Advancements in Clothing Thermal Comfort for Cold Intolerance

Abstract

Due to constantly shifting environmental and personal circumstances, humans have a wide range of thermal comfort needs. Cold intolerance (CI) is a personalized thermoregulation disorder characterized by a persistently cold-feeling problem, regardless of weather conditions. Improvements in clothing thermal comfort can help maintain proper insulation levels, hence reducing excess heat loss brought on by thermoregulation disorders since the wearer’s thermal comfort is impacted by controllable environmental and personal factors. Despite extensive research on cold-proof clothing, no studies have examined the current status of cold protective clothing systems when taking individual considerations into account, particularly those who use them and have cold sensitivity. There is a significant study gap in research on cold intolerance discomfort and advancements in appropriate cold protection apparel applied to individuals with thermoregulation disorders. Accordingly, this paper reviews the occurrence and severity of cold intolerance and its comfort challenges. It also addresses recent developments in cold protective clothing design, aimed at opening pathways for further investigation into adopting this cutting-edge technology for cold intolerance wear design. This review also aims to clarify the existing opportunities for enhancing the thermal insulation capabilities and other comfort factors of cold protection apparel, which are conducted during the stages of garment design and clothing material/textile manufacture. A thorough assessment of the research on introducing novel surface finishing methods in the pretreatment section and modifying the structural properties of garment materials at the fiber/yarn or weaving stage is conducted. Furthermore, we systematically discuss the potential design solutions regarding fit and size as well as stitching technologies during garment development for thermal insulation enhancement of cold protective clothing design.

1. Introduction

As demonstrated by the International Standard Organization standard EN ISO 7730, thermal comfort is described as the “thermal sense experienced by the human body associated with its overall thermal balance” [1]. It is also further defined by the American National Standard for Thermal Environmental Conditions for Human Occupancy as “the condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation” [2]. Recent reviews [3,4,5,6] have shown that in-depth investigation has been conducted on the aspects of clothing thermo-physiological comfort [7]. Even so, due to its dynamic nature, clothing thermal comfort remains a crucial research topic related to its broad range of requirements in many application areas [4,8]. Human thermal comfort with clothing depends on external factors, such as air temperature, mean radiant temperature, air speed, and humidity, along with personal factors, such as metabolic rate, health condition, age, and gender [9]. Thus, the requirement of human thermal comfort expected from clothing varies from person to person [10] depending on each individual’s cold tolerance and the current environmental conditions. In nature, the human body has a homeostatic mechanism to keep the inner body temperature stable regardless of changes in the outside environment [11,12]. This mechanism allows the body to maintain its thermoregulation balance and body temperature within a narrow range (36.5 and 37.5 °C) [12,13,14,15]. The skin, sweat glands, blood arteries, and hypothalamus (brain) are the essential organs in managing the narrow range of body temperature at a comfortable level [16].

However, thermoregulation disorder, which can lead to discomfort and sickness, may originate from a breakdown in one of the organ systems responsible for controlling body temperature. One of the earliest thermoregulation disorders that has been widely documented via numerous studies is cold intolerance (CI) or cold sensitivity (CS) [17,18]. It is normal to feel cold during the cold season as a result of the decrease in temperature. Our bodies adapt to the changing climate as the seasons change, and the air becomes colder, which frequently leads us to look for warmth and extra layers of clothing. This physiological response serves as a reminder of our intimate relationship with the environment, as we naturally adjust to seasonal variations in the weather. However, having a persistently cold feeling regardless of the season is known as cold intolerance or sensitivity to cold [19,20,21]. It is a collection of acquired symptoms that may appear after a severe injury and include pain, altered responsiveness, stiffness, or color changes, in addition to an unusual aversion to cold [19,21]. Various diagnosis reports have confirmed that cold intolerance discomfort is an adverse effect of various injuries and impacts of infectious diseases [22,23]. Research has shown that among infectious diseases, post-poliomyelitis syndrome (PPS) is responsible for the highest incidence of cold intolerance (CI) disorder in individuals who have had poliomyelitis [24].

In recent years, it has been shown that human thermal comfort challenges, including cold intolerance, can be influenced by environmental factors [25,26] and can be treated with different interventions. In addition, patients have been recommended a variety of pharmacological and nonpharmacological techniques to alleviate the discomfort associated with cold intolerance [27]. In this regard, clothing is considered a nonpharmacological intervention tool to aid the thermal comfort balance in human microclimate [3,28,29]. Recent advancements in textile and clothing comfort, including structural modifications of fibers, yarn construction, yarn twist, fabric thickness, fabric design, finishes, and the integration of advanced materials like phase change and smart textiles [30], have significantly enhanced cold protection. These innovations create larger volumes of trapped air within the fabric, thereby improving thermal insulation for the wearer [31]. This emerging trend holds enormous potential for individuals with cold hypersensitivity, offering a promising solution to enhance their thermal comfort and protection [32]. Today, the advancements in cold protective clothing are viewed as a key aspect of threat control, supporting secure and healthy working conditions [33].

This review paper aims to summarize cold intolerance discomfort and assess the state-of-the-art developments in textile and clothing technology to alleviate these issues. Due to the wide range of thermal comfort requirements for clothing, it is quite difficult to group the vast body of literature into distinct categories. A wealth of exceptional studies highlights the role of clothing in regulating human thermal comfort across various applications. Several of these studies provide comprehensive overviews, tracing the advancements in research on clothing and its impact on thermal comfort [34,35], while others focus on particular application areas such as sports clothing for better performance of athletes [25,26,28], thermal comfort adjustment in outdoor or indoor clothing for cold environments [36,37,38], and outdoor or indoor clothing for hot environments [39,40]. Furthermore, investigations have been conducted on the intervention of clothing thermal comfort in protective applications [41,42,43,44]. The majority of the existing literature has thoroughly explored the role of textile and clothing advancements in enhancing in thermal comfort as a response to ecological factors such as temperature, humidity, and wind. These studies have significantly contributed to our understanding of how clothing can function as a barrier or regulator against external climatic conditions. However, research focusing on clothing’s role in addressing personal features that intervene thermal comfort—such as individual metabolism, exercise levels, age, and health status—remains comparatively limited. This gap highlights the need for more targeted studies to develop personalized clothing solutions that optimize thermal comfort based on individual physiological and psychological characteristics.

