Simulating the three-dimensional deformation of in vivo facial skin

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.

Skin viscoelasticity effects on the periodic mechanical stimuli propagation between skin layers

Our daily lives are constantly surrounded by dynamic stimuli, and our skin is deformed in a time-dependent manner. Although skin plays an important role in transmitting stimuli received at the surface to mechanoreceptors, few studies have investigated how differences in skin viscoelasticity affect the mechanical stimuli propagation in the skin. Therefore, using a finite element model, we evaluated the effects and trends of changes in the stiffness and viscoelasticity of the skin on the propagation of mechanical quantities between skin layers where mechanoreceptors are present when subjected to periodic stimuli. First, we constructed a new, sophisticated mathematical model of skin viscoelasticity based on the history-dependent deformation behavior of human skin obtained experimentally. We were able to construct a skin model that thoroughly reproduced the actual human skin deformation behavior at oscillations as fast as 10 Hz by setting viscoelastic parameters with a short time constant (0.001-0.006 s). Then, we calculated how skin material parameters affect the propagation of the mechanical quantities in the skin during the history-dependent skin deformation response to periodic stimuli. The finite element analysis showed that not only stiffness but also viscoelasticity markedly affected the mechanical stimuli propagation in the skin, and the effect differed depending on the layer. In particular, greater immediate responsiveness of the dermis contributed to greater propagation of the mechanical stimulus. Our results indicate that more attention needs to be given to the differences in the time-dependent intradermal mechanical stimuli propagation caused by individual's skin viscoelasticity.

Applied use of biomechanical measurements from human tissues for the development of medical skills trainers: a scoping review

OBJECTIVE

The objective of this review was to identify quantitative biomechanical measurements of human tissues, the methods for obtaining these measurements, and the primary motivations for conducting biomechanical research.

INTRODUCTION

Medical skills trainers are a safe and useful tool for clinicians to use when learning or practicing medical procedures. The haptic fidelity of these devices is often poor, which may be because the synthetic materials chosen for these devices do not have the same mechanical properties as human tissues. This review investigates a heterogeneous body of literature to identify which biomechanical properties are available for human tissues, the methods for obtaining these values, and the primary motivations behind conducting biomechanical tests.

INCLUSION CRITERIA

Studies containing quantitative measurements of the biomechanical properties of human tissues were included. Studies that primarily focused on dynamic and fluid mechanical properties were excluded. Additionally, studies only containing animal, in silico , or synthetic materials were excluded from this review.

METHODS

This scoping review followed the JBI methodology for scoping reviews and the Preferred Reporting Items for Systematic Reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR). Sources of evidence were extracted from CINAHL (EBSCO), IEEE Xplore, MEDLINE (PubMed), Scopus, and engineering conference proceedings. The search was limited to the English language. Two independent reviewers screened titles and abstracts as well as full-text reviews. Any conflicts that arose during screening and full-text review were mediated by a third reviewer. Data extraction was conducted by 2 independent reviewers and discrepancies were mediated through discussion. The results are presented in tabular, figure, and narrative formats.

RESULTS

Data were extracted from a total of 186 full-text publications. All of the studies, except for 1, were experimental. Included studies came from 33 countries, with the majority coming from the United States. Ex vivo methods were the predominant approach for extracting human tissue samples, and the most commonly studied tissue type was musculoskeletal. In this study, nearly 200 unique biomechanical values were reported, and the most commonly reported value was Young's (elastic) modulus. The most common type of mechanical test performed was tensile testing, and the most common reason for testing human tissues was to characterize biomechanical properties. Although the number of published studies on biomechanical properties of human tissues has increased over the past 20 years, there are many gaps in the literature. Of the 186 included studies, only 7 used human tissues for the design or validation of medical skills training devices. Furthermore, in studies where biomechanical values for human tissues have been obtained, a lack of standardization in engineering assumptions, methodologies, and tissue preparation may implicate the usefulness of these values.

CONCLUSIONS

This review is the first of its kind to give a broad overview of the biomechanics of human tissues in the published literature. With respect to high-fidelity haptics, there is a large gap in the published literature. Even in instances where biomechanical values are available, comparing or using these values is difficult. This is likely due to the lack of standardization in engineering assumptions, testing methodology, and reporting of the results. It is recommended that journals and experts in engineering fields conduct further research to investigate the feasibility of implementing reporting standards.

REVIEW REGISTRATION

Open Science Framework https://osf.io/fgb34.

