Experimental method for determining the 2-dimensional mechanical properties of living human skin

Coupling tensile test with LC-OCT and ultrasound imaging: investigation of the skin sublayers mechanical behaviour

The skin is an envelope that covers the entire body. Nowadays, understanding and studying the mechanical, biological and sensory properties of the skin is essential, especially in dermatology and cosmetology. The in-depth study of the skin's mechanical behaviour is a highly intriguing challenge, enabling the differentiation of the behaviour of each layer. An extension device was developed to perform relaxation and extension tests to characterize the skin. The device has also been coupled with imaging tools (LC-OCT and ultrasound), allowing us to observe layer-by-layer deformations during the tests. Relaxation tests revealed significant skin anisotropy, as well as an influence of age and gender on skin viscoelastic parameters calculated from relaxation curves and a skin viscoelastic model. These tests also unveiled their ability to distinguish certain characteristic pathologies that alter the mechanical properties of the skin, such as scleroderma or heliodermatitis. Furthermore, the optical-mechanical coupling and deformation calculation through image analysis demonstrated that the skin layers exhibit distinct mechanical behaviours owing to their different structures. Finally, Poisson's ratio of the skin was obtained by calculating the deformation in two directions for each layer.

Engineering Bioactive Scaffolds for Skin Regeneration

Large skin wounds pose a major clinical challenge. Scarcity of donor site and postsurgical scarring contribute to the incomplete or partial loss of function and aesthetic concerns in skin wound patients. Currently, a wide variety of skin grafts are being applied in clinical settings. Scaffolds are used to overcome the issues related to the misaligned architecture of the repaired skin tissues. The current review summarizes the contribution of biomaterials to wound healing and skin regeneration and addresses the existing limitations in skin grafting. Then, the clinically approved biologic and synthetic skin substitutes are extensively reviewed. Next, the techniques for modification of skin grafts aiming for enhanced tissue regeneration are outlined, and a summary of different growth factor delivery systems using biomaterials is presented. Considering the significant progress in biomaterial science and manufacturing technologies, the idea of biomaterial-based skin grafts with the ability for scarless wound healing and reconstructing full skin organ is more achievable than ever.

In Vitro Measurements of Cellular Forces and their Importance in the Lung-From the Sub- to the Multicellular Scale

Throughout life, the body is subjected to various mechanical forces on the organ, tissue, and cellular level. Mechanical stimuli are essential for organ development and function. One organ whose function depends on the tightly connected interplay between mechanical cell properties, biochemical signaling, and external forces is the lung. However, altered mechanical properties or excessive mechanical forces can also drive the onset and progression of severe pulmonary diseases. Characterizing the mechanical properties and forces that affect cell and tissue function is therefore necessary for understanding physiological and pathophysiological mechanisms. In recent years, multiple methods have been developed for cellular force measurements at multiple length scales, from subcellular forces to measuring the collective behavior of heterogeneous cellular networks. In this short review, we give a brief overview of the mechanical forces at play on the cellular level in the lung. We then focus on the technological aspects of measuring cellular forces at many length scales. We describe tools with a subcellular resolution and elaborate measurement techniques for collective multicellular units. Many of the technologies described are by no means restricted to lung research and have already been applied successfully to cells from various other tissues. However, integrating the knowledge gained from these multi-scale measurements in a unifying framework is still a major future challenge.

Biomechanical Modeling of Human Skin Tissue Surrogates

Surrogates, which precisely simulate nonlinear mechanical properties of the human skin at different body sites, would be indispensable for biomechanical testing applications, such as estimating the accurate load response of skin implants and prosthetics to study the biomechanics of static and dynamic loading conditions on the skin, dermatological and sports injuries, and estimating the dynamic load response of lethal and nonlethal ballistics. To date, human skin surrogates have been developed mainly with materials, such as gelatin and polydimethylsiloxane (PDMS), based on assumption of simplified mechanical properties, such as an average elastic modulus (estimated through indentation tests), and Poisson’s ratio. In addition, pigskin and cowhides, which have widely varying mechanical properties, have been used to simulate human skin. In the current work, a novel elastomer-based material system is developed, which precisely mimics the nonlinear stress–stretch behavior, elastic modulus at high and low strains, and fracture strengths of the natural human skin at different body sites. The manufacturing and fabrication process of these skin surrogates are discussed, and mechanical testing results are presented.

