The modulus of elasticity of a soft contact lens usually applies more to which material category?

• A. Hydrogel
• B. Silicon hydrogel
• C. Both equally
• D. Neither

Answer: (B)

Modulus of elasticity measures how stiff a material is, i.e., how much it resists deformation. Traditional hydrogel soft lenses are highly water-rich and form a very flexible polymer network, giving them a relatively low modulus. Silicone hydrogel lenses, on the other hand, incorporate silicone components that increase rigidity in the polymer network, so their modulus tends to be higher. Because this stiffness difference is a defining characteristic between the two material types, the modulus concept is more applicable to silicone hydrogels. The traditional hydrogel category is generally more compliant, so the modulus isn’t as defining there.