A novel method for visualising and quantifying through-plane skin layer deformations

The Histological and Mechanical Behavior of Skin During Puncture for Different Impactor Sizes and Loading Rates

PURPOSE

The hierarchical structure of skin dictates its protective function against mechanical loading, which has been extensively studied through numerous experiments. Viscoelasticity and anisotropy have been defined for skin in tensile loading, but most puncture studies utilized skin simulants, which lacked natural tension and varying skin thicknesses. The purpose of this study was to define the mechanical behavior and failure thresholds of skin during puncture with various blunt impactor sizes and loading rates.

METHODS

After determining natural tension of porcine skin, 232 isolated skin samples were loaded in puncture. Pre-conditioning, sub-failure, and failure trials were conducted with an electrohydraulic piston actuator loading pre-strained skin samples with a 3-, 5-, or 8-mm spherical impactor at rates of 5 to 1000 mm/s. Generalized linear mixed models were used to determine significant factors and predict probability of puncture.

RESULTS

Increased skin thickness significantly increased RIII stiffness (p = 0.002), failure force (p < 0.001), and strain energy at failure (p = 0.002) and significantly decreased displacement at failure (p = 0.002). Significantly greater force, displacement, strain energy, and stiffness (p < 0.05) at failure were observed with the 8-mm impactor. Loading at 1000 mm/s resulted in significantly greater force (p = 0.026) and stiffness (p < 0.001) at failure compared to 5 mm/s and significantly decreased displacement at failure (p < 0.001). 3D-DIC strain maps displayed anisotropic behavior, and larger elliptical wounds resulted from puncture with an 8 mm impactor (p < 0.001). Quantitative histological analyses revealed collagen re-alignment near the impactor from pre-conditioning and minimal structural damage during sub-failure trials. Initial structural failure occurred in the reticular dermis followed by the papillary dermis and epidermis.

CONCLUSION

The presented failure metrics, with support from histological findings, may be utilized in development of protective clothing, improvement of computational models, and advancement in forensic sciences.

A MPET2-mPBPK model for subcutaneous injection of biotherapeutics with different molecular weights: From local scale to whole-body scale

BACKGROUND AND OBJECTIVE

Subcutaneous injection of biotherapeutics has attracted considerable attention in the pharmaceutical industry. However, there is limited understanding of the mechanisms underlying the absorption of drugs with different molecular weights and the delivery of drugs from the injection site to the targeted tissue.

METHODS

We propose the MPET2-mPBPK model to address this issue. This multiscale model couples the MPET2 model, which describes subcutaneous injection at the local tissue scale from a biomechanical view, with a post-injection absorption model at injection site and a minimal physiologically-based pharmacokinetic (mPBPK) model at whole-body scale. Utilizing the principles of tissue biomechanics and fluid dynamics, the local MPET2 model provides solutions that account for tissue deformation and drug absorption in local blood vessels and initial lymphatic vessels during injection. Additionally, we introduce a model accounting for the molecular weight effect on the absorption by blood vessels, and a nonlinear model accounting for the absorption in lymphatic vessels. The post-injection model predicts drug absorption in local blood vessels and initial lymphatic vessels, which are integrated into the whole-body mPBPK model to describe the pharmacokinetic behaviors of the absorbed drug in the circulatory and lymphatic system.

RESULTS

We establish a numerical model which links the biomechanical process of subcutaneous injection at local tissue scale and the pharmacokinetic behaviors of injected biotherapeutics at whole-body scale. With the help of the model, we propose an explicit relationship between the reflection coefficient and the molecular weight and predict the bioavalibility of biotherapeutics with varying molecular weights via subcutaneous injection.

CONCLUSION

The considered drug absorption mechanisms enable us to study the differences in local drug absorption and whole-body drug distribution with varying molecular weights. This model enhances the understanding of drug absorption mechanisms and transport routes in the circulatory system for drugs of different molecular weights, and holds the potential to facilitate the application of computational modeling to drug formulation.

