Ⅰ. Introduction
As the amount of smart electronic devices and their usage time are increasing, many people complain about discomfort in vision due to refractive errors, and the number of people using contact lenses instead of glasses for convenience, improvement of vision, and cosmetics is increasing.1,2) However, prolonged wear or careless use and misuse of contact lenses can lead to side effects such as dry eye, irritation, and redness.3-5) These eye diseases are caused by hypoxia resulting from reduced oxygen supply to the cornea when contact lenses are worn. To solve this problems, silicone materials are used the most, which is known to have high oxygen permeability.6-8) The silicone contact lens that maximized oxygen permeability was first developed in the U.S. in 1960 and the rigid gas permeable (RGP) contact lens was developed later. However, although the RGP contact lens has excellent oxygen permeability, its wearing comfort is poor due to low water content and wettability.9) Recently, various silicone hydrogel contact lenses composed of silicone monomer and 2-hydroxyethyl methacrylate (HEMA) have been developed, but silicone has very low compatibility with hydrogel. In particular, silicone has the disadvantage of low wettability and poor wearing comfort due to its hydrophobic property, although its oxygen permeability is good because it contains combinations of silicone and oxygen. Furthermore, using silicone alone as a material for contact lens has limitations in properties such as compatibility, high water content and high wettability. To complement these disadvantages, many studies are being conducted using various additives to improve the disadvantages of silicone while maintaining its advantages as much as possible. In addition to hydrogel, synthesizing silicone using hydrophilic monomers and using it as a material for contact lenses is also being researched.10-12)
Styrene, which is an aromatic hydrocarbon that has a structure of benzene combined with a vinyl group, is known to have excellent absorbance and a high strength. Styrene is used as a material for various contact lenses because it is known to improve refractive index, tensile strength and durability, although it is limited as a material for hydrophilic contact lens because adding styrene reduces moisture content and wettability.13,14)
Therefore, this study used a synthetic silicone monomer produced by combining a hydrophilic monomer with silicon, as a contact lens material to produce a functional silicone contact lens with high oxygen permeability and hydrophilicity. In addition, Isobornyl Methacrylate, known as an excellent reactive solvent, was added to improve the compatibility of silicone and hydrophilic monomers.15) Furthermore, the durability of the silicone lens was improved using 2-(trifluoromethyl)styrene, which consists of fluorine compounds with high electronegativity.
Ⅱ. Materials and methods
1. Reagents and materials
A silicone monomer was used as a main material for the silicone contact lens in this study. Silicone hydrogel contact lens was produced by copolymerizing silicone monomer with N,N-dimethylacrylamide (DMA), a hydrophilic monomer, ethylene glycol dimethacrylate (EGDMA), a cross-linking agent, and azobisisobutyronitrile (AIBN), an initiator. In addition, isobornyl methacrylate and 2-(trifluoromethyl) styrene were used separately as an additive. The chemical structure of the silicone and additive are shown in Fig. 1.
2. Polymerization
The lens produced by mixing the silicone monomer with DMA and adding the cross-linking agent EGDMA was named Ref1. Furthermore, to improve the compatibility of silicone and hydrophilic monomer, IBOMA was used as an additive. The Ref1 produced by adding IBOMA in a ratio of 0.5–10% was named -0.5, I-1, I-3, I-5, I-7 and I-10. The mixing ratios of the fabricated contact lens are listed in Table 1.
In addition, 2-(trifluoromethyl)styrene was used as an additive to improve the durability of the fabricated silicone contact lens. Among the produced samples, I-5 was newly named as Ref2. The samples produced by adding 2-(trifluoromethyl)styrene in a ratio of 1–10% to Ref2 were named 2S-1, 2S-3, 2S-5 and 2S-10. The mixing ratios of the contact lenses used in this experiment are listed in Table 2.
For polymerization, the monomers were mixed and stirred using vortex. The mixed monomers were thermal-polymerized at 150℃ for 2 h using a thermal polymerizer. The samples were fabricated using the casting mold method. Each polymerized contact lens sample was hydrated in a 0.9% NaCl saline solution. Then the optical and physical properties were evaluated by measuring spectral transmittance, refractive index, moisture content, oxygen permeability, contact angle and tensile strength.
