NiTi (Nitinol) is a shape memory alloy with distinctive properties, such as shape memory, superelasticity, biocompatibility, and low density. All these unique properties make NiTi a great candidate in different applications. However, the conventional fabrication of NiTi encounters many challenges that significantly limits the practical applications of the alloy. As a solution, the Selective Laser Melting (SLM) which is an Additive Manufacturing (AM) technique has been recently used for the fabrication of NiTi parts. Although complex geometries can be fabricated directly from CAD files via SLM, different process parameters significantly affect the parts’ quality and must be optimized for each application. In most of the potential applications, NiTi components undergo cyclic loads and therefore its structural fatigue must be fully studied and considered in the design process. However, due to the nature of the SLM process, the fatigue behaviour of SLM fabricated NiTi is different from the conventional ones. In this work, as an initial step, the fatigue behaviour of the SLM fabricated NiTi in the horizontal direction is studied and the reasons for failure have been discussed. To this end, strain-controlled fatigue tests were performed on NiTi dog-bone samples, and fractography was used to analyze the different defects which could cause the failure or scatter in the results.

Two unique behaviors of superelasticity and shape memory effect have made shape memory alloy such as NiTi, an interesting alloy for different applications. Recently, additive manufacturing (AM) as a powerful tool for fabricating NiTi has become of interest to make complex geometries. Selective laser melting (SLM) is an AM method that not only provides flexibility to make complex 3D shapes but also it can be possible to tailor the thermomechanical properties of the parts just by changing the process parameters. The non-homogenous microstructure of as-fabricated parts as well as asymmetric mechanical behavior of NiTi, make it important to study the properties of SLM NiTi parts under different loading condition. In this study, Ni50.8Ti (at. %) powder was utilized to fabricate cube, dog-bone, and tube by SLM technique. The transformation temperatures (TTs) of samples were measured by the differential scanning calorimetry (DSC) method and the variation of TTs was discussed. Three coupons were tested mechanically under compression, tension, and torsion. In-situ digital image correlation (DIC) was employed to measure and monitor the strain field of samples during the mechanical tests. The strain distribution showed localized strain for all three samples. The equivalent stress/strain was calculated to compare the result of compressive, tensile, and torsional responses and the significant asymmetric behavior was shown and discussed.

NiTi-based high-temperature shape memory alloys (HTSMAs) such as NiTiHf have been utilized in a broad range of applications due to their high strength and work output, as well as, their ability to increase the transformation temperatures (TTs). Recently, additive manufacturing techniques (AM) have been widely used to fabricate complex shape memory alloy components without any major modifications or tooling and has paved the way to tailor the manufacturing and fabrications of microstructure and critical properties of their final parts. NiTi alloys properties such as transformation temperatures can be significantly altered due to oxidation, which can occur during the manufacturing process or post-processing. In this work, the oxidation behavior of Ni-rich NiTi20Hf shape memory alloys, which was fabricated by the selective laser melting (SLM) method, is evaluated. Thermogravimetric analysis (TGA) is used to assess the kinetic behavior of the oxidation at different temperature ranges of 500, 700, and 900 °C for 20 hours in the air. After oxidation, to evaluate the microstructure and chemical composition X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) was conducted. The isothermal oxidation kinetics of conventional NiTi20Hf alloys were studied, and the results were compared to AM samples. Results show a two-stage oxidation rate at which oxidation increased with the high rate at the initial stage. As the oxidation time increased, the oxidation rate gradually decreased. The oxidation behavior of NiTiHf alloys initially obeyed logarithmic rate law and then followed by parabolic rate law. SEM results showed the formation of a multi-layered oxide scale, including TiO 2 , NiTiO 3 , and Hf oxide.