In the face and neck, keloid scar distribution is related to skin thickness and stiffness changes associated with movement

Keloid scars tend to occur in high-tension sites due to mechanical stimuli that are involved in their development. To date, a detailed analysis of keloid distribution focused specifically on facial and neck areas has not been reported, and limited literature exists as to the related mechanical factors. To rectify this deficiency of knowledge, we first quantified the facial and neck keloid distribution observed clinically in 113 patients. Subsequently, we performed a rigorous investigation into the mechanical factors and their associated changes at these anatomic sites in healthy volunteers without a history of pathologic scarring. The association between keloid-predilection sites and sebaceous gland-dense and acne-prone sites was also examined. To assess skin stretch, thickness and stiffness, VECTRA, ultrasound and indentometer were utilised. Baseline skin stiffness and thickness were measured, as well as the magnitude of change in these values associated with facial expression and postural changes. Within the face and neck, keloids were most common near the mandibular angle (41.3%) and lateral submental (20.0%) regions. These areas of increased keloid incidence were not associated with areas more dense in sebaceous glands, nor linked consistently with acne-susceptible regions. Binomial logistic regression revealed that changes in skin stiffness and thickness related to postural changes significantly predicted keloid distribution. Skin stiffness and thickness changes related to prolonged mechanical forces (postural changes) are most pronounced at sites of high keloid predilection. This finding further elucidates the means by which skin stretch and tension are related to keloid development. As a more detailed analysis of mechanical forces on facial and neck skin, this study evaluates the nuances of multiple skin-mechanical properties, and their changes in a three-dimensional framework. Such factors may be critical to better understanding keloid progression and development in the face and neck.

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.

An evaluation of mechanical and biophysical skin parameters at different body locations

BACKGROUND

Skin is the largest organ in the body, representing an important interface to monitor health and disease. However, there is significant variation in skin properties for different ages, genders and body regions due to the differences in the structure and morphology of the skin tissues. This study aimed to evaluate the use of non-invasive tools to discriminate a range of mechanical and functional skin parameters from different skin sites.

MATERIALS AND METHODS

A cohort of 15 healthy volunteers was recruited following appropriate informed consent. Four well-established CE-marked non-invasive techniques were used to measure four anatomical regions: palm, forearm, sole and lower lumbar L3, using a repeated measures design. Skin parameters included trans-epidermal water loss (TEWL), pH (acidity), erythema, stratum corneum hydration and stiffness and elasticity using Myoton Pro (skin and muscle probe). Differences between body locations for each parameter and the intra-rater reliability between days were evaluated by the same operator.

RESULTS

The results indicate that parameters differed significantly between skin sites. For the Myoton skin probe, the sole recorded the highest stiffness value of 1006 N/m (SD ± 179), while the lower lumbar recorded the least value of 484 N/m (SD ± 160). The muscle indenter Myoton probe revealed the palm's highest value of 754 N/m (± 108), and the lower lumbar recorded the least value of 208 N/m (SD ± 44). TEWL values were lowest on the forearm, averaging 11 g/m2/h, and highest on the palm, averaging 41 g/m2/h. Similar skin hydration levels were recorded in three of the four sites, with the main difference being observed in the sole averaging 13 arbitrary units. Erythema values were characterised by a high degree of inter-subject variation, and no significant differences between sites or sides were observed. The Myoton Pro Skin showed excellent reliability (intra-class correlation coefficients > 0.70) for all sites with exception of one site right lower back; the Myoton pro muscle probes showed good to poor reliability (0.90-017), the corneometer showed excellent reliability (>0.75) among all the sites tested, and the TEWL showed Good to poor reliability (0.74-0.4) among sites.

CONCLUSION

The study revealed that using non-invasive methods, the biophysical properties of skin can be mapped, and significant differences in the mechanical and functional properties of skin were observed. These parameters were reliably recorded between days, providing a basis for their use in assessing and monitoring changes in the skin during health and disease.

