Synthesis and biocompatibility of a novel silicone hydrogel containing phosphorylcholine

European Polymer Journal 47 (2011) 1795–1803

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Synthesis and biocompatibility of a novel silicone hydrogel containing phosphorylcholine Lin Li, Jun-Hua Wang, Zhong Xin ⇑ State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

a r t i c l e

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Article history: Received 18 February 2011 Received in revised form 19 May 2011 Accepted 15 June 2011 Available online 29 June 2011 Keywords: Protein adsorption Platelet adhesion 2-Methacryloyloxyethyl phosphorylcholine Tris(trimethylsiloxy)-3methacryloxypropylsilane

a b s t r a c t A novel poly (methyl methacrylate-co-2-methacryloyloxyethyl phosphorylcholineco-tris(trimethylsiloxy)-3-methacryloxypropylsilane) terpolymer (PMMT) was synthesized and the anti-biofouling properties were studied in order to evaluate the potential of this silicone hydrogel to be used as contact lens material. The chemical structure was characterized by IR, 1H-NMR and elemental analysis. Poly methyl methacrylate (PMMA) and poly (methyl methacrylate-co-tris (trimethylsiloxy)-3-methacryloxypropylsilane) (PMT) were also synthesized to compare with PMMT. Protein adsorption measurement showed that for PMMT-2 membrane (MPC: 16.6 mol%), the amount of adsorbed proteins was decreased by 75.3% and 76.8% compared with PMMA and PMT membranes, respectively. SEM pictures showed clearly that PMMT films suppressed the adhesion of platelets. Water structure in polymers was determined by differential scanning calorimetry, and PMMT-2 possessed more freezing water content than PMMT-1 (MPC: 14.5 mol%). The equilibrium water content of PMMT-2 membrane reached 55%, which might offer comfortable wear feeling, and the introduction of MPC increased the wettability of the polymer surface dramatically. Both the terpolymers PMMT-1 and PMMT-2 exhibited high transparency (relatively constant at approximately 96%) in the visible light wave range. Moreover, oxygen transmissibility (Dk/ t) of the terpolymers met the requirement for a lens to be worn safely overnight. It could be concluded that the novel silicone hydrogel containing MPC unit was an effective candidate material for making biomaterials, particularly for contact lenses. Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.

1. Introduction To design a hydrogel for contact lens application, the candidate materials must satisfy several requirements, including that: the lens must be optically transparent, possess chemical and thermal stability. In addition, since the material is directly in contact with the eye tissue, it should Abbreviations: MPC, 2-methacryloyloxyethyl phosphorylcholine; TRIS, tris(trimethylsiloxy)-3-methacryloxypropylsilane; PMMA, poly methyl methacrylate; PMT, poly(methyl methacrylate-co-tris (trimethylsiloxy)-3-methacryloxypropylsilane); PMMT, poly(methyl methacrylate-co-2-methacryloyloxyethyl phosphorylcholine-co-tris (trimethylsiloxy)-3-methacryloxypropylsilane). ⇑ Corresponding author. Tel.: +86 21 6425 2972; fax: +86 21 6424 0862. E-mail address: [email protected] (Z. Xin).

be tear wettable, biocompatible, biofouling resistant, and oxygen permeable to obtain a lower insult into the eye tissue [1]. Poly methyl methacrylate (PMMA) possesses good optical properties, such as acceptable surface water wettability and good durability, but the extremely low oxygen permeability of this material limits long-term wear. This problem was resolved by copolymerizing MMA with tris(trimethylsiloxy)-3-methacryloxypropylsilane (TRIS), which possessed high oxygen permeability [2]. Both BostonÒ IV (oxygen permeability: 26 barrers, Polymer Technology), Polycon II (oxygen permeability: 12 barrers, PBH) and also Focus Night & DayÒ (oxygen permeability: 140 barrers, Ciba vision) are using TRIS as one of the principle materials to produce commercial contact lens. However, the surface of this material is very hydrophobic,

