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Skin self-adaptation to friction trauma under reciprocal sliding conditions
Tribology International 44 (2011) 1782–1789
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Skin self-adaptation to friction trauma under reciprocal sliding conditions W. Li, S.X. Qu, Y.J. Zheng, Q. Pang, J. Zheng, Z.R. Zhou n Tribology Research Institute, Key Laboratory for Advanced Technology of Materials of Ministry of Education, Southwest Jiaotong University, Chengdu 610031, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 December 2010 Received in revised form 24 June 2011 Accepted 30 June 2011 Available online 8 July 2011
In this study, skin self-rehabilitation and self-adaptation to friction trauma were investigated in vivo by means of friction testing, histological analysis and animal experiments under the simulated prosthetic socket rubbing condition. The denuded dorsal skin of rabbits was used to simulate stump skin. Results showed that after the skin went through several alternations of trauma and rehabilitation process, a keratinization appeared on the skin surface, which decreased the friction coefficient of skin and reduced the skin traumas. These results suggested that during the reciprocal sliding friction process, the rabbit skin suffered from friction trauma, rehabilitation and self-adaptation in turn. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Rabbit skin Friction Trauma Self-adaptation
1. Introduction It is well known that skin is a natural barrier to protect tissue and organ of the body, which comprises primarily of cells and extracellular matrices [1]. In daily life, human skin is often rubbed against external surfaces, which causes many friction problems. The friction between skin and working implements, labor protecting materials, sports appliances, improper footgear, textile materials, etc., may induce skin traumas such as the formation of blister, irritation or sensitization. However, skin has the ability of self-adaptation to friction trauma [2]. The representative example is a foot blister caused by wearing new shoes for the first time. After the blister has healed, the risk of further trauma induced by friction may decrease when wearing the same shoes again. Similar situation occurs for stump skin of amputee in contact with prosthesis socket materials, which usually comes through the process consisting of friction trauma, rehabilitation and gradual adaptation [2]. Coupling between the prosthesis and trans-tibial stump is typically achieved by a socket, which is a critical component for prosthetic performance and the sole means of load transfer between the prosthesis and the stump in current prosthetic practice [3]. Unfortunately, the skin and underlying soft tissues of the stump are not well-suited for load bearing. The interaction between the stump and the prosthetic socket usually causes elevated internal friction injury and pain in the epidermis and muscle tissues of the stump, such as pressure ulcers, blister, cysts, edema, skin irritation, dermatitis, etc. Especially, this
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rubbing injury is very uncomfortable for the amputee during the preliminary phase of wearing prosthesis. This would prevent amputees from wearing their prosthesis for a prolonged time, severely disabling them in their daily activities and reducing the quality of life [4,5]. Till now, only a few studies focused on the friction trauma and self-adaptation of human skin although the initial report on skin friction appeared as early as in 1955 [6]. Previous studies mainly focused on the frictional properties [7–9], deformation behavior [10], surface physical properties [11–13] and influence factors (such as age, gender, anatomical site, hydration and electrical parameters such as capacitance, conductance and impedance) of skin [14–17]. The results were applied to medicine [18], skin cosmetics [17,19,20], textile exploitation [21–24] and others related to skin [25]. It should be noted that previous results are inconsistent with each other. The conflict is due to the complexity of the friction condition between skin and materials and the limitation of measurement technologies [9]. Moreover, the skin was usually regarded as lifeless material in previous studies. Its biological response, such as self-adaptation and self-reparation during the course of friction behavior, was generally neglected. In addition, these studies were mostly carried out on the skin of volunteers. As a result, many analytical methods in engineering cannot be used, which makes tribological studies remarkably difficult. For example, both the wear morphology and wear debris of the skin cannot be observed using SEM. Nowadays, a lack of effective methodology is still a puzzle for skin tribology. Concerning the interface between stump skin and prosthetic devices, most studies paid attention to the stresses at the interface of skin–prosthetic socket by experimental measurement and finite element analysis in the last 40 years [26–28]. The information
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gained not only has been used for the assessment and improvement of prosthetic socket design and fitting, but also has contributed to the understanding of the prosthetic interface [26–28]. However, tribological factors could not be revealed adequately in the limb skin–prosthetic socket interface design and fitting, and the studies on the self-adaptation of stump skin to the friction condition of prosthetic socket are still very limited. In this paper, the self-rehabilitation and self-adaptation of rabbit skin to the friction trauma were investigated in vivo under the simulated prosthetic socket rubbing condition. The skin trauma was examined by histological analysis. The purpose was to find the tribological behavior of skin from friction trauma to gradual self-adaptation under the conditions of reciprocal sliding friction.
2. Materials and methods 2.1. Specimen preparation In order to simulate the stump skin, skin samples were prepared from in vivo dorsal skin of rabbit, which is often used for skin irritation and sensitization test according to ANSI/AAMI/ ISO 10993-10-1995: Biological Evaluation of Medical Devices [29]. The rabbit was used as a model because its epidermic and dermal tissues are almost the same as those of human skin, except for a mass of hair follicles contained within the dermal tissue. Fifteen New Zealand healthy white rabbits with the weight of 2.0 70.4 kg were studied with the approval of Institutional Animal Care and Use Committee, China. The rabbits were supplied by Experimental Animal Culture Center, Sichuan province. All experimental procedures were performed in compliance with Regulations for the Administration of Affairs concerning Experimental Animals, China. For each rabbit, about 10 cm 15 cm dorsal region was carefully shaven without any skin damages with the help of a speed clipper. Six areas were marked on the shaven dorsal skin using indelible ink. Three areas located on the left side, which were used as test sites, while other three on the right side, which were used as control sites, as shown in Fig. 1. All rabbits were acclimatized for 24 h before friction testing. The rabbit was anaesthetized with an intra-abdominal injection of 3% sodium pentobarbital at 1 ml/kg body weight before each friction testing. All testing sites were cleaned with alcohol. Acrylic resin plate, one of the prosthetic socket materials, was chosen as a counterpart. It was supplied by Sichuan Health Recovery Center for Disabled Limb and Trunk, China. A 2.7-mmthick acrylic resin plate was bent into a 15-mm-diameter bottle
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lid shape with a 1.5-mm-radius chamfer and sleeved to the copper cylindrical friction probe [30]. The contact diameter with the rabbit skin was 12 mm. The surface roughness Ra of the acrylic resin plate was 15.1 mm, which was measured using a Confocal Laser Scanning Microscopy (OLS, 1100, Olympus, Japan). 2.2. Friction tests In order to simulate the stump skin–prosthetic socket rubbing interface, two-body in vivo sliding friction tests were carried out in a flat-on-flat configuration using a reciprocating UMT series multi-specimen Biomedical Micro-Tribometer (UMT-II, CETR Corporation, America). The UMT-II is a computer-controlled benchtop instrument adapted to measure tribological parameters on the skin. The counterpart, which consisted of an acrylic resin sleeve and a copper cylindrical friction probe, was attached to a suspension system. It was pressed onto the rabbit skin under a programmed normal force and then moved linearly at a constant speed when testing. The anaesthetized rabbit was immobilized at the bottom stage of the instrument. Test parameters are shown in Table 1. The normal force Fn was 2.0 N, which resulted in a contact pressure of 17.7 kPa when the diameter of the counterpart was 12 mm. Portnoy et al. reported that the principal compressive stress was 3.3–240 kPa between the residual limb and the socket of the prosthesis by experimental measurements and three-dimensional non-linear large-deformation finite-element analysis [26]. Considering that the residual limb muscle was different from the rabbit muscle, where the muscles were larger, the subcutaneous fat overlaying the muscle was thicker, and the fascia were thicker and stronger, large compressive stress on rabbit muscle may be more severe than on human muscle. According to this, the contact pressure of 17.7 kPa was selected in this study, which approach the lower limit as Portnoy et al. reported. The reciprocating amplitude D was 75 mm, which simulated the fluctuating slide between residual limb skin and prosthetic socket surface. The frequency f was 1.0 Hz, which simulated quick step frequency of amputee when walking [27,28]. The average sliding speed was about 20 mm/s according to the reciprocating amplitude and frequency. The reciprocating number of cycles N was 1800 cycles, corresponding to 30 min walking time. Friction coefficients can be measured and recorded by the UMT-II tribometer at a sampling rate of 20 kHz to data files for processing and analysis. A software filter was constructed for the output channels providing a mean value of all friction coefficients at each reciprocating cycle. In order to investigate the self-rehabilitation and self-adaptation ability of skin to friction, the rabbits were divided into five groups. The groups and
Fig. 1. Schematic diagram of test and control sites on the dorsal skin of rabbit.
