The design of the space bioreactor stems from three considerations. First, and foremost, it must sustain cells in microgravity. Closely related is the ability to take advantage of the weightlessness and microgravity. Lastly, it should fit into a bioprocess. The design of the space bioreactor is described in view of these considerations. A flow chart of the bioreactor is presented and discussed

Cross, John H.

English

NASA Technical Reports Server

N88-1?'1?'0 
SPACE BIOREACTOR: DESIGN/PROCESS FLOW 
The 
and 
abi 
John H. Cross 
Technology Incorporated 
design of the space bioreactor stems from three considerations. 
foremost, it must sustain cells in microgravity. 
ity to take advantage of the weightlessness of microgravity. 
First 
Closely related is the 
Lastly, it 
should fit into a bioprocess such as shown in figure 2-1. 
describe the design of the space bioreactor in view of these considerations. 
The flow chart of the bioreactor will then be presented and discussed. 
This paper will 
The conceptual space bioprocess in figure 2-1 yields pure product after six 
steps. The initial step is to select the cells that produce the most prod- 
uct. Studies already conducted in space have shown that some cell fractions 
selected by electrophoresis produce more of a given substance than others. 
The next step is to prepare a bioreactor inoculum from these cells.' This 
step is shown in a dotted box, because it has not been investigated in space. 
The third and fourth steps are those covered by the space bioreactor. The 
bioreactor design consists of two loops. The first loop contains the cell 
culture vessel and all the apparatus necessary to sustain the cells. The 
second loop, isolated from the first, has an ultrafilter or similar device 
for separation of the product from the cell culture media. The fifth step, 
again not tested in space, is concentration to the point the solution of 
product can be introduced into a continuous flow electrophoresis system for 
final purification. This last step has been quite well developed by the 
McDonnell Douglas Astronautics Company, Inc. 
An important driving force for building a space bioreactor is clear from 
figure 2-1. The first and last steps are more efficient in space. 
ing the two with a space bioreactor makes possible a fuller exploration of 
the advantages of space for manufacturing pharmaceuticals. It should also 
be pointed out that the cells for the first step could be grown in space in 
the bi oreactor. 
Connect- 
The next consideration in the design is to take advantage of microgravity. 
Cells cultured in the weightlessness of space will not sediment. The only 
stirring required is to distribute oxygen and nutrients to the cells. 
amount of stirring is expected to be considerably less than is needed to keep 
the cells suspended in 1-g. Less stirring means less turbulence and a lower 
rate of cell death. 
cell concentrations can be cultured.' To allow for the best experimentation 
on this hypothesis, the cell culture vessel has few internal parts. The 
sensors and other parts are instead placed in the first loop mentioned above. 
The medium is pumped around the loop and passes over sensors for oxygen, 
This 
Consequently, it is hypothesized that in space higher 
19 
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carbon dioxide, and pH. The oxygenator and other necessary vessels are also 
in this loop, called the main medium circulation loop. The reactor vessel 
has in it only a stirring vane and a spin filter, which keeps the cells in 
the reactor. 
What type of cells would benefit most from a minimal stirring situation? 
After an assessment of different cell culture systems, it was concluded that 
the answer was mammalian cells on microcarrier beads. Specifically, human 
embryonic kidney cells have been selected for the first flights because they 
have been separated in space by electrophoresis. These cells produce uroki- 
nase, a pharmaceutical that dissolves blood clots. Many other types of mam- 
malian cells could be cultured with minor procedural modifications. Details 
of the reactor design reflect the culture requirements of mammalian cells. 
A second advantage of microgravity is expected to come from the stability of 
foams in space.* On Earth a foam of a gas in a liquid separates rapidly as 
the gas rises to the top. In space this density-driven separation does not 
occur and surface tension forces the gas bubbles from coalescing. One pro- 
posed experiment is to study oxygenation of a cell culture with a foam o f  
tiny air or oxygen bubbles. The modular bioreactor design, a circulation 
loop connecting specialized vessels, allows for insertion of the apparatus 
needed for this experiment. Hopefully, this flexibility will prove to be 
general. Other experiments might be proposed after the data from several 
flights of the bioreactor clarify the effects of microgravity on cell 
cu 1 ture. 
The combination of operating in microgravity and in the Shuttle Orbiter vehi- 
cle places certain constraints on the bioreactor design (table 2-1). 
Headspace will not exist in the cell culture vessel in microgravity. Many 
bioreactors use this headspace as a reservoir for oxygen and carbon dioxide. 
In the space bioreactor, the vessel is kept full of medium. Without head- 
space, the bearings for the spin filter and the stirring vane are immersed in 
the medium. Consequently, a potential exists for grinding cells between the 
bearing surfaces. Designs for the cell culture vessel include versions with 
sleeves that cover the bearings and versions that route cell-free medium 
through the bearings. 
The remainder of the items in table 2-1 are operational constraints. 
plete liquid containment is mandatory, because spills in space cannot be 
easily cleaned up; the bioreactor uses magnetic drives to minimize points for 
leaks and stainless steel tubing instead of plastic tubing. 
possibility of contamination during operation in a nonsterile environment, 
the bioreactor is autoclaved as a sealed unit. 
it with medium and cells. The bioreactor must be compact and energy effi- 
cient to allow the maximum number of experiments to be carried out on each 
