Nanoengineered Stent Surface to Reduce In-Stent Restenosis in Vivo

Nanotexture and crystal phase regulation for synergistic enhancement in re-endothelialization on medical pure titanium surface

Re-endothelialization has been recognized as a promising strategy to address the tissue hyperplasia and subsequent restenosis which are major complications associated with vascular implant/interventional titanium devices. However, the uncontrollable over-proliferation of smooth muscle cells (SMCs) limits the clinical application of numerous modified strategies. Herein, a novel modified strategy involving with a two-step anodic oxidation and annealing treatment was proposed to achieve rapid re-endothelialization function regulated by regular honeycomb nanotexture and specific anatase phase on the titanium surface. Theoretical calculation revealed that the presence of nanotexture reduced the polar component of surface energy, while the generation of anatase significantly enhanced the polar component and total surface energy. Meanwhile, the modified surface with regular nanotexture and anatase phase produced positive effect on the expression of CD31, VE-Cadherin and down-regulated α-SMA proteins expression, indicating excellent capacity of pro-endothelial regeneration and inhibition of SMCs proliferation and migration. One-month in vivo implantation in rabbit carotid arteries further confirmed that modified tube implant surface effectively accelerated confluent endothelial monolayer formation and promoted native-like endothelium tissue regeneration. By contrast, original titanium tube implant induced a disorganized tissue proliferation in the lumen with a high risk of restenosis. Collectively, this study opens us an alternative route to achieve the function that selectively promotes endothelial cells (ECs) growth and suppresses SMCs on the medical titanium surface, which has a great potential in facilitating re-endothelialization on the surface of blood-contacting titanium implant.

Nanodiamond Decorated PEO Oxide Coatings on NiTi Alloy

Cardiovascular diseases (CVDs) remain a leading cause of death in the European population, primarily attributed to atherosclerosis and subsequent complications. Although statin drugs effectively prevent atherosclerosis, they fail to reduce plaque size and vascular stenosis. Bare metal stents (BMS) have shown promise in acute coronary disease treatment but are associated with restenosis in the stent. Drug-eluting stents (DES) have improved restenosis rates but present long-term complications. To overcome these limitations, nanomaterial-based modifications of the stent surfaces have been explored. This study focuses on the incorporation of detonation nanodiamonds (NDs) into a plasma electrolytic oxidation (PEO) coating on nitinol stents to enhance their performance. The functionalized ND showed a high surface-to-volume ratio and was incorporated into the oxide layer to mimic high-density lipoproteins (HDL) for reverse cholesterol transport (RCT). We provide substantial characterization of DND, including stability in two media (acetone and water), Fourier transmission infrared spectroscopy, and nanoparticle tracking analysis. The characterization of the modified ND revealed successful functionalization and adequate suspension stability. Scanning electron microscopy with EDX demonstrated successful incorporation of DND into the ceramic layer, but the formation of a porous surface is possible only in the high-voltage PEO. The biological assessment demonstrated the biocompatibility of the decorated nitinol surface with enhanced cell adhesion and proliferation. This study presents a novel approach to improving the performance of nitinol stents using ND-based surface modifications, providing a promising avenue for cardiovascular disease.

Blood-repellent and anti-corrosive surface by spin-coated SWCNT layer on intravascular stent materials

Despite intravascular bare metallic stents (BMS) being indispensable products in cardiovascular surgery, they face in-stent restenosis (ISR), resulting in stent failure or secondary surgical operation necessity. Accumulation or corrosion processes are key factors that promote ISR development in a vascular pathway, including an intravascular stent. The ISR can be inhibited by increasing the blood-repellency, and electrochemical corrosion resistance features using surface modification techniques on intravascular stent materials. In this study, Single-Walled Carbon Nanotube (SWCNT) structures were deposited using the spin-coating method on stent specimens made of 316L, 316LVM, CoCr-alloy, and Ti-alloy. Hydrophobicity and blood-repellency functions of coated and uncoated specimens were analysed by the Contact Angle (CA) values for distilled water (DIW), glycerol, blood plasma, and total-blood droplets using a computer-controlled goniometer system. Using a potentiostat, the electrochemical corrosion resistance features were analysed from obtained Electrochemical Impedance Spectroscopy (EIS) and Tafel curves in 37 °C Simulated Body Fluid (SBF) mimicking the human blood plasma. Due to the CA values below 90°, the repellency limit for hydrophobicity and blood-repellency, bare specimens performed hydrophilic and blood-philic features. However, SWCNT coating increased the repellency functions to 95° for DIW and 96° for total blood. The electrochemical corrosion resistance analysis showed that 1.433 kΩ cm² polarization resistance and 1.07 kΩ cm² electrochemical impedance of bare specimens increased to 142.8 kΩ cm² and 141.3 kΩ cm² by SWCNT coating. These corrosion resistance enhancements led to ratios of 78.13% inhibition in the corrosion rate and mass loss rate per year for SWCNT-coated 316LVM specimens. The maximum inhibition efficiency was observed for SWCNT-coated 316LVM specimens with a ratio of 87.92%. Obtained results indicate that SWCNT coating of the intravascular stents can inhibit the ISR risks of the BMS group.

