Patent Publication Number: US-2012027972-A1

Title: Fail-closed adaptive membrane structure

Description:
This application is a continuation-in-part of U.S. application Ser. No. 11/584,927, filed Oct. 23, 2006, which in turn claims the benefit of U.S. Provisional Application No. 60/729,193, filed Oct. 21, 2005. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a protective article that includes an adaptive membrane structure comprising three movable membranes and that can be made to change its liquid or vapor permeability in response to surrounding environmental conditions. 
     BACKGROUND 
     There is a growing need for personal protective apparel that guards against toxic chemical and biological agents. These agents may be
         (a) accidentally released in a chemical manufacturing plant, in a scientific or medical laboratory or in a hospital;   (b) released intentionally during wartime by a government to attack the military forces of the opposition; or   (c) released during peacetime by criminal or terrorist organizations with the purpose of creating mayhem, fear and widespread destruction.       

     For this reason, the United States military and other defense organizations of countries all over the world have sought to provide adequate protection against chemical and biological warfare agents. The need for such protective apparel also extends to police departments, fire departments, emergency responders and health care providers. 
     According to the  Handbook of Chemical and Biological Warfare Agents  (D. Hank Ellison, CRC Press, Boca Raton, Fla., 1st edition, 1999), most chemical warfare toxins are fatal at concentrations as low as 1 part per million (ppm). Hence, to provide adequate protection from chemical warfare agents, a protective suit has to be almost impermeable to such chemicals. It is not difficult to devise structures that are impermeable to toxic chemical vapors and liquids, but such structures are also hot, heavy and uncomfortable to wear. The degree of comfort offered by a protective suit is largely determined by the amount of water vapor that can permeate through the protective fabric. The human body continuously perspires water vapor as a method for controlling body temperature. When a protective fabric hinders the loss of water vapor from the body, the transpirational cooling process is hindered, which leads to personal discomfort. When a protective suit allows little or no loss of water vapor, extreme heat stress or heat stroke can result in a short period of time. Hence, in addition to offering the highest levels of protection against toxic chemicals and liquids, a practical chemical and biological protective suit generally should have relatively high water vapor transmission rates. Desirable protective structures are also light in weight and offer the same high level of protection over a long period of time. 
     Some currently available protective garments offer the same constant level of protection at all times, but often, protection is only needed when a toxic chemical or biological agent is present in the environment. Further, comfort is typically sacrificed at the expense of protection or vice versa. A garment is needed that provides a variable and controllable permeability. 
     U.S. Pat. Nos. 7,597,855 and 7,993,606, which are incorporated in its entirety as a part hereof for all purposes, address these problems by providing an adaptive membrane structure having two membranes and means to respond to an actuating stimulus (for example, an electrostatic force) that will move one membrane into contact with the other such that the permeability of the structure to gas, vapor, liquid and/or particulates is decreased. 
     However, should the actuation means fail because of mechanical damage, loss of power, or any other event, protection would be compromised. This is a serious concern when using adaptive membrane structures to protect against highly toxic agents such as those encountered in chemical and biological warfare. It is thus desirable in such situations to consider an alternative adaptive membrane structure wherein a low permeability state is provided should an unexpected failure of the actuation system occur. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention is an article of manufacture which is selected from the group consisting of apparel and enclosures, said article comprising:
         an adaptive membrane structure comprising first, second, and third membranes;   an actuating stimulus and a means to respond to an actuating stimulus, wherein the actuating stimulus and the means to respond to an actuating stimulus are at least one selected from the group consisting of:
           (i) an electrostatic force is the actuating stimulus, and the means to respond to an actuating stimulus is an electrically conductive material that is incorporated in, on, or adjacent to the adaptive membrane structure,   (ii) a magnetic force is the actuating stimulus, and the means to respond to an actuating stimulus is a magnetic material that is incorporated in, on, or adjacent to the adaptive membrane structure; and   (iii) an electrostrictive force is the actuating stimulus, and the means to respond to an actuating stimulus is an electrostrictive material that is incorporated in, on, or adjacent to the adaptive membrane structure and   
           a source of electrical potential;
 
wherein in an unactuated state, the first membrane is in contact with the second membrane, but not in contact with the third membrane such that there is a gap between the first membrane and the third membrane;
 
wherein in an actuated state, the source of electrical potential, the actuating stimulus, and the means to respond to an actuating stimulus are configured to move the first membrane, or a portion thereof, into contact with the third membrane by an attractive force, thereby creating a gap between the first and second membranes; wherein: (a) each membrane has holes, (b) the holes of the first membrane are substantially out of registration, or are out of registration, with the holes of the second membrane, (c) the holes of the first membrane are at least partially in registration with the holes of the third membrane, and (d) each membrane comprises at least one polymer; and
   the means to respond to an actuating stimulus is in physical proximity to the adaptive membrane structure to enable application of the force of the actuating stimulus to move the first membrane or a portion thereof.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a component of an adaptive membrane structure, showing two membranes in contact with each other, and showing an absence of registration of holes on adjacent membranes ( 1 A: perspective view,  1 B: plan view, and  1 C: sectional view). 