A new device for the combined measurement of friction and through-thickness deformation on ex vivo skin samples

Skin irritation is a common phenomenon that becomes a real concern when caused by the use of medical devices. Because the materials used for the design of these devices are usually carefully selected for chemical compatibility with the skin, it is reasonable to assume that the irritations result from the mechanical interaction between the devices and the skin. The aim of this work was to develop a new device to study both the shear strains in the layers of the skin, using Digital Image Correlation (DIC), and the friction behaviour of ex vivo skin interacting with objects. Pig skin samples with various surface preparations were tested in friction experiments involving different contacting materials encountered in the conception of medical devices. The measure of the static and dynamic coefficients of friction as well as the length of adhesion has highlighted the great influence of skin surface conditioning on friction properties. Strain maps obtained through DIC provided insights into the impact of friction and adhesion effects on shear strain distribution in the skin as a function of depth beneath its surface.

Multifunctional Injectable Hydrogel for In Vivo Diagnostic and Therapeutic Applications

Injectable hydrogels show high potential for in vivo biomedical applications owing to their distinctive mode of administration into the human body. In this study, we propose a material design strategy for developing a multifunctional injectable hydrogel with good adhesiveness, stretchability, and bioresorbability. Its multifunctionality, whereupon multiple reactions occur simultaneously during its injection into the body without requiring energy stimuli and/or additives, was realized through meticulous engineering of bioresorbable precursors based on hydrogel chemistry. The multifunctional injectable hydrogel can be administered through a minimally invasive procedure, form a conformal adhesive interface with the target tissue, dynamically stretch along with the organ motions with minimal mechanical constraints, and be resorbed in vivo after a specific period. Further, the incorporation of functional nanomaterials into the hydrogel allows for various in vivo diagnostic and therapeutic applications, without compromising the original multifunctionality of the hydrogel. These features are verified through theranostic case studies on representative organs, including the skin, liver, heart, and bladder.

High-resolution two-photon polymerization: the most versatile technique for the fabrication of microneedle arrays

Microneedle patches have received much interest in the last two decades as drug/vaccine delivery or fluid sampling systems for diagnostic and monitoring purposes. Microneedles are manufactured using a variety of additive and subtractive micromanufacturing techniques. In the last decade, much attention has been paid to using additive manufacturing techniques in both research and industry, such as 3D printing, fused deposition modeling, inkjet printing, and two-photon polymerization (2PP), with 2PP being the most flexible method for the fabrication of microneedle arrays. 2PP is one of the most versatile and precise additive manufacturing processes, which enables the fabrication of arbitrary three-dimensional (3D) prototypes directly from computer-aided-design (CAD) models with a resolution down to 100 nm. Due to its unprecedented flexibility and high spatial resolution, the use of this technology has been widespread for the fabrication of bio-microdevices and bio-nanodevices such as microneedles and microfluidic devices. This is a pioneering transformative technology that facilitates the fabrication of complex miniaturized structures that cannot be fabricated with established multistep manufacturing methods such as injection molding, photolithography, and etching. Thus, microstructures are designed according to structural and fluid dynamics considerations rather than the manufacturing constraints imposed by methods such as machining or etching processes. This article presents the fundamentals of 2PP and the recent development of microneedle array fabrication through 2PP as a precise and unique method for the manufacture of microstructures, which may overcome the shortcomings of conventional manufacturing processes.

In vivo panoramic human skin shape and deformation measurement using mirror-assisted multi-view digital image correlation

Panoramic shape and deformation measurements of human skin in vivo may provide important information for biomechanical analysis, exercise guidance and medical diagnosis. This work proposes the application of an advanced mirror-assisted multi-view digital image correlation (DIC) method for dynamic measurements of 360-deg shape and deformation of human body parts in vivo. The main advantage of this method consists in its capabilities to perform full-panoramic non-contact measurements with a single pair of synchronized cameras and two planar mirrors thus representing a lean yet effective alternative to conventional multi-camera DIC systems in 'surrounding' configuration. We demonstrate the capabilities of this method by measuring the full-panoramic shape of a plastic human head, the deformation of a woman face and the principal strain distribution over the full-360-deg surface of a forearm during fist clenching. The applications of this method can be the most disparate but, given the possibility to determine the full-field strains and derived information (e.g. skin tension lines), we envisage a great potential for the study of skin biomechanics in vivo.

