Cormac Flynn

Mechanical characterisation of in vivo human skin using a 3D force-sensitive micro-robot and finite element analysis

The complex mechanical properties of skin have been the subject of much study in recent years. Several experimental methods developed to measure the mechanical properties of skin in vivo, such as suction or torsion, are unable to measure skin’s anisotropic characteristics. An experiment characterising the mechanical properties of in vivo human skin using a novel force-sensitive micro-robot is presented. The micro-robot applied in-plane deformations to the anterior forearm and the posterior upper arm. The behaviour of the skin in each area is highly nonlinear, anisotropic, and viscoelastic. The response of the upper arm skin is very dependent on the orientation of the arm. A finite element model consisting of an Ogden strain energy function and quasi-linear viscoelasticity was developed to simulate the experiments. An orthogonal initial stress field, representing the in vivo skin tension, was used as an additional model parameter. The model simulated the experiments accurately with an error-of-fit of 17.5% for the anterior lower forearm area, 6.5% for the anterior upper forearm and 9.3% for the posterior upper arm. The maximum in vivo tension in each area determined by the model was 6.2 Nm−1 in the anterior lower forearm, 11.4 Nm−1 in anterior upper forearm and 5.6 Nm−1 in the posterior upper arm. The results also show that a finite element model with a neo-Hookean strain energy function cannot simulate the experiments with the same accuracy.

Simulating the wrinkling and aging of skin with a multi-layer finite element model.

One of the outward signs of the aging process of human skin is the increased appearance of wrinkles on its surface. Clinical studies show that the increased frequency of wrinkles with age may be attributed to changes in the composition of the various layers of skin, leading to a change in mechanical properties. A parameter study was performed on a previously proposed multi-layer finite element model of skin. A region of skin was subject to an in-plane compression, resulting in wrinkling. A number of physical properties of the skin model were changed and the effects these changes had on the size of the subsequent wrinkles were measured. Reducing the moisture content of the stratum corneum by 11% produces wrinkles 25-85% larger. Increasing the dermal collagen fibre density by 67%, results in wrinkles, which are 25-50% larger. A reduction and change in the pre-stress distribution in the skin model, which represents the natural tension and relaxed skin tension lines in real skin, also influences the wrinkle height in a similar manner to real aging skin. Typically, there can be up to a 100% increase in the height of wrinkles as skin ages. This model would be of benefit in the development of cosmetic moisturisers and plastic-surgery techniques to reduce the appearance of aging.

Finite element modelling of forearm skin wrinkling.

Background/purpose: Human skin is a complex multilayered material. Although there are many numerical models of skin in existence, which accurately simulate several of its complex mechanical characteristics, there are very few models that simulate wrinkling – a phenomenon common to all human skin. The purpose of this study was to develop a multilayer model of skin, which could simulate wrinkling more realistically than the existing models in the literature.

Measurement of the force–displacement response of in vivo human skin under a rich set of deformations

The non-linear, anisotropic, and viscoelastic properties of human skin vary according to location on the body, age, and individual. The measurement of skins mechanical properties is important in several fields including medicine, cosmetics, and forensics. In this study, a novel force-sensitive micro-robot applied a rich set of three-dimensional deformations to the skin surface of different areas of the arms of 20 volunteers. The force-displacement response of each area in different directions was measured. All tested areas exhibited a non-linear, viscoelastic, and anisotropic force-displacement response. There was a wide quantitative variation in the stiffness of the response. For the right anterior forearm, the ratio of the maximum probe reaction force to maximum probe displacement ranged from 0.44 N mm(-1) to 1.45 N mm(-1). All volunteers exhibited similar qualitative anisotropic characteristics. For the anterior right forearm, the stiffest force-displacement response was when the probe displaced along the longitudinal axis of the forearm. The response of the anterior left forearm was stiffest in a direction 20° to the longitudinal axis of the forearm. The posterior upper arm was stiffest in a direction 90° to the longitudinal axis of the arm. The averaged posterior upper arm response was less stiff than the averaged anterior forearm response. The maximum probe force at 1.3mm probe displacement was 0.69N for the posterior upper arm and 1.1N for the right anterior forearm. The average energy loss during the loading-unloading cycle ranged from 11.9% to 34.2%. This data will be very useful for studying the non-linear, anisotropic, and viscoelastic behaviour of skin and also for generating material parameters for appropriate constitutive models.

A three-layer model of skin and its application in simulating wrinkling

Human skin is a complex multi-layer material. Many existing numerical skin models accurately simulate several of its complex mechanical characteristics. However, few models simulate wrinkling – a phenomenon common to all human skin. In this study, a multi-layer model of skin was developed to simulate wrinkling. The model consisted of the stratum corneum, dermis and underlying hypodermis. The results of the simulations were compared with results of in vivo wrinkling experiments performed on the volar forearm. The proposed three-layer skin model simulates wrinkling more realistically than models of fewer layers. The size of the wrinkles predicted by the model fell within the range of the wrinkle sizes measured in the experiments. The maximum range and average roughness differed by 34 and 43% from the corresponding mean experimental results, respectively. Applications of the model include simulating skin aging, designing more realistic artificial skin and the development of surgical simulators.

