Characterization of human skin through skin expansion

Abstract

This study characterized human skin of the lower leg and scalp during the surgical process of skin expansion. To our knowledge, this is the first study in this field, which has provided results that considerably improve our understanding of human skin. A detailed in vivo analysis was carried out involving four different patients that allowed for observation during the relaxation process. A comparison between the in vivo and numerical finite elements model of the expansion was used to identify the material elastic parameters of the skin. After a comprehensive search of constitutive equations for describing skin, Delfino's constitutive equation was chosen to model the in vivo results. We considered skin as an isotropic, homogeneous, hyperelastic, and incompressible membrane. The parameters of Delfino's exponential function obtained for the first skin stretch process were a = 40.0 KPa and b = 20.2. As skin is extended, such as with expanders or in other procedures that tighten the skin, the collagen fibers are also extended and cause stiffening in the skin, which results in it being more and more resistant to expansion or further stretching. We observed this phenomenon as an increase in parameters a and b as subsequent expansions continued. The results of this study allow for the quantification of stiffening of the skin after several stretches, when the skin becomes more and more inelastic. These results are very encouraging and provide insight into our understanding of the behavior of stretched skin and maybe other biological tissues, as swollen artery and veins.A list of symbols can be found on page 653.

Growth on demand: Reviewing the mechanobiology of stretched skin

“…The characteristic features predicted by our model are in excellent agreement with the observed phenomena during tissue expansion in animals [9, 10] and in humans [24, 49]. In the tissue expansion literature, our decomposition of the deformation gradient into reversible and irreversible parts F = F e · F g is referred to as mechanical creep and biological creep [29] or, more illustratively, as loan and dividend [7].…”

“…redWhile our first prototype finite element models for growth were fully three dimensional and implicit [11, 72], this manuscript documents our first implementation of the model into a commercially available finite element package. This allows us to use the entire infrastructure of a multi-purpose finite element program, including features such as the fluid filled cavity to model the expander [49], and frictionless sliding to model the expander-skin interface [1]. Our first prototype simulations did not discretize the expander explicitly, but were rather entirely pressure driven [12, 73].…”

Multi-view stereo analysis reveals anisotropy of prestrain, deformation, and growth in living skin

“…Towards these efforts, we are currently designing instrumented expanders to measure the expander pressure before and after expansion [41]. This will eventually allow us to characterize the stiffness of living skin stiffness using an inverse finite element approach [40]. The shape of the expander could also bias the resulting deformation contours [6].…”

“…Computational models are being steadily integrated into medical applications and hold the promise of enhancing medical device design and improving treatments efficacy in reconstructive surgery [8]. Measuring and predicting prestrain, deformation, and growth during a routine skin expansion procedure is practically impossible and the usefulness of a computational approach is unquestionable [40]. However, despite the increasing acceptance of computational tools, their integration into clinically relevant scenarios is slow due to the lack of experimental data to calibrate and validate the models.…”

The Incompatibility of Living Systems: Characterizing Growth-Induced Incompatibilities in Expanded Skin

“…However, it is also of interest to determine the acute deformation. 48,49 It seems reasonable to assume that the acute deformation due to a single inflation step was mainly elastic. To calculate the acute deformation at the inflation step i we took the patch right before the inflation as the reference configuration and the patch immediately after the inflation as the deformed configuration.…”