In vitro hemocompatibility studies on small-caliber stents for cardiovascular applications

The doping of biologically meaningful ions into biphasic calcium phosphate (BCP) bioceramics, which exhibit biocompatibility with human body parts, has led to their effective use in biomedical applications in recent years. Doping with metal ions while changing the characteristics of the dopant ions, an arrangement of various ions in the Ca/P crystal structure. In our work, small-diameter vascular stents based on BCP and biologically appropriate ion substitute-BCP bioceramic materials were developed for cardiovascular applications. The small-diameter vascular stents were created using an extrusion process. FTIR, XRD, and FESEM were used to identify the functional groups, crystallinity, and morphology of the synthesized bioceramic materials. In addition, investigation of the blood compatibility of the 3D porous vascular stents was carried out via hemolysis. The outcomes indicate that the prepared grafts are appropriate for clinical requirements.

Nanostructured Titanium Implant Surface Facilitating Osseointegration from Protein Adsorption to Osteogenesis: The Example of TiO2 NTAs

Titanium implants have been widely applied in dentistry and orthopedics due to their biocompatibility and resistance to mechanical fatigue. TiO₂ nanotube arrays (TiO₂ NTAs) on titanium implant surfaces have exhibited excellent biocompatibility, bioactivity, and adjustability, which can significantly promote osseointegration and participate in its entire path. In this review, to give a comprehensive understanding of the osseointegration process, four stages have been divided according to pivotal biological processes, including protein adsorption, inflammatory cell adhesion/inflammatory response, additional relevant cell adhesion and angiogenesis/osteogenesis. The impact of TiO₂ NTAs on osseointegration is clarified in detail from the four stages. The nanotubular layer can manipulate the quantity, the species and the conformation of adsorbed protein. For inflammatory cells adhesion and inflammatory response, TiO₂ NTAs improve macrophage adhesion on the surface and induce M2-polarization. TiO₂ NTAs also facilitate the repairment-related cells adhesion and filopodia formation for additional relevant cells adhesion. In the angiogenesis and osteogenesis stage, TiO₂ NTAs show the ability to induce osteogenic differentiation and the potential for blood vessel formation. In the end, we propose the multi-dimensional regulation of TiO₂ NTAs on titanium implants to achieve highly efficient manipulation of osseointegration, which may provide views on the rational design and development of titanium implants.

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.

Characterizing the Mechanical Performance of a Bare-Metal Stent with an Auxetic Cell Geometry

This study develops and characterizes the distinctive mechanical features of a stainless-steel metal stent with a tailored structure. A high-precision femtosecond laser was used to micromachine a stent with re-entrant hexagonal (auxetic) cell geometry. We then characterized its mechanical behavior under various mechanical loadings using in vitro experiments and through finite element analysis. The stent properties, such as the higher capability of the stent to bear upon bending, exceptional advantage at elevated levels of twisting angles, and proper buckling, all ensured a preserved opening to maintain the blood flow. The outcomes of this preliminary study present a potential design for a stent with improved physiologically relevant mechanical conditions such as longitudinal contraction, radial strength, and migration of the stent.

