Biomechanical Challenges to Polymeric Biodegradable Stents

Absorbable metal stents for vascular use in pediatric cardiology: progress and outlook

The past five years have yielded impressive advancements in fully absorbable metal stent technology. The desired ultimate ability for such devices to treat a vascular stenosis without long-term device-related complications or impeding future treatment continues to evoke excitement in clinicians and engineers alike. Nowhere is the need for fully absorbable metal stents greater than in patients experiencing vascular anomalies associated with congenital heart disease (CHD). Perhaps not surprisingly, commercially available absorbable metal stents have been implanted in pediatric cardiology patients with conditions ranging from pulmonary artery and vein stenosis to coarctation of the aorta and conduit/shunt reconstructions. Despite frequent short term procedural success, device performance has missed the mark with the commercially available devices not achieving degradation benchmarks for given applications. In this review we first provide a general overview detailing the theory of absorbable metal stents, and then review recent clinical use in CHD patients since the release of current-generation absorbable metal stents around 2019. We also discuss the challenges and our center's experience associated with the use of absorbable metal stents in this pediatric population. Lastly, we present potential directions for future engineering endeavors to mitigate existing challenges.

Multimodal effects of asymmetric coating of coronary stents by electrospinning and electrophoretic deposition

Drug-eluting stents (DESs) are drug-coated vascular implants that inhibit smooth muscle cell proliferation and limit in-stent re-stenosis. However, traditional DESs release a single drug into the blood and cannot cope with complex mechanisms in atherosclerosis and body responses. The present study aimed to develop a novel multimodal stent by fabricating asymmetric coating with electrophoretic deposition and electrospinning. Herein, we use heparin-loaded alginate (Hep/Alg) and atorvastatin calcium-loaded polyurethane (AtvCa/PU) coatings on the stent luminal and abluminal surfaces, respectively. Scanning electron microscopy (SEM) micrographs showed that the alginate coatings had uniformity and thin thickness. Meanwhile, the PU fibers were formed without beads, with an acceptable diameter and suitable mechanical properties. PU nanofiber revealed minimal degradation in a 1-month study. The release of AtvCa and Hep continued for 8 days without a significant initial burst release. None of the stent coatings were cytotoxic or hemolytic, and PU nanofibers supported the survival of human umbilical endothelial cells (HUVEC) with high adhesion and flattened morphologies. The results indicate that electrophoretic deposition and electrospinning have significant potential for achieving asymmetric coating on stents and a promising approach for dual drug release for multimodal effects in vascular stent applications.

InSilc Computational Tool for In Silico Optimization of Drug-Eluting Bioresorbable Vascular Scaffolds

Stents made by different manufacturers must meet the requirements of standard in vitro mechanical tests performed under different physiological conditions in order to be validated. In addition to in vitro research, there is a need for in silico numerical simulations that can help during the stent prototyping phase. In silico simulations have the ability to give the same stent responses as well as the potential to reduce costs and time needed to carry out experimental tests. The goal of this paper is to show the achievements of the computational platform created as a result of the EU-funded project InSilc, used for numerical testing of most standard tests for validation of preproduction bioresorbable vascular scaffolds (BVSs). Within the platform, an ad hoc simulation protocol has been developed based on the finite element (FE) analysis program PAK and user interface software CAD Field and Solid. Two different designs of two different stents have been numerically simulated using this integrated tool, and the results have been demonstrated. The following standard tests have been performed: longitudinal tensile strength, local compression, kinking, and flex 1-3. Strut thickness and additional pocket holes (slots) in two different scaffolds have been used as representative parameters for comparing the mechanical characteristics of the stents (AB-BVS vs. AB-BVS-thinner and PLLA-prot vs. PLLA-plot-slot). The AB-BVS-thinner prototype shows better overall stress distribution than the AB-BVS, while the PLLA-prot shows better overall stress distribution in comparison to the PLLA-plot-slot. In all cases, the values of the maximum effective stresses are below 220 MPa—the value obtained by in vitro experiment. Despite the presented results, additional considerations should be included before the proposed software can be used as a validation tool for stent prototyping.

