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ISSN : 1229-6457(Print)
ISSN : 2466-040X(Online)
The Korean Journal of Vision Science Vol.25 No.4 pp.373-382
DOI : https://doi.org/10.17337/JMBI.2023.25.4.373

Utilization Functional Analysis and Crosslinking Agents of Hydrogel Contact Lens Material according to Polymerization Method

Hye-In Park1), Su-Mi Shin1), 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 : This study analyzed physical properties using 1,6-Hexanediol diacrylate (HDDA) as an additive to improve functionality of hydrogel lens such as durability. It was checked whether HDDA can be applied as a crosslinking agent in place of ethylene glycol dimethacrylate (EGDMA).



Methods : Polymerization used thermal polymerization and photopolymerization methods, and HDDA was added to each polymerization method at a ratio of 1∼20%. Optical and physical characteristics of the manufactured lens were evaluated by measuring spectral transmittance, refractive index, water content, tensile strength, contact angle, and AFM.



Results : Regardless of the polymerization method, the tensile strength ranged from 0.232~0.408 kgf/mm2 in thermal polymerization and from 0.146~0.429 kgf/mm2 depending on the addition ratio of HDDA. In addition, as a result of using HDDA instead of EGDMA, it was confirmed that the physical characteristics of the lens were similar to that of EGDMA, and in the case of tensile strength, HDDA was much improved.



Conclusion : HDDA has been shown to be effective in improving the functionality of hydrogel lens and to improve stability and durability. In addition, it is believed that it can be used in various ways as an ophthalmic material as well as a photopolymerization crosslinking agent.


Crosslinking agent , Tensile strength , 1,6-Hexanediol diacrylate , Hydrogel lens

중합 방법에 따른 교차결합제의 활용 및 친수성 콘택트렌즈 소재로서의 기능성 분석

박혜인1), 신수미1), 성아영2)
1)대구가톨릭대학교 대학원 안경광학과, 학생, 대구
2)대구가톨릭대학교 안경광학과, 교수, 대구

    Ⅰ. Introduction

    Contact lens production methods include mold casting, rotary casting, and shelf cutting. The most commonly used method is mold casting, a method of polymerizing by injecting the lens materials into a frame with the shape of the front and back of the lens by applying ultraviolet, heat, and radiation, which is simple and can be mass-produced at inexpensive.1) Thermal polymerization is a method of polymerizing by high heat by adding a thermal initiator and a crosslinking agent. The advantage is that the process method is simple and there are few impurities in the lens.2) However, there is a disadvantage of consuming a large amount of energy because polymerization takes a high temperature and a long time is required.3) Photopolymerization is a method of polymerization at a low temperature using ultraviolet by adding a photoinitiator and a crosslinking agent.3,4) It is very productive and can be selectively polymerized by controlling UV intensity and irradiation.4) In addition, photopolymerization has the advantage of reducing eye irritation and increasing fit by reducing the thickness of the lens. However, it has the disadvantage of breaking easily due to its weakened durability. Accordingly, research on the manufacture of hydrogel lens with photopolymerization is being actively conducted.2) The material of the contact lens requires optical and physical characteristics such as spectral transmittance, refractive index, water content, tensile strength and contact angle.5-7) In particular, contact lens are medical devices that directly touch the cornea and must have excellent durability and stability. Physical properties such as breaking strength and tensile strength are very important to improve the durability of the contact lens. In general, when the tensile strength increases, the durability of the lens itself increases, so that the lens is not easily torn and can be prevented from being easily damaged.6) The manufacture of hydrogel contact lens requires 2-Hydroxyethyl methacrylate (HEMA), which are the main material, an initiator, and a crosslinking agent. The type or concentration of the initiator and the crosslinking agent affects the physical properties of the lens.1,8) The crosslinking agent tightly binds the polymer chain. Therefore, it is used for polymer polymerization and affects the physical and mechanical strength of the lens depending on the concentration.9) Ethylene glycol dimethacrylate (EGDMA) is a diester formed by the condensation of carboxylic acid (CH2) and ethylene glycol (CH2OH)2, and is the most basic free radical copolymer crosslinking agent used to make hydrogel lens regardless of the polymerization method. 1,6-Hexanediol diacrylate (HDDA) is a bifunctional acrylate ester monomer used in polymer manufacturing that is particularly useful for ultraviolet curing applications, and it is also widely used in adhesive, coating, ink, and dental complex industries because it c an i mprove adhesion, hardness, wear and heat resistance.10,11) In addition, the acrylate group of HDDA can be used as a photo-crosslinking agent in contact lens manufacturing because it can cause a radical polymerization reaction when activated by a photoinitiator and light and form a crosslinking between polymer chains.12,13) Therefore, in this study, HDDA is used as an additive to analyze physical properties to improve the durability and functionality of existing hydrogel lens, and to confirm its applicability as a thermal crosslinking agent in place of EGDMA.