The fast corrosion rate of magnesium (Mg) alloys is the main problem associated with the use of such biocompatible alloys for bone fixation applications. The corrosion resistance of Mg alloys can be improved by different post-fabrication processes such as heat treatment and coating. We have heat-treated a biocompatible Mg-1.2Zn-0.5Ca (wt.%) alloy at optimized heat treatment parameters to achieve the highest mechanical strength and corrosion resistance. Afterwards, the heat-treated alloy was coated with a ceramic layer using micro arc oxidation (MAO) process to further enhance the corrosion resistance. The microstructure of the prepared samples was investigated using optical microscopy and scanning electron microscopy (SEM). The corrosion characteristics were determined by conducting in vitro electrochemical and immersion corrosion tests. The results showed that the heat treatment process successfully improved the mechanical and corrosion properties of the Mg-1.2Zn-0.5Mn (wt.%) alloy. Both the in vitro electrochemical and immersion corrosion tests showed that the MAO-coated samples have a significantly higher corrosion resistance which results in a significantly lower corrosion rate. This study indicated that the biocompatible coating produced by MAO process may be suitable for providing heat-treated Mg-Zn-Ca-based alloys with protection from corrosion towards synthesizing bone fixation materials in clinical application.

NiTi alloys are interesting materials for biomedical implants since they offer unique characteristics such as superelastic behavior, low stiffness (I.e., modulus of elasticity) close to that of the cortical bone, and shock absorption. Thermal treatments are the most common and practical ways to improve the superelasticity of these alloys. In addition to the superelastic behavior of the metallic implants, it is important for the implants to have a stiffness similar to that of cortical bone in order to reduce the risk of failure caused by stress shielding. The cortical bone has a stiffness ranging from 12 to 31 GPa for different patients (e.g., sex, age, mechanical behavior of bone) and various bone locations (e.g., jaw implant, hip implant), while the untreated Ni-rich NiTi has the stiffness equal to 41.37 GPa. One recently used technique to lower the stiffness of NiTi implant is to introduce porosity into the implant. The major problem associated with the imposed porosity is stress concentration on the pore walls and the subsequent implant failure. In this work, the purpose is to tune the stiffness via changing the post-heat treatment conditions, i.e., aging time and aging temperature. In this study, several bulk specimens of Ni-rich NiTi (SLM Ni50.8Ti49.2) were additively manufactured using selective laser melting (SLM) technique. The samples were solution annealed (950 °C, 5.5 h) and subsequently water quenched to provide equilibrium state in the samples. Subsequently, different aging conditions (350 °C and 450 °C for 5 to 18 hours) were applied to the samples. Mechanical testing (compression) was conducted on the samples and the stiffness of each sample was defined to investigate the effect of aging on the stiffness. Our results indicate that the range of 29.9 to 43.7 GPa for stiffness can be achieved through the implant via different time period and temperatures for aging. The modulus of 43.7 GPa is attributed to 10 hours heat treatment under 450 °C and the modulus of 29.9 GPa is attributed to 18 hours heat treatment under 350 °C.

NiTi has been shown to be of great interest for bone implant applications. Introducing porosity to NiTi bone implants is an effective technique to tune their equivalent modulus of elasticity in order to acquire similar value to that of cortical bone. Moreover, such porous implants allow for better tissue ingrowth due to the interconnecting open pore structure. The effect of porosity percentage on the NiTi equivalent modulus of elasticity is well understood. However, the effect of porosity type on NiTi bone implant’s performance, in terms of the geometrical structure and other mechanical properties, has not yet been investigated. To this end, we simulated three porous structures made of shape memory Ti-rich Ni50.09Ti alloy. The effect of porosity type on the NiTi implant’s geometrical structure and mechanical properties was studied using numerical tests. The purpose is to compare three NiTi implants with different kinds of porosities, at a similar level of porosity (i.e., 69 %). The assigned porosity types in this study are Schwartz-type, Gyroid-type, and Diamond-type. Three triply periodic minimal surface (TPMS) models (9mm×9mm×9mm) with the assigned fixed level of porosity (69 %) were designed as CAD files using Solidworks. Each model was meshed, and the convergence study was conducted. The three models were then imported into a finite element package (ABAQUS). A UMAT code developed by IUT (Isfahan University of Technology) group was used to simulate the mechanical behavior of the shape memory NiTi alloy. All boundary conditions and loading conditions were applied to the models. Compressive mechanical tests were simulated in the finite element, and the resultant equivalent modulus of elasticity, elongations, stress, and strain was estimated. The results show anisotropic behavior within the three different porous structures. With the same level of porosity (i.e., 69 %), equivalent modulus of elasticity was observed to be 48.9, 34.8, and 30.2 GPa for Schwartz-type, Gyroid-type, and Diamond-type, respectively. Moreover, the Schwartz-type scaffold was seen to offer the highest stress at plateau start and the lowest residual strain after unloading, in comparison with the other two types of structure.

