Revolutionizing Biomedical Implants: Bioinspired Design and Advanced Metal Printing

Bioinspiration in additive manufacturing for biomedical applications

Bio-inspired design is an approach to designing lattice structures that takes inspiration from natural materials like bone, cork, and wood. These materials have unique structures that can be used as a basis for creating porous materials used in tissue engineering. For example, cancellous bone, which is the spongy part inside our bones, has a cellular structure made up of rods and plates. The arrangement of these structures is graded, with lower porosity near the outer shell and higher porosity towards the inner shell. By mimicking these features, we can design lattice structures that have similar properties.

One advantage of bio-inspired lattice structures is that they can be made using additive manufacturing (AM) technologies, which allow us to create components with smooth transitions and minimize stress at interfaces. These structures can be particularly useful for orthopedic implants used to treat large bone defects that cannot heal on their own. Instead of using tissues from the patient or another donor, which can have complications, we can design and implant materials that mimic the properties of natural bone to facilitate the healing process.

To create these lattice structures, we can use non-destructive imaging methods like computed tomography (CT) or magnetic resonance imaging (MRI) to determine the original configuration. These imaging methods have been widely used in designing implants and prostheses for tissue reconstruction. They can also help create patient-specific implants, where the shape of the implant is based on the individual's bone structure.

Meta-Biomaterials

Two additional characteristics of design for additive manufacturing (AM) are "batch-size-indifference" and "complexity-for-free." These characteristics have been used to create customized implants with special properties using a concept called "designer materials" or mechanical metamaterials. Designer materials are engineering materials that have unique properties based on their internal structure rather than their chemical makeup.

One interesting property of designer materials is the ability to have a negative Poisson's ratio, which means they expand sideways when stretched. There are different types of unit cells that exhibit this property, such as re-entrant, chiral, and rotating cells. These designs have been successfully created using AM technology. The re-entrant unit cell is the most common design and allows us to control the Poisson's ratio by adjusting the angle of the struts.

These designer materials with unique properties have shown potential in promoting tissue regeneration and improving the longevity of orthopedic implants. For example, combining unit cells with positive and negative Poisson's ratios in a hybrid design can prevent loosening of the implant and improve its durability. Additionally, these materials have good mechanical performance and can be used in load-bearing applications like hip stems.

Different geometric designs, both with and without the negative Poisson's ratio, have been explored for use in biomedical devices. Some designs, like cube, diamond, and rhombic dodecahedron, have been used to create space-filling scaffolds.

TPMS-based porous structures, which have similar surface curvature to trabecular bone, are also popular designs for these materials. They provide a good balance between mechanical properties and mass transport characteristics, which is important for tissue regeneration. These structures can be created using multi-material 3D printing and mathematical approaches.

Shape-shifting mechanisms, like self-folding techniques or origami-inspired designs, have also been used to create advanced implants with enhanced properties and functionalities. These mechanisms allow for the creation of deployable implants or foldable lattices.

In summary, by using AM technology and designer materials, we can create customized implants with unique properties and shapes. These materials have the potential to improve tissue regeneration and the durability of orthopedic implants, providing better solutions for patients.

AM in metal printing for biomedical applications

Metals and alloys can be used in various biomedical applications. However, when they come into contact with the body, they can corrode and release ions, which can be harmful. Therefore, it's important for these materials to have good compatibility with the body. Some examples of biocompatible metals and alloys are titanium, stainless steel, cobalt-based alloys, zirconium, niobium, and tantalum. These materials are resistant to corrosion, have good mechanical properties, and are considered biocompatible.

Among these materials, titanium and its alloys, such as Ti6Al4V, have been extensively studied. Titanium alloys like Ti6Al4V are relatively inexpensive but have lower ductility. On the other hand, pure titanium is highly biocompatible but has lower mechanical strength. Stainless steel is another affordable option that is relatively biocompatible. Laser powder bed fusion processes can be used to manufacture stainless steel implants, and they have a higher elastic modulus compared to Ti6Al4V.

Metals and alloys can be used to create porous implants for orthopedic applications. However, their elastic moduli are usually higher than that of the surrounding bones. To avoid stress shielding at the bone-implant interface, the mechanical properties of the implants need to be adjusted. One way to achieve this is by creating graded porous implants. Another approach is to modify the alloy composition by adding specific elements. For example, adding elements like Ta, Nb, Zr, and Mo to titanium can create alloys with lower elastic moduli compared to Ti6Al4V. These modified alloys, such as Ti13Nb13Zr and Ti29Nb13Ta4.6Zr, improve the mechanical compatibility of the implants.

Surface treatments and coatings can enhance the performance of metallic implants in bone tissue regeneration. For instance, introducing bioactive glass or mesoporous bioactive glass to the surface of titanium scaffolds can improve bone tissue regeneration. Surface biofunctionalization processes using techniques like plasma electrolytic oxidation with silver, zinc, or copper nanoparticles can have immunomodulatory effects and reduce implant-related infections. Additionally, decorating the surfaces of metallic implants with nanostructures can control their bactericidal and osteogenic properties. Layer-by-layer coating biofunctionalization is another method that can provide multiple functionalities to metallic implants simultaneously, such as improved tissue growth factors and antibacterial behavior.

In conclusion, metals and alloys play a crucial role in biomedical applications due to their corrosion resistance, good mechanical properties, and biocompatibility. Titanium and its alloys, such as Ti6Al4V, are widely studied materials. While titanium alloys like Ti6Al4V are relatively affordable, they have lower ductility, whereas pure titanium is highly biocompatible but has lower mechanical strength. Stainless steel is another cost-effective and biocompatible option for implants, with a higher elastic modulus compared to Ti6Al4V.

Porous implants made from metals and alloys are commonly used in orthopedic applications. However, their higher elastic moduli compared to surrounding bones can lead to stress shielding at the bone-implant interface. To address this issue, the mechanical properties of the implants can be adjusted. Graded porous implants and alloy modifications with elements like Ta, Nb, Zr, and Mo can decrease the elastic moduli of the implants, improving their mechanical compatibility. Examples of such modified alloys include Ti13Nb13Zr and Ti29Nb13Ta4.6Zr.

Surface treatments and coatings further enhance the performance of metallic implants in bone tissue regeneration. Introducing bioactive glass or mesoporous bioactive glass to the implant surfaces improves bone tissue regeneration. Surface biofunctionalization techniques like plasma electrolytic oxidation with nanoparticles of silver, zinc, or copper have immunomodulatory effects and reduce the risk of implant-related infections. Additionally, incorporating nanostructures on the implant surfaces can control their bactericidal and osteogenic properties. Layer-by-layer coating biofunctionalization enables the simultaneous enhancement of multiple functionalities, such as improved tissue growth factors and antibacterial behavior, in metallic implants.

Overall, the advancements in material design, modification techniques, and surface treatments contribute to the development of biocompatible metallic implants with improved mechanical compatibility, tissue regeneration capabilities, and infection control, paving the way for better biomedical solutions in the field of implantology.

Reference

Mirzaali MJ, Moosabeiki V, Rajaai SM, Zhou J, Zadpoor AA. Additive Manufacturing of Biomaterials-Design Principles and Their Implementation. Materials (Basel). 2022 Aug 8;15(15):5457. doi: 10.3390/ma15155457. PMID: 35955393; PMCID: PMC9369548.