Alisa Sivak, MA(Ed) manages the development of educational publications produced by the Centre for Contact Lens Research at the University of Waterloo’s School of Optometry and Vision Science, Canada.
Manufacturers and scientists continue to develop materials capable of overcoming the challenges associated with contact lens wear. The following is a summary of new materials research presented at the 2012 meeting of the American Academy of Optometry (AAO) and the 2013 meeting of the Association for Research in Vision and Ophthalmology (ARVO).
Antimicrobial lens coatings
At ARVO, Debarun Dutta (Brien Holden Vision Institute) reported that contact lenses coated with Melimine, a synthetic peptide, inhibited the growth of both P. aeruginosa and S. aureus. Cathelicidin, a naturally-occurring peptide, was only active against P. auruginosa.
The incorporation of bacteria-inhibiting components was also described at ARVO by Phat Tran’s team (Departments of Ophthalmology and Visual Science, Microbiology and Cell Biology at Texas Tech University Health Sciences Center, and Selenium Ltd.), which incorporated an organo-selenium polymer into a hydrogel lens. In vitro test results showed that the selenium blocked the formation of S. aureus bacterial biofilm. Both these lenses and selenium-free hydrogel lenses were incubated in a solution containing S. aureus. The test lenses demonstrated total inhibition of the bacteria, which was further confirmed via confocal microscopy.
Improving wettability by incorporating hydroxypropyl guar into lens materials
At AAO, Lakshman Subbaraman (Centre for Contact Lens Research, University of Waterloo) and Heather Sheardown (McMaster University) reported that incorporating hydroxypropyl guar (HPG) into conventional hydrogel and silicone hydrogel lens materials significantly decreased the contact angles of the materials tested, with higher concentrations of HPG significantly lowering contact angles. The incorporation of this agent did not affect the optical transparency of the materials.
Preventing lipid adherence by incorporating hydrophilic substances
Tear film lipids are attracted to the silicone components of contact lens materials, which can cause vision difficulties and decreased wettability leading to discomfort. At ARVO, Holly Lorentz (Chemical Engineering, McMaster University and the Centre for Contact Lens Research, School of Optometry and Vision Science, University of Waterloo) reported that her team was unable to reduce lipid deposition by incorporating hydrophilic substances such as hyaluronic acid, alginate and a silicone surfactant into model silicone hydrogel lens materials.
Coefficient of friction and lubricity
With a water content of more than 80%, the outermost gel layer of delefilcon A daily disposable silicone hydrogel lenses was designed to mimic the surface of the eye. At ARVO, Gregory Sawyer (Mechanical and Aerospace Engineering, University of Florida) reported that the front surface of these lenses maintained their lubricity after mechanical simulation of lens wear.
At AAO, the same team reported that the coefficient of friction of delefilcon A lenses is highly dependent on the force applied: At pressures similar to those encountered with the blinking eye, the coefficient of friction is extremely low; when pressure is increased to many times the normal ocular conditions, the surface is compressed and loses its inherent lubricity.
The composition of a lens material also has an impact on its elastic modulus. The typical modulus of a silicone hydrogel lens is on average between 500 and 700 kPa, and a higher elastic modulus equates to a stiffer material.
At both AAO and ARVO, Thomas Angelini’s team (Mechanical and Aerospace Engineering, University of Florida) reported on the elasticity and viscosity of delefilcon A lenses. These lenses have a very low elastic modulus under normal ocular conditions—considerably less stiff than lenses made from balafilcon A and etafilcon A.
Microrheology, or the study of the flow of matter (or diffusion of particles) within a ‘soft solid’ such as a contact lens polymer, showed that microscopic beads sandwiched between sections of delefilcon A material had significant motion (50-100 nm in 2 seconds) consistent with a water content of over 99%. Similar tests with balafilcon A showed very little movement (10-16 nm over 2 seconds).
Applying increasing pressure to the surface of delefilcon A lenses via colloidal probe indentation yielded higher modulus measurements, indicating that the soft outer layer increases to the modulus of a typical silicone hydrogel lens as the surface is compressed.
Yuchen Huo’s team, also from the University of Florida, determined the compressive surface modulus of various lenses by measuring the force required to penetrate the lens to a depth of 100-300 nm with a colloidal probe. Results presented at AAO showed that balafilcon A lenses, the surface of which has undergone a plasma oxidation process, have an elastic modulus of 2000 kPa; senofilcon A lenses, which have no surface treatment but incorporate internal wetting agents, have an elastic modulus of 700 kPa; delefilcon A lenses have a surface elastic modulus of 14 kPa, which can be attributed to its outermost high water content gel layer.