Blending in with the Body

Chemistry for Everyone edited by

Products of Chemistry

George B. Kauffman California State University Fresno, CA 93740

Blending in with the Body Andrew L. Lewis* and Mike Driver Biocompatibles Ltd., Farnham Business Park, Weydon Lane, Farnham, Surrey GU9 8QL, England; *[email protected]

The Biomaterials Challenge Whether as a result of accident, disease, natural defect, or just plain wear-and-tear, there are occasions when it is necessary to treat an individual by replacement or enhancement of some part of the body. While many materials exist with adequate physical properties to perform the desired function, few can operate without problems when placed within the hostile environment of the body. In recent years, a great deal of research has focused on the improvement of these so-called biomaterials in order that they are better tolerated by the recipient. But this problem has proved fundamentally difficult. Life has had many millions of years over which to evolve, and the human species has developed a very complex and efficient set of mechanisms designed to detect and protect against any marauding foreign body. One approach has been to attempt to fool the body’s self-defense mechanisms into believing that the synthetic material is actually “self ” and not alien. This has been achieved by addressing the chemistry at the very interface where tissue meets biomaterial. Be it a contact lens in the ocular environment, a hip joint replacement, or perhaps a vascular prosthesis within the circulatory system, the initial biological response elicited by the surrounding tissues in reaction to the presence of the device is fundamentally the same. The bodily fluids that bathe the device contain proteins, complex three-dimensional biological entities that have a delicate disposition towards surfaces. Although they may adopt a particular shape (conformation) in their natural surroundings, upon exposure to a foreign surface, there is an energetically favorable drive for these molecules to unfold and change their conformation. Chemical groups previously hidden within the core of the structure are exposed and able to interact with the surface, causing the protein to adhere (usually irreversibly) (1). This may mean that further different groups are expressed at the surface and the protein layer may take on a different level of biological activity (Fig. 1). This binding of proteins is the beginning of the slippery slope that ends with adverse immunological and inflammatory responses and, ultimately, the possible rejection of the device by the body. The designers of medical devices for long-term use have been waging jungle warfare against the body’s defense systems for many years now. Over the past few decades, a new type of camouflage has been developed that helps “soldiers blend in more effectively with the trees”. In the late 1970s the connection was first made between cell membrane structure and biocompatibility (2). The question of why a red blood cell could exist in the blood stream without invoking blood clot formation (thrombosis) was investigated in a series of adroit

experiments. The lipid bilayer of the red cell membrane was isolated and separated into the outer (extracellular) side and the inner (cytoplasmic) side. When the propensity for blood clot formation (thrombogenicity) was determined for the two layers, the former did not induce any clot formation whereas the inner layer was seen to be thrombogenic. This was found to be due to an asymmetry in the proportions of the different phospholipid types on the inner and outer layers (Fig. 2). The key observation here is that the extracellular layer is dominated by the phospholipid dipalmitoylphosphatidylcholine. This lipid has a zwitterionic headgroup (i.e., the polar portion of the molecule has both positive and negative charge, but is overall electrically neutral), whereas the inner layer possesses a greater proportion of charged phospholipids. Chapman and others took this one step further and showed that the zwitterionic phosphorylcholine (PC) headgroup is the key entity in conferring hemocompatibility (Fig. 3) (3). As a result, the PC moiety has been incorporated into a number of synthetic polymer systems and used to modify medical devices in an attempt to improve their compatibility with the body. Our company has taken one such family of materials to commercialization and applied them to a wide variety of medical devices.

Figure 1. Protein adsorption to foreign surfaces. A: The 3-dimensional protein conformation is supported in aqueous solution. B: Upon approaching a foreign surface, the protein unfolds to expose previously buried groups. C: Water molecules are displaced and the groups interact irreversibly with the surface, denaturing the protein.

JChemEd.chem.wisc.edu • Vol. 79 No. 3 March 2002 • Journal of Chemical Education

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Hydrophilic phosphorylcholine headgroup

water molecules

Hydrophobic fatty acid side chains

Figure 3. Structure of the membrane phospholipid dipalmitoylphosphatidylcholine (DPPC). Figure 2. (a) Stylized schematic of a section through a biomembrane lipid bilayer. (b) Representation of the phospholipid asymmetry in the bilayer, and the relative amounts of the major lipid components.

