About the in vivo quantitation of skin anisotropy

Estimating the viscoelastic anisotropy of human skin through high-frequency ultrasound elastography

BACKGROUND

The skin is the largest organ of the human body and serves distinct functions in protecting the body. The viscoelastic properties of the skin play a key role in supporting the skin-healing process, also it may be changed due to some skin diseases.

PROPOSE

In this study, high-frequency ultrasound (HFUS) elastography based on a Lamb wave model was used to noninvasively assess the viscoelastic anisotropy of human skin.

METHOD

Elastic waves were generated through an external vibrator, and the wave propagation velocity was measured through 40 MHz ultrafast HFUS imaging. Through the use of a thin-layer gelatin phantom, HFUS elastography was verified to produce highly accurate estimates of elasticity and viscosity. In a human study involving five volunteers, viscoelastic anisotropy was assessed by rotating an ultrasound transducer 360°.

RESULTS

An oval-shaped pattern in the elasticity of human forearm skin was identified, indicating the high elastic anisotropy of skin; the average elastic moduli were 24.90 ± 6.63 and 13.64 ± 2.67 kPa along and across the collagen fiber orientation, respectively. The average viscosity of all the recruited volunteers was 3.23 ± 0.93 Pa·s.

CONCLUSIONS

Although the examined skin exhibited elastic anisotropy, no evident viscosity anisotropy was observed.

The assessment of natural human skin tension orientation and its variation according to age for two body areas: Forearm and thigh

Human skin has a complex multilayer structure consisting of non-homogeneous, non-linear, viscoelastic and anisotropic materials subjected to in vivo natural pre-tension. This natural tension stems from networks of collagen and elastin fibers. The 3D organization of the collagen and elastin fibers underpins the multidirectional natural tensions in the skin volume while the state of the networks formed influences the surface topography of the skin. This topography depends on the area of the body and on the age of the person. Experiments reported in the literature have been performed ex vivo or on cadavers. By contrast, this work proposes the characterization of the anisotropic natural tension of the human skin in vivo. Experimental tests were performed on the forearms and thighs of 42 female volunteers representing two age groups [20 - 30] and [45-55] years old. Non-contact impact tests and skin-folding tests were conducted using devices developed at the LTDS (Lyon, France). The impact test generated a Rayleigh wave that spread in the skin. The speed of this wave was measured in 7 directions to study the anisotropy of the skin tension. The image of the skin relief at rest and during the skin folding test was reconstructed by optical confocal microscopy and provided the density of the skin lines printed on the outer surface of the skin. Skin folding test enables the clinician's manual procedure to be instrumented to identify tension lines i.e., Langer lines, for better healing during a surgical procedure. The main directions of natural skin tension deduced from the measured wave speed and the densities of skin lines were [40°-60°] for the forearm and [0°-20°] for the thigh, considering that the longitudinal axis of the body is situated at 90° and the transversal axis at 0°. This method shows the remarkable effect of age and body area on the mechanical behavior of human skin in vivo. The elastic properties and natural tension of the skin decrease with age. This decrease is greater in the directions orthogonal to the skin's tension lines, leading to the accentuation of the anisotropic behavior of the cutaneous tissue. The main direction of skin tension is highly dependent on the area of the body and is directed towards a preferred direction which corresponds to the main direction of skin tension.

Probing elastic anisotropy of human skin in vivo with light using non-contact acoustic micro-tapping OCE and polarization sensitive OCT

Skin broadly protects the human body from undesired factors such as ultraviolet radiation and abrasion and helps conserve body temperature and hydration. Skin's elasticity and its level of anisotropy are key to its aesthetics and function. Currently, however, treatment success is often speculative and subjective, and is rarely based on skin's elastic properties because there is no fast and accurate non-contact method for imaging of skin's elasticity. Here we report on a non-contact and non-invasive method to image and characterize skin's elastic anisotropy. It combines acoustic micro-tapping optical coherence elastography (AμT-OCE) with a nearly incompressible transversely isotropic (NITI) model to quantify skin's elastic moduli. In addition, skin sites were imaged with polarization sensitive optical coherence tomography (PS-OCT) to help define fiber orientation. Forearm skin areas were investigated in five volunteers. Results clearly demonstrate elastic anisotropy of skin in all subjects. AμT-OCE has distinct advantages over competitive techniques because it provides objective, quantitative characterization of skin's elasticity without contact, which opens the door for broad translation into clinical use. Finally, we demonstrate that a combination of multiple OCT modalities (structural OCT, OCT angiography, PS-OCT and AμT-OCE) may provide rich information about skin and can be used to characterize scar.

