Impact of nitinol stent surface processing on in-vivo nickel release and biological response

Effects of adsorption of O2 and H2O molecules on the corrosion behavior of the NiTi alloy surface: a DFT investigation

The influence of O2 and H2O adsorption significantly affects the electrochemical corrosion of the NiTi alloy, with unresolved corrosion disparities between the NiTi-B2 and NiTi-B19' phases. Density functional theory (DFT) calculations are utilized in this investigation to explore the adsorption of O atoms at varying coverages on the NiTi-B2(110) and NiTi-B19'(010) surfaces. The goal is to elucidate their oxidation behavior differences. Subsequently, the effect of O adsorption on the dissolution trends of these phases is assessed by inducing Ni/Ti vacancies to simulate alloy dissolution thermodynamically. Additionally, interactions between H2O molecules and O-pre-adsorbed NiTi alloy surfaces are examined to simulate the atomic evolution of the oxidized surface under exposure to humid air and corrosive solutions. The findings indicate a propensity of the NiTi-B19' phase to react with O, forming an oxide film more readily than the NiTi-B2 phase. O adsorption facilitates Ni dissolution and retards Ti dissolution on the alloy surface. Higher O coverage promotes easier dissolution of Ni and Ti atoms on the NiTi-B2(110) surface compared to the NiTi-B19'(010) surface, underscoring the greater corrosion resistance of the NiTi-B19' phase. Both clean and O-pre-adsorbed NiTi alloy surfaces physically adsorb H2O molecules. Notably, an O monolayer substantially mitigates the detrimental effects of H2O molecules on the corrosion resistance of alloy surfaces. This research contributes to a deeper comprehension of the corrosion mechanisms in NiTi alloys.

Cytotoxic effects and comparative analysis of Ni ion uptake by osteoarthritic and physiological osteoblasts

Nickel(Ni)-containing materials have been widely used in a wide range of medical applications, including orthopaedics. Despite their excellent properties, there is still a problem with the release of nickel ions into the patient's body, which can cause changes in the behaviour of surrounding cells and tissues. This study aims to evaluate the effects of Ni on bone cells with an emphasis on the determination of Ni localization in cellular compartments in time. For these purposes, one of the most suitable models for studying the effects induced by metal implants was used-the patient's osteoarthritic cells. Thanks to this it was possible to simulate the pathophysiological conditions in the patient's body, as well as to evaluate the response of the cells which come into direct contact with the material after the implantation of the joint replacement. The largest differences in cell viability, proliferation and cell cycle changes occurred between Ni 0.5 mM and 1 mM concentrations. Time-dependent localization of Ni in cells showed that there is a continuous transport of Ni ions between the nucleus and the cytoplasm, as well as between the cell and the environment. Moreover, osteoarthritic osteoblasts showed faster changes in concentration and ability to accumulate more Ni, especially in the nucleus, than physiological osteoblasts. The differences in Ni accumulation process explains the higher sensitivity of patient osteoblasts to Ni and may be crucial in further studies of implant-derived cytotoxic effects.

A Predictive Toxicokinetic Model for Nickel Leaching from Vascular Stents

In vitro testing methods offer valuable insights into the corrosion vulnerability of metal implants and enable prompt comparison between devices. However, they fall short in predicting the extent of leaching and the biodistribution of implant byproducts under in vivo conditions. Physiologically based toxicokinetic (PBTK) models are capable of quantitatively establishing such correlations and therefore provide a powerful tool in advancing nonclinical methods to test medical implants and assess patient exposure to implant debris. In this study, we present a multicompartment PBTK model and a simulation engine for toxicological risk assessment of vascular stents. The mathematical model consists of a detailed set of constitutive equations that describe the transfer of nickel ions from the device to peri-implant tissue and circulation and the nickel mass exchange between blood and the various tissues/organs and excreta. Model parameterization was performed using (1) in-house-produced data from immersion testing to compute the device-specific diffusion parameters and (2) full-scale animal in situ implantation studies to extract the mammalian-specific biokinetic functions that characterize the time-dependent biodistribution of the released ions. The PBTK model was put to the test using a simulation engine to estimate the concentration-time profiles, along with confidence intervals through probabilistic Monte Carlo, of nickel ions leaching from the implanted devices and determine if permissible exposure limits are exceeded. The model-derived output demonstrated prognostic conformity with reported experimental data, indicating that it may provide the basis for the broader use of modeling and simulation tools to guide the optimal design of implantable devices in compliance with exposure limits and other regulatory requirements.

