To provide virtually unrestricted movements in all kinds of extravehicular activities in space, a Space Activity Suit is proposed, consisting of a powerful elastic leotard to counter the circulatory effects of breathing oxygen at a pressure of 170 mm.Hg. The physiological basis for such a garment is discussed.
A prototype garment has been worn in the laboratory for up to 90 minutes while breathing at pressures of 30, 60, and 100 mm.Hg. Mobility and dexterity were excellent; no circulatory embarrassment appeared. When an arm protected with an elastic gauntlet was exposed to less than 8 mm.Hg absolute for 20 minutes, there was no sign of gaseous swelling, dexterity was unimpeded, and circulation was maintained.
A successful Space Activity Suit promises these advantages over gas-filled pressure suits: complete mobility at small metabolic cost; simplicity and greater reliability; low risk if the garment is torn; and physiological temperature regulation without cooling equipment.
THE FULL PRESSURE SUITS used in U.S. and Soviet space flight are enclosures of gastight cloth, pressurized throughout with gas at 0.25 to 0.4 atmosphere. Physiologically the full pressure suit is correct, but mechanically it is far from satisfactory. Gas bag suits are stiff cylinders except where specific joints or flexible structures are located, and the resulting limited mobility and high metabolic cost of even limited activity while wearing one of these suits cause serious restrictions to extravehicular activity, particularly that which will be needed on the lunar surface.
Many kinds of flexible members have been developed in the past thirty years to make such garments somewhat mobile, and they have been satisfactory for emergency protection in aircraft7 or space cabins. But, in contrast, space activities call for all kinds of movements. A man on the lunar surface must be able to walk easily, to stoop, to climb, to engage in digging and lifting. These motions involve all the joints and flexures of the body—twisting and flexing of the vertebral column from head to pelvis; tilting of the pelvis;
rotation, abduction, adduction, flexion, and extension of the arms and legs; and translation of pivot points such as the movable ball-and-socket shoulder joint. To match these complex motions in a gas-filled suit is a formidable job, one which has not yielded to a concentrated and prolonged effort in research and development.
In order to overcome these problems of awkwardness, limited mobility, and high metabolic cost—which lead to high oxygen cost, high heat production, and rapid onset of fatigue—a new approach to protection in a vacuum seems to be necessary. For the past two
From Webb Associates, Inc., Yellow Springs, Ohio. 376 Aerospace Medicine • April, 1968
years we have been working on such a new approach. We call it the Space Activity Suit, or SAS.
The starting point conceptually for the SAS is that the human skin itself is an ideal pressure suit; the skin is essentially gastight, elastic, and obviously non-restrictive to motion. The whole body may be exposed to a vacuum for brief periods of one to three minutes without any protection at all, the time limit being set primarily by hypoxia. This has been established by exposures of primates and dogs to a nearly complete vacuum, as reported by Bancroft and Dunn2; Cooke and Bancroft3; Koestler12; Rumbaugh and Ternes16; and Stephens, et al.17 In the laboratory Wilson20 has exposed human hands to near vacuum conditions (5 mm.Hg absolute pressure), and swelling from evolution of gas was not evident until at least two minutes and often as late as eight to ten minutes after exposure. There are a number of other reports of harmless swelling in hands exposed to low pressures—e.g., Ernsting, et al.4 Henry, et al.10 and McGuire.13 It would seem that the skin in its natural state exerts a useful degree of elastic counterpressure, and adding mechanical pressure will prevent gas from forming.
Therefore, in order to provide the added mechanical pressure necessary to protect a man in a vacuum, the Space Activity Suit is constructed of a tough elastic mesh, providing a porous restraint garment conforming entirely to the contours of the body, which supports the skin and prevents it from yielding to the potential distortion of gas forming in the tissues. A helmet sealed to the neck provides oxygen for respiration. The effect of positive pressure breathing on the circulation is compensated by the mechanical counterpressure supplied by the elastic net, this requirement in fact calling for more counterpressure than the skin needs for support. At the same time natural thermoregulation takes place by evaporation of sweat without any external cooling equipment, and there should be virtually unlimited mobility at a very small increment in metabolic cost.
We have made a prototype SAS and tested it with -positive pressure breathing at ground level. We have also tested subjects' arms in a near-vacuum while protected with an elastic sleeve and glove. In this report we describe the physiological basis for the suit design and present what assurances there are in our testing with early prototypes that the SAS is feasible for use in the vacuum of space. The remarkable advantages of a successful SAS will be evident from the discussion.
