A N Natali

University of Padova, Padua, Veneto, Italy

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Publications (70)96.32 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The effect of steam on the micro-phase structure and mechanical properties of different block copolymers used in biomedical devices is investigated via FT-IR, tensile tests and dynamic mechanical analysis (DMA). Steam sterilization, commonly performed on medical devices and simulated in this work, affects the copolymers' morphology, due to high temperature and humidity conditions. FT-IR analysis reveals that steam induces a modification in the crystalline conformations of copolymers with a pre-existing hydrogen bonding network, that is, thermoplastic polyurethanes (TPU) and poly(ether-block-amide) (PEBA), while it does not significantly affect the domain conformation in styrenic block copolymers (SEBS), due to weak interaction with water. As a consequence, relevant changes of the mechanical properties, closely related to the microdomain structure, are found for TPU and PEBA after sterilization, while SEBS mechanical behavior remains stable, as demonstrated by tensile tests and DMA results. For this reason, SEBS is suggested as the best choice in terms of durability in biomedical applications. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014
    Journal of Polymer Science Part B Polymer Physics 08/2014; · 2.22 Impact Factor
  • IV Congresso Nazionale GNB, Pavia, Italy; 06/2014
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    ABSTRACT: The aim of this work was to provide computational tools for the characterization of the actual mechanical behaviour of foot skin, accounting for results from experimental testing and histological investigation. Such results show the typical features of skin mechanics, such as anisotropic configuration, almost incompressible behaviour, material and geometrical non linearity. The anisotropic behaviour is mainly determined by the distribution of collagen fibres along specific directions, usually identified as cleavage lines. To evaluate the biomechanical response of foot skin, a refined numerical model of the foot is developed. The overall mechanical behaviour of the skin is interpreted by a fibre-reinforced hyperelastic constitutive model and the orientation of the cleavage lines is implemented by a specific procedure. Numerical analyses that interpret typical loading conditions of the foot are performed. The influence of fibres orientation and distribution on skin mechanics is outlined also by a comparison with results using an isotropic scheme. A specific constitutive formulation is provided to characterize the mechanical behaviour of foot skin. The formulation is applied within a numerical model of the foot to investigate the skin functionality during typical foot movements. Numerical analyses developed accounting for the actual anisotropic configuration of the skin show lower maximum principal stress fields than results from isotropic analyses. The developed computational models provide reliable tools for the investigation of foot tissues functionality. Furthermore, the comparison between numerical results from anisotropic and isotropic models shows the optimal configuration of foot skin.
    Skin Research and Technology 02/2014; · 1.41 Impact Factor
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    ABSTRACT: Numerical models represent a powerful tool for investigating the biomechanical behavior of articular cartilages, in particular in the case of complex conformation of anatomical site. In the literature, there are complex non-linear-multiphase models for investigating the mechanical response of articular cartilages, but seldom implemented for the analysis of high organized structure such as the foot. In the present work, the biomechanical behavior of foot cartilage is investigated by means of a fiber-reinforced hyperelastic constitutive model. The constitutive parameters are obtained through the comparison between in vitro experimental indentation tests on cartilage and numerical analysis data interpreting the specific experimental conditions. A finite element model of the hindfoot region is developed. Particular attention is paid to model cartilage in order to respect its morphometric configuration, including also the synovial capsule. The reliability of the procedure adopted is evaluated by comparing the numerical response of tibio-talar joint model with in vivo experimental tests mimicking the foot response in stance configuration.
    Acta of bioengineering and biomechanics / Wroclaw University of Technology 02/2014; 16(2):57-65. · 0.33 Impact Factor
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    ABSTRACT: The aim of the work is to investigate the biomechanical behaviour of colon tissues by a coupled experimental and numerical approach. The colon wall is composed by different tissue layers. Within each layer, different fibre families are distributed according to specific spatial orientations, which lead to a strongly anisotropic configuration. Accounting for the complex histology of the tissues, mechanical tests must be planned and designed to evaluate the colon wall behaviour along different directions. Uni-axial tensile tests were performed on tissue specimens from fifteen fresh pig colons accounting for six different loading directions (five specimens for each loading direction). The subsequent step of the investigation pertains to the definition of an appropriate constitutive framework and the development of a procedure for the identification of the constitutive parameters. A specific hyperelastic formulation was developed accounting for the multi-layered conformation of the colon wall and the fibre reinforced configuration of the tissues. The parameters were identified by the inverse analyses of the mechanical tests. The comparison of model results with experimental data, together with the evaluation of satisfaction of material thermo-mechanics principles, confirmed the reliability of the analysis developed. This work is the basis of more comprehensive activities that aim at providing computational tools for the interpretation of surgical procedures that refer to the gastrointestinal tract, considering the specific biomedical devices adopted.
