**Acknowledgements**

A novel preparation of gelatin-PHEMA porous scaffolds by freeze-drying technique was developed recently [138]. Their morphology was assessed by SEM and μ-CT (**Figure 8**). In this study, four types of novel hydrogels using different methacrylamide-modified gelatin/2-hydroxyethyl methacrylate ratios between 1/0 and 1/2 (w/w) (samples from C0 to C3) were prepared and the results indicated that the HEMA content in the initial polymerization mixtures modulate the architecture of the porous scaffolds from straightforward, top-to-bottom oriented channels for hydrogels possessing the lowest HEMA content to a complex and dense internal porosity of the channels the case of higher HEMA loaded materials. Besides, the covalently bound gelatin sequences significantly improve the biocompatibility of PHEMA based hydrogels, which is very desirable for tissue engi-

Superporous scaffolds can be also prepared by the salt-leaching technique using NaCl or

By submitting carbon dioxide to supercritical conditions after certain time and then rapidly depressurized is also possible to fabricate porous structures that are related to the super-

such scaffolds has received some attention in the past but many researchers believe that there is limited interconnectivity between the pores, which is required for tissue engineering. However, highly porous (greater than 85%) and well interconnected scaffolds with very promising applications for cartilage repair have been obtained in a blend of poly(ethyl meth-

Probably the most sophisticated techniques to produce scaffolds are electrospinning, 3D printing and bioprinting. Electrospinning is composed of a high-voltage DC power supply, an infusion pumps and a syringe with a needle tip usually with a diameter of 0.5 mm. For example, a three-dimensional aligned nanofibers-collagen type I hydrogel scaffold for controlled non-viral drug/gene delivery to direct axon regeneration in spinal cord injury treat-

3D printing promises to produce complex biomedical devices according to computer design using patient-specific anatomical data. This 3D printing technique has slowly evolved to create one-of-a-kind devices, implants, scaffolds for tissue engineering, and drug delivery systems among other important applications. However, several technological limitations, related to the kind of commercially printable materials available and other technical printing aspects such as printing speed, must still be overcome. The common 3D printing technologies are three-dimensional printing, fused deposition modeling, selective laser sintering, stereolithography, and 3D plotting/direct-write/bioprinting, and are still under deep research for the progress of each technology applied in tissue engineering. Bioprinting is the more advanced 3D printing technology because it consists of printing cells combined with custom 3D scaf-

Mechanical resistance depends both on the material properties and on the interconnected pore structure of the scaffold. This problem is more important in polymer hydrogels, which

critical parameters and to the polymer blend composition [131]. The use of CO<sup>2</sup>

acrylate) and tetrahydrofurfuryl methacrylate [144].

ment has been reported very recently [145].

folds for personalized regenerative medicine [140].

as a porogen [142] or with many other porogenic agents such as ammonium oxa-

to create

neering purposes.

SO<sup>4</sup>

late crystals [143].

(NH4)2

108 Hydrogels

This work was supported by the 2017-231-001UCV grant from the Universidad Católica de Valencia "San Vicente Mártir".
