BIOE 4391

Q1/ what are the necessary requirements for a polymer used in the creation of contact lenses?

In general, some of the requirements for a polymer used in the creation of the contract lenses are

transparency of the lens, Refractive index suitable for bending light rays, and lenses made of polymer

that is easy to insert, remove, clean, and store. Polymers that hydrophilic, oxygen permeable, and

unreactive to chemicals on the eye surface are necessary to increase the biocompatibility of the lenses. A

big requirement of the lens is that it must be lightweight. This allows it to be placed on the eye

comfortably for extended periods of time without causing ocular strain. This is difficult because the lens

must also be strong to avoid tearing and/or scratching, have a reasonably high modulus of elasticity for

ease of handling, and yet still soft and flexible enough to feel comfortable on the eye.

The size and specific gravity of the lens also become important factors for eye comfort. If the

lens is too thick it will interfere with the eyelid and cause discomfort. If the specific weight of the lens

is significantly different from that of tears the lens will have a tendency to move up or down. This

makes the lens unstable on the cornea and makes fitting problematic.

Finally a good contact lens must be affordable. It has to be able to be made cheaply and

efficiently so that it can be purchased at a reasonable cost.

Q2/ would theses requirements be the same for disposable/ reusable contact lens?

The above requirements are general for most contact lenses including disposable and reusable contact

lenses. There is disharmony regarding the definition of a disposable contact lens. The U.S. food and

Drug Administration defines a disposable contact lens as a lens that is worn once and then discarded.

Most practitioners, however, define a disposable contact lens as a lens replaced every 1 to 6 months

intervals. Reusable contact lenses are used for six months or longer. More or less, as the life time and

amount of reusability increases, some of polymer properties need to be enhanced. Durability, oxygen

permeability and resistance to both protein deposition, and chemical degradation should be increased.

Protein or lipid depositions create a biofilm in the lens. This can result in the lens losing its ocular

properties, and the turns turning a yellow color. The yellow color is a result of lens spoilage, a result of

the diffusion of proteins and lipid into the lens. The push for extended wear lenses is limited by the

lenses biocompatibility.

Q3/ What are the differences between a Hard Contact Lens and a Soft Contact Lens?

Soft lenses are also called hydrophilic or hydrogel lenses which owe its softness to its ability to absorb

and bind water. Most soft lenses are made of Hydroxye thylmethacrylate (HEMA), a plastic polymer

with the remarkable ability to absorb water molecules: the polymer consists of three dimensional

network of “HEMA” chains cross linked with ethylene dimethacrylate (EDMA) or ethylene glycol

monogethacrylate (EGDMA). Recent models of hydrogel lenses added the surfactants, N vinyl

pyrollidone (NVP) to increase water content. Seeking to improve soft contact lens permeability scientist

started to make hydrogels from silicone based polymers like polydimethylsiloxane (PDMS).

Hard lenses are made of poly (methyl methacrylate) also known as PMMA. PMMA is an ideal polymer

to be used for hard contact lenses because it is cheap and easy to make. It is moderately hydrophobic,

which also contributes to it repelling proteins effectively. It has a typical oxygen permeability of 0.5

DK, which makes it effectively an impermeable membrane to oxygen and carbon dioxide. This

impermeability is what restricts PMMA lenses from being used more than about 8 hours at a time.

Another type of lenses is Rigid Glass Permeable. RGP lenses are, as the name implies, rigid, but the

plastics of which they are made are somewhat more flexible than hard lenses. Newer RGP lenses offer

the advantage of allowing more oxygen to pass through to the eye. Sometimes they are referred to as

"Oxygen Permeable Lenses". They are available in daily wear and extended wear options.

Q4/ would a polymer in contact lenses be in crystalline or semi-crystalline or amorphous

structure?

As mentioned before, contact lenses have the cross-linked amorphous structure. Crystalline or semi-

crystalline materials are opaque and scatter the lights. Hard contact lenses polymers are below their

glass transition temperature which gives it the hardness. Whereas the soft contact lenses polymers are

above their glass transition temperature which provides the softness in the lenses.

Q5/ what properties required for polymers used to make intraocular lenses are similar to those

required for contact lenses?

Many similar properties are required for intraocular lenses and contact lenses. Some intraocular lenses

are made of Acrylics. Acrylics are polymers synthesised from esters of acrylic or methacrylic acid.

Acrylic IOLs have a refractive index of 1.55 and the hardness of the acrylic is temperature dependent.

The high refractive index gives acrylic lenses the lowest edge thickness of all available lens materials.

At low temperatures the lens feels almost like PMMA and folding is facilitated by warming the lens.

Acrylic lenses fold and unfold slowly and can be handled when wet. If the lens is too warm it can

become sticky and unfolding can be difficult. Another material IOLs are made of is Silicone which is a

biologically inert polymer, polydimethylsiloxane (PDMS), which cannot leach out—unlike silicone in

breast implants. The refractive index of most silicone IOLs is now 1.47 reducing optical thickness and

facilitating folding. Finally, IOL can be made of Hydrogel. The material used is

polyhydroxyethylmethacrylate (PolyHema) with a water content varying from 18% to 30% and a

refractive index of 1.47. Hydrogel lenses fold and unfold faster than acrylic and are more controllable

than silicone. Because of their water content they must be kept hydrated until implantation, making lens

presentation in the operating theatre slightly difficult. Hydrogel lenses are available with a hydrogel

optic of 6 mm bonded to PMMA optics and a single piece lens which is currently in clinical trials.

Q6/ What required properties are different for polymers used to make intraocular lenses in

comparison to those required for contact lenses? Why are they different?

Ultraviolet (UV) radiation can cause phototoxicity, leading to damage the retinal natural structure.

Unlike contact lenses, intraocular lenses incorporated a UV-absorbing chromophore. These UV-

absorbing chromophores typically have a cut off wave length of 400 nm, leaving high transmission of

wavelengths, but negligible transmission of UV wavelengths.