Contact Lens Manufacturing Research Articles

Improved contact lens electrode for corneal ERG recordings in mice

The electroretinogram (ERG) is routinely used to study retinal physiology in the clinic and in research. Due to their outstanding properties, contact lens electrodes (CLEs) are widely used for ERG recordings. Though the procedures for ERG recordings in mice are similar to those used in humans and larger vertebrates, use of CLEs in the mouse has been limited because of difficulties involved with the manufacturing of small contact lenses. We describe a simple instrument and method for manufacturing contact lenses and CLEs for stable ERG recordings in mice. The instrument operates like a hole-punch and is based on slip joint pliers incorporating a ball bearing on one jaw and forming plate on the other. These CLEs are simple to manufacture, inexpensive and provide stable, long-term recordings of corneal ERGs in mice. With minor modifications, these CLEs could be made for other small animals such as rats or fish.

Local oxygen transmissibility of disposable contact lenses

Aim: Despite global standardisation in measurement of oxygen transmissibility (Dk/ t), in our clinic we continue to see limbal vasculature changes in patients wearing some disposable lenses. The use of central Dk/ t as a simple designator of a contact lens oxygen performance has clearly failed, it being acknowledged as often unrelated to the oxygen performance of the lens periphery. This study investigates a new criterion for contact lens oxygen transmissibility, the local Dk/ t. Method: The study measured 20 spherical disposable lens brands (power range: −15.00 to +8.00 D) and 8 toric lens brands (power range: −8.00 to +4.00 D sphere, with a −1.00 D/−1.25 D×180). An electro-mechanical gauge (Heidenhain, Germany) was used to measure lens thickness ( t) at different positions on the lens. Oxygen permeability (Dk) of each lens material was calculated from the equation Dk=1.67 e (0.0397× water content ) of Morgan and Efron [Contact Lens Anterior Eye 21 (1998) 3] who used ISO/ANSI standard methodologies. The local Dk/ t value was calculated from the point of maximum thickness on the lens and compared to the criterion of 12 Fatt units set by Benjamin [Int. Contact Lens Clin. 23 (1996) 188], to provide corneal oxygenation greater than or comparable to that available in the normal closed eye. Results: All lens types with a water content over 65% had a local Dk/ t of 12 Fatt units or more for most or all lens powers: CIBAVision FocusDailies, Bausch & Lomb Soflens66, CooperVision Actifresh400, and CIBAVision PrecisionUV. In addition, some lenses in the 55–62% water content range had some lens powers with a local Dk/ t in excess of 12: CooperVision Proclear, Johnson & Johnson Acuvue 1 and CIBAVision Focus 2 week. None of the disposable toric lenses had a minimum Dk/ t of 12 or more. The silicone-hydrogels, Bausch & Lomb PureVision and CIBAVision Focus Night&Day, comfortably exceeded Dk/ t of 50 for all lens powers. Conclusion: Some current disposable soft lenses have local Dk/ t values below physiological requirements. Contact lens manufacturers and international standards organisations should consider introducing the labelling of local Dk/ t values on lens packaging so that clinicians can make informed prescribing decisions.

Verification of the vertex powers of varifocal rigid contact lenses

Introduction: Increasingly aspherical optics are used in contact lens designs to produce a varifocal effect to correct presbyopia. Whilst no current ISO standard describes a method to verify these products, the vertex power needs to be verified by the manufacturer and optometrist. Visionix claim that the VC2001 instrument is able to resolve the vertex power of complex contact lenses and that it has been adopted by contact lens manufacturers as their “gold standard” method of measure. Practitioners rely on the focimeter for contact lens verification and accept that it has poorer resolution. This raises the question of whether practitioners can verify the vertex powers of varifocal contact lenses to a sufficient sensitivity. The purpose of this study is to determine whether a focimeter can differentiate between various vertex powers incorporating differing degrees of asphericity that are used in contact lens designs. Methods: A range of varifocal contact lenses were used with single vision contact lenses as controls. Three types of focimeters were used: a manual focimeter, a projector focimeter and a computerised focimeter (standard and contact lens modes). The VC2001 assessed the vertex powers over the optical zone and was used as a reference for the focimeters. The measures from the VC2001 and the focimeters were analysed by discrepancy analysis. Results: All discrepancy measures fell outside two standard deviations for either the varifocal contact lenses or the single vision contact lenses when the focimeters were compared to the VC2001. This raises the issue of whether the focimeters are measuring the same parameter as the VC2001. Only the analysis of the repeated measurements from the manual focimeter for single vision contact lenses demonstrated an acceptable measure of precision (±0.21, 2S.D.) with a very small degree of bias (mean difference −0.024, ±0.02 [2S.E.]). Conclusion: These results suggest that the manual focimeter can measure the vertex powers for single vision contact lenses to an acceptable degree of precision. The vertex powers of varifocal lenses cannot be verified using standard practice equipment. The VC2001 has poor precision for power measurements and may not be measuring the same parameter as the focimeters and therefore questions the validity of the VC2001 as the “gold standard” for vertex power measurement.