Optimize PLA/EVA Polymers Blend Compositional Coating for Next Generation Biodegradable Drug-Eluting Stents

In this research work, polymer blends of poly-lactic acid (PLA)/ethylene vinyl acetate (EVA) were prepared as the drug carrier materials for a bi-layer drug-loaded coating film for coronary stents. Different optimum compositions of blends were prepared by using an intense mixer. Then, the blends were hot-pressed and later cold-pressed to prepare for films of different thickness. The changes in weight, surface analysis and biodegradability with increasing time were studied using Scanning electron microscopy (SEM), weight loss and biodegradability tests. The mechanical and thermal properties of drug-loaded films were studied through universal testing machine (UTM) and thermo-gravimetric analysis (TGA). The effects of PLA, EVA and drug contents on in-vitro drug contents were investigated through the Ultraviolet-Visible Spectroscopy (UV-VIS) chemical analysis technique. The results obtained clearly showed that the addition of PLA promoted the unleashing of the drug whereas the addition of EVA nearly did not have the same affect. The mechanical properties of these various films can be tuned by adjusting the contents of blend parts. The factors affecting the unleashing of the drug became a serious matter of concern in evaluating the performance of bio-resorbable drug eluting stents. As a result, today's chemical blends may be useful drug carrier materials for drug-loaded tube coatings capable delivering purgative drug in an incredibly tunable and regulated manner.

Biocompatibility and Mechanical Stability of Nanopatterned Titanium Films on Stainless Steel Vascular Stents

Nanoporous ceramic coatings such as titania are promoted to produce drug-free cardiovascular stents with a low risk of in-stent restenosis (ISR) because of their selectivity towards vascular cell proliferation. The brittle coatings applied on stents are prone to cracking because they are subjected to plastic deformation during implantation. This study aims to overcome this problem by using a unique process without refraining from biocompatibility. Accordingly, a titanium film with 1 µm thickness was deposited on 316 LVM stainless-steel sheets using magnetron sputtering. Then, the samples were anodized to produce nanoporous oxide. The nanoporous oxide was removed by ultrasonication, leaving an approximately 500 nm metallic titanium layer with a nanopatterned surface. XPS studies revealed the presence of a 5 nm-thick TiO₂ surface layer with a trace amount of fluorinated titanium on nanopatterned surfaces. Oxygen plasma treatment of the nanopatterned surface produced an additional 5 nm-thick fluoride-free oxide layer. The samples did not exhibit any cracking or spallation during plastic deformation. Cell viability studies showed that nanopatterned surfaces stimulate endothelial cell proliferation while reducing the proliferation of smooth muscle cells. Plasma treatment further accelerated the proliferation of endothelial cells. Activation of blood platelets did not occur on oxygen plasma-treated, fluoride-free nanopatterned surfaces. The presented surface treatment method can also be applied to other stent materials such as CoCr, nitinol, and orthopedic implants.

Application of in silico Platform for the Development and Optimization of Fully Bioresorbable Vascular Scaffold Designs

Bioresorbable vascular scaffolds (BVS), made either from polymers or from metals, are promising materials for treating coronary artery disease through the processes of percutaneous transluminal coronary angioplasty. Despite the opinion that bioresorbable polymers are more promising for coronary stents, their long-term advantages over metallic alloys have not yet been demonstrated. The development of new polymer-based BVS or optimization of the existing ones requires engineers to perform many very expensive mechanical tests to identify optimal structural geometry and material characteristics. in silico mechanical testing opens the possibility for a fast and low-cost process of analysis of all the mechanical characteristics and also provides the possibility to compare two or more competing designs. In this study, we used a recently introduced material model of poly-l-lactic acid (PLLA) fully bioresorbable vascular scaffold and recently empowered numerical InSilc platform to perform in silico mechanicals tests of two different stent designs with different material and geometrical characteristics. The result of inflation, radial compression, three-point bending, and two-plate crush tests shows that numerical procedures with true experimental constitutive relationships could provide reliable conclusions and a significant contribution to the optimization and design of bioresorbable polymer-based stents.

