Long-term stability of self-assembled monolayers on 316L stainless steel

REDV-Functionalized Recombinant Spider Silk for Next-Generation Coronary Artery Stent Coatings: Hemocompatible, Drug-Eluting, and Endothelial Cell-Specific Materials

Coronary artery stents are life-saving devices, and millions of these devices are implanted annually to treat coronary heart disease. The current gold standard in treatment is drug-eluting stents, which are coated with a biodegradable polymer layer that elutes antiproliferative drugs to prevent restenosis due to neointimal hyperplasia. Stenting is commonly paired with systemic antiplatelet therapy to prevent stent thrombosis. Despite their clinical success, current stents have significant limitations including inducing local inflammation that drives hyperplasia; a lack of hemocompatibility that promotes thrombosis, increasing need for antiplatelet therapy; and limited endothelialization, which is a critical step in the healing process. In this research, we designed a novel material for use as a next-generation coating for drug-eluting stents that addresses the limitations described above. Specifically, we developed a recombinant spider silk material that is functionalized with an REDV cell-adhesive ligand, a peptide motif that promotes specific adhesion of endothelial cells in the cardiovascular environment. We illustrated that this REDV-modified spider silk variant [eADF4(C16)-REDV] is an endothelial-cell-specific material that can promote the formation of a near-confluent endothelium. We additionally performed hemocompatibility assays using human whole blood and demonstrated that spider silk materials exhibit excellent hemocompatibility under both static and flow conditions. Furthermore, we showed that the material displayed slow enzyme-mediated degradation. Finally, we illustrated the ability to load and release the clinically relevant drug everolimus from recombinant spider silk coatings in a quantity and at a rate similar to that of commercial devices. These results support the use of REDV-functionalized recombinant spider silk as a coating for drug-eluting stents.

Green Nanocoatings Prepared by Crosslinking Self-Assembled Fatty Acids on Metals

Self-assembled monolayers (SAMs) are an important element of modern nanotechnology and surface functionalization. However, their application is still limited because they are easily removed from the surface of the object in corrosive environments. Crosslinking would make SAMs more resistant to the corrosive environment they are exposed to. In this work, how to strongly crosslink SAMs made of non-toxic and biodegradable fatty acids on metal surfaces using ionizing radiation has been demonstrated for the first time. The crosslinked nanocoatings are stable over time and have significantly improved properties compared to SAMs. Thus, crosslinking opens up the possibility of using SAMS in a variety of different systems and on different materials for surface functionalization to achieve stable and durable surface properties such as biocompatibility or selective reactivity.

Surface modification of stainless steel for biomedical applications: Revisiting a century-old material

Stainless steel (SS) has been widely used as a material for fabricating cardiovascular stents/valves, orthopedic prosthesis, and other devices and implants used in biomedicine due to its malleability and resistance to corrosion and fatigue. Despite its good mechanical properties, SS (as other metals) lacks biofunctionality. To be successfully used as a biomaterial, SS must be made resistant to the biological environment by increasing its anti-fouling properties, preventing biofilm formation (passive surface modification), and imparting functionality for eluting a specific drug or capturing selected cells (active surface modification); these features depend on the final application. Various physico-chemical techniques, including plasma vapor deposition, electrochemical treatment, and attachment of different linkers that add functional groups, are used to obtain SS with increased corrosion resistance, improved osseointegration capabilities, added hemocompatibility, and enhanced antibacterial properties. Existing literature on this topic is extensive and has not been covered in an integrated way in previous reviews. This review aims to fill this gap, by surveying the literature on SS surface modification methods, as well as modification routes tailored for specific biomedical applications.

STATEMENT OF SIGNIFICANCE

Stainless steel (SS) is widely used in many biomedical applications including bone implants and cardiovascular stents due to its good mechanical properties, biocompatibility and low price. Surface modification allows improving its characteristics without compromising its important bulk properties. SS with improved blood compatibility (blood contacting implants), enhanced ability to resist bacterial infection (long-term devices), better integration with a tissue (bone implants) are examples of successful SS surface modifications. Existing literature on this topic is extensive and has not been covered in an integrated way in previous reviews. This review paper aims to fill this gap, by surveying the literature on SS surface modification methods, as well as to provide guidance for selecting appropriate modification routes tailored for specific biomedical applications.

Immobilization of alendronate on titanium via its different functional groups and the subsequent effects on cell functions

Immobilization of alendronate on orthopedic implants offers the possibility of enhancing osteogenesis without potentially adverse effects associated with systemic administration of this drug. In this work, alendronate was immobilized on titanium (Ti) via either its phosphate (Method 1) or amino (Method 2) groups, and responses of osteoblasts and human mesenchymal stem cells (hMSCs) on these surfaces were investigated. These modified substrates have similar surface roughness and are negatively charged. With similar amounts of immobilized alendronate, these two types of modified substrates showed comparable osteogenic stimulating effects in enhancing osteoblasts' alkaline phosphatase (ALP) activity and calcium deposition for the first 10days. However, alendronate immobilized via its phosphate groups was less stable, and gradually leached into the medium. As a result, its stimulating effect on osteoblast differentiation diminished with time. On the other hand, alendronate immobilized via its amino group stimulated osteoblast differentiation over 21days, and with 1655ng/cm2 of immobilized alendronate on the Ti substrate, calcium deposition by osteoblasts and hMSCs increased by 30% and 69%, respectively, compared to pristine Ti after 21days. The expressions of runt-related transcription factor 2, osterix, osteopontin and osteocalcin in hMSCs cultured on this substrate were monitored. The up-regulation of these genes is postulated to play a role in the acceleration of osteogenic differentiation of hMSCs cultured on the alendronate-modified substrate over those on pristine Ti.

