Partitioning and diffusion of solutes in hydrogels of poly(ethylene oxide).

Hydrogels were created by electron beam irradiation of aqueous solutions of poly(ethylene oxide) (PEO) having a nominal molecular weight of 35,000. The molecular weight between cross-links Mc varied from 3000 to 15,000, and the equilibrium volume fractions of polymer V2,s from 0.01 to 0.08. These hydrogels were exposed to aqueous solutions of solutes: tricyclic antidepressants, cyanocobalamin, four globular proteins and three linear species of PEO. Partition coefficients and diffusion coefficients were determined. For each solute the ratio diffusion coefficient in hydrogel/diffusion coefficient in free solution was determined, and related to the hydrogel parameters Mc and v2,s and to the solute effective radius rE (Einstein radius). The diffusion coefficient ratio is greater for the flexible random coiling PEO than for the 'rigid' solutes at a given set of Mc, v2s and rE, and the disparity increases rapidly as rE increases. Among the globular proteins the diffusion coefficient ratio decreases by orders of magnitude with small changes in rE (20.6-27.6 A) and was found to be nearly zero for albumin (rE = 36.1 A). The tricyclic antidepressants had partition coefficients of around 2, whereas the other solutes had partition coefficients of about unity. By reason of the partition coefficient of around 2, the diffusion coefficient ratio of a tricyclic antidepressant having a value of rE = 5.5 A is half that of the larger cyanocobalamin, for which rE = 8.5 A.

Hydrogels prepared by electron irradiation of poly(ethylene oxide) in water solution: unexpected dependence of cross-link density and protein diffusion coefficients on initial PEO molecular weight.

Under ionizing radiation aqueous solutions of water-soluble polymers become cross-linked and form hydrogels, primarily by radiolysis of water-generating hydroxyl radicals which attack the polymer chains. The chain radicals thus formed create cross-links by coupling. In particular, hydrogels formed from poly(ethylene oxide) are of interest for biomedical applications, including those in which it is necessary to transport large molecules such as growth factors, in addition to nutrients, to cells attached to the surface. We sought a rapid and simple method for estimating diffusion coefficients by observing the diffusion of two red-coloured proteins: cytochrome C and hemoglobin of respective molecular weights 12,000 and 67,000. In the course of this we discovered a previously unreported effect of the primary molecular weight of the polymer, before cross-linking, on the cross-link density finally achieved and on the diffusion coefficient of the proteins.

Activity toward thrombin-antithrombin of heparin immobilized on two hydrogels.

Commercially obtained (Diosynth) heparin was covalently bonded to poly (vinyl alcohol) (PVA) hydrogels and to polyethylene oxide (PEO) hydrogels activated by tresyl chloride. We found that as tresyl chloride activation of PVA increased, the specific activity of the bound heparin toward thrombin and antithrombin decreased by nearly a factor of 10 and that commercial heparin bound to PEO had nearly ten-fold greater activity than when bound to PVA at comparable concentrations. These findings suggest that the long 'leash' provided by PEO hydrogels may give the heparin more access to the thrombin-antithrombin pair than the tight bond to PVA, and that crowding of heparin units on a surface limits access of the thrombin-antithrombin pair.

Hepatocyte culture on carbohydrate-modified star polyethylene oxide hydrogels.

We describe the synthesis and in vitro biological characterization of a new class of carbohydrate-modified hydrogels based on radiation-cross-linked star polyethylene oxide (PEO). Hydrogels were synthesized from either of two types of PEO star molecules in order to vary the terminal hydroxyl content of the gels while keeping other gel properties such as molecular weight between cross-links and water content constant. The resulting gels were covalently modified with monosaccharide ligands and the behaviour of primary rat hepatocytes on the modified gels was evaluated under culture conditions. Hepatocytes exhibited a sugar-specific adhesion to the modified gels, adhering to gels bearing galactose but not glucose. Cell spreading was observed on both types of galactose-modified PEO star gels; moreover, the gels supported long-term (6 d) culture and differentiated function of primary hepatocytes. Further, on comparing the cell spreading behaviour observed on the PEO star gels with that reported previously for galactose-modified polyacrylamide, we find that our gels elicit spreading at ligand concentrations lower by an order of magnitude. A simple mechanistic analysis indicates that this enhanced ability of PEO star gels to support spreading of primary hepatocytes on low concentrations of immobilized galactose derives from freedom of the immobilized ligands to come within sufficiently close proximity to mimic a high-affinity branched oligosaccharide.