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Table 1 Friction parameters and groups of rabbits. Friction test order
Group of rabbit
Number of rabbits
Normal force (N)
Reciprocating amplitude (mm)
Frequency (Hz)
Time (min)
Period of rehabilitation (d)
1 2 3 4 5
1,2,3,4,5 2,3,4,5 3,4,5 4,5 5
15 12 9 6 3
2 2 2 2 2
75 75 75 75 75
1 1 1 1 1
30 30 30 30 30
14 14 14 14 –
Table 2 Evaluation of skin irritation response [29]. Skin irritation
Scoring
Erythema No erthema Very slight erythema, barely perceptible Well defined erythema Moderate to severe erythema Severe erythema, beet redness to slight eschar formation, injuries in depth
0 1 2 3 4
Edema No edema Very slight edema, barely perceptible Slight edema, edges of area well defined by definite raising Moderate edema, area raised approximately 1 mm Severe edema, raised more than 1 mm, and extending beyond area of exposure
0 1 2 3 4
Maximum scoring
8
Fig. 2. Typical Ft–D curve of the rabbit skin.
number of rabbits are also shown in Table 1. For the first group, friction testing was conducted only once. For the second group, friction testing was done twice at the same test site with an interval of 14 days. That is to say, there existed 14-days rehabilitation time for the test skin after each friction testing, which was in accord with the rehabilitation time of traumatic skin [2]. In the same way, for the fifth group, friction testing was done five times at the same test site with an interval of 14 days every two times. For each group, the skin biopsies were taken at 24 h after total friction testing. All the friction tests in this study lasted about 60 days from March to May. All tests were performed at a temperature of 20 75 1C and a relative humidity of 60 710%. Nine repeat tests were performed, and then nine traumatic skin surfaces were obtained under the same testing parameters.
from the right sites to serve as morphological controls. The histological section was made according to the routine method of dermatopathology [1]. First, these samples were immediately immersed in 4% buffered formaldehyde fixative for 24 h. Then, the skin biopsies were rinsed with phosphate-buffered solution, dehydrated in an ethanol grade series, embedded in paraffin and cut into 3.3 mm-thick sections. Then, these sections were mounted on slides coated with gelatin and dried overnight at 50 1C, stained by hematoxylin and eosin (H&E). Subsequently, they were observed by a microscope (DMRX, Leica, Germany). The scab thickness of traumatic skin was measured using a microscope with a 10 objective lens, and the thicknesses of epidermis and stratum corneum were measured with a 40 objective lens. The thickness was continuously measured every 100 mm. Five positions were measured, and the final thickness was the average of 5 positions. The inflammatory cells were counted in a 0.25 mm2 visual field using a microscope with a 10 objective lens. The inflammatory cells of random five visual fields were counted, and the final value was the average of 5 fields.
2.3. Skin trauma evaluation
2.5. Statistical analysis
Skin traumas were observed visually after friction testing, which were described as skin irritation response. The visual scoring system was used to evaluate primary skin irritation manifested as erythema and edema according to Tests for Irritation and Sensitization Guidelines (ANSI/AAMI/ISO 10993-101995: Biological Evaluation of Medical Devices), as shown in Table 2 [29]. Skin irritation was scored immediately after each friction test. The skin irritation response was evaluated by the average value of nine repeated test sites. The maximum scoring was the sum of erythema and edema scores.
In this study, all data were presented as w 7S. The statistical differences of thicknesses and inflammatory cells among different groups were determined by analysis of variance (F test). The level of statistical significance was set to Po0.01 or Po0.05. All the statistical differences resulted from the comparison between the control group and test group, and among the five groups.
3. Results 3.1. Friction behavior
2.4. Skin histological evaluation After friction testing, the rabbits were observed during recovery from the anesthetic, and then returned to holding facility. Before skin biopsy, the rabbits were sacrificed by an intravenous injection of air at 24 h after friction testing. Skin biopsies were taken from the left test sites and the similar biopsies were taken
As mentioned above, friction testing was conducted five times at the same testing site for the fifth group. In all these friction cases, a typical Ft–D (tangential force–imposed displacement) curve in elliptic shape at a reciprocating cycle was obtained as shown in Fig. 2. The relative sliding (dCD) occurred at the interface besides the elastic deformation (dAB–dCD) of rabbit skin and underlying tissue.
