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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.29 No.7 pp.399-407
DOI : https://doi.org/10.3740/MRSK.2019.29.7.399

Analysis of Physical Properties of Hydrogel Lenses Polymer Containing Styrene and PVP

Min-Jae Lee, A-Young Sung†
Department of Optometry & Vision Science, Daegu Catholic University, Gyeongsan, 38430, Gyeongbuk, South Korea
Corresponding author E-Mail : say123sg@hanmail.net (A.-Y. Sung, Daegu Catholic Univ.)
May 21, 2019 July 1, 2019 July 3, 2019

Abstract


This research is carried out to analyze the effects of Styrene and PVP on the properties of silicone hydrogel lenses. Styrene group and PVP(Polyvinylpyrrolidone) are used as additives for a basic combination containing silicone monomer, TSMA(trimethylsilyl methacrylate) and DMA(n,n-dimethylacrylamide) added to the mix at ratios of 1~10 %. Silicone hydrogel lens is produced by cast-mold method. The polymerized lens sample is hydrated in a 0.9 % saline solution for 24 hours before its optical and physical characteristics are measured. Measurement of the physical characteristics of the produced material shows that the refractive index is 1.3682~1.4321, water content 77.11~45.73 %, visible light transmittance 95.14~88.20 %, and tensile strength 0.0652~0.3113 kgf. The results show a decrease of refractive index as the ratio of additives and water content decreases. The result of the stabilization test of polymerization show an increase of extractables along with increase of the ratio of additives, but the difference is not significant for all samples, so it can be judged that the stabilization of the polymer is maintained. Therefore, the additions of styrene and PVP should be taken into consideration for their effects on the physical properties of silicone hydrogel lens.



초록


    Ministry of SMEs and Startups(MSS)

    Korea Institute for Advancement of Technology

    © 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

    For hydrogel lenses, which account for the largest proportion of ophthalmic polymers, biocompatibility is very important because they are directly worn on the cornea. Properties like the refractive index, water content, and optical transmittance are also very important.1-2) Among these, the refractive index is the most important optical property of the material, and the thickness of the lens can be controlled by changing the refractive index. Thin hydrogel lenses provide a comfortable fit, with less foreign-body sensation, easily penetrate oxygen, and help reduce the risk of developing ophthalmologic diseases caused by hypoxia.3) In general, however, the increase of the refractive index increases the optical density of the medium and thus reduces the water content, the most important property of the hydrophilic contact lens.4-5) Styrene, a contact lens material with a high refractive index, is an aromatic hydrocarbon with a structure in which one hydrogen is substituted with a vinyl group in the benzene ring, and thus the refractive index is optically high.6) As it has a hydrophobic property and low flexibility, however, its use for hydrophilic lenses is limited.

    4-fluorostyrene used in this experiment can exhibit a high refractive index and is advantageous for hydrogen bonding due to its fluorine compound with high electronegativity. Thus, it can improve the wettability, thereby resulting in a good fit when applied to lenses. It is chemically stable; has fewer deposits, which cause ophthalmic side effects compared to styrene; and has high oxygen transmissibility.7)

    PVP(polyvinylpyrrolidone) is a light yellowish white powder that has a peculiar smell or is odorless, and is almost tasteless. It is highly soluble in water and in many organic solvents. In addition, it is a water-soluble polymer and generally has high biocompatibility, which not only can increase the dissolution rate of a hydrophobic compound but can also be applied to many medical fields.8-9) It is hygroscopic and is used in various fields, such as in hypoallergenic cosmetics, cleansing agents, water-soluble adhesives, and viscous eyedrops used for the dysfunctional tear syndrome.10) In the molecular structure, a lactam ring(-NCOCH2CH2CH2) is repeated. The structure belongs to the polar polymer mainly used for the modification of a hydrophobic surface or for structure change, and can induce a rapid structural change through the addition of a specific concentration or more.11-13) It has also been widely used of late as a dispersant for nanoparticles.14)