Gamma sterilization of UHMWPE articular implants: an analysis of the oxidation problem. Ultra High Molecular Weight Poly Ethylene.

Gamma irradiation of Ultrahigh Molecular Weight Polyethylene (UHMWPE) leads to long-lived free radicals which react with oxygen. Diffusion of oxygen, occurring over months or years, controlled by the permeability characteristics of the polymer, results in progressive oxidation, breaking of polymer chains, alteration of the crystalline portion of the polymer, and deterioration of the mechanical properties of the polymer. This paper reviews the observations in the literature on this issue and then presents a conceptual model concerning the interplay of radical diffusion, oxygen diffusion, non-uniform permeability, and free-radically driven chain reactions in order to explain these observations. The suggested model is based on literature that is available on the oxidation of linear polyethylenes during and after irradiation. The model directs the attention of researchers in the field of orthopaedic implants to the complexity of the process and the variety of issues and parameters to be considered while studying the long-term effects of radiation sterilization on UHMWPE.

Fibrinogen adsorption and platelet adhesion at the surface of modified polypropylene glycol/polysiloxane networks.

The protein film adsorbed at an artificial surface ultimately affects platelet adhesion and activation. This study examines the role of fibrinogen in platelet adhesion at the surface of crosslinked polypropylene glycol (PPG)/polyglycidoxy propyl methyl siloxane (PGPMS) networks which contain polyethylene glycol monomethyl ether (PEGME) chains. These crosslinked networks were produced by reacting the epoxy groups of PGPMS with the hydroxyl groups of the polyethers. PEGME chains were attached covalently to the network at only one end while PPG chains were attached at both ends. The incorporation of PEGME resulted in a substantial reduction in fibrinogen adsorption as compared to the model network (PPG + PGPMS only), but the expected concomitant decrease in platelet adhesion was not observed.

Crosslinked polyether/polysiloxane networks for blood-interfacing applications.

The interaction of blood with new artificial surfaces is an area of continual medical interest. In this study, a series of polyether/polysiloxane networks were synthesized, characterized in terms of both bulk and surface compositions, and evaluated for blood compatibility. The crosslinked networks were produced by reacting the epoxy groups of polyglycidoxy propyl methyl siloxane (PGPMS) with the hydroxyl end groups of polypropylene glycol (PPG). Blood compatibility was evaluated using an in vitro platelet retention test and fibrinogen adsorption experiments from human plasma and buffered saline. The PPG/PGPMS networks exhibit low fibrinogen adsorption and low platelet activation. Such properties make the networks potentially attractive as materials for blood-interfacing applications.

Challenge to the concept that UHMWPE acetabular components oxidize in vivo.

Severe wear of the ultra-high molecular weight polyethylene (UHMWPE) components of total joint replacements limits their long-term success. In previous studies, the infrared spectra obtained on retrieved UHMWPE components were interpreted as evidence that the UHMWPE oxidizes in vivo. In direct contrast, infrared spectroscopy of the retrieved UHMWPE acetabular components examined in this study demonstrated adsorbed esterified fatty acids, readily extractable by hexane, and no substantial evidence of in vivo oxidation. This emphasizes that special care must be taken when using infrared spectroscopy to assess retrieved components.

Drug partitioning and release characteristics of tricyclic antidepressant drugs using a series of related hydrophilic-hydrophobic copolymers.

A series of crosslinked polymer networks formed from hydrophilic polyethylene oxide (PEO) and a hydrophobic polysiloxane (PGPMDMS) were studied with respect to the partitioning and release of five tricyclic antidepressants (TCAs) at pH 7.4. The TCAs, chemical analogues of one another, have both nonpolar and ionic characteristics at pH 7.4, but differ considerably in hydrophobicity. In PEO-PGPMDMS copolymer networks, the partition coefficient of protriptyline (the TCA studied most extensively) was observed to be higher than in networks of PEO or PGPMDMS singly. This finding, which may represent adsorption of the amphiphilic drug at interfacial sites between hydrophilic and hydrophobic phases within the copolymeric network, shows that in some cases, higher drug loadings of amphiphilic drugs can be obtained with a hydrophilic-hydrophobic copolymer compared with a material made of only one polymer. As the PEO content in PEO-PGPMDMS networks was increased from 20 to 100%, the release rate of protriptyline increased by greater than 1000-fold. Thus, a key variable in achieving a desired release rate is the PEO content of the copolymer. On the other hand, release rates of the five TCAs from PEO-PGPMDMS networks containing 50% PEO varied by a factor of less than 3. Thus, minimal effect on drug release rates was obtained by using a different TCA analogue.