The Development of Design and Manufacture Techniques for Bioresorbable Coronary Artery Stents

Coronary artery disease (CAD) is the leading killer of humans worldwide. Bioresorbable polymeric stents have attracted a great deal of interest because they can treat CAD without producing long-term complications. Bioresorbable polymeric stents (BMSs) have undergone a sustainable revolution in terms of material processing, mechanical performance, biodegradability and manufacture techniques. Biodegradable polymers and copolymers have been widely studied as potential material candidates for bioresorbable stents. It is a great challenge to find a reasonable balance between the mechanical properties and degradation behavior of bioresorbable polymeric stents. Surface modification and drug-coating methods are generally used to improve biocompatibility and drug loading performance, which are decisive factors for the safety and efficacy of bioresorbable stents. Traditional stent manufacture techniques include etching, micro-electro discharge machining, electroforming, die-casting and laser cutting. The rapid development of 3D printing has brought continuous innovation and the wide application of biodegradable materials, which provides a novel technique for the additive manufacture of bioresorbable stents. This review aims to describe the problems regarding and the achievements of biodegradable stents from their birth to the present and discuss potential difficulties and challenges in the future.

Influences of Stent Design on In-Stent Restenosis and Major Cardiac Outcomes: A Scoping Review and Meta-Analysis

Thanks to the developments in implantable biomaterial technologies, invasive operating procedures, and widespread applications especially in vascular disease treatment, a milestone for interventional surgery was achieved with the introduction of vascular stents. Despite vascular stents providing a solution for embolisms, this technology includes various challenges, such as mechanical, electro-chemical complications, or in-stent restenosis (ISR) risks with long-term usage. Therefore, further development of biomaterial technologies is vital to overcome such risks and problems. For this purpose, recent research has focused mainly on the applications of surface modification techniques on biomaterials and vascular stents to increase their hemocompatibility. ISR risk has been reduced with the development and prevalent usage of the art technology stent designs of drug-eluting and biodegradable stents. Nevertheless, their problems have not been overcome completely. Furthermore, patients using drug-eluting stents are faced with further clinical challenges. Therefore, the bare metal stent, which is the first form of the vascular stent technology and includes the highest ISR risk, is still in common usage for vascular treatment applications. For this reason, further research is necessary to solve the remaining vital problems. In this scoping review, stent-based major cardiac events including ISR are analyzed depending on different designs and material selection in stent manufacturing. Recent and novel approaches to overcome such challenges are stated in detail.

Bioresorbable Stent to Manage Congenital Heart Defects in Children

Intravascular stents for pediatric patients that degrade without inhibiting vessel growth remain a clinical challenge. Here, poly(L-lactide) fibers (DH-BDS) at two thicknesses, 250 μm and 300 μm, were assembled into large, pediatric-sized stents (Ø10 - Ø20 mm). Fibers were characterized mechanically and thermally, then stent mechanical properties were compared to metal controls, while mass loss and degradation kinetics modeling estimated total stent degradation time. Thicker fibers displayed lower stiffness (1969 ± 44 vs 2126 ± 37 MPa) and yield stress (117 ± 12 vs 137 ± 5 MPa) than thinner counterparts, but exhibited similar fail strength (478 ± 28 vs 476 ± 16 MPa) at higher strains (47 ± 2 vs 44 ± 2%). Stents all exhibited crystallinity between 51.3 - 54.4% and fiber glass transition temperatures of 88.6 ± 0.5 °C and 84.6 ± 0.5 °C were well above physiological ranges. Radial strength (0.31 ± 0.01 - 0.34 ± 0.02 N/mm) in thinner stents was similar to metal stents (0.24 - 0.41 N/mm) up to Ø14 mm with no foreshortening and thicker coils granted comparable radial strength (0.32 ± 0.02 - 0.34 ± 0.02 N/mm) in stents larger than Ø14 mm. Both 10 mm (1.17 ± 0.02 % and 0.86 ± 0.1 %) and 12 mm (1.1 ± 0.03% and 0.89 ± 0.1%) stents exhibited minimal weight loss over one year. Degradation kinetics models predicted full stent degradation within 2.8 - 4.5 years depending on thickness. DH-BDS exhibiting hoop strength similar to metal stents and demonstrating minimal degradation and strength loss over the first year before completely disappearing within 3 to 4.5 years show promise as a pediatric interventional alternative to current strategies.