Additive manufacturing ( i.e. 3D printing) has only recently be shown as a well-established technology to create complex shapes and porous structures from different biocompatible metal powder such as titanium, nitinol, and stainless steel alloys. This allows for manufacturing bone fixation hardware with patient-specific geometry and properties (e.g. density and mechanical properties) directly from CAD files. Superelastic NiTi is one of the most biocompatible alloys with high shock absorption and biomimetic hysteresis behavior. More importantly, NiTi has the lowest stiffness (36–68 GPa) among all biocompatible alloys [1]. The stiffness of NiTi can further be reduced, to the level of the cortical bone (10–31.2 GPa), by introducing engineered porosity using additive manufacturing [2–4]. The low level of fixation stiffness allows for bone to receive a stress profile close to that of healthy bone during the healing period. This enhances the bone remodeling process (Wolf’s Law) which primarily driven by the pattern of stress. Also, this match in the stiffness of bone and fixation mitigates the problem of stress shielding and detrimental stress concentrations. Stress shielding is a known problem for the currently in-use Ti-6Al-4V fixation hardware. The high stiffness of Ti-6Al-4V (112 GPa) compared to bone results in the absence of mechanical loading on the adjacent bone that causes loss of bone mass and density and subsequently bone/implant failure. We have proposed additively manufactured porous NiTi fixation hardware with a patient-specific stiffness to be used for the mandibular reconstructive surgery (MRS). In MRS, the use of metallic fixation hardware and double barrel fibula graft is the standard methodology to restore the mandible functionality and aesthetic. A validated finite element model was developed from a dried cadaveric mandible using CT scan data. The model simulated a patient’s mandible after mandibular reconstructive surgery to compare the performance of the conventional Ti-6Al-4V fixation hardware with the proposed one (porous superelastic NiTi fixation plates). An optimized level of porosity was determined to match the NiTi equivalent stiffness to that of a resected bone, then it was imposed to the simulated fixation plates. Moreover, the material property of superelastic NiTi was simulated by using a validated customized code. The code was calibrated by using DSC analysis and mechanical tests on several prepared bulk samples of Ni-rich NiTi. The model was run under common activities such as chewing by considering different levels of the applied fastening torques on screws. The results show a higher level of stress distribution on mandible cortical bone in the case of using NiTi fixation plates. Based on wolf’s law it can lead to a lower level of stress shielding on the grafted bone and over time bone can remodel itself. Moreover, the results suggest an optimum fastening torque for fastening the screws for the superelastic fixations causes more normal distribution of stress on the bone similar to that for the healthy mandible. Finally, we successfully fabricated the stiffness-matched porous NiTi fixation plates using selective laser melting technique, and they were mounted on the dried cadaveric mandible used to create the finite element model.

Foot drop usually happens due to neurological and muscular diseases. It limits individuals’ abilities in ankle and toe represented in dorsiflexion during swing phase, and plantar flexion during heel strike. A non-surgical solution to such weakness is the use of ankle foot orthoses (AFOs) which can assist in such abnormal ambulation. The purpose of this work is to develop a new ankle foot orthosis that helps patients to have more normal ankle joint behavior. The proposed AFO device takes advantage of the superelastic behavior of Ni-rich NiTi alloys. In order to evaluate the performance of the Ni-rich NiTi hinged ankle foot orthoses, several motion analysis tests for a normal walking of a healthy subject were conducted. Also, a finite element model were developed to evaluate the performance of superelastic versus stainless steel springs. A Ni-rich NiTi wire was wrapped around a designed rod and the two heads were fixed to the rod (to get the shape of a spring). Then a heat treatment process was performed in a furnace to shape set the NiTi wires and to provide them with the needed superelastic behavior. The produced springs were connected to a designed hinged ankle foot orthoses. Motion analysis was performed on a healthy subject during normal walking in the case of using conventional stainless steel springs, and with using the produced NiTi springs. Joint kinematics and kinetics data of left lower limb (which was equipped with the AFO brace) were collected and calculated to compare normal walking patterns to the resultant walking patterns with the proposed ankle foot orthosis. The CAD file of the AFO, hinge structure and the springs were developed. Each component was meshed and the convergence study were conducted. A finite element model was developed after assembling and introducing all the interactions between parts in Abaqus. The boundary conditions were applied to the system in a way simulating normal walking conditions. Different material properties (stainless steel and superelastic NiTi) were assigned to the springs in the model to evaluate the performance of the system under the aforementioned loading scenario. The results of the motion analysis on a healthy subject during walking indicate that the use of the superelastic NiTi springs causes more normal walk compare to the use of the conventional stainless steel springs, especially during swing phase and heel strike. Moreover, the ankle has closer stiffness profile to the normal walking in the case of using NiTi springs. The results of the finite element analysis show that the super elastic behavior of NiTi results in more hinge rotation while the stress concentration developed on the springs is within the safe levels and cannot cause failure of the NiTi springs. Motion analysis and finite element models were conducted for the proposed hinged AFO and the results were compared with conventional AFO. By taking advantage of the super elastic characteristic of NiTi, more normal walking behavior was observed in the case of using the proposed AFO with Ni-rich NiTi springs.

