCH Verlag Gmbh
In the last five years, flexible textile actuators with applications in smart wearables have undergone rapid development. Great progress has been made in the development of actuating materials for textile actuators. Shape memory alloys (SMAs), a preferred class of thermally or electrothermally responsive materials, have been shown to be powerful and extensively utilised in the development of textile actuators for aerospace suits, mobility-assistive exosuits, and healthcare wearables [20]. Recently, a variety of synthetic polymers, natural materials, and micro- or nanoparticles have been demonstrated to exhibit high actuation capabilities for textile actuators for comfort-adapting clothing. For instance, nanostructured block copolymer hydrogels composed of hydrophilic poly(ethylene oxide) (PEO) and hydrophobic polystyrene (PS) demonstrate an actuation strain of 80% over 100 actuation cycles [21]; natural silk yarn actuated by artificial sweat within 80 s is a viable option for develo** textile actuators that effectively regulate the humidity and temperature levels of human skin [22]; and nylon/Ag/PS heterostructure-based nanocomposite films with rectangular holes can be adhered to garments to expand the thermal comfort zone by 30.7% compared to that of normal static textiles when exposed to human perspiration vapour [23]. Along with the development of actuating materials, textile-forming methods have also been rapidly developed to form desirable structures for textile actuators. To overcome large volume changes in unforeseen dimensions during actuation and reduce time consumption during manufacturing processes, complex textile structures from fibres or yarns to textile levels can be achieved through multilayer knitting and three-dimensional (3D) knitting [24]. To develop high-performance SMA textile actuators, a unique knotting strategy using multiple-unit knots has been proposed to create a novel textile architecture that increases the actuation force and strain [25]. However, the combination of these newly reported actuating materials and textile-forming technologies has not yet been reviewed for use in textile actuators for smart wearables.
Several comprehensive reviews on actuating materials for smart textiles, as well as actuating fibres and textiles for soft robots, have been reported. In 2019, Kongahage et al. outlined a mini-review for actuating materials for smart textiles [26]. In 2020, Sanchez et al. and ** into fibrous substrates to develop textile actuators [42]. For example, MXene nanosheets coated on silk textiles can effectively improve the photothermal conversion efficiency as well as actuation power, thereby resulting in biomimetic deformation and movement of “claw,” “snake,” and even “octopus” [43].
Metal and Metal Compound Micro- or Nano-particles
Metal and metal compound micro- or nano-particles typically exhibit excellent electrical and thermal conductivity, high mechanical performance, and unique magnetic properties. Therefore, these micro- or nano-particles are beneficial for electrothermal, photothermal, and magnetic actuation of fibrous substrates. For example, silver nanoparticles (AgNPs) or nanowires (AgNWs) and gold nanoparticles (AuNPs) have been coated on fibrous substrates to enhance thermal conversion efficiency. In addition, the use of magnetic micro- or nanoparticles composed of metal compounds has been considered an effective strategy for endowing fibrous substrates with magnetic actuation characteristics.
Polymer Membranes
Some polymer materials can also be prepared as membranes instead of fibres or yarns and then combined with existing fabric substrates to form textile actuators, such as rubbers, elastomers, ion gels, etc. For instance, cuttable silicon-based rubber or PU-based elastomer soft membranes are often used to fabricate bladder chambers [25]. As a consequence, four simple knotting unit patterns are specified from 16 basic knotting unit patterns, including one-petal, two-petal, four-petal, and five-petal knotting patterns (Fig. 5b(i)). Among these knotting unit patterns, two criteria for selecting a possible knotting pattern are introduced, including minimising the number of excessive friction crossovers and the interconnection capability of knotting units in the column direction. Finally, only one-petal and two-petal knotting patterns meet both criteria. Figure 5b(ii) illustrates a schematic diagram of a three-step process to fabricate a knotted textile actuator with a two-petal unit. This can achieve high and reliable actuation through a seamless conductive circuit grid.
Bonding
In addition to the textile-forming technologies mentioned above, actuating materials have been proven to adhere to existing fabric substrates through bonding methods [61]. The bonding methods that utilise physicochemical adhesion or mechanical fixation include coating, pasting, heat sealing, embroidery, sewing, etc. Since an existing fabric substrate can provide the textile structure, the bonding technique is simple and does not involve the formation of a new textile structure.
Micro- or nano-particle-based actuating materials dispersed in solution can be integrated with existing textiles through coating, casting, or printing. After the solvent in the solution evaporates, the solute layer containing the actuating materials precipitates on the surface of the textile substrate. For instance, Fig. 6a illustrates the dip-coating process of a solution containing MXene and AgNPs on a commercial Tencel fabric. This imparts excellent electrochemical and photothermal actuation properties to the surface of the fabric substrate [62]. Membrane-based actuating materials can be bonded to the surface of textile substrates using chemical glues and then mechanically pressed or heat-sealed to improve adhesion to the laminated structure. As shown in Fig. 6b, a textile actuator composed of biaxially oriented PP film (55 μm) and a fabric substrate (35 μm) is simply laminated with acrylic glue to a thickness of 90 μm, showing excellent flexibility. By feeding large-size raw materials into a roller press in parallel, large-scale production of textile actuators can be easily achieved [63].
a Coating process of a textile actuator with MXene/AgNPs solution; Reproduced with permission from Ref. [62], Copyright 2022, Springer. b Diagram of pasting between a film actuator and a textile substrate; Reproduced with permission from Ref. [63], Copyright 2020, Wiley. c Embroidered fabric with SMA wires; Reproduced with permission from Ref. [64], Copyright 2021, Wiley
Actuating materials can also be bonded to fabric substrates through embroidery and sewing. Embroidery, as one of the traditional Chinese folk crafts, uses needle and thread to decorate different patterns on textile substrates for making textile actuators. As displayed in Fig. 6c, SMA wires are embroidered into a highly stretchable PET fabric to supply sufficient deformation potential. Thus, the actuating function is implemented for the textile matrix without compromising its mechanical properties [64]. Sewing, including hand sewing and machine sewing, has been widely used in bladder textile actuators. Machine sewing involves the automatic movement of fabric under sewing feet, providing accurate and efficient sewing for large-scale textile actuators. Hand sewing can be used for complicated sewing paths, non-flat fabrics, and even intubation. A bladder textile actuator is fabricated through the sewing process, where a thermoplastic elastomer film bladder is sewn into two pieces of fabric to create an inflatable beam [65]. The beam is then folded and sewn again to control the expansion of the textile actuator when it is inflated to a pre-determined degree of bending.
