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This paper will describe a Nano/micro scale medical robot that is within the range of current Nanotechnology. It is intended for the treatment and/or elimination of medical problems where accumulation of undesired organic substances interferes with normal bodily function, such as,
Blood clots leading to stroke
Accumulation of scar tissue
Localized pockets of infection
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 contrary tissue within the human body that cannot be accessed by other means. 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, means 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.
Nanotechnology is the study, design, creation, synthesis, manipulation, and application of functional materials, devices, and systems through control of matter at the nanometer scale 1-100nanometers, one nanometer being equal to 10-9 of a meter that is, at the atomic and molecular levels, and the exploitation of novel phenomena and properties of matter at that scale.
A Nanorobot is a tiny machine designed to perform a specific task or tasks repeatedly and with precision using Nanotechnology. Nanorobots have potential applications in the assembly and maintenance of sophisticated systems. Nanorobots might function at the atomic or molecular level to build devices, machines, or circuits, a process known as molecular manufacturing. Nanorobots might also produce copies of themselves to replace worn-out units, a process called self-replication.
Nanorobots are of special interest to researchers in the medical industry. This has given rise to the field of nanomedicine. This paper will deal with the problems involved in designing and building a microscale robot that can be introduced into the body to perform various medical activities. The preliminary design is intended for the specific applications like tumors, Arteriosclerosis Blood clots leading to stroke, Accumulation of scar tissue and Localized pockets of infection. The proposed robot will eliminate the damaged cells and tissues from the body and hence cure these diseases completely.
LOADING THE DEVICE INTO BODY
We need to find a way of introducing the nanomachine into the body, and allowing it access to the operations site without causing too much ancillary damage. Gaining access via the circulatory system leaves us with a number of considerations.
The first is that the size of the nanomachine 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. That is the smaller the nanomachine is the better. However, this must be balanced against the fact that the larger the nanomachine 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.
MOVING THE DEVICE INSIDE 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? The idea of using a probe, catheter or umbilicus to move the device around is 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
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.
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.
The other considerations proposed are jet pump, Cilia/flagellae and membrane propulsion which are not discussed here.
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.
NAVIGATING THE DEVICE
The next problem to consider is exactly how to detect the problem tissue that must be treated. We must be able to locate a tumor, blood clot or deposit of arterial plaque closely enough so that the use of short-range sensors is practical. These would be used during actual operations, to allow the device to distinguish between healthy and unwanted tissue. Another important use for sensors is to be able to locate the position of the microrobot in the body. This is particularly true in the initial scenario, where we will only have one device in the body at a given time. Without any way of determining location from internal references, we need to be able to track the device by external means.
First we will examine the various possibilities for external sensors. These will be at least partially external to the microrobot, and their major purpose will be twofold. The first is to determine the location of the operations site; that is, the location of the clot, tumor or whatever is the unwanted tissue. The second purpose is to gain a rough idea of where the microrobot is in relation to that tissue. This information will be used to navigate close enough to the operations site that short range sensors will be useful.
Ultrasonic: This technique can be used in either the active or the passive mode. In the active mode, an ultrasonic signal is beamed into the body, and either reflected back, received on the other side of the body, or a combination of both. The received signal is processed to obtain information about the material through which it has passed. This method is, of course, greatly similar to those used in conventional ultrasound techniques, although they can be enhanced greatly over the current state of the art.
In the passive mode, an ultrasonic signal of a very specific pattern is generated by the microrobot. By means of signal processing techniques, this signal can be tracked with great accuracy through the body, giving the precise location of the microrobot at any time. The signal can either be continuous or pulsed to save power, with the pulse rate increasing or being switched to continuous if necessary for more detailed position information.
In the passive mode, the ultrasonic signal would be generated by means of a signal applied to a piezoelectric membrane, a technology that has been well developed for at least a decade. This will allow us to generate ultrasonic signals of relatively high amplitude and great complexity.
