Medical Plastics and Biomaterials Magazine | MPB Article Index

Originally published July 1996

Y. SAMUEL DING, CHUAN QIN, AND BARRETT E. RABINOW

Over the past few decades, the study of protein-resistant materials has been a very active field of research.^1­18 Investigators have expended a great deal of energy in the attempt to minimize protein adsorption on biomaterial surfaces. The intensity of this effort reflects the importance of a wide range of blood-contacting devices for uses that include antithrombus applications, coatings to reduce biofouling, separation membranes, protein-drug-contacting materials, and immunoassays. Among the effective protein-resistant materials that have been studied are water-soluble polymers such as poly(ethylene oxide) (PEO), polyvinyl alcohol (PVA), and poly(vinyl pyrrolidone) (PVP). The mechanism by which these types of materials prevent protein adsorption has been explained as a combination of the steric stabilization effect, unique solution properties, and the particular molecular conformation in the aqueous solution. The steric repulsion resulting from osmotic pressure and elastic restoring forces competing with the van der Waal's attraction between water-soluble polymers and proteins determines the protein-adsorption process.

Because of their intrinsic nature, these water-soluble polymers cannot be used for direct water-contact applications by themselves. Most researchers involved in immobilizing water-soluble polymers--especially PEO--have resorted to chemical or chemical-physical modifications. Such measures have included covalent grafting, coating of PEO-containing block copolymeric surfactants, block copolymerization, inducing physical interpenetrating surface networks by solution swelling, and synthesizing PEO-containing interpenetrating networks.

In theory, the best way to immobilize water-soluble polymers permanently onto a surface is by covalent bonding processes, but such methods are generally difficult and too costly for commercialization. One alternative involves melt blending of the water-soluble polymer into the base polymer, accompanied by shear processing to drive the water-soluble polymer towards the surface. This technique will be much easier and more economical than covalent bonding, provided that the resulting surface modification can be made sufficiently permanent. In the present article, we provide examples of melt-blended materials that have achieved permanent hydrophilic surfaces, and discuss the factors that control the performance of melt-blended formulations.

EXPERIMENTAL

Materials. Several water-soluble polymers were used as surface modifiers in this study, including PEO (Aldrich Chemicals, five different grades with Mw of about 3400, 10,000, 100,000, 900,000, and 5,000,000); poly(ethyl oxazaline) (PEOX; Dow Chemical, Mw of about 500,000); PVA (Aldrich, 89% hydrolyzed, Mw from 13,000 to 23,000); and PNVP (Aldrich, Mw of about 40,000). The base polymers included ethylene-vinyl acetate copolymer (EVA; EU648, Quantum Chemicals); polypropylene (23M2C, Rexene Corp.); poly(methyl methacrylate) (CP-82, ICI Acrylic); styrene-butadiene copolymer (DR-03, Phillips); polyamide (Nylon-12 and L-20, EMS America Grilon); and polycarbonate (Makrolon, Bayer).

Testing. Procedures employed to evaluate the effectiveness of surface modification included contact-angle measurement, surface-protein-adsorption measurement, and long-term-leaching testing.

Contact-angle measurement--which assesses wettability--was used as the primary screening method to determine the effectiveness of surface modification. Contact angle was measured after the samples had been soaked in water from time zero up to one month, in order to monitor any changes in the specimens' surface hydrophilicity. A Wilhemy-Plate Tensiometric method with a Cahn Electrobalance and an LVDT was used to measure the receding contact angle between the sample films and an HPLC-grade water column.

We performed direct analysis of surface-protein adsorption according to a modified Micro-BCA method.¹⁸ After equilibrating the polymer films with different concentrations (for example, 2.5 ppm) of IgG solution for at least 18 hours, we rinsed the film with distilled water to remove loose protein and measured the level of surface-bound protein.

Film samples were stored in 40°C water for up to 48 days and monitored for organic leachables in the solution. This was done in order to prove that the achieved hydrophilic surface was permanent, and not simply due to continuous migration of water-soluble polymers towards the surface to replenish the lost, loosely bound polymers.

RESULTS AND DISCUSSION

Polymer blends based on different base polymers and water-soluble polymers at various blend ratios were tested for their contact angles in water. Figure 1 shows the contact angles of the EVA blends with water-soluble polymers such as PEO, PVA, PNVP, and PEOX. It is clear that the water-soluble, polymer-modified EVA surfaces are more hydrophilic than the base EVA. Among the four water-soluble polymers, we observed two types of performance. The use of PEO resulted in only a very small improvement in the EVA's surface hydrophilicity, and the change was almost independent of the level of PEO. This indicates that PEO is not an effective surface modifier for the EVA base polymer.

The other three water-soluble polymers--PVA, PEOX, and PNVP--formed a second group. At concentrations of merely 2 to 5 weight percent, their presence caused dramatic improvement in the hydrophilicity of EVA surfaces. It is also interesting to note that these three water-soluble polymers exhibited similar contact angles when added at similar levels in the blends.