Electrospinning
Electrospinning methods are expected to create nonwoven mats for the actuating materials with poor flexibility and discontinuous structures [66]. During the electrospinning process, the spinning solution formed at the tip of the needle elongates and travels towards the counter electrode or collector under an electromagnetic field. Then, a nonwoven structure is formed after the solvent in the solution evaporates. By regulating the viscosity and amount of the spinning solution, supply voltage, feeding and collecting speed, and distance between needle and collector, nonwoven mats with various fibrous nanostructures can be obtained. Recently, electrospinning technology has demonstrated great potential in preparing nonwoven mat actuators with simple textile structures made of nanofibres, such as hydrogel fibres and carbon nanofibres. For instance, Bai et al. employ electrospinning technology to prepare cellulose acetate/CNT nanofibres for a multi-responsive actuator with high heavy-lift capacity. The actuator can lift a load 1050 times heavier than its own weight under thermal and light stimulation [67].
Pros and Cons
In general, knitting, weaving, and braiding are three common textile-forming techniques to achieve seamless integration of actuating materials and fabrics for textile actuators in smart wearables. However, these three different textile-forming technologies have several advantages and disadvantages in terms of the biocompatibility and mechanical properties of textile actuators based on the interlaced density of fibres or yarns [27]. Typically, knitted textiles with the lowest interlaced density offer the highest level of comfort, elasticity (5 ~ 80%), and breathability to well conform to the contour of the human body without producing pressure spots, even when raw yarns are not stretchable. Compared with knitted fabrics, woven textiles with higher interlaced density generally exhibit greater stiffness and greater dimensional stability but lower flexibility (1 ~ 2%). The interlaced density of braided textile surpasses that of knitted fabric but falls short of that of woven fabric. Therefore, the stretchability (1 ~ 3%) of braided textiles is lower than that of knitted fabrics but better than that of woven fabrics, while their stiffness and dimensional stability show the opposite behaviour. Owing to differences in porous structures attributed to the interlaced density of textile surfaces, the breaking force often increases in the order of knitted textiles, braided textiles, and woven textiles. Compared with weaving, knitting and braiding with higher elasticity make it easier to achieve 3D textile structures for textile actuators.
Knotting technology is relatively seldom employed to design textile actuators for smart wearables, yet it shows fascinating potential. Bonding strategies are very simple and easy to implement in multilayer structures for textile actuators, yet they often pose some problems. For instance, a new interface structure between actuating materials and fabric substrates may be introduced through bonding. For coating technology, micro- or nano-particle-based actuating materials are prone to agglomerating in solvents, making it difficult to form uniform and stable dispersions. This will lead to an uneven actuating layer on the surface of the fabric substrate. In addition, simple, effective, and low-cost drying processes are required for the coating strategy. For pasting technology, durability and washability seem to be obstacles to its application in wearables. Besides, electrospinning offers the possibility of multilayered nonwoven structures for textile actuators.
Flexible Textile Actuators
Textiles have been used as structural carriers as well as actuation components for soft actuators for more than a decade. We have conducted a survey on flexible textile actuator literature with high impact factors and citations in the past five years. The actuating behaviours, types, and essential performance metrics of textile actuators are described and summarised in this section.
Actuating Behaviours
Actuating behaviour is a fundamental consideration for textile actuators used in smart wearables. When subjected to environmental stimuli, changes in the structural and material properties of textile actuators can lead to reversible linear, bending, and torsional motions, as pictured in Fig. 7. The structural formation of textile actuators has been elucidated in the preceding section, and thus, the focus will be on the changes in material properties of textile actuators induced by external stimuli. Typically, changes in material properties manifest as changes in molecular chain order, volume, and distance. Different materials can generate various property changes under the same stimulus, whereas the same material can also show different changes in properties in response to multiple stimuli. In contemporary daily life, electricity, heat, light, and humidity are ubiquitous environmental stimuli, while pressure and magnetism can exist in certain environments.
Various actuating behaviours of textile actuators with different stimuli
Theoretically, changes in molecular chain order do not involve or involve only negligible mass exchange and volume changes when subjected to external stimuli such as electricity, heat, or light. For instance, the thickness of DE membranes sandwiched between two electrodes decreases to enlarge their area in the presence of an electric field. This is due to the electrostatic Coulomb force which causes reversible contraction along the static field direction. Also, SMAs or SMPs can be reconfigured upon thermal stimulation, because the interior molecular chains and crystal/molecular networks inside can be reordered and modified. When heated, SMAs or SMPs with deformed shapes can reversibly recover to their initial shapes, indicating the occurrence of the shape-memory effect. In addition, LCEs, one of the most representative SMPs, are composed of anisotropic liquid crystal molecules. These molecules can be reversibly transformed between liquid crystalline and isotropic phases when heated. LCEs with azobenzene groups can also reconfigure their trans structure to a cis structure when exposed to ultraviolet (UV) light. This is called photoisomerization deformation.
