# Biomaterials for Ophthalmic Applications
## 2 Biomaterials in Ophthalmology
### Contact Lenses
Their extensive use is linked to the fact that these contact lenses have the highest oxygen permeability of all contact lens materials. Silicone contact lenses are often durable, attributable to their high Si-O bonding energy, often with a higher modulus than conventional polymer hydrogels. The high modulus is linked to irritation to the eye, such as the conjunctiv of the inner eyel [27]. Discomfort and dryness of silicone-based lenses are two of the main reasons for users' discontinuation [36]. HEMA and related hydrogels are high-water content, oxygen-permeable polymeric materials. These hydrogels can have a water content in the range of 20-80% depending on the comonomers, with a hydrogel composed of only HEMA containing about 38% water. The highly polar properties of HEMA make it attractive for applications where wettability is important, producing very comfortable contact lenses. The oxygen permeability of these gels is suitable for longer wear, but not to the same extent as silicone-based lenses. HEMA is commonly copolymerized with monomers to increase the water content of hydrogels. The mechanical properties can be improved by using a cross-linking molecule, such as ethylene glycol dimethacrylate (EGDMA). EGDMA has two functional groups allowing for the formation of covalent bonds between two individual polymer chains. This increases \begin{table}
\begin{tabular}{c c c} \hline
**Biomaterial** & **Disadvantages** & **Advantages** \\ \hline Poly(vinyl alcohol) (PVA) & Low permeability to oxygen; fixed water contact & Low cost; biocompatible; easy manufacturing \\ Silicon hydrogel & Expensive; abrasive behavior & High permeability to oxygen; high durability \\ Hydroxy ethyl methacrylate & Low permeability to oxygen; protein & Low cost; biocompatible; several copolymer possibilities \\ (HEMA) hydrogel & deposition problems & copolymer possibilities \\ Polymethyl methacrylate (PMMA) & Impermeable to oxygen; not flexible in the eyes; abrasive behavior & Low cost; well-studied polymer \\ \hline \end{tabular}
\end{table} 
Table 1: Advantages and disadvantages of the most commonly used contact lens biomaterials. Figure 1: From the material point of view, contact lenses depend on several materials properties. Final contact lenses require a consideration of wear time, comfort and cost. These characteristics depend on material properties, manufacturing processes and final treatment. the mass of the polymer and improves its ability to form a gel network. However, the crosslinks reduce the polymer-chain motion, which can decrease swelling and oxygen transport. The water content and cross-linking affect the modulus and oxygen permeability of the hydrogels; therefore, a balance must be reached between these parameters when designing a contact lens for a particular application [37]. PMMA has little to no oxygen permeability due to the lack of mobility of polymer chains which prevents the flow of oxygen or internal water to mediate oxygen flow. This occurs due to intermolecular forces, such as dipole-dipole bonding and physical entanglement, which is prevalent between polymer chains. The dipoles are created by the negatively charged (electrochemical negative) oxygen compared with the adjacent positively charged (electrochemical positive) carbon and hydrogen atoms. As a result, neighbouring polymer chains can attract each other to provide thermodynamic stability to the polymer. These intermolecular forces lead to a low free volume in PMMA; therefore, chains do not rotate or move easily. Additionally, PMMA does not contain large pendant chains inhibiting the interaction of neighbouring chains. All these factors together prevent oxygen flow through the polymer. However, functionalization of the PMMA surface can improve hydrophilicity, which can be useful [38]. Soft contact lenses are the most common and most successful commercial application of hydrogels.