Lakshman Subbaraman, PhD, BSOptom, MSc is Head of Biological Sciences at the Centre for Contact Lens Research, School of Optometry and Vision Science at the University of Waterloo.
Contact lens care solutions play a major role in the overall care of contact lenses. A lens care solution is composed of several important components including preservatives (or biocides), surfactants, chelating agents and buffering agents.
Preservatives or biocides are an important component of a lens care product and the primary role of a biocide is to disinfect the lenses so that they can be safely inserted into the eye, following overnight soaking. Modern lens care regimens mainly incorporate one or two of these five biocides – hydrogen peroxide, polyhexamethylene biguanide (PHMB; also called polyhexanide, polyhexidine, polyaminopropyl biguanide), polyquaternium-1 (Trade name: Polyquad), myristamidopropyl dimethylamine (Trade name: Aldox) and alexidine. These biocides have different physical properties and vary in their molecular weight and mechanism of action. 1-5
Green AJ, Phillips SK, Hitchins VM, et al. Material properties that predict preservative uptake for silicone hydrogel contact lenses. Eye & Contact Lens 2012;38:350-357.
Lens uptake of preservatives
When a lens care product interacts with a contact lens, components such as preservatives present in the solution will be taken up by the lens material. 6-8 When these preservatives are released from contact lenses into the eye during lens wear, it can have a significant impact on comfort during lens wear. Furthermore, the uptake of preservatives by soft lenses can affect corneal physiology which is represented by increased staining and reduced antimicrobial activity of the preservative. 8, 9 Preservative uptake from lens care solutions to silicone hydrogel lens materials can be influenced by several properties of the contact lens material and that of the preservative.
Material properties can impact the uptake of preservatives
Green and co-workers assessed contact lens material properties that would impact the uptake of preservatives. The authors evaluated seven different silicone hydrogel contact lenses with varying surface and bulk properties (lotrafilcon B, senofilcon A, enfilcon A, galyfilcon A, comfilcon A, efrofilcon A and balafilcon A) and four different conventional hydrogel lens materials (polymacon, nelfilcon A, phemfilcon A and etafilcon A). The freezable and total water content of these materials was determined using the differential scanning calorimetry technique. The effective pore size of the lens materials was estimated using a probe penetration technique, wherein fluorescent dextrans of varying diameters & molecular weights and fluorescent polystyrene beads were used. The uptake of PHMB by various lens materials was determined using the high performance liquid chromatography (HPLC) technique.
The results from this study showed that all the conventional hydrogel and three of the five silicone hydrogel lenses evaluated (senofilcon A, comfilcon A and efrofilcon A) had a pore size of greater than 170 nm. Lotrafilcon B was shown to have a pore size of greater than 2.8 nm but less than 4.6 nm, while balafilcon A was shown to have a pore size of greater than 29.4 nm but less than 54 nm. This study also showed that low water content and non-ionic lens materials have lower initial PHMB uptake than other lens materials. Furthermore, when all the lenses were grouped based on water content and ionicity, PHMB was taken up by various lenses in the following order: low water content, non-ionic materials < high water content, non-ionic materials < low water content, ionic materials < high water content, ionic materials.
Contact lenses interact differently with biocides depending upon their composition
In conclusion, contact lenses interact differently with biocides depending upon their polymeric composition. This study has shown that the current conventional hydrogel lens grouping system based on two physical properties (water content and ionicity) can predict the material-lens care product interaction. However, these two properties can predict only the uptake of hydrophilic preservatives by silicone hydrogel lens materials. The hydrophobicity of silicone hydrogel contact lenses, although not investigated in the current study, is another distinctive material property that should be evaluated while determining the uptake of hydrophobic biocides. Further work is needed to determine the interaction of other biocides with other contact lens materials. This will be of significant clinical relevance as the mechanisms contributing to symptomatology during lens wear may be attributed to how the components in a lens care product interact with the lens material.
- Broxton P, Woodcock PM, Gilbert P. A study of the antibacterial activity of some polyhexamethylene biguanides towards Escherichia coli ATCC 8739. The Journal of applied bacteriology 1983;54:345-353.
- Broxton P, Woodcock PM, Gilbert P. Injury and recovery of Escherichia coli ATCC 8739 from treatment with some polyhexamethylene biguanides. Microbios 1984;40:187-193.
- Broxton P, Woodcock PM, Heatley F, Gilbert P. Interaction of some polyhexamethylene biguanides and membrane phospholipids in Escherichia coli. The Journal of applied bacteriology 1984;57:115-124.
- Codling CE, Hann AC, Maillard JY, Russell AD. An investigation into the antimicrobial mechanisms of action of two contact lens biocides using electron microscopy. Cont Lens Anterior Eye 2005;28:163-168.
- Codling CE, Jones BV, Mahenthiralingam E, Russell AD, Maillard JY. Identification of genes involved in the susceptibility of Serratia marcescens to polyquaternium-1. The Journal of antimicrobial chemotherapy 2004;54:370-375.
- Rosenthal RA, Dassanayake NL, Schlitzer RL, Schlech BA, Meadows DL, Stone RP. Biocide uptake in contact lenses and loss of fungicidal activity during storage of contact lenses. Eye Contact Lens 2006;32:262-266.
- Powell CH, Lally JM, Hoong LD, Huth SW. Lipophilic versus hydrodynamic modes of uptake and release by contact lenses of active entities used in multipurpose solutions. Cont Lens Anterior Eye 2010;33:9-18.
- Willcox MD, Phillips B, Ozkan J, et al. Interactions of lens care with silicone hydrogel lenses and effect on comfort. Optom Vis Sci 2010;87:839-846.
- Jones L, MacDougall N, Sorbara LG. Asymptomatic corneal staining associated with the use of balafilcon silicone-hydrogel contact lenses disinfected with a polyaminopropyl biguanide-preserved care regimen. Optom Vis Sci 2002;79:753-761.