Patent Publication Number: US-2007104611-A1

Title: Methods for sterilizing silicone hydrogel contact lenses

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of U.S. Provisional Application No. 60/735,407, filed Nov. 9, 2005, the contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to manufacturing and packaging of contact lenses. More particularly, the invention relates to methods for sterilizing silicone hydrogel contact lenses.  
     BACKGROUND  
      Contact lenses have been used successfully to improve vision. Contact lenses can be categorized as hard contact lenses or soft contact lenses. Rigid gas permeable contact lenses are an example of hard contact lenses. Hydrogel contact lenses are examples of soft contact lenses. Silicone hydrogel contact lenses have become popular due to the ability of contact lens wearers to wear such lenses in their eyes for longer times compared to non-silicone hydrogel contact lenses. Examples of silicone hydrogel contact lenses are available from Johnson &amp; Johnson under the tradenames Acuvue Advance and Acuvue Oasys, from Ciba Vision under the tradename Focus Night and Day and O2 Optix, and from Bausch &amp; Lomb under the tradename PureVision. The Acuvue Advance has the United States Adopted Name (USAN) galyfilcon A, the Acuvue Oasys lens has the USAN senofilcon A, the Focus Night and Day lens has the USAN lotrafilcon A, the O2 Optix lens has the USAN lotrafilcon B, and the PureVision lens has the USAN balafilcon A. Additional examples of materials used to make the present silicone hydrogel contact lenses include those materials disclosed in U.S. Pat. No. 6,867,245.  
      Contact lenses, including silicone hydrogel contact lenses, are often packaged during the manufacturing process in sealed plastic containers, sometimes referred to as blister packs. The blister packs are often formed from non-polar resins, which may comprise polymeric materials, such as polyolefin-based materials, including, without limitation, polypropylene, polyethylene, and the like.  
      Before distributing the packaged contact lenses, the packaged contact lenses can be sterilized. A common procedure to sterilize the pre-packaged contact lenses is autoclaving the packages containing the contact lenses. For example, a blister pack containing a contact lens in a volume of liquid can be sterilized using pressurized steam (e.g., steam present at about 121 degrees C.) to denature proteins and lipid complexes and thereby kill microorganisms present in the sealed blister pack. One disadvantage of autoclaving contact lenses is that a relatively high temperature (e.g., 121-132 degrees C.) is needed to obtain a desired degree of sterilization; therefore, the number of materials that can withstand the high temperatures and pressures is limiting.  
      Other methods of sterilizing non-contact lens products include dry heat (e.g., about 140 degrees C. to about 170 degrees C.), ethylene oxide exposure, and radiation (e.g., gamma radiation and accelerated electrons). Some of these procedures are not usable for sterilizing contact lenses and blister packs containing contact lenses. For example, ethylene oxide cannot be used to sterilize contact lenses in blister packs since ethylene oxide can only be applied to dry products. Since contact lenses are usually provided in a liquid in the blister pack when sterilized, ethylene oxide is not a viable alternative. In addition, facilities for sterilizing products using radiation are elaborate and expensive to produce and maintain, therefore, radiation has not been used to sterilize contact lenses.  
      In addition, it is known that high energy sterilization or radiation, such as gamma irradiation, electron beam (e-beam) radiation, and the like, can result in changes to polymers (e.g., molecular weight loss, cross-linking, etc) that may be undesirable to polymeric products. It is also known that these changes are more severe when the product being exposed to radiation is in a hydrated state.  
      Thus, there remains a need for new methods of sterilizing contact lenses, such as silicone hydrogel contact lenses, which can be provided in sealed packages, such as polyolefin based blister packs and the like.  
     SUMMARY  
      The present methods and systems attempt to address this and other needs. The present methods can be practiced to sterilize or terminally sterilize silicone hydrogel contact lenses provided in blister packs and similar containers using radiation, such as gamma radiation or electron beam (e-beam) radiation, without detrimentally changing the material properties of the contact lenses. For example, the silicone hydrogel contact lenses retain one or more properties including a desired size, shape, clarity, wettability, oxygen permeability, modulus, and the like, which enable the lenses to provide a desired vision correction or vision improvement and be worn by a person in need thereof without substantial adverse side effects. For example, silicone hydrogel contact lenses that have been terminally sterilized with gamma radiation or e-beam radiation can still improve a patient&#39;s vision without causing substantial discomfort or other adverse side effects to the patient wearing the lens. The present methods deliver a desired dose of radiation to sufficiently reduce the amount of microorganisms or microbes, including without limitation, bacteria, yeasts, molds, and viruses, present in or on the lens and/or in the lens package liquid without substantially negatively affecting the properties of the silicone hydrogel lenses.  
      In accordance with the description herein, the present methods include exposing one or more silicone hydrogel contact lenses to a sterilizing amount of a high energy radiation, such as gamma radiation or e-beam radiation. The silicone hydrogel contact lens may be provided in a package or container. In certain embodiments, the high energy radiation is used to sterilize a silicone hydrogel contact lens located in a plastic blister pack, such as a polyolefin-based blister pack. In addition, the present methods encompass exposing batches of silicone hydrogel contact lenses. For example, batches of sealed blister packs, each blister pack containing a single contact lens, such as a contact lens in a volume of liquid, can be exposed to high energy radiation to sterilize the entire batch of contact lenses.  
