BY SURFACE OXIDATION TREATMENT

Background

A method is described for coating a material with silk fibroin by surface oxidation and, more particularly, a method of coating a material using a silk protein-based solution.

Silk protein is derived from arthropods, and represents the insect's catchall for external structure. Recent work has demonstrated that the fibroin protein derived from the cocoon of the domesticated silkworm, Bombyx mori, can be solubilized into aqueous solution to produce what is commonly referred to as silk solution. The silk solution can then be used to produce three-dimensional polymer objects. Of acute interest is the use of silk solution to form a coating layer upon a material to allow for enhanced hydration and to act as a binder for attaching other embedded materials within the silk. Of specific interest is the use of silk to coat contact lenses to impart greater comfort for the user, and also allow for the

incorporation of additional materials or therapeutics for clinical benefit.

Current contact lenses are typically produced from hydrophobic silicone rubber, and as result these materials may disrupt the tear film of the cornea. If the tear film is disrupted this may cause discomfort and in some cases injury to the wearer. In an effort to reduce such effects contact lenses may be treated to become more hydrophilic to more readily absorb and retain water. Without such treatment the surface of the contact lens material remains hydrophobic and can disrupt the tear film and has the potential to cause damage to the corneal surface.

Due to the hydrophobic nature of standard contact lens materials, they are typically treated to produce a hydrophilic surface to enhance device performance and comfort. One standard treatment method is the use of oxidative surface methodologies through the use of corona or plasma treatment. Oxidative surface treatments expose the treated material surface to a high voltage charge, typically on the order of 50,000 volts. This has a significant effect on siloxane polymer chains, which are typically composed of an oxygen rich backbone with methyl groups extending towards the surface. Due to these polymers having a high segmental fiexibility and a low glass transition temperature (-120°C), the high voltage causes the polymer chains to rearrange so that the hydrophilic oxygen backbone of the polymer faces the surface and the hydrophobic methyl groups face inward towards the bulk material volume. This rearrangement causes an increase in surface hydrophilicity. The surface oxidation treatment can then be made to last for either a short or extended period of time depending on conditions of the operating environment.

The silk derived protein, fibroin, has been shown to be a biocompatible material within the body, as it does not cause inflammation or immune responses once implanted. As a result fibroin solution has been under investigation for uses in biomedical applications. Of recent, interest has been its use in the eye, specifically in regards to applications relating to the cornea surface. Fibroin has been shown to form films that are highly transparent in nature and nontoxic to the corneal tissue. These films can be used to form a coating layer on a material, and more specifically contact lenses.

In addition, silk fibroin has been shown to maintain activity of labile molecules within the protein matrix. As a result, chemical activity can be maintained for embedded molecules, which has particular utility in the biomedical space. Specifically, clinically relevant drugs and therapeutic entities can be combined within the silk fibroin solution and coated simultaneously. The silk coating will then possess therapeutic drugs and/or other molecules embedded within the material coating. This can then be utilized to enhance a clinical and/or material effect of the coated material. Specifically relating to ophthalmology, antibiotics, anti-inflammatories, nutrients, and/or pain alleviating molecules can be embedded and maintained within the silk fibroin coating to enhance product utility for given application.

Finally, silk protein secondary structure can be controlled through post-processing techniques. This is primarily due to increasing protein secondary structure formations (i.e. beta-sheets and alpha helices). The formation of these molecules changes the

hydrophobicity/ hydrophilicity of the material and also the susceptibility of material dissolution and degradation by aqueous solubilization or enzymatic action. As a result, the silk protein coating can be caused to dissolve immediately when it comes into contact with water or be designed to degrade slowly over a period of months to years. This can allow for a variety of coating designs to allow for the degradation over a variety of time periods for a given application. Summary

A coating method is provided for a material using a silk protein-based solution. The material is first treated using plasma oxidation to alter the material's surface chemistry. Then a silk protein-based solution is applied to the material and coated. The silk protein-based solution is then dried upon the material surface. The silk protein-based coating can then be used to enhance surface wettability or to aid in incorporating other molecules. The silk protein coating can also be processed to degrade over varying periods of time depending on the required application. As a result, other integrated molecules can also be released at varying rates depending on the desired application. In one embodiment, a method for coating a material using a silk protein-based solution is used for the coating of contact lenses.

In one aspect, a method of coating a material comprises the steps of providing a material having a surface, oxidizing the surface of the material, applying a silk protein-based solution to the surface, and drying the surface forming a coating layer on the material.