0014-3057/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2011.06.011

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which results in poor compatibility with the ocular environment [3]: protein adsorption on the lens surface causes lens fouling, decreases the lens visibility, makes it uncomfortable and also reduces the wear lifetime. Many researchers have studied biomaterials based on original bio-inspired polymer containing 2-methacryloyloxyethyl phosphorylcholine (MPC), due to their excellent biocompatibility, blood compatibility and anti-biofouling properties [4–7]. MPC polymers have been used safely in biomedical devices and clinical treatments, such as diagnostic systems, cardiovascular stents, artificial hip joints and implantable blood pumps [8]. What’s more, MPC containing materials are effectively hydrogels due to a large amount of water associated with phosphorylcholine headgroup [9]. The property is ideally suited to design the materials used as soft contact lens. And this led to the advent of the contact lens material containing MPC, this Ò material was marked as Proclear [10] and was registered with the FDA as omafilcon A. Willis et al. [3] have reported the phosphorylcholine (PC)-coated silicone hydrogel contact lens for use in extended wear. They found that good mechanical characteristics were obtained while still achieving sufficiently high oxygen permeability to maintain corneal health during extended wear times. Ishihara and colleagues designed the MPC hydrogel for soft contact lens biomaterials [11,12]. However, this type of MPC polymer provided too high equilibrium water content (EWC > 90%) and this might cause mechanical weaknesses and dehydration of lens. In order to solve the problem, a MPC-based intermolecular crosslinker was synthesized by them and it could not only adjust EWC to that of the cornea (82%), but also enhanced mechanical strength and increased the free water fraction in the MPC polymer [13,14]. More recently, silicone hydrogels from MPC and bis(trimethylsilyloxy)methylsilylpropyl glycerol methacrylate (SiMA) were synthesized [15], and an interpenetrating polymer network (IPN) structure was prepared in order to avoid severe microphase separation. To satisfy the requirements of contact lens materials, a novel silicone hydrogel containing MPC was synthesized to increase both the anti-biofouling properties and wettability of the original material. Polymer PMMA and poly(methyl methacrylate-co-tris(trimethylsiloxy)-3-methacryloxypropylsilane) (PMT) were used as reference. The differences between MPC containing polymers and other polymers for the adsorption of bovine serum albumin (BSA) and the adhesion of platelets were evaluated. In addition, the effect of MPC on the wettability of the polymer surface, transparency of the membranes, mechanical properties and oxygen transmissibility were also studied.

2.2. Instrumentation IR analyses were performed on Magna-IR 550 spectrometer (Nicolet, USA). 1H-NMR analyses were performed on an AVANCE 500 instrument (BRUKER, Germany). The mole fractions of different units in the polymers were determined from the vario EL III elemental analysis (Elementar Analysensysteme GmbH, Germany). The amount of adsorbed proteins on the surface of the polymer membrane was analyzed using the micro-BCA protein assay reagent kit, No. 23235(Pierce, USA) and it was detected by 756CRT ultraviolet–visible spectrophotometer (Shanghai precision & scientific instrument Co., Ltd, China). The morphology of the adherent platelets on the polymer films was observed using the JSM-6360LV scanning electron microscope (JEOL, Japan). Thermal analysis was carried out with a differential scanning calorimeter, Diamond DSC (PerkinElmer, USA) at a cooling rate of 5 °C/min from 40 to 40 °C. 2.3. Synthesis of TRIS TRIS was prepared according to the method previously described [17] as shown in Fig. 1. Briefly, 3-methacryloxypropyltrimethoxysilane (31 g) and chlorotrimethylsilane (81.5 g) were mixed in a 500 mL four-necked, roundbottomed flask equipped with a mechanical stirrer, a thermometer, a drop funnel and a reflux condenser connected to a sodium hydroxide trap for evolving hydrochloride vapors. A solution of methanol (24 g) in water (20.4 g) was added dropwise to the stirred solution over 30 min. The temperature of the reaction mixture was maintained at 25 °C for 12 h. The organic phase was separated and washed with water, then dried over anhydrous sodium sulfate overnight, filtered, and distilled under normal pressure then reduced pressure to give TRIS. Yield: 58%, bp: 114–115 °C/2 mmHg, GC: 95.8%. 1 H-NMR(CDCl3, ppm): d = 0.10 (s, 27H, SiCH3), 0.50 (t, 2H, CH2Si), 1.69 (m, 2H, CH2CH2CH2), 1.91 (s, 3H, CCH3), 4.07 (t, 2H, OCH2CH2), 5.50 and 6.07 (2  m, 1H each, CH2@C); IR: 1639 cm1 (C@C), 1723 cm1 (C@O), 758 cm1, 843 cm1 (Si(CH3)3), 1057 cm1 (Si–O–Si). 2.4. Synthesis of PMMT To prepare PMMT as shown in Fig. 2, the desired amounts of MMA, MPC and TRIS were placed in a glass