Generally, there are two types of volume changes, namely, volume changes owing to thermal expansion or phase transition and volume changes owing to absorption or intercalation. In the former case, the actuating materials undergo thermal expansion or contraction due to density differences in the actuating state; in principle, no mass exchange occurs. Highly twisted and coiled nylon, PE, and CNTs are representative materials of this type and have been widely developed in textile actuators. Melting, vaporisation, and crystallisation are examples of phase transitions that can generate volume changes; examples include bladder textile actuators with chambers containing air or fluid. In the latter case, the absorption/desorption and the intercalation/extraction of small molecules or ions induce mass transfer between the environment and materials, which drives the materials to expand or contract in all directions. In an electrochemical environment, charged ions can be selectively intercalated into or extracted from the pore structure of materials. CNTs, CPs, MXenes, and ionic gels have been used to design these types of textile actuators. In an environment with varying humidity, small molecules can be absorbed into or desorbed from materials owing to variations in concentration and chemical affinity, resulting in asymmetric swelling. To date, many natural or synthetic hydrogels with water-swollen 3D networks have been developed to fabricate humidity-responsive textile actuators.
In electric or magnetic fields, electrostatic or magnetic effects produced by inherent conductivity or magnetism can alter the distance between actuating units. This may lead to changes in the overall architecture of textile actuators. Furthermore, a change in distance may drive the deformation of textile actuators through the expansion or contraction of actuating units. In brief, the change in distance for a textile actuator mostly occurs in the fabric structure formed by actuating fibres or yarns.
Diverse Types
Flexible textile actuators can be mainly divided into six categories based on daily environmental stimulation and some extreme environmental stimulation. These categories include electric textile actuators, thermal textile actuators, optical textile actuators, humidity textile actuators, bladder textile actuators, and magnetic textile actuators.
Electric Textile Actuators
Electric textile actuators use a class of electrical-responsive materials to convert electrical energy into mechanical energy, such as DEs, CPs, carbon-based materials, metal nanomaterials, ion gels, MXenes, SMAs, and SMPs. Electric textile actuators have three main driving modes, including electrostatic effect, electrochemical drive, and electrothermal effect. These modes are based on the various responsive mechanisms of actuating materials to electrical signals. For example, high voltages of several kilovolts commonly generate changes in the molecular chain order of DEs because of electrostatic forces, as illustrated in Fig. 8a(i) [68]. The electrostatic attraction between opposite charges yields a force exerted on the DE layer, contracting its thickness and expanding its area. A DE film is adhered between two conductive fabric electrodes to fabricate a dielectric elastomer actuator (DEA) (Fig. 8a(ii)) [1. For instance, electric textile actuators composed of DEAs, optical textile actuators with azobenzene functional groups, or magnetic textile actuators can exhibit good actuation performance, but they have received little attention. This may be caused by the unsafe and uncommon use of ultra-high voltage, UV light, or magnet fields. Moreover, thermal textile actuators with SMAs, humidity textile actuators, and electrochemical textile actuators may require a longer response time to generate actuation, but they do not require harsh actuating conditions. Besides, the bonding methods are favourable for fabricating bladder textile actuators with high output actuation performance.
Applications of Flexible Textile Actuators in Smart Wearables
Flexible textile actuators have autonomous environmental response ability, excellent textile processability, good breathability, high body compliance, long-term durability, diverse structural patterns, and a lightweight nature. They are increasingly being integrated with garments or auxiliary wearables to design smart wearables that can perceive and respond to human surroundings. Based on the execution of actuation force and deformation, smart wearables with textile actuators have great potential in various applications. In this section, we primarily comment on the applications of textile actuators over the past 5 years in wearable power suits using actuation force, comfort-adapting clothing exerting actuation deformation, and human–machine interaction.
Wearable Power Suits
Textile actuators in wearable power suits may exert various levels of actuation force to meet the power requirements for different applications. This could be achieved through diverse combinations of actuating materials and textile structures. Generally, the actuation forces that aerospace garments, healthcare wearables, and mobility-assistive exosuits exert may decrease in sequence.
Aerospace Garments
In aerospace exploration, astronauts are inevitably subjected to movement constraints against vacuum conditions during spaceflight, as well as orthostatic intolerance that occurs after spaceflight. To alleviate adverse symptoms experienced by astronauts during or after space travel, aerospace garments can supply mechanical countermeasures, promote blood circulation in the cardiovascular system, and prevent blood from accumulating in localised areas of the body. For a long time, pneumatic compression has been the predominant component of spacesuits. For example, the “Space Activity Suit” can provide a mechanical counterpressure of 222 mmHg against space vacuum, and the “American antigravity suit” can provide a pressure of 78 mmHg throughout the body for orthostatic intolerance [101, 102]. However, these traditional spacesuits have some deficiencies in terms of their large size, heavy weight, inconvenient mobility, and cumbersome donning or doffing.
With the advancement of textile actuator technology, several spacesuit prototypes with SMA textile actuators have been reported. In 2014, Newman’s team first integrated SMA springlike coils into elastic fabric to fabricate a lightweight spacesuit, named MIT Knit BioSuit™. This is the first prototype of a compression spacesuit designed with SMA actuators to provide real-time pressure status and thermal regulation during planetary exploration activities. According to theoretical calculations, a spacesuit type designed with SMA textile actuators can provide dynamic actuation pressure of 16 ~ 105 mmHg at the ankle to meet the pressure requirements of orthostatic intolerance after spaceflight [103].
