Nanotribological Properties Of Skin

The study of tribology at the macroscale can simulate the contact of skin withvarious surfaces and has important practical significance. The studies ofnanotribology focus on the microstructure of skin and the interaction of the moleculeson the interface of skin and relative motion surface, and it is very useful to bridge thegap between the nano- and macroscale data, as well as to gain a full understanding ofthe related tribologcial mechanisms. Since the skin care industry continually searchesfor better cream formulations, the studies of the interaction of the molecules of skincream ingredients and skin is useful to the development and assessment of skin careproducts .In this paper, the nanomechanical properties, nanotribological properties, andnanotriboelectrification of virgin rat skin and cream treated rat skin were carried outusing an atomic force microscope (AFM) and a nanoindenter. The nanotribologicalproperties of synthetic skins were also studied. The tribological properties of virgin ratskin and cream treated rat skin was carried out by using a tribometer at the macroscale.The tribological properties of virgin hand skin, hand cream treated skin, andskin-fabrics were carried out by using a multi-speciment test system at macroscale.The conclusions are as following:The study of nanomechanical properties of rat skin shows that the averagescratch depth increases almost linearly with an increase in the normal load for virginskin and cream treated skin. Compared with virgin skin, cream treated skin exhibitsbetter scratch resistance up to a normal load of 15μN. Skin cream treatment cansoften skin, increase the elasticity and ductibility, and reduce the generation offractions (patches) as the strain increases.The study of nanotribological properties of rat skin shows that the skin cream canreduces the surface roughness and increases the hydrophilic properties of skin. Thefilm thickness, velocitiy, normal load, relative humidity, and temperature influence thecoefficient of friction and adhesive force of skin surface. The change of the filmthickness leads to the change of coefficient of friction and adhesive force with theincrease of scan cycles.The study of nanotriboelectrification of rat skin shows that the moisture in theenvironment and skin cream can reduce the static charging effectively. The staticcharges decay through air with time. The static charge is easily generated on virginskin surface and the skin cream treatment can reduce the generation of static charge. The magnitude of the average absolute surface potential change with the macroscalerubbing method is higher than that with microscale rubbing method for virgin skinand cream treated skin. The change of absolute surface potential increases with anincrease of velocity, normal load, and rubbing time for virgin skin and cream treatedskin.The study of the influence of skin creams on the nanotribological properties ofrat skin shows that pure lanolin and pure petroleum jelly have high durability andsticky tactile perception, whereas oil-free cream and common cream have a lowerdurability and slippery tactile perception. Aqueous glycerin has the lowest durabilityand slippery perception. The higher viscosity results in higher friction and durability.The relative humidity affects the film thickness, adhesive force, and Young's modulusof virgin skin and cream treated skin.The study of nanotribological properties of synthetic skins shows that based onsurface and friction properties, synthetic skins are good simulations of rat skin.However, the hardness of synthetic skin-2 is comparable to rat skin and can be a bettersimulation. After treatment with skin cream, the trends of the properties of the twosynthetic skins and rat skin are comparable. The methodology presented here can actas a good reference for researchers to evaluate the surface, frictional, and mechanicalproperties of synthetic skins.The study of tribological properties of rat skin at the macroscale shows that thetribological properties of rat skin is depondent on the scale of study. The coefficient offriction of rat skin at macroscale is larger than that at the nanoscale. The influence offilm thickness, velocity, and normal load on tribological properties of rat skin isdifferent at different study scale. The durability of rat skin is also dependent on thestudy scale.The study of tribological properties of human hand skin at the macroscale showsthat the surface humidity and temperature affect the tribological properties of handskin. In the four parts of hand skin, the decrease magnitude order of the coefficient offriction of skin was nail, hand back, palm, and dactylogram. In the three hand creams,the moistening effect of glycerin hand cream was distinct, and moistening effect thepetroleum jelly hand cream and common hand cream is similar. The petroleum jellyhand cream was highest durability and the durability of glycerin hand cream andcommon hand cream was similar. In the four fabrics, the decrease magnitude order offriction coefficient between fabrics and skin was wool, leather, silk, and cotton.