Again, these techniques (really all the same technique) can be used in both passive and active modes.
The passive mode for the techniques depends on the various tissues in the body generating signals that can be detected and interpreted by external sensors. While the body does generate some very low frequency radio waves, the wavelength is so large that they are essentially useless for any sort of diagnostic purposes of the type we are interested in. The same is true of microwaves. Recent developments, however, in the technology of infrared detection, offer great promise in potentially improving our ability to detect tumors by the increased heat they generate as a result of their increased metabolic state. This technology, however, is in its infancy. We do not know enough about how different cells in the body generate heat to be able to say how useful the technique would be.
In the active mode, a signal is generated from outside the body and is treated the same way that ultrasonics or x-rays are; it is allowed to reflect from/pass through tissues and the result interpreted. However, only infrared has a short enough wavelength to be able to provide the required image resolution for accurate and detailed navigation, and a great deal of image processing would be required to filter out the natural background signal from the body.
In order to use the technique to track the microrobot, a signal would need to be generated by the microrobot, detected outside the body, and interpreted to obtain position information. This is only practical for infrared or higher frequencies could be useful to obtain sufficiently accurate positional information. Recent advances in infrared sensing technology make this more attractive than might otherwise be the case.
And other ways for navigating our robot are using radioactive dye, NMR/MRI (involves the application of a powerful magnetic field to the body) and X-rays. Due to their many drawbacks these techniques are not discussed in this paper.
From the above discussion, we can come to the conclusion that there are two possible choices for our tracking system. We can either generate an ultrasonic signal and track that, or generate enough infrared or heat within the structure of our microrobot and track that. Of the two, the infrared technique is more practical, since there is far less problem of reflections and multi-path problems with infrared than with ultrasonic.
CONTROLLING THE DEVICE
Next, we consider the case of internal sensors. When we say internal sensors, we mean sensors that are an integral part of the microrobot and are used by it to make the final approach to the operation site and analyze the results of its operations. These sensors will be of two types. The first type will be used to do the final navigation. When the device is within a short distance of the operation site, these sensors will be used to help it find the rest of the path, beyond what the external sensors can do. The second type of sensor will be used during the actual operation, to guide the microrobot to the tissue that should be removed and away from tissue that should not be removed.
Chemical: Chemical sensors can be used to detect trace chemicals in the bloodstream and use the relative concentrations of those chemicals to determine the path to take to reach the unwanted tissue. This would require several sensors so as to be able to establish a chemical gradient, and, for the same reason, would require a certain degree of physical separation between sensors. While this is not a strict requirement, the alternative would be to try every path, and retrace a path when the blood chemicals diminish. While it is not difficult to create a solid state sensor for a given chemical, the difficulty increases greatly when the number of chemicals that must be analyzed increases. Consequently, we would probably need a series of microrobots, one for each chemical, or at least a set of replaceable sensor modules. An alternative to solid state sensors is for chemical analysis is described next.
Spectroscopic: This would involve taking continuous small samples of the surrounding tissue and analyzing them for the appropriate chemicals. This could be done either with a high-powered laser diode or by means of an electrical arc to vaporize small amounts of tissue. The laser diode is more practical due to the difficulty of striking an arc in a liquid medium and also due to the side effects possible when sampling near nerve tissue. The diode could be pulsed at regular intervals, with an internal capacitor charging constantly so as to provide more power to the laser diode than the steady output of our power source.
The other devices proposed are TV camera, UHF sonar for resolution texture (involves analyzing the return from an ultrasonic beam) which are not discussed here due to their most disadvantages.
From the above it can be seen that the best choice for short-range sensors is the spectroscopic technique, for the following reasons: The equipment required is all solid state with no moving parts. While there is a certain power requirement, this can be met by using capacitors to store energy over a period of time and discharge it quickly. Another advantage of this technique is that simply by adding power to the diode beam we are destroying the unwanted tissue, thus combining the sensory and treatment requirements into the same equipment. Samples of the blood plasma can be tested inside a closed chamber, which would give us the ability to do a chemical analysis that could detect a wide range of compounds rather than just one or two. Simply by doing chemical tracking with the sampling door closed, and cell analysis with the door open, we can combine both short range sensor requirements using one sensor.
