Hydrophobic modification of polymethyl methacrylate as intraocular lenses material to improve the cytocompatibility
Graphical abstract
Introduction
Cataract surgery has increased rapidly in recent decades. Phacoemulsification combined with intraocular lenses (IOL) implantation is the first choice for cataract treatment on account of small incision, quick recovery and better postoperative vision [1], [2], [3]. However, posterior capsule opacification (PCO) is a common complication after cataract surgery caused by the immune response and residual human lens epithelial cells (HLECs) on the posterior capsule [4]. Wound healing promotes residual HLECs to proliferate, differentiate, and to deposit extracellular matrix, via autocrine and paracrine cell signaling. Although PCO has been extensively studied, there is no unified mechanism to explain the cause. Most current studies hypothesize that a multicellular secondary membrane results from migration and fibrosis of residual HLECs on the posterior capsule, forming elschnig pearls [5]. Others suggest that a single layer of residual anterior capsule epithelial cells migrate onto the posterior capsule and undergo metaplasia into myofibroblasts, pulling the posterior capsule into many tiny folds. Both mechanisms can contribute to the development of PCO [6].
In recent years, several reports have focused on how to prevent PCO. In addition to the position of the capsulorhexis, IOL material and optic design are important factors in the development of PCO [7], [8], [9]. The effect of IOL on PCO has been explained by various suppositions such as the separation of the posterior capsule from the anterior capsule, stretching of the capsule, compression, no space/no cells and adhesiveness of the IOL material. Out of all of the commercial IOLs, hydrophobic acrylic IOL especially poly (methyl methacrylate) (PMMA) has played an important role in cataract surgery soon after its introduction in the mid-1990s [10], [11]. PMMA is the most common commercially available IOL material and is known for long-term stability. It is relatively inexpensive, inert and is well tolerated in the eye with minimal inflammatory reaction. PMMA IOL has good light transmission properties which can transmit a broad spectrum of light including near-ultraviolet light [12]. Unfortunately, the surgery contact can cause considerable HLECs loss between the comparatively hard PMMA IOL surface and the corneal endothelium. Based on the sandwich theory [13], [14], [15] of PCO, the rapid epithelialization of IOL that forms a cell monolayer between IOL and posterior capsule can fill up the space and finally reduce the occurrence of PCO.
The surface properties of a polymer can be modified in order to ensure that it will be better adapted to its final use. Basically, the surface energy of the polymer (hydrophilic vs. hydrophobic nature) can be modified according to two general methods: surface treatment and bulk modification. The surface treatment methods of PMMA mainly include plasma treatment [16], nanoparticles doping [17] and grafting of biological macromolecules [18], [19], [20]. The plasma treatment used as a surface modification method is simple, effective and without safety issues. Using plasma discharge, hydroxyl, carboxyl or other hydrophilic functional groups can be introduced onto the surface of intraocular lenses to improve its biocompatibility. Titanium dioxide nanoparticles have been used to modify IOL to enhance its biocompatibility [17]. Heparin surface modification (HSM) decreases adhesion of cells and inflammations after cataract surgery [18]. However, a recent study shows the ratio of PCO is high using HSM IOL. Recently, a 2-methacryloyloxyethyl phosphorylchoine (MPC) coating was produced, which decreased adhesion of platelet, macrophage, lens epithelial cells and bacteria [19]. Although plasma treatment method can alter the surface wetting properties of IOL, the hydrophilic performance may lose in a short time. Metal oxide nanoparticles and biomolecules modified intraocular lenses always have color and unstable defects. Comparatively, bulk modification is a stable, effective and controllable IOL modified method.
