Non contact method for in vivo assessment of skin mechanical properties for assessing effect of ageing

Contactless mechanical stimulation of the skin using shear waves

The skin, the outermost organ of the human body, is vital for sensing and responding to stimuli through mechanotransduction. It is constantly exposed to mechanical stress. Consequently, various mechanical therapies, including compression, massage, and microneedling, have become routine practices for skin healing and regeneration. However, these traditional methods require direct skin contact, restricting their applicability. To address this constraint, we developed shear wave stimulation (SWS), a contactless mechanical stimulation technique. The effectiveness of SWS was compared with that of a commercial compression bioreactor used on reconstructed skin at various stages of maturity. Despite the distinct stimulus conditions applied by the two methods, SWS yielded remarkable outcomes, similar to the effects of the compression bioreactor. It significantly increased the shear modulus of tissue-engineered skin, heightened the density of collagen and elastin fibers, and resulted in an augmentation of fibroblasts in terms of their number and length. Notably, SWS exhibited diverse effects in the low- and high-frequency modes, highlighting the importance of fine-tuning the stimulus intensity. These results unequivocally demonstrated the capability of SWS to enhance the mechanical functions of the skin in vitro, making it a promising option for addressing wound healing and stretch mark recovery.

Development of a Novel Soft Tissue Measurement Device for Individualized Finite Element Modeling in Custom-Fit CPAP Mask Evaluation

PURPOSE

Individual facial soft tissue properties are necessary for creating individualized finite element (FE) models to evaluate medical devices such as continuous positive airway pressure (CPAP) masks. There are no standard tools available to measure facial soft tissue elastic moduli, and techniques in literature require advanced equipment or custom parts to replicate.

METHODS

We propose a simple and inexpensive soft tissue measurement (STM) indenter device to estimate facial soft tissue elasticity at five sites: chin, cheek near lip, below cheekbone, cheekbone, and cheek. The STM device consists of a probe with a linear actuator and force sensor, an adjustment system for probe orientation, a head support frame, and a controller. The device was validated on six ballistics gel samples and then tested on 28 subjects. Soft tissue thickness was also collected for each subject using ultrasound.

RESULTS

Thickness and elastic modulus measurements were successfully collected for all subjects. The mean elastic modulus for each site is Ec = 53.04 ± 20.97 kPa for the chin, El = 16.33 ± 8.37 kPa for the cheek near lip, Ebc = 27.09 ± 11.38 kPa for below cheekbone, Ecb = 64.79 ± 17.12 kPa for the cheekbone, and Ech = 16.20 ± 5.09 kPa for the cheek. The thickness and elastic modulus values are in the range of previously reported values. One subject's measured soft tissue elastic moduli and thickness were used to evaluate custom-fit CPAP mask fit in comparison to a model of that subject with arbitrary elastic moduli and thickness. The model with measured values more closely resembles in vivo leakage results.

CONCLUSION

Overall, the STM provides a first estimate of facial soft tissue elasticity and is affordable and easy to build with mostly off-the-shelf parts. These values can be used to create personalized FE models to evaluate custom-fit CPAP masks.

Mechanical Models of Collagen Networks for Understanding Changes in the Failure Properties of Aging Skin

Skin undergoes mechanical alterations due to changes in the composition and structure of the collagenous dermis with aging. Previous studies have conflicting findings, with both increased and decreased stiffness reported for aging skin. The underlying structure-function relationships that drive age-related changes are complex and difficult to study individually. One potential contributor to these variations is the accumulation of nonenzymatic crosslinks within collagen fibers, which affect dermal collagen remodeling and mechanical properties. Specifically, these crosslinks make individual fibers stiffer in their plastic loading region and lead to increased fragmentation of the collagenous network. To better understand the influence of these changes, we investigated the impact of nonenzymatic crosslink changes on the dermal microstructure using discrete fiber networks representative of the dermal microstructure. Our findings suggest that stiffening the plastic region of collagen's mechanical response has minimal effects on network-level stiffness and failure stresses. Conversely, simulating fragmentation through a loss of connectivity substantially reduces network stiffness and failure stress, while increasing stretch ratios at failure.