2. Materials and methods 2.1. Materials MPC was prepared by previous method [16]. MMA were purified by distillation before use. Chlorotrimethylsilane, 3-methacryloxypropyltrimethoxysilane, methanol and anhydrous sodium sulfate were used as received. 2,20 Azodiisobutyronitrile (AIBN) and sodium dodecyl sulfate (SDS) were purified by recrystallized thrice from ethanol.

Fig. 1. Reaction scheme for the synthesis of TRIS.

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2.7. Evaluation of protein adsorption on the polymer surface The micro-BCA method was used to determine the amount of adsorbed proteins on the sample surfaces. The protein used in this study was bovine serum albumin (BSA). The concentration of the BSA was 0.5 mg/mL. To equilibrate the membranes surface, the polymer membranes (1  1 cm) were immersed in 5 mL phosphate buffered solution (PBS), pH 7.4, 0.01 mol/L, and remained for 24 h. After PBS was removed, the samples were stored at 37 °C for 3 h in each BSA solution. To rinse these samples, they were immersed in and then removed from the PBS for 5 times. To completely detach the adsorbed proteins on the surface, the samples and a 1% (w/w) PBS solution of SDS were introduced into the test tubes, and then the tubes were sonicated for 20 min using an ultrasonic water bath. Micro-BCA protein assay reagent kit was used to determine the concentration of the proteins in the SDS solution. Fig. 2. Chemical structure of PMMT.

2.8. Evaluation of platelet adhesion pressure bottle for polymerization and dissolved with ethanol, total monomer concentration: 1 mol/L, AIBN was the initiator (10 mmol/L). Argon gas was passed into the mixture to remove oxygen, and the glass bottle was sealed. The polymerization was carried out at 60 °C for 16 h. The reaction mixture was cooled to stop the polymerization, and then poured into a large amount of n-hexane to precipitate the polymer. The precipitated polymer was filtered off and washed with water and n-hexane by Soxhlet extraction, then dried in vacuum at room temperature. For PMMT-1 (yield: 73.8%) and PMMT-2 (yield: 76.2%), the feed mole ratios were 16.6%:16.6%:66.8% and 18.1%: 9.1%:72.8%, respectively. PMT and PMMA were prepared by the similar procedure described above. 2.5. Preparation of the polymer membranes PMMT was dissolved in ethanol (10% w/w), and then the polymer solution was spread on a glass plate at 25 °C under an air atmosphere for 48 h to evaporate most of the solvent, and then dried at 60 °C under a vacuum for 48 h to eliminate the residual solvent. The obtained membrane was peeled off from the glass plate before evaluation. The thickness was controlled by the amount of polymer solution used for membrane preparation and ranged from 0.3 to 0.5 mm. All the other polymer membranes were prepared by the same procedure described above. 2.6. Mechanical properties The mechanical properties of the fully hydrated polymer films were measured by tensile testing using dumbbell-shaped samples cut from the center of the films prepared by evaporation method measuring 20  4 mm. The sample was placed in the jaws of a CMT2203 tensile tester (SANS, PR China). The sample was then pulled apart and the elongation and tensile strength determined using a pull rate of 10 mm/min.

