Poly(dimethylsiloxane)-poly(ethylene oxide)-heparin block copolymers. II: Surface characterization and in vitro assessments

Sung Wan Kim - Early events in blood/material interactions

Sung Wan Kim's initial efforts as an independent investigator were focused on improving the understanding of the early events in blood/material interactions with the goal to develop blood compatible materials for application in medical devices and prostheses. These initial efforts were centered around blood protein adsorption on biomaterials and related mechanisms of thrombus formation (thrombosis). Ultimately, Sung Wan's efforts were expanded to studies of the non-thrombogenic nature of heparinized biomaterials, prostaglandin biomaterials, and block copolymer systems. These studies were supported by two NIH grants for 22 and 19 years, respectively, and a NIH Career Development Award. Moreover, these studies resulted in over 140 peer-reviewed publications and training of many students and postdoctoral scientists. The intent of this paper is to identify key concepts, papers, and contributions by Sung Wan and his colleagues that fall within the four aforementioned research categories. In this context, many of Sung Wan's early efforts contributed directly to Utah's biomaterials efforts and the Total Artificial Heart program at the time, while providing the foundation for the productive international Triangle Collaboration as well as his following work in polymer-controlled drug releasing systems.

Facile synthesis and self-assembly of amphiphilic polydimethylsiloxane with poly(ethylene glycol) moieties via thiol-ene click reaction

A facile and efficient synthesis of polysiloxane-based amphiphilic copolymer via thiol-ene click chemistry.

Bioactive technologies for hemocompatibility

The contact of any biomaterial with blood gives rise to multiple pathophysiologic defensive mechanisms such as activation of the coagulation cascade, platelet adhesion and activation of the complement system and leukocytes. The reduction of these events is of crucial importance for the successful clinical performance of a cardiovascular device. This can be achieved by improving the hemocompatibility of the device materials or by pharmacologic inhibition of the key enzymes responsible for the activation of the cascade reactions, or a combination of both. Different strategies have been developed during the last 20 years, and this article attempts to review the most significant, by dividing them into three main categories: bioinert or biopassive, biomimetic and bioactive strategies. With regard to bioactive strategies, particular attention is given to heparin immobilization and recent related technologies. References from both scientific literature and commercial sites are provided. Future development and studies are suggested.

Surface modification of silicone for biomedical applications requiring long-term antibacterial, antifouling, and hemocompatible properties

Silicone has been used for peritoneal dialysis (PD) catheters for several decades. However, bacteria, platelets, proteins, and other biomolecules tend to adhere to its hydrophobic surface, which may lead to PD outflow failure, serious infection, or even death. In this work, a cross-linked poly(poly(ethylene glycol) dimethacrylate) (P(PEGDMA)) polymer layer was covalently grafted on medical-grade silicone surface to improve its antibacterial and antifouling properties. The P(PEGDMA)-grafted silicone (Silicone-g-P(PEGDMA)) substrate reduced the adhesion of Staphylococcus aureus , Escherichia coli , and Staphylococcus epidermidis , as well as 3T3 fibroblast cells by ≥90%. The antibacterial and antifouling properties were preserved after the modified substrate was aged for 30 days in phosphate buffer saline. Further immobilization of a polysulfobetaine polymer, poly((2-(methacryloyloxy)ethyl)dimethyl-(3-sulfopropyl)ammonium hydroxide) (P(DMAPS)), on the Silicone-g-P(PEGDMA) substrate via thiol-ene click reaction leads to enhanced antifouling efficacy and improved hemocompatibility with the preservation of the antibacterial property. Compared to pristine silicone, the so-obtained Silicone-g-P(PEGDMA)-P(DMAPS) substrate reduced the absorption of bovine serum albumin and bovine plasma fibrinogen by ≥80%. It also reduced the number of adherent platelets by ≥90% and significantly prolonged plasma recalcification time. The results indicate that surface grafting with P(PEGDMA) and P(DMAPS) can be potentially useful for the modification of silicone-based PD catheters for long-term applications.