Surface wettability improvement of silicone elastomers synthesized with water‐soluble polyacrylic acid molds

AbstractThe aim of this work was to evaluate the use of water‐soluble hydrophilic plastic molds for preparing siloxane based random copolymers and for enhancing the surface wettability of resultant polymers, with a view for contact lens manufacture. The random copolymer consisted of silicone monomers and a small amount of N‐vinyl‐2‐pyrrolidone (NVP) along with vinyl acetate and diethyleneglycoldiallylether as a crosslinker. The surface of this copolymer, which faced against a polyacrylic acid (PAA) mold, showed a higher degree of wettability compared to that obtained against a hydrophobic polypropylene (PP) mold. After heating at 80°C for 4 h, the surface of this copolymer became hydrophobic. When it was immersed in water, however, the high degree of surface wettability regained within 30 s, whereas no significant change in wettability was observed for the PP‐facing surface. The results obtained from X‐ray photoelectron spectroscopy indicated that the polar fraction, which is attributed to NVP fractions of the copolymer, concentrated at the vicinity of the PAA facing surface and, in consequence, improved the surface wettability. This surface also showed a dynamic rearrangement of the wettability in response to changes of the surrounding environment. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3786–3789, 2003

Environmental impact of three replacement modalities of soft contact lens wear

Purpose: To illustrate the environmental impact of three contact lens replacement modalities, and to place this into a broader environmental context. Methods: Environmental impact was calculated by sorting the packaging used in the supply of three representative contact lens systems into their constituent materials (liquid, plastic, paper, metal and glass), determining their mass, calculating the annualised consumption by a typical contact lens wearer, and factoring the data against published indices of the environmental impact of the constituent materials. Results: Conventional lens wear had the highest impact and planned replacement lens wear (monthly disposable) had the lowest impact. Conclusions: The overall environmental impact of waste generated thorough the use of contact lens products by the end consumer is insignificant (0.5%) compared with the amount of waste generated in our everyday life.

Clinical performances of two disposable soft contact lenses of different materials on Hong Kong-Chinese

Dryness due to contact lens wear and poor wetting of contact lenses among contact lens wearers is a common observation and contact lens manufacturers have introduced new, better quality soft lens materials that are claimed to be more deposit resistant, and/or dehydration resistant. The purpose of this study was to evaluate and compare the clinical performance of Hydron ActiSoft 60§, made of a new material (HyGMA™) which is claimed to be dehydration resistant with excellent wettability, with ActiFresh 400, made of conventional non-ionic hydrogel material. The experiment was conducted as a double-masked, randomised, cross-over study. Clinical assessment included pre-lens non-invasive tear break up time (PL-NITBUT), logMar visual acuity and contrast sensitivity (CS). Corneal staining, if any, was recorded at each visit. Subjective ratings in regard to dryness, grittiness, lens awareness, vision, comfort, handling (removal), handling (cleaning) and the overall preference of each lens type were obtained at the end of 3 (Phase A) and 6 months' (Phase B) wear. Twenty-eight subjects successfully completed the study. In Phase A, no significant differences were found in both baseline PL-NITBUT and PL-NITBUT after wearing the same lenses for 1 month between the two lens types. In phase B, baseline PL-NITBUT for ActiFresh 400 was significantly shorter than that for ActiSoft 60. However, after 1 month of wear, the difference in PL-NITBUT between the two lens types was no longer significant. PL-NITBUT was significantly reduced after 1 month's wear for both ActiSoft 60 and Actifresh 400, and in both Phase A and Phase B. The two lens types provided comparable logMAR visual acuity, and there was no significant differences in CS (Pelli Robson) between spectacle lens correction (equivalent spheres) and either lens type. The incidence of corneal epithelial staining was significantly higher and more severe with ActiFresh 400 than with ActiSoft 60, both in Phase A and Phase B. Statistically, there were no significant differences in the scores to the set of questions listed in the questionnaire between the two lens types, except for lens awareness and lens handling. Of the subjects having a preference for one lens, more subjects preferred ActiSoft 60 than ActiFresh 400. In terms of overall preference of the two lens types, at the end of Phase A, 50% of the subjects preferred ActiSoft 60 while only 30% preferred ActiFresh 400, the remaining 20% had no preference. However, by the end of Phase B, preferences for ActiSoft 60 and ActiFresh 400 were comparable. This appeared to be due mainly to the ease of lens handling of the latter.

