Biocompatibility of IOLs - Beyond material consideration
In a layman's language biocompatibility refers to acceptance of a foreign article by the body without any significant reaction or irritation. If the biomaterial retards or negatively affects the natural biological process for which it was intended to assist, then it would be largely termed as incompatible. Biocompatibility of IOLs has largely been understood as a function of the biomaterial involved in the making of the IOL - that is in today's context, if the IOL is more of hydrophobic or hydrophillic acrylate material. The hydrophobicity or otherwise is again labelled on the basis of the 'water content' of the IOL, that is, hydrophillic acrylic/acrylate materials are usually having a content of 20 per cent or more.
The purpose of this article is to look into some other aspects involving 'biocompatibility' other than the usual 'hydrophobicity' or 'hydrophilicity' of acrylic IOLs. Some such concepts are, 'chemicophysical property' of the IOL, the surface contact angle (SCA), surface smoothness (or roughness ).
1. Chemicophysical Property of IOL - The Electrostatic charge present at the surface of the IOL emits an electric field which in turn attracts or repels particles/cells/flares that may float in the anterior chamber immediately after surgery. This subject however has got very little attention.
In their study, Topography, Wettability, and Electrostatic Charge Consist Major Surface Properties of Intraocular Lenses, Na Yang et al, reported quite interesting findings with regard to this topic. In this study, the only one I could come across, the Hydrophobic IOLs ( without surface coating or colour ) demonstrated positive charge on the IOL surface while Hydrophillic acrylic IOLs had negative charge. The short wavelength blue light filtering IOLs also recorded a negative chage on the IOL surface. While positive or negative polarity may play an important role in cell adhesion on the IOL optic, more research and studies are required on the affect of biocompatibility with regard to IOL chemicophysical property of its surface.
2. Surface Contact Angle - If you drop a water droplet on the earth, the drop of water would sit on the soil at first. The contact angle of the water with the soil would be high. However, as the soil is porous, water would slowly sip into the soil and the contact angle between the water droplet sitting on the soil would fall. In case of an hydrophobic IOL, a drop of water on the IOL remains in place and does not spread out adequately, thus maintaning a high 'surface contact angle' or what is called as hydrophobicity of IOL surface is high. In case of a hydrophillic acrylic IOL, the water droplet would spread, and the surface contact angle would be lower. The surface hydrophobicity or hydrophilicity ( depending on the surface contact angle of water on IOL surface is also termed as wettability ). The lower the surface contact angle, the more the wettability of the IOL. This wettability of the IOL depends on surface tension of the IOL, and surface tension of the liquid. Schroeder et al, investigated the wettability, that is , surface contact angles of different IOL materials. Higer surface contact angles, that is lower wettability, related to less cell adhesion on the IOL surface. In general, surface contact angles of hydrophobic IOL are more than the hydrophillic IOLs, though significant variation exist between hydrophobic IOLs itself. .Tanaka et al found reduced cell adhesion on hydrophobic acrylic IOLs with a lower surface roughness and higher water contact angle. Heparin surface coated lenses had lower surface contact angles and higher wettability
3. Surface Roughness - As the term suggests, it is an indication of how smooth or rough the IOL surface is. In their study, Lombardo et al, proved greater cell adhesion on IOL optic with greater surface roughness. They postulated that higher surface roughness may relate to higher incidence of PCO. In studies done by Na Yang (2016), as well as in studies done by De Giacinto (2019) and others, surface roughness has been directly related to less hydrophobicity of IOLs. Studies have indicated surface roughness to surface adhessiveness, and thereby more cell attachment. Studies done by Na Yang (2016) noticed more grooves and pores in hydrophillic and PMMA IOLs than hydrophobic IOLs. However, it must be mentioned, surface roughness, like wettability and contace angle, differs widely among different hydrophobic IOLs as found by studies already quoted.
The intention of this article is to shed some light on what may affect biocompatibility of IOLs other than the common discussions of material and square edge optics. The purpose of this article is to cross the common perception that biocompatibility is a function of material itself. On the contrary. the biocompatibility of IOLs may not just be limited to material, but surface property of IOLs, that is the chemicophsical property of the IOL in terms of surface charge, surface contact angle and surface smoothness.