Long-term leaching with water was used to assess whether the surface modification could be the result of the migration of water-soluble polymers towards the surface to replenish the lost modifier. Figure 2 shows the organic leachables detected in the aqueous solution, after the films were soaked for 48 days at 40°C. In comparison with the EVA control, we observed slight increases in the leachables level in the PVA, PNVP, and PEOX systems. In each case, the leached amount was insignificant compared with the total amount of water-soluble polymer in the formulation. However, in the PEO system, we saw a more dramatic increase in leachables, indicating that PEO was very mobile and loosely anchored on the EVA surface and could be easily leached out into the aqueous solution. This explained why PEO could not effectively modify the EVA surface, as shown in Figure 1. (Figures not yet available on-line.)

Once we had succeeded in improving hydrophilicity by adding some water-soluble polymers into the base polymers, we then needed to prove that the enhanced surface also brought about reduced protein adsorption. Results of surface-protein adsorption based on a modified Micro-BCA assay were plotted against the contact angle for the EVA/PVA system, as shown in Figure 3. For this blend, we observed good correlation between the contact angle and the level of protein adsorption. Since a reduction of contact angle indicates improved surface coverage by the water-soluble polymers, it is not surprising to see a correspondingly reduced protein adsorption, given that the water-soluble polymer adsorbs less protein than the base polymer. Similar correlations can be drawn for each water-soluble polymer/base polymer pair.

As discussed, the PEO/EVA system did not successfully improve the surface hydrophilicity, and the subsequent leaching test also showed the inability of the material to hold the PEO additives on the surface. One might suppose that the molecular weight of the PEO could potentially affect the permanency of its surface anchoring. However, we did not find this to be the case, as illustrated in Figure 4, which shows the effect that increasing the molecular weight of PEO had on contact angle. As can be seen, there was practically no difference in contact angle through a wide molecular-weight range.

Figure 5 depicts a schematic of the melt-blending process for achieving surface modification. The water-soluble polymer, which is partially exposed and partially anchored in the matrix, will swell upon exposure to aqueous solutions. The swelling of the exposed chain will exert some pulling force on the anchored segment, which adds to the force generated by the tendency of the water-soluble polymer to migrate into the aqueous phase and dissolve into the solution. The permanency of the anchored water-soluble polymer on the surface of the base polymer depends on the ability of the embedded water-soluble polymer chains to resist the pull force from the exposed swollen segment. In the case of the PEO/EVA blend--where there was little anchoring force to prevent polymer-chain migration--the increase in molecular weight may have slowed down the migration process slightly, but apparently not enough to achieve sufficient permanency.

Reexamination of the data in Figure 1 led us to an interesting observation. The polymers that successfully modified the EVA surface have a glass-transition temperature (T_g) higher than room temperature, whereas the one that failed--that is, PEO--has a T_g far below room temperature. This suggests that reducing mobility or increasing chain friction through the selection of higher-T_g water- soluble polymers could provide the anchored chains with sufficient stability to stay on the polymer surface. Similarly, using a base polymer with a higher T_g could also be a promising method of anchoring the water-soluble polymers.

Figure 6 shows the contact-angle results of PEO blended with various base polymers. Among these base polymers, only the EVA could not hold the PEO onto the surface. The T_g of EVA is approximately room temperature, and the material apparently is too mobile to hold the already very mobile PEO chains. The other polymers, which all have higher T_gs, show strong indications of surface modification with the addition of PEO. These data clearly suggest that a high-T_g matrix will be able to hold the anchored water-soluble polymers in the aqueous environment.

In Figure 7, we compiled the protein-adsorption data of different combinations of water-soluble and base polymers. The results indicate that when either the water-soluble polymer or the base polymer has a T_g higher than room temperature, the surface properties show significant improvement. That is to say, we can modify a base-polymer surface through melt blending simply by ensuring that the T_g of either the base polymer or the modifier polymer is higher than the temperature of the intended application.

The approach described above can be used to reduce protein loss by adsorption that occurs in medical containers, as illustrated in Figure 8. When storing a 2.5-ppm concentration of human IgG protein solution, containers made of polypropylene, polyethylene, EVA, or plasticized PVC will lose 30% of the protein to the surface due to adsorption. Containers made of EVA modified with PVA, however, demonstrate a five- to tenfold reduction in protein loss. The difference can have a significant therapeutic impact on patients and represents substantial cost benefits for this type of expensive, genetically engineered drug.

CONCLUSION

This study demonstrates that a melt-blending method using water-soluble polymer additives can be employed to prepare hydrophilic surfaces. We further determined that increasing the molecular weight of the water-soluble polymer is not sufficient to ensure the durability of the surface modification. Rather, the glass-transition temperatures of both the base polymer and the water-soluble polymer were found to be most important, and should be higher than the end-use temperature.

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Y. Samuel Ding, PhD, is a senior engineering specialist at the Medical Materials Technology Center of Baxter Healthcare Corp. (Round Lake, IL), where he specializes in developing biomedical polymers for disposable medical devices and drug-delivery systems. Chuan Qin, PhD, is an engineering specialist at the same facility, working on polymer structure and physical properties for biomedical material and product development. Barrett E. Rabinow, PhD, is the director of strategic development in Baxter Healthcare's IV Systems Division.