Synthesis of MA POSS–PMMA as an intraocular lens material with high light transmittance and good cytocompatibility

Abstract

Poly(methyl methacrylate) (PMMA) has been widely used for intraocular lenses (IOL) but may lead to posterior capsule opacification (PCO) after implantation due to its undesirable hydrophilicity and surface morphology. A novel methacrylisobutyl polyhedral oligomeric silsesquioxane-co-poly methyl methacrylate copolymer (MA POSS–PMMA) was synthesized by a free radical polymerization method to improve its material properties and cytocompatibility. Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and proton nuclear magnetic resonance spectroscopy (¹H NMR) measurements demonstrated the successful synthesis of MA POSS–PMMA copolymer. The incorporation of MA POSS greatly changed the crystal structure, surface wettability, optical transmission and cytocompatibility of PMMA. XRD peaks at 2θ ∼ 38.5, 44.7 and 66.1° indicated that a portion of the MA POSS molecules had aggregated and crystallized. Furthermore, larger aggregates are formed at higher MA POSS contents. The optical transmission of the copolymers was up to 99%, which was better than pure PMMA. The hydrophilicity and morphology of the IOL surface were characterized by static water contact angle and atomic force microscopy. Results revealed that MA POSS rendered the surface more hydrophobic and with higher roughness than the pure PMMA. Biocompatibility of copolymers with human lens epithelial cells (HLECs) was further evaluated by morphology and activity measurements in vitro. More HLECs adhesion and better spreading morphology on the surfaces of MA POSS–PMMA copolymers than that on PMMA was shown.

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Introduction

Cataract extraction and implantation of intraocular lenses (IOL) is the most common surgical procedure performed in the elderly population to deal with cataracts.^1,2 However, posterior capsule opacification (PCO) develops in approximately 25% of patients after IOL implantation, and is mainly due to the severe loss of the human lens epithelial cells (HLECs) during surgery contact.³ Although PCO has been extensively studied, there is no unified mechanism to explain the cause. Most current studies^4–6 hypothesize that residual HLECs can proliferate and subsequently migrate along the inner surface of the anterior and posterior lens capsule, cloud the capsule and in some cases cause fibrosis and contraction. In the end, this process subsequent causes the reduction of the vision and disrupt of the position and function of IOL. Although most studies have focused on enhancing hydrophilicity of IOL to decrease HLECs adhesion, little attention has been paid to the rapid epithelialization of IOL to bond both IOL and the posterior capsule at the same time. Linnola et al. have proposed a ‘sandwich theory’ on the inhibition of PCO after cataract surgery.^7,8 They consider that if the IOL surface is adherent to posterior capsule early after surgery through HLECs, adhesion of proteins such as fibronectin, hydronectin and extracellular matrix such as collagen IV to IOL can be inhibited, and further HLECs proliferation and PCO can be prevented.

Techniques^9–12 to prevent PCO include physical removal, pharmacological destruction of HLECs, IOL material and optic design and modification, etc. Among these methods, IOL surface modification^13,14 is more adaptable than the other ways for the reason of no need to conduct extra potentially hazardous manipulation (e.g. capsule polishing) during the cataract surgery. Also there is no need to use active and potentially harmful compounds like drug delivery systems.^15–17 Basically, material properties of the IOL (hydrophilic vs. hydrophobic nature) can be modified by two general methods:^18–20 surface treatment and bulk modification. Surface treatment approaches of IOL can be mainly divided into plasma treatment²¹ nanoparticles doping²² and grafting of biological macromolecules.^23,24 Plasma treatment is a simple, effective way to introduce hydroxyl, carboxyl or other hydrophilic functional groups onto the surface of IOL to improve its biocompatibility. Although plasma treatment method can alter the surface wetting properties of IOL, the hydrophilic performance may lose in a short time.^25,26 Hydrophilic modification of IOL with heparin decreases the adhesion of HLECs and inflammations after cataract surgery.²⁰ Other hydrophilic polymers such as 2-methacryloyloxyethyl phosphorylcholine (MPC) coating can decrease the adhesion of platelet, macrophage, HLECs and bacteria.²⁷ However, recent studies show that the ratio of PCO is even higher using heparin modified IOL.²³ More importantly, metal oxide nanoparticles or biomolecules modified IOL always has color and unstable defects. To overcome these disadvantages of the above modification ways, bulk modification is a stable, effective and controllable way to improve the surface performance of IOL.²⁸