      The present invention also relates to silicone hydrogel contact lenses that have been sterilized using the present methods, as well as packages containing such contact lenses, and systems for sterilizing and producing silicone hydrogel contact lenses using the present methods.  
      Any feature or combination of features described herein are included within the scope of the present invention provided that the features included in any such combination are not mutually inconsistent as will be apparent from the context, this specification, and the knowledge of one of ordinary skill in the art. In addition, any feature or combination of features may be specifically excluded from any embodiment of the present invention. Additional advantages and aspects of the present invention are apparent in the following detailed description, drawings, and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a flow chart illustrating one embodiment of the present methods.  
       FIG. 2  is a flow chart illustrating an additional embodiment of the present methods.  
       FIG. 3  is flow chart illustrating an additional embodiment of the present methods.  
       FIG. 4  is an illustration of a system used to practice the present methods.  
       FIG. 5  is an illustration of another system used to practice the present methods.  
       FIG. 6  is an illustration of a sterilized silicone hydrogel contact lens in a blister pack that has been sterilized using the present methods. 
    
    
     DETAILED DESCRIPTION  
      New methods for manufacturing silicone hydrogel contact lenses have been invented. More specifically, methods for sterilizing silicone hydrogel contact lenses have been invented. The present methods include exposing one or more silicone hydrogel contact lenses to high energy radiation, including without limitation, gamma radiation or electron beam radiation (e-beam radiation). The exposure to the high energy radiation is effective to sterilize the contact lens or lenses without substantially negatively affecting one or more properties of the contact lenses.  
      In at least one embodiment, a silicone hydrogel contact lens is provided in a sealed or “pre-sealed” container prior to sterilization. For example, a method may include a step of providing a silicone hydrogel contact lens in a sealed container. The sealed container may be made of any suitable material for packaging silicone hydrogel contact lenses in connection with the manufacture thereof. For example, the sealed container may comprise a plastic base member having a cavity or chamber to contain a silicone hydrogel contact lens and a seal provided over the cavity or chamber. In certain embodiments, the sealed container may be understood to be a blister pack, as understood by persons of ordinary skill in the art. The exposure to the high energy radiation is effective in sterilizing the contents contained in the blister pack without substantially negatively affecting the properties of the blister pack, such as the blister pack&#39;s structural or physical integrity, porosity, and the like.  
      The silicone hydrogel contact lens may be provided in a volume of liquid in the container. For example, the silicone hydrogel contact lens may be provided in a volume of a saline solution, such as a buffered saline solution, suitable for storing silicone hydrogel contact lenses in sterile conditions. The liquid may also contain a surfactant. A surfactant may be understood to be an agent that lowers or reduces the surface tension of a liquid. In certain blister pack and silicone hydrogel contact lens combinations, the surfactant may be provided in an amount effective in reducing adherence of the silicone hydrogel contact lens to the blister pack compared to combinations which include a surfactant-free packaging liquid. In certain combinations, the surfactant may be provided in an amount effective in enhancing ophthalmic comfort of the silicone hydrogel lens by a lens wearer compared to combinations which include a surfactant-free packaging liquid. In certain embodiments, the present methods include exposing a blister pack comprising a silicone hydrogel contact lens in a preservative-free packaging liquid to a sterilizing dose of high energy radiation. In other embodiments, the packaging liquid may be substantially free of preservatives. Examples of these liquids include liquids that are free, or substantially free, of antimicrobial agents. In at least one embodiment, the packaging liquid consists essentially of, or consists entirely of, a buffered saline solution, such as a phosphate buffered or borate buffered saline solution.  
      As discussed herein, the present methods may be used to sterilize batches of silicone hydrogel contact lenses. Therefore, the present methods may be useful in automated manufacturing or production of large quantities of silicone hydrogel contact lenses. In certain methods, batches of blister packs, wherein each blister pack comprises a single silicone hydrogel contact lens in a volume of liquid, are exposed to sterilizing amounts of high energy radiation.  
      As discussed herein, during the manufacture of silicone hydrogel contact lenses, batches of silicone hydrogel contact lenses which have been placed and sealed in blister packs can be sterilized by exposing the batches to sterilizing amounts of high energy radiation and then distributed to the public.  
      Silicone hydrogel contact lenses useful in the present methods, packages, and systems may have one or more properties such as an ophthalmically acceptable oxygen permeability, an ophthalmically acceptable oxygen transmissibility, an ophthalmically acceptable modulus, and/or an ophthalmically acceptable water content. Silicone hydrogel contact lenses can be understood to be contact lenses that comprise a silicone hydrogel material and encompass existing publicly available silicone hydrogel contact lens materials, as well as other silicone hydrogel contact lens materials. For example, silicone hydrogel contact lenses can comprise one or more materials having the United States Adopted Name (USAN): galyfilcon A, lotrafilcon A, lotrafilcon B, balafilcon A, senofilcon A, or comfilcon A.  
      In other words, the lenses used in the present methods may be understood to comprise one or more silicon-containing components, and one or more hydrophilic components.  
