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Application of thiol-ene photopolymerization for injectable intraocular lenses: a preliminary study.

INTRODUCTION

The natural lens, comprising about 65.0% water and 35.0% structural proteins, is a transparent biconvex body covered by the capsule (1). The damages from aging, large stresses, radiation pulse, or sudden physical trauma can change the morphology of lens proteins, resulting in the formation of cataract and opacification of the lens (2). The implantation of the intraocular lenses (IOLs) has been explored and applied to patients to replace the optic function of natural lens. Injectable polymers are promising materials in the application of tissue engineering, such as bone repairing, tissue regenerating and so forth (3), (4). Liquid lens materials could be injected into a lens capsule bag, followed by solidification resulting from photopolymerization, oxidation or ion crosslinking. In this way, an artificial lens can be implanted into the lens capsule with only about 1.0-mm incision. The advantage is that the complication caused by incision is largely reduced (5), (6). Furthermore, by changing the reaction conditions, it is possible to form a low-modulus lens, which can be accommodated by the movement of suspensory ligament in capsule (7).

The research of injectable IOLs has been carried out for over 40 years, including the dimethylsilicone material based on a low-temperature vulcanizing reaction, the poloxamer hydrogels based on the thermo-triggered sol-gel transition and the hydrogels crosslinked by the reversible disulfide bonds (8-10). However, the ideal materials for the injectable IOLs have not been achieved until now. The main reason is that it is very difficult to obtain the injectable materials with the following properties: conveniently injected into a lens bag without any leakage; rapidly transformed into a solid-like physical gel on demand; permanently crosslinked and without swelling; nonexo-thermic or only minimally exothermic during gelling reaction; filterable to ultraviolet (UV) light and highly transparent in the visible region; flexible and soft with low modulus within the range of 1-5 kPa; refractive index close to 1.42; biocompatible and nontoxic. Based on the previous studies, it was found that the selection of properly gelling strategy was the first and vital step to obtain ideal injectable IOLs.

Photopolymerization is one of the most easily controlled ways, through which liquid artificial materials could be solidified within a short period in a transparent lens capsule only by controlling the light radiation on or off (1), (11). Meanwhile, by selecting the proper monomers and adjusting the crosslinking density of a reaction system, it is easy to obtain the suitable materials with good optical and mechanical properties. However, during the implantation of IOLs, the photochemical damage to retina is easily caused by the excess radiance of UV or blue light; moreover, the toxicity of photoinitiator is another potential damage to surrounding cells or tissues. To deal with the aforementioned problems, two methods are suggested. One is the synthesis of polymeric photoinitiators, which can be initiated effectively by blue light; the other is the exploration of new photopolymerizing systems, which can be triggered by good biocompatible photoinitiators at a low concentration and less-radiating intensity. To the former, the copolymer of (4-vinyl-2,6-dimethyl)-benzoyldiphenylphosphine oxide and dimethylacrylamide was used (1); however, the copolymer photoinitiator was difficult to be synthesized because it was highly sensitive to heat or visible light. In this study, the latter method was explored by analyzing the dependence of the thiolene photopolymerization rate upon UV intensity, photoinitiator concentration, and so forth.

The thiol-ene photopolymerization, which was reported as early as middle 70s of last century, shows some advantages, including mild reacting conditions, unnecessary for photoinitiator, reduced oxygen sensitivity, fast polymerization rate and forming crosslinked networks with good physical and mechanical properties. Recently, the thiolene photoinitiated polymerization has been the focus of many studies (12-14). Most of the previous investigations showed similar results that the thiolene photopolymerization could be achieved with a fast reaction rate, of which the reason was more complicated and need to be further explored.

This article made a tentative analysis on the application of the thiolene photopolymerization in injectable IOLs. The influences of photoinitiator concentration and UV intensity on the photopolymerization rate were mainly compared between the thiol-ene system and the mono-component system. Meanwhile, the properties of the bulk samples obtained from above two systems were also analyzed. Based on the advantages of the thiol-ene photopolymerization, that is, the reaction being achieved rapidly and completely with less-photoinitiator content and low-UV intensity, the potential application of the thiolene photopolymerization in injectable IOLs was suggested.

