Contact Lens Materials and Their Modification Methods

Silicone hydrogels are pivotal materials used in contact lenses, biomedicine, and electronic devices, where oxygen permeability and equilibrium water content (EWC) are essential attributes. Generally, an increase in silicone content boosts oxygen permeability but diminishes EWC, presenting a notable trade‐off challenge. In this study, a solvent incorporation method that significantly elevates oxygen permeability without markedly affecting EWC is introduced. Among various solvents, isopropanol incorporation notably enhances oxygen permeability (101 barrer) while maintaining an EWC of 56.0%. This improvement is attributed to enhanced connectivity within the silicone network. Additionally, it is discovered that incorporating siloxane nanoparticles (SNPs) further increases the connectivity of the silicone network, offering a more pronounced improvement in oxygen permeability than solvent alone. However, excessive SNP addition can lead to large phase separation, resulting in decreased transmittance, lower EWC, and increased mechanical strength. Optimizing a transmittance of over 99%, SNP addition achieves a silicone hydrogel with an oxygen permeability of 125 barrer and an EWC of 49.5%, surpassing many advancements reported in the literature.

Influence of water states in hydrogels on the transmissibility and permeability of oxygen in contact lens materials

In this study, soft contact lenses were prepared by the polymerization of N,N-dimethylacrylamide (DMA), 1-vinyl-2-pyrrolidinone (NVP), and silicone nanoparticles (SiNPs). The physical and biochemical characterizations of all samples included scanning electron microscope (SEM), equilibrium water content (EWC), oxygen permeability (Dk), optical transparency, contact angle, mechanical properties, protein deposition, and cytotoxicity. The results indicated that the addition of SiNPs increased the Dk without affecting the EWC of the lenses. With 1.2 wt% of SiNPs, the Dk of p (DMA-co-NVP)-SiNPs was 94.8 barrers while its EWC remained at 85%. In particular, DNS12 exhibited a contact angle around 40°. In addition, protein deposition primarily depended on the negatively charged SiNPs. Higher SiNPs content led to lower human serum albumin (HSA) and higher lysozyme deposition on all samples. Moreover, L929 fibroblasts grew on all hydrogel samples, suggesting non-cytotoxicity. In general, the incorporation of SiNPs with hydrogels significantly increased the Dk without affecting much the optical transparency and moluli. These results may be applicable to develop novel ophthalmic biomaterials for soft contact lens.

ABSTRACTWithin a few minutes of wear, contact lenses become rapidly coated with a variety of tear film components, including proteins, lipids, and mucins. Tears have a rich and complex composition, allowing a wide range of interactions and competitive processes, with the first event observed at the interface between a contact lens and tear fluid being protein adsorption. Protein adsorption on hydrogel contact lenses is a complex process involving a variety of factors relating to both the protein in question and the lens material. Among tear proteins, lysozyme is a major protein that has both antibacterial and anti-inflammatory functions. Contact lens materials that have high ionicity and high water content have an increased affinity to accumulate lysozyme during wear, when compared with other soft lens materials, notably silicone hydrogel lenses. This review provides an overview of tear film proteins, with a specific focus on lysozyme, and examines various factors that influence protein deposition on contact lenses. In addition, the impact of lysozyme deposition on various ocular physiological responses and bacterial adhesion to lenses and the interaction of lysozyme with other tear proteins are reviewed. This comprehensive review suggests that deposition of lysozyme on contact lens materials may provide a number of beneficial effects during contact lens wear.

