Based on the objective function, the distribution of pore features in AM additively fabricated porous structures can be uniform or non-uniform. Homogeneous scaffolds have unit cells of specific shape and porosity, whereas non-uniform (gradient) scaffolds include arrays of unit cells in which pore characteristics vary spatially in the design space to achieve one or more functions in the scaffold.
Gradient implants have been studied recently to address multiple conflicting design requirements by defining constants of the TPMS equation (eg, the offset value of the porosity gradient, C) as a function of a space vector. In this issue, 3D Science Valley will review the main trends and opportunities brought by advanced gradient scaffolds with Friends of Gu.
3D printingImplant-Process-Material-Design-Characterization-Application
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Digital materials bring new developments in orthopaedics
3D printing– Additive manufacturing can adjust local density according to load and other requirements with the help of different material distributions. In addition, with the help of customized digital materials, components can be optimized for weight, cost and production time. Additive Manufacturing (AM) is a breakthrough production technology that enables the efficient production of digital materials due to its geometric freedom and tool-free production.
Gradient porosity and unit cell size
Among the design parameters, the porosity of the scaffold is considered as a key parameter to locally control the mechanical properties and permeability. Porosity gradients are a dominant feature of the internal structure of natural bone, and interestingly, local porosity characteristics change dynamically according to specific patterns of an individual’s daily activities to keep their structure the strongest and mechanically optimized. Therefore, an approach to designing optimal scaffolds is based on lessons learned from bone porosity distribution itself. To this end, CT imaging was used to study the relative density of bone in different sections and to determine its local stiffness from the density-stiffness relationship. The thickness of the struts of the corresponding part of the stent can be designed accordingly.
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By varying the size of the probability sphere to mimic the host tissue, a similar approach can be used to fabricate graded scaffolds through Voronoi-Tessellation, and the pore size and porosity can be engineered in gradient TPMS-based scaffolds to mimic the internal structure of native bone tissue for improved bone tissue growth.
Examples of functionally graded scaffolds designed to meet conflicting requirements in bone architecture. (A) Random porous scaffold with gradient porosity designed based on bone local stiffness. (B) Steps to generate porous scaffolds using probability spheres of various sizes based on the Voronoi-Tessellation method. (C) Sheet-based gyroscopic structure with (i) graded porosity created by varying sheet thickness and (ii) unit cell size. (D) Local stiffness design within the scaffold by varying the sheet thickness to simulate specific tissue types. (E) Bone regeneration within scaffolds of various pore sizes, including (i) 300-500 μm, (ii) 200-600 μm, (iii) 100-700 μm, and (iv) (v) non-porous after implantation Operation. (vi) Bone volume per total volume and (vii) Push-out force (N) of the scaffold.
© Elsevier, American Chemical Society
Conflicting needs can be met by combining the functions of different pore properties in one scaffold structure. For example, it is clear that high relative density is required for high strength; however, bone ingrowth and scaffold integration are favored at low relative density. It has been reported that high porosity (or high permeability) promotes bone formation, whereas cartilage formation occurs at lower porosity. One potential solution to achieve mechanical functionality and high peripheral permeability is based on a radial gradient pattern, where the low relative density at the implant/tissue interface gradually increases towards the center of the scaffold to strengthen the structure along the load axis. Computational and experimental studies on cylindrical scaffolds show that radially graded porous TPMS scaffolds display enhanced fluid biopermeability compared to their uniform porosity counterparts. Studies have shown that the relative plateau strength in hierarchical aluminum-based lattice structures can be twice as high as in homogeneous-based structures. In addition, the graded lattice structure shows a lower relative Young’s modulus, which can better mimic native bone tissue.
In addition to relative densities, the unit cell size has been designed to vary gradually to mimic the natural bone structure, and studies have shown that bone regeneration in P-surface topologies with graded porosity can be approximately three times higher compared to non-porous scaffolds. times. This was further confirmed by the observation of higher push-out forces in porous scaffolds.
