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.