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

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

6.4.3.6 Power Tether Configurations In Vivo

If an application requires only a very small number of nanodevices operating in a very limited tissue volume for a brief period of time, then direct tethered power supply might be preferred. Tethered devices may employ simpler onboard energy conversion systems, hence may be appropriate for devices of more primitive design such as may exist in the early years of nanomedical technology development. Tethered power systems could be useful when working inside a tissue volume that has been drained of indigenous useful chemical energy resources and which has no bloodflow or active homeostatic processes, such as amputated limbs, massively ischemic organs, or cryogenically preserved tissues or bodies. Tethered power might also be acceptable for operations inside tissues with incomplete, non-existent, or low-density vascularization, such as the epidermis, or inside tissues with incompetent or compromised immune systems, such as an embryo or fetus. In cyto slaved nanodevices may transmit/receive power or data to/from an extracellular master nanorobot via transmembrane tethers (Section 9.4.5.6).

The two principal drawbacks to tethered supplies are physical vulnerability and tissue irritation (see also Section 7.3.3). Tethers may kink, break, or become tangled; failures in rotating cable tethers may include "drilling" and "weedwhacker" modes, chemical tethers can detach and leak, and so forth. If billions or trillions of medical nanodevices are deployed, these vulnerabilities are enhanced. In addition, tether volume may equal or exceed nanorobot volume, increasing intrusiveness and reducing nanorobot mobility. For instance, a 10 cm long tether measuring 20 nm in diameter that might be used to power a ~1 micron³ device has a volume of ~30 micron³ -- a clear case of the tail wagging the dog. Limb/organ macromotions and tissue micromovements may produce a microscale sawing action of tether fibers against tissues, causing irritation, inflammation (including leukokine release with macrophage and fibroblast mobilization), and possibly a granulomatous reaction unless tethers can be made with variable compliance to match the stiffness of the tissues through which they pass, adding greatly to design complexity. Mobile tethers deployed inside cells can mechanically excite cytoskeletal elements, eventually triggering unwanted gene expression cascades in the nucleus. Tethers deployed through blood vessel walls or cell membranes may face similar challenges regardless of the chemical biocompatibility of their exterior or sheathing surfaces.

The simplest tethered configuration for powering large numbers of nanorobots is to transmit external power into an energy organ (Section 6.4.4) using a single tether connection. A complete analysis of networks and configurations of more complex tethered nanorobot power supplies is beyond the scope of this book. However, several simple configurations include Cartesian, fractal, and hub systems. A Cartesian system is laid out along a regular gridwork using a rectangular, cylindrical, spherical, helical, or other coordinate system as appropriate to the biotopology. A fractal system resembles a branching vascular-like "power tree" inside the body, with tendrils following blood or lymph vessels and natural tissue graining. A hub system (aka "mother ship") employs an array of isolated distribution nodes, with numerous individual nanorobots directly connected to one or another node.

Tethers must be directed entirely through tissue; placement in the active bloodflow may precipitate prompt thrombosis from microturbulence shear forces or netting action with formed elements. Macrophagic responses characteristic of wound repair (Chapter 24) would likely be stimulated within hours of tissue entry as cellular shear force sensors activate cytokine signaling mechanisms in response to mechanical stresses. To avoid massive granulomatous reaction, tethers cannot be abandoned after use and probably must be spooled out during deployment and respooled during retraction, adding additional potential failure modes and increasing the time required for deployment or retraction, a major drawback in emergency situations.

It might be possible to carefully install a biocompatible electrical or fiberoptic power network throughout a patient's body (Section 7.3.1) as a permanent augmentation without causing irreparable damage. The presence of great numbers of isolated current-carrying wires creates numerous stray electrical fields that could affect cellular processes and possibly stimulate a macrophagic response; however, electric fields are minimal outside a well-shielded 1-micron-diameter coax or twisted-pair triax cable. If ~1 micron stray-field-free cables or optical fibers are used, a total network volume of ~10 cm³ of fiber (~0.01% of human body volume) has ~13,000 kilometers of length; assuming ~1 meter strands and a simple Cartesian distribution, fibers have a mean lateral separation of ~80 microns between nearest neighbors (~4 cell widths).

Last updated on 18 February 2003