Characterization of human female breast and abdominal skin elasticity using a bulge test.
Introduction
Understanding the mechanical behavior of skin is important in many applications, such as cosmetic and reconstructive surgery, healing following surgical operations, skin grafting, and the treatment of skin-based diseases (Boissieux et al., 2000; Evans et al., 2013; Sun et al., 2014). Our particular interest is in breast reconstruction surgery of female breast following mastectomy. An ongoing objective of many research groups is to develop a biomechanical breast model for different applications, ranging from surgical outcome predictions for patients undergoing breast reconstruction surgery to image registration for planning plastic surgery, biopsy and other image guided interventions (Conley et al., 2015; del Palomar et al., 2008; Dmitriev et al., 2013; Eder et al., 2014; Garbey et al., 2013; Haddad et al., 2016; Han et al., 2014; Rajagopal et al., 2008; Roose et al., 2008; Vavourakis et al., 2016). Achieving the goal of developing a physics-based biomechanical model of the human breast requires the determination of material properties of the various tissues constituting the breast. In a recent study (Khatam et al., 2015), our group has established the importance of mechanical properties of skin in such breast simulations due to the large stretch experienced by the breast skin during position change from supine to upright. Breast skin, if included in existing breast deformation models, is mostly characterized using parameters fitted from simple uniaxial experiments that were mostly performed on cadaver or animal skin. However, results that are available in the literature are not adequate for providing calibration of accurate constitutive models of living human female breast skin behavior.
Mechanical properties of skin have been studied by a number of authors [see for example, porcine: (Khatam and Ravi-Chandar, 2013; Shergold et al., 2006); murine: (Muñoz et al., 2008); human: (Agache et al., 1980; Bischoff et al., 2000; Clark et al., 1996; Dunn and Silver, 1983; Hendriks et al., 2003; Kumaraswamy et al., 2017; Kvistedal and Nielsen, 2009; Reihsner and Menzel, 1996; Silver et al., 2001; Tonge et al., 2013a, 2013c; Wan Abas and Barbenel, 1982; Ní Annaidh et al., 2012b, 2012a); and rabbit (Lanir and Fung, 1974a, 1974b)]. These studies were motivated by different objectives such as understanding the mechanics of stabbing (McCarthy et al., 2010, 2007), planning plastic surgery, dermatology, impact biomechanics (Gallagher et al., 2012; Ní Annaidh et al., 2012a), and designing devices that interact with the skin, such as shaving blades (Evans et al., 2013). Investigators have used in vitro (Tonge et al., 2013a, 2013c) and in vivo (Kvistedal and Nielsen, 2009) studies to understand the mechanical behavior of skin under large deformation. However, we are not aware of studies in the literature of skin mechanics that characterize the mechanical properties of freshly harvested human breast skin or the particular regions that are used as donor sites in breast reconstruction surgery such as the Transverse Rectus Abdominis Myocutaneous flap (TRAM). Rather, breast biomechanical models (Azar et al., 2000; Pathmanathan et al., 2004; Vavourakis et al., 2016) typically use material properties obtained from rabbit skin, cat skin, or skin from the back of human cadavers of both genders. However, skin properties obtained from cadavers, especially from other parts of the human body, or from animals may not be representative of the mechanical behavior of living human breast skin (Tonge et al., 2013a). Skin mechanical properties are known to change from their in vivo to in vitro configuration. It is also known from the literature that skin mechanical properties depend on age, race, and the body site from which it is harvested (Escoffier et al., 1989). Therefore, it is imperative to characterize the mechanical properties of relatively fresh female skin specimens from the breast and the TRAM donor site (hereafter referred as ‘abdominal skin’).
In this study, we examine the mechanical response of freshly harvested human skin with a focus on the anisotropic response. The main objective is the collection of a large data set that will enable calibration of constitutive material models suitable for capturing the response of the female breast and abdominal skin. The biaxial test has been recognized for a long time as an important test in characterizing membrane-like tissues. Lanir and Fung (1974a) designed a planar biaxial stretching system that consisted of sutures along the edges of a square specimen for generating arbitrary combinations of biaxiality ratios. Bulge or blister tests have also been used extensively in material characterization; while this is more commonly used in the characterization of metals, and adhesives or coatings, it has also been used to test biological materials as well (Baird et al., 1977; Marra et al., 2006; Mohan and Melvin, 1983; Zioupos et al., 1992). A good overview of the bulge testing is given by Machado et al. (2012). More recently, Tonge et al. (Tonge et al., 2013a, 2013c) presented an evaluation of cadaveric human back skin using a bulge test. The present work utilizes experimental methodology similar to that developed by Machado et al. (2012) and Tonge et al. (2013a); however, for interpretation of the data, we rely on a fully numerical simulation methodology due to the complex nature of the material models to be used.