      A silicone-containing component is a component that contains at least one [—Si—O—Si] group, in a monomer, macromer, prepolymer or polymer. The Si and attached O may be present in the silicone-containing component in an amount greater than 20 weight percent, for example greater than 30 weight percent of the total molecular weight of the silicone-containing component. Useful silicone-containing components may comprise polymerizable functional groups such as acrylate, methacrylate, acrylamide, methacrylamide, N-vinyl lactam, N-vinylamide, and styryl functional groups. Examples of some silicone-containing components which are useful in the present lenses may be found in U.S. Pat. Nos. 3,808,178; 4,120,570; 4,136,250; 4,153,641; 4,740,533; 5,034,461 and 5,070,215, and EP080539.  
      Further examples of suitable silicone-containing monomers are polysiloxanylalkyl(meth)acrylic monomers including, without limitation, methacryloxypropyl tris(trimethylsiloxy) silane, pentamethyldisiloxanyl methyhmethacrylate, and methyldi(trimethylsiloxy)methacryloxymethyl silane.  
      One useful class of silicone-containing components is a poly(organosiloxane) prepolymer such as α,  ω -bismethacryloxypropyl polydimethylsiloxane. Another example is mPDMS (monomethacryloxypropyl terminated mono-n-butyl terminated polydimethylsiloxane). Another useful class of silicone containing components includes silicone-containing vinyl carbonate or vinyl carbamate monomers including, without limitation, 1,3-bis[4-(vinyloxycarbonyloxy)but-1-yl]tetramethylisiloxane 3-(vinyloxycarbonylthio) propyl-[tris (trimethylsiloxysilane]; 3-[tris(trimethylsiloxy)silyl] propyl allyl carbamate; 3-[tris(trimethylsiloxy)vinyl] propyl vinyl carbamate; trimethylsilylethyl vinyl carbonate; and trimethylsilylmethyl vinyl carbonate.  
      One example of a material has the following formula, designated as Formula I:  
                 
 
      Hydrophilic components include components which are capable of providing at least about 20%, for example, at least about 25% water content to the resulting lens when combined with the remaining reactive components. Suitable hydrophilic components may be present in amounts between about 10 to about 60 weight % based upon the weight of all reactive components. About 15 to about 50 weight %, for example, between about 20 to about 40 weight %. Hydrophilic monomers that may be used to make the polymers for the present lenses have at least one polymerizable double bond and at least one hydrophilic functional group. Examples of polymerizable double bonds include acrylic, methacrylic, acrylamido, methacrylamido, fumaric, maleic, styryl, isopropenylphenyl, O-vinylcarbonate, O-vinylcarbamate, allylic, O-vinylacetyl and N-vinyllactam and N-vinylamido double bonds. Such hydrophilic monomers may themselves be used as crosslinking agents. “Acrylic-type” or “acrylic-containing” monomers are those monomers containing the acrylic group (CR′H═CRCOX) wherein R is H or CH 3 , R′ is H, alkyl or carbonyl, and X is O or N, which are also known to polymerize readily, such as N,N-dimethylacrylamide (DMA), 2-hydroxyethyl acrylate, glycerol methacrylate, 2-hydroxyethyl methacrylamide, polyethyleneglycol monomethacrylate, methacrylic acid, acrylic acid and mixtures thereof.  
      Hydrophilic vinyl-containing monomers which may be incorporated into the materials of the present lenses may include monomers such as N-vinyl lactams (e.g. N-vinyl pyrrolidone (NVP)), N-vinyl-N-methyl acetamide, N-vinyl-N-ethyl acetamide, N-vinyl-N-ethyl formamide, N-vinyl formamide, N-2-hydroxyethyl vinyl carbamate, N-carboxy-β-alanine N-vinyl ester. In one embodiment, the hydrophilic vinyl-containing monomer is NVP.  
      Other hydrophilic monomers that can be employed in the present lenses include polyoxyethylene polyols having one or more of the terminal hydroxyl groups replaced with a functional group containing a polymerizable double bond. Examples include polyethylene glycol with one or more of the terminal hydroxyl groups replaced with a functional group containing a polymerizable double bond. Examples include polyethylene glycol reacted with one or more molar equivalents of an end-capping group such as isocyanatoethyl methacrylate (“IEM”), methacrylic anhydride, methacryloyl chloride, vinylbenzoyl chloride, or the like, to produce a polyethylene polyol having one or more terminal polymerizable olefinic groups bonded to the polyethylene polyol through linking moieties such as carbamate or ester groups.  
      Additional examples are the hydrophilic vinyl carbonate or vinyl carbamate monomers disclosed in U.S. Pat. No. 5,070,215, and the hydrophilic oxazolone monomers disclosed in U.S. Pat. No. 4,190,277. Other suitable hydrophilic monomers will be apparent to one skilled in the art. More preferred hydrophilic monomers which may be incorporated into the polymer of the present invention include hydrophilic monomers such as N,N-dimethyl acrylamide (DMA), 2-hydroxyethyl acrylate, glycerol methacrylate, 2-hydroxyethyl methacrylamide, N-vinylpyrrolidone (NVP), and polyethyleneglycol monomethacrylate. In certain embodiments, hydrophilic monomers including DMA, NVP and mixtures thereof are employed.  
      Additional examples of materials used to make silicone hydrogel contact lenses include those materials disclosed in U.S. Pat. No. 6,867,245.  
