# Biomaterials for Ophthalmic Applications
## 2 Biomaterials in Ophthalmology  
The history of ophthalmic biomaterials is relatively brief. [PERSON], in 1862, implanted a foreign material for a purpose similar to that of a biomaterial. It was a glass disc enclosed in a two-ring skirt forming an artificial cornea. This device was tested on the cornea of animals, but it was unable to stay in place for more than a week. Later, [PERSON] attempted to construct an artificial cornea composed of celluloid (a mixture of nitrocellulose and camphor with stabilizers), implanting it in four human patients; however, it was rejected in the first few months. It was not until half a century later that a fully synthetic polymer was used as an implantable ophthalmic biomaterial composed of polyvinyl alcohol (PVA) gel. It was followed by the first artificial cornea made from polymethyl methacrylate (PMMA) [15]. A few years later, a synthetic polymer, poly(1-vinyl-2-pyrrolidone), was implanted in the vitreous cavity as a vitreous substitute. It was mainly thanks to the introduction of synthetic hydrogels that the range of biomaterials to treat ophthalmic lesions developed [1]. The main goal of developing successive generations of biomaterials is to bridge the gaps or defects of previous generations and to improve safety, efficiency and comfort. Innovations were made to improve quality standards, production or production efficiency to reduce costs. Market pressure to reduce costs exists in order to promote competitiveness and provide better accessibility. Ophthalmic biomaterials are now highly sophisticated devices and their usefulness has increased dramatically in recent years [1]. Many important requirements must be met by ophthalmic biomaterials, including the ability to deliver oxygen to tissues, refractive changes, tissue protection during surgery, tissue integration and healing modulation [16; 17]. Contact lenses are especially important as an ophthalmic biomaterial, as they are in contact with ocular surface components, primarily the corneal epithelium. They also, economically speaking, represent the most important category and have undergone the most important and complex evolution [18]. Hydrogels have already been approved for several ophthalmic applications, but currently, several are under investigation. Hydrogels are successfully marketed as soft contact lenses [19], foldable intraocular lenses [20], or in situ gelling vehicles for ophthalmic drug delivery [21]. Efforts are being mounted to improve the sustained release of antibiotics, anti-inflammatory drugs, therapeutic proteins and nucleic acids. Furthermore, hydrogels are also being investigated as potential vitreous substitutes [13]. A wide range of natural, semisynthetic and synthetic polymers can be used as starting materials for hydrogels. Examples of natural origin polymers include alginate, collagen, and hyaluronic acid (HA). Poly(ethylene glycol) (PEG), poly(vinyl alcohol) (PVA), polymers based on acrylate monomers, and siloxanes are examples of synthetic gel-forming materials [10]. Tissue engineering helps to regenerate lost tissue, specifically in severe cases such as burns, ulcers, diabetes, bone defects and liver failure among others. It is based on stem cells, growth factors and biomaterials [22; 23]. The general process of tissue engineering is based on the implantation of living cells in a scaffold specially designed to promote tissue replacement and regeneration [24; 25]. It is therefore considered as the next step in the development of biomaterials. Regenerative medicine relies on interdisciplinary research and applications based on repair, replacement or the regeneration of cells, tissues or organs to restore impaired function. However, adult humans only have a limited capacity for spontaneous regeneration. Induced regeneration has been reported in conjunctiva and cornea. Tissue engineering in the eye has mainly been reported in the anterior segment (cornea and conjunctiva) and significant progress has been made in cell therapies to treat degenerative diseases of the retina.