    In this study, silicone hydrogel lenses were fabricated by adding TSMA, a hydrophilic monomer containing acrylate, and DMA, which are used as hydrogel lens materials, after fabricating ophthalmic lenses using silicon monomer with the hydroxyl group. To compensate for the high refractive index and wettability required for hydrogel hydrophilic ophthalmic lenses, different types of styrene were compared in monomers of basic hydrogel hydrophilic lenses, and 2-trifluoromethylstyrene and 4- fluorostyrene, which have a high refractive index and good wettability, were used to add optically excellent and high wettability. Moreover, PVP, which is widely used as a wetting agent, was added to evaluate the change in the physical property according to each additive. Hydrogel lenses made up of polymers of various monomers under the category of medical devices, and as such, diverse substances can also be extracted from the polymer.15) In this regard, a specific substance that is extracted may pose a risk to the human body; thus, the polymerization stability should be confirmed through extractables testing to assess the potential risk and investigate the usability as a high-functional hydrogel lens material.

    2. Experimental

    2.1 Polymerization

    For the silicon monomer that was synthesized to produce ophthalmic hydrophilic lenses with high wettability, TSMA(trimethylsilyl methacrylate), DMA(N,Ndimethylacetamide), MA(methyl acrylic acid), the crosslinking agent EGDMA(ethyleneglycol dimethacrylate), and the initiator AIBN(azobisisobutyronitrile) were copolymerized. The initiator AIBN that was used in the experiment was a product of JUNSEI, and the crosslinking agent EGDMA and the additives styrene, 2TF(2-trifluorostyrene), 4F(4-fluorostyrene), and PVP were products of Sigma-Aldrich. Ophthalmic lenses were polymerized through the cast molding method, and thermal polymerization was carried out at 130 °C for 2 hours. The polymerized lens samples were hydrated in a 0.9 % saline solution for 24 hours, and the physical properties of the lenses were measured and compared. The structural formula of styrene that was used in the experiment is shown in Fig. 1.

    2.2 Instruments and Analysis

    The refractive index of the prepared hydrophilic lens was measured using an ABBE refractometer[ATAGO, DR-A1(1310)] for the lenses at a hydrated state. The water content was measured using the gravimetric method. The samples were dried using a microwave oven. The weights of the dried and hydrated samples were calculated using a calculation formula, after measuring with the XS205 DualRange balance of METTLER TOLEDO.16) The spectral transmittance was measured using a spectral transmittance meter(Agilent, Cary 60 UV-vis). The transmittance of the UV-B, UV-A, and visible-light regions was measured five times after removing the moisture from the lens surface, and was determined by averaging the values in percent. The wettability was evaluated by measuring the contact angle using a contact angle instrument(Kruss GMBH, DSA30), and the contact angle was measured through the sessile drop method. The oxygen transmissibility was measured using the polarographic method in ISO 18369-4:2006, Ophthalmic optics - Contact lenses - Part 4. The surface of the ophthalmic lens containing styrene and PVP was analyzed using AFM (XE-100, Park Systems). The pH measurement during the extractables test was done to confirm the presence of a dislocated substance by checking the hydrogen ion concentration difference. In addition, the potassium-permanganatereducing material measurement is a test for confirming the presence or absence of an organic or inorganic substance by using potassium permanganate, a strong oxidizing agent. For the extractables test solution, an amount equivalent to a 4 g lens was taken and placed in a suitable container, and 20 mL water was added and heated at 70 ± 2 °C for 24 ± 2 hours, and was then cooled down to room temperature. TGA(thermogravimetric analyzer) analysis was carried out using TGA Q500 of the TA instrument to analyze the thermal stability through the weight change according to the decomposition temperature.

    3. Results and Discussion

    3.1 Selection of Ref

    3.1.1 Fabrication and Polymerization Process

    Hydrogel lenses were fabricated by mixing silicon monomer, TSMA, and DMA, and by using the crosslinking agent EGDMA and the initiator AIBN. To maximize the optical transparency and oxygen transmissibility of the lenses, this study intended to find the optimal combination by varying the amount of each monomer. The optimal ratio of monomers was set and named “Ref.” The mixing ratio of each combination is presented in Table 1.

