The types of crosslinking agents applied in contact lens raw materials (especially for soft contact lenses, including hydrogel and silicone hydrogel lenses) are diverse. Their core function is to connect linear polymer chains into a stable three-dimensional network structure during polymerization, thereby imparting the lens with necessary mechanical strength, shape stability, elasticity, and water content control.
The following are the main categories and specific varieties:

I. Main Categories and Common Varieties

1. Bis(meth)acrylates

This is the most classic and widely used class of crosslinking agents, containing two or more (meth)acrylate double-bond functional groups.

2. Silicon-containing Crosslinkers

Specifically designed for silicone hydrogel lenses. Their molecules contain both siloxane segments (providing oxygen transmission channels) and multiple polymerizable double bonds.

3. Other Types of Crosslinkers

• Vinyl Crosslinkers: Such as divinylbenzene, but due to its hydrophobicity and rigidity, it is less used in soft lenses and may be used in some RGP lens materials.

• Hydrolyzable or Degradable Crosslinkers: Used to manufacture temporary bandage lenses or drug-eluting lenses, which can degrade slowly in tear fluid. For example, certain special diacrylates containing ester bonds.

• Natural Derivative Crosslinkers: In exploratory research, such as dimethacrylate chitosan, used for biodegradable or functional lenses.

II. Key Considerations for Selecting Crosslinkers

1. Reactivity: Must match the polymerization rate of the main monomers.

2. Functionality: Difunctional, trifunctional, or polyfunctional, determining the topology and density of the crosslinked network.

3. Molecular Chain Length and Flexibility: Affects the segmental motion between crosslinking points, thereby influencing the lens's elastic modulus (Young's modulus). Short-chain crosslinkers (e.g., EGDMA) typically make the material harder and more brittle; long-chain crosslinkers (e.g., PEG400DMA or long-chain siloxane crosslinkers) impart better flexibility and tear resistance to the material.

4. Hydrophilicity/Hydrophobicity:

   • Hydrophilic crosslinkers (e.g., PEGDMA) can increase network hydrophilicity, raising water content.

   • Hydrophobic crosslinkers (e.g., siloxane crosslinkers) reduce water content but are crucial for constructing oxygen transmission channels.

5. Biocompatibility: It must be ensured that the crosslinker itself and its residual levels are extremely low, and it does not release harmful substances in the ocular environment.

6. Association with Final Lens Performance:

   • Oxygen Permeability: Siloxane crosslinkers are key.

   • Mechanical Strength: The amount and type of crosslinker directly affect the Young's modulus and tear resistance.

   • Water Content: Hydrophilic crosslinkers can increase it, while high crosslink density typically limits water absorption.

   • Wettability and Comfort: Hydrophilic long-chain crosslinkers help improve surface wettability.

Summary

In contact lens formulation design, the selection and dosage of crosslinkers are the essence of materials science.through precise formulation ratios and polymerization processes, enable these crosslinkers to work synergistically with the main monomers, ultimately producing safe, comfortable, oxygen-permeable, and durable contact lens products.