Brief Description Of The Drawings

For a more complete understanding of the present invention, reference should now be had to the embodiment(s) shown in the accompanying drawing(s) and described below. In the drawings:

Figure 1 is a schematic diagram illustrating steps of (A) applying a oxidative treatment to a material of interest, such as a contact lens, (B) applying the silk solution to the material surface, which may or may not incorporate other molecular entities, (C) in the particular case of contact lens device the coated material is spin cast to produce a uniform coating over the surface, and (D) drying the coating for use. Description

Methods are described including the use of oxidative surface treatments to enhance silk protein adhesion to a material, as well as the use of silk fibroin to coat a contact lens after processing the material with the oxidative surface treatment. The silk fibroin coating is preferably between about 1 nm and 100 μιη in thickness, more preferably between about 100 nm and 50 μιη in thickness, and most preferably between about 1 - 20 μιη in thickness. The oxidative surface treatment includes the use of plasma, corona, ultraviolet radiation, and/or ozone in combination, separately, or in serial use. In one embodiment, the method includes the use of a silk protein coating for binding additional molecules to the material surface. The additional molecules can include lubricating molecules such as hyaluronic acid or carboxymethyl cellulose. Alternatively, the additional molecules can include therapeutic molecules including, but not limited to, an antibiotic, anti-inflammatory, and/or pain reducing agent used in combination or individually. Further, the silk protein coating can be optionally processed to increase or decrease coating degradation rate. The dissolution or degradation of the silk protein coating can be used to release therapeutic agents. Preferably, the degradation rate is between about 1 second and 1 month, more preferably between about 1 minute and 1 day, and most preferably between about 10 minutes and 10 hours. The present invention further includes placing and storing the material processed by oxidative surface treatment within a volume of silk solution including, but not limited to, a packing or processing reservoir, to form a contact lens.

According to one embodiment, a coating for a material comprises a silk protein-based solution. The fibroin protein can be attached to the surface of silicone rubber polydimethyl siloxane (PDMS), which is an example of a material typically used to create a contact lens. A PDMS mold that replicates the curved inner surface of a contact lens is used. The PDMS mold was produced by first machining a two-piece stainless steel mold to cure the PDMS within. The stainless steel mold was produced with dimensions consisting of a diameter of 14.5 mm and a base curvature of 8.6 mm. The two stainless steel mold pieces were assembled together using set-screw placements. A 10: 1 mixture of PDMS potting solution and catalyst were combined and mixed (model number MED-6015 GAL 2PRT, Nusil Technology, Inc.). The solution was then poured into the stainless steel mold and allowed to cure. The PDMS molds were then removed from the stainless steel mold and placed upon a spin-casting machine. Before spin casting, the mold surface treated for 10 seconds using a 50,000 V of plasma with corona treatment device (Model BD-20AC, Electro Technic Products, Inc.).

After treatment with corona, a volume of 100 μΐί of silk solution consisting of 8% fibroin protein and water (w/v) was placed within the PDMS mold. After corona surface treatment it was found that the silk solution more readily flowed over the PDMS mold material, and exhibited lower contact angles indicating an increase in material hydrophilicity after oxidative surface treatment. The molds were then spun at 350 rotations per minute (RPM) for 2 hours. This RPM setting was shown to produce enough centripetal force to coat the entire curved surface of the PDMS mold. The silk solution was then allowed to dry upon the spinning surface. After drying, a silk film was found to form on the PDMS surface.

Adhesion of the silk coating was verified by attempting to manually remove the silk coating from the PDMS surface. The silk coating was adhered so strongly that attempting to remove the coating caused portions of the silicone to tear before the silk coating itself tore or cracked. The silk coating was found to be a thickness of 30-40 pm as determined by micrometer measurement, which is much thicker than what would be required for a contact lens and was used for ease of sample handling and to increase the potential for coating removal.

In addition to a coating material, the silk fibroin protein can be used as a binding agent to contain other molecules within the coating. In one embodiment, the silk solution may include additives such as hyaluronic acid (HA) and cyclosporine. HA is a carbohydrate that readily absorbs water and is a lubricity-promoting molecule. The addition of HA increases water absorption capabilities and produces a more lubricious silk coating surface. The addition of therapeutic molecules to the silk coating surface, such as cyclosporine, can provide a therapeutic effect. Cyclosporine is an anti-inflammatory drug and is regularly used in the treatment of dry eye symptoms. The addition of these additional molecules does not appear to significantly affect silk coating adhesion to the PDMS material.

The addition of both HA and cyclosporine to the silk solution allows for the coating of a contact lens with these ocular health promoting molecules. In one embodiment, a contact lens coated with silk solution, HA, and cyclosporine provides a cocktail of biologically active ingredients for enhancing tear film stability and reducing potential effects of pathological inflammation. As a result, the contact lens can provide a therapeutic effect for a patient suffering from an ocular disorder. For example, the contact lens coated with a silk, HA, and cyclosporine may provide a high clinical benefit for patients suffering from dry eye symptoms.