Recipe for a Living Ink
The goal of 3D bioprinting is an immense technical challenge. For starters, cells don’t make good building blocks.
Without the extracellular matrix, they are too soft. Using them to make a 3D structure is a bit like trying to build a wall out of water-balloons. For this reason, the 3D printing inks being explored in research labs usually contain other components besides the cells themselves. These extra ‘structural’ components are like the ‘cement’ or ‘glue’ which holds the scaffold together while the cells (printed in a precursor form) fuse to make functional tissue. These supporting materials often derive from the naturally occurring or synthetic biopolymers, and may be designed to degrade as the printed cells mature within.
Hydrogels are the jelly-like materials most widely used for tissue engineering due to their cellular compatibility. Many types of living cells can grow happily inside them. However, hydrogels lack the viscosity and strength to hold their shape after printing. Recently, multiple-component composite hydrogels, such as alginate (from-seaweed) blended with gelatin (the foodstuff used in marshmallows), have been developed to improve the gel-like characteristics and enable high resolution printing of free-standing hydrogel constructs. These structures were created using extrusion printing, the ‘tooth-paste squeezing’ technique.
In most current research, the living cells are not incorporated into the hydrogel inks during printing. The cells, such as chondrocytes for cartilage regeneration, can be seeded onto the printed structures in a Petri dish or bioreactor. The goal here is to create a ‘cartilage patch’ suitable for implantation to help heal a defect or injury. Integrating cells at precise locations during printing would have huge advantages over the post-seeding approach. Achieving this will require some clever ways to help the cells survive the printing process.
© University of Wollongong, 3D Bioprinting: Printing parts for bodies, 2014, Wallace, G.G., Cornock, R.C., O’Connell, C.D., Beirne, S., Dodds, S., Gilbert, F.