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236 Biomimetics: Biologically Inspired Technologies

Figure 8.7 The individual self-assembling peptide molecules are 5 nm long (left). The first such peptide, EAK16-
II, was discovered from a yeast protein, zuotin (Zhang et al., 1992). This peptide inspired us to design a large class
of self-assembling peptide construction motifs. Upon dissolving in water in the presence of salt, they spontaneously
assemble into well-ordered nanofibers, further into scaffolds. The AFM image of peptide RAD16-I nanofibers and
PuraMatrix scaffold is shown (right).

We have shown that a variety of tissue cells encapsulated and grown in three-dimensional
peptide scaffolds exhibit interesting functional cellular behaviors, including proliferation, func-
tional differentiation, active migration, and extensive production of their own extracellular
matrices (Kisiday et al., 2002; Zhang, 2003, 2004). When primary rat hippocampal neuron cells
are allowed to attach to the peptide scaffolds, the neuron cells not only project lengthy axons that
follow the contours of the scaffold surface, but also form active and functional synaptic connections
(Figure 8.8). (Holmes et al., 2000). Furthermore, when the peptide scaffold was injected into brain
of animals, it bridged the gap and facilitated the neural cells to migrate across the deep canyon. The
animals regained their visual function. Without the peptide scaffold, the gap remains, and
the animals did not regain visual function (Ellis-Behnke et al., unpublished results).

8.5.4 Designer Peptide Surfactants or Detergents

We designed another new class of peptide surfactants or detergents with short hydrophobic tail and
hydrophilic head (see Figure 8.4, lower panel), taking advantage of the self-assembly properties in
water (Vauthey et al., 2002; Santoso et al., 2002; von Maltzahn et al., 2003). Several peptide
surfactants have been designed using the nature lipid as a guide. These peptides have a hydrophobic
tail with various degrees of hydrophobicity and a hydrophilic head, either negatively charged
aspartic and glutamic acids or positively charged lysine or histidine (Figure 8.9). These peptide
monomers contain 7 to 8 amino acid residues and have a hydrophilic head composed of aspartic
acid and a tail of hydrophobic amino acids such as alanine, valine, or leucine. The length of each
peptide is approximately 2 nm, similar to that of biological phospholipids (Vauthey et al., 2002;
Santoso et al., 2002; von Maltzahn et al., 2003). The length can also be varied by adding more
amino acid, one at a time to a desired length as shown in Figure 8.9.
Although individually these peptide surfactants or detergents have completely different com-
positions and sequences, these peptides share a common feature: the hydrophilic heads have 1 to
2 charged amino acids and the hydrophobic tails have four or more consecutive hydrophobic amino
acids. For example, A 6 D (AAAAAAD), V 6 D (VVVVVVD) peptide has six hydrophobic alanine or
valine residues at the N-terminus followed by a negatively charged aspartic acid residue, thus having
two negative charges, one from the side chain and the other from the C terminus; likewise, G 8 DD
(GGGGGGGGDD), has eight glycines followed by two asparatic acids with three negative charges.
In contrast, KV 6 (KVVVVVV) and V 6 K (VVVVVVK) have one positively charged lysine as the