An overview of thin film nitinol endovascular devices

Recent Advancements in Biomaterials for Chimeric Antigen Receptor T Cell Immunotherapy

Cellular immunotherapy is an innovative cancer treatment method that utilizes the patient's own immune system to combat tumor cells effectively. Currently, the mainstream therapeutic approaches include chimeric antigen receptor T cell (CAR-T) therapy, T cell receptor gene-modified T cell therapy and chimeric antigen receptor natural killer-cell therapy with CAR-T therapy mostly advanced. Nonetheless, the conventional manufacturing process of this therapy has shortcomings in each step that call for improvement. Marked efforts have been invested for its enhancement while notable progresses achieved in the realm of biomaterials application. With CAR-T therapy as a prime example, the aim of this review is to comprehensively discuss the various biomaterials used in cell immunotherapy, their roles in regulating immune cells, and their potential for breakthroughs in cancer treatment from gene transduction to efficacy enhancement. This article additionally addressed widely adopted animal models for efficacy evaluating.

Cell delivery devices for cancer immunotherapy

Adoptive cell therapy (ACT) that leverages allogeneic or autologous immune cells holds vast promise in targeted cancer therapy. Despite the tremendous success of ACT in treating hematopoietic malignancies, its efficacy is limited in eradicating solid tumors via intravenous infusion of immune cells. With the extending technology of cancer immunotherapy, novel delivery strategies have been explored to improve the therapeutic potency of adoptively transferred cells for solid tumor treatment by innovating the administration route, maintaining the cell viability, and normalizing the tumor microenvironment. In this review, a variety of devices for cell delivery are summarized. Perspectives and challenges of cell delivery devices for cancer immunotherapy are also discussed.

Surface finishing of Nitinol for implantable medical devices: A review

Nitinol (NiTi), a nickel-titanium alloy, has been used for various cardiovascular, orthopedic, fracture fixation, and orthodontic devices. As with most other metallic biomaterials, the corrosion resistance and biocompatibility of NiTi are primarily determined by the properties of the surface oxide layer such as thickness, chemical composition, structure, uniformity, and stability. Currently, a number of finishing methods are used to improve the properties of surface oxide of NiTi with an ultimate goal to produce a defect-free, impurity-free, thin homogeneous oxide layer that is stable and composed of only titanium dioxide (TiO₂ ) with negligible amount of Ni species. This review discusses the effects of various surface finishing methods such as mechanical polishing, electropolishing, magnetoelectropolishing, heat treatments at different temperatures, passivation, chemical etching, boiling in water, hydrogen peroxide treatment, and sterilization techniques (steam autoclave, ethylene oxide, dry heat, peracetic acid, and plasma-based treatments) on the properties of a surface oxide layer and how it impacts the corrosion resistance of NiTi. Considering the findings of the literature review, a checklist has been provided to assist with choosing finishing/sterilization methods and relevant rationale and recommendations to consider when selecting a surface finishing process for NiTi used in implantable medical devices.

Fabrication and in-vitro characterization of a polymeric aortic valve for minimally invasive valve replacement

The valve replacement therapy is the standard treatment for severe heart valve diseases. Nowadays, two types of commercial prosthesis are available: mechanical and biological, but both of them have severe limitations. Moreover, alternative therapeutic approach for valve replacement, based on minimally invasive techniques (MIAVR), motivates the search for new valve materials. In this study a polyurethane-based self-expandable tri-leaflets heart valve compatible with MIAVR procedure is proposed. The device is based on the development, fabrication and characterization of three different elements: the leaflets, the polymeric stent for supporting the leaflets, and the external metallic stent for anchoring the valve to the native aortic root. The polymeric stent and the valve leaflets were fabricated using a thermoplastic silicone-polycarbonate-urethane using 3D printing and spray technology while the external metallic stent was made in nickel titanium (Nitinol) to obtain a self-expandable valve after the crimping process. The three elements were assembled in the completed device and tested by crimping, fatigue and fluid-dynamic test. The novel polymeric valve proposed showed promising results about valve crimping capabilities, durability and fluid dynamic performances. This approach could offer advantages such as low cost and to produce a tailor-made device basing on patient's imaging data. Moreover, the selected biomaterial offers the potential to have a device that could need of permanent anticoagulation and lack of calcification.

