Hydrogels based on poly(ethylene oxide) and poly(tetramethylene oxide) or poly(dimethyl siloxane). III. In vivo biocompatibility and biostability

To investigate the effects of polymer chemistry and topology (linear or graft copolymer) on in vivo biocompatibility and biostability based on cage implant system, various hydrogels, composed of short hydrophilic [polyethylene oxide (PEO)] and hydrophobic block, were prepared by polycondensation reaction. Poly(tetramethylene oxide) (PTMO) or poly(dimethyl siloxane) (PDMS) was chosen as a hydrophobic block because of their wide utilization as a biomaterial. By using the specimens retrieved from rats killed after 1, 2, 3, 5, and 7 weeks' implantation, cellular and material responses were assessed. Most hydrogels showed a comparable value of macrophage density to Pellethane(R), control polymer, whereas they did significantly lower foreign body giant cell (FBGC) density and coverage because of the presence of PEO. However, PEO block length and polymer topology did not affect macrophage adhesion and FBGC formation in our polymer composition. The hydrogel based on PDMS alone showed significantly lower macrophage density and FBGC density than Pellethane(R), indicating that PDMS plays a role in inhibiting cellular adhesion. The results obtained from gel permeation chromatography curve and Fourier transform infrared spectra exhibited that all the polymers were susceptible to oxidative degradation in vivo. Although Pellethane(R) revealed surface degradation by 5 weeks in vivo, hydrogels showed rapid degradation in the bulk within 2 weeks because of the penetration of oxidative chemicals released from phagocytic cells into PEO domain of phase-separated hydrogels. The more significant degradation was observed in the hydrogels with longer PEO block and PTMO as a hydrophobic block instead of PDMS. It was evident that the minor degradation could be achieved by grafting PEO and adopting PDMS as a hydrophobic block in the hydrogel.     

The influence of poly(ethylene oxide) grafting via siloxane tethers on protein adsorption

Amphiphilic PEO–silanes (a–c) having siloxane tethers of varying lengths with the general formula α-(EtO)₃Si–(CH₂)₂–oligodimethylsiloxane_n-block-poly(ethylene oxide)₈–OCH₃ [n = 0 (a), n = 4 (b), and n = 13 (c)] were grafted onto silicon wafers and resistance to adsorption of plasma proteins was measured. Distancing the PEO segment from the hydrolyzable triethoxysilane [(EtO)₃Si] grafting group by a oligodimethylsiloxane tether represents a new method of grafting PEO chains to surfaces. Properties of surfaces grafted with a–c were compared to surfaces grafted with a traditional PEO–silane containing a propyl spacer [(EtO)₃Si–(CH₂)₃–poly(ethylene oxide)₈–OCH₃, PEO control]. As the siloxane tether length increased, chain density of PEO–silanes grafted onto oxidized silicon wafers decreased and hydrophobicity of the PEO–silane increased which led to a decrease in surface hydrophilicity. Despite decreased surface hydrophilicity, resistance to the adsorption of bovine serum albumin (BSA) increased in the order: PEO control < a < b ≈ c and to human fibrinogen (HF) increased in the order: PEO control < a < b < c.     

Reactive coupling of poly(ethylene glycol) on electroactive polyaniline films for reduction in protein adsorption and platelet adhesion.

Poly(ethylene glycol) (PEG)-coupled polyaniline (PANI) film surfaces were prepared by incorporating the chlorinie end-capped methoxy PEG (mPEGCl) of molecular weight of about 2000 onto the emeraldine (EM) base form of PANI via N-alkylation. The microstructure and composition of the mPEG-coupled PANI (mPEG-c-PANI) surfaces were characterized by atomic force microscopy, contact angle measurement and X-ray photoelectron spectroscopy. The concentration of surface-coupled mPEG increased with the increase in concentration of the mPEGCl solution. The mPEG-c-PANI film surfaces exhibited enhanced ability to repel protein adsorption, with only an moderate reduction in their electrical conductivity. The mPEG-c-PANI surface with a high concentration of coupled mPEG also exhibited good resistance towards platelet adhesion.     