Hydrogels have peculiar physical and chemical properties and therefore, are used in a variety of biomedical applications including drug delivery agents, prosthetic devices, the repair and replacement of soft tissues, contact lenses, etc. Consequently, investigation of mechanical, physical and chemical properties is crucial in biomedical application of hydrogels. Poly (2-hydroxyethyl methacrylate) (pHEMA), as a biocompatible hydrogel, was first hydrogel used for making soft contact lenses. Many researches have been modified pHEMA with the aim of improving its properties. Application of nanotechnology is one of the possible solutions for improving the characteristics of this biocompatible hydrogel. In this paper, polyhydroxyethyl methacrylate was used as standard material for soft contact lenses (SL 38). This material was incorporated with fullerene C60 (SL38-A), fullerol C60(OH)24 (SL 38-B) and fullerene metformin hydroxylate C60(OH)12(OC4N5H10)12 (SL 38-C), respectively. Three new nanophotonic soft contact lenses were made. The main goal of this research was to develop appropriate process parameters for soft contact lens micro-turning. Also, studying the thermal decomposition of standard soft contact lens, pHEMA, as well as three new nanophotonic soft contact lenses was one of the main objectives. Results have shown that manufacturing process of nanofotonic soft contact lens is considered to be a micro-turning process regarding the cutting depth and tool nose ratio. Thermal stability of all three nanofotonic soft contact lenses was significantly improved comparing to the standard soft contact lens. Still, further research needs to be done so these nonophotonic soft contact lenses could find practical application in the field of biomedical engineering.

The purpose of this study was to examine the in vitro dehydration characteristics of new and worn conventional and silicone-hydrogel contact lenses. Four contact lens materials were investigated: three conventional hydrogels (etafilcon, nelfilcon, omafilcon) and one silicone-hydrogel (narafilcon). Gravimetric data were obtained with analytical balance with 1 min intervals. Quantitative parameters of water content and dehydration rate were calculated allowing quantitative description of dehydration process. Differential scanning calorimetry was used to monitor changes in water states in samples studied. As expected, dehydration behavior of each material is different in terms of mean dehydration rate values and phases of dehydration. Gravimetric data allowed us to distinguish three phases of dehydration. Interestingly, the effect of the osmolality of storing solutions on dehydration was found – lenses stored in hyperosmotic solutions needed more time to achieve equilibrium with the environment. Effect of wearing on dehydration patterns and water properties was confirmed. In worn samples, a decrease in water content was observed. Additionally, there was a change in water structure after 6 h of wearing in all lenses studied. This behavior may be ascribed to tear film components deposition and changes in surface wettability that appear during wearing.

Oxygen permeability, diffusion coefficient of the sodium ions and water flux and permeability in different conventional hydrogel (Hy) and silicone-hydrogel (Si-Hy) contact lenses have been measured experimentally. The results showed that oxygen permeability and transmissibility requirements of the lens have been addressed through the use of siloxane containing hydrogels. In general, oxygen and sodium chloride permeability values increased with the water content of the lens but there was a percolation phenomenon from a given value of water uptake mainly in the Si-Hy lenses which appeared to be related with the differences between free water and bound water contents. The increase of ion permeability with water content did not follow a unique trend indicating a possible dependence of the chemical structure of the polymer and character ionic and non-ionic of the lens. Indeed, the salt permeability values for silicone hydrogel contact lenses were one order of magnitude below those of conventional hydrogel contact lenses, which can be explained by a diffusion of sodium ions occurring only through the hydrophilic channels. The increase of the ionic permeability in Si-Hy materials may be due to the confinement of ions in nanoscale water channels involving possible decreased degrees of freedom for diffusion of both water and ions. In general, ionic lenses presented values of ionic permeability and diffusivity higher than most non-ionic lenses. The tortuosity of the ionic lenses is lower than the non-ionic Si-Hy lenses. Frequency 55 and PureVision exhibited the highest water permeability and flux values and, these parameters were greater for ionic Si-Hy lenses than for ionic conventional hydrogel lenses. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 2218-2231, 2017.