Smart materials in cardiovascular implants: Shape memory alloys and shape memory polymers

Smart materials have intrinsic properties that change in a controlled fashion in response to external stimuli. Currently, the only smart materials with a significant clinical impact in cardiovascular implant design are shape memory alloys, particularly Nitinol. Recent prodigious progress in material science has resulted in the development of sophisticated shape memory polymers. In this article, we have reviewed the literature and outline the characteristics, advantages, and disadvantages of shape memory alloys and shape memory polymers which are relevant to clinical cardiovascular applications, and describe the potential of these smart materials for applications in coronary stents and transcatheter valves.

Microfabricated bioelectrodes on self-expandable NiTi thin film devices for implants and diagnostic instruments

State of the art minimally invasive treatments and diagnostics of neurological and cardiovascular diseases demand for flexible instruments and implants that enable sensing and stimulation of bioelectric signals. Besides medical applications, implantable bioelectronic brain-computer interfaces are envisioned as the next step in communication and data transfer. Conventional microelectrode arrays used for these types of applications are based on polymer substrates that are not suitable for biostable, rigid and self-expanding devices. Here, we present fully integrated bioelectrodes on superelastic NiTi carriers fabricated by microsystem technology processes. The insulation between the metallic NiTi structure and the Pt electrode layer is realized by different oxide layers (SiO_x, TaO_x and Yttrium stabilized Zirconia YSZ). Key properties of bioelectronic implants such as dissolution in body fluids, biocompatibility, mechanical properties and bioelectrical sensing/stimulation capabilities have been investigated by in vitro methods. Particular devices with YSZ are biostable and biocompatible, enabling sensing and stimulation. The major advantage of this system is the combination of medically approved materials and novel fabrication technology that enables miniaturization and integration beyond the state-of-the-art processes. The results demonstrate that this functionalization of superelastic NiTi is an enabling technology for the development of new kinds of bioelectronic devices.

Computational analysis of Nitinol stent-graft for endovascular aortic repair (EVAR) of abdominal aortic aneurysm (AAA): Crimping, sealing and fluid-structure interaction (FSI)

OBJECTIVES

We evaluate the crimping strain, sealing stress and contact forces on a Nitinol stent deployed in the aorta during endovascular aortic (or aneurysm) repair (EVAR) procedures. Nitinol shape memory effect (SME) is used. We also study the fluid-structure interaction (FSI) of the blood flow on the stented aorta.

METHODS

We employ Solidworks to generate a closed-cell honeycomb stent structure used to treat abdominal aortic aneurysm (AAA). We use the commercial Abaqus/Simulia finite element (FEM) simulation package to study the displacements and stresses experienced by the stent during the crimping phase and deployment into the aortic segment. The Nitinol stent is covered with Dacron, a popular graft material. We implement our own user-material (UMAT) subroutines to model the shape memory effect (SME) of Nitinol. The effect of the stent geometry is analyzed. We use the FSI analysis in Abaqus/Simulia to understand the effect of hemodynamic loading on the stent.

RESULTS

Results indicate that the crimping strain increases as the stent strut spacing increases. This is also the case for the radius of curvature. Maximum strains developed on the stent during crimping are in the order of 10%. Stresses exerted by the stent needed to completely seal the aorta are found to be below the yield stress values of Nitinol (700 MPa). Wall shear stresses (WSS) on the stented aorta are close to WSS obtained on the aorta alone.

CONCLUSION

Using Nitinol's thermo-reactivity property as opposed to its superelasticity causes the stent-graft to deploy more gently.

