Soft Contact Lens Materials Research Articles

Oxygen permeability of a new type of high Dk soft contact lens material.

Lotrafilcon A is a biphasic block-copolymer, comprising a highly permeable siloxane-based polymeric phase, coupled with a water phase (hydrogel phase). The high oxygen permeability of this material, and the fact that it is a hydrogel, places it outside the applicability of both the polarographic ISO standard and coulometric ISO draft standards for contact lens Dk determination. The oxygen permeability (Dk) and transmissibility (Dk/t) of lotrafilcon A lenses were determined by an adaptation of the standard coulometric method. Lenses with a thickness (t) range from 30 microns to over 300 microns were measured in a liquid-to-gas and a gas-to-gas configuration in an effort to combine features of the ISO (draft) standards to yield a valid measurement of the intrinsic material's oxygen transmission characteristics. The following results, for lotrafilcon A, are the mean values and SE for the oxygen permeability coefficient (Dk) determined at 34 degrees C: 140 +/- 2 barrer with water overlay (liquid-to-gas) at 2100 rpm stirring speed, 150 +/- 5 barrer with water overlay (liquid-to-gas) at "infinite" stirring speed, and 170 +/- 2 barrer without water overlay (gas-to-gas), where barrer = 10(-11) (mlO2.cm)/(sec.cm2.mm Hg). Clinical lenses produced from this material are expected to be in the 60 to 90 microns thickness range, with an average center thickness of 80 microns. Given that the Dkmaterial is greater than or equal to 140 barrers, for this material, a parallel sided lens of 80 microns central thickness would exhibit a lens transmissibility (Dk/t) of at least 170 x 10(-9) mlO2/(sec.cm2.mm Hg) at 34 degrees C. This transmissibility is well in excess of the 87 x 10(-9) mlO2/(sec.cm2.mm Hg) value postulated to be sufficient to prevent overnight lens-induced corneal swelling and places it in the hyper-permeable material category.

Determination of the total attenuation coefficient for six contact lens materials using the Beer—Lambert law

The Beer—Lambert law has been used to determine the total attenuation coefficient, μt, of three hard and three soft contact lens materials. The three hard contact lens materials were PMMA, Polycon II and Boston IV whereas the 3 soft materials were chosen with differing water contents of 38, 55 and 70%, respectively. The total attenuation coefficients of all six materials were obtained from measurements of the axial transmission at 632.8 nm of a series of plano powered lenses varying in axial thickness from 0.5 to 3.5 mm. The value of the total attenuation coefficient depends on both scattering and absorption and hence PMMA and Boston IV, which both incorporated a handling tint, showed significantly higher values (P < 0.0001) ofμt (0.562 ± 0.010mm−1and 0.820 ± 0.008mm−1, respectively) than Polycon II (μt = 0.025 ± 0.005mm−1). A comparison between Polycon II and the three hydrated soft contact lens materials showed a significant increase (P < 0.02) in the total attenuation coefficients for the 38% and 55% water content materials, and a weakly significant increase for the 70% water content soft lens material (P < 0.1). On the assumption that the absorption coefficients of these four materials are approximately constant, then this change would be due to an increase in the scattering coefficient of the material and could contribute to an increase in intraocular scatter. No significant difference (P > 0.5) was found between any of the hydrated soft contact lens materials tested.

Determination of the total attenuation coefficient for six contact lens materials using the Beer–Lambert law

SummaryThe Beer–Lambert law has been used to determine the total attenuation coefficient, μt, of three hard and three soft contact lens materials. The three hard contact lens materials were PMMA, Polycon II and Boston IV whereas the 3 soft materials were chosen with differing water contents of 38, 55 and 70%, respectively. The total attenuation coefficients of all six materials were obtained from measurements of the axial transmission at 632.8 nm of a series of plano powered lenses varying in axial thickness from 0.5 to 3.5 mm. The value of the total attenuation coefficient depends on both scattering and absorption and hence PMMA and Boston IV, which both incorporated a handling tint, showed significantly higher values (P &lt; 0.0001) of μt (0.562 ± 0.010 mm−1 and 0.820 ± 0.008 mm−1, respectively) than Polycon II (μt = 0.025 ± 0.005 mm−1). A comparison between Polycon II and the three hydrated soft contact lens materials showed a significant increase (P &lt; 0.02) in the total attenuation coefficients for the 38% and 55% water content materials, and a weakly significant increase for the 70% water content soft lens material (P &lt; 0.1). On the assumption that the absorption coefficients of these four materials are approximately constant, then this change would be due to an increase in the scattering coefficient of the material and could contribute to an increase in intraocular scatter. No significant difference (P &gt; 0.5) was found between any of the hydrated soft contact lens materials tested.