Pore gradient
Spatially varying pore topologies naturally exist in porous tissue structures such as trabecular bone. Smooth changes in pore shape allow local control of properties at the micro- and macro-scales, which may not be feasible in homogeneous topologies. By defining a weighting function in the TPMS equation, TPMS design can be used to design porous shape structures with sharp and/or smooth transitions in pore shape.
The study found that the use of gradient hole shape design can solve the loosening problem of hip replacement implants. Implant loosening is primarily due to microscale deformations that often disintegrate the implant from the natural bone. Conventional and auxetic hole topologies were designed in the compression and tension regions of the hip implant, respectively, to prevent retraction from the host bone during physiological activity.
Examples of porous metal biomaterials with pore-shaped gradients. (A) Multiple topologies within the sheet-based TPMS scaffold with smooth transition from diamond shape to gyroscope shape. (B) Hybrid implant designs including conventional and auxetic lattices with various internal angles.
© Elsevier, American Chemical Society
gradient composites
According to the German Fraunhofer Institute-Fraunhofer, the key to future manufacturing competition is materials, providing the behavior of materials in digital form, linking product development with material development, and linking material information to the entire processing application chain through Industry 4.0, Significantly reduces the lifetime application cost of the material.
In order to mimic the physiological properties of native tissues, scaffolds are needed to present the physical and chemical signals of native tissues. Scaffolds with gradients of physical and chemical properties show promise in terms of mechanical and biological properties. For example, scaffolds with functionally graded material distributions can accelerate the healing of osteochondral (OC) defects compared to scaffolds composed of monophasic and multiphase material types that support bone and cartilage formation alone.
Multimaterial 3D printing is one of the well-known methods for fabricating scaffolds with gradients in physical and chemical properties; however, it is mainly used for ink-based 3D printing of multimaterial non-metallic implants. On the metal side, laser directed energy deposition is the most popular method for metal multi-material 3D printing. Researchers have used laser fusion deposition to fabricate functionally graded composites of Ti-6Al-4V, reinforced with TiC particles, and the concentration of TiC is gradually changed from 0% to 50% from bottom to top.
Figure: Example of a metal additive manufacturing (AM) material gradient for biomedical structures. (A) Laser melting deposition of Ti-6Al-4V enhanced TiC particles with TiC concentrations ranging from 0% to 50% from bottom to top. (B) Laser-engineered net-shape O3 and pure Al2O3 of the multi-material structure with layers of pure Ti-6Al-4V, Ti-6Al-4V + Al2.
© Elsevier, American Chemical Society
The amount of unmelted powder and the internal microporosity of the structure increased with increasing TiC concentration. The results show that adding TiC particles can increase the hardness of TiCp/Ti-6Al-4V composites by 94%. Tensile properties are improved by adding up to 5% TiC, while above this amount results in a decrease in tensile properties. A compositionally graded structure was fabricated using the LENS process, including Ti-6Al-4V, Al2O3, and Ti-6Al-4V + Al2O3 layers with different microhardness and elemental composition. In addition, using the LENS fabrication method, the researchers fabricated a compositionally graded structure of vanadium carbide (VC) and stainless steel 304. Various proportions of VC from 5 to 100 wt% are mixed with stainless steel 304 to achieve a wide range of wear resistance and hardness.
Accelerating Digital Materials
To sum up, 3D printing and the development of digital orthopedics complement each other, which makes the development of digital materials in the field of implants embark on the road of acceleration.
Internationally, in order to advance the extraordinary potential of digital materials in industrial applications, the DAP Institute for Digital Additive Production at RWTH Aachen University focuses on the development of innovative and efficient algorithms for generating smart digital materials. The solution developed focuses on the automated integration of production and application-related conditions in the future generation of digital materials, making design easier and smarter. In terms of intelligent digitization of materials, the current main development focus of the DAP Institute for Digital Additive Production at RWTH Aachen University is in the following areas:
- Lattice lattice structure generation algorithms that take into account manufacturing constraints such as critical overhang or minimum achievable feature size
- Adaptive mesh structure generation based on loads and boundary conditions
- Conformal lattice structure generation
- Refinement algorithms for local or global lattice structures
- Topology Optimization Algorithms
l Reference: “Additively manufactured metallic biomaterials”
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