Biomedical Engineering Reference
In-Depth Information
Epidermis layer for structure, protection
Hair follicle for sensing, protection
Veins and arteries for healing, thermal
management, nutrients
Nerves for sensing heat, touch
Sweat glands and ports for thermal
management
Glands for excreting oils
Figure 12.1
Illustration of the many integrated functions within the human skin.
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 efficiency, 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 nature's 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 fiber-reinforced composites
(Christodoulou and Venables, 2003). Individual fibers are coated with cathodic, electrolytic, and
anodic forming layers to create a battery. The use of the surface area of fibers as opposed to that of a
foil in a thin film battery allows greater energy outputs, measured on the order of 50 Wh/kg in a
carbon fiber-reinforced epoxy laminate. These batteries may be deposited on various substrates,
including glass, carbon, and metallic fibers. This research and many others, including our own
research, have been supported by DARPA under the first 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 significant 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.
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