Micro- and nanoscale biophysical cues for cardiovascular disease therapy

After cardiovascular injury, numerous pathological processes adversely impact the homeostatic function of cardiomyocyte, macrophage, fibroblast, endothelial cell, and vascular smooth muscle cell populations. Subsequent malfunctioning of these cells may further contribute to cardiovascular disease onset and progression. By modulating cellular responses after injury, it is possible to create local environments that promote wound healing and tissue repair mechanisms. The extracellular matrix continuously provides these mechanosensitive cell types with physical cues spanning the micro- and nanoscale to influence behaviors such as adhesion, morphology, and phenotype. It is therefore becoming increasingly compelling to harness these cell-substrate interactions to elicit more native cell behaviors that impede cardiovascular disease progression and enhance regenerative potential. This review discusses recent in vitro and preclinical work that have demonstrated the therapeutic implications of micro- and nanoscale biophysical cues on cell types adversely affected in cardiovascular diseases - cardiomyocytes, macrophages, fibroblasts, endothelial cells, and vascular smooth muscle cells.

Surface engineering at the nanoscale: A way forward to improve coronary stent efficacy

Coronary in-stent restenosis and late stent thrombosis are the two major inadequacies of vascular stents that limit its long-term efficacy. Although restenosis has been successfully inhibited through the use of the current clinical drug-eluting stent which releases antiproliferative drugs, problems of late-stent thrombosis remain a concern due to polymer hypersensitivity and delayed re-endothelialization. Thus, the field of coronary stenting demands devices having enhanced compatibility and effectiveness to endothelial cells. Nanotechnology allows for efficient modulation of surface roughness, chemistry, feature size, and drug/biologics loading, to attain the desired biological response. Hence, surface topographical modification at the nanoscale is a plausible strategy to improve stent performance by utilizing novel design schemes that incorporate nanofeatures via the use of nanostructures, particles, or fibers, with or without the use of drugs/biologics. The main intent of this review is to deliberate on the impact of nanotechnology approaches for stent design and development and the recent advancements in this field on vascular stent performance.

Mixed oxide nanotubes in nanomedicine: A dead-end or a bridge to the future?

Nanomedicine has seen a significant rise in the development of new research tools and clinically functional devices. In this regard, significant advances and new commercial applications are expected in the pharmaceutical and orthopedic industries. For advanced orthopedic implant technologies, appropriate nanoscale surface modifications are highly effective strategies and are widely studied in the literature for improving implant performance. It is well-established that implants with nanotubular surfaces show a drastic improvement in new bone creation and gene expression compared to implants without nanotopography. Nevertheless, the scientific and clinical understanding of mixed oxide nanotubes (MONs) and their potential applications, especially in biomedical applications are still in the early stages of development. This review aims to establish a credible platform for the current and future roles of MONs in nanomedicine, particularly in advanced orthopedic implants. We first introduce the concept of MONs and then discuss the preparation strategies. This is followed by a review of the recent advancement of MONs in biomedical applications, including mineralization abilities, biocompatibility, antibacterial activity, cell culture, and animal testing, as well as clinical possibilities. To conclude, we propose that the combination of nanotubular surface modification with incorporating sensor allows clinicians to precisely record patient data as a critical contributor to evidence-based medicine.

Vascular Grafts with Tailored Stiffness and a Ligand Environment via Multiarmed Polymer Sheath for Expeditious Regeneration

The bypass graft is the mainstream of surgical intervention to treat vascular diseases. Ideal bypass materials, yet to be developed, require mechanical properties, availability, clinically feasible manufacturing logistics, and bioactivities with precise physicochemical cues defined to guide cell activities for arterial regeneration. Such needs instigated our fabrication of vascular grafts, which consist of coaxial, nanostructured fibers exhibiting a polycaprolactone (PCL) core and a photoclickable, 4-arm thiolated polyethylene glycol-norbornene (PEG-NB) sheath. The graft strength and bioactivity were modulated by the PCL concentration and the peptides (RGD, transforming growth factor β-1 or TGF-β1) conjugated to thiol-ene of PEG-NB, respectively. Structural, physical, and mechanical characterizations demonstrated that the fibrous grafts mimicked the key features of the native extracellular matrix, including a crosslinked fiber network for structural stability, viscoelasticity emulating arteries, hydration property, and high porosity for cell infiltration. Meanwhile, these grafts displayed strength and toughness exceeding or meeting surgical criteria. Furthermore, the grafts with higher PCL concentration (3 vs 1.8%) showed thicker fibers, lower porosity and pore size, and increased elastic and storage moduli. Graft bioactivity was determined by the mesenchymal stem cell (MSC) behaviors on the grafts and arterial regeneration in vivo using interposition grafting. Results showed that the cell adhesion and proliferation increased with the RGD density (25 vs 5 mM). After 1 week implantation, all peptide-functionalized PCL/PEG-NB grafts with or without MSC preseeding, as opposed to PCL grafts, showed expeditious endothelial lining, abundant vascular cell infiltration, and matrix production. Compared to RGD grafts, RGD/TGF-β1 grafts enhanced MSC differentiation into smooth muscle cells in vitro and developed thicker smooth muscle cell layers in vivo. Overall, the versatile porous vascular grafts offer superior properties and tunability for future translation.

