Several researchers have studied the force required to penetrate solid and some of constitutive models have been developed for the penetration of a soft solid but it seems non of them dealt with sterile needles insertion into human skin.
Understanding the total complexity of bio-mechanical/mechanical properties of the human skin and developing an advanced computational model (e.g. the Finite element skin models) that not differ (or not so much differ!) from experimental data would provide information which could be very useful for surgical training and practical use (special in this project, the goal is to inform the development of optimized device which can be used for effective and reproducible skin penetration in the clinical setting. This project will also provide and make it possible of generating a robust computational and physical model and an excellent technique for measuring skin deformation and in combination with advanced computational/mechanical methods it will also offer many possibilities for in vivo measurements).
For evaluation of simulation of needle insertion into human skin the development of the multilayer cutting will provide the required underlying basis.
Based in scientific and industrial progress,the multilayer cutting of soft solid is scattered over a considerable extent of interests and usefulness.As an application example is the remote robotic surgery or using created computer simulation program for surgical training.
The complex physics and mechanics of cutting process which requires advance knowledge of the fracture mechanics , deformation and friction additionally,can be modeled(3D Mixed-Mode Cracks Model) and imitated using different numerical computation methods such as the extended finite element method (XFEM)[a numerical technique based on the generalized finite element method (GFEM) and the partition of unity method (PUM)] or cohesive elements method.In this study,the method used for simulation of cutting process is based on cohesive elements method.
In considering the complexity of non-linear mechanical behaviour of human skin and the Advanced Measurement Approaches, S Evans* and C A Holt School of Engineering, Cardiff University, UK (2008-2009)[mechanicalpropertiesofhumanskin], after a series of Experimental Measurements on human skin and related computational modelling which was the combination of digital image correlation and advanced Finite element modelling, found evidence to suggest,- due to reduction of the errors-,the applying stochastic optimization algorithms ,because output analysis of stochastic optimized algorithm will produce better result than Simplex algorithm and will enable the method to escape a local optimum and eventually to approach a global optimum.
In other studies by R.B. Grovesa, S.A. Coulmanb, J.C. Birchallb and S.L. Evans School of Engineering, Cardiff University, UK (2011)[Groves2012][Groves2012, hyperelasticmodelforskin][4,7], in order to optimised microneedle device designs, -which is completely depends on understanding of human skin biomechanics under small deformations-, after doing a series of optimized laboratory developed tests and using much more precise model(considering the skin as a multilayer composite)with applying multilayer finite element model( with the results of which show a remarkable degree of success) it could find out that because of not strictly accurate or precision between experimental and FEM measurements the problem with the precise approach! is still exist and it could be solved if some other materials property like viscoelasticity and anisotropy will be considered, which tends to reinforce the belief that optimum development of numerical-experimental procedure and modelling of very complex mechanical behavior of human skin, would require first the perfect understanding of dependency and independency of parts or elements of skin combined with mechanical description which can be used later for computational modeling.
Naturally all these studies were carried out in laboratory conditions with parallel load(Evans and Holt 2009) and perpendicular load( Grovesa, Coulmanb, Birchallb and Evans 2011) to the human skin surface.
Without being affected by complex nature of soft solid penetration, it is worth to say that the existing literature unfortunately provides not much insight the underlying mechanismus of penetration.Generally they indicate the deep penetration involves deformation and cracks and in most case without taking into account the existance of (sliding)friction.
There are two main studies which aim to help to develop this project.The first one is the study by Oliver A.Shergold and Norman A. Fleck (2004)[JonathanWainwright]with development of the deep penetration of a soft solid by a flat-bottomed and by a sharp-tipped cylindrical punch with using one term Ogden strain energy function and considering the skin as an incompressible hyperelastic,isotropic solid and the second one is the study by Mohsen Mahvash and Vincent Hayward (2001)[vincenthayward] by developing the haptic rendering of cutting with a clarifying of the gemotry and mechanism of interaction of tools and sample.
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 Groves. Quantifying the mechanical properties of human skin to optimise future microneedle device design. Comput Methods Biomech Biomed Engin, 15(1):73–82, 2012. Groves, R B Coulman, S A Birchall, J C Evans, S L eng England 2011/07/14 06:00 Comput Methods Biomech Biomed Engin. 2012;15(1):73-82. doi: 10.1080/10255842.2011.596481. Epub 2011 Jul 12.
 Groves. An anisotropic, hyperelastic model for skin: experimental measurements, finite element modelling and identification of parameters for human and murine skin. J Mech Behav Biomed Mater, 18:167–80, 2013. Groves, Rachel B Coulman, Sion A Birchall, James C Evans, Sam L eng Netherlands 2013/01/01 06:00 J Mech Behav Biomed Mater. 2013 Feb;18:167-80. doi: 10.1016/j.jmbbm.2012.10.021. Epub 2012 Nov 19.
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