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Shuai C
sintering. Furthermore, post-treated scaffolds with 70% microstructures of SLM-fabricated Ti6Al4V after heat
porosity possessed a comparable compressive strength treatment [195] . However, although the post-heat treatment
~12.5 MPa to porous trabecular bone. Similar post-heat can reduce thermal stress, it will damage its mechanical
treatment was also reported to treat FDM-derived HA- strength. The reduced strength is mainly due to the grains
based composite scaffolds [190,191] . growth during the long period heat treatment.
Heat treatment has also been reported to be applied in SLS-
produced scaffolds to promote the densification of the 4.2. Surface Treatment
structure. It is believed that SLS-produced parts usually Many surface treatment methods have been explored
have a relatively low densification rate and resultant to improve the biological properties of AM-fabricated
low strength due to its solid or semisolid consolidation porous scaffolds. Among these, coating bioactive ceramics
mechanism. Therefore, a post-heat treatment is necessary on scaffolds is an effective way to improve the surface
for SLS-produced scaffolds, especially for high bioactivity of scaffolds. Zhao et al. [196] achieved a uniform
melting point ceramic scaffolds. Feng et al. [192] applied coating of calcium phosphate on electrospun keratin-PCL
an isothermal heating to secondly sinter the porous scaffolds by immersing the scaffolds into Ca and (PO )
3−
2+
4
HA scaffolds achieved by SLS. It was found that the solutions. Results showed that the incorporated keratin
isothermal heating increased the density by activating provided nucleation sites for the homogeneous deposition
grain-boundary diffusion and grain-boundary migration. of calcium phosphate, which significantly facilitated the
As a result, the compressive strength of scaffolds was cell/matrix interactions. Luo et al. [197] fabricated alginate/
significantly increased from 6.45 to 18.68 Mpa. Other nano-HAP composite scaffolds with a nano-HAP layer
than that, Liu et al. [193] used SLS to construct composites homogeneously and completely coating the surface. The
scaffolds, which exhibited a loose structure and high surface mineralization improved the cell attachment and
surface roughness. The post-heat treatment was then spreading, as well as supported a sustaining protein release
carried out at various temperatures ranging from 1200°C compared to scaffolds without nano-HAP layer. Cell
to 1400°C. After that, the compressive strength was viability studies also demonstrated that polycrystalline
significantly enhanced. Moreover, the surface roughness diamond coating on TI6Al4V scaffolds promoted the
was decreased with pores shrinking. attachment and proliferation of normal CHO mammalian
Heat treatment is also an effective way to reduce the cells and improved osseointegration [198] . Another surface
microstructural defects as well as the residual thermal coating method is electrolytic deposition. It can prepare
stress. In SLM or EBM, the powders fuse and form a a uniform HA layer on porous Ti scaffolds [199] . The
molten pool under the scanning of high-energy laser morphology of the HA deposits could be controlled from
or electron beam, thus completing the fully melting/ plate-like to nanorod-like structure by altering the pulse
solidification mechanism. Nevertheless, the temperature current density. HA coating allowed more adsorption of
distribution at the bottom, inner, and upper region of serum proteins and further enhanced the ALP activity of
the molten pool is considerably different, which results MC3T3-E1 cells. Intriguingly, Chai et al. [200] successfully
in various temperature gradients in different directions. prepared CaP coating on Ti scaffolds using electrolytic
In this condition, the cooling rate in each direction deposition. However, the surface modification with
varies, causing the anisotropic microstructure in the brittle ceramic coating usually has an adverse effect on
built parts. Moreover, in SLM or EBM, there is a certain the elastic modulus [141] . Researchers also explored the
remelting area at each layer, which also presents differed use of a collagen layer coating on the scaffolds. It was
microstructures as compared to the region without reported that collagen coating not only improved its
remelting. On the other hand, SLM and EBM also surface bioactivity but also improved the mechanical
involve an extremely high cooling rate, which usually properties of the scaffold [201,202] .
leads to great residual stresses in the as-built parts. It is Another popular surface treatment is chemical etching,
well known that the residual stresses considerably reduce which is an effective method for modifying surface
the ductility of scaffolds. Therefore, a heat treatment is microstructures by various corrosion methods. It is well
demanded to reduce the anisotropy and relieve the residual known that natural bone has a hierarchical pore structure,
stress existing in the SLM- or EBM-deposited materials. including macroscale, microscale, submicroscale,
Thone et al. [194] investigated the microstructure and and nanoscale pores. Diverse scale pores act different
ductility of SLM-produced Ti6Al4V after heat treatment. biological functions [203] . Nevertheless, the present AM
It was revealed that ɑ-martensite decomposed to uniform technologies are usually applied to prepare the macropore
lamellar ɑ and β structure after heat treatment. Besides, structure of bone scaffolds but come across a difficulty
the elongation at failure increased significantly from 1.6% in fabricating microscale and submicroscale pores due
to 11.6%. Similarly, Wauthle et al. also confirmed the to their limited resolution. To maximally mimic the
considerable enhancement in ductility and homogenized hierarchical pore structure, the post-chemical etching is

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