Nitinol thin films functionalized with CAR-T cells for the treatment of solid tumours

Micropatterned nickel titanium (commonly known as nitinol) thin films with complex designs, high structural resolution and excellent biocompatibility can be cheaply fabricated using magnetron sputtering. Here, we show that these benefits can be leveraged to fabricate micromesh implants that are loaded with tumour-specific human chimeric antigen receptor (CAR)-T cells for the treatment of solid tumours. In a mouse model of non-resectable ovarian cancer, the cell-loaded nitinol thin films spatially conformed to the implantation site, fostered the rapid expansion of T cells, delivered a high density of T cells directly to the tumour and significantly improved animal survival. We also show that self-expandable stents that were coated with T-cell-loaded films and implanted into subcutaneous tumours in mice improved the duration of stent patency by delaying tumour ingrowth. By providing direct access to tumours, CAR-T-cell-loaded micropatterned nitinol thin films can improve the effects of cell-based therapies.

Potential mechanisms of endothelialisation in individuals implanted with a leadless pacemaker systems: An experimental in vitro study

BACKGROUND

Leadless pacemaker technology is a promising upcoming field in clinical rhythmology. Today, the most commonly used system in the clinical setting is the Micra™ leadless pacemaker system (Medtronic). In autopsies of patients who witnessed non-pacemaker associated death, unexpected ingrowth/encapsulation within the wall of the right ventricle was reported. The occurrence of a complete encapsulation was not expected and the process of endothelialisation remains unclear. We hypothesized, that a local inflammatory response might be the cause of these findings. The aim of our experimental in-vitro study was to investigate the effect of the Micra™ system and its single components on inflammatory processes.

METHODS

For this purpose, whole Micra™ pacemakers were incubated in heparin plasma from 25 healthy volunteers for 48 h at 37 °C. Furthermore, 1 g gold, steel, titanium, tungsten and nitinol wires were incubated in heparin plasma for 48 h at 37 °C as well (n = 10). To detect eventual inflammatory processes, interleukin- (IL) 1β, IL-6, and tumor necrosis factor alpha (TNF-α), the chemokine IL-8 were measured using enzyme-linked immunosorbent assay (ELISA). Additionally, the level of transforming growth factor beta 1 (TGF-β1) and vascular endothelial growth factor (VEGF) were analysed.

RESULTS

ELISA analyses showed that the whole Micra system leads to a significant increase in the inflammatory cytokine IL-6 which correlates with the data gained by the incubation of whole blood with the different wires. In particular, 0.5 g of tungsten showed a significant rise of IL-6 which could also be found for IL-1β and IL-8.

CONCLUSIONS

The in vitro study of the Micra system showed that the material composition led to an onset of inflammatory processes in whole blood. Consequently, one may speculate that the composition of Micra pacemaker may have a local inflammatory, though subclinical, effects in patients implanted with a Micra™ pacemakers.

Shape memory of in-body tissue-engineered Biotube® vascular grafts and the preliminary evaluation in animal implantation experiments

BACKGROUND

Tissues formed by in-body tissue architecture (iBTA) are soft and flexible. However, strongly bending iBTA-induced vascular grafts, called biotubes, may cause lumen collapse by kinking, subsequently leading to occlusion after implantation. In this study, we developed a method for biotube shape memory and verified its performance in preliminary animal implantation experiments.

METHODS

Straight biotubes were prepared by subcutaneous embedding of straight molds into beagle dogs for two months. Upon overnight immersion of the obtained straight biotubes in a 70% ethanol solution under U-shape framing, the biotubes maintained their U shape even after washing with saline solution. Additionally, spiral-shaped biotubes formed from goats using spiral molds could be stretched straight via the same alcohol treatment.

RESULTS

Within limited acute-phase animal implantation experiments, U-shaped biotubes functioned as AV shunt grafts in the femoral region of the beagle without deformation of vascular shape (N.=11). In addition, the long straight biotubes derived from spiral molds could be interposed between goat carotid arteries while maintaining their straight shape (N.=2). All implants maintained perfect patency at the 1-month follow-up period without any evidence of vascular deformation.

CONCLUSIONS

By retaining iBTA-induced tissues in an alcohol solution in the target shape, the shape of the tissues was imprinted and maintained even after implantation within a limited acute period. Therefore, in order to obtain tissues of various shapes, it is unnecessary to use a mold design to maintain the individual shape.