Self-Assembled Amphiphilic Macromolecule Coatings: Comparison of Grafting-From and Grafting-To Approaches for Bioactive Delivery

Although drug-eluting stent technologies have significantly improved clinical outcomes over the past decade, substantial issues with postimplantation vessel reocclusion still remain. To combat these issues, bioactive amphiphilic macromolecules (AMs), comprised of a functional end group, a branched hydrophobic domain, and a hydrophilic poly(ethylene glycol) tail, were investigated as a therapeutic coating to reduce smooth muscle cell (SMC) proliferation and platelet adhesion. In this study, grafting-from and grafting-to approaches for AM surface functionalization were compared to determine the effects of fabrication method on bioactive delivery characteristics, including the AM loading, release, and biological activity. Grafted-from coatings were formed by stepwise synthesis of phosphonate AMs, 1pM, on the substrate, first by alkyl phosphonate coordination to stainless steel and subsequent carbodiimide coupling to conjugate the hydrophobic and hydrophilic domains. In contrast, grafted-to monolayers were assembled utilizing presynthesized 1pM in a tethering by aggregation and growth technique. Coatings formed using the grafting-from approach yielded high AM grafting density and a highly ordered layer, which corresponded to a slower release rate and sustained bioactivity over 28 days. In contrast, the grafted-to coatings yielded less dense, heterogeneous layers, which released faster and were therefore less efficacious in suppressing prolonged SMC proliferation. Both coatings significantly reduced platelet adhesion compared to an uncoated control, but similar platelet adhesion results between grafted-from and grafted-to coatings suggest that both surfaces maintained a molecular density favorable for antiplatelet activity. Overall, the grafting-from method produced uniform coatings with improved loading, release, and bioactive properties compared to the grafting-to approach, highlighting the potential of AM controlled release coatings for therapeutic delivery.

Structure and Long-Term Stability of Alkylphosphonic Acid Monolayers on SS316L Stainless Steel

Surface modification of stainless steel (SS316L) to improve surface properties or durability is an important avenue of research, as SS316L is widely used in industry and science. We studied, therefore, the formation and stability of a series of organic monolayers on SS316L under industrially relevant conditions. These included acidic (pH 3), basic (pH 11), neutral (Milli-Q water), and physiological conditions [10 mM phosphate-buffered saline (PBS)], as well as dry heating (120 °C). SS316L was modified with alkylphosphonic acids of chain length (CH2)n with n varying between 3 and 18. While alkylphosphonic acids of all chain lengths formed self-assembled monolayers with hydrophobic properties, only monolayers of chain lengths 12-18 formed ordered monolayers, as evidenced by static water contact angle (SCA), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and infrared reflection absorption spectroscopy (IRRAS). A long-term stability study revealed the excellent stability of monolayers with chain lengths 12-18 for up to 30 days in acid, neutral, and physiological solutions, and for up to 7 days under dry heating. Under strong basic conditions a partial breakdown of the monolayer was observed, especially for the shorter chain lengths. Finally, the effect of multivalent surface attachment on monolayer stability was explored by means of a series of divalent bisphosphonic acids.

Amphiphilic Macromolecule Self-Assembled Monolayers Suppress Smooth Muscle Cell Proliferation

A significant limitation of cardiovascular stents is restenosis, where excessive smooth muscle cell (SMC) proliferation following stent implantation causes blood vessel reocclusion. While drug-eluting stents minimize SMC proliferation through releasing cytotoxic or immunosuppressive drugs from polymer carriers, significant issues remain with delayed healing, inflammation, and hypersensitivity reactions associated with drug and polymer coatings. Amphiphilic macromolecules (AMs) comprising a sugar-based hydrophobic domain and a hydrophilic poly(ethylene glycol) tail are noncytotoxic and recently demonstrated a concentration-dependent ability to suppress SMC proliferation. In this study, we designed a series of AMs and studied their coating properties (chemical composition, thickness, grafting density, and coating uniformity) to determine the effect of headgroup chemistry on bioactive AM grafting and release properties from stainless steel substrates. One carboxyl-terminated AM (1cM) and two phosphonate- (Me-1pM and Pr-1pM) terminated AMs, with varying linker lengths preceding the hydrophobic domain, were grafted to stainless steel substrates using the tethering by aggregation and growth (T-BAG) approach. The AMs formed headgroup-dependent, yet uniform, biocompatible adlayers. Pr-1pM and 1cM demonstrated higher grafting density and an extended release from the substrate over 21 days compared to Me-1pM, which exhibited lower grafting density and complete release within 7 days. Coinciding with their release profiles, Me-1pM and 1cM coatings initially suppressed SMC proliferation in vitro, but their efficacy decreased within 7 and 14 days, respectively, while Pr-1pM coatings suppressed SMC proliferation over 21 days. Thus, AMs with phosphonate headgroups and propyl linkers are capable of sustained release from the substrate and have the ability to suppress SMC proliferation during the restenosis that occurs in the 3-4 weeks after stent implantation, demonstrating the potential for AM coatings to provide sustained delivery via desorption from coated coronary stents and other metal-based implants.