    Ⅱ. Materials and methods

    1. Reagents and materials

    To manufacture hydrogel lens, HEMA (Sigma- Aldrich, USA), the main ingredient of hydrogel lens, AIBN (Junsey, Japan), a thermal initiator, and 2H2M (Sigma-Aldrich, USA) were used. EGDMA (Sigma-Aldrich, USA) and HDDA (Sigma-Aldrich, USA) were used as crosslinking agents, respectively.

    2. Polymerization

    For the polymerization of lens, HEMA, EGDMA, and AIBN were used as basic combinations of thermal polymerization, and HEMA, EGDMA, and 2H2M were used as basic combinations of photopolymerization. HDDA was added at a ratio of 1~20%, and thermal polymerization was performed at 100°C for 1 hour using mold casting, and photopolymerization was performed for 45 seconds. The samples of thermal polymerization used in the experiment were named H-R, H-1, H-5, H-10, and H-20, respectively, and the samples of photopolymerization were named by changing H to U. In addition, when HDDA was used as a crosslinking agent, the samples of thermal polymerization were named H-H and the samples of photopolymerization were named U-H. The composition for manufacturing contact lens is shown in Table 1, and the chemical structural formula of the crosslinking agent used in this study is shown in Fig. 1.

    3. Measuring Device and Methods

    Each of the produced contact lens was hydrated in 0.9% sodium chloride (NaCl) physiological saline for 24 hours and then evaluated for optical and physical properties with spectral transmittance, refractive index, water content, tensile strength, contact angle, and AFM measurements. Spectral transmittance was measured based on ISO 8599:1994, refractive index and water content were measured based on ISO 18369-4:2006, and water content was analyzed using a weight measurement method. Tensile strength was evaluated by applying a force of 0~2.00 kgf to both sides of the sample using a tensile tester (AGS-X 20N, Japan), and measuring the maximum value of lens damage. To analyze the wettability and roughness of the lens, the contact angle was measured using the sessile droop method of the contact angle meter (DSA30, Germany), and the AFM was analyzed using a nuclear microscope (XE-100, Park Systems, Suwon, Korea). In addition, polymerization stability was evaluated with an elution test (KMnO4 reduction test, pH test, absorbance). All measurements presented in this study were repeated more than five times to increase the accuracy of the experiment, compared and analyzed through average values.

    Ⅲ. Results and Discussion

    1. Functional hydrogel lens containing 1,6-Hexanediol diacrylate

    1) Spectral Transmittance

    Spectral transmittance was measured by classifying it into UV-B (280∼315 nm), UV-A (315 ∼380nm), and visible light (380∼780nm). As a result of the measurement, the visible light area was more than 90% in all samples, meeting more than 88% of the basic requirements for contact lens in ANSI Z80.20:2004, and satisfying the basic characteristics of contact lens. A spectral transmittance graph of each sample is shown in Fig. 2.

    2) Refractive index and water content

    As a result of measuring the refractive index and water content of the manufactured hydrogel lens, the refractive index of H-R was 1.4356 and the water content was 35.31%. Depending on the amount of HDDA added, the refractive index was 1.4372~1.4585, and the water content was 34.36∼ 22.35%. The refractive index of U-R was 1.4366 and the water content was 37.15%. Depending on the amount of HDDA added, the refractive index was 1.4404~1.4697, and the water content was 36.07∼19.70%. Accordingly, the refractive index of the manufactured lens was found to increase according to the amount of HDDA added regardless of the polymerization method, and the water content gradually decreased. It was confirmed that the refractive index and the water content were inversely proportional. In addition, it was confirmed that photopolymerization showed a greater rate of increase or decrease in physical properties than thermal polymerization. This is believed to be because HDDA has great usefulness in ultraviolet curing applications. Graphs of the refractive index and water content of each sample are shown in Fig. 3.

    3) Tensile strength

    As a result of measuring the tensile strength of the hydrogel lens manufactured for durability evaluation, the tensile strength of H-R was 0.220 kgf/mm2, and depending on the amount of HDDA added, it was measured as 0.232~0.408 kgf/mm2. The tensile strength of U-R was 0.105 kgf/mm2, and depending on the amount of HDDA added, it was measured as 0.146~0.429 kgf/mm2. Accordingly, the tensile strength of the manufactured lens was found to increase according to the amount of HDDA added regardless of the polymerization method, and it was confirmed that mechanical properties were greatly improved when used with photopolymerization. Since the crosslinking agent firmly binds the polymer chain, the concentration of the crosslinking agent is known to affect the internal and mechanical strength of the lens.14) Therefore, it was confirmed that the refractive index and t ensile s trength increased as HDDA, which is used as a photo-crosslinking agent, was added. A tensile strength graph of each sample is shown in Fig. 4.

