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ISSN : 1229-6457(Print)
ISSN : 2466-040X(Online)
The Korean Journal of Vision Science Vol.25 No.4 pp.383-393
DOI : https://doi.org/10.17337/JMBI.2023.25.4.383

Effect of Alpha Lipoic Acid and Pyridine Group on Silicone Hydrogel Lens

Seon-Young Park1), A-Young Sung2)
1)Dept. of Optometry & Vision Science, Daegu Catholic University, Student, Daegu
2)Dept. of Optometry & Vision Science, Daegu Catholic University, Professor, Daegu
* Address reprint requests to A-Young Sung (https://orcid.org/0000-0002-9441-919X) Dept. of Optometry & Vision Science, Daegu Catholic University, Daegu TEL: +82-53-359-6790, E-mail: say123sg@hanmail.net
November 25, 2023 December 20, 2023 December 26, 2023

Abstract


Purpose : In this study, α-Lipoic acid and pyridine were added to hydrogel materials and silicone hydrogel materials, respectively, to manufacture contact lens to evaluate changes in physical properties.



Methods : N,N-Dimethylacrylamide (DMA), 2,2'-Azobis (2-methylpropionitrile) (AIBN), an initiator, and Ethylene glycol dimethacrylate (EGDMA), crosslinking agent, were used basic mixing ratio, and hydrogel materials were prepared by thermopolymerization at 100°C for 1 hour based on 2-Hydroxyethyl methacrylate (HEMA). Silicone hydrogel materials were prepared by thermopolymerization at 130°C for 2 hours based on Silicone monomer (SID). The properties of contact lenses were evaluated by adding 0.05% of 3-hydroxypyridine N-oxide and (R)-(+)-α-Lipoic acid by ratio to each material.



Results : A lens manufactured by adding alpha lipoic acid to silicone hydrogel materials showed a tendency to increase wettability and decrease refractive index. A lens manufactured by adding pyridine to silicone hydrogel materials showed UV-A blocking ability, and the refractive index tended to increase. In addition, a contact lens manufactured by adding alpha lipoic acid and pyridine together showed antimicrobial properties, reduced surface roughness, and increased wettability and oxygen permeability according to the ratio of alpha lipoic acid addition.



Conclusion : It is judged that alpha lipoic acid and pyridine are expected to improve wettability and oxygen permeability of the lens and to be used as contact lens materials with antibacterial properties.


Alpha lipoic acid , Antimicrobial , Oxygen permeability , Pyridine , Wettability

소재에 따른 알파 리포산과 피리딘이 실리콘 하이드로겔 콘택트렌즈에 미치는 영향

박선영1), 성아영2)
1)대구가톨릭대학교 대학원 안경광학과, 학생, 대구
2)대구가톨릭대학교 안경광학과, 교수, 대구

    Ⅰ. Introduction

    Contact lens have a wider field of view than glasses and have the advantage of no cosmetic shortcomings, so the number of wearers is gradually increasing even in its short history,1) and in addition to these functional purposes, they are also used for beauty purposes. If contact lens are worn for a long time, the wearer easily feels dry due to dehydration of the lens and the comfortable feeling decreases.2) In particular, silicone hydrogel materials that combine highly oxygen-permeable silicone materials and hydrophilic hydrogel materials, which have high oxygen permeability while reducing wettability due to the hydrophobicity of silicone, have been widely used recently.3) Dryness is an important problem related to the oxygen permeability and wettability of contact lens,4) and research is actively underway to solve this problem.5) In recent years, the level of awareness of infectious diseases has been increasing due to the spread of COVID-19, and contact l ens directly contact the cornea, causing infectious diseases.6) Typically, bacteria related to eye disease include Staphylococcus aureus, E. coli, and Pseudomonas aeruginosa. Alpha-lipoic acid (ALA) is a natural compound and exists in the form of two isomers, (S)-(-)-α-Lipoic acid (S-ALA) and (R)-(+)-α -Lipoic acid (R-ALA), of which (R)-(+)-α-Lipoic acid (R-ALA) is more active in body cells.7) In addition, alpha lipoic acid is known to have antibacterial properties against Staphylococcus aureus and E. coli.8) In addition, pyridine is an aromatic heterocyclic compound and is used in various fields such as biosensor materials and ion exchange resins, and is known to have antibacterial properties.9) In particular, among pyridine-based monomers, 3-hydroxypyridine N-oxide is known to have UV absorption ability.10) Ultraviolet rays are one of the causes of various eye diseases in humans, and in particular, wavelengths in the UV-A ( 315-340nm) region a re absorbed by the cornea and lens, causing extreme self-hardening and cataracts.9,11) Therefore, in this study, optical and physical properties were evaluated after using R)-(+)-α-Lipoic acid (R-ALA) and 3-hydroxypyridine N-oxide as additives to manufacture contact lens with high humidity, UV-A blocking, and antibacterial properties.