In vitro study of blood-contacting properties and effect on bacterial adhesion of a polymeric surface with immobilized heparin and sulphated hyaluronic acid

The blood-contacting properties and the effect on bacterial adhesion of a material based on polyurethane and poly(amido-amine) (PUPA), both in its native form and with the anticoagulant molecules heparin or sulphated hyaluronic acid (HyalS3.5) electrostatically bonded to its surface, were evaluated and compared in vitro. The presence of the biological molecules on the surface was revealed by a dye test and ATR/FTIR analysis. Bound heparin was found to maintain its physiological action, in terms of thrombin inactivation, as well as did free heparin. Moreover, it reduced the degree of platelet adhesion. On the contrary, bound HyalS3.5 lost its anticoagulant activity, though it reduced platelet adhesion. The number of platelets on both modified surfaces was low. Their shape distribution, as determined by SEM, did not differ significantly on the two modified surfaces or with respect to the bare PUPA surface. HyalS3.5 and heparin also inhibited adhesion of Staphylococcus epidermidis to the material. A possible relationship between the platelet and bacterial adhesion is ascribed to the mediating role of plasma proteins.

Synthesis and characterisation of phosphorylcholine-based polymers useful for coating blood filtration devices

Copolymers of 2-methacryloyloxyethylphosphorylcholine (MPC) and lauryl methacrylate (LMA) of molar ratios MPC: LMAX where x = 1, 2 or 4, have been synthesised by two different free-radical polymerisation techniques. The solubility characteristics of the resulting materials were investigated in a variety of water: alcohol solvent mixtures and found to be influenced not only by the molar ratio of MPC: LMA, but also the method of synthesis. A window of solubility was observed for certain copolymers and the alcohol used in the solvent mixture was also found to have a profound influence on the solubility profile of the polymers. These materials were soluble in a wider range of aqueous methanol mixtures compared to aqueous mixtures of higher aliphatic alcohols, such as ethanol or isopropyl alcohol, which was rationalised in terms of the affinity of the phosphorylcholine headgroup for the various alcohols relative to water. 1H nuclear magnetic resonance spectroscopy was used to further examine the solution properties of the copolymers in various solvents. The copolymer MPC: LMA2 was coated onto a variety of substrates from both alcohol-only and water: alcohol solvent systems and the surface properties of the films compared by static and dynamic contact angle, atomic force microscopy (AFM) and attenuated internal reflectance Fourier transform infrared spectroscopy (ATR-IR). The coating formed from the water: alcohol solvent was found to be hydrophilic in nature, possessing spontaneous wettability, whereas films formed from alcohol-only solvents were hydrophobic, and only on conditioning with water were more wettable surfaces attained. This phenomenon was applied in the coating of leukocyte filtration material, where the aqueous-based systems demonstrated lower critical wetting surface tension (CWST) and shorter wetting times relative to both uncoated filters and those coated from alcohol-only systems. The haemocompatibility of the coated filters was equivalent for both coating solvent systems. employed, and far superior when compared to the uncoated control.

Surface modification using silanated poly(ethylene glycol)s

Surface-grafted poly(ethylene glycol) (PEG) molecules are known to prevent protein adsorption to the surface. The protein-repulsive property of PEG molecules are maximized by covalent grafting. We have synthesized silanated monomethoxy-PEG (m-PEG) for covalent grafting of PEG to surfaces with oxide layers. Two different trialkoxysilylated PEGs were synthesized and characterized. The first trialkoxysilylated PEG was prepared by direct coupling of m-PEG with 3-isocyanatopropyltriethoxysilane through a urethane bond (silanated PEG I). The other silanated PEG (silanated PEG II) containing a long hydrophobic domain between PEG and a silane domain was prepared by reacting m-PEG with 1,6-diisocyanatohexane and 10-undecen-1-ol in sequence before silylation with 3-mercaptopropyl trimethoxysilane. Silanated PEGs I and II were grafted onto glass, a model surface used in our study. The PEG-grafted glass surfaces were characterized by contact angle, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Although contact angle did not change much as the bulk concentration of silanated PEG used for grafting increased from 0.1 to 20 mg/ml for both PEGs I and II, the surface atomic concentrations from XPS measurements showed successful PEG grafting. Surface PEG grafting increased concentration of surface carbon but decreased silicone concentration. The high resolution C1s spectra showed higher ether carbon with lower hydrocarbon compositions for the PEG-grafted surfaces compared to the control surface. AFM images showed that more PEG molecules were grafted onto the surface as the bulk concentration used for grafting was increased. AFM images of the dried surfaces showed that the surfaces were not completely covered by PEG molecules. After hydration, however, the surface appears to be covered completely probably due to the hydration of the grafted PEG chains. Glass surfaces modified with silanated PEGs reduced fibrinogen adsorption by more than 95% as compared with the control surface. Silanated PEGs provides a simple method for PEG grafting to the surface containing oxide layers.