MEANS OF TREATEMENT
The treatment for each of the medical problems indicated above is the same in general; we must remove the tissue or substance in question from the body. This can be done in one of several ways. We can break up the clump of substance and rely on the body's normal processes to eliminate it. Alternately, we can destroy the substance before allowing the body to eliminate the results. We can use the microrobot to physically remove the unwanted tissue. We can also use the microrobot to enhance other efforts being performed, and increase their effectiveness.
Physical removal: This method can be effective in the treatment of arteriosclerosis. In this case, a blade, probe or edge of some sort can be used to physically separate deposits of plaque from the artery walls. The bloodstream would carry these deposits away, to be eliminated by the normal mechanisms of the body. Since it takes years before the plaque buildup reaches dangerous levels, the small amount not scavenged by the body can be regarded as not immediately significant.
In the case of blood clots, the situation is not so simple. In this case, it is possible that the action of physically attacking the clot could cause it to break away in large chunks, some of which could subsequently cause blockages in the blood flow. If we are going to do this, we need some means of preventing this from happening. We can set up some mechanism to catch these blood clots and further break them up, or we can try to tap into the circulatory system downstream of the clot and filter out the pieces. It behooves us to work out a technique that will crush the blood clot into pieces too small to a danger.
In the case of tumors, the problem is more serious. The act of physically shredding or even just breaking loose clumps of cells can result in the cancer metastasizing throughout the body. Since the mechanism of cancer spreading is unknown, this is a real danger. One possible solution is, as in the case above, to filter the cancerous cells out of the blood immediately downstream of the tumor. Even if it is possible to distinguish cancerous cells from normal cells by filtering, this would not prevent the spread of tumor causing chemicals released by the ruptured cells. In this case, something more drastic is indicated. Again, use of the sampling box described above might be useful, since it does destroy whatever is placed within it. Unfortunately, the act of removing cancerous cells to place them in the box could be dangerous. This leads us to the next alternative.
Physical trauma: Another way of dealing with the unwanted tissues is by destroying them in situ. This would avoid damaging the cancerous cells and releasing chemicals into the bloodstream. In order to do this effectively, we need a means of destroying the cell without rupturing the cell wall until after it is safe. We shall consider a number of methods:
Resonant microwaves/Ultrasonics: Rather than merely apply microwave/infrared or ultrasonic energy at random frequencies, the frequency of the energy could be applied at the specific frequencies needed to disrupt specific chemical bonds. This would allow us to make sure that the tumor producing chemicals created by cancerous cells would be largely destroyed, with the remaining amounts, if any, disposed of by the body's natural defenses.
Microwave: This is a popular method used in diathermy and other techniques. Microwave radiation is directed at the cancerous cells, raising their temperature for a period of time, causing the death of the cells in question. This is normally done by raising the temperature of the cells to just enough above body temperature to kill them after many minutes of exposure. In our case, this would require a means of generating a strong enough microwave signal in a package that is, frankly, pretty small.
The other ways available for the treatment are laser, ultrasonic, chemical and heat which are not discussed here due to their unimportance.
From the above we can see that there is no one best way of treating the unwanted tissue, since the method of treatment is different for each case. Rather than design a microrobot capable of all techniques, we will design a microrobot that can have any of several "treatment modules" installed on it, allowing the same basic design to be used.
One major requirement for our microrobot is, of course, power. We have to be able to get sufficient power to the microrobot to allow it to perform all of its required operations. There are two possible paths we can take for this. The first is to obtain the power from a source within the body, either by having a self-contained power supply, or by getting power from the bloodstream. The second possibility is to have power supplied from a source external to the body.
