Infrared functional textiles
Far infrared (FIR) textiles are a new category of functional textiles that have putative health and wellbeing functionality. FIR exerts strong rotational and vibrational effects at the molecular level with the potential to be biologically beneficial. The underpinning targeted benefit of FIR textiles is the enhancement of blood circulation. These materials are based on the principle of absorbing energy from sunlight and then radiating this energy back onto the body at specific wavelengths.This chapter explores the basic principles of electromagnetic radiation as they relate to FIR, examines the putative mechanism and therapeutic effects of FIR therapy, and surveys what objective evidence currently exists of health benefits. The role of FIR fibres and fabrics in functional textiles is also evaluated, together with existing and potential applications of these textile products. A summary of the benefits, limitations and anticipated future trends of current products and technologies is then provided.
Textiles that offer additional functionality to the user above conventional textiles are increasingly gaining prominence. Far infrared (FIR) textiles are a new category of functional textiles that have putative health and well- being functionality. At the molecular level FIR exerts strong rotational and vibrational effects with the potential to be biologically beneficial. The underpinning targeted benefit of FIR textiles is the enhancement of blood circulation. These materials are based on the principle of absorbing energy from sunlight and then radiating this energy back onto the body at specific wavelengths.
This chapter first explores the basic principles of electromagnetic radiation, and in particular infrared (IR) radiation. Then it examines the putative mechanism and therapeutic effects of FIR therapy, and surveys what objective evidence currently exists of health benefits. The role of FIR fibres and fabrics in functional textiles is then explored, focusing on currently commercially available bio-ceramic-based FIR products. Applications of these textile products are then summarized, along with other, non-FIR, therapy applications of IR properties in textiles. Finally the benefits and limitations of these technologies are highlighted and future trends discussed.
Electromagnetic radiation comprises self-propagating waves, which have electric and magnetic components. The basic unit of electromagnetic radiation is the photon. Electromagnetic waves, or rays, are classified according to their wavelength (Knight, 2004).
Sunlight is made up of both visible and invisible wavelength radiation. The visible range covers red, orange, yellow, green, blue, indigo and violet coloured light. Wavelengths shorter or longer than these colours are invisible to humans and include ultraviolet (UV) light and X-rays at shorter wavelengths, and microwaves and radio waves at longer wavelengths.
Electromagnetic waves between visible light and the microwave region are called IR light. The term infrared literally means below red, because IR has a longer wavelength, and hence lower frequency, than visible red light. The wavelength of IR waves range from 0.75 to 1000 μm. IR is commonly divided into three spectral regions: near (0.75-1.5 μm), mid (1.5-5.6 μm) and FIR (5.6-1000 μm) (Lin et al, 2007). The boundaries between the near, mid and FIR regions are not agreed upon and can vary substantially depending on the context. Figure 8.1 shows the position of IR within the electromagnetic spectrum.
IR radiation is emitted by any object with a temperature above absolute zero. The wavelength at which an object radiates most intensely depends on its temperature. In general, as the temperature of an object cools, it emits farther IR wavelengths.
IR radiation is commonly, but mistakenly, equated to ‘heat’ or ‘thermal radiation’. This misconception is because people often attribute all radiant heating to IR light. In fact, electromagnetic radiation of all frequencies will lead to heating of materials that absorb them. IR radiation from sunlight accounts for half of the heating of the earth, with the other half the consequence of visible light that is absorbed then re-radiated at longer wavelengths (Knight, 2004). Unlike electromagnetic radiation, which can propagate through a vacuum, heat is technically energy that flows due to temperature differences, and is transmitted by thermal conduction or thermal convection.
The FIR electromagnetic spectrum of wavelengths between 5.6 and 1000 μm is not visible to the human eye. As described earlier, any material above absolute zero emits IR. The FIR region of the spectrum is notable in that materials at room temperature emit radiation that is generally concentrated between 8 and 25 μm (Knight, 2004).
On absorption, IR energy in general elicits vibrational excitation in molecules through changes in the molecular dipole moment. FIR radiation can penetrate relatively deeply into the human body, reaching as far as 4–7 cm into the tissues, and is readily absorbed by biological materials (Inoue and Kabaya, 1989; Karu, 1998).
With respect to FIR emission, the concept of radiative emission is significant in understanding the relative IR emissions of materials. Emissivity is a material surface property which describes how radiative emissions deviate from that of a theoretical black body. Two objects can be at the same physical temperature, but can have differing emissivities, meaning that they are emitting different electromagnetic radiation profiles (Knight, 2004).