         FIG. 2  is a schematic diagram of a component of an adaptive membrane structure, showing two membranes separated by a gap between them and showing an absence of registration of holes on adjacent membranes ( 2 A: perspective view,  2 B: plan view, and  2 C: sectional view). 
         FIG. 3  is a schematic diagram of an adaptive membrane structure that contains three membranes showing the positions of the membranes in the absence of an actuating stimulus ( 3 A: perspective view,  3 B: plan view and  3 C: sectional view). 
         FIG. 4  is a schematic diagram of an adaptive membrane structure that contains three membranes showing the positions of the membranes as a result of the application of an actuating stimulus ( 4 A: perspective view,  4 B: plan view and  4 C: sectional view). 
         FIG. 5  is a schematic diagram of a section view of a membrane with a hole of non-circular cross-section. 
         FIG. 6  is a schematic diagram of a component of an adaptive membrane structure that contains an array of protruding features showing the positions of membranes in the absence of an actuating stimulus ( 6 A: perspective view and  6 B: sectional view). 
         FIG. 7  is a schematic diagram of a component of an adaptive membrane structure that contains an array of protruding features showing the positions of membranes as a result of the application of an actuating stimulus ( 7 A: perspective view and  7 B: sectional view). 
         FIG. 8  is a schematic diagram of a membrane of an adaptive membrane structure in which a conductive layer is applied to the membrane in an annular pattern around each hole. 
         FIG. 9  is a schematic diagram of a membrane of an adaptive membrane structure in which a conductive layer is applied to the membrane in parallel lines. 
         FIG. 10  is a schematic diagram of an adaptive membrane structure comprising three substrate membranes, three conductive layers, and three dielectric layers. ( 10 A: perspective view.  10 B: plan view.  10 C: sectional view.) 
         FIG. 11  is a schematic diagram showing a sectional view of three possible states of actuation of an adaptive membrane structure comprising three substrate membranes, three conductive layers, and three dielectric layers. 
         FIG. 12  is a schematic diagram of a portion of a substrate membrane of an adaptive membrane structure in which a patterned dielectric layer is applied to a patterned electrode layer applied to a substrate membrane. 
         FIG. 13  shows a plan view of an adaptive membrane structure that has four subsections and each subsection has an array of holes. 
         FIG. 14  is a schematic diagram of a protective garment, wherein the shaded rectangular sections represent adaptive membrane structures. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention allows the manufacture of protective garments and other protective structures by providing an adaptive membrane structure. The term “adaptive membrane structure” refers to a structure comprising at least two membranes wherein the membranes are movable upon the activation or application of an actuating stimulus such as a force. The membrane structure is thus “adaptive” in the sense that the permeability of the structure can be changed based on the conditions in the external environment. The term “permeability” as used herein is defined as the mass transported per unit area of a membrane or membrane structure, per unit time per unit driving force where the driving force is the difference in concentration of the species of interest on the opposite sides of the membrane or membrane structure. 