    Ⅱ. Materials and Methods

    1. Reagents and Materials

    In this study, AIBN (2,2'-Azobis (2-methylpro pionitrile), Junsey, Japan), the thermal initiator and DMA (N,N-Dimethylacrylamide, TCI-Sejin CI, Korea), EGDMA (Ethylene glycol dimethacrylate, Sigma-Aldrich, USA), cross-linking agent and HEMA (2-Hydroxyethyl methacrylate, Sigma- Aldrich, USA), hydrogel material, SID (Silicone monomer, Sooyangchemtec, Korea), silicone material, used without refining. And R-ALA ((R)-(+)-α-Lipoic acid, Sigma-Aldrich, USA), P (3-Hydroxypyridine N-oxide, Sigma-Aldrich, USA) used as an additive. Fig. 1. shows the structural formula of HEMA, SID, and the additive used.

    2. Polymerization

    AIBN (0.2 wt%), EGDMA (1 wt%), and DMA, a hydrophilic monomer, were used as basic mixing ratios to manufacture contact lens, and HEMA and SID were used, respectively. The hydrogel material contact lens manufactured using HEMA was named H_Ref, and the silicone hydrogel material contact lens manufactured using SID was named S_Ref. In the alpha lipoic acid experiment, contact lens were manufactured by adding R-ALA to each material at ratio of 2%, 5%, and 10%, and samples added to the mixing ratio of H_Ref were named H_R2, H_R5, and H_R10, and samples added to the mixing ratio of S_Ref were named S_R2, S_R5, and S_R10. In the pyridine experiment, a sample added to the mixing ratio of H_Ref was named H_P at a rate of 0.05% with 3-hydroxypyridine N-oxide, and a sample added to the mixing ratio of S_Ref was named S_P. In the alpha lipoic acid pyridine combination experiment, pyridine was fixed at a ratio of 3-Hydroxypyridine N-oxide of 0.05%, R-ALA was added at ratios of 2%, 5%, and 10%, and the samples added to the mixing ratio of H_Ref were named H_PR2, H_PR5, and H_PR10, and the samples added to the mixing ratio of S_Ref were named S_PR2, S_PR5, and S_PR10. The polymerization of contact lens was performed using a cast mold method and was manufactured through thermal polymerization. Contact lens made of hydrogel materials were polymerized at 100°C for 1 hour and polymerized at 130°C for 2 hours. Table 1 summarizes the mixing ratio of the contact lens samples used in the experiment.