Moreover, Fig. 11a presents a dynamic and conformal spacesuit with knitted SMA textile actuators for astronauts returning from space exploration [17]. At low pressure of < 22 mmHg and room temperature, the spacesuit exhibits delayed and controlled actuation, allowing astronauts to alleviate discomfort prior to landing activities. As the temperature momentarily increases, the spacesuit is driven to a high pressure of > 55 mmHg until it is taken off and cooled. This makes it easier for astronauts to don and doff. As seen in Fig. 11b, the design of a spacesuit matches the geometric shape of the human body by adjusting the unit tension of passive and active textiles. Upon heat stimulation, the spacesuit exerts a dynamic pressure of 6.3 ~ 43.6 mmHg on different parts of the body (Fig. 11c). Additionally, the spacesuit can reach a relatively balanced temperature of ~ 30 °C within 30 min without causing body discomfort.
a Aerospace garment using knitted SMA textile actuators for astronauts after spaceflight. b A design strategy based on unit tension. c Dynamic pressure and skin temperature upon stimulation; Reproduced with permission from Ref. [17], Copyright 2022, Wiley
Healthcare Wearables
Compression therapy (15 ~ 50 mmHg) is considered an effective and non-invasive way to treat many diseases. For instance, it can provide a calming effect for patients with sensory processing disorders and improve blood circulation and lymphatic drainage for patients with venous and lymphatic diseases [103]. Wearable compression therapy devices composed of textile actuators can provide the dynamic and customised pressure required for therapy. For instance, a compression vest composed of SMAs can produce a predicted compression of up to 52.5 mmHg on a child’s torso when heated [104]. When a power of 43.8 W is applied, the vest creates an actual pressure of up to 37.6 mmHg, meeting the pressure required for compression treatment. Besides, a compression garment composed of bladder textile actuators is developed to exert a biomechanical force of 5 ~ 25 kPa on the wearer’s lower limb within 25 s for compression therapy [105].
Furthermore, soft exosuits made of textile actuators have been shown to be effective in routine gait rehabilitation training for patients with limb paralysis after stroke [106]. For instance, a soft exosuit with a bladder textile actuator is designed for knee rehabilitation, as shown in Fig. 12a [107]. During the swing phase of gait training, the knee exosuit can increase the knee joint torque by 25% for patients, improving the recovery effect of the femur.
a Soft exosuit with a bladder textile actuator for knee rehabilitation; Reproduced with permission from Ref. [107], Copyright 2018, Frontiers Media Sa. b Soft prosthesis using a magnetic textile actuator; Reproduced with permission from Ref. [16], Copyright 2023, Wiley–VCH Verlag Gmbh. c Massage sleeve with knitted bladder textile actuators; Reproduced with permission from Ref. [49], Copyright 2023, Wiley–VCH Verlag Gmbh. d Smart emergency tourniquet using a humidity textile actuator; Reproduced with permission from Ref. [108], Copyright 2022, Amer Chemical Soc
Besides, several healthcare wearables with textile actuators have been developed. As presented in Fig. 12b, a soft prosthesis with a magnetic textile actuator is expected to be used in genioplasty [16]. Additionally, a massage sleeve with trilayer knitted bladder textile actuators is developed to simulate the acupuncture treatment of therapist (Fig. 12c) [49]. As supply pressure increases from 20 to 50 kPa, the massage intensity that the sleeve exerts increases by more than twofold, which is comparable to bulky massage devices. Uncommonly, a humidity textile actuator is woven into a smart emergency tourniquet (Fig. 12d) [108]. When a wound bleeds, the tourniquet can be curled to stop bleeding and recover to its normal shape after the wound stops bleeding.
Mobility-Assistive Exosuits
Mobility-assistive exosuits with textile actuators can exert extra mechanical forces on the human body to reduce muscle fatigue and protect skeleton joints from injury during movements like heavy lifting, gras**, walking, and running. In general, these exosuits exert certain actuating forces when stimulated to support the activities of the upper and lower extremities. Although these exosuits do not provide enough power to completely replace human activities, they have been demonstrated to significantly reduce the body’s metabolism in daily activities.
Figure 13a presents a portable exosuit composed of a functional waist belt, two thigh warps, electric motors, and Bowden cable sensors, weighing up to 5 kg [109]. The exosuit applies an actuation extension torque around the wearer’s hip joint, resulting in a 9.3% and 4.0% decrease in metabolic rate during walking and running, respectively. Essentially, the exosuit uses an electric motor as the actuation component instead of a textile actuator. To reduce the weight and size of mobility-assistive exosuits, bladder textile actuators are increasingly being employed as actuation components. As exhibited in Fig. 13b, an arm-lift-assistive exosuit is sewn with six bladder textile actuators [15]. Upon a pressure stimulation of 90 kPa, the actuators provide lifting torque to the wearer’s shoulder to assist in raising the arm. A similar arm-lift-assistive exosuit is developed in Fig. 13c [110]. Six bladder textile actuators are firmly affixed to the garment beneath the shoulder joint, providing lifting torque to elevate the arm when applying an inflation force of ~ 25 N. As a result, the wearer’s upper arm raises within 38 s after pressing the set button and descends within 87 s after pressing the reset button.
a Assistive exosuit for walking and running; Reproduced with permission from Ref. [109], Copyright 2019, Amer Assoc Advancement Science. b Arm-lift-assistive exosuit with bladder textile actuators; Reproduced with permission from Ref. [15], Copyright 2022, Amer Assoc Advancement Science. c Arm-lift-assistive exosuit with bladder textile actuators; Reproduced with permission from Ref. [110], Copyright 2022, Natl Acad Sciences
Furthermore, several mobility-assistive exosuits are designed with thermal textile actuators composed of SMAs. For instance, Fig. 14a builds a highly stretchable and lightweight household sleeve knotted with SMA wires, which generates a contract force of 32.3 N under joule heat [25]. The assistive sleeve with excellent wearability weighs 30.7 g and can be easily worn on the arm in daily life to help carry heavy luggage. Upon a low voltage of 6.4 V, the sleeve is activated to complete the entire process of lifting and releasing heavy luggage within 13 s. In addition, an ankle-assistive exosuit is fabricated with SMA textile actuators (Fig. 14b) [111]. The exosuit contracts and provides a rotational torque of 100 N cm to the ankle in 0.5 s when activated with a current of 0.8 A. Figure 14c demonstrates an assistive wrist sleeve with knitted SMA textile actuators based on anatomical and dynamic design. The sleeve can produce a blocking force of up to 308 N m upon stimulation [71]. The sleeve contracts to apply stretching force to the wrist when heated, thereby lifting the wearer’s hand.