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skin is comprised of epidermis and derma. The epidermis (Fig. 6a– b) is composed of two or three layers of cells. Its surface is covered with stratum corneum (SC) layer. The dermis consists of collagen fiber bundles (CFB) (70%), and elastic connective tissue, in a semigel matrix of mucopolysaccharides [31]. Hair follicles, sebaceous glands, a few fibroblast (FB) and capillary vessels can be seen within the dermal tissue. The epidermal thickness (ET), collagen fiber bundle length (CL) and collagen fiber bundle thickness (CT) of these two kinds of skin, which went through rehabilitation 4 times and friction testing 5 times are listed in Table 4. There was significant difference in ET, CL and CT for the control and test skins (Po0.05). As shown in Fig. 6a and b, the test skin showed significant increases in ET and SC compared to the control (P o0.05). But as shown in Fig. 6c and d, the test skin showed significant decreases in CL and CT in comparison with the control (Po0.05). The typical histological photomicrographs of rabbit epidermis for different friction testing times are shown in Fig. 7. The control skin with intact epidermis and stratum corneum is also shown with arrowhead in Fig. 7a as it will be convenient to compare. In the first three times of friction testing (Fig. 7b–d), the rabbit epidermis was almost rubbed out and a lot of inflammatory exudation effused. The surface of the exudation was fibrin. The fibrin was mainly composed of huge amounts of necrotic neutrophilic granulocytes. The worn skin surface shriveled and formed scab when the fibrin was dried (as shown with arrowhead in Fig. 7). The collagen fiber bundles near the necrotic epithelial tissue became swollen and sparse. Serious inflammatory cell infiltration appeared within the derma tissue. The inflammatory exudation and scab thickness gradually reduced with the increasing number of friction testing times. After the fourth and fifth friction testing, the scabs could not be identified (Fig. 7e–f). The average and standard deviation of the scab thickness and the analysis of variance (F test) for different friction testing times are shown in Table 5. The scab thickness gradually reduced with the
The frictional regime was in the intermediate regime from sticking to gross relative sliding regime according to our previous study [30], which was also identical with the result of the friction force changes in the ‘‘stick’’ and ‘‘slip’’ behavior described by Kwiatkowska et al. [10]. Fig. 3 gives the typical curves of friction coefficient (here, mean value of each reciprocating cycle was used) versus time from the first testing to the fifth testing. The evolution trends of all the curves were high at the start of the friction testing, afterwards gradually declined with time when stratum corneum layer was avulsed, or epidermis was scraped accompanied by exudation and bleeding, and finally fluctuated near the stable value. A summary of mean friction coefficient values of the fifth group rabbits for all the number of friction testing times when they fluctuated near the stable value (after 20 min) is provided in Fig. 4. The mean values and standard deviations were calculated from 9 skin samples. The friction coefficient was the highest for the first testing and the lowest for the fifth testing. That is it decreased with the increasing number of friction testing times. 3.2. Skin trauma evaluation With the increasing friction time, three main types of skin trauma, erythema, edema and exudation, were observed. At the beginning of the first friction testing, erythema appeared on the rabbit skin. Then the abnormal exudation infiltrated into the tissue and edema was observed. Subsequently, the rabbit epidermis was scraped accompanied by exudation and bleeding. A typical photo of traumatic skin after the first friction testing is shown in Fig. 5a, a severe erythema was observed on the test skin surface in comparison with the control. Due to the different friction testing times, erythema with various degree hue and sizes were observed on the rabbit skin surface after friction testing. Furthermore, with the number of friction testing times increasing, the start time when the rabbit epidermis was scraped and the exudation and bleeding was delayed. Especially after the fourth and fifth friction testing, the rabbit epidermis almost was not scraped except some stratum corneum layer avulsed, as shown in Fig.5b. Meanwhile, from severe to slight erythema and edema on the friction sites were observed as the number of friction testing times increased. The corresponding scores for erythema, edema and maximum irritation are shown in Table 3. It was observed that the skin irritation scores decreased with the increasing number of friction times. The irritation score was the highest for the first friction testing and the lowest for the fifth friction testing. The levels of skin trauma were consistent with the variations of the friction coefficient for different number of friction testing times.
Friction coefficient
1.0 0.8 0.6 0.4 0.2
3.3. Skin histological evaluation
0.0 2 3 4 Number of friction testing times
1
The histological photomicrographs of the control skin and the test skin of the fifth group, which went through rehabilitation 4 times and friction testing 5 times are shown in Fig. 6. The rabbit
Fig. 4. Mean friction coefficient for different friction testing times (n¼9).
Coefficient of friction
2.0 1st testing
2nd testing
3rd testing
4th testing
5th testing
1.5 1 1.0
2 3
4
0.5 5 0.0 0
5
10
5
15 Time (min)
20
25
Fig. 3. Variations of friction coefficient with time for different friction testing times.
30
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Fig. 5. Photos of traumatic skin and control skin, (a) after the first friction testing, (b) after the fifth friction testing. Table 3 In vivo skin irritation scores for different friction testing times (n¼9, Po 0.01). Friction testing times
Control
1
2
3
4
5
Erythema Edema Maximum irritation score
07 0.0 07 0.0 07 0.0
3.78 70.44 3.33 70.50 7.11 70.47
3.33 7 0.50 2.67 7 0.50 6.007 0.50
2.677 0.50 2.337 0.50 5.007 0.50
1.44 70.53 1.33 70.50 2.78 70.51
1.22 7 0.44 0.677 0.50 1.89 7 0.47
Fig. 6. Histological photomicrographs of control and test skin samples after rehabilitation four times, (a) epidermis and derma of control skin, (b) epidermis and derma of test skin, (c) derma of control skin, (d) derma of test skin (key: E, epidermis; CFB, collagen fiber bundle; FB, fibroblast; SC, stratum corneum) (40 ).
Table 4 Quantitative microscopic alterations between control and test skins of in vivo rabbits which went through rehabilitation 4 times and friction testing 5 times (n¼ 9). Skin sample
Control
Test
P-value
Epidermal thickness, ET (mm) Collagen fiber bundle length,CL (mm) Collagen fiber bundle thickness, CT (mm)
20.27 4.1 43.87 10.2 10.87 1.7
26.4 7 6.8 38.7 7 8.5 8.3 7 1.4
o 0.05 o 0.05 o 0.05
increasing number of friction testing times. Moreover, a significant difference in the scab thickness was observed (Po0.01). The typical histological photomicrographs of inflammatory cell infiltration within the rabbit derma tissue for different friction testing times are shown in Fig. 8 (with arrowhead). The magnified photograph on the left top corner of each photomicrograph was the partial inflammatory cells near the arrowhead using a 100 objective lens. It was seen that the neutrophilic granulocytes within the damaged derma tissue in a given visual field were
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Fig. 7. Histological photomicrographs of rabbit epidermis for different friction testing times, (a) control, (b) first testing, (c) second testing, (d) third testing, (e) fourth testing, (f) fifth testing (10 ).
Table 5 Scab thickness of traumatic skin for different friction testing times and statistical significance analysis (n¼9). Friction testing times
1
2
3
4
5
Scab thickness (mm) F P-value
267.9 711.1
136.4 7 9.4
66.6 77.8 1752.9 o 0.01
14.1 7 2.2
9.97 1.6
more and denser after either the first or the third friction testing (Fig. 8b–d), than after fourth and fifth testing (Fig. 8e–f). The amounts of inflammatory cells within the rabbit subcutaneous tissue for different number of friction testing times are shown in Fig. 9. The amount of inflammatory cells was an average of the nine traumatic skin tissues. Differences of inflammatory cell among means were checked for significance separately by analysis of variance (F test). The amount of inflammatory cells in a given visual field decreased with the increasing number of friction testing times. A significant difference in the amount of inflammatory cells was also observed (Po0.01).
4. Discussion Reciprocal sliding friction on the skin surface would tend to break down the efficiency of the stratum corneum barrier function and induce the skin trauma, especially when these friction behaviors resulted from cyclic mechanical loads if contact pressures and shear forces were high or continue over long periods of time [32]. During the friction course of prosthetic socket material–rabbit skin interface, the SC layer was first avulsed. Subsequently, the epidermal layer was gradually frazzled, and the tissue fluid and blood extravasated. The avulsed SC layer, exudation and blood played the role of lubricant, which resulted in the friction coefficient declining with time. After 1800 reciprocating cycles, various degrees of erythema appeared on the skin surface. Unlike conventional engineering materials, living beings have the particular function of self-rehabilitation to damage. Once trauma occurs to living skin, a series of complex transformations
in shape, biology, physics, etc., which are divided into two phases, namely inflammation eliminating phase and tissue rehabilitating phase, would appear within the skin tissue [1]. In this study, the scab was formed on the traumatic rabbit skin surface during the inflammation eliminating phase. The surface of the scab was fibrin. The underneath of the scab was the cell matrix, which was mainly composed of a lot of necrotic skin tissue and dead neutrophilic granulocytes. The foundation of wound healing was a series of activities of inflammatory cells and rehabilitative cells [1]. Neutrophilic granulocytes were the main incipient inflammatory cells, which accessed the traumatic tissue. They eliminated the necrotic tissue and foreign matter by their functions of phagocytosis, oxygen free radical antibiosis, complement activation, etc., which could protect normal tissue from infection [1]. Substantive neutrophilic granulocytes were observed at the interface of local necrotic tissue and living body tissue within the rabbit traumatic skin tissue after 24 h of friction testing. That was why the skin biopsies were taken at 24 h after each friction testing. The results of cell counting showed that the main cellular infiltration was the neutrophilic granulocytes, which accessed the local traumatic derma tissue. Therefore, the scab thickness on the traumatic skin surface and the amount of neutrophilic granulocytes within the traumatic derma tissue were used to estimate the effects of the friction conditions on the level of skin tissue traumas. In this study, severer skin traumas occurred during the first three times of friction testing, which corresponded to higher friction coefficient. When the test skin went through several alternations of trauma and rehabilitation process, its adaptation ability to friction was built up. That is, during the fourth and fifth
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Fig. 8. Histological photomicrographs of inflammatory cell infiltration within rabbit dermis for different friction testing times, (a) control, (b) first testing, (c) second testing, (d) third testing, (e) fourth testing, (f) fifth testing (40 ).
