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

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

6.3.2 Mechanical Energy Conversion Processes

It has been suggested⁵⁴⁰ that "simple physical shaking" may be sufficient to provide power to nanorobots, much like a self-winding watch that employs mechanical rectification. Material moves through human lymphatic vessels (Section 8.2.1.3) in this manner: The lymphatics have no direct musculature (or cilia) of their own, but depend upon external physiological motion sources such as blood vessel contractions and skeletal movements to induce periodic pressures on the lymphatic vessels, moving material along via oneway valving. Experiments using respiratory chambers found that spontaneous physical activity or "fidgeting" consumed an average of 348 Kcal/day (range 100-700 Kcal/day) or ~17 watts of power²⁹³⁵ and could be a major cause of individual differences in 24-hour energy expenditure.^2936,2937 In a nanorobot, a mechanomechanical transducer would convert environmental motion into mechanical energy for internal storage or immediate utilization.

Consider a medical nanodevice embedded in the hand of a human being who is waving his arm back and forth in a perfect sinusoidal motion of lateral amplitude X_move meters and period t_move seconds per cycle (t_move^-1 Hz). If the mechanomechanical transducer consists of a spring-loaded bob mass of density r_trans, volume V_trans and mass m_trans whose motion is rectified using a ratchet and pawl mechanism, the maximum velocity of the mass is 4 X_move / t_move at the centerpoint of the armswing, the acceleration profile crudely resembles a square wave of amplitude A_move = 16 X_move / t_move², and the maximum extractable nanorobot power P_n is:

(watts) {Eqn. 6.12}

For r_trans = 21,450 kg/m³ (platinum bob), V_trans = 1 micron³, T= 310 K inside the human body, and k = 0.01381 zJ/K (Boltzmann constant), the constant microtwitching of human muscles of ~0.1 mm amplitude and ~10 Hz provides a mechanical energy of ~kT, hence P_n ~ 0 from this source. Ignoring the 10 g, 1-second accelerative impulse due to passage through the beating heart, each ~1 meter circulation of the blood involving a ~60 sec passage from an aortal velocity of ~1 m/sec to a capillary velocity of ~1 mm/sec, provides only P_n ~ 10^-6 pW. The twice per circuit passage through the beating heart raises the full-circuit total to P_n ~ 10^-3 pW; that is, most of the mechanical energy in each circuit is localized in the heart. Nanorobots with a 1 micron³ transducer resident in the diaphragm during normal respiration can obtain P_n ~ 3 x 10^-6 pW assuming X_move ~ 2 cm, t_move ~ 3 sec. In normal walking with X_move ~ 0.6 m and t_move ~ 1 sec, P_n ~ 0.06 pW; for the most violent handwaving exercise with X_move = 0.3 m and t_move = 0.2 sec (5 Hz), P_n = 2 pW. All of these figures optimistically assume 100% mechanical efficiency.

Nanorobots resident in the chest walls experience cyclical displacements of X_move ~ 2.5 cm and t_move ~ 3.3 sec (0.3 Hz) for normal shallow breathing at 10/min, up to X_move ~ 5 cm and t_move = 1 sec (1 Hz) for deep breathing during heavy exertion (Section 8.2.2), a chest velocity range of v_move = 2-10 cm/sec, giving P_n = 3-400 x 10^-6 pW (Eqn. 6.12) for a pendulum transducer. Another alternative in the chest is a simple spring-loaded stretch transducer. If the diaphragm and chest wall muscles cost P_chest = 1-25 watts to operate depending upon exertion level, then the applied force F_chest = P_chest / v_move = 50-250 N; an X = 5 cm displacement for a 90-cm circumference chest cavity gives a d ~ 5% distension. Thus a two-phase L = 100 nm stretch transducer produces P_n = 2 d L e% F_chest / t_move = 150-2500 pW, conservatively taking mechanical efficiency e% = 0.001(0.1%) to account for poor coupling and a highly nonisotropic tissue stress tensor. Similar power levels are available in contractile cardiac tissues, near joints that flex during normal arm or leg motions, and in the pedal dermis during walking.