HISTORY AND THEORY
The idea of using an elastic cloth to provide limb counterpressure during positive pressure breathing is
not entirely new. In the early 1940's Henry,9 who introduced the partial pressure suit, thought of the elastic suit concept, but did not test it Some time later, Hull,11 at the Aeromedical Laboratories at Wright-Patterson Air Force Base, actually had such a suit constructed, but he didn't pursue the idea further because breathing was uncomfortable and the heavy elastic material then available was stiff and hard to bend. But positive pressure breathing with limb counterpressure via capstans in the partial pressure suit was eminently successful in providing get-me-down protection when decompression occurred in aircraft. It also provided adequate protection for brief exposures to a near-vacuum.13
How has the situation changed since the 1940's? The need for unrestricted mobility is far greater for astronauts than for seated pilots in aircraft. In addition there have been impressive postwar developments in fabric technology. Twenty years ago such fibers as spandex, a synthetic elastomer of the polyurethane group, were unknown. Power net and stretch nylon, nowstandard materials for women's foundation garments, were nonexistent. These fabrics can be knitted or woven in a wide range of powers, from the very lightest used in sheer support stockings to heavy weights used in "full control" girdles and in medical support garments. We are confident that even more powerful fabrics can be developed. By layering these fabrics it will be entirely feasible to produce 150-170 mm.Hg counterpressure over those parts of the body that require it, while maintaining excellent flexibility of parts.
The combination of positive pressure breathing and
full body elastic counter-pressure affects a number of physiological systems, most importantly the mechanics of respiration, the systemic and pulmonary circulation, and thermoregulation. Each of these functions has been analyzed and an approach has been developed to maintain homeostasis.
Positive Pressure Breathing and Circulatory Balance
—Positive pressure breathing (PPB) without compensating counterpressure, and then with counterpressure to the trunk and limbs, has a long developmental history in aircraft protective equipment. Routine use of PPB without compensating pressure is limited to 20-30 mm. Hg for any length of time. With counterpressure on the neck and trunk and partial limb pressurization, as in the British jerkin, positive breathing pressures of 80 mm. Hg can be tolerated for 10 minutes and 140mm. Hg can be tolerated for one minute, according to Ernsting.5 The partial pressure suits developed by the Air Force in the 1950's use breathing bladders for respiratory compensation and capstans to draw up the non-stretch suit materials over the limbs and trunk; they provide good protection against decompression but the time of use under maximum PPB is limited. And when the bladders and capstans inflate they seriously decrease mobility. The suits are uncomfortable because of uneven pressure distribution and because extensive body coverage by bladders prevents evaporation of sweat. But the greatest drawback has
been blood pooling and accumulation of fluid in the limbs when the suits are worn pressurized for longer than 20-30 minutes. The capstan partial pressure suit was never designed for prolonged wear pressurized, or for use by active men in space.
For active men in the full vacuum of space, we have chosen 170 mm.Hg (3.3 psi) of oxygen as the design figure for PPB. 150 mm.Hg would approach the sea level partial pressure of oxygen but gives a low alveolar oxygen tension and there would be no safety factor for regulator accuracy, CO2 flushing, and similar equipment hazards. Since PPB to 170 mm. Hg results in a 170 mm. Hg rise in blood pressure, the blood, being essentially a closed fluid system, is everywhere pressurized to 170 mm. Hg Matching tissue pressure must be created all over the body. The circulating blood would rush into any low pressure areas and pool there; small veins offer almost no resistance to distention. If venous engorgement continued, the pressure within the veins and capillaries would begin to rise significantly. Once this pressure became greater than 14 cm of water gage pressure, measurable amounts of excess fluid would be forced through the capillary walls and would begin to accumulate in the tissues, causing swelling (edema). Neither the blood pooling nor the edema would be markedly uncomfortable, but this sequestering of fluid from the main circulation would result in a decrease in circulating blood volume. Ernsting5 has shown that as little as 200 cc lost in this manner, combined with the psychological stresses of PPB, caused fainting, and even the most experienced subjects withstood no more than an 800 cc decrease in circulating blood volume.
In order to provide total body counterpressure, adjusted in magnitude so as to prevent significant pooling or leakage of body fluids, we construct the SAS as a complete leotard of elastic cloth, covering fingers, toes, hands, feet, arms, legs, and torso. We feel from our preliminary studies that the applied force will vary from segment to segment. Probably from 100 to 150 mm.Hg (2-3 psi) of force on the limbs, with up to 170 mm. Hg on the chest and abdomen, will suffice.
Special structures under the elastic material are used to fill large concavities and to allow for complex motions where the elastic material does not slide or stretch readily. Such areas are the central back, the axilla, the groin, and the shoulder. In these areas we place shaped soft spongy pads or liquid filled pads which transmit the force of the elastic garment to the skin. At the shoulder, for example, the pad invests the whole joint, its liquid normally filling the axillary space. When the shoulder pivot slides over the chest, the fluid redistributes, maintaining a nearly constant volume as the inner geometry is rearranged. These pads, or "soft joints," help to maintain a constant pressure over concave areas so that the circulatory balance is not disturbed.
