Lens Water Content Research Articles

Effects of Lens Thickness and Water Content on Dryness and Tear dynamics on Soft Contact Lens

Effects of Lens Thickness and Water Content on Dryness and Tear dynamics on Soft Contact Lens

Water properties of soft contact lens materials

The properties of water in soft contact lenses such as the water content, free-to-bound water ratio, and the extent to which soft lenses dehydrate during wear, are key determinants of their in eye performance and oxygen transmissibility characteristics. This study describes clinical and laboratory experiments that were conducted in order to examine the state of water in eight soft contact lenses manufactured from different materials. Specifically, lenses made from the following eight materials (and nominal water contents) were used: HEMA/VP 40%, HEMA/VP 55%, HEMA/VP 70%, VP/MMA 55%, VP/MMA 70%, HEMA 40%, HEMA/MAA 55% and HEMA/MAA 70% [HEMA = 2-hydroxy-ethyl methacrylate, VP = vinyl pyrrolidone, MMA = methyl methacrylate, MAA = methacrylic acid]. Differential scanning calorimetry (DSC) was used for measuring the free water content in the materials listed above. Some noticeable differences in water properties among soft contact lens materials having approximately the same water contents were revealed. Low water content materials exhibited a simple endotherm and all water melted around 0 °C. On the other hand, medium and high water content materials exhibited multiple melting endotherms, representing a broad range of interactions between water and the polymer. Low water content soft contact lenses have approximately the same amount of bound water as those with much higher water contents. Six subjects were then fitted with the same lenses for one day. In vitro measurements of water content and oxygen transmissibility were taken at 35 °C, both before lens fitting and after 6 h of lens wear. Water content and oxygen transmissibility were correlated with the water properties of the soft contact lens materials. The relative change in lens water content (%ΔWC) and relative change in lens oxygen transmissibility (%ΔDk/ t) were calculated and correlated with the water properties of the eight soft contact lens materials. It was concluded that (a) oxygen transmissibility, free water content and free-to-bound water ratio are increased when the water content of a contact lens is increased and (b) water content, free water content and free-to-bound water ratio cannot be used for the prediction of soft contact lens dehydration in vivo.

Tensile properties of soft contact lens materials

The strength of contact lens materials is an important consideration with respect to resistance to damage during lens handling and long term durability, and may govern some aspects of in-eye lens performance. The tensile properties of hydrogel contact lenses manufactured from eight different materials were examined in a series of clinical and laboratory experiments using the Instron 1122 Universal Testing Instrument. Lenses from the following eight materials (and nominal water contents) were used: HEMA/VP 40%, HEMA/VP 55%, HEMA/VP 70%, VP/MMA 55%, VP/MMA 70%, HEMA 40%, HEMA/MAA 55% and HEMA/MAA 70% (HEMA: 2-hydroxy-ethyl methacrylate, VP: vinyl pyrrolidone, MMA: methyl methacrylate, MAA: methacrylic acid). Tensile strength, elongation-at-break and Young's modulus were measured. A technique was devised that enables three parallel-sided specimens of identical width to be cut from a single contact lens with good accuracy. It was found that materials made from HEMA/MAA—although having a very low tensile strength and elongation-at-break—exhibit only a moderate Young's modulus. Materials made from HEMA/VP exhibit high-to-moderate tensile strength, high elongation-at-break and moderate-to-low Young's modulus. Materials made from VP/MMA exhibit high tensile strength and high-to-moderate elongation-at-break, but the Young's modulus is high for the 55% water content and low for the 70% water content materials. The HEMA 40% material exhibits a moderate tensile strength, a low elongation-at-break and a high Young's modulus. This experiment highlights the necessity of developing an accepted standard test methodology for contact lens material stiffness evaluation, in order to derive useful comparative information. Six subjects were fitted with the same lenses for one day. In vitro measurements of total diameter and back optic zone radius were taken at 35 °C before lens fitting and after 6 h of lens wear. Lens water content, the relative change in lens total diameter (%ΔTD) and relative change in lens back optic zone radius (%ΔBOZR) were calculated and correlated with the tensile properties of the eight hydrogel lens materials. It is concluded that hydrogel materials with high stiffness and strength display less tendency to change their geometric parameters and materials with a high water content do not necessarily have the weakest mechanical properties.