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Given that PC-based systems may be classed as biomimetic (copying nature), there is some debate concerning their underlying mode of action. Current wisdom favors the view that the highly hydrophilic headgroup binds a large number of water molecules around it. When a protein approaches this surface, it contacts the large hydration sphere, and without any formal charge to initiate further interaction, the protein may desorb from the surface without having changed its conformation (4 ). Numerous in vitro protein adsorption assays have demonstrated that PC-coated surfaces adsorb significantly lower amounts of protein (5). Figure 4 exemplifies the extent of this reduction for several proteins involved in the process of thrombus formation within the blood. The fundamental protein–surface interactions that are implicated in the primary stages of the body’s “nonself ” recognition mechanism can thus be controlled by use of PC at the interface. Chemistry to Mimic Biology Many types of synthetic PC polymer have been studied academically, which has proved challenging owing to inherent difficulties in synthesis and handling. It was the development of a PC methacrylate monomer that opened the door to commercialization of these materials. The monomer 2methacryloyloxyethylphosphorylcholine, MPC (5), was first reported by Nakabayashi in Japan (6 ). It is now preferably manufactured using modifications of the method of Nakaya (7) as shown in Figure 5. 2-Hydroxyethylmethacrylate (3) is coupled with 2-chloro-2-oxaphospholane (2) and the inter322

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Figure 4. ELISA results demonstrating the in vitro reduction in adsorption of various plasma proteins by PC coating.

mediate (4) is aminated with trimethylamine. The product is harvested as a white, hygroscopic solid and can be purified further by recrystallization before use. Our company manufactures medical devices that utilize PC Technology™ to ensure that the devices are better tolerated within the body. This has been achieved by the development of a range of PC-containing polymers useful in solutions, as coatings, and as bulk materials. These materials are generally prepared by conventional free-radical polymerization methods that are used widely in industry. For materials suitable as coatings for devices, the polymerization is conducted in solution and the products are isolated by precipitation in a suitable non-solvent (8).

Journal of Chemical Education • Vol. 79 No. 3 March 2002 • JChemEd.chem.wisc.edu


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Figure 5. Reaction schematic for the synthesis of 2-methacryloyloxyethyl phosphorylcholine (MPC).

The preparation of a typical copolymer is shown in Figure 5D. It involves the copolymerization of the hydrophilic MPC monomer with a hydrophobic lauryl methacrylate (LMA) comonomer. The combination of these two very different components is a challenge, but when achieved, it produces a material having the ability to adhere to a variety of substrates and confer upon them a high degree of biocompatibility. The properties of the polymers produced are dependent on the nature and relative proportions of the constituent monomers, which provides a great deal of flexibility and enables a broad range of materials to be prepared. Although these polymers are essentially linear, where appropriate, active chemical groups can be introduced for post-polymerization cross-linking or to generate covalent attachment to the surface of suitable substrates. Where the polymer is the basis of the entire medical device (as is the case for a soft contact lens, for instance), the monomers, initiator, and cross-linking agent are mixed and reacted together using a bulk polymerization method. The material may be molded as a button of plastic that is subsequently shaped into a contact lens by special lathing machinery.

Alternatively, the lens design can be directly formed during the polymerization reaction in an appropriate mold, a process known as cast molding. The unhydrated plastic is known as a xerogel. The xerogel is taken through a number of hydration steps to remove small quantities of unreacted material and to finally attain the fully swollen lens dimension. PC Technology has been investigated in a wide variety of applications where surface fouling is an issue. We are currently focusing on just two application areas in order to best capitalize upon the significant clinical benefits the technology can offer. The remainder of this article describes the advantages PC brings to devices used in the eye and the blood. Future Vision? The use of PC materials in contact lenses may not be immediately obvious. For those wearers who experience lens discomfort or white spot formation however, the need for improved materials is only too clear. The eye is bathed by the complex mixture of proteins, enzymes, and lipids that constitute the tear film. When a foreign body (e.g. a contact


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Figure 7. Scanning electron micrograph (SEM) of the components of a blood clot.

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Figure 6. Clinical data based on comfort. Two groups of patients were asked to wear lenses for 6 weeks and rate them on comfort (1–10). One group started with a PC-containing lens (omafilcon A), the other with a control lens; after six weeks the groups switched lenses. Results show a marked comfort preference for the PCcontaining lens.