Can the CutiScan CS 100® measure anisotropy and viscoelasticity in scar tissue after mastectomy? A reliability and validity study

BACKGROUND

Scars have different biomechanical characteristics, including anisotropy and viscoelasticity compared to healthy skin. To assess these characteristics, the CutiScan CS 100^® can be used. The aim of the present study is to investigate reliability and validity of this device in breast cancer patients.

MATERIALS AND METHODS

Thirty female patients, with scar adhesions following mastectomy were assessed with the CutiScan CS 100^® . Maximal distensibility (pixels) (V1), after-suction return rate (pixels) (V2), and their ratio (%) (V3) at three points on and around the scar were assessed as measures of viscoelasticity. For intra- and interrater reliabilities, the intra-class correlation coefficient (ICC) and its 95% confidence intervals were calculated. The standard error of measurement (SEM) was calculated to interpret reproducibility of these measurements. To investigate criterion validity of the measurement of anisotropy, measurements in the direction of healthy skin were compared with measurements in the direction of the scar, using a paired t-test.

RESULTS

V1, V2, and V3 show poor to moderate intrarater reliability (ICC 0.00-0.72) and interrater reliability (ICC 0.00-0.53). The maximum displacement (V1) on the measurement point above the scar shows the best reliability (ICC 0.33-0.72). The SEM is about the same for all parameters at all three points. The paired sample t-test showed a significant difference (p < 0.05) between V1 in the direction towards the scar versus the measurement towards healthy tissue, on the point below the scar.

CONCLUSION

These first reliability and validity results of the CutiScan CS 100^® for measuring anisotropy and viscoelasticity in scar tissue adhesions after mastectomy seem promising. Further research is needed addressing the limitations of the present study design.

Non-Invasive in Vivo Quantification of Directional Dependent Variation in Mechanical Properties for Human Skin

Skin is the body’s largest organ, and it shows non-linear and anisotropic behavior under the deformation. This behavior of the skin is due to the waviness and preferred orientation (in a particular direction) of collagen fibers. This preferred orientation of collagen fibers results in natural pre-tension and anisotropy of the skin. The knowledge of natural skin pre-tension and anisotropy is essential during incisions and surgery. The available suction-based devices quantify the anisotropy through the displacement field and cannot measure the stress-strain relation in particular directions. Therefore, in the current study, an in vivo full-field measurement suction apparatus was developed to measure the stress and strain of skin in all planar directions through a single experiment. First, this apparatus was tested on silicone substrates of known properties, and then it was used to test the skin of 12 human forearms. Further, to check the effect of hand stability on the measurements, the obtained results of the skin were compared with the results of a standard test performed in the same skin using a steady setup. The consistency between these two results confirms that the stability of the hand does not influence the measurements of skin properties. Furthermore, using the developed apparatus, the skin’s anisotropy and its relation with the Kraissl’s lines orientation was quantified by measuring the toe and linear moduli at an interval of one degree. The minimum and maximum values of the toe and linear moduli were 0.52 ± 0.09 and 0.59 ± 0.11 MPa, and 3.09 ± 0.47 and 5.52 ± 1.13 MPa, respectively. Also, the direction of maximum moduli was found almost similar to Kraissl’s lines’ orientation. These results confirm the contribution of skin pre-tension on the anisotropy of the skin. The present apparatus mimics the tissue expansion procedure, where observation of the test may be helpful in the selection of size and shape of the expander.

Skin biomechanical and viscoelastic properties measured with MyotonPRO in different areas of human body

BACKGROUND

There is still a lack of clinically practical device, which allows to perform rapid and accurate examination of the skin condition. For this reason, suitability of the MyotonPRO for the assessment of skin biomechanical and viscoelastic parameters was evaluated in this study. The aim of the study was to establish the reference values of five parameters measured by MyotonPRO various locations of human skin.