     The adaptive membrane structures disclosed herein enhance the protective performance of an article, having exposed area, which encloses a human being, an animal or an object that needs to be protected from the surrounding environment. The term “exposed area” as used herein refers to the area of the article that is in contact with the surrounding environment. The article can be a protective garment or protective enclosure for protecting, for example, an individual or a collection of human beings; an individual or a collection of animals; or an object, such as the whole or part of a building envelope, machinery, electronic equipment, industrial or military equipment, or agricultural goods. The enclosure or garment can provide protection against various liquid, gas, vapor and particulate species, such as, for example, liquid chemical droplets, toxic vapors and gases, water vapor, aerosols, and biological spores. 
     A “membrane” as the term is used herein is a discrete, thin article that moderates the transport of species in contact with it, such as gas, vapor, aerosol, liquid and/or particulates. Examples of membranes suitable for use in the adaptive membrane structures disclosed herein include film, plastic sheeting, and laminar structures. Woven or knit fabric is not suitable for the membranes used in these adaptive structures because the membrane contains an array of well-defined holes that can be aligned with the holes of another membrane, to overlap those holes completely, partially, or not at all, as discussed below. A membrane can be chemically or physically homogeneous or heterogeneous. A “microporous membrane” is a membrane typically containing pores in the range of 0.1 to 10 micrometers in diameter. Microporous membranes are typically characterized by the fraction of total membrane volume that is porous (i.e. relating to porosity), a term reflecting the average pore length within the membrane as compared with membrane thickness (i.e. relating to tortuosity), and average pore diameter. The term “pore” as used herein denotes an opening that exists in a membrane that may or may not completely traverse the membrane. Typically, the pore size, the pore shape and/or the pore placement is not well defined or controlled, though there may be a relatively reproducible average pore size and/or pore size distribution. 
     The movable membranes used in the structures disclosed herein typically have holes as distinguished from pores, a “hole” being an opening that completely traverses a membrane. In  FIGS. 1C and 2C , membrane  2  has holes  3  and membrane  2 ′ has holes  3 ′. The holes of one membrane may or may not be the same size and shape as the holes of another membrane. Although holes are described herein in terms of their having the shape of a circle, it is not required that a hole have a shape that is perfectly or even approximately circular. 
     The term “porosity” as used herein means the volume occupied by all of the holes or pores in a material divided by the total apparent volume of the material (i.e., volume of the solid material plus volume of pores or holes). When the exposed surface includes the holes or pores and the holes or pores have a uniform cross-sectional area (e.g., are right cylinders), then the porosity simply equals the total cross sectional area of the holes or pores divided by the total exposed surface area (i.e., solid material area plus hole or pore area) of the material. 
     The holes of one membrane can be aligned with the holes of another membrane, in the sense of lines perpendicular or substantially perpendicular to the respective planes of the membranes in the side elevation view of the structure, such that the holes overlap completely, partially or not at all. Holes overlap completely when, if they are the same size, their boundaries are coincident in vertical alignment, or if they are not the same size, the area of the smaller hole fits entirely within the area of the larger hole. Holes do not overlap at all when, again in the sense of vertical alignment, a line perpendicular or essentially perpendicular to the respective planes of the membranes from its point of exit from a hole in one membrane does not enter into any part of a hole on the other membrane. Membranes with holes that have no overlap are shown in  FIGS. 1C and 2C . A line, for example, that is perpendicular or substantially perpendicular to the respective planes of membranes  2  and  2 ′ and that exits from any of holes  3  would not enter into any hole  3 ′. Partial overlap is the intermediate condition when the perpendicular or essentially perpendicular line exiting from a hole on one membrane will enter into only a portion of a hole on the other membrane. 
     In the description in the preceding paragraph, a line that is perpendicular or essentially perpendicular to the plane of a membrane will pass all the way through a hole in such membrane only if the hole is essentially a right circular cylindrical hole. The above description concerning overlap of holes is nevertheless accurate for membranes that have holes in which the axis of the hole is not normal to the plane of the membrane or is tortuous because the portion of a line passing through such hole that is relevant to the determination of overlap of holes is the portion of the line from its exit point from a hole on one membrane to the point of its entry, if any, into a hole on another membrane. That portion of the line may be described as perpendicular or essentially perpendicular to the plane of one or both membranes regardless of the route it has taken in passing through any of the holes. 
