[Image above] Åkermanite is a calcium magnesium silicate compound that has garnered interest as a bioactive material. Credit: Zadehnajar et al., International Journal of Applied Ceramic Technology
As I read through the papers for this month’s ACT @ 20 topic, I felt I could only begin to appreciate the complexity within the field of bioceramics and bioactive glasses. The researchers tackling the many facets of this broad category must consider aspects of ceramic materials and design along with accounting for biological functionality.
From mollusk shells to bones and teeth, nature is amazing at forming natural biological ceramics. The designs throughout the specific organ that enable form and function are highly complex, from the layers of teeth and shells to gradations of bone, both structural and functional. The article “Functionally graded ceramics for biomedical applications: Concept, manufacturing and properties” provides insights into the forms, technologies, and future of graded bioceramics.
It is supremely optimistic to believe that researchers can develop substitutes for these amazing structures. Yet, I and my coworkers can attest to the success of our colleagues in this area because we are the recipients of dental implants and joint replacements.
Though I am not an expert, I find it convenient to parse the broad field of bioceramics and bioactive glasses into four general categories.
- Biomimetic processing strives to duplicate the processes and templating of nature to create new materials and components.
- Antibiotic materials and components are designed to disrupt undesirable biota, such as infectious bacteria in drinking water.
- Bioinert components, such as dental implants, are designed to replace natural bioceramics with minimal interactions with living tissues.
- Bioactive ceramics are designed to be highly interactive with living tissues to form grafts and/or provide support for regrowth of damaged tissues.
The articles chosen by the editors of International Journal of Applied Ceramic Technology for ACT @ 20 this month focus on the latter two topics, highlighting the challenges and successes for the complex balancing of properties, composition, and structures to meet the requirements for biomedical purposes.
For example, “Glass-ceramics in the CaO-MgO-Al2O3-SiO2 system as potential dental restorative materials” highlights the development of bioinert materials with appropriate mechanical properties to replace zirconia. While zirconia prosthetics are among the most common, zirconia tends to have hardness values far exceeding those for natural enamel tooth exteriors. The authors show that additions of alumina to calcium magnesium silicates provides greater inertness with mechanical properties closer to those of the enamel and dentin layers of natural teeth.
Another challenge for medical ceramics is the generation of components with complex shapes to conform to the individual requirements of each patient. While calcium magnesium aluminosilicates are somewhat machinable, net-shape processing is highly desirable, especially for hard materials. Additive manufacturing is a promising technique for net shaping, particularly for complicated parts.
In “Robocasting of silicon nitride with controllable shape and architecture for biomedical applications,” the authors discuss the development of fabrication processes along with the characterization of inks, green parts, and fully sintered structures.
Aimed at fabricating spinal fusion implants, their results demonstrate that “when combined with sinter/HIP, robocasting has the capacity to create Si3N4 implants with precise shape and dimensions to match a patient’s anatomy, high strength to support physiological loads, controllable pore characteristics for bone infiltration, and a fibrous surface morphology for anti-bacterial activity,” they conclude.
Bioactive components are those designed to foster interaction with living tissues. Such components can range from coatings on inert implants to scaffolding designed for tissue engineering (e.g., regrowing bone). Perhaps the most well-known bioactive material is Bioglass 45S5, which can be found today in commercial restorative toothpastes. Many other materials are being studied.
Bioactive materials react in several stages, including partial dissolution, formation of calcium phosphate compounds, and activation of biological processes for forming new cells and new structures. The dissolution provides ions that are the building blocks of the biologically formed ceramics and provide chemical signals to the cells. Cell differentiation—that is, the process of stem cells forming specific other cells, such as blood vessels or bone—relies on these chemical signals as well.
In “Bioactive sol-gel glasses: Processing, properties and applications,” the authors mention that the release rate of ions determines the type of cells, with rapid release leading to bone cells and slower release leading to blood vessels. This article is centered around the use of the sol-gel process for cost-effective and flexible fabrication of bioactive glasses. While control of composition is important, so is control of morphology and adaptability to adapt to multiple fabrications methods. As the authors demonstrate, sol-gel processing has the potential to fulfill these requirements.
Along the same vein, åkermanite, a calcium magnesium silicate compound, has garnered interest as a bioactive material, particularly for bone-forming applications. Much like glasses, it can be produced via solid-state and wet chemical (e.g., sol-gel) methods. The article “Recent advances in akermanite calcium-silicate ceramic for biomedical applications” discusses the chemistry, processing, and performance of åkermanite and composites containing it.
In conclusion, with overviews of application requirements, materials, processing methods, and performance, this month’s ACT @ 20 article line-up provides a wealth of information for both neophytes and seasoned bioceramics researchers. The articles, listed below for your convenience, are free to read for the month of March.
Articles for Bioceramics