Quantified analysis of facial movement: A reference for clinical applications

Most techniques for evaluating unilateral impairments in facial movement yield subjective measurements. The objective of the present study was to define a reference dataset and develop a visualization tool for clinical assessments. In this prospective study, a motion capture system was used to quantify facial movements in 30 healthy adults and 2 patients. We analyzed the displacements of 105 reflective markers placed on the participant's face during five movements (M1-M5). For each marker, the primary endpoint was the maximum amplitude of displacement from the static position (M0) in an analysis of variance. The measurement precision was 0.1 mm. Significant displacements of markers were identified for M1-M5, and displacement patterns were defined. The patients and age-matched healthy participants were compared with regard to the amplitude of displacement. We created a new type of radar plot to visually represent the diagnosis and facilitate effective communication between medical professionals. In proof-of-concept experiments, we collected quantitative data on patients with facial palsy and created a patient-specific radar plot. Our new protocol for clinical facial motion capture ("quantified analysis of facial movement," QAFM) was accurate and should thus facilitate the long-term clinical follow-up of patients with facial palsy. To take account of the limitations affecting the comparison with the healthy side, we created a dataset of healthy facial movements; our method might therefore be applicable to other conditions in which movements on one or both sides of the face are impaired. The patient-specific radar plot enables clinicians to read and understand the results rapidly.

Surface wave analysis of the skin for penetrating and non-penetrating projectile impact in porcine legs

Secondary blast injuries may result from high-velocity projectile fragments which ultimately increase medical costs, reduce active work time, and decrease quality of life. The role of skin penetration requires more investigation in energy absorption and surface mechanics for implementation in computational ballistic models. High-speed ballistic penetration studies have not considered penetrating and non-penetrating biomechanical properties of the skin, including radial wave displacement, resultant surface wave speed, or projectile material influence. A helium-pressurized launcher was used to accelerate 3/8″ (9.525 mm) diameter spherical projectiles toward seventeen whole porcine legs from seven pigs (39.53 ± 7.28 kg) at projectile velocities below and above V₅₀. Projectiles included a mix of materials: stainless steel (n = 26), Si₃N₄ (n = 24), and acetal plastic (n = 24). Tracker video analysis software was used to determine projectile velocity at impact from the perpendicular view and motion of the tissue displacement wave from the in-line view. Average radial wave displacement and surface wave speed were calculated for each projectile material and categorized by penetrating or non-penetrating impacts. Two-sample t-tests determined that non-penetrating projectiles resulted in significantly faster surface wave speeds in porcine skin for stainless steel (p = 0.002), plastic (p = 0.004), and Si₃N₄ ball bearings (p = 0.014), while ANOVA determined significant differences in radial wave displacement and surface wave speed between projectile materials. Surface wave speed was used to quantify mechanical properties of the skin including elastic modulus, shear modulus, and bulk modulus during ballistic impact, which may be implemented to simulate accurate deformation behavior in computational impact models.

Comparison of the accuracy of different handheld-type scanners in three-dimensional facial image recognition

PURPOSE

Handheld-type scanners are widely used in clinical practice. This study examined the accuracy of handheld-type scanners using plaster statues to assess their performance in facial recognition.

METHODS

Twelve 4-mm zirconia balls as measuring points were attached to the facial portions of three types of plaster statue. Six digital facial images of each plaster statue were obtained using one of the following five handheld-type scanners: Artec Eva, Artec Spider, Bellus 3D FaceApp, SNAP, and Vectra H1. Four-millimeter spherical objects were manually placed at the measurement points on the scanned data generated using computer-aided design software and coordinate positions were measured using a contact-type high-resolution three-dimensional measurement device. Consequently, the discrepancy between the distance measured using the contact-type device and that measured using the handheld-type scanner was calculated. The scanning time, processing time, and deviation of the distance between the measuring points were analyzed using two-way analysis of variance and t-test with Bonferroni correction.

RESULTS

The scanning and processing times ranged from 15.2 to 42.2 s and 20.7 to 234.2 s, respectively. Overall, 97% of all measured distances by Spider were within ±1.00% deviation; 79%, Vectra; 73%, Eva; 70%, Bellus; and 42%, SNAP.

CONCLUSIONS

The performance of handheld-type scanners using plaster statues varied among the different scanners. The scanning time of Eva and the processing time of Bellus were significantly shorter than those of other scanners. Furthermore, Spider exhibited the best accuracy, followed by Eva, Vectra, Bellus, and SNAP.

A computer based facial flaps simulator using projective dynamics

BACKGROUND AND OBJECTIVES

Interactive surgical simulation using the finite element method to model human skin mechanics has been an elusive goal. Mass-spring networks, while fast, do not provide the required accuracy.