The various materials which have been used to manufacture intraocular lens implants include polymethylmethacrylate (PMMA), silicone, hydrophobic acrylate, hydrophilic acrylate and collamer.^29,30 Foldable hydrophilic material is especially for microincision cataract surgery lenses because of its plasticity, but has rare optics opacification and higher posterior capsular opacification rates. Silicone is hydrophobic, with a contact angle with water of 99°, which have been suspected to favor bacterial adhesion, with increased risk for postoperative infection. The abrupt opening of silicone IOLs inside the anterior chamber remains a problem for surgeons. PMMA is the most widely used IOL in cataract surgery soon after its introduction in 1949.^31,32 Used as IOL material, PMMA is known for long-term stability, inexpensive, inert and is well tolerated in the eye with minimal inflammatory reaction. Furthermore, PMMA IOL has good light transmission properties which can transmit a broad spectrum of light including near-ultraviolet light.³³ Unfortunately, it is not a perfect IOL biomaterial to promote the rapid epithelialization of IOL surface as the residual HLECs can proliferate and subsequently migrate along the inner surface of the anterior and posterior lens capsule and in some cases cause PCO. Based on the ‘sandwich theory’ of PCO, the rapid epithelialization of IOL can form a uniform HLECs membrane layer and inhibit the irregular growth. So it is necessary to change the surface properties of PMMA to promote the adhesion of HLECs so as to fill up the space between anterior and posterior lens capsule and finally lower the incidence of PCO.

Polyhedral oligomeric silsesquioxane (POSS), the formula of which is [RSiO_1.5]_n, has been studied for the preparation of truly molecularly dispersed composites. It is a novel cage-like structure of the organic–inorganic hybrid molecules^34–36 which consists of two parts: a cage-like inorganic core based on Si–O–Si bonds and the shell composed of eight surrounded organic groups. POSS has regular structure, good biocompatibility,^37,38 small scale and large surface area, which make POSS as one of the most potential next generation biomaterials. POSS can be incorporated into polymer chains to modify the local structure and chain mobility of polymeric materials, which eventually enhances properties of pristine polymer systems such as mechanical, thermal, and biological properties.^39–41 Previous study^42–44 found that POSS and polymer hybrid was capable of forming a colorless transparent material and the toxicity of POSS was very small, almost non-toxic. In another study,⁴⁵ superhydrophobic electrospun POSS–PMMA copolymer fibers with highly ordered nanofibrillar and surface structures were prepared. Therefore, POSS nanomaterial is very suitable to be used to improve the material properties of PMMA IOL for ophthalmology biological repair alternatives.

In this study, polymers of MA POSS–PMMA were prepared using radical random copolymerization method to improve the biocompatibility of PMMA IOL. A schematic of the synthesis procedure is presented in Scheme 1. The number-average molecular weight (M_n) and weight-average molecular weight (M_w) of MA POSS–PMMA copolymers and PMMA were measured by gel permeation chromatography (GPC). The effects of MA POSS on the crystallization, thermodynamic properties, optical performance and surface properties of the polymer were investigated in detail. Furthermore, cell viability assay was performed to investigative biocompatibility of the MA POSS–PMMA copolymers with HLECs by fluorescein diacetate (FDA) and Cell Counting Kit-8 (CCK-8) methods.

Scheme 1 Synthetic route for MA POSS–PMMA copolymer.

Experimental section

Materials

Methacrylisobutyl polyhedral oligomeric silsesquioxane (MA-POSS) from Hybrid Plastics Co. and methyl methacrylate (MMA, 99%), azobisisobutyronitrile (AIBN), ethyl acetate, ethanol and tetrahydrofuran (THF) from Aldrich were used as received. Ultrapure distilled water was used from a Millipore Milli-Q system (USA).