MATERIALS AND METHODS

Materials

Poly(ethylene glycol) diacrylate (PEGDA) with molecular weights of 575 and 700 (PEGDA575 and PEGDA700) were supplied by Aldrich (USA). Pentaeryth-ritol tetrakis(3-mercaplopropionate) (PTMP), N-vinylpyr-rolidone and Irgacure2959 were purchased from Fluka (Germany), Tianjiao Chemical Co., Ltd. (Tianjin, China), and Ciba Specialty Chemicals Corp (Switzerland), respectively. To reduce the weighing errors, 1.0 wt% photoinitiator solution instead of photoinitiator powder was used in the experiments, by dissolving 0.1 g Irgacure2959 into 9.9 g N-vinylpyrrolidone.

Measurement of Double Bond or Thiol Conversion Ratio

A drop of monomers mixture sandwiched between two transparent potassium bromide films was radiated under a UV Floodlight lamp (Labino AB, Sweden). The light density was measured with a radiometer (Photoelectric Instrument of Factory Beijing Normal University, China) at 365 nm. The carbon-carbon double bond (C=C) stretching vibrations in PEGDA molecules at 1636 and 1620 [cm.sup.-1] and the thiol (SH) stretching vibrations in PTMP at 2560 [cm.sup.-1] were used to calculate the conversion ratio of the functional groups with a Fourier transform infrared spectrometer (FTIR) (Spectrum One, PerkinElmer, USA). The conversion ratio of C=C or SH could be calculated by Eqs. 1 and 2 by analyzing the decrease of C=C or SH peak areas compared with the peak area of carbonyl group at about 1735 [cm.sup.-1]

C=C conversion ratio % = [[[A[t.sub.[C=C]]][A[t.sub.[C=O]]]/[[A[0.sub.[C=C]]]/[A[0.sub.[C=O]]]]] x 100% (1)

SH conversion ratio % = [[[A[t.sub.SH]/[A[t.sub.[C=O]]]]/[[A[0.sub.SH]/[A[0.sub.[C=O]]]]] x 100% (2)

where [A0.sub.[c=c]] and [A0.sub.[c=c]] are C=C peak areas before and after "t" min UV radiation, [A0.sub.[c=c]] and [A0.sub.SH] are C=0 peak areas before and after "t" min UV radiation, [A0.sub.SH] and [At.sub.SH] are SH peak areas before and after "t" min UV radiation.

Preparation of Bulk Samples

Bulk samples were prepared using a cylindrical-shaped glass mold with the spacer of 15.0-mm diameter and 2.0-mm thickness. The monomers mixture was injected into the mold and irradiated under the UV Floodlight lamp at 365 nm. During the polymerization, the temperature changes of systems were also detected with a thermal couple sensor connected with a temperature controller (the precise being 0.1[degrees]C) (see Fig. 1). The thermal couple sensor was an armored thermal couple (bottom tangency type) with the top limit of 400[degrees]C. Before polymerization, the tip of the sensor was inserted into the mold and contacted with monomers.

[FIGURE 1 OMITTED]

Characterization of Bulk Samples

The bulk samples were submerged into 37.0[degrees]C water until the swelling equilibrium was achieved. After weighed, the swelled samples were freeze-dried to con- stant weight. The equilibrium water content (EWC) and the mass loss were calculated by the Eqs. 3 and 4:

EWC% = [[[W.sub.s] - [W.sub.d]]/[W.sub.s]] x 100% (3)

Mass loss% = [[[W.sub.b] - [W.sub.d]]/[W.sub.b]] x 100% (4)

where [W.sub.b], [W.sub.s], and [W.sub.d] are the weights of bulk samples before swelling, after swelling and drying, respectively.

Transmittance of bulk samples was measured from 200 to 800 nm with a V-570 spectrophotometer (JASCO Corporation, Japan). Elastic modulus of swelled samples was measured three times for each sample with a SES-1000 materials testing machine (Shimadzu Corporation, Japan) under 400.0 N force and at 1.0 mm/min compressing pace. The elastic modulus was the average value of two same samples.

