Sawed-off stepper motor.

Tyler's new shunt has a fancy valve. It prevents the flow of fluid below a set pressure level. Too much pressure is bad, too little is almost worse. If we were to let all the fluid out of Tyler's ventricles at once the ventricle wall would tear away from the brain and cause bleeding. There is also a link between inter-cranial pressure and skull-growth. Too little pressure and the skull won't grow properly.

Tyler's shunt is programmable. Doctors can adjust the pressure setting without surgery. So as he grows we are less likely to need to go in surgically to adjust the flow of fluid. One down-side of this value is that it is more likely to clog and require surgery to fix. We described how a shunt works here.

This is the device used to program the valve. The system is pretty ingenious. It works just like a stepper-motor cut in half. Most motors just spin when you apply power, stepper-motors only turn one "step" every time you send power to it. You'll find a stepper motor anywhere that exact control of speed or position is required. Printers, scanners, disk-drives, and robots all use steppers. All motors have 2 main parts the rotor and the stator. The rotor is the bit the turns doing the work and the stator is the stationary part. Usually the rotor fits inside the stator. In this case the rotor is now inside Tyler's head, the stator is in the programmer. The rotor has a series of alternating magnets on it. By alternating the magnetic field around it we can turn it in a very precise way.
The Arrow is the valve in the shunt and the 4 magnets outside make up the programming device.
One thing we now need to be careful of with Tyler is magnetic fields. Some studies have shown that a refrigerator magnet can lower the setting (just as bad as raising it). Some rumors I've seen indicate that some cell-phones, old-school telephones, microwaves, speakers etc can change the setting. This may get interesting.