The most common method for mandibular reconstructive surgery is the use of a Ti-6Al-4V fixation device and a fibular double barrel graft. This highly stiff fixation hardware (E = 112 GPa) often shields the bone graft (E = 20 GPa) from carrying the load, which may result in bone resorption. Highly stiff Ti-6Al-4V fixation hardware is also likely to concentrate stress in the fixation plate or at screw threads, possibly leading to hardware cracking or screw pull-out. As a solution for that, we have proposed and studied the effect of using a low stiffness, porous NiTi fixation device [1–4]. Although the stress in the fixation device is increased, using such low stiffness fixation hardware, is preferable to have an even higher stress on the graft in order to minimize the risk of resorption or hardware failure. We assume that preloading screws allows them to better engage the fixation hardware with the plate and the surrounding bone and causes an increased von Mises stress. The fixation device can be patient-specific and additively manufactured, such that the shape would match the outer surface of the cortical bone. In this study, we modeled a healthy cadaver mandible via CT-derived 3D surface data. The mandible was virtually resected in the molar region (M 1−3 ). The model simulated the result of reconstructive surgery under the highest chewing loading regime (i.e., 526 N on first right molar tooth [5, 6]) where reconstruction was done with either Ti-6Al-4V fixation hardware or patient specific, stiffness-matched, porous NiTi fixation hardware. The calibration of the material properties for this simulation was done using experimentally obtained data (DSC and compression tests) of Ni-rich NiTi bulk samples. The analyzed term in the finite element analysis was stress distribution in the cortical and cancellous bone. Porous NiTi fixation devices were also produced using Selective Laser Melting (SLM) using the geometry of the aforementioned cadaver mandible. In this paper we have studied the effect of additional torque or preload on the performance of the fixation plates. The finite element analysis demonstrated that applying a preload to the screws increased the stress on the bone. Under similar levels of applied preload, the porous NiTi fixation device showed an increased level of von Mises stress in the bone, particularly in the graft. Additionally, the analysis indicated the higher level of stress on the bone surrounding the screws for the case of using NiTi, which could contribute to increasing screw stability. The fabricated patient-specific fixation hardware not only matched the shape of cortical bone but also contained the level of porosity that defines the appropriate modulus of elasticity.

Mg alloys are promising materials for bone implant applications mainly due to their low specific density, desirable stiffness and bioresorbability in the human body. Mg-Zn-Ca alloys are among the most promising materials for resorbable orthopedic fixation devices due to their superior biocompatibility. However, the mechanical and corrosion properties of the as-cast Mg-Zn-Ca alloys are insufficient. Heat treatment is a practical approach for strengthening Mg alloys especially after the fabrication of porous structures and 3D-printed components. We have investigated heat treatment of these devices and have studied the resulting microstructure of Mg-1.6Zn-0.5Ca (wt. %) alloys by hardness, compression, scanning electron microscopy (SEM), and electrochemical and immersion corrosion tests. Mg-1.6Zn-0.5Ca alloy was prepared with high purity Mg, Zn and Ca by casting. The cast ingots were solution-treated at 510 °C for 3 h then quenched in water. The quenched ingots were age hardened in an oil bath at 200 °C for 2 h. Pure Mg, as-cast and heat-treated Mg-1.6Zn-0.5Ca alloy ingots were cut into coupons to characterize their mechanical and corrosion properties. In vitro corrosion tests were conducted in modified simulated body fluid (m-SBF) at pH 7.4 and 37 °C. The hardness of the Mg-Zn-Ca alloy was significantly increased from 52.6 to be 66.8 HV after heat treatment. Also, the compression test results revealed that the heat-treated alloy has the highest compressive yield and ultimate strengths without significant change in stiffness and maximum strain. The mass loss of the Mg-Zn-Ca alloy by week 4 of the in vitro immersion test reduced from 174.6 mg/cm 2 for the as-cast alloy to 101.7 mg/cm 2 after the heat-treatment process. Heat-treatment was found to be a powerful post-shaping process not only to enhance the mechanical properties of the Mg-1.6Zn-0.5Ca (wt. %) alloy, but also to significantly improve its biocorrosion properties. Such heat-treated alloys can also be coated with biocompatible ceramics that provide additional protection from corrosion during the bone healing period (3–4 months).

