Peptidic Hydrogels - In-Situ Gelling Polymers: For Biomedical Applications - page 136

number of physical crosslinks of the gel, ultimately affecting the stiffness of the
MAX hydrogel created [ 73 , 94 , 95 ]. The Schneider group has also looked at sub-
stituting all l l-amino (left handed) acids for d l-amino acids (right handed), thereby
reversing the natural chirality of MAX1 [ 85 ]. The assembly kinetics and mechani-
cal stiffness of the hydrogel were found to depend on the chirality of the peptide.
Combining equal amounts of MAX1 and its stereoisomer resulted in a racemic
hydrogel that was an order of magnitude stiffer than the individual stereoisomer
gels. Clearly, there is ample opportunity to alter quaternary structure in peptide
hydrogels through new primary structure design in peptide molecules.
Methods that help to visualize the hydrogel intermolecular quaternary struc-
ture include microscopies such as transmission electron microscopy (TEM), cryo-
TEM, scanning electron microscopy (SEM), and atomic force microscopy (AFM).
TEM, SEM, and cryo-TEM can image at higher resolutions than light microscopes
and routinely image nanostructure of hydrogel networks. TEM and SEM use dried
samples while cryo-TEM uses vitrified samples at cryogenic temperatures to pre-
serve a sample in a hydrated state. TEM images of fibrillar nanostructure can be
seen in Figs. 1 , 2 , 4 , and 5 [ 13 , 50 , 80 , 85 ]. Hydrogel fibrils taken with cryo-TEM
can be seen in Fig. 5 [ 49 ]. AFM uses a mechanical nanoprobe to scan along the
surface of a sample, obtaining information with regards to the surface topography
and nanostructure. Figure 7 shows a nanostructure comparison of the same sample
obtained with both TEM and AFM image [ 85 ].
Rheology is an important technique to identify if a hydrogel network is pre-
sent and to measure properties such as mechanical rigidity and timing of gelation.
Unlike the other characterization techniques, which examine the static structure of
the hydrogel, rheology subjects the hydrogel to shear forces in order to better under-
stand bulk gel mechanical properties and structure such as gel stiffness, flow proper-
ties, assembly time and overall network structure. For the same peptide sequences,
the solution conditions and peptide concentration can cause drastic differences in
gelation time and ultimate gel properties. A simple example of the utility of rheol-
ogy is shown in Fig. 8 [ 86 ]. MAX1 peptide is assembled with the same peptide con-
centration but different salt concentration in Fig. 11 a showing clear differences in
gel stiffness as indicated by different storage moduli, G′, when hydrogelation occurs
with a higher salt concentration in solution. The higher the salt concentration, the
faster the intramolecular folding and secondary structure formation and the faster
the intermolecular quaternary structure hydrogel network formation. Consequently,
the faster the assembly, the more crosslinks in the gel and the stiffer the final net-
work. This observation is only possible with rheology during and after the gelation
process. In Fig. 11 b, the stiffness (G′) and viscous properties (G″ or loss modulus)
are measured for a MAX1 gel versus frequency in order to gain insight into the
material properties relative to time scale of shear applied to the gel [ 28 ]. Rheology
is a critical tool to defining quaternary structures and ultimately peptide hydrogel
properties. Yan et al. have defined and examined the importance and uses for rheol-
ogy to better understand peptide hydrogels as the field expands [ 38 ].
An exciting property of some physical peptide hydrogels due to the entangled
and branched nanostructure is the ability of the solid gel to flow like a liquid when