The platelet-rich plasma (PRP) was prepared by centrifugation (1200 rpm, 15 min) of citrated bovine whole blood. The polymer films (1  1 cm) were equilibrated with PBS for 24 h before test, and the PRP was kept in contact with the polymer films for 60 min at 37 °C. After the films were rinsed by PBS for 3 times, the platelets that adhered on the polymer films were fixed with 2.5% (w/w) glutaraldehyde solution over night at 4 °C. And then the polymer films were rinsed with distilled water, and subsequently immersed into 60%, 70%, 80%, 90% and 100% (v/v) ethanol solution and dried in vacuum, sputtered with gold and observed with SEM. 2.9. Determination of water state in the membranes The water structure in the saturated polymer membrane was estimated by DSC. The films were equilibrated with distilled water for 24 h, and then the excess water on the surface of the films was gently removed with a filter paper. About 3–5 mg of hydrated polymer membrane was placed in an aluminum pan. The pan was sealed tightly to prevent water evaporation during the measurement. An empty aluminum pan was used as the control. DSC analysis was carried out between 40 °C and 40 °C at a cooling rate of 5 °C/min. During the cooling experiment, the sample cell was purged with nitrogen gas at a flow rate of 20 mL/min. The water content (WC) was measured as follows: the dried polymer membrane was immersed in distilled water at 37 °C, and weighed at intervals. The excess water on the surface of the swollen hydrogel was gently removed with a filter paper before the measurement of its weight, and the water content in the membrane was determined from an increase in the membrane weight. WC of the hydrogel can be calculated by the equation: WC% ¼ ½ðMt  M 0 Þ=M t   100%. Here, Mt is the weight of swollen film, and M0 is the weight of dried film.

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2.10. Water contact angle measurement Wettability is often characterized by measuring the water contact angle. This measurement is considered to be a relatively simple, useful, and sensitive tool for assessing surface wettability of solids [18]. PMT, PMMT-1 and PMMT-2 samples were prepared by spin coating of a 1 wt% polymer solution in ethanol onto 13 mm diameter glass cover slips at 1500 rpm for 60 s [19]. And then the samples were dried in a nitrogen stream and immersed in distilled water before measurement. The captive bubble method was utilized, using an OCA20 optical goniometer (DataPhysics, Germany) interfaced with an image-capture software. The glass cell of the captive bubble assembly was filled with distilled water, and the membrane sample covered it. A needle dispensed an air bubble from beneath the sample. The video capture provided an image of the captive bubble (6 lL) for the calculation of the contact angle at 25 °C [20]. Ten contact angles were averaged. 2.11. Transparency measurement

Fig. 3. 1H-NMR spectra of PMT copolymer (CDCl3, d). Peak at d = 3.6 ppm belongs to protons of MMA moieties –OCH3 (m); peaks at d = 0.1 ppm belong to protons of TRIS moieties Si(CH3)3 (o).

The transparency of the membrane was examined using a UV-2500 UV-visible spectrophotometer (SHIMADZU, Japan). Samples were prepared by the solvent evaporation method (central thickness 0.3 mm) and immersed in distilled water for 24 h to reach swelling equilibrium. The measurements were performed from 200 to 700 nm wavelength at room temperature. 2.12. Oxygen permeability The oxygen transmissibility (Dk/t) was determined for the materials under study. Films were submitted to Labthink (Labthink Instruments Co., Ltd., Jinan, PR China) for analysis, using their PERME™ OX2/231 oxygen transmission rate test system for measuring the Dk/t value of the material. Results shown were an average of three films. 3. Results and discussion 3.1. Characterization of the polymers Fig. 4. IR spectra of PMMT terpolymer.