Anti-Biofouling Strategies for Long-Term Continuous Use of Implantable Biosensors

The growing trend for personalized medicine calls for more reliable implantable biosensors that are capable of continuously monitoring target analytes for extended periods (i.e., >30 d). While promising biosensors for various applications are constantly being developed in the laboratories across the world, many struggle to maintain reliable functionality in complex in vivo environments over time. In this review, we explore the impact of various biotic and abiotic failure modes on the reliability of implantable biosensors. We discuss various design considerations for the development of chronically reliable implantable biosensors with a specific focus on strategies to combat biofouling, which is a fundamental challenge for many implantable devices. Briefly, we introduce the process of the foreign body response and compare the in vitro and the in vivo performances of state-of-the-art implantable biosensors. We then discuss the latest development in material science to minimize and delay biofouling including the usage of various hydrophilic, biomimetic, drug-eluting, zwitterionic, and other smart polymer materials. We also explore a number of active anti-biofouling approaches including stimuli-responsive materials and mechanical actuation. Finally, we conclude this topical review with a discussion on future research opportunities towards more reliable implantable biosensors.

Successful Reduction of Neointimal Hyperplasia on Stainless Steel Coronary Stents by Titania Nanotexturing

Bare metal stents (BMSs) of stainless steel (SS) were surface engineered to develop nanoscale titania topography using a combination of physical vapor deposition and thermochemical processing. The nanoleafy architecture formed on the stent surface remained stable and adherent upon repeated crimping and expansion, as well as under flow. This titania nanoengineered stent showed a preferential proliferation of endothelial cells over smooth muscle cells in vitro, which is an essential requirement for improving the in vivo endothelialization, with concurrent reduction of intimal hyperplasia. The efficacy of this surface-modified stent was assessed after implantation in rabbit iliac arteries for 8 weeks. Significant reduction in neointimal thickening and thereby in-stent restenosis with complete endothelial coverage was observed for the nanotextured stents, compared to BMSs, even without the use of any antiproliferative agents or polymers as in drug-eluting stents. Nanotexturing of stents did not induce any inflammatory response, akin to BMSs. This study thus indicates the effectiveness of a facile titania nanotopography on SS stents for coronary applications and the possibility of bringing this low-priced material back to clinics.

One-Step Large-Scale Nanotexturing of Nonplanar PTFE Surfaces to Induce Bactericidal and Anti-inflammatory Properties

Here we demonstrate a simple and scalable nanotexturing method for both planar (films) and nonplanar (tubes) polytetrafluoroethylene (PTFE) surfaces using a commercial desktop oxygen plasma etcher. The simple process can generate semiordered nanopillar structures on both tubular and planar samples with high radial and axial uniformity. We found that the resulting surfaces exhibit good in vitro bactericidal and in vivo anti-inflammatory properties. When tested against Staphylococcus aureus, the nanotextured surfaces showed significantly decreased live bacteria coverage and increased dead bacteria coverage, demonstrating significant bactericidal functionality. Moreover, the etched planar PTFE films exhibited better healing and inflammatory responses in the subcutis of C57BL/6 mice over 7 and 21 days, evidenced by a thinner inflammatory band, lower collagen deposition, and decreased macrophage infiltration. Our results suggest the possibility of using this simple process to generate large scale biomimetic nanotextured surfaces with good antibiofouling properties to enhance the functionality of many implantable and other biomedical devices.