Integration of surface modification and 3D fabrication techniques to prepare patterned poly(L-lactide) substrates allowing regionally selective cell adhesion

Regeneration of organizationally complex tissue requires regulation of spatial distributions of particular cell types in three dimensions. In this paper we demonstrate an integration of polymer processing and selective polymer surface modification using methods suitable for construction of three-dimensional polymer scaffolds which may aid such cell organization. Specifically, the surfaces of degradable polyesters were modified with poly(ethylene-oxide) (PEO)-poly(propylene-oxide) (PPO) copolymers using a process compatible with a solid free-form fabrication technique, the 3DP printing process. We demonstrate inhibition of cell (hepatocyte and fibroblast) adhesion to regions of two-dimensional poly(lactide) (PLA) substrates modified with PEO-PPO-PEO copolymers. We further show that PEO-PPO-PEO-modified surfaces which are not adhesive for hepatocytes or fibroblasts can be made selectively adhesive for hepatocytes by covalent linkage of a carbohydrate ligand specific for the hepatocyte asialoglycoprotein receptor to the PEO chain ends. Our approach may be generally useful for creating regionally selective, microarchitectured scaffolds fabricated from biodegradable polymers, for spatial organization of diverse cell types.

Reduced protein adsorption on plastics via direct plasma deposition of triethylene glycol monoallyl ether

The direct plasma-induced deposition of tri(ethylene glycol) monoallyl ether is reported. RF plasma polymerization of this monomer was carried out under both continuous wave (CW) and pulsed plasma operation. The major focus of this work was optimization of the degree of retention of the C-O-C bonds of the starting monomer during the deposition process. This successfully was accomplished using low RF power during the CW runs and low RF duty cycles during the pulsed plasma experiments. Spectroscopic analysis of the plasma films revealed a strong dependence of film composition on the RF power and duty cycles employed. In particular, an unusually high level of film chemistry compositional control was demonstrated for the pulsed plasma studies, with film composition varying in a steady, progressive fashion with sequential changes in the ratios of plasma on to plasma off times. This film chemistry controllability is demonstrated despite the relatively low volatility of the starting monomer. The utility of this plasma deposition approach in introducing polyethylene oxide (PEO) structures on solid substrates was evaluated via protein adsorption studies. Radiolabeled bovine albumin adsorption was studied on plasma-modified poly(ethylene teraphthalate) (PET) substrates. Dramatic reductions in both initial adsorption and retention of this protein were observed on PET samples having maximal PEO content relative to its adsorption on untreated PET surfaces. Good stability and adhesion of the plasma films to the underlying PET substrates were observed, as evidenced from prolonged immersion of plasma-treated surfaces in aqueous solution. Overall, the results obtained from the present work provide additional support for the utility of one-step plasma process to reduce biological fouling of surfaces via deposition of PEO surface units.

Adsorption of a novel fluorescent derivative of a poly(ethylene oxide)/poly(butylene oxide) block copolymer on octadecyl glass studied by total internal reflection fluorescence and interferometry

We have used total internal reflection fluorescence (TIRF) to measure the adsorption kinetics of a newly synthesized fluorescent derivative of a triblock copolymer comprising two poly(ethylene oxide) arms connected by a poly(butylene oxide) segment. The composition is (EO)400 (BO)55 (EO)400, in which EO represents ethylene oxide, BO represents butylene oxide, and one or both of the terminal OH groups of the two (EO)400 arms are labeled with tetramethylrhodamine. The poly(butylene oxide) segment binds to hydrophobic octadecyl glass, used as a substratum. The TIRF signal is shown to be derived almost entirely from surface-adsorbed polymer. This facilitates calculation of adsorption isotherms from 0.1-0.005% bulk polymer solution by means of diffusion kinetics. Information about the effective thickness of the adsorbed polymer, determined by optical interference microscopy, corresponds with what is known about the conformation of similar molecules at interfaces and indicates monolayer adsorption on the glass.