      Thus, a silicone hydrogel contact lens useful in the present methods may comprise a lens body that includes a hydrogel-forming polymer, such as a water swellable polymer. The hydrogel itself includes such a polymer swollen with water. The water content of the present lenses may be greater than 10% by weight water and less than 100% weight by water. For example, the present lenses may have about 20% by weight water to about 90% by weight water. Certain lenses have about 30% to about 80% by weight water.  
      Reference will now be made in detail to the presently illustrated embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same or similar reference numbers are used in the drawings and the description to refer to the same or like parts. It should be noted that the drawings are in simplified form and are not to precise scale. In reference to the disclosure herein, for purposes of convenience and clarity only, directional terms, such as, top, bottom, left, right, up, down, over, above, below, beneath, rear, front, backward, forward, distal, proximal, anterior, posterior, superior, inferior, temporal, and nasal are used with respect to the accompanying drawings. Such directional terms should not be construed to limit the scope of the invention in any manner.  
      Although the disclosure herein refers to certain illustrated embodiments, it is to be understood that these embodiments are presented by way of example and not by way of limitation. The intent of the following detailed description, although discussing exemplary embodiments, is to be construed to cover all modifications, alternatives, and equivalents of the embodiments as may fall within the spirit and scope of the invention as defined by the appended claims.  
       FIG. 1  is a flow chart illustrating steps involved in a method  100  of manufacturing a silicone hydrogel contact lens. The method  100  comprises a step  110  of placing a silicone hydrogel lens precursor composition into a mold cavity of a contact lens mold. The mold typically comprises a male portion and a female portion, which form a mold cavity in the shape of a contact lens when coupled together. The silicone hydrogel lens precursor composition in the mold cavity of the contact lens mold is cured at step  120  to polymerize components of the lens precursor composition to form a cured contact lens. The cured contact lens is removed from the mold cavity at step  130 . The cured contact lens undergoes a processing step  140  which comprises one or more steps of extracting unreacted and/or unpolymerized products in the cured lens and one or more steps of hydrating the cured/extracted lens to form a water swelled silicone hydrogel contact lens. The silicone hydrogel contact lens so formed is placed in a cavity, typically containing a packaging liquid or packaging solution, of a blister pack at step  150  and is inspected for defects at step  160 . After inspection, the cavity of the blister pack containing the inspected silicone hydrogel contact lens is sealed at step  170 . The sealed blister pack containing the silicone hydrogel contact lens is then terminally sterilized at step  180 . After such sterilization, the blister pack containing the sterilized silicone hydrogel contact lens is ready for distribution to the public.  
       FIG. 2  is a flow chart illustrating a method  200  in accordance with the disclosure herein. The method  200  comprises a step of exposing  210  at least one silicone hydrogel contact lens to a sterilizing amount of high energy radiation. The silicone hydrogel contact lens is exposed to an effective amount of high energy radiation for a sufficient amount of time to sterilize the contact lens without substantially negatively affecting one or more properties of the contact lens, which may reduce the comfort or performance of the lens when worn on an eye of a person.  
      The silicone hydrogel contact lens can be a spherical or aspherical contact lens, and may also be a rotationally stabilized aspherical silicone hydrogel contact lens. The silicone hydrogel contact lens may have a toric region, and/or it may be a monofocal or multifocal, including bifocal, contact lens. The silicone hydrogel contact lenses used in the present methods preferably have one or more surfaces that have an ophthalmically acceptable wettability before exposure to the sterilizing amount of high energy radiation. For example, the exposure to the high energy radiation is not necessary to increase the wettability of the silicone hydrogel contact lenses. As understood by persons of ordinary skill in the art, silicone hydrogel contact lenses and rigid gas permeable contact lenses are different and distinct from each other. Thus, the present methods include exposing a contact lens, which is not a rigid gas permeable contact lens, to sterilizing amounts of high energy radiation. Examples of materials of the silicone hydrogel contact lenses are described herein. Or, in other words, a method comprises exposing a non-rigid gas permeable silicon-containing contact lens to a sterilizing amount of high energy radiation.  
      In certain embodiments, the silicone hydrogel contact lens that is exposed to high energy radiation has an ophthalmically acceptable surface wettability prior to exposure to the high energy radiation. For example, where gamma radiation has been described in U.S. Pat. No. 5,529,727 to improve the surface wettability of rigid gas permeable contact lenses, the present methods expose silicone hydrogel contact lenses without substantially changing the surface wettability of the lenses. In addition, certain of the lenses sterilized using the present methods do not include a surface modification or surface treatment that improves the surface wettability of the lenses. Furthermore, certain of the lenses sterilized using the present methods do not include a polymeric interpenetrating network to achieve the desired surface wettability. However, in other embodiments, surface treated lenses and polymeric interpenetrating network lenses can be sterilized using the present methods.  
      The silicone hydrogel contact lenses used in the present methods and provided in the present packages may be suitable for daily wear or continuous wear. For example, the contact lenses may be daily disposable silicone hydrogel contact lenses, or they may be continuous wear silicone hydrogel contact lenses, such as contact lenses that can be worn continuously on an eye of a person for at least two weeks, and even about thirty days or more.  