In this section, studies on clothing thermal comfort that consider personal factors and their limitations in addressing the needs of the cold intolerance population are reviewed. A study by Z. Wang et al. [10], published in 2018, on the topic of “individual differences in thermal comfort, attempted to consider personal factors such as age, gender/sex, and other elements” is reviewed. However, this research did not address the role of clothing in solving thermal comfort challenges associated with health-related personal factors such as cold intolerance. Similarly, Islam M. et al. [45] provided a comprehensive review on clothing thermo-physiological comfort: “A Textile Science Perspective”. This review focused on how clothing impacts the body’s thermal equilibrium, covering the history of thermo-physiological comfort prediction models, heat and moisture transfer mechanisms, controlling factors, textile materials, and comfort assessment techniques. However, their research was based on healthy individuals and overlooked personal comfort factors influenced by thermo-physiological disabilities. G. Havenith, Ingvar Holmér, and K. Parsons [46] addressed personal factors in thermal comfort assessment, emphasizing the role of clothing properties and metabolic heat production. They discussed the predicted mean vote (PMV) model and its requirements for climate, clothing, and metabolic data. However, the consideration of metabolic heat production was based on healthy individuals. Furthermore, their discussion of the PMV model is suitable for predicting comfort levels based on the average response of a large population [47] but is unable to consider thermo-physiological differences among individuals and predict personal models of thermal comfort. Occupants’ characteristics for the prediction of a thermal comfort model were developed by the authors of [48]. Their research specifically considered personal attributes such as clothing choice, metabolic activity, and drink consumption. Once again, there is a lack of focus on individuals with health conditions that affect thermoregulation. A more relevant research review was conducted by Bogatu et al. [49] on personal thermal comfort based on human physiology related to ventilation, air conditioning, and heating (HVAC). This investigation highlights the development of personal comfort models and the importance of physiological indicators such as skin temperature and heart rate in predicting personal models of thermal comfort. The review discussed the incorporation of anthropometric data such as sex, age, and body mass index to generate personal comfort models, but all issues addressed were according to healthy and normal people.

In conclusion, although significant research has been conducted on general thermal comfort factors, there is still a lack of studies that specifically consider the needs of individuals with health-related conditions such as cold intolerance. This review aims to fill this gap by examining the severity and prevalence of cold intolerance, particularly in PPS, and exploring how modern textile and clothing innovations can help tackle these challenges by creating controlled micro-climates.

2. Occurrence of Cold Intolerance and Its Thermal Discomfort Challenge

The occurrence of intolerance presents a significant challenge in maintaining thermal comfort, particularly in regions where exposure to cold climates is common. Individuals experiencing intolerance often struggle to regulate their body temperature effectively, leading to discomfort and potentially adverse health effects. This phenomenon poses a multifaceted challenge, as it not only impacts personal well-being but also necessitates innovative solutions in clothing design and environmental management to mitigate its effects. Understanding the complexities of intolerance and its implications for thermal comfort is crucial for developing effective strategies to address this pervasive issue. In healthy individuals, the thermoregulation system helps maintain core body temperature regulation by balancing between heat production and heat dissipation, in response to hot and cold environments [12,13]. During warm conditions, sweat glands become active to facilitate heat loss through evaporation [40]. Hair erector muscles in the skin dermis relax to allow hairs to be positioned on the exterior of the skin to allow insensible heat loss through the surface of the skin [41,42]. Blood vessels become wide (dilating) to release increased blood flow to the skin capillaries to facilitate heat loss, called the vasodilation process [43]. Whereas, in a cold environment, the sweat glands tend to reduce their sweat production to control heat loss through evaporation, the hair erector muscles become strengthened, so hair stands erect on the he skin surface and traps a layer of insulating air, and the blood vessels perform the vasoconstriction process to limit the amount of blood flow at skin capillaries [44]. These are the mechanisms of performing a thermoregulation system in a healthy individual to maintain the internal thermal comfort level by responding to external cold or hot environments (Figure 1). However, damage to the fundamental thermoregulation system organs can render the human body unresponsive to external and internal thermal pressures. Damaged to sympathetic vasoconstrictor outflow, for several reasons, can result in inactive dilatation and distension of dermal capacitance beds of veins [50]. This leads to uncontrolled heat loss, cooling of nerves and muscles, and impaired muscle function in the dermis, and results in cold intolerance disorder [51].

However, the exact cause of cold intolerance remains unclear, as its underlying factors are highly diverse. Symptoms are influenced by both individual characteristics and external exposure conditions [17]. Nevertheless, numerous researchers and polio specialists agree that cold intolerance disorder poses a significant challenge for many polio survivors [19,52]. The reason polio survivors have such a challenge with coldness is that the components of the central nervous organism in charge of controlling body temperature are injured by the poliovirus [53]. The poliovirus can damage the brain region responsible for signaling blood vessel constriction, as well as the hypothalamus, the “automatic regulator” of the body’s internal environment [54]. The poliovirus also destroys the neurons in the spinal cord that convey the brain’s signal to the skin’s capillaries to constrict when it becomes cold [55]. As a result, polio survivors cannot prevent warm blood from flowing to the skin’s surface when the external temperature decreases. This leads to the loss of heat from the blood immediate to skin surface sources, causing the skin to cool and discomfort [24]. It is important to note that this damage is often localized to specific body parts, leading to discomfort in those areas [23,27].

