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R O Ritchie - One of the best experts on this subject based on the ideXlab platform.
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an equivalent strain coffin manson approach to multiaxial fatigue and life prediction in superelastic Nitinol medical devices
Biomaterials, 2011Co-Authors: R O Ritchie, Amanda Runciman, David Xu, Alan R PeltonAbstract:Abstract Medical devices, particularly endovascular stents, manufactured from superelastic Nitinol, a near-equiatomic alloy of Ni and Ti, are subjected to complex mixed-mode loading conditions in vivo , including axial tension and compression, radial compression, pulsatile, bending and torsion. Fatigue lifetime prediction methodologies for Nitinol, however, are invariably based on uniaxial loading and thus fall short of accurately predicting the safe lifetime of stents under the complex multiaxial loading conditions experienced physiologically. While there is a considerable body of research documented on the cyclic fatigue of Nitinol in uniaxial tension or bending, there remains an almost total lack of comprehensive fatigue lifetime data for other loading conditions, such as torsion and tension/torsion. In this work, thin-walled Nitinol tubes were cycled in torsion at various mean and alternating strains to investigate the fatigue life behavior of Nitinol and results compared to equivalent fatigue data collected under uniaxial tensile/bending loads. Using these strain-life results for various loading modes and an equivalent referential (Lagrangian) strain approach, a strategy for normalizing these data is presented. Based on this strategy, a fatigue lifetime prediction model for the multiaxial loading of Nitinol is presented utilizing a modified Coffin–Manson approach where the number of cycles to failure is related to the equivalent alternating transformation strain.
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in vitro fatigue crack growth and fracture toughness behavior of thin walled superelastic Nitinol tube for endovascular stents a basis for defining the effect of crack like defects
Biomaterials, 2007Co-Authors: Scott W Robertson, R O RitchieAbstract:Abstract Endovascular stents made of the superelastic nickel-titanium alloy Nitinol are subjected in service to tens of millions of loading cycles and even “single-event” overloads, both of which can potentially result in fracture and/or complete failure of the device. A fracture-mechanics-based methodology can provide a means to quantify relevant material parameters critical to the design against such failures. However, there is a dearth of relevant experimental data in the literature on such fracture-mechanics-based approaches to fatigue in Nitinol; furthermore, that which does exist invariably pertains to product forms that are not appropriate for stent manufacture, e.g., bulk Nitinol bar and strip. Consequently, the current work is focused on characterizing in vitro both subcritical and critical crack growth (fatigue–crack growth and R-curve fracture toughness) behavior in thin-walled (∼400 μm thick) Nitinol tubing similar to that used for medical device manufacture (following shape-setting procedures to flatten the material), with a resultant austenite finish temperature of Af∼25–30 °C, identical to self-expanding Nitinol stents. Fatigue–crack growth behavior, measured in Hanks’ Balanced Saline Solution over a wide spectrum of growth rates (down to 10−10 m/cycle) and at a range of positive load ratios ( R = 0.1 – 0.7 ), revealed significantly higher fatigue thresholds than had been previously reported for bulk Nitinol material. In addition, we examine the critical effect of test frequency, as most fatigue experiments on Nitinol have been performed at 30 Hz or above, despite the fact that this is far in excess of the frequency of physiological loading. Finally, the fracture toughness properties are characterized in thin-section Nitinol and show marked crack-resistance (R-curve) behavior with a dependence on crack-growth angle (with respect to the tube drawing axis); additionally, measured toughnesses are found to be lower than has been previously reported for bulk Nitinol.
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crystallographic texture for tube and plate of the superelastic shape memory alloy Nitinol used for endovascular stents
Journal of Biomedical Materials Research Part A, 2005Co-Authors: S W Robertson, V. Imbeni, Hans-rudolf Wenk, R O RitchieAbstract:The superelastic/shape-memory material, Nitinol, an approximately equiatomic alloy of Ni and Ti, is rapidly becoming one of the most important metallic implant materials in the biomedical industry, in particular for the manufacture of endovascular stents. As such stents are invariably laser-machined from Nitinol tubes or sheets rolled into tubes, it is important to fully understand the physical phenomena that may affect the mechanical behavior of this material. With tubing and plate, one major issue is crystallographic texture, which can play a key role in influencing the mechanical properties of Nitinol. In this article, we present a study on how geometry and heat treatment can affect the texture of Nitinol, with specific quantification of the texture of Nitinol tube used for the production of endovascular stents. © 2004 Wiley Periodicals, Inc. J Biomed Mater Res 72A: 190–199, 2005
