Contact Lenses - Design Principle For Contact Lens Material Using Multifunctionality
Contact lenses (CL) are medical devices that are worn directly on the cornea of the eye and are often used to treat refractive vision problems.
Contact lenses do most of the same things that glasses do for correcting vision, but they are more popular because they look better, are easier to use, and are more comfortable.
Contact lenses, for example, can be made to fit the shape of the eye, giving the wearer a wider field of vision and less blurry vision than glasses.
Contact lenses are also more comfortable to wear during physical activities, and they don't fog up like glasses do, which is a common problem.
In addition to correcting vision, researchers are looking into how contact lenses could be used as smart delivery systems to give medicines a longer time to work and as wearable biosensing platforms.
Additionally, cosmetic colored contact lenses are a popular option for individuals who want to change the look of their eyes.
With a projected worldwide market value of $9 billion in 2019 and more than 200 million customers, the contact lens business is doing very well.
Multifunctional materials are any material-based system that combines multiple functions, allowing several tasks to be carried out through the combination of different functional capabilities. Each function helps with a different physical or chemical process that can improve the whole system.
Human skin has numerous layers of cells that include oil and sweat glands, sensory receptors, hair follicles, blood arteries, and other components with purposes beyond providing structure and protection for interior organs.
Contact lenses incorporate several tasks beyond vision correction to work properly, such as treating glaucoma and age-related macular degeneration, detecting ocular allergies, dry-eye syndrome, etc. Contact lens functions result from storage, handling, disinfection, and biological conditions.
Long-term contact lens wearers must use disinfectants and storage cases. Cleaning methods are necessary for effective and safe contact lens usage. Wet contact lens storage prevents mechanical damage and infection.
Contact lens sterilization is important for hygiene and damage prevention. Blister packs are used to store contact lenses. Before distribution, packed contact lenses may be sanitized to prevent contamination.
Autoclaving denatures proteins and lipid complexes and kills bacteria in sealed blister packs. Other approaches use gamma or electron beam radiation to disinfect contact lens packages without changing their material qualities.
Disinfecting contact lenses is necessary to eradicate microbiological contamination. Contact lens disinfection involves washing, disinfection, and storage. Before disinfection, cleaning removes deposits, dirt, and biofilms. Saline, cleaning chemicals, surfactants, digital rubbing, and rinsing are used. This lowers lens microbes.
Handling contact lenses requires structural stability to withstand damage from repetitive insertion and removal. If there isn't enough mechanical stability, the material could permanently deform or break, which would affect things like optical correction, lubrication, and resistance to wear and tear.
For long-lasting comfort and excellent vision, the lens covers the cornea. Poorly fitted lenses cause mechanical and metabolic damage to the cornea, so lens fitting must permit dynamic fitting.
Applying and removing contact lenses properly prevents lens and eye damage. Wear and tear resistance is needed for repetitive device insertion and removal. The lens should be ion-permeable and have simple blinking movement.
Biomaterial contact lenses must work well in the eye's biological environment. Contact lenses must be compatible with the blinking action, limiting their geometrical form. For ophthalmic compatibility, the lens needs to keep a steady, continuous tear film for clear vision, the right amount of hydration, and the right way for the cornea to work.
The lens must be ion-permeable, sustain mobility, be non-irritating and pleasant, and resist tear film deposition. These needs demand several contact lens functionalities.
The most essential function is optical correction, which is impacted by the lens's contact with the ocular system, such as hydration. The lens surface must be wettable for the tear film, which establishes a new contact with the eyelid, and lubricant to give the eye comfort.
Wettability and tear film stability must be adjusted for blinking, including frequency and contact pressure. Chemical composition, including surface orientation of hydrophobic and hydrophilic macromolecule moieties, determines surface wettability. Biofunctionality of contact lenses comprises oxygen diffusivity, protein resistivity, and antibacterial activity.
System, device, component, and material levels may combine functions. A steel-piston engine drives an automobile system. All levels may have sub-levels depending on system size and complexity.
Materials have hierarchical levels that frequently correspond to length scales, with the number of significant sub-levels varying per class. Each hierarchical level of polymeric materials offers an opportunity to apply material functionalities. Pure liquids lack micro and nanoscale characteristics, whereas metals lack molecular characteristics.
The refracting power of lenses is inversely related to the focal length. A contact lens' refractive index is the ratio of the speed of light in a vacuum to the speed of light in a material and is determined by a contact lens' refracting power.
The Abbé refractometer measures refractive index using the critical angle of total internal reflection. The refractive index is wavelength-dependent; white light offers a spectrum average. Lens refractive indexes vary from 1.3 to 1.74. A higher refractive index means smaller lenses, which improves comfort.
