Current status of three-dimensional printing inks for soft tissue regeneration.

Recently, three-dimensional (3D) printing technologies have become an attractive manufacturing process, which is called additive manufacturing or rapid prototyping. A 3D printing system can design and fabricate 3D shapes and geometries resulting in custom 3D scaffolds in tissue engineering. In tissue regeneration and replacement, 3D printing systems have been frequently used with various biomaterials such as natural and synthetic polymers. In tissue engineering, soft tissue regeneration is very difficult because soft tissue has the properties of high elasticity, flexibility and viscosity which act as an obstacle when creating a 3D structure by stacking layer after layer of biomaterials compared to hard tissue regeneration. To overcome these limitations, many studies are trying to fabricate constructs with a very similar native micro-environmental property for a complex biofunctional scaffold with suitable biological and mechanical parameters by optimizing the biomaterials, for example, control the concentration and diversification of materials. In this review, we describe the characteristics of printing biomaterials such as hydrogel, synthetic polymer and composite type as well as recent advances in soft tissue regeneration. It is expected that 3D printed constructs will be able to replace as well as regenerate defective tissues or injured functional tissues and organs.