Microbivores

Various mechanical subsystems are required to support the principal activities of the microbivore digestive system. These support subsystems include the power supply, external and internal sensors, the onboard computer, structural support, and a ballast system to permit nanapheresis.

Power Supply and Fuel Buffer Tankage

The microbivore is scaled for a maximum power output of 200 pW. The power source is assumed to be an efficient oxyglucose powerplant such as a fuel cell, with net output power density of ~109 W/m3. Each powerplant thus requires an onboard volume of 0.2 micron3. Ten powerplants (each one independently capable of powering the entire nanorobot at its maximum power requirement) are included onboard for redundancy, giving a total powerplant volume requirement of 2 micron3.

The microbivore is initially charged with glucose and compressed oxygen (stored in sapphire-walled tankage), and thereafter absorbs its ongoing requirements directly from the bloodstream. Assuming 50% energy conversion efficiency and a 200 pW continuous power production requirement, each glucose and oxygen molecule that are consumed produce 2382.5 zJ or 397.1 zJ, respectively, indicating a peak burn rate of 8.4 ×107 molecules/sec of glucose and 50 ×107 molecules/sec of O2.

The minimum glucose concentration in normal adult human blood is 2.3 ×10-3 molecules/nm3]. From analysis the required glucose current may be supplied by 13 receptor sites on the device surface at the diffusion limit, assuming device radius ~1 micron and receptor radius ~1 nm. However, at the minimum bloodstream concentration a conventional molecular sorting rotor transports ~106 molecules/rotor-sec, so a minimum of 84 rotors are required to provide the required maximum flow. The present design employs 100 glucose rotors for each of the ten independent powerplants. A small number of glucose rotors could also be positioned for uptake inside the digestion chamber, allowing the scavenging of any microbe-derived glucose before the digesta is expelled; however, this facility is not included in the current design.

The minimum free molecular oxygen concentration in normal adult human blood is 3.0 ×10-5 molecules/nm3 in venous blood and 7.3 ×10-5 molecules/nm3 in arterial blood. From additional analysis, the required oxygen current may be supplied at the diffusion limit by ~1200 receptor sites on the device surface, while in arterial blood; by ~2000 receptor sites assuming an average 50%/50% arterial/venous environment during one complete circulation; or by ~6200 receptor sites in venous blood alone. However, at blood plasma oxygen concentrations a conventional molecular sorting rotor transports ~105 molecules/rotor-sec, so a minimum of ~5000 rotors are required to provide the required maximum flow. The present design employs 7500 oxygen rotors for each of the ten independent powerplants, thus retaining full tenfold redundancy throughout.

Waste products from oxyglucose power generation include water and carbon dioxide. There are 50 ×107 molecules/sec of each waste species produced, which may be ejected from the nanorobot using 500 standard sorting rotors for each species, assuming a transport rate of ~106 molecules/rotor-sec. The present design thus employs 500 rotors each for H2O and for CO2, for each of the ten independent powerplants. However, in an emergency these wastes could alternatively be bulk-vented to the external environment without harmful effect -- the effervescence limit for point releases of bulk CO2 in arterial plasma is ~70 ×107 molecules/sec.

The microbivore design thus includes 86,000 small-molecule sorting rotors for energy-molecule transport with full tenfold redundancy, occupying a total of ~8.6 micron2 of microbivore surface area and 0.103 micron3 of microbivore volume. Energy dissipation by the rotor system, if operated at the maximum 200 pW production rate, is 16 pW assuming the transfer of 158.4 ×107 molecules/sec at an energy cost of ~10 zJ/molecule (net energy cost after compression energy recovery). On the microbivore surface, the energy-molecule transport rotors are arranged as compactly as possible into ten lune-shaped sectors (one for each of the ten powerplants) running from front to back (i.e., from ingestion port to exhaust port), with 8600 rotors/lune.

Diamondoid mechanical cables may transmit internal mechanical energy at power densities of ~6 ×1012 W/m3. Therefore a single cable that can transmit the entire microbivore power output of 200 pW may have a volume of ~3 ×10-5 micron3, or ~5 ×10-5 micron3 including sheathing. To connect every powerplant with each of its 9 neighbors via power cables, permitting rapid load sharing among any pair of powerplants inside the device, requires 45 power cables; assuming 1000 internal power cables to accommodate additional power distribution tasks and for redundancy, total power cable volume is 0.05 micron3. By varying the cable rotation rate, the same power cables can simultaneously be used to convey necessary internal operational information including sensor data traffic and control signals from the computers.

Sensors

The microbivore needs a variety of external and internal sensors to complete its tasks. External sensors include chemical sensors for glucose, oxygen, carbon dioxide, and so forth, up to 10 different molecular species with 100 sensors per molecular species. Each 10 nm × 45 nm × 45 nm chemical concentration sensor with 450 nm2 face area is assumed to discriminate concentration differentials of ~10% and displace ~105 nm3 of internal nanorobot volume. Taking chemical sensor energy cost as ~10 zJ/count with ~104 counts/reading, then 10 readings/sec by each of 1000 microbivore sensors gives a maximum sensor power requirement of ~1 pW by a chemical sensor facility that displaces a total of ~0.1 micron3 of device volume and 0.45 micron2 of device surface area.

Acoustic communication sensors mounted within the nanorobot hull permit the microbivore to receive external instructions from the attending physician during the course of in vivo activities. Assuming (21 nm)3 pressure transducers, then 1000 of these transducers displace ~0.01 micron3 of device volume and 0.44 micron2 of device surface area, producing a small net power input to the device of ~10-4 pW when driven by continuous 0.1-atm pulses.