Parameter stability of soft contact lenses made from different materials

The parameters of soft contact lenses may alter during wear. A series of clinical and laboratory experiments were conducted in order to examine the parameter stability of eight soft contact lenses manufactured from different materials. The following materials (and nominal water contents) were used: HEMA/VP 40%, HEMA/VP 55%, HEMA/VP 70%, VP/MMA 55%, VP/MMA 70%, HEMA 40%, HEMA/MAA 55% and HEMA/MAA 70% (HEMA: 2-hydroxy-ethyl methacrylate, VP: vinyl pyrrolidone, MMA: methyl methacrylate, MAA: methacrylic acid). Two lenses from each of the eight soft contact lens groups were used in experiments concerning the parameter stability. Six subjects were fitted with lenses for 1 day. Verification of back optic zone radius, total diameter, back vertex power, centre thickness and water content was undertaken at 20 °C. In vitro measurements of water content, oxygen transmissibility, total diameter and back optic zone radius were taken at 35 °C before lens fitting and after 6 h of lens wear. Distortion, discolouration and lens quality were assessed before and after lens wear. When the temperature was raised from 20 to 35 °C, a significant reduction in lens water content for all the lens types was observed, as well as a significant reduction in total lens diameter for the majority of the lens types. Water content, oxygen transmissibility, total diameter and back optic zone radius of all the lens types reduced, following a 6 h open eye wearing period. For the majority of the lens types, these changes were found to be statistically significant. Distortion, discolouration and quality of the lenses remained unchanged throughout the study with the exception of the HEMA/MAA 70% lens. Correlating a number of parameters generated in this study, gave the following conclusions. High water content materials exhibit a low relative change in oxygen transmissibility following a 6 h wear period. Soft contact lens dehydration leads to a decrease in oxygen transmissibility and total diameter, following a 6 h wear period. These results will assist practitioners in predicting the alterations that occur in soft contact lens parameters and oxygen performance as a result of lens wear.

Hydrogel contact lens dehydration in controlled environmental conditions.

To ascertain the impact of environmental conditions on hydrogel contact lens dehydration. Six young adult subjects wore an ACUVUE lens in one eye and a Proclear lens in the other eye for 200 minute sessions in arid, temperate, and arctic conditions, maintained in a purpose-built environmental chamber in an aerospace medical facility. Dehydration was determined by measuring lens water content before and after each session using a soft contact lens refractometer. Comfort of the lenses was assessed at the conclusion of each session. Lens dehydration was similar for the three environmental conditions (F2 = 0.3, P = 0.75). There was less dehydration with the Proclear lens compared with ACUVUE lens (F1 = 43.0, P = 0.001). No differences were detected with respect to lens comfort between the three environmental conditions (F2 = 2.0, P = 0.18) or the two lens types (F1 = 1.4, P = 0.29). Soft contact lens dehydration is unaffected by environmental extremes.

Lens aldose reductase inhibiting potential of some indigenous plants

Cataract is the leading cause of blindness worldover. Diabetes is one of the major risk factors for cataractogenesis and aldose reductase (AR) has been reported to play an important role in sugar-induced cataract. In the present study, the AR inhibitory activity of Ocimum sanctum (OS), Withania somnifera (WS), Curcuma longa (CL), Azadirachta indica (AI) were studied together with their effect on sugar-induced cataractogenic changes in rat lenses in vitro. Aqueous extracts of the plants, procured from Dabur, India, were reconstituted with double distilled water to make various dilutions. AR inhibitory activity of these extracts and their anticataract potentials were evaluated in vitro in rat lenses. AR inhibitory activity of the aqueous extract of different plants was calculated considering the AR activity of normal rat lenses as 100%. The concentration of the plant extract that showed maximum AR inhibitory activity was selected to further study its effect on galactose-induced lens swelling and polyol accumulation in vitro. All the four plants were found to inhibit lens AR activity but to different extent. From dose–response curve, OS was found to be the most effective AR inhibitor followed by CL, AI and WS. The IC 50 values of OS, CL, AI and WS were calculated to be 20, 55, 57 and 89 μg/ml, respectively. OS showed a significant inhibition (38.05%) in polyol accumulation followed by CL and AI (28.4 and 25.04%, respectively). WS did not show any effect on polyol level in rat lenses. None of the plant extracts showed any significant effect on lens water content. OS possesses a significant anticataract activity in vitro and its anticataract potential could be related with its AR inhibitory effect.