Protein adsorption and endothelial cell attachment and proliferation on PAPI-based additive modified poly(ether urethane ureas)

To better understand vascular interactions with poly(ether urethane urea) (PEUU) materials, protein adsorption, and endothelial cell attachment and proliferation assays were performed on a base PEUU formulation, on PEUU formulations loaded with hydrophobic and amphiphilic poly(methylene-[polyphenyl isocyanate]) (PAPI) based additives, and on PEUU formulations in which some of the polymer chains had been endcapped with either diisopropylaminoethyl (DIPAA) or decyl (DA) moieties. Protein adsorption experiments with PAPI-based additives showed that additive loaded PEUU formulations adsorbed significantly lower amounts of the studied proteins than did the unloaded PEUU. Protein adsorption to the DA and DIPAA endcapped PEUU films was found not to vary consistently from that of the unloaded PEUU film. Endothelial cell attachment and proliferation experiments with PAPI-DA and polyethylene glycol-PAPI-DA (PEG-PAPI-DA) loaded PEUU films showed that many of the films exhibited attachment and proliferation that was significantly enhanced compared to PEUU A' and that approached or equaled that of the tissue culture polystyrene control. Experiments with PAPI-DIPAA and PEG-PAPI-DIPAA loaded PEUU films exhibited attachment and proliferation data that was often below 10% of the tissue culture polystyrene control values. Experiments with the DA and DIPAA endcapped PEUU films showed endothelial cell attachment and proliferation that was statistically indistinguishable from the PEUU A' values. Contact angle analysis was carried out on the endcapped PEUU films, on the PAPI-based additive loaded PEUU films, and on PEUU A' using the sessile drop method. The advancing and receding contact angle behavior of the PAPI-based additive loaded PEUU films deviated markedly from the behavior of PEUU A', suggesting that the additives were present at the PEUU-water interface. The contact angle behavior of the endcapped PEUUs was similar to that of PEUU A', suggesting that the DA and DIPAA endcap moieties did not exist at the hydrated PEUU surface in appreciable quantities. To explain the differences in protein adsorption and endothelial cell behavior on the air side of additive loaded PEUUs when compared to the base PEUU, it was assumed that the additives near this region of the solvent swollen PEUU matrix may have migrated to, at, or near the PEUU-air interface during film formation, creating an additive enriched PEUU surface region.(ABSTRACT TRUNCATED AT 400 WORDS)

Surface modification of polymeric biomaterials with poly(ethylene oxide), albumin, and heparin for reduced thrombogenicity

Appropriate surface modification has significantly improved the blood compatibility of polymeric biomaterials. This article reviews methods of surface modification with water-soluble polymers, such as polyethylene oxide (PEO), albumin, and heparin. PEO is a synthetic, neutral, water-soluble polymer, while albumin and heparin are a natural globular protein and an anionic polysaccharide, respectively. When grafted onto the surface, all three macromolecules share a common feature to reduce thrombogenicity of biomaterials. The reduced thrombogenicity is due to the unique hydrodynamic properties of the grafted macromolecules. In aqueous medium, surface-bound water-soluble polymers are expected to be highly flexible and extend into the bulk solution. Biomaterials grafted with either PEO, albumin, or heparin are able to resist plasma protein adsorption and platelet adhesion predominantly by a steric repulsion mechanism.

Does polyethylene oxide possess a low thrombogenicity?

Because of the 'bland' nature of polyethylene oxide towards proteins and cells, considerable effort has been devoted to preparing surfaces rich in polyethylene oxide, using block copolymers, surface immobilization or other methods. It is clear that these modifications result in reduced levels of cell (including platelet) adhesion and protein adsorption, when compared to unmodified and typically hydrophobic substrates. It is far less clear whether the reduced adhesion or adsorption is due specifically to the thermodynamic effects of polyethylene oxide or to the increase in surface hydrophilicity after its immobilization. Even more so, it is unclear whether the reduction in such parameters is evidence of a reduced thrombogenicity.