A simplified model of scar contraction

The healing of wounds is a complex process and the contraction of the resulting scar can have a negative impact on the neighbouring skin. A finite element model of skin simulating the contraction of a scar and deformation of the surrounding skin is presented. The skin is represented by an orthotropic-viscoelastic constitutive law, which is validated against experimental data in the literature. A simplified experimental model of a contracting scar in real skin is also developed. The pattern and size of the wrinkles formed around the contracting scar in the finite element model compare favourably with those formed in the experimental model. The orthotropic nature of skin plays a significant role in the behaviour of skin around scars -- the wrinkles have a preferential orientation that corresponds to a direction perpendicular to the Langers lines in the skin. The pre-stress in skin (a property that is ignored in many skin models) is shown to be an important factor in wrinkle formation around scars. The proposed model can be used to analyse the suturing and closure of wounds of various shapes.

Finite element models of wound closure

AIM The achievement of a well-healed wound depends on many factors including its size and location on the body and the properties of the skin. The aim of this study is to develop computational wound closure models and compare the results of using different excision shapes. METHODS Finite element models were developed that simulated the incision, excision and closure of skin. Skin was represented by an orthotropic constitutive law. The size of extrusions, maximum stresses and the force to close wounds with differently shaped excisions were analysed. RESULTS Circular excisions resulted in closed wounds with extrusion heights 76% larger than fusiform or lazy S-plasty excisions. The extrusion length for circular excisions was 50% longer than the lazy S-plasty extrusion length. The maximum stresses around closed wounds with elliptical excisions were between 30 and 40% lower than the maximum stresses around fusiform and lazy S-plasty closed wounds. The force required to close an elliptical wound was between 27 and 66% lower than the closure force of fusiform and lazy S-plasty excisions. The orthotropic nature of skin and the orientation of the excision significantly influence the behaviour of the skin around the closed wound. The in vivo pre-stress, often ignored in wound closure models, influences the size of extrusions. Increasing the pre-stress by a factor of twenty decreased extrusion heights by 40%. A similar change in pre-stress decreased extrusion lengths by 50%. CONCLUSION These models have potential as valuable clinical tools to determine the optimum excision shape that will minimise adverse stress fields and reduce scarring. Models that are patient-specific would be useful to design strategies to ensure favourable healing and improve the quality of life of the person.

Coupled Biomechanical Modeling of the Face, Jaw, Skull, Tongue, and Hyoid Bone

The tissue scale is an important spatial scale for modeling the human body. Tissue-scale biomechanical simulations can be used to estimate the internal muscle stresses and bone strains during human movement, as well as the distribution of force in muscles with complex internal architecture and broad insertion areas. Tissue-scale simulations are of particular interest for muscle structures where the changes in the shape of the structure are functionally important, such as the face, tongue, and vocal tract. Biomechanical modeling of these structures has potential to improve our understanding of orofacial physiology in respiration, mastication, deglutition, and speech production. Biomechanical simulations of the face and vocal tract pose a challenging engineering problem due to the tight coupling of tissue dynamics between numerous structures: the face, lips, jaw, skull, tongue, hyoid bone, soft palate, pharynx, and larynx. In this chapter, we describe our efforts to develop novel tissue-scale modeling and simulation techniques targeted to orofacial anatomy. We will also review our efforts to apply such simulations to reveal the biomechanics underlying orofacial movements.

Characterizing skin using a three-axis parallel drive force-sensitive micro-robot

There is a strong need to measure the complex mechanical properties of soft tissues such as skin. An in vivo experiment characterizing the mechanical response of human skin is presented. A rich set of deformations were applied to several positions on the arm using a novel force-sensitive micro-robot. All sites studied exhibit highly non-linear, anisotropic, and viscoelastic behavior. The experiments determined directions in which the skin response was stiffest. These directions agree with accepted orientations of Langer or relaxed skin tension lines.

Comparison of anisotropic models to simulate the mechanical response of facial skin

Physically-realistic models of the face can contribute to development in several fields including biomedicine, computer animation, and forensics. Face models have benefited from better anatomical representation of the mimetic muscles, and more realistic interactions between soft and bony tissues. These models can also benefit from improved characterisation of the skin layer by having more authentic deformation and wrinkling behaviour. The objective of this work is to compare and evaluate the ability of different constitutive models to simulate the mechanical response of facial skin subjected to a rich set of deformations using a probe. We developed a finite element model to simulate facial skin experiments. Several anisotropic constitutive equations were tested for their suitability to represent facial skin. The finite element model simulated the force-displacement response of facial skin under a rich set of deformations. The variance accounted for between the experimental data and model data ranged from 79% for the Gasser et al. (2006) model to 96% for the Bischoff et al. (2002) model. Estimated pre-stresses ranged from 7 kPa in the lip region to 53 kPa in the central cheek region.