     The term “open area” is used to refer to the extent, expressed as a percentage, to which the respective holes of two membranes overlap, as most easily envisioned in terms of a plan view of the structure from the top. For membranes that do not overlap at all, such as those of  FIGS. 1C and 2C , the open area is defined as 0%. Conversely, an open area of 100% corresponds to the existence of the maximum open area, which is achievable by arranging a particular set of membranes such that the holes completely overlap, and these holes are referred to as “in registration”. A percentage between 0 and 100 indicates partial overlap. The terms “not in registration” and, equivalently, “out of registration” are used herein to indicate that the holes in two membranes do not overlap at all (referring again, for example, to  FIGS. 1C and 2C ); this is equivalent to having an open area of 0%. The terms “substantially out of registration” and “partially in registration” indicate that there is partial overlap, i.e. that the open area of the membrane structure is in the range of from greater than 0% up to, but not including, 50%. In  FIGS. 1C and 2C , for example, holes  3  are out of registration with holes  3 ′. 
     The degree of registration of the holes of two membranes may be described with equal accuracy as set forth above regardless of whether or not the membranes are in contact. The vertical alignment of the holes with respect to each other, in a side elevation view, may be described with equal accuracy in terms of lines perpendicular or substantially perpendicular to the respective planes of the membranes having holes despite the fact that such lines might pass through a gap that separates the two membranes in the case where they are not in contact. 
     The adaptive membrane structure can be “actuated”, which denotes the state of the structure upon the application or operation of a stimulus, such as a force (the “actuating stimulus”), which causes adjacent membranes to move into a position in which the membranes are separated by a gap between them, thereby changing the permeability of the membrane structure. Adjacent membranes, or a side thereof or a layer thereon, are membranes, sides or layers that can be brought into contact with each other. The term “unactuated” thus denotes the state of the adaptive membrane structure before application of the actuating stimulus. The term “deactuated” denotes the state of the adaptive membrane structure after the application and subsequent removal of the actuating stimulus. 
     The term “adaptive barrier system” as used herein denotes a system comprising an adaptive membrane structure in which actuation changes the permeability of the membrane structure to chemical, biological and/or particulate species. 
     Thus, the adaptive membrane structure is capable of displaying a variety of states of gas, vapor, liquid and/or particulate permeability. For example, when the adaptive membrane structure is used for protection against hazardous agents, it can display two different states of permeability. In one state, the “actuated” state, when hazardous environmental conditions do not exist, the membrane structure is highly permeable to water vapor and gases, thereby offering a higher level of personal comfort, than is experienced with a water vapor impermeable garment. When the membrane structure is exposed to a hazardous environment, it is transformed to another state, in which it is substantially impermeable to hazardous chemical and/or biological toxins and/or pathogens, thereby providing protection when it is needed. This is the deactuated state in which it is desired that no gap be provided between membranes in the structure. If the membranes are made from material that is selectively permeable to water vapor while providing a barrier to hazardous chemical and/or biological toxins and/or pathogens, then even in the deactuated or unactuated state, the structure may, remain permeable to water vapor. 
     The conversion of the membrane from one state of permeability to another state of permeability is brought about by the application of a stimulus, such as a force, herein termed an “actuating stimulus”. The actuating stimulus can be any of several forms including pressure, force, change in temperature or ambient concentration of water vapor, voltage, current, magnetic field, and electric field. In one embodiment, the actuating stimulus takes the form of an applied electric field, which causes membranes within the structure to move to convert the structure from an unactuated to an actuated state. 
     The activation and deactivation of the actuating stimulus can be effected with a manually operated switch; that is, an individual receiving a notice or warning in some form (e.g., audible alarm, odor, oral communication, signal from a sensor attached to the protective garment or enclosure) that protection from the environment appears necessary can manually perform whatever operation is needed to deactivate the actuating stimulus. In an alternative embodiment, however, a sensor can detect a change in the environment in which the structure is located, and can automatically activate or deactivate the actuating stimulus. 
