Biomedical Engineering Reference
In-Depth Information
Fig. 11
Showing a difference in rheology data in for 2 wt% MAX1 in a
a
dynamic time sweep
at 20 °C (
circles
) and at 37 °C (
triangles
), and
b
a frequency sweep at 37 °C showing G′ (
(filled)
)
and G″ (
empty
) [
28
]
under stress, otherwise known as shear-thinning. The property allows a gel net-
work quaternary structure to be set and then subsequently processed (i.e. injected
from a syringe) to a secondary site. Rheology is an important tool in understand-
ing and confirming shear-thinning behavior. Many of these materials after the ces-
sation of shear are able to fully recover into solid hydrogel materials similar to
the preshear state [
37
,
43
,
72
,
73
]. The Pochan group studied at length the causes
for shear-thinning behavior in
ʲ
-hairpin peptide hydrogels and how the hydrogel is
able to recover into a solid. The shear-thinning capability comes from the physi-
cal crosslinking, primarily the fibril branching present in the quaternary order.
When the gel experiences shear forces, the fibril network can fracture, breaking
the gel into domains that are able to move past each other. When shear forces
cease, the gel domains come into contact with each other and immediately per-
colate to reform a gel network [
72
]. Figure
10
shows an example of rheology data
for MAX8 where it first gels, then shear-thins under steady-state shear, exhibits
solid properties directly after shear has ceased and ultimately recovers to preshear
stiffness after a short amount of time. This property is critical in defining the prop-
erties of hydrogel-encapsulated payload constructs where one would like to know
the exact properties of the construct, e.g. drug delivery profile [
96
] or encapsulated
cell state, after in vivo injection [
73
,
97
].
5 Hydrogel Properties
Peptide hydrogels have many properties that allow for a wide range of possible
applications from drug delivery to tissue scaffolding. In addition, the customiz-
able synthesis of peptide sequences for use as hydrogels allows for the tailoring of
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