Coincident molecular auxeticity and negative order parameter in a liquid crystal elastomer.

Auxetic materials have negative Poisson's ratios and so expand rather than contract in one or several direction(s) perpendicular to applied extensions. The auxetics community has long sought synthetic molecular auxetics - non-porous, inherently auxetic materials which are simple to fabricate and avoid porosity-related weakening. Here, we report, synthetic molecular auxeticity for a non-porous liquid crystal elastomer. For strains above ~0.8 applied perpendicular to the liquid crystal director, the liquid crystal elastomer becomes auxetic with the maximum negative Poisson's ratio measured to date being -0.74 ± 0.03 - larger than most values seen in naturally occurring molecular auxetics. The emergence of auxeticity coincides with the liquid crystal elastomer backbone adopting a negative order parameter, QB = -0.41 ± 0.01 - further implying negative liquid crystal ordering. The reported behaviours consistently agree with theoretical predictions from Warner and Terentjev liquid crystal elastomer theory. Our results open the door for the design of synthetic molecular auxetics.

Graphene electrodes for adaptive liquid crystal contact lenses.

The superlatives of graphene cover a whole range of properties: electrical, chemical, mechanical, thermal and others. These special properties earn graphene a place in current or future applications. Here we demonstrate one such application - adaptive contact lenses based on liquid crystals, where simultaneously the high electrical conductivity, transparency, flexibility and elasticity of graphene are being utilised. In our devices graphene is used as a transparent conductive coating on curved PMMA substrates. The adaptive lenses provide a + 0.7 D change in optical power with an applied voltage of 7.1 Vrms - perfect to correct presbyopia, the age-related condition that limits the near focus ability of the eye.

Field-induced refractive index variation in the dark conglomerate phase for polarization-independent switchable liquid crystal lenses.

Liquid crystal lenses are an emerging technology that can provide variable focal power in response to applied voltage. Many designs for liquid-crystal-based lenses are polarization dependent, so that 50% of light is not focused as required, making polarization-independent technologies very attractive. Recently, the dark conglomerate (DC) phase, which is an optically isotropic liquid crystalline state, has been shown to exhibit a large change in refractive index in response to an applied electric field (Δn=0.04). This paper describes computational modeling of the electrostatic solutions for two different types of 100 µm diameter liquid crystal lenses, which include the DC phase, demonstrating that it shows great potential for efficient isotropic optical switching in lenses. A feature of the field dependence of the refractive index change in the DC phase is that it is approximately linear in a certain range, leading to the prediction of excellent optical quality for driving fields in this regime. Interestingly, a simulated microlens is shown to exhibit two modes of operation: a positive lens based upon a uniform bulk change in refractive index at high voltages, and a negative lens resulting from the induction of a gradient index effect at intermediate voltages.

Incidence of keratitis of varying severity among contact lens wearers.

AIM: To determine the incidence of non-severe keratitis (NSK) and severe keratitis (SK) among wearers of current generation contact lenses. METHODS: A 12 month, prospective, hospital based epidemiological study was conducted by examining all contact lens wearers presenting with a corneal infiltrate/ulcer to a hospital centre in Manchester. A clinical severity matrix was used to differentiate between NSK and SK, based on the severity of signs and symptoms. The size of the hospital catchment population and the wearing modalities (daily wear (DW) or extended wear (EW)) and lens types being used were estimated from relevant demographic and market data. RESULTS: During the survey period, 80 and 38 patients presented with NSK and SK, respectively. The annual incidences (cases per 10,000 wearers) for each wearing modality and lens type were: DW rigid--NSK 5.7, SK 2.9; DW hydrogel daily disposable--NSK 9.1, SK 4.9; DW hydrogel (excluding daily disposable)--NSK 14.1, SK 6.4; DW silicone hydrogel--NSK 55.9, SK 0.0; EW rigid--NSK 0.0, SK 0.0; EW hydrogel--NSK 48.2, SK 96.4; EW silicone hydrogel--NSK 98.8, SK 19.8. The difference in SK between EW hydrogel and EW silicone hydrogel was significant (p = 0.04). CONCLUSIONS: A clinical severity matrix has considerable utility in assessing contact lens related keratitis. There is a significantly higher incidence of SK in wearers who sleep in contact lenses compared with those who only use lenses during the waking hours. Those who choose to sleep in lenses should be advised to wear silicone hydrogel lenses, which carry a five times decreased risk of SK for extended wear compared with hydrogel lenses.

