AAWSAP DIRD, Aerospace Applications of Programmable Matter, December 2009

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The greatest challenge, however, is temperature. The resistance of an electrical wire
varies with temperature, and while we scarcely notice the difference with the fat copper
wiring in our houses, the delicate films and nanowires of Programmable Matter smart
materials feel it very strongly. Other critical properties, such as the bandgaps of
semiconductors, the optical spacing of photonic crystals, and the internal symmetries of
liquid crystals, are also temperature sensitive and must be managed carefully to avoid
overwhelming the control signals passing through the material.
Early attempts at programmable quantum dot materials were so temperature sensitive
that researchers found that altering temperature, rather than the electric field, was
actually the easiest way to control the response of the materials . In fact, by selecting
materials with higher temperature sensitivity, we were able to enhance these effects to
produce thermally activated smart materials with no need for electric controls at all.
Of course, materials that respond to temperature are not new. Thermochromic (color
changing) liquid-crystal thermometers are well known, and in the mid-1990s, Chinese
researchers developed a thermochromic paint that turns a cool-blue shade when warm,
a warm-red shade when cool, and pale green at room temperature. This was done
mainly for thermal regulation-the paint allegedly could increase the temperature of a
building by about 4°C in winter and decrease it by about s0 c in summer-although the
researchers also claimed an aesthetic benefit to having a home's color match the
season. Even more impressive effects could be achieved if black and white (or clear and
silver) were available color choices.
Although thermochromic building materials were very rare until recently, a number of
other mechanisms-including electrochromics, photochromics, and automatic
mechanical blinds-have been used to create "dynamic windows" that actively control
solar heat gain in buildings and display many of the properties (if not the principles) of
a true smart material. Buildings account for 40 percent of all energy consumed in the
United States, including 71 percent of all electricity and 38 percent of all carbon dioxide
emissions. Managing solar heat gain and controlling the building's thermal envelope are
the most effective ways of reducing energy use. The same principles apply, of course,
to spacecra~, for which solar heat is even more intense and nighttime (in the shadow of
the Earth) is colder.
In 2003, the U.S. Department of Energy's Lawrence Berkeley National Laboratory
(LBNL), on behalf of the American Society of Heating, Refrigeration, and Air
Conditioning Engineers (ASHRAE), performed a study focused on dynamic window
systems capable of darkening or shading automatically such that the solar heat gain
coefficient (SHGC) varied between 0.26 and 0.40. Analyses were performed for eight
U.S. cities: Boston, Denver, Jacksonville, Kansas City, Phoenix, Sacramento, Seattle,
and Washington, D.C.
The ASHRAE study found that across all eight climate zones, buildings with low
emissivity ("low-E") glass saved an average of 8-15 percent on their total annual
energy (heating, cooling, and ventilation) costs, whereas the addition of dynamic
window systems saved an additional 6-19 percent. In other words, the savings
associated with a dynamic window are approximately as large as the savings associated
with low-E. Furthermore, because dynamic windows and low-E coatings save energy
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