a Household sleeve with knotted SMA textile actuators for arm; Reproduced with permission from Ref. [25], Copyright 2022, Wiley–VCH Verlag Gmbh. b Assistive exosuit with SMA textile actuators for ankle; Reproduced with permission from Ref. [111], Copyright 2020, IOP Publishing Ltd. c Assistive sleeve with knitted SMA yarns for wrist; Reproduced with permission from Ref. [71], Copyright 2021, Wiley
Comfort-Adapting Clothing
Textile actuators in comfort-adapting clothing possess the ability to automatically deform in changing environments to improve the wearer’s comfort and convenience. Unlike wearable power suits that focus on large actuation forces, conform-adapting clothing pays more attention to actuation deformation. Two typical representatives of conform-adapting clothing are breathable clothing for thermal management and self-fitting fashion garments.
Breathable Clothing
In the implementation of thermal management regulations, ventilating holes are often designed on the breathable clothing. These holes can automatically open or close in response to sweat and heat transfer generated by human skin, thus regulating the clothing’s breathability. Among the six types of textile actuators, humidity and thermal textile actuators show great potential in manufacturing breathable clothing.
Humidity textile actuators with safe stimulation are well suited for breathable garments. In Fig. 15a, a PPy/PET film engraved with ordered fan-shaped openings is pasted on the fabric substrate of clothing [112]. During exercise, the openings on the clothing promptly switch from closed to open states, driven by sweat, to reduce skin heat and perspiration. Additionally, sodium alginate (SA)/GO/PU/PDMS films with customised shapes are attached to the fabric substrate to design an intelligent T-shirt (Fig. 15b) [85]. Activated by sweat, the actuators on the T-shirt flex, wicking away sweat and cooling down, thereby making the wearer feel more comfortable. Figure 15c shows another piece of breathable clothing with a series of semicircular stomas carved on a woven wool fabric [44]. These carved holes close in dry and cold conditions to keep the wearer warm, while they open in hot and sweaty conditions to evaporate sweat and lower the skin temperature. Also, woven viscose/PET textile actuators are developed for a breathable garment with array flappers (Fig. 15d) [18]. The flappers roll up to form small holes in a wet environment and unfold to close the holes in a dry environment, thereby managing the wearer’s sweat and heat loss. Besides, moisture-responsive flaps made of nylon-Ag heterostructure are pasted on the fabric substrate to form a breathable garment (Fig. 15e) [23]. The flaps open to expel sweat vapour after the wearer starts exercising for ~ 5 s and returns to a closed state after the wearer stops.
a Moisture-sensitive clothing with a PPy/PET film; Reproduced with permission from Ref. [112], Copyright 2021, Amer Chemical Soc. b Sweat response clothing with SA/GO/PU/PDMS films; Reproduced with permission from Ref. [85], Copyright 2023, Amer Chemical Soc. c Humidity-responsive clothing with a wool textile actuator; Reproduced with permission from Ref. [44], Copyright 2022, IOP Publishing Ltd. d Breathable garment with viscose/PET textile actuators; Reproduced with permission from Ref. [18], Copyright 2023, Spring Nature. e Breathable garment with humidity textile actuators composed of nylon-Ag; Reproduced with permission from Ref. [23], Copyright 2021, Amer Assoc Advancement Science
Moreover, thermal textile actuators have also been used to design breathable clothing. For instance, Fig. 16a depicts a breathable shirt composed of an LCE textile actuator with pores [84]. The textile actuator contracts to open the pores after 10 min in a warm environment and expands to close the pores after 5 min at room temperature, thereby effectively regulating skin sweat and temperature. Figure 16b also shows a ventilated garment covered with rectangular thermal textile actuators composed of metalized PE films [113]. The actuators open in a high-temperature environment to enhance body heat dissipation and return to their initial flat shapes in a low-temperature environment to prevent heat dissipation, thus automatically adjusting the temperature change of 2.2 ~ 2.6 °C to adapt to the ambient temperature change of 15 ~ 35 °C. Moreover, Fig. 16c indicates a photothermally driven ventilating garment made of 46 pieces of biaxially oriented PP (BOPP) petals [63]. Under an outdoor sunlight intensity of 0.1 W cm−2, the petals curl like a flower, opening ventilating channels to evacuate skin heat and perspiration. Then, the petals recover to their original shapes in the darkness, covering the stomatal channels.
a Breathable garment with a thermal textile actuator; Reproduced with permission from Ref. [84], Copyright 2019, Amer Chemical Soc. b Thermo-regulating T-shirt with thermal textile actuators; Reproduced with permission from Ref. [113], Copyright 2022, Elsevier. c Ventilated garment with photothermal actuators; Reproduced with permission from Ref. [63], Copyright 2020, Wiley
Self-fitting Fashion
Self-fitting fashion clothing with textile actuators can dynamically deform to adjust shape and size in response to varied environmental gradients. This can automatically improve body fit, aesthetics, and ease of donning or doffing. Humidity textile actuators composed of natural materials and thermal textile actuators made of SMAs have been designed as prototypes for self-fitting clothing.
