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230 Biomimetics: Biologically Inspired Technologies

Table 8.1 What do they have in Common? Machines and Molecular Machines
Machines (Made by Humans) Molecular Machines (Made by Nature)
Car, train, plane, space shuttle Hemoglobin
Assembly lines Ribosomes
Motors or generators ATP synthases or photosystems
Train tracks Actin filament network or intermediate filaments
Train controlling center Centrosome
Digital database Nucleosomes
Copy machines Polymerases
Chain couplers Ligases
Bulldozer or destroyer Proteases or proteosomes
Mail-sorting machines Protein sorting system
Electric fences Membranes
Gates, keys, or passes Ion channels, pumps, or receptors
Internet or World Wide Web Neuron synapses

With these seemingly simple molecules, natural processes are capable of fashioning an enormously
diverse range of fabrication units, which can further self-organize into refined structures, materials
and molecular machines that not only have high precision, flexibility, and error correction, but also
are self-sustaining and evolving.
Indeed, Nature shows a highly-flavored bottom-up design, building up molecular assemblies, bit
by bit, more or less simultaneously on a well-defined scaffold. Take for example egg formation
in oviparous animals. The fabrication of an egg involves not only the creation of the ovum, its
various protective membranes, and accompanying nutritive materials (e.g., yolk) but also simul-
taneous synthesis of the eggshell from an extremely low concentration of calcium and other
minerals, all in a very limited space. Oviparous animals synthesize eggshell against an enormous
ionic and molecular concentration gradient due to the high levels of minerals at the site of eggshell
assembly. Dental tissue formations face similar challenges not only when sharks repeatedly form
new teeth, but also when humans form teeth during early childhood.
Nature accomplishes these feats effortlessly, yet recreating them in the laboratory presents an
enormous challenge to the human engineer. The sophistication and success of natural bottom-up
fabrication processes inspire our attempts to mimic these phenomena with the aim of creating new
and varied structures, with novel utilities well beyond the gifts of Nature.

8.1.1 Two Distinctive and Complementary Fabrication Technologies

Two distinctive and complementary fabrication technologies are employed in the production of
materials and tools. In the ‘‘top-down’’ approach, materials and tools are manufactured by stripping
down an entity into its parts, for example, carving a boat from a tree trunk. This contrasts sharply
with the ‘‘bottom-up’’ approach, in which materials and tools are assembled part by part to produce
supra-structures, for example, building a ship using wooden strips (Figure 8.1) and complex
architectures, construction of a building complex. The bottom-up approach is likely to become
an integral part of materials manufacture in the coming decades. This approach requires a deep
understanding of individual molecular building blocks, their structures, assembling properties, and
dynamic behaviors. Two key elements in molecular material manufacture are chemical comple-
mentarity and structural compatibility, both of which confer the weak and noncovalent interactions
that bind building blocks together during self-assembly. Following nature’s leads, significant
advances have been made at the interface of materials, chemistry and biology, including the design
of helical ribbons, peptide nanofiber scaffolds for three-dimensional cell cultures and tissue
engineering, peptide surfactants, peptide detergents for solubilizing, stabilizing, and crystallizing
diverse types of membrane proteins and their complexes.