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International Journal of Bioprinting 3D bone: Current & future

Figure 11. Powder bed fusion 3D printing. This figure was created using BioRender.

fuses the powder particles at high temperatures. SLS does PCL is used as pellets in DIW extrusion printers and
not require a support system, making it possible to print as filaments in FDM printers. PCL is usually heated
channels and pores with different materials (e.g., metals, to 60–120°C before the printing process at room
ceramics, and composites) (Figure 11). temperature. When the extruded material leaves the
nozzle, it solidifies rapidly and maintains its shape.
4.2. Materials used in bone bioprinting Its mechanical properties are excellent for making a
Bioprinting bone tissues requires both soft and hard bone frame. Although the tensile strength and
hard scaffolds. The former provides the appropriate modulus of PCL are relatively low, its elongation at
microenvironment for cells, and the latter offers the shape break is between 300% and 600% (can even reach
and mechanical strength of the tissue. Soft scaffolds are
commonly hydrogels, while hard scaffolds are usually 1000%), and its flexural modulus is also high. When
PCL is printed in a suitable structure, it retains the
thermoplastic polymers. All components of the scaffolds
need to be biocompatible and biodegradable. In addition, hydrogel. However, PCL may not be suited for cell
it is advisable to use bioactive substances for the hydrogel detachment due to its hydrophobic nature, and PCL
to provide binding sites for the cells and support their features a slow biodegradation rate. Nonetheless,
52-54
growth and differentiation. Likewise, the physical and PCL is easy to print with and is low cost.
chemical properties of the materials are also important in (ii) PLGA: PLGA, also called hyperelastic bone, is a
regulating the printability and mechanical stability of the biocompatible and biodegradable organic polymer
3D construct (Table 5). synthesized from lactic acid and glycolic acid. The
ratios of these two components affect the properties,
4.2.1. Hard scaffold materials
Hard scaffold materials are essential for bone printing as including the biodegradation rate. Higher lactic acid
content would speed up the degradation and increase
hard supporting tissues. The common synthetic polymers
used to create hard scaffolds include polycaprolactone the mechanical strength of the scaffold. However,
(PCL), polylactic acid (PLA), polyglycolide (PGA), and PLGA alone is not sufficient for hard bone scaffolds,
polylactide-coglycolide (PLGA). These materials ensure the as it is not osteoconductive and has weak mechanical
appropriate mechanical properties of the scaffold and are properties. Therefore, PLGA is usually combined
also biocompatible and biodegradable. PCL and PLGA are with other materials, such as PCL, HA, ceramics, or
55,56
suitable for creating a rigid scaffold of bone tissue, as they bioglass, to improve the mechanical properties.
are osteoconductive. In addition, they are approved by the 4.2.2. Hydrogels
US Food and Drug Administration (FDA) as biopolymer Hydrogels are mixtures of several biopolymers. For bones,
materials that can be used for clinical purposes. 50,51 These the base components are typically alginate, gelatin, gelatin
materials are discussed in detail as follows: methacryloyl (GelMA), chitosan, collagen, hyaluronic

(i) PCL: PCL is a biocompatible and biodegradable acid, silk, dECM, cellulose, and polyethylene glycol (PEG)
synthetic thermoplastic polymer widely used for diacrylate (PEGDA). Additionally, inorganic substances,
medicinal purposes. It has a low melting point at 60°C such as HA, calcium, and phosphate, are often added to
and good thermal stability with a decomposition hydrogels to support bone generation (Table 5). 50,51,57 The
temperature of approximately 350°C. In 3D printing, different hydrogel materials are discussed as follows:

Volume 10 Issue 3 (2024) 159 doi: 10.36922/ijb.2056