Ultraviolet transmittance of the vistakon disposable contact lenses

With increasing scientific evidence demonstrating the harmful effects of ultraviolet (UV) radiation on different ocular tissues, contact lens manufacturers are incorporating UV blocking monomers into their contact lenses. This study evaluated the transmittance spectra of Vistakon's disposable soft contact lenses (1 Day Acuvue, Acuvue, Surevue, and Vistavue). All except Vistavue contain a UV blocking agent. Transmittance for each lens was recorded from 200 to 400 nm on a Shimadzu UV 160U Dual Beam Recording Spectrophotometer. The results indicate that lenses which incorporate the UV blocking monomer significantly reduced the transmission of W radiation while untreated lenses did not. Our findings indicate that 1 Day Acuvue, Surevue, and Acuvue meet the American National Standards Institution (ANSI) Standard Z80.20 for Class 2 UV blockers: a maximum of 30% transmittance of UVA wavelengths and 5% transmittance of UVB wavelengths. In contrast, the Vistavue lens demonstrated negligible UV blockage. Our technique, in which contact lenses were placed directly in front of a measuring beam, was accurate and simpler than previously reported methods and may be useful in future studies. We found that UV blocking soft contact lenses can be a viable alternative for spectacles in protecting internal ocular structures from UV radiation. However, the external structures of the eye remain at risk and would continue to benefit from the use of UV blocking sunglasses or spectacle lenses.

A novel method to quantify the edge contour of RGP contact lenses

A new method of accurate profile measurement of rigid gas permeable lenses and edges is described. A purpose built optical profilometer non-destructively scans complete cross-sections in several meridians of any rigid lens. The accuracy of the instrument was verified to be better than ±2.5 μm. The profile data are obtained and stored in digital format for further analysis. The seven most commonly used edge features were defined by means of mathematical algorithms to extract unique numerical values for these parameters from the measured profiles. Applications of the new edge quantification method in clinical contact lens research and manufacturing quality control are shown.

Curing of polymer composites—I. NMR characterization of component monomers

The curing of monomers is a widespread procedure in the commercial use of polymers. This paper discusses the way in which NMR may be used to probe the in situ curing of monomer mixtures commonly encountered in the manufacture of contact lenses. Spectral lines are identified which monitor curing with time of individual components in the monomer mix. This information underpins an understanding of the overall curing dynamics and provides important guidance in establishing suitable protocols in the manufacturing process. As a precursor to this goal, this paper reports on the characterization of a number of commonly encountered component materials by high resolution 1H and 13C NMR. The efficacy of the overall procedure is demonstrated.

Contact Lens-Related Microbial Keratitis

The literature is reviewed concerning the pathophysiologic effects of contact-lens wear, the microbiology of contact-lens wear, the change in microflora with contact-lens wear, the contamination of contact lenses and contact-lens products, patient compliance, and corneal interaction with the contact lens. Hypoxia and hypercapnia are the most significant changes in the cornea as a result of contact-lens wear. Changes take place in the conjunctival flora in patients with contact lenses. Compliance of patients and contamination of contact lenses and contact-lens products are significant risk factors. The corneal interaction with the contact lens can overwhelm the protective mechanisms of the cornea, increasing the ability of microbes to adhere to the cornea and progress to microbial keratitis. Some of the factors associated with microbial keratitis are modifiable and should stimulate the contact-lens industry to develop better contact lenses and contact lens products and also permit ophthalmologists to obtain better informed consent from their patients.

