Article pubs.acs.org/JPCB
The Relationship between the Hydrophilicity and Surface Chemical Composition Microphase Separation Structure of Multicomponent Silicone Hydrogels Zheng-Bai Zhao,† Shuang-Shuang An,† Hai-Jiao Xie,† Xue-Lian Han,‡ Fu-He Wang,‡ and Yong Jiang*,† †
School of Chemistry and Chemical Engineering, Southeast University, Jiangning, Nanjing, Jiangsu 211189, P. R. China R&D Center, Hydron Contact Lens Co., Ltd, Danyang, Jiangsu 212331, P. R. China
‡
S Supporting Information *
ABSTRACT: Three series of multicomponent silicone hydrogels were prepared by the copolymerization of two hydrophobic silicon monomers bis(trimethylsilyloxy) methylsilylpropyl glycerol methacrylate (SiMA) and tris(trimethylsiloxy) 3-methacryloxypropylsilane (TRIS) with three hydrophilic monomers. The surface hydrophilicity of the silicone hydrogels was characterized by contact angle measurements, and an interesting phenomenon was found that the silicone hydrogels made from less hydrophobic monomer SiMA possess more hydrophobic surfaces than those made from TRIS. The surface properties such as morphology and elemental composition of the silicone hydrogels were explored by scanning electron microscopy (SEM) imaging and energy dispersive spectrometry (EDS) analysis, and their relationships with the surface hydrophilicity were investigated in details. The results show neither the surface morphology nor the elemental composition has obvious impact on the surface hydrophilicity. Atomic force microscopy (AFM) imaging revealed that SiMA hydrogel had a more significant phase separation structure, which also made its surface uneven: a lot of tiny holes were observed on the surface. This surface phase separation structure made SiMA hydrogel more difficult to be wetted by water or PBS buffer, i.e., more hydrophobic than TRIS hydrogel. On the basis of these results, we propose that the phase separation structure as well as the nature of silicon monomers might be the fundamental reasons of surface hydrophilicity. These results could help to design a silicone hydrogel with better surface properties and wider application.
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INTRODUCTION Biomaterials are compounds of natural and artificial origin that can mimic, store, or come into close contact with living biological cells or fluids.1 Over the last several decades, the application of biomaterials has shown steady growth, particularly in the areas of medicine, biology, material science and engineering. Specific applications of biomaterials include contact lenses, intraocular lens, blood-contacting medical devices, and artificial tissue like bone, or tendon, or even arteries.2−12 Hydrogels have been one of the most important biomaterials because of their hydrophilic property and potential to be biocompatible.13−22 They have been generally considered as three-dimensionally cross-linked hydrophilic polymer networks capable of swelling and retaining possibly large amounts of water in the swollen state.23−28 Although hydrogels possess many advantages and have been used in a variety of areas including gas separation, liquid separation, contact lenses and intraocular lens,29−31 their development is still restricted because of the poor mechanical strength, low oxygen permeability and so on. It is difficult to improve the oxygen permeability of hydrogels simply by increasing their water contents because the increase in water contents will lead to the sharp decrease in the mechanical strengths. Then, a new type of hydrogel emerged, i.e., the silicone hydrogel that prepared by © XXXX American Chemical Society
copolymerizing of hydrophilic monomers with hydrophobic silicon monomers. The silicone hydrogels have attracted increasing interests among scientists and became widely applied in many fields, especially contact lenses manufacturing.32−37 High oxygen permeability is an obvious advantage of the silicone hydrogels compared to the conventional hydrogels. However, one critical disadvantage is their innate surface hydrophobicity, which will affect the biocompatibility of the biomaterial. Normally, a biomaterial with a high degree of wettability would be more biocompatible.38 The two usual methods to improve surface hydrophilicity are plasma treatment39 and grafting40 of hydrophilic polymers onto the biomaterials. Lopez-Alemany et al. have found that in the case of plasma treatment, the oxygen present in the plasma oxidized the siloxane groups into silicate. The silicate pieces increased the hydrophilicity of the silicone hydrogels without affecting the oxygen permeability because the silicate pieces did not cover the entire surface.41,42 Thissen et al. produced a coating on the silicone hydrogel by depositing a thin film of allylamine plasma polymer as a reactive interlayer for the high Received: May 1, 2015 Revised: June 28, 2015