Amount of inflammatory cells
800 514 600
442
400
289 196
200
157
0 1
2 3 4 Number of friction testing times
5
Fig. 9. Comparison of the amount of inflammatory cells within rabbit dermis for different friction testing times (n¼ 9, Po 0.01).
friction tests, the level of traumas and the friction coefficient decreased synchronously. Histopathological examinations demonstrated that the scab thickness and the amount of inflammatory cells decreased with the increasing number of friction testing times, suggesting that the level of rabbit skin traumas gradually reduced. That is to say, the rabbit skin gradually adapted to the friction condition. The adaptation ability of the skin to friction was in connection with the changes of skin microstructure. After the rabbit skin went through several alternations of trauma and rehabilitation process, the stratum corneum layer and epidermal could thicken. That is, keratinization could appear on the skin surface [2]. These changes can be seen in Fig. 6. For the skin samples after the fourth rehabilitation (Fig. 6 b), compared with the control (Fig. 6 a), the stratum corneum layer and epidermal thickened. On one hand, the thickened stratum corneum layer and epidermal could protect skin from rubbing injury. On the other hand, the thickened stratum corneum layer might be partially avulsed, and then acted as a boundary lubrication film during the friction process, which could help to decrease the friction coefficient of the skin. In addition, for the skin samples after the fourth rehabilitation (Fig. 6d), the CFB were also found to become shorter than the
control (Fig. 6c). For the control skin, the collagen fibers were bundled together and hence appeared coarse and aggregated (Fig. 6c). For the test skin (Fig. 6d), the CFB were found to become thin and rarefied. In histological section, collagen fibers appear wrapped over each other in the form of bundle. The sizes of CFB were measured according to the method of Singh and Singh [33]. Double adjustable crosshairs were put around the main collagen fiber bundle excluding branched CFB with the help of video caliper. These crosshairs formed a rectangle whose diagonal was taken as an approximation of the length of CFB. Although this was not the exact length of CFB, it indicated the effect of friction on CFB. Collagen fibers are fundamental to animal development because they provide a mechanical basis for molecular and cell attachment and stabilize the shape and form of growing tissues [34]. In this study, only normal load of 2 N could induce elastic deformation of rabbit skin. The changes in collagen fiber bundles due to alternations of friction trauma and rehabilitation process might decrease the extent of skin elastic deformation. Thus, the friction energy which would overcome the elastic deformation could decrease, which would also help to reduce friction coefficient of the skin. Summarising, when the rabbit skin went through several alternations of trauma and rehabilitation process, the thicknesses of stratum corneum and epidermal increased, and the length and thickness of collagen fibers’ bundles decreased, which could decrease the friction coefficient of skin, and correspondingly, reduce the skin traumas. The rabbit skin would go through the tribological behavior from friction trauma to gradual self-adaptation under the reciprocal sliding friction condition. Although there are some differences in tissue between rabbit and human skin, our main objective of using rabbit skin instead of human skin was to explore an effective method to study the friction behavior and trauma of skin. Based on the above results, there would exist an adaptative period to wear prosthesis for new amputee. The adaptative time could depend upon friction behavior of prosthetic materials– residual limb interface, traumas of the residual limb skin and individual difference. For the new amputee who is undergoing mobility capability training, it would be the best policy to reduce walking time at a time and increase training time every day. In
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this way, the self-adaptation period to prosthetic socket rubbing condition would be shortened, and the risk of skin trauma would be decreased.
5. Conclusion The skin self-rehabilitation and self-adaptation to friction trauma were investigated in vivo by means of friction testing, histological analysis and animal experiments under the simulated prosthetic socket rubbing condition. Based on the given test conditions, the conclusions can be summarized as follows: (1) With the number of friction testing times increasing, the friction coefficient of rabbit skin decreased. Correspondingly, the skin traumas induced by friction were changed from severe to slight erythema and edema, and the scab thickness and the amount of inflammatory cells gradually reduced. (2) When the rabbit skin went through several alternations of trauma and rehabilitation process, the thicknesses of stratum corneum and epidermal increased, and the length and thickness of collagen fiber bundles decreased, which could decrease the friction coefficient of skin, and correspondingly, reduce the skin traumas. (3) During the reciprocal sliding friction process, the rabbit skins would suffer from friction trauma, rehabilitation and selfadaptation in turn.
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[11] Pailler-Mattei C, Nicolic S, Pirot F, Vargiolu R, Zahouani H. A new approach to describe the skin surface physical properties in vivo. Colloids Surf B Biointerfaces 2009;68:200–6. [12] Asserin J. Measurement of the friction coefficient of the human skin in vivo quantification of the cutaneous smoothness. Colloids Surf B Biointerfaces 2000;19:1–12. [13] Zahouani H, Vargiolu R, Boyer G, Pailler-Mattei C, Laquie ze L, Mavon A. Friction noise of human skin in vivo. Wear 2009;267:1274–80. [14] Cua AB, Wilhelm K-P, Maibach HI. Skin surface lipid and skin friction: relation to age, sex, and anatomical region. Skin Pharmacol 1995;8:246–51. [15] Cua AB, Wilhelm K-P, Maibach HI. Friction properties of human skin: relation to age, sex and anatomical region, stratum corneum hydration and transepidermal water loss. Br J Dermatol 1990;123:473–9. [16] Elsner P, Wilhelm D, Maibach HI. Friction properties of human forearm and vulva skin: influence of age and correlation with trans-epidermal water loss and capacitance. Dermatology 1990;181:88–91. [17] Sivamani RK, Wu GC, Gitis NV, Maibach HI. Tribological testing of skin products: gender, age, and ethnicity on the volar forearm. Skin Res Technol 2003;9:299–305. [18] Lode´n M, Olsson H, Axe´ll T, Linde YW. Friction capacitance and transepidermal water loss in dry atopic and normal skin. Br J Dermatol 1992;126: 137–41. [19] Johnson SA, Gorman DM, Adams MJ, Briscoe BJ. The friction and lubrication of human stratum corneum, thin films in tribology. In: Proceedings of the 19th Leeds-Lyon symposium on tribology, Amsterdam, BV; 1993. p. 663–72. [20] Adams MJ, Briscoe BJ, Johnson SA. Friction and lubrication of human skin. Tribol Lett 2007;26:239–53. [21] Kenins P. Influence of fiber type and moisture on measured fabric-to-skin friction. Text Res J 1994;64:722–8. [22] Bueno MA, Lamy B, Renner M, Viallier-Raynard P. Tribological investigation of textile fabrics. Wear 1996;195:192–200. [23] Derler S, Schrade U, Gerhardt LC. Tribology of human skin and mechanical skin equivalents in contact with textiles. Wear 2007;263:1112–6. [24] Ramkumar SS, Wood DJ, Fox K, Harlock SC. Developing a polymeric human finger sensor to study the frictional properties of textiles, part I: artificial finger development. Text Res J 2003;73:469–73. [25] Bhushan B, Wei GH, Haddad P. Friction and wear studies of human hair and skin. Wear 2005;259:1012–21. [26] Portnoy S, Yizhar Z, Shabshin N, Itzchak Y, et al. Internal mechanical conditions in the soft tissues of a residual limb of a trans-tibial amputee. J Biomech 2008;41:1897–909. [27] Mak AFT, Zhang M, Boone DA. State-of-the-art research in lower-limb prosthetic biomechanics-socket interface: a review. J Rehabil Res Dev 2001;38:161–74. [28] Zhang M, Turner-Smith AR, Tanner A, Roberts VC. Clinical investigation of the pressure and shear stress on the transtibial stump with a prosthesis. Med Eng Phys 1998;20:188–98. [29] Biological Evaluation of Medical Devices-Part 10: Tests for Irritation and Sensitization, ANSI/AAMI/ISO 10993-10-1995. [30] Li W, Qu SX, Zhou ZR. Reciprocating sliding behavior of human skin in vivo at lower number of cycles. Tribol Lett 2006;23:165–70. [31] Roberts MS, Walters KA. The relationship between structure and barrier function of skin. New York: Marcel Dekker; 1998. [32] Li W, Kong M, Liu XD, Zhou ZR. Tribological behavior of scar skin and prosthetic skin in vivo. Tribol Int 2008;41:640–7. [33] Singh S, Singh J. Dermal toxicity and microscopic alterations by JP-8 jet fuel components in vivo in rabbit. Environ. Toxicol Pharmacol 2004;16:153–61. [34] Kadler KE, David FH, Helen G, Tobias S. Tip-mediated fusion involving unipolar collagen fibrils accounts for rapid fibril elongation, the occurrence of fibrillar branched networks in skin and the paucity of collagen fibril ends in vertebrates. Matrix Biol 2000;19:359–67.
