POLYMERIC INTRAOCULAR LENS MATERIAL HAVING IMPROVED SURFACE PROPERTIES

Abstract: An improved intraocular lens material which includes a polymer or glass substrate modified by a gas plasma deposition of a fluorocarbon coating producing a rigid, low-energy surface having significantly reduced cell damage with respect to corneal endothelial tissues. The preferred lens material is produced by exposing a polymeric substrate to a gas plasma of a fluorinated carbon.

Description: Description POLYMERIC INTRAOCULAR LENS MATERIAL HAVING IMPROVED SURFACE PROPERTIES Technical Field The field of the invention is bioco patible poly¬ meric materials. More particularly, the invention relates to such polymers having modified surface properties that substantially improve suitability for intraocular implants. Background of The Invention Adhesive contact between the most commonly used intraocular lens material, poly (methyl methacrylate) (PMMA) , and the corneal endothelium during surgical im¬ plantation results in the loss of endothelial cells which do not regenerate. Cell loss has been directly related to the number of times the intraocular lens contacts the endo¬ thelium during surgery, with approximately 20% loss result- ing from each contact. The relationships between cell adhesion and polymer surface properties such as surface energy, surface chemistry and surface rigidity have been studied by many investigators. However, corneal endotheli¬ al cell adhesion has not been heretofore directly correlat- ed with the properties of the intraocular lens materials examined. Surface modification of the PMMA surface sub¬ strate has been shown to alter lens adhesiveness to cells. Investigators have used gamma radiation grafting to poly- merize hydroxyethyl methacrylate (HEMA) and vinyl pyrroli- done (VP) onto a PMMA substrate. Using a laboratory "touch test" between the modified lens material and rabbit corneas, it was discovered that PMMA alone induces 10-30% damage, a PMMA/HEMA graft about 10% damage and a PMMA/VP graft less than 10% cell damage. However, these "touch tests" have been discovered to be relatively arbitrary and nonreproducible . Another investigator used an 18 gram weight to press sample intraocular lens and rabbit cornea together - for 10 seconds to produce a consistent force on the endo¬ thelium. It was reported that PMMA caused "considerable damage", silicone resin lenses induced "less damage" than PMMA, and silicone elastomer created "far less damage". No quantitative comparisons of cell damage between the samples - • - were possible. In yet another study, the investigator construct¬ ed and employed an instrument directly measuring the force of adhesion between rabbit corneal endothelium and intra¬ ocular material samples. The average stress calculated for 15 PMMA was 0.66 g/cιr-2 which was shown to be the highest of all materials studied. A plasma-deposited VP coating on PMMA and a conventional coating of Healon™ (manufactured by Pharmacia Inc. ) on PMMA each lowered the stress to 0.19 g/cπ-2. Two hydrogels poly (HEMA) and Duragel™ (Soflex), 0 exhibited the lowest stresses, 0.09 and 0.14 ^ g/cπ-2 respec¬ tively of the materials tested. • Thus, prior workers have modified PMMA surface properties by coating with a number of materials such as HEMA and VP . They have discovered that such coatings 5 reduce cell damage to values below that measured for the conventional PMMA intraocular lens material. The best state-of-the-art coatings appear to result from surfaces that are soft, hydrophilic hydrogels. Hydrogels such as HEMA and VP demonstrate low 0 cell damage relative to the PMMA substrate and other coatings tried. However, the hydrogels exhibit a number of disadvantages. For example, the coatings are soft and easily damaged. They are difficult to package and diffi¬ cult to hydrate properly at surgery. Further, hydrogels 5 are prone to calcification and bacterial contamination. Disclosure of the Invention It ' is an object of the invention to provide a surface-modified polymer that has improved biocompatibility. It is particularly/, an object of the invention to provide a surface modified poly (methyl methacrylate) intraocular lens that causes a reduction in adhesion damage of corneal endothelium cells during surgical implantation. It is an object of the invention to provide an improved, modified intraocular lens material which includes a rigid, low-energy surface having reduced adhesion char¬ acteristics with corneal cells. The improved intraocular lens material of the invention is an intraocular lens material polymer substrate including a fluorocarbon polymer coating bound covalently to the substrate. The fluorinated coating will protect the lens from degradation by the body by virtue of the dense and impermeable cross-linked nature