Biomedical Engineering Reference
In-Depth Information
these characteristics, physical hydrogels are suited for a plethora of applications as
functional smart materials [
25
-
29
].
Physical chain crosslinking can be achieved by two different approaches. In
one approach, junctions are formed directly between precursor polymer chains by
mutual reversible association of suitable moieties on the polymer backbone [
30
,
31
]. In another approach, supramolecular junctions are formed by chain associa-
tion mediated by an additional linker [
32
,
33
]. To realize these different strategies,
different crosslinker motifs have been reported, including those based on hydrogen
bonding [
34
,
35
], metal complexation [
36
,
37
], or ionic interaction [
38
,
39
]. These
different motifs have specific pros and cons, depending on the desired application
of the hydrogel. For example, applications in the medicinal sector, such as tissue
engineering [
40
,
41
] or drug delivery [
42
,
43
], are impaired by chain crosslinking
through metal complexation, because many metal ions are toxic [
44
,
45
]. Instead
hydrogels used for these purposes are better crosslinked by hydrogen bonding or
ionic interactions. However, hydrogels crosslinked by metal complexation find
various use in other areas, including application as superabsorbers [
46
], optical
devices [
47
,
48
], soft semiconductors [
49
,
50
], or fuel cells [
51
,
52
].
Several starting materials can be used to prepare supramolecular polymeric
hydrogels, including biopolymers, synthetic polymers, or hybrids of both. Many
biomolecules, such as alginate [
53
-
55
], gelatin [
56
-
58
], or chitosan [
59
-
61
] can
form hydrogels even without chemical modification of the polymer. In addition,
these materials are biocompatible, bioavailable, biodegradable, and cheap. This
makes them ideal candidates for applications in the medicinal sector. However, a
downside of biopolymers is their batch-to-batch variation [
62
-
64
], entailing vari-
ation of the physical properties of gels that are formed from them. To avoid this
problem, synthetic polymers like polyethylene glycol [
65
-
67
], polyhydroxyethyl-
methacrylate [
68
-
70
], and polyglycerol [
71
-
73
] can be used to substitute biopol-
ymers. In their native form, these polymers are incapable of forming hydrogels.
Thus, suitable chemical modification must be applied to use them for this pur-
pose. This approach often entails increased costs and a high workload to prepare
the hydrogels, but it introduces the advantage that chemical modification can be
applied in a custom and versatile principle, thereby tailoring the precursor poly-
mers in rational materials design. In another approach, hybrid gels that consist of
biopolymers
and
synthetic polymers combine the utility of both [
74
-
76
].
The topic of supramolecular polymeric hydrogels is a wide field. In this chap-
ter, we summarize some recent work on the formation, characterization, and appli-
cation of hydrogels crosslinked by hydrogen bonding, metal complexation, or
electrostatic interaction. The selected examples represent a cross-section of the
recent research and development on physical polymeric hydrogels for biomedical
applications. We emphasize on different approaches using biopolymers, synthetic
polymers, or hybrids of both. A special focus is on the physical-chemical features
and the resulting macroscopic properties of these functional materials in biotech-
nological applications.
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