    Experimental physiology 01/2014; · 3.17 Impact Factor
  • E L Carniel, A Rubini, A Frigo, A N Natali
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    ABSTRACT: An integrated experimental and computational procedure is provided for the evaluation of the biomechanical behaviour that characterizes the pressure-volume response of gastrointestinal regions. The experimental activity pertains to inflation tests performed on specific gastrointestinal conduct segments. Different inflation processes are performed according to progressively increasing volumes. Each inflation test is performed by a rapid liquid in-flaw, up to a prescribed volume, which is held constant for about 300s to allow the development of relaxation processes. The different tests are interspersed by 600s of rest to allow the recovery of the specimen mechanical condition. A physio-mechanical model is developed to interpret both the elastic behaviour of the sample, as the pressure-volume trend during the rapid liquid in-flaw, and the time-dependent response, as the pressure drop during the relaxation processes. The minimization of discrepancy between experimental data and model results entails the identification of the parameters that characterize the viscoelastic model adopted for the definition of the behaviour of the gastrointestinal regions. The reliability of the procedure is assessed by the characterization of the response of samples from rat small intestine.
    Computer methods and programs in biomedicine 01/2014; 113(1):338-345. · 1.56 Impact Factor
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    P G Pavan, P Pachera, C Stecco, A N Natali
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    ABSTRACT: The attention is focused on the viscoelastic behavior of human plantar aponeurosis tissue. At this purpose, stress relaxation tests were developed on samples taken from the plantar aponeurosis of frozen adult donors with age ranging from 67 to 78 years, imposing three levels of strain in the physiological range (4%, 6%, and 8%) and observing stress decay for 240 s. A viscohyperelastic fiber-reinforced constitutive model with transverse isotropy was assumed to describe the time-dependent behavior of the aponeurotic tissue. This model is consistent with the structural conformation of the tissue where collagen fibers are mainly aligned with the proximal-distal direction. Constitutive model fitting to experimental data was made by implementing a stochastic-deterministic procedure. The stress relaxation was found close to 40%, independently of the level of strain applied. The agreement between experimental data and numerical results confirms the suitability of the constitutive model to describe the viscoelastic behaviour of the plantar aponeurosis.
    Computational and Mathematical Methods in Medicine 01/2014; 2014:530242. · 0.79 Impact Factor
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    ABSTRACT: The effect of steam on chemical structure and mechanical properties of renewable poly(ether-block-amide)s (PEBAs) is investigated by different characterization techniques, i.e. FT-IR, TGA, DSC, DMA, and BES. Steam sterilization is a mandatory process for materials used in medical applications. This process, employed during clinical practice and replicated in this study, affects polymer structure and morphology. Steam induces an increase of polyamide (PA) crystallinity in PEBAs with a majority of PA domains, due to the conformational transition from α-helix to parallel and anti-parallel β-sheet, with stronger hydrogen bonding. In PEBAs with longer polyether (PE) blocks, steam induces an increase of random PA domains and the formation of a more extended hydrogen bonding network between ether and amide moieties of the two segments. As a consequence of these microdomain conformational variations, relevant changes occur in molecular relaxations as demonstrated by DMA and BES results. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014
    Journal of Polymer Science Part B Polymer Physics 12/2013; · 2.22 Impact Factor
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    ABSTRACT: Renewable poly(ether-block-amide)s (PEBAs), with different polyamide/polyether (PA/PE) ratios, are characterized for the development of biomedical devices. The effect of the chemical composition on the mechanical properties is studied via Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), tensile tests, dynamic mechanical analysis (DMA), and broadband electric spectro­scopy (BES). FTIR spectroscopy evidences the presence of different PA crystalline domains depending on the PA/PE ratio, as also evidenced by TGA and DSC. Such differences influence the molecular dynamics, as shown by DMA and BES, and the mechanical properties characterized by tensile tests. Indeed, PEBAs with a higher PA/PE ratio are stiffer due to a higher crystalline degree.
