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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.28 No.12 pp.685-690
DOI : https://doi.org/10.3740/MRSK.2018.28.12.685

Fabrication and Processing Method of Ophthalmic Hydrogel Tinted Lens Containing Indium Tin Oxide-Composited Materials

Min-Jae Lee1,Kyung-Mun Lee2,A-Young Sung1†
1Department of Optometry & Vision Science, Catholic University of Daegu, Gyeongbuk 38430, Republic of Korea
2Devision of Research & Development, Vision Science Co., Ltd, Daegu 41059, Republic of Korea
Corresponding author
E-Mail : say123sg@hanmail.net (A. Y Sung, Catholic Univ. of Daegu)
September 27, 2018 October 18, 2018 November 15, 2018

Abstract


In this study, a multifunctional ophthalmic lens material with an electromagnetic shielding effect, high oxygen permeability, and high water content is tested, and its applicability is evaluated. Metal oxide nanoparticles are applied to the ophthalmic lens material for vision correction to shield harmful electromagnetic waves; the pyridine group is used to improve the antibacterial effect; and silicone substituted with urethane and acrylate is employed to increase the oxygen permeability and water content. In addition, multifunctional tinted ophthalmic lens materials are studied using lens materials with an excellent antibacterial effect (2,6-difluoropyridine, 2-fluoro-4-pyridinecarboxylic acid) and functional (UV protection, high wettability) lens materials (2,4-dihydroxy benzophenone, 2-hydroxy-4-(methacryloyloxy)benzophenone). To solve problems such as air bubbles generated during the polymerization process for the manufacturing and turbidity of the lens surface, polymerization conditions in which the defect rate is minimized are determined. The results show that the polymerization temperature and time are most appropriate when they are 110 °C and 40 minutes, respectively. The optimum injection amount of the polymerization solution is 350 ms. The turbid phenomenon that appears in lens processing is improved by 10 to 95% according to the test time and conditions.



초록


    Ministry of SMEs and Startups

    © Materials Research Society of Korea. All rights reserved.

    This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    1. Introduction

    Recently, hydrogel lenses are used mainly for cosmetic reasons. In particular, as color soft lenses are colored with hydrophobic pigments on the surface, they may have adverse effects on the health of the eyes by inducing a feeling of dryness, thereby resulting in the congestion of the eyes, keratitis, and conjunctivitis when worn for a long time.1-4) Among the physical properties that hydrogel lenses should have, wettability plays an important role in eye comfort, in the physiological adaptation of the eyes, and in tear layer maintenance when lenses are worn.5-6) In addition, electromagnetic waves cause eye fatigue and redness as well as lower working efficiency and mental fatigue, and may cause a change in the brain if severe.7-11) Ophthalmic materials with an electromagnetic shielding function have optical properties that are transparent in the visible light region and that reflect in the infrared light region; thus, they are sometimes used as coatings for building windows, etc..12) Pyridine, which is widely used in various fields, such as in biosensor materials, microfiltration membranes, and ion exchange resins, has also been applied as a material for medical polymers because of its antibacterial properties.13-14)

    Especially, an excellent antibacterial effect was shown when it was used as an additive for ophthalmic hydrogel lenses.15) In addition, it can be effectively applied to the hydrogel lenses that are currently being developed because it is widely used as base dissolving agents and as synthesis materials for dyes, medicines, and the like. Due to the technology development and the diversity of the materials, the diversity of the ophthalmic lens development processes has been widening. Depending on the development process, there is a certain ratio of defects in the process of mass production while contact lenses go through various processes. Defective lenses may cause not only a decrease in the production rate but also a feeling of discomfort and foreign-body sensation when they are worn. Therefore, many studies have been conducted to minimize the defect rate through process improvement and stabilization.16-17) Moreover, the number of people using hydrogel lenses is increasing, and as such, the safety of contact lenses must be secured for commercialization.18) Hydrogel lenses are classified as medical devices, and the Ministry of Food and Drug Safety(MFDS) has set guidelines for the stability and safety evaluation of medical devices to specify the standard specifications of contact lenses.19) This study aimed to fabricate functional ophthalmic lenses with a harmful-wave-shielding feature, and to optimize the ophthalmic lens fabrication process. Additionally, an experiment to improve the surface turbidity at the time of lens production was conducted.