The structure of the constituents of skin is important for determining its response to mechanical stress. A succinct summary of skin composition is provided by Sanders et al. (1995): skin is composed of collagen (27–39% by volume, 75–80% of fat-free dry weight), elastin (0.2–0.6% by volume, 4% of fat-free dry weight), glycosaminoglycans (0.03–0.35% by volume), and water (60–72% by volume). Elastin fibers form a network and provide the ability to recoil; this network is embedded in the network of crimped collagen fibers that are themselves cross-linked. While early research suggested that the collagen fibers are initially randomly oriented (see Fig. 5 of Dunn et al. (1985)), more recent work has provided measurements that indicate a systematic orientation distribution (Bancelin et al., 2015; Ní Annaidh et al., 2012b). Nevertheless, a sharp increase in stiffness with deformation is generated as the average stretch increases beyond some threshold, primarily due to uncrimping and reorientation of the collagen fibers with deformation. While the initial low stiffness response of the skin is mainly governed by the non-collagenous matrix and the elastin network, the mechanical stiffening behavior is primarily attributed to the dermal layer. The remaining constituents, water and the glycosaminoglycans, provide viscous properties to skin. This composite structure of skin results in nonlinear, anisotropic, time-dependent mechanical behavior that can include elastic response, viscoelasticity, and damage (Bischoff et al., 2000; Dunn et al., 1985; Muñoz et al., 2008; Sanders et al., 1995; Silver et al., 2001).
Various phenomenological, structural and structurally based constitutive material models have been adopted or specifically developed to model the mechanical behavior of skin observed in in vivo and in ex vivo experiments. Several of these models treat the skin as an isotropic elastic material with a nonlinear response. Although, some of these models were capable of predicting the stiffening effect observed in experiments, they fail to represent the anisotropic behavior widely observed in experiments (Evans and Holt, 2009; Flynn et al., 2011b; Hendriks et al., 2006; Kumaraswamy et al., 2017). In previous work, our group examined the mechanical properties of female breast skin, but under uniaxial loading conditions (Kumaraswamy et al., 2017). Two different constitutive models – one phenomenological (Hart-Smith, 1966) and another microstructurally inspired (Rausch and Humphrey, 2015) – were used to interpret the mechanical response observed in these experiments. Remarkably, it was found that the model parameters that characterize dependence on previous maximum stretch (or preconditioning) exhibited specimen-independent universal behavior. In the last few decades, there have been significant efforts to develop constitutive models to measure the anisotropic behavior exhibited by skin and other collagenous tissues (Bischoff et al., 2002; Flynn et al., 2011a; Gasser et al., 2006; Lanir, 1983, 1979; Limbert, 2011). In particular, Lanir (Lanir, 1983, 1979) proposed a structural model that treats skin as a composite material composed of a non-collagenous matrix and bundles of collagen fibers distributed within the matrix according to some chosen orientation dependence. Gasser et al. (2006) developed a generalized structure tensor model to capture the orthotropic hyperplastic behavior of arterials tissues that has also been adopted for skin. These models attempt to capture the uncrimping of the collagen fibers using an exponential law for the strain energy density function.