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Tribology International journal homepage: www.elsevier.com/locate/triboint
Skin self-adaptation to friction trauma under reciprocal sliding conditions W. Li, S.X. Qu, Y.J. Zheng, Q. Pang, J. Zheng, Z.R. Zhou n Tribology Research Institute, Key Laboratory for Advanced Technology of Materials of Ministry of Education, Southwest Jiaotong University, Chengdu 610031, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 December 2010 Received in revised form 24 June 2011 Accepted 30 June 2011 Available online 8 July 2011
In this study, skin self-rehabilitation and self-adaptation to friction trauma were investigated in vivo by means of friction testing, histological analysis and animal experiments under the simulated prosthetic socket rubbing condition. The denuded dorsal skin of rabbits was used to simulate stump skin. Results showed that after the skin went through several alternations of trauma and rehabilitation process, a keratinization appeared on the skin surface, which decreased the friction coefficient of skin and reduced the skin traumas. These results suggested that during the reciprocal sliding friction process, the rabbit skin suffered from friction trauma, rehabilitation and self-adaptation in turn. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Rabbit skin Friction Trauma Self-adaptation
1. Introduction It is well known that skin is a natural barrier to protect tissue and organ of the body, which comprises primarily of cells and extracellular matrices [1]. In daily life, human skin is often rubbed against external surfaces, which causes many friction problems. The friction between skin and working implements, labor protecting materials, sports appliances, improper footgear, textile materials, etc., may induce skin traumas such as the formation of blister, irritation or sensitization. However, skin has the ability of self-adaptation to friction trauma [2]. The representative example is a foot blister caused by wearing new shoes for the first time. After the blister has healed, the risk of further trauma induced by friction may decrease when wearing the same shoes again. Similar situation occurs for stump skin of amputee in contact with prosthesis socket materials, which usually comes through the process consisting of friction trauma, rehabilitation and gradual adaptation [2]. Coupling between the prosthesis and trans-tibial stump is typically achieved by a socket, which is a critical component for prosthetic performance and the sole means of load transfer between the prosthesis and the stump in current prosthetic practice [3]. Unfortunately, the skin and underlying soft tissues of the stump are not well-suited for load bearing. The interaction between the stump and the prosthetic socket usually causes elevated internal friction injury and pain in the epidermis and muscle tissues of the stump, such as pressure ulcers, blister, cysts, edema, skin irritation, dermatitis, etc. Especially, this
n
Corresponding author. Tel.: þ86 28 87600971; fax: þ 86 28 87603142. E-mail address: [email protected] (Z.R. Zhou).
0301-679X/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2011.06.027
rubbing injury is very uncomfortable for the amputee during the preliminary phase of wearing prosthesis. This would prevent amputees from wearing their prosthesis for a prolonged time, severely disabling them in their daily activities and reducing the quality of life [4,5]. Till now, only a few studies focused on the friction trauma and self-adaptation of human skin although the initial report on skin friction appeared as early as in 1955 [6]. Previous studies mainly focused on the frictional properties [7–9], deformation behavior [10], surface physical properties [11–13] and influence factors (such as age, gender, anatomical site, hydration and electrical parameters such as capacitance, conductance and impedance) of skin [14–17]. The results were applied to medicine [18], skin cosmetics [17,19,20], textile exploitation [21–24] and others related to skin [25]. It should be noted that previous results are inconsistent with each other. The conflict is due to the complexity of the friction condition between skin and materials and the limitation of measurement technologies [9]. Moreover, the skin was usually regarded as lifeless material in previous studies. Its biological response, such as self-adaptation and self-reparation during the course of friction behavior, was generally neglected. In addition, these studies were mostly carried out on the skin of volunteers. As a result, many analytical methods in engineering cannot be used, which makes tribological studies remarkably difficult. For example, both the wear morphology and wear debris of the skin cannot be observed using SEM. Nowadays, a lack of effective methodology is still a puzzle for skin tribology. Concerning the interface between stump skin and prosthetic devices, most studies paid attention to the stresses at the interface of skin–prosthetic socket by experimental measurement and finite element analysis in the last 40 years [26–28]. The information
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gained not only has been used for the assessment and improvement of prosthetic socket design and fitting, but also has contributed to the understanding of the prosthetic interface [26–28]. However, tribological factors could not be revealed adequately in the limb skin–prosthetic socket interface design and fitting, and the studies on the self-adaptation of stump skin to the friction condition of prosthetic socket are still very limited. In this paper, the self-rehabilitation and self-adaptation of rabbit skin to the friction trauma were investigated in vivo under the simulated prosthetic socket rubbing condition. The skin trauma was examined by histological analysis. The purpose was to find the tribological behavior of skin from friction trauma to gradual self-adaptation under the conditions of reciprocal sliding friction.