Continuous wear silicone hydrogel contact lenses and microbial keratitis.

Like other lens types, the new generation of silicone hydrogel contact lenses can be associated with a spectrum of ocular complications. Most tend to be very minor, but serious and sight-threatening complications can occur. We present four such cases with microbial keratitis following extended wear of these lenses. Cultures were positive for Pseudomonas aeruginosa in three cases and all three of these suffered lasting visual impairment. We describe our findings and discuss possible risk factors.

The minimum stimulus energy required to produce a cooling sensation in the human cornea.

PURPOSE: To estimate the minimum stimulus energy required to stimulate the corneal nerves, using a thermally cooling stimulus. METHODS: The localised temperature change produced in the pre-corneal tear film by the air-pulse stimulus of the Non-Contact Corneal Aesthesiometer (NCCA) was analysed using a thermal imaging camera. Assuming that the cornea behaves as a near perfect black-body, the threshold for energy loss was calculated by combining Stefan-Boltzmann's law and Kirchoff's law for imperfections in black-body radiation, taking into account the likely thermal conductivity of the human cornea. RESULTS: The average normal subject has a threshold for detecting a cooling effect when the local corneal tear film surface temperature drops from a mean of 33.2-32.9 degrees C (i.e. 0.3 degrees C), over a 1 mm diameter circular area (0.785 mm2), and a stimulus time duration of 0.9 s. This gives rise to a sensation threshold for perceiving a loss of heat energy of 7.0 ergs, or 7.0 x 10(-7) joules. CONCLUSION: The cornea possesses stimulus specific receptors, which have an exquisite sensitivity to a rapid, thermally cooling stimulus.

Thermal consequences of photorefractive keratectomy.

PURPOSE: Photorefractive keratectomy (PRK) is still a widely used method for the correction of refractive error despite the advent of laser in situ keratomileusis (LASIK). However, both procedures are associated with significant side effects such as corneal haze and regression. Several factors have been implicated in the etiology of haze, one of which is thermal loading of the cornea. The purpose of this study was to investigate the temperature changes occurring during PRK when performed at different ablation depths. METHODS: Noncontact, color-coded ocular thermography was performed with an infrared detector apparatus during PRK ablation on 19 ovine corneas. Five different refractive corrections were carried out ranging from -2.00 diopters (D) (ablation depth, 31.2 microm) to -10.00 D (ablation depth, 137.9 microm). RESULTS: A temperature rise at the corneal surface was demonstrated in all 19 corneas. The mean rise in temperature was 7.35 +/- 1.13 degrees C with a maximum rise in temperature of 8.97 degrees C. A positive correlation was found between the refractive correction and the peak rise in temperature (r2 = 0.57, p< 0.0001). The rate of temperature change was greater for smaller treatments than for larger treatments (r2 = 0.79, p < 0.0001). Corneas undergoing larger treatments were subject to greater rises in temperature for longer periods of time. CONCLUSIONS: This study suggests that the cornea undergoes a significant rise in temperature as a result of the PRK process. Further investigation is required to determine what effect this thermal loading has on the corneal wound healing response after PRK.

Validation of grading scales for contact lens complications.