of the film coating. Also, the fluorinated film will protect leaching of' fusible components from the lens into the body. The preferred intraocular lens material is a poly (methyl methacrylate) substrate including a fluorocarbon polymer coating. The surface coating is deposited by exposing the substrate polymer to a gaseous monomer and an electrical field. The field ionizes the monomer gas, creat- ing a fluorocarbon plasma. The plasma reacts with the polymer substrate resulting in the simultaneous deposition and polymerization of the fluorocarbon groups and their attachment onto the substrate surface. The preferred process for making a modified poly- meric intraocular lens material includes placing poly (methyl methacrylate) substrate material in a chamber including a gaseous perf luoropropane monomer. A radio- frequency generator then induces an oscillating electric field within the chamber polymerizing the monomer and attaching it to the surface of the substrate. System para¬ meters such as RF power, system pressure and length of reaction control the nature and depth of the deposition as desired. Brief Description of the Drawings Figure 1 is a schematic diagram of an instrument for contacting treated intraocular lens material with corneal tissue. Figure 2 presents percent corneal cell damage as a function of initial force of contact with tested intra- ocular lens materials. Best Mode for Carrying Out the Invention The surface modified intraocular lens material of the invention was produced by coating disks of a substrate polymer, poly (methyl methacrylate), with a fluorocarbon material. In the preferred process, deposition was accomplished by exposing the substrate to a gas plasma of perfluoropropane. The plasma reactor was of conventional design including a glass chamber for holding the -substrate ma¬ terial to be treated. The chamber was surrounded by capaci¬ tance plates coupled to a radio frequency generator which established an oscillating electric field within the chamber. In operation, the sample disks of substrate ma¬ terial were placed in the chamber. The samples were then etched by exposure to an argon plasma. The samples were then exposed to a plasma of the monomer to be deposited on the substrate surface. In addition to the perfluoropropane monomer of the invention, tests were run using ethylene oxide (EO), N-vinyl-2-pyrrolidone ( NVP ) and hydroxyethyl methacrylate (HEMA) . System parameters of RF power, chamber pressure and reaction time were varied to control the nature of the reaction and depth of depositions. Para- meters selected depended upon the character of each monomer tested. The resulting products were characterized in terms of surface chemistry and surface energy. Cell damage resulting from contact of the sample of the material of the invention with corneal tissue was measured and compared with the untreated substrate and other treated surfaces. The measured surface characteristics of the material of the invention as a function of cell damage were compared with measurements of the same qualities for the other surfaces. Electron Spectroscopy for Chemical Analysis (ESCA) was used to determine elemental composition and bond¬ ing states of the outermost 100 Angstroms of the polymer surface. A survey scan at 1-1000 eV was taken to determine the various elements present. Then, scans in specified eV ranges were made to obtain the spectra of the elements C, 0, F and N. As a means of characterizing the surface energy of the material of the invention and comparable materials, the "critical surface energy" was determined using the method of Zis an, as described in "Relation of the Equili- briu Contact Angle to Liquid and Solid Constitution" , Ad¬ vances in Chemistry Series No; 43, Fowkes, Editor, American Chemical Society, Washington, D.C. pp. 1-51 (1964). The analysis required measuring contact angles of various puri¬ fied liquids on each type of surface at atmospheric condi- tions. Cosines of the measured angles were then plotted as a function of the surface tensions of the test liquids, resulting in a Zisman plot from which the critical surface energy was calculated. Actual interaction of the modified surface of the invention with corneal endothelium was determined by employ¬ ing an apparatus which produced consistent and quanita- tively comparable results between samples. Referring to Figure 1, the device used in this test is shown schematically. A disk 10 of the material to be tested is secured in a holder 11 mounted on the lever arm 12 of a microforce detector 17. A freshly excised rabbit cornea 13 is mounted in a holder 14 on a base 15 opposed to the disk 10 to be tested. A micrometer 16 is provided to advance the lever arm 12 and, the disk sample 10 into contact with the corneal button 13 at a measurable