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12
Multifunctional Materials
Sia Nemat-Nasser, Syrus Nemat-Nasser, Thomas Plaisted,
Anthony Starr, and Alireza Vakil Amirkhizi
CONTENTS
12.1
Introduction ........................................................................................................................................ 309
12.1.1
Multifunctional Concepts .................................................................................................... 310
12.2
Multifunctional Composites ............................................................................................................... 311
12.2.1
Electromagnetic Functionality ............................................................................................ 312
12.2.1.1 Thin-Wire Plasmonic Composites ...................................................................... 312
12.2.1.2 Coiled Wire Plasmon Media Composites ........................................................... 314
12.2.1.3 Braided Composite Manufacturing ..................................................................... 318
12.2.1.4 Controlling the Effective Magnetic Permeability............................................... 321
12.2.1.5 Negative Refractive Index Composites .............................................................. 323
12.2.2
Heating Functionality .......................................................................................................... 324
12.2.2.1 Simulation and Testing ....................................................................................... 324
12.2.3
Healing Functionality .......................................................................................................... 328
12.2.3.1 Polymer Healing.................................................................................................. 329
12.2.3.2 Thermo-Reversibly Cross-Linked Polymer ........................................................ 329
12.2.3.3 Healing Experiments ........................................................................................... 330
12.2.3.4 Healing Summary................................................................................................ 332
12.2.4
Sensing Functionality .......................................................................................................... 332
12.2.4.1 Integrating Sensing into Composites .................................................................. 333
12.2.4.2 Sensor Communications and Power ................................................................... 333
12.2.4.3 Mechanical Integration........................................................................................ 333
12.2.4.4 Data Management ............................................................................................... 335
12.2.4.5 Preliminary Results ............................................................................................. 335
12.2.4.6 Sensors for Structural Health Monitoring........................................................... 337
12.3
Summary............................................................................................................................................. 337
Notes ............................................................................................................................................................... 338
References....................................................................................................................................................... 338
12.1
INTRODUCTION
Multifunctional structural materials possess attributes beyond the basic strength and stiffness that
typically drive the science and engineering of the material for structural systems. Structural
materials can be designed to have integrated electrical, magnetic, optical, locomotive, power
generative, and possibly other functionalities that work in synergy to provide advantages that
309
BIOMIMETICS: Biologically Inspired Technologies
Edited by Yoseph Bar-Cohen
CRC Press (2005)
Preprint reach beyond that of the sum of the individual capabilities. Materials of this kind have tremendous
potential to impact future structural performance by reducing size, weight, cost, power consump-
tion, and complexity while improving efciency, safety, and versatility.
Nature offers numerous examples of materials that serve multiple functions. Biological
materials routinely contain sensing, healing, actuation, and other functions built into the primary
structures of an organism. The human skin, for instance (see Figure 12.1), consists of many layers of
cells, each of which contains oil and perspiration glands, sensory receptors, hair follicles, blood
vessels, and other components with functions other than providing the basic structure and protection
for the internal organs. These structures have evolved in nature over eons to the level of seamless
integration and perfection with which they serve their functions. Scientists now seek to mimic these
material systems in designing synthetic multifunctional materials using physics, chemistry, and
mathematics to their advantage in competing with the unlimited time frame of natures evolutionary
design process. The multifunctionality of these materials often occurs at scales that are nano through
macro and on various temporal and compositional levels.
12.1.1 Multifunctional Concepts
In recent years, wide arrays of multifunctional material systems have been proposed. Each of these
systems has sought to integrate at least one other function into a material that is capable of bearing
mechanical loads and serves as a structural material element. Researchers at ITN Energy Systems
and SRI International have integrated a power-generating function into ber-reinforced composites
(Christodoulou and Venables, 2003). Individual bers are coated with cathodic, electrolytic, and
anodic forming layers to create a battery. The use of the surface area of bers as opposed to that of a
foil in a thin lm battery allows greater energy outputs, measured on the order of 50 Wh/kg in a
carbon ber-reinforced epoxy laminate. These batteries may be deposited on various substrates,
including glass, carbon, and metallic bers. This research and many others, including our own
research, have been supported by DARPA under the rst generation of Synthetic Multifunctional
Materials Initiative (Figure 12.2).
Other power-generating schemes integrated into structural composites have been proposed,
where the composite structure is consumed to generate power after its structural purpose is
complete (Joshi et al., 2002; Thomas et al., 2002; Baucom et al., 2004; Qidwai et al., 2004).
Physical Sciences Inc. have incorporated oxidizers into thermoplastic matrix composites and
demonstrated signicant energy output from directly burning the material (Joshi et al., 2002).
Such a material would be useful for instance in a space application, where weight saving is critical,
and structures required only for launch could provide an energy source once the structure is no longer
needed.
Hair follicle for sensing, protection
Nerves for sensing heat, touch
Glands for excreting oils
Sweat glands and ports for thermal
management
Veins and arteries for healing, thermal
management, nutrients
Epidermis layer for structure, protection
Figure 12.1
Illustration of the many integrated functions within the human skin.
310
Biomimetics: Biologically Inspired Technologies The integration of sensing into materials has made many advances in recent years. Much of the
research has been conducted under the context of Structural Health Monitoring, or SHM. In line
with the overall theme of this book, researchers seek to make a material sense its environment, feel
internal damage, and signal an alert that repair is needed, essentially mimicking the behavior of
biological organisms. Later in this chapter we provide an approach to integrating sensing into
composite materials. For a comprehensive overview of the eld, the reader is directed to a recent
review paper on the subject (Mal, 2004).
This brief introduction to multifunctional materials only scratches the surface of the various
multifunctional concepts developed to date. The remaining sections of this chapter will detail a
further example of a multifunctional material under development at University of California, San
Diego (UCSD), in the rst authors laboratories. The functionalities of this material include
integrated structural, electromagnetic, thermal, healing, and sensing capabilities. While this ma-
terial in no way encompasses all of the possible functionalities that may be integrated into a
material, it offers an example of how such an integration may be achieved while maintaining the
structural integrity of the overall material. Particular attention is given to the interplay and resulting
synergy between the various elements that contribute these functionalities.
12.2
MULTIFUNCTIONAL COMPOSITES
We focus on the issues that relate to integrating multiple functions into ber-reinforced
polymers to create composites with basic structural attributes that can also perform other functions.
We discuss various methods that have been used to control the mechanical, electromag-
netic (EM), and thermal properties of the material, while introducing self-healing, and environ-
mental-sensing and prognostic capabilities into the material. The polymer matrix of these
composites has the ability to covalently heal microcracks at rates that can be facilitated by
moderate heating through thin conductors which are also used to control the EM properties of
the material. The same conductors can also be used to create sensor-integrated electronic
networks within t