Noninvasive Experimental Methods Exploring Sensitive Skin
Subjective tactile sensory comfort
Dew point determination
Skin capacitance mapping
Corneocyte chemical reactivity
The fabric sensory comfort is a complex psychological process. Tactile or sensory comfort is thought to be directly unrelated with any particular nerve ending. When the fabric is held, handled, touches lightly, moves over, or is packed tightly to the skin, the individual describes how the skin feels and responds to fabrics (17). The same stimulus possibly elicits quite different responses from different individuals. Descriptors of the sensory attributes of tactile or sensory comfort such as soft, harsh, smooth, rough, sticky, damp, prickly, clammy, cool, warm, heavy, comfortable, or uncomfortable are used for describing the feelings. The intensity of the sensations defines the fabric feel ranges from soft to uncomfortable.
Two distinct situations are contrasted when assessing (a) perceived sensory attributes, including softness, smoothness, prickliness, and (b) other characteristics evaluating the fabric tactile comfort. Both test evaluations are possibly performed either by handling the fabrics or after wearing the fabrics on other parts of the body. In fact, the feel of a textile contact is frequently different on the palms from that on elsewhere. Indeed, the nature and density of neural receptors in the skin differ according to the body site. In addition, other skin structures are likely involved in abnormal sensory responses to contact with cleaned, laundered, or softened fabrics (9). This might suggest that a causative mechanism is associated with the chemicals involved in fabric processing. Changes in surface friction in part result from treating fabric with enzymes and softeners. The SC reactivity to cleaning agents for textiles is impaired in some subjects claiming SSI (13,14). Basically, two distinct chemical mechanisms, acting either singly or in combination, are involved in this process. On the one hand, surfactant interactions with regular corneocytes are unusual in their intensity, releasing a variety of mediators including cytokines, prostaglandins, and leukotrienes (15). In turn, these biomolecules boost the release of neuromediators from distinct cells leading to specific nerve ending stimulation (29). On the other hand, any impaired SC barrier function to surfactants possibly allows some xenobiotics to directly stimulate sensory nerve endings. Some chemicals are indeed released near the nerve endings. According to such hypothesis, SSI might be induced by subtle changes in the SC structure including a thinner layer with or without alteration of the corneocyte structure (30) and desquamation rate (31). Similarly, a reduced corneocyte size was suggested to enhance the penetration of water-soluble xenobiotics. Still another possibility involves an individual lowered threshold for skin nerve stimulation. Indeed, free nerve endings and specialized nerve corpuscules are under the influence of both excitation stimuli and antagonist signs (29,30,32). When the latter activity is lowered, it has been postulated that the efferent neurosensorial input is amplified and perceived as an SSI manifestation. As a result, lifestyle including psychological stress and emotions possibly influences the reactive skin condition.
Some subjects with SSI feel discomfort including itching and stinging when wearing garments. Unlike the majority of mild cleansers, some more aggressive cleaning agents including household cleaning products and laundering products possibly induce skin tightness, after about 5–10 minutes (14). Such unpleasant perception is ascribed to the physical attack of the upper SC by rapid water evaporation from the skin surface. In fact, harsh surfactants create an immediate corneocyte overhydration and swelling, followed by rapid water evaporation leading to an SC moisture level below the presurfactant treatment level (24). The overhydration process followed by the abated hydration level is responsible for a higher rate in skin surface water evaporation. Thus, a differential stress is created inside the upper SC. Such a condition is in part linked to epidermal lipid removal combined with surfactant binding to SC proteins and subsequent changes in the overall electrical charges at the skin surface.
In any case of surfactant-induced SSI, regional variability in the intensity of response is commonly present on different body sites (33). Moreover, the complex molecular composition of the cleansing products influences each individual skin surface overreactivity. In distinct individuals, these features are commonly restricted to limited sets of compounds. In addition, the SSI status is influenced by age, gender, and ethnicity, as well as by specific environmental and seasonal geoclimatic conditions (34). In particular, the negative geoclimatic influence manifests itself when the environmental dew point modifications in winter alter the SC physiology (18,19,35,36). The influence of the environmental dew point possibly participates to the variability in time of any reactive skin perception (18).
Similar to the decline in irritation reactivity with age, SSI to surfactants appears less frequent in older adults. In general, fair skin is believed to be more susceptible to SSI than darker skin. However, the situation is probably more complex when interpreting the heterogeneity in the mottled subclinical melanoderma (MSM) (37,38). This condition is likely due to the chronic effect of UV light exposure (38,39). The complaints about SSI appear to be more frequent when the extent in MSM pattern is enlarged (4).
Mechanical Skin Irritation
Fabrics are rubbed on the skin, sometime on preconditioned (inflamed) skin (17,20,21,40,41). The effects of gently rubbing semioccluded swatches of cotton terrycloth fabric are tested on the skin. Most mechanical irritation tests require panelists to wear laundered garments each day for several days. Alternatively, laundered fabric is worn in close proximity to the skin at a limited test site, such as in the diaper test and the axilla, the back of the knee, and the wrist ban wear test. The continuous friction through normal movement results in mechanical irritation. Of note, similar tests are used for assessing the skin effects associated with wearing fabrics washed with commercial laundry detergents with or without softeners. These procedures combine the potential for the mechanical and chemical irritations.
Chemical Skin Allergy and Irritation
For fabrics that remain in prolonged contact with the skin, two considerations are important with regard to their chemical composition: (a) the inherent irritation potential of the material and (b) a potential immune reaction promoting allergic reactions (4–61).
Bioengineering and Textile Skin Impact