People have long believed that exposure to sunshine can maintain and enhance health and wellbeing. Phototherapy, or light therapy, is a rapidly growing, but often controversial, area of health treatment. In particular, low-level laser therapy (LLLT), also known as ‘photobiomodulation’, is a relatively new therapeutic option used to relieve pain and inflammation and promote wound healing (Desmet et al, 2006; Karu, 1989). It has been known for many years that low levels of either coherent (waves are in phase with each other, e.g. laser light) or non-coherent (waves are out of phase with each other, e.g. sunlight) radiation in the red or near-IR region can in some situations accelerate some phases of wound healing, reduce pain, inflammation and swelling, and prevent tissue death (Karu, 1998), but the benefits of LLLT in medicine in general are still debated. The Food and Drug Administration (FDA) cleared the first LLLT device for hair growth in January 2007, and light treatment is now being used for a wide range of therapeutic applications (Basford et al, 1998).
The number of clinical studies in the general area of phototherapy is increasing, with mixed results in terms of observed efficacy (Capon and Mordon, 2003; Karu, 2003; Smith, 2005; Yu et al, 2006). Many researchers claim to have achieved positive clinical and pre-clinical results with LLLT, and the evidence for positive therapeutic effects under selected conditions is certainly mounting (Desmet et al, 2006; Zhang et al, 2009b), but others remain sceptical. This scepticism can probably be largely attributed to the introduction into the market of self-treatment light devices with little or no supporting clinical data. However, it seems certain that selected forms of light therapy will continue to grow in popularity and to gain increasing mainstream acceptance and medical usage. It is within this context that FIR therapy must be considered.
The systemic biological effects of FIR radiation on whole organisms are currently poorly understood. However, the basic principles are established. FIR wavelengths have relatively high penetrability in most biological materials. Absorption by biomolecules, such as proteins and lipids, leads to stimulation of intramolecular bond vibrations and rotations, and this increased molecular kinetic energy leads to an elevation in temperature (Lubart et al, 1992).
The underlying principle of FIR therapeutic approaches is that FIR absorption can be used to elevate local tissue temperature, leading to dilation of blood vessels and therefore enhancing blood microcirculation for targeted regions of the body. Microcirculation refers to the flow of blood through the vascular network lying between the arterioles and venules. The main function of the circulating blood is to transport oxygen and nutrients, and to remove carbon dioxide and other waste materials.
FIR rays in the 4–14 μm range are considered beneficial for cellular growth, and this radiation has been termed by some as the ‘rays of life’ (Voeikov, 2006). FIR can penetrate deeply into tissue and induce a higher skin blood flow. When FIR penetrates through the skin into subcutaneous tissue, it induces the generation of heat energy through resonance effects, essentially stimulating intramolecular micro-vibrations. It can be said that the metabolic rate of an individual is an indicator of health. This resonance process is believed to invigorate cellular activities through expanding capillaries and increasing the circulation of nutrient-rich, oxygenated blood, thereby improving overall metabolism. Other beneficial effects may include assisting the regenerative ability of tissues, activating the immune system and enhancing the removal of cellular waste materials.
On this basis, FIR therapy therefore has a wide range of claimed health and wellbeing benefits. A summary list of the major proposed ways in which FIR therapy can improve human health is presented here (Burton Goldberg, 1993):
The question can be asked – What are the advantages of utilising IR over full spectrum sunlight? The potential advantage is that sunlight contains a range of radiation types including UV, visible light and IR. Different wavelengths have different biological effects, with some positive and some negative (Knight, 2004). IR can penetrate tissues (the longer the wavelength, the deeper the penetration) and in principle specifically target selected biomol- ecules, avoiding damage associated with exposure to other electromagnetic wavelengths (Inoue and Kabaya, 1989; Karu, 1998).
Health conditions that are particularly targeted by FIR therapy, and from which there is significant anecdotal feedback of efficacy, include arthritis, muscle pain and spasm, joint stiffness and Raynaud’s syndrome.
The most notable application to date is probably the treatment of arthritis. Arthritis refers to a wide range of different muscle/skeletal conditions, including osteoarthritis, gout, rheumatoid arthritis and fibromyalgia. The symptoms of most forms of arthritis are joint inflammation, pain and stiffness. Arthritis pain and inflammation occurs in nearly everyone as they age, with the majority of people over the age of 50 exhibiting some signs of arthritis as joints degenerate. FIR therapy is believed to be particularly useful for arthritis because it can be applied directly to the area of pain.
