Chronic kidney disease is a growing health concern that affects 10 percent of the population worldwide and there is a large demand for more accurate cell-based models of the kidney. Researchers in the EU-funded BIRDIE consortium have now, by combining microfluidic bioprinting and new biomaterials, successfully created detailed models of the tubular structures that are an essential part of the kidney function. The results, which are published in the scientific journal Bioactive Materials, make it possible to construct more accurate in vitro kidney models, creating important new opportunities for research and drug development.
The nephron, the functional part of the kidney, is a highly complex biological tissue, with an intricate cellular architecture. Recently, considerable effort has been made to recreate the cell-laden tubular structures that play an essential role in the kidney’s ability to filter the blood from toxins. Many of the most promising approaches to this are based on a combination of biomaterials with patient-derived kidney epithelial cells. However, many of the biomaterials used suffer from high batch-to-batch variability, resulting in poor reproducibility which makes the models unusable for screening applications.
New biomaterials increase accuracy and repeatability
To address the reproducibility issues associated with commonly used biomaterials, the team of researchers at Maastricht University created a alginate-based biomaterial, Alg-Norb—a natural polymer derived from seaweed commonly used in medical applications—which they functionalized with peptides to create a bioactive environment enabling cell attachment.
“We have created new material formulations for bioprinting where we can vary crosslinker composition and peptide functionalization to be able to finely tune both cell interaction and hydrogel properties of the bioink. When using this new biomaterial formulations, we were able to obtain endothelial cells displaying tubule formation and epithelial cells forming cystic aggregates. Our results demonstrate Alg-Norb as a highly tunable and more reproducible alternative to Matrigel for tubule formation and tubuloid cultures” says Carlos Mota, assistant professor at Maastricht University and principal investigator in the additive manufacturing and bioprinting group at the MERLN Institute.
Moving from 2D to 3D with microfluidic bioprinting
Bioprinting is an essential tool in the research group led by Carlos Mota that enables a transition from simplistic two-dimensional cell cultures to complex 3D structures that more closely resemble human tissues and organs. To achieve tissue constructs that mimic the tubular structures of the nephron, the researchers used microfluidic coaxial bioprinting, which makes it possible to produce multi-layered hollow fibers in a way that maintains high cell viability.
A common method to stabilize the bioinks following printing is to use a fluid containing calcium ions that forms an ionic crosslink holding the gel together. This technique has a major drawback since it commonly results in unstable gels that expose the cells to high ionic concentration. “We were able to remove the need for ionic crosslinking by optimizing the Alg-Norb crosslinker composition and by adapting our printer with UV-irradiation ability at the nozzle. Using our setup, we were able to achieve rapid reaction kinetics enabled and the bioprinting of cell-laden tubular structures, using microfluidic bioprinting without the need for ionic crosslinking. The bioprinted hollow fibers showed the formation of monolayers, as promising preliminary results in the use of Alg-Norb to model kidney in vitro” says Francesca Perin, former researcher at Maastricht University.
A functional model of the human kidney
Being able to construct thin hollow fibers in a precise and reliable way using cell-friendly materials is an important step towards faithfully mimicking the tubular structures of the nephron in vitro. The next step for the researchers is now further functionalize the bioprintined tubules. “Future steps will aim at improving the model by perfusing the bioprintied fibers. We expect perfusion to set the inner lumen as preferential monolayer formation site and to stimulate tight junction formation in epithelial cells, creating a functional interface that resembles the structure in the native kidney,” says Carlos Mota.
The research results are a key piece of a larger puzzle aimed at creating a complete model of the human renal tubular tubulointerstitium. “In the BIRDIE project, we combine organ-on-chip technology, high precision bioprinting and microphysiological modeling to create a completely new type of research solution that can provide a detailed understanding of kidney disease and drug interaction, from the cellular scale to the organ level” Carlos Mota concludes.
More about the research:
- The article Bioprinting of Alginate-Norbornene bioinks to create a versatile platform for kidney in vitro modeling is published in Bioactive Materials with Carlos Mota of the MERLN Institute and Faculty of Health Medicine and Life Sciences at Maastricht University as corresponding author.
- The research is part of the project Bioprinting on-chip microphysiological models of humanized kidney tubulointerstitium (BIRDIE), which is supported by the European Union’s Horizon 2020 FET Open programme under grant agreement No 964452.
BIRDIE Project Website 