Biocompatibility of new materials based on nano-structured nitinol with titanium and tantalum composite surface layers: experimental analysis in vitro and in vivo

A technology for obtaining materials from nanostructured nitinol with titanium- or tantalum-enriched surface layers was developed. Surface layers enriched with titanium or tantalum were shown to provide a decrease in the formation of reactive oxygen species and long-lived protein radicals in comparison to untreated nitinol. It was determined that human peripheral vessel myofibroblasts and human bone marrow mesenchymal stromal cells grown on nitinol bases coated with titanium or tantalum-enriched surface layers exhibit a nearly two times higher mitotic index. Response to implantation of pure nitinol, as well as nano-structure nitinol with titanium or tantalum-enriched surface layers, was expressed though formation of a mature uniform fibrous capsule peripherally to the fragment. The thickness of this capsule in the group of animals subjected to implantation of pure nitinol was 1.5 and 3.0-fold greater than that of the capsule in the groups implanted with nitinol fragments with titanium- or tantalum-enriched layers. No signs of calcinosis in the tissues surrounding implants with coatings were observed. The nature and structure of the formed capsules testify bioinertia of the implanted samples. It was shown that the morphology and composition of the surface of metal samples does not alter following biological tests. The obtained results indicate that nano-structure nitinol with titanium or tantalum enriched surface layers is a biocompatible material potentially suitable for medical applications.

Immobilization of antibacterial metallic cations (Ga3+, Zn2+ and Co2+) in a polypyrrole coating formed on Nitinol

Gallium, zinc and cobalt species were immobilized in hollow rectangular-sectioned microtubes of polypyrrole (PPy) electrosynthesized on Nitinol (NiTi) alloy by means of two different methods. One of them involved the immobilization after the PPy electropolymerization and the other one during the electrosynthesis process. The antibacterial activity of the coating against Escherichia coli (E. coli) was evaluated and the best results were obtained with gallium species. Characterization results demonstrated that gallium is incorporated into the PPy matrix as Ga³⁺ ions. The PPy film with gallium species incorporated during the electropolymerization exhibited a good corrosion protection performance.

Miniaturizing Floating Traps to Increase RF Safety of Magnetic-Resonance-Guided Percutaneous Procedures

OBJECTIVE

MRI in the area of cardiovascular catheter-based interventional procedures is an active field. A common intervention-revascularization of chronic total occlusions, requires a conductive guidewire for revascularization. The mechanical properties of guidewires are paramount to the successful execution of such procedures. Furthermore to benefit from MRI techniques, additional conductors are required to transmit signal from the tip of a catheter. Long thin conductors in MRI systems pose a safety risk in the form of RF heating due to induced RF currents on the conductors. Unfortunately many existing techniques to mitigate this risk require physical modification of the conductors, inevitably resulting in detrimental mechanical tradeoffs in the guidewire. This manuscript proposes a novel application and miniaturization of an existing device, the floating RF trap. The RF trap couples strongly to any thin conductor passing through the trap lumen inducing significant series impedance. This results in reduction of induced RF currents, and thus, heating.

METHODS AND RESULTS

This study shows theoretical and experimental analysis of induced impedance as well as theoretical reduction in heating due to various distributions of traps along the length of a catheter. Results of measuring induced current and heating in phantom experiments are also presented. Through comparison with commercial simulation packages and results of phantom experiments, it is shown that miniaturized RF traps can be modeled accurately, including their induced series impedance and effect on induced RF current.

CONCLUSION AND SIGNIFICANCE

It was demonstrated that floating RF traps present a feasible method to mitigate RF heating while maintaining important mechanical properties of guidewires.