    2. Experimental

    2.1 Fabrication and Polymerization Process

    Silicone monomers synthesized to fabricate silicone hydrogel ophthalmic lenses including Silicone, DMA (N,N-dimethylacrylamide), and HPMA(hydroxypropyl methacrylate) and additives including VP(vinyl pyrrolidone), 2-4P(2-fluoro-4-pyridinecarboxylic acid), 2H4M(2-hydroxy- 4-(methacryloyloxy)benzophenone), and ITO(indium tin oxide) were used. Also, a crosslinking agent, EGDMA (ethyleneglycol dimethacrylate), and an initiator, AIBN (azobisisobutyronitrile), were used for polymerization with the cast mold method. The structure of synthesized silicone monomer and final mixing ratio of the monomers and additives that were used was determined and are shown in Fig. 1 and Table 1. Additionally, the optimal injection amount of polymerization solution, the polymerization time, the polymerization temperature of the lens were analyzed.

    2.2 Improvement of turbidity

    In the experiment with the PBS(phosphate buffer saline) solution, the lens turbidity removal effect was observed according to the stirring time. PVA(Mw = 31,000~50,000) and PVA group for Mowiol(Mw = 67,000) were added to the PBS solution for each concentration, and the turbidity of the lens were analyzed.

    3. Results and Discussion

    3.1 Fabrication and Polymerization Process

    3.1.1 Fabrication and Optimization of polymerization conditions

    For the fabrication of the samples to be used in the experiment, the mixing ratios of monomers were variously set for testing purposes. For the result of the polymerization that was conducted after determining the final sample combinations, the degree of polymerization varied according to the polymerization temperature of the polymer, the polymerization time, and the polymerization solution injection amount. The cases of low polymerization, such as the cases where polymerization defects occur(in which the shape is not normal), where optically opaque turbidity appears, and where round deformity occur(in which pores are generated on the surface), were classified as “defects.” To polymerize the ophthalmic lens using a synthesized silicon material, a high temperature of 120 °C or higher and a polymerization time of 1 hour or more are generally required. To find the optimum polymerization conditions, polymerization was conducted based on a 120 °C polymerization temperature and 1 hour polymerization time, and as a result of the experiment by changing the conditions variously, the optimum conditions for polymerization were determined to be 110 °C and 40 minutes. When the lens was polymerized for a long time at low temperatures, the optical transparency of the center portion was decreased due to the low polymerization state of the lens. When the polymerization was carried out at a high temperature of 120 °C or higher for a long time, a round defect occurred. Table 2 shows polymerization results according to the polymerization temperature and time and the lens shape according to each state. Physical property, oxygen permeability and electromagnetic shielding effect of fabricated sample shown in Table 2 and 3, Fig. 2 and 3.20)

    3.1.2 Optimization according to the Injection Amount of the Polymerization Solution

    Experiments were carried out by varying the solution injection amount within the 450-330 ms range for lens fabrication. The amount of injection was measured by the injection time and represented by the injection time in millisecond(ms). The defect rate gradually decreased from 450 to 360 ms, but it was very high overall. When the injection amount was 350 ms, the defect rate was very low compared with that with the other injection amounts. The defect rate sharply increased again from 340 ms of injection amount. Therefore, the 350 ms polymerization solution injection amount is considered optimal. The defect rates according to the polymerization solution injection amount are shown in Fig. 4.

    3.1.3 Improvement of turbidity after lens hydration

    The surface of the lens appeared to be cloudy in the process of hydration after the lens was polymerized with silicon addition, and after its separation from the mold. It was more clearly seen when it was observed by scattering strong light and then named “turbidity”. Table 4 shows the phenomenon of turbid that occurred according to the type and condition of the lens.

    After hydration, the turbidity was removed by eliminating the substances that were suspected of being mold extracts attached to the inner surface. The principle that the turbidity can be removed through natural rubbing or friction was applied. That is, 300 ml PBS was added to the glass beaker, followed by stirring at about 800 RPM using a magnetic bar. This is a method of preventing the adhesion of the lenses to each other while being hydrated. It was considered that as the amount of friction between the lenses increased according to the stirring time, the lens turbidity removal effect would increase. Thus, lenses were sampled at regular time intervals, and were observed. Table 5 shows the turbidity removal rate results according to the stirring time.

    In the experiment with the PBS solution, the lens turbidity removal effect was observed according to the stirring time, but the problem of lens adhesion still occurred because the PBS solution itself does not have its own lubrication and moisturizing properties. Therefore, to increase the performance efficiency of the stirred solution, a PVA and Mowiol with good moisturizing, wettability, and lubrication properties were added to the PBS solution for each concentration, and the effect was observed. Additionally, a kind of surfactant, Poloxamer 407 1 %, was added. PVA attracts numerous water molecules with polarity due to the numerous -OH functional groups present at the terminal, and has moisturizing and wettability properties, which are harmless to the human body. The basic structure of PVA and Poloxamer 407 are shown in Fig. 5.

