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Drug-eluting stents (DESs) are used to treat cardiovascular diseases such as atherosclerosis. The anti-proliferative drug released from the DES suppress the proliferation of smooth muscle cells and reduced in-stent restenosis. However, a burst release of the drug in the early stages and degradation morphology of the polymer coating represent major disadvantages, which might increase the incidence of in-stent restenosis and/or thrombosis under in vivo clinical studies. To solve these problems, in this study, a double-layer coating system composed of poly(lactide) (PLLA) bottom layer and poly(lactide-co-glycolide) (PLGA) top layer are used for the fabrication of DES. PLLA bottom layer was firstly coated on the metal surface followed by oxygen ion beam treatment. It was found that increasing the ion beam exposure time, increased the roughness of PLLA surface with a nanoscale pocket (or hole)-like structure. The top layer coating represented a mixture of PLGA and paclitaxel (PTX) with 5, 10, and 20% PTX contents. The coating was performed through ultrasonic spray technique, and the morphology showed not only a smooth and uniform surface but also no irregularities were observed at zero day. The drug release and degradation morphology for single-layer (PLGA/PTX) and double-layer (PLLA/PLGA/PTX) coatings were compared. The drug release from the double-layer stainless steel (SS) group showed a slower and controlled drug release for all PTX content samples as compared to single-layer SS group. Moreover, the degradation morphology of double-layer SS group presented a smoother and uniform surface after 12 weeks of degradation under physiological conditions. Therefore, an oxygen ion beam technique with double-layer coating system could effectively control the drug release, i.e., prevent initial burst drug release, and improve the degradation morphology of biodegradable polymer-based DESs.

Silicone rubber is widely used in various biomedical fields. However, the silicone surface is known to cause biofouling and thrombosis issues due to its higher hydrophobic property. In this study, to improve its antifouling property and blood compatibility, silicone rubber was modified by coating of hyaluronic acid (HA) or poly(ethylene glycol) (PEG). To enhance adhesive property of HA and PEG on the silicone surface, HA and PEG were modified by introducing the amino groups of dopamine (DA) to the carboxyl group of each polymer. The modified HA (HA-DA) and PEG (PEG-DA) were confirmed by 1H nuclear magnetic resonance (NMR) and the coated surfaces were characterized with contact angle, X-ray photoelectron spectroscopy (XPS), and micro-bicinchoninic acid (micro-BCA) analyses. To compare antifouling property and blood compatibility, adsorbed proteins, adhered platelets, and anticoagulation on the modified surfaces were evaluated by fluorescence microscopy, scanning electron microscopy, and activated partial thromboplastin time (APTT) assay, respectively. The property of cell adhesion on the modified surface was evaluated by NIH 3T3 fibroblast as a model cell system. Protein, platelet, and fibroblast showed low adsorption and adhesion on the modified silicone rubber surfaces compared to the unmodified one. Especially, the third coated silicone rubber surfaces were found to strongly resist protein adsorption as well as platelet and fibroblast adhesion. These results indicated that the antifouling property and blood compatibility were enhanced significantly on HA-DA and PEG-DA immobilized silicone rubbers. Therefore, these modified silicone rubbers can be used in various biomedical applications.

The restoration of a damaged endothelium might be a fascinating way to reduce significantly late-thrombosis and restenosis in the stent treatment. It has been recently reported that the recruitment of endothelial progenitor cells (EPCs), capable for repairing a damaged endothelium, can be important in the vascular stent application. Therefore, we focused on the hypothesis that stromal cell-derived factor-1α (SDF-1α) acts as a key chemokine for the mobilization and recruitment of EPCs to the damaged endothelium lesion. In this study, the effect of SDF-1α released from a cobalt-chromium alloy (Co-Cr) plate, vascular stent material, was investigated on the recruitment of EPCs in vitro. Dopamine-conjugated heparin was synthesized to introduce heparin onto Co-Cr. Heparin-coated Co-Cr surfaces were examined by water contact angle and attenuated total reflectance-Fourier transform infrared (ATRFTIR) to prove successful coating of heparin. Finally, SDF-1α was bound to the coated heparin derivative. Amounts of loaded and released SDF-1α were measured by ELISA, and the recruited EPC by released SDF-1α was evaluated using two different migration assays. The quantity of SDF-1α bound to the heparin-modified surface increased in a concentration-dependent manner and SDF-1α was released over 28 days. Transwell migration assay revealed that soluble SDF-1α released from the heparin-coated Co-Cr induced significantly more EPC recruitment than bare Co-Cr and heparin-coated Co-Cr. More importantly, fibrin gel migration assay further demonstrated that EPCs evidently respond to heparin-based substrate with SDF-1α and this type of behaviors was surprisingly found at as low level as less 1 ng/day of SDF-1α. These results are strongly encouraging for further in vitro and in vivo studies. Open image in new window