      As discussed herein, the high energy radiation used in the present methods, includes without limitation, gamma radiation and e-beam radiation. The present methods include exposing the silicone hydrogel contact lens to gamma radiation for a time sufficient to deliver a dose of gamma radiation in a range of at least 0.005 Megarads (Mrads) and up to 10 Mrads, and more preferably in the range of about 1 to about 4 Mrads. In one embodiment, the methods include exposing the contact lens to gamma radiation for a time to deliver a dose of gamma radiation greater than 2 Mrads (20 kGy) and less than 3 Mrads (30 kGy). For example, the dose of gamma radiation may be 2.5 Mrads (25 kGy). In certain embodiments, however, the amount of high energy radiation used to sterilize the lens or lenses is different than an amount of such radiation used to make a surface of rigid, gas permeable lenses more wettable. When using e-beam radiation, the time of exposure is usually shorter than the time for gamma radiation, but the time is effective to deliver a dosage from 0.005 Mrad to 5 Mrad or more. For example, a method may comprise exposing a silicone hydrogel contact lens to e-beam radiation for a time sufficient to achieve a dose of at least 2.5 Mrads (25 kGy). Another method may comprise exposing a silicone hydrogel contact lens to e-beam radiation for a time sufficient to achieve a dose from about 1.5 Mrads to about 3.5 Mrads. The e-beam radiation kills microorganisms by ionizing key cellular components, such as cellular DNA. One advantage provided by the present methods is that the number of parameters needed to properly sterilize the silicone hydrogel contact lenses is reduced relative to autoclaving sterilization procedures. For example, sterilization of silicone hydrogel contact lenses can be achieved by just controlling the sterilization time, for example, by using a time sufficient to deliver a dose of at least 25 kGy to the silicone hydrogel contact lenses. In accordance with the present methods, the time is sufficient to sterilize the silicone hydrogel contact lens without negatively affecting the ophthalmic acceptability of the silicone hydrogel contact lens. Another advantage is that the choice or type of materials which may be used to package such contact lenses is wider or greater since high temperatures are not required for radiation sterilization, as disclosed herein.  
      As used herein, the term “high energy radiation” denotes radiation in the form of gamma rays and/or accelerated electrons. As used herein, high energy radiation does not include ultraviolet (UV) energy or radiation. For example, high energy radiation, as used herein, may be understood to refer to radiation or energy emitted or provided at a wavelength less than 10 nm, such as a wavelength less than about 1 nm.  
      Accordingly, the present methods encompass exposing one or more silicone hydrogel contact lenses to a sterilizing amount or amounts of high energy radiation other than UV radiation. Or, stated in different words, a method includes exposing the silicone hydrogel contact lens or lenses to non-UV high energy radiation. Certain embodiments of the present methods include exposing the silicone hydrogel contact lenses to radiation at a wavelength less than about 10 −2  nm, and even as low as 10 −6  nm.  
      Generally, the high energy radiation has an energy per particle or per quantum of from about 15×10 6  electron volts (15 Mev) to about 0.003×10 6  electron volts (0.003 Mev). Several known high energy radiation sources are listed below.  
                                               ENERGY PER           WAVELENGTH   PARTICLE       RADIATION   (1 × 10 −10  m)   (or per quantum, Mev)                  x-rays   0.008 to 40   1.5 to 003       Gamma rays   0.0014 to 1.6   9 to 0.008       Accelerated electrons   0.05 to 0.0008   15 to 0.25       Neutron particles   0.05 to 0.0008   15 to 0.25       Alpha particles   0.05 to 0.0008   15 to 0.25                  
 
      The dosage of the high energy radiation is preferably chosen to effect sterilization of the lens or lenses without causing any significant structural change, for example, any significant molecular change, to the polymeric components of the lens. For example, the amount of the high energy radiation is selected so as to not cause any significant loss of mechanical properties of the polymers of the lens body, including without limitation tensile strength, elastic modulus, impact strength, shear strength, and elongation. Embrittlement and of the lens body should also be avoided, as well as any change in crystallinity and thus density characteristics thereof. Discoloration of the lens should also be avoided.  
      It will be appreciated by those of skill in the art that the effective dose for achieving sterilization of the lens refers to how much radiation energy is absorbed by the lens body. This will depend on measurable factors such as the source, strength, and size of the radiation field, the distance between the lens and the source, and the type of radiation utilized. Generally, for electron and gamma sources of the same strength, the dose rate of the electron source is many times greater than that of the gamma source. This is because the electron beam is unidirectional and is concentrated in a much smaller region, and because the interaction of electrons with other electrons is much stronger than with photons.  
      Sources for gamma radiation include conventional sources based on cobalt-60 or cesium-137. Many x-ray sources are available and are known to those of skill in the art.  
      The optimum dosage used to sterilize the silicone hydrogel contact lenses, and accordingly, the time of exposure during the sterilization procedure can be determined using routine methods known to persons of ordinary skill in the art. For example, the sterilization effects can be validated after exposing the silicone hydrogel contact lenses to the high energy radiation.  
      Validation of the gamma sterilization can include one or more steps. For example, a method of validating the gamma sterilization method described herein can include determining presterilization bioburden; establishing a verification dose; performing a verification dose experiment; and interpreting the verification dose experiment. As understood by persons of ordinary skill in the art, bioburden refers to the number or amount of viable microorganisms present in the product to be sterilized, such as the number of viable microorganisms on or in the contact lens or on or in the contact lens package.  