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stack polypropylene molds for 3 h at the same temperature. The newly formed copolymer was purified by extraction with ethanol for 16 h and then hydrated in boiling water for 4 h. The obtained silicone hydrogels were preserved in phosphate buffered saline. Totally, three series of silicone hydrogels were prepared using the formula lists in Tables 1, 2, and 3. For all the formulations, the amounts of the cross-linking agent EGDMA and the initiator D-1173 used were 1 and 0.2 wt %, respectively.48 And for each formula, at least 30 pieces of hydrogels were prepared for the next step measurements. Measurement of Contact Angle. The surface contact angles on the silicone hydrogels were determined using a contact angle system (KSV CAM-200, KSV Ins). A high speed camera with 2 ms frame interval was used to capture the momentary droplet images when the water droplet (2 μL) was collided on the surfaces. Additionally, the contact angles were calculated using a well-known method. Measurement of Surface Morphology and Elemental Composition. The surface micromorphology was characterized by a scanning electron microscope (SEM FEI Inspect F50). The surface elemental composition was determined by energy dispersive X-ray spectroscopy (EDS) equipped with the same SEM. The ratios of the elements were calculated based on at least three measurements. Measurement of Surface Phase Structure. The surface microphase separation structure was characterized by an atomic force microscope (AFM Dimension ICON). The hydrogel sample was cut into a small piece and immobilized on the mica surface. Both the height and phase images were collected.
density grafting of poly(ethylene oxide) dialdehyde, which had previously shown complete resistance to protein adsorption.43 Wavhal et al. grafted hydrophilic poly(acrylic acid) onto the surface of biomaterial membrane after plasma treatment and found it to be an effective method to improve the hydrophilicity.44 Most of these previous studies were focused on the development of new silicone hydrogel materials with good surface hydrophilicity for application purposes. Many factors for surface hydrophilicity have been given attention, such as surface roughness, surface energy, topography, and so on. However, little concern has been focused on the fundamental relationship between the hydrophilicity and the surface chemical composition microphase separation structure of the silicone hydrogels. Some researchers have reported that the phase structure or surface chemical composition has an influence on the silicones’ surface hydrophilicity.45−47 However, the underlying mechanism has not been studied in detail. Meanwhile, the effect of different silicon monomers on the surface hydrophobicity of silicone hydrogels is not yet clear. We have done a great deal of work on multifunctional hydrogels.48,49 Several series of silicone hydrogels were prepared by the copolymerization of silicon monomers with different hydrophilic monomers. Their optical performance, permeability, and mechanical properties were studied in detail.48 The oxygen permeating coefficients (Dk) were measured, and their relationships with equilibrium water content (EWC) and internal morphologies were discussed in detail.49 In this work, several series of silicone hydrogels were designed to study their surface hydrophobicity. The hydrophilicity of hydrogel surface was evaluated by a contact angles test. The surface morphology was characterized by scanning electron microscopy (SEM), and the elemental composition of the surface was analyzed by energy dispersive spectrometry (EDS). The surface microphase separation structure was characterized by an atomic force microscopy (AFM). The relationship of surface hydrophilicity with surface morphology, elemental composition, and phase separation structures was discussed in detail. Finally, the basic mechanism that silicone hydrogels made from more hydrophobic monomer TRIS were less hydrophobic than those made from SiMA was proposed.