METHODS

This paper presents an interactive, cognitive, facial flaps simulator based on a projective dynamics computational framework. Projective dynamics is able to generate rapid, stable results following changes to the facial soft tissues created by the surgeon, even in the face of sudden increases in skin resistance as its stretch limit is reached or collision between tissues occurs. Our prior work with the finite element method had been hampered by these considerations. Surgical tools are provided for; skin incision, undermining, deep tissue cutting, and excision. A spring-like "skin hook" is used for retraction. Spring-based sutures can be placed individually or automatically placed as a row between cardinal sutures.

RESULTS

Examples of an Abbe/Estlander lip reconstruction, a paramedian forehead flap to the nose, a retroauricular flap reconstruction of the external ear, and a cervico-facial flap reconstruction of a cheek defect are presented.

CONCLUSIONS

Projective dynamics has significant advantages over mass-spring and finite element methods as the physics backbone for interactive soft tissue surgical simulation.

Multiscale Computational Model Predicts Mouse Skin Kinematics Under Tensile Loading

Skin is a complex tissue whose biomechanical properties are generally understood in terms of an incompressible material whose microstructure undergoes affine deformations. A growing number of experiments, however, have demonstrated that skin has a high Poisson's ratio, substantially decreases in volume during uniaxial tensile loading, and demonstrates collagen fiber kinematics that are not affine with local deformation. In order to better understand the mechanical basis for these properties, we constructed multiscale mechanical models (MSM) of mouse skin based on microstructural multiphoton microscopy imaging of the dermal microstructure acquired during mechanical testing. Three models that spanned the cases of highly aligned, moderately aligned, and nearly random fiber networks were examined and compared to the data acquired from uniaxially stretched skin. Our results demonstrate that MSMs consisting of networks of matched fiber organization can predict the biomechanical behavior of mouse skin, including the large decrease in tissue volume and nonaffine fiber kinematics observed under uniaxial tension.

Alternatives and preferences for materials in use for pressure ulcer prevention: An experiment-reinforced literature review

Alleviation of localised, sustained tissue loads and microclimate management are the most critical performance criteria for materials in use for pressure ulcer prevention, such as in prophylactic dressings, padding or cushioning. These material performance criteria can be evaluated by calculating the extents of matching between the material stiffness (elastic modulus) and the thermal conductivity of the protective dressing, padding or cushioning with the corresponding properties of native skin, separately or in combination. Based on these bioengineering performance criteria, hydrocolloids, which are commonly used for prophylaxis of medical device-related pressure ulcers, exhibit poor stiffness matching with skin. In addition, there is remarkable variability in the modulus and thermal conductivity matching levels of different material types used for pressure ulcer prevention, however, it appears that among the materials tested, hydrogels provide the optimal matching with skin, followed by gels and silicone foams. The stiffness matching for hydrocolloids appears to be inferior even to that of gauze. This article provides quantitative performance criteria and metrics for these evaluations, and grades commonly used material types to biomechanically guide clinicians and industry with regards to the selection of dressings for pressure ulcer prevention, both due to bodyweight forces and as a result of applied medical devices.

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.

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.

Evaluation of facial tissue stresses under medical devices post application of a cyanoacrylate liquid skin protectant: An integrated experimental-computational study

Medical device‐related pressure ulcers (PUs) (injuries) are a subclass of PUs, associated with pressure and/or shear applied by a medical device onto the skin. Clinical application of a cyanoacrylate liquid skin protectant (CLSP) under the contours of skin‐contacting medical devices to shield an intact skin from the sustained mechanical loads that are applied by medical devices is a preventative option, but no computer modelling work has been reported to assess the biomechanical efficacy of such interventions. Here, we investigated the biomechanical protective effect of a polymerised cyanoacrylate coating using three‐dimensional, anatomically realistic finite element models of the ear with oxygen cannula and the mouth with endotracheal attachment device, informed by experimental studies. We have compared tissue stress exposures under the devices at these facial sites between conditions where the cyanoacrylate skin protectant has been applied or where the device was contacting the skin directly, without the shielding of the cyanoacrylate coating. The CLSP considerably reduced the skin stress concentration levels and overall tissue stress exposures under the aforementioned medical devices. This demonstrates strong biomechanical effectiveness of the studied cyanoacrylate‐based skin protectant in prevention of facial medical device‐related injuries at small, curved and thereby difficult to protect facial sites.