    Macromolecular Chemistry and Physics 07/2013; 214(18):2061-2072. · 2.39 Impact Factor
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    ABSTRACT: The mechanical properties of deep fasciae strongly affect muscular actions, development of pathologies, such as acute and chronic compartment syndromes, and the choice of the various fascial flaps. Actually, a clear knowledge of the mechanical characterization of these tissues still lacks. This study focuses attention on experimental tests of different regions of human crural fascia taken from an adult frozen donor. Tensile tests along proximal-distal and medial-lateral direction at a strain rate of 120 %/s were performed at the purpose of evaluating elastic properties. Viscous phenomena were investigated by applying incremental relaxation tests at total strain of 7, 9 and 11 % and observing stress decay for a time interval of 240 s. The elastic response showed that the fascia in the anterior compartment is stiffer than in the posterior compartment, both along the proximal-distal and medial-lateral directions. This result can explain why the compartment syndromes are more frequent in this compartment with respect to posterior one. Furthermore, the fascia is stiffer along the proximal-distal than along medial-lateral direction. This means that the crural fascia can adapt to the muscular variation of volume in a transversal direction, while along the main axis it could be considered as a structure that contributes to transmitting the muscular forces at a distance and connecting the different segments of the limb. The stress relaxation tests showed that the crural fascia needs 120 s to decrease stress of 40 %, suggesting a similar time also in the living so that the static stretching could have an effect on the fascia.
    Anatomia Clinica 06/2013; · 0.93 Impact Factor
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    ABSTRACT: The molecular relaxations of two poly(ether-block-amides) with different polyamide/polyether ratios were studied to evaluate the effect of the chemical structure on the mechanical and electrical properties and to investigate the modification of these properties by an ageing process at high relative humidity. A specific treatment is designed to simulate an accelerated degradation of the material to evaluate the effect of freezing and melting thermal cycles of residual adsorbed water. The effects of the polyamide/polyether ratio on the polymer properties and the consequences of the degradation treatment are studied by correlating the results of FT-IR, TGA, DSC, DMA and BES. The analysis of DMA and BES data highlights the presence of various relaxation events: αPA, αPE, βPA, βPE and γ, assigned respectively to polyamide (αPA) and polyether (αPE) glass transitions, local fluctuations of the dipole associated with the polyamide (βPA) and the polyether (βPE) chains and local fluctuations of the CH2 groups (γ) along the polymer chains. The ageing treatment results in an increased crystallinity in PEBAs with a high polyamide content due to the transition of the polyamide chains from a parallel to anti-parallel β-sheet conformation which forms a stronger hydrogen bonding network. In contrast in PEBAs with high polyether content, the ageing treatment induces the transition of polyamide chains from a parallel β-sheet to an α-helix conformation resulting in the formation of weaker inter-chain interactions.
    Polymer Degradation and Stability 03/2013; 98(6):1126. · 2.77 Impact Factor
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    ABSTRACT: The aim of this work is to provide a computational tool for the mechanical characterization of the hindfoot ligaments. The investigation is performed by a coupled numerical and experimental approach. For this purpose, a numerical model that represents the complex structural configuration of the hindfoot and the typical features of the mechanical behaviour of the ligament tissue is developed. The geometrical analysis of the anatomical site is performed starting from the processing of computed tomography and magnetic resonance images. Accounting for morphometric measurements, the virtual solid model provides an averaged configuration of the hindfoot structure. In order to specify the mechanical behaviour of the ligament tissue, a fibre-reinforced visco-hyperelastic model is adopted. The formulation accounts for the anisotropic configuration, geometric non-linearity, non-linear elasticity and time-dependent phenomena. Numerical analyses are performed to evaluate the biological tissues and structure mechanics with regard to physiological boundary conditions, accounting for dorsiflexion and plantarflexion movements. In order to evaluate the reliability of the numerical model developed, the experimental data are compared with the numerical results. The numerical results are in agreement with the range of values obtained by experimental test confirming the accuracy of the procedure adopted.