Research progress of metal biomaterials with potential applications as cardiovascular stents and their surface treatment methods to improve biocompatibility

Facing the growing issue of cardiovascular diseases, metallic materials with higher tensile strength and fatigue resistance play an important role in treating diseases. This review lists the advantages and drawbacks of commonly used medical metallic materials for vascular stents. To avoid post-procedural threats such as thrombosis and in-stent restenosis, surface treatments, and coating methods have been used to further improve the biocompatibility of these materials. Surface treatments including laser, plasma treatment, polishing, oxidization, and fluorination can improve biocompatibility by modifying the surface charges, surface morphology, and surface properties of the material. Coating methods based on polymer coatings, carbon-based coatings, and drug-functional coatings can regulate the surface properties, and also serve as an effective barrier to the interaction of metallic biomaterial surfaces with biomolecules, which can be used to improve corrosion resistance and stability, as well as improve their biocompatibility. Biocompatibility serves as the most fundamental property of cardiovascular stents, and maintaining the excellent and stable biocompatibility of cardiovascular stent surfaces is a current research bottleneck. Few reviews have been published on metallic biomaterials as cardiovascular stents and their surface treatments. For the purpose of advancing research on cardiovascular stents, common metal biomaterials, surface treatment methods, and coating methods to improve biocompatibility and comprehensive properties of the materials are described in this review. Finally, we suggest future directions for stent development, including continuously improving the durability and stability of permanent stents, accelerating the development of biodegradable stents, and strengthening feedback to improve the safety and reliability of cardiovascular stents.

Porous NiTi Dental Implant Fabricated by a Metal Injection Molding: An in Vivo Biocompatibility Evaluation in an Animal Model

This study assessed the corrosion resistance, intracutaneous reactivity, acute systemic toxicity, and in situ tissue effect of the implantation of porous NiTi fabricated by metal injection molding in animal models. For the intracutaneous reactivity study, five intracutaneous injections were administered per site with and without the tested extract in polar and nonpolar solutions. The extract was also delivered via intravenous and intraperitoneal routes for acute systemic toxicity. TiAl6 V4 (control) and porous NiTi were implanted in rabbit femora for a period of 13 weeks to evaluate the in situ tissue response. Corrosion was evaluated through open and cyclic polarization in PBS, while biocompatibility was investigated by assessing the general conditions, skin irritation score (edema and erythema), and histopathology. No active dissolution or hysteresis loop was observed in the corrosion study. None of the animals exhibited death, moribundity, impending death, severe pain, self-mutilation, or overgrooming. No edema was observed at injection sites. Only the positive control showed an erythematous reaction at 24, 48, and 72 h observations (p < 0.001). Porous NiTi showed a low in situ biological response for inflammation, neovascularization, and fibrosis in comparison to the control implant (p = 0.247, 0.005, and 0.011, respectively). Porous NiTi also demonstrated high pitting corrosion resistance while causing no acute hypersensitivity or acute systemic toxicity. The study concludes that porous NiTi implants were unlikely to cause local sensitization, acute systemic toxicity, or chronic inflammatory reactions in an animal model. Porous NiTi also exhibited osseointegration equivalent to Ti6AI4 V of known biocompatibility.

On the development of physiologically based toxicokinetic (PBTK) models for cardiovascular implants

Local and systemic contamination caused by metal ions leaching from medical device materials is a significant and continuing health problem. The increasing need for verification and validation, and the imposition of stringent government regulations to ensure that the products comply with the quality, safety, and performance standards, have led regulatory bodies worldwide to strongly recommend the use of modeling and simulation tools to support medical device submissions. A previously published physiologically based toxicokinetic (PBTK) model, is here expanded and enriched by an additional separate tissue compartment to better resemble normal physiology and by the introduction of time-dependent functions to describe all biokinetic parameters. The new model is exercised in conjunction with state-of-the-art probabilistic, Monte Carlo methodology to calculate the predictions' confidence intervals and incorporate variability associated with toxicological biodistribution studies. The quantitative consistency of the model-derived predictions is validated against reported data following the implantation of nickel-containing cardiovascular devices in humans and minipigs. Finally, a new methodology for compartmental toxicological risk assessment is presented that can be used for forward or reverse dosimetry. Our work is aimed at providing a computational tool to optimize the device design characteristics and safeguard that the substances released do not exceed permissible exposure limits.

