Photon correlation spectroscopy applied to tear analysis

Viscoelastic properties of the human tear film

When considering eye blinking from a tribological perspective, tear viscosity is expected to play a fundamental role. The application of rheological techniques to describe the tear film dates back to the late 1980s, but there has been a continuous need of reappraisal due to new findings in tear film biochemistry and to the development of new methods of physico-chemical characterization. This review provides an overview on tear rheological behavior by analyzing the peer-reviewed literature on this topic. Specifically, examples of in-vitro and in-vivo viscosity measurements are detailed, highlighting experimental criticalities and the need of a standard convention for rheological techniques to compare data across different studies, of analyses on tears of single individuals even within the limits of the low volume available, and of a further development of in-vivo techniques. Then, the controversial role of specific tear components on viscosity is discussed, together with the alterations associated to dry eye disease and contact lens wear. Finally, an updated focus is reported on the viscosity of artificial tears formulations.

Effects of Aqueous-Supplementing Artificial Tears in Wearers of Biweekly Replacement Contact Lenses vs Wearers of Daily Disposable Contact Lenses

PURPOSE

To compare the effects of artificial tears (ATs) in wearers of biweekly replacement silicone hydrogel contact lenses (BW-Ws) and wearers of daily disposable contact lenses (DD-Ws) of the same material.

MATERIALS AND METHODS

The aqueous-supplementing ATs, OPTOyalA and OPTOidro, were assigned to be used for 2 weeks to healthy and young subjects: 1) 20 (8 and 12, respectively) BW-Ws wearing silicone hydrogel somofilcon A CLs (Clariti Elite), 2) 18 (9 and 9, respectively) DD-Ws wearing silicone hydrogel somofilcon A CLs (Clarity 1 Day), and 3) a control group of 33 (16 and 17, respectively) N-Ws. Ocular symptoms and comfort, tear volume and stability, and ocular surface condition were assessed by Ocular Surface Disease Index (OSDI), 5-Item Dry Eye Questionnaire (DEQ5), tear meniscus height (TMH), non-invasive tear break-up time (NIBUT), and evaluation of ocular redness (OR). The assessment was performed before and after 15 days of use of the ATs in the 3 groups (BW-Ws, DD-Ws, and N-Ws).

RESULTS

No clear significant difference was noted in symptoms and signs between OPTOyalA and OPTOidro irrespectively of the group of people studied. ATs use for 15 days produced a significant improvement in DEQ5 and OR in DD-Ws (Δ=-34%, p=0.006; Δ=-23%, p<0.001) and in N-Ws (Δ=-21%, p=0.001; Δ=-10%, p=0.006) but not in BW-Ws (Δ=-5%, p=0.072; Δ=-2%, p=0.257). No significant change was noted for TMH.

CONCLUSION

In young and healthy subjects, the aqueous-supplementing effect of the ATs under consideration is more a rinsing and tear replacem ent effect than an increase in tear volume, and it produces an improvement of the eye redness and ocular symptoms. Contact lens wear influenced the effectiveness of ATs in a way which is correlated with the CL replacement schedule.

Zeta potential of tear samples: A tool to explore the effects of wear of contact lenses

PURPOSE

The aim was to develop a method to assess the electrostatic properties of human tear samples, and to evaluate their modifications induced by the wear of contact lenses (CLs).

METHOD

The barrier method was developed for the measurement of the isoelectric point (IEP) on relatively small quantities. The method was applied to compare three groups: tears (T_NW) of non-wearers, tears (T_{W_etaf}) of regular wearers of etafilcon A CLs, and tears (T_{W_omaf}) of regular wearers of omafilcon A CLs. Zeta potential (ζ) as a function of pH was measured by a Zetasizer Nano ZS90 (Malvern Instruments) on 40%-diluted samples, obtained by mixing 57 μL of tears of different subjects of the same group with 85 μL of HCl aqueous solution. IEP was deduced as the pH at which ζ is zero, i.e. the net electric charge on tear constituents being neutralized.

RESULTS

Within an error of about 0.05, IEPs were found to be 2.90 (T_NW), 2.80 (T_{W_omaf}), and 3.16 (T_{W_etaf}). On average, a lower H⁺ concentration is needed to neutralize the surface charge of the tear components of etafilcon A wearers, compared to both T_NW and T_{W_omaf}.

CONCLUSION

IEP measurements on tear samples of wearers of different types of CLs are proposed in order to enhance the knowledge on the modifications of the profile of charged species in tears. The T_{W_etaf} results, compared to those of the other groups, are compatible with an increase, due to the wear of etafilcon A CLs, of the relative concentration of high-IEP proteins.

Differences between tears of contact lens wearers studied by photon correlation spectroscopy

PURPOSE

The purpose was to evaluate if there are differences between tears of contact lens (CL) wearers of different materials detectable by measuring the hydrodynamic diameter of tear components through photon correlation spectroscopy (PCS).

METHODS

Tears of 59 CL wearers and tears of 39 non-wearers were collected by glass capillary. Wearers were divided into groups depending on the CL material: (i) hydrogels of II FDA group (H-II, 15 subjects), (ii) hydrogels of IV FDA group (H-IV, 13 subjects), (iii) silicone hydrogels (SH, 31 subjects). PCS analyses were performed at 25 °C on samples diluted with deionized water with tear concentration (10 ± 2)% ^V/_W to obtain, for each subject, the average hydrodynamic diameter (d_H,avg) of tear components by analyzing intensity fluctuations in time of scattered light.

RESULTS

Means of d_H,avg calculated on each group were found, on increasing order, to be 256 nm (std dev 18 nm) for non-wearers, 297 nm (std dev 45 nm) for H-II, 360 nm (std dev 76 nm) for SH, and 391 nm (std dev 85 nm) for H-IV with statistical differences between each group of wearers compared to non-wearers and between groups of wearers except between SH and H-IV.

CONCLUSIONS

PCS reveals the differences between tears of CL wearers of different materials, not only between tears of wearers and non-wearers.