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RESULTS AND DISCUSSION In this paper, UV radiation was adopted to initiate the copolymerization since UV-initiated polymerization is much faster and easier to control compared to thermal initiated polymerization. For the formulations in Table 1, bulk Table 1. Copolymerization Formulations for the SiMA Hydrogels with Different Percentages of Hydrophobic Monomer SiMA
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EXPERIMENTAL SECTION Materials. 3-Methacryloxy propyl tris(trimethylsiloxy) saline (TRIS, CAS No:17096-07-0) was obtained from Alfa Aesar Chemical Co. 3-Bis(trimethylsilyloxy) methylsilylpropyl glycerol methacrylate (SiMA, CAS No: 69861-02-5) was bought from Cornelius Specification Ltd. N-Vinylpyrrolidone (NVP) and 2-hydroxy-2-methylbenzene acetone (D-1173) were purchased from TCI (Shanghai) Development. N,NDimethyl acrylamide (DMA) was bought from Aldrich Chemical Co. 2-Hydroxyethyl methacrylate (HEMA) was obtained from BASF Chemical Co. Ethylene glycol dimethacrylate (EGDMA) was got from TCI (Shanghai) Development. 1-Hexanol was obtained from Sino Pharm Chemical Reagent Co. Some of the chemicals were purified by distillation under reduced pressure before use if needed. Sample Preparation. For making silicone hydrogels, the photo initiator D-1173 and cross-linking agent EGDMA were added in flask first. Then, HEMA, NVP, and silicone monomers were added in orders. After that, a certain amount of DMA was added into the mixture. At last, the solvent hexanol was added. The formulation mixture was stirred for 2 h in the dark at 25 °C, and then it was UV cured (≥15 mV/cm2) in the double
hydrogel
S0
S10
S20
S30
S40
S50
SiMA (wt %) HEMA (wt %) NVP (wt %) DMA (wt %) EGDMA (wt %) D-1173 (wt %)
0 36 54 10 1 0.2
10 32 48 10 1 0.2
20 28 42 10 1 0.2
30 24 36 10 1 0.2
40 20 30 10 1 0.2
50 16 24 10 1 0.2
polymerization was adopted because these monomers were miscible together without help of solvent. Six different types of hydrogels named from S0 to S50 were prepared using the formulations listed in Table 1. The percentages of SiMA increased from 0 to 50 wt %, and the percentage of NVP and HEMA decreased correspondingly. Figure 1 shows the relationship between the contact angle and SiMA content. When the SiMA content increased from 0 to 10 wt %, the contact angel increased quickly from 45 ± 8° to 106 ± 9°, which means the surface of the SiMA hydrogel turned into hydrophobic immediately. However, after that, the contact angle almost had no change with the increasing of SiMA content from 10 to 50 wt %. It demonstrates that the surface hydrophobicity of the silicone hydrogel did not change with the increase of SiMA content at this range. B
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Figure 1. Contact angle of silicone hydrogels as the function of the SiMA content.
Figure 3. Element contents of C, N, O, and Si as a function of the SiMA content.
Figure 2 shows the SEM images of the silicone hydrogels. The surface of the hydrogel became rougher with the increase
content, which means that the hydrophobic silicone groups can attach to the surface because the silicon group has a lower surface free energy than those of the hydrophilic groups. We have observed and analyzed the morphologies and chemical compositions of the cross sections of silicone hydrogel with different SiMA contents by SEM and EDS before.49 The results showed that the internal compositions were also not homogeneous, and the content of silicon element in bulk hydrogel is obviously less than that on the surface. The surface of silicone hydrogel became hydrophobic mainly due to the enrichment of silicon group. The two most important factors that affect the surface hydrophobicity are surface free energy and surface roughness. On the basis of the result in Figure 2, the tiny change of surface roughness did not affect the hydrophilicity of silicone hydrogel. And as shown in Figure 3, the silicon content on the surface of hydrogel would reach about 20% when 10 wt % SiMA was added. At this range, this silicon content would lower the surface free energy to make the surface hydrophobic. However, the surface free energy would not be further lowered with increasing of the silicon content. Therefore, the wettability of the silicone hydrogel did not change with the increase of SiMA from 10 to 50 wt %. In order to compare the surface hydrophilicity of the hydrogels made from different silicon monomers, another series of hydrogels named S30, S15-T15, and T30 were prepared using the formulations listed in Table 2. As seen from Scheme 1, TRIS is a more hydrophobic monomer that cannot be miscible with those hydrophilic monomers because it has one more silicon element and no hydroxyl group compared with
Figure 2. SEM images of the surface of silicone hydrogels with SiMA percentages of (A) 0 wt %, (B) 10 wt %, (C) 30 wt %, and (D) 50 wt %.