    4) Contact angle and Surface analysis by AFM

    As a result of measuring the contact angle and AFM of the hydrogel lens manufactured for wettability and roughness evaluation, the contact angle of H-R was 54.17°, and it measured from 50.50°∼29.07° depending on the amount of HDDA added. The contact angle of U-R was 44.83°, and it measured 49.21°∼65.12° depending on the amount of HDDA added. In addition, the average surface roughness value (Ra) of the lens was identified as H-R 3.8 μm, H-20 1.6 μm, U-R 1.5 μm, and U-20 3.3 μm, respectively. Accordingly, it was confirmed that the contact angle of the lens decreased as the addition ratio of HDDA increased in thermal polymerization, and that photopolymerization increased in an inversely proportional relation to each other. AFM was similar to the contact angle results. The surface roughness of the lens is known to affect the contact angle.15) AFM and contact angle images of each sample are shown in Fig. 5.

    2. Hydrogel lens containing 1,6-Hexanediol diacrylate as a crosslinking agent

    1) Physical characteristics

    As a result of the physical characteristics of the manufactured hydrogel lens, the hydrogel lens manufactured using HDDA showed similar values to the hydrogel lens manufactured using EGDMA regardless of heat and photopolymerization. Accordingly, it was confirmed that HDDA maintains the basic physical characteristics of the lens without harming it. For tensile strength, H-H was measured to be 0.271kgf/mm2 , and U-H was measured to be 0.157kgf/mm2. Therefore, it was confirmed that the lens manufactured with HDDA has improved mechanical properties compared to the lens using EGDMA. A tensile strength graph of each is shown in Fig. 6.

    2) Stability test

    A KMnO4 reduction test and pH test were performed and absorbance was measured to evaluate the addition of HDDA according to the polymerization method and polymerization stability as a crosslinking agent. The lens was extracted by heating at 70°C for 24 hours. The eluate was performed as an experimental group and tertiary distilled water was performed as a control group. The amount of HDDA added was selected as 10%.

    (1) KMnO4 reduction test, pH measurement, absorbance measurement

    The KMnO4 reduction test, pH and absorbance test results are shown in Fig. 7. In the KMnO4 reduction test, the extracted test fluid of the control group was 21.11, and the difference from the control group was measured to be less than 2ml in all groups. In the pH test, the difference between the experimental group and the control group was measured to be less than 1.5 in all samples. Accordingly, excellent polymerization stability was shown regardless of the presence or absence of additives. As a result of measuring absorbance, light was absorbed the most at 205 nm, and absorbance was less than 0.30 in all groups, indicating excellent polymerization stability. In particular, lens using HDDA measured less eluate than lens using EGDMA, confirming that polymerization stability was better.

    Ⅳ. Conclusion

    This study compared and analyzed the properties of each of thermal polymerization and photopolymerization using 1,6-hexanediol acrylate (HDDA) as an additive to HEMA, a hydrogel main material, instead of EGDMA, which is widely used as a crosslinking agent for contact lens. We tried to find out the usefulness of HDDA as a crosslinking agent. As a result of measuring the physical properties of contact lens using HDDA as an additive, it was found that the refractive index and tensile strength gradually increased depending on the addition ratio regardless of the polymerization method, and the contact angle was contradictory to each other depending on the polymerization method. In addition, when HDDA was used as a crosslinking agent, it was found to be similar to the physical properties of the lens using EGDMA, and it was confirmed that the tensile strength was much improved. Therefore, hydrogel lens with HDDA are effective in improving stability and durability, and it is believed that they can be used in various ways as ophthalmic materials as well as photo-crosslinking agents.

    Figure

    KJVS-25-4-373_F1.gif

    Chemical structures of crosslinking agent. (a): EGDMA, (b): HDDA

    KJVS-25-4-373_F2.gif

    The effect of Spectral transmittances of samples. (a): thermal polymerization, (b): photo polymerization

    KJVS-25-4-373_F3.gif

    The effect of Refractive Index and water content of samples. (a): thermal polymerization, (b): photo polymerization

    KJVS-25-4-373_F4.gif

    The effect of Tensile strength graph of samples. (a): H-R, (b): H-20, (c): U-R, (d): U-20

    KJVS-25-4-373_F5.gif

    The effect of AFM and contact angle images of samples. (a): H-R, (b): H-20, (c): U-R, (d): U-20

    KJVS-25-4-373_F6.gif

    The effect of tensile strength graph of samples. (a): H-H, (b): U-H

    KJVS-25-4-373_F7.gif

    Elution test of samples. (a): KMnO4 reduction test, (b): pH test, (c): absorbance

    Table

    Compositions of hydrohel contact lens (Unit: %)

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