Tissue Engineering: Focus on the Musculoskeletal System - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 214

A 4

2

3

1

5

B

C

FIGuRE 9.5
(A) Chemical structure of the peptide amphiphile, highlighting five key structural features.
Region 1 is a long alkyl tail that conveys hydrophobic character to the molecule and, when com-
bined with the peptide region, makes the molecule amphiphilic. Region 2 is composed of four
consecutive cysteine residues that when oxidized may form disulfide bonds to polymerize the
self‐assembled structure. Region 3 is a flexible linker region of three glycine residues to provide
the hydrophilic head group flexibility from the more rigid cross‐linked region. Region 4 is a
single phosphorylated serine residue that is designed to interact strongly with calcium ions and
help direct mineralization of hydroxyapatite. Region 5 displays the cell adhesion ligand RGD.
(B) Molecular model of the PA showing the overall conical shape of the molecule going from
the narrow hydrophobic tail to the bulkier peptide region. Color scheme: C, black; H, white; O,
red; N, blue; P, cyan; S, yellow. (C) Schematic showing the self‐assembly of PA molecules into
a cylindrical micelle. (From Science 294: 1684-1688, 2001. With permission from the Publisher.)

To address this problem, Cui and coworkers [101] synthesized an amphiphilic
peptide with diacetylene moiety at the hydrophobic site. After self-assembly,
the as-formed nanofibers were crosslinked using UV light and reinforced
the nanofiber network. The photocrosslinking among peptides, however,
may lead to non-biodegradable scaffolds. The need for acidic solution to
induce self-assembly is also an issue for encapsulating cells. Alternatively,
Greenfield et al. [102] used calcium ions to induce nanofiber assembly by
forming salt bridges between the peptides. Rheological testing showed that
these calcium-bound nanofibers have a higher rigidity compared to nanofi-
bers assembled by hydrogen bonds.