lens) is placed within the tear fluid, proteins and lipids adsorb onto the lens surface. This process may be governed by many factors, but the lens chemistry is one of the most critical. Fouling of the lens may lead to its dehydration and instability of the tear film, resulting in lack of tolerance in the wearer. Furthermore, there is an increased potential for proteinmediated bacterial colonization and increased risk of visionthreatening infections. Oxygen permeability is another important characteristic of contact lenses. The cornea has no blood supply of its own and relies on diffusion of atmospheric oxygen through the tear film. When a contact lens is worn, it is important that it does not act as a barrier to oxygen transport to the cornea. Soft contact lenses achieve this by virtue of their chemical composition: they are made from polymers known as hydrogels, which absorb, but are not dissolved by, a large amount of water. The aqueous phase of the hydrogel acts as the vehicle for oxygen transport, and the more water that is present, the higher the oxygen transport capabilities. The water content is generally governed by the nature of the constituent monomers, and hydrophilic compounds such as vinyl pyrrolidone and methacrylic acid are common choices as comonomers in hydrogel contact lens formulations. There is, however, a price to pay for the increased water content, namely, an increased susceptibility to protein and/or lipid adsorption to the materials. A PC-containing contact lens formulation has been developed (omafilcon A) that not only possesses a high water content, but also the corresponding adsorption of tear components to this material is less than 5% that of many other lenses (9). PC has a high affinity for water and its presence within the material also resists the natural tendency of the lens to dehydrate on-eye. This means that the lens should be able to maintain higher oxygen permeability for longer wear periods and this translates into improved comfort. Clinical trials have in fact shown this to be the case (Fig. 6); just 1% of the water 324

is lost during wear, compared to 5% for a market-leading product, Acuvue. The PC-containing lens also promotes tearfilm stability and in particular, may be suitable for patients with so-called “dry eyes” (10). There is little doubt that the use of PC in ocular devices can bring significant benefits. Previous work on implantable ophthalmic devices such as intraocular lenses (IOLs) and glaucoma shunts supports this view, and research into a PC 30-day extended-wear lens is in progress. PC has a large part to play in our vision of the future. Stopping the Clot Blood is a complex fluid and a particularly hostile opponent for any prospective device that must come into contact with it. Again, rapid adsorption of plasma proteins, such as fibrinogen, induces a domino effect, which activates the clotting cascade and results in the creation of an insoluble network of fibrin strands, activated platelets, and white cells— that is, thrombus (Fig. 7). Ordinarily, the anticoagulant heparin would be administered to prevent the blood’s clotting upon exposure to the extraneous surface. This in itself can bring about complications, as an antidote must be administered at the end of any procedure, which can lead to lowering of platelet counts and bleeding problems. Any material that can function without inducing an overly adverse reaction within the blood (i.e., exhibits improved hemocompatibility), is potentially very useful. PC coatings are being used in the clinic to improve the performance of extracorporeal circuit components (including oxygenators, heat exchangers, and arterial filters), which are used in heart bypass surgery. Figures 8a and 8b show SEMs of typical blood filters after use. The presence of thrombi on the uncoated filter (Fig. 8a) indicates activation of the blood components and results in reduced filter performance. The PCcoated filter however, is completely clear (Fig. 8b). Coating performance on other devices such as thoracic drain catheters, vascular grafts, and dialysis membranes has also been evaluated. Percutaneous transluminal coronary angioplasty (PTCA) is a minimally invasive alternative to heart bypass surgery in which various devices are passed through an artery (usually in the leg) and steered through the vasculature to the heart by use of a guide wire. The target is a narrowing (lesion) within a coronary artery that impedes blood flow. When such



Figure 8. (a) Uncoated blood-contacted filter material. (b) PC-coated blood-contacted filter material. (c) Uncoated coronary guide wire after a 28-minute procedure. (d) PC-coated guide wire after a 108minute procedure.

a lesion is located, the wire is passed through the blockage. A balloon catheter is passed over the wire to the site of the lesion and expanded to reopen the vessel, allowing blood to flow freely again. A small metallic tubular structure, known as a stent, may be expanded in the area of the lesion using the balloon to help prevent the vessel’s re-narrowing. PTCA is conducted under local anaesthetic. It is less traumatic for the patient (it can be performed as an outpatient procedure) and less expensive for the hospital (the cost is significantly lower than that of bypass surgery). PC coatings on the devices used during PTCA reduce the complications that may occur during and after this procedure. Figures 8c and 8d compare the tip of a PC-coated guide wire

and an uncoated product. Clearly the latter has significant thrombus deposits upon its surface, whereas the PC-coated wire remains unchanged. Even more impressive are the clinical data being generated with the use of PC-coated stents. They compare very favorably with data published for market-leading stents and have shown a very low incidence of stent-related thrombotic events. The battle to control restenosis has been taken one stage further. A strategy has been developed in which the benefits of a bio-inert PC coating are combined with the potential for local drug therapy direct to the site of the lesion. The chemistry and thickness of the PC coating on the stent can be controlled to produce a hydrogel layer that is an effective

PC Coatings for Devices around the Body Contact lenses† Extended wear lenses* Tympanostomy tubes* Interocular lenses Synthetic ear Punctum plugs Glaucoma shunts Dentures Teeth Extracorporeal circuits (oxygenators, etc.)† Blood filters

Coronary stents† Guide wires† Catheters Vascular grafts* Needles/cannulae Introducers

Figure 9. Examples of where PC-based materials have found use in the body. †Indicates commercially available. *Indicates under development.