Experimental study on tissue phantoms to understand the effect of injury and suturing on human skin mechanical properties

Skin injuries are the most common type of injuries occurring in day-to-day life. A skin injury usually manifests itself in the form of a wound or a cut. While a shallow wound may heal by itself within a short time, deep wounds require surgical interventions such as suturing for timely healing. To date, suturing practices are based on a surgeon's experience and may vary widely from one situation to another. Understanding the mechanics of wound closure and suturing of the skin is crucial to improve clinical suturing practices and also to plan automated robotic surgeries. In the literature, phenomenological two-dimensional computational skin models have been developed to study the mechanics of wound closure. Additionally, the effect of skin pre-stress (due to the natural tension of the skin) on wound closure mechanics has been studied. However, in most of these analyses, idealistic two-dimensional skin geometries, materials and loads have been assumed, which are far from reality, and would clearly generate inaccurate quantitative results. In this work, for the first time, a biofidelic human skin tissue phantom was developed using a two-part silicone material. A wound was created on the phantom material and sutures were placed to close the wound. Uniaxial mechanical tests were carried out on the phantom specimens to study the effect of varying wound size, quantity, suture and pre-stress on the mechanical behavior of human skin. Also, the average mechanical behavior of the human skin surrogate was characterized using hyperelastic material models, in the presence of a wound and sutures. To date, such a robust experimental study on the effect of injury and sutures on human skin mechanics has not been attempted. The results of this novel investigation will provide important guidelines for surgical planning and validation of results from computational models in the future.

A structural model of the forced compression of the fingertip pulp

The fingertip pulp modulates the force transmitted to the underlying musculoskeletal system during finger contact on external bodies. A model of the fingertip pulp is needed to represent the transmission of forces to the tendons, muscles, and bone during these contacts. In this study, a structural model of the in vivo human fingertip was developed that incorporates both the material inhomogeneity and geometry. Study objectives were to determine (1) if this fingertip model can predict the force-displacement and force contact area responses of the in vivo human fingertip during contact with a flat, rigid surface, and (2) if the stresses and strains predicted by this model are consistent with the tactile sensing functionality of the in vivo human fingertip. The in vivo fingertip pulp was modeled as an inflated, ellipsoidal membrane, containing an incompressible fluid, that is quasi-statically compressed against a flat, frictionless surface. The membrane was assigned properties of skin (Veronda and Westmann, 1970) and when inflated, possessed dimensions approximating those of a human fingertip. Finite deformation was allowed. The model was validated by the pulp force-displacement relationship obtained by Serina et al. (1997) and by measurements of the contact area when the fingertip was pressed against a rigid surface with contact forces between 0.25 and 7.0 N. Model predictions represent the experimental data sufficiently well, suggesting that geometry, inhomogeneous material structure, and initial skin tension appear to represent the nonlinear response of the in vivo human fingertip pulp under compression. The predicted response of the fingertip pulp is consistent with its functionality as a tactile sensor.

An in vivo method for measuring the mechanical properties of the skin using ultrasound

In this study, we report a new and original device called the "echorheometer," comprising a suction system with an ultrasound scanner (A-mode, TM-mode and B-mode) that enables the simultaneous visualization and measurement of the deformation of skin structures in vivo. With the scanner described here, high resolution is obtained using a strongly focused, wide-band 20-MHz center frequency transducer, with an axial resolution of 0.07 mm. This device can determine, noninvasively, not only those skin structures that are involved in the deformation, but also their morphological variation and their extent of involvement with the degree of stress applied. Using this device, the behavior of the dermis and subcutaneous fat, while under suction, was investigated on the volar forearm of 10 volunteers. The results showed that the resistance to the applied vertical stress is essentially due to the dermis rather than the subcutaneous fat, and that there is a certain amount of infiltration of fluid into the tissues under suction. In addition, it was shown that the dermal response to an applied suction is initially due to its own natural tension and that, with increasing deformation, the intrinsic dermal elasticity has a greater contribution to the resistance of stress. With this information, we hope to develop a mechanical model to define appropriate mechanical parameters for skin. This will allow the evaluation of changes in the dermis and also enable therapeutic intervention to be assessed. Furthermore, it could also be applied to studies of skin ageing and the assessment of cosmetic product efficacy (emolliency, hydratation, etc.).