Synthesis of MA POSS–PMMA copolymers

MA POSS–PMMA copolymers containing 0.05–0.50 weights% of the MA POSS monomers have been synthesized by free-radical polymerization. The typical synthesis process (0.10 MA POSS–PMMA) is described as follows: in a 50 mL round bottom flask, MA POSS (0.222 g, 0.235 mmol), MMA (2.0 g, 20.0 mmol) and AIBN (0.025 g, 0.15 mmol) were dissolved in ethyl acetate (16 mL) and THF (4 mL) under a nitrogen atmosphere. The mixture was heated to 60 °C under constant magnetic stirring to initiate the polymerization reaction, and the polymerization was then carried out at the elevated temperature for 24 h. After reaction, the solution was dropped into excess ethanol to precipitate the polymer and then dissolved in ethyl acetate/THF (v/v = 4 : 1). The polymer was then purified via three dissolving/precipitating cycles, and finally dried at 30 °C in vacuum for 24 h.

Preparation of material surfaces

The material surfaces of PMMA and MA POSS–PMMA were spin-coated onto many kinds of substrates including glass slide, silicon wafer, PET sheet and quartz plate (1 × 2 cm²) from ethyl acetate (0.5% (w/w) at 1500 rpm for 60 s). The coatings were dried at 25 °C for 24 h and under vacuum at 30 °C for 12 h. Glass slide, silicon wafer and quartz plate used for coating preparation were cleaned in “piranha” (7 : 3 (v/v) H₂SO₄/H₂O₂) for 1 min and water for 10 min respectively, and then dried with N₂.

Characterizations

Gel permeation chromatography (GPC)

Molecular weights and distributions of all polymer samples were characterized by GPC performed in THF (1.0 mL min⁻¹). Calibration was carried out using a series of near-monodisperse polystyrene standards.

Nuclear magnetic resonance (NMR)

¹H NMR was measured with a Bruker 400 NMR spectrometer at 25 °C, using deuterio-chloroform (CDCl₃) as solvent and tetramethylsilane (TMS) as the internal standard.

Fourier transforms infrared spectroscopy (FT-IR)

Fourier transform infrared spectra (FT-IR) were measured on a FT-IR spectrometry (Bruker Optics). The samples were prepared as KBr disk.

X-ray diffraction (XRD)

XRD was performed on a powder diffractometer (Philips 1140/90) using Cu radiation with a wavelength of 1.54 Å. The samples were analyzed at room temperature over a 2θ range of 5–50° with sampling intervals of 0.04°.

Thermal properties

Thermogravimetric analysis (TGA) was performed with an instrument from Thermal Analysis Incorporation (TA-TGA 2050) at a heating rate of 10 °C min⁻¹ under nitrogen atmosphere from room temperature to 550 °C. The phase transformation behavior of the experimental specimens was characterized by differential scanning calorimetry (DSC), using a PerkinElmer Diamond calorimeter with a heating and cooling rate of 20 °C min⁻¹.

Atomic force microscopy (AFM)

Surface morphology was measured by AFM (SPA 400, Seiko instrument Inc.). AFM images were performed in the tapping mode under ambient conditions using a commercial scanning probe microscope, equipped with a silicon cantilever, nanosensors, typical spring constant 40 N m⁻¹.

Water contact angle (WCA)

Surface wettability of the films was measured by Drop Shape Analysis (KRŰSS, DSA10-MK2). The sessile dropping method was used to detect surface of the film with different times after the ultrapure water droplet contacted the film. The contact angle formed between the sample surface and droplet was measured using built-in microscope and software provided by manufacturer. All the measurements were performed at least in triplicate and the data were presented as mean ± standard deviation.

Optical transmission (OT)

The OT of PMMA and MA POSS–PMMA surfaces was performed on a quartz plate followed by measurement of its transmission by using of UV-Vis spectrophotometer (Evolution 200, Thermo Fisher Scientific) from 350 to 800 nm. The quartz plate was used as reference. Film samples were used for measurement of OT and the results were expressed as mean value.