Light affects how our eyes work. When there is a lot of light around, like during the day, our vision is controlled by three cone photoreceptor classes (L, M, and S-cones). This is called photopic vision, and it allows us to see colors and has the highest spectral sensitivity at 555 nm.In low light (at night), scotopic vision is mediated by rods.
Only rods are active at 507 nm in the blue area. Between photopic and scotopic vision is mesopic vision, which is when rods become more sensitive while cones are still working.
Contact lenses are classified by location and shape (surface form). Comfort relies on a contact lens' shape, especially its edge, thickness, and diameter. Spherical and aspherical lenses are types of contact lenses. Spherical contact lenses feature a sphere-shaped surface and uniform refractive power. Aspherical lenses feature varied refractive powers in various meridians to treat astigmatism. Diameter-wise, contact lenses are corneal, mini-scleral, and scleral. Scleral lenses are large-diameter gas-permeable contact lenses that cover the whole ocular surface.
Surface qualities determine a material's interaction with its environment. The contact lens surface interacts with tear film, ions, proteins, and corneal tissue.
The surface's chemical composition determines features like hydrophilicity and roughness. Surface qualities contribute to device wettability and lubrication. Wettability refers to the tear film's ability to spread and retain itself on a contact lens, influencing the device's interaction with the ocular surface and eyelid.
Hydrophilicity influences the intensity of attraction between a liquid and a lens surface. Hydrogel contact lenses comprise hydrophobic and hydrophilic polymers. As a lens dries, the hydrophilic ends migrate toward the inner matrix, and the hydrophobic ends turn outward. When the surface of the lens dries out between blinks, it makes water-repellent patches that irritate the eyelid.
Several variables affect a contact lens' comfort, including its modulus, thickness, water content, design, and perimeter. The surface coefficient of friction and lubricity correlate highly with lens comfort.
During blinking, the eyelid slides over the tear-lubricated cornea. The presence of a contact lens material between the cornea and eyelid disrupts the natural function of the tear film, causing changes in the fluid circulation between the pre and post-lens tear films (tear-exchange rate), which disrupts the activity of lipids and proteins in the lubricant as they interact with the lens surface.
Physical contact with the lens thins and compresses the ocular surface glycocalyx, which strains the eyelid epithelium during the blink cycle. Lubrication and maintenance of lubricated surfaces are important for both the eye and the contact lens to work well at the point where two moving surfaces meet. Any problem will increase frictional resistance and cause these structures to break down.
The mechanical stability of contact lenses affects damage resistance during handling and long-term durability. Blinking, rubbing for cleaning, incorrect fitting, and frequent insertion and removal from the eye may cause deformation or fracture, leading to optical performance loss and discomfort.
Contact lens hydrogels are viscoelastic. The elastic component causes physical deformation or shape changes when stressed, whereas the viscous component controls the rate of recovery when the tension is removed. Excessive stress may deform materials permanently. Softer materials are often more pleasant on the eyes, while lenses with inadequate rigidity are harder to use and cause more pain.
Contact lens materials must be oxygen permeable since the cornea gets most of its oxygen from the air. Oxygen permeability (Dk) is the product of oxygen diffusivity (D) and solubility (k) and specifies how much oxygen travels through a substance.
Oxygen transmissibility (Dk/t) considers the material's thickness (t) related to oxygen transport. Oxygen transmissibility in contact lens materials is the velocity of oxygen transport through a lens for a given thickness. Oxygen transfer through a lens depends on pH, temperature, and ionic strength.
Protein adsorption is the major cause of soft contact lens deterioration. Proteins adsorb to most surfaces, and whereas hydrophobic (non-polar) amino acids are concealed within protein molecules, hydrophilic (polar) amino acids interact freely with their surroundings. If charged side chains meet an oppositely charged surface, adsorption is strengthened.
When exposed to a solid surface, proteins reorganize and promote adsorption, lowering the system's Gibbs energy. Unfolded proteins can't execute their normal functions and may interact with other proteins and cells.
Denatured proteins may induce aggregation or immunological responses. The deposition rate depends on lens surface chemistry, surface charge, and water content. Surface charge affects protein adsorption on contact lenses.
Depending on the interdependence between multiple functions, the design and implementation of functions in any material could result in (a) decoupled functions, where the output of one function does not influence the input of the other function, so both are independent; or (b) coupled functions, which link several functionalities so that the output of one function serves as the input for another function.
By dispersing key properties across hierarchical levels or geographical separation, decoupling functions in multifunctional materials may be performed. By using different post-functionalization and surface treatment methods, the wetness of contact lenses can be changed locally without changing how the material works as a whole.