     A schematic of an illustrative embodiment of a two-membrane component of an adaptive barrier system according to the present invention, in an unactuated state, is shown in  FIGS. 1A ,  1 B and  1 C. Planar membranes  2  and  2 ′ that are largely parallel to each other, each membrane further comprising a geometric array of holes such as those denoted  3  and  3 ′ in  FIGS. 1A ,  1 B and  1 C. The holes completely traverse the thickness of the membranes. 
     In the absence of an actuating stimulus, the adjacent surfaces  4  and  4 ′ of the membrane pair  2  and  2 ′ are typically in contact with each other. In an alternative embodiment of the unactuated state, however, a small gap may exist between the membranes that is not sufficient for a substantial amount of permeability of the structure, and thus does not permit a desirable amount of transport across the structure of harmless species such as water vapor. When an actuating force is applied to an adaptive barrier system of which membranes  2  and  2 ′ are a component, surfaces  4  and  4 ′ of the membrane pair  2  and  2 ′ are forced apart and away from each other, thus creating a gap  5  between  2  and  2 ′, as shown in  FIGS. 2A ,  2 B and  2 C. In the actuated state, the membranes  2  and  2 ′ have been moved to a position in which they are separated by a gap  5  between them. Because the holes completely traverse the thickness of the membranes, actuation gives rise to a path of increased convection and/or diffusion of a chemical, biological, or particulate species across the membrane thickness, as compared to convection and/or diffusion of the same species through the material from which the membrane is made at a location surrounding the hole. 
     The arrays of holes in at least two of the membranes are such that the openings of the array of holes on the adjacent membrane surfaces  4  and  4 ′ are substantially out of registration with each other. That is, the degree of hole overlap is such that, in the unactuated or deactuated state, the open area is reduced to less than about 50%. It is preferred that the open area be reduced to about 10% or less, more preferred that it be reduced to about 1% or less, and most preferred that the open area be reduced to 0%. In this embodiment (0% open area), no hole opening on the surface  4  of the membrane  2  will come in contact with a hole opening on the adjacent surface  4 ′ of the membrane  2 ′ when the membranes are in contact. When the two adjacent membranes  2  and  2 ′ are in contact, the holes of each membrane are thus effectively sealed, and there is no continuous porous path for convection and/or diffusion of chemical, biological, or other particulate species across two adjacent membranes, as seen in  FIG. 1C . However, in the actuated state, when the two adjacent membrane surfaces  4  and  4 ′ are not in contact, chemical, biological, or other particulate species may traverse one membrane through its holes (see, e.g., flow path  6  in  FIG. 2C ), enter the gap between the non-contacting membranes, and then traverse the second membrane through its holes (see, e.g., flow path  6  in  FIG. 2C ). The convection and/or diffusion of species will be greatly enhanced through membranes that are separated by a gap through action of the actuating force as compared to convection and/or diffusion of the same species when the same membranes are in contact, or are nearly so, in the absence of the actuating stimulus. 
     The adaptive membrane structure described herein contains at least three membranes. Consider an adaptive membrane structure containing only two membranes having holes out of registration, each incorporating, for elements (e.g., metal stripes or a metal coating) that makes them conductive. The conductive membranes are then connected to a source of electrical potential through a switch, in hopes of developing like charges on the membrane surfaces facing each other, thereby driving them apart by a repulsive force. However, if the two conductive membranes carry the same sign of charge, that is, both positive or both negative, the membranes will feel no force of repulsion or attraction. This is a consequence of Gauss&#39; Law. Instead, all of the excess charges on each conductive membrane will go to the surface that is away from the other conductive membrane (i.e., the outer surface), leaving the surfaces of the conductive membranes that are facing each other with no charge. There is thus no electric field between the conductive membranes and hence no appreciable force developed between them. Further, the electric field from the charges on the outer surfaces of the conductive membranes cannot penetrate the conductive material of the conductive membranes (the electric field inside of a conductor is always zero). Thus, the charges on each conductive membrane do not feel the presence of the charges on the other conductive membrane via any electric field and the membranes do not move apart. In short, such a system is inoperable. This is demonstrated in Comparative Example A. 
