Hyaluronan-Modified Surfaces for Medical Devices

Medical Device & Diagnostic Industry Magazine
MDDI Article Index

An MD&DI February 1999 Column

SPECIAL SECTION

A unique biopolymer with therapeutic applications is also gaining recognition as a versatile surface-treatment material for improving device biocompatibility.

Hyaluronan is a naturally occurring biopolymer used in medical applications ranging from cataract surgery and postsurgical adhesion prevention to hydrophilic coatings. The purpose of this review is to make readers aware of this increasingly important biomaterial and to survey the techniques for improving the biocompatibility of medical devices by modifying synthetic material surfaces with hyaluronan.

WHAT IS HYALURONAN?

A unique biopolymer, hyaluronan is one of a number of polysaccharides that occur in the body's mucous membranes and are known as mucopolysaccharides. It was first isolated from the vitreous body of the eye in 1934 by Karl Meyer, who called it hyaluronic acid.^1,2 The term hyaluronan is attributed to Endre Balazs, who coined it to encompass the different forms the molecule can take—for example, the acid form, hyaluronic acid, and the salts, such as sodium hyaluronate, which form at physiological pH.³

After 65 years, quite a lot is known about the appearance of the hyaluronan molecule; its behavior; its occurrence in different tissues and body fluids; the manner in which it is synthesized by the cells, metabolized, and cleared from the body; and the nature of some of the functions it performs.

Hyaluronan and related polysaccharides are called glycosaminoglycans. These substances are made up largely of repeating disaccharide units containing a derivative of an aminosugar. The most abundant glycosaminoglycans in the body are chondroitin sulfates; others are keratin sulfate, heparin and heparan sulfate, and dermatan sulfate.

Figure 1 shows the disaccharide unit of hyaluronan, consisting of alternating glucuronic acid and N-acetylglucosamine units, which are repeated over and over to form long chains. Each repeating disaccharide unit has one carboxylate group, four hydroxyl groups, and an acetamido group. Hyaluronan differs from the other major glycosaminoglycans in that it does not have sulfate groups.

In the body, hyaluronan is synthesized by many types of cells and extruded into the extracellular space where it interacts with the other constituents of the extracellular matrix to create the supportive and protective structure around the cells. It is present as a constituent in all body fluids and tissues and is found in higher concentrations in the vitreous humor of the eye and the synovial fluid in the joints. In mammals, the highest reported concentration is found in the umbilical cord.

Figure 1. Hyaluronan molecule.

CHARACTERISTICS OF HYALURONAN

Hyaluronan possesses a unique set of characteristics: its solutions manifest very unusual rheological properties and are exceedingly lubricious, and it is very hydrophilic.

Rheological Properties. In solution, the hyaluronan polymer chain takes on the form of an expanded, random coil. These chains entangle with each other at very low concentrations, which may contribute to the unusual rheological properties. At higher concentrations, solutions have an extremely high but shear-dependent viscosity. A 1% solution is like jelly, but when it is put under pressure it moves easily and can be administered through a small-bore needle. It has therefore been called a "pseudo-plastic" material.

Lubricity. The extraordinary rheological properties of hyaluronan solutions make them ideal as lubricants. There is evidence that hyaluronan separates most tissue surfaces that slide along each other. Solutions of hyaluronan are extremely lubricious and have been shown to reduce postoperative adhesion formation following abdominal and orthopedic surgery.

Hydrophilicity. As mentioned, the polymer in solution assumes a stiffened helical configuration, which can be attributed to hydrogen bonding between the hydroxyl groups along the chain. As a result, a coil structure is formed that traps approximately 1000 times its weight in water.^4,5

COMMERCIAL AVAILABILITY

The classical sources for the isolation of hyaluronan have been either from mammalian tissues or from certain strains of cultured bacteria. At one time, the material was isolated from human umbilical cords collected in hospitals. One company, Pharmacia AB (Uppsala, Sweden), developed a special strain of roosters with very luxuriant combs, from which the compound was isolated. More recently, submerged cell-culture techniques using certain strains of streptococci have been developed to grow hyaluronan. The commercially available material comes in molecular weights ranging from less than 1 million to as high as 8 million.

Hyaluronan-based coatings are used on a variety of medical devices such as guidewires. (Photos courtesy of Lake Region Mfg. Inc.; Chaska, MN.)