Grafting of PEO to polymer surfaces using electron beam irradiation.

A new method was developed for binding poly-(ethylene oxide) (PEO) to polymer surfaces that involves the use of electron beam irradiation in two steps. In the first, methacrylic acid was grafted and polymerized to a polymer surface, changing it from hydrophobic to hydrophilic. Exposure of this surface to aqueous PEO solutions resulted in strong hydrogen bonding of the PEO, which was covalently grafted in a second radiation step. The PEO grafts were stable; they could not be removed with extensive washing with water, soaking in basic solution, or gentle mechanical scraping. Both monolayers and multilayers of PEO were formed. The density of the monolayers were found to have little dependence on the molecular weight or concentration of the PEO solution; multilayers could be controlled by varying the viscosity of the PEO solution and the method of application. The PEO-grafted monolayers were tested for their ability to prevent protein adsorption of cytochrome-c, albumin, and fibronectin. Monolayers of star PEO were the most effective, at best showing a 60% decrease in adsorption from untreated controls. One million molecular wight linear PEO monolayers were almost as effective as star monolayers, and 35,000 g/mol linear PEO was bound too closely to the surface, owing to its small size, to have much impact in preventing protein adsorption. The reason for the continued protein adsorption was believed to be due to a close grafting of the PEO to the surface, as well as the grafted methacrylic acid chains being long enough to extend through the PEO monolayer, thus being accessible on the surface.

Surface hydroxylation of styrene-butadiene-styrene block copolymers for biomaterials.

This work pertains to the development of high strength elastomers potentially useful as nonthrombogenic cardiovascular prostheses. Triblock copolymers of the styrene-butadiene-styrene type have been subjected to surface hydroxylation which provide reactive sites at the surface for the subsequent coupling of heparin while retaining the unique mechanical properties of the SBS copolymers. Curves of hydroxyl content versus the copolymer film thickness demonstrate the effect of swelling in the surface region on the product distribution and on the time dependence of the hydroxylation process. In addition, the effect of time, temperature, and the composition of the reaction bath on the diffusion/reaction process is shown. Finally, the general applicability of this surface modification scheme to the development of biomaterials is discussed.

Development of semicrystalline poly(vinyl alcohol) hydrogels for biomedical applications.

Swollen crosslinked poly(vinyl alcohol) (PVA) networks were developed and tested as potential biomaterials. They were prepared by electron-beam irradiation of aqueous PVA solutions at various temperatures and doses of irradiation. These materials were characterized by low mechanical properties and especially by low elongation at break and ultimate tensile strength. Reinforcement was achieved by a two-stage dehydration-annealing process, introducing crystallites in the polymeric network. Improved mechanical properties were achieved due to the presence of this crystalline PVA phase. Due to their structure, these hydrogels are proposed to be used as biomembranes for selective transport of macromolecules and as biomaterials for synthetic articular cartilage applications. Surface heparinization was accomplished by reacting heparin to the hydroxyl groups of the PVA surface, through covalent acetal bridges.

In vivo tissue reactivity of radiation-cured silicone rubber implants.

Silicone rubber implants are clinically used in large numbers and elicit a milk tissue reaction. An occasional patient develops an accentuated reaction, an observation which has stimulated clinicians to try to understand this process more fully. The chemistry of medical silicone implants, including quantitative composition, is reviewed. This in vitro laboratory study show less tissue reaction-cured silicone with SiO2 filler. A proposed system for fabrication and curing of a silicone implant, with the qualities of strength and diminished tissue reactivity, is discussed.

Does the conformation of adsorbed fibrinogen dictate platelet interactions with artificial surfaces?

