From a Gummi Bear to a Skin Model

Developing a Frictional Skin Model for In Vitro Measurements

  • Fig. 1: A gelatine-based skin model; due to its water-responsivity, the surface of the skin model becomes smoother, shinier and swollen after water exposure.Fig. 1: A gelatine-based skin model; due to its water-responsivity, the surface of the skin model becomes smoother, shinier and swollen after water exposure.
  • Fig. 1: A gelatine-based skin model; due to its water-responsivity, the surface of the skin model becomes smoother, shinier and swollen after water exposure.
  • Fig. 2: Water makes the skin model smoother, softer and swollen. Due to the hydration-related changes in its properties, the gelatine-based skin model undergoes the same changes in frictional behavior as skin.
  • Fig. 3: Cause-and-effect chain for the exposure of skin to water: (a) three pathways of water diffusion, (b) effects of prolonged contact with water, (c) effects of increased skin hydration level, (d) friction-related consequences of increased skin hydration, (e) main characteristics of the gelatine-based human skin model. *μ-coefficient of friction, SC-stratum corneum.
It is the first interface of contact between our bodies and the environment. It is our protective armor, keeping us safe, healthy and comfortable every single day. It is our skin.
Skin, with a surface of some 2 m2 and a mass equal to around 15% of total body mass, is the largest single organ of the human body and plays a variety of vital functions [1]. First of all, skin protects internal body parts from various external triggers, such as mechanical injuries, radiation, chemicals and contamination, or microorganisms [2]. It is also responsible for temperature regulation and fulfills this role by means of both insulation and active regulation, for example sweating and modification of blood flow [3]. Skin is also an impressive sensory organ, able not only to feel pain or temperature change, but also to distinguish objects with different surface morphologies [4]. All of these are simply examples of the unnumerable features and abilities of skin. 
Skin Friction
In everyday life, human skin is in almost continuous contact with other materials, such as clothes, tools, household items, music instruments and sports or medical equipment. The safety, usefulness and comfort of use of various objects are strongly related to the friction between them and human skin. The need to determine the interaction between human skin and everyday materials in a safe and practical way is one of the main reasons for the development of the frictional skin models [2]. Interaction between skin and other materials can be measured in vivo or in vitro. In vivo measurements entail the involvement of volunteers. This type of investigation is expensive, time consuming and can be challenging to perform. In vitro measurements may be performed with the use of cadavers or animal skin, which are both unstable and can raise ethical issues, or on artificial skin models. 
The Story Behind the Model
In order to develop an appropriate skin model, the desirable characteristics of human skin need to be investigated.

A key conclusion can be extracted from an in vivo study: the properties and function of skin strongly depend on water [5]. Water can appear on the skin surface due to sweating, sweat accumulation or even rainfall or increased air humidity. In real-life situations, skin is almost always covered with a thin layer of water. Human skin interacts with water and absorbs it, becoming smoother, thicker and more easily deformable. According to many skin tribologists, interaction with water leads an increase in real contact area between skin and other objects and, therefore, to increased friction coefficient [6]. In the presence of a limited amount of water, lying within the physiologically relevant range related to sweating and sweat accumulation, friction forces between skin and a large number of materials are significantly higher than in the case of dry skin. This phenomenon may have beneficial consequence and improve grip, for example when holding a tool. Unfortunately, it can also lead to blisters, irritation and mechanical damage of the skin surface. To make things even more complicated, it is not only a question of friction-coefficient values. The relationship between the friction coefficient and the normal load also differs between dry and damp skin. Under completely wet conditions, lubrication starts taking place and friction-coefficient values decrease again. 