Figure 1. Scheme of thermoregulation mechanism in healthy individuals. Source: [56], reprinted with copyright under open access publication (CC BY/4.0).

Figure 1. Scheme of thermoregulation mechanism in healthy individuals. Source: [56], reprinted with copyright under open access publication (CC BY/4.0).

Medical experts have provided their findings on the relationship between the impact of post-polio disorder and the occurrence of intolerance in polio survivors. For example, Dr. Silver (Director of the International Rehabilitation Center for Polio in Framingham, New England, Framingham, MA, USA) justifies that polio survivors experience sensitivity to cold because atrophied muscles fail to contract effectively, limiting their ability to support blood vessels in delivering warm blood to the extremities [57]. Dr. Owen, Emeritus (Medical Director of the Sister Kinney Institute, St. Paul, MN, USA), was one of the leading specialists to designate the relationship between post-polio and cold intolerance. According to his explanation, the poliovirus not only harms human motor neurons, causing paralysis of their muscles, but also encounters sympathetic nervous system neurons within the spinal cord that manipulate the blood flow into veins and arteries [58]. When veins are unable to contract, they become too open, causing blood to “pool” in the feet, giving the skin a bluish tint and causing puffy swelling. As a result, patients with post-polio disorder become colder than someone who did not have polio, since sympathetic neurons are damaged. Dr. Bruno, a clinical psychophysiologist [59], also observed that the skin on the affected arm of his first polio patient was cold to the touch, suggesting a problem with blood flow to the limb. As Dr. Bruno explored additional patients, he unearthed the same diagnosis findings. He presumed that the magnitude of the polio survivors’ skin blood vessels could not be controlled accurately because the poliovirus killed off the sympathetic neurons in the spinal cord [60]. These are responsible for creating the muscles covering blood vessels to contract.

To date, there is no clear statistical evidence on the prevalence of cold intolerance worldwide. However, some cluster-based population studies have been conducted to determine the current prevalence and associated factors of cold intolerance. A study conducted in northwest Iran, Tabriz, found that 47% of the studied population experienced cold-related symptoms, with 11% reporting cold intolerance discomfort [61]. Another study [62] in China titled “Prevalence, characteristics and natural history of cold intolerance after the reverse digital artery flap” reported that 60% of fingertip amputation patients experienced cold intolerance discomfort, out of 87 patients. Additionally, a study conducted by author(s) in Ref. [63] on the frequency and seriousness of cold intolerance assessed 129 patients at the Erasmus MC in Rotterdam, The Netherlands. The assessment revealed that 38%, 56%, 70%, and 83% of affected persons experienced cold intolerance discomfort after hand injury, specific nerve damage, combined nerve damage, and replantation, respectively. Furthermore, research conducted in the UK [27] on hand-injured patients found that 83% experienced cold intolerance in follow-up patients. The severity and prevalence of cold sensitivity vary across body parts, gender, and individual differences. A nationwide survey study conducted by the works in Ref. [64] on the Korean adult population found that women are more prone to hypersensitivity to cold in body parts such as hands, feet, and abdomen compared to men. This findings highlight a significant difference in cold sensitivity prevalence between women and men (Figure 2), with variations in distribution across different body parts in both genders.

Another exploratory study conducted on the prevalence of “cold hypersensitivity in Japanese women: genetic associations and somatic symptom burden” revealed that 54% of the studied population reported experiencing discomfort from cold hypersensitivity [65]. Based on the available data, the readers can infer that the prevalence of cold sensitivity discomfort disorder varies among individual due to factors such as health conditions, gender differences, age, genetic associations, and specific body parts.

3. Advancements in Textile and Clothing Development for Cold Intolerance

Current advancements in textile and clothing development play a promising pivotal role in addressing challenges associated with intolerance to various environmental conditions, including cold, heat, and humidity [66,67,68,69]. Researchers and industry professionals have made considerable progress in improving the comfort, functionality, and adaptability of clothing for people with specific intolerance requirements by utilizing innovative materials, designs, and manufacturing techniques [70]. These developments improve the quality of life for affected persons and contribute to the broader goal of establishing inclusive and accessible clothing solutions for various groups. The human body has a thermoregulation mechanism, the vasoconstriction process of blood vessels, to prevent unnecessary heat loss during cold exposure [71,72]. In assisting the natural vasoconstriction process, wearing the desired clothing ensemble is one means of managing extra heat loss and keeping the human microclimate controllable [31]. Thus, clothing plays a significant part in preserving the body’s thermal balance by maintaining normal levels of skin temperature, heat regulation, airflow, and humidity at the surface, all of which contribute to comfort [73].

This section provides a comprehensive understanding of recent advances in textile and clothing technologies for intervention options in maintaining the thermal comfort balance of cold-intolerant people. However, there is a shortage of studies on clothing specifically designed for individuals with cold intolerance problems. Liu et al. [36] conducted a systematic review on “Quantitative characterization of clothing’s cold protective capability to achieve thermal comfort”. The review included research publications from the years 2000 to 2021. Their results showed that while most prior research has focused on adult clothing, children, elders, and individuals with cold intolerance disorders have not received as much attention in terms of cold-proof clothing. Additionally, the skin temperatures of individuals with cold intolerance are significantly lower than healthy individuals [74,75], making them more vulnerable to the negative impacts of cold environments on their health [76]. Furthermore, the correlation between the thermal insulation of clothing worn by individuals with cold intolerance disorders and ecological factors, such as normal temperature, radiation temperature, relative humidity (RH), and wind speed, has not yet been studied. Therefore, this review suggests that recent research on cold protective clothing overlooks wearers with cold intolerance disorder who have different boundary conditions at the skin interface than healthy individuals. Given the existing research gaps, this section discusses advances in cold-proof clothing development from a broad thermal comfort perspective. Specifically, it aims to highlight the potential adaptation of current technologies to meet the requirement of cold-intolerant individuals through further research. The first part addresses recent advances in thermal insulation improvement of cold protective clothing at the textile stage, including the selection and modification of material types, alteration in fabric structure from a technical textile design perspective, and innovative surface treatments at the filament/fiber, yarn, and fabric levels. The second part reviews the available innovations in clothing design solutions for enhancing thermal insulation at the garment development design stage.

