On Possibilities and Limits of a New Experimental Method for Determining Skin Elastic

The 2nd International Conference Computational Mechanics and Virtual Engineering COMEC 2007 11 – 13 OCTOBER 2007, Brasov, Romania

ON POSSIBILITIES AND LIMITS OF A NEW EXPERIMENTAL METHOD FOR DETERMINING SKIN ELASTIC PROPERTIES

Ileana ROŞCA 1 1Transilvania University, Braşov, ROMANIA, [email protected]

Abstract: It is well known that skin thickness and elastic properties can characterize its health and the health of other parts of human body. The knowledge of this kind of properties has also a great importance from the aesthetic point of view, and the research in this domain finds a lot of applications in beauty products industry. More the skin is thickened or changed in its internal structure, more its mechanical elastic properties are modified and a correlation with different diseases and degeneration levels can be done. In order to perform simpler and non-invasive tests, we conceived a new method that presents many advantages because of its simplicity and its non-invasive character but there are also some disadvantages: it is strictly necessary to calibrate the device and this needs to create several samples of artificial tissues with well-determined elastic properties by mechanical testing. Also, sensitive problems are force intensity control and estimation of errors due to natural reaction of the human subject to external action.. Keywords: skin, elasticity, and testing, mechanical properties.

1. INTRODUCTION

The skin is the biggest and the most visible organ of the human body. It ensures its first defending line and plays a vital role in body’s integrity. Mechanical behavior of skin is very nonlinear, anisotropy and non homogenous. The most important skin’s mechanical properties are: extensibility, friction resistance and the specific answer to lateral compressive charges that are variable according to age, exposure, moisture, obesity, health state, orientation. Other properties are viscoelasticity, incompressibility and plasticity. Aging changes considerably the mechanical properties of the skin – old skin is much less extensible and elastic than those of a young person. It changes with the age, wrinkles appear and become more deep; they depend on the skin nature and on muscles’ contraction. Skin is a consistent structure organized on three principal layers: epidermal, dermal, and hypodermal. Its most evident functions are to protect the body by preventing water loose and penetration of undesired substances due to impermeability of the epidermal and to shock absorber character of dermal. The less evident but equally important is the immunity answer to unknown materials – inside of skin there are little organs playing sensors’ functions and detecting the stimuli by touch, pressure, heat, coldness and pain. The skin plays also an essential role in social communication by its external image, by touching it and by its odor. Traditional mechanical characteristics of homogenous materials are not characteristic for skin. Its Young modulus has not a unique value and the shearing modulus is also variable – all these properties changing continuously according to the applied loading. Stress–strain diagram of biological materials has a linear or non-linear elastic zone and another that is not elastic and depends on loading history.

2. IN VITRO TESTING ON SKIN. ITS LIMITS AND SOME EXAMPLES

The most practiced in vitro experiments on skin are traction – extension testing, but really they present some particular difficulties. For testing a sufficient quantity tissue is necessary to be conveniently exploited and the multiplayer structure of the skin is not suitable for quantitative cut. The shape and the dimensions of the specimen impose the use of special experimental equipments. In the same time, the experiment reproducibility is not always ensured; the direction of specimen cut of can influence the results. These are a part of reasons that make scientists to choose other different methods to test the characteristics of soft tissue. If a mono-axial loading is applied to a skin sample, it will be extended in the force direction and contracted in perpendicular direction. If skin extension is done “in vitro” a diminution of extended sample volume can occur when

175 the water is physically ejected from it. Initially in traction loading, a little strain is developed but another follows this phase when similar extensions are produced by much greater loadings. The thin elastic fibers are associated with collagen fibers in a secondary network connected with the collagen. This network probably acts in the sense to replace the collagen in initial alignment after low forces action. Some researches show us that the elastic fibers produce the recoil after deformation. They are immersed in water, proteins and macromolecules producing lubricating effect during deformation. The replacement of collagen fibers in initial alignment needs the replacement of the ejected liquid. “In vivo”, this fluid is available in neighbor tissues. The detached or too tensed skin cannot replace the fluid and, consequently, cannot recover its initial state. It was shown that the elastic fibers are more elastic and in greater number than the collagen ones that break at an elongation of 10 % in comparison with 100 % for elastic fibers. Many explanations for properties of skin were done essentially based on observations of the reaction to variables loadings of this kind of fibers. Previous researches resumed the skins' extensibility characteristics as follows: the dermal collagen do not react identically many times, more fibers becomes aligned on the direction of loading and these parallel fibers are extended only under great charges. Some time ago, in literature was described a natural collagen fibers' organization in form of a star that allows the individual fibers to be continuously resettled as to resist to major extension coupled with the normal activity. Apparently, the fibers are randomly settled but when an increasing charge is applied they become parallel. After the stress is removed, the fibers do not recover the initial state because the fluid was eliminated from the star pattern.

