NANOSCALES
3.5 CONCLUSION AND OUTLOOK
Despite significant efforts to duplicate the structural and functional properties of biologi-cal hard and stiff materials, only a few success-ful implementations of these mechanisms in biomimetic materials have been reported [48, 54]. Also, none of these studies have been able to develop micro-/nanocomposites with the high level of structural organization observed in natural composites. Finally, the high level of mineral concentration in hard biological materials (e.g., 95 %w/w for nacre, 99 %w/w for enamel) has not been achieved in their syn-thetic counterparts.
Experimental investigations show that high mineral concentrations result in poor mechani-cal properties, particularly toughness, in biomi-metic materials [46, 86, 87] whereas models for fracture toughness of biological composites pre-dict the opposite trend [29]. This moderate suc-cess of biomimetic materials can be explained by the following limitations:
(1) In biomimetic materials, the mineral tablets are not organized in a controlled fashion in the polymer matrix [88], whereas the min-eral tablets in natural composites like bone and nacre are arranged in a well-organized, staggered structure. This lack of configura-tion in biomimetic materials necessitates the use of highly directed fabrication methods rather than simple mixing of the phases [89].
This also restricts fabrication of these materi-als to higher-length scales so that controlled arrangements of tablets can be more easily achieved.
(2) The bonding between mineral and polymer phases is not as efficient as it is in natural composites. In nacre, the sacrificial ionic bonds at the interface, which can reform, are one of the keys to its superior mechani-cal properties. This encourages fundamen-tal research to incorporate such bonding in biomimetic materials. The presence of these bonds prevents catastrophic failure as the broken bonds at the interfaces can reform so that the integrity of the structure is maintained, even at high levels of strains.
The challenging question of whether this break-reform fashion can be engaged in other bonds such as covalent bonds remains open [87].
Meanwhile, composites with well-organized structures made of macroscale inclusions have been developed relatively easily at the expense of losing the advantages of using inclusions of small size. Therefore, although reducing the size of the inclusions is beneficial according to the design guidelines for staggered composites, it may not result in the expected high perfor-mance due to the limitations of the fabrication processes at small scales, i.e., poor structural organization. There is therefore a trade-off between the size of inclusions and scalability of well-designed structure. State-of-the-art biomi-metic studies, however, aim to explore innova-tive and promising fabrication methods to develop structures with high levels of struc-tural organization made of micro/nano inclusions.
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ABOUT THE AUTHORS
Mohammad Mirkhalaf was born in Isfahan, Iran, in 1985. He received his BS and MEng in mechanical engineering at Isfahan University of Technology (IUT) and Nanyang Technological University (NTU), respectively. He is now a PhD candidate in mechanical engineering at McGill University. His main research interests include structure and performance of biological materi-als, design and development of biomimetic materials, and micro/nano fabrication methods.
Mr. Mirkhalaf has won several scholarships and awards, namely A*STAR scholarship at NTU, the McGill Engineering Doctoral Award (MEDA), and the James McConnell Fellowship at McGill.
Francois Barthelat is an associate professor in the Department of Mechanical Engineering at McGill University (Montreal, Canada). In 2006 he created the Biomimetic Materials Laboratory, which focuses on the structure and mechanics of natural materials and on the design, fabrication, and Deju Zhu is a postdoctoral fellow in the Depart-ment of Mechanical Engineering at McGill Univer-sity, Montreal. He received his BS and MS in civil engineering from Northeast Forestry University (China) in 2001 and 2004, respectively, and his PhD in civil engineering from Arizona State University in 2009. He is a member of the American Concrete Institute and the Society for Experimental Mechan-ics. His research interests include structure and mechanics of natural materials and novel bio-inspired materials, dynamic behavior of fabrics and textile-reinforced cement composites, experi-mental techniques of impact and high strain-rate testing, and computational simulation of woven fabrics subjected to ballistic impact.
ABOUT THE AUTHORS 79 testing of engineering materials inspired from
nature. Projects include mechanics of seashells, bone, natural interfaces and adhesives, and fish scales. Professor Barthelat and his team have also developed new nacre-like materials and bio-inspired fibers for composites. He has won a num-ber of awards, including the Hetenyi Award for the best research article in the journal Experimental
Mechanics (2003), the Award for the Best Paper by a Young Researcher at the 12th International Con-ference on Fracture (2009), and the Best Paper Award at the 2011 Society for Experimental Mechanics Annual Conference (Biological Sys-tems and Materials Division). He currently serves on the Editorial Boards of Bioinspiration and Biomi-metics and Experimental Technique.
Engineered Biomimicry 81 © 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/B978-0-12-415995-2.00004-0
Ranjan Vepa
School of Engineering and Materials Science, Queen Mary, University of London, London E1 4NS, United Kingdom
4
Biomimetic Robotics
Prospectus
Some basic features of biomimetic robotics and the technologies that are facilitating their development are discussed in this chapter. The emergence of smart materials and structures, smart sensors and actua-tors capable of mimicking biological transducers, bio-inspired signal-processing techniques, modeling and control of manipulators resembling biological limbs, and the shape control of flexible systems are the primary areas in which recent technological advances have taken place. Some key applications of these technological developments in the design of morphing airfoils, modeling and control of anthropo-morphic manipulators and muscle activation mod-eling, and control for human limb prosthetic and orthotic applications are discussed. Also discussed, with some typical examples, are the related develop-ments in the application of nonlinear optimal control and estimation, which are fundamental to the suc-cess of biomimetic robotics.
Keywords
Biomimetic robots, Electromyography, Electro-active polymer, Electrorheological fluids, Kalman filter, Magnetorheological fluids, Morphing, Nernst equa-tion, Nonlinear optimal control, Orthotics, Prosthetics, Shape control, Shape-memory alloys, Smart structures