A multiscale four-layer finite element model to predict the effects of collagen fibers on skin behavior under tension

Human skin is a complex multilayered multiscale material that exhibits nonlinear and anisotropic mechanical behavior. It has been reported that its macroscopic behavior in terms of progression of wrinkles induced by aging is strongly dependent on its microscopic composition in terms of collagen fibers in the dermis layer. In the present work, a multiscale four-layer 2D finite element model of the skin was developed and implemented in Matlab code. The focus here was to investigate the effects of dermal collagen on the macroscopic mechanical behavior of the skin. The skin was modeled by a continuum model composed of four layers: the Stratum Corneum, the epidermis, the dermis, and the hypodermis. The geometry of the different layers of the skin was represented in a 2D model with their respective thicknesses and material properties taken from literature data. The macroscopic behavior of the dermis was modeled with a nonlinear multiscale approach based on a multiscale elastic model of collagen structure going from cross-linked molecules to the collagen fiber, combined with a Mori-Tanaka homogenization scheme. The model includes the nonlinear elasticity of the collagen fiber density, the fiber radius, the undulation, and the fiber orientation. An axial tension was applied incrementally to the lateral surfaces of the skin model. A parametric study was performed in order to investigate the effect of the collagen constituents on the macroscopic skin mechanical behavior in terms of the predicted macroscopic stress-strain curve of the skin. The results of the FE computations under uniaxial tension showed that the different layers undergo different strains, leading to a difference in the transversal deformation at the top surface. In addition, the parametric study revealed a strong correlation between macroscopic skin elasticity and its collagen structure.

What makes a hydrogel-based dressing advantageous for the prevention of medical device-related pressure ulcers

The synergistic influences of geometrical, mechanical and thermal mismatches between a skin‐contacting medical device and the skin may cause tissue stress concentrations and sharp temperature gradients, both of which contribute to the risk for medical device‐related pressure ulcers. In this work, we developed an innovative, integrated experimental bioengineering approach encompassing mechanical stiffness, friction and thermal property studies for testing the biomechanical suitability of a hydrogel‐based dressing in prophylaxis of injuries caused by devices. We characterised the viscoelastic stress relaxation of the dressing and determined its long‐term elastic modulus. We further measured the coefficient of friction of the hydrogel‐based dressing at dressing‐device and skin‐dressing interfaces, using a tilting‐table tribometer. Lastly, we measured the thermal conductivity of the dressing, using a heat‐flow meter and infrared thermography‐based method. All measurements considered dry and moist conditions, the latter simulating skin perspiration effects. Our results revealed that the long‐term stiffness and the thermal conductivity of the hydrogel‐based dressing matched the corresponding properties of human skin for both dry and moist conditions. The dressing further demonstrated a relatively high coefficient of friction at its skin‐facing and device‐facing aspects, indicating minimal frictional sliding. All these properties make the above dressing advantageous for prevention of device‐related injuries.

Improving reconstructive surgery design using Gaussian process surrogates to capture material behavior uncertainty

To produce functional, aesthetically natural results, reconstructive surgeries must be planned to minimize stress as excessive loads near wounds have been shown to produce pathological scarring and other complications (Gurtner et al., 2011). Presently, stress cannot easily be measured in the operating room. Consequently, surgeons rely on intuition and experience (Paul et al., 2016; Buchanan et al., 2016). Predictive computational tools are ideal candidates for surgery planning. Finite element (FE) simulations have shown promise in predicting stress fields on large skin patches and in complex cases, helping to identify potential regions of complication. Unfortunately, these simulations are computationally expensive and deterministic (Lee et al., 2018a). However, running a few, well selected FE simulations allows us to create Gaussian process (GP) surrogate models of local cutaneous flaps that are computationally efficient and able to predict stress and strain for arbitrary material parameters. Here, we create GP surrogates for the advancement, rotation, and transposition flaps. We then use the predictive capability of these surrogates to perform a global sensitivity analysis, ultimately showing that fiber direction has the most significant impact on strain field variations. We then perform an optimization to determine the optimal fiber direction for each flap for three different objectives driven by clinical guidelines (Leedy et al., 2005; Rohrer and Bhatia, 2005). While material properties are not controlled by the surgeon and are actually a source of uncertainty, the surgeon can in fact control the orientation of the flap with respect to the skin's relaxed tension lines, which are associated with the underlying fiber orientation (Borges, 1984). Therefore, fiber direction is the only material parameter that can be optimized clinically. The optimization task relies on the efficiency of the GP surrogates to calculate the expected cost of different strategies when the uncertainty of other material parameters is included. We propose optimal flap orientations for the three cost functions and that can help in reducing stress resulting from the surgery and ultimately reduce complications associated with excessive mechanical loading near wounds.

How stiff is skin?

