3D printing of cell-laden constructs using gelatin-alginate-collagen
The printing of cell-laden constructs consisted of several steps; the printing setup, procedure and printing constructs are schematically summarised in Fig. 1.
To better mimic corneal-specific ECM, collagen was added to the gelatin/alginate printing materials. The largest concentration of collagen that can be added to the gelatin/alginate materials to be printed and form a stable 3D hydrogel network was determined. As shown in Fig. 2a, we added four amounts of collagen to 10% gelatin/1% alginate materials and found that adding collagen concentrations lower than 0.82 mg/ml resulted in complete homogeneity of gelatin/alginate/collagen. Otherwise, the cell-laden solution appeared stratified, indicating that, in this case, collagen could not totally dissolve in the gelatin/alginate solution, undoubtedly reducing the precision of the printing scaffolds. Thus, we chose 0.82 mg/ml collagen for the subsequent experiments. Figure 2(b,c) shows the 3D-printed HCECs/gelatin/alginate/collagen constructs, which had a clear and stable structure with interconnected channels and macroporous networks. The fibres of the 3D printing constructs were uniform and smooth with a mean thread diameter of 445.6 ± 8.0 μm (Fig. 2 (d)). The interconnectivity of the different layers of scaffolds in the Z-direction is shown in Fig. 3. The thickness of the scaffolds can be precisely controlled by regulating the thickness of one layer or printing different layers (shown in Fig. 3a–c). The printing bioink (collagen/gelatin/alginate materials) has suitable mechanical properties to self-support for layer-by-layer fabrication (shown in Fig. 3d) and is suitable to be used in extrusion bioprinting. Moreover, as shown in Fig. 3e, the school logo image under the scaffold is clearly visible, and the result of light transmittance under the 630-nm wavelength of the scaffold is 62.2 ± 8.4%, indicating that the optical characteristics of the hydrogel construct is good and can be used as engineered corneal epithelium.