3.1. Clothing Thermal Insulation Improvement at the Textile Stage

Increasing thermal insulation in clothing during the textile stage is essential for enhancing comfort and protection against elevated temperatures. Researchers and manufacturers aim to optimize the thermal performance of clothing by focusing on advanced textile materials and manufacturing techniques. These advancements are crucial in ensuring that clothing can effectively retain body heat in cold environments and facilitate efficient heat dissipation in warmer conditions. Understanding the strategies, mechanisms, and tactics involved in enhancing thermal insulation during the textile stage is key to developing functional and comfortable clothing solutions that cater to diverse environmental and human requirements.

As reported by the standard EN ISO 15831:2004, the thermal insulation ability of clothing materials is defined as how well the whole garment or combination of garments insulates against the cold [77]. Therefore, improving the thermal insulation properties of textile materials plays a significant role in the manufacturing of cold protective clothing [78]. Previous studies have been conducted to enhance the insulation ability of textiles for use in cold protective clothing systems. The textile material properties that should be considered when designing cold protective clothing include thermal insulation, water vapor resistance, thermal resistance, air permeability, and protection against external moisture [78,79,80]. Factors such as the fiber properties, including cross-section, fineness, length, and crimp [81,82,83], alterations in fabric structure design [84,85,86], and the applications of innovative fabric treatment techniques in the finishing stage [87,88], are all advanced options to enhance the thermal insulation capabilities of textile materials for use in cold protective clothing.

Innovative research indicates that the physical characteristics of textile materials can be changed by structural alterations in the fiber/yarn [89] or textile stage to increase the thermal insulation capacity of garment materials [90,91,92]. A well-known methodology used to maximize the textile material’s insulation ability and thermoregulation feature is created with the multilayer materials through sandwiching an inner material (thermo-insulator) between two outer-layer fabrics, known as the padding technique [93]. The inner padding material (thermal insulator) is conventionally made from goose feathers, duck feathers, or artificial materials at the microfiber level [80]. In this technique, the thermal insulation and water vapor transmission performance could vary depending on the style of filling materials [94], hence natural or synthetic-based fillers have different effects [95]. However, due to the direct correlation between thermal insulation ability and the padding material width [96], this method has a critical drawback in terms of wearing comfort, specifically in weight and bulkiness.

Accordingly, researchers have developed an inner thermal barrier by integrating advanced, low-thickness insulating materials to address current comfort challenges. Agnieszka Greszta, et al. [97] created an effective and thin insulating nonwoven insert by combining aerogel and phase change material (PCM) microcapsules. Their experimental results confirmed that these newly developed, low-thickness insulating inserts greatly improve the ergonomics comfort and wearability of cold protective clothing while maintaining sufficient thermal insulation performance. Due to their superior thermal insulation properties to traditional materials, aerogels have become a promising material for high-performance textiles. These materials insulate by trapping air inside their structure, utilizing the low heat conductivity of air to create a comfortable environment for the wearer, unlike standard insulating materials, which often require thick layers that can restrict mobility and lose their insulating ability when compressed or wet.

Aerogels offer a significant boost in thermal insulation without the bulkiness of traditional materials, as they are extremely light solids with very low thermal conductivity. Despite their advantages, aerogels have faced challenges such as brittleness and high production cost [98]. However, Chao Sun et al. [99] developed a cold protective clothing material filled with reflective nano-fibrous material. They found that the new filler materials were successful in providing optimal insulation for the protective jacket without compromising breathability or adding extra weight.

While phase change materials (PCM) and nanofillers have been used to create a low-thickness, low-weight insulating material for improved ergonomic comfort, the moisture content of clothing insulation materials, which are typically composed of multiple layers of padding, causes inefficient thermal management [100]. The thermal insulation for such goods is significantly decreased when the padding is unprotected from water and the fabric surface is not appropriately separated; it decreases to zero. To overcome the state-of-the-art limitations, more recent innovators have introduced other ways of enhancing textile fabric insulation, such as altering the structure of fiber/yarn or fabric weave. Rocco Furferi et al. [101] introduced an innovative thermal insulating fabric where the insulation is obtained instantly from the structure of its weave rather than the padding technique (Figure 3). Their new fabric was constructed from a volumized yarn having 8% more volume than the conventional wool yarn. The lowest quantity of wool for volumized yarn was found to be 75%. The thermal insulation performance of their newly developed fabric was 1.6 Clo with 350 g/m² weight. This is comparable with Prima Loft’s [102,103] patented thermal performance fabric with lightweight and excellent thermal insulation that effectively protects against extreme cold.