J. Ro - One of the best experts on this subject based on the ideXlab platform.
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Optimal vibration control of Nitinol-reinforced composites
Composites Engineering, 2008Co-Authors: J. RoAbstract:Abstract Shape-memory fibers, made of a NIckel-Titanium alloy (Nitinol), are embedded inside smart composite beams in order to control the dynamic characteristics and the damped response of these beams when subjected to external excitations. The Nitinol fibers are tuned by adjusting their initial tension and operating temperature to achieve an optimal balance between the thermal softening of the composite matrix, the stiffening effect imparted by the activated fibers, and the enhanced damping of the matrix as it is heated towards its glass transition region. A finite element model is developed to model the dynamics of damped Nitinol-reinforced composite beams. The model is utilized to compute the natural frequencies, the modal loss factors and the frequency response functions of this class of SMART beams. The frequency responses of the Nitinol-reinforced beams are compared with those of the unreinforced beams in order to emphasize the importance of the Nitinol reinforcement and its optimal tuning in significantly attenuating the vibration of these beams.
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Thermo-dynamic characteristics of Nitinol-reinforced composite beams
Composites Engineering, 2008Co-Authors: J. RoAbstract:Abstract The dynamic characteristics of flexible composite beams are controlled by heating sets of shape memory alloy (Nitinol) fibers embedded along the neutral axes of these beams. The activation of the shape memory effect of the fibers increases the elastic energy and enhances the stiffness of the composite beams. With such capabilities, the vibration modes of the beams can be tailored and shifted away from the excitation frequency band in order to avoid undesirable vibrations. Emphasis is placed, in the present study, on the effect of intentional electrical heating of a selected subset of the Nitinol fibers on the overall dynamics of the beams. The effect of the associated thermal energy propagating through the composite on the unintentional thermal activation of additional subsets of the Nitinol fibers is accounted for. Such an effect is not only significant but also essential to the thorough understanding of the operation of Nitinol-reinforced composites. Finite element models are developed to describe the interaction between the thermal and dynamic characteristics of the Nitinol composites as well as the interaction between the intentional and unintentional activation of the Nitinol fibers. The models are experimentally validated and close agreement is obtained between the theoretical predictions and the experimental results. The mathematical models and procedures described in this paper provide an invaluable means of predicting realistic performance of Nitinol-reinforced composites.
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Nitinol-reinforced plates: Part II. Static and buckling characteristics☆
Composites Engineering, 2008Co-Authors: J. RoAbstract:Abstract The static and buckling characteristics of flexible fiberglass Nitinol-reinforced composite plates are controlled by activating optimal sets of Nitinol fibers embedded along the midplane of these plates. The Nitinol fibers are pre-tensioned and activated to generate significant phase recovery forces in order to increase the membrane strain energy which in turn increases the critical buckling load of the Nitinol-reinforced plates. With such control capabilites, the plates can be manufactured from light weight sections without compromising their elastic stability. This feature is invaluable in building light weight structures that have high resistance to failure due to buckling. The Nitinol fibers are trained to memorize the shape of the unbuckled plate and when the plate is deflected under the action of external compressive loads, the controller activates the Nitinol fibers by heating them above their transformation temperature. The generated phase recovery forces bring the plate back to its memorized undeflected position. A finite element model of Nitinol-reinforced plates is developed to describe the interaction between the external loads, operating conditions and the geometrical and physical parameters of the composite plate and the Nitinol fibers. This model predicts the critical buckling loads of Nitinol-reinforced plates. The predicted loads are compared with results available in the literature for symmetrically isotropic, orthotropic and anisotropic laminates. The mathematical model described in this paper provides an invaluable means of predicting realistic performance of Nitinol-reinforced composites.
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Nitinol-reinforced plates: Part III. Dynamic characteristics
Composites Engineering, 2008Co-Authors: J. RoAbstract:Abstract The dynamic characteristics of Nitinol-reinforced composite plates are controlled by heating sets of Nitinol fibers embedded inside these plates. The activation of the shape memory effect of these Nitinol fibers increases the elastic energy, enchances the stiffness of the composite plates and modifies their modal characteristics. One of the objectives of the resulting modal modification is to shift the modes of vibration of the plates away from the excitation frequencies in order to avoid undesirable resonances. In this way, the modal characteristics can be tailored in response to the external disturbances acting on the plates. The classical finite element approach is used to form the equations of motion of the assembly of Nitinol-reinforced plate elements and the appropriate boundary conditions are then applied. The solution of the eigenvalues of the resulting homogeneous equations gives the natural frequencies of the Nitinolreinforced plate as influenced by the properties of the composite matrix and the Nitinol fibers. It is important to note that these properties are influenced by the temperature distribution inside the composite plate which is developed by virtue of activating and de-activating the Nitinol fibers. Emphasis is placed on the effect of intentional electrical heating of a selected subset of the Nitinol fibers on the overall dynamics of the plates. The effect of the associated thermal energy propagating through the composite on the unintentional thermal activation of additional subsets of the Nitinol fibers is accounted for. Such an effect is not only significant, but also essential to the thorough understanding of the operation of the Nitinol-reinforced plates.
