**3.1 Structural properties**

Scaffold micro and macro architecture critically influences cell survive and surface adhesion, but also cellular proliferation, differentiation, vascularization, and specific gene expression [10].

If on the one hand, a scaffold may be strong enough to support the physiological load of the body and to allow surgical handling during implantation, on the other hand, it is important to obtain a porous structure to avoid cellular colonization. It is clear that a balance between mechanical strength and high porosity is a significant challenge in scaffold production.

## *3.1.1 Pore interconnection, porosity, and pore size*

 Pore interconnection, porosity, and pore size represent very important parameters for the scaffold production. All three features allow cellular penetration, vascularization, adequate diffusion of nutrients and oxygen to cells within the construct, and neo-formed extracellular matrix ensuring cell viability [5, 11].

 In particular, pore size is a key element for the scaffold efficiency. In fact, the pores must be large enough to allow cells to penetrate and migrate within the scaffold structure, but also small enough to allow the binding of a critical number of cells at the same. Pores can be classified into micropores (0.1–2 nm), mesopores (2–50 nm), and macropores (>50 nm) according to their dimension. All the scaffolds used for tissue engineering may have a macroporous structure with a specific pore size as a function of the type of host tissue. In particular, a pore size of 20 micron is required for hepatocyte and fibroblast growth, while the dimension is around 20–150 micron for soft tissue healing. For bone tissue engineering, researchers propose a pore size range between 200 and 400 micron.

The most common techniques used to obtain a porous structure are gas foaming, salt leaching, phase separation, sintering, and freeze-drying.

## **3.2 Mechanical properties**

The scaffold for tissue engineering must have adequate mechanical integrity, so that it can offer support from the time of implantation until the remodeling process is fully completed.

## *3.2.1 Mechanical strength and stiffness*

The mechanical strength depends on the bonding forces that hold together the atoms in scaffold architecture. It is an important parameter to avoid the

solid structure deformation due to cellular loading on the scaffold or caused by scaffold handling.

Another important feature of the scaffold surface is the stiffness that is measured by Young's modulus. Cells respond to scaffold stiffness *via* different mechanisms such as activation of ion channels or protein unfolding, and by this way, stiffness affects cell proliferation and differentiation. Hadjipanayi *et al.* demonstrated that the increasing of free-floating collagen matrix stiffness led to a higher proliferation rate for human dermal fibroblasts [12].