Electroassisted Functionalization of Nitinol Surface, a Powerful Strategy for Polymer Coating through Controlled Radical Surface Initiation

Coating Nitinol (NiTi) surfaces with a polymer layer has become very appealing in the past few years owing to its increased attraction in the biomedical field. Although its intrinsic properties helped ensure its popularity, its extensive implementation is still hampered by its nickel inclusion, making it sensitive to pitting corrosion and therefore leading to the release of carcinogenic Ni²⁺ ions. Among all recent ways to modify NiTi surfaces, elaboration of self-assembled monolayers is of great interest as their high order confers a reinforcement of the metal surface corrosion resistance and brings new functionalities to the metal for postmodification processes. In this work, we compare the electroassisted and thermally assisted self-assembling of 11-(2-bromoisobutyrate)-undecyl-1-phosphonic acid (BUPA) to the classical immersion process on NiTi surfaces initially submitted to a hydrothermal treatment. Among all tested conditions, the electroassisted grafting of BUPA at room temperature appears to be the most promising alternative, as it allows grafting in very short times (5-10 min), thus preventing its degradation. The thus-formed layer has been proven to be sufficient to enable the surface-initiated atom transfer radical polymerization (SI-ATRP) of 2-(dimethylamino)ethyl methacrylate.

Interfacial Engineering of Hierarchically Porous NiTi/Hydrogels Nanocomposites with Exceptional Antibiofouling Surfaces

Seamlessly bridging the hard and the soft, a strategy to fabricate hierarchically porous NiTi/hydrogels nanocomposites is reported. The nanocomposite surface can hold high-content water while keeping its hierarchical nanoscale topography, thus showing exceptional antibiofouling performance. This strategy will lead to antibiofouling alloy (e.g., NiTi)/hydrogel nanocomposites for improved stents and other blood-contacting implants and medical devices.

Use of Micropatterned Thin Film Nitinol in Carotid Stents to Augment Embolic Protection

Stenting is an alternative to endarterectomy for the treatment of carotid artery stenosis. However, stenting is associated with a higher risk of procedural stroke secondary to distal thromboembolism. Hybrid stents with a micromesh layer have been proposed to address this complication. We developed a micropatterned thin film nitinol (M-TFN) covered stent designed to prevent thromboembolism during carotid intervention. This innovation may obviate the need or work synergistically with embolic protection devices. The proposed double layered stent is low-profile, thromboresistant, and covered with a M-TFN that can be fabricated with fenestrations of varying geometries and sizes. The M-TFN was created in multiple geometries, dimensions, and porosities by sputter deposition. The efficiency of various M-TFN to capture embolic particles was evaluated in different atherosclerotic carotid stenotic conditions through in vitro tests. The covered stent prevented emboli dislodgement in the range of 70%–96% during 30 min duration tests. In vitro vascular cell growth study results showed that endothelial cell elongation, alignment and growth behaviour silhouettes significantly enhance, specifically on the diamond-shape M-TFN, with the dimensions of 145 µm × 20 µm and a porosity of 32%. Future studies will require in vivo testing. Our results demonstrate that M-TFN has a promising potential for carotid artery stenting.

Ultrathin Metal Films with Defined Topographical Structures as In Vitro Cell Culture Platforms for Unveiling Vascular Cell Behaviors

Implanted material surfaces make direct contact with body tissues to work on its own purpose. Therefore, studies of the surface properties of implantable materials that determine cell fate are very important for successful implantation. Although numerous studies have addressed the relationship between cells and material surfaces, nonmetallic surfaces and metallic surfaces likely produce different cellular responses because of their intrinsic differences in surface energy, roughness, and chemical composition. Moreover, given the nontransparent property of metal materials, which hampers the real-time imaging of cellular behavior, a detailed cellular-level analysis at the metal-tissue interface has not been performed. In this study, metal-based cell culture platforms (MCPs) with defined microscale topographical patterns are developed using a combination of photolithography and direct current magnetron sputtering techniques. The MCPs allow to observe vascular cells on metals in real time and identify the selective regulation of human aortic smooth muscle cells and Human umbilical vein endothelial cells (HUVECs) by metallic surface topography. Additionally, atomic force microscopy, contact angles, and energy-dispersive X-ray spectroscopy analyses show that the MCPs exhibit nearly identical chemical properties with their bulk counterparts, demonstrating that MCPs can be utilized as an in vitro cell culture platform system for understanding the cellular behavior on metal substrates.