MATERIALS AND METHODS

Oscillation frequency, dynamic stiffness, logarithmic decrement, mechanical stress relaxation and creep were measured at three different skin locations (clavicula, volar forearm and shin), using L-shape short and medium arm probes in 32 young female volunteers. Mean values of obtained parameters recorded by both probes were compared among three skin locations while reliabilities of measurements were assessed. Additionally, relationships between all recorded parameters were examined RESULTS: There were no statistically significant differences between the mean values of five measured parameters obtained with both probes in all investigated areas. However, statistically significant differences of mean values of almost all parameters measured among three places examined were found. Despite considerable differences in mean values of obtained parameters, there were visible strong correlations between some studied parameters in all three investigated areas of skin.

CONCLUSION

It was demonstrated in all locations studied that the higher value of oscillation frequency corresponds to the higher value of dynamic stiffness, moreover such tissue recovers faster to its initial shape, and it was characterized by lower creep values. Such results indicate the existence of identical relationships between the same studied parameters in different areas of skin.

Taxifolin as a Promising Ingredient of Cosmetics for Adult Skin

Active substances, effective in the reduction in or delay of skin changes caused by aging occurring in natural compounds, are desirable. Taxifolin (TXF), a flavonoid of strong antioxidant activity found in the plant Stizolophus balsamita (S. balsamita), has been tested for its biological effects on adult human skin. The aim of the study was to investigate the effects of two creams: 3% S. balsamita extract and 3% TXF on the function of adult skin. In total, 97 Caucasian women with clinical signs of skin aging were investigated. The biophysical and biomechanical skin parameters were measured before and after applying the creams, using Colorimeter CL400, Mexameter MX16, Skin-pH-Meter PH900, Skin-Thermometer ST 500, Glossymeter GL200, and Cutiscan SC100. Patch tests were performed with the investigated products to assess their potential irritant properties. The percutaneous penetration of creams was examined with the use of electrospray ionization mass spectrometry (ESI-MS) and confocal Raman spectroscopy. The 3% S. balsamita extract cream reduced hyperpigmentation, erythema, and elevated pH. All the tested preparations were proven to be nonirritant. A higher penetration rate was revealed for the 3% TXF cream than for the 3% S. balsamita extract cream. A total of 3% TXF cream improved skin viscoelasticity. The obtained results suggested that S. balsamita extract and TXF may be considered as ingredients of skincare products for adults.

Skin anisotropy: Finding the optimal incision line for volar forearm in males and females

PURPOSE

Proper understanding of skin biomechanics, viscoelasticity and investigation of skin tension vectors is necessary to find optimal incision lines. Great tension across a healing wound after any surgical procedure might lead to forming hypertrophic scars. The aim of the study was to investigate tension lines in volar forearm skin in young males and females, in order to ensure best incision line.

METHODS

Five biomechanical and viscoelastic parameters were measured using a hand-held myotonometer: Oscillation Frequency [Hz], Dynamic Stiffness [N/m], Logarithmic Decrement of tissue's natural oscillation, Mechanical Stress Relaxation Time [ms], and Creep. Measurements were taken in four different directions; Along Forearm, Across Forearm, Along Langer's Line and Across Langer's Line.

RESULTS

Significant main effects for direction were found for Oscillation Frequency (p < 0.001, η² = 0.371) [Hz], Dynamic Stiffness (p < 0.001, η² = 0.522) [N/m], Logarithmic Decrement (p < 0.001, η² = 0.083), Mechanical Stress Relaxation Time (p < 0.001, η² = 0.494) [ms] and Creep (p < 0.001, η² = 0.480). For each parameter except for logarithmic decrement results obtained Along Langers Line and Across Forearm were significantly different to Across Langers Line and Along Forearm (p < 0.001, d = -2.76 - 2.66). Significant main effects for sex were found for logarithmic decrement Along Forearm (p < 0.001, d = 1.698) and Across Langer's Line (p = 0.021, d = 1.697).

CONCLUSIONS

Our results suggested that optimal incision line for this age group in males and females could potentially be performed diagonally i.e. Across Langer's Line or parallel i.e. Along Forearm to forearm axis. These directions would provide the lowest tension across a healing wound and possibly minimalize the risk of hypertrophic scarring post incision.