    Proceedings of the Institution of Mechanical Engineers Part H Journal of Engineering in Medicine 03/2013; · 1.42 Impact Factor
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    ABSTRACT: The aim of this study was to investigate the viscoelastic behaviour of the human heel pad by comparing the stress-relaxation curves obtained from a compression device used on an in vivo heel pad with those obtained from a three-dimensional computer-based subject-specific heel pad model subjected to external compression. The three-dimensional model was based on the anatomy revealed by magnetic resonance imaging of a 31-year-old healthy female. The calcaneal fat pad tissue was described with a viscohyperelastic model, while a fibre-reinforced hyperelastic model was formulated for the skin. All numerical analyses were performed to interpret the mechanical response of heel tissues, with loading conditions and displacement rate in agreement with experimental tests. The heel tissues showed a non-linear, viscoelastic behaviour described by characteristic hysteretic curves, stress-relaxation and viscous recovery phenomena. The reliability of the investigations was validated by the interpretation of the mechanical response of heel tissues under the application of three pistons with diameter of 15, 20 and 40 mm, at the same displacement rate of about 1.7 mm/s. The maximum and minimum relative errors were found to be less than 0.95 and 0.064, respectively.
    Proceedings of the Institution of Mechanical Engineers Part H Journal of Engineering in Medicine 03/2013; 227(3):334-42. · 1.42 Impact Factor
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    ABSTRACT: The gastrointestinal tract is a primary district of the living organism that shows a complex configuration in terms of biological tissues and structural conformation. The investigation of tissues mechanical functionality in healthy and degenerative conditions is mandatory to plan and design innovative diagnostic and surgical procedures. The aim of this work is to provide some tools for the mechanical analysis of gastrointestinal structures. Computational methods allow for evaluating tissues behaviour and interaction phenomena between biomedical devices, prosthetic elements and tissues themselves. The approach envisages a strong integration of expertise from different areas, proceeding from medicine to bioengineering, computational and experimental biomechanics, bio-robotics and materials science. The development of computational models of gastrointestinal structures requires data from histological analysis and mechanical testing, together with engineering and mathematical skills for the definition of constitutive formulations and numerical procedures. An outline of the computational mechanics approach to the investigation of the gastrointestinal tissues and structures response is reported. A general formulation is presented together with specific applications to oesophageal and colonic tissues. Preliminary results from the numerical analysis of interaction phenomena between colonoscopy devices and tissues are also proposed to address to aspects that allow for an evaluation of feasibility and reliability of the proposed approach.
    Technology and health care: official journal of the European Society for Engineering and Medicine 01/2013; 21(3):271-83. · 0.64 Impact Factor
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    ABSTRACT: A combined experimental and numerical approach is used to investigate the interaction phenomena occurring between foot and footwear during the heel strike phase of the gait. Two force platforms are utilised to evaluate the ground reaction forces of a subject in bare and shod walking. The reaction forces obtained from the experimental tests are assumed as loading conditions for the numerical analyses using three dimensional models of the heel region and of the running shoe. The heel pad region, as fat and skin tissues, is described by visco-hyperelastic and fibre-reinforced hyperelastic formulations respectively and bone region by a linear orthotropic formulation. Different elastomeric foams are considered with regard to the outsole, the midsole and the insole layers. The mechanical properties are described by a hyperfoam formulation. The evaluation of the mechanical behaviour of the heel pad tissues at the heel strike in bare and shod conditions is performed considering different combinations of materials for midsole and insole layers. Results allow for the definition of the influence of different material characteristics on the mechanical response of the heel pad region, in particular showing the compressive stress differentiation in the bare and shod conditions.
    Medical Engineering & Physics 07/2012; · 1.78 Impact Factor
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    ABSTRACT: The biomechanical behaviour of the ankle ligaments is investigated by means of a combined experimental and computational approach. In particular this works deals with the constitutive formulation of the ankle ligaments tissues and recalls the procedure for identifying constitutive parameters through the comparison between experimental data and analytical or numerical models results.
    Congresso Nazionale di Bioingegneria; 06/2012
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    A Forestiero, E L Carniel, A N Natali
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    ABSTRACT: This study was aimed at the definition of a constitutive formulation of ankle ligaments and of a procedure for the constitutive parameters evaluation, for the biomechanical analysis by means of numerical models. To interpret the typical features of ligaments mechanical response, as anisotropic configuration, geometric non-linearity, non-linear elasticity and time-dependent behaviour, a specific fibre-reinforced visco-hyperelastic model is provided. The identification of constitutive parameters is performed by a stochastic-deterministic procedure that minimises the discrepancy between experimental and computational results. A preliminary evaluation of parameters is performed by analytical models in order to define reference values. Afterwards, solid models are developed to consider the complex histo-morphometric configuration of samples as a basis for the definition of numerical models. The results obtained are adopted for upgrading parameter values by comparison with specific mechanical tests. Assuming the new parameters set, the final numerical results are compared with the overall set of experimental data, to assess the reliability and efficacy of the analysis developed for the interpretation of the mechanical response of ankle ligaments.