Selective laser melting (SLM) is an additive manufacturing technique in which complex parts can be fabricated directly by melting layers of powder from a CAD model. SLM has a wide range of application in biomedicine and other engineering areas and it has a series of advantages over traditional processing techniques. A large number of variables including laser power, scanning speed, scanning line spacing, layer thickness, material based input parameters, etc. have a considerable effect on SLM process materials. The interaction between these parameters is not completely studied. Limited studies on balling effect in SLM, densifications under different processing conditions, and laser re-melting, have been conducted that involved microstructural investigation. Grain boundaries are amongst the most important microstructural properties in polycrystalline materials with a significant effect on the fracture and plastic deformation. In SLM samples, in addition to the grain boundaries, the microstructure has another set of connecting surfaces between the melt pools. In this study, a computational framework is developed to model the mechanical response of SLM processed materials by considering both the grain boundaries and melt pool boundaries in the material. To this end, a 3D finite element model is developed to investigate the effect of various microstructural properties including the grains size, melt pools size, and pool connectivity on the macroscopic mechanical response of the SLM manufactured materials. A conventional microstructural model for studying polycrystalline materials is modified to incorporate the effect of connecting melt pools beside the grain boundaries. In this model, individual melt pools are approximated as overlapped cylinders each containing several grains and grain boundaries, which are modeled to be attached together by the cohesive zone method. This method has been used in modeling adhesives, bonded interfaces, gaskets, and rock fracture. A traction-separation description of the interface is used as the constitutive response of this model. Anisotropic elasticity and crystal plasticity are used as constitutive laws for the material inside the grains. For the experimental verification, stainless steel 316L flat dog bone samples are fabricated by SLM and tested in tension. During fabrication, the power of laser is constant, and the scan speed is changed to study the effect of fabrication parameters on the mechanical properties of the parts and to compare the result with the finite element model.

The aim of this article is to investigate the effect of two different fixation hardware materials on bone remodeling after a mandibular reconstruction surgery and to restore the mandible’s function, healthy appearance, mastication, swallowing, breathing, and speech. The hypothesis is that using fixation hardware with stiffness close to that of the surrounding bone will result in a more successful healing process in the mandible bone. The finite element model includes the material properties and forces of the cancellous bone, cortical bone, ligaments, muscles, and teeth. The reconstruction surgery is modeled by including the fixation hardware and the grafted bone. In the sectioned mandible, to best mimic the geometry of the mandible, two single barrel grafts are placed at the top of each other to form a double barrel graft set. Two different materials were used as the mandibular fixation parts, stiff Ti-6Al-4V, and porous superelastic Nickel-Titanium (NiTi) alloys. A comparison of these two alloys demonstrates that using porous NiTi alloy as the fixation part results in a faster healing pace. Furthermore, the density distribution in the mandibular bone after the healing process is more similar to the normal mandible density distribution. The simulations results indicate that the porous superelastic NiTi fixation hardware transfers and distributes the existing forces on the mandible bone more favorably. The probability of stress shielding and/or stress concentration decrease. This type of fixation hardware, therefore, is more appropriate for mandible bone reconstruction surgery.