    3.2 Styrene group

    3.2.1 Polymerization

    The groups made by adding styrene, 2TF-styrene, and 4F-styrene at a ratio of 1~10 % to the Ref were named S1, S3, S5, and S10; 2TF1, 2TF3, 2TF5, and 2TF10; and 4F1, 4F3, 4F5, and 4F10, respectively. In addition, the groups that were polymerized by adding PVP at the ratio of 3 % to S1 and S10 were named “S1P” and “S10P,” and the groups that were polymerized by adding PVP at a ratio of 3 % to 4F1 and 4F10 were named “4F1P” and “4F10P.” The mixing ratio of each combination is presented in Table 2.

    3.2.2 Analysis of synthesized polymer by TGA

    TGA analysis was carried out to investigate the thermal properties according to the mixing ratio(2TF1 and 2TF10) of the polymerized sample. The pyrolysis of 2TF1 started at 192.05 °C, and 90 and 80 % of the weight remained at 228.74 and 276.23 °C, respectively. The pyrolysis of 2TF10 started at 184.47 °C, and 90 and 80 % of the weight remained at 221.06 and 274.60 °C, respectively. Both of the combinations showed a graphical pattern similar to the pyrolysis of a single substance as a copolymer of several monomers, but 2TF1 is judged to have better thermal stability because its pyrolysis initiation temperature is higher. The mass residual ratio at 800 °C was 6.13 and 5.27 %, respectively, and the 2TF1 combination seems to have excellent thermal stability. The TGA analysis result of each sample is presented in Fig. 2.

    3.2.3 Extractable test

    To evaluate the polymerization condition of the prepared ophthalmic lens, as extractables testing, two items − the pH difference and the potassium-permanganate-reducing material difference − were tested. After manufacturing the lens, it was immersed in distilled water and heated for 5 minutes to remove unreacted monomers. 2TF1, 2TF10, 2TF1P, and 2TF10P samples were compared to determine the difference in stability according to the amount of styrene added and the presence or absence of PVP. In the test results, the pH difference was 0.07 in 2TF1, 0.11 in 2TF10, 0.15 in 2TF1P, and 0.15 in 2TF10P, showing a slightly increasing tendency according to the styrene and PVP contents. It was considered that the extractables had no influence as the pH difference was not more than 1.5, the reference value, in 2F group. As a result of the measurement of the potassium-permanganatereducing material difference, the difference from the control group was 1.05 ml for 2TF1, 1.41 ml for 2TF10, 1.95 ml for 2TF1P, and 2.00 ml for 2TF10P. The values increased according to the styrene and PVP contents, but it is considered that there would be no influence because the difference was not more than 2 ml, the reference value, in 2F group. The excess styrene and PVP would rather reduce the polymerization stability, so it would be necessary to select an appropriate ratio depending on the physical properties. The results of the extractables testing according to the pH and reducing materials are presented in Fig. 3.