RESULTS AND DISCUSSION

Influence of C=C/SH Molar Ratio on the Thiol-Ene Photopolymerization

Two kinds of PEGDA monomers with different molecular weights were used in experiments because PEGDA monomers show high reactivity and their refractive indexes are close to that of natural lenses. Meanwhile, the resulted polymers or copolymers showed high transparency, good biocompatibility, and could be used for tissue engineering (15) and drug delivering (16). In this study, the monomers were polymerized under the UV radiation in period, during which their FTIR spectra were measured. Figure 2 showed the changes of FTIR spectra of PEGDA575-PTMP system with the UV radiation time under the conditions that UV intensity was 4.5 mW/[cm.sup.2], Irgaeure2959 concentration was 0.005 wt% and the molar ratio between double bond (C=C) in PEGDA and thiol (SH) in PTMP was 1.0. It could be seen from the FTIR curves that the stretching vibration peak of SH group appeared at 2560 [cm.sup.-1] and the stretching vibration peaks of C=C were at 1636 and 1620 [cm.sup.-1]. With the increase of the cumulative radiation time, the peak areas corresponding to SH and C=C groups both decreased. The decaying degree of SH or C=C peak areas could effectively reflect the conversion ratios of SH or C=C during the polymerization (17), (18), Figures 3 and 4 showed the plots of radiation time versus the conversion ratios of C=C and SH, respectively. The experiments were carried out under the conditions that UV intensity was 4.5 mW/ [cm.sup.2] and Irgacure 2959 was concentration 0.005 wt%. It could be seen that the reactivity of C=C in PEGDA system was very high, and the conversion ratio of C=C was close to 80.0% within 3.0 min. Meanwhile, for the systems of PEGDA575-PTMP and PEGDA700-PTMP the polymerization showed the similar tendency, that is, the polymerization rate increased with the rise of SH content within the first 3.0 min. By comparing PEGDA and PEGDA-PTMP systems, it could be seen that the conversion ratio of C=C was only 0.9% for PEGDA575 with the cumulative radiation time of 0.5 min, while the ratio increased to 72.0% for PEGDA575-PTMP mixture with C=C/SH molar ratio of 1.0 in the same radiation conditions. As the cumulative radiation time prolonged to 1.0 min, the conversion ratio of C=C increased from 26.9% for PEGDA700 to 80.0% for PEGDA700-PTMP. The reason that the C=C conversion ratio increased with the rise of SH content in PEGDA-PTMP system could be postulated that the SH group decayed into thiyl and hydrogen radicals under the UV radiation. The influence of SH on the reaction rate was also proved by the fact that PEGDA-PTMP mixture could be polymerized spontaneously into solid in ambient conditions within several hours even without photoinitiator. The advantage of the thiol-ene reaction that was less sensitive to the inhibitor effect of oxygen was reported in a previous work (12). However, it was found in the experiments that the photo-polymerization of PEGDA-PTMP system was inhibited obviously, as the monomers mixture was exposed to air and radiated directly by UV light.

[FIGURE 2 OMITTED]

The conversion rate of SH groups was also fast for PEGDA-PTMP system, in which the maximum conversion ratio was achieved within 3.0 min for all samples. However, the final conversion ratio of SH decreased with the rise of SH amount. For instance, the final conversion ratio of SH was close to 100.0% when the C=C/SH molar ratio was over 6.0, while it was below 60.0% as the C=C/SH molar ratio was lower than 1.0. Meanwhile, the experimental results showed that the conversion ratio of SH changed little when the radiation time was over 5.0 min. The results seemed to be contrary to the fact that the SH decayed easily into radicals under the UV radiation. The reason was that SH monomers did not homopolymerize, as a result, thiol radicals would simply undergo hydrogen abstraction and reform unreacted SH groups.