3. Instruments and Analysis
1) Measurement and analysis of physical properties
The spectral transmittance of the fabricated lenses was measured for the UV-B, UV-A, and VIS regions using a spectral transmittance meter (Agilent, Cary 60 UV-VIS) and the average of the percentage values was determined. The refractive index was measured for the hydrated lenses by focusing based on the refractive index of water using the ABBE refractometer (ATAGO NAR IT, Japan). The moisture content was determined as a percentage after the weight of the hydrated sample and the weight of the sample dried in a microwave oven were measured using an electronic scale (OHAUS, PAG 214 cm USA) using the gravimetric method. The oxygen permeability was measured inside a thermos-hygrostat that was maintained in constant conditions of 35±0.5℃, which is similar to the temperature of snow and a humidity of 98±1% using the constant temp & humidity chamber for lens (LAB Co., LTD., South Korea). For the thickness of the lens for measuring oxygen permeability, the center thickness of the contact lens was measured using an optical thickness gauge (OTG-137, USA). For the tensile strength, the strength of the fabricated silicone contact lens was measured using AGS-X 20 (Shimadzu, Japan). To enhance the accuracy of this experiment, the physical properties were measured at least five times for each experimental group and represented by average values.
2) Stability evaluation
To evaluate the stability of the fabricated contact lens, the eluate was measured and tested using potassium permanganate reduction test and pH test. To evaluate the stability of pH, the pH difference between the control and experiment groups was determined by checking the hydrogen ion concentration difference and the existence or absence of substances with a potential using a pH tester of Mettler Toledo. The potassium permanganate reduction test checks the existence of organic or inorganic substances in a lens test solution using potassium permanganate. The stability was evaluated by calculating the difference in the consumption of potassium permanganate reducing substances between the control and experimental groups.
Ⅲ. Results and Discussion
1. Polymerization and fabrication
The measurement results of the basic properties of Ref1 fabricated using silicone monomer as the main material showed that the spectral transmittance was 50.92% in the UV-B region, 68.67% in the UV-A region, and 77.73% in the visible spectrum. The measurement results of the refractive index and water content showed that the refractive index was 1.3967 and the water content was 65.22%. Furthermore, when the eluate was measured to evaluate the stability of the lens, the pH values of the control and experimental groups were 4.60 and 4.61, respectively. Thus, the pH difference between the control and experimental groups was less than 1.5, indicating high stability of pH. In the case of the potassium permanganate reduction test, it was 19 ml for the control group and 8.58 ml for the experimental group. Thus, the difference between the control and experimental groups was larger than 2 ml, indicating low stability for organic and inorganic substances. Consequently, opaque contact lenses with low transmittance were fabricated, and the stability for organic and inorganic substances was low. Thus, to improve the degree of polymerization and compatibility of the lens, IBOMA was added in a ratio of 1~10%, and the optical and physical properties of the lens were evaluated after the lenses were fabricated.
2. Change of the physical properties of silicone contact lens containing IBOMA
As a result of measuring the physical properties of the fabricated silicone contact lenses, the spectral transmittances in the UV-B, UV-A, and VIS regions were 50.92%, 68.67%, and 77.73%, respectively for Ref1; 60.67%, 79.40%, and 86.43% in I-1; 60.17%, 80.16%, and 88.00% for I-3; 62.02%, 82.11%, and 90.10% for I-5; 59.80%, 82.32%, and 90.77% for I-7; 61.85%, 83.37%, and 90.57% for I-10. Thus, the spectral transmittance gradually increased with the addition of IBOMA. The shapes of the lenses fabricated to compare the transparency of the lenses by adding IBOMA in different ratios is changed, and the measurement results for spectral transmittance are shown in Fig. 2. The measured refractive index was 1.3967 for Ref1, 1.3991 for I-0.5, 1.4007 for I-1 1.4007, 1.4032 for I-3, 1.4047 for I-5, 1.4057 for I-7 and 1.4087 for I-10. Thus, the refractive index increased with the addition of IBOMA compared to the Ref1. When the moisture content was measured for the same samples, it was 65.22% for Ref1, and according to the added amount of IBOMA, it was 63.94% for I-0.5, 63.85% for I-1, 63.85% for I-3, 62.31% for I-5, 60.76% for I-7, and 58.72% for I-10. Thus, the water content showed a gradually decreasing trend. As the IBOMA was added, the polymerization density of monomers increased, which increased the refractive index and decreased the water content of the lens. The refractive index and water content results of the fabricated lenses are shown in Fig. 3.
