Biomedical Micro-Trobometer Application Notes for Skin Studies
(both in-vivo and in-vitro)
Goals
The skin acts as the body's first line of defense against its surroundings. Over the course of time a person's skin will undergo changes, and to maintain skin health, it is important to quantitatively follow these changes. Sometimes these skin changes are visible, such as wrinkles, blemishes or rashes. In other cases, the changes may not be easily discernable without a quantitative assessment of some skin property. Currently, there is no widespread diagnostic technique or method available to quantitatively relate skin properties to skin health. The knowledge of skin's tribological properties, namely, surface coefficient of friction and surface contact electrical impedance, provides a quantitative assessment of skin health.
The bulk of observation in dermato-pharmarcology and dermato-toxicology is based on qualitative visual or papillatory data. The experience with skin bioengineering proves the sharpness of the biometric focus provided by appropriate instrumentation and execution. Skin tribology has not been widely utilized as appropriate, robust, and facile instrumentation (and validation was not available). The unique UMT multi-sensing technology should fill this void in the fields of dermato-physiology, dermato-pharmocology, dermato-toxicology and cosmetology.
Friction and electrical measurements have already been utilized as analytical techniques for skin health assessment, primarily in research settings. This project will allow the adoption the most advanced and patented technology for use in a clinical setting. Utilization of CETR proprietary technology in multiple sensors and real-time high frequency data acquisition allows for dramatic improvement in data quality of the tribological in vivo and in-vitro assessment of human skin. The results of this will allow for a fast and quantitative assessment of such skin conditions as dryness and moisturization, early diagnosis of skin diseases, or deterioration in skin functions at a stage that may not be easily discernable visibly. It may be instrumental in developing and testing skin cosmetics and medicine.
Friction Measurements
In all the different methods used to quantify the friction property of skin, the general process was to press a probe perpendicular to the skin surface and to either pull or push along the skin. Some experiments have dealt with static friction at the moment the probe just begins to move, but most of the past work has reported values for dynamic friction. Experiments on skin friction coefficient are summarized in Table 1.
The range in the friction coefficient observed in Table 1 may reflect the subject-to-subject variability in skin, but other factors have influenced the variability, too: a variety of probe sizes, shapes, and materials and differences in measuring techniques.
There are two dominant types of designs for the test apparatus. One design, first used by Naylor, incorporated linear motion, wherein a probe was pressed onto the surface and dragged across the skin in a straight line. The other design was rotational and consisted of a probe pressed onto and rotated against the skin surface. For linear unilateral movement, a probe is continually rubbing an “untested” skin as it moves in its linear path. For rotational movement a probe is continually rotating over the same patch of skin, thus, rubbing a “tested” skin. Some of the differences in the measured friction values may arise from studying the “untested” versus “tested” skin.
Knowing or controlling the normal force on the probe is absolutely essential for getting accurate and repeatable friction coefficient measurements. Two methods were used to set the normal force: static weights and springs. In either case there was no real-time recording of the normal load and it was incorrectly assumed that the normal load stays constant during the test. Maintenance of the normal force is a source of data variation, as the skin is not totally flat, the probe encounters dips (valleys) and raises (peaks), and the normal force fluctuates during the test.
Table 1.
Values of the dynamic coefficient of friction ( µ ) of untreated skin in vivo
Previous studies have focused on correlating the friction with age, gender, anatomical site, and hydration. Age-related friction studies have reached disparate conclusions. Cua reported that there were no friction coefficient differences across age. Asserin and Elsner found that young subjects had a higher forearm friction coefficient than older subjects. However, Elsner also analyzed vulvar skin and found no age-related differences. No significant differences have been shown between the genders. The friction coefficient seems to vary considerably with anatomical site. Friction increases on skin hydrated either with anatomical site. Friction increases on skin hydrated either with water or creams and moisturizers. Addition of water typically elicits an immediate rise in the friction coefficient that returns to pre-hydration values in minutes. The creams and moisturizers elicit a slower rise in friction, some have the immediate effect of decreasing friction, but the change in friction is sustained over a longer period.
Electrical Measurements
Another measurement technique used to assess the skin surface is based on its electrical properties: capacitance, conductance, and impedance. The dry stratum corneum acts like a dielectric medium. Addition of water makes the stratum corneum responsive to an electrical field, Typically, electrical methods are utilized for assessment of skin hydration.
The capacitance measurements involve two oppositely charged plates held in close proximity. An electric field is formed between them, and the maximum charge on each plate is known as the capacitance. When dielectric materials are introduced into the gap between the two plates, they increase this capacitance value. A Corneometer is used for comparative studies of age, gender, anatomical site, clinical skin dryness, and moisturizer applications, outlined in Table 2.
Table 2.
Findings in skin capacitance comparative studies
Conductance measurements of the skin are performed by recording the skin's resistance to an electrical current. These currents are low (a few microamperes), so that the skin is not harmed in the measurement process. Studies in skin conductance are summarized in Table 3.