There are a large number of hyaluronan producers around the world. Biomatrix Inc. (Ridgefield, NJ), a U.S. company, operates a plant that produces hyaluronan from mammalian sources in Canada. Anika (Woburn, MA), Genzyme Corp. (Framingham, MA), and Lifecore Biomedical (Chaska, MN) are other domestic suppliers. Pharmacia produces hyaluronan in Sweden, Fidia Advanced Biopolymers (Brindisi) in Italy, Bio-Technology General Corp. (Iselin, NJ) in Israel, and a number of companies, including Kibun Food Chemifa Co. and Seikagaku Corp. (both Tokyo), in Japan.

MEDICAL APPLICATIONS

Balazs and his coworkers have written extensively on the medical applications of hyaluronan and its derivatives.⁶ The major application of the material in the United States has been as a viscoelastic in ophthalmic surgery, primarily during the implantation of intraocular lenses in patients with cataracts. Japan supports a large market for hyaluronan because it is used there as an injectable for arthritis. Recently, Biomatrix and Fidia have obtained FDA approval for that use in the United States.

Drug release represents another interesting application, and formulations of hyaluronan and its derivatives have been developed as topical, injectable, and implantable vehicles for the controlled and localized delivery of biologically active molecules.⁷

With the advent of cross-linked gels and films of hyaluronan, there are now a number of products on the market for the prevention of adhesions after abdominal surgery. Genzyme developed the Seprafilm and Sepracoat product lines, based on cross-linked hyaluronan to prevent adhesions after abdominal surgery. Lifecore has developed a gel form for similar indications.

Another prominent application of hyaluronan is its use in hydrophilic coatings for a variety of medical devices, including catheters, guidewires, and sensors. Use of such coatings can improve device biocompatibility and lubricity and reduce fouling and tissue abrasion.

An interesting application not yet commercialized—although some human clinical trials have been completed—is the injection of a hyaluronan gel, compounded with thrombin and other materials, for percutaneous embolization.⁸

PATENTS

The patent literature is extensive and offers evidence of a great deal of development work in this field in recent years. Of course, it is not always possible to know whether a particular patented invention has practical uses, and in some patents hyaluronan is mentioned as only one of a class of compounds (e.g., mucopolysaccharides or glycosaminoglycans) that can be used. The major inventors in this field and their corporate affiliations are summarized in Table I.

Many different processing techniques and uses for hyaluronan have been invented and patented by Balazs, Leshchiner, and their coworkers. There is the important Balazs patent, issued in 1979 and now expired, on hyaluronan isolated from animal tissue that does not cause an inflammatory response when tested in the eye of the owl monkey.⁹ The process involves extracting hyaluronan from the blood, deproteinizing the extract, and then treating it with chloroform. The result (marketed by Pharmacia as Healon) is described as a sterile, pyrogen-free, nonantigenic and noninflammatory, high-molecular-weight fraction of hyaluronan that is essentially free of proteins, peptides, and nucleic acid impurities. Various therapeutic uses are described for this material, including improvement of pathological joint function; prevention of postoperative adhesion of tissues, tendons, and their sheaths; and various uses in the eye.

CROSS-LINKING

There are many ways in which hyaluronan can be cross-linked to produce insoluble gels.⁶ The Balazs patent on chemically modified hyaluronan describes cross-linking with small amounts of an aldehyde (e.g., formaldehyde) to produce a unique soluble polymer fluid with very high viscoelastic properties.¹⁰ Also discussed is cross-linking of hyaluronan with divinyl sulfone to obtain a jellylike material.

In a 1990 U.S. patent assigned to Genzyme, Hamilton describes water-insoluble derivatives of polysaccharides that are activated with carbodiimides and reacted with an amino acid.¹¹ Others have also reported on the cross-linking of hyaluronic acid with a water-soluble carbodiimide to produce water-insoluble films.¹² Cross-linking can also be achieved with polyvalent cations (ferric, aluminum, etc.) and aziridines (e.g., cross-linker CX-100).

DERIVATIVES

In addition to cross-linking, various chemical modifications of the hyaluronan polymer have been reported and patented over the years. According to Balazs, the earliest synthesized derivative of hyaluronan was its sulfate ester, which showed resistance to hyaluronidase and anticoagulant activity.⁶ More recently, a group of researchers at the University of Siena in Italy has published extensively on sulfated hyaluronic acid. They report that introducing sulfate groups in the hyaluronan molecule converts it to a heparin-like material with antithrombogenic properties and also makes it resistant to enzymatic digestion.^13,14

There are several patents assigned to Fidia in which della Valle describes esters of hyaluronic acid, in which all or only a portion of the carboxylic groups of the acid are esterified by treatment of the free hyaluronic acid with alcohols in the presence of a catalyst.^15–17 These patents disclose the many different types of alcohols that can be employed, the use of salts of the partial esters with metals and with pharmacologically active organic bases, and the differing degrees of esterification.

