Without our musculoskeletal system, which consists of bones, joints, and muscles, we would not be able to live. Our bones are responsible for the protection of our organs, the support of our body, and they enable our mobility. Therefore, it is important to keep them healthy. This is done by cells who repair small cracks and fractures caused by our daily activities through continuous remodeling of the skeleton. However, severe bone damage and defects can occur, for example, due to trauma (e.g., car accidents) and bone tumor resection. In this case, the defects are too large for the cells to repair and surgical intervention is required to support the bone regeneration process. Bone substitutes or porous biomaterials are used to fill these defects to help the cells to regenerate the bone. Bone substitutes require implantation via open surgery due to their large dimensions and rigidity. This causes great damage to the body, which results in a long recovery time for the patient and increases the risk of infections. To reduce the invasiveness of the implantation process, minimally invasive surgery (MIS) could be used. MIS techniques make it possible to perform surgical treatments through specific minimally invasive tools that are inserted into the body through small incisions. In order to make minimally invasive implantation possible, the dimensions of porous biomaterials should be reduced to fit through these small incisions. In addition, it has been demonstrated that the bone regeneration process can be optimized and infections could be prevented by applying precisely controlled nanopatterns to the surface of bone substitutes. However, surface patterning techniques can only be applied to flat surfaces. Therefore, it is not possible to apply surface patterns to the inner surfaces of three-dimensional porous structures, such as those fabricated through 3D printing techniques. To resolve @en

Porous biomaterials are often used to treat large bony defects or fractured vertebras. Most of such biomaterials are made of metals and their alloys and have a pre-defined, fixed shape. Due to their predefined fixed shape, however, they are not suitable for implantation through minimally invasive surgical procedures. To overcome this problem, we designed three different deployable non-assembly mechanisms, which were manufactured using selective laser melting. These deployable geometries, including a bicapped cube, a bicapped trigonal antiprism, and a bicapped square antiprism, possess a large aspect ratio in their retracted state. Upon the application of an external force, they expand radially into their deployed load-bearing configuration. Using non-assembly manufacturing, revolute joints, wavelike elements, rigid rods and restrictions could be integrated into the design. The designs were manufactured in such a way that the least amount of support structures was required during the fabrication process. Additionally, the deployable structures were functional immediately after printing. Mechanical tests were performed to determine the forces required to deploy the designed structures and to determine their failure load. A maximum change of 322 ± 7% in the circumdiameter was found for the bicapped trigonal antiprism while the bicapped square antiprism showed the largest reduction in the height (61 ± 1%). A maximum force of 10.3 ± 1.6 N was required during the deployment process of the bicapped square antiprism 3. The bicapped antiprisms could support up to 1212 ± 45.5 N before they failed, while the bicapped cubes failed under a force of 232 ± 5.5 N. The elongated geometry of our designs makes them ideal for implantation using minimally invasive surgical procedures. Given the fact that these are the first non-assembly deployable bone substitutes manufactured using selective laser melting, further studies are required to make them suitable as orthopedic implants.

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Deployable meta-implants aim to minimize the invasiveness of orthopaedic surgeries by allowing for changes in their shape and size that are triggered by an external stimulus. Multi-stability enables deployable implants to transform their shape from some compact retracted state to the deployed state where they take their full sizes and are load-bearing. We combined multiple design features to develop a new generation of deployable orthopaedic implants. Kirigami cut patterns were used to create bi-stability in flat sheets which can be folded into deployable implants using origami techniques. Inspired by Russian dolls, we designed multi-layered specimens that allow for adjusting the mechanical properties and the geometrical features of the implants through the number of the layers. Because all layers are folded from a flat state, surface-related functionalities could be applied to our deployable implants. We fabricated specimens from polylactic acid, titanium sheets, and aluminum sheets, and demonstrated that a deployment ratio of up to ≈2 is possible. We performed experiments to characterize the deployment and load-bearing behavior of the specimens and found that the above-mentioned design variables allow for adjustments in the deployment force and the maximum force before failure. Finally, we demonstrate the possibility of decorating the specimens with micropatterns.