Cell cultivation

The HLECs (HLE-B3, American Type Culture Collection (ATCC) number: CRL-11421™) were grown in Dulbecco's Modified Eagle Media: Nutrient Mixture F-12 (DMEM/F12, 1 : 1) mixed medium supplemented with 10% fetal bovine serum, 100 U mL⁻¹ penicillin, and 100 μg mL⁻¹ streptomycin in a 5% CO₂ incubator (Thermo Scientific Series 8000) at 37 °C. Confluent cells were digested using 0.25% trypsin–0.02% EDTA, followed by centrifugation (1000g for 3 min) to harvest the cells. Subsequently, the single cell suspension was used for cell number calculation using haemocytometer (Jipad, Qi3537). After confluence, cells were digested and resuspended for cultivation on the materials. The HLECs were seeded onto the specimens at a density of 1.0 × 10⁴ cells per sample by using 96-well tissue culture plate as the holder, cultivation was conducted for 24 h. Then, FDA and CCK-8 assays were used for the viability and morphology studies of cells grown on the resulting films.

Cell viability assay

A cell count kit-8 (CCK-8 Beyotime, China) was employed in this experiment to quantitatively evaluate the cell viability. After HLECs were inoculated on the film coated dishes for 24 h, the original medium was replaced by 100 μL 10% FBS DMEM/F12 (1 : 1) mixed medium contains 10 μL CCK-8. It was incubated at 37 °C for 2 h to form water dissoluble formazan. Then 100 μL of the above formazan solution were taken from each sample and added to one well of a 96-well plate, six parallel replicates were prepared. The absorbance at 450 nm (calibrated wave) was determined using a microplate reader (Multiskan MK33, Thermo electron corporation, China). Tissue culture plates (TCPS) without any film were used as a control.

Cell morphology assay

FDA (Sigma) is an indicator of membrane integrity and cytoplasm esterase activity. The cell monolayer on different surface was stained with FDA for fluorescence microscope investigation (Zeiss, Germany) at 10 × magnification in fluorescein filter, 488 nm excitation. Stock solutions were prepared by dissolving 5.0 mg mL⁻¹ FDA in acetone. The working solution was freshly prepared by adding 5.0 μL of FDA stock solution into 5.0 mL of PBS. FDA solution (20 μL) was added into each well of a 96-well plate and incubated for 5 min. The sheets were then washed twice with PBS and placed on a glass slide for fluorescence microscope examination. The 488 nm wavelength of the laser was used to excite the dye. Cells incubated into wells that did not contain films were used as controls.

Results and discussion

Synthesis of MA POSS–PMMA copolymers

The synthetic route to the preparation of the MA POSS–PMMA polymer was illustrated in Scheme 1 via free radical polymerization using AIBN as the initiator, MA POSS and MMA as monomers. The ¹H NMR spectrum of 0.05 MA POSS–PMMA is shown in Fig. 1. The signals at δ 0.82, 2.05 and 3.60 ppm were assigned to the protons of PMMA, the signals at δ 0.60, 1.03, 1.25, 1.82, 1.94, 2.04 and 3.73 ppm attributed to the protons of MA POSS.

Fig. 1 ¹H-NMR spectrum of 0.05 MA POSS–PMMA copolymer.

The M_w/M_n and polydispersity (PDI) of PMMA and MA POSS–PMMA copolymers were calculated from GPC using THF as eluent. The characterization of PMMA and the copolymers is displayed in Table 1. The M_w/M_n for 0.05 MA POSS–PMMA copolymer was found to be 424 300/183 700 g mol⁻¹ with a PDI of 2.31, typical for copolymers synthesized by free radical polymerization. The M_w/M_n for the 0.50 MA POSS–PMMA copolymer was found to be 509 000/224 700 g mol⁻¹ with a PDI of 2.27 which had a higher M_w/M_n and PDI. For the pure PMMA, the M_w/M_n and PDI were found to be 448 600/210 400 g mol⁻¹ and 2.13, respectively (see Fig. 2).