Along with these therapeutic applications, some claim farther-reaching health benefits, including anti-cancer, anti-obesity and anti-pathogen effects. There are even claims that FIR rays can be used not only to inhibit cancer growth, but also to kill existing cancerous cells.
There is an increasing number of supporting studies on the therapeutic effects of FIR therapy, but these are still few in number. For example, one clinical study came to the conclusion that there are significant improvements in subjective measures of pain and discomfort associated with Raynaud’s syndrome (Ko and Berbrayer, 2002). Raynaud’s syndrome is a disorder that results in discoloration of the fingers and toes. This phenomenon is believed to be the result of decreased blood supply, with emotional stress and cold the classic triggers (Anderson et al, 2007; Hirschl et al, 2006).
Among FIR therapy’s putative healing benefits is its ability to relieve inflammation (Lin et al., 2008). Studies have found that FIR therapy can exert a strong anti-inflammatory effect by inducing haem oxygenase-1, an enzyme which catalyses the oxidative degradation of haem. It is noteworthy that induction of haem oxygenase-1 production is also associated with anti- proliferative, antithrombotic and antioxidant effects through generation of carbon monoxide and bilirubin.
Another notable research study investigated whether FIR treatment could improve wound healing in rat models (Toyokawa et al, 2003). This study measured skin wound area, skin blood flow and skin temperature before and during FIR irradiation. Wound healing was observed to be significantly more rapid with than without FIR. Interestingly, skin blood flow and skin temperature were not observed to change significantly. However, histological evaluation revealed greater collagen regeneration and infiltration of fibroblasts that expressed transforming growth factor-β1 (TGF-β1) in wounds in the FIR group. Stimulation of the secretion of TGF-β1 or the activation of fibroblasts can be considered as a possible mechanism for the promotion of wound healing.
Taken as a whole, there is still a relative paucity of objective scientific and clinical data to provide comprehensive support for FIR therapy health benefits in humans, although those studies that have been conducted indicate significant potential. In addition, detailed information on the molecular mechanisms by which FIR exerts beneficial effects is scarce. It is undoubtedly for these reasons that, currently, FIR therapy is generally regarded as an alternative medical approach, and has not yet been fully accepted by mainstream medicine. As further research is conducted and the general field of phototherapy takes on greater acceptance, the indications are that FIR therapy will continue to grow and become established as a viable therapeutic approach.
With respect to the more extreme claims of therapeutic benefits, such as FIR efficacy in killing cancerous cells and reversing ageing effects, currently there does not appear to be sufficient objective data in support, and these claims must be treated with due caution until further robust clinical trials are undertaken.
Textiles represent an excellent means to deliver localized treatment to the human body. FIR fibres are fibres that can transform absorbed energy into the emission of FIR rays. FIR fabrics are generally derived from traditional fibres, but have been functionalized by the incorporation of a material with appropriate electromagnetic absorption and emission properties.
So how can a fibre emit FIR radiation? The human body is continuously emitting thermal energy. The principle behind FIR textiles is that when the fabric is stimulated by this thermal energy, it absorbs the heat and converts it via emission of FIR rays, which are directed back into the human body. So, in theory, FIR textiles are powered by the wearer.
A simplified way in which this putative process can be explained is that the layer of FIR-active material acts as a mirror to the human body, with heat emitted sent back as FIR within a specific range of wavelengths.
FIR functionality can be incorporated into textile products in a variety of ways. The main route for manufacturing an FIR textile is usually to blend the fibre polymer and an FIR ceramic powder together to make the FIR yarn, with the FIR yarn subsequently incorporated into various FIR textile products. The FIR-active materials can be adhered to the textiles or yarn by techniques including impregnating, printing, coating, covering and laminating (Lin et al, 2008). FIR materials can be added during the dyeing or finishing treatment process of an ordinary fabric.
After absorbing either sunlight or heat from the human body, FIR textiles are designed to transform the energy into FIR radiation with a wavelength of 4–14 μm and pass it back to the body. Fabrics with FIR function can effectively achieve thermal retention and are therefore ideal materials for activities that require warmth retention, such as mountaineering and hiking.
It has been reported that skin temperature increases are more rapid for radiant heating than for equivalent conductive or convective heating (Nakajima et al., 2002). This corresponds to increased skin sensitivity and therefore needs to be taken into consideration in the design of textiles with radiant heating functionality.
