Biomedical Engineering Reference
In-Depth Information
hydrogel properties. Some of these supramolecular hydrogel properties, like cyto-
compatibility or customizable gelation conditions, are properties seen regularly in
peptide hydrogels and are necessary for biomedical peptide hydrogel applications.
Like many other nature-inspired materials, peptides and proteins are often
cytocompatible. This makes peptide hydrogels excellent candidates in provid-
ing growth conditions that mimic the natural, in vivo environment of cells.
Cytocompatibility is a broad, general term that describes whether or not a cell is
able to survive near, in, with, or on a peptide hydrogel. Many peptide hydrogels,
regardless of structure and composition, have shown good cytocompatibility with
many different cell lines such as mesenchymal stem cells (MSCs) [
73
,
97
,
98
],
embryonic stem cells (ESCs) [
84
,
99
], rat adrenal pheochromocytoma cells [
100
],
or macrophages [
101
]. Many of these cell types have not only stayed alive while
encapsulated, but also continued to grow and proliferate, indicating a high degree
of cytocompatibility in the peptide hydrogels.
Self-assembly methods and the solid structure of hydrogels, combined with
biological compatibility, leads to many biomedical applications specific to drug,
cell, or molecule delivery. In addition, these materials are excellent candidates as
scaffolding for tissue regeneration or for 3D cell growth environments. Drug, pro-
tein, and molecule delivery systems include targeted treatment for eradication of
cancers [
6
,
55
,
96
], diabetes [
43
], and other illnesses [
37
,
38
,
102
,
103
]. As the fol-
lowing examples will discuss, successfully encapsulated drugs and molecules are
varied in size, hydrophobicity, charge, origin, and pH.
The Zhang group encapsulated biologically native proteins of varying charge
and hydrophobicity to better understand diffusion kinetics through the Ac-
(RADA)
4
-CONH
2
peptide hydrogels in vitro [
103
]. Using a single molecule
approach, the kinetics showed potential for sustained release of proteins, where
diffusion of the protein was dependent on size and not charge. The Schneider
group also encapsulated proteins of varying charge and hydrophobicity in MAX8
hydrogels, but instead examined bulk release after syringe delivery [
104
]. Unlike
the Zhang group, the charge of the protein greatly affected the release and diffu-
sion profile, suggesting the importance of the native hydrogel environment for
molecule encapsulation. Instead of looking at proteins, the Pochan group exam-
ined drug molecule release through encapsulation of hydrophobic curcumin, a
derivative of the naturally occurring Indian spice turmeric [
96
]. In this study, the
hydrophobic curcumin was encapsulated and protected from quick degradation
in a mostly aqueous situation in the MAX8 peptide hydrogel. The encapsulated
curcumin remained active after release well beyond its chemical stability half-
life of 8 h. In addition, the compound continued very low, sustained concentra-
tions of release over 2 weeks, enough for cancer cell eradication in vitro. Since
many chemotherapeutics can be equally detrimental to healthy cells, this ability to
release chemotherapeutic compounds locally and at low but effective concentra-
tions is advantageous for minimizing side effects while maintaining meaningful
treatments.
Peptide hydrogels can be delivered into the body through several different
methods, depending on the gel assembly mechanism and desired application of the
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