Bone regeneration strategies: Engineered scaffolds, bioactive molecules and stem cells current stage and future perspectives.

Additive manufacturing (AM) or 3D printing has revolutionized the manufacturing world through its rapid and geometrically-intricate capabilities as well as economic benefits. Countless businesses in automotive, aerospace, medical, and even food industries have adopted this approach over the past decade. Though this revolution has sparked widespread innovation with single material usage, the manufacturing world is constantly evolving. 3D printers now have the capability to create multi-material systems with performance improvements in user-definable locations. This means throughout a single component, properties like hardness, corrosion resistance, and environmental adaptation can be defined in areas that require it the most. These new processes allow for exciting multifunctional parts to be built that were never possible through traditional, single material AM processes. AM of metals, ceramics, and polymers is currently being evaluated to combine multiple materials in one operation and has already produced never-before-produced parts. While multi-material AM is still in its infancy, researchers are shifting their mindset toward this unique approach showing that the technology is beginning to advance past a research and development stage into real-world applications. This review is intended to highlight the range of 3D printed polymer-based, metal-metal, and metal-ceramic applications while discussing advantages and challenges with additively manufactured multi-material structures.

Additive manufacturing of multi-material structures

Propulsion system development requires new, more affordable manufacturing techniques and technologies in a constrained budget environment, while future in-space applications will require in-space manufacturing and assembly of parts and systems. Marshall is advancing cuttingedge commercial capabilities in additive and digital manufacturing and applying them to aerospace challenges. The Center is developing the standards by which new manufacturing processes and parts will be tested and qualified. Rapidly evolving digital tools, such as additive manufacturing, are the leading edge of a revolution in the design and manufacture of space systems that enables rapid prototyping and reduces production times. Marshall has unique expertise in leveraging new digital tools, 3D printing, and other advanced manufacturing technologies and applying them to propulsion systems design and other aerospace materials to meet NASA mission and industry needs. Marshall is helping establish the standards and qualifications “from art to part” for the use of these advanced techniques and the parts produced using them in aerospace or elsewhere in the U.S. industrial base.

Additive manufacturing

Additive manufacturing of biomaterials.

3D printing alias additive manufacturing can transform 3D virtual models created by computer-aided design (CAD) into physical 3D objects in a layer-by-layer manner dispensing with conventional molding or machining. Since the incipiency, significant advancements have been achieved in understanding the process of 3D printing and the relationship of component, structure, property and application of the created objects. Because hydrogels are one of the most feasible classes of ink materials for 3D printing and this field has been rapidly advancing, this Review focuses on hydrogel designs and development of advanced hydrogel-based biomaterial inks and bioinks for 3D printing. It covers 3D printing techniques including laser printing (stereolithography, two-photon polymerization), extrusion printing (3D plotting, direct ink writing), inkjet printing, 3D bioprinting, 4D printing and 4D bioprinting. It provides a comprehensive overview and discussion of the tailorability of material, mechanical, physical, chemical and biological properties of hydrogels to enable advanced hydrogel designs for 3D printing. The range of hydrogel-forming polymers covered encompasses biopolymers, synthetic polymers, polymer blends, nanocomposites, functional polymers, and cell-laden systems. The representative biomedical applications selected demonstrate how hydrogel-based 3D printing is being exploited in tissue engineering, regenerative medicine, cancer research, in vitro disease modeling, high-throughput drug screening, surgical preparation, soft robotics and flexible wearable electronics. Incomparable by thermoplastics, thermosets, ceramics and metals, hydrogel-based 3D printing is playing a pivotal role in the design and creation of advanced functional (bio)systems in a customizable way. An outlook on future directions of hydrogel-based 3D printing is presented.

3D printing of hydrogels: Rational design strategies and emerging biomedical applications

Laser Engineered Net Shaping (LENS™) is a commercially available additive manufacturing technique that was used for one step manufacturing of bimetallic structures of stainless steel and Ti6Al4V (Ti64) alloy. Two approaches were adopted for manufacturing these bimetallic structures. In the first approach, direct deposition of Ti64 on SS410 substrate and compositionally graded bimetallic structures were attempted without any intermediate bond layer. In the second approach, an intermediate NiCr bond layer (of thickness ∼750 μm) was deposited to minimize thermal and residual stresses for these bimetallic structures. Direct deposition of Ti64 was successful only for a couple of layers before the structures were delaminated. Compositionally graded bonding was unsuccessful with the formation of brittle intermetallics and related residual stresses causing delamination. Using an intermediate NiCr layer, bimetallic structures were successfully fabricated. Our work is focused on LENS™ based processing approach and related microstructural evolution towards bimetallic structures.

Stainless steel to titanium bimetallic structure using LENS

Applications of porous metallic implants to enhance osseointegration of load-bearing implants are increasing. In this work, porous titanium implants, with 25 vol.% porosity, were manufactured using Laser Engineered Net Shaping (LENS™) to measure the influence of porosity towards bone tissue integration in vivo. Surfaces of the LENS™ processed porous Ti implants were further modified with TiO2 nanotubes to improve cytocompatibility of these implants. We hypothesized that interconnected porosity created via additive manufacturing will enhance bone tissue integration in vivo. To test our hypothesis, in vivo experiments using a distal femur model of male Sprague–Dawley rats were performed for a period of 4 and 10 weeks. In vivo samples were characterized via micro-computed tomography (CT), histological imaging, scanning electron microscopy, and mechanical push-out tests. Our results indicate that porosity played an important role to establish early stage osseointegration forming strong interfacial bonding between the porous implants and the surrounding tissue, with or without surface modification, compared to dense Ti implants used as a control.

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In Vivo Response of Laser Processed Porous Titanium Implants for Load-Bearing Implants.

Additive manufacturing of Ti-Si-N ceramic coatings on titanium

Laser remelting of commercially pure titanium (CP-Ti) surface was done in a nitrogen rich inert atmosphere to form in situ TiN/Ti composite coating. Laser surface remelting was performed at two different laser powers of 425 W and 475 W. At each power, samples were fabricated with one or two laser scans. The resultant material was a nitride rich in situ coating that was created on the surface. The cross sections revealed a graded microstructure. There was presence of nitride rich dendrites dispersed in α-Ti matrix at the uppermost region. The structure gradually changed with lesser dendrites and more heat affected α-Ti phase maintaining a smooth interface. With increasing laser power, the dendrites appeared to be larger in size. Samples with two laser scans showed discontinuous dendrites and more α-Ti phase as compared to the samples with one laser scan. The resultant composite of TiN along with Ti2N in α-Ti showed substantially higher hardness and wear resistance than the untreated CP-Ti substrate. Coefficient of friction was also found to reduce due to surface nitridation. Leaching of Ti(4+) ions during wear test in DI water medium was found to reduce due to laser surface nitriding.

Himanshu Sahasrabudhe

Papers

Additive manufacturing of biomaterials.

Stainless steel to titanium bimetallic structure using LENS

In Vivo Response of Laser Processed Porous Titanium Implants for Load-Bearing Implants.

Additive manufacturing of Ti-Si-N ceramic coatings on titanium

Laser processing of in situ TiN/Ti composite coating on titanium.