ATTEMPTS to grow artificial organs in the lab have so far been thwarted, largely because building complex structures out of the necessary variety of cells is so difficult. But adapting techniques from the silicon chip industry promise to make the process much easier.
Until now, researchers have only managed to build relatively simple tissues such as cartilage and skin in the lab. To make these, they start with a biodegradable polymer 鈥渟caffold鈥� roughly the same dimensions as the tissue they want, and plunge it into a solution of cells. The cells work their way into the scaffold, which is then immersed in a nutrient-rich broth. As the cells grow and multiply, the scaffold breaks down, leaving an appropriately shaped clump of cells ready for transfer to the patient.
But this approach fails for complex organs, says Sangeeta Bhatia, a tissue engineer at the University of California in San Diego. A liver, for example, contains several different types of cell, arranged in a very specific way. 鈥淚t鈥檚 going to be very difficult to achieve that just by building a complicated plastic architecture and seeding cells onto it,鈥� she says.
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Instead, Bhatia and her colleague Valerie Liu developed an alternative technique for building complex structures from different types of cells. They start by mixing a batch of one cell type with a liquid polymer called polyethylene glycol diacrylate. Then they squirt the cell-laden solution onto a Teflon tray where it forms a thin film.
Next, the polymer film is covered with a glass sheet before a template is placed on top. Like 鈥渕asks鈥� used in the microchip industry, the template has an opaque pattern on it. When UV light is shone onto the glass, it only falls on certain regions while others stay covered.
The liquid polymer contains an additive that makes its molecules link together when exposed to UV. This makes exposed regions of polymer become stiffer, finally taking on the feel of a soft, flexible plastic. But the shaded areas remain liquid and can be washed away to leave just the solidified cell-laden pattern.
By using different templates and adding layers on top of each other, Bhatia and Liu can build up complex 3D structures containing well-defined regions of different cells. For example, they could make layers of cartilage on top of layers of bone cells.
So far, they have created interconnecting strands of polymers filled with different types of cells. To demonstrate the technique鈥檚 versatility, they also made squares, triangles and star shapes (Biomedical Microdevices vol 4, p 257). 鈥淚t鈥檚 a tool for [building] any kind of tissue,鈥� says Bhatia. Because the polymer is biocompatible and permeable, the cells should function healthily within the structure.
Bhatia has already shown that liver cells survive being mixed with the liquid polymer and exposed to UV, but cells from other organs might not be so hardy. 鈥淗ow well cells tolerate photo-polymerisation will vary with cell types,鈥� she says.
