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Filament Structure, 3D printing, Bone Repair Scaffolds
scaffolds . In the past few years, researchers have the ability to promote cell adhesion ability to promote cell
[52]
developed COL-containing hybrid printing and sacrificial adhesion. Mahdi et al.[58] modified the hydrophobicity
material printing methods to improve the rheological and surface properties of PCL scaffolds by coating them
properties of bioink by improving the printability of with peptide hydrogel or polydopamine to improve cell
the slurry [53,54] . Shim et al. prepared a scaffold for adhesion. Another candidate bone tissue engineering
[55]
application in cartilage tissue regeneration using COL, material PLA is an aliphatic polyester, mainly derived
supramolecular hyaluronic acid and PCL loading from plant starch, with good biocompatibility and
(hTMSCs), bone morphogenetic protein 2 (BMP 2) degradability, which survives in the human body as
and transforming growth factor-beta (TGF-β), and the soluble lactic acid after hydrolysis. Its main advantages
results showed that the hydrogel/PCL layered printing are low melting point, low-viscosity, and excellent
method successfully prepared multilayer cell-carrying mechanical properties, but there are problems such as
scaffolds with high mechanical stability, circumventing high brittleness and high glossiness. Yi et al.[59] modified
the negative effects of chemical cross-linking agents and HA using poly(L-lactide)/β-cyclodextrin/citrate (PLA/β-
physical cross-linking while showing higher bone repair CD/citrate), and the modified HA had significantly
performance than pure hydrogel scaffolds in animal improved bioactivity and mechanical properties, with
experiments. Alginate, a natural polymer derived from better cell adhesion and higher viability for rat bone
algae, has properties similar to extracellular matrix with marrow mesenchymal stem cells (MSCs). Silicone
good biocompatibility and printability, and has many elastomer can be formulated to have low elastic modulus,
applications in the field of 3D printed bioinks. Almarza high extensibility and toughness, excellent thermal and
et al. used scaffolds prepared by adding polyglycolic oxidative stability, and chemical inertness[60,61]. Luis
[56]
acid to natural alginate for culturing temporomandibular et al.[62] used a two-part Ecoflex silicone resins for 3D
joint cells, and found that a large amount of COL was printing a bionic scaffold for meniscus structure using
produced by the cells inside the scaffold after 4 weeks a thermosetting extrusion method. The results of the
of incubation, confirming the good biocompatibility of quantitative cell proliferation test showed low cytotoxicity
alginate. and good biocompatibility of silicone.
To date, polymer materials and their composites
have been commonly used in clinical treatment of 2.3. Composites
bone defective diseases. These materials are widely The structure of scaffolds prepared from any single
available and have good biocompatibility and excellent material can be affected by the defects that exist in the
mechanical properties. Natural polymer materials are material itself. For example, ceramic scaffolds may
limited in clinical use because of different sources and undergo very fast or difficult in vivo degradation and
forms. Their chemical structures are more complex, and have poor sintering quality due to material differences,
their physicochemical properties are highly variable. and polymer scaffolds have insufficient mechanical
Compared to natural polymer materials, many synthetic strength and may collapse during printing, resulting in
polymer materials have received attention due to their low porosity [63,64] . The emergence of composite materials
superior mechanical strength and processing flexibility. has provided a new strategy to solve this problem, and
At present, the common synthetic polymer materials researchers have developed a series of composites with
used in bone tissue engineering are poly (lactic acid- excellent properties through extensive experiments .
[65]
hydroxyacetic acid) (PLGA), PCL, PLA, silicone, The results showed that the performance of composites is
polyurethane (PU), and so on. These materials have superior and more comprehensive than that of individual
superior biocompatibility, biodegradability, and usually components, and they are rapidly attracting widespread
the degradation products which belong a class of green attention in the field of bone repair [66,67] . Among them,
eco-polymers are non-toxic. bioceramic materials and polymeric materials are favored
PCL has good biodegradability, biocompatibility, for their material properties; the former have good
and non-toxicity and is typically used as a medical biocompatibility, excellent bone regeneration properties,
biodegradable material. It has high crystallinity and and high mechanical strength, whereas the latter have
low melting point, and its excellent rheological and high printability, notable toughness, and the ability to
viscoelastic properties endow it with good melt encapsulate cells for bioprinting .
[29]
printability. In addition, scaffolds prepared from PCL have Composites are constructed from two or more
high mechanical strength and are popular in bone tissue different materials (e.g. bioceramics and bioceramics,
engineering systems. Li et al.[57] achieved simultaneous polymers and bioceramics, and polymers and
repair of bone and cartilage tissue defects by coating a polymers.) [68-70] . BCP, which is a new composite
self-assembled peptide hydrogel on a PCL scaffold and bioceramic material, was synthesized by HA and β-TCP.
blank controls, and confirmed that PCL scaffolds lacked The degradation rate of β-TCP is too fast, resulting in the

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