The 1H-NMR spectra (CDCl3, d) of PMT was indicated in Fig. 3. Peak at d = 3.6 ppm belongs to protons of MMA moieties OCH3 (m), and peaks at d = 0.1 ppm belong to protons of TRIS moieties Si(CH3)3 (o). The characteristic signals of unsaturated protons at d = 5.5–6.1 ppm were not observed, which indicated that the copolymerization happened. Fig. 4. was the IR spectra of PMMT polymer. IR: 1740 cm1 (C@O), 1240 cm1 (P@O), 1170 cm1 (P–O), 966 cm1 (N+(CH3)3), 758 cm1, 843 cm1 (Si(CH3)3), 1059 cm1 (Si–O–Si). Fig. 5 was 1H-NMR spectra (CDCl3, d) of PMMT polymer: except for the peaks attributed to MMA and TRIS which appeared at the same chemical shifts in Fig. 3, the peak at d = 3.4 ppm attributed to MPC moieties N+(CH3)3 (n) appeared. From the results of IR and 1H-NMR we could reasonably deduced that PMMT was synthesized successfully. Table 1 summarized the mole fraction of different units in the polymers. For PMT, the mole fractions of TRIS and

MMA were calculated from the integral intensities at d = 0.1 ppm (o) and 3.6 ppm (m) in the 1H-NMR spectra. The feed mole ratio of TRIS: MMA was 25.0%:75.0%. For PMMT polymers, the peaks attributed to MPC (n) and MMA (m) were crossed as shown in Fig. 5, it could not be calculated accurately by the integral intensities, so elemental analysis was adopted and it provided the confidence in supporting the predicted polymer formulae. 3.2. Effect of MPC on the adsorption of proteins Interactions between various body fluid components and an artificial, non-biological surface are important elements in the design of synthetic materials for biological applications. The interaction between contact lenses and the components from the tear film is a typical example

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Fig. 6. Amount of adsorbed BSA on polymer membranes. The concentration of the BSA solution: 0.5 mg/mL; Phosphate buffered solution (PBS), pH 7.4, 0.01 mol/L; 37 °C for 3 h. Fig. 5. 1H-NMR spectra of PMMT terpolymer (CDCl3, d). Peak at d = 3.6 ppm belongs to protons of MMA moieties –OCH3 (m); peaks at d = 0.1 ppm belong to protons of TRIS moieties Si(CH3)3 (o); peak at d = 3.4 ppm attributed to MPC moieties –N+(CH3)3 (n).

Table1 Mole fractions of the different units in the polymers (mol%). Samples

MPC

TRIS

MMA

PMMA PMT PMMT-1 PMMT-2

0 0 14.5 16.6

0 24.7 18.3 10.7

100 75.3 67.2 72.7

[21]. The adhesion and activation of proteins are affected by surface characteristics. In general, proteins adsorb on a surface within a few minutes after the material comes in contact with tears [22]. Which in turn, influence the inflammatory response to the cornea. Bacterial adhesion to the contact lens material is the first stage in contact lens-associated microbial keratitis, which may ultimately lead to corneal ulceration [23,24]. Thus much of the work must been done directly towards developing materials with an improved anti-fouling property. As shown in Fig. 6, the amounts of BSA adsorbed on PMMA and PMT membranes were 17.06 lg/cm2 and 18.20 lg/cm2, respectively. While in the case of PMMT-1 and PMMT-2 silicone hydrogels with MPC unit, the values were 5.31 lg/cm2 and 4.22 lg/cm2, which decreased dramatically. It demonstrated that both the MPC containing polymers showed excellent anti-biofouling property. Especially for PMMT-2, it was decreased by 75.3% and 76.8% compared with PMMA and PMT membranes, respectively. Furthermore, this value was only 11.7% of commercial disposable soft contact lens (1 DAY ACUVUEÒ, etafilcon A, Johnson&Johnson Vision Care) [1]. For the only different component in PMT and PMMT polymers was MPC, this result denoted that the amount of adsorbed proteins on the surface of PMMT membranes was reduced by the introduction of MPC unit. Furthermore, there were little differences between PMMT-1 and PMMT2 with regards to the mole fraction of MPC (14.5% and