Contact lens practice in Hong Kong in the new millennium

Purpose: We evaluated the contact lens services provided by practitioners in Hong Kong, the fitting habits of the contact lens practitioners and their attitudes regarding the use of orthokeratology (ortho‐k) and silicone hydrogel (SH) lenses for overnight wear.Methods: Questionnaires were sent to all Hong Kong optometrists licensed to prescribe contact lenses.Results: Two hundred and seventy‐five questionnaires (21 per cent) were returned. Most of die respondents (96 per cent) were employed in optical shops and worked in practices that provided contact lens services. The number of new/refit cases per practice per month was about 55, with die percentages of new and refit cases being 48 per cent and 52 per cent respectively. Eighty‐eight per cent of the patients were fitted widi soft lenses and 10 per cent were fitted widi rigid gas permeable (RGP) lenses. Fifty‐nine per cent of soft lens patients were fitted widi planned replacement (frequent replacement and disposable) soft lenses. Fifty per cent of soft lenses were low water content lenses and 85 per cent of RGP lenses were in low DK (≤ 40) material. Multipurpose solutions and soaking solutions with separate cleaner were die most commonly prescribed care regimens for soft and rigid lenses, respectively. Thirteen per cent of die practitioners provided ordio‐k services and die mean number of ortho‐k cases per mondi per ortho‐k practitioner was six, die majority of patients being children. DreimLens was die lens design most commonly used and Boston XO die most used material. However, 96 per cent of die responding practitioners had reservations about die practice of ortho‐k and die use of SH lenses for overnight wear.Conclusions: The contact lens market in HK is still dominated by soft lenses, particularly planned replacement lenses, whereas die percentage of RGP fits has changed little in the past decade. Few respondents used overnight ortho‐k and SH lenses in view of concerns about their efficacy and long‐term safety.

Corneal staining in hydrogel lens wearers.

The purpose of this study was to determine the factors that contribute to corneal staining in hydrogel lens wearers by examining the following variables: type of lens worn, wearing time and lens replacement schedule, lens care system, and topical and systemic medication use. Five hundred full-time, successful hydrogel contact lens wearers were evaluated for corneal fluorescein staining at 20 clinical sites. Corneal staining was evaluated in five regions and was graded on a scale of 0 (no staining) to 4 (severe staining) in 0.5 steps for severity. Corneal fluorescein staining was present in at least one eye in 55.7% of the hydrogel lens wearers examined in this study, and 8.0% had moderate-to-severe staining (defined as a cumulative staining score of > or = 3 with at least one quadrant score > or = 2). Staining was most severe for the inferior portion of the cornea (mean +/- SD; OD: 0.34 +/- 0.57, OS: 0.30 +/- 0.54). All other areas of the cornea showed lesser amounts of staining, all of which were approximately equal (mean range, 0.13 to 0.16). Staining was observed in two or more quadrants in 24% of right eyes and 22.4% of left eyes. When questioned about care system compliance, 81.1% of subjects reported compliance. Patients who were noncompliant with their lens care system (chi 2 p = 0.0037), used rewetting drops (chi 2 p = 0.0005), or wore conventional lenses without a planned replacement schedule (chi 2 p = 0.0008) were more likely to have some degree of corneal staining. Noncompliance with care system (chi 2 p = 0.0147), replacement schedule (chi 2 p = 0.0039), and lens power (chi 2 p = 0.0224) were associated with moderate-to-severe staining. The following factors were not significantly associated with corneal staining: patient age, gender, medication use including oral contraceptives, type of wear (extended, flexible, or daily), average wearing time, wearing time at the examination, lens type (spherical or toric), water content of lenses (high or low water content), type of care system, use of enzymes, and pinching the contact lens directly off the cornea. Staining occurs to some extent in many hydrogel contact lens patients, and is influenced by many factors. Moderate-to-severe staining, which may be more clinically significant, is associated with noncompliance with care system, a conventional replacement schedule, and lens power.

