For any type of ophthalmic lens inserted into the eye, whether to enhance vision or simply for cosmetics, wear comfort is based on two primary factors: oxygen permeability and wettability.
For manufacturers, achieving these characteristics means utilizing new advanced materials, such as silicone hydrogels. Unfortunately, hydrogels are also inherently “hydrophobic,” meaning they have poor wettability characteristics that can cause discomfort during lens wear.
Surface modification by plasma treatment during the manufacturing process can help overcome this problem, rendering the lenses more wettable to tear fluid. A flexible process, plasma treatment is also used to inhibit biofilm accumulation, smooth out lens machining marks, promote bonding and other tasks.
The use of plasma treatment is growing, and leading equipment providers are able to modify existing, mature tools and technology, complete with fixturing, to deliver what are essentially drop-in solutions for contact and intraocular lens manufacturers. Some providers even provide access to on-site research and development equipment as well as engineering expertise.
Access to this knowledge base also facilitates new entrants into the market, as in the case of a unique application of contact lens being utilized for next generation virtual and augmented reality images.
“The elegance of these [plasma treatment] solutions is that they leverage existing technology and know-how, as opposed to creating something that is completely new,” says Michael Barden of PVA TePla, a system engineering firm that designs plasma systems. “The advantage for lens manufacturers is they can move quickly and get fast results.”
Contact Lens Applications
Whether to improve vision, or for cosmetic reasons, contact lenses are typically made out of acrylates and siloxanes.
In recent years, advances in silicone hydrogel contact lenses have improved oxygen permeability, enabling up to five times more oxygen to reach the cornea compared to regular soft hydrogel alternatives. As a result, silicone hydrogel lenses now account for approximately two-thirds of all contact lens fittings performed in the United States.
Given the hydrophobic nature of silicone hydrogels, the surface characteristics of the lens must be altered by plasma treatment to render it wettable, or hydrophilic.
According to Barden, one method of achieving this is by plasma oxidation of the silicone hydrogel to form thin silicate islands across the surface that remain intact when the hydrogel is hydrated (causing it to swell by 10 – 20% by volume) and autoclaved.
Another method involves first plasma activating the lens material, followed by a plasma induced polymerization process of an organic species, which is then oxidized on the surface of the contact lens by an additional plasma step.
Plasma treatment provides other positive benefits beyond comfort, most notably the prevention of “cloudy” spots on the lens caused by lipid deposits.
“Tear fluid carries lipids and proteins which can deposit on the surface of the lens,” says Barden. “The accumulated deposition of these materials can result in cloudy spots, as well as biocompatibility issues.”
In addition to improving patient wear comfort, the hydrophilic plasma treatment of hydrogel contact lenses also reduces their affinity for lipids.
Intraocular Lens Applications
Plasma treatment also plays a critical role by modifying the surface of foldable intraocular lenses used to replace the natural lenses of eyes which have been damaged by trauma or disease, such as cataracts.
Advances in technology have brought about the use of silicone and acrylic, both of which are soft foldable inert materials. This allows the lens to be folded and inserted into the eye through a relatively small incision.
However, this type of lens material has a tendency to adhere to itself, as well as to handling tools. A tacky lens requires more manipulations by the surgeon and increases surgery time.
Plasma surface modification of intraocular lenses removes the surface tackiness, ensuring safe insertion and automatic unfolding of the intraocular lens to its original configuration once inside the eye.
In addition, the delivery device used by surgeons to insert the lens is often coated with a lubricious coating to facilitate the release of the folded lens. The cartridge or tube can instead be plasma treated to modify the surface, or apply a carbohydrate coating, to make it lubricious.
Plasma surface treatment is also used to remove the spiral pattern of lines on the optic portion of intraocular lens caused during the manufacturing process, which are traditionally removed by an extensive polishing procedure.
Plasma Treated Lenses for Augmented-Reality
Technology startup Innovega is using contact lenses in a whole new way. Its iOptik augmented reality system combines contact lens with a low-profile head mounted display for next generation viewing of virtual and augmented reality images. For this use, a uniquely designed contact lens was the most critical component.
This type of eyewear utilizes optical components that allow the wearer to perceive they are viewing a large computer display in the foreground, while simultaneously allowing them to view real objects at normal distances.
However, conventional video eyewear requires the optics to be built into the device, which compromises style, adds weight and even limits interaction with others. The designers at Innovega felt these barriers were limiting the mainstream acceptance of these products.
So the company decided to eliminate the optics from the eyewear and replace it with a contact lens that incorporated a micro-lens within it. The contact lens could be prescriptive, if necessary.
This design creates a bifocal effect that allows the wearer to simultaneously focus on virtual content from the eyewear and on the entire spectrum of activities in the real world. As a result of moving the optics to the contact lens, the iOptik eyewear is much more comfortable and stylish.
“Our contact lens is novel because it is a bifocal, but the bifocal has such an extreme step in prescription that you can’t see things unless they’re placed about a half inch from your eye,” says Jay Marsh, VP of Engineering at Innovega.
This shift in approach, however, meant the company needed to immerse itself in the world of contact lens manufacturing.
The first step was hiring an expert in contact lens design and manufacturing. Because wear comfort was a primary concern, using plasma treatment to increase wettability quickly became part of the discussion.
“We were aware that wetting modification was typical in the contact lens industry,” says Marsh. “We were also concerned about preventing cloudy spots on the lens caused by lipid deposits.”
To increase its knowledge and expertise in plasma treatment options, Innovega began working with PVA TePla, a company that designs plasma systems for surface activation, functionalization, coating, ultra-fine cleaning and etching.
Technicians from Innovega visited the PVA TePla lab to brainstorm with their technical team of Ph.D scientists, chemists and plasma physicists and run experiments.
This evolved into regular, monthly visits by technicians, who ultimately incorporated plasma treatment into a multi-step, lens molding process.
The final system incorporates plasma for surface activation of the molds to facilitate release of the lenses during manufacturing, for bonding micro-components to the lens, and for surface modification to increase wettability.
– Jeff Elliott is a Torrance, Calif.-based technical writer. He has researched and written about industrial technologies and issues for the past 20 years.