Ординатура / Офтальмология / Английские материалы / Biomaterials and regenerative medicine in ophthalmology_Chirila_2010

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10

Bioinspired biomaterials for soft contact lenses

T. Goda, T. Shimizu and K. Ishihara,

The University of Tokyo, Japan

Abstract: This chapter describes representative bioinspired biomaterials, synthetic phospholipid polymers, for use as soft contact lens materials with advanced biocompatibility. The chapter first reviews how the phospholipid polymer was inspired from natural phospholipids in the cell membrane in order to develop biocompatible materials and why the phospholipid polymer shows superior biocompatibility from the physicochemical and biological viewpoints. The chapter then describes the application of materials containing the phospholipid polymer in daily-wear, daily-disposable, extended-wear, and continuous-wear soft contact lenses with enhanced protein repellency, water wettability, and biocompatibility.

Key words: phospholipid polymer, bioinertness, silicone hydrogel, oxygen permeability, water wettability.

10.1Introduction

In many recent clinical trials of artificial materials, very few candidates turned out to be compatible with living organisms and tissues. That is, the living body rejects most of the naturally occurring and synthetically delivered artificial materials by inducing acute biological reactions such as inflammation, immune responses, and blood coagulation; or by producing toxicity, unfavorable deterioration, or corrosion of the materials themselves (Ratner et al., 2004). Besides being biologically compatible, biomaterials must meet several physicochemical requirements for therapeutic use in specific clinical situations. As for soft contact lenses, for example, the fundamental requirements of the candidate material are that they must possess optical properties suitable for vision correction. Hence, it is difficult to develop biomaterials that meet both biological and physicochemical requirements. Therefore, biologically inspired or biomimetic materials that mimic molecular designs, functionality, or nanostructures found in nature have been developed for successful therapeutic application (Shin et al., 2003). Hereafter, we discuss the use of a representative bioinspired material, the phospholipid polymer, as soft contact lens material. The natural properties of the phospholipid polymer make it capable of mediating mild interaction with biomolecules and cell membrane surfaces to prevent undesired biological reactions inside

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the living body. The bioinertness of this material may be of great use for the development of a soft contact lens material with superior biocompatibility. With the increase in the popularity of contact lenses in the world, great interest has been generated in the potential complications of contact lens usage and in the means to reduce the frequency and severity of such complications. For example, the risk of infectious complications associated with the frequent use of contact lenses results in the pathophysiologic changes that predispose the eye to microbial invasion and replication (Stapleton et al., 2007).

Pseudomonas aeruginosa and Staphylococcus epidermidis are major causative agents of infectious keratitis in patients using soft contact lenses (Ladage et al., 2004). Adhesion of these bacteria to soft contact lenses is influenced by lens surface properties because the adhesion originates in adhesive protein in the extracellular matrices produced by the bacteria themselves. Therefore, the bioinertness of the contact lens material impairs the susceptibility to biofilm formation. In addition, the phospholipid polymer exhibits a good processability, which further justifies its use in the manufacture of soft contact lenses. Introduction of the phospholipid polymer into an appropriate material by means of a wide variety of chemical methods facilitates the regulation of the physicochemical properties required for soft contact lenses.

10.2Bioinspired phospholipid polymer

As described above, biocompatibility is a key criterion for the preparation of biomaterials. Generally, non-specific protein adsorption on to a biomaterial triggers a series of biological cascade reactions (Colman and Schmaier, 1997). The bioinert natural cell membrane surface is the best example of a material capable of preventing non-specific protein adsorption. A cell membrane as a self-assembly with a bilayer structure is organized by amphiphilic phospholipids as a unit in association with membrane proteins, glycoproteins, and glycolipids (Singer and Nicolson, 1972). Of these, phosphatidylcholine is the main component of the phospholipids in the human erythrocyte cell membrane. This compound is composed of a hydrophilic phosphorylcholine headgroup and two hydrophobic long alkyl-chain tails. The hydrophilic headgroup is oriented in the outer leaflet of the erythrocyte, which is believed to prevent non-specific protein adsorption on to the cell membrane. Applying the natural cell membrane itself directly to the biomaterial surface, however, is a problem because the cell membrane cannot be stably immobilized on a material surface for a specific duration because of the weak interaction of the membrane with the material surface and because of the membrane’s selfassembled structure. Furthermore, covalent bonding between the phospholipid molecules is not possible because of their lack of chemical reactivity. This obstacle has been overcome by a chemical innovation. Inspired by the molecular structure of the headgroup in the phosphatidylcholine, a new compound named

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of the headgroup due to intramolecular salt formation (Xu et al., 2007). The electroneutrality of the phosphorylcholine group may also stabilize the dimensions and solubility of poly(MPC) regardless of the ionic strength in the aqueous media (Matsuda et al., 2006). In the following section, we describe the use of the bioinspired phospholipid polymer for enhancement of biocompatibility and wettability; these factors being strongly related to comfort issues of soft contact lenses.