Nanohybrid dual-network chitosan-based hydrogels: Synthesis, characterization, quicken infected wound healing by angiogenesis and immune-microenvironment regulation

Infectious wounds are difficult to heal because of vascular damage and immune imbalance. The multi-functional hydrogel dressing can regulate vascular regeneration and immune microenvironment through continuous supply of bioactive ingredients to the wound site, which can effectively accelerate the healing speed of infected wounds. In this work, a multifunctional dual-network hydrogel (QCMOD) with good injectability, stability, self-healing and adhesion was designed by combining methacrylic anhydride-modified quaternized chitosan (QCM) with oxidized dextran (OD) via Schiff base reaction and photo-crosslinked polymerization. Subsequently, MgO/Icariin composite nanoparticles with icariin coating were prepared and loaded in QCMOD hydrogel to establish nanohybrid dual-network chitosan-based hydrogels (QCMOD@MI), which possessed a controlled release of Mg2+ and icariin as well as the ability of guiding physiological behavior in wound healing progress. In vitro results showed the nanohybrid hydrogel reduced bacterial infection and possessed multiple physiological functions including promoting cell migration, angiogenesis and reducing secretion of inflammatory factors. In vivo, the nanohybrid hydrogel showed excellent pro-healing abilities for infected full-thickness wounds by reducing bacterial infections and improving the microenvironment of ischemia and inflammation. This study provides a new paradigm for the design of multifunctional bioactive hydrogels and the obtained hydrogel is expected to become a new type of functional dressing.

Dill Extract Preserves Dermal Elastic Fiber Network and Functionality: Implication of Elafin

INTRODUCTION

Elastic skin fibers lose their mechanical properties during aging due to enzymatic degradation, lack of maturation, or posttranslational modifications. Dill extract has been observed to increase elastin protein expression and maturation in a 3D skin model, to improve mechanical properties of the skin, to increase elastin protein expression in vascular smooth muscle cells, to preserve aortic elastic lamella, and to prevent glycation.

OBJECTIVE

The aim of the study was to highlight dill actions on elastin fibers during aging thanks to elastase digestion model and the underlying mechanism.

METHODS

In this study, elastic fibers produced by dermal fibroblasts in 2D culture model were injured by elastase, and we observed the action of dill extract on elastic network by elastin immunofluorescence. Then action of dill extract was examined on mice skin by injuring elastin fibers by intradermal injection of elastase. Then elastin fibers were observed by second harmonic generation microscopy, and their functionality was evaluated by oscillatory shear stress tests. In order to understand mechanism by which dill acted on elastin fibers, enzymatic tests and real-time qPCR on cultured fibroblasts were performed.

RESULTS

We evidence in vitro that dill extract is able to prevent elastin from elastase digestion. And we confirm in vivo that dill extract treatment prevents elastase digestion, allowing preservation of the cutaneous elastic network in mice and preservation of the cutaneous elastic properties. Although dill extract does not directly inhibit elastase activity, our results show that dill extract treatment increases mRNA expression of the endogenous inhibitor of elastase, elafin.

CONCLUSION

Dill extract can thus be used to counteract the negative effects of elastase on the cutaneous elastic fiber network through modulation of PI3 gene expression.

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.

The Shape of Corneal Deformation Alters Air Puff–Induced Loading

Purpose: To determine the dynamic modification of the load exerted on the eye during air-puff testing by accounting for the deformation of the cornea.

Methods: The effect of corneal load alteration with surface shape (CLASS) was characterized as an additional component of the load produced during the concave phase where the fluid outflow tangential to the corneal surface creates backward pressure. Concave phase duration (t_CD), maximum CLASS value (CLASS_max), and the area under CLASS-time curve (CLASS_int) are calculated for 26 keratoconic (KCN), 102 normal (NRL), and 29 ocular hypertensive (OHT) subjects. Tukey’s HSD tests were performed to compare the three subject groups. A p-value less than 0.05 was considered statistically significant.

Results: Accounting for CLASS increased the load by 34.6% ± 7.7% at maximum concavity; these differences were greater in KCN subjects (p < 0.0001) and lower in OHT subjects (p = 0.0028) than in NRL subjects. t_CD and CLASS_int were significantly longer and larger, respectively, for KCN subjects than those in the NRL and OHT groups (p < 0.0001).