An internal temperature sensor capable of detecting 0.3°C temperature change may have a volume of (~46 nm)3 ~ 10-4 micron3; positioning ten such sensors near each of the 10 independent powerplants for redundancy implies a total internal temperature sensor volume of ~0.01 micron3. An additional 0.03 micron3 of unspecified internal sensors are included in the microbivore design, bringing the total for all sensors to 0.15 micron3.

Onboard Computers

Starting with Drexler's benchmark (400 nm)3 gigaflop mechanical nanocomputer [93], the microbivore computer is scaled as a 0.01 micron3 device in principle capable of >100 megaflops but normally operated at <~1 megaflop to hold power consumption to <~60 pW. Assuming ~5 bits/nm3 for nanomechanical data storage systems and a read/write cost of ~10 zJ/bit at a read/write speed of ~109 bits/sec, then 5 megabits of mass memory to hold the microbivore control system displaces a volume of 0.001 micron3 and draws ~10 pW while in continuous operation. The current microbivore design includes ten duplicate computer/memory systems for redundancy (with only one of the ten computer/memory systems in active operation at a time), displacing a total of 0.11 micron3 and consuming <~70 pW.

Structural Support

The external microbivore hull is taken as a 50-nm thick diamondoid surface of surface area 24.885 micron2 (again excluding the 2 micron2 of ports), a materials volume of 1.2443 micron3. The buckling pressure of a circular diamondoid cylinder of similar dimensions, subjected to crushing forces, is ~300 atm. However, an ellipsoidal hull is considerably weaker than a circular hull so some internal cross-bracing (not included in the present design) might be necessary to resist the ~50 atm force of dental grinding. An additional 0.3799 micron3 of unspecified mechanisms and support structure are included in the present design, which is summarized in the table below.

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Microbivore Baseline Design: External Surface Area, Internal Volume, and Maximum Power Allocations
Microbivore Subsystem	Nanorobot Hull Area Allocation	Internal Volume Allocation	Maximum Power Draw*
	(micron2)	(micron3)	(pW)
Reversible Microbobial Binding Sites
20,000 Receptor Blocks	8.82	0.0882	0.02
Telescoping Grapples
300 Grapple Arms in Silos	0.589	0.177	180
Ingestion Port
Ingestion Port Door	1.0	0.01	3
Port Inlet Excluded Volume	----	0.5	----
Morcellation Chamber
Morcellation Chamber Cylinder	----	2.0	----
Morcellation Chamber Walls	----	0.073	----
10 MC Chopping Blades	----	0.05	100
MC Chopping Blade Housings	----	0.1	----
10 MC Blade Drive Motors	----	0.1	----
MC Ejection Piston	----	0.02	2
MC/DC Interchamber Door	----	0.01	3
Digestion Chamber/Exhaust Port
Digestion Chamber Cylinder	----	2.0	----
Digestion Chamber Walls	----	0.11	----
80,000 Enzyme-Transp. Rotors	----	0.5	2.4
Annular DC Ejection Piston	----	0.02	2
Annular DC Exhaust Port Door	1.161	0.01	3
Waste heat of hydrolysis	----	----	( <5 )
Power Supply and Buffer Storage
10 Powerplants	----	2.0	----
Power Distribution Cables	----	0.05	----
1000 Glucose Import Rotors	0.1	0.0012	0.84
75,000 Oxygen Import Rotors	7.5	0.09	5
10,000 Exhaust Export Rotors	1.0	0.012	10
Glucose Buffer Storage Tank	----	1.0	----
Oxygen Buffer Storage Tank	----	1.3	----
Sensors
External sensors	0.45	0.1	1
Acoustic sensors	0.44	0.01	----
Internal sensors	----	0.04	0.4
Computers
Computer and Memory Storage	----	0.11	< 70
Structural Support
External Microbivore Hull	----	1.2443	----
Unspecified Other Structure	5.825	0.3799	----
TOTALS	26.885	12.1056	< 382.66
Microbivore dry mass	12.2 pg
Microbivore wet mass	17.0 pg
Not all systems are operated at peak power requirement simultaneously; normal power usage is typically 50-150 pW.

 

 

Ballasting for Nanapheresis

The microbivore can alter its overall density to achieve approximately neutral buoyancy, thus allowing convenient removal from the patient's body via nanapheresis after the therapeutic purpose is complete. Nanapheresis in medical nanorobotics refers to the removal of bloodborne medical nanorobots from the body using aphersis-like processes - or where the machines are separated from the blood using centrifugal techniques. Density is altered by exhausting the onboard O2 buffer tank and then pistoning the MC and DC empty, thus establishing a vacuum in both chambers. If either or both of the pistons have failed, the device can still be prepared for nanapheresis by venting the compressed oxygen into the MC and DC, blowing the two chambers clear of fluid and filling this volume with gas, which is substantially similar in density to vacuum from the standpoint of ballasting.

Assuming a mean density of 1900 kg/m3 for diamondoid nanomechanical structure, the "dry weight" of a microbivore is ~12.2 pg, giving a minimum achievable density of ~1000 kg/m3. The density of a fully charged microbivore with both chambers loaded is ~17.0 pg, a net density of ~1400 kg/m3. The mean atomic weight per atom in simple nanomechanical system designs available in 2001 ranged from 7.5-18.8 daltons/atom of structure, with an average of 12 daltons/atom; taking the average figure, the microbivore consists of 610 billion structural or permanent atoms, plus ~15 billion molecules of oxygen when fully charged at 1000 atm and 135 billion molecules of water (solvating 2.52 billion glucose molecules) with both chambers flooded.