    Computer Methods in Biomechanics and Biomedical Engineering 05/2012; · 1.39 Impact Factor
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    ABSTRACT: The aims of the present work were to build a 3D subject-specific heel pad model based on the anatomy revealed by MR imaging of a subject's heel pad, and to compare the load-displacement responses obtained from this model with those obtained from a compression device used on the subject's heel pad. A 30 year-old European healthy female (mass=54kg, height=165cm) was enrolled in this study. Her left foot underwent both MRI and compression tests. A numerical model of the heel region was developed based on a 3D CAD solid model obtained by MR images. The calcaneal fat pad tissue was described with a visco-hyperelastic model, while a fiber-reinforced hyperelastic model was formulated for the skin. Numerical analyses were performed to interpret the mechanical response of heel tissues. Different loading conditions were assumed according to experimental tests. The heel tissues showed a non-linear visco-elastic behavior and the load-displacement curves followed a characteristic hysteresis form. The energy dissipation ratios measured by experimental tests (0.25±0.02 at low strain rate and 0.26±0.03 at high strain rate) were comparable with those evaluated by finite element analyses (0.23±0.01 at low strain rate and 0.25±0.01 at high strain rate). The validity and efficacy of the investigation performed was confirmed by the interpretation of the mechanical response of the heel tissues under different strain rates. The mean absolute percentage error between experimental data and model results was 0.39% at low strain rate and 0.28% at high strain rate.
    Medical Engineering & Physics 01/2012; 34(9):1253-9. · 1.78 Impact Factor
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    ABSTRACT: The present paper pertains to the definition of a numerical model of the calcaneal fat pad region, considering a structure composed of adipose and connective tissues organized in fibrous septae and adipose chambers. The mechanical response is strongly influenced by the structural conformation, as the dimension of adipose chambers, the thickness of connective septae walls and the mechanical properties of the different soft tissues. In order to define the constitutive formulation of adipose tissues, experimental data from pig specimens are considered, according to the functional similarity, while the mechanical response of connective tissue septae is assumed with regard to the mechanical behaviour that characterize ligaments. Different numerical models are provided accounting for the variation of chambers dimensions, septae wall thickness and tissues characteristics. The spiral angles of collagen fibres within the septae influence the capability of the structure to withstand the bulging of chambers. The analysis considers different orientation of the fibres. The response of calcaneal fat pad region is evaluated in comparison with experimental data from unconfined compression tests. The present work provides a preliminary approach to enhance the correlation between the structural conformation and tissues mechanical properties towards the biomechanical response of overall heel pad region.
    Journal of the mechanical behavior of biomedical materials. 01/2012; 5(1):216-23.
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    ABSTRACT: The mechanical characteristics of respiratory system tissues include visco-elastic behaviour. In particular, after mechanical unloading, i.e., a reduction in respiratory system volume, the lower stress achieved slowly increases, approaching higher constant value, due to visco-elastic stress recovery. We performed experiments in which constant deflation flow arrest was applied in rats to study the successive pressure-time course, which defines the visco-elastic stress recovery. To investigate the possible effects of temperature changes, measurements were performed at two body temperatures, 36.6 ± 0.3 and 39.0 ± 0.1 °C. We found that stress recovery is reduced by increasing body temperature. Pressure-time curves after deflation arrest were fitted by specific mathematical model, and a good agreement was found. Model parameters exhibited significant changes with body temperature variations, suggesting that temperaturedependent micro-structural rearrangement phenomena in the tissues of alveolar wall were involved in the stress recovery decrement with body temperature increase. Thus, visco-elastic phenomena in respiratory system tissues of mammals exhibit temperature dependence. The stress recovery changes with body temperature suggest that expiration is expected to be easier in condition of physiological body temperature than in the case of increased temperature.
    Acta of bioengineering and biomechanics / Wroclaw University of Technology 01/2012; 14(3):59-66. · 0.33 Impact Factor

Publication Stats

485 Citations
96.32 Total Impact Points

Institutions

  • 1989–2014
    • University of Padova
      • • Department of Industrial Engineering
      • • Center of Mechanics of Biological Materials
      Padua, Veneto, Italy
  • 2013
    • University of Florence
      Florens, Tuscany, Italy
  • 1995
    • Swansea University
      • Department of Civil Engineering
      Swansea, WLS, United Kingdom