Surface wettability and hydrophilicity of soft contact lens materials, before and after wear

Surface wettability and hydrophilicity of pHEMA soft contact lenses were investigated utilising adherent liquid/laser method (ALLM) and adherent liquid/balance method (ALBM). The measurements were carried out before and after periods of 15min to 2 h contact lens wear. Following the lens removal, the wetting angle of the contact lens of the left eyes and hydrophilicity (maximum force) of the contact lens of the right eyes of the five subjects were immediately measured, respectively, without cleaning (one value for pre-lens wear and eight values for post-lens wear). The wettability of soft contact lenses significantly increased after 15min of wearing and stabilised at a maximum level after 30 min in vivo. Contact lens hydrophilicity was found to decrease as the lens was worn. The reduction continued for up to 1.5 h of wearing, for male cases, and for the female cases, the reduction continued until the end of the experiment.

Surface wettability and hydrophilicity of soft contact lens materials, before and after wear

SummarySurface wettability and hydrophilicity of pHEMA soft contact lenses were investigated utilising adherent liquid/laser method (ALLM) and adherent liquid/balance method (ALBM). The measurements were carried out before and after periods of 15 min to 2 h contact lens wear. Following the lens removal, the wetting angle of the contact lens of the left eyes and hydrophilicity (maximum force) of the contact lens of the right eyes of the five subjects were immediately measured, respectively, without cleaning (one value for pre‐lens wear and eight values for post‐lens wear). The wettability of soft contact lenses significantly increased after 15 min of wearing and stabilised at a maximum level after 30 min in vivo. Contact lens hydrophilicity was found to decrease as the lens was worn. The reduction continued for up to 1.5 h of wearing, for male cases, and for the female cases, the reduction continued until the end of the experiment.

Poly(vinyl alcohol) hydrogels as soft contact lens material

A new type of soft contact lens was developed from the poly(vinyl alcohol) (PVA) hydrogel prepared by a low temperature crystallization technique using a water-dimethyl sulfoxide mixed solvent. The PVA contact lens materials had a water content of 78% and a tensile strength of 50 kg/cm2, five times as strong as that of commercial poly(2-hydroxyethyl methacrylate) soft contact lens. The amount of proteins adsorbed to the PVA soft hydrogel material was half to one thirtieth of that of conventional soft contact lenses. Histological and scanning electron microscopic observation of rabbit eyes which had worn the PVA soft contact lens for 12 weeks showed no difference in corneal epithelium and cell arrangement in the corneal epithelium from the non-wearing eyes.

Evaluation of polyvinyl alcohol hydrogel as a soft contact lens material

We prepared a transparent polyvinyl alcohol (PVA) hydrogel from a PVA solution in a mixed solvent consisting of water and a water-miscible organic solvent by cooling. The physical properties of the hydrogel were evaluated in various mixed solvents and compared with those of commercially available soft contact lens materials, such as polyhydroxyethyl methacrylate (PHEMA) and copolymers of methylmethycrylate (MMA) and N-vinyl pyrrolidone (N-VP). The PVA hydrogel showed higher tensile strength and elongation before breaking than did the other materials. Also, the PVA hydrogel was comparable in its high water content and its oxygen permeability with the MMA/VP copolymers. The protein adsorption of the PVA hydrogel was much lower than that of the other materials. Soft contact lenses of PVA hydrogel were applied to rabbit eyes for 12 weeks. The effects of the lenses on the cornea were studied by biomicroscopy, ultrasonic pachymetry, and histopathologic examination. No abnormal findings were noted, suggesting that the PVA hydrogel may be promising as a new material for use in soft contact lenses.

