Microscopic contact area and friction between medical textiles and skin

Hydrophobicity and Macroscale Tribology Behavior of Stearic Acid/Hydroxypropyl Methylcellulose Dual-Layer Composite

Hydroxypropyl methylcellulose (HPMC) and stearic acid (SA) are integrated to fabricate a double-layer thin film composite material with potential applications in sustainable packaging and coating materials. The effect of SA concentration on the moisture and wear resistance at the macroscale of the composite are studied. The amount of SA on the surface (>SA5H) is beneficial in increasing anti-wear behavior and reducing the friction coefficient by 25%. The petal-shaped crystals formed by SA are distributed on the surface of the double-layer film, increasing its hydrophobicity. When subjected to wear, the SA crystals on the surface of the double-layer film are fractured into debris-like abrasive particles, forming an optimal third-body of moderate shape and particle size, and imparting anti-wear and lubricating characteristics.

Changes in the three-dimensional microscale topography of human skin with aging impact its mechanical and tribological behavior

Human skin enables interaction with diverse materials every day and at all times. The ability to grasp objects, feel textures, and perceive the environment depends on the mechanical behavior, complex structure, and microscale topography of human skin. At the same time, abrasive interactions, such as sometimes occur with prostheses or textiles, can damage the skin and impair its function. Previous theoretical and computational efforts have shown that skin’s surface topography or microrelief is crucial for its tribological behavior. However, current understanding is limited to adult surface profiles and simplified two-dimensional simulations. Yet, the skin has a rich set of features in three dimensions, and the geometry of skin is known to change with aging. Here we create a numerical model of a dynamic indentation test to elucidate the effect of changes in microscale topography with aging on the skin’s response under indentation and sliding contact with a spherical indenter. We create three different microrelief geometries representative of different ages based on experimental reports from the literature. We perform the indentation and sliding steps, and calculate the normal and tangential forces on the indenter as it moves in three distinct directions based on the characteristic skin lines. The model also evaluates the effect of varying the material parameters. Our results show that the microscale topography of the skin in three dimensions, together with the mechanical behavior of the skin layers, lead to distinctive trends on the stress and strain distribution. The major finding is the increasing role of anisotropy which emerges from the geometric changes seen with aging.

Nondestructive Quantitative Evaluation of Yarns and Fabrics and Determination of Contact Area of Fabrics Using the X-ray Microcomputed Tomography System for Skin-Textile Friction Analysis

In different mechanical conditions, repetitive friction in combination with pressure, shear, temperature, and moisture leads to skin discomfort and imposes the risks of developing skin injuries such as blisters and pressure ulcers, frequently reported in athletes, military personnel, and in people with compromised skin conditions and/or immobility. Textiles next to skin govern the skin microclimate, have the potential to influence the mechanical contact with skin, and contribute to skin comfort and health. The adhesion-friction theory suggests that contact area is a critical factor to influence adhesion, and therefore, friction force. Friction being a surface phenomenon, most of the studies concentrated on the surface profile or topographic analysis of textiles. This study investigated both the surface profiles and the inner construction of the fabrics through X-ray microcomputed tomographic three-dimensional image analysis. A novel nondestructive method to evaluate yarn and fabric structural details quantitatively and calculate contact area (in fiber area %) experimentally has been reported in this paper. Plain and satin-woven fabrics with different thread densities and made from 100% cotton ring-spun yarns with two different linear densities (40 and 60 Ne) were investigated in this study. The measurements from the tomographic images (pixel size: 1.13 μm) and the fiber area % analysis were in good agreement to comprehend and compare the yarn and fabric properties reported. The fiber area % as reported in this paper can be used to evaluate the skin-textile interfaces and quantitatively determine the contact area under different physical, mechanical, and microclimatic conditions to understand the actual skin-textile interaction during any physical activity or sports. The proposed method can be helpful in engineering textiles to enhance skin comfort and prevent injuries, such as blisters and pressure ulcers, in diversified application areas, including but not limited to, sports and healthcare apparel, military apparel, and firefighter's protective clothing. In addition, the images were capable of precisely evaluating yarn diameters, crimp %, and packing factor as well as fabric thickness, volumetric densities, and cover factors as compared with those obtained from theoretical evaluation and existing classical test methods. All these findings suggest that the proposed new method can reliably be used to quantify the yarn and fabric characteristics, compare their functionality, and understand the structural impacts in an objective and nondestructive way.

Interpersonal differences in the friction response of skin relate to FTIR measures for skin lipids and hydration

Understanding the mechanical response of skin to contact is of importance when developing products that interact with the skin. The shear forces that arise due to friction in the interface are a key aspect of skin interactions, because shear is known to contribute to discomfort and tissue injury. However, the frictional response of skin shows large variations between people. It has been hypothesised that these variations relate to differences between people in the physiological properties of their skin, but the underlying mechanisms are not well understood. In order to gain new insights into these interpersonal differences in friction behaviour, in vivo FTIR measurements and in vivo friction measurements were performed on the same patch of skin. Quantitative analysis of the various peaks in the FTIR spectra provided information on the moisture content of the stratum corneum and the amount and mechanical properties of the lipids on the skin. The lipid viscosity, as characterised by the width of the 2920 cm^-1 peak, correlates with the friction, whilst, interestingly, no relationship was found between the quantity of lipids on the skin surface and the coefficient of friction. Additionally, and as expected, a fairly strong correlation was obtained between the moisture content, as characterised by the height of the Amide I peak and the coefficient of friction. The presented results show that spectroscopy techniques can be used in as a non-invasive method to identify people who may show elevated levels of friction and thus are at increased risk of developing shear induced tissue injury.

