CH391L/S13/Cell Scaffolding and Printing
Cellular scaffolding is used in biomedical engineering to support tissue growth, usually during the process of tissue regeneration or tissue engineering. There are several requirements that the scaffold needs to fulfill in order to be a good support for cells and tissues.
1) The cells need to be able to be placed precisely onto the scaffold. Tissue-specific stem cells need to be placed in the right location onto or into a scaffold or else the cells may develop abnormally. The cells also need to be able to adhere to the scaffold through secretion of extra cellular matrix (ECM) proteins and saccharides.
2) The cells need to be able to survive on the scaffold (the scaffold should be biocompatible). This includes that the scaffold should be able to facilitate cellular signaling by signaling molecules or mechanical ques. One challenge faced by tissue engineers is the lack of known biocompatible scaffolding material with the particular properties needed in applications such as tissue printing and rapid prototyping techniques.
3) The scaffold needs to be rigid enough to support tissues, but porous enough to diffuse oxygen and nutrients to the cells inside. If the scaffold is poorly designed, it could collapse or starve the cells growing inside the scaffold.
4) The scaffold should be able to guide the development of new tissues. (Read the Tamjid and solorio papers)
Cells can also be grown in vivo onto scaffolds by implanting scaffolding directly into the patient. This has been used for bone regeneration if large parts of bone are missing.
Scaffold Fabrication Methods
These methods for scaffold construction are just a few of the many techniques used in tissue engineering. Other techniques in use but not discussed include: phase-separation, freeze dry, self-assembly and fused deposition modeling scaffold construction.
In this process, high voltage is applied to a capillary tube filled with a polymer solution. The polymer is repulsed by the electric field and when the field is intense enough, the polymer solution flies out the end. The solvent evaporates and the polymer hardens into a tangled web. The diameter of the threads can be adjusted anywhere from the nanometer to micrometer range.
3D Printing/Rapid Prototyping
The scaffold is designed in a CAD program and printed using a 3D printer. This process is also called “Additive Manufacturing” since structures created by this process are built layer by layer. This method is relatively quick and cheap in that it greatly reduces the cost of creating a single copy of a particular scaffolding design and the scaffold can be created within a day. A drawback in 3D printing is that it is difficult to deposit material over an empty cavity, so a scaffold for the scaffold is needed. Another is that this process is limited the number of available biomaterials that can be used to create these scaffolds.
In this process, organs are extracted from animals (sheep, mice, ferrets, etc) and the cells are removed by washing with mild detergents (usually by pumping the solution through the vascular system). This kills and washes away the cells without damaging the extracellular matrix (ECM). The ECM is then reseeded ex vivo with human or animal cells. They repopulate the ECM and create a new, functional organ that can be transplanted. A limitation for this procedure (semi-xenotransplantation) is the lack of scaffolding for solid organs. Animal experiments, such as transplanting functional rat penile tissue, have been successful but this technology is far away from entering clinical studies. 
Hydrogel is a network of superabsorbent polymer chains immersed in a fluid solution. They can used in combination with cell printing to use as a scaffold for tissue growth. They are currently used mostly in connective tissue studies, such as cartilage tissue engineering, since they are more compliant (easier to bend and transfer mechanical forces throughout itself). 
In vivo Printing
In vivo Printing Cells Onto Mice
The Skin Spray Gun
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