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Additively manufactured (AM) biodegradable porous zinc exhibits great potential as a promising bone-substituting biomaterial. However, there is no information whatsoever available regarding its corrosion fatigue behavior. In this study, we used direct metal printing to fabricate topologically ordered biodegradable porous zinc based on a diamond unit cell. We compared the compression-compression fatigue behavior of AM porous zinc in air and in revised simulated body fluid (r-SBF). The fatigue strength of AM porous zinc was high in air (i.e., 70% of its yield strength) and even higher in r-SBF (i.e., 80% of its yield strength). The high value of the relative fatigue strength in air could be attributed to the good ductility of pure zinc itself. The formation of corrosion products around the strut junctions might explain the higher fatigue strength of AM zinc in r-SBF. Furthermore, we compared the fatigue behavior of a uniform design of the AM porous zinc with a functionally graded design. The functionally graded structure exhibited higher relative fatigue strengths than the uniform structure. The inspection of the fatigue crack distribution revealed that the functionally graded design controlled the sequence of crack initiation, which occurred early in the thicker struts and moved towards the thinner struts over time. The theoretical fatigue life models suggest that optimizing the functionally graded structure could be used as an effective means to improve the fatigue life of AM porous zinc. In conclusion, the favorable fatigue behavior of AM porous zinc further highlights its potential as a promising bone-substituting biomaterial. Statement of Significance: Additively manufactured (AM) biodegradable porous zinc exhibits great potential for the treatment of large bony defects. However, there is no information available regarding its corrosion fatigue behavior. Here, we compared the fatigue behavior of AM porous zinc in air and in revised simulated body fluid (r-SBF). The fatigue strength of AM porous Zn was even higher in r-SBF than in air, which were attributed to the formation of corrosion products. Furthermore, we found that the functionally graded structure controlled the sequence of crack initiation in differently sized struts and exhibited higher relative fatigue strengths than the uniform structure, suggesting that optimizing the functionally graded structure could be an effective means to improve the fatigue life of AM porous Zn.

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Additively manufactured (AM) biodegradable porous zinc exhibits great potential as a promising bone-substituting biomaterial. However, there is no information whatsoever available regarding its corrosion fatigue behavior. In this study, we used direct metal printing to fabricate topologically ordered biodegradable porous zinc based on a diamond unit cell. We compared the compression-compression fatigue behavior of AM porous zinc in air and in revised simulated body fluid (r-SBF). The fatigue strength of AM porous zinc was high in air (i.e., 70% of its yield strength) and even higher in r-SBF (i.e., 80% of its yield strength). The high value of the relative fatigue strength in air could be attributed to the good ductility of pure zinc itself. The formation of corrosion products around the strut junctions might explain the higher fatigue strength of AM zinc in r-SBF. Furthermore, we compared the fatigue behavior of a uniform design of the AM porous zinc with a functionally graded design. The functionally graded structure exhibited higher relative fatigue strengths than the uniform structure. The inspection of the fatigue crack distribution revealed that the functionally graded design controlled the sequence of crack initiation, which occurred early in the thicker struts and moved towards the thinner struts over time. The theoretical fatigue life models suggest that optimizing the functionally graded structure could be used as an effective means to improve the fatigue life of AM porous zinc. In conclusion, the favorable fatigue behavior of AM porous zinc further highlights its potential as a promising bone-substituting biomaterial. Statement of Significance: Additively manufactured (AM) biodegradable porous zinc exhibits great potential for the treatment of large bony defects. However, there is no information available regarding its corrosion fatigue behavior. Here, we compared the fatigue behavior of AM porous zinc in air and in revised simulated body fluid (r-SBF). The fatigue strength of AM porous Zn was even higher in r-SBF than in air, which were attributed to the formation of corrosion products. Furthermore, we found that the functionally graded structure controlled the sequence of crack initiation in differently sized struts and exhibited higher relative fatigue strengths than the uniform structure, suggesting that optimizing the functionally graded structure could be an effective means to improve the fatigue life of AM porous Zn.