      Determination the actual bioburden in the materials (such as the blister packs containing the silicone hydrogel contact lenses) submitted to radiation can be carried out using methods such as those contained in ANSI/AAMI/ISO 11737-1 (Association for the Advancement of Medical Instrumentation. New Standard. Sterilization of Medical Devices—Microbiological Methods—Part 1: Estimation of the Population of Microorganisms on Product. ANSI/AAMI/ISO 11737-1, 1995). The average bioburden estimate per product unit (e.g., per blister pack) is typically established for at least 10 random samples from a minimum of three production batches, as well as for all product units sampled for exposure to gamma radiation.  
      For example, the maximum bioburden allowed per unit may be 1000 cfu. The calculated absorbed dose to attain a sterility assurance levels (SAL) of about 10 −6  for a bioburden of 1000 cfu per unit is 24.9 kGy. The SAL can be defined as the probability that a given product to be sterilized will remain nonsterile following a sterilization process. A SAL of 10 −6  indicates that one of a one million products will remain nonsterile after sterilization. Therefore, the substantiation of a 25-kGy dosimetric release is overkill for all units with bioburden of less than or equal to 1000 cfu. As discussed herein, a 25 kGy dose may provide the desired sterilization of the packaged silicone hydrogel contact lenses without substantially negatively affecting the properties of the silicone hydrogel contact lenses or the packages containing the lenses.  
      After determining the bioburden, the verification dose can be established. Based on the average unit bioburden, the verification dose is statistically calculated to provide a SAL of about 10 −2  in accordance with AAMI TIR27:2001 (Association for the Advancement of Medical Instrumentation. Procedure. Sterilization of Health Care Products—Radiation Sterilization—Substantiation of 25 kGy as a Sterilization Dose—Method Vdmax. AAMI TIR27:2001, Sections 5.3, 5.3.4, 5.3.5, March 2001). The verification dose experiment states that for a SAL of about 10 −2 , there should be no more than one positive out of 10 samples. If the 10 −2  test is successful, then the substantiation of 25 kGy as a sterilization dose for SAL that is 10 −6  is verified.  
      As described in AAMI TIR27:2001, 10 units from the three production lots used for the bioburden estimate or from a new batch manufactured under conditions representative of normal production are randomly sampled and irradiated at the appropriate verification dose (±10%). Dosimeters are placed in predetermined locations throughout the equipment to check for absorbed doses to be ±10% of the verification dose. This step is also used to identify zones receiving minimal doses and define the worst-case locations during routine sterilization cycles. During product runs, dosimeters should be placed in first, middle, and last product containers.  
      Each sample is tested for sterility after exposure to the verification dose. If no more than one positive test of sterility is obtained in the 10 tests, then a sterilization dose of 25 kGy (to provide a SAL=10 −6 ) is substantiated. If two positives are found, the verification dose experiment can be repeated. If zero positives are found (a maximum total of two out of 20), then a sterilization dose of 25 kGy (to provide a SAL=10 −6 ) is substantiated. If three or more positives are found in the first 10-20 tests, then the sterilization dose of 25 kGy is not substantiated, and alternative dose setting methods may be required.  
      Thus, after practicing the present methods, a high energy radiation-sterilized silicone hydrogel contact lens is provided. This lens is suitable for wearing on a person&#39;s eye to provide vision improvement without substantial discomfort.  
       FIG. 3  is a flow chart of a further example of a method  300  in accordance with the present invention. The method  300  includes a step  310  of providing a silicone hydrogel contact lens in a package, such as a sealed blister pack. The lens may be provided in a volume of liquid, such as any conventional lens packaging liquid, such as an aqueous liquid. In certain embodiments, the liquid is saline. In certain embodiments, the liquid is buffered saline. As one example, the liquid may be phosphate buffered saline. Certain embodiments include a silicone hydrogel lens in a liquid that comprises a surfactant. Certain embodiments include a preservative free liquid or a liquid that is free of anti-microbial agents. Certain embodiments include a lubricant-free liquid, or a liquid that is free of a lubricant, such as a polyanionic polymer that does not lower the surface tension of an aqueous liquid. For example, the packaging liquid may be free of carboxymethylcellulose (CMC). In addition, certain embodiments include a packaging liquid that is free of a contact lens cleaning agent or disinfectant. Some examples of liquids include a solution of water and sodium chloride, potassium chloride, one or more other tonicity agents, and the like, and mixtures thereof. The pH of the liquid is preferably neutral, and may be from about 7.0 to about 8.0. For example, in certain embodiments, the pH of the medium is 7.2.  
      The package in which the silicone hydrogel contact lens is provided may be of any suitable material. For example, the package may be made of glass or plastic. When blister packs are used to package the silicone hydrogel contact lens, the package typically comprises a plastic base member having a cavity for holding a volume of liquid, and a sealing member disposed over the cavity. In certain embodiments, one silicone hydrogel contact lens is provided in one sealed blister pack. For example, one silicone hydrogel lens that has been extracted and hydrated is placed in a volume of packaging liquid in one blister pack, is inspected, and is then sealed in the blister pack by placing a sealing member over the cavity of the blister pack.  