     An embodiment of an adaptive membrane structure of the present invention that contains three membranes is shown in  FIGS. 3A ,  3 B and  3 C, wherein there is an electrostatic actuating force. Membranes  2 ′ and  2 ″ have arrays of holes, such as those denoted  3 ′ and  3 ″, that are at least partially in registration, having an open area preferably equal to 100%, but each of these hole arrays is out of registration with the array of holes denoted by  3  in membrane  2 . In the unactuated state, membranes  2  and  2 ′ are in contact along the respective adjacent surfaces thereof  4  and  4 ′, having been moved into contact with each other by means for moving membrane  2 ′ into contact with membrane  2 . Such means for moving a membrane may include a passive, constant force (not shown) that does not require activation, such as ferromagnetism, which is particularly useful for this purpose because it continues to hold membranes  2  and  2 ′ in contact with each other after contact has first been made. Because the hole arrays of membranes  2  and  2 ′ are at least substantially out-of-registration, this state is one of low permeability or impermeability, similar to that depicted in  FIG. 1 . 
     The respective surfaces of membranes  2 ′ and  2 ″ that are not directly opposed to (i.e., are not facing) each other are coated with conducting layers  7 ′ and  7 ″. As depicted in  FIGS. 3A and 3C , the two coated layers  7 ′ and  7 ″ are further connected in series with each other through conductors  8  to a switch  9  and a source of electrical potential  10  which may include a battery or other power source. When the switch  9  is closed, the adaptive membrane structure attains its actuated state as depicted in  FIGS. 4A ,  4 B and  4 C. 
     In this actuated state, the current is the actuating stimulus, and the conductor is the means to respond to the actuating stimulus. When the switch  9  is closed, an attractive electrostatic force exists between charges which develop on layers  7 ′ and  7 ″ thereby moving membrane  2 ′ into a position in which it is separated from membrane  2  by a gap  5  between them, and also bringing membranes  2 ′ and  2 ″ in contact along their adjacent surfaces. Membrane  2 ′ is drawn into contact with membrane  2 ″ by way of an attractive electrostatic force, and since the arrays of holes  3 ′ and  3 ″ of membranes  2 ′ and  2 ″ are in-registration, this is a state of high permeability. 
     Note that membranes  2 ′ and  2 ″ essentially comprise a continuous or patterned electrode (not shown), and that the force of the means for moving membrane  2 ′ into contact with membrane  2 , and to hold them together in that idle state, would have to be overcome by the force generated by the response of the responding means to the actuating stimulus, in this case the response of conductors  7 ′ and  7 ″ to the electrical current. As shown in  FIGS. 4A and 4C , a gap  5  now exists between the membranes  2  and  2 ′, and chemical, biological or particulate species may traverse membrane  2  through its holes  3 , enter the gap  5  between membranes  2  and  2 ′, and then traverse membranes  2 ′ and  2 ″ through their holes which are in registration (see, e.g., flow path  6  in  FIG. 4C ) thereby making the actuated state of this structure one of high permeability. 
     Although the holes depicted in  FIGS. 1 through 4  are right circular cylindrical holes with linear axes normal to the plane of the membranes, the holes are not limited to circular or cylindrical geometry.  FIG. 5  shows a section view of part of a membrane according to embodiments of the invention, with a hole with non-circular cross-section, which changes in shape and size as it traverses the membrane along a general tortuous path. The optimum hole diameter will vary depending on the specific application of the structure, particularly how much flow or diffusion is desired through holes in the actuated state. In all cases, the holes are large enough to allow transport to occur in the actuated state. 
     Although the hole arrays depicted in  FIGS. 1 through 4  comprise the same regular square pitch pattern, the hole arrays of the adaptive membrane structures disclosed herein are not limited to a regular square pitch pattern. In particular, the array pattern on membrane  2  may be different from the pattern on membranes  2 ′ and/or  2 ″, and either pattern may comprise any regular or non-regular pitch pattern provided that the patterns are such that the holes of membrane  2  will be at least substantially out of registration, if not actually out of registration, with the corresponding holes of the membrane  2 ′. In contrast, the hole array geometries incorporated in membranes  2 ′ and  2 ″ are preferably identical. 