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Additively manufactured (AM) biodegradable porous zinc exhibits great potential as a promising bone-substituting biomaterial. However, there is no information whatsoever available regarding its corrosion fatigue behavior. In this study, we used direct metal printing to fabricate topologically ordered biodegradable porous zinc based on a diamond unit cell. We compared the compression-compression fatigue behavior of AM porous zinc in air and in revised simulated body fluid (r-SBF). The fatigue strength of AM porous zinc was high in air (i.e., 70% of its yield strength) and even higher in r-SBF (i.e., 80% of its yield strength). The high value of the relative fatigue strength in air could be attributed to the good ductility of pure zinc itself. The formation of corrosion products around the strut junctions might explain the higher fatigue strength of AM zinc in r-SBF. Furthermore, we compared the fatigue behavior of a uniform design of the AM porous zinc with a functionally graded design. The functionally graded structure exhibited higher relative fatigue strengths than the uniform structure. The inspection of the fatigue crack distribution revealed that the functionally graded design controlled the sequence of crack initiation, which occurred early in the thicker struts and moved towards the thinner struts over time. The theoretical fatigue life models suggest that optimizing the functionally graded structure could be used as an effective means to improve the fatigue life of AM porous zinc. In conclusion, the favorable fatigue behavior of AM porous zinc further highlights its potential as a promising bone-substituting biomaterial. Statement of Significance: Additively manufactured (AM) biodegradable porous zinc exhibits great potential for the treatment of large bony defects. However, there is no information available regarding its corrosion fatigue behavior. Here, we compared the fatigue behavior of AM porous zinc in air and in revised simulated body fluid (r-SBF). The fatigue strength of AM porous Zn was even higher in r-SBF than in air, which were attributed to the formation of corrosion products. Furthermore, we found that the functionally graded structure controlled the sequence of crack initiation in differently sized struts and exhibited higher relative fatigue strengths than the uniform structure, suggesting that optimizing the functionally graded structure could be an effective means to improve the fatigue life of AM porous Zn.

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Topological design provides additively manufactured (AM) biodegradable porous metallic biomaterials with a unique opportunity to adjust their biodegradation behavior and mechanical properties, thereby satisfying the requirements for ideal bone substitutes. However, no information is available yet concerning the effect of topological design on the performance of AM porous zinc (Zn) that outperforms Mg and Fe in biodegradation behavior. Here, we studied one functionally graded and two uniform AM porous Zn designs with diamond unit cell. Cylindrical specimens were fabricated from pure Zn powder by using a powder bed fusion technique, followed by a comprehensive study on their static and dynamic biodegradation behaviors, mechanical properties, permeability, and biocompatibility. Topological design, indeed, affected the biodegradation behavior of the specimens, as evidenced by 150% variations in biodegradation rate between the three different designs. After in vitro dynamic immersion for 28 days, the AM porous Zn had weight losses of 7-12%, relying on the topological design. The degradation rates satisfied the desired biodegradation time of 1-2 years for bone substitution. The mechanical properties of the biodegraded specimens of all the groups maintained within the range of those of cancellous bone. As opposed to the trends observed for other biodegradable porous metals, after 28 days of in vitro biodegradation, the yield strengths of the specimens of all the groups (σy = 7-14 MPa) increased consistently, as compared to those of the as-built specimens (σy = 4-11 MPa). Moreover, AM porous Zn showed excellent biocompatibility, given that the cellular activities in none of the groups differed from the Ti controls for up to 72 h. Using topological design of AM porous Zn for controlling its mechanical properties and degradation behavior is thus clearly promising, thereby rendering flexibility to the material to meet a variety of clinical requirements.

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Additively manufactured (AM) functionally graded porous metallic biomaterials offer unique opportunities to satisfy the contradictory design requirements of an ideal bone substitute. However, no functionally graded porous structures have ever been 3D-printed from biodegradable metals, even though biodegradability is crucial both for full tissue regeneration and for the prevention of implant-associated infections in the long term. Here, we present the first ever report on AM functionally graded biodegradable porous metallic biomaterials. We made use of a diamond unit cell for the topological design of four different types of porous structures including two functionally graded structures and two reference uniform structures. Specimens were then fabricated from pure iron powder using selective laser melting (SLM), followed by experimental and computational analyses of their permeability, dynamic biodegradation behavior, mechanical properties, and cytocompatibility. It was found that the topological design with functional gradients controlled the fluid flow, mass transport properties and biodegradation behavior of the AM porous iron specimens, as up to 4-fold variations in permeability and up to 3-fold variations in biodegradation rate were observed for the different experimental groups. After 4 weeks of in vitro biodegradation, the AM porous scaffolds lost 5–16% of their weight. This falls into the desired range of biodegradation rates for bone substitution and confirms our hypothesis that topological design could indeed accelerate the biodegradation of otherwise slowly degrading metals, like iron. Even after 4 weeks of biodegradation, the mechanical properties of the specimens (i.e., E = 0.5–2.1 GPa, σ_y = 8–48 MPa) remained within the range of the values reported for trabecular bone. Design-dependent cell viability did not differ from gold standard controls for up to 48 h. This study clearly shows the great potential of AM functionally graded porous iron as a bone substituting material. Moreover, we demonstrate that complex topological design permits the control of mechanical properties, degradation behavior of AM porous metallic biomaterials. Statement of Significance: No functionally graded porous structures have ever been 3D-printed from biodegradable metals, even though biodegradability is crucial both for full tissue regeneration and for the prevention of implant-associated infections in the long term. Here, we present the first report on 3D-printed functionally graded biodegradable porous metallic biomaterials. Our results suggest that topological design in general, and functional gradients in particular can be used as an important tool for adjusting the biodegradation behavior of AM porous metallic biomaterials. The biodegradation rate and mass transport properties of AM porous iron can be increased while maintaining the bone-mimicking mechanical properties of these biomaterials. The observations reported here underline the importance of proper topological design in the development of AM porous biodegradable metals.