Conclusion: Load characterization is an essential step in assessing the cornea’s biomechanical response to air-puff–induced deformation. The dynamic changes in the corneal surface shape significantly alter the load experienced by the corneal apex. This implies a subject-specific loading dynamic even if the air puff itself is identical. This is important when comparing the same eye after a surgical procedure or topical medication that alters corneal properties. Stiffer corneas are least sensitive to a change in load, while more compliant corneas show higher sensitivity.

Biochemical, structural and physical changes in aging human skin, and their relationship

Skin is the largest organ of the human body, having the purpose of regulating temperature, protecting us from microbes or mechanical shocks, and allowing the sensations from touch. It is generally accepted that aging induces profound changes in the skin's biochemical, structural and physical properties, which can lead to impaired biological functions and/or diverse diseases. So far, the effects of aging on these skin properties have been well documented. However, very few studies have focused exclusively on the relationship among these critical properties in the aging process, which is this review's primary focus. Many in vivo, ex vivo, and in vitro techniques have been previously used to characterize these properties of the skin. This review aims to provide a comprehensive overview on the effects of aging on the changes in biochemical, structural, and physical properties, and explore the potential mechanisms of skin with the relation between these properties. First, we review different or contradictory results of aging-related changes in representative parameters of each property, including the interpretations of the findings. Next, we discuss the need for a standardized method to characterize aging-related changes in these properties, to improve the way of defining age-property relationship. Moreover, potential mechanisms based on the previous results are explored by linking the biochemical, structural, and physical properties. Finally, the need to study changes of various functional properties in the separate skin layers is addressed. This review can help understand the underlying mechanism of aging-related alterations, to improve the evaluation of the aging process and guide effective treatment strategies for aging-related diseases.

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.

Effects of Kinesio taping on skin deformation during knee flexion and extension: a preliminary study

Background

Kinesio Taping (KT) is proved useful to many musculoskeletal disorders. But the mechanism remains unclear. The kinesio tape works by sticking to the skin surface. So exploring the interaction between the tape and the skin and analyzing its biomechanical influence may be an effective way to explore the mechanism of the tape.

Objectives

This study aimed to investigate the effect of Kinesio taping and taping methods on skin deformation during knee joint flexion and extension motion and further explore its possible functional mechanisms.

Methods

Ten healthy and pain-free subjects (4 males, 6 females) were recruited in this study. The skin observation area on the anterior side of the right thigh of the subjects was divided into 11 segments by 12 reflective marker points for distance measurement, from the distal knee to the proximal knee, the length of the interval was L1 to L11, and the total length was L0. Subjects were treated with no KT (NT), resting positive taping (RPT), resting negative taping (RNT), stretching positive taping (SPT), and stretching negative taping (SNT). A Qualisys infrared high-speed three-dimensional spatial coordinate capture system was used to observe changes in the length of the observed skin surface on the right anterior thigh during right knee flexion and extension in the sitting position.

Results

During right knee flexion and extension in the seated position in 10 subjects, all skin segment deformations produced significant differences between intervention groups (P < 0.05), except for L1 during flexion (P = 0.07). During right knee flexion and extension, total length, L0, and spacing lengths, L1, L6, and L11, were longer in the NT group than in all other groups. L0 and L1 were both longer in the stretched position than in the rest position; L11 also showed this trend.

Conclusions

The usage of the KT had an effect on the biomechanical changes of the skin, resulting in changes in skin deformation. I-tape, natural tension taping can shorten the skin distance between the two ends of the tape. Limb position during taping may influence the KT’s effects. However, the change in taping direction showed no significant effects on skin deformation during exercise. KT may apply a pre-stress in the biomechanics of the skin.

Mechanical modeling and characterization of human skin: A review

This paper reviews the advances made in recent years on modeling approaches and experimental techniques to characterize the mechanical properties of human skin. The skin is the largest organ of the human body that has a complex multi-layered structure with different mechanical behaviors. The mechanical properties of human skin play an important role in distinguishing between healthy and unhealthy skin. Furthermore, knowing these mechanical properties enables computer simulation, skin research, clinical studies, as well as diagnosis and treatment monitoring of skin diseases. This paper reviews the recent efforts on modeling skin using linear, nonlinear, viscoelastic, and anisotropic materials. The work also focuses on aging effects, microstructure analysis, and non-invasive methods for skin testing. A detailed explanation of the skin structure and numerical models, such as finite element models, are discussed in this work. This work also compares different experimental methods that measure the mechanical properties of human skin. The work reviews the experimental results in the literature and shows how the mechanical properties of human skin vary with the skin sites, the layers, and the structure of human skin. The paper also discusses how state-of-the-art technology can advance skin research.