2. Materials and methods 2.1. Specimen preparation In order to simulate the stump skin, skin samples were prepared from in vivo dorsal skin of rabbit, which is often used for skin irritation and sensitization test according to ANSI/AAMI/ ISO 10993-10-1995: Biological Evaluation of Medical Devices [29]. The rabbit was used as a model because its epidermic and dermal tissues are almost the same as those of human skin, except for a mass of hair follicles contained within the dermal tissue. Fifteen New Zealand healthy white rabbits with the weight of 2.0 70.4 kg were studied with the approval of Institutional Animal Care and Use Committee, China. The rabbits were supplied by Experimental Animal Culture Center, Sichuan province. All experimental procedures were performed in compliance with Regulations for the Administration of Affairs concerning Experimental Animals, China. For each rabbit, about 10 cm 15 cm dorsal region was carefully shaven without any skin damages with the help of a speed clipper. Six areas were marked on the shaven dorsal skin using indelible ink. Three areas located on the left side, which were used as test sites, while other three on the right side, which were used as control sites, as shown in Fig. 1. All rabbits were acclimatized for 24 h before friction testing. The rabbit was anaesthetized with an intra-abdominal injection of 3% sodium pentobarbital at 1 ml/kg body weight before each friction testing. All testing sites were cleaned with alcohol. Acrylic resin plate, one of the prosthetic socket materials, was chosen as a counterpart. It was supplied by Sichuan Health Recovery Center for Disabled Limb and Trunk, China. A 2.7-mmthick acrylic resin plate was bent into a 15-mm-diameter bottle
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lid shape with a 1.5-mm-radius chamfer and sleeved to the copper cylindrical friction probe [30]. The contact diameter with the rabbit skin was 12 mm. The surface roughness Ra of the acrylic resin plate was 15.1 mm, which was measured using a Confocal Laser Scanning Microscopy (OLS, 1100, Olympus, Japan). 2.2. Friction tests In order to simulate the stump skin–prosthetic socket rubbing interface, two-body in vivo sliding friction tests were carried out in a flat-on-flat configuration using a reciprocating UMT series multi-specimen Biomedical Micro-Tribometer (UMT-II, CETR Corporation, America). The UMT-II is a computer-controlled benchtop instrument adapted to measure tribological parameters on the skin. The counterpart, which consisted of an acrylic resin sleeve and a copper cylindrical friction probe, was attached to a suspension system. It was pressed onto the rabbit skin under a programmed normal force and then moved linearly at a constant speed when testing. The anaesthetized rabbit was immobilized at the bottom stage of the instrument. Test parameters are shown in Table 1. The normal force Fn was 2.0 N, which resulted in a contact pressure of 17.7 kPa when the diameter of the counterpart was 12 mm. Portnoy et al. reported that the principal compressive stress was 3.3–240 kPa between the residual limb and the socket of the prosthesis by experimental measurements and three-dimensional non-linear large-deformation finite-element analysis [26]. Considering that the residual limb muscle was different from the rabbit muscle, where the muscles were larger, the subcutaneous fat overlaying the muscle was thicker, and the fascia were thicker and stronger, large compressive stress on rabbit muscle may be more severe than on human muscle. According to this, the contact pressure of 17.7 kPa was selected in this study, which approach the lower limit as Portnoy et al. reported. The reciprocating amplitude D was 75 mm, which simulated the fluctuating slide between residual limb skin and prosthetic socket surface. The frequency f was 1.0 Hz, which simulated quick step frequency of amputee when walking [27,28]. The average sliding speed was about 20 mm/s according to the reciprocating amplitude and frequency. The reciprocating number of cycles N was 1800 cycles, corresponding to 30 min walking time. Friction coefficients can be measured and recorded by the UMT-II tribometer at a sampling rate of 20 kHz to data files for processing and analysis. A software filter was constructed for the output channels providing a mean value of all friction coefficients at each reciprocating cycle. In order to investigate the self-rehabilitation and self-adaptation ability of skin to friction, the rabbits were divided into five groups. The groups and
Fig. 1. Schematic diagram of test and control sites on the dorsal skin of rabbit.
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Table 1 Friction parameters and groups of rabbits. Friction test order
Group of rabbit
Number of rabbits
Normal force (N)
Reciprocating amplitude (mm)
Frequency (Hz)
Time (min)
Period of rehabilitation (d)
1 2 3 4 5
1,2,3,4,5 2,3,4,5 3,4,5 4,5 5
15 12 9 6 3
2 2 2 2 2
75 75 75 75 75
1 1 1 1 1
30 30 30 30 30
14 14 14 14 –
Table 2 Evaluation of skin irritation response [29]. Skin irritation
Scoring
Erythema No erthema Very slight erythema, barely perceptible Well defined erythema Moderate to severe erythema Severe erythema, beet redness to slight eschar formation, injuries in depth
0 1 2 3 4
Edema No edema Very slight edema, barely perceptible Slight edema, edges of area well defined by definite raising Moderate edema, area raised approximately 1 mm Severe edema, raised more than 1 mm, and extending beyond area of exposure
0 1 2 3 4
Maximum scoring
8
Fig. 2. Typical Ft–D curve of the rabbit skin.
number of rabbits are also shown in Table 1. For the first group, friction testing was conducted only once. For the second group, friction testing was done twice at the same test site with an interval of 14 days. That is to say, there existed 14-days rehabilitation time for the test skin after each friction testing, which was in accord with the rehabilitation time of traumatic skin [2]. In the same way, for the fifth group, friction testing was done five times at the same test site with an interval of 14 days every two times. For each group, the skin biopsies were taken at 24 h after total friction testing. All the friction tests in this study lasted about 60 days from March to May. All tests were performed at a temperature of 20 75 1C and a relative humidity of 60 710%. Nine repeat tests were performed, and then nine traumatic skin surfaces were obtained under the same testing parameters.
from the right sites to serve as morphological controls. The histological section was made according to the routine method of dermatopathology [1]. First, these samples were immediately immersed in 4% buffered formaldehyde fixative for 24 h. Then, the skin biopsies were rinsed with phosphate-buffered solution, dehydrated in an ethanol grade series, embedded in paraffin and cut into 3.3 mm-thick sections. Then, these sections were mounted on slides coated with gelatin and dried overnight at 50 1C, stained by hematoxylin and eosin (H&E). Subsequently, they were observed by a microscope (DMRX, Leica, Germany). The scab thickness of traumatic skin was measured using a microscope with a 10 objective lens, and the thicknesses of epidermis and stratum corneum were measured with a 40 objective lens. The thickness was continuously measured every 100 mm. Five positions were measured, and the final thickness was the average of 5 positions. The inflammatory cells were counted in a 0.25 mm2 visual field using a microscope with a 10 objective lens. The inflammatory cells of random five visual fields were counted, and the final value was the average of 5 fields.
2.3. Skin trauma evaluation
2.5. Statistical analysis
Skin traumas were observed visually after friction testing, which were described as skin irritation response. The visual scoring system was used to evaluate primary skin irritation manifested as erythema and edema according to Tests for Irritation and Sensitization Guidelines (ANSI/AAMI/ISO 10993-101995: Biological Evaluation of Medical Devices), as shown in Table 2 [29]. Skin irritation was scored immediately after each friction test. The skin irritation response was evaluated by the average value of nine repeated test sites. The maximum scoring was the sum of erythema and edema scores.
In this study, all data were presented as w 7S. The statistical differences of thicknesses and inflammatory cells among different groups were determined by analysis of variance (F test). The level of statistical significance was set to Po0.01 or Po0.05. All the statistical differences resulted from the comparison between the control group and test group, and among the five groups.
3. Results 3.1. Friction behavior
2.4. Skin histological evaluation After friction testing, the rabbits were observed during recovery from the anesthetic, and then returned to holding facility. Before skin biopsy, the rabbits were sacrificed by an intravenous injection of air at 24 h after friction testing. Skin biopsies were taken from the left test sites and the similar biopsies were taken
As mentioned above, friction testing was conducted five times at the same testing site for the fifth group. In all these friction cases, a typical Ft–D (tangential force–imposed displacement) curve in elliptic shape at a reciprocating cycle was obtained as shown in Fig. 2. The relative sliding (dCD) occurred at the interface besides the elastic deformation (dAB–dCD) of rabbit skin and underlying tissue.