Multilevel Assessment of Stent-Induced Inflammation in the Adjacent Vascular Tissue

A recent U.S. Food and Drug Administration report presented the currently available scientific information related to biological response to metal implants. In this work, a multilevel approach was employed to assess the implant-induced and biocorrosion-related inflammation in the adjacent vascular tissue using a mouse stent implantation model. The implications of biocorrosion on peri-implant tissue were assessed at the macroscopic level via in vivo imaging and histomorphology. Elevated matrix metalloproteinase activity, colocalized with the site of implantation, and histological staining indicated that stent surface condition and implantation time affect the inflammatory response and subsequent formation and extent of neointima. Hematological measurements also demonstrated that accumulated metal particle contamination in blood samples from corroded-stetted mice causes a stronger immune response. At the cellular level, the stent-induced alterations in the nanostructure, cytoskeleton, and mechanical properties of circulating lymphocytes were investigated. It was found that cells from corroded-stented samples exhibited higher stiffness, in terms of Young's modulus values, compared to noncorroded and sham-stented samples. Nanomechanical modifications were also accompanied by cellular remodeling, through alterations in cell morphology and stress (F-actin) fiber characteristics. Our analysis indicates that surface wear and elevated metal particle contamination, prompted by corroded stents, may contribute to the inflammatory response and the multifactorial process of in-stent restenosis. The results also suggest that circulating lymphocytes could be a novel nanomechanical biomarker for peri-implant tissue inflammation and possibly the early stage of in-stent restenosis. Large-scale studies are warranted to further investigate these findings.

DFT investigation of the dissolution trends of NiTi alloys with the B2 and B19' phases during the initial oxidation stage

The selective corrosion of NiTi alloys was studied using density functional theory calculations, and the dissolution trends of the NiTi-B₂ and NiTi-B19' phases in the initial oxidation stage were compared to predict their corrosion difference. The dissolution process of Ni and Ti was simulated by creating Ni or Ti vacancies on the unoxidized and oxidized NiTi alloy surfaces. The results show that the surface vacancy formation energy of Ti vacancies is higher than that of Ni vacancies, indicating that Ti is more difficult to dissolve than Ni. Furthermore, oxidation promotes and impedes the dissolution of Ni and Ti, respectively. This study improves the fundamental understanding of the corrosion mechanism of NiTi alloys.

Mediation Effect of Body Mass Index on the Association of Urinary Nickel Exposure with Serum Lipid Profiles

The objective of this study was to determine the relationship of urinary nickel (U-Ni) exposure to serum lipid profiles and the mediation effect of body mass index (BMI) in a US general population. We analyzed the cross-sectional data from 3517 participants in the National Health and Nutrition Examination Survey (NHANES) (2017-March 2020). Multivariable linear regression and restricted cubic spline (RCS) regression were conducted to explore the association of U-Ni with four serum lipids and four lipids-derived indicators. Mediation analysis was performed to examine the effect of BMI on the relationship between U-Ni levels and serum lipid profiles. Compared with the lowest quartile, the β with 95% confidence intervals (CIs) in the highest quartile were - 12.83 (- 19.42, - 6.25) for total cholesterol (TC) (P for trend < 0.001), - 12.76 (- 19.78, - 5.74) for non-high-density lipoprotein cholesterol (non-HDL-C) (P for trend = 0.001) and - 0.29 (- 0.51, - 0.07) for TC/HDL-C (P for trend = 0.007) in the fully adjusted model. RCS plots showed the linear association of log₂-transformed U-Ni levels with TC, non-HDL-C and TC/HDL-C (P for nonlinearity = 0.294, 0.152, and 0.087, respectively). Besides, BMI decreased monotonically in correlation with increasing U-Ni levels (P for trend < 0.001). Mediation analysis revealed that BMI significantly mediated the relationship of U-Ni to TC, non-HDL-C and TC/HDL-C with mediated proportions of 11.17%, 22.20% and 36.44%, respectively. In summary, our findings suggest that BMI mediates the negative association of U-Ni with TC, non-HDL-C, and TC/HDL-C in the US general population.