Another innovation mentioned in Ref. [104] involves the development of thermally insulating textiles from silkworm cocoons, inspired by polar bear hair. The “freeze-spinning” technique was utilized in their experiment to achieve endless and mass-scale production of fibers with permeable structures that mimic polar bear hair (Figure 4). Experimental findings show that textile structures with biometric fibers have excellent thermal insulation capabilities, appropriate breathability, and wearability [105]. These properties of biometric fibrous structure textiles make them a promising clothing material for individual thermal management in cold exposure without compromising ergonomic comfort in terms of clothing weight and thickness [106,107]. From the fabric structure parameters, the thickness, density, and porosity of fabrics are three principal factors that determine the insulation performance of textile materials [108]. These fabric structure components have a considerable influence on the insulating properties of textiles. The fabric’s ability to trap air, which is crucial for protection against temperature extremes, is affected by its thickness. Increased fabric density reduces airflow through the cloth, enhancing thermal resistance. Additionally, porosity plays a key role in controlling heat transmission; materials with lower porosity typically offer better insulation by reducing airflow. Understanding and refining these crucial elements facilitates the creation of fabrics with higher thermal insulation capabilities, ensuring increased comfort and protection for users in various environmental conditions.

Because fabric structure parameters highly influence the insulation performance of cold protective clothing materials [85,109], recent researchers and designers have focused on altering the structural changes of textile materials through the technical textile technology perspectives [110]. Many researchers agree that securing an air layer is crucial to improving the insulation ability of textiles [111,112]. The 3D spacer fabric is a dominant technical textile that has been introduced and characterized by researchers. Its structure is available in woven and knitted (warp and weft knitted) constructions (Figure 5) [113,114,115]. The 3D spacer cloth is a one-piece interpose structure containing two outer coats and upright and slopped spacer monofilaments. It acts like a padding material with straight, plateau, and compaction stages under compression. It is attributed to lightweight, high-strength characteristics, and has the potential to increase the trapping of air inside the cross-section of the structured space due to the occlusion in the surface, which helps to establish the thermal barrier between the human body and the atmosphere.

All of these properties make it an interesting structural parameter to enhance the insulation function of cold protective clothing [110,116]. Ran-I Eom et al. [117] evaluated the thermal properties of 3D spacer technical materials in a cold environment. In their experiment, they constructed four 3D spacer technical object structures with varying pore sizes and thicknesses. The samples were exposed to a cold climate chamber (temperature 5 ± 1 °C, relative humidity (60 ± 5)%, wind velocity ≤ 0.2 m/s) and placed on a heating plate set to 30 °C. Their findings indicate that the 3D spacer technical material with large pores and a thick air layer exhibited superior insulation compared to other materials [89]. However, when the heat supply was stopped, the air gap significantly affected the insulation performance of 3D technical fabrics, regardless of pore size. Therefore, increasing the air layer space is a more crucial structural parameter than increasing pore size for enhancing the insulation of textile materials.

Meanwhile, the thermal insulation performance of spacer material can be influenced by its material type and surface characteristics, including yarn linear density and porosity [89]. To address the inefficiencies of conventional spacer fabric, state-of-the-art improvements have been outlined in various research. Mao N. Russell S. [116] demonstrated an enhancement in thermal insulation of spacer fabric by introducing hydroentangling technology. They attached a thin, woolen mesh to one side of preformed knitted spacer fabrics. The resulting hydroentangled wool fiber web-spacer fabric structures showed a significant reduction in thermal conductivity with minimal change in fabric density. However, the new integrated structure was found to increase the trapping of air within the spacer fabric cross-section due to the obstruction of large openings on the surface. While clothing thermal insulation can be improved by utilizing technical textile weaving and knitting technology, achieving satisfactory thermal insulation in clothing material solely through limited structural parameters remains challenging.

In line with this, the enhancement of thermal insulation performance in clothing materials has been expanded to incorporate a contemporary fabric treatment technology to address the thermal insulation inadequacies caused by structural parameters. A popular trend in this area is the use of aerogel-based fiber, yarn, or fabric treatments, which have garnered significant scientific attention for their ability to improve the thermal insulation of clothing products. These treatments are valued for their special properties, such as ultrahigh porosity, large specific surface area, and ultralow thermal conductivity, making them suitable for various applications in cold protective clothing [118,119,120]. Ghane and Mohammad [121] conducted a study on the thermal insulation properties of 3D-knitted spacer fabrics integrated with ceramic-powder-impregnated fabrics. Their experiment involved a three-layered fabric system that consisted of a three-dimensional knitted polyester spacer fabric in the middle flanked by two layers of knitted cotton fabrics soaked with silica on both sides. This construction aimed to enhance the thermal properties of the fabric and prevent air flow through the conventional spacer fabric. The three-layer construction model demonstrated a significant improvement in thermal resistance compared to the pristine spacer fabric.

Amir Abbas et al. [122] claimed experimental findings that improve the heat-retaining properties of cotton fabric through surface coating. Their experiment used aluminum oxide (Al₂O₃), zirconium oxide (ZrO₂), and fumed silica as insulating materials. The results showed that thermal conductivity was lowered by 19.1–44.5% compared to conventional cotton fabric, with a considerable improvement in thermal insulation ability. However, limitations such as poor fastness, coating duration, and release of nanoparticles were detected. Another experimental study by Zhou, Z., et al. investigated the insulation performance of 3D spacer jacquard fabric treated with silica aerogel [110]. The experiment began by modifying the electronic jacquard machine for weaving with a spacing of 5 mm. Furthermore, the 3D spacer jacquard fabric was treated with silica aerogel to further enhance the thermal insulation function of the 3D spacer fabric, as shown in Figure 6. The results indicated that the insulation performance of the treated fabric significantly increased as the aerogel content increased. However, the fabric’s toughness decreased with increasing aerogel content due to the poor mechanical properties of aerogel [121,123]. They found that the optimal insulation achievement of the 3D spacer jacquard fabric occurred at 3.3% aerogel content.