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skin is comprised of epidermis and derma. The epidermis (Fig. 6a– b) is composed of two or three layers of cells. Its surface is covered with stratum corneum (SC) layer. The dermis consists of collagen fiber bundles (CFB) (70%), and elastic connective tissue, in a semigel matrix of mucopolysaccharides [31]. Hair follicles, sebaceous glands, a few fibroblast (FB) and capillary vessels can be seen within the dermal tissue. The epidermal thickness (ET), collagen fiber bundle length (CL) and collagen fiber bundle thickness (CT) of these two kinds of skin, which went through rehabilitation 4 times and friction testing 5 times are listed in Table 4. There was significant difference in ET, CL and CT for the control and test skins (Po0.05). As shown in Fig. 6a and b, the test skin showed significant increases in ET and SC compared to the control (P o0.05). But as shown in Fig. 6c and d, the test skin showed significant decreases in CL and CT in comparison with the control (Po0.05). The typical histological photomicrographs of rabbit epidermis for different friction testing times are shown in Fig. 7. The control skin with intact epidermis and stratum corneum is also shown with arrowhead in Fig. 7a as it will be convenient to compare. In the first three times of friction testing (Fig. 7b–d), the rabbit epidermis was almost rubbed out and a lot of inflammatory exudation effused. The surface of the exudation was fibrin. The fibrin was mainly composed of huge amounts of necrotic neutrophilic granulocytes. The worn skin surface shriveled and formed scab when the fibrin was dried (as shown with arrowhead in Fig. 7). The collagen fiber bundles near the necrotic epithelial tissue became swollen and sparse. Serious inflammatory cell infiltration appeared within the derma tissue. The inflammatory exudation and scab thickness gradually reduced with the increasing number of friction testing times. After the fourth and fifth friction testing, the scabs could not be identified (Fig. 7e–f). The average and standard deviation of the scab thickness and the analysis of variance (F test) for different friction testing times are shown in Table 5. The scab thickness gradually reduced with the
The frictional regime was in the intermediate regime from sticking to gross relative sliding regime according to our previous study [30], which was also identical with the result of the friction force changes in the ‘‘stick’’ and ‘‘slip’’ behavior described by Kwiatkowska et al. [10]. Fig. 3 gives the typical curves of friction coefficient (here, mean value of each reciprocating cycle was used) versus time from the first testing to the fifth testing. The evolution trends of all the curves were high at the start of the friction testing, afterwards gradually declined with time when stratum corneum layer was avulsed, or epidermis was scraped accompanied by exudation and bleeding, and finally fluctuated near the stable value. A summary of mean friction coefficient values of the fifth group rabbits for all the number of friction testing times when they fluctuated near the stable value (after 20 min) is provided in Fig. 4. The mean values and standard deviations were calculated from 9 skin samples. The friction coefficient was the highest for the first testing and the lowest for the fifth testing. That is it decreased with the increasing number of friction testing times. 3.2. Skin trauma evaluation With the increasing friction time, three main types of skin trauma, erythema, edema and exudation, were observed. At the beginning of the first friction testing, erythema appeared on the rabbit skin. Then the abnormal exudation infiltrated into the tissue and edema was observed. Subsequently, the rabbit epidermis was scraped accompanied by exudation and bleeding. A typical photo of traumatic skin after the first friction testing is shown in Fig. 5a, a severe erythema was observed on the test skin surface in comparison with the control. Due to the different friction testing times, erythema with various degree hue and sizes were observed on the rabbit skin surface after friction testing. Furthermore, with the number of friction testing times increasing, the start time when the rabbit epidermis was scraped and the exudation and bleeding was delayed. Especially after the fourth and fifth friction testing, the rabbit epidermis almost was not scraped except some stratum corneum layer avulsed, as shown in Fig.5b. Meanwhile, from severe to slight erythema and edema on the friction sites were observed as the number of friction testing times increased. The corresponding scores for erythema, edema and maximum irritation are shown in Table 3. It was observed that the skin irritation scores decreased with the increasing number of friction times. The irritation score was the highest for the first friction testing and the lowest for the fifth friction testing. The levels of skin trauma were consistent with the variations of the friction coefficient for different number of friction testing times.
Friction coefficient
1.0 0.8 0.6 0.4 0.2
3.3. Skin histological evaluation
0.0 2 3 4 Number of friction testing times
1
The histological photomicrographs of the control skin and the test skin of the fifth group, which went through rehabilitation 4 times and friction testing 5 times are shown in Fig. 6. The rabbit
Fig. 4. Mean friction coefficient for different friction testing times (n¼9).
Coefficient of friction
2.0 1st testing
2nd testing
3rd testing
4th testing
5th testing
1.5 1 1.0
2 3
4
0.5 5 0.0 0
5
10
5
15 Time (min)
20
25
Fig. 3. Variations of friction coefficient with time for different friction testing times.
30
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Fig. 5. Photos of traumatic skin and control skin, (a) after the first friction testing, (b) after the fifth friction testing. Table 3 In vivo skin irritation scores for different friction testing times (n¼9, Po 0.01). Friction testing times
Control
1
2
3
4
5
Erythema Edema Maximum irritation score
07 0.0 07 0.0 07 0.0
3.78 70.44 3.33 70.50 7.11 70.47
3.33 7 0.50 2.67 7 0.50 6.007 0.50
2.677 0.50 2.337 0.50 5.007 0.50
1.44 70.53 1.33 70.50 2.78 70.51
1.22 7 0.44 0.677 0.50 1.89 7 0.47
Fig. 6. Histological photomicrographs of control and test skin samples after rehabilitation four times, (a) epidermis and derma of control skin, (b) epidermis and derma of test skin, (c) derma of control skin, (d) derma of test skin (key: E, epidermis; CFB, collagen fiber bundle; FB, fibroblast; SC, stratum corneum) (40 ).
Table 4 Quantitative microscopic alterations between control and test skins of in vivo rabbits which went through rehabilitation 4 times and friction testing 5 times (n¼ 9). Skin sample
Control
Test
P-value
Epidermal thickness, ET (mm) Collagen fiber bundle length,CL (mm) Collagen fiber bundle thickness, CT (mm)
20.27 4.1 43.87 10.2 10.87 1.7
26.4 7 6.8 38.7 7 8.5 8.3 7 1.4
o 0.05 o 0.05 o 0.05
increasing number of friction testing times. Moreover, a significant difference in the scab thickness was observed (Po0.01). The typical histological photomicrographs of inflammatory cell infiltration within the rabbit derma tissue for different friction testing times are shown in Fig. 8 (with arrowhead). The magnified photograph on the left top corner of each photomicrograph was the partial inflammatory cells near the arrowhead using a 100 objective lens. It was seen that the neutrophilic granulocytes within the damaged derma tissue in a given visual field were
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Fig. 7. Histological photomicrographs of rabbit epidermis for different friction testing times, (a) control, (b) first testing, (c) second testing, (d) third testing, (e) fourth testing, (f) fifth testing (10 ).