Directional dependent variation in mechanical properties of planar anisotropic porcine skin tissue

Nonlinear and anisotropic mechanical behavior of skin is essential in various applications such as dermatology, cosmetic products, forensic science, and computational studies. The present study quantifies the mechanical anisotropy of skin using the bulge method and full-field imaging technique. In bulging, the saline solution at 37 °C mimics the in vivo body temperature and fluid conditions, and all experiments were performed in the control environment. Assumption of thin spherical shell membrane theory and imaging techniques were implemented to obtain the anisotropic stress strain relations. Further, stress strain relations at an interval of 10° were calculated to obtain the variation in modulus with direction. Histological examinations were performed to signify the role of the collagen fibers orientation on the mechanical properties. The maximum and minimum linear modulus and collagen fiber orientation intensity were found in good agreement. The angular difference between maximum and minimum linear modulus and orientation intensity was found 71° ± 7° and 76° ± 5° respectively, and the percentage difference was 43.4 ± 8.2 and 52.5 ± 6.4 respectively. Further, a significant difference in the maximum and minimum collagen orientation intensity between the untested and tested specimens indicates the realignment of the fibers. Additionally, a cubic polynomial empirical relation was established to calculate the quantitative variation in the apparent modulus with the directions, which serves for the anisotropic modeling of the skin. The experimental technique used in this study can be applied for anisotropic quantification of planar soft tissues as well as can be utilized to imitate the tissue expansion procedure used in reconstructive surgery.

Linking microvascular collapse to tissue hypoxia in a multiscale model of pressure ulcer initiation

Pressure ulcers are devastating injuries that disproportionately affect the older adult population. The initiating factor of pressure ulcers is local ischemia, or lack of perfusion at the microvascular level, following tissue compression against bony prominences. In turn, lack of blood flow leads to a drop in oxygen concentration, i.e, hypoxia, that ultimately leads to cell death, tissue necrosis, and disruption of tissue continuity. Despite our qualitative understanding of the initiating mechanisms of pressure ulcers, we are lacking quantitative knowledge of the relationship between applied pressure, skin mechanical properties as well as structure, and tissue hypoxia. This gap in our understanding is, at least in part, due to the limitations of current imaging technologies that cannot simultaneously image the microvascular architecture, while quantifying tissue deformation. We overcome this limitation in our work by combining realistic microvascular geometries with appropriate mechanical constitutive models into a microscale finite element model of the skin. By solving boundary value problems on a representative volume element via the finite element method, we can predict blood volume fractions in response to physiological skin loading conditions (i.e., shear and compression). We then use blood volume fraction as a homogenized variable to couple tissue-level skin mechanics to an oxygen diffusion model. With our model, we find that moderate levels of pressure applied to the outer skin surface lead to oxygen concentration contours indicative of tissue hypoxia. For instance, we show that applying a pressure of 60 kPa at the skin surface leads to a decrease in oxygen partial pressure from a physiological value of 65 mmHg to a hypoxic level of 31 mmHg. Additionally, we explore the sensitivity of local oxygen concentration to skin thickness and tissue stiffness, two age-related skin parameters. We find that, for a given pressure, oxygen concentration decreases with decreasing skin thickness and skin stiffness. Future work will include rigorous calibration and validation of this model, which may render our work an important tool toward developing better prevention and treatment tools for pressure ulcers specifically targeted toward the older adult patient population.