There are numerous studies of the effect of freezing on the mechanical properties of different types of tissues; however, a clear picture has yet to emerge. The works of Rosset et al. (1996) who examined the common carotid artery and the superficial femoral artery and Pukacki et al. (2000) who considered iliofemoral arteries and veins and Venkatasubramanian et al. (2006) who tested porcine femoral arteries provide contradicting conclusions on the effect of freezing. Specifically with respect to skin, Foultz (Foutz et al., 1992) tested rat skin specimens and concluded that “freezing did not affect the resistance of the skin to tensile deformation”. More recently, Caro-Betellene et al. (Caro-Bretelle et al., 2016, 2015) examined different preservation methods and concluded that cryopreservation following a specific protocol is the only way to maintain the mechanical behavior of fresh samples of porcine skin. Ranamukhaarachchi et al. (2016) compared fresh and frozen porcine and human skin specimens, under conditions of microindentation and microneedle insertion and found significant influence of freezing. Some rationalization of the different observations and conclusions reached by different investigations may be found in the work of Pegg (2006): “For some tissues, the preservation of mechanical properties is crucial: for others it is not. These considerations are crucial for the design of preservation methods for specific tissues: bone tendon and skin can provide useful grafts in the absence of living cells and this may even be true of cardiac valves: the crucial requirement here is that the mechanical properties remain intact. Simply freezing at around −80 °C may be sufficient.” Based on this, one would presume that freezing does not influence mechanical response of skin. We will explore the influence of freezing on the mechanical behavior of human skin.
This manuscript is organized as follows. The formulation of two constitutive material models adopted in this work to model the mechanical behavior of skin is summarized in Section 2, with additional details provided in the Appendix. The experimental methods, data collection procedures for the bulge test, the numerical simulation methodology and the optimization procedure for calibration of the material constitutive models are discussed in Section 3. The main experimental results related to preconditioning, freezing, and patient-specific variability in the mechanical response of human female breast skin and abdominal skin are then presented in Section 4. This section also describes the results of the optimization procedure used for calibrating the material models. In Section 5, comparison between the experimental results and the numerical results obtained from both calibrated material models are discussed. Finally, concluding remarks are given in Section 6.
Section snippets
Theory: constitutive material models
As described in the introduction, skin is a complex non-homogenous structure with time-dependent and anisotropic mechanical behavior that varies with position. The time dependence arises from the viscoelasticity of the material. The non-homogeneity arises both from the multi-layer structure and the spatial variation of the preferred fiber orientation along the Langer lines. The anisotropic response stems mainly from the in-plane orientation distribution of collagen fibers. However, in keeping
Sample collection and preparation
Skin specimens were collected and tests were performed in accordance with the institutional review board-approved protocols at The University of Texas MD Anderson Cancer Center and The University of Texas at Austin. Human skin specimens with diameter in the range of 4.5–7 cm were obtained from breast cancer patients who underwent mastectomy and/or reconstructive surgery at MD Anderson Cancer Center and who provided informed consent to donate the specimen for mechanical testing. The inclusion
Experimental results
We will examine the results in three stages. First, the experimental variation of the out of plane Z-displacement of the apex normalized by the specimen thickness, , as a function of the inflation pressure, , for the two different protocols (P1 & P2) will be examined in an effort to determine the effect of preconditioning. Second, the effect of freezing for durations from about 1 day to about 100 days will be examined, once again through a comparison of, vs . Third, the
Experimental results
The pressurized bulge test has been used in the present work to obtain the biaxial mechanical behavior of human skin specimens. 3D DIC method was used to determine the biaxial deformation of the skin as a function of the pressure level; this technique provides a measurement of the bulged geometry of the specimen, as well as the amplitudes of the strain at selected points on the specimen. The primary results discussed in the present work are the measurements of the variation of the out-of-plane
Conclusion
The mechanical response of human female breast skin and abdominal skin from a TRAM procedure was determined through a biaxial bulge test method. A large data set has been generated with 17 specimens collected from 12 subjects. Our results show that
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Cycling at small pressure levels shows negligible preconditioning, while cycling at greater pressure levels shows measurable preconditioning effect.
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Storage in a freezer at −20 °C for up to about 40 days does not lead to any measurable changes in the
Declaration of competing interestCOI
The authors declare that the research was conducted in the absence of any potential conflict of interest.
Acknowledgements
This study was supported by grants R01CA143190 and R01CA203984 from the National Institutes of Health. This study was approved by The University of Texas MD Anderson Cancer Center (protocol number 2015-1118) and by The University of Texas at Austin (protocol number 2010-05-0098). The authors also would like to acknowledge the help received from June Weston, Mary Catherine Bordes, Norma Lau, and Cynthia Branch-Brooks at The University of Texas MD Anderson Cancer Center in recruiting participants
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