Figure 17a demonstrates the application of a humidity textile actuator made of woven silk yarns in self-fitting sleeves [114]. The sleeves shrink by ~ 45% when exposed to moisture and then return to their original length after drying, making them suitable for automatic shape deformation of clothing under wet or dry conditions. Figure 17b presents a similar self-adapting sleeve made of a woven chitosan and CNT fabric [94]. In high humidity, the sleeve generates a large contraction within 39 s, thereby shortening the length of the sleeve. Also, a cotton-hydrogel textile actuator is rolled into a cylindrical shape to form a toy “dress” (Fig. 17c) [115]. Initially, the “dress” does not fit the size of the toy’s arm. After applying water mist, the “dress” generates super contractions to fit the toy’s arm.
a Self-fitting sleeve with a woven silk textile; Reproduced with permission from Ref. [114], Copyright 2019, Wiley–VCH Verlag Gmbh. b Self-fitting sleeve with a woven chitosan and CNT fabric; Reproduced with permission from Ref. [94], Copyright 2021, Wiley–VCH Verlag Gmbh. c Self-adapting dress using homochiral cotton-hydrogel; Reproduced with permission from Ref. [115], Copyright 2022, Springer Nature. d Self-fitting sleeve with a knitted SMA textile actuator; Reproduced with permission from Ref. [83], Copyright 2019, Wiley. e Dynamic fashion dress with bladder textile actuators; Reproduced with permission from Ref. [98], Copyright 2020, Wiley. f Wearable light-driven ornaments; Reproduced with permission from Ref. [116], Copyright 2021, Amer Chemical Soc
Furthermore, SMA wires are knitted into a thermal textile actuator and used in a self-fitting wrist sleeve, as outlined in Fig. 17d [83]. The self-fitting process of the sleeve involves five states. They are pre-donned martensite state with an oversized shape, deformed martensite state with pulling force, relaxed martensite state, fitted-partially austenite state with heat contraction, and tight-partially austenite state with conforming to wrist curvature. Besides, textile actuators are also used to design dynamic and fashionable clothing. For example, a dynamic fashion dress composed of bladder textile actuators is exhibited in Fig. 17e [98]. Also, a wearable artificial flower and an eagle made of MXene/polyethylene films are integrated on the fabric substrates of shirts (Fig. 17f) [116]. Under sunlight, the petals of flower bow down to blossom, and the eagle spreads its wings, endowing the clothing with fashionable and aesthetic function.
Human–Machine Interaction
Human–machine interaction refers to the exchange of information between humans and machines through Internet technologies. Textile actuators in human–machine interaction can perceive external stimuli and provide interactive responses to the wearers, which have garnered growing attention. Wearable manipulative robots and haptic feedback gloves are two classic applications of textile actuators in human–machine interaction.
Wearable Manipulative Robots
Wearable manipulative robots with textile actuators are developed to provide safer, smarter, and more mechanised functions to replace humans in hazardous, heavy, complex, or tedious activities. As shown in Fig. 18a, a claw gripper with knitted bladder textile actuators is connected to a robotic arm. The arm can be actuated by a pressure of 55 kPa to grasp and rotate within 12 s [48]. Furthermore, Fig. 18b proposes a wearable gripper with electrothermal actuator films made of graphite paper and polyimide, which can generate 250° bending within 10 s at a voltage of 6 V [117]. The gripper worn on a robotic hand has outstanding manoeuvrability and can hold a plastic foam sheet of 122 mg, moving forth and back on command.
a Motions of a multi-chamber claw gripper with knitted bladder textile actuators; Reproduced with permission from Ref. [48], Copyright 2023, Wiley–VCH Verlag Gmbh. b Grip** process of a smart robot with electrothermal actuators; Reproduced with permission from Ref. [117], Copyright 2020, Elsevier Science Sa. c Virtual reality glove with SMA textile actuators; Reproduced with permission from Ref. [81], Copyright 2022, Korean Fibre Soc. d Output force signals and haptic feedback of an actuating glove based on hand gestures; Reproduced with permission from Ref. [19], Copyright 2023, Wiley–VCH Verlag Gmbh
Haptic Feedback Gloves
Haptic feedback gloves with textile actuators can sense and provide tactile feedback vibrations for the wearer to interact with their surroundings. For instance, Fig. 18c validates an interactive system of a virtual reality glove with stitched SMA textile actuators. By touching the contour of an object through three various actions, a dynamic feedback force of 0.8 ~ 0.4 N can be provided to the wearer at the sensing point within 5.2 ~ 20 s [81]. In Fig. 18d, an air-permeable vibrotactile glove composed of electric textile actuators can convert electrical signals into mechanical vibrations through electrostatic effects. The glove can generate a dynamic output force of 0.02 ~ 0.06 N [19]. The actuator glove worn on the hand can perceive the vibration patterns of gestured words or sentences and transmit them from a sensor glove within 28 ~ 50 ms. After being seamlessly integrated with an ordinary garment, the actuator glove utilises coding rules to deliver controlled vibration signal feedback in real time by flexing fingers at different positions on the wearer’s body.
Design Challenges for Next-Generation Smart Wearables
Table 2 tabulates the application data of textile actuators in smart wearables. Several clear trends can be inferred. For example, wearable power suits, including aerospace garments, healthcare wearables, and mobility-assistive exosuits, perform with high actuation forces. They are primarily composed of bladder textile actuators and SMA thermal textile actuators. Furthermore, comfort-adapting clothing often employs humidity or thermal textile actuators to exert high actuation deformation, including breathable clothing and self-fitting fashion clothing. Unlike the applications in comfort-adapting clothing, textile actuators are always paired with other electronic devices in human–machine interaction. In line with these results, the five challenges associated with next-generation smart wearables composed of textile actuators are summarised in this section.
Safety
Although some textile actuators in smart wearables exhibit good output actuation performance, they are triggered by hazardous or uncommon external stimuli. For example, electric textile actuators composed of DEAs always require several kilovolts to generate actuation. Additionally, thermal textile actuators are typically heated beyond 45 °C to achieve thermal actuation. Besides, the pressure stimulation required to generate a high actuation force for bladder textile actuators normally exceeds 50 kPa. Obviously, these stimuli may bring risks or cause discomfort to the human body; thus, the application of textile actuators in smart wearables must focus on safety. In addition, electricity is often introduced into wearable power suits and human–machine interaction, which may cause harm to the wearer in the event of electrical leakage.