      The method  300  includes another step  320  of exposing the package comprising the silicone hydrogel contact lens to a sterilizing dose of high energy radiation, as described herein. The sterilized packaged silicone hydrogel contact lens is ready for distribution to the public.  
      The present methods may also include one or more optional steps, as described herein.  
      In certain embodiments, a method may include a step of transporting the silicone hydrogel contact lens, or one or more batches of silicone hydrogel contact lenses, to a high energy radiation sterilization facility. For example, high energy sterilization facilities are typically expensive to produce, maintain, and operate. Therefore, in the course of manufacturing contact lenses, it may be desirable to utilize the services of a pre-existing sterilization facility. Thus, batches of silicone hydrogel contact lenses can be transported from a lens manufacturing facility to a sterilization facility that is physically located away from the lens manufacturing facility. One example of a sterilization facility is Isotron (United Kingdom).  
      Methods of the present invention may also include a step of cooling the silicone hydrogel contact lens, or a package containing the contact lens prior to exposing the silicone hydrogel contact lens to the sterilizing dose of high energy radiation. For example, the packages containing the lenses can be cooled to reduce microbial growth in the package between the time when the package is sealed and when the package and lens are sterilized. Cooling the contact lens provides an advantage of reducing growth of the bioburden present in the package and therefore helps ensure that a sufficient dose of radiation is used to sterilize the contact lens. The cooling of the lens or lens package can be accomplished by reducing the temperature in or around the package. For example, the lens or package can be placed in a refrigerated container or similar device. Alternatively, the packaging solution can be selectively cooled to reduce the temperature of the immediate environment of the contact lens. When a method includes transporting the contact lens to a sterilization facility, the method may include transporting the contact lens in a refrigerated compartment. In certain embodiments, the contact lens is cooled below 10 degrees C., for example the contact lens may be cooled to about 4-5 degrees C. The contact lens can be maintained at a cool temperature (e.g., a temperature less than room temperature, such as about 20 degrees C.) for extended periods of time. For example, batches of silicone hydrogel contact lenses can be stored at reduced temperatures for at least 30 minutes. In addition, the lenses can be stored at reduced temperatures for more than 2 hours or even more than 1 day if desired. Typically, the lenses will be stored at a reduced temperature for as little time as possible prior to sterilizing the lenses to enhance the amount of lenses that can be manufactured in a given amount of time.  
      As described herein, sterilization facilities may be utilized to sterilize the silicone hydrogel contact lenses with high energy radiation. An example of such a facility is described herein and is illustrated in  FIG. 4 . In such facilities, a conveyor system can be used to direct silicone hydrogel contact lenses into a sterilization room and to direct the sterilized lenses out of the sterilization room. Thus, the present methods may include a step of conveying the silicone hydrogel contact lenses through a sterilization facility.  
      A method may also include a step of immediately or substantially immediately sterilizing the silicone hydrogel contact lenses as soon as they arrive at a sterilization facility. Such an immediate sterilization step may be helpful in reducing potential microorganism growth that may occur during the time between packaging and sterilization.  
      In addition, as described herein, the present methods may also include one or more inspection steps. For example, a method may include inspecting the silicone hydrogel contact lenses for defects prior to sealing the package in which a silicone hydrogel contact lens is placed. The inspection can be performed on hydrated or unhydrated silicone hydrogel contact lenses, and can be performed manually or using an automated inspection system. Since the silicone hydrogel contact lenses will be located in a volume of liquid in the package, it may be desirable to utilize a “wet” inspection process in which the contact lens is located. In addition, or alternatively, a method may include inspecting a sample of sterilized silicone hydrogel contact lenses after exposing the silicone hydrogel contact lenses to the high energy radiation. Such a method may be helpful in ensuring that the sterilization process is sufficient to sterilize the lenses and packages, and to ensure that the lens properties are maintained.  
      Systems for sterilizing are also provided by the present invention. A system useful in practicing the present methods is schematically illustrated in  FIG. 4 . The system  400  comprises a sterilization facility  410  which includes a sterilization room  412  surrounded by walls  414  that are impermeable to high energy radiation. The walls  414  may be a portion of the walls of the sterilization facility  410 . The sterilization room  412  includes a high energy radiation source  416 . For example, the high energy radiation source  416  may include cobalt-60 or cesium-137 provided on a holding rack. The sterilization room  412  may include a pit containing a volume of water for storing the rack when sterilization is not occurring. Alternatively, or in addition, the high energy radiation source may include an x-ray device or a electron accelerator. The sterilization room  412  includes an opening  418 , which can be used as sterilization room entrance and exit, and the sterilization facility  410  may include an entrance  420  and an exit  422 . A conveyor system  424  extends through the entrance  420  into the sterilization room  412  through the opening  418 , and out of the sterilization room through opening  418  and out of the exit  422 .  
      As shown in  FIG. 4 , the conveyor system  424  does not proceed along a perfectly straight path from the entrance  520  into the sterilization room  412  and from the opening  418  to the exit  422 . For example, the configuration between the entrance  420  and exit  422  and the opening  418  may be understood to be an “F-bin”. In other words, the path between the entrance  420  and the opening  418 , and the path between the opening  418  and the exit  422  can have one or more portions oriented at right angles to the other portions. In this configuration, inadvertent escape of high energy radiation from the sterilizing room  412  is minimized.  