    3.2.4 Physical Property

    The refractive index of the prepared lens was measured to be 1.3682 in the Ref. It was 1.3921~1.4321 in the S group, where styrene was used as an additive to the Ref; 1.3700~1.3973 in the 2TF group; and 1.3820~1.4091 in the 4F group. In the case where PVP was added to S1 and S10, it was 1.3831 and 1.4209, respectively, and in the case where PVP was added to 4F1 and 4F10, it was 1.3794 and 1.4061 respectively. The results showed that styrene increased the refractive index and PVP slightly decreased it. The water content of the prepared lens was measured to be 71.69 % in the Ref. It was 49.12~64.10 % in the S group, where styrene was used as an additive to the Ref; 65.67~76.12 % in the 2TF group; and 61.02~77.11 % in the 4F group. In the case where PVP was added to S1 and S10, it was 58.81 and 45.73 %, respectively, and in the case where PVP was added to 4F1 and 4F10, it was 62.79 and 50.87 %. The results varied according to the styrene content. Especially, when 2TF and 4F were added at a ratio of 1 %, the water content increased and then decreased again from 3 %. This is because the fluoro group attached to styrene is polymerized with other monomers to modify the surface to make it hydrophilic, but when an excessive amount of it is added, the water content seems to decrease due to the intramolecular or intermolecular bonding of styrene. The addition of PVP also decreased the water content. In general, PVP is widely used for the surface modification of materials, and increasing the water content increases the wettability of the surface, thereby increasing the water content and the wettability simultaneously. As shown in the experiment in this study, however, the addition of PVP decreases the water content, and the interaction between styrene and PVP affects the lens surface, resulting in a change in the water content. The mean tensile strength of the prepared contact lens was measured to be 0.0652 kgf in the Ref. It was 0.1236~ 0.3113 kgf in the S combinations, where styrene was added at the ratio of 1~10 %; 0.1033~0.1993 kgf in 2TF; and 0.1360-0.2413 kgf in 4F. In the case where PVP was added to S1 and S10, it was 0.1133 and 0.1997 kgf, respectively, and in the case where PVP was added to 4F1 and 4F10, it was 0.1336 and 0.2290 kgf. The experiment results showed that the tensile strength increased with the increasing ratio of styrene added, and that the addition of PVP slightly decreased the tensile strength. According to the experiment results, the tensile strength of the lens increased irrespective of the water content, suggesting that the molecular structure of styrene directly affected the water content. The oxygen permeability (Dk) of the prepared silicone hydrogel lens was calculated after measuring the oxygen transmissibility(Dk/t) and the thickness of the center. The oxygen transmissibility of the prepared sample was 24.38×10-9 (cm/sec) (mlO2/ml× mmHg) in the Ref. It was 10.84-21.68×10-9 (cm/sec) (mlO2/ml×mmHg) in the S combinations, where styrene was added at each ratio; 17.82-22.87×10-9 (cm/sec) (mlO2/ml×mmHg) in 2TF; and 16.34-21.68×10-9 (cm/sec) (mlO2/ml×mmHg) in 4F. In the case where PVP was added to S1 and S10, it was 14.78×10-9 and 16.93×10-9 (cm/sec) (mlO2/ml×mmHg), respectively, and in the case where PVP was added to 4F1 and 4F10, it was 19.36× 10-9 and 13.07×10-9 (cm/sec) (mlO2/ml×mmHg). The unique property of the material, oxygen permeability (Dk), was 40.48×10-11 (cm2/sec) (mlO2/ml×mmHg) in the Ref. It was 22.66~32.75×10-11 (cm2/sec) (mlO2/ml×mmHg) in the S combinations, where styrene was added at each ratio; 24.01-36.25×10-11 (cm2/sec) (mlO2/ml×mmHg) in 2TF; and 18.57~35.42×10-11 (cm2/sec) (mlO2/ml×mmHg) in 4F. In the case where PVP was added to S1 and S10, it was 29.52×10-11 and 18.37×10-11 (cm2/sec) (mlO2/ml×mmHg), respectively, and in the case where PVP was added to 4F1 and 4F10, it was 27.11×10-11 and 28.57×10-11 (cm2/sec) (mlO2/ml×mmHg). Overall, the oxygen permeability and oxygen transmissibility decreased with the increasing ratio of styrene added. The combinations with 2TF and 4F at 1%, however, showed a significant increase in oxygen permeability, demonstrating the same change pattern as the water content. Styrene is generally known to have a high refractive index and high strength, and to reduce the water content.6) In addition, the study of Efron et al. suggested that the oxygen permeability increases in proportion to the water content in hydrogel lenses while the study of Maldonado-Codina et al. suggested that the oxygen transmissibility decreases with increasing water content.17-18) From the experiment results, it can be seen that when hydrogel lenses are fabricated by adding styrene with a functional group capable of hydrogen bonding for the stabilization of the physical properties, the addition of a small amount (about 1 %) can increase the water content while greatly increasing the oxygen transmissibility at the same time; thus, it can be used for the production of contact lenses with a high water content and high oxygen transmissibility. In addition, the combination of styrene and PVP showed a decrease in both water content and DK. The combination of 4Fstyrene and PVP, however, showed an increase in DK despite the decrease in water content, as seen in the 4F10P combination. It is considered that the molecular structure obtained oxygen transmissibility through the binding of styrene containing more than a specific amount of fluoro group with PVP, contrary to the report in the previous study that the oxygen transmissibility increases due to the water content. The physical properties of each combination are shown in Table 3, the relationship between the refractive index and the water content of each combination is presented in Fig. 4, the tensile strength is presented in Fig. 5, and the relationship between oxygen transmissibility and water content is presented in Fig. 6.

