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Respirocytes

A Mechanical Artificial Red Cell:
Exploratory Design in Medical Nanotechnology

by Robert A. Freitas Jr.

Research Fellow, Institute for Molecular Manufacturing (IMM)
Palo Alto, California USA

© Copyright 1996-1999, Robert A. Freitas Jr.
All rights reserved.

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3. Baseline Design

Many specific design issues must next be confronted, including tank configuration, rotor and glucose engine placement, subsystem scaling, and the redundancy level required to produce acceptable system reliability. Centralized systems have less complexity, but less redundancy hence less reliability. The final design represents a compromise among many competing factors.

3.1 Power

Onboard power is provided by a mechanochemical engine that exoergically combines glucose and oxygen to generate mechanical energy to drive molecular sorting rotors and other subsystems, as demonstrated in principle in a variety of biological motor systems. Glucose engine design -- possibly involving a ballistic turbine driven by rotor-combustion ejecta operating near ~1000 atm -- is a critical research issue. Drexler [2] estimates engines can be designed to operate at >99% efficiency. However, since natural cellular metabolic pathways using the glycolysis and tricarboxylic acid (TCA) cycles achieve only 68% efficiency, we adopt a more conservative 50% efficiency for the present study. Sorting rotors absorb glucose directly from the blood and store it in a fuel tank. Oxygen is tapped from onboard storage.

The power system is scaled such that each glucose engine can fill the O2 tank from a fully empty condition in 10 seconds, requiring a peak continuous output of 3 x 10-13 watts. This pumping rate, ~108 molecules/sec for the gases, is not diffusion limited because [86] maximum diffusion current J = 4 p R C D ~ 109 molecules/sec, for gas diffusion coefficient D ~ 2 x 10-5 cm2/sec for O2 and CO2 in distilled water at 20 °C [87], C = 7.3 x 1022 molecules O2/m3 (arterial blood), and R = 0.5 micron. Taking Drexler's estimate of 109 watts/m3 for mechanochemical power conversion [2], a glucose engine could measure 42 nm x 42 nm x 175 nm in size, comprising 108 atoms (~10-18 kg).

The glucose fuel tank is scaled such that one tankful of fuel drives the glucose engine at maximum output for 10 seconds, consuming 5% of the O2 gas stored onboard and releasing a volume of waste water approximately equal to the volume of the glucose consumed. Such a fuel tank can measure 42 nm x 42 nm x 115 nm in size comprising <108 atoms (<10-18 kg), hold ~106 glucose molecules and be filled using ~10-3 sec of engine output. Power is transmitted mechanically or hydraulically using an appropriate working fluid, and can be distributed as required using rods and gear trains, or using pipes and mechanically operated valves, controlled by the computer.

3.2 Communications

The attending physician can broadcast signals to molecular mechanical systems deployed in the human body most conveniently using modulated compressive pressure pulses received by mechanical transducers embedded in the surface of the respirocyte. Converting a pattern of pressure fluctuations into mechanical motions that can serve as input to a mechanical computer requires transducers that function as pressure-driven actuators. Mechanisms proposed by Drexler [2] to transmit data at < 10 MHz (~107 bits/sec) using peak-to-trough 10-atm pressure pulses (the same as medical pulse-echo diagnostic ultrasound systems [88]) may be ~(21 nm)3 in size, comprising 105 atoms (~2 x 10-21 kg). Such signals attenuate only ~10% per 1 cm of travel, so whole-body broadcasts should be feasible even in emergency field situations. Pressure transducers consume minimal power because the input signal drives the motion; 0.1-atm MHz-pulse pressure sensors (10 nm)3 in size proposed by Merkle [9] consume only 10-20 joules/pulse detected.

Internal communications within the respirocyte may be achieved by impressing modulated low-pressure acoustical spikes on the hydraulic working fluid of the power distribution system, or via simple mechanical rods and couplings.

