Recent Advances in Bioink Design for 3D Bioprinting

Despite recent advances in tissue engineering, there remains a lack of tissues and organs for transplantation and a shortage of tissue models for drug discovery and testing. Some of the hindrances involve conventional techniques, such as porogen-leaching, injection molding, and electrospinning due to the limited control over scaffold architecture, pore shape, composition, size, and distribution.

3D bioprinting overcomes these barriers by enabling fabrication of scaffolds, devices, and tissue models with high complexity. Using computer-aided design, 3D printing facilitates construction of tissues from commonly used medical imaging like x-rays, MRIs, and CT scans.

3D Bioprinting Technologies

Demand is growing for alternative fabrication approaches for developing tissues and organs that are biocompatible, stable, and cost-effective. Tissue engineering is a multidisciplinary field currently focused on two major areas: 

• developing new methods to repair, regenerate, and replace damaged tissues and organs
• creating in vitro tissue models to better understand tissue development, disease development, and progression, and to develop and screen drugs

3D bioprinting combines the fields of developmental biology, stem cells, and computer and materials science to create complex bio-hybrid structures which are then used for various applications. Its unique feature is that it can precisely place different cell types, biomaterials, and biomolecules together in a predefined position to generate printed composite architectures.

This is an additive manufacturing approach that uses a "bioink" to fabricate devices and scaffolds in a layer-by-layer manner. It allows printing of a cell suspension into a tissue construct with or without scaffold support. A bioink can be defined as an ink formulation that allows printing of living cells.

While currently available 3D printing technologies across many applications use diverse ink formulations to print a wide range of materials, the strict requirements of 3D bioprinting necessarily prohibit the use of certain types of techniques and materials. 

Bioink Design

The ideal bioink formulation must satisfy specific material and biological requirements.

Material properties:

• printability
• mechanics
• degradation
• functionalizability

Biological requirements:

• biocompatibility
• cytocompatibility
• bioactivity

When considering material properties, printability is most important. This comprises the processability of the bioink formulation and the print fidelity associated with the mechanical strength of the resulting construct to self-sustain a 3D structure post-printing.

Other factors of printability affecting the end result are solution viscosity, surface tension, and cross-linking properties. Viscosity is a critical parameter for bioink formulation because it affects cell encapsulation efficiency and print fidelity. 

Degradation is important in vivo for the functional integration of the printed construct by enabling cells to gradually replace it with their extracellular matrix. 

Currently Available Bioinks

Cell-laden Hydrogels

These are the most commonly used bioinks because they are easily formulated for extrusion-based (DIW), droplet-based (inkjet), and laser-based (SLA and LIFT) bioprinting technologies. They utilize natural hydrogels:

• alginate
• agarose
• chitosan
• collagen
• gelatin
• fibrin
• hyaluronic acid (HA)

And synthetic hydrogels:

• pluronic (poloxamer)
• polyethylene glycol (PEG)
• or blends of both

The challenges of working with hydrogels include instability, unreliable bioactivity, and undesirable viscosities. To compensate for these shortfalls, blends of synthetic and natural polymers have been used to develop mechanically tunable hydrogels with user-defined bioactivity. Another fine-tuning technique involves incorporating a small amount of nanoparticles into bioink formulation.

Cell Suspension Bioinks

This bioink formulation consists of cell aggregates in the form of monocellular or multicellular spheroids which undergo a fully biological self-assembly, either with or without a temporary support layer. The technique relies on tissue liquidity and fusion, whereby cell-to-cell interactions direct the cells to self-assemble and fuse.

This innovation is guiding development of vasculature models. The capability to print 3D cellular tubes is a critical indicator of the feasibility of successful organ printing.

Fibroblast (3T3 cell)-based tubes with an overhang structure have been successfully fabricated using an inkjet bioprinting system. A study by researchers at Clemson University yielded fabrication knowledge that will help print tissue-engineered blood vessels with complex geometry.

dECM-based Bioinks

Decellularized extracellular matrix-based bioinks are obtained by removing the cells of a tissue of interest while preserving the ECM. The ECM is then crushed into powder form and dissolved in a cell-friendly buffer solution to formulate the bioink.

As in mixing paints, a carrier may be used to increase solubility, change the viscosity, or to enhance post-cross-linking of the bioink. The processing of decellularized tissue increases the cost of these bioinks.

One promising example using this approach is a 3D printed cardiac patch which promoted strong vascularization and tissue matrix formation in vivo. 

Conclusion

Further study and advancements in this rapidly developing field continues to be driven by the goal of achieving the "ideal' bioink for each bioprinting technology. The other challenges to overcome are the need for high-resolution bioprinters, cost factors, and regulatory issues.