Director/Professor, Institute of Advanced Biomedical Engineering and Science, Tokyo Women’s Medical University
Professor, University of Utah
Cell sheets functionally connectable to organs
—Nano tissue engineering leads to new medical treatments—
Pulsating tissues inside a culture dish are living cardiac muscle cells. Individual cells that make up the cell sheet are functionally and structurally connected to each other and show functions inherent to organs and tissues. Prof. Okano has established “cell sheet engineering” as a method for new medical treatments.
Cells normally attach strongly to hydrophobic surfaces but weakly to hydrophilic surfaces. When cells are cultured in a normal polystyrene dish, they proliferate and make a monolayer sheet. The cells are structurally connected to each other by intercellular junction proteins and adhere to the surface of the culture dish with adhesive proteins. In order to detach the cell sheet from the dish, protein-splitting enzymes must be used to disrupt the adhesive proteins; however, they also damage intercellular junction proteins at the same time.
During Prof. Okano’s research on Drug Delivery Systems (DDS), he worked with polymers that change their structure depending on temperature. He wanted to make a system that releases drugs when the body temperature is increased and stops releasing them when the temperature is decreased again. He, then, came up with the idea of detaching cells from a polystyrene culture dish by grafting the surface of the dish with temperature-responsive polymers, which change their surface properties when the temperature is changed. He therefore tried using poly (N-isopropyl acrylamide) (PIPAAm) because its affinity for water drastically changes below 32ºC. He discovered that the surface of a dish grafted evenly with PIPAAm with a nanometer-scale thickness is hydrophobic at 37ºC, which is the ideal temperature for culturing cells, and thus, the cells attach to the dish and proliferate. After proliferation, when the temperature is lowered below 32ºC, the surface becomes hydrophilic, and the cells can be detached from the dish as a cell sheet.
Prof. Okano says, “If the PIPAAm is grafted too thickly, cells do not attach to the dish even at 37ºC. Because normal polystyrene culture dishes are hydrophobic, complexes that exclude water are created by strong hydrophobic bonding in the area of surface contact between polystyrene and PIPAAm at 37ºC. When the grafted PIPAAm film is thin, the PIPAAm-grafted surface is hydrophobic enough for the cells to attach due to the influence of the interface between polystyrene and PIPAAm. When the PIPAAm layer is thick, cells do not attach to the surface because there is less influence from the interface on the grafted surface.” He has prepared PIPAAm films with a uniform thickness of 30 nm by controlling the density of monomer solution and using appropriate solvents to graft the dish surface evenly with monomers. After irradiation by electron beam, the PIPAAm is simultaneously fixed to the surface and polymerized. This technique has led to the establishment of cell sheet engineering.
“Some people may question the effectiveness of this method, which does not make the entire organ, but parts of the organs or tissues, for regeneration, but I don’t agree,” says Prof. Okano. Cell sheets have already been used as parts of organs or tissues in medicine. A cell sheet that was prepared from 2 mm x 2 mm portions of corneal epithelial cells can be detached from the culture dish surface and transplanted with its own adhesive proteins. This cell sheet adheres to the surface of an eye in about 5 minutes without sutures. The sheets can also be layered to create a three-dimensional structure with complex functions. He says, “Cell sheets can be connected to organs or tissues not only structurally but also functionally. If we could prepare various cell sheets and fully utilize the abilities of the cells, transplantation medicine without donors may be possible. Now, medicine can be synthesized from proteins and hormones instead of chemicals. Medical treatment using highly functionalized aggregates such as cells and tissues is also expanding and regenerative medicine that enables drastic medical treatments has attracted worldwide attention,” says Prof. Okano.
“How do we combine the concepts and technologies of various research fields in order to fully establish regenerative medicine? When setting up a research collaboration in a new field, such as medicine-engineering collaboration, some people may think working in research fields other than their specialized areas is somebody else’s job, and there is no need for them to put an effort into it. However, if you want to create something new in the world, you have to go beyond the normal collaboration into interdisciplinary research,” says Prof. Okano. In his lab, there are researchers specializing in medicine, engineering, and basic science, and their aim is to establish advanced medical technology. He says, “I would like to make it so that we are active throughout our lifetimes rather than only prolonging our life expectancy. So we can live actively without anxiety.”