Moreover, G. Rosace et al. also studied the effect of silica aerogel-based finishing and textile structure on textiles’ thermal insulation properties [124]. Their findings, again, proved that aerogel-based finishing shows poor mechanical properties and complex preparation processes [125]. This indicates that there is a need for further research to improve the mechanical properties of aerogel-based materials for further application in cold protective clothing systems [126,127]. In this regard, Karami Kamkar S. et al. [128] conducted review research on the advancement of silica-based aerogel manufacture towards enhanced mechanical properties such as customized morphology and multifunctionality. According to their review findings, five options were researched to enhance the mechanical properties of aerogel, the use of flexible silica predecessors in the silica gel backbone [129,130,131], surface-crosslinking of silica particles with a polymer [132,133,134], prolonged aging step in different solutions [135], allocation of flexible nanofillers into the silica solution before gelation [136], and, most recently, polymerizing the silica predecessor earlier than gelation [137].

To enhance thermal insulation performance, all of these options improve the mechanical qualities of silica-based treatment fabrics. This review section informs readers that the current research efforts to enhance the thermal insulation performance of cold protective clothing at the textile level have primarily focused on altering the types of material, architecture of the fabric type [138], and the application of sophisticated treatment techniques. As further evidence, Ullah et al. [139] conducted a review of recent research endeavors aiming at innovative approaches for managing moisture and/or thermal regulation at the material, filament/fiber, yarn, and fabric levels. Their review findings concluded that achieving adequate thermoregulation in textiles depends on modifying fiber/yarn properties, fabric construction, and the application of surface treatments on fiber/yarn and fabric.

3.2. Clothing Thermal Insulation Improvement at the Garment Design Stage

Improving the thermal insulation of clothing during the garment design phase is crucial for maximizing comfort and protecting against temperature changes. At this point, designers utilize their expertise to make clothing that effectively traps body heat in colder climates and allows it to escape in warmer ones. By carefully considering elements such as fabric choice, construction techniques, and ergonomic fit, designers aim to enhance the overall thermal efficiency of the clothing. Understanding thermal insulation principles and incorporating creative design ideas enable the creation of clothing that offers exceptional comfort and functionality in various environmental conditions.

In addition to the fabric’s properties, the thickness of the air space and the area of interaction between the body and the garment also have a significant impact on heat and mass transmission between the human body and the environment [140]. Clothing and air layers in the microclimate greatly affect heat dissipation from the body and are therefore vital considerations for thermal comfort [141]. Enhancing and improving garment design factors in the development of cold protective clothing can greatly increase its thermal insulation value [142]. Garment fit, size, type of seams and stitching, accessories, and fasteners used for dressing management are fundamental factors to consider in the design of cold protective garments [79].

3.2.1. Considerations in Clothing Fit and Size

Garment fit levels of the entire silhouette directly determine the amount of air gap (air layer depth) between the individual body and the clothing layer [142,143]. In conventional pattern engineering, the desired garment fit is generated from basic blocks drafted based on specific body shapes [144]. Typically, garment fit is categorized (Figure 7) as loose fit (5 inches bigger than the body’s hip and chest girth measurements), semi-fit (2–4 inches bigger than the body hip and chest measurement), and fitted (snug around the body) [145]. However, analyzing fit for quantifying of air layer thickness and its effect on clothing thermal insulation value through traditional pattern engineering is difficult and almost impossible. For over a decade, three-dimensional (3D) body scanning technology with manikin support has been commonly used as a modern approach to analyze garment fit and quantitatively measuring available air gaps in 3D garment fit. Y. Lu et al. [146] used three-dimensional (3D) body scanning technology for fit analysis to determine the air gap size and its distribution between protective clothing and the human body.

Based on their research results, the air space size was randomly allocated on the body, and was strongly correlated with body functions, garment fit, and fabric properties. Additionally, Emer Melt et al. [147] evaluated the allocation of the air gap thickness and the interaction area for the male lower body in relation to garment fit and style using a manikin and 3D body scanning technology. Their results confirmed that the variation in air gap thickness and the contact area due to garment fit had a greater impact than garment style. The influence of fit and dimensions on Ivana Spelic also studied thermal insulation of garments and ensembles [148]. Through their experiment, a 3D body scanner was used as an accurate tool to determine the microclimatic air volume. A resting thermal manikin was utilized to investigate the thermal insulation level of selected garments and ensembles.

According to their results, the dimensions of the garment had an incredible influence on the thermal insulation improvement for both the jackets and the complete ensembles. The overall ensemble’s insulation increased with the length enlargement due to the presence of air volume along the garment length. A. Magdalena Mlynarczyk et al. [149] conducted a pilot study on the effect of clothing size on thermal insulation. Through their research, total thermal insulation (dynamic and static conditions) against entire air volume and air clearance inside clothing in varied sizes were evaluated. Their findings indicated that the thermal insulation value improved with increasing clothing air volume and air gap size both in dynamic and static conditions (Figure 8). Furthermore, the volume of thermal insulation increased with the dimension of the garment increasing. This is due to the linear increment of thermal insulation with air clearances until there is heat transmission by convection. Hence, when the size of the air gaps is too large due to convective heat transfer, the insulating effect of the air clearances would be weakened [142].

From the available research, clothing thermal insulation is highly affected by the garment fitting status quo. Both loose-fit and tight-fitted garments have an undesirable relationship with the thermal insulation capacity of protective apparel. In a tight-fitted ensemble, heat transfer through conduction is a major factor that weakens the insulation performance of the clothing system, creating discomfort in the microclimate. On the other hand, heat transmission from the physical body to the surroundings via convection is a major issue in loose-fitted garment silhouettes due to the aeration of the trapping air layer, which allows heat to escape through openings such as cuffs, collars, and hems, also known as the chimney [150]. Thus, the determination of an optimum air layer thickness between the human body and the clothing layer through advanced fit analysis techniques is necessary to improve the thermal insulation value of cold protective clothing in terms of fit and size considerations. However, regardless of fit and size, the thermal insulation performance of a properly fitting garment is also affected by the presence of wind and its speed [73,151,152]. Due to the constantly changing wind speed and availability, it is difficult to find an optimum air layer thickness in a specific clothing fit that consistently maintains the wearer’s thermal comfort level in the microclimate.

