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A new dimension to biomaterials
OPINION
A new dimension to biomaterials Moving from two to three dimensions for growing cells will improve toxicity assays, antibody production, and tissue engineering. Shang-Tian Yang and Robin Ng | The Ohio State University | [email protected] Mammalian cells usually need to adhere to a differentiation, gene expression, and tissue function. surface to grow. In the laboratory, tissue flasks and However, very little has been done to ‘quantify’ roller bottles providing two-dimensional surfaces the three-dimensional spatial effects on tissue are commonly used for simplicity. For industrialdevelopment. Now that developments in polymer scale processes, microcarriers can be used to microfabrication and soft lithography can produce provide larger surface areas for cell attachment tissue scaffolds with well-defined three-dimensional in suspension cultures, but this is limited by the structures1, we can study the effect of structure on difficulty in seeding the microcarriers with cells. As the organization of cells and their interactions, which an alternative, hollow-fiber membranes, polymer could lead to the optimal design of tissue scaffolds. gels and foams, and fibrous matrices are being Although current used for growing applications of threecells. These systems The benefits of threedimensional cultures have advantages dimensional scaffolds are mostly limited to because their threetissue engineering, dimensional structures are clear a three-dimensional mimic the in vivo cell perfusion culture system environment, which is can be used to produce recombinant proteins more of critical importance to cellular function and their efficiently for therapeutic purposes. For example, applications. the higher cell density in a three-dimensional cell Three-dimensional cultures provide high surface culture can produce more monoclonal antibody at areas on which cells can attach and arrange a higher titer2. It is not known, though, if protein themselves. For a similar reactor volume, a threeglycosylation differs in three-dimensional cultures dimensional culture can provide a surface area three from those in two dimensions. Three-dimensional orders of magnitude larger than the flat surface of perfusion cultures can also mass-produce cells, a two-dimensional culture. In three dimensions, such as pluripotent stem cells, without needing cells are not limited by contact inhibition and can subculturing or passaging of the cells to maintain reach a higher density closer to that found in tissues, their activity and normal cellular function3. For provided nutrients can be efficiently transported. example, cow luteal cells cultured in a threeRecent research has revealed that cells behave very dimensional bioreactor can maintain their normal differently when they are grown in three dimensions. function for a longer period than cells in twoThe third dimension provides another direction for dimensional T-flasks. The ability of cells to maintain the cell interactions, migration, and morphogenesis, their normal function is critical in the development which are all important in regulating cell cycle and of cell-based assays for drug screening. tissue function. Three-dimensional bioreactors are generally better Cells attach along the matrix fibers, form bridges than two-dimensional models in predicting drug spanning adjacent fibers, and aggregate in the void treatment efficacy. Colon cancer cells from threespace, all depending on the matrix porosity and dimensional cultures showed up to a 180-fold pore size, factors which affect cell proliferation, increase in drug resistance compared with cells
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cultured in two dimensions. A 1000-fold decrease in the cytotoxicity of gemcitabine was found in multilayer cultures of colon and ovarian cancer cells, while two-dimensional monolayer cultures erroneously predicted gemcitabine to be an effective proliferation inhibitor. Bioreactor models using twodimensional cultures for cytotoxicity analyses are inherently prone to error because of their lack of three-dimensional structural support for cell growth and tissue function. In addition, cell-based sensors using two-dimensional cultures are difficult to implement in online monitoring because of the low signal intensities that result from the small number of cells on two-dimensional surfaces. On the other hand, three-dimensional cultures of cells expressing green fluorescent protein have been developed for cell proliferation and cytotoxicity assays that can be used for high-throughput drug screening. Although the benefits of three-dimensional scaffolds in cell culture and tissue engineering are clear, directing cell function using biomaterials is challenging. Nanostructured polymers offering tunable ‘surface’ properties similar to the natural extracellular matrix can be incorporated into threedimensional scaffolds. The higher surface energy stemming from nanoscale surface roughness may positively affect cell adhesion, proliferation, and function. Therefore, three-dimensional scaffolds with nanoscale features offer great promise for enhancing the biological performance of cell cultures, but they are not yet fully exploited. REFERENCES 1. Yang, Y., et al., Biomaterials (2005) 26, 2585 2. Yang, S. T., et al., in Adv. Biochem. Eng./ Biotechnol., Zhong, J. J., (ed.), Springer, Berlin, Germany (2004), 87, 61 3. Ouyang, A., and Yang, S. T., Stem Cells (2006) doi: 10.1634/stemcells.2006-0322