Implantable Microfluidic Device: An Epoch of Technology

Implantable microfluidic devices are milestones in developing devices that can either measure parameters like ocular pressure and blood glucose level or deliver various components for therapeutic needs or behavioral modification. Researchers are currently focusing on the miniaturization of almost all its tools for a better healthcare platform. Implantable microfluidic devices are a combination of various systems including, but not limited to, microfluidic platforms, reservoirs, sensors, and actuators, implanted inside the body of a living entity (in vivo) with the purpose of directly or indirectly helping the entity. It is a multidisciplinary approach with immense potential in the area of the biomedical field. Significant resources are utilizing on for the research and development of these devices for various applications. The induction of an implantable microfluidic device into an animal would enable us to measure the responses without any repeated invasive procedures. Such data would help in the development of a better drug delivery profile. Implantable microfluidic devices with reservoirs deliver specific chemical or biological products to treat situations like cancers and diabetes. They can also deliver fluorophores for specific imaging inside the body. Implantable microfluidic devices help provide a microenvironment for various cell differentiation procedure. These devices know no boundaries, and this article reviews these devices based on their design and applications.

Extracting the elasticity of the human skin in microscale and in-vivo from atomic force microscopy experiments using viscoelastic models

Detecting mechanical properties of the intact skin in-vivo leads to a novel quantitative method to diagnose skin diseases and to monitor skin conditions in clinical settings. Current research and clinical methods that detect skin mechanics have major limitations. The in-vitro experiments are done in non-physiological conditions and in-vivo clinical methods measurer unwanted mechanics of underneath fat and muscle tissues but report the measurement as skin mechanics. An ideal skin mechanics should be captured at skin scale (i.e., micron-scale) and in-vivo. However, extreme challenges of capturing the in-vivo skin mechanics in micron-scale including skin motion due to heart beep, breathing and movement of the subject, has hindered measurement of skin mechanics in-vivo.This study for the first time captures micro-scale mechanics (elasticity and viscoelasticity) of top layers of skin (i.e., the stratum corneum (SC) and stratum granulosum (SG)) in-vivo. In this study, the relevant literature is reviewed and Atomic Force Microscopy (AFM) was used to capture force-indentation curves on the fingertip skin of four human subjects at a high indentation speed of 40 μm/s. The skin of the same subject were tested in-vitro at 10 different indentation speeds ranging from 0.125 to 40 μm/s by AFM. This study extracts the in-vivo elasticity of SC and SG by detecting time-dependency of tested tissue using a fractional viscoelastic standard linear model developed for indentation. The in-vivo elasticity of SC and SG were smaller in females and in-vitro elasticity were higher than that of in-vivo results. The results were consistent with previous observations.

Surface wave investigation and phenomenological analysis: Application on in vivo human cutaneous tissue

The wave phenomenon in free surface media stems from the propagation of mode grouping. Due to the nature of propagation in a given medium, this phenomenon expresses different types of dependence on the medium's properties and represents its mechanical admittance. In contrast with body wave propagation, dependencies related to surface propagation in a medium can be described by spatial-temporal characteristics. These characteristics can be obtained by performing appropriate experiments and do not require prior knowledge of the physical properties of the medium. In this study, we propose an original surface wave investigation and a phenomenological analysis approach adapted to the mechano-bio-structural states evaluation of in vivo human skin. Two objectives are sought with the method proposed: the first concerns the development of a non-invasive device for generating and tracking surface waves in human skin called Free-Skin-Surface-Wave (FSSW); the second concerns the adaptation of the Multi-Chanel Analysis of Surface Waves (MASW) method to evaluate the mechano-bio-structural states of human cutaneous tissue in vivo on the basis of the propagating phenomena observed. As an illustration of the proposed method application, we have done an in vivo evaluation, on intern-forearm of female volunteers population. In addition, we proposed a study of the aging effect and a comparison with ultrasound B-Mode technique, to validate the method sensitivity to follow the mechano-morphological properties of the in vivo human skin. In this study, our medium of application was human skin in vivo, but it is conceivable to extend this application to other soft biological media.