Human Body Fit
Most applications of textile actuators in smart wearables involve tiny or local prototypes, such as compression, assistive or deformation sleeves, assistive or haptic feedback gloves, and breathable clothing with holes on the back. Additionally, due to shape deformation or size changes caused by the actuation process of textile actuators, the pre-determined sizes of smart wearables may become inappropriate for the human body. Thus, the implementation and commercialization of textile actuators in actual smart wearables require solving the customisation problem of fitting the human body shape.
Some textile actuators composed of actuating fibres or yarns, including nylon, PET, and natural fibres, exhibit remarkable compatibility and friendliness to human skin. However, they do not possess strong actuation capabilities and cannot effectively serve as autonomous actuation components for smart wearables. To improve the actuation performance of textile actuators, supplementary actuating materials, such as SMA wires, CNT yarns, and micro- or nanoparticles, are often doped. Nevertheless, these actuating materials are not common components of clothing textiles, and their incorporation may reduce the skin affinity of smart wearables. These may lead to a reduction in body compatibility, skin friendliness, and breathability. In addition, these actuating materials may also introduce new structural interfaces between smart wearables and human skin. The interfaces may not only introduce discomfort or potentially severe skin damage during the actuation process but also unstable actuation performance.
Flexible Textile Actuator Technology
Thermal textile actuators and bladder textile actuators are widely used in smart wearables, accounting for nearly 33.3% and 30.8%, respectively. Nevertheless, most bladder textile actuators require connections to rigid power sources and control boards to provide the necessary pressure, which are not perfect candidate textile actuators for smart wearables. Most thermal textile actuators in smart wearables are composed of self-actuating SMA wires, which are suitable for integration into various kinds of smart wearables through textile-forming technologies. However, they must be manually reprogrammed to be driven repeatedly. Moreover, humidity textile actuators are popular for comfort-adapting clothing, but their low output force and slow response speed are troublesome issues for other types of smart wearables.
Actuating materials are essential for the development of textile actuators. However, potential difficulties may arise when using some actuating materials. For instance, the unsafety of activated high-voltage for DEs, the operational barriers to pairing electrochemically responsive materials with electrolyte media, and the harmful effects of UV radiation on human body. Thus, it is likely that only electrothermally or NIR photothermally actuating materials are suitable for develo** electrically or photothermally thermal textile actuators for smart wearables. Thanks to the rarity of magnetic stimulation, magnetic-responsive materials for smart wearables have seldom been reported. Therefore, actuating materials confront various challenges in the development of textile actuators for smart wearables, like safety concerns, operational difficulties, poor output performance, etc.
The integration of fabrics and actuating materials relies heavily on textile structure formation technologies. They have a significant impact on the structural morphology, mechanical anisotropy, shape, size, and actuating behaviour of textile actuators in smart wearables. However, these textile-forming technologies possess their own pros and cons. For example, knitted textile actuators exhibit high flexibility but a tendency to slide at intersections during the actuation process. This may lead to suboptimal force transfer and uneven deformation. Besides, woven textile actuators show remarkable structural stability, but their maximum actuating deformation potential cannot be fully realized. Compared with knitted or woven textile actuators, braided textile actuators exhibit a more stable fabric structure but demonstrate a smaller actuation strain. In contrast, knotted textile actuators generally provide superior actuation performance, but the knotting process involves a greater degree of manual operation. Bonding techniques have been proven to effectively integrate actuating materials with the existing fabrics, but they introduce challenges for the interfaces between actuating materials and fabric substrates.
Output Performance
Textile actuators in smart wearables yield different output performance metrics. Nevertheless, there are several limitations in terms of the output performance, including inadequate output force, long response time, improper strain, and uncertain durability. For instance, the maximum output force of a thermal textile actuator using SMAs can reach 67 mmHg. The force is suitable for orthostatic intolerance (40 ~ 80 mmHg) after spaceflight but significantly lower than mechanical counterpressure of 222 mmHg during space extravehicular activities. Besides, the response speed of textile actuators in breathable clothing is not fast enough; often, tens of minutes are needed to reduce the skin temperature by 1 ~ 3 °C. In addition, the bending strain or contraction of textile actuators in mobility-assistive wearables is diverse, ranging from 30° to 163° or from 2 to 30%. Thus, these existing output performances pose challenges to the application of textile actuators in smart wearables.
Other performance indicators, including washability, weight, durability, and stability, are also important for smart wearables composed of textile actuators. However, these indicators are seldom discussed in the literature due to a lack of attention. For instance, significant advancements have been made in breathable clothing, but ventilation holes are always designed on the back of garments, which may compromise their durability during machine washing. Additionally, the shape, size, or weight of bladder textile actuators with inflated chambers is often large, which impedes the feasibility of develo** portable or lightweight smart wearables.
Integrated Microelectronic System
Textile actuators in comfort-adapting clothing are typically not paired with other microelectronics. They can sense the wearer’s surrounding environment or skin status, automatically adjust skin temperature and perspiration, or automatically alter garment shapes to fit the wearer’s body. However, textile actuators in other smart wearables cannot autonomously execute the entire process without involving other microelectronic components, such as sensors, power supplies, control boards, and interconnections. As a result, the integration of these microelectronic components with textile technology is also urgent and important.
Sensors can detect various physical, chemical, or biological information and transform the information into electrical or digital signal output. They play a vital role in establishing the connection between human body and smart wearables, as well as facilitating communication between smart wearables and the surrounding environment [129]. For instance, several simple sensors have been developed to control the actuation process of electric or bladder textile actuators in smart wearables. Moreover, compatible combinations of sensors and actuators have been designed to achieve precise and efficient feedback and control, improving the human–machine interaction capability of smart wearables. Nevertheless, the key challenge for integrating sensors into smart wearables lies in the high performance and reliable stability of textile sensors.