Live endothelial cells on plasma-nitrided and oxidized titanium: An approach for evaluating biocompatibility

We evaluated the effects of titanium plasma nitriding and oxidation on live endothelial cell viscoelasticity. For this, mechanically polished titanium surfaces and two surfaces treated by planar cathode discharge in nitriding (36N₂ and 24H₂) and oxidant (36O₂ and 24H₂). Surfaces were characterized regarding wettability, roughness and chemical composition. Rabbit aortic endothelial cells (RAECs) were cultured on the titanium surfaces. Cell morphology, viability and viscoelasticity were evaluated by scanning electron microscopy (SEM), methyl thiazolyl tetrazolium (MTT) assay and atomic force microscopy (AFM), respectively. Grazing Incidence X-ray Diffraction confirmed the presence of TiN_0,26 on the surface (grazing angle theta 1°) of the nitrided samples, decreasing with depth. On the oxidized surface had the formation of TiO₃ on the material surface (Theta 1°) and in the deeper layers was noted, with a marked presence of Ti (Theta 3°). Both plasma treatments increased surface roughness and they are hydrophilic (angle <90°). However, oxidation led to a more hydrophilic titanium surface (66.59° ± 3.65 vs. 76.88° ± 2.68; p = 0.001) due to titanium oxide films in their stoichiometric varieties (Ti₃O, TiO₂, Ti₆O), especially Ti₃O. Despite focal adhesion on the surfaces, viability was different after 24 h, as cell viability on the oxidized surface was higher than on the nitrided surface (9.1 × 10³ vs. 4.5 × 10³cells; p < 0.05). This can be explained by analyzing the viscoelastic property of the cellular cytoskeleton (nuclear and peripheral) by AFM. Surface oxidation significantly increased RAECs viscoelasticity at cell periphery, in comparison to the nucleus (2.36 ± 0.3 vs. 1.5 ± 0.4; p < 0.05), and to the RAECs periphery in contact with nitrided surfaces (1.36 ± 0.7; p < 0.05) and polished surfaces (1.55 ± 0.6; p < 0.05). Taken together, our results have shown that titanium plasma treatment directly increased cell viscoelasticity via surface oxidation, and this mechanobiological property subsequently increased biocompatibility.

Enhanced interfacial adhesion and osseointegration of anodic TiO2 nanotube arrays on ultra-fine-grained titanium and underlying mechanisms

The poor adhesion of anodic TiO₂ nanotubes (TNTs) arrays on titanium (Ti) substrates adversely affects applications in many fields especially biomedical engineering. Herein, an efficient strategy is described to improve the adhesion strength of TNTs by performing grain refinement in the underlying Ti substrate via high-pressure torsion processing, as a larger number of grain boundaries can provide more interfacial mechanical anchorage. This process also improves the biocompatibility and osseointegration of TNTs by increasing the surface elastic modulus. The TNTs in length of 0.4 µm have significantly larger adhesion strength than the 2.0 µm long ones because the shorter TNTs experience less interfacial internal stress. However, post-anodization annealing reduces the fluorine concentration in TNTs and adhesion strength due to the formation of interfacial cavities during crystallization. The interfacial structure of TNTs/Ti system and the mechanism of adhesion failures are further investigated and discussed. STATEMENT OF SIGNIFICANCE: Self-assembled TiO₂ nanotubes (TNTs) prepared by electrochemical anodization have a distinct morphology and superior properties, which are commonly used in photocatalytic systems, electronic devices, solar cells, sensors, as well as biomedical implants. However, the poor adhesion between the TNTs and Ti substrate has hampered wider applications. Here in this study, we describe an efficient strategy to improve the adhesion strength of TNTs by performing grain refinement in the underlying Ti substrate via high-pressure torsion (HPT) processing. The interfacial structure of TNTs/Ti system and the mechanism of adhesion failure are systematically studied and discussed. Our findings not only develop the knowledge of TNTs/Ti system, but also provide new insights into the design of Ti-based implants for orthopedic applications.