The measurement of the mechanical properties of skin (such as stiffness, extensibility and strength) is a key step in characterisation of both dermal ageing and disease mechanisms and in the assessment of tissue-engineered skin replacements. However, the biomechanical terminology and plethora of mathematical analysis approaches can be daunting to those outside the field. As a consequence, mechanical studies are often inaccessible to a significant proportion of the intended audience. Furthermore, devices for the measurement of skin function in vivo generate relative values rather than formal mechanical measures, therefore limiting the ability to compare studies. In this viewpoint essay, we discuss key biomechanical concepts and the influence of technical and biological factors (including the nature of the testing apparatus, length scale, donor age and anatomical site) on measured mechanical properties such as stiffness. Having discussed the current state-of-the-art in macro-mechanical and micromechanical measuring techniques and in mathematical and computational modelling methods, we then make suggestions as to how these approaches, in combination with 3D X-ray imaging and mechanics methods, may be adopted into a single strategy to characterise the mechanical behaviour of skin.

The regional-dependent biaxial behavior of young and aged mouse skin: A detailed histomechanical characterization, residual strain analysis, and constitutive model

Skin fulfills several vital functions, many of which are dependent on its mechanical properties. Therefore, as mice have become an invaluable model for skin research, determining murine skin's mechanical properties is important. Specifically, skin's mechanical properties are important for functional tests as well as for prognostic and diagnostic purposes. Additionally, computational simulations of skin behavior are becoming commonplace, rendering accurate models of murine skin's constitutive behavior necessary. To date, our knowledge of mouse skin mechanics shows significant gaps. For example, there are no comprehensive reports correlating skin's mechanical properties with region, age, and direction. Moreover, mouse skin's residual strain behavior has not been reported on. In our current work, we set out to fill these gaps. Based on histology, 2-photon microscopy, and planar biaxial testing, while accurately tracking various reference configurations, we report on differences in gross structure, microstructural organization, and constitutive response of skin, and cast those properties into a versatile Fung-type hyperelastic constitutive law for three reference configurations. Our data is the most comprehensive report contrasting the mechanical properties of young (12 weeks) and aged (52 weeks) mouse skin and will, thus, be valuable to basic science as control data, and provide accurate constitutive laws for mouse skin modeling. STATEMENT OF SIGNIFICANCE: Our findings are significant as they fill several gaps in our understanding of mouse skin mechanics. This is particularly important as mouse skin is becoming a frequent and critical model of human skin for cosmetic and medical science. Specifically, we quantified how mechanical properties of mice skin vary with age, with location, and with direction. Additionally, we cast our findings into constitutive models that can be used by others for predictive computer simulations of skin behavior.

Surface deformation tracking and modelling of soft materials

Many computer vision algorithms have been presented to track surface deformations, but few have provided a direct comparison of measurements with other stereoscopic approaches and physics-based models. We have previously developed a phase-based cross-correlation algorithm to track dense distributions of displacements over three-dimensional surfaces. In the present work, we compare this algorithm with one that uses an independent tracking system, derived from an array of fluorescent microspheres. A smooth bicubic Hermite mesh was fitted to deformations obtained from the phase-based cross-correlation data. This mesh was then used to estimate the microsphere locations, which were compared to stereo reconstructions of the microsphere positions. The method was applied to a 35 mm × 35 mm × 35 mm soft silicone gel cube under indentation, with three square bands of microspheres placed around the indenter tip. At an indentation depth of 4.5 mm, the root-mean-square (RMS) differences between the reconstructed positions of the microspheres and their identified positions for the inner, middle, and outer bands were 60 µm, 20 µm, and 19 µm, respectively. The usefulness of the strain-tracking data for physics-based finite element modelling of large deformation mechanics was then demonstrated by estimating a neo-Hookean stiffness parameter for the gel. At the optimal constitutive parameter estimate, the RMS difference between the measured microsphere positions and their finite element model-predicted locations was 143 µm.

A model of human skin under large amplitude oscillatory shear

Skin mechanics is of importance in various fields of research when accurate predictions of the mechanical response of skin is essential. This study aims to develop a new constitutive model for human skin that is capable of describing the heterogeneous, nonlinear viscoelastic mechanical response of human skin under shear deformation. This complex mechanical response was determined by performing large amplitude oscillatory shear (LAOS) experiments on ex vivo human skin samples. It was combined with digital image correlation (DIC) on the cross-sectional area to assess heterogeneity. The skin is modeled as a one-dimensional layered structure, with every sublayer behaving as a nonlinear viscoelastic material. Heterogeneity is implemented by varying the stiffness with skin depth. Using an iterative parameter estimation method all model parameters were optimized simultaneously. The model accurately captures strain stiffening, shear thinning, softening effect and nonlinear viscous dissipation, as experimentally observed in the mechanical response to LAOS. The heterogeneous properties described by the model were in good agreement with the experimental DIC results. The presented mathematical description forms the basis for a future constitutive model definition that, by implementation in a finite element method, has the capability of describing the full 3D mechanical behavior of human skin.