Modification of gold surface by grafting of poly(ethylene glycol) for reduction in protein adsorption and platelet adhesion

Gold surfaces were first treated in an alkanethiol solution to form self-assembled monolayers (SAMs). The thiolated Au surface was then subjected to Ar plasma pretreatment, followed by air exposure and UV-induced graft polymerization of poly(ethylene glycol) methacrylate (PEGMA) macromonomer. In comparison with the 3-mercaptopropionic acid-2-ethylhexyl ester (MPAEE) SAM, the (3-mercaptoproply)trimethoxysilane (MPTMS) SAM on Au exhibited higher stability under the conditions of Ar plasma pretreatment. The graft concentration of the PEGMA polymer on SAMmodified Au surface increased with increasing PEGMA macromonomer concentration and UV-graft polymerization time. The modified-Au surfaces were characterized by X-ray spectroscopy (XPS), atomic force microscopy (AFM), and water contact angle measurement. The Au surface with a high concentration of grafted PEGMA polymer could completely repel protein adsorption and platelet adhesion.     

Modification of gold surface by grafting of poly(ethylene glycol) for reduction in protein adsorption and platelet adhesion

Gold surfaces were first treated in an alkanethiol solution to form self-assembled monolayers (SAMs). The thiolated Au surface was then subjected to Ar plasma pretreatment, followed by air exposure and UV-induced graft polymerization of poly(ethylene glycol) methacrylate (PEGMA) macromonomer. In comparison with the 3-mercaptopropionic acid-2-ethylhexyl ester (MPAEE) SAM, the (3-mercaptoproply)trimethoxysilane (MPTMS) SAM on Au exhibited higher stability under the conditions of Ar plasma pretreatment. The graft concentration of the PEGMA polymer on SAM-modified Au surface increased with increasing PEGMA macromonomer concentration and UV-graft polymerization time. The modified-Au surfaces were characterized by X-ray spectroscopy (XPS), atomic force microscopy (AFM), and water contact angle measurement. The Au surface with a high concentration of grafted PEGMA polymer could completely repel protein adsorption and platelet adhesion.     

Modification of Si(100) surface by the grafting of poly(ethylene glycol) for reduction in protein adsorption and platelet adhesion

The modification of argon plasma-pretreated single-crystal Si(100) wafer surfaces via the UV-induced graft polymerization of poly(ethylene glycol) methacrylate (PEGMA) macromonomer (molecular weight approximately 340) for biomaterials applications was explored. The modified Si(100) surfaces were characterized by X-ray photoelectron spectroscopy and atomic force microscopy. Surface peroxide concentrations resulting from the argon plasma treatment and subsequent atmospheric exposure were determined by a coupling reaction with diphenylpicrylhydrazyl. The results suggested that a short plasma treatment time of 10 s and brief air exposure were sufficient for generating an optimum amount of peroxides and hydroperoxides for the subsequent UV-induced graft polymerization. The graft concentration of the PEGMA polymer increased with increasing PEGMA macromonomer concentration for the graft polymerization and with increasing UV graft polymerization time. The PEGMA graft-polymerized silicon surface with a high poly(ethylene glycol) graft concentration was very effective in preventing protein adsorption and platelet adhesion. The grafted PEGMA polymer layer on the Si(100) surface exhibited fairly good stability during storage in a buffer solution.     