Ureteric stents† Nephrostomy tubes

Thoracic drainage tubes

Urological catheters Foley catheters Intermittent catheters

Peripheral stents* Wound dressings

Bone fixation screw


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reservoir for loading and release of selected therapeutic agents. The stent can be placed into a solution of a chosen drug and the coating allowed to soak up a quantity of the compound as it hydrates. Once the stent is in place within the diseased artery, the release of the drug can be controlled so that it is delivered directly to the affected tissue where it is needed. This allows the use of potent agents that would have severe systemic toxicity if administered any way but site specifically. This combined technology has the potential to significantly reduce the occurrence of restenosis following stent placement. As the current rate of repeating the procedure (re-PTCA) is extremely costly, any benefit that reduces this figure by even a small amount could save of millions of dollars and reduce the suffering of many patients.

PC-based materials significantly reduce biofouling and interaction with cells, including bacteria, most appropriately in actual-use situations as shown by a growing body of supporting clinical and consumer data (12). With our ever-increasing understanding of these systems, “bio-inert” PC coatings may be an ideal foundation on which to build “smart” surfaces, reducing nonspecific interactions and providing the prospect of eliciting a truly specific biological response. The first coated devices with the ability to deliver therapeutic agents are emerging and mark the direction in which we will develop this technology in the future, driven by our goal of everimproving clinical benefits for the patient.

Other Application Areas and Future Directions

1. Rouhi, A. M. Chem. Eng. News 1999, 77 (Jan 18), 51. 2. Zwaal, R. A. F.; Comfurius, P.; van Deenen, L. L. M. Nature 1977, 268, 358. 3. Hayward, J. A.; Chapman, D. Biomaterials 1984, 5, 135. 4. Ishihara, K.; Nomura, H.; Mihara, T.; Iwasaki, Y.; Nakabayashi, N. J. Biomed. Mater. Res. 1998, 39, 323. 5. Campbell, E. J.; O’Byrne, V.; Stratford, P. W.; Quirk, I.; Vick, T. A.; Wiles, M. C.; Yianni, Y. P. ASAIO J. 1994, 40, 853. 6. Kadoma, Y.; Nakabayashi, N.; Masuhara, E.; Yamauchi, J. Kobunshi Ronbunshu 1978, 35, 423. 7. Umeda, T.; Nakaya, T.; Imoto, M. Makromol. Chem., Rapid Commun. 1982, 3, 457. 8. Lewis, A. L.; Hughes, P. D.; Kirkwood, L. C.; Leppard, S. W.; Redman, R. P.; Tolhurst, L. A.; Stratford, P. W. Biomaterials 2000, 21, 1847. 9. Young, G.; Bowers, R. J. W.; Hall, B.; Port, M. CLAO J. 1997, 23, 249. 10. Lemp, M. A.; Caffery, B.; Lebow, K.; Lembach, R.; Park, J.; Foulks, G.; Hall, B.; Bowers, R. J. W.; McGarvey, S.; Young, G. CLAO J. 1999, 25, 40. 11. Russell, J. C. J. Endourol. 2000, 14, 39. 12. Lewis A. L. Colloids Surf., B 2000, 18, 261.

Although the business is currently focusing on the applications described, a glance at Figure 9 reveals some of the other areas in which PC polymers have demonstrated benefits over the past decade or so; additionally, there are many nonmedical applications. These are possible because the mechanisms of fouling processes are similar in many cases. For instance, the use of PC coatings for combating microbial infection of devices is certainly gaining momentum (11). Beneficial reduction of bacterial adhesion to contact lenses, orthopedic fixation screws, and oral prostheses such as teeth and dentures has been demonstrated in vitro. The effect has been confirmed in vivo for ear grommets and a variety of urological devices, and PC-coated devices are now available in these applications. We have shown that there may be further useful properties of the hydrophilic PC group. For example, PC coatings can improve the lubricity of devices such as guide wires, introducer sheaths, and hypodermic syringe needles, adding an additional performance attribute to complement the nonthrombogenic properties.

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Blending in with the Body

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