Applications of these compounds can include use in pharmaceutical preparations, cosmetics, and medical and surgical devices. It is claimed that the compounds qualitatively possess the same or similar physical-chemical, pharmacological, and therapeutic properties as hyaluronic acid, but that they are considerably more stable, especially with regard to enzymatic degradation by hyaluronidase. It is also claimed that most of the esters, unlike hyaluronic acid itself, have a certain degree of solubility in organic solvents, such as dimethylsulfoxide (DMSO), and poor solubility in water. These characteristics make it possible to form articles ranging from film and sheet to thread, sponges, and ophthalmic lenses.

Another example of a hyaluronan derivative—a conjugate with the naturally occurring, free-radical scavenger superoxide dismutase—was reported to have greater antiinflammatory activity in vivo than hyaluronan or superoxide dismutase.¹⁸ Amino groups of the superoxide dismutase were coupled with carboxyl groups in the hyaluronan molecule using carbodiimide.

Table I. U.S. patents related to hyaluronan (HA).

COMPOSITES

Hyaluronan lends itself to compounding or complexing with other materials to produce biomedically useful composites. Several Balazs patents on hyaluronan-modified polymeric articles describe how materials such as polyHEMA, polyurethanes, polyesters, or polyolefins can be rendered biocompatible by inclusion of or coating with hyaluronan.^19,20 For example, the patent on cross-linked gels discloses mixtures of hyaluronan with substances that include other hydrophilic polymers, polysaccharides, proteins of various types, and synthetic water-soluble polymers.²¹

A recent patent, issued in 1997 to Giusti et al., describes a biomaterial comprising an interpenetrating polymer network in which one of the components is an acidic polysaccharide or a semisynthetic derivative thereof and the second component a synthetic polymer.²² The polysaccharide can be hyaluronic acid, a total or partial ester, or a salt of hyaluronic acid with an organic base.

One of the earlier patents, by Yannas at the Massachusetts Institute of Technology, was issued in 1981 and describes composites of collagen and mucopolysaccharides, including hyaluronic acid, that are cross-linked with aldehydes, carbodiimides, azides, and diisocyanates.²³ Also discussed is dehydrothermal cross-linking, in which the material is first dehydrated and then heated. Many of the cross-linked composites were found to have mechanical and biocompatibility characteristics superior to those of collagen alone. The collagen and mucopolysaccharide can either be mixed together, or an article can first be coated with collagen and then have the mucopolysaccharide applied to it.

IMMOBILIZATION

There are two basic methods for immobilizing hyaluronan to produce biomedically useful coatings. The material can either be reacted with or coupled to functional groups present or introduced on the surface, or a photoreactive group can be attached to the hyaluronan molecule, which then reacts with the surface upon being illuminated (i.e., photoimmobilization). There are many variations on these two schemes, depending on the nature of the substrate and the functional requirements of the coating.

Functional Groups. Halpern and Beavers describe a bilaminar graft configuration to immobilize hyaluronan (and other mucopolysaccharides) when suitable functional groups are not present on the substrate.^24–26 An adhesive polymer coating is first applied. This first coat provides functional groups (for example, diisocyanates) on its surface, which can then be used to covalently bind the second coat of hyaluronan. In this process, the polysaccharide molecules, depending on their length and shape, may be tied down at multiple points along the chain and probably also through entanglement and interaction between polymer chains.

A recent patent by Beavers et al. discloses a process for producing the pure acid form of hyaluronan, which readily undergoes chemical reactions with substances such as epoxides, aziridines, and alcohols.²⁷

Patents issued in 1986 and 1989 to Larm describe the "Carmeda process" (Carmeda AB, Stockholm) for the covalent coupling of conjugates of substances such as polysaccharides with specific reference to heparin (partially deacetylated), hyaluronic acid, dermatan sulfate, and chitosan.^28,29 In this process, fragments of the substance are created that have reactive terminal aldehyde groups. These aldehyde groups are then reacted with amino groups on the substrate to form unstable Schiff's bases, which are converted to stable secondary amines with a suitable reducing agent, such as cyanoborohydride.