In vitro biostability of poly(dimethyl siloxane/hexamethylene oxide)-based polyurethane/layered silicate nanocomposites

We have prepared a number of silicone-based thermoplastic polyurethane (TPU) nanocomposites and demonstrated an enhancement of in vitro biostability against metal-ion-induced oxidation for potential use in long-term implantable medical devices. Organoclays based on both low-aspect-ratio hectorites and high-aspect-ratio fluoromicas were evaluated after being dual-modified with two quaternary alkyl ammonium salts with differing degrees of polarity. The resultant nanocomposites were tested for in vitro biostability using physiologically relevant oxidizing conditions. Subsequently, the effects of oxidative treatment on the surface degradation and bulk mechanical integrity of the nanocomposites were investigated and compared with the parent TPUs to identify nanocomposites with the most desirable features for long-term implantation. Here, we demonstrate that the low-aspect-ratio organohectorite was delaminated and well dispersed in the nanocomposites. Importantly, these factors gave rise to the enhanced oxidative stability. In addition, the mechanical properties of all nanocomposites were less adversely affected by the oxidative treatment compared to their parent TPUs. These results suggest the potential for improved mechanical integrity and biostability when suitable dual modified organoclays are incorporated in a silicone-based TPU.     

Protein adsorption to poly(ethylene oxide) surfaces.

Surfaces containing poly(ethylene oxide) (PEO) are interesting biomaterials because they exhibit low degrees of protein adsorption and cell adhesion. In this study different molecular weight PEO molecules were covalently attached to poly(ethylene terephthalate) (PET) films using cyanuric chloride chemistry. Prior to the PEO immobilization, amino groups were introduced onto the PET films by exposing them to an allylamine plasma glow discharge. The amino groups on the PET film were next activated with cyanuric chloride and then reacted with bis-amino PEO. The samples were characterized by scanning electron microscopy, water contact angle measurements, gravimetric analysis, and electron spectroscopy for chemical analysis (ESCA). The adsorption of 125I-labeled baboon fibrinogen and bovine serum albumin was studied from buffer solutions. Gravimetric analysis indicated that the films grafted with the low-molecular-weight PEO contained many more PEO molecules than the surfaces grafted with higher-molecular-weight PEO. The high-molecular-weight PEO surfaces, however, exhibited greater wettability (lower water contact angles) and less protein adsorption than the low-molecular-weight PEO surfaces. Adsorption of albumin and fibrinogen to the PEO surfaces decreased with increasing PEO molecular weight up to 3500. A further increase in molecular weight resulted in only slight decreases in protein adsorption. Protein adsorption studies as a function of buffer ionic strength suggest that there may be an ionic interaction between the protein and the allylamine surface. The trends in protein adsorption together with the water contact angle results and the gravimetric analysis suggest that a kind of "cooperative" water structuring around the larger PEO molecules may create an "excluded volume" of the hydrated polymer coils. This may be an important factor contributing to the observed low protein adsorption behavior.     

Crystallization behaviour of poly(ethylene terephthalate) and poly(tetramethylene terephthalate) blends

The isothermal and non-isothermal crystallization behaviour of blends of poly(ethylene terephthalate) (PET) and poly(tetramethylene terephthalate) (PBT) is investigated at low percentage of the second component. The melting behaviour of the isothermally crystallized sample shows that the crystallization behaviour in the blend is governed by the mobility of PBT. Below 200°C the crystallization process is hindered, whereas above 200°C the PET crystals are larger in case of added PBT. The non-isothermal crystallization behaviour shows that the crystallization process is hindered when the PBT content in the blend is less or higher than 6 wt.-%.     

Diffusion of p-nitroaniline in poly(tetramethylene oxide)-poly(tetramethylene terephthalate) block copolymers

Diffusion coefficients of p-nitroaniline in poly(tetramethylene ether)glycol terephthalate-poly(tetramethylene terephthalate) (4GT) block copolymers, containing 4GT wt fraction of 0,345, 0,514, and 0,712, have been determined from sublimative desorption kinetics in the range from 60 to 120°C. The temperature dependence of diffusion coefficients in each diffusion system has been found to be a Williams-Landel-Ferry (WLF) type. When the glass transition temperatures (T_g) of the soft segments are selected as reference temperatures, the WLF parameters of C and C in the case of diffusion at temperatures above T_g+100 K can be obtained from a new method, proposed in the present paper, on the basis of Fujita's expression of diffusion coefficient in terms of free volume. Parallel studies of dynamic mechanical properties of these copolymers and the assumption of iso-free volume state at T_g provide the values of free volume parameters of B_d and expansion coefficients of free volume. Both the quantities so obtained decrease to a small extent with increasing 4GT wt fraction. Alternatively, the temperature dependence of diffusion coefficients in the present study can be expressed exactly by Arrhenius plots. The activation energies of diffusion based on Eyring's rate processes are almost independent of, but the activation entropies decrease significantly with, increasing 4GT wt fraction. An expression of the latter in terms of the free volume parameters is presented.     

