**2.1 Properties and selection criteria for ideal nanocomposite scaffolds for tissue engineering**

In the past research, various nanocomposite scaffolds have been prepared using different manufacturing techniques. In a broad sense, the ideal nanocomposite scaffold should meet the following demands:


Of course, the final criteria for choosing a nanocomposite scaffold depends on the type of tissue to be treated. These nanocomposite scaffolds must be resistant to damage during the implantation process, as damage can lead to necrosis and inflammation. For example, nanofibers scaffolds for tendon tissue engineering should be arranged in parallel to imitate the natural tissues arrangement structure. In cardiac tissue engineering, nanocomposite scaffold materials should have a certain electrical conductivity, and gradually promote the repair and regeneration of myocardial infarction damaged tissue under the conditions of satisfying electrophysiology [23]. For neural tissue engineering, scaffolds with higher flexibility are needed to allow cells to adhere, migrate and differentiate, and they need to mimic the geometry of

#### *Nanocomposite Biomaterials for Tissue Engineering and Regenerative Medicine Applications DOI: http://dx.doi.org/10.5772/intechopen.102417*

natural nerves [24]. The scaffold required for bone tissue engineering should have the characteristics of providing multiple growth factors for different stages of bone tissue regeneration, corrosion resistance, osteoconductivity, and shape controllability [25, 26]. It also needs to create multiscale layered structures at the nanometer level to add growth factors that promote vascularization and provide a surface for stem cells to initiate bone repair and regeneration [15].

According to the type of tissue to be repaired, degradation rate and surface morphology are also important criteria for stent selection. There should be coordination between the degradation time of the scaffold and the time required for natural tissue replacement, it can be implanted for a long time without multiple operations and avoid subsequent surgical removal. The degradation rate can be controlled by introducing nanoparticles into the scaffold. On the other hand, the scaffolds used for skin and heart repair usually require planar scaffolds, the scaffolds used for bone tissue repair are usually cube-shaped or disc-shaped, while the tissue regeneration of nerves, blood vessels, and trachea require tubular scaffolds [27].

#### **2.2 Types of nanocomposite scaffolds**

#### *2.2.1 Hydrogel*

Hydrogel, a highly hydrated 3D network, has a similar structure and composition with ECM and become preferred tissue engineering scaffolds due to their adjustable network structure, outstanding biocompatibility, effective mass transfer, and the ability to encapsulate cells and biological factors [28]. These properties are affected by the degree of crosslinking of polymer chains, molecular arrangement, and the amount of water they absorb [29]. Hydrogels typically exhibit a hydrophilic network porous structure with interconnected pores (>10 μm) to allow gas penetration, nutrient delivery and provide sufficient space for cell attachment and interaction. In addition, the porosity of the hydrogel can be controlled by preparation methods (phase separation, gas foaming, solvent evaporation, etc.), types of raw materials, and the polymers concentrations [30]. The hydrogel nanocomposite scaffold can be designed in different sizes and shapes, such as patches, microspheres, sheets, hollow tubes, to satisfy their unique practical application [31]. From the perspective of regeneration medicine, hydrogels can be used as the cell niche and provide stimuli to accelerate tissue regeneration, and able to act as the delivery vehicle for biologically active molecules.

Nanocomposite hydrogels were first reported in 2002 when Haraguchi et al. introduced exfoliated clay flakes into poly(N-isopropylacrylamide) (PNIPAM) to form a unique organic/inorganic network [32]. Compared to conventional ones, nanocomposite hydrogels have enhanced chemical, physical, electrical, and biological properties, which are mainly attributed to the improved interaction between the polymer network and nanoparticles. However, nanoparticles themselves lack some essential properties, such as biodegradation and stimulated reactivity, which can be ameliorated by incorporating multiphase in nanocomposite hydrogels (**Figure 2**).

#### *2.2.1.1 Inorganic nanomaterial based nanocomposite hydrogels*

Over the last decades, significant progress has been made in the study of the impact of introducing inorganic nanomaterials into tissue engineering applications in natural and synthetic polymer networks. Inorganic nanomaterials, such as HAp, metal oxide nanoparticles, bioglass, and calcium phosphate NPs, are based

#### **Figure 2.**

*Hydrogels for different applications such as tissue engineering and other biomedical researches [33].*

on materials found in biological tissues and show significant biological activity. NPs confer and enhance signal (mechanics, bioelectricity, etc.) conductivity, while polymers provide flexibility and stability to nanocomposite structures. Surface chemistry favorable for protein adsorption and enhanced matrix stiffness makes these nanocomposites ideal for tissue engineering scaffolds.