TiO2-Based Nanotopographical Cues Attenuate the Restenotic Phenotype in Primary Human Vascular Endothelial and Smooth Muscle Cells

Coronary and peripheral stents are implants that are inserted into blocked arteries to restore blood flow. After stent deployment, the denudation of the endothelial cell (EC) layer and the resulting inflammatory cascade can lead to restenosis, the renarrowing of the vessel wall due to the hyperproliferation and excessive matrix secretion of smooth muscle cells (SMCs). Despite advances in drug-eluting stents (DES), restenosis remains a clinical challenge and can require repeat revascularizations. In this study, we investigated how vascular cell phenotype can be modulated by nanotopographical cues on the stent surface, with the goal of developing an alternative strategy to DES for decreasing restenosis. We fabricated TiO2 nanotubes and demonstrated that this topography can decrease SMC surface coverage without affecting endothelialization. In addition, to our knowledge, this is the first study reporting that TiO2 nanotube topography dampens the response to inflammatory cytokine stimulation in both endothelial and smooth muscle cells. We observed that compared to flat titanium surfaces, nanotube surfaces attenuated tumor necrosis factor alpha (TNFα)-induced vascular cell adhesion molecule-1 (VCAM-1) expression in ECs by 1.8-fold and decreased TNFα-induced SMC growth by 42%. Further, we found that the resulting cellular phenotype is sensitive to changes in nanotube diameter and that 90 nm diameter nanotubes leads to the greatest magnitude in cell response compared to 30 or 50 nm nanotubes.

Ta ion implanted nanoridge-platform for enhanced vascular responses

Bare metal stents are commonly used in interventional cardiology; they provide successful treatment because of their excellent mechanical properties, expandability ratios, and flexibility. However, their insufficient vascular affinity can induce the development of neointimal hyperplasia following arterial injury and subsequent smooth muscle cell overgrowth in the lumen of a stented vessel. Nanoengineering of the bare metal stent surface is a valuable strategy for eliciting favorable vascular responses. In this study, we introduce a target-ion-induced plasma sputtering (TIPS) technique to fabricate a platform with a favorable endothelial environment. This technique enables the simple single-step production of a Ta-implanted nanoridged surface on a stent with a complex 3D geometry that shows a clear tendency to become oriented parallel to the direction of blood flow. Moreover, the nanoridges developed show good structural integrity and mechanical stability, resulting in apparently stable morphologies under high strain rates. In vitro cellular responses to the Co-Cr, such as endothelialization, platelet activation, and blood coagulation, are considerably altered after TIPS treatment; endothelium formation is rapid and surface thrombogenicity is low. An in vivo rabbit iliac artery model is used to confirm that the nanoridged surface facilitates rapid re-endothelialization and limits the formation of neointima compared to the bare stent. These results indicate that the Ta ion implanted nanoridge platform fabricated using the TIPS technique has immense potential as a solution for in-stent restenosis and ensuring the long-term patency of bare metal stents.

Ni-free, built-in nanotubular drug eluting stents: Experimental and theoretical insights

Stents used for cardiovascular applications are composed of three main elements; a metal, polymer coating and the specific drug component. Nickel-based metals and polymer coatings currently used in the stent market have increased the recurrence of in-stent restenosis and stent failure due to inflammation. In this study, a Ti-8Mn alloy was used to fabricate a nanostructured surface that can be used for drug eluting stents to overcome the hypersensitivity of metals that are currently used in stent making as well as introducing a new built-in nano-drug reservoir instead of polymer coatings. Two different systems were studied: titanium dioxide nanotubes (NTs) and Ti-8Mn oxides NTs. The materials were characterized using field emission electron microscope (FESEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), roughness, wettability and surface energy measurements. Nanoindentation was used to evaluate the mechanical properties of the nanotubes as well as their stability. In-vitro cytotoxicity and cell proliferation assays were used to study the effect of the nanotubes on cell viability. Computational insights were also used to test the blood compatibility using band gap model analysis, comparing the band gap of the materials under investigation with that of the fibrinogen, in order to study the possibility of charge transfer that affects the blood clotting mechanism. In addition, the drug loading capacity of the materials was studied using acetyl salicylic acid as a drug model.