    3. Analysis

    The manufactured contact lens samples were hydrated in 0.9% sodium chloride physiological saline for 24 hours and used in the experiment, and optical transmittance, refractive index, water content, oxygen transmittance, antibacterial property, surface analysis by AFM and contact angle were measured to evaluate optical and physical properties. To analyze the optical properties of the manufactured samples, Agilent (Cary 60 UV-vis, USA) was used to measure the spectral transmittance of the UV-B region (280–315 nm), the UV-A r egion (315-380nm), and t he v isible light region (380-780 nm). An ABBE Refractometer (ATAGO, Japan) was used to analyze the physical properties of the manufactured sample, and the refractive index was measured based on ISO 18369-4:2006. The water content was measured using an Ohaus electronic scale (PAG 214C, USA), and the weight measurement method of ISO 18369-4:2006 was used. Oxygen permeability was measured according to the polarographic method in ISO 18369-4:2006. The manufactured sample was placed in an LIB-030M thermohygrostat (General Incubator, Korea) and maintained at a temperature of 35±0.5°C and a humidity of 98±1%, and measured using the Opermeometer Model 201T (Createch, USA) of the Development Company. A 3M Petrifilm medium was used to evaluate antimicrobial activity against S. aureus and E. coli, and the prepared contact lens sample was hydrated with 0.9% sodium chloride physiological saline for 24 hours with bacteria, the water was removed from the samples, and it was put in a saline solution nine times the weight of the lens and quenched to prepare a solution. Each 1 mL of the prepared solution was smeared on a dry film and cultured at 36±1°C for 24 hours. Surface roughness analysis was measured with XE-100 atomic force microscope equipment (Park Systems, Korea). The contact angle was measured to evaluate the wettability of the prepared sample. The contact angle was measured by the Sessile drop method using a DSA30 (KrussGMBH, Germany) device. In order to increase the reliability of the measured values, all experiments were measured five times per sample and the average value was calculated and used.

    Ⅲ. Results

    1. Properties of Alpha Lipoic Acid

    1) Optical Transmittance

    The optical transmittance for each wavelength band of the contact lens sample prepared by adding α-lipoic acid was measured. As a result of the measurement, the addition of alpha lipoic acid did not affect the optical transmittance regardless of the material, and visible optical transmittance of more than 88% was shown in all samples, which is the optical transmittance standard (ANSI Z80.20). The optical transmittance of the samples is presented in Fig. 2.

    2) Refractive Index and Water Content

    The refractive index affects the refractive power of the contact lens, and the water content is one of the important physical properties that affect the wettability and comfort of the contact lens. The results of measuring the refractive index and water content of the manufactured contact lens samples are presented in Fig. 3.

    In hydrogel materials, H_Ref, a basic sample without alpha Lipoic acid, has a refractive index of 1.4214 and a water content of 47.77%. The refractive indices of H_R2, H_R5, and H_R10, which are samples with alpha lipoic acid, increased according to the ratio to 1.4216, 1.4224, and 1.4246, respectively, and the water content decreased to 47.02%, 46.61%, and 46.01%. This is analyzed as a decrease in water content due to the hydrophobic characteristics of alpha lipoic acid.12) In hydrogel materials, alpha lipoic acid is thought to be able to be used to manufacture high refractive hydrogels. In silicone hydrogel materials, S_Ref, a basic sample without alpha lipoic acid, has a refractive index of 1.3815, and a water content of 70.42. The refractive indices of S_R2, S_R5, and S_R10 with alpha lipoic acid decreased according to the ratio to 1.3852 1.3828 and 1.3821, respectively, and the water content increased according to the ratio to 73.24%, 75.93%, and 76.89%, respectively.

    2. Properties of Pyridine

    1) Optical Transmittance

    The optical transmittance of the contact lens samples prepared by adding pyridine was measured by wavelength band. The optical transmittance of samples is presented in Fig. 4.

    The addition of pyridine did not affect the optical transmittance of the contact lens. On the other hand, when added to a silicone hydrogel material, it showed high UV protection ability in the UV-A area without affecting visible optical transmittance. This is judged to be the effect of the UV absorption capacity of 3-Hydroxypyridine N-oxide.10) If pyridine is used together with a silicone hydrogel material, it is judged that it can be used as a UV-A blocking functional material. Regardless of the material, all samples showed a visible optical transmittance of 88% or more, which is the optical transmittance standard (ANSI Z80.20).

    2) Refractive Index and Water Content

    The refractive index and water content of the contact lens samples prepared by adding pyridine were measured, and the results of measuring the refractive index and water content of the manufactured contact lens samples are presented in Fig. 5.