ガラス多孔質体中のアイスレンズの観察 アイスレンズの成長と含水比について

ガラス多孔質体中のアイスレンズの観察 アイスレンズの成長と含水比について

The influence of water content of hydrogel contact lenses when fitting patients with ‘tear film deficiency’

THe fitting of patients presenting a deficient tear film with hydrogel contact lenses offers a challenge to the professional who must choose between materials having either a low or a high water content. Conflicting conclusions from previous studies concerning the better option led the authors to design a single-centre, double masked, clinical study. Twenty-six subjects deemed to have ‘tear film deficiency’ were fitted with a low water content hydrogel lens on one eye and a high water content one on the other eye. Each subject wore the lenses on a daily wear basis (about 10 hours a day) for 30 days. A detailed questionnaire about subjective considerations, such as comfort and ocular appearance, was completed by the subjects at the end of every day of contact lens wear. The data obtained from these questionnaires and those recorded at three follow-up visits led to the final conclusions. THe questionnaires showed no statistically significant difference between the two eyes in dryness sensation, redness, light sensitivity, blurred vision or overall comfort. During the follow-up visits, no observed sign showed a clinically significant difference between the two eyes. Accordingly, the authors conclude that the water content of hydrogel lens is not a key factor when fitting hydrogel lenses to patients with ‘tear film deficiency’.

Characteristics of fungal growth in soft contact lenses

Six fungal species ( Fusarium sp, Exophiala jeanselmei, Penicillium spp, Aspergillus niger, Paecilomyces spp, and Alternaria spp) were used to inoculate soft contact lenses. Four types of soft lenses were used: high-water (58%) and low-water (38%) content lenses and lenses that were unworn or worn for 1 day. The fungi displayed a range of macroscopic and microscopic features that allowed differentiation of species. There was no statistically significant effect of lens water content on growth rate and only Penicillium spp showed significantly higher growth for worn versus unworn lenses. A number of the fungi showed secretions, thought to be enzymes, which potentially aid in the process of lens penetration.

Cataract Development in 12-Month-Old Rats Fed a 25% Galactose Diet and Its Relation to Osmotic Stress and Oxidative Damage

We attempted to clarify the pattern of cataract development in 12-month-old rats fed a 25% galactose diet and to assess the relation of cataract development with osmotic stress and oxidative damage. In lenses of 12-month-old male Wistar rats fed a 25% galactose diet over an 8-month period, suture accentuation appeared at 6 months of galactose feeding and then opacities developed from the anterior subcapsular cortex toward the posterior subcapsular cortex, reaching the nuclear region at 8 months of galactose feeding. Increases in lens galactitol and lipid peroxide contents and a decrease in lens reduced glutathione content occurred at 4, 6 and 8 months of galactose feeding. The increase in lens lipid peroxide content and the decrease in lens reduced gluthatione content were accelerated with an increase in feeding period, while the increase in lens galactitol content was decelerated. An increase in lens water content and a decrease in lens protein content occurred at 6 and 8 months of galactose feeding. The lens vitamin E content increased at 6 months of galactose feeding and this increase was concomitant with increases in serum vitamin E and total cholesterol concentrations. The serum lipid peroxide concentration increased at 4 and 6 months of galactose feeding. The present results indicate that in lenses of 12-month-old rats fed a 25% galactose diet, suture accentuation appears initially and then opacities develop from the anterior subcapsular cortex toward the posterior subcapsular cortex, finally reaching the nuclear region. These results also suggest that in the galactosemic aged rats, osmotic stress would mainly contribute to cataract formation, while oxidative damage could be linked to both cataract formation and progression, although an increase in lens vitamin E content occurs during the cataract development.

An apparent pH-induced effect on extended-wear hydrogel lens water content: Lattimore MR: Optom Vis Sci 1996;73:689–694

An apparent pH-induced effect on extended-wear hydrogel lens water content: Lattimore MR: Optom Vis Sci 1996;73:689–694

Near-infrared spectroscopy for monitoring water permeability of optical coatings on plastics

Near-infrared spectroscopy has been applied to determine the water content of plastic lenses. An analytical method is presented for monitoring the water permeability of thin layers on plastic optics by utilizing the reversible moisture absorption of organic polymers. As an example, scratch-resistant and antireflective layers on poly[diethylenglycol-bis(allylcarbonate)] lenses are investigated. The measurements demonstrate the relatively high water barrier of coatings deposited by plasma-ion-assisted deposition compared with classical physical vapor deposition coatings and polysiloxane dip coatings.

