© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

8.4.1.2 Thermal Demarcation

Thermographic navigation to a target site is possible without the use of any maps. Simple thermal demarcation is a good example of this. In a controlled room-temperature clinical setting, the relatively narrow homeostatically-maintained temperature band of 36-38°C allows easy demarcation of selected target regions by applying thermal stimuli lying just outside the normal band. Examples might include hot (T > 38°C) or cold (T < 36°C) packs applied to the skin; diathermic tissue heating using different frequencies to achieve selective heating at various tissue depths (Section 6.4.2); adaptive phased array (APA) heating of small target volumes deep inside the body;¹⁵¹⁸ thermally active catheter probe tips inserted transdermally or otherwise, directed to specified internal targets; or multiple intersecting focused infrared, visible photon, or ultrasound beams directed to specific convergence volumes for maximum heating effect. Depending upon the technique employed and the target tissue, volumes as small as ~1 mm³ may be demarcated in this manner. Nanorobots of relatively simple design may be injected near the target or allowed to circulate in the bloodstream, and will detect the outlier temperature data and gather in the vicinity of the target at a statistically determined accumulation rate and number density.

Outside of controlled environments or during prolonged treatment periods, simple demarcation becomes less reliable for several reasons:

1. Error in the physical placement of the instrument of demarcation will produce erroneous nanorobot localization, which may increase the likelihood of treatment error. Such iatrogenic mistakes are especially likely during time-critical or emergency crises, or in situations where the diagnosis is unclear, or in cases of self-administered medical care.

2. Even when initially correctly placed, the artificial hot or cold region will diffuse outward from the original spot, enlarging the radius of action in imprecisely known directions (due to nonisotropic tissue conductance), again increasing the risk of iatrogenic effects.

3. Load error in the human thermoregulatory control system is ~0.2-0.5°C.⁸⁶⁵ Any thermal deviation of this magnitude or larger from local temperatures will provoke a natural counteractive response to restore the local temperature, including capillary sphincter action (both vasodilator and vasoconstrictor activity), increased sweat gland secretions, accelerated counter-current exchanges, more rapid heart rate, or local thermogenesis.

4. Increased susceptibility to spoofing, as when a patient enters a hot shower or bath³³³¹, lies down on snow or on a cold concrete surface, washes his hands and face with hot water, drinks a hot or cold beverage, basks in the sun, and so forth. Temperatures hot or cold enough to definitively prevent spoofing might damage tissues if maintained for very long. This problem can be avoided by employing an oscillating artificial thermal gradient instead of a time-invariant source (especially across layers of highly insulating subcutaneous fat), but at the cost of increased required overall nanorobot sophistication to detect the oscillating demarcation. For energy delivered to a target L ~ 1 mm from the source in a medium of heat capacity C_V and thermal conductivity K_t, the maximum frequency that may be resolved is n_thermal ~ K_t / C_V L² ~ 0.15 Hz for aqueous tissue at 37°C.

General positional information is also available to nanorobots equipped with thermal sensors in the absence of any maps. For instance, assuming the temperature of the external environment has been measured and disseminated via the communication network (Section 7.3), a nanorobot traveling within a microvolume of blood can crudely infer its distance from the epidermis based on the local blood temperature, especially when passing through the thermal shell of the body surrounding the core. Other cues including velocity measurements or histonavigational data may allow further refinement of the estimate.

Last updated on 20 February 2003