The last two decades have seen a number of fascinating innovations in biomaterial scaffolds development. This comes from the growing realization that 2D culture of cells can only do so much in terms of mimicking physiological niches of the body. To encourage stem cells towards a particular mature cell fate, or to mimic disease pathologies like cancer, 3D culture environments are critical. Hence, we now have technologies like electrospinning polymers into fibrous meshes and 3D printing of polymer scaffolds. In fact, the lab I work in has a 3D printer humbly positioned next to its older, more familiar cousin (a regular 2D printer). It feels like a juxtaposition of old technology and new.
While our regular printer may not “grow” scaffolds from the bottom up, the paper with which it is loaded happens to be a type of biological scaffold that people have been manufacturing for centuries. Composed primarily of cellulose fibres, it has a natural porosity, and a depth, that can be appreciated when you zoom into the microscale.
Dr. Jianhua Qin’s Shanghai-based group makes this argument in their recent paper published in Lab on a Chip. In their article, they compare the ability of three types of paper – print paper, filter paper and nitrocellulose paper – to grow human induced pluripotent stem cell-derived cardiomyocytes (iPS-CMs).
To create a multi-well arrangement (a flat plate with multiple “wells” used as small test tubes), the investigators first bound a PDMS sheet to the paper that left patterned gaps for cells to grow.
In their first series of experiments, they seeded pre-differentiated iPS-CMs onto the paper wells and showed that the cells had positive immunofluorescence staining for cardiac Troponin T after several days in culture. They noted that filter and printer paper lead to more tightly packed iPS-CM aggregates, which they propose as an explanation for why they were able to achieve spontaneous beating in culture vs. the more loosely packed iPS-CM clusters grown on nitrocellulose membranes.
Intriguingly, they then tested whether they could grow iPS cells on their paper substrates and directly differentiate them into cardiomyocytes. To facilitate iPS growth, the wells were first coated with a layer of gelatin and Matrigel. The investigators witnessed growth on all three types of paper and showed that the pluripotency markers SOX2 and OCT4 were kept until five days of culture.
When they substituted in media to induce cardiac differentiation, they noted positive cardiac troponin staining for those grown on filter and print paper and, after 7-14 days, tissues with a beat frequency of 40-70 beats per minute could be seen on the print paper. The investigators posit that differentiation was more successful on the print paper because of the high cell densities achieved (smaller pore sizes). These tissues could maintain spontaneous beating activity up to at least three months, after which they were digested for further analysis.
Curiously, the nitrocellulose membranes seemed to cause the differentiation of iPS cells into a retinal pigment epithelium-like lineage (i.e. cells relevant to the eye) when using their media that normally differentiates iPS cells into cardiomyocytes.
This article not only demonstrates how creative thinking can lead to the use of common materials in new ways, but it also clearly reflects the importance of scaffold structure on stem cell fate.
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