Thermoregulation—The third body system that must be maintained is thermoregulation. In the vacuum of space thermoregulation is a problem of heat dissipation. Since an astronaut's heat production will be sizable
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and variable if he is to cany out useful activities, heat dissipation should be rapidly variable and matched to the requirement of maintaining thermal balance. In the Space Activity Suit, heat dissipation is via the nicely regulated mechanism of sweating.
The porous net of an SAS permits instantaneous evaporation of any water which comes through the skin. (Overgarments for micrometeorite protection and for control of thermal radiation can be left loose and unsealed, like normal coveralls.) Evaporation cools the skin, heat is dissipated, and sweating decreases.
The steady diffusional water loss (insensible perspiration) in a vacuum should not exceed about 100 grams of water lost per hour per man, an estimate based on extrapolating the data of Hale, et al.8 and Webb, et al.19 This represents a heat loss of 56 kcal/ hr, which is roughly half the metabolic heat production of a man standing at rest. If there is water produced on the skin from thermal sweating, it will be instantly evaporated and cooling will take place at the rate of 0.56 kcal/gm of water lost. Not only is evaporative cooling from sweating constantly available and under full control by the body, but also it requires no additional machinery or power. The only limit to this cooling approach is dehydration, and this can be prevented by drinking during rest periods. If the astronaut's work rate were maintained at a steady 300 kcal/hr (1200 Btu/hr) for four hours, the total loss of water would be just over 2100 grams for the whole period. It all evaporates and cools; there is no need for the body to secrete 1.5 to 2 times the sweat required for cooling, as is often true in an air environment on Earth.
Properties of Human Skin—The tensile strength of human skin is more than adequate to prevent serious
Fig. 1. The high heat production (metabolic cost) of walking slowly in pressurized full pressure suits, from data of Wortz et al21 and Roth,15 compared to the normal heat production of walking at common speeds without encumbrance (data of Alt-man, et al1).
378 Aerospace Medicine • April, 1968
deformation or rupture in the small spaces between the strands of elastic netting. According to Yoshi-mura,22 the average tension required to tear adult human skin is 1600 gms/mm2, while the maximum force of 170 mm. Hg which might be developed wearing the SAS is 2.3 gms/mm.2 The elastic materials we are now using show spaces as big as 1 mm2 only when grossly overstretched.
The skin as a barrier to gaseous diffusion has been studied enough to permit educated guesses as how much water vapor, oxygen, nitrogen, and carbon dioxide would be lost during exposure to a vacuum. Our estimate of the rate of diffusion of water vapor was given in the preceding section—about 100 gms per hour. This may well be too generous, since the barrier layer of the skin increases its resistance to diffusion as the skin becomes drier, and the immediate effect of exposure to vacuum will be maximal drying of the superficial horny layer.
Moyer, et al.14 feel that the skin remains gastight when the transcutaneous pressure exceeds 760 mm.Hg. Diffusion of individual gases through the skin has been reported by Fitzgerald6 as negligible for oxygen and about 180 ml(STPD)/hr for CO2 when there is no transcutaneous pressure gradient. Dissolved nitrogen will be at a low value from the standard procedure of breathing oxygen before going into the vacuum environment. We estimate the nitrogen diffusion through the skin may be 10-20 ml/hr.
All of these estimates of gaseous diffusion rates may have to be revised when experimental measurements can be made during exposures to full vacuum. How-ever, on the basis of present information, we do not expect any problem from excessively high loss by transcutaneous diffusion.
Other Features— Reliability of the SAS is high due to its drastic simplification compared to full pressure suits. There are far fewer parts to fail, as witness the elimination of cooling equipment and the absence of mechanical joints. The backpack is simplified to a package containing only oxygen for breathing. Safety is enhanced, since a tear in the tough elastic material would not represent the catastrophe that a tear in a gas-filled suit means. The complete suit (except for the outer coverall) will be small enough to tuck into the interior of the helmet for storage.
But it must be kept in mind that the first and major advantage of the Space Activity Suit is that it gives total flexibility for all body parts. Since mobility essentially unimpeded, the incremental energy cost of doing useful activities in the SAS should be small Total mobility allows use of all the familiar actions of body parts in carrying out a task—not the constrained and unfamiliar motions used when "walking," for example, in a full pressure suit. The bending forces in the elastic net are greater than those for normal clothing, but these forces are expected to be small compared to the forces the muscles normally generate when a man walks, climbs, stoops, digs, and weights. We estimate that the metabolic cost of carrying out a full schedule of walking and climbing activities will not increase by more than 10 to 20 percent
in the SAS.
