**1. Introduction**

The cornea is an avascular, transparent, and multilayered tissue whose primary function is to act as a lens to direct light entering the eye toward the retina. It consists of three cellular layers, an epithelium, stroma, and endothelium, and two acellular layers, the Bowman's and Descemet's membrane [1]. A third acellular layer, the Dua membrane, has also been described. The epithelium forms the cornea's outermost layer and comprises stratified epithelial cells. These cells act as a barrier to protect the eye from pathogens and allow the diffusion of essential nutrients and oxygen. The stroma constitutes 90% of the cornea's total thickness and contains keratocyte cells interspersed in lamellae but forming a connected network. The cornea is responsible for focusing approximately 75% of light into the eye for vision. It also protects the eye while preserving its transparency by self-renewal of its epithelium in particular [2]. However, injury, aging, or disease could lead to irreversible loss of clarity and corneal blindness.

Corneal blindness is one of the significant causes of vision loss leading to 23 million patients who have unilateral blindness and 4.9 million patients who have bilateral blindness globally [3]. Although corneal transplant is the most common

transplanted tissue worldwide, corneal blindness is still rising, with 2 million cases yearly [4]. Due to the increasing demand, eye banks cannot provide the cornea to all the patients, leaving an estimated 12.7 million people on the waiting list for corneal transplantation [5]. Corneal transplant with a human donor cornea has been the go-to treatment for corneal blindness since the twentieth century. In penetrating keratoplasty or full-thickness grafts, a full-thickness button of healthy donor corneal tissue replaces excised pathologic corneal tissue. Only the damaged layers are replaced in lamellar keratoplasty or partial thickness grafts. This helps maintain the integrity of the cornea and surrounding tissue, leading to faster healing and vision improvement. Complications of corneal transplantation include graft failure, of which graft rejection is a leading cause. Disease transmission from graft to host is rare but has been reported, e.g., endophthalmitis, due to donor-to-host microbial transmission [6]. Other issues include complications in immune-compromised patients, infectious diseases, and astigmatism associated with corneal transplants. However, proper donor screening and processing may reduce the risk of infection and related post-surgical complications. However, the donor cornea shortage is a major issue, especially in developing nations. It is even difficult to carry out complex surgical procedures in some developing countries due to insufficient resources/facilities. Thus, there is an urgent need to develop new therapies or biomaterials as alternatives to donor transplantation.

In this chapter, we focus on the biomaterials rather than fabrication techniques such as electrospinning or 3D bioprinting. We refer the reader to Patel et al. 2021 for a review of electrospinning in corneal constructs [7] and to Bin et al. 2019 and Fuest et al. 2020 for reviews on bioprinting of corneal implants [8, 9].