The validity of use of two artist-rendered and two photographic sets of grading scales (grading 'systems') designed for gauging the severity of contact lens-related ocular pathology was assessed in terms of precision and reliability. Thirteen observers each graded 30 images--by interpolation or extrapolation to the nearest 0.1 increment--of each of the three contact lens complications (corneal staining, conjunctival redness and papillary conjunctivitis) that were common to all four grading systems. This entire procedure was repeated approximately two weeks later, yielding a total data base comprising of 9360 individual grading estimates. Analysis of variance revealed statistically significant differences in both precision and reliability between systems, observers and conditions (p < 0.03 for system reliability; p = 0.0001 for all other combinations). The artist-rendered systems generally afforded lower grading estimates and better grading reliability than the photographic systems. Corneal staining could be graded less reliably than conjunctival redness and papillary conjunctivitis. Grading reliability was generally unaffected by the severity of the condition being assessed. Notwithstanding the above differences, all four grading systems are validated for clinical use and practitioners can initially expect to use these systems with average 95% confidence limits of +/- 1.2 grading scale units (observer range +/- 0.7 to +/- 2.5 grading scale units). In view of the significant between-system differences revealed in this study, it is advisable to consistently use the same grading system. It may be possible to reduce between-observer differences by applying personalised correction factors to normalise grading estimates.

The role of tear physiology in ocular surface temperature.

PURPOSE: To determine whether the more rapid cooling of the tear film in dry eyes is related to other tear film parameters, a battery of tear physiology tests was performed on dry eye patients and control subjects. METHODS: Tear evaporation rate was measured with a modified Servomed (vapour pressure) evaporimeter and ocular temperature with an NEC San-ei 6T62 Thermo Tracer in 9 patients diagnosed as having dry eye and in 13 healthy control subjects. Variability in temperature across the ocular surface was described by the temperature variation factor (TVF). Lipid layer structure and tear film stability were assessed with the Keeler Tearscope and tear osmolality was measured by freezing point depression nanolitre osmometry. RESULTS: The data were explored by principal component analysis. The subjects with and without dry eye could be separated into two distinct groups entirely on the basis of their tear physiology. Dry eye patients exhibited higher tear evaporation rates, osmolalities and TVF, lower tear film stabilities and poorer-quality lipid layers than the control subjects. A significant linear relationship was found to exist between tear evaporation rate and TVF for all subjects (R2 = 0.242, p = 0.024). CONCLUSIONS: Rapid cooling of the tear film in dry eyes appears to be related to the reduced stability of the tears and the increased rate of evaporation. The higher latent heat of vaporisation, associated with the increased evaporation in dry eyes, may account for the increased rate of cooling of the tear film in this condition.

Hydrogel contact lens ageing.

PURPOSE: To determine if hydrogel lens water content decreases over wearing periods of up to four weeks; that is, demonstrate an 'ageing' effect. METHODS: Six subjects (4 males, 2 females; age range: 34.8 +/- 11.0 years) wore each of the following lenses for one month: Acuvue, Frequency 38, Gentle Touch and Proclear. Pre-insertion and post-removal lens water content was measured daily by the subjects using an Atago CL-1 Soft Contact Lens Refractometer for a 28 day period. Post-insertion and pre-removal comfort was recorded over the same period. RESULTS: A significant lens ageing effect was noted whereby the pre-insertion lens water content decreased significantly over the 28 day cycle for all lenses (P < 0.05). The pattern for daily dehydration remained consistent for the 28 day period for the Acuvue lens and the Gentle Touch lens but decreased for the Frequency 38 lens and the Proclear lens. Comfort decreased significantly by the end of the day for three of the four lens types investigated. CONCLUSIONS: It is apparent that the extent of lens dehydration changes over time, and that this pattern of change is different for different lens types. The associated loss of oxygen performance with dehydration is demonstrated. These data will assist practitioners in understanding the clinical effects of soft lens dehydration over time.

Blunt thoracic aortic injuries: initial evaluation and management.