     Holes, for the adaptive membrane structures disclosed herein can be formed by any hole manufacturing process known in the art, including mechanical punching, laser or electron beam drilling, and chemical etching. Once the holes have been created, the membranes can be further processed to reduce any surface distortion that may have resulted due to hole formation process. 
     Again referring to  FIGS. 3A ,  3 B,  3 C,  4 A,  4 B and  4 C, the membranes  2 ,  2 ′ and  2 ″ can be fabricated from the same or different materials or combinations of materials and, furthermore, each membrane of the pair can have the same or different thickness. The materials from which the membranes are fabricated are selected to impart desirable levels of permeability to one or more species, which can come in contact with the membrane in use. For example, the material comprising the membrane can be selected to have high permeability to water vapor but very low permeability to one or more human toxic or poison agents or pathogens as may be encountered by military personnel subjected to a chemical warfare attack. 
     The materials that can be used to create membranes  2 ,  2 ′ and  2 ″ can be chosen from any sheet structure, but it is preferred that the sheet structure be flexible, and it is also preferred, although not necessary, that the materials used are polymeric in nature. Preferably, the flexible sheet structure can be prepared from at least one polymer component. Such polymer sheets or films, used to create membranes  2 ,  2 ′ and  2 ″, may be continuous (i.e. containing no microvoids or micropores) or microporous. Methods for creating polymer sheets or films are well known in the art. Such polymer sheets or films can be prepared from a large variety of polymers. 
     Continuous polymer films to be used to create membranes  2 ,  2 ′ and  2 ″ can also be semipermeable in nature. Semipermeable polymer membranes and their manufacture are known, for example, from sources such as U.S. Pat. No. 4,515,761 (Plotzker) and U.S. Pat. No. 6,579,948 (Tan). 
     The starting materials to create the membranes used in the adaptive membranes disclosed herein are not limited to continuous polymer films. Suitable starting materials may also have microvoids or micropores such as those present in microporous membranes, in which the typical pore size is about 0.1 to 10 micrometers. 
     The membrane can also contain materials to adsorb, absorb or react with harmful and undesired species. Hence, the membrane can include activated carbon, high surface silica, molecular sieves, xerogels, ion exchange materials, powdered metal oxides, powdered metal hydroxides, antimicrobial agents, and the like, which can be in the form of nanoparticles if so desired. Such materials would typically be mixed into the membrane material during the membrane formation process, such as, which can include a process such as extrusion compounding or solution casting. 
     Embodiments of the present invention as described above, those for example shown in  FIGS. 3 and 4  involve at least two largely planar membranes that, as a result of being moved by the actuating stimulus, are separated from each other along adjacent, largely planar, surfaces  4  and  4 ′ and thereby create a gap  5  that had not previously existed, or extend a small gap that had previously existed, between these surfaces in the unactuated state. The contact of the membranes in the unactuated state also eliminates paths such as path  6  that, in the actuated state, would permit enhanced permeation, convection and/or diffusion associated with the array of holes incorporated in the membranes  2  and  2 ′. An alternative embodiment for a component of the adaptive membrane structure is shown in  FIGS. 6 and 7  in the unactuated and actuated states, respectively. In this embodiment, one or both of two adjacent membranes contain an array of protruding members  21  in the form of a post, knob or bump. In the actuated state of this embodiment, depicted in  FIG. 7 , the adjacent membranes are separated from each other such that paths for enhanced permeation, convection and/or diffusion such as path  6  exist. However, as shown schematically in  FIG. 6 , each protruding member  21  in the array is shaped and positioned so as to be insertable in and enter a hole in the adjacent membrane upon deactuation when one or both membranes are moved toward each other. As each protruding member enters its corresponding hole, it contacts the inner surface of the hole in such a way as to create a seal between the protruding member and its mating hole, thereby eliminating paths  6  for permeation, convection and/or diffusion. 
