4.5.2. Core–shell structure of filament
The scanning electron microscopic (SEM) image of the cross-section of the gel filament after freeze-drying is shown in Figure 8; the porous structures were observed in all the samples. The porous structure is important for tissue engineering scaffold. The high porosity provides sufficient surface area to support cell adhesion and growth. Moreover, interconnected networks are considered essential for cell proliferation, nutrition transport, and vascularisation. The pore size of the KC-SA-15kPa scaffold was relatively uniform, with 82% of pore sizes between 80 and 110 μm. With an increase in the printing pressure, delamination occurs along the radius. Coaxial rings near the surface were found to be more compact than those near the axis forming the core–shell structure, which was caused by gradient shear stress and cross-linking. Bioink near-surface was subjected to the maximum shear stress, and the chain segments were arranged regularly, to release more free volume and retaining less water (water in the hydrogel forms ice crystals and sublimates to form pores during freeze-drying). The kc-s scaffold also has different cross-linking degrees between the shell and inner layers, which is in accordance with the diagram shown in Figure 7. The shear stress exhibits a gradient distribution from the shell to the core; therefore, the bridged ions migrate inside the gel, resulting in a lower crosslink density and a higher porosity in the core. The shear stress is isotropic along the circumferential direction, and hence, delamination occurred along the radius.
The core–shell structure was noted in KC-SA-25kPa; the shell layer was dense with small pores, and the core area had large pores. Furthermore, the KC-SA-35kPa scaffold showed a more obvious core–shell structure, with 85% of the pore size between 30 and 70 μm and a large pore present in the centre area.
The SEM images of KC-SA-C-15kPa, KC-SA-C-25kPa, and KC-SA-C-35kPa are shown in Figures 9 and 10. The porous structure of KC-SA-C was similar to that of KC-SA. With increasing printing pressure, the core–shell structure became more obvious, with the central pore of KC-SA-C-0.3 scaffold increasing to 321 µm at 35 kPa of the printing pressure, and the thickness of the shell layer increased obviously with increasing printing pressure (Figure 10).