Analysis of the colleting method of producingspherical surfaces in contact lens blanks using finite element and experimental methods

In contact lens manufacturing, the collet chuck is one of the most common methods of holding a lens blank in alathe in order to cut a back optic zone. In this method the lens blank is compressed by a colleting force, and the effects of this force on the back optic zone radius of the lens blank are not well known. If these effects are known, improved back optic zone radius dimensions may be obtained. In this paper, the finite element and experimental results obtained for the effects of colleting on the back optic zone radius are presented and discussed.

Surface chemical structure for soft contact lenses as a function of polymer processing.

The surface chemistry and topography of cast-molded Etafilcon-A and doubled-sided lathed Etafilcon-A soft contact lenses were determined to be significantly different. The variations in surface chemical and morphologic structure between the two lenses were the result of contact lens manufacturing methods. The surface of the cast-molded Etafilcon-A had a consistently less rough surface compared to the doubled sided lathed Etafilcon-A as determined by atomic force microscopy. The surface of the doubled sided lathed Etafilcon-A contained primarily silicone and wax contamination in addition to minute amounts of HEMA. The cast-molded Etafilcon-A had an elemental and chemical content which was consistent with the polymer stoichiometry. Contact angle wettability profiles revealed inherent wettability differences between the two lenses types. The cast-molded Etafilcon-A had an inherently greater water wettability, polarity, and critical surface tension. This means that these two lenses cannot be compared as similar or identical lens materials in terms of surface composition. The manufacturing method used to produce a soft contact lens directly determines the surface elemental and chemical structure as well as the morphology of the finished lens material. These results suggest possible differences in the clinical comfort, spoilage, and lubricity felt during patient wear.

Philippine contact lens demographics 1995

A survey among Philippine Optometric Association members was undertaken to determine the level of contact lens practice in the country. The membership was surveyed on the types of contact lenses prescribed, how many patients per month were fitted with contact lenses, wearing mode prescribed, and lens care regimen prescribed. Eighty-six questionnaires were returned and analyzed. The respondents were largely young practitioners, who have been in practice for less than 5 years, had taken a specific course in contact lens fitting, and benefitted from seminars and lectures given by various contact lens manufacturers. The majority (54%) were fitting soft contact lenses to their patients. The conventional soft contact lens (i.e., worn for at least 1 year) is 51.4% of the total number of soft contact lenses prescribed.

Low energy photon attenuation measurements of hydrophilic materials for tissue equivalent phantoms

The object of the study was to measure the linear attenuation coefficients of hydrophilic materials with the aim of investigating their suitability as tissue equivalent materials. Hydrophilic materials are used in the ophthalmic industry for the manufacture of soft contact lenses. Hydrophilic materials have the trade name “Biogel” and are commonly known as hydrogels. Two types of hydrophilic material were tested, ED4C (72% water uptake by weight) and EDIS (60% water uptake by weight). The measurements were obtained using γ-ray photons of energy 59.5 keV, and x-ray photons of energies 44.23 and 17.44 keV. Measurements were made for material types ED4C and ED IS in both the dry and fully hydrated state. Measurements were also made on powdered samples of ED4C at different hydration levels using a photon energy of 17.44 keV and powdered samples of ED IS at different hydration levels using a photon energy of 59.5 keV. The precision of the measurements was approx. 1%. It was found that material ED4C has linear attenuation coefficients that closely match those of the calculated values for soft tissue across the range of energies used.

