PEG based materials have low glass transition temperatures and as such the polymer chains are very mobile at body temperature. This chain mobility enables oxygen molecules to permeate through the material. Ocutec has made contact lenses from PEG based materials which contain no silicone and has tested their Oxygen Permeability in 3 different laboratories (Ocutec, Contamac and UMIST). Tests show the Dk values to be significantly higher than those for a standard hydrogel at the same water content. This class of materials has a natural non-silicone enhanced high Dk and so delivers oxygen to the eye without introducing a hydrophobic compound to the material which can compromise the quality of the product in other areas.
Clinical testing against a standard hydrogel product at Visioncare Research Ltd has shown a significantly lower level of limbal hyperaemia for Ocutec’s lens. Ocutec intends to test its product against silicone hydrogels and is expecting to see no difference in limbal hyperaemia as the Ocutec lens is delivering sufficient oxygen to the cornea to prevent an adverse physiological response.
Ocutec has successfully made lenses with a modulus of 0.50 MPa. This is a low enough value so that no mechanical staining occurs and so that the lens drapes over the vast majority of corneal topographies. It is high enough, however, so as to avoid any difficulties with handling on insertion. Many materials on the market today are made using materials with too high a modulus, this is often a problem seen with silicone hydrogels. Too high a modulus can lead to discomfort, staining and difficulty when removing the lens from the eye. A good balance in material flexibility is required to make a successful soft contact lens.
PEG materials bind water very strongly using hydrogen bonding – the strongest molecular electrostatic force. This binding means the lens rapidly rehydrates leading to minimal dehydration even when the conditions are forced. This attribute prevents lens shrinkage and surface drying, both of which lead to discomfort. Many people who wear contact lenses experience discomfort after several hours of wear and in fact as many as a half of all wearers discontinue wearing lenses due to this issue. The PEG material is fundamentally different from standard lens materials and Ocutec intends to prove this by in-vitro measurements and through clinical testing.
PEGs are capable of forming strong hydrogen bonding interactions with water . The interaction between the lone pair of electrons on the ether oxygen atoms and the hydrogen atoms in water molecules (which carry a positive charge) can form directly orientated hydrogen bonds. This strong interaction with water makes these polymers highly hydrophilic and makes water cling to the polymer surface. This attribute help the eye’s tear film to move effortlessly across the surface of the lens and to remain stable between blinks. This in turn makes the lens feel very comfortable as it fits naturally into the ocular environment.
This hydrophilic neutrally charged surface also prevents lipids, proteins and bacteria from adhering to the surface. Adherence can lead to drying and discomfort and increases the potential for infection.