Harvard scientists have made tiny sculptures from cells, pieced together one by one like building bricks. This technique is not simply an exercise in micro-aesthetics: it could one day be used to make biological sensors, or even replacement organs.
George Whitesides and his co-workers used delicate tools based on laser beams, called optical tweezers, for their feats of 'cell construction'. They stuck polystyrene beads to the surface of red blood cells ('erythrocytes') and moved them into position, as they describe in the journal
Optical tweezers were devised in the late 1980s. Although seemingly insubstantial, light can exert a force on matter because of electrical interactions. This force is exploited in an optical trap, where two or more intersecting laser beams set up such a strong 'light field' at their crossing point that small objects within this field are immobilized. The trap becomes a set of tweezers when the beams are moved, dragging the object with it.
To move a cell or single molecule in this way entails attaching a larger 'handle' to it, such as a microscopic plastic bead, which is more readily trapped. If the bead is pulled, the molecule or cell comes with it.
Red blood cells have molecules dangling from their surfaces, from which Whitesides' group hooked beads. They next fixed a protein extracted from wheat germ to the beads that recognizes and latches onto the cell-surface molecules of erythrocytes and other cells.
Thus, each bead could stick to one or more cells, allowing the researchers to build a variety of simple arrangements, such as chains and rings. Once assembled, these were so strongly bound that they could not be pulled apart again using the tweezers.
But the researchers could dissolve their sculptures by adding free proteins that bound to the wheat-germ protein. These displaced the cells from their points of attachment to the polystyrene beads.
The researchers did not stop at simple 'flat' assemblies, but also made three-dimensional objects from the disk-shaped erythrocytes, such as a pryramid-like tetrahedron and a chain-shaped structure.
The method is not limited to cells. In principle, any small object could be manipulated in this way, and joined up with appropriate molecular linkers. But there are good reasons to move and assemble cells one by one in this manner.
The resulting structures could function as miniature biological sensing devices, for example, and could be used to develop a new understanding of how cells assemble into real tissues. It might even be possible to make new tissues and organs this way, automating the linking together of different cell types one at a time in precisely the order and the positions necessary.