In contrast, the data for operating in standard pressure garment assemblies show extreme incremental increased with even mild activity. Some typical energy cost values for walking at very slow speeds in pressure suits at 1 g are given in Figure 1. Increases of three times the cost of walking unpressurized are not unusual. When we compare this metabolic cost with the anticipated 10 to 20 percent increment of the SAS, and when we recognize that many simple activities are virtually impossible to do in conventional pressure suits, the advantages of the SAS are compelling.
In order to demonstrate the mobility inherent in the SAS concept and to study the effects of prolonged PPB with elastic counterpressure, we have made an elastic leotard of several layers so that increasing power is applied as layers are added. Initial demonstrations of mobility, dexterity, and lack of circulatory effects have been carried out with this prototype garment. In addition we have exposed the arm and hand of two subjects protected with a double elastic sleeve to a near-vacuum in an arm chamber, with no swelling and no harmful effects.
First Prototype Garment—The first prototype garment was carefully tailored to the individual measurements of one subject. The power applied by donning each layer of this garment was accurately calculated and achieved by the techniques developed by the Jobst Institute of Toledo, Ohio, a supplier of medical support garments. The material used was a special bobbinet woven with cotton-covered 50-core rubber strands in the warp direction running circumferentially around the limb and 100-denier nylon in fill. The material has two-way stretch but the main power is developed circumferentially in the direction of the rubber strands. Three layers were made, each consisting of a pair of garments, one member of which ran from the foot to the shoulders, without arms, and the other member from the crotch to the base of the fingers, without legs. Each pair when donned provided single coverage on the arms and legs and double coverage on the trunk. Elastic gloves and footlets completed the assembly. The first two pairs- of garments developed powers of 30 mm.Hg and the third pair developed a power of 40 mm.Hg. The applied mechanical pressure of each layer was checked and verified to be 30, 30, and 40 respectively, and the combination gave approximately 100 mm.Hg by our technique of measurement.
Measuring the applied power was accomplished with small flat inflatable pads fitted with two electrical leads and a small pneumatic connection. The flat pads, or envelopes, were made of 10-gage vinyl sheet and were cut in a roughly oval shape measuring approximately 1 X ¾ inches. The inside surfaces of the two layers were fitted with thin electrical contacts. Inflation with a syringe through the pneumatic connector caused the contacts to separate. When the pads were placed under the elastic material and against the skin, the pressure required to break the contacts was our index of the
force applied by the elastic garment.
The suit was always worn in the laboratory with positive pressure breathing delivered by a helmet which was connected to a simple flow-through compressed air supply. The helmet sealed around the face and supplied pressure to the neck and most of the head by means of a bladder. The suit and helmet are seen as worn in Figure 2.
A single layer of the prototype garment developing 30 mm.Hg was worn with 30 mm.Hg PPB for 90 minutes with complete comfort. Two layers of the prototype garment, which together develop 60 mm.Hg pressure, have been worn twice; the first period was 90 minutes and the second 45 minutes. During the first and longer trial, the subject complained of fatigue of the abdominal muscles from unusual respiratory effort. A double-layered elastic corset was added to the lower torso to aid in respiration and this relieved the discomfort. Three layers were worn together to develop a pressure of approximately 100 mm.Hg for 10 minutes.
Mobility and dexterity—While it was evident to the subject and to the observers that there was excellent mobility in the first prototype garment with either one, two, or three layers, we devised a simple set of body motions to demonstrate that there was good flexibility. Since the subjects in current full pressure suits have difficulty in keeping up with a treadmill, and greater difficulty in stooping, climbing, and other activities involving trunk flexion, we asked our subject to do the following things, which he did happily for a period of about 45 minutes. First, on the treadmill, the subject walked at 3 mph on a level, then at 3 mph up a 5 percent grade, then at 3 mph level with a 30-lb. backpack, and then at 4 mph with a 25 percent grade with the backpack. This last heavy exercise was maintained for 30 seconds without great difficulty. The subject donned and buckled his own backpack. Next the subject was asked to do a number of calisthenic
Fig. 2. Subject wearing 30 mm.Hg suit with 30 mm.Hg breathing pressure via air line being attached to helmet.
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activities including sit-ups, push-ups, trunk bends in all directions, squat jumps; knee bends, and toe touching. Again these were accomplished without difficulty. Finally, he was asked to shovel sand from one container to another using a long-handled shovel; he was asked to climb a 14-foot ladder, up and down several times; and he was asked to crawl on the treadmill at 1.2 mph both level and up a grade. All of these activities were carried out without difficulty. There was no excessive increase in heart rate nor excessive respiratory distress. While direct metabolic data were not taken, it was clear that the metabolic cost of doing these activities, some of which are virtually impossible in a full pressure suit, was only a little greater than would be measured if he had had no suit on at all. Figures 3, 4, and 5 are photographs of the subject doing some of the activities described.
To demonstrate hand dexterity we asked the subject