Nanomedicine, Volume I: Basic Capabilities
© 1999 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999
8.1 Navigating the Human Body
It is difficult to imagine any significant application of medical nanodevices which does not involve navigation, however crude. Devices intended to monitor somatic states, assemble artificial internal structures, remove tumors or foreign matter, combat infections, or perform repairs, must normally be extremely tissue- or cell-specific. Navigation is also required to execute many control protocols (Chapter 12), to locate dedicated energy, communication, or navigational helper organs, or to stationkeep and coordinate with other nanodevices. Even bloodborne nanorobots intended to operate solely at the systemic level -- such as nanobiotics or immunocytes (Chapter 19) and respirocytes (artificial red cells;1400 Chapter 22) -- must know if they have been prematurely ejected from the vasculature so that they may cease functioning or at least modify their activities.
Perhaps the most important challenge of in vivo navigation is to determine how physicians may best direct nanorobots to specific target sites needing treatment within the human body. Two alternative strategies appear most likely to produce the best clinical results (both of which will be considered in this Chapter).
The first strategy is positional navigation, in which the nanorobot knows its position inside the human body to ~micron accuracy at all times in some clinic-centered or body-centered coordinate grid system. The nanodevice relies upon dead reckoning, cartotaxis, microtransponder network alignment, or triangulation on external beacon signals to establish its position continuously. This method requires some onboard computation, at least a basic set of sensors (e.g., acoustic), and probably also a good clock (Section 10.1). However, it is hardly foolproof -- if the target coordinates are poorly specified or the beacon signals are misaligned or poorly calibrated, the nanorobots may go to the wrong place.
The second strategy is functional navigation, in which nanodevices seek to detect subtle variations in their environment, comparing diverse sensor readings with the profile of the target tissue or cell and congregating wherever this very precisely defined set of preconditions exists. These preconditions may be thermal, acoustic or barostatic, cytochemical or immunochemical, mechanical or topological, or even genetic. The crudest forms of functional navigation may be called demarcation, wherein the doctor manually creates detectable artificial conditions at or near the target site such as dermal hot spots, injected chemical plumes, or focused ultrasonic beam spots of appropriate magnitudes and frequencies. Demarcation strategies can be implemented using extremely simple onboard sensors and control devices, possibly not even requiring a nanorobot computer, thus may prove useful early in nanomedical technology development.
More sophisticated forms of functional navigation can be extraordinarily flexible because targets may be specified without the physician having to know their exact physical location in the body -- e.g., nascent cancer tumors, T cells reactive to specific antigens, infected deep-thoracic lymph nodes, bacteria of a particular species, broken capillary vessels, or virus particles having a specified protein coat chemistry. The physician need not know the exact number of targets, nor even if any targets are present at all. These forms of functional navigation require onboard computation, a resident database of relevant parameters and operational details, and a wider assortment of sensors and control protocols. But they also offer the greatest benefit at lowest risk for the patient and thus must be regarded as the preferred approach once the technology is available.
This Chapter opens with a survey of human somatography (Section 8.2), followed by general discussions of positional (Section 8.3) and functional (Section 8.4) navigation, cytonavigation (Section 8.5), and finally ex vivo navigation (Section 8.6).
Last updated on 19 February 2003