A knee-ankle-foot orthosis (KAFO), which covers the knee, ankle and foot, can mitigate abnormal walking pattern caused by weak quadriceps. Several types of KAFOs are currently available in the market: passive KAFOs, stance-control KAFOs and dynamic KAFOs. In passive KAFOs, the knee joint keeps being locked during standing and walking, and can be unlocked manually to allow free rotation for sitting. Stance-control KAFOs (SCKAFOs) allow free knee motion during swing phase when the braced leg is unloaded. Dynamic KAFOs are able to reproduce normal walking ability throughout whole gait cycle. This research is directed at using superelastic alloys to develop a dynamic knee actuator that can be mounted on a traditional passive KAFO. The actuator stiffness can match that of a normal knee joint during the walking gait cycle. This proposed knee actuator utilizes a storing-releasing energy method to apply functional compensation to the knee joint, controlling the knee joint during both stance and swing phases. Fundamentally, the knee actuator is composed of two distinct parts which are connected with the thigh and shank segments, respectively. There are two superelastic actuators that are housed within these two parts and activated independently. Each actuator is developed by combining a superelastic rod and a rotary spring in series. When neither actuator is engaged, the knee joint is allowed to rotate freely. The stance actuator works only in the stance phase and the swing actuator is active for the swing phase. The conceptual design of the knee actuator is verified using numerical simulation and a prototype is developed through additive manufacturing for confirming the concept.

The pseudoelastic and shape memory effects of NiTi can be used in passive or active actuation systems. Often used in the aerospace industry, the use of NiTi for actuation is also growing in the biomedical fields and elsewhere. However, it’s potential in industry is currently limited by the inability to produce complex NiTi parts. Conventional manufacturing processes are complicated by the extreme difficulty associated with machining NiTi. Furthermore, the transformation temperatures which drive the unique behavior of NiTi as a shape memory alloy are extremely sensitive to the relative concentrations of nickel and titanium. Therefore, exceptionally tight compositional control during production is necessary to guarantee ideal material behavior. Additive manufacturing (AM) is a near-net-shaping technology which allows for the direct fabrication of complex metallic components. By utilizing the AM processing principle, the poor machinability of NiTi is no longer an issue. Using AM also enables production of 3D geometries that are not possible using traditional techniques. Furthermore, direct CAD fabrication reduces the timescale of the concept-to-prototype transition. In the present work, an SLM machine (Phenix Systems PXM) is used to develop NiTi components directly from powder. The thermal characteristics and shape memory functionality of SLM NiTi components is demonstrated.

Dynamic KAFOs are developed to recover the normal walking ability during both stance and swing phases. Three types of dynamic KAFOs have been reported in the literature. Various actuation mechanisms including spring, pneumatic and hydraulic systems have been used. These devices can improve walking disability and compensate lower leg muscle deficiency. However, they are bulky, in some cases need complex control systems and do not recreate the normal gait pattern. These shortcomings have limited the application of dynamic KAFOs in daily life. The purpose of this paper is to develop a novel knee actuator for a dynamic KAFO that is actuated easily by employing shape memory materials. Such an actuation system makes the KAFO lightweight and has a greater potential to restore the normal gait. Torsional superelastic alloys are used in this actuator in order to match the stiffness of the knee joint of the KAFO with that of a normal knee joint during the walking gait cycle. There are two distinct parts in the knee actuator, acting independently to mimic the two phases of the gait cycle. One engages only in the stance phase and the other works in the swing phase. Each part is developed by combining a superelastic rod and a stiff rotary spring, in series. According to numerical simulation, such combination reproduces the varying knee stiffness during the whole walking gait. Also mechanical experiments have been conducted to further verify the conceptual design. The simulation and experimental results show that the actuator is able to reproduce the stiffness of the normal knee joint during the gait cycle.

The reduction of consumption and emissions is a key factor in modern vehicle design. The overall vehicle efficiency is pursued in several ways, including the reduction of consumption of the auxiliary devices (e.g. water pumps, oil pumps, vacuum pumps etc.). In this paper, using two different smart materials (i.e. magnetorheological fluids and shape memory alloys) a device aimed at disengaging the vacuum pump was developed. The conceived device is composed of a magnetorheological (MR) clutch excited by permanent magnets coaxially manufactured with a sliding spline sleeve moved by shape memory alloys (SMA) springs. In the MR clutch, the magnet can move under the effect of a passive pneumatic system. The magnetic field in the fluid varies with the magnets displacement and two steady positions are possible: the engaged clutch (ON) and the disengaged clutch (OFF). The torque in the ON configuration is high enough to drive the vacuum pump during normal operating conditions, whereas the low torque value in the OFF condition guarantees power saving up to 150 W. In particular operating conditions (i.e. at low environmental temperatures) the torque necessary to start the vacuum pump may exceed the maximum transmissible torque of the MR clutch. For this reason a sliding spline sleeve, led by two SMA springs which counteract the force of two traditional springs was developed. A dummy plain sliding sleeve with SMA and conventional springs was numerically developed and tested. The fairly good agreement of the results confirmed the feasibility of the combined SMA/MR device.