10.3Requirements for biocompatible soft contact lenses

Before describing the applications of the phospholipid polymer for soft contact lenses, let us review the conditions necessary for designing soft contact lens materials. The candidate polymer must satisfy the following requirements: the material must be transparent with suitable optical properties such as transparency and sphere power availability, must possess chemical and thermal stability, and must exhibit a high tensile and tear strength for lenshandling durability. In addition, since the material is directly in touch with the corneal epithelium, the material should be wettable to allow coverage with tear film, biocompatible, protein adsorption-resistive, electrolyte permeable, and oxygen permeable so as to avoid insult to the eye tissue. The potential hydrophobicity of a material increases the risk of dehydration of the lens as well as excessive lipid deposition. Protein adsorption on lenses concurrently decreases visual acuity, makes the lenses uncomfortable, and also reduces the wear lifetime. The contact lenses should transmit as much oxygen from the atmosphere into corneal tissues as in a no-lens situation (at least 87 barrer/mm) because, owing to the lack of blood circulation in transparent cornea, insufficient oxygen transport through the lens causes corneal edema resulting in a number of adverse physiological responses, such as excessive corneal swelling in overnight wear (Harvitt and Bonanno, 1999). Finally, from the manufacturer’s viewpoint, the material must be suitable for bulk polymerization and for manufacture using current production techniques.

The importance of the irreversible interaction of tear components with soft contact lenses has long been recognized. Two of the most significant variables in the development of lens biofouling are duration of wear and individual patient tear chemistry. The wear period is now much more carefully controlled with the advent of daily-wear disposable soft contact lenses. However, the development of materials with enhanced antibiofouling properties needs further improvement. The contributions of the material’s surface to the inflammatory response is becoming increasingly important with the advent of continuous-wear and long-term-wear soft contact lenses, as previously mentioned in the Introduction. Adhesions and activations of neutrophils are affected by surface characteristics which, in turn, influence

the inflammatory response to the lens. Bacterial adhesion to soft contact lens materials is the first stage in contact lens-associated microbial keratitis, which may ultimately lead to corneal ulceration. The candidate materials should possess protein adsorption resistance, biocompatibility, and tear wettability as well as bulk polymer characteristics for continuous and extended wear. Thus, the use of a bioinspired phospholipid polymer is one of the effective ways to enhance the antifouling properties and biocompatibility of the lens. Recently, soft contact lenses have been developed mainly for use as daily, daily disposable, extended, and continuous wear. The phospholipid polymer has succeeded in enhancing the overall biocompatibility of all types of soft contact lenses, as described in the following sections.

10.4Phospholipid polymer for daily-wear soft contact lenses

Daily-wear soft contact lenses with routine maintenance have a longer history of use than the other types of lenses. They have been available since the development of poly(2-hydroxyethyl methacrylate) (poly(HEMA)) hydrogels as a soft contact lens material (Wichterle and Lim, 1960). The hydrogel contains 38% water, has reasonable wettability, and offers the wearer comfort. Various types of derivative hydrogels containing different hydrophilic monomer units – such as methacrylic acid, N-vinylpyrrolidinone (NVP), and glyceryl methacrylate – have been developed with increased water content, subsequent oxygen permeability, and surface water wettability. Many attempts have focused on the enhancement of oxygen transport by either using a hydrophilic polymer or by making the hydrogel lens thinner. However, these hydrogels did not generate ideal results because of the induction of mechanical weakness and a tendency to adsorb tear proteins and lipids. For example, US Food and Drug Administration (FDA) II Group materials that contain NVP tend to adsorb more lipids than other conventional soft contact lens materials (Jones et al., 1997). Spoliation occurs more easily for materials with higher water content, not because of the water content itself, but because of the presence of NVP. In addition, soft contact lenses with relatively high water content tend to increase the occurrence of bacterial adhesion (Dang et al., 2003). In the case of ionic materials, e.g. methacrylic acid which is commonly introduced to elevate the oxygen permeability, lysozyme – a major protein of the tear fluid working as an enzyme that damages the bacterial cell wall by catalyzing hydrolysis of peptidoglycan and chitodextrins − is extensively adsorbed on materials that contain carboxylate groups (Keith et al., 2003). Lysozyme adsorption should originate in the electrostatic attraction between the negatively charged methacrylic acid and the positively charged lysozyme. Although it has been reported that the adsorbed lysozyme on the lens does not generate an adverse clinical reaction owing to the enzyme’s

(

HEMA (

(

MPC (