Another example of mechanomechanical transduction is a simple gear train, which transmits mechanical rotational power from one location to another. In one example given by Drexler,¹⁰ a 17 nm³ steric gear pair transmits 1 nanowatt of mechanical power, giving a power density of 6 x 10¹⁶ watts/m³ with a mechanical efficiency of 99.997%. Complex hectomicron-scale gear trains have been fabricated.⁵⁵⁸ Sandia's Microelectronics Development Laboratory massproduces 100-micron motors and gears.¹²⁵⁹ Properly designed molecular bearings will have lifetimes that are not limited by wear but only by the static lifetime of the bearing (e.g., due to radiation damage).

In 1998, there was at least one unconfirmed experimental report of direct mechanochemical energy transduction,²⁹²⁴ in which the mechanical energy of stirring of water over a catalyst bed of powdered cuprous oxide was alleged to produce H₂ and O₂ gas effluent.

Mechanoacoustic transduction may be achieved by operating the pressure-driven actuators described in Section 6.3.3 in reverse, using an oscillating mechanical energy input such as a reciprocating rod to drive the piston. See Section 7.2.2.1 for a more complete treatment of the vibrating piston acoustic radiator.

Mechanoelectric transduction is commonplace in atomic force microscope (AFM) sensors using piezoresistive cantilevers that produce a varying electrical potential in response to changing mechanical loads. Typical coupling constants (e.g., mechanoelectric efficiency) are 11% for polyvinylidene fluoride (PVDF), and 35-59% for lead zirconate titanate (PZT).⁹⁹³ Transduction of mechanical to electrical signals can also be achieved by mechanically modulating a tunneling contact junction as a mechanically controlled electrical switch, as occurs at the tip of a scanning tunneling microscope (STM) in response to varying physical loads. Fullerene molecules also change their electrical resistance in response to mechanically-applied loads, providing mechanoelectrical switching (Section 10.2.2). Alternatively, operating the nanoscale electrostatic DC motor¹⁰ described in Section 6.3.5 in reverse converts it into a mechanoelectric generator, transforming rotational mechanical energy into electrical current at high efficiency. Cochlear outer hair cells are capable of both mechanoelectrical and electromechanical transduction.³⁵⁹⁷

Cardiac mechanoelectric transducers have also been proposed or tested.^{593,722,723,3513-3517} In 1966, Ko⁶³³ fabricated a mechanical converter that converted heart movement into the vibration of a piezoelectric rod to generate AC electric power that was rectified and used to power a pacemaker; such devices were installed on dogs' hearts and generated 30 microwatts for several months. Electropaced skeletal muscles (latissimus dorsi) have been used to compress an implanted plastic balloon counterpulsation device to produce pulmonary artery diastolic pressure augmentation.^3519-3521 There are proposals to use piezoelectric bimorphs to transform the mechanical energy of diaphragm motions during breathing^592,3516 and of the circumferential movements of the aorta and other elastic arteries during each pulse cycle^593,3522 into electrical energy, but transducer fatigue and biocompatibility issues were problematical.^{590,3517,3518} Piezoelectric fields developed in moving bones^3090-3095 and collagenous tissues^1939-1942 possibly could be tapped. Indeed, many biological materials have been found to be piezoelectric, including tendon, elastin, silk, dentin, ivory, wood, aorta, trachea, intestine, and even nucleic acids.³⁰⁸⁹ A gait-powered battery-charging system exploited ground reaction forces associated with the heel-strike and toe-off phases to convert the autologous mechanical energy of walking into electrical energy using a piezoelectric array embedded in the midsole of a shoe.³⁵²³ Shoe manufacturers have already put LEDs in sneakers (LED flashing is powered by footfall energy), and handcranked power supplies for laptop computers were being developed by Freeplay Power Group in 1999.³¹⁸⁵ Keyboard typing produces 0.130 milliwatts/finger (Section 7.4.2.1).

Piezoluminescent crystals provide mechanooptical power transduction, and micromachined optical shutters ⁵⁴⁶ have demonstrated mechanical switching of optical signals.

Last updated on 18 February 2003