    In hydrogel materials, H_Ref, a basic sample without pyridine, has a refractive index of 1.4216 and a water content of 47.75%. The refractive index of H_P, a sample with pyridine, was 1.4218 and the water content was 47.69%, showing no change in physical properties. In silicone hydrogel materials, S_Ref, a basic sample without pyridine, has a refractive index of 1.3811, and a water content of 70.12%. The refractive index of S_P, a sample to which pyridine was added, was 1.3893, which was increased by the addition of pyridine, and the water content was 70.48%, which did not change. In silicone hydrogel materials, pyridine is judged to be a functional material that can increase the refractive index without changing the water content.

    3. Properties of Alpha Lipoic Acid Pyridine Combination

    1) Optical Transmittance

    The optical transmittance of the contact lens samples prepared by adding alpha lipoic acid and pyridine were measured by wavelength band, The optical transmittance of samples is presented in Fig. 6.

    The UV-A blocking ability of pyridine was lost in a lens manufactured by adding alpha lipoic acid and pyridine to silicone hydrogel materials, and the loss rate increased as the ratio of alpha lipoic acid increased. It is judged that the compatibility between alpha lipoic acid and pyridine is low in UV protection characteristics.

    2) Refractive Index and Water Content

    The refractive index and water content of the contact lens samples prepared by adding alpha lipoic acid and pyridine were measured, and the results of measuring the refractive index and water content of the manufactured contact lens samples are presented in Fig. 7.

    The refractive index of S_Ref, a basic sample without alpha lipoic acid and pyridine, is 1.3817, and the water content is 70.30%. When alpha lipoic acid and pyridine were added, the refractive index increased to 1.3852∼1.38.21 and then decreased again, which was analyzed to have increased the refractive index due to the addition of pyridine, which improves the refractive index in silicone hydrogel materials, and then decreased again as the ratio of alpha lipoic acid, which reduces the refractive index, increased. When alpha lipoic acid and pyridine were added, the water content tended to increase gradually to 73.27~76.89%, which is analyzed to increase the water content as the ratio of alpha lipoic acid, which improves the water content in silicone hydrogel materials, increased. It is judged that the refractive index and water content can be adjusted in various ways by varying the addition ratio of alpha lipoic acid and pyridine to silicone hydrogel materials.

    3) Oxygen Permeability

    The oxygen permeability of contact lens is an important characteristic that affects corneal metabolism. The oxygen permeability of the prepared contact lens samples was measured, and the measurement results are presented in Fig. 8,9.

    The oxygen permeability of S_Ref, a basic sample without alpha lipoic acid and pyridine, was 24.6Dk/t, and when alpha lipoic acid and pyridine were added, the oxygen permeability increased to 25.25 to 29.7 Dk/t. It is analyzed that the oxygen permeability increased as the water content increased due to the increase in the ratio of alpha lipoic acid addition.

    4) Antimicrobial Test

    Antimicrobial activities against Staphylococcus aureus and E. coli were evaluated for contact lens samples to which alpha lipoic acid and pyridine were added. The measurement results are presented in Fig. 10.

    Compared to S_Ref (control), a basic sample without alpha lipoic acid and pyridine, the antimicrobial property of samples with alpha lipoic acid and pyridine was excellent, and the number of bacteria decreased as the rate of alpha lipoic acid added increased. Zhao G et al.8) suggested that alpha lipoic acid has antibacterial properties against Staphylococcus aureus and E. coli, and showed similar results to the results of this study. It is thought that a contact lens with antibacterial properties can be manufactured by adding alpha lipoic acid and pyridine to the contact lens.

    5) Surface Analysis by AFM

    The surfaces of the samples were observed through AFM to determine the roughness of the lens surfaces. The AFM measurement of S_Ref, a basic sample without alpha lipoic acid and pyridine, was 3.652nm, and the AFM measurement of samples with alpha lipoic acid and pyridine was 1.194nm. It is judged that the addition of alpha lipoic acid and pyridine reduced the surface roughness value of the lens.