3. IN VIVO TESTING ON SKIN

Is it not always possible or convenient to cut off a tissue or an organ to analyze its biomechanical parameters. On the other hand, most «in vitro» methods are destructive for the tested tissue or organ, and this question is not acceptable from the deontological point of view. Finally, most often we do not try to determine the behavior of an insulated organ or of a unique tissue layer but mostly their behavior in biological environment. Practical appliance of such tests is delicate; even the access to organ is not difficult we must take care not to affect the neighbor organs by the identification devices. The experimental device must be sure, ergonomic, and sterile and the operational protocol has to be exactly defined as to keep unmodified the anatomic behavior. Human skin is material presenting a very complex behavior. Many present days studies dedicated to a better understanding of living materials are based on finite elements method. A finite elements model needs, generally some hypothesis and some well-defined steps. The geometry and the mechanical behavior have to be as close as possible to real state. These data can be obtained by “in vivo” measurements (ultrasounds, IRM) that are more difficult to perform but they give much more accurate results on mechanical and geometrical parameters. One of “in vivo” method to analyze its mechanical characteristics is based on extensometer tests. In principle, two devices are fixed on skin and further the experimental results are compared with finite elements method calculus results. The comparison is done by an inverse method using extended Kalman filters and, thus the representative equivalent elastic parameters are obtained. The advantages of such a method are of three kinds. Firstly the method cab be easily adapted to other kinds of tests. The elements’ comparison makes directly possible every individual test. Further, it is a sure (adapted) method that does not need a time consuming study as for standard algorithms. Finally, it has the advantage to obtain rapidly the parameters because all numerical simulations are done at the beginning. For the mono-axial skin stressing an aspiration – suction a pneumatic pistol and the pressure apply technique is progressively reduced through its tube. For deformations reading two techniques and the adapted recording equipment for the deformation's evolution in real time are commonly used: a laser captor placed in the center of the pistol tube or a camera and a mirrors’ system. The last solution allows determining the integer deformation profile that can be further integrated in a finite element model. This is its advantage versus the other method using a laser sensor and which is measuring only a punctual deformation. These two techniques are simply enough and they are often used in biomechanics because they allow with the same instrument two types of testing – in traction (by air aspiration) and in relaxation (depression followed by abrupt admission of air).

4. CONSIDERATIONS ON NEW EXPERIMENTAL METHOD

Charging/discharging technique uses the next principle: with a simple surgical instrument, a simple cylinder (called indenter) or a round plate stick on skin, endowed with a stress captor and an execution element, the stresses and strains of tissue (according with the time or with imposed oscillation frequency). Taking as a model the measurement of the ocular tonus a new method was designed to test the elastic properties of the skin. This method consists in applying a load F on skin through an indenter maintained in vertical position (figure 1). It will produce a depression having the deepness d dependent on: initial stiffness of the skin, rigidity of the superficial layer and the curvature of the indented surface.

R1 R2   d

R Figure 1: Configuration for measuring elastic properties of skin by indentation

The skin will answer with a reaction force R = F that could be decomposed in two components R = R1 + R2 on the two sides of the indenter. Deeper the depression less the angle α of two components with the vertical direction will be. This measured angle is put in concordance with the elastic properties of the skin. Taking into account that the method is inspired from another class of experimental testing – those practiced on the eye, the conceived device must to be calibrated in order to obtain results possible to be used in further processes. Calibration was a long and not easy operation to accomplish. Firstly we needed some material samples to simulate the skin as close as possible to real tissue. For this purpose we choose plastic materials with a chemical composition mixed with collagen in a certain percentage determined after many testing and tentative. From this mixture we made a series of samples with increasing values of mechanical properties established to compose a scale. Further, this scale was used as comparison base for the experimental results obtained on skin. Mechanical properties of different plastic samples were determined by classical methods used, generally, in strength of materials: traction, torsion, bending and composed loading of these basics.

5. LIMITS OF THE METHOD. FURTHER DIRECTIONS OF RESEARCH

The experimental method described above in its principle showed us, in practice, its limits. Without mentioning the above-described problem of device calibration and executing and measuring plastic samples, we observed physiological problems during real testing on human subjects. It is well known that the measurements of ocular tonus are made under anesthesia as to avoid pain and mostly the reaction of subject. During skin tests we observed that a defense reaction is manifest for all subjects on all locations on their bodies. This reaction is a strong source of errors because it determines a local stiffening of the tissue and also a certain acceleration of local deformation, increasing the solicitation instantaneously. Taking into account the demonstrated limits of the presented method, we already considered another alternative for the method. It will be performed with another type of action performing the force F as to avoid the defense reaction of subject.

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