## 1 Introduction
### Options for Delivery of Cultured CECs  
At the moment, there are two methods of cell delivery described in the literature--tissue engineered endothelial keratoplasty (TE-EK) and direct corneal endothelial cell injection (CE-CI) [67]. The TE-EK approach builds upon established EK surgeries, such as DSEK and DSAEK, for graft delivery. Firstly, the cultured CECs are grown to confluence, dissociated into single-cell suspension, and seeded densely onto a thin biological scaffold carrier at a density of 3000 cells/mm\({}^{2}\)[67]. After allowing the TE-EK graft to stabilise for up to 7 days, it is transplanted into the eye [68]. As for, CE-CI approach, after dissociation into a single-cell suspension, cultivated CECs are administered via direct injection into the recipient's anterior chamber [69]. In a study comparing the techniques for treating bullous keratopathy in a rabbit model, animals receiving TE-EK grafts initially exhibited an increase in central corneal thickness (CCT) to >1000 \(\upmu\)m post-operatively. However, from week 1 onwards, improvements in corneal clarity were observed, with CCT measuring 484.3 \(\pm\) 73.7 \(\upmu\)m by week 3. In contrast, rabbits treated with CE-CI maintained corneal clarity throughout the study period, with CCT measuring 582.5 \(\pm\) 171.5 \(\upmu\)m at week 3. These findings underscore the efficacy of both TE-EK and CE-CI therapies in restoring corneal clarity and function in the rabbit model, with CE-CI showing a quicker initial response [67]. Due to its simpler procedure and reduced invasiveness compared to TE-EK, direct injection of cells into the anterior chamber is often preferred. Numerous animal studies have demonstrated the potential of CE-CI, often in combination with ROCKi, to restore corneal endothelial function in models of endothelial dysfunction [70]. The effectiveness of this approach has been demonstrated in multiple models, including rabbits and monkeys; however, complete functional recovery of the corneal endothelium could not be clearly demonstrated following CE-CI in in a feline model, possibly due to species-specific differences and variations in the techniques used to induce CEC injury and culture the cells [67; 71; 72; 73]. In a rabbit model, a study investigating CE-CI for corneal endothelial deficiency revealed challenges in cell attachment. Injected cells, contrary to previous findings, did not effectively reduce corneal oedema in an upright eye position; rather, they adhered to the posterior corneal surface only when the eye was positioned downward, relying on gravitational forces for attachment [74; 75]. The promising findings from preclinical studies have laid the groundwork for the progression of the cell injection technique to human clinical trials. In 2018, [PERSON] et al. released an influential study demonstrating the efficacy of injecting 300 \(\upmu\)L HCECs cultured from donor cornea supplemented with ROCKi in 11 patients with bullous keratopathy [69]. All 11 eyes achieved a CEC density greater than 500 cells/mm\({}^{2}\) at 24 weeks (100%; 95% confidence interval [CI], 72 to 100). Corneal thickness decreased significantly to less than 630 \(\upmu\)m in 10 out of 11 patients (91%; 95% CI, 59 to 100). Visual acuity improved in most cases, with an improvement in best-corrected visual acuity of two lines or more recorded in 9 of the 11 treated eyes (82%; 95% CI, 48 to 98). A follow-up study confirmed the safety and efficacy of cultured HCEC injection therapy for up to 5 years after surgery with a mean endothelial cell density of 1257 cells/mm\({}^{2}\)[76]. Following this pioneering research, other research groups are aiming to re-create this work. AurionBiotech is the first clinical trial currently underway trying to recreate these findings outside of Japan [77]. [PERSON] et al.