

Sometime around the turn into the 20th century, medical extraordinaire Paul Ehrlich coined the word zauberkugel or “magic bullet” to describe new drugs he was working on to cure syphilis and cancer. In theory, such drugs would leave healthy tissue intact while targeting only the diseased. Psychologists later appropriated this term to describe the phenomenally widespread panic that ensued when H. G. Well’s epic 1938 thriller The War of the Worlds was broadcast to an unsuspecting American public.

Incidentally, these psychologists also liked to refer to their magic bullet theory as the “hypodermic syringe model,” reflecting the media’s new found ability to inject a radical concept directly into the minds of a captive audience with pinpoint accuracy. While neither of the highly idealized magic bullets we just alluded to may be entirely realistic conceptions, a real magic bullet that can be guided at will throughout the interior of the body has recently become a credible concept. We are not referring to a deadly kind of bullet that often follows a highly contorted trajectory. Instead, we are talking about a device fired by a much more controllable kind of gun — namely, something called a Gauss gun, after the famous mathematician of the same name.

Similar to a coil or rail gun, a Gauss gun linearly accelerates an object using electromagnetic fields. It can also be configured to store potential energy in the positions of objects inside its bore, and then amplify the kinetic energy of an incoming particle by converting that potential energy into a larger kinetic energy added to an exiting object. A simple video probably illustrates the basic principle much more clearly:

Researchers Aaron Becker, Ouajdi Felfoul, and Pierre Dupont have built a proof-of-principle Gauss gun that could propel a tiny device they call a millibot throughout the body. While several researchers have already hot-rodded standard MRI machines or built dedicated ‘octomags’ to magnetically steer catheters and other devices throughout the liquid compartments of the body, this is the first indication that forces high enough to traverse solid barriers in the body could be attained magnetically.


The main trick behind the scheme is to preload the chamber of a hypodermic needle with a series of magnetizable steel balls and spacers. The beauty of using alloy steel balls instead of your typical high-strength neodymium magnets is twofold: Neodymium permanent magnets have a lower magnetic saturation (at only 77% that of steel they can only produce 43% of the equivalent magnetic force of steel), and their magnetism cannot be turned off. On the other hand, with steel electromagnets, the force goes away when you turn off the electromagnet. If you were to introduce permanent magnets into the body, by eating them for example, your bowels would quickly be cinched together and the magnets would inexorably bore themselves directly through tissue in mutual attraction.

The key insight made by the researchers is that the coils of an MRI machine could be used to implement a Gauss gun. There are different ways to configure and run an MRI, but for our purposes here we can think of it as an electric motor. The MR scanner acts as the stator and generates propulsive torques on an actuator rotor containing the ferroelectric material. To generate maximum torque, the smaller gradient coils (as opposed to the large static field of the MRI) need to be put under closed loop control, or in motor lingo — commutation. If done properly, it should even be possible to control more than one object inside the bore.

The Gauss gun would enable tiny devices to breach barriers between fluid chambers or even go through solid tissue itself. This capability would be critical for accessing remote corners of the ventricular system of the brain, for example, and would have immediate applications in conditions like hydrocephalus, where proper flow through these chambers is disrupted. The beauty of the Gauss gun is that the MRI magnets do everything — position the components, charge them, and fire them. After the ballistic component of a millibot surgical procedure is done, control of the millibot would theoretically transfer over to standard low power MRI navigation.

Lest the reader think that this is all just pie in the sky, we should give some hard numbers. The authors note that the maximum gradient available in most clinical scanners is in around 20-40mT/m. This would produce a force on a magnetized steel particle equal to 36-71% of its gravitational force. In other words, not a whole lot of force to work with. Custom high-strength gradient coils up to a 400mT/m coil have been tried, but they are not practical retrofits to most MRI machines. For a general comparison, pushing a 18-gauge needle through 10mm muscle requires about 0.6N of force.

We asked corresponding author Pierre Dupont directly what the Gauss gun could put out. He said that they have already demonstrated up to 15mm penetration depth into a brain tissue phantom using an 18gauge needle. We should note that real brains are basically lipid and cytoskeletal protein composites that should be expected to behave nonlinearly with regards to impacts. In other words, like a pool surface, impact speed should greatly affect the material stiffness that is felt by a penetrating object.

The main picture at the top shows a commercial magnetic navigation system already used for advanced cardiac procedures. The operators don’t even sit in the surgical amphitheater, but rather run the show from a separate control room. This device, called the Niobe Remote Magnetic Navigation System, steers a catheter through the vasculature by bending it at various control points that react to the magnetic field. While already making itself indispensable in the OR, when devices like the Niobe eventually add beefy Gauss gun style attachments, remote robotic surgery will have entered a new era.

Some time ago, we discussed some of the finer points of installing and manipulating neural hardware in the ventricular system of the brain. Of the 1700ml or so available space in our skull, 1400ml of that is the brain itself, 150ml is for the blood, and 150ml for the cerebrospinal fluid (CSF) in which the brain floats. An additional 30ml of CSF circulates inside a network of chambers in the center of the brain known as the ventricular system. That’s a fairly roomy working environment. The fine membranes that separate these spaces are precisely the targets a Gauss gun could work on. Of note, we would offer that one of the key procedures would be making or stitching passageways between the brain and the larger immune and lymphatic systems of the body.

We won’t say much more here other than to mention that just a week ago, hardly anyone would have imagined the central nervous system had any classical lymphatic system, to speak of. Now everyone wants to know how to control and access it to ensure the continued health and power of the brain.