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Nitinol-reinforced plates: Part I. Thermal characteristics☆
Composites Engineering, 2008Co-Authors: J. RoAbstract:Abstract The static and dynamic characteristics of Nitinol-reinforced composite plates are influenced primarily by the temperature distribution inside the composite matrix. Such distribution arises from the electrical heating of Nitinol fibers embedded along the neutral plane of these composite plates. When temperatures are developed above the martensite transformation temperature of the Nitinol fiber, the elastic modulus of the fibers increases approximately fourfold and significant phase recovery forces are generated. Such thermal activation of the Nitinol fibers increases the elastic energy of the fibers and enchances the stiffness of the plates, provided that the phase recovery forces are high enough to compensate for the loss of the modulus of elasticity of the composite and counterbalance the generated thermal loads. Understanding the interaction between the thermal, static and dynamic characteristics of the Nitinol-reinforced plates is essential to tailoring the performance of these plates to match changes in the operating conditions. Such an interaction is influenced primarily by the temperature distribution inside the plates during the activation and de-activation of the Nitinol fibers. In this study, a thermal finite element model is developed to determine steady-state and transient temperature distributions inside Nitinol-reinforced composite plates resulting from different activation strategies of the Nitinol fibers. The theoretical predictions are compared with experimental measurements in order to validate the thermal finite element model. The resulting temperature distribution can be used to determine an average modulus of elasticity of the composite. The average temperature rise above ambient can also be used to compute the axial thermal loading on the composite plate. Such predictions are utilized in computing the static and dynamic characteristics of Nitinol-reinforced plates which are presented in Parts II and III of this paper, respectively.
Harlan Reitan - One of the best experts on this subject based on the ideXlab platform.
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shape memory stapes prosthesis for otosclerosis surgery
Laryngoscope, 2005Co-Authors: Glenn W Knox, Harlan ReitanAbstract:Objectives: The aim of this study was to determine the efficacy of a shape-memory alloy, Nitinol, as a component of an improved stapes prosthesis. Study Design: Prospective laboratory and clinical study to develop a Nitinol stapes prosthesis. Methods: Various diameters of Nitinol wire and temperature transition variants were analyzed with regard to ease of deformation, response to heating, and strength. The size and geometry of the closed hook was determined by measurement of 50 incus cadaver bones. Several heat sources for activating the shape memory were evaluated, including electrocautery, lasers, and warm water. Trial surgeries were then performed on human temporal bones in the laboratory. The closure characteristics of the Nitinol loop were studied. Magnetic resonance imaging (MRI) testing at 1.5 Tesla was performed to determine safety during MRI studies. Preliminary human subject trials were then instituted. Results: In all cases, a low heat condition was ample to activate the shape memory characteristics of the hook and return it to a closed position after it had been opened. Laser power was generally set well below the power needed for removing bone. The Nitinol loop closed snugly around the incus with application to the top of the hook with a low temperature laser setting. Almost any heat source was effective. MRI testing at 1.5 Tesla showed no movement of the prosthesis. Preliminary results in human subjects showed excellent air-bone closure. The Nitinol loop holds uniform contact around the incus. Conclusions: The Nitinol piston greatly simplifies the stapedectomy procedure by taking the need for a hand operated instrument out of the surgeon's hands. Because of the nature of the Nitinol wire, it can never over-crimp. All these characteristics make the prosthesis advantageous for otosclerosis surgery.
Marcus D Atlas - One of the best experts on this subject based on the ideXlab platform.
