We must consider contamination control inside and outside of the cleanroom. In fact, some of the frontiers of contamination control efforts are taking place far outside the confines of the typical fab facility – outer space. Space exploration efforts bring into sharp focus the limits of contamination control, both biological and non-biological, and the related issue of contamination measurements, particularly technique sensitivity and measurement near the detection limits.
Mars and Ecosystem Management
Three probes are due to land on Mars during January, 2004; two from the U.S. and one from the European Space Agency. The mission statement for the two U.S. landers is to “follow the water.” Because water is necessary for life, the working hypothesis is that if evidence of water is found, life is or was possible.
One overall contamination minimization goal in space programs is referred to as ìplanetary protection,î preventing cross-contamination between Earth and entities in space. Introduction of microbes or other contaminants from spacecraft to another planet is referred to as ìforward contamination.î ìBack contaminationî refers to contamination of earth by returning spacecraft. The need for interplanetary contamination control has been known for over three decades. However, our increasing understanding of the potential for contamination makes these goals an ever increasing challenge.
One issue is the ìwallî of absolute cleanliness. Complete, utter cleanliness is a goal toward which we approach, but one which will probably never be achieved. At the May, 2003 ASTM conference to develop cleanliness standards for biomedical devices, many speakers and attendees commented that it is impossible to define zero contamination.1
The same principle applies to hardware for space exploration. Detecting the presence of one single particle, and then eliminating that particle, would not seem to be reasonable. Similarly, we cannot detect contamination down to one spore or one cell. While biologists tacitly assume that a certain di minimus contamination can be tolerated, the potential (albeit very low) of cross-contaminating interplanetary ecosystems calls for perhaps unprecedented efforts to minimize viable contaminants. Certainly, cross-fertilization of the space exploration community with the medical community might prove productive. For example, as with ASTM efforts, it may be possible to extrapolate from contamination studies on successful, long-term biomedical implants. NASA standards for managing outbound hardware and for containing returning materials have been established.
Solar Wind and Low-level Detection
The Genesis mission is a current effort to collect atomic particles from the sun, the solar wind, over a two year period. The samples will be returned to Earth for analysis in 2004. The project illustrates techniques required to determine low levels of analyte and illustrates problems associated with chemical analysis near the detection limits.
The technique of collecting samples for long periods of time to obtain a significant, representative sample of low levels of material is analogous to the technique of collecting witness samples for analysis of Airborne Molecular Contamination (AMC) in fabrication facilities for wafers, optics, and biomedical devices.2
Spurious interferences near the detection limits presents yet another challenge in contamination control. In the solar wind project, for example, plasticizers in the walls of a nitrogen cabinet were found to outgas at sufficiently high levels to interfere with analysis. A specially designed cabinet is required to minimize potentially interfering contaminants.
The potential for contamination is inseparable from efforts to fabricate, measure, or explore, at the microscopic or intergalactic level. Approaches to achieving accurate sample collection in space and preventing interplanetary ecosystem contamination have many commonalities with preparation of a small stent or of an implantable pharmaceutical delivery device with well-defined surface properties. Both communities require accurate, precise, sensitive measurements. Each community can benefit from the experiences and approaches used by the other. Such cross-fertilization has the potential for obvious benefits both in and out of the cleanroom.
Thanks to Mary Sue Bell, Lockheed Martin, Science, Engineering, Test, and Analysis at Johnson Space Center, Houston, TX for her helpful contributions to and review of this column.
1. Kanegsberg and Kanegsberg, A2C2, October, 2003
2. Kanegsberg and Chawla, A2C2, April, 2001.