Towards An Artificial Cornea: Surface Modifications of Optically Clear, Oxygen Permeable Soft Contact Lens Materials by Ammonia Plasma Modification Technique for the Enhanced Attachment and Growth of Corneal Epithelial Cells

The advent of high water content, oxygen permeable contact lens materials has made the intracorneal implants more feasible. A major obstacle encountered is the regrowth of a stable epithelium over the implant. Therefore, ammonia gaseous plasma modification technique was used to modify the surface chemical properties of soft contact lens material such as poly(2-hydroxyethyl methacrylate and methacrylic acid), PHEMA-MAA copolymer, in an attempt to enhance the cell attachment and growth of rabbit corneal epithelial cells.

The oxygen permeability of rigid gas permeable contact lens materials: reaching a concensus

Over the past two years, significant advances have been made in our understanding of the proper experimentalmethods and analytical procedures for deriving the intrinsic oxygen permeability (Dk) of rigid and soft contact lens materials. This paper traces, in chronological sequence, the vigorous scientific discussions which have emerged from the solid foundations of contact lens Dk theory laid down by Professor Irving Fatt. The inevitable merging of the optometric and chemical engineering literature has brought us to the point where the true intrinsic Dk of contact lens materials can be derived. Efforts are currently underway to develop and distribute a standard set of contact lenses with a view to constructing a universally acceptable framework for evaluating contact lens oxygen permeability.

Contact lenses of soft material (HEMA) for afocal microscopic study of the fundus

Soft contact lens material aids the examination of the fundus with afocal and focal fundus microscopy during vitreoretinal surgery and outpatient examinations.

Echo reference standards in ophthalmic ultrasonography

The use of different echo standards in ophthalmic ultrasonography is discussed. The acoustical properties of the W38 Hema reflector made out of soft corneal contact lens material are determined and verified in several independent measurements. It is shown that this material meets the requirements for plane echo interfaces which are mandatory for reflectors considered to serve as echo standards. A conversion diagram is given for echo references presently used in ophthalmic ultrasonography. Finally, an example for an application of the W38 reflector in performance control and calibration measurements on ultrasonic instruments is presented.

The adsorption of cationic antimicrobial agents onto polyhema

The effect of alkyl-chain length on the uptake of cationic antimicrobial agents onto and into poly(hydroxyethyl methyl methacrylate) soft contact-lens material and lenses, has been studied using benzalkonium and cetrimide analogues. The results obtained support the theory that lens uptake is dependent on the alkyl-chain length and the nature of the preservative.

The adsorption of cationic antimicrobial agents onto polyhema

The effect of alkyl-chain length on the uptake of cationic antimicrobial agents onto and into poly(hydroxyethyl methyl methacrylate) soft contact-lens material and lenses, has been studied using benzalkonium and cetrimide analogues. The results obtained support the theory that lens uptake is dependent on the alkyl-chain length and the nature of the preservative.

Evidence for Lack of Antigenicity of Polymacon Contact Lens Material in Guinea Pigs

In an experiment designed to test one parameter of the antigenic potential of the hydrogel soft contact lens material polymacon (delayed hypersensitivity as shown by skin test), we divided 20 guinea pigs into four groups of 5. 5 guinea pigs were immunized with ground polymacon in complete Freund’s adjuvant. These animals and 5 uninjected control animals were skin-tested with the same lens material. As a positive control, another 5 animals were immunized with bovine serum albumin in complete Freund’s adjuvant. These animals and 5 uninjected control animals were skin-tested with bovine serum albumin. Only the animals immunized and skin-tested with bovine serum albumin had positive skin-test results for delayed hypersensitivity. The lens material was tolerated without inflammation in immunized and nonimmunized skin-tested animals. There was no histologic evidence for antigenicity of polymacon in guinea pig tissues. We conclude that polymacon is unlikely to act as an antigen for delayed hypersensitivity in the guinea pig. We suggest that conjunctivitis associated with the wearing of polymacon lenses is not caused by antigenicity of the lens itself, but by that of some material adhering to the lens.

Approaches to the development of improved soft contact lens materials.

Approaches to the development of improved soft contact lens materials.