The spectral transmittance of the lens fabricated by adding IBOMA increased, the water content decreased, and the refractive index increased compared to Ref1 without IBOMA. However, the lenses with 0.5~3% IBOMA showed very low strengths because it was difficult to maintain their shape. I-7 and I-10 showed the highest spectral transmittance and maintained their lens shapes, but the moisture content reduction was somewhat higher than that of Ref1. Meanwhile, I-5 showed a high spectral transmittance and a relatively high water content. Thus, it was determined that 5% addition of IBOMA was most appropriate. Thus, I-5 with 5% IBOMA was newly named as Ref2. To improve the durability of lens, 2- (trifluoromethyl)styrene was added in 1–10% to Ref2 and the optical and physical properties of the fabricated lenses were evaluated.
3. Evaluation of the properties of silicone contact lens with improved durability
1) Spectral transmittance
The measurement of the spectral transmittance of the fabricated silicone contact lenses in the UV-B, UV-A, and VIS regions are shown in Fig. 4. The spectral transmittances were 55.76%, 76.10%, and 84.66% for Ref2. Depending on the ratio of 2-(trifluoromethyl)styrene, the spectral transmittances were 58.02%, 78.65%, and 86.37% for 2S-1; 60.95%, 82.63%, 90.33% for 2S-3; 61.51%, 84.17%, and 91.63% for 2S-5; and 53.80%, 74.74%, and 80.48% for 2S-10. Thus, the spectral transmittance increased as the ratio of 2-(trifluoromethyl)styrene increased from 1% to 5%. The spectral transmittance improved with the addition of 2-(trifluoromethyl)styrene, but it became somewhat lower in 2S-10 with 10% of 2-(trifluoromethyl)styrene.
2) Refractive index and water content
The measurement result of the refractive index of the fabricated lenses was 1.4044 for Ref2, 1.4059 for 2S-1, 1.4073 for 2S-3, 1.4078 for 2S-5, and 1.4108 for 2S-10. Thus, the refractive index gradually increased with the addition of 2-(trifluoromethyl)styrene. The measurement result of the water content gradually decreased from 62.04% for Ref2 to 59.44% for 2S-1, 57.11% for 2S-3, 56.69% for 2S-5, and 53.85% for 2S-10, showing an opposite result to the refractive index. The measurement results of refractive index and water content are shown in Fig. 5.
3) Oxygen permeability
The measurement of the oxygen permeability of the fabricated silicone contact lenses was 33.22 ×10-11(cm2/s)(mlO2/ml×mmHg) for Ref2. According to the added amount of 2-(trifluoromethyl)styrene, the oxygen permeability values of the samples were 31.29~26.81×10-11(cm2/s)(mlO2/ml×mmHg). Thus, the oxygen permeability decreased somewhat with the addition of 2-(trifluoromethyl)styrene, but the oxygen permeability was excellent owing to the effect of silicone. The Dk measurement results are outlined in Table 3.
4) Tensile strength
The tensile strength was measured to evaluate the durability of the contact lenses fabricated with different added amounts of 2-(trifluoromethyl) styrene. As a result, the tensile strength of Ref2 was 0.1054 kgf/mm2. With the addition of 2- (trifluoromethyl)styrene, the tensile strength gradually increased from 0.1061 kgf/mm2 for 2S-1, 0.1317 kgf/mm2 for 2S-3, 0.1795 kgf/mm2 for 2S-5, and 0.1893 kgf/mm2 for 2S-10. These results indicated that with the addition of 2-(trifluoromethyl)styrene, the oxygen permeability decreased due to the higher refractive index of the lens, but the tensile strength increased. The tensile strength results according to the added amount of 2-(trifluoromethyl) styrene are graphically represented in Fig. 6, and the graphs of Dk and tensile strength results are shown in Fig. 7.
Ⅳ. Conclusions
Functional silicone hydrogel contact lenses were fabricated by adding silicone monomer synthesized with a hydrophilic materials. The lens fabricated with silicone monomer and DMA showed low stability for spectral transmittance, organic and inorganic substances. To improve the stability and compatibility of the lens, IBOMA was added in different ratios. The spectral transmittance and refractive index increased with the addition of IBOMA, but the moisture content decreased gradually. Thus, the addition of 5% of IBOMA was found to be most appropriate. Furthermore, 2-(trifluoromethyl)styrene was added in different ratios to improve the durability of the lens. The test results showed that the addition of 2-(trifluoromethyl)styrene improved spectral transmittance, refractive index and tensile strength. Therefore, IBOMA and 2-(trifluoromethyl)styrene were found to be appropriate materials for silicone contact lens because adding appropriate amounts of these two materials improved the stability, compatibility and tensile strength of the lens.