of SiMA content. Because SiMA is a hydrophobic monomer, it will separate with hydrophilic monomers to form microphases during the copolymerization process, which might lead to a rough surface. However, the hydrophobicity of the surface has small change with the SiMA content increasing from 10 to 50 wt %. This implied that the influence of surface roughness on the hydrophilicity of silicone hydrogel was negligible. According to previous reports, many silicone groups could form embossments on the surface of the silicone hydrogel.33,41,50,51 Here, EDS was used to analyze the content of elements on the surface and the results indicated that the elements content has obviously changed when SiMA was added from 0 to 50 wt %. According to Figure 3, we find that the content of the nitrogen element has become 0 wt % when the content of SiMA increased to 10 wt %. This indicated that the hydrophilic group pyrrolidone cannot accumulate on the surface of the silicone hydrogel. However, the content of silicon element increased all the time with the increase of SiMA
Table 2. Copolymerization Formulations for the SiMA-TRIS Hydrogels with Different Kinds of Silicone Monomer SiMA or/and TRIS
C
hydrogel
S30
S15-T15
T30
SiMA (wt %) TRIS (wt %) HEMA (wt %) NVP (wt %) DMA (wt %) EGDMA (wt %) D-1173 (wt %) 1-hexanol (wt %)
30 0 24 36 10 1 0.2 20
15 15 24 36 10 1 0.2 20
0 30 24 36 10 1 0.2 20
DOI: 10.1021/acs.jpcb.5b04202 J. Phys. Chem. B XXXX, XXX, XXX−XXX
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Table 3. Copolymerization Formulations for the TRIS Hydrogels with Different Percentages of Hydrophobic Monomer TRIS hydrogel
T10
T20
T30
T40
T50
TRIS (%) HEMA (%) NVP (%) DMA (%) EGDMA (%) D-1173 (%) 1-Hexanol (%)
10 32 48 10 2 0.2 20
20 28 42 10 2 0.2 20
30 24 36 10 2 0.2 20
40 20 30 10 2 0.2 20
50 16 24 10 2 0.2 20
a (A) N,N-dimethyl acrylamide (DMA); (B) N-vinyl pyrrolidone (NVP); (C) 2-hydroxyethyl methacrylate (HEMA); (E) 3-bis(trimethylsilyloxy) methylsilyl propyl glycerol methacrylate (SiMA); (F) 3-methacryloxy propyl tris(trimethylsiloxy) saline (TRIS).
SiMA. And in this case, solvent was added to help these monomers mix together. In Table 2, the total percentage of silicone monomer was fixed at 30 wt % and three different hydrogels labeled S30, S15-T15, and T30 were prepared. Figure 4 shows the contact angles of the three different kinds of silicone hydrogels. The S30, S15-T15, and T30 hydrogels
Figure 5. Contact angle value of silicone hydrogels as a function of the content of silicon monomers: SiMA (black square), TRIS (red circle).