Conformability and irritancy of adhesive tapes on the skin

The relationship between the conformability of adhesive tape, the mechanical property of the adhesive tape not to interfere with skin movement, and irritation of the skin was investigated. In order to assess the conformability of adhesive tape to the skin, a uniaxial method employing film strips connected to a strain gauge was used to measure the elastic property of the skin, with or without application of various elastic tapes. The tension loaded on the strain gauge was measured while the skin was extended by 15% of its original length in a direction across the humeral axis on the flexor side of the upper right arm. The most elastic adhesive tape showed the best conformity to the skin. The same adhesive tapes were applied on the flexor side of both upper arms so that the tape held a piece of sanitary cotton in place for 24 h. Dermal irritation was not so remarkable in the skin under the inner part of the tapes. On the other hand, the skin reaction was much more severe on the skin under the edge portion of the applied tape which showed poor conformability to the skin. These findings seemed to indicate that the skin reaction was caused by localized distortion of the skin under the edge portion of the applied tapes, during movement of the underlying muscle. Actually, the distortion of the skin surface was great in the areas immediately outside the edge of the applied tape. In conclusion, adhesive tape conformable to skin movement reduced localized distortion of the skin during application, resulting in low irritation at the edge of the applied tape.

The pipette aspiration applied to the local stiffness measurement of soft tissues

A simple method of identifying the initial slope of the stress-strain curve (i.e., Young's modulus of the soft tissue) by introducing the pipette aspiration technique is presented. The tissue was assumed to be isotropic and macroscopically homogeneous. Numerical simulations by the linear finite element analysis were performed for the axisymmetric model to survey the effects of friction at the tissue-pipette contact boundary, pipette cross-sectional geometry, relative size of the specimen to the pipette, and the layered inhomogeneity of the specimen tissue. The friction at the contact region had little effect on the measurement of Young's modulus. The configuration of the pipette was shown to affect the measurement for small pipette wall thickness. The measurement also depended on the relative size of the specimen to the pipette for relatively small specimens. The extent of the region contributing to the measurement was roughly twice the inside radius of the pipette. In this region, the maximum stress did not exceed the level of the aspiration pressure, with only minor exceptional locations. Calculation of strain energy components indicated that the major contributions to the deformation under pipette aspiration were by the normal extension and shear deformation in pipette axial direction. Experimental verification of the present method for the isotropic, homogeneous artificial material is also presented.

A method for determination of the anisotropic properties of biomembranes

A methodology was developed for potential determination of the anisotropic properties of biomembranes. This method is based on the theoretical discretization of a continuous membrane used for finite element analysis and the simultaneous measurement of the displacement of nodes on the surface of a membrane. From the given loads and measured nodal displacements, one can assemble the resulting stiffness matrix and approximate the material properties associated with the membrane. Mathematical estimations and computer simulations were performed to determine the perturbation of load and displacement errors on the resulting material properties. The results indicated that the material properties are particularly sensitive to displacement errors. The displacement measurements may require an accuracy of 20 microns for a 4 x 4 cm2 specimen. Significant inaccuracies occur close to the points of load application.

A method for the evaluation of tensile properties of skin equivalents

In vitro production of anchored skin equivalent is a new therapeutical option for burn patients. A skin equivalent is a combined culture of dermal and epidermal layers. The dermal layer provides important mechanical properties, such as tensile resistance and nonlinear elasticity, to the skin equivalent during its development. Prior to any in vivo human transplantation, the tensile properties of cutaneous equivalents have to be evaluated as a function of its structural components, in view of establishing the culture conditions leading to the best mechanical resistance and stretchability characteristics. However, the handling and clamping of skin equivalents are frequent causes of tearing and lack of repeatability in the measuring of tensile properties. A new indentation method involving a specially designed culture dish has been developed to minimize the risk of damage. Using this new culture dish, cutaneous equivalents were installed on an indentation apparatus. The central loading of a spherical tip was transmitted to the central area of a circular anchored cutaneous equivalent and was recorded with tip position. The tests were achieved at a constant low deflection rate of the tip. This new and accurate method gave repeatability in three central load-deflection characteristics of anchored dermal equivalent: the high-modulus (0.15 g mm-1), the central load of rupture (1.49 g), the rupture deflection (0.470 mm). This indentation test is expected to be an efficient tool in the evaluation of various skin equivalent models tensile properties.