16.6%), and little change of BSA adsorption between PMMT-1 and PMMT-2 was observed, while the difference of TRIS mole fraction in these polymers was evident (18.3% and 10.7%). This result indicated again that MPC unit contributed to the reduction of BSA adsorption. 3.3. Evaluation of platelet adhesion on the polymers The material’s surface to the inflammatory response is becoming increasingly important with the advent of continuous-wear and long-term wear contact lens. Contact lens need to possess anti-biofouling properties and biocompatibility for continuous and extended wear. Platelet adhesion and activation, which may lead to thrombus formation, occur as blood comes in contact with the material surfaces [25]. Both the surface platelet adhesion density as well as the morphology of adhered platelets are considered useful measures of the biocompatibility of the biomaterials. Fig. 7 shows SEM pictures of the different polymer membranes after 60-min contact with the PRP. Many adherent platelets were observed on the hydrophobic PMMA and PMT membranes, and a morphological change was induced and some aggregates were also observed. In contrast, when the mole fraction of the MPC unit in the polymer was only 14.5% (PMMT-1), platelet adhesion was suppressed on the silicone hydrogel membrane. Previous article [26] reported that the reduction of cell adhesion on the PC containing film was due to the suppression of protein adsorption on the surface. The detection of proteins adsorbed on the polymer membranes has been performed as described above, and it was demonstrated that the same effect had been found. These results clearly showed that PMMT polymers had excellent biocompatibility. There are many speculative mechanisms about biocompatibility of MPC polymers, and one of them can be explained as this: when blood contacts with MPC surface, phospholipids in blood are adsorbed and accumulate originally on the surface and then they rearrange by themselves forming a biomimetic membrane [27]. Another explanation is that the characteristics of water in the

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Fig. 7. SEM images of polymer membranes after PRP exposure: (a) PMMA, (b) PMT, (c) PMMT-1 and (d) PMMT-2. PRP was kept in contact with the polymer films for 60 min at 37 °C.

material or on the surface of the material are important to recognize the interactions between proteins and polymeric materials [22]. Generally, the water structure in the polymer hydrogel can be distinguished into ‘free water’, ‘freezing bound water’, and ‘non-freezing bound water’ [28,29]. Free water does not take part in hydrogen bonding with polymer molecules. It has a similar transition temperature, enthalpy and DSC curves as pure water. Freezing bound water interacts weakly with polymer molecules. Nonfreezing bound water is complex with the polymer chain through hydrogen bonds [13]. Ishihara et al. [9] proposed that the total amount of the freezing bound water and free water content might be the key parameter for biocompatibility, which was proved in this paper. Fig. 8 demonstrated DSC curves of the hydrated polymer membranes. Each MPC polymer had only one peak near 15 °C, which was the exothermic peak corresponding to the freezing of free water and freezing bound water. What’s more it was noticed that, corresponding to the increase of MPC mole fraction, the integration area of the exothermic peak of PMMT-2 hydrogel membrane was larger than PMMT-1, which indicated that the content of freezing water in PMMT-2 was more than PMMT-1. Thus the conformation of proteins did not change even when they were adsorbed on the surface or in contact with the surface [30], which resulted in the excellent anti-biofouling property of PMMT-2 films. 3.4. Water content and wettability of the polymer surface The water content is one of the important indexes which can reflect the hydrophilic property of the materials. Take two commercial silicone hydrogel contact lens

Fig. 8. DSC cooling curves of the silicone hydrogels. Cooling rate 5 °C/min, the sample cell was purged with nitrogen gas at a flow rate of 20 mL/min.

