Color-Changing Materials in Fibers: Opportunities and Challenges

The integration of color-changing materials into fibers represents a significant area of intersection between materials science and textile engineering. This endeavor focuses on incorporating materials with specific color-changing propertiessuch as photochromic, thermochromic, and electrochromic functionalities-into fiber matrices. This combination endows textiles with “sensing-responsive” intelligent characteristics, applicable across diverse scenarios, including apparel, home textiles, medical care, protection, and decoration. The following analysis provides a detailed examination of the specific applications, technical attributes, and typical scenarios associated with colorchanging materials in fibers, beginning with an exploration of the various types of these materials.

Color-changing materials exhibit various triggering mechanisms, including light, temperature, electric fields, and humidity, which influence their applications and functional roles in fibers. Presently, the primary categories of color-changing materials that have reached a more advanced stage of commercialization and research are photochromic, thermochromic, electrochromic, and emerging structural color-changing materials [1]. The following four categories are currently the most developed in terms of commercialization and research.

“Change with light” refers to the primary visual interaction and sun protection provided by photochromic materials, which undergo reversible structural changes when exposed to specific wavelengths of light. Compounds such as spirogyra, spiroxamine, and azobenzene exhibit these transformations under ultraviolet and visible light, resulting in color changes, such as from colorless to purple or from yellow to blue. Upon removal of the light source, these materials revert to their original state [2,3]. In the case of spirogyras, the ring-opening reaction occurs through the cleavage of the spiro carbon-oxygen bond, yielding π-conjugated perthiocyanogen isomers that facilitate the observed color change [4].

The application of fibers necessitates addressing the challenges of “washability” and “light stability.” Traditional small molecule materials are prone to migration and fading. Current methodologies involve the fabrication of nanoparticles or the blending of these materials with fiber polymers, such as polyester and nylon, through co-spinning techniques. Additionally, coating and printing methods are employed to secure these materials onto the fiber surface [2,5]. For instance, one study demonstrated a significant enhancement in the color transition stability of fibers subjected to stretch, sweat, and detergent solutions by effectively immobilizing photochromic microcapsules on the surface of spandex fibers, utilizing an outer layer of polydimethylsiloxane (PDMS) [6]. The following introduces several application scenarios of photochromic materials.
a) Smart Clothing: outdoor apparel, including T-shirts in basic colors, is designed to withstand strong ultraviolet (UV) radiation. The darkening of cuffs and collars serves as an intuitive indicator of UV intensity, fulfilling a protective function by warning wearers about sun exposure [6]. Additionally, children’s clothing featuring photochromic patterns changes color in sunlight, enhancing enjoyment while also signaling the presence of acidic and alkaline vapors through color alterations. This expands the applicability of multi-stimulus responsive materials [7].
b) Sun-protective textiles: sun umbrellas and sunscreen curtains can enhance their ultraviolet (UV) protection capabilities by deepening fiber color, which improves UV absorption and reflection. This modification can increase the sun protection factor (UPF) to 50 or higher, while also considering light transmission without restoring the original color of the light [8].
c) Anti-counterfeit textile labels: anti-counterfeiting markings for high-end clothing and home textiles only show color under specific wavelengths (e.g., UV lamps), and are colorless under ordinary light, making them difficult to forge [9,10].
d) Wearable Displays: wearable interactive multicolor photochromic fibers can be mass-produced by thermal stretching technology, and their PMMA light-conducting layer and PVDF/CaS fluorescent composite layer achieve uniform multicolor light control, which can be used in daily wearable devices, automotive interiors, underwater lighting and emotionally interactive displays, and provide a new pathway for future human-computer fusion [11,12].

Thermochromic materials, including liquid crystals, organic small molecules, and inorganic composites, exhibit sensitivity to temperature variations. When the temperature reaches the “color-changing threshold,” these materials undergo alterations in molecular arrangement or crystalline transformation, resulting in a visible color change. For instance, red can transition to colorless, while blue may shift to pink. This property can be effectively utilized for comfort monitoring and practical applications.