Mechanical and topographical anisotropy for human skin: Ageing effect

Skin ageing is a complex process which strongly impacts the three skin layers (epidermis, dermis, hypodermis) both functionally and structurally. Of particular interest are the effects of ageing on the dermis biomechanics and how this evolution can impact the reorganization of the cutaneous lines which compose the skin relief. It has been argued that the skin relief could reflect the underlying mechanical condition of the skin. Nevertheless, there is not yet conclusive evidence of the existence of such a link. This work aims at experimentally studying, in vivo, the correlation between the anisotropy of human skin biomechanics and skin topography as a function of ageing. The study was conducted on a panel of 20 men divided into 4 groups according to age (from 23 to 64 years old). The measurements were performed on the right volar forearm of each volunteer. For the biomechanical measurements, an innovative contactless bio-rheometer was developed. It allows access to the mechanical behaviour of the skin in several directions. This device generates an air blast without any contact with the skin area and measures its dynamic response (evaluation of speed of wave propagation) with a linear laser. Moreover, a turntable enables measurements to be made in different angular directions. To analyse the topography of skin relief, we proposed a new method, based on watershed and linear radon transformations. First, an optical analysis of a replica of the skin relief is performed. Then, from the skin image obtained, the density of the cutaneous lines is calculated in different directions using watersheld transformation. The orientation of the detected lines is then estimated with an algorithm based on linear radon transformation. The results observed show a good correlation between the skin relief and the mechanical properties of the skin all along the ageing process. For both topography and mechanical properties, there is a transition from an almost isotropic mechanical behaviour to an anisotropic one as a function of ageing process. Thus, we might conclude that the skin relief reflects the underlying mechanical conditions of the skin.

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.

An Economic, Modular, and Portable Skin Viscoelasticity Measurement Device for In Situ Longitudinal Studies

A indentation-based device to measure tissue mechanical property was designed and built using over-the-counter and 3D-printed parts. The device costs less than 100 USD and is capable of measuring samples of various geometry because of its modular design. The device is light-weight, thus portable, for measurements that can be performed at different sites. It was demonstrated that the measurement results obtained using our device are comparable to previous observations. The elastic shear modulus of the human skin was in the range of 2 kPa to 8 kPa, and skin tissues in old mice were stiffer than young mice. Mechanical properties of the skin tissues belonging to the same test subject varied depending on the location of the measurement. In conclusion, because our device is economic, modular, portable, and robust, it is suitable to serve as a standard measurement platform for studying tissue mechanics.

Skin aging as a mechanical phenomenon: The main weak links

From a mechanical point of view, human skin appears as a layered composite containing the stiff thin cover layer presented by the stratum corneum, below which are the more compliant layers of viable epidermis and dermis and further below the much more compliant adjacent layer of subcutaneous white adipose tissue (sWAT). Upon exposure to a strain, such a multi-layer system demonstrates structural instabilities in its stiffer layers, which in its simplest form is the wrinkling. These instabilities appear hierarchically when the mechanical strain in the skin exceeds some critical values. Their appearance is mainly dependent on the mismatch in mechanical properties between adjacent skin layers or between the skin and sWAT, on the adhesive strength and thickness ratios between the layers, on their bending and tensile stiffness as well as on the value of the stress existing in single layers. Gradual reduction of elastic fibers in aging significantly reduces the skin's ability to bend, prompting an up to 4-fold reduction of its stability against wrinkling, thereby explaining the role of these fibers in skin aging. While chronological and extrinsic aging differently modify these parameters, they lead to the same end result, reducing the critical strain required for the onset of instabilities. Comparing of mechanical properties of the skin presented as a bi-, tri- or tetra-layer structure demonstrates the particular importance of the papillary dermis in skin aging and provides the arguments to consider the undulations on the dermal-epidermal and dermal-sWAT interfaces as the result of mechanical bifurcation, leading to structural instabilities inside of the skin. According to this model, anti-aging strategies should focus not as much on the reinforcement of the dermis, but rather aim to treat the elastic mismatch between different adjacent layers in the skin and sWAT as well as the adhesion between these layers.