Com- 
To minimize the 
It is opened only to charge 
20 
Shuttle flight. The lessons learned from these design constraints may give 
industrial biochemists clues for reducing capital and operating costs. Auto- 
matic operation of the bioreactor is another consequence of trying to maxi- 
mize the number of experiments on every flight; the crewmembers can spend 
only so much time with every experiment. The space bioreactor design in- 
cludes an advanced process control scheme (described later) that provides 
for operating the system, logging data, and controlling the dynamics of a 
biological system. 
This completes the discussion of the design considerations for the biore- 
actor. The next subject is a description of the flow chart. To begin this, 
table 2-2 summarizes the essential elements of any bioreactor. 
discussion will show how these elements are expressed in this particular 
design. Detailed descriptions of individual components are given in the 
paper by William Bowie. 
The following 
The culture vessel, labeled reactor vessel, is a 500 ml perfused vessel with 
a new version of a spin filter from Virtis Company (fig. 2-2 and table 2-3). 
The cells are, as stated, anchored on microcarrier beads and suspended in the 
culture vessel. To get the desired gentle mixing, a stirring vane similar to 
that described by Feder and Tolbert3 will be tested initially. The reactor 
is designed to operate with no headspace. 
circulation loop, Stream 1, is pumped into the reactor vessel and withdrawn 
through the spin filter, which allows the medium to pass, but retains the 
microcarrier beads with the cells. 
(do,), CO,, and redox potential before and after the reactor in the medium 
circulation loop provides for differential measurements across the reactor 
and reduces turbulence and shear within the reactor. 
Medium from the main medium 
Locating sensors for pH, dissolved oxygen 
In Stream 1 are vessels for oxygenation, separation o f  depleted medium (upper 
left, Waste or Sample Tap), replenishment of medium, and pH control. A con- 
nection to the product separation loop, Stream 3, is also provided. A dial- 
ysis unit to separate wastes is contemplated, but has not been incorporated. 
Provision is made to maintain the entire loop at 37" f 1°C. The oxygenator 
has a membrane that is permeable to oxygen and carbon dioxide. A gaseous 
mixture of nitrogen and oxygen (possibly room air) is passed through one 
side o f  the membrane. Carbon dioxide from metabolism passes from the medium 
to the gas mixture and is carried away. This design allows for addition of 
CO, if desired. System pressure is designed for 5 psig with a maximum of 15 
psig; four transducers are incorporated in the system. Concentration of the 
medium is done in Stream 3 by ultrafiltration or hollow fiber filtration. 
This loop will be run every 3 or 4 days to provide a solution of urokinase 
and other cell products for analysis. (In a complete bioprocess, this loop 
would be designed to provide crude urokinase to a purification unit, figure 
2-1, steps 5 and 6.) 
21 
The process control scheme has two levels o f  control (fig. 2-3). 
depicted at the top maintains set levels of pH, do,, CO,, and temperature. 
These variables determine the environment o f  the cells. The environmental 
controller is being built in the JSC labs from a SPD bus system. This system 
has been used successfully for the McDonnell Douglas continuous flow electro- 
phoresis system and the 3M crystallization unit. In addition to operating 
the system, the STD bus system logs data from the biochemical sensors, the 
flow meters, and the pressure transducers. This part o f  the process control 
system will fly. In many processes, controlling the environmental variables 
would control the process. A biological system, however, is more complex. 
To obtain the extra control required, a second level of control has been 
designed to monitor the metabolism of the cells. 
The level 
This function, termed an "expert system," will supervise the process con- 
troller by adjusting the environmental set points. The metabolic parameters 
now thought to be the most important are the respiratory quotient (dO,/CO, 
concentration ratio), concentration of NADH, and the type and quantity of 
cell secretory products, such as urokinase. Some of the data used by the 
expert system will be from online sensors, but results from reactor samples 
and subjective evaluations o f  previous experience will be incorporated using 
"fuzzy m~delling."~ An IBM AT computer will be used. The expert system 
computer will be used for development and other ground-based operations. 
More details are given in the paper by Bill Hall. 
REFERENCES 
1. Morrison, D. R., "Bioprocessing in Space-An Overview," in the World 
Biotech 1984, Vol. 2: USA. Published by Online Conference Inc., N.Y. 
ISBN-0-86353-004-4, pp. 557-571, 1984. 
2. Fester, D. A., Eberhardt, R. N., and Tegart, J. R., "Behavior o f  Fluids 
i n  a Weightless Environment," Colloquium on Bioprocessing in Space, pp. 
37-52, Johnson Space Center, Houston, Texas, March 10-12, 1976. 
3. Feder, J. and Tolbert, W. R., "The Large Scale Cultivation of Mammalian 
Cells,Il Scientific American, 248:36-43, 1983. 
4. Turunen, I., Nyberg, T., Jarvelainen, J., Linko, Y-Y., Lindo, P., and 
Dohnal, M., "Fuzzy Modeling in Biotechnology: Sucrose Inversion," The 
Chemical Engineering Journal, 30:B51-B60, 1985. 
22 
TABLE 2-1.- HARDWARE CONSTRAINTS 
No Headspace 
Complete Liquid Containment 
Autoclave 
Compact 
Energy E f f i c i e n t  
Automatic 
TABLE 2-2.- ESSENTIAL ELEMENTS 
Oxygenat e 
Resupply Nutrients 
Remove Wastes 
Remove Product 
Collect Biochemical Data 
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ADJUSTED 
ENVIRONMENTAL 
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VARIABLES 
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S E T  P O I N T S  
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DATA 
REDUCTION 
CONTROL SENSORS 
ACT I O N S  
Figure 2-3.- Process control scheme. 
27 


Space bioreactor: Design/process flow

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