Functional Gradient Metallic Biomaterials: Techniques, Current Scenery, and Future Prospects in the Biomedical Field

Abstract

Functional gradient materials (FGMs), as a modern group of materials, can provide multiple functions and are able to well mimic the hierarchical and gradient structure of natural systems. Because biomedical implants usually substitute the bone tissues and bone is an organic, natural FGM material, it seems quite reasonable to use the FGM concept in these applications. These FGMs have numerous advantages, including the ability to tailor the desired mechanical and biological response by producing various gradations, such as composition, porosity, and size; mitigating some limitations, such as stress-shielding effects; improving osseointegration; and enhancing electrochemical behavior and wear resistance. Although these are beneficial aspects, there is still a notable lack of comprehensive guidelines and standards. This paper aims to comprehensively review the current scenery of FGM metallic materials in the biomedical field, specifically its dental and orthopedic applications. It also introduces various processing methods, especially additive manufacturing methods that have a substantial impact on FGM production, mentioning its prospects and how FGMs can change the direction of both industry and biomedicine. Any improvement in FGM knowledge and technology can lead to big steps toward its industrialization and most notably for much better implant designs with more biocompatibility and similarity to natural tissues that enhance the quality of life for human beings.

Production and Characterization of a 316L Stainless Steel/β-TCP Biocomposite Using the Functionally Graded Materials (FGMs) Technique for Dental and Orthopedic Applications

“…Healing bone fractures requires a precise alignment with a minimum motion of the bone in situ [26]. However, metallic devices used for these purposes have some disadvantages related to their Young's modulus, which is much higher than that of natural bone [9]. Therefore, FGM composites were produced to overcome these disadvantages, due to their ability to imitate the natural gradation seen in bone tissue.…”

“…In addition, knowing that the design of the implant can directly affect fracture healing, FGMs are considered interesting, since that they can provide a bending stiffness, while their Young's modulus remains close to the bone's Young's modulus, avoiding stress shielding effect [11,27]. In addition, FGMs provide morphological properties that lead to good mechanical and physical characteristics due to the related parameter control [9].…”

“…However, metallic implants have a limitation due to their Young's modulus. Cortical bone presents a Young's modulus value between 5 and 30 GPa, while, for 316L stainless steel, it is about 190 GPa [9,10]. This difference can lead the implant to instability and failure due to the stress shielding effect, which causes non-union between implant and bone, bone necrosis and delay in the healing process [11].…”

“…Initially, FGMs were developed to solve thermal problems in space shuttles. However, researchers found that this route was extremely interesting for the production biomaterials since the functional gradient simulates the natural condition of human bone [9,22].…”

Recent Progress in Surface Modification of Mg Alloys for Biodegradable Orthopedic Applications

“…However, when the implant is intended for a temporary application, they necessitate a second surgery. Also, the high elastic modulus of the metallic materials affects bone remodeling and leads to osteoporosis (Shi et al, 2021;Zhao et al, 2021). There were significant efforts to develop bioresorbable metallic implants for temporary orthopedic applications.…”