      When the sterilization room  412  includes a gamma radiation source, the gamma radiation source is raised from the water in the pit and emits radiation throughout the sterilization room  412 . Batches of silicone hydrogel contact lenses can be placed on the conveyor system at a loading station  426  and directed through the entrance  420  into the sterilization room  412  through opening  418  and directed out of the sterilization room  412  through opening  418  and out of exit  422  to an unloading station  428  to provide a gamma sterilized batch of silicone hydrogel contact lenses.  
      Another system is illustrated in  FIG. 5 .  FIG. 5  is an illustration of a high energy radiation sterilization system  500  which uses electron beams to sterilize silicone hydrogel contact lenses, or packages containing silicone hydrogel contact lenses. The system  500  includes an electron accelerator  510 . The electron accelerator  510  generates a scanning electron beam through which the silicone hydrogel contact lenses or packages containing such lenses can pass and become sterilized. The electron beam is achieved by using a tungsten filament gun to emit the electrons and directing the electrons through a tube in which they become accelerated. The electron accelerator  510  is located in a sterilization room  512 , which may be similar to that shown in  FIG. 4 . A conveyor system  514  can be used to direct the silicone hydrogel contact lenses or packages, or batches thereof, into a sterilization facility  516  which includes an electron accelerator  510  in a sterilization room  512 . Using an electron beam sterilization system, silicone hydrogel contact lenses can be effectively sterilized with doses in the range of about 25 kGy (2.5 Mrads) to about 35 kGy (3.5 Mrads), for example.  
      Although the system in  FIG. 5  appears physically similar to that in  FIG. 4 , the present systems can have different physical and structural features and arrangements than those illustrated. For example, additional systems may have a conveyor system that passes through the sterilization facility in a more direct manner, and may pass by the radiation source in fewer or more configurations to control the dose of radiation used to sterilize the contact lenses. In addition, the present systems can include more than one conveyor system, and can include a conveyor system that has different movement rates, which can be controlled to provide a desired sterilizing amount of radiation to the contact lenses. Furthermore, the system could include more than one electron accelerator to provide multiple electron beam curtains through which the contact lenses or packages pass and become sterilized.  
      The present invention also relates to one or more contact lens packages containing a silicone hydrogel contact lens that has been sterilized with high energy radiation, as described herein. As shown in  FIG. 6 , one embodiment of such a package is a blister pack  600 . As described herein, the blister pack  600  includes a base member  612  having a cavity  614 . The cavity  614  retains a volume of liquid  616 , in which a silicone hydrogel contact lens  618  is placed. A sealing member  620  is attached to the base member  612  to seal the cavity. For example, the sealing member  620  can be heat sealed to the base member  612 . Any suitable blister pack design can be used in the foregoing methods and systems, for example, the blister pack may have a curved bottom surface, one or more curved sidewall surfaces, one or more depending sidewalls extending around or near the perimeter of the blister pack. The present blister packs may be disposable packages, or in other words, the blister packs cannot be reused by a lens wearer to store the silicone hydrogel contact lens in a sterile environment after opening the blister pack.  
      In view of the present disclosure, one example of the present methods includes providing a batch of blister packs. Each blister pack contains a volume of liquid in a sealed cavity and a silicone hydrogel contact lens located in the volume of liquid, such as phosphate buffered saline, which may or may not include a surfactant.  
      The batch of blister packs is transported to a sterilization facility, such as a building, that is spaced apart from the manufacturing facility used to produce the blister packs. Thus, the method includes transporting the batch of blister packs to the sterilization facility.  
      The sterilization facility includes a sterilization room, such as that shown in  FIG. 4 . The sterilization room includes a pit filled with a volume of water. A cobalt-60 radiation source rack is stored and shielded in the water prior to a sterilization procedure. The room is defined by concrete walls having a thickness of about six feet. The sterilization room has an entrance and an exit through which a conveyor system can deliver the batch of blister packs containing the silicone hydrogel contact lenses into the sterilization room and out from the sterilization room. The entrance and exit of the sterilization room are structured to prevent radiation from being emitted from the sterilization room to the surrounding environment.  
      The batch of blister packs is placed on the conveyor and is directed into the sterilization room. The cobalt-60 is raised from the water and radiation is emitted therefrom. The radiation is directed to the batch of blister packs to sterilize the silicone hydrogel contact lens and the packaging liquid.  
      The batch of blister packs is transported through the sterilization room at a rate effective to deliver a sterilizing amount of gamma radiation to the blister packs and contact lenses located therein. The terminally sterilized blister packs are then transported out of the sterilization room and are processed for distribution and use by lens wearers.  
      The present methods may also be practiced in combination with other sterilization techniques, if desired. For example, the present lenses may also be autoclaved, especially if improvements are made to autoclaving technologies. Or, the present methods can include sterilizing the silicone hydrogel contact lenses without exposing the silicone hydrogel contact lenses to sterilizing amounts of ultraviolet light. In addition, the present methods may include one or more additional steps of exposing the lenses to a preservative or sterilizing agent, and if desired, one or more washing steps to remove the preservative or sterilizing agent from the lens.  
      A number of publications and patents have been cited hereinabove. Each of the cited publications and patents are hereby incorporated by reference in their entireties.