In Situ Mechanical Characterization of Multilayer Soft Tissue Using Ultrasound Imaging

In this paper, we report the development of a technique to characterize layer-specific nonlinear material properties of soft tissue in situ with the potential for in vivo testing. A soft tissue elastography robotic arm system comprising of a robotically manipulated 30 MHz high-resolution ultrasound probe, a custom designed compression head, and load cells has been developed to perform compression ultrasound imaging on the target tissue and measure reaction forces. A multilayer finite element model is iteratively optimized to identify the material coefficients of each layer. Validation has been performed using tissue mimicking agar-based phantoms with a low relative error of ∼7% for two-layer phantoms and ∼10% error for three layer phantoms when compared to known ground-truth values obtained using a commercial material testing system. The technique has then been used to successfully determine the in situ layer-specific mechanical properties of intact porcine stomach. The mean C10 and C20 for a second-order reduced polynomial material model were determined for the muscularis (6.41 ± 0.60, 4.29 ± 1.87 kPa), submucosal (5.21 ± 0.57, 3.68 ± 3.01 kPa), and mucosal layers (0.06 ± 0.02, 0.09 ± 0.24 kPa). Such a system can be utilized to perform in vivo mechanical characterization, which is left as future work.

Biophysical properties of the human finger for touch comprehension: influences of ageing and gender

The human finger plays an extremely important role in tactile perception, but little is known about how age and gender affect its biophysical properties and their role in tactile perception. We combined studies on contact characteristics, mechanical properties and surface topography to understand age and gender effects on the human finger. The values obtained regarding contact characteristics (i.e. adhesive force) were significantly higher for women than for men. As for mechanical properties (i.e. Young's modulus E), a significant and positive correlation with age was observed and found to be higher for women. A positive correlation was observed between age and the arithmetic mean of surface roughness for men. However, an inverse age effect was highlighted for women. The age and gender effects obtained have never been reported previously in the literature. These results open new perspectives for understanding the weakening of tactile perception across ages and how it differs between men and women.

Semianalytical Solution for the Deformation of an Elastic Layer under an Axisymmetrically Distributed Power-Form Load: Application to Fluid-Jet-Induced Indentation of Biological Soft Tissues

Fluid-jet-based indentation is used as a noncontact excitation technique by systems measuring the mechanical properties of soft tissues. However, the application of these devices has been hindered by the lack of theoretical solutions. This study developed a mathematical model for testing the indentation induced by a fluid jet and determined a semianalytical solution. The soft tissue was modeled as an elastic layer bonded to a rigid base. The pressure of the fluid jet impinging on the soft tissue was assumed to have a power-form function. The semianalytical solution was verified in detail using finite-element modeling, with excellent agreement being achieved. The effects of several parameters on the solution behaviors are reported, and a method for applying the solution to determine the mechanical properties of soft tissues is suggested.

Water jet indentation for local elasticity measurements of soft materials

We present a novel elastography method for soft materials (100Pa-100kPa) based on indentation by a μm-sized water jet. We show that the jet creates a localized deformation ("cavity") of the material that can be easily visualized. We study experimentally how cavity width and depth depend on jet speed, height, incidence angle and sample elasticity. We describe how to calibrate the indenter using gels of known stiffness. We then demonstrate that the indenter yields quantitative elasticity values within 10% of those measured by shear rheometry. We corroborate our experimental findings with fluid-solid finite-element simulations that quantitatively predict the cavity profile and fluid flow lines. The water jet indenter permits in situ local stiffness measurements of 2D or 3D gels used for cell culture in physiological buffer, is able to assess stiffness heterogeneities with a lateral resolution in the range 50-500μm (at the tissue scale) and can be assembled at low cost with standard material from a biology laboratory. We therefore believe it will become a valuable method to measure the stiffness of a wide range of soft, synthetic or biological materials.

