Recent Advances in Smart Textiles: Composition and Characteristics

Introduction

The smart textile market is gradually becoming a new engine for 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 by 2026, with a compound annual growth rate of 28.7%. In the next decade, during the Internet of Things era, smart textiles are likely to change human life alongside artificial intelligence, human-computer interfaces, and cloud technology. This publication has set up a special column on recent advances in smart textiles, aiming to provide readers with a deeper understanding of smart textiles through comprehensive introductions.

Continuous and stable operation is a fundamental requirement for smart textile systems, and the power supply issue poses a significant challenge.To address the power supply problem of electronic textile devices, current solutions mainly focus on developing energy storage and energy harvesting textile devices.The main types of textile-based flexible energy storage devices are shown in Figure 1.

Figure 1 Textile-based Flexible Energy Storage Devices

Energy Storage Textile Devices

Flexible batteries and supercapacitors are currently the two most widely studied types of flexible energy storage devices, where lithium-ion batteries have high energy density and operating voltage, but they are prone to safety issues such as electrode material shedding, electrolyte leakage, and short circuits under external forces; supercapacitors achieve high power density and excellent safety through a combination of non-Faradaic and fast reversible Faradaic reactions. Among these, flexible electrodes are crucial components of both types of flexible energy storage devices. Currently, the substrates for flexible electrodes mainly include metallic materials, fabrics, polymer films, nano-carbon, and carbon nanotube films. Among them, textile materials have natural flexibility, excellent mechanical properties, and a large specific surface area, allowing them to withstand multi-directional bending and folding, and can be cut into any size and shape, thus being considered an ideal substrate for flexible energy storage devices.

Currently, textile-based supercapacitors exhibit characteristics such as lightweight, high power density, long cycle life, good thermal stability, and environmental stability, but still face challenges such as low volumetric capacitance, low energy density, significant self-voltage drop, and difficulties in packaging. To achieve commercialization and large-scale application of textile-based flexible energy storage devices, future research and development goals and focuses should include:

(1) Selecting suitable packaging materials and processes to maximize the flexibility and electrochemical performance of electrode materials, ensuring the durability of flexible energy storage devices;

(2) Researching issues related to the interface between electrode materials and textile substrates, as well as the flow of liquid electrolytes, to further improve electrochemical stability and eliminate safety hazards during the use of flexible energy storage devices;

(3) Evaluating and comparing the flexibility and wearable-related performance of flexible energy storage devices to lay a foundation for commercialization and standardization.

Energy Harvesting Textile Devices

Energy harvesting technology is a green and sustainable technology that converts environmental energy (solar, wind, thermal, mechanical, electromagnetic, etc.) into electrical energy, providing ubiquitous, environmentally friendly, and sustainable energy solutions for wearable electronic products in the Internet of Things era. Various forms of energy that can be converted into electrical energy exist in the human body and its surrounding environment, including biomechanical motion energy, body heat, biochemical energy, and solar energy (Figure 2), enabling power generation through the integration of textiles with energy generation.

Figure 2 Schematic Diagram of Energy Harvesting Textile Devices

Nano-generators, as a new type of self-powered device, combine flexible textile materials with nano-generators to obtain textile-based nano-generators, preserving the basic characteristics of textiles while adding self-powering functions of nano-generators, making it one of the hot research directions. Currently, four types of generators based on piezoelectric, triboelectric, thermoelectric, and photovoltaic principles have been developed.

Textile-based Triboelectric Nano-generator

The essence of textile-based triboelectric generation is the physical contact between two different materials with different electron affinities, generating opposite static charges on the contact surface. Gaps or disturbances created by external mechanical forces can establish a potential difference between the two charged surfaces, thus generating voltage and polarization-induced current. Converting environmental mechanical motion into electrical energy can serve as both sustainable energy and as a self-powered active sensor. Currently, textile triboelectric nano-generators (TENGs) manufacturing technologies mainly include layer stacking, yarn crossing, and 3D printing. The triboelectric materials used include polydimethylsiloxane (PDMS), polyester (PET), nylon 6 (PA6), and polytetrafluoroethylene (PTFE), while electrode materials include metals like copper, silver, nickel, or carbon-based textile materials, with power densities reaching up to 2 W/cm². However, several challenges limit the development of TENGs: (1) Packaging: Moisture or water in everyday environments significantly hinders the output power of textile TENGs. Systematic research on related hydrophobic materials and existing waterproof yarns or fabrics, as well as a deeper understanding of the manufacturing technologies involved, is crucial for achieving long-term reliability; (2) Mechanical durability: The adhesion between fiber/fabric electrodes and active triboelectric materials is another bottleneck for achieving long-term mechanical durability. Research should focus on the reasonable selection of conductive materials and corresponding interface design to ensure that the devices can withstand the accompanying deformation during daily use; (3) Mass production: Considering that textile TENGs consist of multiple components and require layered assembly, existing manual, laboratory-scale, centimeter-length weaving methods have limited potential for scalable manufacturing. Therefore, it is necessary to redesign functional fibers to adapt to industrial weaving/knitting machines or develop corresponding specialized weaving/knitting equipment, but currently, there is little research focusing on the latter.