Table 1 Copolymerization of MMA and MA POSS with free radical polymerization

Polymer	PMMA	0.05 MA POSS + PMMA	0.10 MA POSS + PMMA	0.25 MA POSS + PMMA	0.50 MA POSS + PMMA
a The number-average molecular weight (Mn) was determined by gel permeation chromatography (GPC) performed in THF (1.0 mL min^−1).b Weight-average molecular weight (Mw) was determined by gel permeation chromatography (GPC) performed in THF (1.0 mL min^−1).c The molecular weight distributions were calculated as Mw/Mn.
Mn^a/g mol^−1	210 400	183 700	217 700	201 400	224 700
Mw^b/g mol^−1	448 600	424 300	466 500	491 500	509 000
PDI^c	2.13	2.31	2.14	2.44	2.27

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Fig. 2 GPC chromatogram of PMMA copolymer.

In addition, FT-IR spectroscopy was also used to confirm the successful synthesis of MA POSS–PMMA. There were two strong vibration bands at 1638.1 and 1736.2 cm⁻¹, attributable to C=O and C–O–C stretching vibrations from PMMA. The FT-IR spectrum of the MA POSS–PMMA copolymer showed combined features for PMMA and MA POSS (see Fig. 3). Comparing with the spectrum of PMMA, an obvious signal at 1115.8 cm⁻¹ which is the characteristic peak of the Si–O–Si stretching vibration can be observed in the spectrum of MA POSS–PMMA, indicating that POSS cage had been successfully incorporated into the structure of MA POSS–PMMA copolymer.⁴⁶

Fig. 3 FT-IR spectra of PMMA and 0.05 MA POSS–PMMA copolymer.

Furthermore, the XRD scattering was employed to characterize the crystallization properties of MA POSS–PMMA and the profiles of PMMA and MA POSS–PMMA copolymers are shown in Fig. 4. The pure PMMA exhibited reflections 2θ's at 14° but no obvious absorption peak indicating PMMA molecules existed in amorphous form.⁴⁶ As shown in Fig. 4, obvious reflections (2θ = ca. 38.5°, 44.7° and 65.2°) could be easily observed, which certified the ordered packing of MA POSS chains. As the MA POSS content increased from 0.05 to 0.50, the absorption peaks at the above locations increased in intensity and sharpness. From this observation, it was confirmed that the MA POSS cages formed the aggregation and crystalline domain.

Fig. 4 X-ray diffraction patterns of PMMA and MA POSS–PMMA copolymers.

TGA was used to evaluate the thermal stability of the PMMA and PMMA with MA POSS in the main chains and the TGA curves were shown in Fig. 5. For comparison, it can be observed that PMMA displayed a higher thermal stability than the MA POSS–PMMA in terms of the initial degradation temperatures (0–350 °C) and the yields of decomposition residues. All the MA POSS–PMMA copolymers displayed TGA profiles similar to the pure PMMA, suggesting that the incorporation of MA POSS into the main chains did not significantly alter the degradation mechanism. It is noted that the temperatures of initial degradation of the copolymers were slightly lower than PMMA and the 0.1 MA POSS–PMMA copolymer exhibited the lowest initial degradation temperatures. This observation suggested that the incorporation of MA POSS with small amount in the main chains of the copolymers improved the random-chain scission. For the 0.05 MA POSS–PMMA copolymer, the content of MA POSS was too little to affect the chain movement of the copolymer. Also the MA POSS–PMMA copolymers with MA POSS contents higher than 0.25, MA POSS occupied great ingredients of the chain, which increased thermal stability of the copolymers due to reduced random-chain scission.

Fig. 5 TGA derivative plots of PMMA and MA POSS–PMMA copolymers.