The measurement of photodamage

The use of non-invasive and invasive techniques for the assessment of human photodamaged skin is reviewed. Physical changes during photodamage and its treatment are best scored using a visual analogue scale rather than a short, non-equal interval scale. Epidermal thickness can be measured by histometric methods but dermal thickness can be measured non-invasively using pulsed A-scan and B-scan ultrasound techniques. These approaches are not effective in detecting any changes due to photodamage. Mechanical properties of the dermis can be determined using either a static or a dynamic test mode. The authors have used extensometry to provide a measure of the laxity of skin. Replicas of the crow's foot areas have been taken before and after tretinoin treatment, and the replicas have been inspected by optical profilometry. Reductions of blood flow in photodamaged skin have been established using laser Doppler measurements, the effect being reversed by topical tretinoin. Invasive biochemical techniques have the disadvantage that they generally require large amounts of tissue. Cytochemical techniques, however, have shown increased glucose-6-phosphate dehydrogenase activity in the granular cell layer of patients with non-melanoma skin cancer, premalignant epidermal lesions, sun-damaged epidermis and artificially irradiated skin. This technique may provide an important model for the study of photodamage. It is concluded that there is no single method available to quantify the degenerative changes associated with photodamage and the effects of tretinoin.

An improved method of in vivo wound disruption and measurement

Biomechanical studies of wound strength are important because of new investigations in growth factors, cytokines, and fetal wounds. We compared two traditional methods of wound disruption measurement with a novel computerized model designed for in vivo experiments. An Instron tensiometer (INSTS) and an air insufflated positive pressure device (AIPPD) were compared with a vacuum-controlled wound chamber device (VCWCD). The VCWCD produced vacuum at the wound site and wound disruption was monitored with two video camera/recorders. Rats were marked with a template guide for a 2.5 cm, full-thickness, abdominal incisional wound. Rats were divided into three groups and studied at 2, 7, or 14 days after wounding. The recorded images were computer digitalized to generate wound strength curves from a three-dimensional model. A comparison of the wound disruption curves demonstrated that the VCWCD was comparable to the INST or AIPPD in normal wound healing (P greater than .40). The VCWCD provided data with less standard error at 2 days after wounding (P less than 05). In separate series of experiments, VCWCD was tested in the early phases of healing and was found to be sensitive to change at intervals of 48 hr after wounding (P less than .005). The INST or AIPPD methods could not perform this task because of an unacceptable level of random error after tissue manipulation. The VCWCD system was considered superior for evaluating early wound healing because it was an in vivo method which required minimal wound manipulation.

Surface deflection of primate fingertip under line load

A study of the biomechanics of the skin and the subcutaneous soft tissues is of fundamental importance in understanding the process of transduction at the mechanoreceptive nerve terminals responsible for the sense of touch. In the present investigation, the fingertips (distal phalanges) of three adult humans and four monkeys were indented in vivo using a line load delivered by a sharp wedge. The resulting skin surface deflection profile was photographed and used as a clue to infer the mechanical nature of the materials that make up the fingertip. It is shown that the modified Boussinesq solution used by Phillips and Johnson (1981), applicable when the fingertip is modeled as an elastic half-space in a state of plane strain, predicts a skin surface deflection profile that can only roughly approximate the empirically observed profiles. As an alternative, a simple model which views the fingertip as an elastic membrane filled with an incompressible fluid (like a 'waterbed') under plane strain conditions is proposed. It is shown that the predictions of this model, which takes into account the finite deformations that occur, agree very well with the photographed profiles in the region of interest (up to about 3 mm from the load).

Biomechanical properties of dermis

Data obtained from a wide variety of test methods both in vitro and in vivo and histological studies of stressed skin have led to an understanding of the mechanical properties of the dermis and the relation of these properties to the structure of the collagen and elastin fiber networks of the dermis. The mechanical properties are found to be well adapted to the mechanical function of the dermis. The viscoelastic nature of the skin shows that a simple structural model based only on collagen and elastin is not adequate for a full understanding of this tissue. The role of proteoglycans in bonding collagen fibrils into large fibers and in connecting these fibers into the fibrous net of the dermis is not well understood. Mechanical testing can yield information on the nature of bonds at this level of structure.