In vitro and ex vivo platelet interactions with hydrophilic-hydrophobic poly(ethylene oxide)-polystyrene multiblock copolymers

Hydrophilic-hydrophobic multiblock copolymers synthesized from telechelic oligomers of poly(ethylene oxide) (PEO) and polystyrene (PS) have been used to study the influence of hydrophilic and hydrophobic balance on interfacial interactions of these surfaces with blood components. In vitro coagulation assays show no inherent ability of these amphiphilic surfaces to affect contact activation or coagulation factors. In vitro platelet adhesion and release reactions from rabbit platelet-rich plasma were shown to be greatest on Biomer and PS homopolymer surfaces and least on cross-linked PEO surfaces, with the PEO-PS block copolymers demonstrating intermediate responses. These same substrates were tested in a new low-flow, low-shear arterio-artery shunt system in rabbits. Whole blood occlusion times were not a direct function of hydrophilic content as both PEO and PS homopolymers and Biomer showed short occlusion times, while PEO-PS block copolymers prolonged occlusion times considerably, depending on composition. Overall, results suggest that PEO-PS block copolymers promote unique whole blood responses in contrast to homopolymer and Biomer controls which are more complex than direct correlations to bulk hydrophilic and hydrophobic contents.     

In vitro drug release studies from organoclay/poly(dimethyl siloxane) nanocomposite matrices

Silicone elastomers are versatile biomaterials and have been used for fabrication of drug release systems, usually incorporating lipophilic drugs. However, attempts have been made to extend the use of these biomaterials to the delivery of hydrophilic drugs. Furthermore, the need to improve mechanical properties of silicones led, among others, to the incorporation of organoclay nanoparticles and, therefore, has introduced some new parameters to be investigated regarding their effect on the release profile. In this work, the delivery of 2-methyl-5-nitroimidazole-1-ethanol (metronidazole) from nanocomposites with silicone matrix based on condensation cured elastomers with different molecular weights was investigated in various surrounding liquids. The results showed that incorporation of organic modified montmorillonite (OMMT) generally decreases the drug release rate and restricts the initial burst effect. Interestingly, OMMT concentrations of 2 phr in low MW silicone systems seem to enhance drug release and, independently of interpretation, it might indicate a route for the adjustment of diffusivity through the nanoclay concentration. Maximum drug release rates can rather be achieved with low MW PDMS than with the higher MW elastomers. ? 2012 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2012.     

Synthesis and characterization of copolyesters based on poly(tetramethylene terephthalate) and p-hydroxybenzoic acid

Copolyesters of poly(tetramethylene terephthalate) (PTMT) and p-hydroxybenzoic acid (PHB) are obtained by means of transesterification in the molten state using different ratios of the starting reagents. After treatment with trifluoroacetic acid, the total reaction product could be divided into a soluble part and an insoluble one, differing in the tetramethylene terephthalate/oxybenzoate (TMT/OB) units ratio. It was proved by ¹H NMR that the fraction of OB units in the soluble part of the copolyester varies from 3 to 12 mol-% for PHB feed contents of 10–80 mol-%. There is no systematic influence of the PHB feed content on the copolyester composition. The maximum OB content was achieved at 20 mol-% PHB in the starting mixture. The results from ¹³C NMR, DSC, WAXS and crystallization kinetics measurements suggest the presence of blocks of TMT units in the macrochains. The observed thermal properties and crystalline structure are explained in terms of the inhomogeneity of both chemical composition and units sequence in the total reaction products. This leads to the coexistence of at least two phases differing mainly in their content of OB units.     