In the thiol-ene polymerization, it is expected that C=C and SH groups could be depleted completely within a short time. Figures 3 and 4 showed that the conversion ratios of C=C and SH were close to 90.0% and 100.0% respectively, within 3.0 min as the C=C/SH molar ratio was close to 6.0.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Influence of UV Intensity on Thiol-Ene Photopolymerization

In this study, the thiol-ene photopolymerization was introduced into the filed of injectable IOLs, to reduce the damages of UV radiation and photoinitiator toxicity to cells or tissues of eyes during the polymerization. The influence of UV intensity on the thiol-ene photopolymerization was first compared between PEGDA and PEGDA-PTMP systems. For the PEGDA-PTMP system, the monomers mixture with C=C/SH molar ratio of 6.0 was used in the following experiments because the ultimate conversion ratio of SH was close to 100.0% within a short radiation time as shown in Fig. 4. The decay of C=C peaks on the FTIR curves was detected after 2.5- and 10.0-min radiation, respectively. Figure 5 showed the plots of C=C conversion ratio with UV intensity under the condition that Irgacure 2959 concentration was 0.005 wt%. ft could be seen that the conversion ratio of C=C showed the similar tendency for PEGDA575 and PEGD700 systems. The conversion ratio was very low for PEGDA monomers within 2.5 min as the UV intensity was lower than 1.2 mW/[cm.sup.2], while the ratio increased obviously with the rise of radiation time or UV intensity. The influences of radiation time and UV intensity on the conversion ratio of C=C showed the similar results for PEGDA systems. The high-conversion ratio of C=C could be achieved by prolonging radiation time at low-UV intensity or increasing UV intensity within a short radiation time. Shortening the radiation time and reducing the UV intensity seemed to be contrary to achieve high C=C conversion ratio at the same experimental conditions. While with the addition of SH groups, the conversion ratio of C=C obviously increased even with the low-UV intensity and short radiation time. For instance, the conversion ratio was increased from about 0.1% for PEGDA 575 to 85.5% for PEGDA575-PTMP mixture with the UV intensity of 1.2 mW/[cm.sup.2] and radiation time of 2.5 min. Similarly, the ratio increased from 1.5% for PEGDA700 to 86.8% for PEGDA700-PTMP system in the aforementioned experimental conditions. Afterwards, prolonging the radiation time or enhancing the UV intensity did not increase the conversion ratio of C=C because most of C=C bonds were consumed. The fact that the conversion ratio of C=C was high even at low-UV intensity and short irradiation time could be postulated that the SH groups were sensitive to UV radiation and easily decayed into thiyl and hydrogen radicals even under low-UV intensity. With the increase of the radical concentration, the rate of polymerization was promoted and the conversion ratio of C=C reached a high level within short time.

[FIGURE 5 OMITTED]

Influence of Photoinitiator Concentration on the Thiol-Ene Photopolymerization

Most of photoinitiators are unsuitable for bio-application because of their toxicity to the surrounding tissue or cells. The side effects of various photoinitiators were reported in the pilot study of injectable IOLs, such as conjunctival injection, corneal opacity, anterior chamber reaction, and hypopyon (9). In this study, lrgacure2959 was used because of its less toxicity; however, previous study showed that the corneal was easily caused opacity as the Irgacure2959 concentration was above 0.01% (9). In this study, the comparison between homopolymeriza-tion and thiol-ene photopolymerization was analyzed at different photoinitiator concentration with the UV intensity of 4.5 mW/[cm.sup.2]. As expected, the PEGDA-PTMP system showed higher reactivity even with the low Irgacure2959 concentration, for example, 78.3% of C=C was depleted with 0.001% photoinitiator and 2.5 min UV radiation for PEGDA575-PTMP monomers mixture, while only 5.5% of C=C bonds were consumed at the same conditions for PEGDA575 as shown in Fig. 6. The thiol-ene polymerization could also be achieved even without photoinitiator, for instance, 74.5% conversion ratio of C=C was achieved with 10.0 min UV radiation and 4.5 mW/[cm.sup.2] UV intensity for PEGDA575-PTMP system without photoinitiator; however, the similar conversion ratio would require at least 0.01% photoinitiator for PEGDA575 system in the same conditions mentioned earlier. It could be postulated that SH groups acted as the photoinitiator in the PEGDA-PTMP system, in which SH groups decayed into radicals thus the polymerization was accelerated under UV radiation. Furthermore, it was interesting to find the fact that the plot of C=C conversion ratio for PEGDA575 with 10.0 min of UV radiation was similar to that of PEGDA575-PTMP monomers mixture with 2.5-min radiation, indicating that the same polymerization results could be achieved within shorter radiation time by adding SH groups into PEGDA system. The similar results were also obtained for PEGDA700-PTMP system. The experimental results indicated that the thiol-ene photopolymerization could be used in the field of injectable materials because there would be less damage from UV light and photoinitiator toxicity to the filled site during the photopolymerization.