Source within the body: There are a number of possible mechanisms for this scenario. The basic idea is that the microrobot would carry its power supply within itself. It would need enough power to move to the site of the operation, perform its functions, which might be very power intensive, and then exit the body. There are three basic scenarios for on-board power supplies.
Power from the bloodstream: There are three possibilities for this scenario. In the first case, the microrobot would have electrodes mounted on its outer casing that would combine with the electrolytes in the blood to form a battery. This would result in a low voltage, but it would last until the electrodes were used up. The disadvantage of this method is that in the case of a clot or arteriosclerosis, there might not be enough blood flow to sustain the required power levels. Also, if the electrodes were ever embedded in anything that blocked their access to the blood, power would drop to zero and stay there. This means that a backup would be required.
The second way to get power from the bloodstream is by means of a fuel cell, or simply by burning blood chemicals. This is similar to a battery except that rather than obtain power from current flow between electrodes, we would obtain power by causing chemical reactions to take place at a controlled rate and obtaining power from this. This is much the same way that the body gets its own power by consuming fuel chemicals from the bloodstream. This has the same problem as the electrode method; it will stop working if access to the blood is blocked, or if the chemicals are not replenished.
Carrying the energy required directly onboard: The third method is simply to carry the full amount of energy required directly onboard. The first case is one in which we use conventional chemical batteries. Unfortunately, the power to weight ratio of chemical batteries is extremely low, and a battery of such small size would be of limited use.
The second method is to use high-voltage capacitors to store a charge and use it gradually. As capacitor technology improves this may become practical, but at the moment the power to weight ratio is again too low. If we could manufacture body-temperature ceramic superconducting power storage coils (a distinct possibility, given the scale involved), this method becomes very attractive.
The third, and by far the most practical method, is to use an onboard nuclear power source. This would be relatively easy to shield given the amount of fuel involved, and it has other advantages as well. For one thing, the same radioactive material could be used for power and tracking, since the casing must be hotter than body temperature to produce power. This would have the effect of greatly reducing the complexity of the microrobot. For another, there would be no worries about running out of power, or insufficient power to get the job done. At the micro scale, shielding and power conversion are relatively easy, making this method extremely practical. The only major problem with this method is the social and political objections that would take place.
External to the body: In this case, the power would be transmitted to the microrobot from outside the body. This can be done in a number of different ways, but it boils down to two possibilities. The first is to transmit the power by means of a physical connection, and the second, of course, is to transmit it without a physical connection.
Physical connection: In the first case, we would need some sort of wire or cable to carry power between the microrobot and the outside power source. There are a number of problems with this approach. The first, of course, is that the wire needs to be able to reach inside the body to where the microrobot is. This means that it must be thin enough to fit down every blood vessel that the microrobot can enter. If the wire is deployed from outside the body, the friction of the outer casing must be low enough to allow the wire to move freely within the blood vessels without cutting into the walls at any change of direction. The wire must also be flexible enough to be able to withstand abrupt changes of direction without fatiguing, kinking or breaking. If the wire is deployed from the microrobot, we must have enough stored on the microrobot for it to be able to reach all the way to the operating site. We must also have a means of deploying the wire without tangles, and a means of retracting it back into the microrobot. Of course, if the wire is strong enough, it would greatly ease our movement problems, since the microrobot would then be deployed on a tether, with only navigational capabilities required, rather than long range movement. Similarly, removing it from the body would be greatly eased since it could simply retrace its path.
The next question is how the power would be transmitted. There are two possibilities: electricity and light. In the case of electricity, we must take several factors into account. The first is that the electricity needs a return path. This means that we must deploy a two-conductor cable, or use the body itself for the return path. Given the small amounts of power required, this is possible. Another consideration to take into account is that due to the small diameter of the wire, there would inevitably be some heating of the wire, and therefore the surrounding tissue and this would have to be taken into account. The blood, of course, would act to carry away most of the heat. We could also use the wire for high-speed two-way communications, making that job much easier.
