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).