PEO enhancement of platelet deposition, fibrinogen deposition, and complement C3 activation

Whereas it has been commonly thought that adding polyethylene oxide PEO to a surface would diminish the capacity of the surface to cause deposition of platelets and of fibrinogen, and to activate complement C3, we present data showing exactly the opposite. These unexpected results are obtained with low molecular weight (2000) PEO, and are not found with higher molecular weight (20,000) PEO.

Reduction of fibrinogen adsorption on PEG-coated polystyrene surfaces

Reduction of protein adsorption by coating surfaces with polyethylene glycol (PEG) is well documented. The present work has four goals related to these previous studies: first, to develop chemistry providing densely packed, covalently bound PEG on polystyrene (PS); second, to determine the ability of these modified surfaces to reject fibrinogen; third, to compare the protein-rejecting ability of branched and linear PEGs; and fourth, to examine the utility of an ELISA-type procedure for measuring protein adsorption. It was found that PEG-epoxide could be readily coupled to amine groups of poly(ethylene imine) (PEI), which had been preadsorbed onto an oxidized PS surface. The PEG groups on branched PEGs appear to act as an excluded volume to repel proteins, similar to arguments previously raised for linear PEGs. The results of protein adsorption studies showed that fibrinogen adsorption is significantly reduced by coating polystyrene with either linear or branched PEGs of 1500 to 20,000 in molecular weight. The ELISA technique was found to be equivalent in sensitivity to radiolabeled fibrinogen for estimating adsorption levels. It is expected that PEG-coated PS will have much utility in a variety of biomedical applications.

Glow discharge plasma deposition of tetraethylene glycol dimethyl ether for fouling-resistant biomaterial surfaces

The glow discharge plasma deposition (GDPD) of tetraethylene glycol dimethyl ether is introduced as a novel method for obtaining surfaces that are resistant to protein adsorption and cellular attachment. Analysis of films by x-ray photoelectron spectroscopy and several biological assays indicate the formation of a fouling-resistant, PEO-like surface on several substrata (e.g., glass, polytetrafluoroethylene, polyethylene). Adsorption of 125I-radiolabelled proteins (fibrinogen, albumin and IgG) from buffer and plasma was very low (typically less than 20 ng/cm2) when compared to the untreated substrata, which exhibited much higher levels of protein adsorption. Not all coated substrata adsorbed equal amounts of protein (e.g., coated glass samples typically adsorbed more protein than coated polyethylene or coated polytetrafluoroethylene samples), suggesting that the substratum used may affect the amount of protein adsorbed. Measurement of dynamic platelet adhesion, using epifluorescent video microscopy, and endothelial cell attachment further demonstrates the short-term nonadhesiveness of these surfaces.

Surface characterization of 2-hydroxyethyl methacrylate/styrene copolymers by angle-dependent X-ray photoelectron spectroscopy and static secondary ion mass spectrometry

The surface composition and structure of three structurally distinct amphiphilic copolymers of 2-hydroxyethyl methacrylate (HEMA) and styrene have been examined with angle-dependent X-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectrometry (SIMS). The phase-separated block copolymer made by anionic living polymerization, HSH-A50, showed significant surface enrichment of styrene. The outermost 2-3 A appeared to be approximately 100% styrene, with the styrene concentration decreasing to its bulk value at a depth of approximately 50 A from the surface. However, HEMA was detected in the outer 20 A of this copolymer. The presence of HEMA in the surface region implies this copolymer may undergo significant restructuring when hydrated in a hydrophilic environment (as opposed to the hydrophobic environment in which the sample was prepared and analyzed). The phase-separated block copolymer made by telechelic coupling of free radical polymerized functionalized oligomers, HSH-B60, showed only slight styrene enrichment at the surface. Both HEMA and styrene were detected at all sampling depths, including the outermost surface layer, consistent with the presence of discrete HEMA and styrene domains at the copolymer surface. Since both components are already present at the surface under hydrophobic conditions, the degree of restructuring this copolymer may undergo upon hydration should be minor. The random HEMA--styrene copolymer made by conventional free radical initiation techniques, HS-RAN50, had a surface composition that was similar to the bulk composition and independent of depth, as expected for a homogeneously mixed copolymer film.