3.3 Sensors

Various sensors are needed to acquire external data essential in regulating gas loading and unloading operations, tank volume management, and other special protocols. For instance, sorting rotors can be used to construct quantitative concentration sensors for any molecular species desired. One simple two-chamber design (Figure 3; exhaust manifold not shown) uses an input sorting rotor running at 1% normal speed synchronized with a counting rotor (linked by rods and ratchets to the computer) to assay the number of molecules of the desired type that are present in a known volume of fluid. The fluid sample is drawn from the environment into a fixed-volume reservoir with 104 refills/sec using two paddlewheel pumps. At typical blood concentrations, this sensor, which measures 45 nm x 45 nm x 10 nm comprising ~500,000 atoms (~10-20 kg), should count ~100,000 molecules/sec of glucose, ~30,000 molecules/sec of arterial or venous CO2, or ~2000 molecules/sec of arterial or venous O2.

Figure 3. Molecular Concentration Sensor

contains loading pumps and three molecular sorting rotors
larger image (700x520 pixels, 130 K)

It is also convenient to include internal pressure sensors to monitor O2 and CO2 gas tank loading, ullage (container fullness) sensors for ballast and glucose fuel tanks, and internal/external temperature sensors to help monitor and regulate total system energy output.


Respirocytes Table of Contents

 

3.4 Onboard Computation

An onboard computer is necessary to provide precise control of respiratory gas loading and unloading, rotor field and ballast tank management, glucose engine throttling, power distribution, interpretation of sensor data and commands received from the outside, self-diagnosis and activation of failsafe shutdown protocols, and ongoing revision or correction of protocols in vivo.

A 104 bit/sec computer can probably meet all computational requirements, given the simplicity of analogous chemical process control systems in factory settings [89-90]. That's roughly the computing capacity of a transistor-based 1960-vintage IBM 1620 computer, or about 1/50th the capacity of a 1976-vintage Apple II microprocessor-based PC. Both the IBM 1620 and the Apple II used ~105 bits of internal memory, but even the early PCs typically had access to 106 bits (0.1 megabyte) of external floppy drive memory.

Assuming ~500 bits/sec/nm3 and 1018 bits/sec/watt for nanomechanical computers, and ~5 bits/nm3 for nanomechanical mass storage systems [2], each 104 bit/sec CPU is allocated a volume of ~104 nm3 and consumes ~10-14 watt (3% of the power output of one glucose engine), while 500 kilobits of memory requires ~105 nm3. The use of reversible logic significantly reduces power consumption [2, 91].


Respirocytes Table of Contents

 

3.5 Baseline Configuration

The artificial respirocyte is a spherical nanomedical device 1 micron in diameter consisting of 18 billion precisely arranged structural atoms plus 9 billion temporarily resident molecules when fully loaded. Allocations of device volume and mass were determined by specifying equal O2 and CO2 tank volumes, glucose tank volumes as described in Section 3.1, ballast volume as a variable, and all structural mass as ~diamondoid in density. The ballast system was scaled such that a full water tank achieves neutral buoyancy with all gas and glucose tanks empty, and an empty water tank achieves neutral buoyancy with all gas and glucose tanks full, producing the gross allocations of Table 3 and the detailed allocations of Table 4.

Table 3. Gross Volume and Mass Allocations in Respirocyte Baseline Design

Component   Volume
(% of Device)
  Volume
(meter3)
  Minimum "Dry"
Mass (kg)
  Loaded
Mass (kg)
  No. Atoms or
Molecules
O2 Gas Tank   22.73 %   1.19 x 10-19   0   8.00 x 10-17   1.51 x 109
CO2 Gas Tank   22.73 %   1.19 x 10-19   0   9.72 x 10-17   1.33 x 109
Glucose Tank   0.46 %   2.43 x 10-21   0   3.80 x 10-18   1.27 x 107
Ballast Tank   34.72 %   1.82 x 10-19   0   1.81 x 10-16   6.07 x 109
All Structure   19.36 %   1.01 x 10-19   3.56 x 10-16   3.56 x 10-16   1.79 x 1010
                     
TOTAL   100 %   5.24 x 10-19   3.56 x 10-16   7.18 x 10-16   2.68 x 1010

 

Table 4. Specific Component Allocations in 12-Station Respirocyte Baseline Design

