Nanomedicine, Volume I: Basic Capabilities
© 1999 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999
9.3.1 Nanoscale Manipulators
126.96.36.199 Biological Cilia
Biological cilia are motile, hairlike cylindrical processes typically 200-300 nm in diameter, 2-20 microns in length, and ~10-1610-15 kg in mass.338,396,936,939,1394,1449 They are found in large numbers on some cell surfaces. Unicellular organisms use cilia both for locomotion and for food collection, while multicellular organisms use cilia primarily for mass transport of the environment past the cell.
Each cilium is bounded by an extension of the cellular plasma membrane (Fig. 9.2A), rising from a cytoskeletally-anchored centriole-like basal body consisting of an array of 9 triplets of common-walled microtubules ~150 nm in diameter and 300-500 nm in length. Sprouting upward from the basal body is the primary ciliary structure, called the axoneme. The axoneme is a cylinder of microtubules (Section 188.8.131.52) with 9 outer doublets of microtubules and two additional complete microtubules in the center, often called the central pair (Fig. 9.2B). The 9 outer doublets are extensions of two of the three microtubules comprising each of the nine basal body triplets. Each outer doublet of the axoneme consists of the A tubule (a complete 25-nm outer-diameter microtubule with the usual 13 protofilaments made of tubulin dimers), the B tubule (an incomplete microtubule with only 10-11 protofilaments), and a common wall containing the protein tektin (an intermediate-filament-like protein with high tensile strength) (Fig. 9.2C).
A set of 1-2 megadalton dynein side arms project outward from each of the A tubules, reaching clockwise toward the B tubules of the adjacent doublet (Fig. 9.2D). The dynein arms occur in pairs, one inner arm and one outer arm, spaced at regular ~30 nm intervals along the microtubule. Nexin protein interdoublet links at wider intervals join the outer doublets making circumferential rings, and radial spokes project inward from each of the 9 microtubule doublets. Microtubules do not change length during ciliary movement. Rather, the stalk of each dynein arm attaches to and detaches from the adjacent B tubule in an ATP-driven cyclic process,3551-3554 pulling the stalk the length of one dimer along the tubule with a velocity of ~14 microns/sec.1450 The maximum force generated by a dynein arm has been measured as ~1 pN.452 The differential stress of adjacent doublet columns causes the axoneme to flex. Selective sidearm bonding and unbonding allows the cilium to generate a bend anywhere along its length.
In early experiments, bullfrog gullet ciliary surfaces developed a mechanical motive power of ~0.01 watts/m2;526 assuming 1-10 micron2/cilium across the gullet surface gives an estimated mechanical power output Pcilium ~ 0.01-0.1 pW/cilium. Similarly, hair cell cilia of the human inner ear446 respond to a minimum 2 x 10-5 N/m2 (2 x 10-10 atm) pressure with a minimum 0.1 nm deflection at an applied force of ~10-3 pN, indicating a mechanical stiffness of ~10-5 N/m. (Axoneme bending moment is ~7 x 10-17 N-m.1449) Assuming a maximum Dp ~ 105 watts/m3 power density in nonmuscular working tissue (Table 6.8) and a ciliary volume of ~0.1-1 micron3 also gives an estimated mechanical power output of Pcilium ~ 0.01-0.1 pW/cilium.
A simple cilium 12 microns long may beat at a frequency ncilium ~ 30 Hz with an angular velocity of 12 deg/millisec and a tip velocity of 2500 microns/sec in the effective stroke, while the bend in the recovery stroke propagates at ~350 microns/sec.1394 (A 600-micron long compound cilium beats at ~20 Hz with angular velocity 10 deg/millisec and a 17 mm/sec recovery stroke.1394) Taking Pcilium ~ 0.05 pW and vcilium ~500 microns/sec , then Fcilium ~ Pcilium / vcilium ~ 0.1 nN.
Last updated on 20 February 2003