Effects of foot position on skin structural deformation

As the largest and most superficial organ, the skin is well positioned for receiving sensory information from the environment. It is conceivable that changes in posture could result in deformations of the skin and subsequent changes in skin material properties. Specifically, the ankle and metatarsophalangeal joints have the capability to undergo large postural alterations with the potential to induce large structural deformations in the skin of the foot. The purpose of this study was to determine the extent to which alterations in foot posture may influence measures of foot sole and dorsum skin stretch, hardness, and thickness in vivo. Ten young and healthy individuals were tested while three static foot postures (plantar flexion, neutral and dorsiflexion) were maintained passively. Skin stretch deformation was quantified across each posture using an 11 × 4 point matrix of 3D kinematic markers affixed to the skin of the foot sole and dorsum. Skin hardness was assessed across each posture at specific locations of the foot sole (1st metatarsal, 5th metatarsal, medial arch, lateral arch and heel) and foot dorsum (proximal, middle and distal) using a handheld Shore durometer. Skin (epidermal + dermal) thickness was measured in each posture from the same test locations using ultrasound images obtained for the foot sole and dorsum. In the plantar flexion ankle posture, the foot sole skin was observed to relax/retract on average (± standard errorr of the mean (SEM) by 9 ± 2% to become both 20 ± 6% softer and 10 ± 6% thicker. In this posture, the foot dorsum skin stretched on average by 7 ± 2% resulting in 84 ± 8% harder and 5 ± 4% thinner skin. In the dorsiflexion ankle posture, the skin of the foot sole was observed to stretch on average by 5 ± 1% to become both 20 ± 8% harder and 4 ± 7% thinner. In this posture, the skin of the foot dorsum relaxed/retracted on average by 9 ± 1% resulting in the skin becoming 27 ± 12% softer and 7 ± 5% thicker. Notably, all of the sites responded with movement in a similar direction, but each site responded to a variable extent. Importantly, it was clear that the majority of skin structural deformation of the foot sole occurred within the 1st metatarsal, 5th metatarsal, and medial arch regions, while deformation was more evenly distributed across regions of the foot dorsum. The results suggest there is location specificity in the retraction and stretch characteristics of the foot skin. While not tested directly, this may suggest that local stretch distributions could be in part due to the underlying dermal and hypodermal structures in these foot regions. With these observed changes in the mechanical structure of the foot sole and dorsum skin tissue matrix, it is possible that corresponding posture-dependent changes in cutaneous mechanoreceptor activation may be present.

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.

Biomaterial stiffness determines stem cell fate

Stem cells have potential to develop into numerous cell types, thus they are good cell source for tissue engineering. As an external physical signal, material stiffness is capable of regulating stem cell fate. Biomaterial stiffness is an important parameter in tissue engineering. We summarize main measurements of material stiffness under different condition, then list and compare three main methods of controlling stiffness (material amount, crosslinking density and photopolymeriztion time) which interplay with one another and correlate with stiffness positively, and current advances in effects of biomaterial stiffness on stem cell fate. We discuss the unsolved problems and future directions of biomaterial stiffness in tissue engineering.

The hyperelastic and failure behaviors of skin in relation to the dynamic application of microscopic penetrators in a murine model

In-depth understanding of skin elastic and rupture behavior is fundamental to enable next-generation biomedical devices to directly access areas rich in cells and biomolecules. However, the paucity of skin mechanical characterization and lack of established fracture models limits their rational design. We present an experimental and numerical study of skin mechanics during dynamic interaction with individual and arrays of micro-penetrators. Initially, micro-indentation of individual skin strata revealed hyperelastic moduli were dramatically rate-dependent, enabling extrapolation of stiffness properties at high velocity regimes (>1ms^-1). A layered finite-element model satisfactorily predicted the penetration of micro-penetrators using characteristic fracture energies (∼10pJμm^-2) significantly lower than previously reported (≫100pJμm^-2). Interestingly, with our standard application conditions (∼2ms^-1, 35gpistonmass), ∼95% of the application kinetic energy was transferred to the backing support rather than the skin ∼5% (murine ear model). At higher velocities (∼10ms^-1) strain energy accumulated in the top skin layers, initiating fracture before stress waves transmitted deformation to the backing material, increasing energy transfer efficiency to 55%. Thus, the tools developed provide guidelines to rationally engineer skin penetrators to increase depth targeting consistency and payload delivery across patients whilst minimizing penetration energy to control skin inflammation, tolerability and acceptability.

