The field of 3D bioprinting has exploded in recent years largely due to the advances in additive manufacturing technology along with progress in material chemistry and tissue engineering techniques. The key ingredients that make up the complex bioprinted structures are comprised of hydrogel-based biomaterial/s often coined as ‘bioinks’ and a cellular component that serves as building blocks to create a 3D printed biological tissue. Clearly, the choice of the cell source, proteins and other biological ingredients depends largely on the desired final application. For the purpose of this blog, we will only focus on the desire to create bioprinted tissue for clinical translation.
Which cell should I use?
Cells for clinical use can be derived from the patient (autologous) or a donor (allogeneic). In many tissue engineering applications, stem cells are used due to their properties in self-renewal and differentiation. Adult stem cells, currently being the most clinically viable solution, include hematopoietic, mesenchymal, neural and epithelial. While hematopoietic stem cell transplantation is widespread, it has limited utility in tissue engineering applications due to its limited differentiation capabilities primarily toward blood lineages. Neural stem cells are most effective in neural regeneration but the limited source makes it very challenging to become clinically relevant. Mesenchymal stem/stromal cells (MSCs), on the other hand, has a significant advantage due to its tri-lineage differentiation ability and immunomodulatory functions thus it has been widely used in various therapeutic indications including brain trauma, graft-versus-host disease and cardiovascular disease. Moreover, MSCs have already demonstrated clinical safety in > 800 clinical trials treating over 30,000 patients to-date.
How many cells do I need?
In order to print a 3D tissue, we need a significant number of cells seeded at a relatively high density to achieve full tissue mimicry. Exactly how many cells would one require? Let us take a look at a simple example of the knee meniscus. The figure below (left) illustrates a 3D model of an adult meniscus with rough dimensions of 3.5 cm in diameter and 5.3 mm in height. If we were to print this structure at 30% infill, it would require a total volume of ~2.5 mL of bioink. Several studies in bioprinting cartilage tissues have reported that cells would need to be seeded at high densities, a minimum of 10-25 million cells/mL in order to form cartilage in vivo[1, 2]. Thus, in this print, we would require roughly 62.5 million MSCs for a single print. For an intervertebral disc with a diameter of 4 cm and height of 10 mm (Figure on the right), the total volume of bioink required to perform a print would be 4 mL thus the total number of cells required would be 100 million for a single print. These two examples serve to illustrate the number of cells required to bioprint a simple 3D tissue. For larger tissues or organs, one can imagine that you will need a significantly higher number of cells, in the orders to 10 billion or more, to achieve desirable cell distribution through the tissue.
1. Cohen et al., 2018. Tissue engineering the human auricle by auricular chondrocyte-mesenchymal stem cell co-implantation. PLoSOne13(10): e0202356. doi: 10.1371/journal.pone.0202356
2. Moller et al., 2017. In vivochondrogenesis in 3D bioprinted human cell-laden hydrogel constructs. Plast Reconstr Surg Glob Open. 5(2): e1227. Doi: 10.1097/GOX.000000000000127