Biomedical Engineering Reference
In-Depth Information
An exciting alternative approach to cell delivery for tissue engi-
neering is the use of polymers (i.e., hydrogels) that can be injected
into the body. This approach enables the clinician to transplant
the cell and polymer combination in a minimally invasive manner.
Hydrogels have structural similarity to the macromolecular-based
components in the body and are considered biocompatible.
14
Hydrogels have found numerous applications in tissue engineering
as well as in drug delivery. Tissue engineering is the most recent
application of hydrogels, in which they are used as scaffolds to engi-
neer new tissues (Fig. 8.1).
15
8.2 Photopolymerization of Hydrogels
Visibleorultraviolet(UV)lightcaninteractwithlight-sensitivecom-
pounds called photoinitiators to create free radicals, which can ini-
tiate polymerization to form cross-linked hydrogels.
16
The use of
light to polymerize or cure materials
in vivo
has been practiced
extensively in dentistry to form sealant and dental restorations
in
situ
.
17
,
18
Photopolymerizationhasalsobeenusedinelectronicmate-
rials, printing materials, optical materials, membranes, polymeric
materials, and coatings and surface modifications.
16
Photopolymer-
ization has several advantages over conventional polymerization
techniques. These include spatial and temporal control over poly-
merization,fastcuringrates(lessthanasecondtoafewminutes)at
roomorphysiologicaltemperatures,andminimalheatproduction.
19
One major advantage of photopolymerization is that hydrogels
canbecreated
in situ
fromaqueousprecursorsusingphotopolymer-
ization in a minimally invasive manner, for example, using laparo-
scopic devices,
20
,
21
catheters,
22
,
23
or a subcutaneous injection with
transdermalillumination.
24
Fabricationofpolymers
in situ
isattrac-
tive for a variety of biomedical applications because this allows
one to form complex shapes that adhere and conform to tissue
structures. Polymerization conditions for
in vivo
applications, how-
ever, are di
cult since biological systems require a narrow range
of acceptable temperatures and pH, as well as the absence of toxic
materials, such as most monomers and organic solvents (Table 8.1).
Some photopolymerization systems can overcome these limitations
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