STATEMENT OF SIGNIFICANCE

The mechanics of skin penetration by dynamically-applied microscopic tips is investigated using a combined experimental-computational approach. A FE model of skin is parameterized using indentation tests and a ductile-failure implementation validated against penetration assays. The simulations shed light on skin elastic and fracture properties, and elucidate the interaction with microprojection arrays for vaccine delivery allowing rational design of next-generation devices.

Multiscale Approach to Characterize Mechanical Properties of Tissue Engineered Skin

Tissue engineered skin usually consist of a multi-layered visco-elastic material composed of a fibrillar matrix and cells. The complete mechanical characterization of these tissues has not yet been accomplished. The purpose of this study was to develop a multiscale approach to perform this characterization in order to link the development process of a cultured skin to the mechanical properties. As a proof-of-concept, tissue engineered skin samples were characterized at different stages of manufacturing (acellular matrix, reconstructed dermis and reconstructed skin) for two different aging models (using cells from an 18- and a 61-year-old man). To assess structural variations, bi-photonic confocal microscopy was used. To characterize mechanical properties at a macroscopic scale, a light-load micro-mechanical device that performs indentation and relaxation tests was designed. Finally, images of the internal network of the samples under stretching were acquired by combining confocal microscopy with a tensile device. Mechanical properties at microscopic scale were assessed. Results revealed that adding cells during manufacturing induced structural changes, which provided higher elastic modulus and viscosity. Moreover, senescence models exhibited lower elastic modulus and viscosity. This multiscale approach was efficient to characterize and compare skin equivalent samples and permitted the first experimental assessment of the Poisson's ratio for such tissues.

3D nanomechanical evaluations of dermal structures in skin

Skin is a multilayered multiscale composite material with a range of mechanical and biochemical functions. The mechanical properties of dermis are important to understand in order to improve and compare on-going in vitro experiments to physiological conditions, especially as the mechanical properties of the dermis can play a crucial role in determining cell behaviour. Spatial and isotropy variations in dermal mechanics are thus critical in such understanding of complex skin structures. Atomic force microscopy (AFM) based indentation was used in this study to quantify the three dimensional mechanical properties of skin at nanoscale resolution over micrometre length scales. A range of preparation methods was examined and a mechanically non-evasive freeze sectioning followed by thawing method was found to be suitable for the AFM studies. Subsequent mechanical evaluations established macroscale isotropy of the dermis with the ground substance of the dermis dominating the mechanical response. Mechanical analysis was extended to show significant variation in the elastic modulus of the dermis between anatomical locations that suggest changes in the physiological environment influence local mechanical properties. Our results highlight dependence between an isotropic mechanical response of the dermal microenvironment at the nanoscale and anatomical location that define the variable mechanical behaviour of the dermis.

Computational modeling of skin: Using stress profiles as predictor for tissue necrosis in reconstructive surgery

Local skin flaps have revolutionized reconstructive surgery. Mechanical loading is critical for flap survival: Excessive tissue tension reduces blood supply and induces tissue necrosis. However, skin flaps have never been analyzed mechanically. Here we explore the stress profiles of two common flap designs, direct advancement flaps and double back-cut flaps. Our simulations predict a direct correlation between regions of maximum stress and tissue necrosis. This suggests that elevated stress could serve as predictor for flap failure. Our model is a promising step towards computer-guided reconstructive surgery with the goal to minimize stress, accelerate healing, minimize scarring, and optimize tissue use.

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.