Porous Scaffold Design for Additive Manufacturing in Orthopedics: A Review

Abstract

With the increasing application of orthopedic scaffolds, a dramatically increasing number of requirements for scaffolds are precise. The porous structure has been a fundamental design in the bone tissue engineering or orthopedic clinics because of its low Young's modulus, high compressive strength, and abundant cell accommodation space. The porous structure manufactured by additive manufacturing (AM) technology has controllable pore size, pore shape, and porosity. The single unit can be designed and arrayed with AM, which brings controllable pore characteristics and mechanical properties. This paper presents the current status of porous designs in AM technology. The porous structures are stated from the cellular structure and the whole structure. In the aspect of the cellular structure, non-parametric design and parametric design are discussed here according to whether the algorithm generates the structure or not. The non-parametric design comprises the diamond, the body-centered cubic, and the polyhedral structure, etc. The Voronoi, the Triply Periodic Minimal Surface, and other parametric designs are mainly discussed in parametric design. In the discussion of cellular structures, we emphasize the design, and the resulting biomechanical and biological effects caused by designs. In the aspect of the whole structure, the recent experimental researches are reviewed on uniform design, layered gradient design, and layered gradient design based on topological optimization, etc. These parts are summarized because of the development of technology and the demand for mechanics or bone growth. Finally, the challenges faced by the porous designs and prospects of porous structure in orthopedics are proposed in this paper.

Keywords: additive manufacturing; cellular design; mechanical property; orthopedic scaffolds; porous structure design.

FIGURE 1

Units of various non-parametric and parametric designs. (A) The body-centered cubic (BCC) unit and its modified units. a¹: BCC unit. (a²) A type of modified BCC unit by Adding vertical stiffeners through the center of the BCC/OC unit. (a³) The BCCZ unit. (a⁴) The pillar BCC. (B) The Diamond unit. (C) The Octet unit. (D) The rhombic dodecahedron unit. (E) The rhombic cube octahedron unit. (F) The Honeycomb units. (f¹) The commonly Honeycomb unit. (f²) The modified Honeycomb unit. (G) A new type Honeycomb unit. (g¹) Positional relationship of adjacent layers of new type Honeycomb unit. (g²) Structural characteristics of the layered slice and rod-connected structure of new type Honeycomb unit. (H) The Voronoi units. (I,J) The classification of TPMS. (I) The surface dominated by stretching. (i¹) Primitive surface. (i²) I-WP surface. (J) The surface dominated by bending. (j¹) Diamond surface. (j²) Gyroid surface.

FIGURE 2

Stress distribution and failure mechanism analysis of non-parametric design and parametric designs. (A) From left to right, the stress distribution of body-centered cubic (BCC), modified BCC unit, pillar BCC and diamond units. The red part represents the stress concentration area. Failure modes of (B) BCC, (C) Diamond. The stress distribution of (D) Voronoi, (E) TPMS. Reprinted: (A) from Zhang B. et al. (2018) with permission from Elsevier; (B) from Onal et al. (2018) under the terms of the Creative Commons Attribution License; (C) from Zhang et al. (2019) with permission from Elsevier; (D) from Sharma et al. (2019) under the terms of the Creative Commons Attribution License; (E) from Maskery et al. (2018b) under the terms of the Creative Commons Attribution License.

FIGURE 3

The other field designs and their mechanical property. (A) 2D cuttlefish bone model and its mechanical property. Young’s modulus for cuttlefish bone 2D model: (a¹) Ex (planar), (a²) Ey (planar), and (a³) Ez (thickness). (B) The turtle carapace structure and its mechanical property. (b¹) The various microscopic features of the turtle carapace, including the layered rib structure, the perisuture, and keratin scutes. The elastic moduli shown were calculated from nanoindentation measurements performed under wet conditions, reflecting physiological conditions. (b²) Representative quasi-static compressive performance of dry ribs taken from carapaces of the box turtle (Terrapene carolina). The specimens, containing the whole three layers, or alternatively only individual cortex layer, were tested under various strain-rates. Specimens containing the whole three layers show a unique deformation behavior involving a pronounced plateau region, corresponding to buckling and fracturing of the trabeculae forming the cancellous interior. Reprinted: (A) from Hu et al. (2017) with permission from Elsevier; (B) from Achrai and Wagner (2017) with permission from Elsevier.

FIGURE 4

The classification of whole design. (A) The uniform design. (a¹) e.g., BCC. (a²) e.g., Voronoi. (B) The layered Gradient design. (b¹) e.g., BCC. (b²) e.g., Voronoi. (C) The continuous gradient design. (c¹) e.g., BCC. (c²) e.g., Voronoi. (D) The design based on TO. Taking the knee joint gasket as an example to describe the design based on TO.

FIGURE 5

Fluorescence micrographs on the unit and gradient BCC. Fluorescence micrographs representing merged Hoechst stained nucleus (blue) and actin cytoskeleton (red) of preosteoblast cells on the uniform and gradient BCC structures after culturing for (A) 4 h, (B) 4 days and (C) 7 days. The top represents the side where cells were seeded onto the samples. Reprinted from Onal et al. (2018) under the terms of the Creative Commons Attribution License.

FIGURE 6

The relationships of various parameters in gradient Voronoi structure. Schematic showing the unit cell of gradient Voronoi scaffolds, the relationships between parameters, the mechanical properties in different porosity, and the porous structures with irregularities. Reprinted from Wang et al. (2018) with permission from American Chemical Society.