The Future of Functional Clothing for an Improved Skin

 

Abstract
The skin microbiome has become a hot field of research in the last few years. The emergence of next-generation sequencing has given unprecedented insights into the impact and involvement of microbiota in skin conditions. More and more cosmetics contain probiotics or bacteria as an active ingredient, with or without scientific data. This research is also acknowledged by the textile industry. There has been a more holistic approach on how the skin and textile microbiome interacts and how they influence the pH, moisture content and odour generation. To date, most of the ingredients have a broad-spectrum antibacterial action. This manuscript covers the current research and industry developments in the field of skin and textiles. It explores the nature of antimicrobial finishing in textiles which can disrupt the skin microbiome, and the benefits of more natural and microbiome friendly therapies to combat skin conditions, malodour and skin infection.



1. Introduction: Background and Driving Forces
The clothing and fashion industry is a multi-billion market with a fast pace of innovation. In the years 1990 to 2000, laundering habits have shifted from chemical-based detergents and hot washes towards enzyme-based detergents and cold washes, driven by environmental and economic reasons [1]. The side effect of the lower washing temperatures, enzyme-based cleaning technology, decreased water consumption and absence of bleaching agents is a reduced microbial and volatile removal in the laundry process [2]. Polyester and other synthetic fibres have become a mainstream yarn in clothing textiles. On average, two-thirds of new textiles produced are synthetic and over half are made from fossil fuel-based polyester [3]. However, polyester is prone to selective bacterial growth and odour development [2].

Clothing textiles absorb skin sweat and microbes, which can lead to an increased odour generation and bacterial colonisation. Textile microorganisms can cause staining, fabric deterioration and even physical irritation, such as skin allergies or infections [4]. In recent decades, a wide range of textile finishes, antimicrobial techniques, nanoparticle applications and bioactive remedies have been developed to answer this need.
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2. Skin & Textiles
2.1. The Skin

The skin is the largest organ in the human body [5]. Its complex surface and semipermeable membranes interact with the immune system and inner body as well as the outer environment. The ability for water to permeate the skin’s surface can be considered one of the most crucial skin functions that make human life possible [6]. This porous surface is a highly efficient self-repairing barrier designed"to keep in the inside in and outside out" [7]. The skin gives protection against chemical, physical and biological hazards. Chemical hazards include irritating agents, while physical and biological hazards are UV radiation and pathogens or insects, respectively. Yet, the different protective functions of the skin are often linked or even co-regulated [8,9]. In addition, the skin has a regulatory role as well. Examples of this include moisture release, dehydration prevention, blood pressure regulation and body temperature control [8,10]. It is clear that the skin has an indispensable function where its importance can be seen when confronted with skin diseases [8].

Despite the skin’s semi-porous characteristic, it is generally perceived by society as a rigid "symbolic surface between self and world" [11]. This functional and visible interface is regarded by many as the basis of personal identity. As such, skin does not contribute solely to physical properties, but also to emotional human well-being as skin appearance affects one’s self-esteem. The skin’s biology, often regarded as the superficial layer or trivialised as vanity, is now well-recognised as a real interdisciplinary research topic within sciences [6]. In this way, the cultural attitude towards the skin, compared with the physical qualities, are quite contradictory. While society considers skin as an absolute barrier, this major organ is working hard to allow substances to pass through to protect and preserve the body’s equilibrium.

2.2. The Skin Microbiome
"Our skin serves as a primary defence, a sensory and an excretory organ" [12]. The skin is also home to an estimated 1012 microorganisms that live and feed on the skin secretions and desquamations [12]. These billions of microorganisms, including bacteria, fungi, protists and viruses, form the so-called skin microbiome [13,14]. It is known that these organisms are more numerous in comparison to the body's own cells: there are approximately 30 trillion body cells in comparison to 39 to 100 trillion microorganisms. The skin covers an outside surface area of approximately two square metres, and the average microbial density is estimated to be 106 per square centimetre [8,9,15]. In reality, the skin covers about 25 m2, including invaginations, sweat glands and hair follicles [16]. Actual microbial density differs per body site, with the lowest counts found on the volar forearm, trunk and legs (102/cm2) and the highest counts in the underarms, umbilicus and toe web spaces (107/cm2) [17].

In general, at least 19 phyla are characterized within the microbiome, of which four phyla dominate the skin: Actinobacteria (51.8%), Firmicutes (24.4%), Proteobacteria (16.5%) and Bacteroidetes (6.3%). Furthermore, three genera represent 60% of the skin microbiome: Staphylococcus spp., Corynebacterium spp. and Cutibacterium spp. [8,13,14,15]. Humans acquire a skin microbiome at birth, but as people grow up, dynamic changes in the microbiome take place [18]. Stabilisation of this microbiome occurs when becoming adults; however, changes as a reaction to environmental factors are not excluded [14,15]. This variation and evolution in the human skin microbiome can be described in four different ways: as topographical, interpersonal, intrapersonal and temporal variation. Topographical variation is defined as the different skin niches, which determine the composition of the present community. As such, a difference can be made between sebaceous (oily), moist and dry niches. In fact, the skin is not an appropriate environment for the bacterial species to live. This is due to the acidic pH, low moisture content, presence of salts and antimicrobial molecules as free fatty acids, sphingosine, nitric oxide, immunoglobulins, etc. Yet, a lot of bacterial species are present on the skin because of this topographical variation. Each topographical niche differs in pH, hair density, sebaceous, apocrine and eccrine activity. This creates microenvironments that select for particular microorganisms. Moreover, some microorganisms have developed specific strategies to survive on the skin [15,16]. Next to topographical variation, there is interpersonal variation, which refers to the differences in skin microbiome between people. Differences in the microbiome within an individual is described by intrapersonal variation. Temporal variation is the change in skin microbiome that might occur within an individual over time.

