Using stereolithography for the fabrication of human corneal stroma tissue

The human cornea acts as an optical window of the eye. In our previous publication, “Fabricating corneas in the lab”, we discussed in more detail the function of this tissue and presented various approaches that are recently being explored to build 3D bioengineered corneal tissues. 1

Figure 1: The human cornea 2.

3D bioprinting aims to develop a corneal model that is geometrically accurate, mechanically stable, and physiologically relevant for the development of eye drugs. However, creating this biocompatible structure still represents a significant challenge in the production of corneal substitutes. 3

In 2018, a first study by Isaacson and colleagues proposed to generate complex 3D corneal equivalent within gelatin slurry using the Freeform Reversible Embedding of Suspended Hydrogels (FRESH) method with the extrusion 3D bioprinting method. They highlighted that careful adjustment of print parameters such as printing speed, needle diameter, and bioink viscosity could ensure that both mechanical stability and print accuracy were achieved4. However, this method has limitations as decreasing the nozzle diameter, which is recommended for achieving higher resolution, can affect cell viability by creating higher extrusion forces.

More recently, stereolithography printing (SLA), a light-based approach of rapid 3D prototyping in which a liquid resin is cured by a UV or visible light to form a solid object, has been shown to be a promising technique to print the corneal stroma5.

Kutlehria et al. used the SLA technique to print a scaffold to help maintain the curvature of the cornea during printing and to mold high-resolution features that would be difficult to achieve by extrusion alone1. As a matter of fact, the resolution of the print with SLA printing can reach a feature size of 25 µm, as opposed to a maximum resolution of 160 μm when relying on nozzle gauge alone as described in the Isaacson study4. The precision of the SLA resolution is ideal for printing highly curved structures as more layers can be printed, resulting in more apparent curvature. 1,5 They also showed that this support scaffold could facilitate high-throughput printing of 6–12 corneas and enabled using bioinks of optimized mechanical properties to minimize printing time1.

Optimization of the bioink is a crucial challenge for corneal stromal equivalent printing. Gelatin Methacrylate (GelMA) has been widely used as a tissue-engineered scaffold material because of its excellent biocompatibility and tunable physical properties.

In a study developed by Mahdavi et al., the team used a visible-light SLA 3D bioprint system with a bioink composed of GelMA and corneal stroma cells to bioprint the precise geometry of the human corneal stroma. Different formulations of the bioink (at concentrations of 7.5 and 12.5 %) were used to manufacture and test the corneal scaffold. Regarding the water content and optical transmittance of bioprinted scaffolds, it was observed that the 12.5% ​​scaffold was more similar to the native corneal stromal tissue5. Subsequently, cell proliferation, gene, and protein expression of human corneal stromal cells encapsulated in bioprinted structures were investigated. The cytocompatibility in 12.5% ​​GelMA scaffolds was significantly higher than those in 7.5% GelMA scaffolds.

In addition to corneal stromal tissue, the stereolithography method can be used in several biomedical applications, such as soft tissue printing and transplantable devices. The new TissueRay ™ developed by TissueLabs is the first masked stereolithography (MSLA) 3D bioprinter on the market. Its 4K light-based system allows the creation of microfluidic devices, organs on chips, cell-loaded constructions, and scaffolding, offering the perfect combination of high resolution and high throughput.

Figure 2: The new TissueRay ™ 3D bioprinter developed by TissueLabs.

REFERENCES

1. Kutlehria, S. et al. High-throughput 3D bioprinting of corneal stromal equivalents. J. Biomed. Mater. Res. B Appl. Biomater. 108, 2981–2994 (2020).

2. Credit javi_indy: https://www.freepik.com/free-photo/close-up-beautiful-little-girl-brown-eye_1319043.htm#query=eye%20close%20up&position=2&from_view=search

3. Ulag, S. et al. Recent developments and characterization techniques in 3D printing of corneal stroma tissue. Polym. Adv. Technol. 32, 3287–3296 (2021).

4. Isaacson, A. et al. 3D bioprinting of a corneal stroma equivalent. Exp Eye Res. Aug; 173: 188–193 (2018).

5. Mahdavi, S. S. et al. Stereolithography 3D Bioprinting Method for Fabrication of Human Corneal Stroma Equivalent. Ann. Biomed. Eng.48, 1955–1970 (2020).