Biomedical Engineering Reference
In-Depth Information
2 Spontaneously Forming Reversible Hydrogels
In situ
gelling hydrogels can be subdivided into two main categories: systems that
are created upon irradiation with light, and systems in which the polymer molecules
self-assemble [
17
]. Photoinduced polymerization can form a hydrogel in situ;
however, they are not self-gelling. Spontaneously forming hydrogels based on
molecular interactions of polymer components are formed spontaneously or after
certain triggers under biological conditions, such as a change in temperature,
pH, ionic strength or molecular interactions. Also, reversibility of the gelation/
dissociation process is important for polymer scaffolds applied in cell and tissue
engineering. Table
3
summarizes the types of spontaneously forming reversible
hydrogel systems [
21
-
25
,
27
,
30
,
31
,
33
-
39
,
42
-
51
].
PEG-based hydrogels are the most widely used materials. PEG-based hydrogels
are nontoxic and nonimmunogenic, and they can be covalently crosslinked
using various methods to form hydrogels. It was reported that PEG-poly(propylene
glycol) (PEG-PPG) triblock copolymers (Pluronics or Poloxamers) turn into hydro-
gels at physiological temperature by forming a liquid crystalline phase [
42
].
PEG-peptide bioconjugates are amenable to proteolytic degradation in response
to secreted proteases, such as matrix metalloproteases (MMPs) from cells [
33
,
34
].
Another important synthetic polymer is PVA. PVA hydrogels are stable and elastic
gels that can be prepared by repeated freezing and thawing cycles [
31
]. The
physically crosslinked PVA hydrogels are biodegradable, and thus can be used
for various applications in cell engineering.
Table 3 Classification of spontaneously forming and reversible hydrogels
Hydrogel
Crosslinking mode
Dissociation signal
References
Collagen/gelatin
Entanglement
Temperature
[
18
,
19
]
Poly(
L
-lysine)-based hydrogel
Ionic interaction
pH
[
20
]
Poly(lactic acid)-based
hydrogel
Stereocomplex formation
pH
[
21
-
25
]
Hydrophobic interaction
Hydrolysis
[
26
]
Alginic acid/divalent cation
complex
Ionic interaction
pH
[
27
]
Agarose hydrogel
Entanglement
Hydrolysis
[
28
]
Chitosan hydrogel
Ionic interaction
pH
[
29
]
PNIPAAm-based hydrogel
Hydrophobic interaction
Temperature
[
30
]
PVA hydrogel
Hydrogen bonding
Temperature
[
31
]
Boronate/diol reaction
Chemical stimulation
(e.g., sugar)
[
32
]
PEG-based hydrogel
Biological interaction
Protease
[
33
,
34
]
MPC polymer-based hydrogel
PMA/PMB hydrogel
Hydrogen bonding
pH
[
35
-
39
]
PMBV/PVA hydrogel
Boronate/diol reaction
Chemical stimulation
(e.g., sugar)
[
40
,
41
]
PMA
poly(MPC-
co
-methacrylic acid),
PMB
poly(MPC-
co
-BMA)