[FIGURE 6 OMITTED]

Polymerization of Bulk Samples

The bulk polymers with the size of 2.0-mm thickness and 15.0-mm diameter were prepared in glass mold. The polymerization time was defined as the period of time from mobile liquid monomers to solid polymer. Figure 7 showed the plots of polymerization time on photoinitiator concentration with 4.5 mW/[cm.sup.2] UV intensity. The polymerization time decreased with the rise of photoinitiator concentration; meanwhile, the time for PEGDA-PTMP system was obviously shorter than that of PEGDA in the same experimental conditions. As the concentration of Irgacure2959 was 0.005%, the polymerization time was about 2.5 min for PEGDA575-PTMP and PEGDA700-PTMP systems, whereas it was about 17.5 min and 5.0 min for PEGDA575 and PEGDA700, respectively. With decreasing the Irgacure2959 concentration to 0.001%, the mixture of PEGDA575-PTMP monomers was polymerized within 10.0 min, whereas PEGEDA575 could not be polymerized within 30.0 min. Similarly, the polymerization time was shortened from 30.0 min for PEGDA700 to 5.0 min for PEGDA700-PTMP system as the concentration of Irgacure2959 was 0.001%. Moreover, PEGDA700-PTMP system could be polymerized within 10.0 min UV radiation even without photoinitiator, whereas solid polymers were not obtained within 30.0 min UV radiation for PEGDA700 without Irgacure2959. Figure 8 showed the influence of UV intensity on the polymerization time of PEGDA and PEGDA-PTMP systems with the Irgacure-2959 concentration of 0.005 wt%. It could be seen that the polymerization time increased with the decrease of UV intensity. PEGDA575-PTMP system could be cured within 10.0 min with UV intensity of 2.5 mW/[cm.sup.2]; however, PEGDA575 was not polymerized in the same conditions.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

The aforementioned results indicate that PEGDA-PTMP system could be photopolymerized with less-UV intensity and photoinitiator content than PEGDA system. It could be attributed to the sensitivity of thiol groups to UV radiation. Radicals decayed from thiol groups were beneficial to the polymerization of double bonds. The fact that the polymerization time was not consistent with the conversion rate of C=C could be attributed to the impact of thickness of samples. Whereas the bulk sample and the thin film showed the similar tendency, that is, photopoly-merization was accelerated with the addition of PTMP into PEGDA systems.

However, one side effect of fast polymerization was that lots of heat was released within a short period and a thermal front (a sudden increase of temperature) was produced. For instance, the temperature rose from 28.0 to 50.0[degrees]C for PEGDA575-PTMP system and to 46.0[degrees]C for PEGDA700-PTMP system as the UV intensity was 4.5 mW/[cm.sup.2] and the concentration of Irgacure2959 was 0.005% (sec Fig. 9). However, the thermal front was not the unique phenomenon for the thiol-ene photopolymeri-zation. There was also an obvious increase of temperature in PEGDA systems during the photopolymerization. For instance, the temperature increased from 28.0 to 44.0[degrees]C within 5.0 min for PEGDA700 system when the concentration of Irgacure2959 was 0.005% and the UV intensity was 4.5 mW/[cm.sup.2]. Decreasing the polymerization rate properly by reducing the UV intensity or the photoinitiator concentration could limit the formation of the thermal front. For instance, the temperature in PEGDA700-PTMP system increased about 13.0[degrees]C when the UV intensity and the concentration of Irgacure2959 were 1.5 mW/[cm.sup.2] and 0.005%, respectively. While the curing temperature increased about 9.0[degrees]C when the UV intensity and the concentration of Irgacure2959 were 1.5 mW/[cm.sup.2] and 0.001%, respectively. However, the method of decreasing the polymerization rate to reduce the thermal front is very limited, because the heat produced during polymerization is mainly dependent on the amount of double bonds reacted in the system; meanwhile, a proper polymerization rate is necessary for the injectable materials. By comparing the Fig. 9a and b, it could be also found that the released heat of PEGDA700-PTPM system was less than that of PEGDA575-PTMP system. The main reason was that PEGDA700 possesses higher molecular weight and less double bond content compared with PEGDA575, thus the reacted double bonds was less for PEGDA700-PTPM system during the polymerization. So in the future research, the selection of monomers with high-molecular weight and less double bond content is necessary to avoid lots of heat release during the thiol-ene photopolymerization.

