On The Friction Behaviour Of Medical Compression Stockings Against Human Skin

Venous diseases of the human lower limbs are one of the most common vascular diseases and a considerable percentage of the population worldwide has suffered from some form of vein disorder. The wearing of medical compression stockings has been widly used as an essential and effective method for the treatment of venous disease. The amount of pressure required is mainly dependent on the severity of the diseases. The working principle is to provide an external and controlled pressure gradient along the leg, helping to reduce the effects of venous insufficientcy and improving the lower limb venous blood return. Howerver, dispite meeting the medical functions, medical compression stockings would be faced some common problems in practical use, which were the putting on process and uncomfortability after long-time wearing. The stockings imposes a high contact pressure on the foot skin during stretched, leading to a high friction force which make it difficult to pull on the stockings, especially for elderly persons. Moreover, the friction for frequently mechanical contact between leg skin and medical compression stockings and perspiration during long time wearing would cause perceptual discomfort for human beings, even skin injuries, such as pressure pattern, blisters or ulcers. It can be seen that the main reason of these two practical problems would be attributed to the friction. However, most studies focused on the compressive properties and therapeutic effects of medical compression stockings, few research about the friction properties of compression stockings were only testing the friction between stockings against with steel and hard probe, disobeying the state of soft skin, or just measureing the friction under the nature condition, without considering the stretched state for putting on process and sweaty after long-time wearing. Therefore, the friction behaviour of MCSs on skin was not clearly understood.In view of the weakness on the research of friction properties between medical compression stockings and skin, the contact and friction behaviour of medical compression stockings on skin were investiagted by combining the specifications of medical compression stockings on the basis of skin tribology in the current study. The work was carried out from the fricion difference of medical compression stockings among various pressure classes and parts, materials composition, structures and strains to the influence of interface water and surface topography on the friction of MCSs, in order to systematic understand the contact and friction behaviour of medical compression stockings, and provide useful information to improve the putting on process and comfortability of MCSs. The work can be divided into the following three parts:(1) According to the functions of medical compression stockings, the friction variation of differnt pressure classes and parts were compared by in-vtro friction measurements under dry, moist and wet conditions. It was found that under dry condition there was no significant difference on the friction of MCSs among different pressure classes and the friction coefficient on ankle region was bigger than calf region. However, a slight decrease on COF was presented with the increasing pressure classes and a higher COF was revealed in calf region under both moist and wet conditions. Then the relationship between the pressure classes, regions and friction of MCSs were established.Three kinds of medical compression stockings, which have the the same material composition and structures, with increasing pressure classes from low to high intensity(class 1 to 3) were selected in the current study. Two parts(ankel and calf region) were chosen for the in-vitro friction experiments against a skin model with simliar texture and mechnical properties to the skin under dry, moist and wet conditions. It was found that the friction coefficients are increased from dry, moist to wet conditions, which are 0.28-0.35,0.41-0.51,0.45-0.54 respectively. Under dry condition, though the highest friction coefficient was observed on the MCS for pressure class 2, there are no significant difference on the friction of medical compression stockings among different pressure classes, while a higher friction coefficient was observed of medical compression stockings for the ankle regions, due to the bigger densities of inlaid yarns, causig an increasing real contact area. On the case of moist and wet conditions, a slight decrease on friction coefficient was presented with the increasing pressure classes, it can be explained by the thicker inlaid yarn for higher pressure classes, resulting a smaller gap for obsorbing and decreasing friction coefficient. Meanwhile, a higher friction coefficient was revealed in calf region for all medical compression stockings, that is because there were more space for calf region to gather water and softem the skin or skin model, producing a bigger contact area, so as the friction coefficient.(2) In order to understand the influence of material composition and base structure on the friction between MCSs and skin, a comparative study on the friction of five kinds of medical compression stockings(one traditional and four modified) with a series of normal loads under dry and wet conditions were carried out. It was revealed that cotton and Tencel fiber in the inlaid yarn, and also the plain jersey can reduce the friction coefficients effeciently by improving the water absorbing and transportation capacity.In vivo friction measurements between five kinds of MCSs and forearm skin of four subjects, which were chosen to substitute the leg skin, were carried out in the present study, the normal loads were designed from 0-10 N. Then the friction coefficients were compared. It can be seen that COF was decreasing with the increasing normal loads, indicating that the friction mechanism of MCS was predominated by adhesion. Morever, the friction coefficients of dry fabrics showed no significant difference between a traditional MCS and versions with modified material composition and base structure, mainly because of the special structure of MCS fabrics leading to similar contact geometries. However, under wet conditions, the addition of cotton and Tencel fiber lead the COFs were decreased by 30%, and 60% for the plain jersey base structure. The μ-CT measurements provided the evidence that the water transportation properties were strengthened by the variation of material composition and base structure, resulting a relative dry interface and lower friction coefficient.(3) In order to explore the influence of surface topography on the friction property of MCS/skin, friction measurements of skin model and MCSs with a series of strains under two directions were carried out on the basis of stretched state of MCS in practical use. It was found the surface structure of medical compression stockings was considerably changed when varying the strain state, while friction coefficients remained comparable. The strain-independent friction behaviour can be explained by Hertz model that the strain-induced changes in surface topography and compression properties have a balancing effect on the number of contact points and the contact area of single asperity.According to practical use, a series of strains(0%-75%) under two directions(along with the inlaid yarn and perpendicular to the inlaid yarn, called i-direction and p-direction) were designed to lead the surface topographies vary evenly and systematically. Then image processing technology was applied to extract three parameters on the topography images of MCSs. The surface topography was characterized by the three parameters, which are the heights of peak-to-valley, the average distance between two inlaid yarns and the diameter of inlaid yarn. The results were that the heights of peak-to-valley were increased by around 50% and the average distance was also increased with the increasing strains in p-direction. When it was stretched in i-direction, the heights were declined over 40%, but the distance was barely changed. The diameter under two directions remains the same as the strains were increased. The friction coefficients were from 0.69 to 2.82 under dry condition and from 0.95 to 5.1 under wet condition. Under a certain normal load, the friction coefficients were comparable for the MCS samples of different strains. Furthermore, the relationship between material ratio and penetration depth drived from the compression measurement was represented a realistic estimate for the contact area under small vertical deformations. Then two Hertz contact model(round surface asperity and cylindrical fiber) related to the textile construction and composing fibres based on different scale were established to estimate the real contact area. It was revealed that the real contact area was relatively constant between MCS fabrics and PUR for different strains and directions.At the end, the study were found that there were a big difference on the friction coefficient between MCSs and skin under dry and wet conditions, the friction coefficient was increased with mositure content before stabilizing at a certain level. But COF were decreased with the increasing normal loads at the beginning, then remained constant, indicating that the friction of MCS was predominated by adhesion. In addition, the friction coefficients were influenced by the design parameters such as pressure classes, regions, material composition, base structures and strains. Under dry condtion, there were no signficant difference among the MCSs with different pressure classes, different regions, different material composition and structures, and different strains. These can be explained that the COF was depended on the surface topography under dry condition. Simliar geometrical structurs lead a relative constant contact area. In the case of moist and wet conditions, the COF has a big difference, due to the water absorbing and transpation capacity between the gaps and materials. That is to say, it is an efficient way to reduce the friction between MCSs and skin by reducing the interficial water, leading a dry interfce. These findings were helpful for choosing an ideal MCS product of the patients and developing an design of MCSs for the manufacturers.