Textile-based Piezoelectric Nano-generator

The essence of textile-based piezoelectric generation lies in the movement of internal positive and negative charge centers in certain types of materials in response to applied mechanical stress, thereby generating an internal electric field. Integrating piezoelectric materials with textiles can be used for harvesting mechanical energy from human motion. Since the discovery of the direct piezoelectric effect in 1880, a large number of piezoelectric materials have been reported, including organic materials like polyvinylidene fluoride (PVDF), polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE), and inorganic materials like zinc oxide (ZnO), lead zirconate titanate (PZT), and barium titanate (BaTiO₃). Depending on the device structure and preparation process, textile piezoelectric devices can be realized through multilayer stacking and yarn crossing methods. In addition to the commonly used electrospinning process, there has also been research focusing on using thermal stretching spinning processes to prepare filaments with piezoelectric effects. However, the energy efficiency and material preparation of such piezoelectric textile devices still need improvement, and the recently adopted triboelectric-piezoelectric hybrid energy harvesting smart textile devices may be an effective way to address this challenge.

Textile-based Thermoelectric Nano-generator

Thermal energy is the most common energy source in daily life. In the wearable field, smart textiles can harvest energy from body heat. Compared to intermittent biomechanical energy that requires body movement, body heat is continuously present in the human body even at rest. An adult can release approximately 100 to 525 W of body heat each day. Two working mechanisms can be used to collect human thermal energy, including utilizing time temperature differences and spatial temperature differences through the thermoelectric effect. Although the surface temperature of the human body does not vary much over time (staying around 33.5 ℃), the spatial temperature difference between adults and the environment can generate thermal flows of up to 10 mW/cm², where the thermoelectric effect plays a significant role. Textile thermoelectric devices typically rely on two structures: the textile substrate and the yarn as the building block. Constructing thermoelectric devices on textile substrates is an effective way to collect human thermal energy. Textile materials not only exhibit good compliance, effectively contacting the curved skin to absorb heat, but also have low thermal conductivity. Textile-based thermoelectric units show many attractive features due to insulation and relative compliance with human skin. However, this configuration requires coating/printing/sewing TE arms onto the textile substrate, which largely compromises the breathability of smart textiles. Using TE yarns as building units to construct thermoelectric textiles through weaving or knitting is a better strategy to address the aforementioned issues, where the design of yarn-type TE units is a crucial determinant of the thermoelectric effect of textile devices.

Textile-based Photovoltaic Nano-generator

Solar cells can directly convert solar energy into electrical energy and are currently the most mature energy harvesting devices. Flexible textile-based solar cells can be integrated directly with textile fabrics using textile technology, truly achieving one-piece formation, thus providing optimal comfort and functionality. The conversion efficiency of flexible textile-based solar cells ranges from 0.1% to 5%. In comparison, silicon-based solar cells have much higher efficiency (10% to 20%). The methods for manufacturing flexible textile-based solar cells are generally divided into two types: one is to directly prepare fibrous solar cells, and then weave them using textile technology to create textile-structured solar cells; the other is to use textile materials as substrates to directly produce fabric solar cells. The preparation process of fabric-based planar solar cells is relatively mature, and many processes can integrate solar cells into textile coatings. Currently, researchers also face the challenge of applying micro-scale wafer thin layers that make up solar cells (bottom electrodes, photovoltaic layers, and top electrodes) onto rough fabrics. Compared to planar solar cells, flexible fibrous solar cells break through the limitations of the substrate, featuring lightweight and bendable characteristics, making them more suitable for wearable electronic devices. However, flexible fibrous solar cells still require technological breakthroughs to be woven using traditional textile techniques such as weaving or knitting.

In recent years, an increasing number of scholars have begun to focus on wireless radio frequency energy harvesting technology. The antennas of energy harvesting devices receive electromagnetic energy through electromagnetic coupling technology and generate alternating current (AC) within the antennas; rectifiers convert AC into direct current (DC) for direct use by sensing nodes or for storage in supercapacitors. Energy harvesting antennas require wide bandwidth and high gain characteristics. Although the output power of fabric-based radio frequency energy harvesting systems can reach the milliwatt level, higher than the power consumption of LED lights, their output power still cannot meet the demands of energy-intensive smart textiles (such as electrically heated fabrics and heated underwear). Furthermore, existing research has rarely evaluated the environmental adaptability of systems, and there is a contradiction between the flexibility and performance stability of fabric-based radio frequency energy harvesting systems, making the trade-off balance a significant challenge. The focus of future research will be on developing small-sized, low-cost, interference-resistant, low-loss, and high-output power fabric-based radio frequency energy harvesting systems through antenna structure design and the development of new textile materials.

Figure 3 Radio Frequency Energy Harvesting System and Circularly Polarized Antenna

In summary, although research on textile-based energy storage and harvesting devices is increasing, and performance is continually being optimized, they still face a series of challenges such as low energy density and energy conversion efficiency, high costs, inability to mass-produce, and poor durability, leaving a considerable distance from practical applications.

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