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

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

9.4.7.1 Objectives of Cytocarriage

One of the most important utilities of cytocarriage is the "cell herding" function.⁹ Nanorobots can marshall significant resources drawn from the body's natural immune and wound repair systems, and redirect them to particular sites that are in urgent need of assistance. In this role, medical nanorobots may act as immune system or wound repair homeostatic accelerants, regardless of the nanorobots' own additional inherent reparative functions, which may be considerable. Nanorobots may also seize control of pathogen and parasite mobility systems, then guide them to natural disposal sites within the body.

While nanorobot control of the natural motility apparatus might slightly improve its performance, as a practical matter the maximum speed of cytovehicles being driven through tissues is probably limited to near the maximum observed natural speeds of in vivo progression of uncontrolled cells (e.g., ~1-10 microns/sec). This will normally be considerably slower than the maximum transtissue speeds available to self-propelled medical nanodevices (e.g., 100-1000 microns/sec). Also, cytocarriage is usually energetically inefficient compared to self-propelled medical nanorobots. However, in some circumstances a strategy of cytocarriage may avoid the need to include certain propulsive mechanisms in the nanorobot design for some segments of the mission pathway, thus reducing nanodevice design complexity and freeing up scarce onboard storage volume for additional consumables or mission-critical machinery. Cytovehicles (e.g., leukocytes) engineered to display organ-specific tissue homing could be preloaded with passenger nanorobots prior to injection into the patient. Once injected, the nanorobots would be delivered to the intended destination with acceptable reliability. Of course, surface-placed antigen semaphores (Section 5.3.6) may directly confer a similar homing ability on bloodborne nanorobots, with equal or superior reliability.

Due to the small size of most readily available cytovehicles, conservatively only one or at most a few nanorobots may be able to safely enter and occupy the interior of each commandeered motile cell without triggering unwanted cellular responses (Chapter 15). However, additional nanorobot passengers may adhere to the exterior of the plasma membrane of the cytovehicle (after disabling certain natural haptotactic reactivities), or may be towed by the motile cell, allowing the motile cell to serve as a "pack animal" or "biological tractor." Interestingly, protozoa called mixotrichs that are present in the termite gut are propelled by several thousand bacterial spirochetes attached to the protozoan as obligate symbiotes;^2025-2027 S. Vogel observes that "the protozoa have adopted bacteria as engines the way a human might use a team of horses."²⁰²²

Even heavily burdened cytovehicles operate at very low Reynolds numbers (Section 9.4.2.1), so the total mass of passengers (M_pass, determining the inertial load) is relatively unimportant compared to the total surface area of the passengers (A_pass, determining the viscous load); M_pass ~ N_pass (the number of passengers per cell), but A_pass ~ N_pass^2/3 for efficiently packed passengers. Given a towing strength of 165 nN for the human fibroblast,¹⁴⁶¹ a properly harnessed single fibroblast cell traversing interstitial fluid at a speed of 10 microns/sec can in theory tow behind it a 20-micron wide, 1000-micron long cylindrically-packed aggregate of N_pass ~ 300,000 1-micron³ nanorobot passengers with a towing force of only F_nanoP = 0.01 nN (Eqn. 9.75). (A fibroblast pulling 165 nN at 10 microns/sec consumes ~2 pW of power.)

Last updated on 21 February 2003