Ocular Biocompatibility of a Nitinol Capsular Tension Ring (CTR)

Introduction: The biocompatibility of nitinol in the human body has extensively been demonstrated. Although nitinol is already being used for intraocular surgeries such as lens fragmentation and foreign body extraction, little is known about its intracapsular, long-term behavior. The purpose of this study is to evaluate the long-term uveal and capsular biocompatibility of a nitinol CTR placed in the capsular bag after cataract surgery in an animal model.

Method: After approval of the study by the Institutional Animal Care and the Ethics Committee, bilateral phacoemulsification was performed in 6 rabbits; 1 eye received a nitinol CTR and the other a control polymethylmethacrylate (PMMA) open-ended ring. Ophthalmic evaluation for the presence of infections in all 12 eyes was performed after 7 days, 4 weeks, 3 months, and 6 months follow-up period. After a follow-up period of 6 months, the eyes were enucleated, and a histopathologic evaluation was performed.

Results: Neither of the groups showed any clinical signs of posterior capsule opacification (PCO) or inflammation. The nitinol group showed slightly less inflammation during histopathologic examination compared to the PMMA group.

No biocompatibility issues have been observed in this animal study.

Conclusions: There were no histological differences between eyes implanted with nitinol and eyes implanted with PMMA rings. Nitinol has proven to show high biocompatibility when implanted in the capsular bag of the rabbit eye.

Nitinol Release of Nickel under Physiological Conditions: Effects of Surface Oxide, pH, Hydrogen Peroxide, and Sodium Hypochlorite

Nitinol is a nickel-titanium alloy widely used in medical devices for its unique pseudoelastic and shape-memory properties. However, nitinol can release potentially hazardous amounts of nickel, depending on surface manufacturing yielding different oxide thicknesses and compositions. Furthermore, nitinol medical devices can be implanted throughout the body and exposed to extremes in pH and reactive oxygen species (ROS), but few tools exist for evaluating nickel release under such physiological conditions. Even in cardiovascular applications, where nitinol medical devices are relatively common and the blood environment is well understood, there is a lack of information on how local inflammatory conditions after implantation might affect nickel ion release. For this study, nickel release from nitinol wires of different finishes was measured in pH conditions and at ROS concentrations selected to encompass and exceed literature reports of extracellular pH and ROS. Results showed increased nickel release at levels of pH and ROS reported to be physiological, with decreasing pH and increasing concentrations of hydrogen peroxide and NaOCl/HOCl having the greatest effects. The results support the importance of considering the implantation site when designing studies to predict nickel release from nitinol and underscore the value of understanding the chemical milieu at the device-tissue interface.

Metallic Implants Used in Lumbar Interbody Fusion

Over the last decade, pedicle fixation systems have evolved and modifications in spinal fusion techniques have been developed to increase fusion rates and improve clinical outcomes after lumbar interbody fusion (LIF). Regarding materials used for screw and rod manufacturing, metals, especially titanium alloys, are the most popular resources. In the case of pedicle screws, that biomaterial can be also doped with hydroxyapatite, CaP, ECM, or tantalum. Other materials used for rod fabrication include cobalt-chromium alloys and nitinol (nickel-titanium alloy). In terms of mechanical properties, the ideal implant used in LIF should have high tensile and fatigue strength, Young's modulus similar to that of the bone, and should be 100% resistant to corrosion to avoid mechanical failures. On the other hand, a comprehensive understanding of cellular and molecular pathways is essential to identify preferable characteristics of implanted biomaterial to obtain fusion and avoid implant loosening. Implanted material elicits a biological response driven by immune cells at the site of insertion. These reactions are subdivided into innate (primary cellular response with no previous exposure) and adaptive (a specific type of reaction induced after earlier exposure to the antigen) and are responsible for wound healing, fusion, and also adverse reactions, i.e., hypersensitivity. The main purposes of this literature review are to summarize the physical and mechanical properties of metal alloys used for spinal instrumentation in LIF which include fatigue strength, Young's modulus, and corrosion resistance. Moreover, we also focused on describing biological response after their implantation into the human body. Our review paper is mainly focused on titanium, cobalt-chromium, nickel-titanium (nitinol), and stainless steel alloys.

