Advances in tissue engineering: cell printing.

Tissue engineering may be defined as the science and engineering of functional tissues and organs for the replacement of diseased body parts. Traditionally, this has been done by the seeding of cellular material onto a suitable scaffold material to create 3-dimensional constructs. However there are a number of drawbacks to this technique. The degree of cellular penetration is variable and does not proceed uniformly through the scaffold. Organs consist of varied cell types in specific locations, and this is hard to replicate with this technique. Preformed, rigid scaffolds are not suitable for engineering contractile tissues, such as myocardium or vascular conduits. Perhaps the single most limiting factor with solid scaffold design is that of providing the developing structure with a vascular supply. Many of the top-down fabrication techniques that have been developed relate to the manufacture of microelectromechanical devices and are therefore unsuitable for biologic systems. It is therefore necessary to develop other strategies for assembling tissuelike constructs, a strategy that allows the creation of structures with distinct shapes and functions that are nurtured by vascular connectivity incorporating methods of vascularizing large, living, 3-dimensional tissue-engineered constructs. Adapting bottom-up approaches to tissue engineering is a genuine challenge. Since the first application of fused deposition modeling for tissue engineering scaffolds, considerable effort has been focused on printing synthetic biodegradable scaffolds. Concurrently, a variety of rapid prototyping techniques have been developed to define macroscopically the shapes of deposited biomaterials, including photolithography, syringe-based gel deposition, and solid freeform fabrication. That these approaches have not yet led to the construction of harmonically organized complex tissues may be due to the difficulty of embedding the various cell types within the intricate designs. Our tissue engineering approach combines rapid prototyping procedures with microencapsulation to print viable freeform structures with custom-modified ink-jet printers. Inspired by developmental biology, this approach may provide the necessary cues, rules, and framework for hierarchic self-assembly. With this innovative technique, it is possible to