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Lin, et al.
inability to provide structural support for new bone after of 3D printed bone repair scaffolds are also increasing.
degradation, so it is especially important to control the In addition to the need for continuous improvement,
degradation rate and mechanical strength by synthesizing mapping and configuration of printing materials, the
two materials . By comparing the compressive strength spatial filament structure of 3D printed scaffolds which
[71]
of BCP scaffolds with different ratios, Zyman et al. can directly affect the porosity and mechanical properties
[72]
showed that the compressive strength of the material of the scaffolds has drawn much attention, indicating that
increased with the increase of β-TCP content. Sánchez- the structure could be used in biomaterials. Therefore,
Salcedo et al. [73,74] investigated the degradation rate of it is important to design and develop microfilament
BCP slurry in an in vitro test by testing different ratios of structured scaffolds that are appropriately sized and meet
BCP slurry and showed that the dissolution rate of BCP clinical needs . This section explores the latest state
[84]
material was between HA and β-TCP, and the dissolution of research on the filament structure of 3D printed bone
rate increased with increasing β-TCP content. repair scaffolds and summarizes and lists the physical
Polymer-ceramic composites combine the excellent structure as well as the application characteristics of bone
properties of two different chemical compositions, repair scaffolds (Table 2).
including the high wear resistance of ceramic materials
and the high toughness of polymers [75-78] . The incorporation 3.1. Classic structure
of ceramic particles and bioglass particles into the initial The classical scaffold structure defined in this paper is
material effectively enhances the mechanical strength of the most widely used 3D printed bone repair scaffold
the composite, and its bioactivity gives the material the structure, in which the scaffold fibers are single
ability to regenerate bone . This can also be applied cylindrical and cross-arrayed at a certain angle between
[79]
in the fabrication of biphasic porous scaffolds to repair layers, and assembled into a 3D scaffold after the
the regenerated damaged tissues. Inzana et al. used printing parameters are regulated. The classical scaffold
[80]
Darvan821-A as a size controlling agent and dispersant structure is characterized by easily adjustable printing
for the 1 time during HA synthesis to prevent the parameters, simple scaffold preparation, high printability,
st
formation of particle aggregates throughout the COL high potential for secondary processing, and good
matrix, resulting in COL-nHA scaffolds with excellent development prospects. However, the classical structure
rheological properties and great potential for precise of the scaffold type is single and cannot simulate the tissue
tailoring of scaffold shape. Li et al. incorporated COL structure more accurately. The printing slurry is mostly
[81]
into calcium phosphate slurry at low temperature to prepared by direct mixing, and thus, the performance of
maximize the cytocompatibility and mechanical strength the material cannot be maximized, and it is still necessary
of the scaffold. Compared to the difficult degradation to improve the scaffold performance by improving the
problem of conventional HA powder, nano-scale HA printing technology.
(nHA) possesses a faster degradation rate in vivo without Classical monolayer scaffold structures are mostly
affecting osteogenesis . However, nHA single-phase based on bioceramic materials with the auxiliary addition
[82]
materials are not able to mimic the composition, structure of certain binders or dispersants to the slurry. Shao
and properties of natural bone, and researchers need et al. [85] conducted a detailed study of the composition-
to compensate this deficiency by introducing another structure-strength relationship of the ceramic scaffold
material. Wang et al. prepared the scaffold by adding process using a one-step/two-step method (Figure 1A),
[83]
polyamide (PA) to HA which has excellent mechanical which showed that the overall mechanical strength of the
properties, and the addition of PA did not produce adverse scaffold could be better balanced and the degradability
effects in in vitro experiments. In vivo experiments could be adjusted using a two-step sintering method.
showed that the nHA/PA composite scaffold had good Treatment of cartilage defects remains a great challenge in
biocompatibility and osteoconductivity with host bone. clinical practice, and Deng et al. successfully prepared
[86]
High water content, low-viscosity hydrogels provide a bioactive (BRT) scaffolds with controlled surface
superior environment for cell growth, but the mechanical micro/nanostructures (Figure 1B), which significantly
strength properties they provide are often insufficient to improved the scaffold’s compressive strength and
support in vivo analysis. Therefore, attempts have been promoted the simultaneous regeneration of cartilage and
made to create composite bioinks that can integrate subchondral bone tissue, providing a sensible strategy for
the mechanical strength of viscous hydrogels with the inducing cartilage regeneration. Wei et al. [87] successfully
biocompatibility provided by low-viscosity hydrogels. constructed hexagonal microarrays on the surface of 3D
3. Scaffold filament structure printed HA porous scaffolds by hydrothermal reaction
and added Sr to replace the crystal phase of HA in the
2+
With the development of technology and the demand of surface microarrays (Figure 1C) to improve the surface
clinical applications, the overall performance requirements morphology and chemical properties of the scaffolds, and

International Journal of Bioprinting (2021)–Volume 7, Issue 4 47