Schematic illustration of a Stewart platform. The lower (blue) triangle forms the base, while the upper (green) triangle froms the platform.
The position and orientation of the platform with respect to the base can be controlled in six degrees of freedom (X, Y, Z, rool, pitch, and yaw) by adjusting the lengths of the six gray struts.
Today's SPMs are large, relatively slow, and will never make mole quantities of product. If we really want positional assembly to make products in the volume that ribosomes make proteins, we must have small, fast positional devices7. Yet it seems unlikely that biotechnology will directly give us a molecular robotic arm.
Which brings us to the Stewart platform8,9,10. This device, basically an octahedron six of whose struts can be lengthened or shortened under programmatic control (see illustration), provides six degree of freedom positional control for the "platform," (the green triangle at the top of the octahedron) with respect to its base (the blue triangle at the bottom of the octahedron). The ability to make an octahedron does not seem beyond the capabilities of biotechnology (in the broad sense of the term), particularly when the ability to self assemble a truncated octahedron has already been demonstrated by Seeman2.
All we need are twelve stiff struts, some way to make their ends stick together, and some way to lengthen or shorten six of those struts. As the latter seems the harder problem, we discuss one possible approach to solving it.
Consider a single strut: how can we change its length? One way would be to use two struts that overlap, and then make them slide past each other in a controlled fashion. Suppose that the first strut is made of three repeat units, ABCABCABCABCABC...., while the second strut is also made of three repeat units, XYZXYZXYZXYZ.... If we want to combine these two struts into one long strut, then we have to join them together. Suppose we use "joiners" that have two ends: one end binds to the A units of the first strut, while the other end binds to the X units of the second strut. Then, as illustrated above, the two struts would be held together by this a-x joiner to form a single longer strut.
But how can we change the position of the ABC strut with respect to the XYZ strut? First, we add a c-y joiner. These new joiners will bridge between the C and Y units of the ABC and XYZ struts. They will at first be strained, but as we add more and more c-y joiners they will start to balance out the a-x joiners. If we now wash the a-x joiners out of solution (the simplest arrangement would be to anchor the octahedron in place by a tether, and flow a solution with an appropriate concentration of joiners past them), then the c-y joiners will dominate the linkage between the two struts leading to the results in the final illustration, below. At this point, the ABC and XYZ struts have moved past each other by one monomer.
If we repeat the whole process again, this time washing in a b-z joiner and washing out the c-y joiner, we can again move the two struts over by one monomer. Finally, if we wash in an a-x joiner and wash out the b-z joiner, we are back where we started. By repeating the whole cycle, we can move the ABC strut past the XYZ strut as much as we want. By running the cycle in reverse, we can reverse the motion. In essence, we have a three-phase linear motor. While this is slow (it's limited by the speed at which we can wash the joiners into and out of position) it does provide a flexible means of controlling the length of the strut, and does not seem hopelessly difficult.
The essential point here is not that this particular approach is the right one or even necessarily a good one, but that biotechnology and self assembly can be used to make positional devices. This is just one possible way: there are a great many more.
The position and orientation of the platform with respect to the base can be controlled in six degrees of freedom (X, Y, Z, rool, pitch, and yaw) by adjusting the lengths of the six gray struts.
Today's SPMs are large, relatively slow, and will never make mole quantities of product. If we really want positional assembly to make products in the volume that ribosomes make proteins, we must have small, fast positional devices7. Yet it seems unlikely that biotechnology will directly give us a molecular robotic arm.
Which brings us to the Stewart platform8,9,10. This device, basically an octahedron six of whose struts can be lengthened or shortened under programmatic control (see illustration), provides six degree of freedom positional control for the "platform," (the green triangle at the top of the octahedron) with respect to its base (the blue triangle at the bottom of the octahedron). The ability to make an octahedron does not seem beyond the capabilities of biotechnology (in the broad sense of the term), particularly when the ability to self assemble a truncated octahedron has already been demonstrated by Seeman2.
All we need are twelve stiff struts, some way to make their ends stick together, and some way to lengthen or shorten six of those struts. As the latter seems the harder problem, we discuss one possible approach to solving it.
Consider a single strut: how can we change its length? One way would be to use two struts that overlap, and then make them slide past each other in a controlled fashion. Suppose that the first strut is made of three repeat units, ABCABCABCABCABC...., while the second strut is also made of three repeat units, XYZXYZXYZXYZ.... If we want to combine these two struts into one long strut, then we have to join them together. Suppose we use "joiners" that have two ends: one end binds to the A units of the first strut, while the other end binds to the X units of the second strut. Then, as illustrated above, the two struts would be held together by this a-x joiner to form a single longer strut.
But how can we change the position of the ABC strut with respect to the XYZ strut? First, we add a c-y joiner. These new joiners will bridge between the C and Y units of the ABC and XYZ struts. They will at first be strained, but as we add more and more c-y joiners they will start to balance out the a-x joiners. If we now wash the a-x joiners out of solution (the simplest arrangement would be to anchor the octahedron in place by a tether, and flow a solution with an appropriate concentration of joiners past them), then the c-y joiners will dominate the linkage between the two struts leading to the results in the final illustration, below. At this point, the ABC and XYZ struts have moved past each other by one monomer.
If we repeat the whole process again, this time washing in a b-z joiner and washing out the c-y joiner, we can again move the two struts over by one monomer. Finally, if we wash in an a-x joiner and wash out the b-z joiner, we are back where we started. By repeating the whole cycle, we can move the ABC strut past the XYZ strut as much as we want. By running the cycle in reverse, we can reverse the motion. In essence, we have a three-phase linear motor. While this is slow (it's limited by the speed at which we can wash the joiners into and out of position) it does provide a flexible means of controlling the length of the strut, and does not seem hopelessly difficult.
The essential point here is not that this particular approach is the right one or even necessarily a good one, but that biotechnology and self assembly can be used to make positional devices. This is just one possible way: there are a great many more.