Two kinds of magnesium hydroxide (Mg(OH)2) rods (Mg-Rod, 150 and 350 nm in size) and plates (Mg-PL, 60 and 300 nm) were prepared, and blended with poly(L-lactide) (PLLA) to obtain PLLA/Mg(OH)2 composites to investigate the effect of the shape and size of Mg(OH)2 particles. The structure, morphology, pH change, thermal and mechanical properties, cytotoxicity, and inflammation of Mg(OH)2 control and PLLA/Mg(OH)2 composites were evaluated. PLLA/Mg-Rod150 (30%) composite showed a 50% higher tensile strength and a 45% improved modulus as compared with PLLA/Mg-PL300 30% composite. Although Mg-Rods displayed similar cell viability (above 80%) as compared to Mg-PLs, the expression levels of TNF-α from Mg-PL60 gradually increased with increasing concentrations from 1 to 300 μg. This indicates that Mg-PL60 had a potential cytotoxicity due to endocytosis. In addition, the byproduct of PLLA/Mg-Rods composite was more effectively neutralized than that of the PLLA/Mg-PLs composite, but cell viability and the expression levels of TNF-α were similar. Therefore, the use of our PLLA/Mg-Rod composite system would be a promising strategy to prevent the current fatal problems in biomedical applications including biodegradable implants such as stents.

Poly(D,L-lactic-co-glycolic acid) (PLGA) has been widely used as a biodegradable polymer in the fabrication of porous polymer scaffolds, but it is hydrolyzed into acidic by-products such as glycolic acid and lactic acid in the human body. Magnesium hydroxide nanoparticles (Mg-NPs) were incorporated into a PLGA scaffold in order to neutralize the acidic environment caused by the hydrolysis of PLGA, thereby reducing the cytotoxicity and inflammatory response. In this study, three-dimensional porous scaffolds blended with 30% Mg-NP were fabricated using gas foaming (PLGA/Mg/NaHCO3), salt leaching (PLGA/Mg/NaCl), and freeze drying (PLGA/Mg/Ice), and their structures, morphologies, pH change, thermal properties, and mechanical properties were analyzed by Fourier transform infrared spectroscopy, scanning electron microscopy, pH meter, thermogravimetric analysis, and a universal testing machine. The porosity of the PLGA/Mg/Ice scaffold was higher at about 10-13 wt% than those of the PLGA/Mg/NaCl or PLGA/Mg/NaHCO3 scaffolds. The Mg-NP content of the PLGA/Mg/NaHCO3 scaffold remained lower than those of the other scaffolds at about 63%. As a result of this loss of Mg-NP, the PLGA/Mg/NaHCO3 scaffold was confirmed to have lower cell viability (about 70%) than the PLGA/Mg/Ice scaffold (about 100%), owing to the reduced neutralizing effect. Although the PLGA/Mg/Ice and PLGA/Mg/NaCl scaffolds showed similar cell viability, the NaCl of the PLGA/Mg/NaCl scaffold exhibited slight toxicity in the body. The expression level of interleukin-6 (IL-6) was significantly decreased in the PLGA/Mg/Ice scaffold than in the PLGA/Ice scaffold, but the PLGA/Mg/Ice scaffold exhibited an IL-6 expression level that was about 10% lower than that of the PLGA/Mg/ NaCl scaffold. Consequently, the addition of Mg-NP/Ice could conceivably reduce the expression level of IL-6 in PLGA scaffolds. This anti-inflammatory PLGA/Mg/Ice scaffold is therefore expected to show great promise when used as a template in tissue engineering. Open image in new window

A series of hybrid nanocomposites containing poly(L-lactide) (PLLA) and polyhedral oligomeric silsesquioxane (POSS) was prepared by solvent casting method for the enhancement of the mechanical properties of PLLA. One of them was mixed by only a physical blending between PLLA and POSS to form PLLA/POSS nanocomposite that did not show considerable improvement. In the other hand, two types of PLLA-POSS additives were synthesized via the ring-opening polymerization of L-lactide in the presence of POSS in different content. On these additives, POSS played as not only an initiator for the ring-opening polymerization reaction due to its hydroxyl functional groups on the surface but also a type of hybrid filler for PLLA/PLLA-POSS nanocomposite after mixing with PLLA matrix. PLLA/PLLA-POSS composite showed significant improvement in mechanical properties because of covalent bonding between PLLA and POSS. In particular, the tensile modulus has been apparently increased in PLLA/5PLLA-5POSS nanocomposites (1,449 MPa) with respect to PLLA control (498 MPa). SEM and XRD data indicated that the dispersion of PLLA-modified POSS was better than that of POSS in PLLA matrix. Although the contact angle was not significantly different, the protein adsorption and platelet adhesion are reduced slightly in PLLA/PLLA-POSS nanocomposite as compared to those of PLLA control and PLLA/POSS. Therefore, it is expected that biodegradable POSS nanocomposite may be helpful to be utilized in biomedical devices such as drug-eluting stents and artificial implants. Open image in new window