The goal of this study was to analyze how various incubation times affect the uptake of cholesterol on silicone hydrogel (SH) and conventional hydrogel (CH) daily disposable (DD) contact lens materials using an in vitro radiochemical detection method. Three SH (somofilcon A, delefilcon A, and narafilcon A) and four CH (etafilcon A, nesofilcon A, ocufilcon A, and nelfilcon A) contact lenses were incubated in an artificial tear solution that contained major tear film components and a portion of radioactive C-cholesterol. Lenses (N = 4) were incubated for four incubation times (2, 6, 12, or 16 h) to assess the effects on cholesterol deposition. Subsequent to the incubation, the lenses were extracted using 2:1 chloroform:methanol, and the extracts were analyzed in a beta counter and (in nanograms per lens) extrapolated from standard curves. In general, cholesterol deposited statistically significantly more on SH lenses than CHs (p ≤ 0.033), with the exception of somofilcon A and nesolfilcon A materials (p = 0.067). Within the SH materials, narafilcon A accumulated the largest quantity of cholesterol (p < 0.05) and somofilcon A the lowest (p < 0.05). The uptake of cholesterol ranged from 22.63 ± 2.98 ng/lens to 97.94 ± 4.18 ng/lens for all lens materials. The accumulation of cholesterol was shown to be continuous throughout the 16 h of incubation, without reaching a plateau (p < 0.001). For the periods that DD lens materials are worn, cholesterol deposits significantly more onto SH contact lenses than CHs. This could have implications for wearers who have higher levels of lipid in their tears that are fitted with SH DD materials.

Since the initial launch of silicone hydrogel lenses, there has been a considerable broadening in the range of available commercial material properties. The very mobile silicon-oxygen bonds convey distinctive surface and mechanical properties on silicone hydrogels, in which advantages of enhanced oxygen permeability, reduced protein deposition, and modest frictional interaction are balanced by increased lipid and elastic response. There are now some 15 silicone hydrogel material variants available to practitioners; arguably, the changes that have taken place have been strongly influenced by feedback based on clinical experience. Water content is one of the most influential properties, and the decade has seen a progressive rise from lotrafilcon-A (24%) to efrofilcon-A (74%). Moduli have decreased over the same period from 1.4 to 0.3 MPa, but not solely as a result of changes in water content. Surface properties do not correlate directly with water content, and ingenious approaches have been used to achieve desirable improvements (e.g., greater lubricity and lower contact angle hysteresis). This is demonstrated by comparing the hysteresis value of the earliest (lotrafilcon-A, >40°) and most recent (delefilcon-A, <10°) coated silicone hydrogels. Although wettability is important, it is not of itself a good predictor of ocular response because this involves a much wider range of physicochemical and biochemical factors. The interference of the lens with ocular dynamics is complex leading separately to tissue-material interactions involving anterior and posterior lens surfaces. The biochemical consequences of these interactions may hold the key to a greater understanding of ocular incompatibility and end of day discomfort.

The main weakness of non-silicone hydrogel contact lenses is their low oxygen permeability (Dk). Hence, we have tried to optimize their Dk using various concentrations and lengths of the poly (ethylene glycol) dimethacrylate crosslinker in a mixture of N,N-Dimethylacrylamide and Cyclohexyl methacrylate monomers. After synthesizing the different contact lenses, we evaluated their chemical, optical, and mechanical properties. The resultant non-silicone hydrogel contact lenses presented similar high water contents (75.69–80.60%) and adequate optical (e.g., a transmittance ranging from 85.91% to 99.91% and a refractive index between 1.3630 and 1.3740) and elongation at break (178.95–356.05%) characteristics for clinical applications. Conversely, they presented high contact angles (81.00–100.00°) and a low Young’s modulus (0.066–0.167 MPa). Regarding the impact of the crosslinking modifications, the water content, contact angle, refractive index, transmittance, and Young’s modulus of the synthesized lenses were slightly affected by crosslinker conditions. In contrast, the elongation at break (178.95–356.05%) and, more importantly, the oxygen permeability, which reached values of up to 73.90 Fatt units, were considerably impacted by the crosslinker conditions. To our knowledge, this study demonstrates for the first time that, in addition to water, other usual hydrogel components, like crosslinkers, can modulate the Dk of non-silicone contact lenses. It also provides a simple and scalable method to fabricate more permeable non-silicone lenses.