Skin mechanical properties and modeling: A review

The mechanical properties of the skin are important for various applications. Numerous tests have been conducted to characterize the mechanical behavior of this tissue, and this article presents a review on different experimental methods used. A discussion on the general mechanical behavior of the skin, including nonlinearity, viscoelasticity, anisotropy, loading history dependency, failure properties, and aging effects, is presented. Finally, commonly used constitutive models for simulating the mechanical response of skin are discussed in the context of representing the empirically observed behavior.

Multidirectional In Vivo Characterization of Skin Using Wiener Nonlinear Stochastic System Identification Techniques

A triaxial force-sensitive microrobot was developed to dynamically perturb skin in multiple deformation modes, in vivo. Wiener static nonlinear identification was used to extract the linear dynamics and static nonlinearity of the force-displacement behavior of skin. Stochastic input forces were applied to the volar forearm and thenar eminence of the hand, producing probe tip perturbations in indentation and tangential extension. Wiener static nonlinear approaches reproduced the resulting displacements with variances accounted for (VAF) ranging 94-97%, indicating a good fit to the data. These approaches provided VAF improvements of 0.1-3.4% over linear models. Thenar eminence stiffness measures were approximately twice those measured on the forearm. Damping was shown to be significantly higher on the palm, whereas the perturbed mass typically was lower. Coefficients of variation (CVs) for nonlinear parameters were assessed within and across individuals. Individual CVs ranged from 2% to 11% for indentation and from 2% to 19% for extension. Stochastic perturbations with incrementally increasing mean amplitudes were applied to the same test areas. Differences between full-scale and incremental reduced-scale perturbations were investigated. Different incremental preloading schemes were investigated. However, no significant difference in parameters was found between different incremental preloading schemes. Incremental schemes provided depth-dependent estimates of stiffness and damping, ranging from 300 N/m and 2 Ns/m, respectively, at the surface to 5 kN/m and 50 Ns/m at greater depths. The device and techniques used in this research have potential applications in areas, such as evaluating skincare products, assessing skin hydration, or analyzing wound healing.

Scalp simulation - A novel approach to site-specific biomechanical modeling of the skin

OBJECTIVES

This study aimed to determine the hardness of the human scalp in vivo in order to identify an appropriate scalp simulant, from a range of commercially available silicone materials, for force impact assessment. Site-dependent variation in scalp hardness, and the applicability of contemporary skin simulants to the scalp were also considered.

MATERIALS AND METHODS

A Shore A-type durometer was used to collected hardness data from the scalps of 30 human participants (five males and five females in each of the three age categories: 18-30, 31-40, 41-50) and four commercially available silicones (light, medium, and heavy-bodied PVS, and duplication silicone). One-sample t-tests were used to compare the mean hardness of simulants to that of the scalp. Site-dependent variation in the hardness of the scalp was assessed using a mixed-model repeated measures ANOVA.

RESULTS

Mean human scalp hardness derived from participants was 20.6 Durometer Units (DU; SD = 3.4). Analysis revealed only the medium-bodied PVS to be an acceptable scalp simulant when compared to the mean hardness of the human scalp (p = 0.869). Scalp hardness varied significantly anteroposteriorly (with an observable linear trend, p < 0.001), but not mediolaterally (p = 0.271). Comparisons of simulants to site-specific variation in scalp hardness anteroposteriorly found the medium-bodied PVS to be only suitable in the central region of the scalp (p = 0.391). In contrast, the duplication silicone (p = 0.074) and light-bodied PVS (p = 0.147) were only comparable to the posterior region.

CONCLUSIONS

Contemporary skin simulants fail to accurately represent the scalp in terms of hardness. There is strong support for the use of medium-bodied PVS as a scalp simulant. Human scalp hardness varies significantly anteroposteriorly, but not mediolaterally, corresponding to regional anatomical variation within the scalp. A number of materials were identified as potential simulants for different regions of the scalp when more site-specific simulant research is required.