To address this limitation, W. Song, Y. Lu, W. Su et al. [153] developed an innovative air-inflatable garment using real air as the thermal insulator. The garment is able to adjust the amount of air volume as required to regulate thermal insulation. Their new garment was developed from a polyester fleece material/thermoplastic polyurethane (TPU) film laminated internal layer and an extreme-density polyester fabric/TPU laminated outer layer by creating a series of connected air chambers uniformly distributed in the garment except for the sleeves (Figure 9). Their findings showed that the thermal insulation of the air-inflatable garment can be adjusted with the increase in air inflation volumes. The presence of the sleeves increases the air inflation volume inside the garment, significantly improving its thermal insulation. However, increasing air velocity critically disables thermal insulation performance due to the growing local dry convective heat transfer coefficient, through air inflatable garments. It is important to note that other parameters affecting the wearer’s thermal comfort, such as water vapor absorptivity (moisture management) and ergonomic issues during body movement, were not considered in this innovation.

3.2.2. Considerations in Stitching and Seams

As is common knowledge, the process of creating a three-dimensional garment silhouette involves superimposing seams that are connected by stitching. The formation of the stitch is created mostly by the joining process of the upper (needle thread) and the lower (bobbin thread) inserted via the needle [154,155]. The degree of thread tension during the interlacing process results in the compression effect of materials along the stitching line. In addition, the needle insertion creates structural damage at the seamline [156,157,158,159]. Various types and classes of seams and stitches are reasonably selected to develop a certain clothing article [160]. Depending on the type of stitches (lockstitch, chain-stitch, zigzag-stitch), seams [161,162], and type of needle, the impact of sewing on the original material functionality performance can differ [159,163,164]. For a decade, researchers have proved that the thermal insulation value of cold protective clothing is influenced by stitching due to the compression effect and thickness reduction along the sewing line [165]. As a result, the heat loss from the human body to the surrounding environment via the clothing layer is facilitated by cold spots created at each needle insertion point alongside the stitching direction. As discussed in other sections, the efficacy of thermal insulation is mostly dependent on the amount of trapped air in the insulation material and its thickness. However, due to localized compression at the needle insertion positions, the trapped air within the insulating material is pushed to escape via the permeable structure.

Recently, new stitching technologies have been introduced to improve the drawbacks of conventional stitching that hinder the thermal resistance and other performance of the original clothing material [166]. Hassan Saeed et al. [167] created a new stitching model to overcome the effect of traditional stitching on the thermal insulation capacity of 3D spacer fabric (Figure 10). Their new stitching formation was inspired by the existing lock stitch orientation, but they created a dual-level feeding system instead, by introducing an extra raised stitch plate on top of the original stitch plate, to keep the distance between the upper and bottom layers of the spacer fabric (Figure 10). In their experiment, the 3D spacer fabric trials were stitched with a traditional lock stitch and the new spacer stitching method. Using an infrared (IR) camera, they found that samples sewn by the new spacer stitching method had 10–14% better thermal resistance than samples sewn by conventional lock stitch. However, in the development of this new spacer sewing technique, the researchers failed to evaluate other mechanical quality parameters such as stitch size, stitch tension, stitch sequence, elongation, resilience, elasticity, etc. Furthermore, due to the ignorance of stitching’s impact on thermal insulation performance, new stitching innovations for protective clothing have not been sufficiently researched so far.

4. Advancements in Moisture Management of Cold Protective Clothing

When designing cold protective clothing, it is crucial to consider the principles of moisture management due to assorted reasons. When wearing higher thermal insulation clothing, the wearer’s body will accumulate sweat in the microclimate [168]. If this moisture is not effectively managed, it can lead to damp clothing, which loses its insulation and can cause feelings of coldness [6]. Unmanaged moisture trapped in the microclimate can also create a high-humidity environment, leading to discomfort and increasing the risk of cold-related injuries such as hypothermia [169]. Furthermore, the insulation performance of cold protective clothing can be degraded by moisture remaining on the clothing material [170]. For example, down insulation loses its loft and insulating ability when wet. Addressing these challenges involves using advanced materials and designs that enhance moisture wicking, breathability, and thermal regulation. This ensures that the wearer remains dry, warm, and comfortable, even in harsh cold conditions.

The development of moisture management in cold protective clothing has evolved significantly over the years, driven by advancements in textile technology and a deeper understanding of human physiology. Recent advancements in moisture management for cold protective clothing have focused on enhancing thermal comfort and efficiency. Some key developments are smart textiles, membrane-based systems, nanotechnology, layering techniques, and performance evaluations. Smart textiles have the ability to dynamically respond to environmental changes, adjusting their properties to manage moisture and temperature more effectively [171]. Membrane-based systems offer both waterproof and breathable properties, allowing water vapor to escape while preventing external moisture from entering [6]. Applications of nanomaterials, such as graphene and nanotubes, helps improve moisture-wicking and thermal regulation of fabrics. In addition, the development of multilayered clothing systems has been developed to optimize moisture management. Each layer serves a specific function, such as moisture wicking, insulation, and protection from external elements [172]. The Moisture Control System, MCS^®, technology improves the hydrophilic properties of fabric, allowing moisture to dissipate or transport along the yarn to dry faster, keeping the wearer dry and comfortable. In cool environments, MCS^® Adaptive technology moves moisture more gradually to provide and maintain warmth, as shown in Figure 11. Performance evaluation methods help in understanding the interaction between heat and moisture management and their impact on thermal comfort. These advancements aim to provide better protection and comfort for individuals working in cold environments, ensuring they stay dry and warm.

