What Is the Difference between Hard and Soft Contact Lenses

6 What Is the Difference between Hard and Soft Contact Lenses  

The number of options for contact lens wearers has been transformed by innovative advances in the chemistry of the polymeric materials used to create these vision products. 

Most everyone is familiar with the contrasting resilient, pliable nature of soft contact lenses and the brittle character of hard lenses. Both lenses are constructed from polymers, but the differing chemical composition of each polymer leads to considerably different physical properties. 

Hard contact lenses are composed of a polymer that repels water because the constituent repeating units (the monomers that link together to form the polymer) are nonpolar, hydrophobic segments. The first hard contact lens was constructed in 1948 from the monomer known as methyl methacrylate (MMA), yielding the polymer poly(methyl methacrylate) or PMMA. This material offers durability, optical transparency, and acceptable wettability for optimal comfort. Today the rigid lens material of hard contact lenses is often constructed by combining MMA with one or more additional hydrophobic monomers to provide better gas permeability. 

The first soft contact lenses were also constructed with a polymeric material containing a single monomeric unit. The added pliability of the soft lens was derived from the more hydrophilic nature of the monomer, enhancing the ability of the polymer to absorb water and provide greater comfort to the lens wearer. This monomer is a derivative of MMA known as hydroxyethyl methacrylate (HEMA). A number of hydrophilic monomers are used in soft lenses today these materials are referred to as hydrogels because of their ability to absorb significant amounts of water yet remain insoluble. 

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What is the difference between hard and soft contact lenses See p. 221. 

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