Bioprinting

3D bioprinting is the process of creating cell patterns in a confined space using 3D printing technologies, where cell function and viability are preserved within the printed construct. Generally, 3D bioprinting utilizes the layer-by-layer method to create tissue-like structures that are later used in medical and tissue engineering fields. Bioprinting covers a broad range of materials. Currently, bioprinting can be used to print tissues and organs to help research drugs and pills. In addition, 3D bioprinting has begun to incorporate the printing of scaffolds. These scaffolds can be used to regenerate joints and ligaments.

Bioprinting is printing with biological materials. Think of it as 3D printing, but with squishier ingredients. There’s a lot of work being done at research labs and big companies like Organovo on 3D printing of human tissues for drug testing, or even of whole human organs for transplantation. But the basic underlying technologies are surprisingly accessible: it’s all based on inkjet printing or 3D printing—technologies that are readily available to the DIY scientist.

The limitation of inkjet printers is that they only print in two dimensions -- along the x- and y-axes. A 3-D printer overcomes this by adding a mechanism to print along an additional axis, usually labeled the z-axis in mathematical applications. This mechanism is an elevator that moves a platform up and down. With such an arrangement, the ink head can lay down material from side to side, but it can also deposit layers vertically as the elevator draws the platform down and away from the print head. Fill the cartridge with plastic, and the printer will output a three-dimensional plastic widget. Fill it with cells, and it will output a mass of cells.

For 3D printers need to start with say a Makerbot and then :

Elevator -- A metal track running vertically at the back of the machine, the elevator, driven by the z-axis motor, moves the print heads up and down. This makes it possible to stack successive layers of material, one on top of the next.

Platform -- A shelf at the bottom of the machine provides a platform for the organ to rest on during the production process. The platform may support a scaffold, a petri dish or a well plate, which could contain up to 24 small depressions to hold organ tissue samples for pharmaceutical testing. A third motor moves the platform front to back along the y-axis.

Reservoirs -- The reservoirs attach to the print heads and hold the biomaterial to be deposited during the printing process. These are equivalent to the cartridges in your inkjet printer.

Print heads/syringes -- A pump forces material from the reservoirs down through a small nozzle or syringe, which is positioned just above the platform. As the material is extruded, it forms a layer on the platform.

Triangulation sensor -- A small sensor tracks the tip of each print head as it moves along the x-, y- and z-axes. Software communicates with the machine so the precise location of the print heads is known throughout the process.

Microgel -- Unlike the ink you load into your printer at home, bioink is alive, so it needs food, water and oxygen to survive. This nurturing environment is provided by a microgel -- think gelatin enriched with vitamins, proteins and other life-sustaining compounds. Researchers either mix cells with the gel before printing or extrude the cells from one print head, microgel from the other. Either way, the gel helps the cells stay suspended and prevents them from settling and clumping.

Bioink -- Organs are made of tissues, and tissues are made of cells. To print an organ, a scientist must be able to deposit cells specific to the organ she hopes to build. For example, to create a liver, she would start with hepatocytes -- the essential cells of a liver -- as well as other supporting cells. These cells form a special material known as bioink, which is placed in the reservoir of the printer and then extruded through the print head. As the cells accumulate on the platform and become embedded in the microgel, they assume a three-dimensional shape that resembles a human organ.

Alternatively, the scientist could start with a bioink consisting of stem cells, which, after the printing process, have the potential to differentiate into the desired target cells. Either way, bioink is simply a medium, and a bioprinter is an output device. Up next, we'll review the steps required to print an organ designed specifically for a single patient. 

For simple organs, such as bladders, researchers don't print the living tissue directly. Instead, they print a 3-D scaffold made of biodegradable polymers or collagen. To determine the exact shape of the scaffold, they first build a 3-D model using computer-aided design (CAD) software. They usually define the exact x-, y- and z-coordinates of the model by taking scans of the patient using computerized tomography (CT) or magnetic resonance imaging (MRI) technology. 

Next, researchers get the cells they need by taking a biopsy of the patient's bladder. They then place the cell samples in a culture, where they multiply into a population sufficiently large enough to cover the scaffold, which provides a temporary substrate for the cells to cling to as they organize and strengthen. Seeding the scaffold requires time-consuming and painstaking handwork with a pipette. It generally takes about eight weeks before such artificial bladders are ready for implantation. When doctors finally place the organ in the patient, the scaffold has either disappeared or disappears soon after the surgery.

The procedure above works because bladder tissue only contains two types of cells. Organs like kidneys and livers have a far more complex structure with a greater diversity of cell types. While it would be easy enough to print a scaffold, it would be almost impossible to recreate the three-dimensional structure of the tissue manually. A bioprinter, however, is ideally suited to complete such a time-consuming, detail-oriented task.

Here are the steps to print a complex organ:

<ul>

<li>First, doctors make CT or MRI scans of the desired organ.</li>

<li>Next, they load the images into a computer and build a corresponding 3-D blueprint of the structure using CAD software.</li>

<li>Combining this 3-D data with histological information collected from years of microscopic analysis of tissues, scientists build a slice-by-slice model of the patient's organ. Each slice accurately reflects how the unique cells and the surrounding cellular matrix fit together in three-dimensional space.</li>

<li>After that, it's a matter of hitting File > Print, which sends the modeling data to the bioprinter.</li>

<li>The printer outputs the organ one layer at a time, using bioink and gel to create the complex multicellular tissue and hold it in place.</li>

</ul>

Finally, scientists remove the organ from the printer and place it in an incubator, where the cells in the bioink enjoy some warm, quiet downtime to start living and working together. For example, liver cells need to form what biologists call "tight junctions," which describes how the cell membrane of one cell fuses to the cell membrane of the adjacent cell. The time in the incubator really pays off -- a few hours in the warmth turns the bioink into living tissue capable of carrying out liver functions and surviving in a lab for up to 40 days.

Bioprinting

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