In this study, a bioactive hydrogel was prepared from a chitosan derivative and Arg-Gly-Asp (RGD)-conjugated polypseudorotaxane, which is a cell-adhesive extracellular matrix. Chitosan was modified with 4-hydroxy phenyl acetic acid to obtain a water-soluble product for enzymatic cross-linking. Tyramine-terminated polypseudorotaxane (PRx) was prepared from the inclusion complex of a tyramine-terminated poly(ethylene glycol) backbone, and α-cyclodextrin (α-CD). Gly-Arg-Gly-Asp-Ser (GRGDS) was conjugated to the PRx using 4-nitrophenyl chloroformate (NPC) and partially carboxylated with succinic anhydride. The structure of the PRx-RGD and 4-hydroxylphenylacetamide chitosan (CHPA) was characterized by 1H NMR and FTIR spectroscopy. The RGD content in PRx-RGD was determined to be 0.19%. PRx-RGD and CHPA solution was crosslinked to form a bioactive hydrogel in the presence of horseradish peroxidase (HRP) and hydrogen peroxide (H2O2), which exhibited rapid gelation (∼20 sec). An in vitro cell culture was carried out with L929 mouse fibroblasts for 1 and 3 days. The results showed that fibroblasts adhered better and appeared to be more biocompatible on the RGD-conjugated hydrogel than the hydrogel without RGD. The combined results highlight the potential use of this bioactive hydrogel as an injectable scaffold in tissue engineering applications. Open image in new window

Heparin has not been used as a macromolecule to prepare nanocarriers for drug delivery, even though it has advantageous structural features, such as a negatively charged structure and presence of specific binding domains of proteins. In this study, a nanogel of a Pluronic-conjugated heparin was prepared by direct dissolution to develop a versatile drug nanocarrier composed of heparin. The aqueous nanogel solution of the heparin-Pluronic (HP) conjugate, exhibited higher stability than the micelles and smaller hydrodynamic size than 100 nm. The drug encapsulation test demonstrated that the HP nanogel encapsulates efficiently both hydrophobic small molecules and proteins. The release profile of indomethacin (IMC) was almost zero-order without a burst due to the presence of heparin, which appears to be advantageous comparing to polymeric micelles. Circular dichroism (CD) and gel electrophoresis demonstrated that the stability of monoclonal antibodies (3D8 scFv) encapsulated into the HP nanogel had been maintained. Overall, the self-assembled HP nanogel may be a versatile and promising nanocarrier for a range of drugs in drug delivery applications.

Targeting endothelial progenitor cells (EPC) for in vivo endothelialization is an emerging and promising approach for the development of cardiovascular medical devices. This study examined the efficacy of capturing CD34 positive EPC onto polyurethane (PU) immobilized with CD34 monoclonal antibodies (mAbs) (a biomecial polymer for cardiovascular devices). Electrospun PU matrices were fabricated and heparin was immobilized along with CD34 mAb. The modified PU surfaces at each step were characterized by contact angle measurements, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). XPS showed that each surface was modified, as expected in terms of the chemical composition. The amine-terminated poly(ethylene glycol)(PEG)-PU surface was considerably more hydrophilic than the PU surface. In addition, its surface roughness was similar to the PU surface, indicating that PEG was sufficiently and evenly grafted onto the PU surface. The CD34 mAb-immobilized PEG-PU surface was less hydrophilic than PEG-PU and extremely rough as compared to the other two surfaces. These results demonstrate that relatively large CD34 mAbs were immobilized on the PU surface. The surface density of the immobilized CD34 mAb, which was quantified using an enzyme-linked immune-sorbent assay (ELISA), was increased to ∼40 ng/cm2 by varying the feed amount up to ∼200 ng/cm2 and co-immobilizing with heparin. These results suggest that the co-immobilization with heparin can provide two benefits: inhibiting initial occlusion and improving the surface density of CD34 mAb. The in vitro cell study also demonstrated that the CD34 mAb-immobilized PU surface was favorable for cell attachment and proliferation. Therefore, in this study, a novel approach was developed to achieve endothelialization for cardiovascular applications by immobilizing CD34 onto PU, and the synergistic effects of co-immobilization with heparin on the bioactivity of the PU surfaces was demonstrated.