Polymer-Free Drug-Eluting Stents: An Overview of Coating Strategies and Comparison with Polymer-Coated Drug-Eluting Stents

Clinical evaluations have proven the efficacy of drug-elution stents (DES) in reduction of in-stent restenosis rates as compared to drug-free bare metal stents (BMS). Typically, DES are metal stents that are covered with a polymer film loaded with anti-inflammatory or antiproliferative drugs that are released in a sustained manner. However, although favorable effects of the released drugs have been observed, the polymer coating as such has been associated with several adverse clinical effects, such as late stent thrombosis. Elimination of the polymeric carrier of DES may therefore potentially lead to safer DES. Several technologies have been developed to design polymer-free DES, such as the use of microporous stents and inorganic coatings that can be drug loaded. Several drugs, including sirolimus, tacrolimus, paclitaxel, and probucol have been used in the design of carrier-free stents. Due to the function of the polymeric coating to control the release kinetics of a drug, polymer-free stents are expected to have a faster drug elution rate, which may affect the therapeutic efficacy. However, several polymer-free stents have shown similar efficacy and safety as the first-generation DES, although the superiority of polymer-free DES has not been established in clinical trials.

Effect of surface chemistry on the integrin induced pathway in regulating vascular endothelial cells migration

The migration of vascular endothelial cells (ECs) is essential for reendothelialization after implantation of cardiovascular biomaterials. Reendothelialization is largely determined by surface properties of implants. In this study, surfaces modified with various chemical functional groups (CH3, NH2, COOH, OH) prepared by self-assembled monolayers (SAMs) were used as model system. Expressions and distributions of critical proteins in the integrin-induced signaling pathway were examined to explore the mechanisms of surface chemistry regulating EC migration. The results showed that SAMs modulated cell migration were in the order CH3>NH2>OH>COOH, determined by differences in the expressions of focal adhesion components and Rho GTPases. Multiple integrin subunits showed difference in a surface chemistry-dependent manner, which induced a stepwise activation of signaling cascades associated with EC migration. This work provides a broad overview of surface chemistry regulated endothelial cell migration and establishes association among the surface chemistry, cell migration behavior and associated integrin signaling events. Understanding the relationship between these factors will help us to understand the surface/interface behavior between biomaterials and cells, reveal molecular mechanism of cells sensing surface characterization, and guide surface modification of cardiovascular implanted materials.

Tribological durability of silane monolayers on silicon

We report the frictional performance and long-term tribological stability of various alkyl silane monolayer films on silicon by using pin-on-disk tribometry at ambient conditions. We show that the durability of monolayers derived from n-alkyltrichlorosilanes on silicon increases exponentially with the chain length of the silane precursor, which we relate to the cohesive energy of these monolayers through molecular dynamics simulations. X-ray photoelectron spectroscopy (XPS) was used to show that tribological damage consisted of the loss of molecular components that could be partially replaced upon exposure to a solution containing perfluorinated silane precursors. For monolayers derived from n-octadecyltrichlorosilane, a critical load was identified to be approximately 250 mN (200 MPa), above which failure of films occurred within 100 cycles of testing. Monolayers with hydroxyl surfaces exhibited reduced stabilities due to stronger tip-surface interactions. Monolayers with the capability for cross-linking exhibited much greater stabilities than monolayers where cross-linking was limited or prevented. Collectively, these results demonstrate that the mechanical durability of monolayers when subjected to a tribological load is greatly enhanced by maximizing dispersional interactions and cross-linking and minimizing tip-surface interactions.

Biocompatible carbohydrate-functionalized stainless steel surfaces: a new method for passivating biomedical implants

A convenient method for passivating and functionalizing stainless steel is described. Several methods of coating stainless steel (SS) samples with silica were investigated and of these methods, a thin (less than 15 nm thick) layer of silica created by atomic layer deposition (ALD) was found to give superior performance in electrochemical testing. These interfaces were then used as a platform for further functionalization with molecules of biological interest. Specifically, the SS samples were functionalized with biologically significant carbohydrates [N-acetyl-D-glucosamine (GlcNAc) and D-galactose (Gal)] that contain trialkoxysilane derivatives as chemical handles for linking to the surface. The presence and biological availability of these moieties on the silica coated SS were confirmed by XPS analysis and an enzyme-linked lectin assay (ELLA) using complementary lectins that specifically recognize the surface-bound carbohydrate. This method has the potential of being adapted to the functionalization of stainless steel biomedical implants with other biologically relevant carbohydrates.