TABLE 1.

Three main classes of the microstructure of the bone scaffolds.

Time period	Representative microstructure		Advantages and disadvantages	References
Early stage	Regular periodic unit		Advantages: Easy for the design and optimization	Gómez et al. (2016)
	Cubes	Hexagons
	Spheres	Cylinders	Disadvantages: Poor mechanical and flow properties
Approximately 2012 to present	TPMS based unit		Advantages: Large surface-to-volume ratio, fully connected, diverse types and controllability	Al-Ketan and Abu (2019)
	Diamond	Gyroid
	Schwarz P	Fischer-Koch S	Disadvantages: The clinical needs still cannot be fully met and there is room for improvement
Approximately 2016 to present	Nonperiodic, irregular unit		Advantages: The complex and anisotropic microstructures of bone tissues can be simulated	Wang et al. (2018a)
			Disadvantages: The design and optimization are challenging