If the power is transmitted in the form of light, which is then either used directly or converted to electricity, the problems are different. There is no requirement for a return path, nor is there any significant leakage along the length of a fiber-optic cable of such a short length. On the other hand, the problem of brittleness is much more significant at the diameters required. This is especially true if the fiber-optic cable is stored in, and deployed from, the microrobot itself. There is also a problem in that the conversion of light to electricity would require more on-board equipment.
Of the two techniques, electricity is the better choice at this state of the art, and this is unlikely to change for this application.
Without physical connection
In this scenario, we are transmitting power to the microrobot without the use of wires or any sort of physical means to transfer the power. Here we have a number of choices.
Ultrasonic: This technique is similar to that of the microwaves, except that since water is such a good conductor of sound, most of the energy would not go into heating up the tissues in the path of the beam. Instead, they would tend to dissipate, and would be absorbed by the body as a whole, with much less attendant danger. A piezoelectric membrane would be used to pick up the ultrasonic waves and convert them to electricity. This membrane, of course, could be modulated at the same time to act as a communications device (two-way) and for a sensor device, as well.
Induced magnetic: In this case, the body is surrounded by a magnetic field. This field would induce currents within a rotating closed conducting loop in the microrobot, which it would then use for power. The frequency of the resulting power is dependent on the rotational speed of the pickup loop, and so alternating the rotational frequency (mechanical FM modulation) would provide a communications path as well. By switching the current through a relatively high resistance path, we would obtain a pinpoint heat source, which could be used for treatment as well.
From the above descriptions, we can see that if we can maintain the physical connection, a wire deployed from the microrobot itself would be very useful, and solve many of the problems we would encounter. However, if no physical connection can be maintained, either ultrasonics or magnetic induction could be used, with ultrasonics appearing to be somewhat more effective.
We need to steer the microrobot to where the sensors tell us it needs to be. As always, the two choices are internal control and external. The following are considerations:
Need to know where to go: This does not necessarily mean that we have a detailed map of the body that the microrobot is following. It simply means that the microrobot must be able to proceed to the location of the unwanted tissue within the specified time constraints, if any. If the microrobot is permanently introduced into the body to circulate and remove unwanted tissues as they are detected, this requirement is largely unnecessary.
Need to know the route: This is different from the above requirement in that some places are more difficult than others to reach. For example, a tumor deep within the brain can be located by various means, but it cannot be accessed by conventional surgical techniques. We may be able to locate the tumor by means of conventional techniques, but the sequence of blood vessels that we need to follow may be more difficult to determine. Again, for a maintenance routine, this requirement may be unnecessary as long as the microrobot covers the entire bloodstream, or at least all the sections that it can access, in a reasonable time.
Need to be able to correct if drawn off course: This requirement is only necessary if there is a specific track that must be followed to reach the necessary location. This can be true for two different scenarios. The first is when a preplanned route exists and must be followed. Due to the complexity of the circulatory system, this will generally be the case only when the tumor can be accessed from the larger and more obvious blood vessels. The second scenario is when the microrobot is using long range sensors, specifically chemical sensors, to locate the tumor. In this case, the microrobot would be functioning in a manner similar to a bloodhound. A reduction of the chemical trail used to locate the tumor would indicate a "wrong turn" in the bloodstream, and the need to backtrack to the point where the chemical traces started to diminish. In this case, we must also be sure not to be fooled by eddies in the bloodstream that cause a momentary reduction of the chemicals that are detected.
Need to be able to apply treatment effectively: Once we have reached the location of the tumor, clot or deposit of arterial plaque, we must be able to apply the appropriate treatment without making matters worse. We do not want to cause tumor producing chemicals or cells to scatter throughout the bloodstream. Similarly, we do not want clots to break up into large chunks, precipitating the very strokes we are trying to prevent; nor do we want to pierce the wall of an artery rather than simply remove plaque deposits.
