Moreover, although collagen has been used in various tissue-engineering applications for skin, bone and cartilage because of its good biocompatibility and low antigenicity, its application for 3D-bioprinting still has limitations. In previous studies, collagen was most often used in inkjet bioprinting, which prints materials with low viscosity but rarely in extrusion bioprinting. Extrusion bioprinting needs the bioink to be self-supporting for layer-by-layer fabrication; additionally, the bioink must be temperature-sensitive and have the ability to gel rapidly on the printing substrate with high viscosity for printing definition. Thus, in extrusion bioprinting, gelatin rather than collagen, has often been used as a bioink because its property fits the requirement mentioned above. However, to better mimic tissue-specific extracellular matrixes (ECM), collagen, which is the most abundant component of ECM, is used as printing materials in extrusion bioprinting.
Human corneal epithelial cells (HCECs), which are native resident cells of the cornea connected to form a dense plasma membrane, were chosen to print in this matrix. HCECs were applied for seeding onto some substrates, such as amniotic membranes, collagen gel and polycarbonate membrane, to generate a bioengineered corneal epithelium. However, thus far, there are no reports on the 3D printing of such cells. Considering that the advances in 3D bioprinting have enabled the direct assembly of cells and ECM to form cellular models for 3D in vitro biology, we used the 3D hydrogel-based cell-laden technique to print HCECs and fabricated a 3D-engineered corneal epithelium.
Therefore, in this study, we report the 3D printing of HCECs into cell-laden constructs using an extrusion-based 3D cell-printing machine that is considered to be suitable for rapid prototyping and quick fabrication of 3D organic materials. Briefly, to make an environment capable of 3D cell printing, the low-temperature forming room, refrigeration function, cleaning function and sterile function need to be integrated as shown in Fig. 1. A nozzle system was developed that had a wide range of temperatures (−5 °C–150 °C) and precise temperature control (±0.1°), so that the material can be not only heated but also cooled. To obtain a construct with a good condition, we optimised the process parameters, temperature and extrusion speed that were presented in previous papers. To better mimic corneal-specific ECM, the additional amount of collagen that can be added into the alginate/gelatin system to print and form a stable 3D hydrogel macroporous network was determined in this study. Immediately after printing, the viabilities of the printed cells were evaluated. To accelerate the degradation rate of the alginate/gelatin/collagen gel, we tested whether the addition of different amounts of sodium citrate could result in different degrees of degradation of the gel and determined the relation curve between them. Finally, the effect of sodium citrate on proliferation and other key biological functions, such as specific marker protein synthesis, on HCECs printed within the constructs was evaluated.