In most of the existing SMA constitutive models, it is assumed that transformation starts when a thermodynamic driving force reaches a specified amount regardless of loading history. In this work, a phenomenological approach is used to develop an enhanced one-dimensional constitutive model in which loading history is directly considered as one of the main parameters affecting the transformation start conditions. To generalize the model to three-dimensional cases, a microplane formulation based on volumetric-deviatoric is employed. A free energy potential is defined at the microplane level, integrated over all orientations at a material point to provide the macroscopic free energy. Experiments are carried out on Nitinol superelastic tubes to validate the newly proposed constitutive model. In these experiments, interruptions are applied during transformations to show the effects of loading history on transformation start conditions. Numerical results are compared with the experimental data to demonstrate the accuracy of the enhanced model.

Shape memory alloys (SMA) when subjected to deformation at low temperature can recover their original shape by heating above a temperature called Austenite transformation temperature. This original shape is sustained till the material is deformed again by an applied stress. This property makes the SMA a unique actuator, which doesn’t require any other components. Also, the material’s resistance changes with deformation. Thus the change in resistance can be used to sense the deformation, which eliminates the requirements of additional sensors. This can make the system more compact and reduce the cost. In our study, a binary Nickel-Titanium alloy is used as a rotary actuator. The actuation is controlled by adjusting the temperature through controlled joule heating by varying the electric current. The manipulator used in this research is a single degree-of-freedom, bias type actuator. SMA actuation in this system is under a varying stress, thereby creating a complex thermo-mechanical condition which affects the transformation temperatures, significantly. Also, the resistance change during heating and cooling paths exhibit hysteresis behavior. This paper investigates the use of artificial neural network (ANN) in establishing relationship between resistance and angular position of the manipulator. To model the hysteresis behavior of the SMA, in addition to resistance of the SMA, other electric properties like voltage etc are given as input to the ANN. The obtained ANN model is able to determine the angular position of the rotary manipulator with good accuracy.

Mandibular segmental defect reconstruction is most often necessitated by tumor resection, trauma, infection, or osteoradionecrosis. The standard of care treatment for mandibular segmental defect repair involves using metallic plates to immobilize fibula grafts, which replace the resected portion of mandible. Surgical grade 5 titanium (Ti-6Al-4V) is commonly used to fabricate the fixture plate due to its low density, high strength, and high biocompatibility. One of the potential problems with mandibular reconstruction is stress shielding caused by a stiffness mismatch between the Titanium fixation plate and the remaining mandible bone and the bone grafts. A highly stiff fixture carries a large portion of the load (e.g., muscle loading and bite force), therefore the surrounding mandible would undergo reduced stress. As a result the area receiving less strain would remodel and may undergo significant resorption. This process may continue until the implant fails. To avoid stress shielding it is ideal to use fixtures with stiffness similar to that of the surrounding bone. Although Ti-6Al-4V has a lower stiffness (110 GPa) than other common materials (e.g., stainless steel, tantalum), it is still much stiffer than the cancellous (1.5–4.5 GPa) and cortical portions of the mandible (17.6–31.2 GPa). As a solution, we offer a nitinol in order to reduce stiffness of the fixation hardware to the level of mandible. To this end, we performed a finite element analysis to look at strain distribution in a human mandible in three different cases: I) healthy mandible, II) resected mandible treated with a Ti-6Al-4V bone plate, III) resected mandible treated with a nitinol bone plate. In order to predict the implant’s success, it is useful to simulate the stress-strain trajectories through the treated mandible. This work covers a modeling approach to confirm superiority of nitinol for mandibular reconstruction. Our results show that the stress-strain trajectories of the mandibular reconstruction using nitinol fixation is closer to normal than if grade 5 surgical titanium fixation is used.