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

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

7.4.6.7 Macroscale Outmessaging Transducers

Inmessaging transducers (Section 7.4.2.7) may also be employed as outmessaging interfaces between in vivo nanorobots and patients. For example, a simple transduction organ (e.g., macroscale transdermal needle) inserted into the bloodflow could chemically or acoustically interrogate passing nanodevices and obtain information from them as they travel by, without the need for nanodevice removal or locomotion. This information may then be transferred out through the skin to a small external receiving device accessible to the patient.

In vivo nanodevices can also join together to form coordinated aggregates which may be large enough to emulate the action of a macroscale communications device. In some instances, this approach will produce only energy-inefficient marginally-feasible results. For example, consider the concept of the subdermal nanospeaker or "talking tattoo." The minimum acoustic power output that can be heard as a whisper is ~0.1 microwatts (Section 4.9.1.5). From Eqn. 7.7, a 1-micron acoustic radiator driven by 1000 pW of input power at 3000 Hz produces ~4 x 10^-18 watts/radiator. The sound waves then lose 99.9% of their power passing through the skin-air interface (Section 4.9.1.6), so ~22 trillion devices are required to generate the required ~0.1 microwatt (by coherent emission) in order to be heard. But this many 1000-pW devices operated simultaneously produce 22,000 watts, well in excess of the suggested 100-watt whole-body thermogenic maximum (Section 6.5.2). However, if the radiators position themselves near the skull to employ bone conduction into the tympanum, most of the skin-air interface loss is avoided reducing the required radiator count to 22 billion and total power to 22 watts, which might be acceptable. Such radiator nanorobots occupying 10% of local tissue volume make a patch ~350 microns deep covering the area of a postage stamp (~1 inch²). (Skin-air interface losses may also be avoided by acoustic nanoradiators positioned exclusively supra-epidermally.) The "talking wristwatch" approach (Section 7.4.2.7) is another possibility.

A more efficient nanorobot-aggregate user interface is the programmable dermal display. Pigment tattoos,⁹⁶ port-wine stains, strawberry marks (common hemangiomas), and other birthmarks constitute an existence proof that small biocompatible particles can be permanently implanted in the dermis and do not migrate on timescales of decades or longer. This suggests that dermal displays can be positionally stable over very lengthy periods of time.

Consider a population of ~3 billion display nanorobots embedded 200-300 microns below the surface of the epidermis covering a 6 cm x 5 cm rectangle on the flat part of the back of the hand or on the smooth medial surface of the forearm. The nanorobots are ~1 micron³ in volume and occupy only 1% of the 300 mm³ local tissue deployment volume. Each device consumes ~10 pW when generating visible photons of desired colors at a comfortable visible intensity of ~1 pW/micron² or ~1 watt/m² (Section 5.3.7), assuming an improved 10% ergooptical conversion efficiency. Visible photons are completely scattered in 10-100 microns but almost none are absorbed, producing a diffuse glow as ~50% of the scattered photons eventually exit the surface of the skin. For installation and stationkeeping, display nanodevices require at least limited mobility, and a few additional nanodevices may be required to assist with computation, data storage and external communications, and other housekeeping chores.

If powered by continuously available chemical fuels, the display would only be operable for 0.1-10 sec before exhausting the entire oxyglucose supply present in the limited tissue deployment volume. Consequently, it is necessary to include a large energy storage buffer constituting 40% of device volume (assuming 10¹⁰ joule/m³ stored energy density; Section 6.2.3) which allows ~1000 sec of operation if only 20% of all pixels are radiating at any given time. This power can be reabsorbed from local oxyglucose supplies in ~1 day, giving a long-term ~1% duty cycle for the display (~14 min/day) although the buffer can operate the display for up to ~21 minutes before being exhausted. This power restriction may be entirely avoided by adding supplemental power from, say, a wristwatch-sized extradermal acoustic source (Section 6.4.1), a dedicated transvascular energy organ (Section 6.4.4), photoelectric collectors (~30 pW/micron² in cloudless noontime sunlight, ~1 pW/micron² in a well-lighted room; Sections 4.9.4 and 6.3.6), or by employing a passive (reflective) display which might be satisfactory for some purposes.

The array of 3 billion nanorobots may be programmed to adopt any of many thousands of different displays. Each display configuration is capable of

1. presenting output data received from the larger in vivo nanorobot population scattered throughout the body (via a communication network), and

2. accepting input data from the patient to be conveyed (through the communication network) to appropriate internal nanorobot subpopulations.

Full-motion animation or video may also be projected, up to the 10⁷ bit/sec maximum limit of the mobile communication network; fiber networks may allow up to ~10⁹ bit/sec data transfer rates (Section 7.3.1). Figure 7.7A shows a small sample of alternative displays (at actual size) from among the many thousands that are conceivable. Acquisition of the information displayed requires a population of sensory nanodevices distributed throughout the body and linked by a communications network. For instance, the "Vital Signs" panel requires at least one telemetering nanorobot stationed in each of the several hundred arteries of the human body (or possibly behind each vein valve cusp (Fig. 8.3) throughout the venous system), with each device reporting local blood pressure conditions on a periodic basis.

As for the display itself, in the first example ("Messager/Calculator") there are 60 input keys measuring (0.5 cm)² each and a 2 cm x 6 cm output panel that can print 7 rows of 30 characters, each character measuring (2 mm)² with 1 mm of blank space between each line. Line segments used to write characters are 500 microns wide, more than ten times the limit of human visual acuity at a reading distance of 30 cm; ~500 million active nanorobots participate in drawing the images of the 60 input keys. Display nanorobots have a mean separation of ~5 microns in the tissue but remeasure their relative positions at least ten times per second; a finger-touch on an input key (Figure 7.7B) depresses the skin by ~500 microns, producing massive and easily detectable displacements of the underlying nanodevices from which the chosen key may be inferred. Non-message related skin stretchings and proximate limb flexures are readily distinguished from input fingerings. The display is activated or deactivated by using finger tapping on the epidermis, temporally coded handclaps or other similar means, and becomes invisible under the epidermis when unilluminated and not in use.

The display may be partly obscured by excessive body hair or particularly dark dermal pigmentation. Post-installation bruises, scabs, incisions, scars, or even wrinkling on the back of the hand may slightly or temporarily impair readability. But nanorobot stationkeeping allows maintenance of near-perfect feature geometry during any trauma short of major avulsion or excision wounds involving significant loss of tissue, deep burns, or outright dermal excoriation or flaying.

In 1998 the closest analogous technology was an all-digital chip-mounted ~1 cm² projection display, created by Texas Instruments, which operated by independently twisting 307,200 individual tiny mirrors, each measuring 16 microns square, through + 10^o, reflecting pulses of colored light onto a screen.^1062,1974 Wristwatch-sized personal status monitors for regular biomonitoring were being developed by the Defense Advanced Research Projects Agency (DARPA) in the United States³³⁰¹ and in the private sector for sports medicine.³³²³

Last updated on 4 June 2011