Until now, available artificial skin models were not able to undergo the characteristic changes in the skin that are caused by the presence of water and, therefore, could not be used as skin models under hydrated conditions. 
From a Gummi Bear to a Skin Model
Keratin is the substance mainly responsible for this skin-specific water interaction [7]. It is a fibrous structural protein, present in the cells of the stratum corneum (SC), the outer layer of human skin. From a practical point of view, keratin is not the best choice for a macroscopic skin model, since it is very expensive. 
Gelatine has similar properties to keratin, but its price is only a small fraction of that of keratin. Gelatine has already been applied as a skin model in various applications, such as ballistic tests, elastography or cosmetics [2]. Some researchers even use gummi bears as skin models for preliminary investigations.
Striving Towards the Skin Model
Bloom number is a measure of the strength of a gelatine-based gel. With increasing Bloom number, gel strength and stiffness of gelatine increases as well. After preliminary results, gelatine with a Bloom number of 300 was chosen as the basic component of the skin model, since its properties were the most similar to the ones of human skin. Preliminary investigations have also shown that the presence of an additional layer of cotton substrate provides the necessary mechanical support for the brittle gelatine film and allows long friction measurements to be performed on a macroscopic scale. 
Three layers of a gelatine coating (water-based, 10 wt.% gelatine solution) were applied on top of cotton fabric and left to dry for 24 h between each new layer. Then, the last preparation step was performed. Gelatine in its natural state is water-soluble. In order to be applicable as a properly functioning skin model for hydrated conditions, gelatine has to be crosslinked before use. Thanks to the crosslinking process, performed in a controlled-pH bath containing glutaraldehyde as the crosslinking agent, the polymeric chains of gelatine were connected together through a chemical reaction. This still allows the final material to absorb water and swell, but prevents it from the undesirable property of dissolving. After crosslinking, the skin model needs to be dried under load, in order to avoid surface inhomogeneities caused by drying-related contraction.
Ready to Go?
Figure 1 presents the appearance of the final skin model. It is brownish and stiff and looks remarkably like the horny layer of skin. Besides the general characteristics of the gelatine-based skin model, its water-responsiveness may also be observed in figure 1. The ellipsoidal area in the middle of the figure is clearly much shinier and smoother than the rest of the skin model’s surface. This significant change in the appearance was caused by water exposure of this particular area immediately prior to taking the photograph. 
The skin model is water-responsive and therefore it takes up water. Due to the water uptake, the surface becomes smoother and shinier and the thickness of the material increases as a consequence of swelling. After 60-minutes water exposure, the surface roughness (Sa) of the skin model dropped from 2.3 ± 0.3 μm when dry to 0.8 ± 0.1 μm under hydrated conditions [8]. The significant decrease in surface roughness after water exposure corresponds to the trends observed for human skin under the same conditions [5]. Prolonged water exposure leads to swelling and, therefore, an increase in thickness for both the horny  layer of human skin and the skin model. For both, a maximum increase in thickness of 21% has been reported [5, 8]. Similarly, comparable changes in stiffness have been observed for the horny layer of human skin and the skin model. In both cases, a significant decrease in Young’s modulus values (from the GPa to the MPa range) has been observed [5, 8]. 
As a result of all the above-mentioned, hydration-triggered changes in human skin properties, the frictional behavior of human skin in contact with other materials changes significantly. In vivo measurements performed on the volar forearm rubbed against a Martindale reference textile have shown that the presence of water on human skin leads to increased friction-coefficient values. Moreover, the mechanism of friction changes as well, resulting in a stronger dependency of the friction-coefficient values on the applied normal load. According to these general trends, the skin model undergoes the same changes in frictional behavior as skin. Moreover, the friction-coefficient values obtained from in vitro measurements lie within the range of values obtained from measurements performed on human skin.
Figure 2 summarizes the influence of water on the properties of the gelatine-based, water-responsive skin model. 
Possible Applications
Various technologies and markets could benefit from such a skin model. Measurements characterizing the interaction between human skin and other materials are necessary for the development of products that are designed to stay in prolonged contact with skin or for which the skin-material interaction is crucial for performance. Conventionally, such measurements are performed in vivo, which involves human participants, leading to high costs and complex procedures. Thus, validation processes for various tools, medical items, sports and professional clothing, household items, sports equipment, patches or wound dressings can all benefit from the possibility of using an inexpensive skin model with a long shelf-life, instead or in parallel with conventional in vivo measurements.
Next Steps
What would make the skin model even more similar to real skin? The ability to sweat! Therefore, as a next step, an independent, load-triggered sweating system is going to be implemented, in order to provide artificial sweat release within the physiologically relevant range and to mimic human skin even better.
Figure 3 summarizes the effect of water on the properties of both human skin and the gelatine-based skin model. They both interact with water, becoming smoother, softer and thicker. In addition, the frictional behaviour of both human skin and the skin model changes in the presence of water, following the same trends and ranges of friction-coefficient values. 
A. Dabrowska1, F. Spano1, C. Affolter1, G. Fortunato1, S. Lehmann1, S. Derler1, N. D. Spencer1 and R. Rossi1
1 EMPA Materials Science and Technology, Laboratory for Biomimetic Membranes and Textiles, St.Gallen, Switzerland
EMPA Materials Science and Technology
Laboratory for Biomimetic Membranes and Textiles
St.Gallen, Switzerland

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