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

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

9.3.2 Nanoscale End-Effectors and Tool Tips

An end-effector is usually placed at the tip of a manipulator in order to focus and redirect the application of forces. Examples of simple force redirection include a screwdriver head or bolting socket, a cutting blade, a perforative needle, and levers, pry bars or wedges.

End-effectors can also be used to achieve very fine motion control at the tip. Perhaps the simplest task such an end-effector can perform is the grasping or gripping of target objects in the environment. Figure 9.11 offers a small sampling of the many possible schematic concepts for grasping endeffectors. Mechanical grippers as in (A) have been fabricated with a jawspan of ~10 microns, tines ~2 microns thick, and a ~5 KHz resonant frequency, and have been used to grasp individual 2.7-micron polystyrene spheres, dried red blood cells, and even various protozoa including a 7-micron diameter, 40-micron long Euglena.¹²⁶⁷ Pneumatic grippers (B) have been built and operated on a larger scale.¹²³² Figure 9.11(C) illustrates how high-frequency acoustic point sources or ultrasonic resonance fields¹⁶²⁷ can be used to reversibly secure a small target object in a small space; electrodynamic fields, electrophoretic forces,^1628,1629 and optical tweezers^{1247,1630,1631} employ similar principles. The suction gripper (D) works on any target immersed in a gas or liquid environment (Eqn. 9.22),^1619,1620 whereas the magnetic gripper (E) only works on magnetic materials. The electrostatic gripper (F) induces attractive Coulombic, image force, or dipole charge effects, and is helped by the increased breakdown field strength of very small gaps due to the Paschen effect. The van der Waals gripper shown in Figure 9.11(G) uses variable nanoscale surface roughness to regulate the van der Waals attractive force; fluid surface tension or related forces may be modulated to reversibly grip the target. Finally, any soft object in the environment can be impaled by a narrow articulating spike, which then drops anchor inside the object until the gripper is ready to withdraw (Fig. 9.11(H)).

Most of the end-effectors shown in Figure 9.11 involve direct physical contact with the target object. Undesired or accidental surface adhesivity (Section 9.2) of grippers and manipulators is a serious design issue that must be addressed with respect to all possible environments and target objects likely to be encountered by the manipulation mechanism during the performance of its mission. Complementary diamondoid surfaces in vacuo may adhere with a contact tensile strength up to ~10⁴ atm, representing ~1 nN of contact force per nm² of contact area and releasing ~5400 zJ/nm² of energy when the surfaces are joined.¹⁰ However, noncomplementary surfaces in fluid environments can in theory be engineered to display almost any interfacial adhesivity desired -- including negative adhesivity or repulsion (Section 9.2.3). Gripper/object adhesivity may vary according to many factors that are subject to design control. For example, all else equal, passivated diamond is 5-10 times more "sticky" in the van der Waals regime than glass or plastic (Table 9.1). Manipulator mechanism surfaces should in general be highly noncomplementary to container surfaces, biological surfaces, nanorobot surfaces, and the surfaces of adjacent manipulators. Contact electrification is minimized by using materials with a small contact potential difference between the gripper and the target object.¹¹⁴⁷

A wide variety of specialized complex nanomedical tool tips are readily imagined, although an exhaustive listing or description is beyond the scope of this book. Among the more interesting possible functions (offered without regard to specific implementation) are the following:

1. Injectors -- pipettes, glue applicators, aerators/foamers, and compound grasper/fluid injectors.

2. Adhesion Antennae -- partially selective binding tips that are swept through the environment, whereupon desired moieties or particles adhere and can be removed from the environment or drawn into the nanodevice (i.e., selective biochemical "dust mops"); transmembrane in cyto sensory appendages.

3. Core Sampler -- a rapidly-spinning hollow cylindrical cutting tool that is driven into nearby tissue, whereupon suction is applied, an irising diaphragm closes at the distal end which separates the sample from the main tissue mass, and then the tool is withdrawn, extracting a full cylindrical core sample of neatly excised tissue that is ready for further analysis or transport. A similar tool may be required for ice burrowing (Section 10.5.2).

4. Noncovalent Biochemical Welders -- rejoins noncovalently linked biological structures that have become detached (Section 9.4.4.3); closes gaps in lipid bilayer membranes after being breached by the passage of an nanomechanical appendage or entire nanorobot device (Section 9.4.5.6); reattaches vesicles and organelles to microtubular tracks; reseals intercellular gap junctions.

5. Joining and Spooling Tools -- bobbins, spinning and spooling tools; fiber manipulators including stitching, stapling and threading devices; rivet pin inserters.

6. Compression and Compaction Tools -- extruders, compactors, compression rollers, beam benders, crimpers, and compression fitters including shrink fitters and press fitters.

7. Coarse Shearing Tools -- microscale slitting, scissoring, piercing, punching, trimming, shaving, notching, and perforating; "eggbeater" tools for rapid pulping or liquefaction of semifluid biological materials (Section 10.4.2.5.3).

8. Coarse Machining Tools -- microscale drilling, lathing, sawing, filing, planing, grinding and polishing.

9. Pattern Impression and Coating Tools -- transfer molding, pantographic tracing; surface laminators, abstractors, decorators and other surfacing tools.

10. Membrane Manipulation Tools -- cell and organelle lipid membrane bending, stretching, tearing, and joining; electric field-induced lipid bilayer membrane demixers;¹⁶¹³ transmembrane channel inserters.

11. Molecular Tools and Jigs -- tool tips designed to allow manipulation of individual molecules or molecular structures, perhaps using enzyme-like functionalizations for grasping (e.g., binding sites), cutting (e.g., pepsin, trypsin, collagenases, proteases, lipases, nucleases), joining (e.g., ribozymes, ligases), splicing (e.g., spliceosomes), folding (e.g., molecular chaperones,⁴⁶⁶), copying (e.g., DNA polymerase, reverse transcriptases), appending (e.g., transferases), and packing or pumping (e.g., hexagonal pRNA packs coiled DNA into viral capsids¹⁷²³). The engineering of "wet" enzymes to accurately and reliably function when attached to stiff manipulators will be challenging because any attachment methods (multiple bonds) which will give the tip a high positional accuracy might also interfere with enzyme function. Enzymes generally require nanoscale movement to effect catalysis -- the more tightly the tip is held, the less likely the enzyme will retain functionality.

Tool exchange ports and tool tip garages should lie within the accessible work volume of the nanomanipulator system, to allow convenient and watertight tool tip changeout, or else tool tips must be transferrable internally within the manipulator (e.g., Section 9.3.1.4).

Last updated on 21 February 2003