phobic monomer TRIS were less hydrophobic than the hydrogels made from SiMA monomer. It also confirmed that the surface hydrophilicity of silicone hydrogel was not related to the content of the silicon monomers but strongly depended on the types of silicon monomers. The surfaces of all the hydrogels that we prepared were measured and analyzed by SEM and EDS. Figure S1 in the Supporting Information showed the SEM images of these hydrogels. There was no obvious difference on the surface among these silicon hydrogels, and many embossments could be found on different silicone hydrogels. It demonstrates that the different silicon monomers had similar influences on the surface of hydrogels. Then, we analyzed the elemental composition of the embossment area and the flat area of different silicone hydrogels using EDS, and the results were shown in Figure 6. For all the silicone hydrogels, the percentage of silicon element increased, and the percentages of oxygen and nitrogen elements decreased as the content of silicon monomer increased at different areas. And for all the hydrogels, the percentages of silicon element on the embossments were always higher than those on the flat areas, which indicated that the silicon element was not equally distributed on the surface but enriched on the embossments. According to the above results, it is clear that the hydrophilicity of TRIS hydrogel is better than that of SiMA hydrogel. However, this difference is due to neither the difference of surface morphology nor the difference of elemental composition between these two serials of hydrogels. According to the current results and the former findings,49 we think it may be caused by the phase separation morphology on the surface of the silicone hydrogels.
Figure 4. Contact angle images of silicone hydrogels listed in Table 2. (A) The silicone hydrogel S30 that contained 30 wt % SiMA had a contact angle of 99.7°. (B) The silicone hydrogel S15-T15 containing 15 wt % SiMA and 15 wt % TRIS had a contact angle of 85.4°. (C) The silicone hydrogel T30 containing 30 wt % TRIS had a contact angle of 78.2°.
had a contact angle of 99 ± 6°, 83 ± 4°, and 75 ± 4°, respectively. The detailed results of 10 different samples for each silicone hydrogels are shown in Table S1 in the Supporting Information. An interesting phenomenon was found from Figure 4 and Table S1 in that the surface hydrophilicity of TRIS hydrogels was better than that of SiMA hydrogels. In order to get more specific and accurate results, a series of silicone hydrogels were synthesized using TRIS monomer corresponding to SiMA hydrogel. As listed in Table 3, five different types of TRIS hydrogels named from T10 to T50 were obtained. As shown in Figure 5, the contact angles of TRIS hydrogels did not change and were always about 30° smaller than that of SiMA hydrogels no matter how much silicone monomers were added. These results confirmed the former findings that the silicone hydrogels made from more hydroD
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Figure 6. EDS analysis shows the element contents of different areas on different silicone hydrogels. (A,B) Embossments and flat area of the SiMA hydrogels, respectively; (C,D) embossments and flat area of the TRIS hydrogels; (E,F) embossments and flat area of the SiMA-TRIS hydrogels.
during the contact angle measurement, which made the surface of SiMA hydrogel more hydrophobic. However, the phase separation structure on the surface of TRIS hydrogel was unobvious as shown in Figure 7D. One possible reason is that TRIS is more hydrophobic monomer than SiMA. When TRIS was copolymerized with hydrophilic monomers, silicon monomers tend to polymerize together. This led to a bigger hydrophobic phase field. Therefore, tiny holes that could store air bubbles did not exist, and the surface of TRIS hydrogel was more homogeneous than that of SiMA hydrogel. The other possible reason is that, based on our former observation,52 the phase structure of silicone phase in the SiMA hydrogel was spherical, and the silicone structure in the TRIS hydrogel was fibrous. This might indicate that the surface phase structure may have an influence on the surface hydrophilicity of the multicomponent silicone hydrogel. The fibrous structure in the TRIS hydrogel might be less hydrophobic than the spherical phase structure in the SiMA
In order to prove our assumption, the surface structures of these two silicone hydrogels were observed by AFM imaging. The images are shown in Figure 7 and Figure S2 in the Supporting Information. Figure 7A,B were the height and phase images showing the same area of SiMA hydrogel S30. From the phase image shown in Figure 7B, it is clear that the SiMA hydrogel has a significant phase separation structure on the surface. According to the formula of this hydrogel, 30 wt % of SiMA monomer was added in this hydrogel. So the dark areas were hydrophobic SiMA phase and the size of this phase region was about 100 nm, which was consistent with the result observed using TEM.49 These phase separation structures made the surface of the hydrogel uneven, meanwhile, many tiny holes formed on the surface of SiMA hydrogel as shown in Figure 7A. Figure 2C showed the SEM image of the same SiMA hydrogel. However, the scan area was too big to resolve this tiny phase separation structure. These tiny holes could store the air bubbles when the surface was wetted by water or PBS buffer E
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composition. AFM imaging revealed that SiMA hydrogel had a more significant phase separation structure, which made its surface uneven also, a lot of tiny holes were observed on the surface. This surface phase separation structure made SiMA hydrogel more hydrophobic than TRIS hydrogel. On the basis of these results, we propose that the phase separation structure and the nature of silicon monomers might be the fundamental reasons for surface hydrophilicity. These results could help to design a silicone hydrogel with better properties and wider applications.
