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

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

7.3.3 Networks Assessment

Fiber networks excel at high-capacity long-distance communication, with additional capacity readily added to the design by multiplying the number density or bundle density of fibers. Acoustic mobile relay networks have relatively low and fundamentally limited bandwidth. An additional advantage of the fiber network is speed -- point-to-point fiber messaging takes ~10^-4 sec vs. up to ~10 sec for the mobile network. Both of these advantages may be important in transdermal communication and whole-body diagnostic monitoring applications.

However, fiber and mobile networks may provide roughly equivalent local transmission speed and capacity if both must employ acoustic nodal access, as this provides the highest endogenously transmitted bit rate per unit power consumption for mobile nanorobots that are arbitrarily positioned network users. Thus in applications requiring predominantly local information transfers, the choice of basic network architecture may be driven by factors other than system total capacity and speed.

Fiber networks also have many significant disadvantages. Both fibers and mobile nanodevices can be made immunochemically biocompatible, but fibers are physically vulnerable and can cause serious physical tissue and cellular irritation (Section 6.4.3.6). It is well-known in dermatology that subdermally-placed, but incompletely absorbed, "dissolvable" stitches can work their way back to the surface of the skin; fiber network elements could be similarly ejected from the body. Some of these mechanical difficulties possibly may be overcome using a helical coil or a fiber which is coupled to a biocompatible sheathing using compliant internal springs; large-diameter fibers could be installed within bones where they would experience minimal flexing and immune system exposure, although transmissions across joints or tissues with high flexure or vascularization might still be required to be acoustic.

Networks with mobile components are easier and quicker to install, to reconfigure, to upgrade while in vivo, or to remove in the event of malfunction or change in medical objectives, than fiber networks with immobile components. On the other hand, communication protocols may need to be substantially more complicated to provide the necessary reliability with drifting nodes.

The fixed physical positions of fiber network nodes is also a major disadvantage because tissue movement is ubiquitous inside the human body, causing progressive fiber misalignment. Examples of such movement include untreated metastasizing tumors or benign growing tumors, angiogenesis in injured or heavily exercised tissue, enlargement and shrinkage of tissue channels or prelymphatics after wounding or edema, adipocyte deposition, and the breakdown or buildup of muscle tissue; cell loss and tissue modifications in the brain, sclerotic livers, radiation-damaged bone marrows, chancres and blisters, hematomas, and the uterus and related tissues during pregnancy; normal periodic movements in muscle-laden tissues such as the biceps, cardiac tissue, arterial walls, and the thoracoabdominal diaphragm; and other nonpathological but irregular movements such as tissues which may rapidly inflate with fluid, including the sex organs and related tissues, the bladder, and to a lesser extent the lungs. With such movement, nodal positional assignments quickly become obsolete and nodes may be carried out of acoustic range. Fiber spatial distributions may develop undesirable clumping and rarefactions. Fiber-embedded tissues which slough off in the normal course of events (e.g., epidermal, endometrial, placental, gastrointestinal) may carry fibers out into the uterine or intestinal canals, or expose them to air in the case of the epidermis.

The paradoxical conclusion is that overall fiber network reliability may be improved by adding nodal mobility. One crude biological analog is the filopodia of embryonic axons which can drag their fibrous neural cargo through embryonic tissue at ~0.1 micron/sec.³²⁹⁶ While adult tissue may in theory have a more solidified extracellular matrix (ECM) and greater intercellular connectivity, making dragging more difficult, leukocyte and fibroblast mobility through ECM is typically 0.05-0.70 microns/sec (Section 9.4.4.2).

Last updated on 19 February 2003