Nano-Robotics in Medicine

Nano-Robots:

While much speculation has been published on possible far-future applications of nanotechnology using advanced materials and manufacturing techniques, relatively little has been published on applying existing engineering technology to the problems in order to create a solution that can be incrementally improved as the technology becomes available. In this paper, we will describe a mobile robot that can be created with existing technology, that can be used to seek out and destroy inimical tissue within the human body that cannot be accessed by other means.

The construction and use of such devices would result in a number of benefits. Not only would it provide either cures or at least a means of controlling or reducing the effects of a number of ailments, but it will also provide valuable empirical data for the improvement and further development of such machines. Practical data garnered from such operations at the microscopic level will allow the elimination of a number of false trails and point the way to more effective methods of dealing with the problems inherent in operation at that level.

We will address and propose solutions to problems such as size, method of entry into the body, means of propulsion, means of maintaining a fixed position while operating, control of the device, power source, means of locating substances to be eliminated, mans of doing the elimination and how to remove the device from the body afterward. During the course of this we will also discuss the appropriate manufacturing techniques for the construction of the device.

The preliminary design is intended for the following specific applications:

Tumors: We must be able to treat tumors; that is to say, cells grouped in a clumped mass. While the technique may eventually be used to treat small numbers of cells in the bloodstream, this is normally done effectively by white blood cells and antibodies, and this technique is not intended to replace that. The specified goal is to be able to destroy tumorous tissue in such a way as to minimize the risk of causing or allowing a recurrence of the growth in the body. The technique is intended to be able to treat tumors that cannot be accessed via conventional surgery, such as deep brain tumors. However, since the technique is extremely effective and much less debilitating than conventional surgery, it should be used, if possible, as a replacement for conventional surgery in this application.

Arteriosclerosis: This is caused by fatty deposits on the walls of arteries. The device should be able to remove these deposits from the artery walls. This will allow for both improving the flexibility of the walls of the arteries and improving the blood flow through them. In view of the years it takes to accumulate these deposits, simply removing them from the artery walls and leaving them in the bloodstream should allow the body’s natural processes to remove the overwhelming preponderance of material.

Blood clots: The cause damage when they travel to the bloodstream to a point where they can block the flow of blood to a vital area of the body. This can result in damage to vital organs in very short order. In many if not most cases, these blood clots are only detected when they cause a blockage and damage the organ in question, often but not always the brain. By using a micro robot in the body to break up such clots into smaller pieces before they have a chance to break free and move on their own, the chances of ensuing damage are reduced greatly.

We must consider the following factors when designing our microrobot.

• How do we introduce the device into the body?
• How do we move the device around the body?
• How do we know where the device should go?
• How do we control the device?
• How is the device powered?
• What does the device do when it gets there?
• How do we remove the device when its job is done?

How do we introduce the device into the body?

The first is that the size of the nano-machine determines the minimum size of the blood vessel that it can traverse. Not only do we want to avoid damaging the walls of whatever blood vessel the device is in, we also do not want to block it too much, which would either cause a clot to form, or just slow or stop the blood flow, precipitating the problem we want to cure in the first place. What this means, of course, is that the smaller the nano-machine the better. However, this must be balanced against the fact that the larger the nano-machine the more versatile and effective it can be. This is especially important in light of the fact that external control problems become much more difficult if we are trying to use multiple machines, even if they don't get in each other's way.

The second consideration is an even simpler one; we have to get it into the body without being too destructive in the first place. This requires that we gain access to a large diameter artery that can be traversed easily to gain access to most areas of the body in minimal time. The obvious candidate is the femoral artery in the leg. This is in fact the normal access point to the circulatory system for operations that require access to the bloodstream for catheters, dye injections, etc., so it will suit our purposes nicely.

How do we move the device around the body?

 We start with a basic assumption: we will use the circulatory system to allow our device to move about. We must then consider two possibilities: should it be carried to the site of operations, or should it be propelled? We will start by dismissing the idea of using a probe, catheter or umbilicus to move the device around since this would be very difficult to make versatile enough.

The first possibility is to allow the device to be carried to the site of operations by means of normal blood flow. There are a number of requirements for this method to be practical. We must be able to navigate the bloodstream; to be able to guide the device so as to make use of the blood flow. This also requires that there be an uninterrupted blood flow to the site of operations. In the case of tumors, there is very often damage to the circulatory system that would prevent our device from passively navigating to the site. In the case of blood clots, of course, the flow of blood is dammed and thus our device would not be carried to the site without the capability for active movement. Another problem with this method is that it would be difficult to remain at the site without some means of maintaining position, either by means of an anchoring technique, or by actively moving against the current. While the above objections do not eliminate any possibility of ! using this technique, they do point out the need for at least a supplementary means of locomotion.

There are a number of means available for active propulsion of our device.

Propeller:

The very first Feynman prize in Nanotechnology was awarded to William McLellan for building an electric motor that fit within a cube 1/64th of an inch on a side. This is probably smaller than we would need for our preliminary microrobot. One or several of these motors could be used to power propellers that would push (or pull) the microrobot through the bloodstream. We would want to use a shrouded blade design so as to avoid damage to the surrounding tissues (and to the propellers) during the inevitable collisions

Cilia/flagellae:

In this scenario, we are using some sort of vibrating cilia (similar to those of a paramecium) to propel the device. A variation of this method would be to use a fin-shaped appendage. While this may have its attractions at the molecular level of operation, an electric motor/propeller combination would be more practical at the scale we are talking about.

Electromagnetic pump:

This is a device with no moving parts that takes conductive fluid in at the front end and propels it out the back, in a manner similar to a ramjet, although with no minimum speed. It uses magnetic fields to do this. It would require high field strengths, which would be practical with high capacity conductors. At the scale we are talking about, room (or body) temperature ceramic superconductors are practical, making this a possibility.

Jet Pump:

In this scenario, we use a pump (with moving parts) to propel blood plasma in one direction, imparting thrust in the opposite direction. This can either be done with mechanical pumps, or by means of steam propulsion, using jets of vaporized water/blood plasma.

Membrane propulsion:

A rapidly vibrating membrane can be used to provide thrust, as follows: Imagine a concave membrane sealing off a vacuum chamber, immersed in a fluid under pressure, that is suddenly tightened. This would have the effect of pushing some of the fluid away from the membrane, producing thrust in the direction toward the membrane. The membrane would then be relaxed, causing the pressure of the fluid to push it concave again. This pressure would impart no momentum to the device, since it is balanced by the pressure on the other side of the device. At the macro scale, this thrust is not significant, but at the micro scale it is a practical means of propulsion.

Crawl along surface:

Rather than have the device float in the blood, or in various fluids, the device could move along the walls of the circulatory system by means of appendages with specially designed tips, allowing for a firm grip without excessive damage to the tissue. It must be able to do this despite surges in the flow of blood caused by the beating of the heart, and do it without tearing through a blood vessel or constantly being torn free and swept away.

For any of these techniques to be practical, they must each meet certain requirements:

• The device must be able to move at a practical speed against the flow of blood.
• The device must be able to move when blood is pooling rather than flowing steadily.
• The device must be able to move in surges, so as to be able to get through the heart without being stuck, in the case of emergencies.
• The device must either be able to react to changes in blood flow rate so as to maintain position, or somehow anchor itself to the body so as to remain unmoving while operating.
• The device must be able to change direction laterally, so as to navigate the bloodstream.

From consideration of the above requirements, we can see that the most practical solution at present is one or more electric motors turning propellers. This solution is simple, well understood, and the technology has existed since 1960. The manufacturing techniques are relatively easy, as are methods for integrating it with the rest of the microrobot.