Solid Freeform Fabrication in Tissue Engineering
A Technology for Directly Fabricating 3D Living Tissue...
Creating living tissue in complex geometries is a challenging issue facing the tissue engineering community. Traditional tissue engineering techniques result in living tissue of simple, often 2D geometries. By harnessing the capabilities of Solid-Freeform Fabrication (SFF) – also known as Rapid Prototyping (RP) – we can create living tissue of arbitrary 3D shapes directly from computer-aided design (CAD) data.
Not only can patient-specific living implants be created directly from medical imaging data, but the complex 3D multi-tissue configurations of native structures can be more accurately reproduced by using SFF. For example, intervertibral discs have a very specific 3D overall shape and spatial distribution of multiple cell types; traditional tissue engineering techniques would have great difficulty reproducing these types of structures.
The "printing ink" is a cell-seeded alginate hydrogel. The alginate hydrogel is similar to that used for injection molding tissue engineering, but has been modified to be compatible with extrusion through a printing deposition tool. The material is also stiff enough to prevent material sag, hold its shape and be manipulated.
To print the alginate in prescribed 3D geometries, we used a custom-built gantry robot platform. The gantry moves the syringe deposition tool (with algiante hydrogel contained within the syringe) and extruded the alginate along specific paths.
These paths were calcualted by taking a CAD model and slicing it vertically, then planning paths within each layer. This form a path planning is a standard within the SFF community. More advanced path planning is being explored to support complex 3D, multiple material prints involving embedded geometries and difficult overhanging structures.
The cells seeded within the printed alginate was shown to be viable, as demonstrated in the Live/Dead assay image below. The Live/Dead assay confirms that the printing process does not have a substantial adverse affect on the cell viability. In addition to cell viability, cell function needed also be tested.
Cell function was assessed by checking for the production of extracellular matrix (i.e. the bulk material of cartilage produced by living cells). After time in culture, the printed structures demonstrated significant levels of GAG and collagen, two markers of cartilage extracellular matrix production. These markers indicated that the printed material developed into cartilage in ways similar to samples produced by alternative tissue engineering techniques.
Once cell viability and cell function were assessed using biochemical assays, the printing of 3D structures could be focused on. Using the algiante material and the gantry robot platform, various 3D geometries were pritned directly from CAD data. In the image below: 1) a crescent, 2) a first-order approximation of an intervertibral disc, and 3) an ovine mensicus directly from a CT scan. The overall shapes of the printed contructs closely resembled the intended geometries, which were directly prescribed from 3D CAD models.