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Additively manufactured (AM) functionally graded porous metallic biomaterials offer unique opportunities to satisfy the contradictory design requirements of an ideal bone substitute. However, no functionally graded porous structures have ever been 3D-printed from biodegradable metals, even though biodegradability is crucial both for full tissue regeneration and for the prevention of implant-associated infections in the long term. Here, we present the first ever report on AM functionally graded biodegradable porous metallic biomaterials. We made use of a diamond unit cell for the topological design of four different types of porous structures including two functionally graded structures and two reference uniform structures. Specimens were then fabricated from pure iron powder using selective laser melting (SLM), followed by experimental and computational analyses of their permeability, dynamic biodegradation behavior, mechanical properties, and cytocompatibility. It was found that the topological design with functional gradients controlled the fluid flow, mass transport properties and biodegradation behavior of the AM porous iron specimens, as up to 4-fold variations in permeability and up to 3-fold variations in biodegradation rate were observed for the different experimental groups. After 4 weeks of in vitro biodegradation, the AM porous scaffolds lost 5–16% of their weight. This falls into the desired range of biodegradation rates for bone substitution and confirms our hypothesis that topological design could indeed accelerate the biodegradation of otherwise slowly degrading metals, like iron. Even after 4 weeks of biodegradation, the mechanical properties of the specimens (i.e., E = 0.5–2.1 GPa, σ_y = 8–48 MPa) remained within the range of the values reported for trabecular bone. Design-dependent cell viability did not differ from gold standard controls for up to 48 h. This study clearly shows the great potential of AM functionally graded porous iron as a bone substituting material. Moreover, we demonstrate that complex topological design permits the control of mechanical properties, degradation behavior of AM porous metallic biomaterials. Statement of Significance: No functionally graded porous structures have ever been 3D-printed from biodegradable metals, even though biodegradability is crucial both for full tissue regeneration and for the prevention of implant-associated infections in the long term. Here, we present the first report on 3D-printed functionally graded biodegradable porous metallic biomaterials. Our results suggest that topological design in general, and functional gradients in particular can be used as an important tool for adjusting the biodegradation behavior of AM porous metallic biomaterials. The biodegradation rate and mass transport properties of AM porous iron can be increased while maintaining the bone-mimicking mechanical properties of these biomaterials. The observations reported here underline the importance of proper topological design in the development of AM porous biodegradable metals.

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Buckling, which was once considered the epitome of design failure, has been harnessed during the last few years to develop mechanical metamaterials with advanced functionalities. Soft robotics in general and soft actuators in particular could greatly benefit from such designer materials. Unlocking the great potential of buckling-driven materials is, however, contingent on resolving the main limitation of the designs presented to date, namely the limited range of their programmability. Here, we present multi-material buckling-driven metamaterials with high levels of programmability. We combined rational design approaches based on predictive computational models with advanced multi-material additive manufacturing techniques to 3D print cellular materials with arbitrary distributions of flexible and stiff materials in the central and corner parts of their unit cells. Using the geometry and spatial distribution of material properties as the main design parameters, we developed soft mechanical metamaterials behaving as mechanisms whose actuation force and actuation amplitude could be adjusted both independently and concomitantly within wide ranges. Our designs also resulted in the emergence of a new lowest instability mode, i.e. double-side buckling, in addition to the already known modes of side-buckling and symmetric compaction. Finally, we proposed a general approach to pre-dispose our soft mechanical metamaterials such that they can reliably actuate their higher instability modes without any need for additional boundary conditions or fixtures. To demonstrate this approach, we created a cellular mechanism with a rotational buckling pattern that clones the functionality of mechanical machines. The potential of the presented designs in robotics is then demonstrated by applying them as a force switch, kinematic controllers, and a pick and place end-effector.