materials (PureVisionÒ, Bausch and Lomb; Focus Night & DayÒ, Ciba vision) for example, the equilibrium water content (EWC) are 36% and 24%, respectively. Fig. 9 shows the relations between the water content of the silicone hydrogels with the immersion time. The hydration degrees increased rapidly before 10 min for both the hydrogels and gradually reached the EWC within 30 min. The large difference in EWC (25% and 55%) of PMMT-1 and PMMT2, despite the relatively small difference in the MPC content (14.5% and 16.6%) may be explained by the big difference of the hydrophobic unit in the terpolymers (TRIS: 18.3% and 10.7%). As described by Ishihara et al. [16], in the series copolymers poly (MPC-co-n-butyl methacrylate),

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3.5. Mechanical properties of the polymers

Fig. 9. Water content (WC) of the different membranes: PMMT-1 (d), PMMT-2 (N), distilled water, 37 °C.

though the EWC increased with MPC composition, there was a dramatic change when MPC mole fraction was 20%, which indicated that the relationship between EWC and the MPC mole fraction in the copolymer might not be linear, the hydrophobic unit in the polymer system also affected the hydrated degree. It was concluded that MPC was incorporated into the polymers to overcome the reduced wettability of the silicone containing materials. The EWC of PMMT-2 was close to the commercial HEMAbased contact lens material 59% (ProclearÒ, Cooper vision). It possessed good wettability and may offer a comfortable wear feeling. The contact angle of the films was measured by the captive bubble method. The advantage of this method was the complete hydration of the sample, thus the surface energy between water and the film should not change with the time of measurement. The siloxane component was hydrophobic which could form the surface with low energy, thus the surface of the MPC free polymer (PMT) turned out to be hydrophobic. The surfaces of the terpolymers containing MPC unit (PMMT-1 and PMMT-2) were hydrophilic as shown in Table 2. Many researchers have studied the dynamics of phosphorylcholine functional polymers [19,31–33], they demonstrated that in response to the polar environment (water or blood), the polymer surface rearranged to present the phosphorylcholine functionalities at the top surface to end up in a new minimum interfacial energy state. Thus it might be explained by the reorientation phenomenon that for PMMT-1 and PMMT-2 membranes, the PC head groups rearranged on the upper surface of the polymer and increased the wettability of the surface dramatically. Table 2 Water contact angles of the different membranes measured by the captive bubble method, distilled water, 6 lL, 25 °C. Samples

Contact angle (°)

PMT PMMT-1 PMMT-2

90.3 38.9 32.6

For a material to be used in a contact lens application, it must possess suitable mechanical properties. The ultimate strength and elongation will affect handling and tearing characteristics. In order to study the effect of MPC on the mechanical properties of the synthesized hydrogels, tensile testing was performed. Table 3 listed the mechanical properties of the hydrated terpolymers. The value of elongation to break of PMMT-2 was much higher than PMMT-1 with an increase in EWC, however, the ultimate tensile strength was decreased. As described by previous article [3], the elongation to break and the energy to break of PureVisionÒ (Bausch and Lomb) were 200% and 2.3 MPa, respectively. The data showed that the mechanical properties of PMMT-2 were close to the commercial lens. The formulation of monomers, crosslinking levels and also the water content will affect the final mechanical properties of the materials [3]. A series of crosslinkable polymers which containing MPC and trimethoxysilylpropyl methacrylate(TSMA) were synthesized by Lewis [34], and it was concluded that at TSMA contents <5%, the membranes did not form properly or were ‘tacky’; with an increase in TSMA content, 10% TSMA was shown to cause severe embrittlement of the membrane. This implies that we may control the mechanical property of the terpolymer by adjusting the mole ratio of the different units. 3.6. Transparency of the films The synthesized polymers must be transparent in order to be suitable for contact lenses. Fig. 10 shows the pictures of fully hydrated PMMT-1 and PMMT-2 films, meanwhile, the light transmittance was compared between the two films and a commercial contact lens (ACUVUEÒ, etafilcon A, Johnson&Johnson) in Fig. 10. In the visible light wave range of 400–700 nm, both the terpolymers PMMT-1 and PMMT-2 exhibited high transparency (relatively constant at approximately 96%), which was comparable to that of ACUVUEÒ. Monomers employed in contact lenses should be as compatible as possible to avoid phase separation. There would be a critical problem with the hybridization of these organic materials because a methacrylate monomer bearing silicone moiety in the side chain is a hydrophobic monomer, while MPC is a hydrophilic monomer. Such incompatible compounds may cause microphase separation and form opaque materials. One solution to this problem-an interpenetrating polymer network (IPN) structure, has been proved to be a promising approach [15], and the other solution-preparation of similar hydrophilic side-chain siloxanes, indicated that the polymers derived

