Building Living Tissues: The advancement of Bioinks in 3D Bioprinting

In the 21st century, organ failure and severe tissue damages has been one of the most common and urgent medical issues, mostly due to the aging population and a severe global shortage of donor organs. As millions of patients everywhere remain on organ transplant waiting lists, researchers have found that traditional 2D cell cultures often fail to accurately replicate the complex microenvironment of the human tissue. These limitations would pose risks to translational medicine due to 2D cell culture not having the ability to replicate the same scenario towards three-dimensional cell-to-cell interactions (Kapałczyńska et al., 2016).

While groundbreaking discoveries in stem cell biology help to address cellular mechanisms, the world desperately needs to develop functional, lab-grown tissues. To achieve a true breakthrough in regenerative medicine, researchers are turning to one of biofabrication's most critical components: advanced bioinks.

Bioinks

Traditional tissue engineering scaffolds that are derived from aliphatic polyesters (PLA or PGLA) or ceramics (hydroxyapatite) come with major bottlenecks such as its risks of having toxic chemical residues during chemical processing while also lacking built-in biological signals. This puts them at a disadvantage when trying to support living cells (Adel et al., 2022). Current advanced bioinks circumvent these issues as the specialised hydrogel formulations mimic the natural extracellular matrix found in vivo, which provides the environment that supports cell adhesion, proliferation and differentiation.

Figure 1. 3D bioprinting with composite bioinks (Maan et al., 2022).

A composite bioink is created by mixing three distinct components:

1. Base Polymers such as Alginate, Gelatin, Synthetic Polymers

2. Living cells specifically on the targeted tissue.

3. Bioactive inorganic fillers that support cell growth and act as a structural buffer to avoid the introduction of damaging shear forces during the extrusion process.

Once placed into a grid-like scaffold, these composites can be customized depending on the medical application necessary, such as bone regeneration, osteochondral repair, vascularization promotion and drug release.

Smart Additives

To achieve specific tissue engineering goals, various micro- and nanotechnologies are incorporated into standard hydrogels, transforming them into "smart bioinks" capable of active responses and controlled release:

Addictive Type	Key Characteristic	Primary Function
Nanoparticles	Made of bioactive glasses or graphene oxide.	Delivers drugs or growth factors to influence stem cell differentiation into tissues.
Microparticles & Microspheres	Spherical microparticles  in sizes from  1 to 1000 µm.	Acts as a physical cushion to protect cells from shear stress and localized biomolecules release.
Microswimmers	Magnetically responsive helical microstructures capable of active liquid navigation	Highly cell-compatible and biodegradable method for micro-level interventions in hydrogel.

Challenges and limitations

Despite their immense potential, several obstacles currently hinder large-scale clinical commercialization:

●      The Printability problem: High-concentration hydrogels retain their shape well but require high shear stress during extrusion, which can compromise the structural integrity of the living cell membranes encapsulated inside.

●      Batch Variability: The chemical composition of natural biopolymers varies depending on the biological source and extraction environment, making industrial standardization complicated.

Future Prospects: Transitioning to 4D Bioprinting

Overall, the continuous development of stimuli-responsive smart bioinks is paving the way for bioprinting. For the next-generation approach, the printed 3D biological constructs can dynamically change their shape or behavior over time in response to external environmental triggers, such as shifts in temperature or pH levels. This collaboration between materials science and regenerative biology promises a much more dynamic and personalized future for medicine.

Citations:

Adel, I. M., ElMeligy, M. F., & Elkasabgy, N. A. (2022). Conventional and Recent Trends of Scaffolds Fabrication: A Superior Mode for Tissue Engineering. Pharmaceutics, 14(2), 306. https://doi.org/10.3390/pharmaceutics14020306

Kapałczyńska, M., Kolenda, T., Przybyła, W., Zajączkowska, M., Teresiak, A., Filas, V., Ibbs, M., Bliźniak, R., Łuczewski, Ł., & Lamperska, K. (2016). 2D and 3D Cell Cultures – a Comparison of Different Types of Cancer Cell Cultures. Archives of Medical Science, 14(4). https://doi.org/10.5114/aoms.2016.63743

Maan, Z., Masri, N. Z., & Willerth, S. M. (2022). Smart Bioinks for the Printing of Human Tissue Models. Biomolecules, 12(1), 141. https://doi.org/10.3390/biom12010141

 Building Living Tissues: The advancement of Bioinks in 3D Bioprinting