The development of an artificial skin model and its frictional interaction with wound dressings

Human skin interacts with various materials in our daily life. The interaction between human skin and contacting materials is very important for the development of skin contacting products. Owing to the ethic and different testing results because of the using of in vivo or ex vivo skin, it is important to develop an artificial skin model (ASM) for the study. Therefore, an ASM mimicking the deformation and friction behavior of in vivo human skin was designed based on the evaluation of in vivo human skin behavior, and its frictional interaction with wound dressings was studied. The ASM was prepared by the combination of hydrophilic network carboxyl chitosan (CC) and hydrophobic network polydimethylsiloxane (PDMS). The influence of ingredient ratio, including PDMS/CC and curing agent/PDMS ratio, on the mechanical property of ASM was determined firstly. By adjusting the curing agent/PDMS ratio, the water absorption swelling rate (WASR) of ASM could be controlled to mimic different hydration status of human skin. With the changing of ingredient ratio and hydration level, the elastic modulus and viscoelasticity of ASM can be tailored to be similar to that of in vivo human skin. When the PDMS/CC ratio was 7:3, and PDMS/curing agent ratio was smaller than 1:30, the elastic modulus of ASM was in the range of in vivo inner forearm, and with the increasing of indentation depth, the elastic modulus decreased. Based on the ASM, the frictional interaction between skin/wound dressing/mattress was studied. It was found that although the friction using ASM was slightly higher than that using in vivo inner forearm, but the friction decreasing trend was the same for different kinds of wound dressings. In addition, the friction tested with ASM was less fluctuation, more reliable and reproducible than that tested with in vivo skin, indicating that the ASM was suitable to be used for the studying of frictional interaction between skin and product, such as wound dressings.

An experimental study of friction between volar forearm skin and nonwoven fabrics used in disposable absorbent products for incontinence

Incontinence-associated dermatitis is common among wearers of absorbent incontinence products and friction between product materials and skin is thought to be a contributing factor, but the details of its role are unclear. In this study, friction was measured between the dry volar forearm of 19 women (20-95 years) and five nonwovens typical of those in commercial disposable products. Euler's model/Amontons' law held to high precision for all person-fabric pairs for both static and dynamic friction, despite substantial variations in forearm size, soft tissue compliance and skin smoothness between subjects, sometimes substantial lateral contraction in fabric strips, and skin rucking beneath them. For a given subject, the highest coefficients of friction among the fabrics exceeded the lowest by ∼30% to 75%, while - for a given fabric - the highest coefficients of friction among the subjects exceeded the lowest by ∼55% to 85%. The order of coefficient of friction values across fabrics was similar for each subject, and across subjects for each fabric. There was no systematic variation with subject age. The data were well modelled by estimating the coefficients of friction for a given person-fabric combination as the product of the mean coefficient of friction across all fabrics for that person, and the mean coefficient of friction across all persons for that fabric, normalised to the mean coefficient of friction across all person-fabric combinations. Predicted values were within 10% of measured figures for ∼97% of person-fabric combinations. Stick-and-slip behaviour was observed with seven person-fabric combinations, but especially strongly for two subjects with each of two fabrics. It is not clear why and further investigation is merited. Comparison of the data with results from earlier work with the same fabrics and a skin surrogate (Lorica Soft) suggests that measurements with Lorica Soft may be helpful to screen, evaluate and compare candidate materials preparatory to human studies.

Assessing the accumulated stickiness magnitude from fabric-skin friction: effect of wetness level of various fabrics

Increasing skin wetness tends to increase fabric-skin adhesion and friction, resulting in wear discomfort or skin injuries. Here, the magnitude estimation approach was used to assess the stickiness sensation perceived in fabrics. Seven fabric types were wetted by putting onto wet 'skin' surface and dried for different durations to achieve different wetness levels, simulating wearing conditions during the recovery period after sweating. Results showed that the relationship between magnitude estimates of stickiness and amount of water present in fabric demonstrated a power function. The exponents and constant from power regression show the growth rate of stickiness sensation with moisture intensity and the perceived stickiness under fixed stimulus intensity, respectively. A novel parameter, accumulated stickiness magnitude (ASM), describing how much discomfort a wetted fabric offered throughout the drying period, was developed. Thin cotton fabrics (fabric W01 and W03), having higher saturation level after contacting with wetted skin surface, arouse stronger stickiness feeling and their ASM is remarkably higher. The difference in stickiness estimates is due to the difference in chemical composition and surface geometry. This study suggests us the way to predict perceived stickiness in fabrics with different wetness levels which is useful for applications like sportswear, intimate apparel or healthcare products.

Body-monitoring with photonic textiles: a reflective heartbeat sensor based on polymer optical fibres

Knowledge of an individual's skin condition is important for pressure ulcer prevention. Detecting early changes in skin through perfusion, oxygen saturation values, and pressure on tissue and subsequent therapeutic intervention could increase patients' quality of life drastically. However, most existing sensing options create additional risk of ulcer development due to further pressure on and chafing of the skin. Here, as a first component, we present a flexible, photonic textile-based sensor for the continuous monitoring of the heartbeat and blood flow. Polymer optical fibres (POFs) are melt-spun continuously and characterized optically and mechanically before being embroidered. The resulting sensor shows flexibility when embroidered into a moisture-wicking fabric, and withstands disinfection with hospital-type laundry cycles. Additionally, the new sensor textile shows a lower static coefficient of friction (COF) than conventionally used bedsheets in both dry and sweaty conditions versus a skin model. Finally, we demonstrate the functionality of our sensor by measuring the heartbeat at the forehead in reflection mode and comparing it with commercial finger photoplethysmography for several subjects. Our results will allow the development of flexible, individualized, and fully textile-integrated wearable sensors for sensitive skin conditions and general long-term monitoring of patients with risk for pressure ulcer.