    3.2.5 Optical Property

    The transmittance of each lens sample in the UV-B, UV-A, and visible-light regions are as follows. As for the Ref, the transmittance was 59.36 % in the UV-B region, 90.71 % in the UV-A region, and 95.14 % in the visiblelight region. As for the S combinations, where the optical transmittance was measured according to the amount of styrene added, it was 44.95~53.81 % in the UV-B region, 84.85~90.82 % in the UV-A region, and 91.61~94.97 % in the visible-light region. As for 2TF, it was 42.57~55.79 % in the UV-B region, 78.84~89.04 % in the UV-A region, and 89.96~93.97 % in the visible-light region. As for 4F, it was 38.28-62.51 % in the UV-B region, 75.45~ 90.17 % in the UV-A region, and 88.32-93.15 % in the visible-light region. In the case where PVP was added to S1 and S10, it was 26.20-43.19 % in the UV-B region, 68.34~81.63 % in the UV-A region, and 87.90~92.48 % in the visible-light region. In the case where PVP was added to 4F1 and 4F10, it was 29.73-46.55 % in the UVB region, 70.48-82.69 % in the UV-A region, and 88.20- 91.80 % in the visible-light region. The overall amount of light was reduced according to the amounts of styrene and PVP added, and especially, PVP slightly lowered the transmittance of the UV-B and UV-A regions. Therefore, it is thought that adding PVP to a hydrogel hydrophilic lens without a UV shielding function can provide the lens with a UV shielding function. The optical transmittance change graph of each combination is shown in Fig. 7.

    3.2.6 Surface Property

    For the results of the measurement using the sessile drop method, the contact angle was 78.34° in the Ref. It was 80.47~108.22° in the S group, where styrene was added at 1~10 %, and 93.14~99.51° in the 4F group, indicating that the wettability decreased with the increasing ratio of styrene. In the case where PVP was added to S1 and S10, it was 83.51 and 87.86°, respectively, and in the case where PVP was added to 4F1 and 4F10, it was 88.41 and 99.30°. It was found that the addition of styrene decreased the wettability while the addition of PVP increased it, but the two did not really greatly affect the wettability.

    To confirm the effects of the presence of styrene, the styrene content, and the interaction of styrene with PVP on the surface conditions, such as the roughness of the lens surface, the AFM was measured. The roughness was 1.470 nm in the Ref, 1.810nm in 4F10, and 2.535 nm in 4F10P, showing that the surface became rough according to the styrene content, and that the surface roughness further increased when PVP was added. The measured contact angle results for each group are presented in Table 3 and Fig. 8, and the AFM analysis results are shown in Fig. 9.

    4. Conclusion

    The basic physical properties of ophthalmic lenses using styrene and PVP(polyvinylpyrrolidone) as additives satisfied the general reference values. In terms of wettability, the addition of styrene significantly improved it while the addition of PVP did not affect it. In particular, 4F-styrene used in the experiment can be utilized as an ophthalmic lens material because it has a high water content and high oxygen transmissibility when polymerized with PVP at an appropriate ratio. Moreover, when small amounts(about 1 %) of 2TF-styrene and 4F-styrene are combined with PVP, the mixture can be used as a lens material with a high water content by further increasing the water content. When both styrene and PVP were added to the basic ophthalmic lens material, various changes occurred to the water content and oxygen transmissibility depending on the addition ratio. The results of the extractables test showed that it partially blocked the UV-B and UV-A regions while satisfying the polymerization stability. Therefore, it was found that the interaction with PVP varies depending on the type and number of functional groups attached to styrene, and as a result, the physical properties vary. Thus, to obtain the optimal physical properties in lens fabrication, it is necessary to pay attention to the possibility of bonding depending on the molecular structure of the additive.