Age-related changes in male forearm skin-to-fat tissue dielectric constant at 300 MHz

Prior research suggests that tissue dielectric constant (TDC) values are useful to assess localized skin water in females for early diagnosing breast cancer treatment-related lymphoedema and TDC values in young adults have shown gender differences. However, no TDC data are available for older males nor have ageing effects been studied despite known shifts in water state and other skin age-related changes. Thus our goals were to (i) characterize TDC values at various skin depths in young and older males, (ii) determine the dependence of these values on body composition parameters and (iii) establish inter-arm TDC ratios for use as normal male reference values. TDC measurements were made to depths of 0·5, 1·5, 2·5 and 5·0 mm bilaterally on volar forearm skin in 60 males in three groups of 20 that had mean ages ± SD of 24·0 ± 0·9, 40·0 ± 12·9 and 71·0 ± 8·0 years. Total body fat and water percentages were determined via bioimpedance at 50 KHz. Results showed that (i) for all age groups TDC values decreased with increasing depth, (ii) TDC values were not statistically different among age groups except at a depth of 0·5 mm, (iii) TDC values were highly negatively correlated with total body fat and (iv) inter-arm ratios varied little among age groups and depths. It is concluded that (i) age-related larger TDC values at only the shallowest depth is consistent with skin water shifting state from bound to more mobile in the oldest group and (ii) inter-arm ratios at any depth provide a basis to test for unilateral oedema.

Changes of skin collagen orientation associated with chronological aging as probed by polarized-FTIR micro-imaging

During chronological skin aging, alterations in dermal structural proteins cause morphological modifications. Modifications are probably due to collagen fiber (type I collagen) rearrangement and reorientation with aging that have not been researched until now. FTIR microspectroscopy appears as an interesting method to study protein structure under normal and pathological conditions. Associated with a polarizer, this vibrational technique permits us to probe collagen orientation within skin tissue sections, by computing the ratio of integrated intensities of amide I and amide II bands. In this study, we used the polarized-FTIR imaging to evaluate molecular modifications of dermal collagen during chronological aging. The data processing of polarized infrared data revealed that type I collagen fibers become parallel to the skin surface in aged skin dermis. Our approach could find innovative applications in dermatology as well as in cosmetics.

Measurement of the axial and circumferential mechanical properties of rat skin tissue at different anatomical locations

Skin tissue is not only responsible for thermoregulation but also for protecting the human body from mechanical, bacterial, and viral insults. The mechanical properties of skin tissue may vary according to the anatomical locations in the body. However, the linear elastic and nonlinear hyperelastic mechanical properties of the skin in different anatomical regions and at different loading directions (axial and circumferential) so far have not been determined. In this study, the mechanical properties during tension of the rat abdomen and back were calculated at different loading directions using linear elastic and nonlinear hyperelastic material models. The skin samples were subjected to a series of tensile tests. The elastic modulus and maximum stress of the skin tissues were measured before the incidence of failure. The nonlinear mechanical behavior of the skin tissues was also computationally investigated through a constitutive equation. Hyperelastic strain energy density function was calibrated using the experimental data. The results revealed the anisotropic mechanical behavior of the abdomen and the isotropic mechanical response of the back skin. The highest elastic modulus was observed in the abdomen skin under the axial direction (10 MPa), while the lowest one was seen in the back skin under axial loading (5 MPa). The Mooney-Rivlin material model closely addressed the nonlinear mechanical behavior of the skin at different loading directions, which can be implemented in the future biomechanical models of skin tissue. The results might have implications not only for understanding of the isotropic and anisotropic mechanical behavior of skin tissue at different anatomical locations but also for providing more information for a diversity of disciplines, including dermatology, cosmetics industry, clinical decision making, and clinical intervention.

Corneal viscoelastic properties from finite-element analysis of in vivo air-puff deformation

Biomechanical properties are an excellent health marker of biological tissues, however they are challenging to be measured in-vivo. Non-invasive approaches to assess tissue biomechanics have been suggested, but there is a clear need for more accurate techniques for diagnosis, surgical guidance and treatment evaluation. Recently air-puff systems have been developed to study the dynamic tissue response, nevertheless the experimental geometrical observations lack from an analysis that addresses specifically the inherent dynamic properties. In this study a viscoelastic finite element model was built that predicts the experimental corneal deformation response to an air-puff for different conditions. A sensitivity analysis reveals significant contributions to corneal deformation of intraocular pressure and corneal thickness, besides corneal biomechanical properties. The results show the capability of dynamic imaging to reveal inherent biomechanical properties in vivo. Estimates of corneal biomechanical parameters will contribute to the basic understanding of corneal structure, shape and integrity and increase the predictability of corneal surgery.

