Stem Cells: Promising Opportunities but Highest Demands

Stem cells are expected to be among the most powerful medicines of the future. They have considerable potential for novel cell-based therapies, tissue engineering, and innovative drug discovery. Some therapies, like transplanting bone marrow or grafting skin tissue are already in use. Several others, targeting heart, immune or degenerative neural diseases, for example, are currently in clinical trials. A lot of research remains to be done, however, to improve the existing approaches and develop new methods. One of the major challenges is to develop scalable and reproducible cultivation procedures that produce sufficient numbers of cells.

Stem cells are diverse

Stem cells have the unique capability to develop into several different cell types in the body. This is essential during early development for the proper growth of tissues and organs in the embryo. Later, tissue-specific stem cells guarantee the continuing maintenance of vital organs and blood cells, and functioning of the immune system. Depending on their origin and their potential for differentiation, stem cells are classified as embryogenic or adult. While embryogenic stem cells are pluripotent, and can develop into multiple cell types, adult stem cells are tissue specific and can only produce cells of the same type.
The third, quite recently developed class of stem cells are induced pluripotent stem cells (IPSCs). They are generated in the laboratory by reprogramming tissue-specific adult stem cells into pluripotent ones. IPSCs offer all the many advantages of pluripotent embryogenic stem cells but they are ethically innocuous because they don’t derive from early human embryos.

Hierarchy of stem cells (adapted from Arizona Pain Stem Cell Institute)

Dishes, flasks, and shaker—semi-controlled cultivation

Like other human and animal cells, stem cells can be grown outside the body in cell cultures. In most laboratories, these cultures are grown in small volumes with special culture ware. Depending on the needs of the individual cell line, the cells are grown floating freely as cell-only aggregates in suspension, bound to microcarriers or attached to the surface of the culture vessel. The culture dishes, T-flasks or shake flasks used are usually placed in incubators to ensure proper conditions to support cell viability and expansion.

Stem cells in bioreactors—easy, efficient, and highly reliable

Large numbers of cells, usually undifferentiated, are needed for cell-based therapies and drug discovery.  The methods described above are limited in terms of scalability, maintaining defined cultivation conditions and overall efficiency. When it comes to industrial-scale applications, stirred-tank bioreactors might be the better choice:

  • Key cultivation parameters such as pH and dissolved oxygen (DO) can be monitored and controlled individually in real time, making it easy to establish robust and reliable processes.
  • Cell viability and metabolic activity can be monitored directly.
  • Advanced feeding strategies such as medium perfusion can be implemented.
  • Bioreactors need significantly less working space relative to their cultivation volume, compared to T-flasks or shake flasks.
  • Automation possibilities and integrated software systems streamline analysis and documentation, minimizing the time, cost, and labor required.

Stem cells are among the most sensitive animal cells in culture. Shear forces, shifts in pH, temperature or oxygen tension can directly influence their viability and expansion. Moreover, the isolation of stem cells is a cumbersome process, and for embryogenic stem cells can be ethically problematic. This makes them a highly valuable good that should be handled in a sustainable way and with the greatest care. Stirred-tank bioreactors, with the above mentioned advantages in monitoring, control, and automation help achieving these favorable conditions.

Schematic overview of a stirred-tank bioreactor

Trust is good but control is better

The cultivation of stem cells in bioreactors offers full control of all process parameters, processing according to pre-defined protocols, real-time analysis of cell expansion and metabolism, as well as direct visualization of parameters and test results. Unlike semi-controlled cultivation in dishes or flasks, bioreactors provide valuable information about the status of the stem cell culture in real time.
A constant pH, for instance, is vital to the expansion and differentiation of stem cells. Bioreactor systems tightly control the pH. Industry-standard sensors measure the pH value and precisely control it by addition of acid and base or by regulating the CO2 supply.

Shear stress is an important factor, too because it can harm sensitive stem cells. Soft and especially in the lower range precise agitation with an impeller design that matches the demands of the specific cells provides the best conditions for effective mixing. Testing reveals the appropriate stirring conditions in which the cell aggregates or microcarriers can be held in suspension while the cells are safe from damage.

Impact of impeller design and agitation speed on microcarrier distribution (after 10 min stirring).

The proliferation and differentiation of stem cells is also strongly dependent on the oxygen tension during cultivation. Depending on the origin tissue of the cultivated cells they prefer hypoxic or normoxic conditions. Bioreactor systems enable users to set up optimal oxygen conditions for their specific cells, including the gassing cascades and DO, to hypoxic levels.

Example for a potential influence of oxygen tension on pluripotent stem cells (adapted from AIMS Bioengineering Volume 2, Issue 2, 15-2)

Size counts—from R&D to production

Stirred-tank bioreactors are available for a broad range of working volumes, with many choices of equipment and designs. They can be made of glass or stainless steel, but are also produced in polymeric single-use versions. The latter offer great potential for industrial applications in which a good manufacturing practice (GMP) environment is required.
Each stem cell line has its own specific requirements, which need to be figured out in the research and development phase. Early-stage experiments, for example, can be carried out in bioreactors with small working volumes (e.g., 100 mL) to save valuable cell material. Intelligent bioreactor design allows for smooth scale-up to larger volumes, because of similar geometries, control systems, and comprehensive software. This ensures consistency throughout all development phases. Optimum growth conditions can be determined and seamlessly scaled to larger working volumes to produce sufficient amounts of stem cells for therapeutic or diagnostic applications.

Perfusion: The next big thing in stem cell cultivation?

Recent studies indicate that perfusion cultivation, a feeding strategy quite well established in classical cell culture, will be suitable to grow iPSCs and other stem cells.
In perfusion culture, fresh medium is continuously added to the bioreactor while at the same time cell-free medium is withdrawn from the cultivation. In this way, the culture volume remains constant and cells experience an unchanging environment over a long period. Unlike continuous cultivations in general, perfusion techniques require retention of the cells using a filter or by binding them within the bioreactor using fibers or membranes.

For a deeper insight

The integration of external analytical equipment such as cell counters, nutrient analyzers or autosamplers into a bioreactor system offers useful automation possibilities. Integrated analytics give even more detailed information on the cultivation process, cell behavior, and cell vitality. Moreover, it allows for set-up of closed-loop feedback control mechanisms.