Healthy skin depends on the diversity of bacterial species that live in close proximity and in relation to one another. The competition between different bacteria constitutes the body's first line of defence against bacteria and viruses [14]. The microorganisms create an environment that enables the skin to be less susceptible to diseases and pathogenic invasion [19]. A specific example of how bacteria play a role in defence is the production of the Esp protease by Staphylococcus epidermidis. This protease inhibits colonisation by the pathogenic S. aureus [15]. However, besides microbe–microbe interactions, there are also microbe–host interactions. These interactions can be divided into three categories, the first one being commensalism, where one species has an advantage and the other is unaffected. The second type of interaction is mutualism, which can be seen as a win–win situation for both parties. Lastly, a detrimental interaction is an interaction where one species obtains an advantage while adversely affecting the other party. Advantages for the microorganisms include the presence of nutrients on the skin. Therefore, by allowing the microbial community to evolve, the possibility of the skin microbiome to respond in a fast way to environmental changes is improved. As such, the skin forms a better protective barrier, which is, in turn, advantageous for the host [13].

The human skin microbiome faces many challenges. Over the past few decades, there has been an increase in the prevalence of allergic diseases present in Western countries [20]. This links with the'hygiene hypothesis’, a theory initiated by Erika Von Mutius, which indicates that children who are kept in very clean environments are more likely to develop hay fever, asthma and a range of other conditions [21]. Environments with high levels of microbial components found in nature can be preventative against the development of allergies long-term [20]. The ideologies on cleanliness, sterility and urbanisation have created a difficult environment for many beneficial organisms to survive on skin [22]. Nonetheless, microbial diversity is the key to a healthy immune system [23]. The skin microbiome has an important role in controlling the skin inflammation and tuning resident T lymphocyte function [19]. The urban environment has made it difficult to re-establish or maintain the beneficial bacteria that the body would regularly be exposed to in a more natural environment [24]. This modern chemistry and shift in lifestyle habits have undermined this ecosystem, making it less diverse and leaving the skin more susceptible to skin pathologies [25].

2.3. Odour Formation in Textiles
Clothing textiles are important contributors to odour development. The bacteria in textiles convert sweat secretions, sebum, skin desquamations and other external molecules into volatile compounds. Shelley et al. (1953) stated that odour intensity is potentially more intense in clothes as compared to the axillary skin [26]. Dravnieks et al. (1968) described axillary skin odour as primary odour, and axillary clothing odour as secondary odour [27]. Bacteria attaching and living on and in the fibres use carbon molecules or sebum compounds as substrate and generate volatiles as by-products. This odour can be vastly different from the odour from the axillary skin. There are two main reasons for this: the build-up of sweat secretions in the textiles, and the microbiome converting these molecules into malodour [28].

Three main types of sweat glands are present in the dermis: eccrine, apocrine and apoeccrine sweat glands. The different glands secrete fluids from which composition differs among types of glands [29]. Eccrine sweat glands are the most abundant and can be found on the entire body: on glabrous as well as on non-glabrous surfaces [29,30]. Apocrine glands are the second most important type of glands. The highest concentrations of apocrine sweat glands are found in the underarms and the anogenital area. However, the secretion is viscous and odourless and is composed of proteins, sugars, ammonia, ferric ions, steroids and lipids [8,29,31]. Apoeccrine glands develop during puberty from eccrine-like precursor glands and produce extremely copious sweat, which is secreted directly on the skin surface [29,31,32,33].

Over time, the metabolite load can increase in the fibres, as more sweat and sebum secretions are transferred to the clothes and are insufficiently removed by the laundering process. Specifically hydrophobic molecules, such as apocrine and sebaceous secretions, can adhere and bind to clothes, which leads to a build-up of metabolite precursors [28]. This might be an important explanation for permastink clothes. Not only the load of sweat but also the type of sweat is of influence. It was shown that a mutation in the human ABCC11 gene, which is prominent among east-Asian people, leads to less apocrine sweat secretions [34] and, as such, less malodour build-up in clothes. Additionally, gender, hygiene habits, diet and body mass index can have an impact on odour build-up in clothes.
2.4. The Textile Microbiome

The microorganisms living in clothing textiles have been a major target for odour control and disease prevention. The interaction between the wearer and clothing is an opportunity for skin bacteria to attach to the textile surface, which can lead to the growth of certain strains [28]. During this process, the absorption of the wearer’s sweat, sebum and bacterial metabolites to clothing can contribute to the outgrowth of pathogenic strains, contribute to malodour generation or trigger skin diseases [28,35]. The growth of these pathogenic or odour-causing strains are, however, dependent on textile composition, the individual and the activities of the wearer [28].