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Metal additive manufacturing (AM) techniques are solid free-form, three-dimensional fabrication processes of metals and their alloys. Recent developments of metal AM processes have resulted in the fabrication of functional parts with reliable, reproducible, and predictable properties. Metal AM processes have received a tremendous attention from both academic researchers and industry.Powder bed fusion (PBF) (e.g, selective laser melting (SLM), electron beam melting (EBM)) and direct energy deposition (DED) techniques are the most important categories of the AM technologies used for processing of metals and their alloys. We will therefore address some of the critical aspects of both PBF and DED techniques including the effects of processing parameters, material types, and geometrical design on the quality, functionality, and properties of the final products. Finally, we conclude with a discussion of the challenges, limitations, and future outlooks for metal powder-based AM. @en

Metal additive manufacturing (AM) techniques are solid free-form, three-dimensional fabrication processes of metals and their alloys. Recent developments of metal AM processes have resulted in the fabrication of functional parts with reliable, reproducible, and predictable properties. Metal AM processes have received a tremendous attention from both academic researchers and industry.Powder bed fusion (PBF) (e.g, selective laser melting (SLM), electron beam melting (EBM)) and direct energy deposition (DED) techniques are the most important categories of the AM technologies used for processing of metals and their alloys. We will therefore address some of the critical aspects of both PBF and DED techniques including the effects of processing parameters, material types, and geometrical design on the quality, functionality, and properties of the final products. Finally, we conclude with a discussion of the challenges, limitations, and future outlooks for metal powder-based AM. @en

Meta-biomaterials exhibit unprecedented or rare combinations of properties not usually found in nature. Such unusual mechanical, mass transport, and biological properties could be used to develop novel categories of orthopedic implants with superior performance, otherwise known as meta-implants. Here, we use bi-stable elements working on the basis of snap-through instability to design deployable meta-implants. Deployable meta-implants are compact in their retracted state, allowing them to be brought to the surgical site with minimum invasiveness. Once in place, they are deployed to take their full-size load-bearing shape. We designed five types of meta-implants by arranging bi-stable elements in such a way to obtain a radially-deployable structure, three types of auxetic structures, and an axially-deployable structure. The intermediate stable conditions (i.e. multi-stability features), deployment force, and stiffness of the meta-implants were found to be strongly dependent on the geometrical parameters of the bi-stable elements as well as on their arrangement.

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Porous biomaterials that simultaneously mimic the topological, mechanical, and mass transport properties of bone are in great demand but are rarely found in the literature. In this study, we rationally designed and additively manufactured (AM) porous metallic biomaterials based on four different types of triply periodic minimal surfaces (TPMS) that mimic the properties of bone to an unprecedented level of multi-physics detail. Sixteen different types of porous biomaterials were rationally designed and fabricated using selective laser melting (SLM) from a titanium alloy (Ti-6Al-4V). The topology, quasi-static mechanical properties, fatigue resistance, and permeability of the developed biomaterials were then characterized. In terms of topology, the biomaterials resembled the morphological properties of trabecular bone including mean surface curvatures close to zero. The biomaterials showed a favorable but rare combination of relatively low elastic properties in the range of those observed for trabecular bone and high yield strengths exceeding those reported for cortical bone. This combination allows for simultaneously avoiding stress shielding, while providing ample mechanical support for bone tissue regeneration and osseointegration. Furthermore, as opposed to other AM porous biomaterials developed to date for which the fatigue endurance limit has been found to be ≈20% of their yield (or plateau) stress, some of the biomaterials developed in the current study show extremely high fatigue resistance with endurance limits up to 60% of their yield stress. It was also found that the permeability values measured for the developed biomaterials were in the range of values reported for trabecular bone. In summary, the developed porous metallic biomaterials based on TPMS mimic the topological, mechanical, and physical properties of trabecular bone to a great degree. These properties make them potential candidates to be applied as parts of orthopedic implants and/or as bone-substituting biomaterials.@en

The success of bone substitutes used to repair bone defects such as critical sized defects depends on the architecture of the porous biomaterial. The architectural parameters and surface properties affect cell seeding efficiency, cell response, angiogenesis, and eventually bone formation. The relevant parameters include pore size and porosity, pore shape and fibre orientation, surface properties, and mechanical properties. For example, small pores are preferable for cell seeding, but limit cell viability, cell proliferation and differentiation. Moreover, the pore size and geometry affect the alignment of cells and the structure of the regenerated bone. This paper presents an overview of the effects of porous biomaterial architecture including pore size and porosity, pore shape and fibre orientation, surface topography and chemistry, and structure stiffness on cell seeding efficiency, cell response, angiogenesis, and bone formation.@en