General theory of skin reinforcement

Macroscopic mechanical properties of human skin in vivo cannot be considered independent of adjacent subcutaneous white adipose tissue (sWAT). The layered system skin/sWAT appears as the hierarchical structural composite in which single layers behave as fiber-reinforced structures. Effective macroscopic mechanical properties of such composites are mainly determined either by the properties of the skin or by those of the sWAT, dependent on the conditions of mechanical loading. Mechanical interactions between the skin and the adjacent sWAT associated with a mismatch in the mechanical moduli of these two layers can lead to production of the skin wrinkles. Reinforcement of the composite skin/sWAT can take place in different ways. It can be provided through reorientation of collagen fibers under applied loading, through production of new bonds between existing collagen fibers and through induction of additional collagen structures. Effectiveness of this type of reinforcement is strongly dependent on the type of mechanical loading. Different physical interventions induce the reinforcement of at least one of these two layers, thus increasing the effective macroscopic stiffness of the total composite. At the same time, the standalone reinforcement of the skin appears to be less effective to achieve a delay or a reduction of the apparent signs of skin aging relative to the reinforcement of the sWAT.

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.

A surface wave elastography technique for measuring tissue viscoelastic properties

A surface wave elastography method is proposed to study the viscoelastic properties of skin by measuring the surface wave speed and attenuation on the skin. Experiments were carried out on porcine skin tissues. The surface wave speed is measured by the change of phase with distance. The wave attenuation is measured by the decay of wave amplitude with distance. The change of viscoelastic properties with temperature was studied at room and body temperatures. The wave speed was 1.83m/s at 22°C but reduced to 1.52m/s at 33°C. The viscoelastic ratio was almost constant from 22°C to 33°C. Fresh and decayed tissues were studied. The wave speed of the decayed tissue increased from 1.83m/s of fresh state to 2.73m/s. The viscoelastic ratio was 0.412/mm at the decayed state compared to 0.215/mm at the fresh state. More tissue samples are needed to study these viscoelastic parameters according to specific applications.

A nonlinear anisotropic inverse method for computational dissection of inhomogeneous planar tissues

Quantification of the mechanical behavior of soft tissues is challenging due to their anisotropic, heterogeneous, and nonlinear nature. We present a method for the 'computational dissection' of a tissue, by which we mean the use of computational tools both to identify and to analyze regions within a tissue sample that have different mechanical properties. The approach employs an inverse technique applied to a series of planar biaxial experimental protocols. The aggregated data from multiple protocols provide the basis for (1) segmentation of the tissue into regions of similar properties, (2) linear analysis for the small-strain behavior, assuming uniform, linear, anisotropic behavior within each region, (3) subsequent nonlinear analysis following each individual experimental protocol path and using local linear properties, and (4) construction of a strain energy data set W(E) at every point in the material by integrating the differential stress-strain functions along each strain path. The approach has been applied to simulated data and captures not only the general nonlinear behavior but also the regional differences introduced into the simulated tissue sample.

Automatic Segmentation of Mechanically Inhomogeneous Tissues Based on Deformation Gradient Jump

Variations in properties, active behavior, injury, scarring, and/or disease can all cause a tissue's mechanical behavior to be heterogeneous. Advances in imaging technology allow for accurate full-field displacement tracking of both in vitro and in vivo deformation from an applied load. While detailed strain fields provide some insight into tissue behavior, material properties are usually determined by fitting stress-strain behavior with a constitutive equation. However, the determination of the mechanical behavior of heterogeneous soft tissue requires a spatially varying constitutive equation (i.e., one in which the material parameters vary with position). We present an approach that computationally dissects the sample domain into many homogeneous subdomains, wherein subdomain boundaries are formed by applying a betweenness based graphical analysis to the deformation gradient field to identify locations with large discontinuities. This novel partitioning technique successfully determined the shape, size and location of regions with locally similar material properties for: (1) a series of simulated soft tissue samples prescribed with both abrupt and gradual changes in anisotropy strength, prescribed fiber alignment, stiffness, and nonlinearity, (2) tissue analogs (PDMS and collagen gels) which were tested biaxially and speckle tracked (3) and soft tissues which exhibited a natural variation in properties (cadaveric supraspinatus tendon), a pathologic variation in properties (thoracic aorta containing transmural plaque), and active behavior (contracting cardiac sheet). The routine enables the dissection of samples computationally rather than physically, allowing for the study of small tissues specimens with unknown and irregular inhomogeneity.