A new dimension to biomaterials Moving from two to three dimensions for growing cells will improve toxicity assays, antibody production, and tissue engineering. Shang-Tian Yang and Robin Ng | The Ohio State University | [email protected] Mammalian cells usually need to adhere to a differentiation, gene expression, and tissue function. surface to grow. In the laboratory, tissue flasks and However, very little has been done to ‘quantify’ roller bottles providing two-dimensional surfaces the three-dimensional spatial effects on tissue are commonly used for simplicity. For industrialdevelopment. Now that developments in polymer scale processes, microcarriers can be used to microfabrication and soft lithography can produce provide larger surface areas for cell attachment tissue scaffolds with well-defined three-dimensional in suspension cultures, but this is limited by the structures1, we can study the effect of structure on difficulty in seeding the microcarriers with cells. As the organization of cells and their interactions, which an alternative, hollow-fiber membranes, polymer could lead to the optimal design of tissue scaffolds. gels and foams, and fibrous matrices are being Although current used for growing applications of threecells. These systems The benefits of threedimensional cultures have advantages dimensional scaffolds are mostly limited to because their threetissue engineering, dimensional structures are clear a three-dimensional mimic the in vivo cell perfusion culture system environment, which is can be used to produce recombinant proteins more of critical importance to cellular function and their efficiently for therapeutic purposes. For example, applications. the higher cell density in a three-dimensional cell Three-dimensional cultures provide high surface culture can produce more monoclonal antibody at areas on which cells can attach and arrange a higher titer2. It is not known, though, if protein themselves. For a similar reactor volume, a threeglycosylation differs in three-dimensional cultures dimensional culture can provide a surface area three from those in two dimensions. Three-dimensional orders of magnitude larger than the flat surface of perfusion cultures can also mass-produce cells, a two-dimensional culture. In three dimensions, such as pluripotent stem cells, without needing cells are not limited by contact inhibition and can subculturing or passaging of the cells to maintain reach a higher density closer to that found in tissues, their activity and normal cellular function3. For provided nutrients can be efficiently transported. example, cow luteal cells cultured in a threeRecent research has revealed that cells behave very dimensional bioreactor can maintain their normal differently when they are grown in three dimensions. function for a longer period than cells in twoThe third dimension provides another direction for dimensional T-flasks. The ability of cells to maintain the cell interactions, migration, and morphogenesis, their normal function is critical in the development which are all important in regulating cell cycle and of cell-based assays for drug screening. tissue function. Three-dimensional bioreactors are generally better Cells attach along the matrix fibers, form bridges than two-dimensional models in predicting drug spanning adjacent fibers, and aggregate in the void treatment efficacy. Colon cancer cells from threespace, all depending on the matrix porosity and dimensional cultures showed up to a 180-fold pore size, factors which affect cell proliferation, increase in drug resistance compared with cells
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MARCH 2007 | VOLUME 10 | NUMBER 3
cultured in two dimensions. A 1000-fold decrease in the cytotoxicity of gemcitabine was found in multilayer cultures of colon and ovarian cancer cells, while two-dimensional monolayer cultures erroneously predicted gemcitabine to be an effective proliferation inhibitor. Bioreactor models using twodimensional cultures for cytotoxicity analyses are inherently prone to error because of their lack of three-dimensional structural support for cell growth and tissue function. In addition, cell-based sensors using two-dimensional cultures are difficult to implement in online monitoring because of the low signal intensities that result from the small number of cells on two-dimensional surfaces. On the other hand, three-dimensional cultures of cells expressing green fluorescent protein have been developed for cell proliferation and cytotoxicity assays that can be used for high-throughput drug screening. Although the benefits of three-dimensional scaffolds in cell culture and tissue engineering are clear, directing cell function using biomaterials is challenging. Nanostructured polymers offering tunable ‘surface’ properties similar to the natural extracellular matrix can be incorporated into threedimensional scaffolds. The higher surface energy stemming from nanoscale surface roughness may positively affect cell adhesion, proliferation, and function. Therefore, three-dimensional scaffolds with nanoscale features offer great promise for enhancing the biological performance of cell cultures, but they are not yet fully exploited. REFERENCES 1. Yang, Y., et al., Biomaterials (2005) 26, 2585 2. Yang, S. T., et al., in Adv. Biochem. Eng./ Biotechnol., Zhong, J. J., (ed.), Springer, Berlin, Germany (2004), 87, 61 3. Ouyang, A., and Yang, S. T., Stem Cells (2006) doi: 10.1634/stemcells.2006-0322