Bioresorbable Polymeric Scaffold in Cardiovascular Applications

Advances in material science and innovative medical technologies have allowed the development of less invasive interventional procedures for deploying implant devices, including scaffolds for cardiac tissue engineering. Biodegradable materials (e.g., resorbable polymers) are employed in devices that are only needed for a transient period. In the case of coronary stents, the device is only required for 6-8 months before positive remodelling takes place. Hence, biodegradable polymeric stents have been considered to promote this positive remodelling and eliminate the issue of permanent caging of the vessel. In tissue engineering, the role of the scaffold is to support favourable cell-scaffold interaction to stimulate formation of functional tissue. The ideal outcome is for the cells to produce their own extracellular matrix over time and eventually replace the implanted scaffold or tissue engineered construct. Synthetic biodegradable polymers are the favoured candidates as scaffolds, because their degradation rates can be manipulated over a broad time scale, and they may be functionalised easily. This review presents an overview of coronary heart disease, the limitations of current interventions and how biomaterials can be used to potentially circumvent these shortcomings in bioresorbable stents, vascular grafts and cardiac patches. The material specifications, type of polymers used, current progress and future challenges for each application will be discussed in this manuscript.

Fabrication and Characterization of Bioresorbable Drug-coated Porous Scaffolds for Vascular Tissue Engineering

Bioresorbable polymers have been studied for several decades as attractive candidates for promoting the advancement of medical science and bio-technology in modern society. In particular, with a well-defined architecture, bioresorbable polymers have prominent advantages over their bulk counterparts for applications in biomedical and implant devices, such as cell delivery, scaffolds for tissue engineering, and hydrogels as well as in the pharmaceutical fields. Biocompatible implant devices based on bioresorbable materials (for instance, bioresorbable polymers that combine the unique advantages of biocompability and easy handling) have emerged as a highly active field due to their promising applications in artificial implant systems and biomedical devices. In this paper, we report an approach to fabricate porous polycaprolactone (PCL) scaffolds using a 3D printing system. And its surface was treated to a hydrophilic surface using plasma treatment. Then, the aspirin and atorvastatin calcium salt mixture was dip coated onto the surface. The drug coating technology was used to deposit the drug material onto the scaffold surface. Our porous PCL scaffold was coated with aspirin and atorvastatin calcium salt to reduce the blood LDL cholesterol and restenosis. These results suggest that our approach may provide a promising scaffold for developing bioresorbable drug-delivery-biomaterials. We further demonstrate that our bioresorbable medical device can be used as vascular scaffolds to provide a wide range of applications for the design of medical devices.

The Crimping and Expanding Performance of Self-Expanding Polymeric Bioresorbable Stents: Experimental and Computational Investigation

Abstract: Polymeric bioresorbable stents (PBRSs) are considered the most promising devices to treat cardiovascular diseases. However, the mechanical weakness still hampers their application. In general, PBRSs are crimped into small sheathes and re-expanded to support narrowed vessels during angioplasty. Accordingly, one of the most significant requirements of PBRSs is to maintain mechanical efficacy after implantation. Although a little research has focused on commercial balloon-expanding PBRSs, a near-total lack has appeared on self-expanding PBRSs and their deformation mechanisms. In this work, self-expanding, composite polymeric bioresorbable stents (cPBRSs) incorporating poly(p-dioxanone) (PPDO) and polycaprolactone (PCL) yarns were produced and evaluated for their in vitro crimping and expanding potential. Furthermore, the polymer time-reliable viscoelastic effects of the structural and mechanical behavior of the cPBRSs were analyzed using computational simulations. Our results showed that the crimping process inevitably decreased the mechanical resistance of the cPBRSs, but that this could be offset by balloon dilatation. Moreover, deformation mechanisms at the yarn level were discussed, and yarns bonded in the crossings showed more viscous behavior; this property might help cPBRSs to maintain their structural integrity during implantation.