Table 3 Mechanical performance of the hydrated terpolymers. Samples

Elongation to break (%)

Tensile strength (MPa)

PMMT-1 PMMT-2

35.5 165.4

4.1 2.3

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Fig. 10. Pictures of fully hydrated PMMT-1 and PMMT-2 films; the light transmittance was compared between the two films and a commercial contact lens (ACUVUEÒ, etafilcon A, Johnson&Johnson).

from them were more wettable and that the siloxanes were compatible with hydrophilic monomers [35]. For PMMT-1 and PMMT-2 films, the high transparency might be explained by the separation length between the phases: if mutual disliking groups were part of the same molecule, as existed in siloxane hydrogel, molecular entanglements and hydrophobic interactions limited the maximal separation length between the phases. Separation lengths of about 100 nm or less generally led to microphase separated systems. Such systems then were optically clear as the phase size was significantly less than the wavelength of light [35]. The transmittance for ACUVUEÒ suddenly decreased below 350 nm because of the UV protection treatment. Usually, benzotriazole or benzophenone is incorporated to absorb the UV light [13]. MPC polymer may also have UV protection abilities if they are incorporated. The MPC terpolymer membrane was assessed as being optically transparent, which indicated that it was useful contact biomaterials in terms of light transmittance in the range of visible light wavelengths. 3.7. Oxygen permeability of the films For the contact lenses, the oxygen transmissibility (Dk/t) value (barrer/mm), defined as the Dk value (barrer) divided by the thickness t (mm), has been the typical index. Dk/t is generally viewed as a representation of the ability of a specific lens to deliver oxygen to the cornea [35]. It was suggested that the oxygen transmissibility

should be 87–125 barrer/mm for a human cornea [35,36]: 87 barrer/mm was required for a lens to be worn safely overnight, and 125 barrer/mm was needed to completely avoid low oxygen-related effects. The values of oxygen transmissibility (Dk/t) of the terpolymers PMMT-1 and PMMT-2 were 114 barrer/mm and 103 barrer/mm, respectively. It was obvious that TRIS unit played a valuable role in the terpolymers to improve the oxygen transmissibility, and the values were different according to the different mole ratios of TRIS. Thus the oxygen transmissibility may be controlled in the future by adjusting the mole ratio of the different units. 4. Conclusions A novel PMMT silicone hydrogel was synthesized with a focus on the anti-biofouling properties for new biomaterials, especially for contact lens materials. IR, 1H-NMR and elemental analysis confirmed the chemical structure of PMMT, and the yields were 73.8% and 76.2% for PMMT-1 and PMMT-2, respectively. For PMMT-2, the amount of adsorbed proteins was decreased by 75.3% and 76.8% compared with PMMA and PMT membranes, respectively. SEM observation displayed that the adhesion of platelets was dramatically reduced compared to the polymers without MPC unit. The equilibrium water content of PMMT-2 membrane reached 55%, which may offer comfortable wear feeling, and the introduction of MPC increased the wettability of the polymer surface dramatically. In the visible light wave

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