The influence of the MA-POSS content on the phase transition was evaluated by testing DSC of PMMA homopolymer and the MA POSS–PMMA copolymers. Fig. 6 showed the DSC heating traces of neat PMMA and MA POSS–PMMA copolymers at 20 °C min⁻¹ from the amorphous state. Pure PMMA exhibited the glass transition temperature (T_g) around 103.7 °C. Incorporation of MA POSS into PMMA caused a slight decrease of T_g, while maintaining thermal stability. The T_gs of 0.05–0.50 MA POSS–PMMA copolymers were 98.1, 100.3, 99.6 and 91.5 °C, respectively. It is proposed that the increased free volumes owing to the introduction of MA POSS led to the decrease of T_g. The introduction of MA POSS into the main chains of the PMMA resulted in the increase of free volume in copolymers. The increased free volume is responsible for the introduction of bulky and vacant cage-like structure of the POSS macromer, which caused the polymer chains to be unable to densely pack in the glassy state.³⁹

Fig. 6 DSC thermograms of PMMA and MA POSS–PMMA copolymers.

Characterization of the surface properties

To calculate the hydrophilicity of the observed copolymers, water contact angles (WCAs) of PMMA and MA POSS–PMMA copolymers were measured with MA POSS at different concentrations from 5 to 960 s. From Fig. 7, we can see that the WCAs of the PMMA and MA POSS–PMMA copolymer films were gradually decreased during the initial 480 s and reached plateau after that time. The surface of PMMA film showed hydrophobic property with WCAs of 97.0 ± 0.7°at 15 s and 67.9 ± 1.2° at 960 s, while films consisting of MA POSS showed higher WCA values than PMMA film. The WCAs of the 0.05 MA POSS–PMMA copolymer films were 102.1 ± 0.3° 15 s and 72.6 ± 0.8° at 960 s respectively, which were higher than PMMA film. With the increase of MA POSS from 0.05 to 0.50, the WCAs of the copolymer films gradually increased. The WCA of 0.50 MA POSS–PMMA were 83.1 ± 5.7° at 960 s which was 16° higher than PMMA film surface. Similar conclusions are reported by other researchers.⁴⁷

Fig. 7 WCAs of the PMMA and MA POSS–PMMA copolymer surfaces with the change of time.

To further understand the effect of material composition on the membrane hydrophobicity, we used AFM to study the morphology of the film surfaces. Fig. 8 showed the 3D images of the copolymers and pure PMMA films. It can be observed that the MA POSS–PMMA copolymer films had very low root-mean-square (RMS) roughness which confirmed the good homogeneity of the films. The pure PMMA film (5 × 5 μm²) also had low RMS roughness of 2.04 ± 0.38 nm. As the adding of MA POSS changing from 0.05 to 0.50, the RMS roughness of the films increased from 3.67 ± 0.43 to 4.04 ± 0.38 nm (3.67 ± 0.43, 3.40 ± 0.51, 3.88 ± 0.29 and 4.04 ± 0.38 nm, respectively). The increase of RMS roughness of the surface contributed to the hydrophobicity increase of copolymer films.^48,49

Fig. 8 AFM images of films of (a) PMMA, (b) 0.05 MA POSS–PMMA, (c) 0.10 MA POSS–PMMA, (d) 0.25 MA POSS–PMMA and (e) 0.50 MA POSS–PMMA.

The OT of PMMA plays an important role when used as optical material, especially used as IOL, which has a great influence on the visual experience of patients. As examined in Fig. 9 and 10, the transmittance of pristine PMMA in visible region (400–700 nm) was good, with light transmission of 90%. It is noteworthy that the OTs of the copolymer films were higher than 99% which were even better than PMMA. The OT of film is mainly depended on intermolecular compatibility and molecular arrangement of MA POSS and PMMA. As shown in Scheme 1, the section containing double bonds in the structure of MMA and MA POSS monomers was similar, which was conducive to the formation of regular structures. Furthermore, MA POSS polymer itself has a high OT due to the high inorganic component. As the adding of MA POSS from 0.05 to 0.50, the MA POSS greatly improved segments arrangement of the copolymers and OT did not change much with POSS content. Other kind of POSS with different section containing double bonds has been copolymerized with MMA which showed even lower OTs.⁴⁴ As found in XRD and AFM measurements, the MA POSS–PMMA polymer was in regular arrangement and also formed into to crystalline state.

Fig. 9 Optical transmittance of the PMMA and MA POSS–PMMA copolymer films.

