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

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

5.1 Flexible Form and Function

It has been asserted that nanomechanical systems fundamentally differ from systems of biological molecular machinery in their basic architecture -- specifically, that nanomechanical components are supported and constrained by stiff housings, while biological components often can move freely with respect to one another.¹⁰ As regards medical nanodevices, this may be a somewhat artificial distinction. The likelihood that most nanoscale components will be connected in rigid arrays does not imply that the nanomachines themselves must be entirely rigid in shape, nor does it rule out the possibility that some major nanomachine components may be designed to allow periodic reconfiguration and repositioning.

Why might a flexible shape be useful in nanomedicine? While cell membranes are self-sealing, large wall breaches during cell repair operations can be problematical. A flexible shape makes it easier for a cell repair nanodevice to enter the cytoplasm with minimum disruption, for instance by elongating and narrowing so as to present the narrowest possible aspect during plasma membrane and cytoskeletal penetration (Section 9.4.5). When navigating through narrow passages in hard biological substances such as bone or enamel, a nanorobot with a rigid shape is more likely to scrape or jam than is a more flexibly-shaped device. Extensible volumes allow the projection of compliant mechanical pseudopods of various sizes and shapes from the nanomachine surface. Deformable bumpers make it easier to establish and maintain reliable multidevice linkages in large-scale cooperative nanorobotic architectures.

Flexibility expands the options available for nanodevice mobility to include amoeboid and pulsatile peristaltic locomotion (Section 9.4.3), surface deformation natation (Section 9.4.2.5.1), and circumvascular tissue diving or nanorobot diapedesis (Section 9.4.4). Fluidlike surfaces are ubiquitous among motile microorganisms. Malleable nanodevices situated on a cardiac or arterial luminal surface can adopt minimum fluid drag configurations.

Microhydrodynamic stability is another factor. For example, when placed in a bloodflow of constant speed in a tubular vessel, a rigid sphere, rod, or disk will tumble as it travels, due to the differential axial velocity field. But external fluid stresses distort a deformable object like an emulsion droplet from its original spherical shape into an ellipsoid oriented at a constant angle to the direction of flow. The fluid stresses are transmitted across the droplet interface; the surface and interior fluid circulates about the particle center in a tank tread motion.³⁸⁶ Shape changes can also be employed for steering and orientational control during active nanorobot swimming (Section 9.4.2.5): Hard-shelled particles without active mobility don't marginate laterally in blood vessels; deformable surfaces can radially migrate.³⁶² Flexible surfaces may also be used to minimize the increment to blood viscosity caused by the presence of bloodborne nanorobots (Section 9.4.1.4).

A flexible or "metamorphic" surface is a nanodevice exterior surface comprised of independently controllable elements that can translate or rotate their relative positions, thus enlarging or contracting total surface area of the device, or changing its shape, with or without altering the membership of elements in the surface, with or without altering the enclosed volume of the entire nanomachine, while maintaining continuous structural integrity and nonpermeability of the surface. The range of possible designs is enormous. Metamorphic surfaces may include integument systems with semirigid components; hinged elements with fixed relative position and area, allowing variable volume; partially mobile surfaces having unit elements of fixed size but variable position relative to their neighbors, allowing control of surface area as well as volume; and even fully metamorphic surfaces with surface elements free to rotate, alter shape or orientation, slide, or even change membership, permitting maximum surface/volume flexibility.

Device shape is driven by task requirements which may or may not demand surface flexibility, as described in Section 5.2. Section 5.3 presents a number of design alternatives for metamorphic surfaces and manipulators, and briefly considers the impact of surface flexibility on internal configurational design. Section 5.4 examines the challenges of metamorphic bumpers which serve as interdevice fasteners and junctions. Discussion of the biocompatibility of nanodevice surfaces³²³⁴ is deferred to Chapter 15.

Last updated on 17 February 2003