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The Space Between: Biomaterials and Much Ado About Nothing(ness)
When it comes to the effectiveness of the next generation of biomaterials, the functionality is as much about what is not there as what is there. Designing voids of specific sizes becomes an integral part of the successful incorporation of a bioscaffold within the body. Biomaterials implanted in a body are in the form of a scaffold, i.e., these materials provide the framework for the regrowth of new tissue. Biomaterials are not just simple replacement “components” for damaged bone or tissue. They facilitate regeneration.
What can voids actually do?
We define a void as an empty volume adjacent to a solid material, independent of size and shape. Based on this definition there are many material properties that can be attributed to or enhanced by designed voids and biomaterials represent one important example. With today’s technology, one can control the design of a material (and voids) and resulting functionality, down to the nanometer range or even smaller.
How do tailored voids help solve the problem for biomaterials?
During the 1960s and 1970s, the first generation of biomaterials was chosen mainly for chemical inertness i.e. biocompatibility. In other words, implant materials would not be attacked and rejected by the body. However, no account was taken of the aspects of healing and the growth of new tissue. An important example of this is the use of titanium. Subsequent generations of biomaterials, referred to as bioactive materials, are open to increasing interactions with the body. Today, biomaterials are bioactive and biodegradable, in addition to being biocompatible. We use the term biomimetic to describe them.
We define biomimetic as a material or material process that replicates one in nature or biology. Porosity plays an important role in the tissue regeneration process by facilitating growth of cellular and extracellular matrix (ECM). ECM is a term used to describe all of the supporting materials required to allow the cells to grow and function. Functions of ECM can be, for example, structural and biochemical.
To regenerate bone, for example, we need three specific sizes of porosity for optimum functionality. A material with designed porosity on several scales is said to have a hierarchical porosity. Each scale of porosity has essential interactions with the body that are required for successful tissue regeneration. It is important to have interconnected porosity (open cell) larger that 100 μm and, in certain cases, as large as 300 μm so that new bone can grow throughout the scaffold. The maximum cell size on this level is usually controlled by mechanical properties. If cell size gets too large, the strength and fracture toughness are degraded to a point where the material is no longer suitable for scaffolding applications. Pores in the range < 10 μm are important for intensifying adsorption of cell differentiation inducing factors and ion exchange (Holzapfel, Reichert et al. 2013).
In addition, an increase in surface area is needed for the proliferation and differentiation of anchorage-dependent cells for tissue regeneration. Nanoscale texture and surface features, such as porosity, facilitate interactions between host cells and the biomaterial. Surface features and properties determine the organization of adsorbed protein layers, which in turn determine specific cellular responses.
The design of voids is essential to the proper functionality of these biomaterials implanted into a body. In this book, Voids in Materials: From Unavoidable Defects to Designed Cellular Materials we provide details on the design , processing, characterization, and functionality derived from atomic scale defects up through macroscale porosity. The basic premise is that,at some level, all solids contain voids, whether intrinsic or intentional. Sometimes the voids are ignored, at other times they are taken into account, and other times they are the focal point of the research. In this book, we take due notice of all of these occurrences of voids, whether designed or unavoidable defects. We define these voids (or empty spaces in materials), categorize them, characterize them, and describe the effect they have on material properties.
Gladysz and Chawla’s upcoming book Voids in Materials: From Unavoidable Defects to Designed Cellular Materials is now available for pre-0rder on the Elsevier Store. Use discount code “STC3014” and save up to 30% on your very own copy!
About the Authors
Dr. Gary Gladysz (Twitter: @GMGladysz) is an Associate at Empyreus Solutions, LLC, Seattle WA, USA, where he consults and leads university and government technical interactions. He received his PhD from the New Mexico Institute of Mining & Technology where he participated in the NATO Collaborative Program with the German Aerospace Institute (DLR). Since receiving his PhD, he has led research efforts in university, government and industrial settings. He has extensive research experience designing and characterizing fibrous composite materials, ceramic composites, polymers, composite foams, and thin films.
As a Technical Staff Member at Los Alamos National Laboratory (LANL), he was technical lead for Rigid Composites and Thermoset Materials. In 2005 he was awarded the LANL Distinguished Performance Group Award for his work leading materials development on the Reliable Replacement Warhead Feasibility Project. He has served on funding review boards for LANL, National Science Foundation, ACS, and the Lindbergh Foundation. He has been guest editor on four issues of leading materials science journals, including Journal of Materials Science and Materials Science & Engineering: A. Dr. Gladysz has organized five international conferences/symposia on syntactic foams and composite materials. He started and currently chairs the ECI international conference series on Syntactic & Composites Foams. Dr. Gladysz currently lives near Boston, Massachusetts, USA.
Krishan Chawla is Professor Emeritus of Materials Science and Engineering at the University of Alabama at Birmingham, USA. He received his Ph.D. from the University of Illinois at Urbana-Champaign. His research interests encompass processing, microstructure, and mechanical behavior of materials. He has taught and/or done research at several universities around the world. Professor Chawla has served as a Program Director for metals and ceramics in the Division of Materials Research, National Science Foundation.
He is a fellow of ASM International. Among his other awards are: Distinguished Researcher Award at New Mexico Tech, Distinguished Alumnus Award from Banaras Hindu University, Eshbach Society Distinguished Visiting Scholar award at Northwestern University, Faculty Fellow award at Oak Ridge National Laboratory. Professor Chawla is the author or coauthor of various textbooks in the area of materials and serves on the editorial boards of a number of journals. He is editor of the journal International Materials Reviews.
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