Preclinical Testing of a Novel Thin Film Nitinol Flow-Diversion Stent in a Rabbit Elastase Aneurysm Model

BACKGROUND AND PURPOSE

Thin film nitinol can be processed to produce a thin microporous sheet with a low percentage of metal coverage (<20%) and high pore attenuation (∼70 pores/mm(2)) for flow diversion. We present in vivo results from the treatment of experimental rabbit aneurysms by using a thin film nitinol-based flow-diversion device.

MATERIALS AND METHODS

Nineteen aneurysms in the rabbit elastase aneurysm model were treated with a single thin film nitinol flow diverter. Devices were also placed over 17 lumbar arteries to model perianeurysmal branch arteries of the intracranial circulation. Angiography was performed at 2 weeks (n = 7), 1 month (n = 8), and 3 months (n = 4) immediately before sacrifice. Aneurysm occlusion was graded on a 3-point scale (grade I, complete occlusion; grade II, near-complete occlusion; grade III, incomplete occlusion). Toluidine blue staining was used for histologic evaluation. En face CD31 immunofluorescent staining was performed to quantify neck endothelialization.

RESULTS

Markedly reduced intra-aneurysmal flow was observed on angiography immediately after device placement in all aneurysms. Grade I or II occlusion was noted in 4 (57%) aneurysms at 2-week, in 6 (75%) aneurysms at 4-week, and in 3 (75%) aneurysms at 12-week follow-up. All 17 lumbar arteries were patent. CD31 staining showed that 75% ± 16% of the aneurysm neck region was endothelialized. Histopathology demonstrated incorporation of the thin film nitinol flow diverter into the vessel wall and no evidence of excessive neointimal hyperplasia.

CONCLUSIONS

In this rabbit model, the thin film nitinol flow diverter achieved high rates of aneurysm occlusion and promoted tissue in-growth and aneurysm neck healing, even early after implantation.

Biocompatibility and Systemic Safety of a Novel Implantable Annuloplasty Ring for the Treatment of Mitral Regurgitation in a Minipig Model

Prosthetic annuloplasty rings are a common treatment modality for mitral regurgitation, and recently, percutaneous implantation techniques have gained popularity due to their favorable safety profile. Although in common use, biocompatibility of annuloplasty rings has been reported only sparsely in the literature, and none of these reports used the percutaneous technique of implantation. We report on the biocompatibility and the systemic safety of a novel transcatheter mitral valve annuloplasty ring (AMEND™) in 6 minipigs. This device is composed of a nitinol tube surrounded by a braided polyethylene terephthalate fabric tube. The device produced no adverse inflammatory response, showing gradual integration between the metal ring and the fabric by normal host fibrocellular response, leading to complete neoendocardium coverage. There was no evidence for adverse reactions, rejection, or intolerance in the valvular structure. In 2 animals, hemopericardium resulted from the implantation procedure, leading to right-sided cardiac insufficiency with pulmonary edema and liver congestion. The findings reported herein can serve as a case study for the expected healing pathology reactions after implantation of transcatheter mitral valve annuloplasty rings.

A novel low-profile thin-film nitinol/silk endograft for treating small vascular diseases

Since the introduction of various endovascular graft materials such as expanded polytetrafluoroethylene (e-PTFE) and Dacron^® polyester, they have been rapidly applied in endovascular devices for treating a variety of clinical situations. While present endovascular grafts have been successful in treating large blood vessels, there are still significant challenges and limitations for small and tortuous vessels to their use. Recently, our group has demonstrated the potential to use thin-film nitinol (TFN) as a novel material to develop endografts used in the treatment of a wide range of small vascular diseases because TFN is ultralow profile (that is, a few micrometers thick), relatively thromboresistant, and superelastic. While TFN has shown superior thromboresistance, its surface endothelialization is not rapid and sufficient. Therefore, our laboratory has been exploring the feasibility of using thin-film silk as a novel coating for facilitating rapid and confluent endothelial cell growth. The purpose of this study is to fabricate a low-profile composite endograft using thin layers of nitinol and silk, and to evaluate both thrombogenicity as well as endothelial cell and smooth muscle cell responses. This study also evaluates the functionality of the composite endograft using an in vitro blood circulation model. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 575-584, 2017.