Need to be able to reach outlet from body: This is a problem if the microrobot has been introduced into the body in order to perform a specific task. In such a case, it will need to be removed, which means that either it must obtain egress from the circulatory system, or it must pass through an already existing port of exit. It can either proceed to a point where it can be removed easily, or it can backtrack to where is first entered the body.
In the case of semi-permanent introduction of the microrobot into the bloodstream for maintenance purposes, the problem is the exact opposite; we want to avoid the removal of the microrobot from the body unless it is done deliberately. One way of doing this is by means of chemical sensors. When the chemicals that accompany the breakdown of platelets and the formation of a blood clot are detected, the microrobot would swim "upstream"; away from the clot formation. Of course, the microrobot is supposed to seek out clots in order to destroy them. We would have to find some way of distinguishing between clots that are caused by an opening in the circulatory system large enough for the microrobot to exit, and those that are small and growing only gradually. This is especially true since if we do not distinguish between them, the microrobot will constantly be breaking up clots around a wound and reopening it, causing a particularly ironic form of hemophilia.
Need to compensate for the unexpected: Certainly while the techniques are being developed, there will be many unexpected events. Even after the control techniques are perfected, there will be many occasions where it will be necessary to have external decision making introduced into the control loop; i.e. we are not going to be creating an autonomous microrobot any time soon. There are two ways we can handle this problem. Either the microrobot is autonomous for simple things, and calls for help when something unexpected happens, or it can be completely externally controlled, greatly reducing the complexity of the on-board processing power. The above requirements cannot even be met for a car navigating in a city, which is a similar but much less complex problem. However, not all of these requirements are entirely binding. Let us consider each one in sequence.
The only real thing that we need to know about where to go is that there is tissue to be treated along the route from introduction to egress of the microrobot. This can be accomplished in several ways. Introduction of the microrobot into the bloodstream at the correct point will allow it to move to the target by means of simply following the blood vessels appropriately. In a maintenance program, the microrobot does not even need a destination, but simply goes where it will while trying to sense target tissues and act appropriately. While it would be more effective to know the shortest or most effective route to the target tissue, this is only a constraint if there is a time constraint as well. For a maintenance routine, it does not matter where the microrobot is, although a chemical tracking system of some sort would improve performance greatly over random sampling. If we do not have a specific location as goal, we need not worry about the route, save that we do not want the microrobot to wind up in some sort of eddy with no escape. For that matter, we do not want it to wind up embedded in the wall of a vein, or in sludge of arterial plaque. We do need some means of detecting and avoiding such an occurrence.
We can see from the above that even though we have reduced some of the control requirements for our microrobot, the remaining considerations are well beyond the capabilities of modern programming techniques. If we had thousands or millions of nanorobots in the bloodstream, this would be a serious obstacle. However, with only a very few microrobots to control at once, we can actually (assuming sufficient communications bandwidth) have a person controlling the microrobot directly.
MEANS OF RECOVERY
Given sufficiently accurate control of the nanomachine, or a tether, this is not a problem; we can just retrace our path upstream. However, it would be a lot easier, and recommended, to steer a path through the body that traverses major blood vessels and winds up at a point where we can just filter the nanomachine out of the bloodstream. This will reduce the possibilities for difficulties, and also cause less wear and tear on the nanomachine. Of course, either scenario is a possibility, depending on where the actual operation site is. Another possibility is to have the nanomachine anchor itself to a blood vessel that is easily accessible from outside, and perform a small surgical operation to remove it.
The construction and use of these 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. As can be seen from the above, most or all of the engineering technologies to create a series of practical and effective microrobots already exist. Rather than keep our eyes fixed on the far future, let us start now by creating some actual working devices that will allow us to cure some of the most deadly ailments known, as well as advance our capabilities directly, rather than as the side effects of other technologies. A concerted development effort could have a working model of the Nanorobot ready within a year or two, and this would certainly advance the development of nanotechnology. All in all, nanotechnology should have a positive effect on our world in the not-so-distant future.