Polyhedral oligomeric silsesquioxane (POSS) is a novel cage-like structure of the organic–inorganic hybrid molecules [21], [22], [23]. The main structure of POSS consists of two parts: a cage-like inorganic core based on SiOSi bonds and the shell composed of eight surrounded organic groups, which may be designed according to needs. The inner diameter of POSS is about 0.53 nm and the outer diameter is generally between 1 nm and 3 nm due to different organic functional groups. POSS has regular structure, good biocompatibility, small scale and large surface area, which make POSS as one of the most potential next generation biomaterials. POSS macromers with different shells have been doped into polystyrene [24], PMMA [25], [26], polyurethane [27], polyethylene [28], ethylene–propylene [29], etc. by the way of copolymerization with other polymer monomers to change its mechanic, thermodynamic, surface or biological properties. Previous study [25] found that POSS and polymer hybrid is capable of forming a colorless transparent material, which does not affect the light transmittance. The cytotoxicity of POSS was also investigated and it was found that the toxicity of POSS is very low, almost non-toxic. Therefore, POSS nanomaterial is more suitable than other materials for ophthalmology biological repair alternatives. However, there is almost no reference about the studies on POSS used for IOL modification.
In order to improve the biocompatibility of PMMA used as IOL material and achieve the rapid epithelialization, surface characteristics of PMMA can be changed through bulk modification. In this work, polymer of allyl POSS–PMMA was prepared using radical random copolymerization method. A schematic of the synthesis procedure is presented in Scheme 1. The number-average molecular weight (Mn) and weight-average molecular weight (Mw) of allyl POSS–PMMA copolymer and PMMA were measured by gel permeation chromatography (GPC). The effect of POSS on the crystallization, thermodynamic properties, optical performance and surface properties of allyl POSS–PMMA were studied in detail with various techniques. Furthermore, cell viability assay was performed to determine biocompatibility of the allyl POSS–PMMA copolymer with HLECs by fluorescein diacetate (FDA) and Cell Counting Kit-8 (CCK-8) methods.
Section snippets
Materials and reagents
Isobutyl (allyl)-POSS(R), [(allyl)(isobutyl)7Si8O12] (allyl POSS) from Hybrid Plastics Co. and methyl methacrylate (MMA), azobisisobutyronitrile (AIBN), ethyl acetate, ethanol and tetrahydrofuran (THF) from Aldrich were used as received.
Synthesis of allyl POSS–PMMA copolymers
Allyl POSS–PMMA copolymers containing 0.01 or 0.02 weight of the allyl POSS monomers have been synthesized by free-radical polymerisation. The typical synthesis process (0.02 allyl POSS–PMMA) is described as follows: in a 50 mL round bottom flask, allyl POSS (0.34
Synthesis of allyl POSS–PMMA copolymers
The allyl POSS–PMMA copolymer was synthesized via free radical polymerization using AIBN as the initiator, allyl POSS and MMA as monomers. The successful synthesis of copolymers was proved by 1H NMR. Fig. 1. is the typical 1H NMR spectra of 0.02 allyl POSS–PMMA copolymer recorded in CDCl3 with the relevant signals labeled. The characteristic resonance signals at 0.60 ppm, 0.91 ppm, 1.81 ppm, and 1.85 ppm were attributed to allyl POSS, while signals at 0.85 ppm, 1.03 ppm, 1.25 ppm, 1.60 ppm and 3.60 ppm
Conclusion
Allyl POSS–PMMA copolymer was synthesized by radical polymerization and made into film as IOL material. FT-IR, XRD and 1H NMR measurements indicated the successful synthesis of allyl POSS–PMMA copolymer. By incorporating of ally POSS into the PMMA main chain results in good thermodynamic properties and high transparency of PMMA-based polymeric material. Characterizations of morphology and surface hydrophility suggested that the allyl POSS–PMMA copolymer film had a higher hydrophobicity and a
Acknowledgments
Financial supports from National Natural Science Foundation of China (81271703, 51203120), Natural Science Foundation of Zhejiang Province (LQ12E03001), Medical & Health Technology Program of Zhejiang Province (2013KYA133, 2014KYA149), the International Scientific and Technological Cooperation Projects (2012DFB30020) and the Specialized Research Fund for Science and Technology Major Projects of Higher Education of China (ZD2007006) are greatly acknowledged.
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