Platelet activation by polymer surfaces is thought to require preliminary adsorption of fibrinogen and perhaps changes in fibrinogen conformation. We measured fibrinogen adsorption by a series of polymers by two methods, using either 125I-labeled fibrinogen or 125I-labeled antifibrinogen antibodies, and correlated the results with platelet reactivity (retention and secretion) in columns of beads coated with the polymers. For polyalkyl methacrylates with 1 to 4 carbon side chains, platelet reactivity varied directly with increasing length of the alkyl side chain and with the quantity of bound fibrinogen recognizable by antifibrinogen antibody but not with the total quantity of fibrinogen adsorbed. The same pattern of results was seen with five antibody preparations, including affinity-purified Fab fragments against the D or E domain of fibrinogen. Tests of platelet retention and fibrinogen binding to four polyalkyl acrylates and to three unrelated polymers (polystyrene, polymethyl methacrylate, and a polyether polyurethane) indicated that platelet retention correlated positively with both total fibrinogen binding and with the amount of antibody-recognizable fibrinogen bound. Drugs that block platelet aggregation, but not adhesion, did not alter the hierarchy of platelet retention to the polyalkyl methacrylates. These data suggest that, contrary to previous views, platelet adhesion to artificial surfaces increases with increasing surface coverage of adsorbed fibrinogen if the bound fibrinogen maintains a conformation such that its functional domains remain recognizable by antibody probes.

Platelets and artificial surfaces: the effects of drugs.

Contact of blood with a foreign surface activates platelets and leads to their consumption. This property is shared by most non-biological materials, including air, but can be reduced by an optimal balance of hydrophobicity and hydrophilicity, minimal capacity for hydrogen bonding, avoidance of crystallinity, maintenance of polymer backbone mobility, and other manipulations of the chemistry of the polymer. None the less, no totally non-thrombogenic artificial surface has been developed. Attention has therefore turned to suppression of platelet-surface interaction by drugs that alter platelet function. Agents that block cyclo-oxygenase inhibit surface-induced secretion and aggregation but have no effect on platelet adhesion. Drugs that increase platelet cyclic AMP levels have a dose-related effect, which at high concentrations can eliminate adhesion to surfaces. The most successful agent, prostacyclin, has achieved total protection of platelets during cardiopulmonary bypass, with preservation of normal platelet number and function. Associated vasodilatation is a notable side effect, and hypotension may prove to be a significant problem in clinical practice. The development of more selective analogues with minimal vasodepressor activity is to be encouraged.

Surface chemical analysis of Avcothane and Biomer by Fourier transform IR internal reflection spectroscopy.

During the solvent casting process, one side of the polymer film is exposed to air while the other side is in contact with a substrate, used as a mold. We have studied the effect of this difference in exposure during casting on the chemical composition of two types of segmented polyurethane, Biomer and Avcothane, by using Fourier transform IR internal reflection spectroscopy. Also, a depth-composition profile was obtained by placing a thin barrier film between the reflection plate and the polymer film. In Avcothane, the air side, which is the blood-contact side, contains a greater amount of the soft segment than the substrate side, and this is more pronounced in the layer closer to the surface. The anisotropy in composition is more drastic when the silicone content is compared. In a layer about 1.5 mu thick, one can detect a greater amount of silicone in the substrate side than in the air side. However, when one averages the concentration in a layer of about 0.8 microns the trend in reversed; i.e., the greater amount of silicone is now present in the air side than in the substrate side. In Biomer films, the anisotropy in chemical composition is less pronounced. Only a modest increase in the relative content of the soft segment/hard segment is observed in the air side when a depth-composition profile is obtained.

Platelet-compatible hydrophilic segmented polyurethanes from polyethylene glycols and cyclohexane diisocyanate.

A new type of segmented polyurethane (SPU) was synthesized from alpha, omega polyethylene oxide diols (PEG) of MW varying from 600 to 4500, by end capping with 1,4 trans cyclohexane diisocyanate (CHDI) and chain extending with ethylene diamine (ED) in toluene with dibutyl tin dilaurate as catalyst. These SPU are cast as films and coatings from hexafluroisopropanol (HFIP). Depending on PEG MW, these SPU swell two- to tenfold in water. Examined by an in vitro platelet retention test, these SPU are more bland (platelet retention -rho around 0.05) than most other polymers, whereas an alternating copolymer of CHDI and ED shows -rho around 0.80 (very active); x-ray photoelectron spectroscopy shows that the surfaces of these SPU to a depth of about 40 Ao are nearly pure PEO, unlike SPU synthesized from aromatic diisocyanates TDI and MDI.

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.