Making Artificial Tissues With Bioprinters

Brian Derby from the University of Manchester is using inkjet technology to distribute cells onto scaffolds that are shaped as a particular organ. Inkjet and laserjet technologies can build three-dimensional scaffolds that are coated with cells that will grow into the scaffold, assume its shape and degrade the scaffold, leaving only the tissue in its place.

This type of technology, which involves the simultaneous placement of biodegradable scaffold and cells in a three-dimensional structure that resembles that of an organ is called additive manufacture and it might very well be the future of replacement organ production.  Additive manufacture recreates the biological structure in a three-dimensional, digital image, from which two-dimensional, digital slices are taken and fashioned one layer at a time.  The summation of all the digital slices eventually produces a three-dimensional structure.

Inkjet technology dispenses the material that makes the scaffold in very small droplets that quickly solidify.  The materials is loaded into an actual inkjet printer cartridge that is sprayed onto the surface.  More droplets are placed on top of previous droplets in a very specific pattern and this repetitive distribution of droplets develop into a pattern that is very complex and forms a scaffold that nicely mimics the conditions inside the body.  The scaffold also provides a surface the for cells to adhere, grow and thrive.  The scaffold and its internal structure control the behavior and maintain the health of the cells embedded in the scaffold.  This method of distributing cells onto a surface through a printer is called “bioprinting.”

In his article, published in the journal Science, Derby examines experiments in which porous structures are made by means of bioprinting.  Bioprinting uses inkjet and laserjet technologies to distribute cells or molecules onto a surface in a desired pattern.  In the case of porous structures, cells interweave throughout the scaffold and such cell-encrusted scaffolds can be placed in the body to encourage cell growth.  Depending on the composition of the scaffold and the cells embedded in it, the scaffold can become a part of the body or the cells will dissolve it.   Such a treatment can help heal patients with particular injuries such as cavity wounds.

Bioprinted cells can also be deposited onto scaffolds with various other chemicals, such as hormones, growth factors, or small molecules that influence the behavior of the cells.  The inclusion of such molecules with the scaffold can coax cells to differentiate into distinct cell types, such as, for example, bone- or cartilage-producing cells.

Cells do suffer some damage during bioprinting, and the rule of thumb is the more energy is used to deposit the cells onto the scaffold, the lower the viability of the cells after bioprinting.  To deposit and pattern cells in a scaffold there are three techniques that are used:  inkjet printing, microextrusion, which is also known as filament plotting, and laser forward transfer.  Bioprinting has probably the highest viability rates, and that has come after the techniques have been precisely worked out to ensure a minimum of damage.  Microextrusion shows extremely variable rates of cell survival after the cells are deposited.  Laser forward transfer suffers from the need for higher energy lasers to more precisely and efficiently deposit the cells, but this same higher energy kills off the cells.

Even though this technology has come a long way, it has a way to go before it is ready for the clinic.  Scaffolds are being used in clinical trials, but scaffold synthesis suffers from inconsistency, and until a consistent high-quality is delivered, scaffold production will not be ready for commercial production.

Despite these caveats, there have been some successes.  For example, D’Lima and others used an solution of chemicals in water (poly(ethylene glycol) dimethacrylate to be exact) that also contained cartilage-making cells (chondrocytes).  They printed this suspension a bone defect in a cultured bone and then used a chemical not unlike what dentists use to harden tooth plastic called a photoinitiator.  Such chemicals crosslink and bond together in response to particular wavelengths of light, and D’Lima used light to crosslink the chemicals to make a wet gel that contained the cells.  After several days, this printed structure appeared to have integrated into the surrounding tissue.  This experiment demonstrates that this technology is at least feasible.  The hanging issue is the toxicity of the photoinitiator chemicals to cells (X. Cui, et al Tissue Eng. A 18, 1304 (2012).  However, this has been studied, and it turns out the susceptibility to these chemicals is very cell type-specific.  Thus, picking the right photoinitiator could potentially make this technique rather safe (see C. G. Williams, et al Biomaterials 26,1211 2005).

(A) Schematic of bioprinting a cartilage analog structure, combining inkjet printing with a poly(ethylene glycol) dimethacrylate (PEGDMA) solution containing cells in suspension with a simultaneous photopolymerization process. (B) Light microscopy image of cell-containing polyethylene hydrogel printed into a defect formed in an osteochondral plug (scale bar, 2 mm). After culture, the cells within the printed material express ECM similar to those in the adjacent tissue

Scaffolds, however, can also be used to make external tissues, for example, skin patches.  Derby is working with ear, nose, and throat surgeons at the Manchester Royal Infirmary.  His goal is to use bioprinting to make patches that can be implanted into the inside of the nose or throat.

Derby explains: “It is very difficult to transplant even a small patch of tissue to repair the inside of the nose or mouth.  Current practice, to transplant the patient’s skin to these areas, is regarded as unsatisfactory because they transplants do not possess mucous generating cells or salivary glands.  We are working on techniques to print sheets of cells that are suitable for implantation in the mouth and nose.”

Derby hopes that someday bioprinting can be used to grow tumors in realistic cultures that will make superior models for drug testing and drug development.