Blood compatible aspects of poly(2-methoxyethylacrylate) (PMEA)--relationship between protein adsorption and platelet adhesion on PMEA surface

Platelet adhesion and spreading is suppressed when a poly(2-methoxyethylacrylate) (PMEA) surface is used, compared with other polymer surfaces. To clarify the reason for this suppression, the relationship among the amount of the plasma protein adsorbed onto PMEA, its secondary structure and platelet adhesion was investigated. Poly(2-hydroxyethylmethacrylate) (PHEMA) and polyacrylate analogous were used as references. The amount of protein adsorbed onto PMEA was very low and similar to that absorbed onto PHEMA. Circular dichroism (CD) spectroscopy was applied to examine changes in the secondary structure of the proteins after adsorption onto the polymer surface. The conformation of the proteins adsorbed onto PHEMA changed considerably, but that of proteins adsorbed onto PMEA differed only a little from the native one. These results suggest that low platelet adhesion and spreading are closely related to the low degree of the denaturation of the protein adsorbed onto PMEA. PMEA could be developed as a promising material to produce a useful blood-contacting surface for medical devices.     

Stearyl poly(ethylene oxide) grafted surfaces for preferential adsorption of albumin

An ideal surface for many biomedical applications would resist non-specific protein adsorption while at the same time triggering a specific biological pathway. Based on the approach of selectively binding albumin to free fatty acids, stearyl groups were immobilized onto poly(styrene) backbone via poly(ethylene oxide) side chains. X-ray photoelectron spectroscopy (XPS) analysis indicates substantial surface enrichment of the stearyl poly(ethylene oxide) (SPEO). In an aqueous environment, the surface rearrangement is limited, as proved by dynamic contact angle tests. The comb-like copolymer presents a special hydrophobic surface with high SPEO surface density, which may be due to the 'tail like' SPEO architecture at the copolymer/water interface. Protein adsorption tests confirm that the comb-like surfaces adsorb high levels of albumin and resist fibrinogen adsorption very significantly. The surfaces prepared in this research attract and reversibly bind albumin due to the synergistic action of the PEO chains and the stearyl end groups.     

In vitro studies of platelet adhesion, activation, and protein adsorption on curcumin-eluting biodegradable stent materials

A major complication of coronary stenting is in-stent restenosis (ISR) due to thrombus formation. We hypothesized that locally released curcumin from coronary stent surface would inhibit ISR due to thrombus formation because of antithrombosis of curcumin. In the present work, curcumin-eluting polylactic acid-co-glycolic acid (PLGA) films were fabricated and their properties in vitro were investigated. The in vitro platelet adhesion and activation, as well as protein adsorption on curcumin-loading PLGA films were investigated to evaluate the blood compatibility of curcumin-eluting films. The structure of curcumin-eluting PLGA film and control was examined by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy indicating that the peaks of curcumin did not shift in curcumin-eluting films. The results of contact angle and surface free energy indicated that loading curcumin in PLGA would make PLGA become more hydrophilic, which contributed to the increase of polar fraction of surface free energy. With the increase of curcumin in films, platelets adhering to the curcumin-eluting films decreased significantly. The number of activation platelets decreased after incorporating curcumin in PLGA films. Loading curcumin in PLGA film can markedly reduce the fibrinogen adsorption. All results indicated that incorporating curcumin in PLGA film can improve the blood compatibility of PLGA films. It can be used to fabricate drug-eluting stent to prevent thrombosis formation.     