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ASSOCIATED CONTENT
S Supporting Information *
Table S1 shows the results of the contact angles of three different silicone hydrogels. Figure S1 shows the SEM images for the surface of eight different silicone hydrogels. Figure S2 shows the AFM images for the surface of four different silicone hydrogels. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcb.5b04202.
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Figure 7. AFM images of the silicone hydrogels. (A,B) Height and phase images of SiMA hydrogel S30; (C,D) height and phase images of TRIS hydrogel T30. Scan sizes are 2 μm.
AUTHOR INFORMATION
Corresponding Author
*Mailing address: School of Chemistry and Chemical Engineering, Southeast University, No. 2 Dongnandaxue Road, Jiangning District Nanjing, Jiangsu, 211189, P. R. China. Phone: +86 139 139 931 09; E-mail:
[email protected]; Web: http://jianglab.net.
hydrogel. As the result, the surface of TRIS hydrogel shown in Figure 7C was flatter than the surface of SiMA hydrogel shown in Figure 7A. The surface hydrophilicity of TRIS hydrogel was only affected by the surface composition. So considering the surface chemical composition and surface phase separation structures together, the hydrogel made from less hydrophobic monomer SiMA had a more hydrophobic surface. Figure S2 in the Supporting Information shows the relationship between the surface structures of another two silicone hydrogels and their silicon monomer contents. Figure S2A,B,C, D shows the height and phase images showing the same area of SiMA hydrogels S10 and S50. With the silicon monomer content increased, more tiny holes appeared on the surface of silicone hydrogels. The spherical phase-separation structures have arranged more tightly with the increase of the SiMA content. Figure S2E,F,G,H shows the height and phase images showing the same area of TRIS hydrogels T10 and T50. There were not obvious phase-separation structures on the surface of TRIS hydrogels with the TRIS content increased. According to the results in Figure S2, the phase structures on the surface of the silicone hydrogels were similar to that shown in Figure 7. This means these phase structures was not related to the contents of the silicon monomer. The nature of silicon monomer has further influence on the surface phase structure than their contents.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC) with Grant Number 21174029 to Y.J. and the Industry Academia Cooperation Innovation Fund of Jiangsu Province with Grant Number BY2014127-07 to Y.J.
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CONCLUSION Three series of silicone hydrogels were prepared using two kinds of silicon monomers, and their surface hydrophilicity was discussed in detail. The contact angles of the hydrogels reached to maximum immediately when the content of silicon monomer increased to 10 wt %, and the contact angles did not change when the content of silicon monomer increased further. The hydrophilicity of silicone hydrogels made from more hydrophobic silicon monomer TRIS is better than those made from SiMA. Meanwhile, SEM imaging and EDS analysis showed that the hydrophilicity of silicone hydrogel neither related to the surface morphology nor the surface elemental F
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DOI: 10.1021/acs.jpcb.5b04202 J. Phys. Chem. B XXXX, XXX, XXX−XXX
