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232 Biomimetics: Biologically Inspired Technologies

The Great Wall was program-assembled
one brick at a time (~5,600 km!)
Each brick has a dimension ~10 x 20 x 30 cm
The Great Wall used ~3 billion bricks!
Self-assembly is ubiquitious in Nature
Each fish in about 5–50 cm in length

(a) (b)
Figure 8.2 The programmed assembly and self-assembly. (a) The Great Wall was program-assembled over
2200 years ago, one brick at a time, with a defined plan, thus an ordered structure, using approximately 3 billion
bricks (similar in number to the DNA bases in the human genome). (b) On the other hand, numerous individual fish
self-assembled into well-ordered structure without external instructions. The power and ubiquitousness of self-
assembly is witnessed everywhere in nature (images of the fish are courtesy of the National Geographic Society).

What do they have in common? Stone walls & Proteins
Molecular Self-assembly of Tetrameric
Hemoglobin Polypeptides

Deoxy α β 1
2

6.4 nm in diameter β α 1
2
(b)
(a)
Figure 8.3 (See color insert following page 302) (a) The stone wall is built one stone at a time with different
sizes and colors of stones. It has a defined function. (b) The protein — hemoglobin consisting of four chains — is
built one amino acid at a time with 20 amino acids of all shapes and chemical properties. It also has a defined
function to carry oxygen.

construction units by many orders of magnitude into nano-scale, such as structurally well-ordered
protein fragments, or peptides (Fields, 1999; Yu et al., 1997), we can apply similar principles to
construct molecular materials and devices, through molecular self-assembly and programmed
molecular assembly. This field is growing at a rapid pace and it is impossible to summarize all
aspects of the work being done by others in this limited space, and hence this chapter focuses on
a few examples especially from the author’s laboratory.
In this chapter, two distinct classes of self-assembling peptide construction motifs are described
(Figure 8.4). The first class belongs to amphiphilic peptides that form well-ordered nanofibers (Zhang
et al., 1993, 1995). The first member of such self-assembling peptides, EAK16, was discovered in a
segment fromyeastprotein,zuotin(Zhang etal.,1992).Thesepeptideshavetwodistinctivesides,one
hydrophobic and the other hydrophilic. The hydrophobic side forms a double sheet within the fiber
and hydrophilic side the outside of the nanofibers that interact with water molecules. The hydrophilic
side can form extremelyhigh water content hydrogel,containing as high as 99.9% water similar as the
water content of a jhellyfish. At least three types of molecules can be made, with , þ, /þ on the