Nickels and tines: the myth of nickel allergy in intracranial stents

BACKGROUND

Most intracranial stents contain nickel alloy, and nickel allergy or hypersensitivity is common. Neurological injury following endovascular treatment with a nickel containing intracranial stent has been reported in patients with purported nickel allergy, but it is unclear whether these reactions represent true nickel hypersensitivity. We quantified nickel release from commonly used intracranial stents to investigate whether such stents should be avoided in patients with nickel allergy.

METHODS

We examined nickel release from seven commonly used intracranial stents: Enterprise, LVIS Jr, Neuroform, Wingspan, Zilver, Pipeline Flex Embolization Device, and Surpass Evolve. We incubated each stent in human plasma-like media for 30 days. Dimethylglyoxime (DMG) spot testing was performed on each stent to detect released nickel at 0 and 30 days. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was then used to quantify the nickel concentration of the media at 30 days. Nickel currency and nickel standard for atomic absorption spectrometry were used as positive controls.

RESULTS

DMG spot tests indicated nickel release only from nickel currency at 0 and 30 days of incubation. No nickel release was detected from any stent at 30 days using ICP-OES.

CONCLUSIONS

Nickel release from commonly used intracranial stents is negligible. These results suggest that previously reported hypersensitivity to these stents may be misattributed to nickel allergy, and that patients with nickel allergy may be safely treated with select nickel-containing stents.

ASIA (Shoenfeld's syndrome) due to hysteroscopic Essure sterilization

Essure (TM, Bayer; Leverkusen, Germany) may act as a potential cause of autoimmune/inflammatory syndrome by adjuvants (ASIA). Essure is a device hysteroscopically inserted into the fallopian tubes to elicit a local inflammatory response for permanent sterilization. Patients with ASIA present with a constellation of symptoms including fatigue, cognitive impairment, and arthralgias. It is well known that ASIA is triggered by implantation of foreign material such as breast implants and mesh for hernia repair. In the current study, we present a retrospective cohort of 33 patients electing to remove Essure due to pelvic pain and systemic symptoms consistent with an ASIA diagnosis, and detail a case report of an Essure patient. Furthermore, we reviewed the existing literature on adverse events associated with Essure and studies assessing outcomes following explantation. The concept that Essure may trigger ASIA is further supported by both in vivo and in vitro studies demonstrating immunostimulatory effects of the material components of the device. We conclude that the existing evidence is sufficient to recommend screening of Essure recipients for ASIA symptoms, and where indicated, discussion of the risks and potential benefits of surgical removal.

The role of surface oxide thickness and structure on the corrosion and nickel elution behavior of nitinol biomedical implants

Biocompatibility is an important factor for metallic medical device implants, and corrosion resistance of implantable alloys is one aspect of biocompatibility. Corrosion behavior of nitinol is strongly dependent upon the nature of the surface oxide, which forms during processing. The surface oxide is comprised of a mixture of titanium and nickel oxides, and subsequent thermal exposure (e.g., during shape setting) and surface removal (e.g., electropolishing, mechanical polishing, etching, etc.) influence its structure. Corrosion behavior is often assessed through testing methods such as cyclic potentiodynamic polarization (e.g., ASTM F2129) and nickel ion release. Studies have suggested that a correlation exists between oxide thickness and nickel ion release, with thicker oxides eluting more nickel. It is hypothesized that the composition of the surface oxide, and not only its thickness, influences the corrosion performance of nitinol. To investigate this, nitinol wire samples were processed to produce surface oxides with different structures both in terms of thickness and composition. These samples were tested per ASTM F2129 and nickel ion release testing.

