READER AND INDUSTRY FORUM
THE ROAD TO MODERN DAY CONTACT LENSES
AGUSTIN GONZALEZ, OD
The early foundations of what the public recognizes as modern day contact lenses originated in the late 1800s. Originally made of glass and only worn for short time periods, those contact lenses were very uncomfortable and never really became a viable option for vision correction (Schifrin and Rich, 1984).
With the development of plastic polymers in the 1930s, long-chain polymer molecules made up of identical smaller units called monomers allowed for designs that where much more flexible and tolerable in the eye. This laid the foundation for the development and fundamentals of what we consider modern day contact lens technology. These polymers permitted much thinner contact lens designs to be fitted on the corneal surface and allowed modifiable optics to provide vision correction (Kemsley, 2008).
The First Polymer
The first polymer used to design contact lenses was polymethyl methacrylate (PMMA). With similar clarity to glass and being more lightweight, designs that conformed to the corneal radius could be worn much more comfortably compared to other available options (Kemsley, 2008). However, these hard lenses still had problems; they did not allow oxygen to pass through, which can cause adverse effects on the eye, though this was not known at the time. More importantly, the designs were rustic, and the lenses were uncomfortable (Rinehart, 2010).
It was not until the 1950s that a breakthrough in polymer science led to the contact lenses as we know them today. Polyhydroxyethyl methacrylate was developed and used to create the first flexible hydrogel lenses.
This polymer had the added advantages of being much more flexible compared to PMMA, thus increasing comfort, and much more permeable to oxygen, creating a product that could be worn for longer time periods (Maldonado-Codina and Efron, 2003).
Hydrogel lens polymers consist of networks of cross-linked molecules that are both insoluble in water but hydrophilic. This property is facilitated by the presence of highly electronegative oxygen atoms exposed in the polymer, which allows this oxygen to form hydrogen bonds with water. This property also allowed the polymer to be able to contain exponentially more water than its dry weight, providing the previously mentioned breakthrough in higher comfort and oxygen transmission compared to the PMMA lenses (Maldonado-Codina and Efron, 2003).
Even with the great promise that these new polymers delivered, it was still a hurdle to develop a lens that could be more physiologically friendly to the cornea and could be worn overnight or for extended periods of time. But, even with the expanding knowledge of corneal physiological responses to contact lens wear and a better understanding of oxygen requirements and design thickness aimed to improve oxygen delivery, oxygen transmission still had limits. At that point, the industry clearly understood that polymer gas transmission limitations were a hurdle that was affecting both physiological response and extended wear time.
To achieve higher oxygen transmission, new, more permeable materials had to be developed, and alternate types of polymers were required; therefore, the era of the poly-siloxanes or silicone hydrogels (SiHys) was the next breakthrough (Lai and Friends, 1997).
The poly-siloxanes or SiHy polymers allowed for extremely high oxygen transmission, which then permitted overnight safer wear of contact lenses. With a higher oxygen permeability than that of water, contact lenses could now be designed and worn overnight or for extended periods of time with less oxygen deprivation and alterations to corneal physiology (Lai and Friends, 1997).
But this benefit did not come without challenges, as these high-oxygen-transmission polymers are hydrophobic in nature. In addition, wearing comfort of the early designs was often not as good as with previously available hydrogel lenses.
In fact, many of the first generation of silicone hydrogel lenses used surface modifications (gas plasma and plasma oxidation) to improve wearability and wetting angles (González-Méijome et al, 2006).
SiHy lenses introduced later included surfactants in blister packs to improve wetting. Surfactants work by having a hydrophobic head and a hydrophilic tail, thus attaching themselves to the silicone on one end and decreasing the surface tension of liquids on the other.
Second-generation SiHy lenses also use a different type technology that commonly consists of a wetting agent that binds well to polar molecules. One common compound is polyvinyl pyrrolidone (PVP) that can surround the silicone and improve both hydrophilicity and water-retaining characteristics of the lens.
Similar to the hydrogel polymers, PVP has a highly electronegative oxygen atom exposed that bonds to water molecules. Other polymer developments incorporate modified siloxane polymers designed to make the silicone more hydrophilic (Szczotka-Flynn, 2008).
With clinical emphasis being shifted to a healthy physiological response and acknowledging the advent of increased market share of daily disposable lenses, it is interesting to speculate what the future evolution of contact lens polymer technology will be.
It is important to recognize that daily disposable lenses are far healthier compared to multiuse units. But, it also is fun to speculate about an ideal chemical composition that would maximize vision, physiological corneal response, and comfort—all in an affordable and healthy manner. CLS
For references, please visit www.clspectrum.com/references and click on document #252.
Dr. Gonzalez is a graduate from the Inter American University School of Optometry in Puerto Rico (IAUPR) and has been in practice in Texas since 1995. He is a recognized industry expert in adoption and usage of ophthalmic medications by optometrists and is a board certified Clinical Medical Optometrist who has lectured extensively, including at AOA’s Optometry’s Meeting and the American Academy of Optometry.