Table 3.
Findings in skin conductance comparative studies
Most of the electrical assessment has involved measurements of either capacitance and conductance, but a few studies have looked at a parameter that combines them both: impedance. Nicander investigated impedance differences of skin across anatomical locations, age, and gender. They found that the impedance varied by anatomical site, but no significant differences due to gender. In a subsequent study, Nicander and Ollmar looked at how electrical impedance measurements related to the different seasons. They found seasonal variations in the electrical impedance on all of the tested anatomical sites except for the neck.
State-of-the-Art Tribo-Metrology
CETR's commercially available, portable bench-top Biomedical Micro-Tribometer is shown in Figure 1. It provides diverse tribological measurements for many different types of samples in a variety of scientific and engineering applications. For example, it has been utilized for measurements of bathroom tissue on skin and soap on skin, for durability tests of surgical staples and sutures, for tribological tests of shaving blades, after-shave lotions and tooth brushes. With hand and arm rests on the tribometer's lower stage, in vivo skin tests for these limb sites can be easily accommodated.
Figure 1.
The Biomedical
Micro-Tribometer
The Biomedical Micro-Tribometer has significant technological and design advantages over previous devices used to measure friction coefficient. Figure 2 shows the UMT in use for the skin measurements. In performing its test functions, the UMT is capable of providing precision linear, rotational and reciprocating motions with speeds ranging from 0.1 µm/s to 10 m/s. A normal load is applied and tightly controlled with a closed-loop servomechanism in the instrument's upper stage, and can be either kept constant or linearly increasing, ranging from 0.1 mN to 1,000N. Friction force, normal load, electrical contact resistance, capacitance and impedance can all be measured and recorded at a total sampling rate of 20 kHz. Wear depth, skin deformation, contact acoustic emission, and digital video with magnifying optics are also readily available.
Figure 2.
The Biomedical Micro-Tribometer Measures Skin Friction Coefficient In Vivo
The computer-controlled tribometer functions offer several advantages over past methods in assuring more consistent results. As noted earlier, measuring the normal load is critically important. These previous devices required the skin to be flat, since there was no monitoring or correction of the normal load. In the tribometer the normal load and friction force are measured simultaneously, and the resulting friction coefficient is calculated and monitored real-time; thus, when the normal load changes due to an uneven surface, the calculation of the friction coefficient gives a true value. The friction coefficient and the electrical measurements are monitored simultaneously in the Biomedical Micro-Tribometer. The sensors and amplifiers used in the friction coefficient and the electrical impedance measurements are proprietary and patented. In addition the proprietary multi-channel data acquisition system records and displays the measurements in real-time. The upper stage holds a probe and translates it across the skin surface at a constant speed. For in vivo skin tests, the subject's hand or arm rests in fixtures attached to the lower stage, so that the desired anatomical site is properly positioned and held under the probe. If in vitro skin tests are performed, the skin is held on the lower stage in a simple, constant tension fixture. The universal lower stage can also translate in a direction perpendicular to the probe motion, so that the skin sample can be indexed laterally to allow multiple tests on it.
The UMT is perfectly safe and suitable for in vivo skin studies. The loading force of 50 mN (5 gm) used in some measurements is equivalent to the gravitational force of a nickel (5 cents) resting on the skin. This force is barely perceptible to the test subject and even a force ten times larger will pose no problems. Once the instrument operator sets the test parameters, the probe motions are completely controlled by the system computer, so operator influence/error is not a factor in test results.
In Vitro Studies
The UMT comparative test results regarding skin hydration are graphically presented in Figure 3. In vitro test of skin was conducted wherein the skin was exposed to water for three minutes and then blotted off to remove excess water that had not been absorbed into the skin. Hydration of the skin changed the skin coefficient of friction by 150% immediately after treatment with water. After some time (~15 minutes) the skin coefficient of friction returned to its dry state value.
Figure 3.
Water Exposure to Skin Increases Coefficient of friction. In vitro test of skin wherein the skin was exposed to water for three minutes and then blotted off to remove water that had not been absorbed into the skin.
In Vivo Studies
A test was also carried out by inducing dryness in the skin with the application of isopropyl alcohol. The induced dryness was visibly noticeable on the skin surface and these results are shown in Figure 4.
A comparison of three different lotions was performed with simultaneous electrical and friction measurements. One can clearly distinguish performance characteristics of the lotions (Figure 5).
Figure 4.
Exposure to Isopropyl Alcohol Decreases Skin Coefficient of Friction : In vivo test of skin shows a lowered coefficient of friction after alcohol, which visibly dries out skin, was applied to the skin surface.
Figure 5A
Figure 5B
Figure 5C
Figure 5.
Effect of lotions on skin properties.
Conclusions
The effective skin assessment instrumentation has been developed and utilized on the bench-top scale. Both the consistency and comprehensiveness of test data is much higher than that typically obtained on the previously known skin measuring instruments.
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