Providing multiple functional roles simultaneously, the composition of seashells and bones gradually vary throughout their structure making them part of a group of materials called Functionally Graded Materials (FGMs). This classification can also be applied to a diverse range of artificially engineered materials, the thermal and mechanical properties of which can be appropriately tuned making them integral to the operation of countless optoelectronic device, sensor, and battery applications.
The key lies in the careful composition of composites where several ceramic layers are filled in-between with graphene, defining their porosity and conductivity. Producing different FGM stacks to realize novel composites containing graphene, hexagonal boron nitride (hBN) as well as other ceramics such as silicon carbide and zirconia is the core activity of the Graphene Flagship Partnering Project (CERANEA).
The CERANEA project brings together researchers from a wide range of Institutes and Centres from countries such as Hungary, Germany and Slovakia to analyze a broad spectrum of graphene-containing materials from the macro to the micro-level while gathering information about their homogeneity, porosity and morphology to arrive at the desired performances for particular composites.
Here the use of ceramics and graphene for opening up new possibilities for bone-like materials is highlighted, where the integration of hierarchical structure, functionality, and chemical activity is important in taking these materials to the next level in terms of compatibility and incorporation in for example implants and their use within tissue-engineered scaffolds.
Graphene Foam as a Bio-Scaffold
Functionally Graded Materials (FGMs) can be constructed with ceramics as the bottom layer, a mixture of ceramics and graphene as the intermediate layer and a foam-like structure of graphene on top. Graphene foam itself has already found use as a bio-scaffold material supporting tissue development. Able to support 3,000 times its own weight with outstanding damping capabilities, it is ideal as it can foster and generate stem cells and cartilage tissue. Cartilage in joints in combination with graphene can dissipate very high impact forces.
Despite recent advances in the bio-fabrication of cartilage, mimicking native bone tissue by constructing biomimetic bone scaffolds is proving a greater challenge, one fundamentally rooted in material science.
Developing FGMs with Graphene in Ceramic Matrices
As a highly mineralized tissue, bone forms in a continuous crystallization process under ambient conditions. Bone-producing cells are incorporated into an organic matrix while density varies within different parts of bone as changes in composition and morphology naturally occur in its evolution.
Replicating bone-like structures is possible by altering the amounts of graphene in ceramics-graphene composites and mimicking different porosities in the process. This makes bone reconstruction possible and the use of composites (containing various mixes of graphene, silicon nitride or silicon nitride-zirconia for example) into novel biomedical implants increasingly exciting.
In the case of CERANEA partners, conventional powder technologies are exploited to produce graphene. Typically, a process called attrition milling is used to reduce particle size. Ceramics and grinding media are placed together in a tank facilitating homogenous particle dispersion by shear action forces between the different components resulting in size reduction and material of uniform composition.
Utilizing 3-10 mm media, materials ranging in size from approximately 1 to 10 microns tend to be produced. In terms of increasing ceramic density, a manufacturing process called Hot Isostatic Pressing is something of an industry standard and also the choice of the partnership. Using this type of pressing in a one-step sintering process, solids are formed without melting, allowing CERANEA to manufacture FGMs of differing layers and varying compositions, usually 5% to 30% graphene by weight.
Multi-layered graphene (MLG) from commercially available micro-sized graphite powder gets prepared uniquely, promoting the intercalation and exfoliation of graphite into multi-layered graphene particles.
Establishing that a two-to-three-fold improvement in mechanical properties was possible in a silicon nitride, zirconia, and graphene “sandwich” was a significant advancement. The group established this by altering the configuration; a layer of 30% MLG by weight sandwiched between two layers of 5% MLG replacing the opposite ratio (30-5-30 wt% MLG). Findings such as these have underpinned new developments. The production of the first gradient-structured silicon nitride (Si3N4) ceramic with MLG by a team of researchers at the Institute of Technical Physics and Materials Science at the Centre for Energy Research of the Eötvös Loránd Research Network (EK MFA, a CERANEA partner) is a great example. Their work is now pioneering the use of silicon nitride as an implant in orthopedic surgery.
Ceramic-Graphene FGMs for Orthopaedics
The correction of deformities in bones or muscles is a branch of medicine known as orthopedics. The researchers have produced a composite with a Si3N4 bulk ceramic outer and with the same mechanical properties as solid bone.
With the addition of graphene, a porous Si3N4-graphene ceramic composite with open and closed porosities can accurately mimic bone. Such artificial morphologies open up the exploration of bioactivity in tissue response, in other words, can tissue bond biologically to these natural bone alternatives? Results seem promising.
Seeking tough and strong alternative materials to ensure reliability is also key in orthopedics material science optimization. The ceramic-graphene sandwich structure developed by the researchers as well as being very similar to human bone has the added advantage of addressing the intrinsic brittleness of ceramic materials with the extreme hardness and toughness of Si3N4. Real possibilities lie when the mechanical properties of the ceramic matrix can be adjusted by regulating the mix of graphene, MLG or graphene oxide (GO).
Bone-Like FGMs with 3D Printing
Now that bone-like structures can seemingly be created with living cells at room temperature, opening up the possibility of 3D printing of bone-like material directly into the body, there may be a scope to combine these advances with the breakthroughs in the material science research emerging from CERANEA and its ceramic and graphene composites. The question is where this combination might take us.
References and Further Reading
Diamante, L. (2021) Ceramics and graphene open-up new possibilities for bone like-materials [Online] Graphene Flagship. Available at: https://graphene-flagship.eu/graphene/news/ (Accessed on 9 June 2021).
McAleese, J. (2020) The thinnest of materials may hold a cure for osteoarthritis [Online] Scientia. Available at:https://www.scientia.global/the-thinnest-of-materials-may-hold-a-cure-for-osteoarthritis (Accessed on 9 June 2021).
Institution of Mechanical Engineers (2021) 3D printing technique could print bone-like structures directly within the body [Online]. Professional Engineering. Available at: https://www.imeche.org/news/news-article/3d-printing-technique-could-print-bone-like-structures-directly-within-the-body (Accessed on 9 June 2021).