    6) Contact Angle

    The contact angle was measured to measure the wettability of the prepared samples. The contact angle measurement of S_Ref, the basic sample without alpha lipoic acid and pyridine, was 103.77°, and the contact angle measurement of the sample with alpha lipoic acid and pyridine was 121.11°. It is judged that the water content increased due to the addition of alpha lipoic acid, and the contact angle decreased as the surface roughness value decreased.

    Ⅳ. Discussion

    In this study, alpha lipoic acid did not affect the transmittance of the lens, and when added to a silicone hydrogel material, the water content increased and the refractive index decreased. In addition, in hydrogel materials, the water content decreased and the refractive index tended to increase. This is judged to be due to the amphiphilic characteristics of alpha lipoic acid.13) Pyridine did not affect the physical properties of the lens when added to the hydrogel material, and showed UV protection ability when added to the silicone hydrogel material. This is judged to be the result of pyridine's UV absorption ability. Also, pyridine increased the refractive index of the lens. When alpha lipoic acid and pyridine were added to the silicone hydrogel material together, pyridine's UV protection ability was lost. The refractive index decreased and the water content increased according to the alpha lipoic acid addition ratio. In addition, as the water content increased, the oxygen permeability increased.14) Samples to which alpha lipoic acid and pyridine were added showed antimicrobial properties,15) and the contact angle was reduced. This is judged to be the result of a decrease in surface roughness.

    Ⅴ. Conclusion

    In this study, the physical properties were compared after manufacturing contact lens by adding the natural compounds alpha lipoic acid and pyridine to hydrogel materials and silicone hydrogel materials, respectively. The alpha lipoic acid increased the refractive index and decreased the water content in hydrogel materials, while silicone hydrogel materials increased the water content and decreased the refractive index. When pyridine was added to a silicone hydrogel material, it increased the refractive index and exhibited a UV-A blocking function. When alpha lipoic acid and pyridine were added to the silicone hydrogel together, the surface roughness decreased and an antimicrobial property was shown. In addition, wettability and oxygen permeability increased as the ratio of alpha lipoic acid added increased, and the refractive index and water content could be adjusted according to the presence or absence of pyridine and the ratio of alpha lipoic acid added. If alpha lipoic acid and pyridine are combined with silicone materials, refractive index, wettability, oxygen permeability, and surface roughness can be adjusted according to different ratios, and it is judged that they can be used as functional materials with UV-A blocking and antibacterial properties.

    Figure

    KJVS-25-4-383_F1.gif

    Chemical structures of Reagents. (a) 2-hydroxyethyl methacrylate, (b) silicone monomer, (c) (R)-(+)-α -lipoic acid, (d) 3-hydroxypyridine N-oxide

    KJVS-25-4-383_F2.gif

    Optical transmittance of R-ALA samples. (a) hydrogel material, (b) silicone hydrogel material

    KJVS-25-4-383_F3.gif

    Refractive index and water content of R-ALA samples. (a) hydrogel material, (b) silicone hydrogel material

    KJVS-25-4-383_F4.gif

    Optical transmittance of pyridine samples. (a) hydrogel material, (b) silicone hydrogel material

    KJVS-25-4-383_F5.gif

    Refractive index and water content of pyridine samples. (a) hydrogel material, (b) silicone hydrogel material

    KJVS-25-4-383_F6.gif

    Optical transmittance of samples.

    KJVS-25-4-383_F7.gif

    Refractive index and water content of samples.

    KJVS-25-4-383_F8.gif

    Oxygen permeability and water content of samples.

    KJVS-25-4-383_F9.gif

    Probe current and temperature versus time in optic zone of (a) S_Ref, (b) S_PR10.

    KJVS-25-4-383_F10.gif

    Antimicrobial test of samples. (a) staphylococcus aureus test of S_Ref, (b) staphylococcus aureus test of S_PR10, (c) escherichia coli test of S_Ref, (d) escherichia coli test of S_PR10

    KJVS-25-4-383_F11.gif

    AFM image of (a) S_Ref, (b) S_PR10.

    KJVS-25-4-383_F12.gif

    Contact angle image of (a) S_Ref, (b) S_PR10.

    Table

    Percent compositions of samples (Unit : wt%)

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