    Acknowledgement

    This research was financially supported by the Ministry of SMEs and Startups(MSS), Korea, under the “Regional Specialized Industry Development Program(R&D, P0004853)” supervised by the Korea Institute for Advancement of Technology (KIAT).

    Figure

    MRSK-29-7-399_F1.gif

    Chemical structures of styrenes. (a) Styrene, (b) 2TFstyrene, (c) 4F-styrene.

    MRSK-29-7-399_F2.gif

    Typical TGA thermogram analysis. (a) 2TF1, (b) 2TF10.

    MRSK-29-7-399_F3.gif

    Extractables test of 2TF group.

    MRSK-29-7-399_F4.gif

    Refractive index & water content of samples. (a) S group, (b) 2TF group, (c) 4F group, (d) SP group, (e) 4FP group.

    MRSK-29-7-399_F5.gif

    Tensile strength of samples. (a) S1, (b) S1P, (c) S10, (d) S10P, (e) 4F1, (f) 4F1P, (g) 4F10, (h) 4F10P.

    MRSK-29-7-399_F6.gif

    DK & water content of samples. (a) S group, (b) 4F group.

    MRSK-29-7-399_F7.gif

    Optical transmittance distribution of samples.

    MRSK-29-7-399_F8.gif

    Contact angle image of samples. (a) 4F1 (b) 4F1P (c) 4F10, (d) 4F10P.

    MRSK-29-7-399_F9.gif

    AFM image of lens samples for surface analysis. (a) Ref, (b) 4F10, (c) 4F10P.

    Table

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

    Percent composition of samples. (Unit: wt%)

    Physical and surface property of samples.

    Reference

    1. T. H. Kim and A. Y. Sung, J. Korean Chem. Soc., 54, 105 (2010).
    2. S. A. Cho, T. H. Kim and A. Y. Sung, J. Korean Chem. Soc., 55, 283 (2011).
    3. N. A. Brennan, N. Eforn and B. A. Holden, Ophthalmic Physiol. Opt., 7, 485 (1987).
    4. N. A. Brennan, J. Int. Contact Lens Clin., 10, 357 (1983).
    5. G. Y. Mousa, M. G. Callender, J. G. Sivak and D. J. Edan, J. Int. Contact Lens Clin., 10, 31 (1983).
    6. T. H. Kim, K. H. Ye and A. Y. Sung, J. Korean Chem. Soc., 53, 755 (2009).
    7. T. H. Kim and A. Y. Sung, J. Korean Chem. Soc., 54, 317 (2010).
    8. F. I. Kanaze, E. Kokkalou, I. Niopas, P. Barmpalexis, E. Georgarakis and E. Bikiaris, Drug Dev. Ind. Pharm., 36, 292 (2007).
    9. I. S. Han, Y. M. lim, H. J. Kwon, J. S. Park and Y. C. No, Polymer, 35, 13 (2011).
    10. Y. S. Chun, J. Korean Med. Assoc., 50, 842 (2007).
    11. M. P. Zheng, M. Y. Gu, Y. P. Jin, H. H Wang, P. F. Zu, P. Tao and J. B. He, Mater. Sci. Eng., B87, 197 (2001).
    12. T. Thirugnanam, J. Nanomater., 2013, 7 (2013).
    13. Z. Zhang, B. Zhao and L. Hu, J. Solid State Chem., 121, 105 (1996).
    14. D. Wang, V. L. Dimonie, E. D. Sudol and M. S. ELAasser, J. Appl. Polym. Sci., 84, 2721 (2002).
    15. General Standard for Biological Safety of Medical Devices (KFDA Notification No. 2014-115).
    16. International Organization for Standardization, 2006, Ophthalmic opt ics contact lenses – Par t 4: Physicochemical properties of contact lens materials, ISO 18369-4.
    17. N. Efron, P. B. Morgan, Optom. Vis. Sci., 84, E328 (2007).
    18. C. Maldonado-Codina and N. Efron, Optometry in Practice, 4, 101 (2003).