Polyurethane (PU) is widely used as a cardiovascular biomaterial due to its good mechanical properties and hemocompatibility, but it is not adhesive to endothelial cells (ECs). Cell adhesive peptides, GRGDS and YIGSR, were found to promote adhesion and spreading of ECs and showed a synergistic effect when both of them were used. In this study, a surface modification was designed to fabricate an EC-active PU surface capable of promoting endothelialization using the peptides and poly(ethylene glycol) (PEG) spacer, The modified PU surfaces were characterizedin vitro. The density of the grafted PEG on the PU surface was measured by acid-base back titration to the terminal-free isocyanate groups. The successful immobilization of peptides was confirmed by amino acid analysis, following hydrolysis, and contact angle measurement. The uniform distribution of peptides on the surface was observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). To evaluate the EC adhesive property, cell viability test using human umbilical vein EC (HUVEC) was investigatedin vitro and enhanced endothelialization was characterized by the introduction of cell adhesive peptides, GRGDS and YIGSR, and PEG spacer. Therefore, GRGDS and YIGSR co-immobilized PU surfaces can be applied to an EC-specific vascular graft with long-term patency by endothelialization.

Injectable hydrogels are quite promising materials due to their potential to minimize invasive implantation and this provides versatile fitness irrespective of the damaged regions and facilitates the incorporation of bioactive agents or cells.In situ gel formation through stereocomplex formation is a promising candidate for injectable hydrogels. In this paper, a new series of enantiomeric, four-arm, PEG-PLA block copolymers and their stereocomplexed hydrogels were prepared by bulk ring-opening polymerization of D-lactide and L-lactide, respectively, with stannous octoate as a catalyst. The prepared polymers were characterized by1H nuclear magnetic resonance (NMR) spectroscopy, Fourier-transform infrared (FT IR) spectroscopy, gel permeation chromatography (GPC) and thermal gravitational analysis (TGA), confirming the tailored structure and chain lengths. The swelling and degradation behavior of the hydrogels formed from a selected copolymer series were observed in different concentrations. The degradation rate decreased with increasing polymer content in the solution. The rheological behavior indicated that the prepared hydrogel underwentin situ gelation and had favorable mechanical strength. In addition, its feasibility as an injectable scaffold was evaluated using a media dependence test for cell culture. A Tris solution was more favorable forin situ gel formation than PBS and DMEM solutions were. These results demonstrated thein situ formation of hydrogel through the construction of a stereocomplex with enantiomeric, 4-arm, PEG-PLA copolymers. Overall, enantiomeric, 4-arm, PEG-PLA copolymers are a new species of stereocomplexed hydrogels that are suitable for further research into injectable hydrogels.

A RGD (Arg-Gly-Asp) conjugated chitosan hydrogel was used as a cell-supporting scaffold for articular cartilage regeneration. Thermosensitive chitosan-Pluronic (CP) has potential biomedical applications on account of its biocompatibility and injectability. A RGD-conjugated CP (RGD-CP) copolymer was prepared by coupling the carboxyl group in the peptide with the residual amine group in the CP copolymer. The chemical structure of RGD- CP was characterized by H NMR and FT IR. The concentration of conjugated RGD was quantified by amino acid analysis (AAA) and rheology of the RGD-CP hydrogel was investigated. The amount of bound RGD was 0.135 µg per 1 mg of CP copolymer. The viscoelastic parameters of RGD-CP hydrogel showed thermo-sensitivity and suit- able mechanical strength at body temperature for cell scaffolds (a > 100 kPa storage modulus). The viability of the bovine chondrocyte and the amount of synthesized glycosaminoglycans (GAGs) on the RGD-CP hydrogels were evaluated together with the alginate hydrogels as a control over a 14 day period. Both results showed that the RGD- CP hydrogel was superior to the alginate hydrogel. These results show that conjugating RGD to CP hydrogels improves cell viability and proliferation, including extra cellular matrix (ECM) expression. Therefore, RGD conju- gated CP hydrogels are quite suitable for a chondrocyte culture and have potential applications to the tissue engi- neering of articular cartilage tissue.

A series of novel 6-arm PLLA-PEG copolymers was synthesized for drug delivery system. This block copolymer was prepared by conjugating PEG to 6-arm PLLA. The 6-arm PLLA-PEGs formed micelle at aqueous solution, showing different sizes and loading efficiency of IMC by changing PLLA chain length. Overall release profiles of IMC from 6-arm PLLA-PEG micelles showed no initial burst and prolonged release time. Particularly, 6-arm PLLA-PEG micelles with longer PLLA chain showed more sustained release behaviors. The release rate of IMC can be modulated with the chain length of PLLA.