In at least one large study, the average time from arrival at the emergency department to arrival in the operating room was nearly 6 hours. That 30% of survivors will die in the same amount of time underscores the need for rapid diagnosis and treatment. In blunt thoracic aortic injury, beta-blockers have been shown to reduce the incidence of rupture, and their use is rarely contraindicated. A working knowledge of the mechanisms of injury likely to produce this lesion, commonly associated injuries, clinically relevant and easily recognizable chest film findings, and appropriate use of beta-blockade can have a significant impact on mortality. Any physician responsible for evaluation of trauma patients should be familiar with this information.

Ocular temperature in carotid artery stenosis.

PURPOSE: To explore the potential application of wide-field, color-coded infrared ocular thermography in the investigation of carotid artery stenosis (CAS). METHODS: Ocular thermography and color duplex ultrasonography were undertaken in a masked study of 24 asymptomatic, consecutively presenting patients who were examined for vascular occlusive disease. RESULTS: Linear regression analysis indicated that there was a significant negative correlation between ocular surface temperature and the degree of CAS (r = -0.67, p < 0.001). Similar results were demonstrated between the relative difference in CAS (right - left) and the relative difference in ocular surface temperature (right - left) (r = -0.67, p < 0.001). CONCLUSIONS: These results indicate that the circle of Willis and the other anastomoses within the brain are unable to compensate fully for the reduction in blood flow on the affected side. Noncontact ocular temperature measurement has potential as both a screening test and a supplementary diagnostic clinical test for CAS.

Hydrogel contact lens dehydration and oxygen transmissibility.

PURPOSE: Oxygen transmissibility is a key determinant of the physiological response of the cornea to contact lens wear. Because transmissibility is related to hydrogel water content, we conducted a study to determine the change in water content during lens wear and to quantify the impact any such change would have on transmissibility. METHODS: In a double masked clinical investigation, two subjects each wore 17 different pairs of contact lenses. Water content was measured before lens wear at 35 degrees C and immediately after 4 hours of contact lens wear. Contact lens oxygen transmissibilities were calculated on each occasion. RESULTS: The absolute changes in water content for the 17 lenses varied from +0.5% to -5.3%. For some lenses, this change in water content altered the lens oxygen transmissibilities to a clinically significant degree. The changes in water content and oxygen transmissibilities were greatest with FDA Group IV lenses. CONCLUSIONS: Dehydration during contact lens wear can alter the oxygen transmissibility of hydrogel lenses, and in some situations, this factor may be clinically significant.

Corneal surface temperature change as the mode of stimulation of the non-contact corneal aesthesiometer.

PURPOSE: The non-contact corneal aesthesiometer (NCCA) assesses corneal sensitivity by using a controlled pulse of air, directed at the corneal surface. The purpose of this paper was to investigate whether corneal surface temperature change was a component in the mode of stimulation. METHODS: Thermocouple experiment: A simple model corneal surface was developed that was composed of a moistened circle of filter paper placed on a thermocouple and mounted on a glass slide. The temperature change produced by different stimulus pressures was measured for five different ambient temperatures. Thermal camera experiment: Using a thermal camera, the corneal surface temperature change was measured in nine young, healthy subjects after exposure to different stimulus air pulses. Pulse duration was set at 0.9 s but was varied in pressure from 0.5 to 3.5 millibars. RESULTS: Thermocouple experiment: An immediate drop in temperature was detected by the thermocouple as soon as the air flow was incident on the filter paper. A greater temperature change was produced by increasing the pressure of the incident air flow. A relationship was found and a calibration curve plotted. Thermal camera experiment: For each subject, a drop in surface temperature was detected at each stimulus pressure. Furthermore, as the stimulus pressure increased, the induced reduction in temperature also increased. A relationship was found and a calibration curve plotted. CONCLUSION: The NCCA air-pulse stimulus was capable of producing a localized temperature change on the corneal surface. The principal mode of corneal nerve stimulation, by the NCCA air pulse, was the rate of temperature change of the corneal surface.

Corneal surface temperature decreases with age.