Of course, the FIR emissivity value of the final product is critical with respect to its function. Studies on the emissivity of various FIR textiles have been performed and have arguably shown that relatively high emissions can be obtained (Zhang, 1994; Zhang et al., 2009a). The critical question with regard to therapeutic applications is – What level of emissivity is required to achieve measurable benefits to the wearer? This is the R&D area where currently there is a paucity of robust supporting data with respect to reallife applications and where the potential to significantly boost the image of FIR textiles exists.
Bio-ceramics is a term applied to ceramics with biological functionality, including those that can emit FIR (Richerson, 1992; Shackleford, 1998). The ceramic is therefore the functional component of the functional textile, and the amount and means by which the bio-ceramic can be incorporated into the fibres determines the overall fabric FIR emissivity (Koo et al., 2007).
FIR textiles are often made through adding nano- or micro-sized ceramic powder to polymers prior to spinning. Bio-ceramic powders that can be incorporated into the structure of textiles to add FIR effects include magnesium oxide, zirconium, iron oxide, silicon carbide and germanium-based compounds (Lee and Lee, 2006; Park et al., 2006). These materials are believed to retain emitted body heat and re-emit as FIR deep into joints and surrounding tissues to increase blood flow.
A common way to manufacture FIR fibres is through blending polypropylene with the active bio-ceramic. The bio-ceramic is ground into micro- or nano-particles and is either inseminated into the polypropylene during the fibre forming process or impregnated through a soaking or coating process. In one example of a coating process, the Fir-Tex line of products are derived from a treated polyamide fabric with a film of composite material based on polyurethane and the chosen bio-ceramic (http://fir-tex.com/).
A key challenge in the manufacture of bio-ceramic/polymer-based FIR fibres is to optimize the density of active material within the polymer matrix and thereby maximize the FIR emissivity. As the tensile strength of the fibre polymer tends to decrease with increased ceramic content and because the nozzles often used are abrasively worn down by the ceramic powder, the content of FIR bio-ceramic powder in textiles is limited. In one manufacturing protocol, this potential issue is claimed to be overcome by a sputtering method that forms a transparent and uniform FIR bio-ceramic thin film on the surface of the substrate (Lin et al., 2008).
Another class of FIR fibres are bamboo-carbon modified polyesters (BCMP). Aged bamboo is converted to bamboo-carbon though thermal dissociation processes. The materials thus generated are very dense but porous, with high surface area. When ground into powders, these can then be incorporated into a polymer matrix, in this case polyester. BCMPs are reported to have good FIR emissivity properties and it is claimed that use of BCMP fabrics can raise body temperature by 4–7 °C (Qi et al., 2006).
Bio-ceramic FIR textiles have also been designed by incorporating phase-change material microcapsules (PCMMcs) in combination with silicon carbide to provide additional thermal storage and insulation properties (Koo et al.,2007).
As discussed, the key targeted effect of FIR therapy is improved blood microcirculation in order to reduce inflammation and pain in muscles and joints, and to promote regeneration and fast healing. The ideal application for FIR textiles, therefore, has been in a full range of products for localized pain relief and support. This includes therapeutic knee bands, elbow bands, wrist bands, waist bands, gloves, socks, shirts, back-belts and long johns. However, applications for IR textiles are broad. The major categories are summarized below.
FIR medical supports and orthotics worn during exercise are purported to help increase sweating, removal of lactic acid and toxin breakdown. Traditionally, athletes have used supporting textiles made of fibres such as myelin, latex and rubber to achieve some benefit. However, since these fabrics do not breathe well, deleterious effects such as skin irritation and inflammation can result. In contrast, FIR textiles can, in principle, maximize body heat and thereby prevent muscles from feeling over-worked, potentially improve muscle tone and mitigate soreness and muscle spasms while reducing the risk of injury (Babu, 2008). FIR textile products can also assist with the relief of injured or over-worked muscles, tendons and ligaments. Due to their enhanced absorption properties, FIR textiles can also help minimize sun damage during sport and recreation.
The chief therapeutic function targeted by FIR textile products is promotion of microcirculation. Textiles can be made to adhere closely to body surfaces and are therefore ideal for delivering localized functionality, such as in joint supports. FIR therapeutic products are therefore often designed to relieve arthritis pain, such as rheumatoid arthritis, knee pain, shoulder pain, neck pain, and all kinds of chronic pains.
The range of therapeutic targets for FIR textiles currently on the market is very broad and includes Raynaud’s syndrome, sprains, fractures, bunions, gout, athlete’s foot, psoriasis, poor circulation, chilblains and RSI (http://www.farinfraredhealth.com/). Essentially, FIR textiles are marketed with therapeutic claims to match those of FIR therapy in general.