Synthesis and bioactivity of copolymers with fragments of heparin

A new type of biocompatible copolymer comprising small fragments of heparin, (octa- to dodecasaccharides) copolymerized with a synthetic monomeric component, viz. acrylamide, has been prepared. The heparin fragments are produced by enzymatic or chemical means and are copolymerized, directly or after suitable derivatization, with acrylamide as the major polymerizable component. The polymeric material incorporates the heparin segments as pendant moieties such that their essential functional groups and structural features for specific binding with the selective serine protease coagulation factor inhibitor antithrombin III are preserved. An important feature of this copolymer is its biocompatibility which relates specifically to its antithrombotic and antithrombogenic activity derived from those of heparin fragments. The biological activity of heparin fragments and copolymers thereof are determined in terms of APTT and anti-Xa activity, their antithrombotic potential being expressed as a ratio of anti-Xa activity to APTT. The copolymers reported have biological activities similar to equivalent amounts of respective heparin fragments, and show higher antithrombotic activity compared to intact heparin or commercially available low-molecular-weight heparin (4,000-6,000 Da).

Poly(dimethylsiloxane)-poly(ethylene oxide)-heparin block copolymers. III: Surface and bulk compositional differences

Previously observed bioactivity of poly(dimethylsiloxane)-poly(ethylene oxide)-heparin (PDMS-PEO-Hep) triblock copolymers has prompted studies of the surface and bulk character of this copolymer using angular-dependent electron spectroscopy for chemical analysis (ADESCA), static secondary mass spectroscopy (SIMS), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Because the low-energy PDMS phase dominates surfaces of this copolymer when solvent cast under air or vacuum conditions, attempts were made to explain surface restructuring and rearrangements induced in hydrated or aqueous environments that permit surface accessibility and bioactivity of heparin moieties. Based on comparisons with PDMS, PEO, and heparin homopolymers, PEO/heparin blends, and an unheparinized PDMS-PEO diblock copolymer, PDMS-PEO-heparin demonstrates both phase-mixed and phase-separated regions in DSC analysis. During annealing cycles above the Tg values of the copolymer constituents, phase-mixed regions become increasingly phase separated and PEO enriched. TGA analysis confirmed the presence block copolymer constituents and presented evidence of intermolecular segmental interactions, hence phase-mixing in the copolymers. ADESCA analysis indicates that the outer 5 A of both the PDMS-PEO and PDMS-PEO-Hep copolymers is essentially pure PDMS. However, significant amounts of PEO are detected 5 to 20 A below the surface. Static SIMS also detects the presence of PDMS at the surfaces of the PDMS-PEO and PDMS-PEO-Hep copolymers. Compositional models based on ADESCA, SIMS, and DSC data are presented for desiccated and hydrated copolymer surfaces.

Synthesis and characterization of poly(dimethylsiloxane)-poly(ethylene oxide)-heparin CBABC type block copolymers

Heparin and poly(ethylene oxide) were coupled to a central anchoring block of poly(dimethylsiloxane) in order to investigate its blood compatible properties. Diamino telechelic poly(dimethylsiloxane) (PDMS-(NH2)2, Mw = 20,000) was first modified to isocyanate functionalities using toluene 2,4-diisocyanate. This modified PDMS was then coupled to diamino-telechelic poly(ethylene oxide) (PEO-(NH2)2, Mw = 2000, 4000, 6000) to create BAB type copolymers having terminal free amino groups. These amino groups were covalently coupled to heparin containing terminal aldehyde groups using sodium cyanoborohydride to yield a bioactive, CBABC type block copolymer. The physical characterization of these copolymers was performed with IR, NMR, sulphur elemental analysis, Wilhelmy plate contact angle, and differential scanning calorimetry (DSC). CBABC block copolymer surfaces demonstrated heparin bioactivity in in vitro evaluation, and improved nonthrombogenic properties during ex vivo A-A shunt experiments.