force. An electronic circuit of the microforce detector 17 measures the contact force, reporting the measurement on a chart recorder 18. In operation the test disk 10 was brought into contact with the corneal tissue by means of the micrometer 16 for a period of 40-60 seconds. The initial maximum force under which contact was made was recorded. The corneal buttons were then removed, stained and examined under a low power microscope. The percent damaged cells were counted and recorded. The details of the process and results achieved are reported in the following examples. Example 1 PMMA lens material, PERSPEX™ fabricat¬ ed by CooperVision of Seattle, Washington, in the form of 10mm diameter disks was selected. Monomers of perf luoropro- pane and ethylene oxide both manufactured by Matheson Chemical Company of Newark, California, N-vinyl-2-pyrroli- done manufactured by Alfa Products of Danvers, Massachu¬ setts and hydroxyethyl methacrylate manufactured by Hydron Industries were prepared. Sample disks were placed approximately 6cm down¬ stream of the capacitance plates and the continuous system was evacuated at 0.035 Torr. Argon was introduced at 0.25 Torr. RF generator power at 50 watts was continued for five minutes to allow etching of the PMMA substrate. After evacuating the reactor, the fluorocarbon monomer gas was introduced at the desired pressure. For perfluoropro¬ pane monomer pressure was 0.25 Torr and generator power was at 30 watts for 10 minutes. Afterwards, the chamber was evacuated to 0.035 Torr and then gradually brought up to the atmospheric pressure by introduction of Argon and air. Two ESCA spectrometers, a Hewlett Packard Model 5950-B at University of Utah and a Surface Science Model SSX-100 at University of Washington were used to determine the chemical nature of the surfaces of the intraocular lens material samples. Spectra from the HP-5950-B were resolved using a DuPont 310 curve resolver, while spectra from the SSX-100 were resolved with a peak-fitting routine on an HP-9836-C computer. The C(ls) hydrocarbon peak was assign¬ ed to 285eV and used as a reference peak to correct for any energy shifts. The ESCA results of the samples measured appear in section 2 of the following table. TABLE 1 Elemental and Bonding Ratios 1. Stoichiometry based on monomer structure 0 0 Compound C/0 ' C/N C/F CH C-0 C=0 C-0 C-N C-N PMMA 2.5 60 20 20 HEMA 2.0 50 33.3 - 16.7 - NVP 6.0 6.0 - 50 • - - - 33.3 1-6.7 Ethylene Oxide 2.0 - - - 100 CF3 CF2 CFCF n CF C-CFn Perfluoropropane - - 0.375 33.3 33.3 - 33.3 2. " ESCA Results Compound C/O C/N C/F -CH C-0 C=0 C-0 C-N C-N PMMA 3.0 - - 58 23 - .19 pHEMA (a) 2.4 - - 46 38 - 16 HEMA (b) 2.2 - - 46.5 31 - 23.5 pNVP (c) 8.0 8.0 - 43.2 - - 29.7 27.1 NVP (b) 5.7 3.1 - 53 - 34 13 Ethylene Oxide (b) 4.8 -*- - ~ ~ 61 21 9 9 CF3 CF2 CFCF n CF C-CF n Perfluoropro-- pane (b) - - 0.52 4.0 28.0 27.6 15.6 13.3 11.5 Notes (a) Poly(HEMA) spun on glass coverslips (2% in DMF). (b) Plasma-deposited film on PMMA disk. (c) Poly(NVP) spun on glass coverslip (2% in methanol). The contact angles of various purified liquids on each type of surface were measured under atmospheric conditions using a Rame-Hart goniometer. Model 110-00-OONRL Experimentally determined values of the critical surface energy ^ c , for the several surfaces tested are presented in Table II. Teflon 01 and Mylar*" were examined as reference surfaces. TABLE II Critical Surface Tensions. ( ) of Solid Surfaces Surface " $ c (ergs cm-2) Teflon 1 " (TFE) 20.0+1.3 Mylar 1 " 46.7+0.3 PMMA 37.8+3.2 Perfluoropropane film 8.1+3.1 Ethylene Oxide film 45.4+2.7 HEMA film 49.4+4.1 NVP film 48.0+4.3 Example 2 The samples produced and characterized in Ex¬ ample 1 were contacted with corneal tissue by means of the apparatus of Figure 1. Each cornea, rimmed by 2-3 mm of sclera, was excised from a 2-3 kg New Zealand white rabbit and immedi¬ ately placed in RPMI 1640 media with HEPES buffer, L-Glutamine, and penicillin-streptomycin (Grand Island Biological). The cornea in solution was placed in a Forma Scientific Hydro jac CO2 incubator for at least 30 minutes. In preparation for a test, the cornea was removed from this solution, rinsed in a 0.9% NaCl solution, placed on a con¬ cave Teflon 1 " block, and trephined to form a 9-mm-diameter button. The cornea was then placed endothelial side up, in a convex stainless steel holder of the apparatus of Figure 1. A circle of endothelium 7 mm in diameter was exposed with the center projected 2 mm above the level of the holder edge. A 0.9% NaCl solution was dropped intermit- tently onto the cornea to keep the cells cόntinuousLy moist. A 10-mm-di ameter sample disk was mounted in the stainless steel holder of the apparatus which clamped the edges, leaving a 9-mm-diameter exposed planar surface.