Collagen Hydrolysate Prepared from Chicken By-Product as a Functional Polymer in Cosmetic Formulation

Chicken stomachs can be processed into collagen hydrolysate usable in cosmetic products. The aim of the study was to verify the effects of a carbopol gel formulation enriched with 1.0% (w/w) chicken hydrolysate on the properties of the skin in the periorbital area after regular application twice a day for eight weeks in volunteers ageed 50 ± 9 years. Skin hydration, transepidermal water loss (TEWL), skin elasticity and skin relief were evaluated. Overall, skin hydration increased by 11.82% and 9.45%, TEWL decreased by 25.70% and 17.80% (always reported for the right and left area). Generally, there was an increase in skin elasticity, a decrease in skin roughness, as the resonance times decreased by 85%. The average reduction of wrinkles was 35.40% on the right and 41.20% on the left. For all results, it can be seen that the longer the cosmetic gel formulation is applied, the better the results. Due to the positive effect on the quality and functionality of the skin, it is possible to apply the cosmetic gel formulation in the periorbital area. The advantage of the product with chicken collagen hydrolysate is also the biocompatibility with the skin and the biodegradability of the formulation.

EEMCO Guidance for the in vivo Assessment of Biomechanical Properties of the Human Skin and Its Annexes: Revisiting Instrumentation and Test Modes

Biomechanics of the skin is an important subject in skin research. It has been studied for many decades involving various technologies and methods to characterize and quantify mechanical properties of the skin under different in vivo conditions. The present EEMCO paper reviews the current rel-evant information, providing practical orientation to researchers dedicated to in vivo assessment of biomechanics of skin and its annexes. We discuss the available non-invasive instruments, including their principles and variables. A correspondence between the descriptors nomenclature proposed by Agache and the designation for the suction-based standard instruments is proposed. The addressed properties include skin softness/stiffness, firmness, elasticity, elastic and viscoelastic properties, extensibility, resilience, anisotropy, acoustical shock wave hardness, friction (in relation to topographic properties), thickness, fiber/stress mechanics (bending, cyclic, tensile, fatigue, or torsion), and hardness. We provide the relation of these properties to biomechanical descriptors and in some cases to SI units. Practical guidance for the proper use of these instruments, limitations, and possible interpretations are provided, while discussing the meaning of descriptive or "phenomenological" variables. For studies intended to quantify the effect of an intervention with regard to mechanical properties, we recommend a minimum of 30-40 participants, based on normal distribution of the data sets. Some important limitations are recognized, including the lack of standardization of procedures and calibration of instruments, which compromises the relevance and real nature of the descriptors/parameters obtained with these devices. The present work highlights an approach to a better practice and a science-supported biomechanical assessment of human skin, hair, and nails.

Engineering biological gradients

Biological gradients profoundly influence many cellular activities, such as adhesion, migration, and differentiation, which are the key to biological processes, such as inflammation, remodeling, and tissue regeneration. Thus, engineered structures containing bioinspired gradients can not only support a better understanding of these phenomena, but also guide and improve the current limits of regenerative medicine. In this review, we outline the challenges behind the engineering of devices containing chemical-physical and biomolecular gradients, classifying them according to gradient-making methods and the finalities of the systems. Different manufacturing processes can generate gradients in either in-vitro systems or scaffolds, which are suitable tools for the study of cellular behavior and for regenerative medicine; within these, rapid prototyping techniques may have a huge impact on the controlled production of gradients. The parallel need to develop characterization techniques is addressed, underlining advantages and weaknesses in the analysis of both chemical and physical gradients.

Mathematical and computational modelling of skin biophysics: a review

The objective of this paper is to provide a review on some aspects of the mathematical and computational modelling of skin biophysics, with special focus on constitutive theories based on nonlinear continuum mechanics from elasticity, through anelasticity, including growth, to thermoelasticity. Microstructural and phenomenological approaches combining imaging techniques are also discussed. Finally, recent research applications on skin wrinkles will be presented to highlight the potential of physics-based modelling of skin in tackling global challenges such as ageing of the population and the associated skin degradation, diseases and traumas.