Silicon hydrogel contact lenses

Introduction

Contact lenses have a rich and extensive history dating back over 500 years; when, in 1508, Leonardo Di Vinci first conceived the idea. It was not until the late 19th century that the concept of contact lenses as we know them now were realised.  In 1887 F.E.Muller was credited with making the first eye covering that could improve vision without causing any irritation. 

This eventually led to the first generation of hydrogel-based lenses as the development of the polymer, hydroxyethyl methacrylate (HEMA), allowed Rishi Agarwal to conceive the idea of disposable soft contact lenses. 

Silicon hydrogel contact lenses dominate the contemporary market. Their superior properties have extended wear options and have transformed the landscape of vision correction. These small but complex items continue to evolve, benefiting wearers worldwide. This evolution is such that the most recent generation of silicon hydrogel lenses have recently been released and aim to phase out all the existing products.

Benefits of silicon hydrogel lenses 

There are many benefits to this material’s use in this application. For example, the higher oxygen permeability improves user comfort and experience through relatively increased oxygen transmissibility that the material offers. These properties are furthered by the lens’ moisture retention which allows for longer wear times without compromising on comfort or eye health. Hence, silicon hydrogel lenses aimed to eradicate the drawbacks of traditional hydrogel lenses including: low oxygen permeability, lower lens flexibility and dehydration causing discomfort and long-term issues. 

This groundbreaking invention has revolutionised convenience and hygiene for users.

The structure of silicon hydrogel lenses

Lenses are fabricated from a blend of the two materials: silicon and hydrogel. The silicon component provides high oxygen permeability, while the hydrogel component contributes to comfort and flexibility.

Silicon is a synthetic polymer and is inherently oxygen-permeable; it facilitates more oxygen to reach the cornea, promoting eye health and avoiding hypoxia-related symptoms. Its polymer chains form a network, creating pathways for oxygen diffusion. Whereas hydrogel materials are hydrophilic polymers that retain water, keeping the lens moist and comfortable as it contributes to the lens’s flexibility and wettability. Both materials are combined using cross-linking techniques which stabilise the matrix to make the most of both properties and prevent dissolution. (See Figure 1).

There are two forms of cross-linking that enable the production of silicon hydrogel lenses: chemical and physical. Chemical cross-linking involves covalent bonds between polymer chains, enhancing the lens’s mechanical properties and stability. Additionally, physical cross-links include ionic interactions, hydrogen bonding, and crystallisation. Both techniques contribute to the lens’s structure and properties and can be enhanced with polymer modifications. In fact, silicon hydrogel macromolecules have been modified to optimise properties such as: improved miscibility with hydrophilic components, clinical performance and wettability. 

The new generation of silicon hydrogel contact lenses

Properties

Studies show that wearers of silicon hydrogel lenses report higher comfort levels throughout the day and at the end of the day compared to conventional hydrogel lenses. This is attributed to the fact that they allow around 5 times more oxygen to reach the cornea. This is significant as reduced oxygen supply can lead to dryness, redness, blurred vision, discomfort, and even corneal swelling. 

What’s more, the most recent generation of lenses have further improved material properties, the first of which is enhanced durability and wear resistance. This is attributed to their complex and unique material composition, maintaining their shape and making them suitable for various lens designs. Additionally, they exhibit a balance between hydrophilic and hydrophobic properties which have traditionally caused an issue with surface wettability. This generation of products have overcome this through surface modifications improving comfort by way of improving wettability. Not only this, but silicon hydrogel materials attract relatively fewer protein deposits. Reduced protein buildup leads to better comfort and less frequent lens replacement. 

Manufacturing

There are currently two key manufacturing processes that silicon hydrogel materials are made with. Most current silicon hydrogel lenses are produced using either cast moulding or lathe cutting techniques. In lathe cutting, the material is polymerised into solid rods, which are then cut into buttons for further processing in computerised lathe - creating the lenses. Furthermore, surface modifications are employed to enhance this concept. For example, plasma surface treatments enhance biocompatibility and improve surface wettability compared to earlier silicon elastomer lenses.

Future innovations

There are various future expansions related to this material and this application. Currently, researchers are exploring ways to create customised and personalised lenses tailored to an individual’s unique eye shape, prescription, and lifestyle. One of the ways they are aiming to do this is by using 3D printing and digital scanning to allow for precise fitting. Although this is feasible, there are some challenges relating to scalability and cost-effectiveness while ensuring quality. 

Moreover, another possible expansion is smart contact lenses which aim to go beyond just improving the user's vision. For example, smart lenses are currently being developed for glucose and intraocular pressure monitoring to benefit patients with diseases including diabetes and glaucoma respectively. The challenges associated with this idea are data transfer, oxygen permeability and therefore comfort. (See Figure 2).

Conclusion

In conclusion, silicon hydrogel lenses represent a remarkable fusion of material science and engineering. Their positive impact on eye health, comfort, and vision correction continues to evolve. As research progresses, we can look forward to even more innovative solutions benefiting visually-impaired individuals worldwide.

Written by Roshan Gill

REFERENCES

Optical Society of India, Journal of Optics, Volume 53, Issue 1, Springer, 2024 February

Lamb J, Bowden T. The history of contact lenses. Contact lenses. 2019 Jan 1:2-17.

Ţălu Ş, Ţălu M, Giovanzana S, Shah RD. A brief history of contact lenses. Human and Veterinary Medicine. 2011 Jun 1;3(1):33-7.

Brennan NA. Beyond flux: total corneal oxygen consumption as an index of corneal oxygenation during contact lens wear. Optometry and vision science. 2005 Jun 1;82(6):467-72.

Dumbleton K, Woods C, Jones L, Fonn D, Sarwer DB. Patient and practitioner compliance with silicon hydrogel and daily disposable lens replacement in the United States. Eye & Contact Lens. 2009 Jul 1;35(4):164-71.

Nichols JJ, Sinnott LT. Tear film, contact lens, and patient-related factors associated with contact lens–related dry eye. Investigative ophthalmology & visual science. 2006 Apr 1;47(4):1319-28.

Jacinto S. Rubido, Ocular response to silicone-hydrogel contact lenses, 2004.

Musgrave CS, Fang F. Contact lens materials: a materials science perspective. Materials. 2019 Jan 14;12(2):261.

Shaker LM, Al-Amiery A, Takriff MS, Wan Isahak WN, Mahdi AS, Al-Azzawi WK. The future of vision: a review of electronic contact lenses technology. ACS Photonics. 2023 Jun 12;10(6):1671-86.

Kim J, Cha E, Park JU. Recent advances in smart contact lenses. Advanced Materials Technologies. 2020 Jan;5(1):1900728.