When utilized in fibers, the “color-changing temperature range” must be tailored to specific scenarios. For example, it should be calibrated between 32℃ and 38℃ for applications related to human body temperature, and between 0℃ and 25℃ for outdoor environments. Additionally, integration into the fibers should be achieved through microcapsule coating technology [13,14], which prevents the material from interacting with the fiber matrix and enhances wash resistance. These thermochromic fibers exhibit significant potential for application across various fields, including temperature monitoring and home textile products. The color of the fiber in temperature monitoring wristbands changes in response to human body temperature. Specifically, it appears green when the body temperature is within the normal range (36℃ to 37℃), yellow during a fever (> 37.5℃), and red in cases of high fever (> 38.5℃). This feature allows for visual assessment of body temperature without the necessity of electronic devices. When baby pajamas contact a baby’s skin, they change color to blue in low ambient temperatures and to pink when the temperature is appropriate, thereby helping parents accurately assess warmth retention. Thermochromic fibers are incorporated in areas susceptible to sweating, such as the soles of the feet. Following perspiration, the local temperature increases, causing the fibers to lighten in color, which signals the need to change socks to prevent athlete’s foot. Additionally, temperature-controlled mattress fabrics and baby sleeping bags in home textiles can visually indicate the surface temperature of the bedding through color changes, preventing conditions that are either too cold or too hot. Furthermore, flexible thermochromic fabrics can facilitate dynamic displays and camouflage, enabling applications such as the dynamic display and recognition of QR codes, thus offering a novel approach to intelligent information interaction [14], as well as providing visible light/infrared synchronous camouflage [15].

“Power on is change.” Focusing on dynamic regulation and highend applications, electrochromic materials-such as tungsten oxide, vanadium oxide, and conductive polymers-undergo redox reactions when subjected to an electric field, enabling reversible modulation of color or transparency. These materials are characterized by rapid color change responses, ranging from milliseconds to seconds, and exhibit high color contrast, allowing for precise control [16].

The application of electrochromic materials in fiber technology must address the challenges of “conductivity” and “flexible integration.” Typically, these materials are coated onto the surfaces of conductive fibers, such as silver-plated nylon or carbon nanotubemodified polyester, or they are combined with elastic conductive substrates to create bendable “electrochromic fibers”[16,17]. These fibers can subsequently be woven into textiles. For instance, carbon nanotube fibers exhibiting electrochromic properties can be fabricated by depositing WO₃ thin films onto carbon nanotube (CNT) substrates using magnetron sputtering. The resulting color change relies on a thin film interference mechanism, and these fibers have been successfully assembled into reversible colorchanging electrochromic devices [18].

Electrochromic materials are utilized in various applications, including wristbands, e-sports apparel, smart sunshades, and flexible textile displays. By controlling the current via a mobile phone application, users can adjust the fabric’s color, enabling transitions between black, white, and blue to achieve “personalized dressing”. Certain products can also indicate battery levels through color intensity, displaying green when the battery is fully charged and red when it is low. Clothing designed to resemble game character skins employs electrochromic fibers to simulate color changes that occur when “skills are triggered” in the game. For instance, the fabric may partially brighten or change color upon skill activation, thereby enhancing the user’s sense of immersion. Once powered on, the sunshade’s transparency can be modified to regulate the amount of light entering a space, effectively replacing traditional manual sunshades. In vehicles, the light-transmitting mode of a sunroof’s sunshade can be swiftly adjusted using buttons located inside the car. Flexible textile display screens integrate electrochromic fibers into large-area fabrics, which can serve as advertising screens or stage backdrops to showcase dynamic text or patterns. For example, these textile advertisements can be displayed in shopping mall windows and can switch content in real time.