Interface Engineering of Fully Metallic Stents Enabling Controllable H2O2 Generation for Antirestenosis

Despite significant advances in the design of metallic materials for bare metal stents (BMSs), restenosis induced by the accumulation of smooth muscle cells (SMCs) has been a major constraint on improving the clinical efficacy of stent implantation. Here, a new strategy for avoiding this issue by utilizing hydrogen peroxide (H₂O₂) generated by the galvanic coupling of nitinol (NiTi) stents and biodegradable magnesium-zinc (Mg-Zn) alloys is reported. The amount of H₂O₂ released is carefully optimized via the biodegradability engineering of the alloys and by controlling the immersion time to selectively inhibit the proliferation and function of SMCs without harming vascular endothelial cells. Based on demonstrations of its unique capabilities, a fully metallic stent with antirestenotic functionality was successfully fabricated by depositing Mg layers onto commercialized NiTi stents. The introduction of surface engineering to yield a patterned Mg coating ensured the maintenance of a stable interface between Mg and NiTi during the process of NiTi stent expansion, showing high feasibility for clinical application. This new concept of an inert metal/degradable metal hybrid system based on galvanic metal coupling, biodegradability engineering, and surface patterning can serve as a novel way to construct functional and stable BMSs for preventing restenosis.

Coaxially-structured fibres with tailored material properties for vascular graft implant

Readily-available small-diameter arterial grafts require a great combination of materials properties, including high strength, compliance, suturability, blood sealing and anti-thrombogenicity, as well as anti-kinking property for those used in challenging anatomical situations. We have constructed grafts composed of coaxially-structured polycaprolactone (PCL)/gelatin nanofibres, and tailored the material structures to achieve high strength, compliance and kink resistance, as well as excellent water sealing and anti-thrombogenicity. Coaxially-structured fibres in the grafts provided mechanical stability through the core, while flexibility and cell adhesion through the sheath. Results showed that graft compliance increased while strength decreased with the concentration ratio between core and sheath polymers. Compared to pure PCL fibrous surfaces, coaxial PCL/gelatin fibrous surfaces potently inhibited platelet adhesion and activation, providing excellent anti-thrombogenicity. To render sufficient burst strength and suturability, an additional layer of pure PCL was necessary to cap the layer of coaxial PCL/gelatin fibres. The two-layered grafts with the wall thickness comparable to native arteries demonstrated artery-like compliance and kink resistance, properties important to arteries under complex mechanical loading. The in vivo evaluation was performed using the interposition carotid artery graft model in rabbits for three months. Interestingly, results from ultrasonic imaging and histological analysis demonstrated that the two-layered grafts with a thinner outer PCL layer, which possessed higher compliance and kink resistance, showed increased blood flow, minimal lumen reduction and fibrosis. All vascular grafts exhibited patency and induced limited cell infiltration. Together, we presented a facile and useful approach to fabricate vascular grafts with superior graft performances, biomechanical properties, and blood compatibility. Grafts with artery-like compliance and flexibility have demonstrated improved implantation outcomes.

Cardiovascular stents: overview, evolution, and next generation

Compared to bare-metal stents (BMSs), drug-eluting stents (DESs) have been regarded as a revolutionary change in coronary artery diseases (CADs). Releasing pharmaceutical agents from the stent surface was a promising progress in the realm of cardiovascular stents. Despite supreme advantages over BMSs, in-stent restenosis (ISR) and long-term safety of DESs are still deemed ongoing concerns over clinically application of DESs. The failure of DESs for long-term clinical use is associated with following factors including permanent polymeric coating materials, metallic stent platforms, non-optimal drug releasing condition, and factors that have recently been supposed as contributory factors such as degradation products of polymers, metal ions due to erosion and degradation of metals and their alloys utilizing in some stents as metal frameworks. Discovering the direct relation between stent materials and associating adverse effects is a complicated process, and yet it has not been resolved. For clinical success it is of significant importance to optimize DES design and explore novel strategies to overcome all problems including inflammatory response, delay endothelialization, and sub-acute stent thrombosis (ST) simultaneously. In this work, scientific reports are reviewed particularly focusing on recent advancements in DES design which covers both potential improvements of existing and recently novel prototype stent fabrications. Covering a wide range of information from the BMSs to recent advancement, this study mostly sheds light on DES's concepts, namely stent composition, drug release mechanism, and coating techniques. This review further reports different forms of DES including fully biodegradable DESs, shape-memory ones, and polymer-free DESs.

