Figure 41 The perspective AFM and sectional-profile images of hybrid
resist (a) imprinted, (b) after direct etching without transfer film and
(c) after bilayer etching with PMMA as transfer
film.[136]
3.4. Hydrogel
Hydrogel, as a class of hydrophilic polymer with three-dimensional
network, possesses the properties of absorbing and retaining a large
amount of water due to many hydrophilic functional groups such as
hydroxyl (-OH), carboxyl (-COOH), amino (-NH2), sulfonic acid (-SO3H)
and so on, at the same time, maintaining their three-dimensional network
structure.[116, 137-141] Hydrogel materials are
widely utilized in biomedical engineering and pharmaceutical industry
due to biocompatibility, biodegradability, easy to synthesize and other
merits.[140-143]
At present, the formation methods of hydrogel mainly include physical
and chemical crosslinking. The chemical crosslinking by means of forming
chemical bond among monomer or oligomer is more stable than physical
crosslinking through weak interactions such as hydrogen
bond.[144] Chemical crosslinking includes Michael
addition, Schiff base, enzymatic reaction, click chemistry, and
photopolymerization.[144] Among them,
photopolymerization is probably the most effective and commonly
crosslinking route. To form hydrogel by photopolymerization has many
merits, firstly, the photopolymerization process is extremely rapid
which could be completed in a few minutes or even seconds, secondly, the
photopolymerization allows spatial and temporal control over the
cross-linking process, this feature is particularly exploited by
stereolithography and 3D-(bio)-printing, thirdly, there is no necessity
for high temperature or extreme pH value because the photopolymerization
process only requires lower energy and
temperature.[144-145] The mentioned-above
technology has been specifically applied in the biological field.
For the last few years, as a 3D-(bio)-printing technology, Digital Light
Processing (DLP) getting more and more attention because it can create
more complex structure of tissue and
organ.[146-148] Hong et
al.[149] used the silk fibroin (SF) which modified
by glycidyl methacrylate (GMA) to construct the chondrocyte-laden
hydrogel scaffold using DLP 3D-printing technology by UV light source at
365 nm. The schematic diagram for modification of SF with GMA and
bioprinting of chondrocyte with Silk-GMA DLP are illustrated in Figure
42. The Silk-GMA shows strong effectiveness for chondrogenesis in vitro
and in vivo transplantation, meanwhile, suggests the good
biocompatibilities and mechanical advantages for defected tissue
regeneration.