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long term audiometric and clinical outcomes following stapedectomy with the shape memory Nitinol stapes prosthesis
Otology & Neurotology, 2019Co-Authors: Rebecca Heywood, Mark Quick, Marcus D AtlasAbstract:OBJECTIVE: To assess long-term hearing outcomes following stapedectomy using a self-crimping shape memory Nitinol prosthesis. The results were compared with those of a group of patients who received a conventional prosthesis. STUDY DESIGN: Retrospective case review. SETTING: Tertiary referral center. PATIENTS: All patients who underwent stapedectomy for otosclerosis between July 2001 and November 2008 with a minimum dataset of preoperative, early postoperative (≤2 yr), and late postoperative (≥6 yr) audiometry were included. Fifty-six patients with a Nitinol prosthesis and 27 patients with a titanium prosthesis met the inclusion criteria. INTERVENTION: Stapedectomy using a Nitinol or conventional prosthesis. MAIN OUTCOME MEASURE(S): Hearing outcomes by audiological assessment. RESULTS: Mean duration of follow up was 9.5 (standard deviation [SD] 1.4) years in the Nitinol group and 12.6 (SD 2.1) years in the titanium group. The early and late mean postoperative air-bone gaps (ABGs) were 9.7 and 9.8 dB in the Nitinol group and 11.0 and 12.6 dB in the titanium group, respectively. The proportion of patients achieving an ABG less than or equal to 20 dB at early and late follow up was 96% and 96% in the Nitinol group and 92% and 86% in the titanium group respectively. CONCLUSIONS: The excellent closure of the ABG achieved at early follow up remains remarkably stable up to 12 years using a self-crimping shape memory Nitinol prosthesis. There is no evidence that firm fixation of the hook around the long process of incus has a detrimental effect in the long-term.
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in vivo performance of the Nitinol shape memory stapes prosthesis during hearing restoration surgery in otosclerosis a first report
Journal of Biomedical Materials Research, 2005Co-Authors: Gunesh P Rajan, Robert H Eikelboom, Keith S Anandacoomaraswamy, Marcus D AtlasAbstract:The limitations of manual prosthesis crimping in hearing restoration surgery for otosclerosis are thought to have a key role in the occurrence of incomplete postoperative elimination of conductive hearing loss and postoperative recurrences of conductive hearing loss. To eliminate manual crimping, the self-crimping, shape-memory alloy Nitinol stapes piston was introduced in nine otosclerosis patients. The results were compared with those in a database of surgeries performed with conventional titanium pistons. The effects of the self-crimping Nitinol prosthesis on the postoperative elimination of conductive hearing loss and its postoperative variations were investigated. The variations of postoperative residual conductive hearing loss were significantly smaller and the extent of conductive hearing-loss elimination greater in the Nitinol group. The mean postoperative residual conductive hearing loss was smaller in the Nitinol group. The postoperative stability of conductive hearing loss elimination was similar in both patient groups. Preliminary results suggest that the self-crimping shape-memory alloy Nitinol stapes piston overcomes the drawbacks of manual crimping in hearing restoration surgery for otosclerosis.
Jorma Ryhänen - One of the best experts on this subject based on the ideXlab platform.
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Biocompatibility of Nitinol
Minimally Invasive Therapy & Allied Technologies, 2009Co-Authors: Jorma RyhänenAbstract:This review links together recent information on the biocompatibility of Nitinol. Fundamental aspects of biological responses to Nitinol and its alloy components are clarified. The clinical advantages of using this functional biomaterial are evident. Although most studies support the good biocompatibility of Nitinol, there are still a lot of unanswered questions. The long-term in vivo performance of this material has not been well demonstrated, and host-Nitinol interactions at cell and molecular level are mostly unknown. However, the experimental and clinical data strongly support Nitinol as a safe biomaterial, at least as good as stainless steel or titanium alloys.
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Biocompatibility Evaluation of Nickel- Titanium Shape Memory Metal Alloy
Surger, 1999Co-Authors: Jorma RyhänenAbstract:Nickel-titanium shape memory alloy (Nitinol) has properties that could be very useful in surgical applications. Thermal shape memory, superelasticity, and high damping properties make such alloys behave differently compared to other implant metals. There has previously been a lack of sufficient evidence on the biocompatibility of Nitinol. The purpose of this study was to evaluate general soft tissue response and biocompatibility to Nitinol in vivo, and to clarify neural and perineural responses, previously unreported. Seventy-five rats were randomized into three groups. Test specimens were implanted into paravertebral muscle and near the sciatic nerve. A comparison was made between Nitinol, stainless steel, and Ti-6Al-4V. The animals were euthanized at 2, 4, 8, 12, and 26 weeks after implantation. General morphologic and histologic observations were made under light microscopy. Semiautomatic computerized image analysis was used to measure the encapsule membrane thickness around the implants. The muscular tissue response to Nitinol was clearly nontoxic, regardless of the time period. The overall inflammatory response to Nitinol was very similar to that of stainless steel and Ti-6Al-4V alloy. There were no necroses, granulomas, or signs of dystrophic soft tissue clacification. The immune cell response to Nitinol remained low. Only a few foreign-body giant cells were present. The detected neural and perineural responses were also clearly nontoxic and nonirritating with Nitinol. No qualitative differences in histology between the different test materials could be seen. At 8 weeks, the encapsule membrane of Nitinol was thicker than that of stainless steel (mean 62 25 microns vs. 41 8 microns). At the end of the study, the encapsule thickness was equal to all the materials tested. We concluded that Nitinol had good in vivo biocompatibility after intramuscular and perineural implantation in rats in the 26-week follow-up. Based on the results of the present study, Nitinol appears to have good potential for clinical use.