Corrosion resistance of a Nitinol ocular microstent: Implications on biocompatibility

Nitinol is commonly used in medical implants due to its unique thermomechanical properties of shape memory and superelasticity. Free nickel has the potential to induce biological responses that may be a concern for permanent implants manufactured from nickel-containing alloys. Although there are extensive reports on the effects of surface treatments on corrosion behavior in cardiovascular Nitinol implants, there is a lack of data on corrosion resistance and impact on biocompatibility for ocular implants. Therefore, the objective of this study was to determine localized corrosion and nickel elution resistance of an electropolished Nitinol-based ocular device (Hydrus Microstent, Ivantis, Inc.) intended for patients with primary open angle glaucoma. Pitting corrosion susceptibility was characterized by potentiodynamic polarization testing per ASTM F2129. In addition, nickel ion release was quantified with immersion testing to 63 days. The results indicated high localized corrosion resistance as all samples reached polarization potentials of 800 mV without pitting initiation. Maximum nickel elution rates per device were less than approximately 1.1 ng/device/day after the first day of immersion and reduced to less than 0.1 ng/device/day after 7 days. For a patient with bilateral microstents, these nickel concentrations are ×10,000 lower than previously published tolerable intake levels for systemic toxicity. Overall, these corrosion results are in good agreement with literature values of well processed and biocompatible Nitinol devices indicating adverse systemic biological responses are not expected in vivo.

On the High Sensitivity of Corrosion Resistance of NiTi Stents with Respect to Inclusions: An Experimental Evidence

In this study, the electrochemical breakdown potentials (E _b) of NiTi stents were assessed in correlation to their nonmetallic inclusion fractions in the extra low inclusion (ELI) range (inclu.% < 1% in area fraction, average size <39 μm). Quantitative investigations were performed to study the role of nonmetallic inclusions during pitting corrosion. Two stent samples with different inclusion fractions were fabricated using commercial NiTi tubes for studying the corrosion and mechanism. A survey of seven commercial stents in Europe was also conducted. Dependence was observed between the breakdown potentials and the inclusion fractions in the ELI stent (inclu.% = 0.2-0.8%), in which the breakdown potentials were found to be inversely proportional to inclusion fractions and densities (E _b dropped from ∼800 to ∼400 mV). No breakdown occurred on the samples using high-purity NiTi materials (inclu.% < 0.1%). The roles of inclusions in pitting mechanisms were investigated using scanning electron microscopy (SEM) characterizations. The microstructural evidence showed that the impact of TiC and Ti₂NiO _x was very different in the pitting process. A maximum inclu.% ≤ 0.9% was required for obtaining E _b ≥ 600 mV to meet the Food and Drug Administrations (FDA's) in vivo safety acceptance (low risk up to 6 months postimplantation). The high-purity stents (inclu.% < 0.1%) did not exhibit corrosion susceptibility until 1000 mV, suggesting superior corrosion resistance and thus long-term in vivo safety.

Cardiovascular therapies utilizing targeted delivery of nanomedicines and aptamers

Cardiovascular ailments are the foremost trigger of death in the world today, including myocardial infarction and ischemic heart diseases. To date, extraordinary measures have been prescribed, from the perspectives of both conventional medical therapies and surgeries, to enforce cardiac cell regeneration post cardiac traumas, albeit with limited long-term success. The prospects of successful heart transplants are also grim, considering exorbitant costs and unavailability of suitable donors in most cases. From the perspective of cardiac revascularization, use of nanoparticles and nanoparticle mediated targeted drug delivery have garnered substantial attention, attributing to both active and passive heart targeting, with enhanced target specificity and sensitivity. This review focuses on this aspect, while outlining the progress in targeted delivery of nanomedicines in the prognosis and subsequent therapy of cardiovascular disorders, and recapitulating the benefits and intrinsic challenges associated with the incorporation of nanoparticles. This article categorically provides an overview of nanoparticle-mediated targeted delivery systems and their implications in handling cardiovascular diseases, including their intrinsic benefits and encountered procedural trials and challenges. Additionally, the solicitations of aptamers in targeted drug delivery with identical objectives, are presented. This includes a detailed appraisal on various aptamer-navigated nanoparticle targeted delivery platforms in the diagnosis and treatment of cardiovascular maladies. Despite a few impending challenges, subject to additional investigations, both nanoparticles as well as aptamers show a high degree of promise, and pose as the next generation of drug delivery vehicles, in targeted cardiovascular therapy.