Matrices rigides permeables a l'oxygene: application aux lentilles de contact

Results concerning the synthesis and characterization of a new class of oxygen permeable, optically transparent and rigid polymers are related. These polymers have been used in the manufacture of rigid hard contact lenses. They are of a network type structure, the dimensional stability of which is caused by a high crosslink density, short elastic chains and by the chemical nature of the components. The synthesis consists of a two-step reaction of two (or more) polyfunctional monomeric molecules (each monomer could be either the crosslink or the elastic chain). In the first step, we have synthesized a rigid matrix, the permeability of which is optimized in a second step by incorporating various amounts and types of polysiloxanes. The mesh size of the network is related to the monomer size. The crosslinking reaction consists of the hydrosylation of a carbon-carbon double bond with a silicon hydride function.

Systemic and local tolerability of ophthalmic drug formulations. An update.

All eye drops raise problems of local tolerance, but with variable frequencies. They can induce pain on instillation, allergic reactions, delayed healing, punctate keratitis, disturbances of lacrimal secretion, disturbances of accommodation (especially the parasympathomimetics) and local pigmentation after prolonged use. Corticosteroids are associated with 2 major risks: chronic glaucoma and cataract, initially reversible if treatment is stopped. There is still a major risk of corneal herpes with corticosteroids. It is important to be aware of these local problems as they are responsible for poor patient compliance. The systemic effects essentially concern the agonists and antagonists of the autonomic nervous system. beta-Blocker eye drops can cause bronchospasm, heart failure, syncope and psychiatric disorders, especially at high doses and with nonselective beta-blockers. These consequences are usually related to failure to comply with the prescribing precautions. alpha-Adrenergic agonists, which exert dose-dependent effects, can induce hypertensive crises or angina attacks. Apart from patients at risk (children under the age of 30 months and the elderly), parasympathomimetics cause few systemic adverse effects; anticholinesterases, which have curare-like properties, are contraindicated for 6 weeks before general anesthesia. In the very young and the very old, atropinic eye drops carry a risk of cardiovascular collapse and neuropsychiatric disturbances. Problems may also occur with other classes of drugs such as anti-infectives, antispectics, anti-inflammatories and contact lens products. Nevertheless, it is clear that this form of treatment is generally very well tolerated in relation to the volume of eye drops prescribed by ophthalmologists each day.

Ulcerative Keratitis and Disposable Lenses-Reply

In Reply. —We agree with Levy that the selection of suitable controls is the most difficult and most important aspect in the design of a case-control study. His main objection to our study design is that we did not match our cases and controls with regard to contact lens wear schedule. This was intentional, since we were not primarily interested in the lens material itself as a risk factor for ulcerative keratitis in isolation from the interaction of patients with contact lens products. Rather, we were interested primarily in the more clinically relevant question of the risk of ulcerative keratitis among users of different contact lens types. In a case-control study, cases and controls are compared with respect to the attributes or exposures thought to be related to the disease under consideration. Controls should ideally be matched to cases in all respects except for the risk factor(s) of interest. Any

The surface treatment of polypropylene molds and its effect on the quality of cast contact lenses.

An investigation of the surface by XPS photoelectron spectroscopy has shown that the process of production of cast contact lenses based on poly(2-hydroxyethyl methacrylate-co-diethyleneglycol methacrylate) is accompanied by mass transfer at the lens-mold boundary. This phenomenon, which impairs the compatibility of the lens during its application, can be considerably suppressed by employing a suitable surface modification of polypropylene molds. The surface treatment consisting in the oxidation of the mold surface by an AC corona discharge in the oxygen atmosphere increases hydrophilicity of the material, thus facilitating separation of the lens from the mold. The results of the XPS study were also confirmed microscopically by employing the SEM method.

Two pioneers in contact lens development and manufacturing have been lost

Two pioneers in contact lens development and manufacturing have been lost

Study on the kinetics and influence factors of radiation synthesis hydrogel material

This paper presents mainly the study of hydrogel material synthesized by high dose rate electron beam. The results are as follows: 1. In the kinetic curve, the gel is produced when the conversion attains to 20% approximately. 2. The relation between the initial polymerization rate (V ∘) and dose rate (I) are as follows: HEMA + EGDMA system, V ∘∝I 0.53; HEMA + IOMA + EGDMA system, V ∘∝I 0.48; HEMA + IOMA + EGDMA + ROH system, V ∘∝I 0.5. 3. The hydrogel material prepared by electron beam have been used for the manufacture of soft contact lenses.