- This holder was then attached to the micro-force detector 12, Deflection Sensor Cartridge, Model DSC3, manufactured by Imperial Controls. The system was calibrated using 1-20 gram weights, depending on the force anticipated for each test. The test range was: 4000-20,000 dynes. The corneal holder was placed directly beneath the lens sample, and the two surfaces were brought into contact with the micrometer attachment. After 40-60 seconds, the cornea and the sample were separated. The cornea was immediately placed in a 0.9% NaCl solution. The initial and maximum force with which contact was made was recorded. Controls for the test were corneal buttons which remained in the holder for 20 minutes and were kept moist with 0.9% NaCl solution. These corneas were subjected to all handling except for actual contact with a. sample disk. To inspect the endothelium, the staining method of Spence and Peyman, described in "A New Technique for the Vital Staining of The Corneal Endothelium, " Invest . Ophtalmol , 15, No .12 (1000) 1976, involving a combination of Trypan Blue and Alizarin Red S stains provided by Sigma Chemical of St. Louis, Missouri was used. The cornea was then examined under a low power X100 microscope. A central 9 c area, divided into 900 grids, was observed consistent¬ ly for each cornea. The undamaged and damaged cells were counted. The PMMA substrate and the treated disk were measured for cell damage over a range of contact forces. The results were then plotted showing cell damage as a func¬ tion of contact force. Table III reports the best fit curves for the data for each sample. The data are also plotted in Figure 2. TABLE III Best Fits for Cell Damage Data Surface Slope Intercept Average PMMA 0.00190+0.000494 -1.65+5.4 HEMA 0.000498+0.000190 1.99+2.2 NVP 0.000646+0.000239 1.27+2.7 Ethylene 0.001240+0.000565 17.44+6.1 Oxide Perfluoro¬ propane 6.96+3.83 Referring to Table I of Example 1, the ESCA results of both conventional and plasma deposited polymers are compared with stoichiometry of the monomers. The un¬ treated PMMA disk exhibits C/O ratios close to the expected ratio from stoichiometry. The types of bonding present in the sample also compare closely. Each of the HEMA and NVP plasma-deposited coating shows a close resemblance in co po- sition and bonding to the stoichiometry and. to the model poly(HEMA) and poly (NVP) conventional films spun on glass. The peaks comprising the ESCA spectra of the conventional HEMA and NVP polymer surfaces are more distinct than those in the spectral envelopes of the plasma-deposited films. The more ill-defined spectra of the plasma-deposited films reflect the wider distribution of structures, the increased number of chemical species, and the increased crosslinking, all of which are characteristic of plasma depositions. However, the similarity of these plasma deposits to the conventional polymers may indicate a higher level of mo¬ lecular polymerization (i.e. free radical polymerization through the double bond) than atomic polymerization. This similarity also suggests a comparable regularity of struc¬ ture which is important to the hydrogel character of conven- tional poly(HEMA) and poly(NVP). Hence, the coatings de¬ posited by the HEMA and NVP plasma exhibit hydrogel behavior. Coatings deposited by ethylene oxide and per¬ fluoropropane plasmas are vastly altered from the monomer structures. These plasmas undergo complex reactions involv¬ ing "atomic polymerization", a type of deposition in which the molecular structure of the monomer is not retained in the polymer. As a result, a regularity in the structure of the ethylene oxide plasma coating is not likely to occur because the bonding environments noted by ESCA are so dis¬ similar to those in conventional poly-(ethylene oxide). • Referring to Table II of Example 1, the plasma-altered surfaces exhibit definite changes in wet¬ tability compared to the PMMA substrate. The fluorinated plasma rendered the disk nonwettable. The other three plasma-deposited surfaces show an increase in wettability as the critical surface tension increases. These results are consistent with the trends reported in the literature, to the effect that wettability decreases with an increase in f luor.ination and increases with an increase in nitrogen or oxygen bonded to carbon. In Figure 2, percent cell damage is plotted as a function of initial force for each type of surface contact¬ ed with corneal tissue by means of the apparatus shown in Figure 1 and described in Example 2. The solid horizontal line at 3.5+2.7% represents the average damage associated with the control corneas. The lines shown for the other samples are the best curves for the force-damage data, representing either a least squares fit or an average of cell damage where damage appears independent of force. able