    Table 6 shows the turbidity removal results of the stirred solution. As shown in the photographs, the turbidity removal effect of the Mowiol significantly diffed over time. At 30-minute stirring, more than 30 % of the turbidity was removed; at 45-minute stirring, more than 50 %; and at 60-minute stirring, almost 100 %. There was a difference in the degree, however, revealing a very clear state. Additionally, when the Mowiol concentration was reduced by half in the stirred solution at the observation test on turbidity removal, the effect was reduced to half compared to the stirred solution with the standard amount added. Based on the results of the experiment, it was determined that the factor affecting the turbidity removal performance was the concentration of the Mowiol added. Considering that the Mowiol and PVA are similar substances, similar results can be predicted for PVA. As the Mowiol, however, has a greater molecular weight and is heavier than PVA due to the difference in unit molecular structure length between the two, it is thought to affect the higher turbidity removal effect than PVA.

    Table 7 shows the state of turbidity removal by the PVA solution. Compared to the above results, there was a minor difference, but a 30-40 % turbidity removal rate was observed in the photograph of the lens sample that was stirred for 30 minutes. In the photograph of the lens sample that was stirred for 60 minutes, a 90-100 % turbidity removal rate was observed. Therefore, the difference in properties between PVA and the Mowiol due to the basic molecular structures of the two was found to be not that great. If there is a difference between PVA and the Mowiol, it is that the viscosity of the Mowiol is slightly higher, which is considered due to the difference in the molecular weight. The reason that the Mowiol and PVA have such a turbidity removal effect seems to be that the long carbon chain with -OH groups attached thereto causes a similar effect as a surfactant with polar and non-polar properties. It is judged that the structure similar to the surfactant dissolves and removes the turbidity-causing material expected to be extracted from the mold by exerting a similar effect as a surfactant. The extractable material that can be from the mold is thought to be the oil that comes out during plastic molding. In addition, it was found that the turbidity removal effect in the stirred solution in which the amount of PVA was reduced by half decreased compared to the stirred solution with the standard amount of PVA. Its turbidity removal rate, however, was slightly higher than that of the stirred solution in which the Mowiol was added by half. In PVA, as in the Mowiol, it is considered that the factor affecting turbidity removal is the concentration of PVA contained by the stirred solution, but the ability of PVA to maintain its turbidity removal performance is somewhat higher at lower concentrations than the Mowiol. The reason for this is that PVA has a smaller molecular weight than the Mowiol, and as such, more particles are present in the stirred solution even at the same weight.

    4. Conclusion

    In this study, multifunctional tinted ophthalmic lenses were fabricated using a lens material with high oxygen permeability and a high water content and electrowaveshielding function. The optimum polymerization temperature and time for the contact lens fabrication were found to be 110 oC and 40 minutes, respectively, and it was confirmed that the optimum polymerization solution injection amount for reducing the defect rate was 350 ms. In addition, turbidity occurred on the surface of the lens after the lens’s hydration, and a method of addressing this was experimented on. Moreover, it is considered that the fabricated lens can be used as a multifunctional contact lens with high oxygen permeability and a high water content by shielding harmful waves through the application of ITO nanoparticles, improving the antibacterial effect through the application of the pyridine system, and applying the silicon substituted with urethane and acrylate.

    Acknowledgement

    This research was financially supported by the Ministry of SMEs and Startups(MSS), Korea, under the “Regional Specialized Industry Development Program(R&D, R000- 6451)” supervised by the Korea Institute for Advancement of Technology(KIAT).

    Figure

    MRSK-28-685_F1.gif

    Structure of synthesized silicone monomer(Sil-D).

    MRSK-28-685_F2.gif

    Probe current and temperature versus time in peripheral zone. [(a) Ref. (b) ITO].

    MRSK-28-685_F3.gif

    Electromagnetic shielding effect of fabricated sample. [(a) Ref. (b) ITO].

    MRSK-28-685_F4.gif

    Defect rates according to the polymerization solution injection amount.

    MRSK-28-685_F5.gif

    Chemical structure of additives. [(a) PVA (b) PLOLXAMER 407].

    Table

    Final mixing ratio of the samples(Unit: wt%).

    Polymerization results according to the polymerization conditions.

    Physical property of fabricated sample.

    Turbidity according to type and condition of the lens.

    Turbidity removal rates according to the stirring time.

    Removal effect of the Mowiol solution.

    Removal effect of the PVA solution.

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