In addition to the three mainstream materials mentioned above, several color-changing materials have been specifically developed for particular applications. Although these materials are relatively niche in fiber applications, they serve precise functions that address specific functional requirements. For example, wet-chromic materials, including cobalt salts and certain organic dyes, undergo color changes when exposed to water or high humidity, reverting to their original color upon drying. These materials can be employed in “waterproof testing”; for instance, wet-chromic fibers used in the stitching areas of outdoor footwear can indicate seal failure by turning red upon water leakage. The outer layer of baby diapers is constructed from moisture-chromic fiber, which changes to blue when wet, signaling a need for change. Hydrogel-based humidityresponsive fibers can contract or expand in response to moisture absorption, facilitating the development of adaptive textiles. These fibers have potential applications in humidity sensors or alarms and may also be utilized in biomedical fields, such as drug delivery systems and smart hemostatic bandages [19].
a) pH-chromogenic materials (e.g. phenolphthalein, bromocresol green): sensitive to environmental pH, the color of these fibers’ changes in response to acidity and alkalinity. Primarily utilized in medical applications, wound dressings incorporate pH-induced chromogenic fibers. When a wound becomes infected and the pH of the exudate increases, the fibers transition from pink to blue, thereby alerting healthcare professionals to take timely action [20].
b) Mechanically discolored materials: color changes in response to mechanical stress are notable phenomena. For instance, twisted fibers coated with thermochromic dyes serve as optical strain sensors, signaling the strain experienced by the fibers through observable color changes [21]. Additionally, fibers possessing photonic crystal structures alter their lattice spacing under stress, leading to a blue shift in the structural color, thereby facilitating the visual detection of strain [22].
c) Structural color materials: the application of light interacting with micro- and nano-structures to produce colors offers several advantages, including environmental friendliness, resistance to fading, and high brightness. This approach demonstrates significant potential for coloring carbon-based fibers [18]. For instance, photonic crystal structures created on the surfaces of carbon fibers through atomic layer deposition (ALD) technology can yield photonic crystal carbon fiber yarns and fabrics that exhibit tunable structural colors alongside excellent mechanical properties [23]. Conversely, magnetic field-responsive structural color fibers employ magnetic nanoparticles that assemble into a one-dimensional chainlike structure in response to a magnetic field. This assembly facilitates dynamic changes in structural color, presenting innovative possibilities for smart textiles, sensors, and anticounterfeiting applications [24].

Despite the wide range of applications, the scale-up of colorchanging materials in fibers is still facing some technical bottlenecks, while moving towards smarter and more practical directions [25].

Most color-changing materials, particularly small-molecule photochromic and thermochromic substances, are susceptible to degradation from water washing and friction, which adversely impacts their performance and longevity [18,26]. Currently, technologies such as microcapsule coating and chemical bonding are employed to enhance fixation; however, it remains essential to strike a balance between “washability” and “color change sensitivity”[6,3,5]. While the response speed of electrochromic fibers is relatively advanced, photochromic and thermochromic fibers exhibit slower color-changing responses and a limited number of cycles under extreme conditions, including low temperatures and intense light [27,4].

Electrochromic materials, such as tungsten oxide and conductive fibers, tend to be costly and are primarily utilized in highend applications [17]. The large-scale production of photochromic and thermochromic fibers necessitates the optimization of spinning or coating processes to reduce costs and cater to the mass consumer market [14,13]. In addition to achieving efficient thermal management, maintaining the breathability, moisture absorption, softness, and overall comfort of textiles presents a significant challenge for intelligent thermal conductive fiber materials [28].

The single color-changing function has been enhanced to a “multi-trigger” system, exemplified by “light-temperature dual-response fibers.” These fibers change color in response to ultraviolet rays outdoors, signaling sun protection, and also react to body temperature, indicating perceived warmth, making them suitable for complex environments [6,22,18,7]. Most traditional color-changing materials consist of organic synthetic compounds. Future developments are likely to incorporate more natural colorchanging substances, such as anthocyanins derived from plants, which alter color in response to ph or temperature changes. This shift aims to reduce environmental hazards and align with sustainable development principles [29,26,18]. Additionally, these materials are being integrated with other intelligent functions. For example, color-changing fibers can be combined with antibacterial, moisture-wicking, and energy storage capabilities. These innovations not only facilitate color change but also function as flexible batteries to power smart devices, thereby broadening application scenarios [28,16,30]. By synthesizing knowledge from diverse fields such as bionics, materials science, optics, and physics, and employing artificial intelligence for material design and performance optimization, new solutions can be developed to tackle existing challenges and explore novel application domains, particularly in health monitoring [31-34].

The integration of color-changing materials into fibers fundamentally transforms traditional textiles from a state of “passive use” to one of “active response” through the combination of functional materials and textile carriers. This advancement relies on breakthroughs in materials science to develop more stable and cost-effective color-changing materials. Additionally, innovation in textile technology is essential, particularly in creating more efficient fiber composites and refining weaving processes. As technology matures and costs decrease, color-changing smart textiles are expected to transition from high-end applications to everyday consumer use, ultimately becoming a significant component of “smart wear” and “functional textiles”.

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The author declares that there is no conflict of interest.

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