From textiles to humans: the role of textile moisture transfer properties on human physiological and perceptual responses

2018-06-21T10:15:33Z (GMT) by Margherita Raccuglia
Clothing provides the body with a protective barrier from environmental factors, such as rain, snow, wind and solar radiation. Beside this imperative protective function, the interaction between clothing and the human body has implications in terms of temperature regulation and comfort. Specifically, wetness at the skin-clothing interface represents one of the highest sources of discomfort when wearing clothing, which could even contribute to reductions in human performance and, in extreme environments, impact human health. To maximise heat and mass transfer through the clothing barrier, the textile and clothing industry constantly works on apparel innovations. Textile test methods allow assessments of objective improvements in material performance; however it is often unknown whether improvements at material level have an impact on human physiological and/or perceptual responses. Therefore, the aim of this research was to adopt an integrative paradigm in which textile and clothing moisture transfer parameters are instrumentally characterised and, subsequently, assessed in human physiological as well as sensorial experiments. In this thesis, the current literature review focuses on the interactions occurring between the thermal environment, the human body and the clothes worn by the person (Chapter 1). The test methods applied to evaluate textile and clothing parameters are reviewed and discussed (Chapter 1). This is followed by an outlined of the methodological developments adopted in the current research to measure human responses when interacting with textiles and clothing, both during rest and exercise conditions (Chapter 2). In the first laboratory study (Chapter 3), a skin regional experiment (fabrics applied on a restricted body area) was conducted to study the role of fabric thickness and fibre type on human cutaneous wetness perception, in condition of static fabric contact with the skin. In the same study, the approach adopted to characterise fabric moisture content, i.e. absolute (same µL of water per area (cm2)) versus relative (same µL of water per unit of fabric volume (cm3)) was studied and the implications that fabric total saturation has on skin wetness perception were explored. The results showed the role of fabric thickness as major determinant of fabric absorption capacity and also wetness perception. In fabrics presenting same saturation percentage (same water content per volume) a positive relation between fabric thickness and wetness perception was observed and this was independent of fibre type. When applying the same relative to volume water content (same saturation percentage) thicker fabrics were perceived wetter than the thinner ones. Conversely, when applying the same absolute water amount, thicker fabrics were perceived dryer compared to thinner fabrics, given that thinner fabrics were more saturated. These findings indicate that human wetness perception responses between fabrics with different volume/thickness parameters should be interpreted in light of their saturation parameters rather than considering the absolute moisture content. In the same study, it was observed that the weight of the fabric in wet state can also modulate wetness-related perceptual responses. Specifically, heavier fabrics were perceived wetter than lighter ones, despite using the same fabric and applying the same level of physical moisture. This phenomenon was explained in light of the synthetic nature of wetness perception, specifically through the effect of fabric weight on cutaneous perceived pressure which was associated with higher physical wetness in fabrics. In a following skin regional experiment (inner forearm), the individual and combined role of fabric surface texture (contact points with the skin) and fabric thickness on wetness perception as well as stickiness sensation was studied (Chapter 4). In contrast to Study 1, in this experiment, fabrics were examined in dynamic contact conditions with the skin. It was observed that, when pre-wetted (same relative water content, corresponding to 50% of their maximum absorption capacity), fabric materials with a smoother surface (higher contact) resulted in greater skin wetness perception and stickiness sensation compared to the rougher fabric surfaces. Interestingly, the power of wetness perception prediction became stronger when including, together with stickiness, fabric thickness, indicating the important role of these two parameters when developing next to skin clothing. In the same dynamic application, to assess whether texture data can be used as predictors of fabric stickiness sensation, fabric surface texture was quantified using the Kawabata Evaluation System. The results showed that the Kawabata Evaluation System failed to predict stickiness sensation of wet fabrics commonly assumed to be associated with fabric texture, thus a different way to define fabric texture may be needed in order to represent this link (stickiness and texture). Moving from this first research stage, where the impact of textile properties on human perceptual responses was investigated using a mechanistic approach, in the second research phase a more applied approach was adopted. The aim was to study textile parameters and clothing performance in conditions of exercise-induced sweat production as opposed to laboratory-induced wetness conditions. Before investigating human sensorial responses in transient exercise conditions, in Study 3 (Chapter 5) we addressed potential biases which can occur when sensorial scores of temperature, wetness and discomfort are repeatedly reported in transient exercise conditions. We pointed out that, when repeatedly reported, previous sensorial scores can be set by the participants as reference values and the subsequent score may be given based on the previous point of reference, the latter phenomenon leading to a bias which we defined as anchoring bias . Indeed, the findings showed that subsequent sensorial scores are prone to anchoring biases and that the bias consists in a systematically higher magnitude of sensation expressed, as compared to when reported a single time only. As such, the study allowed recognition and mitigation of the identified error, in order to improve the methodological rigour of the following research involving sensorial data in transient exercise conditions. Following from Study 2, where the impact of stickiness sensation on wetness perception was highlighted, in the fourth laboratory study (Chapter 6) we aimed to investigate the combined effect of garment contact area, sweat content and moisture saturation percentage, in conditions of exercise-induced sweat production. Furthermore, the influence that both stickiness sensation and wetness perception have on wear discomfort was studied. The findings showed that fabric saturation percentage mainly affected stickiness sensation of wet fabrics, dominating the impact of fabric contact area and absolute sweat content. On the contrary, wetness perception was not different between garments. This indicated that stickiness sensation and wetness perception are not always strongly related; as such they should both be measured and considered individually. Texture and stickiness sensation presented the best relation with wear discomfort at baseline and during exercise, respectively. Due to the impact of fabric moisture saturation percentage on stickiness sensation and wear discomfort, identified in Study 1 and Study 2, in Study 5 (Chapter 7) we aimed to quantifying temporal and regional sweat absorption in cotton and synthetic upper body garments. Sweat production was induced in male athletes during 50 minutes of running exercise, performed in a warm environment. Considerable variations in sweat absorption were observed over time and between garment regions. Based on these data, we provided temporal and spatial sweat absorption maps which could guide the process of clothing development, using a sweat mapping approach. In Study 5 a destructive gravimetric method was developed to quantify local garment sweat absorption. While this currently is the only methodology that permits direct and analytical measurements of garment regional sweat absorption, the latter approach is time-consuming and expensive, therefore of limited applicability. As such, in study 6 (Chapter 8), it was assessed whether infrared thermography could be used as an indirect method to estimate garment regional sweat absorption, right after exercise, in a non-destructive fashion. Spatial and temporal sweat absorption data, obtained from Study 5, were correlated with spatial and temporal temperature data (also obtained from study 5) measured with an infrared thermal camera. The data suggested that infrared thermography is a good tool to qualitatively predict regional sweat absorption in garments at separate individual time points; however temporal and quantitative changes are not predicted well, due to a moisture threshold causing a temperature limit above which variations in sweat content cannot be discriminated by temperature changes any further. In conclusion, the textile parameters identified in this PhD research as major determinants of fabric absorption capacity and related perceptions are thickness/volume, wet weight, moisture saturation percentage, surface area and surface texture. These textile factors influence wetness-related sensations and perceptions over time, in relation to the over-time changes in human thermophysiological responses (such as metabolic rate and sweating) and to the environmental conditions the person is exposed to. This clearly shows that in a multifactorial system such as the environment-human-clothing one, the strength of different cutaneous moisture-related stimuli, triggered by various textiles parameters, should be considered. Finally, this indicates that, to obtain a better understanding of clothing performance and its impact on human sensations