In connection with hyaluronic acid, Larm states: "By covalent coupling of hyaluronic acid to plastic implants for, for example, eye surgery, the implants can acquire better tissue affinity. In this manner, one avoids complementary activation and activation of the mononuclear cell system. . . ."^28,29

Larsson reported on the biocompatibility of surfaces prepared by immobilized heparin and hyaluronate.³⁰ In creating the immobilized hyaluronate surfaces, carbodiimide chemistry was used to react carboxyl groups on the hyaluronate molecule to primary amine groups.

Two recently issued patents, assigned to Cordis (Miami), describe immobilization of polysaccharides to metallic surfaces, such as those used for stents. In the first of these patents, an organic polysilane coating with amine functionality is first applied, followed by the application of the polysaccharide, using carbodiimide as the coupling agent.³¹ In the second patent, a coat of hexafluorobutylmethacrylate is applied by RF plasma deposition, followed by RF plasma treatment with water vapor to create functional groups on the surface; carbodiimide chemistry can then be used to tie down the polysaccharide.³² Although heparin is the material used as an example in the patent, these processes may also be applicable to hyaluronan.

Photoimmobilization. A number of patents assigned to SurModics (Eden Prairie, MN) disclose methods for attaching all kinds of molecules—including heparin and hyaluronic acid—to substrates using spacer molecules with photochemically and thermochemically reactive groups.^33–35 One of the patents, issued in 1990, describes biocompatible coatings for solid surfaces, in which the biocompatible agents, including hydrophilic polymers such as hyaluronic acid, are covalently bonded to the solid surface.³⁵

Chen et al. have employed a photoimmobilization method for sulfated hyaluronic acid, using a water-soluble carbodiimide to attach photoreactive groups to carboxyl groups of the hyaluronic acid. The photoreactive groups then bond directly to a polymeric surface via UV activation.³⁶

Benefits of hyaluronan-based coatings include reduced device fouling and tissue abrasion.

BIOCOMPATIBILITY

As one might expect for a material that is ubiquitous in the body, the biocompatibility of hyaluronan-modified surfaces has been well established. Various in vitro and in vivo tests have been conducted over the years by Biocoat Inc. (Fort Washington, PA) and its licensees that have demonstrated the biocompatibility of hyaluronan coatings. It has been shown that coated surfaces exhibit a marked reduction or absence of cellular attachment and fouling and of bacterial growth, compared with uncoated surfaces.

Larsson used various cellular systems and in vitro blood analyses to compare immobilized heparin surfaces with immobilized hyaluronate. He concluded that the two surfaces were indistinguishable when evaluated for short-term cellular compatibility—that is, for platelet activation and cell adhesion in contact with blood. However, the heparin surface could be clearly distinguished from the hyaluronate surface on the basis of its capacity to adsorb and inactivate thrombin.³⁰

It is interesting to compare this finding with that of Burns, who postulated that hyaluronan is capable of interfering with the interaction of von Willebrand factor with platelets and components of the subendothelial matrix to inhibit platelet aggregation and adhesion.³⁷ Burns also mentions that the coating of devices with hyaluronan to inhibit the interaction of platelets with the surface carries a substantially reduced risk of affecting overall hemostasis compared with heparin and warfarin.

Additional research has examined the resistance of hyaluronate coatings to hyaluronidase.³⁸ Results indicate that coatings prepared by covalent binding with diisocyanates are not degraded by the enzyme hyaluronidase, in contrast with hyaluronan in solution, which is rapidly degraded by hyaluronidase.

A recently published study of various photochemically immobilized polymeric coatings on silicone rubber compared hyaluronic acid with synthetic materials such as polyacrylamide, polyethylene glycol, and polyvinyl pyrrolidone.³⁹ The study included protein adsorption assays, fibroblast growth assays, leukocyte adhesion assays, and subcutaneous implantation in rats to study inflammation and fibrous-capsule formation.

CONCLUSION

Hyaluronan is a unique biomaterial that lends itself to cross-linking and immobilization in various ways to produce hydrophilic, lubricious, and biocompatible surfaces. The ability to derivatize and complex hyaluronan with other substances makes it possible to create a range of bioactive surfaces. Promising device applications might include using such surfaces, for example, to impart antithrombogenic and antibacterial properties or to interact preferentially with certain proteins and cells.