A mechanistic insight into the mechanical role of the stratum corneum during stretching and compression of the skin

The study of skin biophysics has largely been driven by consumer goods, biomedical and cosmetic industries which aim to design products that efficiently interact with the skin and/or modify its biophysical properties for health or cosmetic benefits. The skin is a hierarchical biological structure featuring several layers with their own distinct geometry and mechanical properties. Up to now, no computational models of the skin have simultaneously accounted for these geometrical and material characteristics to study their complex biomechanical interactions under particular macroscopic deformation modes. The goal of this study was, therefore, to develop a robust methodology combining histological sections of human skin, image-processing and finite element techniques to address fundamental questions about skin mechanics and, more particularly, about how macroscopic strains are transmitted and modulated through the epidermis and dermis. The work hypothesis was that, as skin deforms under macroscopic loads, the stratum corneum does not experience significant strains but rather folds/unfolds during skin extension/compression. A sample of fresh human mid-back skin was processed for wax histology. Sections were stained and photographed by optical microscopy. The multiple images were stitched together to produce a larger region of interest and segmented to extract the geometry of the stratum corneum, viable epidermis and dermis. From the segmented structures a 2D finite element mesh of the skin composite model was created and geometrically non-linear plane-strain finite element analyses were conducted to study the sensitivity of the model to variations in mechanical properties. The hybrid experimental-computational methodology has offered valuable insights into the simulated mechanics of the skin, and that of the stratum corneum in particular, by providing qualitative and quantitative information on strain magnitude and distribution. Through a complex non-linear interplay, the geometry and mechanical characteristics of the skin layers (and their relative balance), play a critical role in conditioning the skin mechanical response to macroscopic in-plane compression and extension. Topographical features of the skin surface such as furrows were shown to act as an efficient means to deflect, convert and redistribute strain-and so stress-within the stratum corneum, viable epidermis and dermis. Strain reduction and amplification phenomena were also observed and quantified. Despite the small thickness of the stratum corneum, its Young׳s modulus has a significant effect not only on the strain magnitude and directions within the stratum corneum layer but also on those of the underlying layers. This effect is reflected in the deformed shape of the skin surface in simulated compression and extension and is intrinsically linked to the rather complex geometrical characteristics of each skin layer. Moreover, if the Young׳s modulus of the viable epidermis is assumed to be reduced by a factor 12, the area of skin folding is likely to increase under skin compression. These results should be considered in the light of published computational models of the skin which, up to now, have ignored these characteristics.

Linear elastic properties of the facial soft tissues using an aspiration device: towards patient specific characterization

Biomechanical modeling of the facial soft tissue behavior is needed in aesthetic or maxillo-facial surgeries where the simulation of the bone displacements cannot accurately predict the visible outcome on the patient's face. Because these tissues have different nature and elastic properties across the face, depending on their thickness, and their content in fat or muscle, individualizing their mechanical parameters could increase the simulation accuracy. Using a specifically designed aspiration device, the facial soft tissues deformation is measured at four different locations (cheek, cheekbone, forehead, and lower lip) on 16 young subjects. The stiffness is estimated from the deformations generated by a set of negative pressures using an inverse analysis based on a Neo Hookean model. The initial Young's modulus of the cheek, cheekbone, forehead, and lower lip are respectively estimated to be 31.0 kPa±4.6, 34.9 kPa±6.6, 17.3 kPa±4.1, and 33.7 kPa±7.3. Significant intra-subject differences in tissue stiffness are highlighted by these estimations. They also show important inter-subject variability for some locations even when mean stiffness values show no statistical difference. This study stresses the importance of using a measurement device capable of evaluating the patient specific tissue stiffness during an intervention.

An inverse method to determine the mechanical properties of the iris in vivo

BACKGROUND

Understanding the mechanical properties of the iris can help to have an insight into the eye diseases with abnormalities of the iris morphology. Material parameters of the iris were simply calculated relying on the ex vivo experiment. However, the mechanical response of the iris in vivo is different from that ex vivo, therefore, a method was put forward to determine the material parameters of the iris using the optimization method in combination with the finite element method based on the in vivo experiment.

MATERIAL AND METHODS

Ocular hypertension was induced by rapid perfusion to the anterior chamber, during perfusion intraocular pressures in the anterior and posterior chamber were record by sensors, images of the anterior segment were captured by the ultrasonic system. The displacement of the characteristic points on the surface of the iris was calculated. A finite element model of the anterior chamber was developed using the ultrasonic image before perfusion, the multi-island genetic algorithm was employed to determine the material parameters of the iris by minimizing the difference between the finite element simulation and the experimental measurements.

RESULTS

Material parameters of the iris in vivo were identified as the iris was taken as a nearly incompressible second-order Ogden solid. Values of the parameters μ1, α1, μ2 and α2 were 0.0861 ± 0.0080 MPa, 54.2546 ± 12.7180, 0.0754 ± 0.0200 MPa, and 48.0716 ± 15.7796 respectively. The stability of the inverse finite element method was verified, the sensitivity of the model parameters was investigated.

CONCLUSION

Material properties of the iris in vivo could be determined using the multi-island genetic algorithm coupled with the finite element method based on the experiment.