Table 5 Scab thickness of traumatic skin for different friction testing times and statistical significance analysis (n¼9). Friction testing times
1
2
3
4
5
Scab thickness (mm) F P-value
267.9 711.1
136.4 7 9.4
66.6 77.8 1752.9 o 0.01
14.1 7 2.2
9.97 1.6
more and denser after either the first or the third friction testing (Fig. 8b–d), than after fourth and fifth testing (Fig. 8e–f). The amounts of inflammatory cells within the rabbit subcutaneous tissue for different number of friction testing times are shown in Fig. 9. The amount of inflammatory cells was an average of the nine traumatic skin tissues. Differences of inflammatory cell among means were checked for significance separately by analysis of variance (F test). The amount of inflammatory cells in a given visual field decreased with the increasing number of friction testing times. A significant difference in the amount of inflammatory cells was also observed (Po0.01).
4. Discussion Reciprocal sliding friction on the skin surface would tend to break down the efficiency of the stratum corneum barrier function and induce the skin trauma, especially when these friction behaviors resulted from cyclic mechanical loads if contact pressures and shear forces were high or continue over long periods of time [32]. During the friction course of prosthetic socket material–rabbit skin interface, the SC layer was first avulsed. Subsequently, the epidermal layer was gradually frazzled, and the tissue fluid and blood extravasated. The avulsed SC layer, exudation and blood played the role of lubricant, which resulted in the friction coefficient declining with time. After 1800 reciprocating cycles, various degrees of erythema appeared on the skin surface. Unlike conventional engineering materials, living beings have the particular function of self-rehabilitation to damage. Once trauma occurs to living skin, a series of complex transformations
in shape, biology, physics, etc., which are divided into two phases, namely inflammation eliminating phase and tissue rehabilitating phase, would appear within the skin tissue [1]. In this study, the scab was formed on the traumatic rabbit skin surface during the inflammation eliminating phase. The surface of the scab was fibrin. The underneath of the scab was the cell matrix, which was mainly composed of a lot of necrotic skin tissue and dead neutrophilic granulocytes. The foundation of wound healing was a series of activities of inflammatory cells and rehabilitative cells [1]. Neutrophilic granulocytes were the main incipient inflammatory cells, which accessed the traumatic tissue. They eliminated the necrotic tissue and foreign matter by their functions of phagocytosis, oxygen free radical antibiosis, complement activation, etc., which could protect normal tissue from infection [1]. Substantive neutrophilic granulocytes were observed at the interface of local necrotic tissue and living body tissue within the rabbit traumatic skin tissue after 24 h of friction testing. That was why the skin biopsies were taken at 24 h after each friction testing. The results of cell counting showed that the main cellular infiltration was the neutrophilic granulocytes, which accessed the local traumatic derma tissue. Therefore, the scab thickness on the traumatic skin surface and the amount of neutrophilic granulocytes within the traumatic derma tissue were used to estimate the effects of the friction conditions on the level of skin tissue traumas. In this study, severer skin traumas occurred during the first three times of friction testing, which corresponded to higher friction coefficient. When the test skin went through several alternations of trauma and rehabilitation process, its adaptation ability to friction was built up. That is, during the fourth and fifth
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Fig. 8. Histological photomicrographs of inflammatory cell infiltration within rabbit dermis for different friction testing times, (a) control, (b) first testing, (c) second testing, (d) third testing, (e) fourth testing, (f) fifth testing (40 ).
Amount of inflammatory cells
800 514 600
442
400
289 196
200
157
0 1
2 3 4 Number of friction testing times
5
Fig. 9. Comparison of the amount of inflammatory cells within rabbit dermis for different friction testing times (n¼ 9, Po 0.01).
friction tests, the level of traumas and the friction coefficient decreased synchronously. Histopathological examinations demonstrated that the scab thickness and the amount of inflammatory cells decreased with the increasing number of friction testing times, suggesting that the level of rabbit skin traumas gradually reduced. That is to say, the rabbit skin gradually adapted to the friction condition. The adaptation ability of the skin to friction was in connection with the changes of skin microstructure. After the rabbit skin went through several alternations of trauma and rehabilitation process, the stratum corneum layer and epidermal could thicken. That is, keratinization could appear on the skin surface [2]. These changes can be seen in Fig. 6. For the skin samples after the fourth rehabilitation (Fig. 6 b), compared with the control (Fig. 6 a), the stratum corneum layer and epidermal thickened. On one hand, the thickened stratum corneum layer and epidermal could protect skin from rubbing injury. On the other hand, the thickened stratum corneum layer might be partially avulsed, and then acted as a boundary lubrication film during the friction process, which could help to decrease the friction coefficient of the skin. In addition, for the skin samples after the fourth rehabilitation (Fig. 6d), the CFB were also found to become shorter than the
control (Fig. 6c). For the control skin, the collagen fibers were bundled together and hence appeared coarse and aggregated (Fig. 6c). For the test skin (Fig. 6d), the CFB were found to become thin and rarefied. In histological section, collagen fibers appear wrapped over each other in the form of bundle. The sizes of CFB were measured according to the method of Singh and Singh [33]. Double adjustable crosshairs were put around the main collagen fiber bundle excluding branched CFB with the help of video caliper. These crosshairs formed a rectangle whose diagonal was taken as an approximation of the length of CFB. Although this was not the exact length of CFB, it indicated the effect of friction on CFB. Collagen fibers are fundamental to animal development because they provide a mechanical basis for molecular and cell attachment and stabilize the shape and form of growing tissues [34]. In this study, only normal load of 2 N could induce elastic deformation of rabbit skin. The changes in collagen fiber bundles due to alternations of friction trauma and rehabilitation process might decrease the extent of skin elastic deformation. Thus, the friction energy which would overcome the elastic deformation could decrease, which would also help to reduce friction coefficient of the skin. Summarising, when the rabbit skin went through several alternations of trauma and rehabilitation process, the thicknesses of stratum corneum and epidermal increased, and the length and thickness of collagen fibers’ bundles decreased, which could decrease the friction coefficient of skin, and correspondingly, reduce the skin traumas. The rabbit skin would go through the tribological behavior from friction trauma to gradual self-adaptation under the reciprocal sliding friction condition. Although there are some differences in tissue between rabbit and human skin, our main objective of using rabbit skin instead of human skin was to explore an effective method to study the friction behavior and trauma of skin. Based on the above results, there would exist an adaptative period to wear prosthesis for new amputee. The adaptative time could depend upon friction behavior of prosthetic materials– residual limb interface, traumas of the residual limb skin and individual difference. For the new amputee who is undergoing mobility capability training, it would be the best policy to reduce walking time at a time and increase training time every day. In
W. Li et al. / Tribology International 44 (2011) 1782–1789
this way, the self-adaptation period to prosthetic socket rubbing condition would be shortened, and the risk of skin trauma would be decreased.
5. Conclusion The skin self-rehabilitation and self-adaptation to friction trauma were investigated in vivo by means of friction testing, histological analysis and animal experiments under the simulated prosthetic socket rubbing condition. Based on the given test conditions, the conclusions can be summarized as follows: (1) With the number of friction testing times increasing, the friction coefficient of rabbit skin decreased. Correspondingly, the skin traumas induced by friction were changed from severe to slight erythema and edema, and the scab thickness and the amount of inflammatory cells gradually reduced. (2) When the rabbit skin went through several alternations of trauma and rehabilitation process, the thicknesses of stratum corneum and epidermal increased, and the length and thickness of collagen fiber bundles decreased, which could decrease the friction coefficient of skin, and correspondingly, reduce the skin traumas. (3) During the reciprocal sliding friction process, the rabbit skins would suffer from friction trauma, rehabilitation and selfadaptation in turn.
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