[FIGURE 9 OMITTED]

Characterization of Bulk Samples

After the polymerization of PEGDA-PTMP systems, the transparent copolymers were obtained. The addition of PTMP into PEGDA induced a little heteropolymerization; however, the influence of PTMP on the transparency of copolymers is very limited. The transmittances of PEGDA-PTMP copolymers were both over 80.0% in the visible light range. Even after immersed in 0.9 wt% NaCl aqueous solution for about 4 weeks, the copolymers still remained the high transparency as shown in Fig. 10. The aforementioned results indicate that highly transparent polymers could also be obtained by the thiol-ene photopolymerization, just like those obtained by the homopolyme-rization (1).

[FIGURE 10 OMITTED]

The EWCs of PEGDA and PEGDA-PTMP (C=C/SH molar ratio of 6.0) bulk samples were measured as shown in Table 1. The bulk samples were obtained under the conditions that the concentration of Irgacure2959 was 0.01 wt%, the UV intensity was 4.5 mW/[cm.sup.2] and the radiation time was 5.0 min. The EWC of PEGDA575-PTMP copolymer was about 30.5%, which was lower than that of PEGDA700-PTMP copolymer (about 38.4%). It was due to the reason that the molecular chain of PEGDA575 between two double bonds is shorter than that of PEGDA700. After polymerization, the crosslinking density of PEGDA575-PTMP copolymer was higher than that of PEGDA700-PTMP copolymers. As a result, the mesh size in PEGDA575-PTMP copolymer was smaller than that of PEGDA700-PTMP, and there are less water molecules absorbed into PEGDA575-PTMP compact networks. Moreover, the EWCs for PEGDA575 and PEGDA700 polymers were about 31.0 and 39.0%, which were very close to those of PEGDA575-PTMP and PEGDA700-PTMP copolymers, respectively. These results demonstrated that the hydrophilicity of PEGDA-PTMP copolymers was similar to that of PEGDA polymers. The mass losses of PEGDA575-PTMP and PEGDA700-PTMP copolymers were also measured. The results showed that the mass losses of those two kinds of copolymers were both <0.75%. While the mass losses for PEGDA polymers were close to 1.0%, which was consistent with the previous study (1). The aforementioned results indicate that the mixture of PEGDA-PTMP monomers could be photopolymerized more completely than PEGDA monomers under the same experimental conditions.
TABLE 1. EWC and mass loss values of PEGDA and PEGDA-PTMP (C=C/SH molar
ratio 6.0) bulk samples.

Samples                 EWC (%)         Mass loss (%)

PEGDA575         31.0 [+ or -] 1.0    0.9 [+ or -] 0.20
PEGDA700         39.0 [+ or -] 1.0    1.2 [+ or -] 0.25
PEGDA575-PTMP    30.5 [+ or -] 1.0    0.4 [+ or -] 0.15
PEGDA700-PTMP    38.4 [+ or -] 1.0    0.6 [+ or -] 0.15


