Dogan et al., investigate photoinitiator (PI) design criteria in cell-laden gelatin methacryloyl (GelMA) bioprinted constructs. The study focuses on two types of PI, type I such as lithium phenyl (2,4,6-trimethyl-benzoyl) phosphinate (LAP) and 2-hydroxy-1-(4-36 hydroxyethyl-phenyl)-2-methyl-1-propanone (Irgacure), and type II, such as Eosin Y. It has been concluded that high PI type I concentration increases hydrogel’s elastic modulus but reduces cell viability due to reactive oxygen species (ROS) production, and type II PI affect hydrogel’s mechanical properties in a “fluctuating manner”. The conclusions are based on conducting mechanical tests, swelling assessment, SEM imaging of dried scaffolds, ROS measurement, and in vitro cell culture.

The study is based on a reasonable rationale that a tradeoff between PI concentration, scaffold mechanical properties, and cell viability may exist, so the conducted experiments are mostly sound; however, there are several major issues that require careful consideration:

1) This manuscript does not include any experiments or discussion related to bioprinting. Therefore, the title and focus should strictly reflect the actual scope, specifically "photocrosslinked GelMA hydrogels," unless bioprinting data are provided. Additionally, the authors utilized 5% GelMA, a concentration that would pose significant challenges for extrusion-based bioprinting due to insufficient viscosity.

2) The influence of photoinitiator concentration on hydrogel properties, including those reported in this study, has been extensively investigated in previous literature. Therefore, the authors must clearly articulate how their results offer novel or differentiated insights compared to the substantial existing literature on this topic. A comprehensive discussion placing their findings in context with earlier work is required. For example, the following references specifically address the same problem and should be discussed accordingly:

a. Nguyen, A.K., Goering, P.L., Reipa, V. and Narayan, R.J., 2019. Toxicity and photosensitizing assessment of gelatin methacryloyl-based hydrogels photoinitiated with lithium phenyl-2, 4, 6-trimethylbenzoylphosphinate in human primary renal proximal tubule epithelial cells. Biointerphases, 14(2).

b. Pahoff, S., Meinert, C., Bas, O., Nguyen, L., Klein, T.J. and Hutmacher, D.W., 2019. Effect of gelatin source and photoinitiator type on chondrocyte redifferentiation in gelatin methacryloyl-based tissue-engineered cartilage constructs. Journal of materials chemistry B, 7(10), pp.1761-1772.

c. Xu, H., Casillas, J., Krishnamoorthy, S. and Xu, C., 2020. Effects of Irgacure 2959 and lithium phenyl-2, 4, 6-trimethylbenzoylphosphinate on cell viability, physical properties, and microstructure in 3D bioprinting of vascular-like constructs. Biomedical Materials, 15(5), p.055021.

d. Effect of Photoinitiation Process on Photo-Crosslinking of Gelatin Methacryloyl Hydrogel Networks. Doğukan Duymaz, İsmail Can Karaoğlu, Seda Kizilel. bioRxiv 2025.04.10.648118; doi: https://doi.org/10.1101/2025.04.10.648118

3) Th mechanism of each reaction and the effect of variables, such as light intensity, need to be shown and explained in detail.

4) The SEM images of dry scaffolds do not accurately represent the pore architecture of hydrated hydrogels. Drawing conclusions about pore size from dehydrated samples is therefore misleading and scientifically unsupported. To obtain reliable measurements of hydrogel pore size in the hydrated state, the authors should use established techniques as outlined in the literature. These methods preserve or probe the native hydrogel structure and should be employed to validate any claims regarding pore size.

Jayawardena, I., Turunen, P., Garms, B.C., Rowan, A., Corrie, S. and Grøndahl, L., 2023. Evaluation of techniques used for visualisation of hydrogel morphology and determination of pore size distributions. Materials Advances, 4(2), pp.669-682.

5) Statistical analyses should be added to Figures 2, 3A, 3B, and 3Cii. The discussion needs to be informed by these analyses.

The authors have addressed my previous comments. I appreciate their effort in improving the quality of this manuscript.