Textiles with strong FIR emission properties make excellent fabrics for warmth retention products. FIR fabrics can keep the body warm in cold environments. For instance, FIR sleeping pads are available offering instant warmth even in cool air temperatures. Similarly, wools and cottons impregnated with ceramic insulating powder are available as socks, gloves, jackets and car seats to promote warmth. For marketing purposes these products are promoted in conjunction with the offer of passive FIR health benefits.
Similar FIR fibres are also used in some advanced heated clothing. For such heated clothing, an additional power source in the form of a battery is used to stimulate the warmth radiating properties of the FIR fibres. This form of clothing is designed to provide a significant heating effect in very cold environments.
Preliminary evidence is emerging of beneficial health effects from FIR therapy. Particularly for arthritis and pain relief, textiles represent an excellent potential means of delivering localized treatment.
However, FIR textile technology is currently limited by the lack of reputable scientific and clinical trial support for the proposed therapeutic effects of FIR. For this reason, treatment using FIR textiles is still regarded as an alternative therapy, and currently lies outside mainstream health and medical treatments. Further targeted and objective studies are required to provide support for genuine therapeutic claims.
The benefits of IR textiles in non-therapeutic applications are significant and relatively well demonstrated. In particular, utilization in sportswear and warmth clothing offers an excellent route to providing additional functionality over and above conventional fabrics.
A limitation of current FIR textile products and technologies is the amount of FIR-active material that can be incorporated into the fibre, and the level of FIR that can be therefore be directed back to the body. A further limitation are the consumer-care properties. Ceramic pores can be blocked by dirt and sweat, but repeated washing can damage the ceramic layers and limit function. For instance, for the Fir-Tex range of breathable FIR fabrics, the manufacturer recommends that the products are not washed above 30 °C and that fabric softener be avoided (http://fir-tex.com/). Development of fibre technologies that are increasingly robust to consumer routine will facilitate enhanced consumer satisfaction with FIR textile products.
The general field of phototherapy is well on the path to mainstream acceptance for those applications where efficacy is being robustly demonstrated. It is notable that a recent paper in Nature outlined a potential mechanism of actions that could underpin the biological effects of low-level light therapy (Lane, 2006). This study presents evidence that red or near IR light may be able to relieve chronic inflammatory conditions.
Recent studies are also beginning to provide preliminary evidence for potential specific beneficial effects of FIR in particular. Selected forms of FIR therapy are anticipated to gradually take on increasing acceptance in mainstream medicine and science. Lack of full acceptance to date can be attributed to a combination of both general unsubstantiated claims and the fact that many studies have not been conducted with appropriate scientific rigour and methodology (Smith, 2005).
The further development of FIR textiles is likely to parallel that of FIR therapy in general. FIR textile products have an existing consumer base and will continue to occupy a niche in the textile market. To break out of this niche, however, robust data will need to be generated showing therapeutic efficacy. On the other hand, for non-therapeutic applications such as sportswear and warmth retention, FIR textiles have excellent potential for increased utilization and market uptake. This will be supported by continued innovation in FIR-active materials, in particular those technologies allowing increased active density and quantum yield.
As mentioned at the outset of this chapter, IR textiles represent a growing and exciting area of functional textiles. As new technologies and further supporting studies become available, this area will undoubtedly continue to mature and generate ongoing interest.
General information on electromagnetic radiation can be found in the ‘Electromagnetic Waves’ section of the Centre for Remote Imaging, Sensing and Processing website (http://www.crisp.nus.edu.sg/ research/tutorial/em.htm) and a historical summary of IR is on the Omega website (http://www.omega.com/literature/transactions/volume1/historical1.html). There is also a simplified explanation of IR on the NASA website for children at http://science.hq.nasa.gov/kids/imagers/ems/infrared.html.
The American Society of Photobiology website provides objective background information with respect to the area of phototherapy (http://www.pol-us.net/ASP_Home/index.html), and also provides excellent background information on the nature and effects of electromagnetic radiation in a biological context.
Sources of general information around FIR usage in putative therapeutic applications include the websites http://www.farinfraredhealth.com/, http://www.toolsforwellness.com/far-infrared.html and www.earthtym.net/ref-far-infared.htm.
There are still relatively few sources of information specific to FIR textiles. However, the following websites provide a general indication of current FIR textile products available on the market – www.fir-tex.com, www.firheals.com/catalog/infrared_clothing and www.therapygloves.com.
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