Determination of water content in bovine lenses using near-infrared spectroscopy.

Near-infrared spectral data of bovine nuclear lens tissue was collected in the 3,500- to 6,500-cm(-1) frequency range. The intensity ratios of the water peak at 5,194 cm(-1) to characteristic protein peaks at 4,862, 4,599, and 4,368 cm(-1) were determined for the nucleus of bovine lens samples. The water and protein mass percentages were determined by using a calibration curve established by spectra obtained from aqueous solutions with various concentrations of bovine serum albumin. The bovine fetal nucleus was found to contain 61.2 +/- 1.6, 61.2 +/- 1.9, and 58.4 +/- 1.9 water using the ratio of the water peak intensity to each of the three characteristic protein peaks within each spectrum. These results are consistent with other analytical techniques, thus showing the accuracy of this technique for obtaining chemical information on lens tissue.

On the nature of hereditary cataract in strain 13/N guinea pigs

PURPOSE. Congenital nuclear cataracts in strain 13/N guinea pigs are caused by a single splice-site mutation in the zeta-crystallin gene. Very little is known of the physical factors involved in lens opacification of this system. The aim of this study is to elucidate the biophysical processes causing the nuclear turbidity. METHODS. Normal, homozygous and heterozygous mutant guinea pig lenses were studied. Polarized light scattering measurements were performed on thin sections of lenses as a function of scattering angle. Scattering intensities were collected in two modes, I" and I+. The total water content of lenses was determined by thermogravimetric analysis. The nonfreezable (bound) water content was obtained by differential scanning calorimetry. The morphology of lenses was investigated by scanning electron microscopy. RESULTS. Normal lenses scatter 5- to 10-fold less light than cataractous lenses at wide angles in both modes. The intensity ratios of the two modes imply that most of the scattering comes from density fluctuations; 10–20% of the turbidity may be contributed by orientation fluctuations. The nucleus of heterozygous cataractous lenses contain less total water than normal lenses, whereas the cortex has the same hydration as the normal lens. The nonfreezable water content of the cataractous nucleus is higher than that of the normal lens. Scanning electron microscopy showed frequent truncation of the fiber cells, cavitations and occasional longitudinal splitting resulting in hollow cylinder formation in the nucleus of the cataractous lens. CONCLUSION. Mutation of zeta-crystallin in guinea pigs causes a congenital cataract. A number of supramolecular events contribute to the turbidity. The mobile water leaves the nuclear fiber cells, causing a collapse of supramolecular structures. Both the size of the aggregates and their refractive index increase by this dehydration process, contributing to the turbidity. The truncation and hollowing of fiber cells causes the orientation fluctuations that also increase turbidity.

The therapeutic use of disposable contact lenses

Introduction: Disposable contact lenses (generally used as part of planned replacement schemes) have become a major part of the general contact lens armamentarium. Because of this widespread acceptance in general optometric practice, it was considered useful to assess their use as therapeutic devices. Several previous studies have considered low water content lenses; the present study reports on the use of a high water contact lens.Materials and methods: A 74% water content cast‐moulded lens (8.70/14.40 plano) was fitted to 66 eyes of 60 patients over an eight‐month trial period. Indications for lens fitting were wide ranging and the lens was used as a first choice unless there was a definite contraindication to its use. Periods of wear ranged from two days to permanent wear, with exchange every few months.Results: The lens was found to fit satisfactorily in 56 eyes (85%). With regard to ocular abnormality, there was no consistent reason for poor fitting in the remaining 10 eyes. Lens wear was discontinued in three eyes due to lens‐related complications. The success rate from the use of this lens was, therefore, calculated to be 53 out of 66 eyes (80%).Conclusion: The relatively successful use of a high water content disposable lens for therapeutic purposes was considered very encouraging and confirms the results of other studies using lower water content lenses. The cost benefits and ready availability of such lenses make them a very useful product for hospital contact lens practice. In the Nottingham Unit they have become the lens of first choice unless there is a specific indication for a different type of lens.Further details of indications, fitting problems and complications will be given in the poster.