A NONLINEAR HYPERELASTIC BEHAVIOR TO IDENTIFY THE MECHANICAL PROPERTIES OF RAT SKIN UNDER UNIAXIAL LOADING

Skin is a thin membrane which provides many biological functions, such as thermoregulation and protection from mechanical, bacterial, and viral insults. The mechanical properties of skin tissue are extremely hard to measure and may vary according to the anatomical locations of a body. However, the mechanical properties of skin at different anatomical regions have not been satisfactorily simulated by conventional engineering models. In this study, the linear elastic and nonlinear hyperelastic mechanical properties of rat skin at different anatomical locations, including back and abdomen, are investigated using a series of tensile tests. The Young's modulus and maximum stress of skin tissue are measured before the incidence of failure. The nonlinear mechanical behavior of skin tissue is also experimentally and computationally investigated through constitutive equations. Hyperelastic strain energy density functions are adjusted using the experimental results. A hyperelastic constitutive model is selected to suitably represent the axial behavior of the skin. The results reveal that the maximum stress (20%) and Young's modulus (35%) of back skin are significantly higher than that of abdomen skin. The Ogden model is selected to closely address the nonlinear mechanical behavior of the skin which can be used in further biomechanical simulations of the skin tissue. The results might have implications not only for understanding of the mechanical behavior of skin tissue at different anatomical locations, but also to give an engineering insight for a diversity of disciplines, such as dermatology, cosmetics industry, clinical decision making, and clinical intervention.

Assessment of the in-plane biomechanical properties of human skin using a finite element model updating approach combined with an optical full-field measurement on a new tensile device

Human skin is one of the most important organ of the body. The assessment and knowledge of its properties are very useful for clinical or cosmetic research. Many techniques are used to measure the mechanical properties of this organ, like suction, indentation, torsion or tension tests. The aim of this paper is to present a new device based on tension technique and combining mechanical and optical measurements. The whole procedure used to assess the displacement field as described, and first results of tests performed in vivo are shown.

Computational and experimental characterization of skin mechanics: identifying current challenges and future directions

The characterization of skin mechanics has many clinical implications and has been an active area of research for the past few decades. Biomechanical models have evolved from earlier empirical models to state-of-the-art structural models that provide linkage between tissue microstructure and macroscopic stress-strain response. To maximize the accuracy and predictive capabilities of such computational models, there is a need to reliably identify often a large number of unknown model parameters. This is critically dependent on the availability of experimental data that cover an extensive range of different deformation modes, and quantification of internal structural features, such as collagen orientation. To this end, future challenges should include the ongoing development of noninvasive instrumentation and imaging modalities for in vivo skin measurements. We highlight the important concept of tightly integrating computational models, instrumentation, and imaging modalities into a single platform to investigate skin biomechanics.

Large amplitude oscillatory shear properties of human skin

Skin is a complex multi-layered tissue, with highly non-linear viscoelastic and anisotropic properties. Thus far, a few studies have been performed to directly measure the mechanical properties of three distinguished individual skin layers; epidermis, dermis and hypodermis. These studies however, suffer from several disadvantages such as skin damage due to separation, and disruption of the complex multi-layered composition. In addition, most studies are limited to linear shear measurements, i.e. measurements with small linear deformations (also called small amplitude oscillatory shear experiments), whereas in daily life skin can experience high strains, due to for example shaving or walking. To get around these disadvantages and to measure the non-linear mechanical (shear) behavior, we used through-plane human skin to measure large amplitude oscillatory shear (LAOS) deformation up to a strain amplitude of 0.1. LAOS deformation was combined with real-time image recording and subsequent digital image correlation and strain field analysis to determine skin layer deformations. Results demonstrated that deformation at large strains became highly non-linear by showing intra-cycle strain stiffening and inter-cycle shear thinning. Digital image correlation revealed that dynamic shear moduli gradually decreased from 8kPa at the superficial epidermal layer down to a stiffness of 2kPa in the dermis. From the results we can conclude that, from a mechanical point of view, skin should be considered as a complex composite with gradually varying shear properties rather than a three layered tissue.