Hard and soft-lens polymers must be chemically stable and polymerizable. First, contact lens macromolecular components must be stable in the eye's physiological environment. Double backbone bonds must be avoided because they react with ultraviolet light and oxygen to generate unstable peroxide bonds. Solution, suspension, or emulsion polymerization is used to make contact lens polymers.
After polymerization, minor quantities of unreacted cross-linking agents and initiators remain in the mixture. Residues may transfer from the polymer to the biological environment and create long-term difficulties.
Additional purification stages or methods are needed to eliminate residual pollutants. Irradiation with ultraviolet light might begin polymerization, but cross-linking is more challenging. Different monomers react under proper circumstances to generate random copolymers for contact lens materials.
Two ways to make silicone hydrogel lens surfaces hydrophilic are outlined. These surface treatments shield the hydrophobic silicone from the tear film and increase surface wettability and protein resistance without affecting the bulk material's functionalities. First, plasma oxidation is followed by hydrolysis.
Second, ultrathin polymer films or surfactants are deposited or grafted on the lens surface. Physical adsorption or covalent bonding may accomplish this. In plasma polymerization/functionalization, tiny molecules are injected into the plasma chamber, which subsequently polymerizes to produce a thin layer on the lens.
The thin coating modifies the lens' surface characteristics and adds new functionalities locally without affecting the material's existing functions.
Gas plasma methods were used to provide a thin, homogeneous, high refractive index, 25 nm hydrophilic coating on Polydimethylsiloxane and Tris(trimethylsiloxy)-3-methacryloxypropylsilane-modified silicone contact lenses. This thin layer of polymer is easier to wet than the bulk material, but it doesn't change the way the bulk material lets oxygen through.
Combining high-level mechanical, physical, and chemical qualities in a single material system is difficult. Multiple components in one system, each contributing to a distinct purpose, may achieve multifunctionality.
This technique may influence pre-existing activities if new compounds contribute further functions. Methacrylic acid increases the water content of poly(2-hydroxyethyl methacrylate)-based hydrogels, which affects the material's oxygen permeability (a previously inherited function). Methacrylic acid’s carboxyl groups bind positively charged proteins like lysozyme, reducing its protein resistance.
Because silicone is hydrophobic, it is incompatible with the ocular surface. Hydrophobic lens surfaces are uncomfortable because they destabilize the tear film and promote deposit buildup.
The hydrophilicity of silicone hydrogel lenses is improved by soluble polymers (e.g., internal wetting agents) that provide an interface between the lens and the tear film. Poly(vinylpyrrolidone) and n-vinyl pyrrolidone are internal wetting agents used to promote hydrophilicity and minimize hydrogel surface friction.
The first commercial non-surface-treated silicone hydrogel containing long-chain high-molecular-weight internal wetting agents was polydimethylsiloxane copolymerized with dimethylacrylamide, hydroxyethyl methacrylate, and poly(vinylpyrrolidone). This gives the silicone a hydrophilic coating, lowering its hydrophobicity.
Although contacts seldom damage the cornea, sleeping with contacts that are not designed for prolonged usage increases the risk of corneal infection and even ulceration. In a 2018 paper, doctors highlighted the cases of six people who wore their soft contact lenses while sleeping and had significant eye infections.
Compared to spectacles, contacts provide a larger field of vision, adhere to the curve of the eye, and create fewer visual distortions and blockages. Sports and exercise don't interfere with wearing contact lenses. Normally, contacts are unaffected by the weather and won't fog up in the cold.
Simply put, the answer to this query is "no"—wearing contact lenses shouldn't be painful. If they do, you need to see your eye doctor right away. When you first get contacts, they might feel a little strange until your eyes get used to them, but they shouldn't hurt.
Sleeping while wearing contacts is dangerous. Experts say that sleeping in your contacts makes you more likely to get a corneal infection, which is an inflammation of the clear layer that covers the colored part of your eye.
The device's performance is improved by giving contact lens material pertinent functionalities as contact lenses gain in popularity. The better knowledge of the system environments during storage, handling for insertion and removal, and the biological environment is largely responsible for advancements in the functional behavior of contact lenses.
In this article, topics for function realization using chemical processes and associated design strategies were covered along with pertinent examples. Contact lenses are restricted to the form of a basic thin slab of material, so all the functions must be provided by the material itself in order for them to fit between the eye and eyelid.
The difficulty of integrating all functions into a single material system has now increased. It must be verified that none of the inherited or established functions will be adversely impacted when further functions are added to the same material. Molecular design, multi-component systems, surface treatment, and the incorporation of nanoparticles or small molecules are used to create and understand the functional behavior of the material building blocks in order to implement numerous functionalities in contact lens devices.
Similar design ideas and methods for incorporating multifunctionality into contact lenses are a useful example of how to accomplish these functionalities in other biomedical devices due to their demonstrated effectiveness in a broad application.