III reports the parameters and errors of the fits of these data. The choice of fit was based on results from the "f test for chi-square" which determines whether adding a term dependent on force (least squares line) is an improve¬ ment from the average damage line. PMMA, ethylene oxide, NVP and HEMA surfaces displayed a trend of increasing damage with increasing force, and the least squares fits were plotted. Since the slope of the perfluoropropane force-damage curve was not significantly different from zero, the average cell damage value was plotted. The parameter values shown in Table III were used to calculate the significance of the differences in the cell adhesion associated with the five surfaces. Relative to PMMA, each of the four data sets was found to have greater than 97% probability that the difference between the fit for PMMA and for an altered surface fit is significant. The fluorinated surface of the invention induced the lowest endothelial damage over the entire force range investigated. Damage appears to be independent of force. The HEMA and NVP surfaces were also associated with decreased cell damage, compared to the unmodified PMMA. However, both HEMA and NVP surfaces induced increasing cell damage .with increasing force. The ethylene oxide coating caused significantly greater adhesion damage to the cornea than PMMA. As shown, the degree of cell adhesion damage is significantly changed by modifying the ζMMA surface. Changes in the surface chemistry and surface energy have been documented by the ESCA and contact angle studies reported in Example 1. In summary, the data demonstrate a change in surface properties of poly (methyl methacrylate) material modified by RF plasma deposition. The degree of endotheli¬ al cell adhesion to the surface is considerably altered. The percent cell damage as a function of the initial force of contact for each modified surface was significantly different than that induced by PMMA. The results suggest that a rigid, low-energy fluorinated surface is desirable for reduced cell adhesion. Any glass or polymeric substrate for which low-cell adhesion properties are desired and upon which a fluorocarbon monomer may be plasma deposited can be improv¬ ed by modifying its surfaces according to this invention. Such substrates include glass, poly-propylene and silicone polymers, for example. Fluorocarbon monomers or mixtures thereof which can be deposited on a substrate using the gas plasma technique are within the scope of the invention. Preferred monomers include perfluoropropane, perf luoropro- pene, hexaf luoroethane, and tetraf luoroethylene, for example.

Claims: Claims 1. An improved intraocular lens material which causes low corneal ' endothelial cell damage, comprising: a substrate selected from the group consisting of glass and a polymeric material; and a fluorocarbon polymer coating bound covalently to said substrate. 2. The lens material of claim 1 wherein said polymer substrate is poly (methyl methacrylate) . 3. The lens material of claim 1 wherein said fluoro¬ carbon polymer coating is a fluorinated hydrocarbon. 4. The lens material of claim 1 wherein said fluoro¬ carbon polymer coating is perfluoropropane. 5. The lens material of claim 1 wherein said fluoro¬ carbon polymer is perf luoropropene. 6. An improved intraocular lens material, which causes comparatively low corneal endothelial cell damage, comprising: a poly (methyl methacrylate) substrate; and a surface coating of perfluoropropane, resulting from exposing said substrate to a monomer of perfluoropropane and an electric field which ionizes said monomer, forming a plasma gas of said monomer which polymerizes and covalently bonds to said substrate. 7. The lens material of claim 6 wherein said gas plasma deposition is produced by means of an inductively coupled radio-frequency generated discharge. 8. The lens material of claim 6 wherein said gas plasma deposition is produced by means of a microwave generated discharge. 9. A method of improving intraocular lens material, reducing corneal endothelial cell adhesion, comprising: selecting a substrate from the group consisting of glasS oly (methyl methacrylate), polypropylene and silicone polymers; and modifying the surface of said substrate ' polymer by plasma deposition of a fluorocarbon monomer selected from the group consisting of perfluoropropane, perf luoropropene, hexa- f luoroethane, tetraf luoroethylene and mixtures thereof, whereby a rigid, low-energy surface results wherein said contact cell adhesion is substantially reduced from that of said substrate polymer.

Inventors: MATEO NANCY B, RATNER BUDDY D, MATEO, NANCY, B., RATNER, BUDDY, D.
Applicants: WASHINGTON RES FOUND
Application year: 1986
Main classification: A61L27
IPC codes: A61F2, A61L27, A61L33, B05D7, C08J7