Synthesis and characterization of poly(ethylene oxide) and poly(perfluorohexylethyl methacrylate) containing triblock copolymers

A new series of poly(perfluorohexylethyl methacrylate)‐block‐poly(ethylene oxide)‐block‐poly(perfluorohexylethyl methacrylate), PFMA‐b‐PEO‐b‐PFMA triblock copolymers has been synthesized by atom transfer radical polymerization using bifunctional PEO macroinitiators. The molecular structure of the block copolymers was confirmed by ¹H NMR spectroscopy and SEC. X‐ray scattering studies have been carried out to investigate their bulk properties. SAXS has shown cubic arrangement of spheres (bcc), hexagonally packed cylinders (hpc) and lamellar microdomain formation in the melt of triblock copolymers investigated, depending on composition. Crystallization was, however, found to destroy the ordered melt morphology and imposes a lamellar crystalline structure. WAXS, DSC and polarized light microscopy measurements confirmed the crystallization of PEO segments in block copolymers. Long PFMA blocks were found to have significant effect on PEO crystallization.

Synthesis of triblock copolymers of EO and FMA by ATRP.     

Novel amphiphilic branched copolymers based on polystyrene and poly(ethylene oxide)

Newly designed polystyrene (PS)/poly(ethylene oxide) (PEO) branched copolymers were obtained by a sequence of reactions involving both cationic and anionic polymerizations. Stars made of six PS-b-PEO arms and PS/PEO copolymers constituted of an inner PS part and an arborescent outer layer of PEO are two novel amphiphilic branched architectures that are described in this paper. The investigation of the solution properties of these copolymers by size exclusion chromatography (SEC) using tetrahydrofuran and an aqueous medium revealed that PS/PEO arborescent copolymers are more inclined to self-organize into unimicellar systems than their star homologues.     

Reverse thermo-responsive poly(ethylene oxide) and poly(propylene oxide) multiblock copolymers

Aiming at developing new reverse thermo-responsive polymers, poly(ethylene oxide)-poly(propylene oxide) multiblock copolymers were synthesized by covalently binding the two components using carbonyl chloride and diacyl chlorides as the coupling molecules. The appropriate selection of the various components allowed the generation of systems displaying much enhanced rheological properties. For example, 15 wt% aqueous solutions of an alternating poly(ether-carbonate) comprising PEO6000 and PPO3000 segments, achieved a viscosity of 140,000 Pas, while the commercially available Pluronic F127 displayed 5,000 Pas only. Furthermore, the structure of the chain extender played a key role in determining the sol-gel transition. While poly(ether-ester)s containing therephtaloyl (para) and isophtaloyl (metha) coupling units failed to gel at any concentration, a 15 wt% aqueous solution of the polymer chain-extended with phtaloyl chloride (ortho) gelled at 43 degrees C. The water solutions were also studied by dynamic light scattering and a clear influence of the PEO/PPO ratio on the aggregate size was observed. By incorporating short aliphatic oligoesters into the backbone, prior to the chain extension stage, reverse thermal gelation-displaying biodegradable poly(ether-ester-carbonate)s, were generated.     

Characterization of poly(ethylene oxide) brushes on glass surfaces and adhesion of Staphylococcus epidermidis

Poly(ethylene oxide) brushes have been covalently bound to glass surfaces and their presence was demonstrated by an increase in water contact angles from fully wettable on glass to advancing contact angles of 54 degrees, with a hysteresis of 32 degrees. In addition, electrophoretic mobilities of glass and brush-coated glass were determined using streaming potential measurements. The dependence of the electrophoretic mobilities on the ionic strength was analyzed in terms of a softlayer model, yielding an electrophoretic softness and fixed charge density of the layer. Brush-coated glass could be distinguished from glass by a 2-3-fold decrease in fixed charge density, while both surfaces were about equally soft. Adhesion of Staphylococcus epidermidis HBH276 to glass in a parallel plate flow chamber was extremely high and after 4 h, 19.0 x 10(6) bacteria were adhering per cm2. In contrast, the organisms did not adhere to brush-coated glass, with numbers below the detection limit, i.e. 0.1 x 10(6) per cm2. These results attest to the great potential of polymer brushes in preventing bacterial adhesion to surfaces.