Improvement of the surface wettability of silicone hydrogel contact lenses via layer-by-layer self-assembly technique

Applanation tonometry in silicone hydrogel contact lens wearers

More is demanded from ophthalmic treatments using contact lenses, which are currently used by over 125 million people around the world. Improving the material of contact lenses (CLs) is a now rapidly evolving discipline. These materials are developing alongside the advances made in related biomaterials for applications such as drug delivery. Contact lens materials are typically based on polymer- or silicone-hydrogel, with additional manufacturing technologies employed to produce the final lens. These processes are simply not enough to meet the increasing demands from CLs and the ever-increasing number of contact lens (CL) users. This review provides an advanced perspective on contact lens materials, with an emphasis on materials science employed in developing new CLs. The future trends for CL materials are to graft, incapsulate, or modify the classic CL material structure to provide new or improved functionality. In this paper, we discuss some of the fundamental material properties, present an outlook from related emerging biomaterials, and provide viewpoints of precision manufacturing in CL development.

To evaluate the mechanical properties of commonly available soft contact lens materials and compare results using custom-built MicroTensometer. The Young modulus, parameters for stress relaxation, and toughness of 18 types of single vision soft contact lenses were measured using custom-built MicroTensometer. Five lenses of each type were soaked in standard phosphate buffered saline and measured at a temperature of 35°C. Each lens was flattened and sliced into a rectangular strip sample using two parallel blades. The Acuvue Moist 1-Day and SofLens Daily lenses measured lowest moduli, whereas Air Optix Night & Day Aqua and Premio measured the highest. The measured moduli for silicone hydrogel materials were generally higher compared with the hydrogels except for Dailies AquaComfort Plus. The exponential curve fitted over the decay in stress showed a consistent time constant of approximately 10 sec for most lens types measured. However, the amplitude constant varied from 2.84% for SofLens Daily to 22.39% for Acuvue TruEye 1-Day. The toughness results showed that Dailies AquaComfort Plus is strong but not necessarily tough. The mechanical properties of commonly prescribed soft contact lens materials were measured using a dedicated instrument. Its reliability was demonstrated, and modulus results were compared against published data from manufacturers and other research groups. Agreement was generally good, with only a few exceptions exceeding 15% difference. The more recently released silicone hydrogel lens types have reduced modulus, approaching that of medium or high water content hydrogel materials.

Background: Rigid gas permeable (RGP) and silicone hydrogel (SH) contact lenses with specific designs are currently being used to improve visual function in patients with keratoconus. However, there are minimal data available comparing the effects of these lenses on visual function in patients with keratoconus. The objectives of this study were to compare visual acuity and contrast sensitivity using spectacles, RGP lenses, and SH lenses, and to evaluate the effects of RGP and SH lenses on higher-order aberrations and visual quality in eyes with keratoconus. The relationship between visual outcomes, aberration, and visual quality were also examined. Methods: This was a pilot study involving 13 eyes from nine subjects with keratoconus. Subjects were fitted with RGP and SH contact lenses. Visual acuity and contrast sensitivity were measured using Snellen and Pelli-Robson charts, respectively. Ocular aberrations and visual quality were measured using an OPD-Scan II device. All measurements were conducted before and after contact lens wear. Results: Significantly better visual acuity was obtained with RGP lenses than with spectacles or SH lenses ( P <0.001). No significant difference in contrast sensitivity values was detected between RGP and SH lenses ( P =0.06). Both SH and RGP lenses significantly reduced total ocular and higher-order aberrations ( P <0.001) when compared with spectacles, but RGP lenses reduced trefoil, coma, and spherical aberrations more than SH lenses. No significant difference in astigmatic aberrations was found between RGP and SH lenses ( P =0.12). Negative correlations were found between visual acuity and coma aberration and contrast sensitivity with higher-order aberrations and coma, trefoil, and astigmatic aberrations. Regarding visual quality, no significant difference was found between RGP and SH lenses, but RGP significantly improved visual quality when compared with spectacles ( P =0.04). Conclusion: RGP lenses provided better visual acuity and greater reduction of aberrations than spectacles or SH lenses in patients with keratoconus. However, more data are needed to confirm these findings. Keywords: visual acuity, contrast sensitivity, aberration, visual quality, keratoconus