Fig. 10 Optical microscope image of PMMA and 0.05 MA POSS–PMMA copolymer.

Cell viability and morphology assays

The cell viability of PMMA and MA POSS–PMMA copolymer films were investigated against HLECs. Fig. 11 showed quantitative assessment of the cytotoxicity by CCK-8 assay over a period of 24 h. It can be observed that the cell viability of PMMA and MA POSS–PMMA copolymers was much lower than that of TCPS. In comparison, PMMA film had the highest cytotoxicity compared with TCPS and MA POSS–PMMA copolymers (48.1%). With the increase of MA POSS, the cell viability increased from to 59.1 to 72.2% for 0.05 and 0.50 MA POSS–PMMA copolymer films respectively.

Fig. 11 The cell viability assay of HLECs cultured on the surfaces of (a) TCPS, films of (b) pristine PMMA, (c) 0.05 MA POSS–PMMA, (d) 0.10 MA POSS–PMMA copolymer, (e) 0.25 MA POSS–PMMA and (f) 0.50 MA POSS–PMMA for 24 h. The absorbance of the diluted Cell Counting Kit solution has been deducted from each data point and the statistical significance is indicated by different letters (p < 0.05).

The adhered HLECs on the surface were photographed with an inverted fluorescence microscope after FDA staining, and the results are shown in Fig. 12. After 24 h incubation, it could be seen that HLECs cultured on the TCPS had undergone great extent of proliferation, grew into monolayer and maintained normal spreading morphology suggesting the HLECs were in the growth status (Fig. 12(a)). The HLECs cultured on the PMMA and MA POSS–PMMA copolymer films had undergone some degree of proliferation but much fewer than that on TCPS. Furthermore, the surface of MA POSS–PMMA copolymer films had more HLECs and better spreading morphology than that on the surface of PMMA indicating much better cytocompatibility with HLECs. This was consistent with the results of CCK-8.

Fig. 12 Growth and morphology of HLECs stained with FDA after 24 h of incubation on (a) TCPS, films of (b) pristine PMMA, (c) 0.05 MA POSS–PMMA, (d) 0.10 MA POSS–PMMA copolymer, (e) 0.25 MA POSS–PMMA and (f) 0.50 MA POSS–PMMA under fluorescence microscopy (the magnification is 10×).

Surface chemistry especially wettability and morphology of IOL can severely mediate the adhesion, proliferation and migration of HLECs. Previous measurements of WCA and AFM indicated that MA POSS–PMMA copolymers had a higher hydrophobicity and roughness. The hydrophobic property of HLECs membrane facilitates the adhesion on hydrophobic material surfaces. According to ‘sandwich theory’ of PCO,^7,8 if IOL is in favor of HLECs adhesion, it would allow a single HLECs layer to bond both to the IOL and the posterior capsule at the same time. This would produce a sandwich pattern including the IOL, the cell monolayer and the posterior capsule. The sealed sandwich structure would prevent further epithelial ingrowth and reduce the occurrence of PCO.

Conclusion

MA POSS–PMMA copolymers were synthesized by radical polymerization and made into film as IOL material. FT-IR, XRD and ¹H NMR measurements indicated the successful synthesis of MA POSS–PMMA copolymers. The observed hybrid copolymers showed better transparency than pure PMMA and good thermodynamic stability, which have superiority to be used as IOL biomaterial. Characterizations of morphology and hydrophilicity illustrated that the MA POSS–PMMA copolymer film had a higher hydrophobicity and a higher roughness. The surface of MA POSS–PMMA films had better cell viability and better spreading morphology with HLECs than pure PMMA film. In summary, the incorporation of MA POSS significantly promoted the epithelialization of HLECs on the surface, which may be of great interest to be used as IOL material to reduce the occurrence of PCO.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (51403158, 81271703, 51203120), the International Scientific & Technological Cooperation Projects (2012DFB30020), Medical & Health Technology Program of Zhejiang Province (2013KYA133, 2014KYA149), Natural Science Foundation of Zhejiang Province (LQ12E03001), Science & Technology Program of Wenzhou (Y20140177) are greatly acknowledged.

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