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

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

8.4.1.4 High-Resolution Thermographics

High-resolution maps can be assembled in designated tissue monitoring volumes, using a network of fixed thermographicytes whose physical positions are accurately known (Section 8.3.3). If L_therm = 100 microns is the mean spacing between devices, then for a maximum transorgan temperature differential DT ~ 1°C for an organ with scale size d_organ ~ 10 cm (Table 8.9) the largest endogenously generated temperature differential between neighboring thermographicytes is DT L_therm / d_organ ~ 10,000 microkelvins; the smallest detectable thermal variation of ~1 microkelvin (Section 4.6.1) corresponds to a fluctuation of ~60 watts/m³ in power density within the 0.001 mm³ monitoring volume (the equivalent power output of ~100 mitochondria). Recording a single temperature measurement to microkelvin accuracy requires at most ~24 bits.

Many useful configurations of such a network are readily conceived. As a simple example, consider a monitoring unit consisting of 100 thermographicytes reporting at ~1 KHz to one centrally located data processing nodule. The thermographicytes transmit a total of 2.4 megabits/sec to the nodule from within their 0.1 mm³ patrol volume. A nodule with ~8 micron³ of storage tape can retain up to ~1 day's worth of historical data generated within its monitoring unit. Nodule nanocomputers can scan the data for a wide range of specified anomalies and thresholds, emitting an appropriate alert message (containing all relevant details) when one is found. In principle, an expanding (warm) tumor could be detected* after growing to a volume of at most L_therm³, a maximum pathological cell count of ~100. Each monitoring unit consumes at most ~20,000 pW, including 100 pW/thermographicyte and 10,000 pW in the nodule for a ~1 micron³ computational facility drawing ~1000 watts/m³-Hz (Section 6.5.6 (E)) and operating internally at a ~10 MHz clock speed. For continuous surveillance of an entire organ to microkelvin accuracy, up to ~16 million monitoring units are required, dissipating at most ~0.3 watts, which must also be taken into account. (The liver, for example, generates ~10 watts metabolically.)

* Tumors which are metabolically less active are harder to detect by this means. Also, measuring heat from tumors located in organs having high metabolic activity (e.g., liver, kidney) is even more difficult, especially given the large caloric variations after consuming food or drink, or during normal hormonal responses, and given the relatively high normal rate of cell division in some organs (e.g., liver, gut) that must be distinguished from abnormal tumor growth.

Patients may also receive thermographicytes installed in a mobile configuration, with the nanodevices constantly traversing the tissues looking for anomalies, outmessaging only when such anomalies are detected, and accurately fixing their location to within ~3 microns at all times by interrogating a separate microtransponder positional navigation network (Section 8.3.3). At the extreme limits of accuracy, nanorobot waste heat may contribute significant error to thermal measurements. A nanorobot generating waste heat P_heat = 100 pW produces a DT = 10 microkelvin temperature differential over an X = P_heat / K_t DT = 16-micron aqueous path; this nanorobot heat is dissipated by thermal conduction to below measurable levels in ~C_V X² / K_t ~ 2 milliseconds, approximating the ~1 KHz sampling time.

Last updated on 20 February 2003