Component   Dimensions
(nm)
  Component
Mass (kg)
  No. per
Station
  Surface Area
per Station
  Total
No.
  Total
Mass (kg)
Computer   ~ 58 (diam.)   3.51 x 10-19   ----   ----   10   3.51 x 10-18
Press. Transducers   21 x 21 x 21   2 x 10-21   2   882 nm2   24   4.78 x 10-20
Sorting Rotors   7 x 14 x 14   2 x 10-21   2,430   79,380 nm2   29,160   5.82 x 10-17
   O2   "   "   144 x 3 stages   14,112 nm2   5,184   1.04 x 10-17
   CO2   "   "   144 x 3 stages   14,112 nm2   5,184   1.04 x 10-17
   H2O   "   "   504 x 3 stages   49,392 nm2   18,144   3.61 x 10-17
   Glucose   "   "   18 x 3 stages   2   648   1.29 x 10-18
External Sensors   14 x 42 x 42   ~ 1 x 10-20   24   14,112 nm2   288   2.88 x 10-18
   O2   "   "   10   5,880 nm2   120   1.20 x 10-18
   CO2   "   "   10   5,880 nm2   120   1.20 x 10-18
   H2O   "   "   2   1,176 nm2   24   2.40 x 10-19
   Glucose   "   "   2   1,176 nm2   24   2.40 x 10-19
   Temperature   "   "   2   1,176 nm2   24   2.40 x 10-19
Internal Sensors   14 x 42 x 42   ~ 1 x 10-20   ----   ----   150   1.60 x 10-18
   O2 Tank   "   "   ----   ----   10   1.00 x 10-19
   CO2 Tank   "   "   ----   ----   10   1.00 x 10-19
   H2O Tank   "   "   ----   ----   10   1.00 x 10-19
   Glucose Tank   "   "   2   ----   24   2.40 x 10-19
   Temperature   "   "   ----   ----   10   1.00 x 10-19
Glucose Engine   42 x 42 x 175   1.46 x 10-18   1   1,764 nm2   12   1.75 x 10-17
Glucose Tank   42 x 42 x 115   8.01 x 10-19   1   ----   12   9.61 x 10-18
Glucose Flues   7 x 14 x 14   2 x 10^-21   3   294 nm2   36   7.20 x 10-20
Other Structure   ----   ----   ----   ----   1   2.63 x 10-16
                         
TOTAL   1000 (diam.)   ----   ----   97,603 nm2   12   3.56 x 10-16

The system presented here has at least tenfold redundancy in all components, excluding the pressure tanks which, because of their compartmentalized structure (Section 3.6), may be regarded as having even greater redundancy. This multiplicity was chosen following Drexler's suggestion [2] that tenfold redundancy in nanomechanical systems should suffice to reduce system failure to negligible levels.

Twelve pumping stations are spaced evenly along an equatorial circle. Each station has its own independent glucose engine, glucose tank, environmental glucose sensors, and glucose sorting rotors (Figure 4). Each station alone can generate sufficient energy to power the entire respirocyte. Detailed reliability simulations will be required to determine whether stations should run (1) at peak power on a rotating schedule, (2) at partial power on a continuous basis, or (3) one at a time until failure, switching to the next backup. Power is transmitted hydraulically to local station subsystems and also along a dozen independent interstation trunk lines that allow stations to pass hydraulic power among themselves as required, permitting load shifting and balancing.

Figure 4. Glucose Rotor, Tank, Engine and Flue Assembly in 12-Station Respirocyte Baseline Design

top view and side view
larger image (904x1252 pixels, 371 K)

For tenfold redundancy, ten duplicate computer/mass-memory sets are located at the center of the device in a spherical 106 nm3 volume. This location offers maximum shielding from environmental insults and centralized access to all surface components including communications links, external sensors, and distributed power supply. Any of the 10 computers at the core can receive power or communications directly from any of the 12 pumping stations along hard links in protected utility conduits.

Each pumping station has an array of 3-stage molecular sorting rotor assemblies for pumping O2, CO2, and H2O into and out of the ambient medium. The number of rotor sorters in each array is determined both by performance requirements and by the anticipated concentration of each target molecule in the bloodstream (Table 5). Any one pumping station, acting alone, can load or discharge any storage tank in ~10 sec (typical capillary transit time in tissues), whether gas, ballast water, or glucose. Gas pumping rotors are arrayed in a noncompact geometry to minimize the possibility of local molecule exhaustion during loading (Figure 5A).

