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Summary: fashion to release previously hidden internal
stresses in the material, thus destroying the
carefully planned geometry of the desired
machine.
The unwanted deformations of micro-
machines were subsequently brought under
control, and even turned to good use, by delib-
erately engineering stresses into materials to
generate a preferred direction for bending.
Thiscanbeachievedbyconsideringtheatomic
structures of the materials. Every crystalline
solid has its own preferred spacing between
constituent atoms -- the atoms in germanium,
forexample,aremorewidelyspacedthanthose
in silicon. If two sheets of different crystalline
materials are layered to form a thin bilayer
sheet in which the atoms across the interface
are mutually aligned, then the layer that prefers
a larger inter-atomic spacing is placed under
compression, whereas the other layer is placed
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