Furthermore, in order to prevent unnecessary heat loss from the human body in cold conditions, cold protective clothing is typically made from materials with excellent insulating materials such as merino wool and synthetic fibers [99]. These materials are specifically designed to provide efficient insulation and prevent excessive heat dissipation from the human body [78]. However, when heat is retained by the clothing insulation, and the wearer engages in intensive physical activities, the skin may produce sweat, leading to the fabric becoming wet. If the sweat remains trapped inside the fabric, it can adhere to the skin and cause discomfort in the body’s microclimate. For cold protective clothing, it is important to consider moisture management and permeability to regulate heat transfer through evaporation. Therefore, a well-designed moisture management system is essential to maintain an optimal microclimate that is free from both cold sensations and wetness [36]. These advancements represent the forefront innovations in clothing moisture management, from the polymer to the fabric level.

5. Summary and Outlook

In the first section of this review, the prevalence and severity of intolerance to thermal comfort challenges caused by disabilities in thermoregulation mechanisms were highlighted. Secondly, although this review intended to assess recent innovations in clothing development, particularly those designed for people with cold-intolerance problems, unfortunately, the current research conducted in this area overlooks the special comfort needs of cold-intolerant people. Due to the identified gaps, this paper reviewed recent research findings and state-of-the-art developments around cold protective clothing in general that could be applied to aid cold-intolerant people. This was aimed at providing input for future researchers and designers to create clothing for cold-intolerant people and to indicate adaptation options that could be taken from the current state, considering the nonstandard ways that cold-intolerant people react to thermal changes.

Some novel methods used to enhance the thermal insulation capabilities of cold protective clothing at the material/textile manufacturing stage were discussed. Structural alterations in fibers or yarns, weaving designs, incorporating smart materials into conventional textiles, and utilizing contemporary surface improvement techniques in the pretreatment stage are some of the state-of-the-art intervention options for developing superior clothing materials. Studies have been conducted to create clothing materials that can trap a maximum air layer for optimal thermal insulation performance within a specific weight and thickness.

In the first phase of research in this field, multilayer padding technology was commonly used to enhance the thermal insulation of clothing materials. However, several researchers noted that this method led to ergonomic discomfort for the wearer due to increased clothing thickness. To address this issue, thin inserts made of nonwoven insulating materials incorporating aerogel and PCM microcapsules were developed. While these inserts improved thickness and weight, the overall thermal insulation performance of padding materials decreased when exposed to moisture. On the other hand, researchers found that clothing materials with better thermal insulation performance were obtained through innovative structural designs rather than padding. By using volumized yarn and porous structures, a sufficient air trap could be achieved on the textile surface. This led to the development of 3D spacer fabric, a lightweight, low-thickness, and effective thermal insulation material produced using technical textile manufacturing technology. Various structures of high-level air-trapped spacer fabric are currently being produced; advanced surface finishing technologies are being used to address any thermal insulation weaknesses resulting from the material type. Previous efforts to enhance cold protective clothing focused solely on the textile/clothing material stage. However, the insulation capacity of cold protective clothing is also influenced by design factors during the garment development stage. This review paper discussed innovations and design solutions at the garment development stage. Size and fit analysis to achieve an optimal air layer depth and volume between the human body and clothing layer, as well as the applications of new stitching technology for sewing cold protective clothing, are among the design solutions highlighted in this review.

In conclusion, future research directions in the design of cold protective clothing are recommended. While significant progress has been made in understanding and addressing thermal comfort factors in various application areas, most existing studies have focused on the thermo-physiological responses of healthy individuals. However, individuals with cold intolerance (CI) often experience cold sensations despite wearing multiple layers of winter clothing, such as trousers. This highlights a critical gap in the research, emphasizing the need to develop innovative clothing solutions tailored to the unique thermal comfort requirements of CI individuals. People or groups who demand special consideration due to their health conditions have been mostly ignored. In this review, the prevalence of cold intolerance among individuals currently facing critical thermal discomfort challenge was investigated. However, this paper did not find research on clothing thermal comfort specifically addressing these issues. Individuals who experience symptoms of cold intolerance are exposed to continuous and excessive heat loss due to their thermoregulation disorder. Seasonal weather conditions are not significantly important for developing cold protective clothing for such wearers. Furthermore, the skin temperature of individuals with cold intolerance is lower than that of healthy individuals, regardless of environmental conditions. Therefore, they are more vulnerable to the detrimental effects of cold environments on their health.

To better transform existing research results into practical products that meet the needs of people with cold intolerance, it is crucial to consider the thermo-physiological parameters specific to this group. Researchers play a key role in developing a baseline understanding of these parameters, studying the unique challenges in regulating body temperature, identifying effective insulation, and heating solutions. Textile and clothing technologists contribute by developing and incorporating advanced materials and technologies into clothing and accessories that provide effective insulation and heating for optimal thermal performance. Designers integrate these materials into practical, comfortable, and aesthetically pleasing clothing and accessories. Medical experts offer valuable insights into the health implications of cold intolerance and advise on the safety and efficacy of the products. Through collaboration, these professionals can develop prototypes, conduct rigorous testing with cold-intolerant individuals, and ensure affordability and accessibility. Continuous improvement based on user feedback, along with effective education and marketing strategies, will further enhance the adoption and effectiveness of these products in improving thermal comfort for those with cold intolerance.