The only needed quality that skin did not provide on its own was pressure retention, as its natural flexibility allows internal fluids to collect in areas of low pressure. If mechanical pressure could be allowed to counteract this effect, most of the body could be left unpressurized, only the head would have to be provided with gas pressure, both for breathing as well as protecting the ears and eyes. Additionally, such a suit would almost certainly be lighter than a pressurized suit, and much safer; punctures would not impair the operation of the suit as a whole, and no cooling system was needed as long as the material did not significantly block perspiration.
Original research was done by Paul Webb, who published a paper titled "The Space Activity Suit: An Elastic Leotard for Extravehicular Activity" in the April 1968 issue of Aerospace Medicine. The research was funded by NASA, and as the report appeared to be extremely positive, further work was contracted in order to test various design concept. Between 1968 and 1971 ten designs of increasing sophistication were built, leading eventually to a series of successful tests in vacuum chambers. The longest test was two hours and forty-five minutes.
In general terms the tests were successful, and the basic idea that mechanical counter-pressure could create a usable spacesuit was demonstrated conclusively. The energy needed to move about was considerably reduced compared to conventional designs, a major improvement for long-duration spacewalks. Tests of punctures showed that up to a square millimeter of skin could be directly exposed to vacuum for extended periods with no permanent effect, unlike conventional suits which would lose pressure and breathing air along with it. On top of all this, it weighed half as much as the most modern suits then available, those used on Program Apollo.
Unfortunately, a number of problems also turned up, primarily related to the problem of keeping the suit in strong mechanical contact at every point on the body. Concavities or small folds in the fabric could lead to fluid pooling in the gaps, and the crotch area proved extremely difficult to tailor successfully. To fill out these areas, small pads of polyurethane foam were inserted into concavities and were successful in most problem areas. The suits also had to be tailored to each individual, although the same was true of the earlier Apollo suits. The largest difficulty was donning and removing the suit. In order to effectively provide the minimum pressure of necessary for human physiology, the suit must be extremely tight-fitting, making donning and doffing a highly strenuous task.
In 1971, Webb, along with James F. Annis, published their findings in NASA CR-1892, "Development of a Space Activity Suit" The report remained positive, and the researchers felt that further improvements were possible. Quoting the Report:
In conclusion, the SAS at its present stage of development will protect man from the effects of the vacuum environment, in a garment, which permits improved mobility and natural body movements. Physiologically the approach is sound, and although there remain many problems to be solved, they are principally mechanical in nature. It has been suggested that solution of the mechanical problems, combined with careful tailoring based upon biomechanical analysis, plus the development of specific elastic fabrics, could eventually lead to a space qualified version of the SAS.
NASA discontinued research into the space activity suit, and pressurized suits are still used as of 2006. However, research is under way at the Massachusetts Institute of Technology (MIT) on a "Bio-Suit" System in preparation for President Bush's Vision for Space Exploration.
The suit as a whole consisted of several layers of the fabrics along with additional layers for other purposes. Starting at the skin, a "slip layer" of light powernet was used to allow the outer layers to slide over the skin without binding. Under this layer a number of foam pads were placed on various concavities on the body to keep them in contact with the suit. On top of this was the counter-pressure bladder, part of the breathing system. On top of this were up to six additional layers of powernet over the trunk with bobbinet arms and legs, or all-bobbinet garments covering the trunk only. The garments were put on like a normal bodysuit with a large zipper closing the front, with additional drawstrings at some points to help close the garment. Zippers on alternating layers were offset.
The positive-pressure breathing system consisted of three main portions; a pressurized helmet, the breathing bladder, and the tankage system in a backpack. The bladder and helmet were connected together to pump air out of the bladder over the torso when the user breathed in, reducing the amount of pressure on the user's chest. The helmet was held on by connecting it to a non-elastic garment of nomex cloth covering the torso, creating a lock under the arms and additionally held in place by the pressure of the elastic layers above and below it.
Newman has worked extensively in biomechanics, especially in the field of computerized measurement of human movement. Applying this knowledge, Newman's team looked for points on the body where stretching motions did not take place by painting a series of circles over a portion of the body and then watching their deformations as the wearer walked around or performed various tasks. The circles deform into ellipses as the skin stretches over the moving musculature, and these deformations were recorded. After a huge number of such measurements the data is then examined to find all of the possible deformations of the circles, and more importantly, the non-moving points on them where the original circle and the deformed ellipse intersect (at four points per circle). By mapping these points over the entire body, a series of lines are produced that Newman refers to as "lines of non-elongation". These lines generally follow the musculation, for instance there is a prominent line of non-elongation running from the shoulder area, down the front of the body, then curving under the armpit. This follows the line where the subscapularis connects to the skeleton.
The primary structure of the BioSuit is built by placing an elastic cord along a line of non-elongation. As the cord will not be stretched along these lines, at least in normal movements, whatever pressure they provide will be constant even as the wearer moves. In this way they can very accurately control the mechanical counter-pressure being applied by the suit. The rest of the suit is then built up from spandex lying between the primary pressure cording. The Bio-Suit team has thus far constructed a number of lower leg prototypes using different materials, including nylon-spandex, elastic, and urethane-painted foam. In one experimental design, kevlar fabric was used between the cording, in areas where the expansion was limited. Each suit has to be custom tailored for the wearer, but the complexity of this task is greatly reduced through the use of whole-body laser scans.
The result is a one-layer version of the SAS, it is lighter than the original and considerably more flexible, allowing much more natural motion and decreasing the energy cost of motion. Current versions of portions the BioSuit have consistently reached , and the team is currently aiming for for a baseline design. As mechanical counterpressure has proven difficult for small joints such as those in the hands, the BioSuit baseline design uses gas-filled gloves and boots in addition to a gas-filled helmet.
Space activity suits have featured prominently in much space fiction. Writers including Stephen Baxter and Larry Niven have made use of these suits in their stories. The potential for greater mobility and simpler operation with space activity suit make this type of space suit an attractive choice for fiction, where flexibility of use can be a boon to plot development. The aesthetic qualities of a sleek, form-fitting space activity suit also contrast the traditional image of rigid, diving-suit-style spacesuits, lending costumes in fiction a futuristic look.