Clinical comparison of opaque tint soft contact lenses

The study was designed to compare five types of blue opaque tint lenses (PBH Natural Touch; WJ Durasoft D3-OP, D2-OP, and D3-CO; and Ciba Illusions) with respect to vision, fit, comfort, and ocular physiologic response. Performance reliability was evaluated, at least in part, by comparing one of the lenses (Natural Touch) across repeated wearing sessions. Ten adapted soft contact lens patients wore the Natural Touch lens on one eye, paired in random successive order with each of the other lenses on the contralateral eye, over four separate wearing days. On each day, spectacle refraction and high- and low-contrast/illumination logMAR acuities were measured, followed before and after lens wear, by the evaluation of corneal thickness (pachometry), topography, and corneal/conjunctival staining, as well as lens fit, surface properties, logMAR acuities with over-refraction, and comfort. The opaque tinted lenses all yielded less than optimal visual acuity (average, two to three letters below spherical equivalent spectacle acuity) and occasional reports of haziness or poor quality vision. The lenses were similar in fitting characteristics, with lens centers sometimes deviating up to 0.4 mm from the anatomic pupillary midpoint, despite excellent limbal centration. Although surface properties and mean comfort were acceptable and comparable for all five lenses, the DuraSoft 3 lens tended to become significantly less comfortable with wear. Corneal swelling (average, 1.8%) was induced by all of the low-water-content lenses (Natural Touch, D2-OP, Illusions), whereas transient corneal flattening (superior) and corneal staining (inferior) were associated with one of the higher water content lenses (D3-CO). The clinical response measures were all highly reliable for the Natural Touch lens across repeated wearing days. The results suggest that the opaque tint lenses evaluated do not differ substantially from one another in shortterm daily wear clinical performance. Clinicians can therefore prescribe opaque tint lenses on the basis of cosmetics and the proposed wearing schedule, with awareness that the quality or sharpness of vision may be reduced, especially under low-contrast/illumination conditions.

Evaluation of soft contact lens fitting characteristics.

The assessment of soft contact lens fit by observation of the lens behavior on the eye is one of the most common procedures undertaken in contact lens practice. Nevertheless, the guidelines for doing so have been derived empirically with little or no research into the usefulness of these observations. Over 2000 evaluations of soft contact lens fit were analyzed to determine the predictive value of various indicators of contact lens fit, namely: subjective comfort, lens centration, postblink movement (PBM), version and upgaze lag, and tightness assessed by the push-up test. The receiver-operator characteristic (ROC) curves were prepared for each variable by plotting sensitivity values against false positive rates (FPRs) for various cut-off points. Sensitivity was defined as the proportion of "bad" fits correctly identified by a given variable; FPR was defined as the proportion of "good" fits incorrectly identified as bad fits. With each fitting variable, the optimum cut-off point and its corresponding sensitivity and FPR were determined from the ROC curves. Subjective comfort was found to be of little value in identifying tight fitting lenses but was of some limited value in identifying loose fitting lenses, particularly high water content (HWC) lenses. The push-up test was the most accurate single test of lens fit acceptability having a high sensitivity for both tight (92%) and loose (80%) lens fits. Assessment of PBM was a sensitive indicator of tight fitting lenses (95%), but was a poor indicator of loose fitting lenses, having a low sensitivity (51%). Assessment of version lag showed better sensitivity than upgaze lag in assessing loose fits, whereas upgaze lag showed better sensitivity for tight lens fits. Lens centration was of no predictive value in identifying tight fitting lenses but, as expected, was of some value in identifying loose fitting lenses. The results of this study enable a critical reassessment of the procedures that practitioners follow when fitting soft contact lenses.

Water transport in dehydrating hydrogel contact lenses: implications for corneal desiccation.

This article provides direct evidence that the rate of evaporation from the anterior surface of a hydrogen contact lens is related to the bulk water transport within the contact lens, analogous to diffusion-controlled evaporation. The evaporative flux of water from the anterior surface of a contact lens, with free access to water at its posterior surface, was greater for contact lenses of greater overall water content. Blockage of the water entry to the posterior surface of the contact lens reduced the evaporative loss from the anterior surface by 44%. These results suggested that a decrease in the rate of lens bulk water transport during wear would lead to a reduced evaporation rate from the anterior lens surface, for example, if the contact lens water content was reduced due to lipid and mucus adsorption. In addition, a rise in the external temperature of 1.3 degrees was required to produce a significant increase (2 SD) in the evaporative flux from the contact lenses. This suggests that the steady-state water content in vivo may not be significantly influenced by temperature cycling during blinking.