Infrared temperature measurement has found numerous applications in the detection and diagnosis of ocular and systemic disease. In the study of ocular temperature it is important to consider all parameters which might influence recorded measurements. Examples of such parameters include room temperature, time of day and age of subject. This communication discusses the effect of age on ocular temperatures measured with a modern, wide field, colour-coded infrared detector. It is demonstrated that ocular surface temperature decreases by -0.010 degrees C per year throughout life, although the rate of change increases after middle age.

Efficacy of topical nonsteroidal drops as pain relief after excimer laser photorefractive keratectomy.

PURPOSE: To compare the efficacy of topical aqueous indomethacin and ketorolac for pain relief following excimer laser photorefractive keratectomy (PRK). METHODS: We conducted a double masked, randomized trial involving 120 patients. All patients received a standard regimen of one of the trial drugs and chloramphenicol ointment to be used four times daily for 2 days. Pain levels and quality were assessed on a 10 cm linear analogue scale prior to each instillation of the topical medication with the McGill pain questionnaire. Pain threshold was evaluated using a modified State Trait Anxiety Inventory questionnaire. Patients were advised to use paracetomol for additional pain relief but were allowed to use any oral analgesic (except nonsteroidal agents) which they documented (type and quantity). RESULTS: There was no significant difference in pain scores between the groups using indomethacin and ketoralac. CONCLUSION: We found no significant difference between topical ketoralac and aqueous indomethacin as pain relief following excimer laser PRK. Both drugs seemed to significantly reduce pain.

The oxygen performance of contemporary hydrogel contact lenses.

The oxygen performance of a hydrogel contact lens is arguably its most important property when considering the possible physiological response of the eye. However, information about this aspect of a contact lens can frequently be difficult to obtain, and there are numerous pitfalls in the interpretation and application of the available data. This paper presents the oxygen permeability and oxygen transmissibility of 17 different lens types as measured by one technician in one laboratory across a short time interval. The clear relationship between water content and oxygen permeability is confirmed, and the transmissibilities of three lens types across a wide power range are reported.

Corneal temperature changes during photorefractive keratectomy.

PURPOSE: Corneal temperature changes in photorefractive keratectomy (PRK) have been implicated in the aetiology of subepithelial haze. This study was undertaken to quantify the temperature change during this surgical procedure. METHODS: Non-contact, colour-coded ocular thermography was performed by using an infrared detection system during PRK on a group of 12 subjects. RESULTS: Mean (+/-SD) central ocular surface temperature (OST) after epithelial debridement was 29.15 +/- 0.39 degrees C. Mean peak OST during PRK was 37.77 +/- 0.67 degrees C, with most of the temperature increase occurring during the first 15 s. Factors such as ablation depth, optical correction, and procedure duration were not demonstrated to have a significant effect on corneal temperature during the procedure, suggesting a complex relation between-PRK parameters and the change in corneal temperature. CONCLUSIONS: Previous work suggested that corneal collagen denatures at approximately 39 degrees C, and it has been demonstrated that corneal temperature may be elevated to this level during routine PRK. Further research is indicated into the effects of preoperative cooling and pausing in treatment to reduce temperature changes.

Infrared thermography of the tear film in dry eye.

Infrared ocular thermograms were recorded for a group of 36 dry eye patients and for 27 age- and sex-matched controls. Mean ocular surface temperature was greater in the dry eye group (32.38 +/- 0.69 degrees C) compared with the control group (31.94 +/- 0.54 degrees C; p < 0.01). In addition, there was a greater variation of temperatures across the ocular surface in the dry eye group, illustrated by the difference in temperature between the limbus and the centre of the cornea (0.64 +/- 0.20 degrees C in dry eye patients compared with 0.41 +/- 0.20 degrees C in the control group; p < 0.001). This parameter was also shown to be greater in dry eye patients who displayed either a fast tear break-up time or a poor Schirmer's test result. Infrared thermography is a non-invasive and objective technique that may prove a useful research tool for study of the tear film, its deficiencies and its various treatment modalities.