Table 5. Typical Blood Plasma Concentrations of Relevant Molecules

Blood
Gas Molecule
  Concentration
(molecules/m3)
O2 (arterial)   7.3 x 1022
O2 (venous)   3.0 x 1022
CO2 (arterial)   4.1-10.8 x 1023
CO2 (venous)   4.5-11.4 x 1023
H2O (plasma)   3.1 x 1028
Glucose   2.3-3.5 x 1024
N2   2.1 x 1023

Figure 5A. Pumping Station Layout (One Dodecant (30°) of Sphere Surface)

layout of one twelfth of surface
larger image (1352x672 pixels, 277 K)

The design allows for significant numbers of outbound impurity return rotors because the gas rotor systems are actually scaled for greater than 10:1 redundancy. For O2 we only need 4530 rotors for 10:1 redundancy; we have 5184 rotors, so 654 rotors or 12.6% of them (54.5 per station) are not necessary to meet the 10:1 redundancy requirements and can be allocated for impurity return. For CO2, we need 3990 but have 4320, so 330 or 7.6% of them (27.5 per station) can be used for impurity return. The glucose rotors are grossly overdesigned to ensure energy supply even in the most hypoglycemic patients. We need only 40 rotors for 10:1 redundancy but have 216, so even if half are required for impurity return we achieve ~27:1 redundancy.

Each station also includes three glucose engine flues for discharge of CO2 and H2O combustion waste products, 10 environmental oxygen pressure sensors distributed throughout the O2 sorting rotor array to provide fine control if unusual concentration gradients are encountered, 10 similar CO2 pressure sensors on the opposite side, 2 external environment temperature sensors (one on each side located as far as possible from the glucose engine to ensure true readings), and 2 fluid pressure transducers for receiving command signals from medical personnel.

The equatorial pumping station network (Figure 5B) occupies ~50% of respirocyte surface. On the remaining surface, a universal "bar code" consisting of concentric circular patterns of shallow rounded ridges is embossed on each side, centered on the "north pole" and "south pole" of the device (Figure 5C). This coding permits easy product identification by an attending physician with a small blood sample and access to an electron microscope, and may also allow rapid reading by other more sophisticated medical nanorobots which might be deployed in the future.

Figure 5B. Equatorial Cutaway View of Respirocyte

view cut through water and gas ballast tanks
larger image (968x1038 pixels, 313 K)

 

Figure 5C. Polar Cutaway View of Respirocyte

cutaway view from the top
larger image (804x849 pixels, 138 K)


Respirocytes Table of Contents

 

3.6 Tank Chamber Design

Each storage tank is constructed of diamondoid honeycomb or a geodesic grid skeletal framework for maximum strength. Thick diamond bulkheads separate internal tankage volumes. Available structural mass is equivalent to a 10-nm thick (~60 carbon atoms) 2.2 micron x 2.2 micron diamond sheet, enough material for 1000 compartments ~(40 nm)3 in size for all tanks. Compartment walls are perforated with sufficient holes of varying sizes to allow gas to flow easily between them, with larger compartments nearest the rotors graduating to smaller compartments more distant from the rotors to encourage isobaric entrainment.

The present design includes separate O2 and CO2 chambers. In theory, these gases could be stored mixed in a single chamber. A single chamber design can effectively double the O2-carrying capacity of each respirocyte by allowing the entire gas tank volume to be initially charged with oxygen at 1000 atm. There are four minor drawbacks to this approach: (1) Respiration is controlled by CO2, not O2, levels, requiring maintenance of sizable CO2 inventories at all times, reducing surplus volume available for O2 storage; (2) respirocytes may be deployed to reverse serious tissue CO2 overloading, requiring significant available storage volume to absorb this gas; (3) the rate of binding for outbound transport by sorting rotors may be lower for mixed gases, reducing maximum outgassing rate; and (4) inability to emergency vent pure gas.

© Copyright 1996-1999, Robert A. Freitas Jr. All rights reserved.

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