Bacteria are transferred in high numbers from armpits to clothing fibres due to their close contact. However, axillary and textile microbiomes of one person are not the same and differ in composition. Important odour-causing bacteria in armpits, Corynebacterium spp., could not be isolated from worn textiles [36,37]. Particularly low abundant skin microorganisms can selectively enrich in synthetic textiles [38]. Rather, the textile microbiome is composed of Staphylococcus, Micrococcus, Bacillus, Enterobacteriaceae and Acinetobacter, among others [36]. Selective enrichment of particular odour-associated taxa on different fibre types is an explanation of why some fibres, such as polyester, smell less pleasant. Micrococcus has been shown to be enriched on malodorous synthetic clothes, and are known to have the enzymatic potential to cause malodour from apocrine sweat [38,39]. Moraxella and Pseudomonas were similarly found on washed laundry and associated with malodour formation [40]. Unlike the microbial composition, the microbial load does not seem to be a determining factor in odour formation in clothes [37]. Nonetheless, most techniques to prevent odour or pathogen formation are making use of broad-spectrum antimicrobial agents.

2.5. Skin Conditions & Textile Relationship
Our clothing is consistently in contact with the human skin; thus, textiles are an important consideration when studying the cutaneous environment. Therefore, textiles are essential players in the potential causation and treatment of various types of dermatitis and skin diseases [41]. Different factors such as textile structure, pH, breathability, and microbiome should be considered for functional outcomes.

The relationship between textiles and skin is a new focus on how certain textiles and their microbiome can alleviate skin disorders. A good example is for atopic dermatitis, a chronic relapsing skin condition, associated with skin barrier dysfunctions, moisture loss, allergy/immunology, and pruritus symptoms [42]. Atopic dermatitis can be exacerbated by the colonisation of Staphylococcus aureus on the affected skin [43]. Atopic dermatitis is in part an environmental-related disease, and textiles and skin contact are an influential part of the cutaneous environment [41]. Research has found that wool and synthetic fibres, such as polyester and nylon, can worsen atopic dermatitis symptoms [41,44,45]. Cotton, however, is a recommended fabric for patients with such skin conditions [46]. Studies have also explored the use of silver-coated, chitosan-coated and cellulose-based textiles for their antibacterial effect on S. aureus, and have found promising outcomes [35,47].

The epidermis is characterised by a slightly acidic film, which is called the “acid mantle”, and has a pH value of about 5 or lower [43,48]. The resident skin microbiome is dominated by S. epidermidis at a pH of 4.7, and the growth of S. aureus is inhibited under these acidic conditions [48]. The skin microbiome is altered if the pH value increases towards a more neutral range. Skin diseases characterised by faulty barriers such as atopic dermatitis, candidal intertrigo and acne are all associated with aberrant pH values [49]. Recent research has explored the possibilities of citric acid-coated textiles as a therapeutic strategy for these chronic skin conditions [43]. In this study, a cellulose-based textile was coated with a citric acid finish in order to lower the pH of the textile surface. It was concluded that the citric acid-coated textiles, when worn next to the skin, reduced symptoms of atopic dermatitis, such as itching, and improved the skin barrier. Therefore, textiles with a lower skin pH might reduce microbial colonisation of atopic skin when worn next to the body. The citric acid-coated fabric might provide a basis for new preventive and therapeutic options in atopic dermatitis and other pH-related skin diseases [43].

Similarly, contact dermatitis is a common skin disorder caused by the direct contact with an agent or surface to the skin. The symptoms are marked by erythematous, vesicular, papular or lichenified pruritic skin lesions [50]. It is caused by irritant triggers in 80% of the cases, such as textiles or chemical agents, and allergic triggers, in the remaining 20% of patients. There are many chemicals used in the production of textiles which have been linked to contact dermatitis. However, disperse azo dyes, frequently used for colouring synthetic textiles and known for its poor adhesion to fabric, have often been found to cause allergic textile dermatitis. This could be due to the lipophilic molecules which can easily migrate onto the skin [51,52].

Other allergens include dust mites, which are found in clothing and bedding, represent a prevalent risk factor for asthma and skin conditions. There are many species of dust mites; however, the most common in homes across the world are Dermatophagoides pteronyssinus, Dermatophagoides farinae and Euroglyphus maynei [53]. There have been many studies linking atopic dermatitis with dust mites. A recent study has linked levels of vitamin D3 and the severity of atopic dermatitis symptoms from dust mite allergies [54]. However, long-term trials of house dust mite reduction methods are needed [53]. In order to remove these allergy-associated dust mites in clothing, it is generally advised to wash fabrics at minimum 55 °C. An alternative for more natural methods for reducing dust mites are being studied. Research on the use of eucalyptus oil on textiles, as a natural alternative to kill the dust mites, is being explored [55].

♦ References

► www.ncbi.nlm.nih.gov

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