3D BIOPRINTING TECHNIQUES - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 60

3D Bioprinting processes begin with a digital model definition of the cellular and tissue construct
architecture to be fabricated. In the case of 2D patterns, the digital files can be directly coded into the
process control interface to drive the motion of printheads to help physically reproduce the architecture.
This digital modeling of a complex 3D construct is typically performed in a computer-aided design
(CAD) software environment. Both the external and internal architecture for the 3D construct can be
designed with initial data obtained from CT/MRI images of the patient in need of a tissue replacement
strategy. Image-based 3D reconstruction procedures can be carried out to help define the 3D digital
model of the tissue replacement construct. Tools available in the software environment can help iden-
tify different material regions which specify the placement of the biomaterial matrix, biological mol-
ecules, and living cells. Process algorithms are written to convert the digital model to printhead path
instructions necessary to drive the hardware systems. The exact format for machine instructions in such
computer-aided manufacturing (CAM) depends on the printing technique and hardware configuration
utilized. Once the signal for printing is activated, the process control system drives the bioprinting sys-
tem hardware components for the physical realization of the printed construct.
Complex engineered tissue and cellular construct-based products will be made from spatially pat-
terned cellular layers which ultimately will become large aggregates for a specialized tissue function.
The entire operation must take place in sterile conditions to limit contamination of both source raw
material and the final construct. If cells are involved in the fabrication process, the total time needed
to produce a construct can be critical. The amount of time available is dependent on the cell type used.
Unless printing conditions are well suited for cell maintenance, time to fabricate constructs should not
exceed an hour. Longer times will result in reduced cellular viability and abnormally higher cellular
stress, which will lead to degraded function.
We describe the main bioprinting techniques used by the research community to print biomateri-
als including cells and biomolecules to form 3D constructs. These 3D constructs can be used as tissue
models for drug screening, as disease models to study cancer, and as constructs meant for animal or
human implantation. Due to the size scale of cells, which are generally in the 5-20 m m size range, all
bioprinting processes work at dimensional scale levels larger than the cell type utilized.
3.3.1 INK-JET-BASED BIOPRINTING
Ink-jet bioprinting is a noncontact printing process involving the precise deposition of picoliter to nano-
liter droplets of “bioink” (a low-viscosity suspension of living cells, biomolecules, growth factors, etc.)
onto a “biopaper” (a hydrogel substrate, culture dish, etc.) in a digitally controlled pattern. It is a direct
adaptation of the conventional ink-jet printing process, and a majority of current ink-jet bioprinting ac-
tivities continue to be conducted using partially modified commercially available desktop ink-jet print-
ers. There are two fundamental approaches to ink-jet printing: continuous (CIJ) and drop-on-demand
(DOD). In the CIJ approach, an uninterrupted stream of droplets is produced by forcing the ink through
a microscopic nozzle orifice under pressure and deflecting it onto the substrate using an electrostatic
field. Where droplet deposition is not required in the digital pattern, the droplets are steered into a gutter
and collected for reuse. In the DOD approach, the ink droplets are ejected through the nozzle orifice by
creating a pressure pulse inside a microfluidic chamber only when required. The DOD approach is of
primary interest in bioprinting due to the pulsed nature of printing. The CIJ approach is not well suited
to bioprinting due to the need for conductive ink formulations and the risk of contamination due to ink
recirculation, among other reasons.