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


4.2.6 Spatial Concentration Gradients

Medical nanodevices can detect both extremely steep and extremely shallow spatial concentration gradients. For example, a receptor array sensor (Section 4.2.1) consisting of 10 steric probe units has a total volume of ~1000 nm3 including its prorated share of drive systems, housings, etc. In just a few measurement cycles, adjacent sensors can detect a maximum concentration change from 10-2 nm-3 to 10-11 nm-3 across a mean nanosensor separation of 10 nm on a nanodevice surface, a steep chemical spatial gradient of ~1010% per nanometer. J. Soreff points out that maintaining a steep concentration gradient of a freely diffusing species implies a power dissipation of ~kT per molecule per time to diffuse a concentration-halving distance. For a common, small, tethered molecule with cligand ~ 3 x 10-3 molecules/nm3 that diffuses ~1 nm in tdiff ~ 1 nanosec, implied power density Dmol ~ kT cligand / tdiff ~ 1013 watts/m3, a power density that might possibly be encountered in the vicinity of chemoelectric transducers (Section (Such high power densities never occur on macroscale dimensions, and heat rapidly diffuses at nanoscale dimensions; Sections 4.6.1.)

Similarly, counting rotor sensors (Section 4.2.3) for small common serum molecules like glucose deployed at opposite ends of a 1-micron nanodevice can detect a concentration differential of ~10-4 over a ~1 sec measurement period, a shallow chemical spatial gradient of only ~10-5 % per nanometer. Even lower gradients may be detected by communication (Chapter 7) between physically separated cooperating nanodevices.


Last updated on 17 February 2003