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important because of its high resolution and versatility. Electrophoresis is based on the electrical characteristics of materials in solution or
suspension and is mostly used for the separation of complex biological mixtures.
Space and time do not permit a thorough technical review of the reasons that electrophoresis was singled out for this detailed study. Some of these were presented in my prior testimony to the Subcommittee on Space Science and Applications (1), and others are included in the reprint submitted in the Appendix. It will suffice to state that electrophoresis requires stabilization of fluid against gravity-dependent disturbances. This stabilization is achievable on the ground by a variety of subterfuges, but the weightless condition prevailing in orbiting spacecraft offers an entirely new approach to the control of these gravitydependent phenomena. The possibility of developing new electrophoretic techniques based on weightlessness proved to be of considerable attraction to a number of scientists, who have cumulatively spent a great deal of time serving as consultants, committee members and contractors to the NASA project. As a result of this international effort, the potential advantages of weightlessness for electrophoresis have been confirmed in pilot experiments conducted aboard the Apollo 16 (2), the Skylab (3), and the Apollo-Soyuz flight (4,5). One of these experiments (5) was developed by the German government, as part of the European Space Research
The current interlude between these pioneering experiments and the advent of the Shuttle is permitting a thorough reevaluation of the means and objectives of this program, in terms of equipment and candidate materials for space processing:
a. Optimization of Equipment. It is important to emphasize that electrophoresis is a term which encompasses a family of interrelated tech
niques and instrumentation.
The reason for this multiplicity of approaches
is to be found in the very versatility of the basic process of electrophoresis which can be adapted in manifold ways to suit particular needs.
The prototype instruments flown in prior space missions were developed to meet the severe constraints of time, space, and facilities available aboard the Apollo spacecraft, and should not be adopted for the Shuttle without thorough multidisciplinary reevaluation. This has been partly accomplished already due to the mathematical modelling and theoretical evaluation of the performance of continuous flow instruments in zero and one-G environments by Ostrach (6), Saville (7) and Giannovario and Griffin (8). Their conclusions will surely be incorporated into future flight instruments.
Of more serious concern is that all instruments used in the past space experiments have been adaptations of ground based equipment. Bier et al (9) and Strickler (10) have proposed instruments more specifically designed for the weightless environment, and other such instruments were, no doubt, incorporated into various proposals submitted to NASA. While it may be difficult to pursue all alternatives, the development of new concepts should be encouraged. Only through innovative approaches taking full advantage of the weightless environment can true breakthroughs in space processing be realized.
The above instrumental design considerations are nevertheless relatively simple to evaluate as they fall into the province of engineering. More complex is the selection of condidate materials for space processing, for this demands value judgments and crystal gazing into future needs in a
rapidly changing state of art. This requires separate discussion of electrophoresis of living cells and electrophoresis of proteins and other soluble biomolecules, for these two groups of candidate materials have different instrument requirements.
b. Cell Electrophoresis. At the inception of the NASA program, some six or seven years ago, the main emphasis was on the development of a space facility for the electrophoretic separation of cells. This application appeared particularly timely. Rapid advances in cellular immunology and the recognition of the manifold functions of lymphocytes have shown the need to obtain functionally pure cell subpopulations. Electrophoresis appeared particularly promising as ground-based techniques were lagging and the field was nearly dormant.
The situation has changed significantly in the last few years as characterization of cell subpopulations has become central to much of present immunological research. Several powerful non-electrophoretic techniques for cell separation have been developed such as electrostatic cell sorting, sedimentation techniques, affinity chromatography, phase partition with polymer systems, etc. There have also been significant new developments in both analytical and preparative cell electrophoresis. To a large degree these advances were the result of the stimulus provided by the NASA program and can be considered a major benefit of the space program.
The fact that there are now several new methods for ground-based separation and characterization of cells does not invalidate the premise that cell electrophoresis may benefit from the microgravity environment. To the contrary, the new methods permit clearcut focusing on the most promising areas of space exploration and a thorough evaluation of candidate
materials. These will also obviate the need for much of the space experimentation and will result in a significant saving of funds in the develop
ment of useful space processes.
For cell electrophoresis, the crucial point is to obtain solid data on the relationship between cell mobilities and the desired cell functions, whether these be immunological, physiologic, metabolic or synthetic. It should be emphasized that the establishment of the relationship between surface characteristics of living cells, as reflected by their electrophoretic mobilities, and cell functions is not a trivial matter of only pragmatic value for the space program. Quite the contrary, it is an important scientific problem for cell biology, which merits fundamental research in its own right, i.e., independently of the space program. It is increasingly evident that cell surfaces play a key role in many cell functions including immunologic recognition, cell-cell interaction or organ differentiation.
Summarizing the case for cell electrophoresis in space, we wish to emphasize that: (1) Studies of cell subpopulations are an important area of scientific endeavor and there is great need for simple methods for their separation. Electrophoresis can make a significant contribution. (11) Current ground based techniques for cell electrophoresis have some inherent problems, such as poor resolution in the continuous flow method, sedimentation and artifacts due to polymeric materials in density stabilization methods. More rapid means for cell separation are also needed, as well as the possibility of using more physiological buffers. There is no doubt that these problems can be significantly alleviated or completely avoided in a weightless environment. (iii) There is urgent need for more groundbased research to develop artificial means for increasing resolution of
cell electrophoresis and to obtain better data on mobility distributions in a variety of cell populations.
c. Protein electrophoresis. The situation is rather different for protein electrophoresis, where there are a variety of well established techniques which are in world-wide use in literally thousands of laboratories. There are no major unresolved scientific problems and the usefulness of electrophoresis is based on the fact that there is an exquisite correlation between the molecular structure of the protein and its electrophoretic characteristics. As a result, electrophoresis is used for various analytical and micropreparative purposes in the most diverse areas of biological research, clinical medicine, and quality control in industry. Despite numerous efforts, ground-based technology has failed in one important respect: scaling up the capacity of the instruments for the separation of meaningful quantities of purified components. There are no theoretical reasons that electrophoresis could not be used on the largest possible industrial scale, only limitations in present instrument designs. Some of these limitations may be gravity-dependent, due to the need for fluid stabilization against gravity driven convection.
We are currently actively engaged in an effort to overcome these limitations, using a new principle of recycling isoelectric focusing. Isoelectric focusing is generally recognized as a variant among electrophoretic methods giving the highest resolution, but it offers small throughputs useful only in research. The recycling principle has already shown promise for far greater throughputs, possibly even of industrial capacity. These studies are only in their initial phase, and it is not yet clear to what degree this new process could benefit from weightlessness.
Should our present ground-based studies confirm the expectation that