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International Journal of Bioprinting Continuous gradient TPMS bone scaffold

therapies involving repair technologies become necessary. regeneration. Thus, the scaffold needs to have sufficient
Repairing bone tissue has always been a significant mechanical strength to accommodate and stabilize the
challenge in modern medicine. According to research, the cells, but not be excessively strong. Sychov et al. used
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global demand for bone substitutes and grafts is projected polylactic acid (PLA) as raw material to prepare four TPMS
to reach US$3.4 billion by 2022, with a growth rate of structures of Gyroid, Schwartz diamond, Neovius surface,
4.8%. This poses a substantial financial burden on most and D-prime surface by fused deposition modeling (FDM)
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countries and regions worldwide. and studied their compression resistance. The results
The process of bone remodeling can be divided into four showed that D-type TPMS structure has the best elastic
stages: hematoma formation, fibroblast formation, callus modulus but with poor energy absorption performance.
formation, and bone remodeling. In this complex process, The G-type TPMS structure has good compressive
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various cytokines, growth factors, and other signaling strength and energy absorption capacity. Yan et al. studied
molecules are involved in guiding the repair of various the mechanical properties of Ti-6Al-4V and Al-Si10-Mg
cellular defects. This repair process is highly intricate, and Schoen Gyroid (SG) TPMS unit cell structures prepared
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in order to enhance repair efficiency and achieve better by powder bed fusion (PBF) under uniaxial compression
healing outcomes, it is necessary to create a more stable tests, and fitted the compression test results with the
environment, such as surface folding, to accommodate all Gibson–Ashy model to guide biological applications and
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signaling molecules and facilitate cell proliferation on its implant matching. Furthermore, the structures should
surface. Traditional bone scaffolds generally adopt regular have an elastic modulus similar to that of natural bone to
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shapes, such as squares, hexagons, and circles. However, prevent the stress shielding effect. The elastic modulus of
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these regular shapes are unable to accurately mimic bone cortical bone is approximately 1–15 GPa. Therefore, the
final bone scaffold should possess a mechanical strength
structures. With the continuous advancement of three- within this range.
dimensional (3D) printing technology, the concept of
minimal surface structures has emerged. Among them, the The permeability of the bone scaffold is a crucial
triply periodic minimal surface (TPMS) scaffold stands factor that significantly influences the outcomes of bone
out as an optimal choice. Mathematically speaking, a regeneration. The permeability is typically quantified
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minimal surface is characterized by zero mean curvature. using Darcy’s law, with the Darcy permeability value (k)
TPMS structures represent minimal surfaces with complex being measured. 21,22 Permeability refers to the ability of
topological spatial structures. They possess advantageous the scaffold to facilitate fluid flow. Good permeability,
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features, including high surface area, high porosity, indicated by a higher k value while maintaining
and periodic variations. The interconnected interior satisfactory mechanical properties, not only facilitates
and smooth surface of TPMS scaffolds make them ideal the diffusion of essential nutrients, but also determines
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structures for orthopedic implants. When designing the internal shear force and pressure within the scaffold.
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TPMS scaffolds for bone regeneration, one of the crucial Studies have demonstrated that these factors can affect
factors to consider is pore design. This is because the cell differentiation and adaptation by enhancing alkaline
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design of pores directly affects the outcome of bone phosphatase (ALP) activity and upregulating osteogenic
regeneration. Firstly, an interconnected porous structure marker genes. Since porosity, pore size, connectivity,
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promotes the diffusion of nutrients to cells, thereby curvature, direction, and shape collectively influence the
assisting the bone regeneration process. Secondly, natural permeability and hydrodynamics of bone scaffolds, it is
bone tissues possess porosity, with a range from 30% to advantageous to have bone scaffolds with high porosity,
90% in trabecular bone. 11,12 Therefore, when designing a sufficient pore connectivity, and appropriate pore size and
bionic porous scaffold as a bone implant, it is necessary to shape to ensure good permeability, which is beneficial for
optimize the pore size and porosity to ensure that the pores bone tissue engineering. 27,28 Montazerian et al. analyzed
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are large enough to facilitate nutrient diffusion and tissue the biological permeability of different TPMS units at
growth, while still maintaining a sufficiently large surface different relative densities. The relationship between
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area for cell attachment. In addition to directly affecting cell normalized permeability and volume fraction and the
behavior, the pores of the TPMS structure also indirectly velocity contour of biological fluid were obtained by curve
impact other important characteristics crucial in the bone fitting. Varley et al. placed the bone scaffold in different
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regeneration process. For instance, mechanical strength humidity conditions and analyzed its permeability through
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is a vital property of a bone scaffold. The process of bone experimental and theoretical simulation analysis. It is found
remodeling depends largely on the ability of bone tissue that the experimental value and the theoretical value are
to perceive and adapt to mechanical load. Consequently, a similar in numerical results, providing a theoretical basis
good mechanical environment is key to the success of bone for us to use computer simulation to evaluate permeability.

Volume 10 Issue 2 (2024) 313 doi: 10.36922/ijb.2306