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in vivo biocompatibility evaluation of nickel titanium shape memory metal alloy muscle and perineural tissue responses and encapsule membrane thickness
Journal of Biomedical Materials Research, 1998Co-Authors: Jorma Ryhänen, Matti Kallioinen, J Tuukkanen, J Junila, Elina H Niemela, P Sandvik, W SerloAbstract:Nickel-titanium shape memory alloy (Nitinol) has properties that could be very useful in surgical applications. Thermal shape memory, superelasticity, and high damping properties make such alloys behave differently compared to other implant metals. There has previously been a lack of sufficient evidence on the biocompatibility of Nitinol. The purpose of this study was to evaluate general soft tissue response and biocompatibility to Nitinol in vivo, and to clarify neural and perineural responses, previously unreported. Seventy-five rats were randomized into three groups. Test specimens were implanted into paravertebral muscle and near the sciatic nerve. A comparison was made between Nitinol, stainless steel, and Ti-6Al-4V. The animals were euthanized at 2, 4, 8, 12, and 26 weeks after implantation. General morphologic and histologic observations were made under light microscopy. Semiautomatic computerized image analysis was used to measure the encapsule membrane thickness around the implants. The muscular tissue response to Nitinol was clearly nontoxic, regardless of the time period. The overall inflammatory response to Nitinol was very similar to that of stainless steel and Ti-6Al-4V alloy. There were no necroses, granulomas, or signs of dystrophic soft tissue calcification. The immune cell response to Nitinol remained low. Only a few foreign-body giant cells were present. The detected neural and perineural responses were also clearly nontoxic and nonirritating with Nitinol. No qualitative differences in histology between the different test materials could be seen. At 8 weeks, the encapsule membrane of Nitinol was thicker than that of stainless steel (mean 62 ± 25 μm vs. 41 ± 8 μm). At the end of the study, the encapsule thickness was equal to all the materials tested. We concluded that Nitinol had good in vivo biocompatibility after intramuscular and perineural implantation in rats in the 26-week follow-up. Based on the results of the present study, Nitinol appears to have good potential for clinical use. © 1998 John Wiley & Sons, Inc. J Biomed Mater Res, 41, 481–488, 1998.
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biocompatibility of nickel titanium shape memory metal and its corrosion behavior in human cell cultures
Journal of Biomedical Materials Research, 1997Co-Authors: Jorma Ryhänen, Elina H Niemela, P Sandvik, W Serlo, E Niemi, Hannu Pernu, Tuula SaloAbstract:Nickel-titanium alloy (Nitinol) is a metallic biomaterial that has a unique thermal shape memory, superelasticity, and high damping properties. Nitinol is potentially very useful in orthopedic surgery, for example. At present, there are not enough confirmative biocompatibility data available on Nitinol. The aim of our study was to clarify the primary cytotoxicity and corrosion rate of Nitinol in human cell cultures. Comparisons were made with stainless steel (Stst), titanium (Ti), composite material (C), and control cultures with no test discs. Human osteoblasts (OB) and fibroblasts (FB) were incubated for 10 days with test discs of equal size, 6 × 7 mm. The cultures were photographed and the cells counted. Samples from culture media were collected on days 2, 4, 6, and 8, and the analysis of metals in the media was done using flameless atomic absorption spectrophotometry. The proliferation of FB was 108% (Nitinol), 134% (Ti) (p < 0.02), 107% (Stst), and 48% (C)(p < 0.0001) compared to the control cultures. The proliferation of OB was 101% (Nitinol), 100% (Ti), 105% (Stst), and 54% (C) (p < 0.025) compared to the controls. Initially, Nitinol released more nickel (129–87 μg/L) into the cell culture media than Stst (7 μg/L), but after 2 days the concentrations were about equal (23–5 μg/L versus 11–1 μg/L). The titanium concentrations from both Nitinol and Ti samples were all <20 μg/L. We conclude that Nitinol has good in vitro biocompatibility with human osteoblasts and fibroblasts. Despite the higher initial nickel dissolution, Nitinol induced no toxic effects, decrease in cell proliferation, or inhibition on the growth of cells in contact with the metal surface. © 1997 John Wiley & Sons, Inc. J Biomed Mater Res, 35, 451–457, 1997.