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

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

5.3.7 Chromatic Modification

The external surface color of nanodevices and their macroscopic aggregates is also subject to design and operational control, which may be useful in some special applications. Pure diamondoid in the flawless crystalline form will be clear and largely colorless; impurities commonly found in commercial and investment gemstones that impart bluish (~0.1% B substituted for C), yellowish (~0.1% N substituted for C), or other (e.g., blood-red, kelly green)³⁵²⁴ hues are unlikely in nanomanufactured materials, as are graphitic deposits that produce black streaking or onyxlike coloration.²⁶⁴¹ Perception of chromaticity is limited by such factors as the minimum spatial resolution of the human eye (~ 0.5 arcmin or ~30 microns at closest focus of 10 cm), the maximum wavelength of visible light (~0.7 micron), and the degree of physical proximity of individual colored nanodevices when aggregated.

The simplest method for imparting a specific surface color is to add a thick coating of engineered corundum. Pure corundum (aluminum oxide) is colorless and has a hardness and chemical inertness only slightly inferior to diamond. Sapphire is the best-known crystalline form. Sapphire can be manufactured in a full spectrum of blues, pinks, yellows, oranges, teals, lavenders, greens, grays, whites, and all intermediate hues. More colors with greater intensity exist with sapphires than with any other gemstone. The broad color palette is achieved by replacing aluminum atoms with ~0.1% iron atoms and ~0.01% titanium atoms. Rubies, also corundum crystals, achieve their vivid reds by replacing a few aluminum atoms with atoms of chromium. The biocompatiblity of sapphire and ruby coatings is largely unexplored. If this should present a problem, a thin overcoating of transparent diamond preserves the color effects while presenting a potentially bioinactive interface³²³⁴ (Section 15.3.1) to the external environment. Given the differences in crystal structures and atomic lattice sizes between diamond and sapphire, there may be some sacrifices in materials properties at such interfaces; D.W. Brenner [personal communication, 1999] is unaware of any computational modeling studies having been performed on the atomically-bonded diamond-corundum interface as of 1998.

In larger devices or in macroscopic aggregates, color may be created and dynamically manipulated by embedding micron-scale light-emitting solid-state lasers in the surface. For a surface to appear a certain color, embedded monochromatic optical lasers must emit light of sufficient brightness to compete with other illuminated surfaces that may be present in the visual field. The required intensity is probably ~1 watt/m² (Section 4.9.4) or ~1 pW/micron² -- a feasible energy emission budget for micron-scale nanorobots. For example, Shen⁷³² has constructed 0.86-micron thick three-color electrically-tunable organic light-emitting devices²⁵⁶⁰ which generate ~0.5 pW/micron² with ~1% energy efficiency; red-only LEDs have achieved efficiencies up to 16%,¹⁰⁵⁴ and larger diode lasers have achieved up to 60-66% efficiencies.^3144,3145 Organic light-emitting devices (OLEDs) using small organic molecules can have high brightness (2-4 pW/micron²), half-lifetimes of >4000 hours, and can be made with a wide range of emission colors in a ~300 nm thick sandwich.³¹⁸⁸ White-light organic electroluminescent devices ~100 nm thick have produced ~3 pW/micron² at 15 volts;¹⁰⁴⁸ 0.1-1% energy efficiency is typical.¹⁰⁴⁹ (Commercially available LEDs like Hewlett-Packard's gallium arsenide VCSEL blue-light laser chip have active layers ~10 microns thick and ~11% energy efficiency. LED chips typically have lifetimes ~10⁸ sec, and other microcavity lasers are well-known.³²⁵⁵

If surface color is a serious design objective, arrays of frequency-selective absorbers or emitters such as rhodopsin, fluorescein, carotenoids, luciferins, or engineered porphyrins can be placed below a thin transparent diamondoid window. Active manipulation of covershades to block or expose these windows could permit rapid modulation of chromatic surface characteristics (e.g., polychromatic sparkling). Diverse surface pattern morphologies are theoretically available;^1051,3447 dynamic texture arrays can produce marked changes in color and other optical properties.¹⁰⁴¹ Coherent modulation of the optical characteristics of aggregated nanorobots over large areas allows the creation and control of optical patterns visible to the human eye (e.g., epidermal displays; Section 7.4.6.7) with ~micron resolution at frequencies up to ~10 KHz consistent with a conservative ~1 cm/sec covershade speed. The visibility of subdermal chromomorphic devices is greatly reduced by photon scattering and absorption processes at greater depths in tissue (Section 4.9.4). Pure suspensions of spherical nanorobots in clear fluid will partake of the nanorobot surface color; suspensions of nonspherical, irregularly-shaped devices will appear milky due to scattering effects. Another design consideration is photochemical stability, especially for nanodevices that must operate in open sunlight or near other sources of ultraviolet (UV) radiation. To avoid photochemical damage, such devices may require a UV-opaque surface or components purposely designed for photochemical stability¹⁰ (see Chapter 13).

Last updated on 16 April 2004