Most smart wearables rely on electrical energy to power textile actuators through electricity, pressure, joule heat, or NIR radiation stimulation. Hence, energy power systems, including textile-based generators and energy storage devices, are being rapidly incorporated into textiles. Textile-based generators can collect various forms of energy from human body or ambient environment, like frictional energy, humidity, wind energy, or solar energy, and convert the energy into electrical energy. For example, many triboelectric generators have been integrated with commercialised fabrics to harvest energy from human motion and wind via contact-electrification and electronic induction [130, 131]. Recently, emerging moist electric generators that harvest energy from atmospheric moisture have attracted great attention in self-powered textiles [132]. Besides, thermoelectric generators that harvest energy from waste heat have been combined with fabric platforms [133]. Nevertheless, these textile-based generators often exhibit microwatt levels and instantaneous low-grade energy, which cannot meet the energy requirements of smart wearables. As a result, energy storage devices are expected to effectively store the energy produced by these textile-based generators to form textile-based power management systems. Textile-based supercapacitors, Li-ion batteries, Zn-ion capacitors, and Zn-ion batteries have become promising energy storage devices [134, 135]. However, the power, capacity, durability, and compliance of these textile self-powered systems still need to be further improved to satisfy the demands of smart wearables.
Perspectives
The employment of flexible textile actuators in smart wearables exemplifies some discernible trends. Over the past few decades, various theories and technologies have been developed for soft actuators in soft robotic applications. Many of these strategies may be applied to textile actuators for smart wearables. Nevertheless, there are inherent disparities between textile actuators and soft actuators in the integration of structural platforms and actuating materials. To better apply textile actuators in smart wearables, solutions or improvements to the above challenges should be gradually explored in the future. The following insightful perspectives are proposed as follows:
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(1)
Innovative, flexible textile-forming technology merits exploration. The integration of actuating materials and fabric structures includes, but is not limited to, actuating materials and textile-forming methods. These common textile-forming technologies often create 2D fabric structures for textile actuators, leading to insufficient output performance. A composite textile-forming technique may contribute to a novel 3D textile structure for textile actuators in smart wearables, improving output performance and human body fit. Textile machines, textile structures, actuating unit connections, and integrated strategies of other microelectronics should also be considered. Furthermore, twisting and coil insertion into fibres, yarns, or wires during the textile-forming process may greatly improve the output performance of textile actuators. In addition, surface modification may contribute to the immobilisation of micro- or nanoparticle-based actuating materials on the surface of fibrous substrates and the stability of output performance.
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(2)
Endeavours can be made to develop novel actuating fibres or yarns. Although non-fibrous actuating materials can be bonded with the existing fabrics to form textile actuators, actuating fibres, or yarns as textile units are more attractive. The actuating fibres or yarns may have a balanced compromise in terms of stretchability, skin affinity, and actuation performance. The introduction of new micro- or nanoparticle-based actuating materials into fibre substrates prior to spinning is regarded as an innovative approach for designing high-performance smart wearables. For example, modified LCEs with thermal memory behaviour are expected to be applied to smart wearables owing to their superior skin affinity and photo-responsive features. Besides, fresh natural fibres or polymer fibres with actuating properties, excellent skin affinity, and weavability are worth exploring. Furthermore, when applied to smart wearables, such as sleeves and back vents, the shape and size features often limit the output performance of textile actuators. To improve the output performance, the size and proportion of textile actuators in actual smart wearables should be customised from small to large areas to fit the human body shape.
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(3)
The seamless integration of other textile-based microelectronics into smart wearables remains an ongoing concern. Typically, sensors and power supplies are two important microelectronics used in smart wearables. Once loaded onto textile platforms, the raw performance of sensors and power supplies may be significantly degraded due to the stretchability of fabric substrates. To improve performance, attention should be given to the integration strategies of microelectronics and fabrics. Many strategies, such as textile architectures, sensing mechanisms, and textile electrodes, have been proposed for textile-based sensors. For power supplies, some green textile generators have been paired with textile energy storage devices to achieve energy management for smart wearables. However, these devices are still unable to provide sufficient energy for smart wearables in real time due to the low amount of output energy and extended charging time. Connecting multiple textile-based generators and ion batteries in series can meet the energy needs of smart wearables over short periods of time. Increasing the size but controlling the weight of power supplies and develo** new electrode or electrolyte materials with high energy are also effective methods.
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(4)
Safety protection systems need to be implemented in smart wearables. Recently, high-intensity stimuli such as UV radiation, temperatures greater than 45 °C, and input voltages beyond 36 V have been applied to textile actuators. For these unsafe stimuli, robust electrical and thermal insulation is required when applied to smart wearables. In addition, reducing the intensity of stimulation can achieve safe stimulation. Accordingly, the actuation behaviour of textile actuators in smart wearables should be reassessed after adding supplementary protective components or reducing the stimulus intensity.
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(5)
Smart wearables with long-term output performance and washability are in high demand. Daily conform-adapting clothing that requires laundering can be worn every day, whereas other forms of smart wearables are not disposable, even if they do not require frequent laundering. However, to date, little research has been conducted into these aspects of smart wearables. When evaluating the longevity and launderability potential of smart wearables, a special attention should be given to the damage, waterproofing, performance degradation, positional stability, and structural stability of textile actuators. By consistently prioritising the durability and washability of smart wearables, the prospects for large-scale commercialization may be significantly enhanced.
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Acknowledgements
The authors would like to acknowledge The Hong Kong Polytechnic University for funding support (Project No. G-YWA2, 1-YXAK, and 1-WZ1Y) of this work.
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Fang, C., Xu, B., Li, M. et al. Advanced Design of Fibrous Flexible Actuators for Smart Wearable Applications. Adv. Fiber Mater. 6, 622–657 (2024). https://doi.org/10.1007/s42765-024-00386-9
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DOI: https://doi.org/10.1007/s42765-024-00386-9