Measurement of oxygen flux into the cornea by pressing a sensor onto a soft contact lens on the eye

Pharmaceutical-loaded contact lenses as an ocular drug delivery system: A review of critical lens characterization methodologies with reference to ISO standards

Therapeutic contact lenses for ocular drug delivery have received considerable interest as they can potentially enhance ocular bioavailability, increase patient compliance, and reduce side effects. Along with the successful in vitro and in vivo studies on sustained drug delivery through contact lenses, lens critical properties such as water content, optical transparency and modulus have also been investigated. Aside from issues such as drug stability or burst release, the potential for the commercialization of pharmaceutical-loaded lenses can be limited by the alteration of lens physical and chemical properties upon the incorporation of therapeutic or non-therapeutic components. This review outlines advances in the use of pharmaceutical-loaded contact lenses and their relevant characterization methodologies as a potential ocular drug delivery system from 2010 to 2020, while summarizing current gaps and challenges in this field. A key reference point for this review is the relevant ISO standards on contact lenses, relating to the associated characterization methodologies. The content of this review is categorized based on the chemical, physical and mechanical properties of the loaded lens with the shortcomings of such analytical technologies examined.

Oxygen permeability of the pigmented material used in cosmetic daily disposable contact lenses

Purpose

To evaluate the individual contributions of pigment colorant and packing solution containing polyvinyl pyrrolidone (PVP) on the oxygen permeability (Dk) of a cosmetic printed etafilcon A daily disposable contact lens packaged with PVP.

Method

The oxygen transport of a contact lens is evaluated through the central optical zone of the lens. Cosmetic printed contact lenses contain pigment colorant in the periphery or mid-periphery of the lens. Therefore, to assess the impact of cosmetic print on oxygen permeability, special lenses need to be produced that contain the colorant within the central optical zone. This technique was used to obtain multiple measurements of nonedge-corrected Dk/t of both the center pigmented lens and its nonpigmented equivalent, using a polarographic measurement described in International Organization for Standardization (ISO) 18369-4:2006(E), and the Dk derived for each measurement is corrected for edge effect. In addition, the edge-corrected Dk values of lenses made from the same monomer batch were measured. The lenses were packaged and autoclaved with and without proprietary technology which embeds PVP in the contact lens during autoclaving. The resulting Dk value of the printed lens material was then used with thickness data to generate true Dk/t profiles for a given lens power.

Results

The edge-corrected Dk of the printed etafilcon A lens with offset pigment colorant was measured to be 19.7×10⁻¹¹ (cm²/s) (mL O₂/mL·mmHg) at 35°C. This was within ±20% tolerance range as specified in ISO 18369-2:2012(E) for the edge-corrected Dk of the nonpigmented etafilcon A control lens evaluated during the same session, 19.5×10⁻¹¹ (cm²/s) (mL O₂/mL·mmHg). The edge-corrected Dk values of the lenses packaged with PVP (mean 20.1, standard deviation [SD] 0.3) were also within the ±20% tolerance range compared to those packaged without PVP (mean 20.0, SD 0.3).

Conclusion

The pigment colorant and PVP embedded in the contact lens during autoclaving were not found to influence the oxygen permeability of the etafilcon A material.

Factors influencing the measurement of oxygen shortfall of the human cornea: Sequencing of test conditions

PURPOSE

The effects of sequencing of test conditions, in this case contact lens thicknesses, on the measurement of the oxygen shortfall of human corneas were studied.

METHODS

Corneal oxygen uptake rates were measured with a Clark-type polarographic electrode on the central, unanesthetized right corneas of 14 human subjects. Measurements were made under the following conditions: (1) the normal open eye; (2) after 5 min of static (without blinking) wear of each of seven rigid gas permeable lenses of seven center thicknesses (0.18, 0.12, 0.16, 0.20, 0.24, 0.28, and 0.32 mm); (3) after 5 min of static wear of a polymethylmethacrylate (PMMA) contact lens. Lens thicknesses were randomly assigned numbers, which were sequenced in seven cycles. Two subjects were assigned to each sequencing cycle, and each subject participated in two identical sessions.

RESULTS

The interaction of order x thickness was determined to be insignificant (F=0.99; p=0.5101). The effect of lens order was also insignificant (F=0.76; p=0.6239), indicating that the order of lens placement did not affect the measured corneal oxygen shortfall. Not surprisingly, the analysis indicated a significant effect of lens thickness on corneal oxygen shortfall (F=3.94; p=0.0032).

CONCLUSIONS

The sequencing of lenses of various thicknesses on the cornea does not affect the measurement of corneal oxygen shortfall.

Corneal oxygen uptake: A review of polarographic techniques, applications, and variables

Factors that influence the polarographic measurement of the oxygen uptake of the cornea are reviewed. These factors include the technique, electrode assembly, oxygen reservoir, membrane material and thickness, oxygen tension of the corneal environment, duration of exposure to environmental conditions and time to application of the probe all influencing measured oxygen uptake rates. Subject factors include lid position, palpebral aperture size, blinking, corneal thickness, and corneal integrity. Contact lens wear influences corneal oxygen uptake, with lens material and design parameters influencing rates obtained both under static (without blinking) and dynamic (with blinking) conditions. Measurement of corneal oxygen uptake rates remains an excellent method to quantify the oxygen supply in contact lens systems that include the contact lens, the tears, and the cornea, in which oxygen flux is influenced by the thickness and diffusion characteristics of each component.

A noninvasive optical method to measure oxygen tension at the corneal epithelium

Corneal redox fluorometry is presented as an alternative method to noninvasively measure the oxygen tension in equilibrium with the tear film. The application of this method to the study of the effects of a contact lens on the tear film oxygen tension is described. The development of a clinical corneal redox fluorometer would permit noninvasive measurements of the effects of contact lens induced corneal hypoxia on corneal physiology. These studies could investigate the effects of corneal hypoxia at the cellular level and result in a new level of understanding of the processes involved.

Clinical aspects of the measurement of oxygen flux into the cornea

A comparison was firstly done of the oxygen flux through the Bausch and Lomb Plano T lens, the Bausch & Lomb U3 lens, and the Duragel 75 lens with a sensor on the normal eye. The results showed that the Bausch and Lomb Plano T lens was the best for use with the sensor probe. The sensor was then used to obtain a comparison with the normal oxygen flux through a therapeutic lens on keratoplastic eyes and eyes with bullous keratopathy. The results showed that both conditions had a lower oxygen consumption than normal and that eyes with bullous keratopathy predictably had the lowest value.