Biomimetic soft fibrous hydrogels for contractile and pharmacologically responsive smooth muscle

The ability to assess changes in smooth muscle contractility and pharmacological responsiveness in normal or pathological-relevant vascular tissue environments is critical to enable vascular drug discovery. However, major challenges remain in both capturing the complexity of in vivo vascular remodeling and evaluating cell contractility in complex, tissue-like environments. Herein, we developed a biomimetic fibrous hydrogel with tunable structure, stiffness, and composition to resemble the native vascular tissue environment. This hydrogel platform was further combined with the combinatory protein array technology as well as advanced approaches to measure cell mechanics and contractility, thus permitting evaluation of smooth muscle functions in a variety of tissue-like microenvironments. Our results demonstrated that biomimetic fibrous structure played a dominant role in smooth muscle function, while the presentation of adhesion proteins co-regulated it to various degrees. Specifically, fibre networks enabled cell infiltration and upregulated expression of actomyosin proteins in contrast to flat hydrogels. Remarkably, fibrous structure and physiologically relevant stiffness of hydrogels cooperatively enhanced smooth muscle contractility and pharmacological responses to vasoactive drugs at both the single cell and intact tissue levels. Together, this study is the first to demonstrate alterations of human vascular smooth muscle contractility and pharmacological responsiveness in biomimetic soft, fibrous environments with a cellular array platform. The integrated platform produced here could enable investigations for pathobiology and pharmacological interventions by developing a broad range of patho-physiologically relevant in vitro tissue models.

STATEMENT OF SIGNIFICANCE

Engineering functional smooth muscle in vitro holds the great potential for diseased tissue replacement and drug testing. A central challenge is recapitulating the smooth muscle contractility and pharmacological responses given its significant phenotypic plasticity in response to changes in environment. We present a biomimetic fibrous hydrogel with tunable structure, stiffness, and composition that enables the creation of functional smooth muscle tissues in the native-like vascular tissue microenvironment. Such fibrous hydrogel is further combined with the combinatory protein array technology to construct a cellular array for evaluation of smooth muscle phenotype, contraction, and cell mechanics. The integrated platform produced here could be promising for developing a broad range of normal or diseased in vitro tissue models.

Progress in TiO2 nanotube coatings for biomedical applications: a review

Titanium dioxide nanotubes (TNTs) have drawn wide attention and been extensively applied in the field of biomedicine, due to their large specific surface area, good corrosion resistance, excellent biocompatibility, and enhanced bioactivity. This review describes the preparation of TNTs and the surface modification that entrust the nanotubes with better antibacterial property and enhanced osteoblast adhesion, proliferation, and differentiation. Considering the contact between TNTs' surface and surrounding tissues after implantation, the interactions between TNTs (with properties including their diameter, length, wettability, and crystalline phase) and proteins, platelets, bacteria, and cells are illustrated. The state of the art in the applications of TNTs in dentistry, orthopedic implants, and cardiovascular stents are introduced. In particular, the application of TNTs in biosensing has attracted much attention due to its ability for the rapid diagnosis of diseases. Finally, the difficulties and challenges in the practical application of TNTs are also discussed.

Strategy of Metal-Polymer Composite Stent To Accelerate Biodegradation of Iron-Based Biomaterials

The new principle and technique to tune biodegradation rates of biomaterials is one of the keys to the development of regenerative medicine and next-generation biomaterials. Biodegradable stents are new-generation medical devices applied in percutaneous coronary intervention, etc. Recently, both corrodible metals and degradable polymers have drawn much attention in biodegradable stents or scaffolds. It is, however, a dilemma to achieve good mechanical properties and appropriate degradation profiles. Herein, we put forward a metal-polymer composite strategy to achieve both. Iron stents exhibit excellent mechanical properties but low corrosion rate in vivo. We hypothesized that coating of biodegradable aliphatic polyester could accelerate iron corrosion due to the acidic degradation products, etc. To demonstrate the feasibility of this composite material technique, we first conducted in vitro experiments to affirm that iron sheet corroded faster when covered by polylactide (PLA) coating. Then, we fabricated three-dimensional metal-polymer stents (MPS) and implanted the novel stents in the abdominal aorta of New Zealand white rabbits, setting metal-based stents (MBS) as a control. A series of in vivo experiments were performed, including measurements of residual mass and radial strength of the stents, histological analysis, micro-computed tomography, and optical coherence tomography imaging at the implantation site. The results showed that MPS could totally corrode in some cases, whereas iron struts of MBS in all cases remained several months after implantation. Corrosion rates of MPS could be easily regulated by adjusting the composition of PLA coatings.