Blending Electronics with the Human Body: A Pathway toward a Cybernetic Future

At the crossroads of chemistry, electronics, mechanical engineering, polymer science, biology, tissue engineering, computer science, and materials science, electrical devices are currently being engineered that blend directly within organs and tissues. These sophisticated devices are mediators, recorders, and stimulators of electricity with the capacity to monitor important electrophysiological events, replace disabled body parts, or even stimulate tissues to overcome their current limitations. They are therefore capable of leading humanity forward into the age of cyborgs, a time in which human biology can be hacked at will to yield beings with abilities beyond their natural capabilities. The resulting advances have been made possible by the emergence of conformal and soft electronic materials that can readily integrate with the curvilinear, dynamic, delicate, and flexible human body. This article discusses the recent rapid pace of development in the field of cybernetics with special emphasis on the important role that flexible and electrically active materials have played therein.

Biodegradable coronary scaffolds: their future and clinical and technological challenges

Angioplasty and stenting are standard treatment options for both stabile occlusive coronary artery disease and acute myocardial infarctions. Over the last years, several biodegradable stent systems have entered pre-clinical and clinical evaluation and into clinical practice. A strong supporting scaffold is necessary after angioplasty to prevent elastic recoil of the vessel but in the long term a permanent metallic stent will only impair normal physiology of the artery wall. Thus, the main advantage of a resorbable system is the potential for better vessel recovery and function in the long term. The new stent systems differ from traditional stents in size and biological responses and questions have risen regarding their mechanical strength and increased risk of stent thrombosis. Here, we present current treatment options with biodegradable scaffolds, discuss further key areas for improvements and review novel technological advances in the context of all up-to-date clinical trial information. New material choices are also covered as well as special considerations for pre-clinical testing.

The future of heart valve replacement: recent developments and translational challenges for heart valve tissue engineering

Heart valve replacement is often the only solution for patients suffering from valvular heart disease. However, currently available valve replacements require either life-long anticoagulation or are associated with valve degeneration and calcification. Moreover, they are suboptimal for young patients, because they do not adapt to the somatic growth. Tissue-engineering has been proposed as a promising approach to fulfil the urgent need for heart valve replacements with regenerative and growth capacity. This review will start with an overview on the currently available valve substitutes and the techniques for heart valve replacement. The main focus will be on the evolution of and different approaches for heart valve tissue engineering, namely the in vitro, in vivo and in situ approaches. More specifically, several heart valve tissue-engineering studies will be discussed with regard to their shortcomings or successes and their possible suitability for novel minimally invasive implantation techniques. As in situ heart valve tissue engineering based on cell-free functionalized starter materials is considered to be a promising approach for clinical translation, this review will also analyse the techniques used to tune the inflammatory response and cell recruitment upon implantation in order to stir a favourable outcome: controlling the blood-material interface, regulating the cytokine release, and influencing cell adhesion and differentiation. In the last section, the authors provide their opinion about the future developments and the challenges towards clinical translation and adaptation of heart valve tissue engineering for valve replacement. Copyright © 2016 John Wiley & Sons, Ltd.

Human Endothelial Cell Models in Biomaterial Research

Endothelial cell (EC) models have evolved as important tools in biomaterial research due to ubiquitously occurring interactions between implanted materials and the endothelium. However, screening the available literature has revealed a gap between material scientists and physiologists in terms of their understanding of these biomaterial-endothelium interactions and their relative importance. Consequently, EC models are often applied in nonphysiological experimental setups, or too extensive conclusions are drawn from their results. The question arises whether this might be one reason why, among the many potential biomaterials, only a few have found their way into the clinic. In this review, we provide an overview of established EC models and possible selection criteria to enable researchers to determine the most reliable and relevant EC model to use.