To compare the differences of the elastic modulus between PEGDA bulk polymers and PEGEDA-PTMP bulk copolymers, the bulk samples were obtained under the conditions that the concentration of Irgacure2959, the UV intensity and the radiation time were 0.01 wt%, 4.5 mW/[cm.sup.2] and 5.0 min, respectively. By the addition of PTMP into PEGDA, the elastic modulus of PEGEDA-PTMP copolymers decreased. The value of the elastic modulus decreased from 11.54 [+ or -] 0.28 MPa for PEGDA575 polymer to 7.44 [+ or -] 0.30 MPa for PEGDA575-PTMP copolymer; similarly, the value decreased from 9.77 [+ or -] 0.27 MPa for PEGDA700 polymer to 4.89 [+ or -] 0.24 MPa for PEGDA700-PTMP copolymer. Meanwhile, it was found that the PEGDA-PTMP copolymers were more flexible than PEGDA polymers, which was proved by the fact that PEGDA polymers were easily cracked when exposed in air for a short period, whereas PEGDA-PTMP copolymer remained intact at the same conditions. Unfortunately, the elastic modulus values of PEGDA-PTMP or PEGDA bulk samples are far higher than that of natural lens (about 1-5 kPa), and the shapes of samples are hardly accommodated through the micro-force just like the force of suspensory ligament contraction in eyes. For the polymer materials, the high-elastic modulus values usually results from the high-crosslinking density, which was proved by the fact that PEGDA575 or PEGDA-575 bulk sample has higher value than that of PEGDA700 or PEGDA700-PTMP bulk one. Just like the strategy of decreasing heat release as mentioned earlier, the selection of the monomers with high-molecular weight and less double bond content is also an effective way to reduce the crosslinking density, and thus to decrease the elastic modulus of the polymerized samples.

CONCLUSIONS

To explore the application of the thiol-ene photopoly-mcrization in the field of injectable IOLs, the differences between the homopolymerization and the thiol-ene photo-polymerization were compared. It was found that thiol-ene photopolymerization was more suitable for the injectable IOLs than homopolymerization, because the former could be achieved with less-UV intensity and photoinitia-tor content, thus the damages to eye from the UV radiation and photoinitiator toxicity were greatly reduced. The PEGDA-PTMP (C=C/SH molar ratio being 6.0) bulk samples and the PEGDA bulk ones were obtained through the thiol-ene photopolymerization and the homophotopo-lymerization, respectively. The PEGDA-PTMP bulk samples showed the similar transparency, hydrophilicity, and EWCs as the PEGDA ones; while the flexibility of PEGDA-PTMP bulk samples was improved and their elastic modulus was decreased. Meanwhile, some draw- backs were founded in the experiments, such as lots of heat released during polymerization and the PEGDA-PTMP or PEGDA bulk samples with far higher elastic modulus than that of natural lenses. The experimental results indicated that the thiol-ene photopolymerization is one potential method to obtain the ideal injectable IOLs materials, although many works have to be done in the future research, such as selecting proper monomers, testing the cytotoxicity and biocompatibility of bulk samples.

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Correspondence to: Huai Yang; e-mail: yanghuai@mater.ustb.edu.cn or Siquan Zhu; e-mail: siquanzhu@sina.com

Contract grant sponsor: Chinese National Science and Technology; con-tract grant number: 2006BA103A09; contract grant sponsor: National High Technology Research and Development Program ("863" Program) of China; contract grant number: 2008AA03A318.

DOI 10.1002/pen.21465

Guoguang Niu, (1) Li Song, (1) Hongbin Zhang, (1) Xiaopeng Cui, (1) Miki Kashima, (1) Zhou Yang, (1) Hui Cao, (1) Guojie Wang, (1) Yudong Zheng, (1) Siquan Zhu, (2) Huai Yang (1)

(1) Department of Materials Physics and Chemistry, School of Materials Science and Engineering, University of Science and Technology Beijing, China

(2) Department of Ophthalmology, Tongren Hospital, Capital Medical University Beijing, China
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Author:Niu, Guoguang; Song, Li; Zhang, Hongbin; Cui, Xiaopeng; Kashima, Miki; Yang, Zhou; Cao, Hui; Wang, G
Publication:Polymer Engineering and Science
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Geographic Code:9CHIN
Date:Jan 1, 2010
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