The effect of thermal treatment on the mechanical properties of PLLA tubular specimens

Conventional permanent stent systems for vascular applications are associated with long-term risks, such as restenosis and thrombosis. To overcome these limitations, novel approaches using various biodegradable materials for stent construction have been investigated. In this context, thermal treatment of polymer materials is investigated to adjust the mechanical properties of biodegradable stents. In this work polymeric tubular specimens of biodegradable poly(L-lactide) (PLLA) were extruded and subjected to a molding process using different temperatures above glass transition temperature TG. Physicochemical properties of the molded samples were analyzed using DSC measurements and uniaxial tensile tests. The molding process resulted in a weakening of the PLLA tubular specimens with a simultaneous increase in the degree of crystallinity (χ).

Heart Valve Replacements with Regenerative Capacity

The incidence of severe valvular dysfunctions (e.g., stenosis and insufficiency) is increasing, leading to over 300,000 valves implanted worldwide yearly. Clinically used heart valve replacements lack the capacity to grow, inherently requiring repetitive and high-risk surgical interventions during childhood. The aim of this review is to present how different tissue engineering strategies can overcome these limitations, providing innovative valve replacements that proved to be able to integrate and remodel in pre-clinical experiments and to have promising results in clinical studies. Upon description of the different types of heart valve tissue engineering (e.g., in vitro, in situ, in vivo, and the pre-seeding approach) we focus on the clinical translation of this technology. In particular, we will deepen the many technical, clinical, and regulatory aspects that need to be solved to endure the clinical adaptation and the commercialization of these promising regenerative valves.

Feasibility of using bulk metallic glass for self-expandable stent applications

Self-expandable stents are widely used to restore blood flow in a diseased artery segment by keeping the artery open after angioplasty. Despite the prevalent use of conventional crystalline metallic alloys, for example, nitinol, to construct self-expandable stents, new biomaterials such as bulk metallic glasses (BMGs) are being actively pursued to improve stent performance. Here, we conducted a series of analyses including finite element analysis and molecular dynamics simulations to investigate the feasibility of using a prototypical Zr-based BMG for self-expandable stent applications. We model stent crimping of several designs for different percutaneous applications. Our results indicate that BMG-based stents with diamond-shaped crowns suffer from severe localization of plastic deformation and abrupt failure during crimping. As a possible solution, we further illustrate that such abrupt failure could be avoided in BMG-based stents without diamond shape crowns. This work would open a new horizon for a quest toward exploiting superior mechanical and functional properties of metallic glasses to design future stents. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1874-1882, 2017.

In vitro performance investigation of bioresorbable scaffolds - Standard tests for vascular stents and beyond

BACKGROUND/PURPOSE

Biodegradable polymers are the main materials for coronary scaffolds. Magnesium has been investigated as a potential alternative and was successfully tested in human clinical trials. However, it is still challenging to achieve mechanical parameters comparative to permanent bare metal (BMS) and drug-eluting stents (DES). As such, in vitro tests are required to assess mechanical parameters correlated to the safety and efficacy of the device.

METHODS/MATERIALS

In vitro bench tests evaluate scaffold profiles, length, deliverability, expansion behavior including acute elastic and time-dependent recoil, bending stiffness and radial strength. The Absorb GT1 (Abbott Vascular, Temecula, CA), DESolve (Elixir Medical Corporation, Sunnyvale, CA) and the Magmaris (BIOTRONIK AG, Bülach, Switzerland) that was previously tested in the BIOSOLVE II study, were tested.

RESULTS

Crimped profiles were 1.38±0.01mm (Absorb GT1), 1.39±0.01mm (DESolve) and 1.44±0.00mm (Magmaris) enabling 6F compatibility. Trackability was measured depending on stiffness and force transmission (pushability). Acute elastic recoil was measured at free expansion and within a mock vessel, respectively, yielding results of 5.86±0.76 and 5.22±0.38% (Absorb), 7.85±3.45 and 9.42±0.21% (DESolve) and 5.57±0.72 and 4.94±0.31% (Magmaris). Time-dependent recoil (after 1h) was observed for the Absorb and DESolve scaffolds but not for the Magmaris. The self-correcting wall apposition behavior of the DESolve did not prevent time-dependent recoil under vessel loading.

CONCLUSIONS

The results of the suggested test methods allow assessment of technical feasibility based on objective mechanical data and highlight the main differences between polymeric and metallic bioresorbable scaffolds.