     As seen in  FIG. 6  in this embodiment, to provide a good seal between membranes  2  and  2 ′, the adjacent membrane surfaces  4  and  4 ′ need not be in contact in the absence of the application or operation of the actuating stimulus and, a small gap  5  between these surfaces can persist in the unactuated state. In this embodiment, a good seal is provided by a tight fit between a protruding member on one membrane and the corresponding hole on the other membrane. Although the protruding member  21  is shown as a truncated cone in  FIGS. 6 and 7 , other shapes for the protruding member can be used, limited only by the need to form a seal against the mating hole surface. Furthermore, although  FIGS. 6 and 7  depict an array of identical protruding members deployed in a regular square pitch array, no two protruding members in the array need have identical geometry, and the array pattern for the protruding members is governed by the array pattern of the holes in the adjacent membrane. 
     A preferred actuating stimulus for use in the adaptive membrane structures disclosed herein is the force produced by electrostatics. The preferred electrostatic force can be applied to the structure by incorporating electrically conducting materials in or onto specific regions of at least two and possibly more membranes such that upon action of appropriate circuitry, the conducting regions on at least two membranes become oppositely charged, thereby creating an attractive force which brings two membranes into contact. In any of these electrically stimulated embodiments, therefore, means to respond to an actuating stimulus can include such electrically conducting materials, and the features, lines and patterns into which they can be formed, on which an electrostatic force may operate. 
     Alternatively, the conducting layers need not cover the entire surface of a substrate membrane but instead may be selectively applied in a pattern, which only partially covers the substrate membrane surface. One such example is shown in  FIG. 8 , in which a conductive layer  7  is applied to a substrate membrane  2  in an annular pattern around each hole  3 . Another possible patterned electrode is shown in  FIG. 9 , in which the conducting layer  7  comprises two arrays of parallel electrically conducting lines applied to the substrate membrane  2  traversing the space between the holes. In the particular pattern shown in  FIG. 9 , it is seen that any line from either array intersects and is perpendicular to the lines in the other array. Again, all points in this network of lines may be held at a single electrical potential by appropriate connection to a voltage source. The use of a patterned conducting layer as shown in  FIGS. 8 and 9 , as opposed to a continuous electrode as shown in  FIG. 4 , can increase the desirable permeability of the structure in the actuated state to species such as water vapor, since the barrier afforded by the electrode material to transport of these desirable species is removed over much of the substrate membrane surface. There are many other geometric patterns that could be used to provide electrodes. 
     The method for laying down conductive features, lines and patterns onto surfaces is well known in the electronic manufacturing art. Some of the processes that may be used for creating conductive features include without limitation letterpress printing, screen printing, gravure printing, offset lithography, flexography, electrophotography and laser jet printing. Several additional variations of lithographic printing for laying down micron and submicron conductive features onto surfaces are also well known in the art. 
     Additionally, the conductive layers on each membrane may be coated with one or more dielectric layers, which can impart additional features to the membrane structure. In particular, these layers may serve to insulate the conductive layers from the environment thereby eliminating or minimizing the potential for undesirable shorting or arcing of the charged conductive layer to surrounding conductive objects. The dielectric layers may comprise the same or different materials and thickness. Furthermore, in general, the dielectric layers may be the same material or a different material than that comprising the substrate membranes. Note further that a dielectric layer may be installed on the side of membrane that is adjacent to another membrane, or on the opposite side, 
       FIGS. 10A ,  10 B and  10 C show an adaptive membrane structure comprising three substrate membranes  2 ,  2 ′ and  2 ″, conducting layers  7 ,  7 ′ and  7 ″; and associated dielectric layers  11 ,  11 ′ and  11 ″. Two potential sources  10  and  10 ′, switches  9  and  9 ′, and conductors  8  are provided to permit electrostatic actuation of the conductors in the system. Thus, three different actuation states can be achieved with this system by closure of switch  9  and/or switch  9 ′, as shown in  FIGS. 11A ,  11 B, and  11 C. 
     In some embodiments, the holes, in addition to fully penetrating the substrate membranes, also fully penetrate the conductive layers and also fully penetrate the dielectric layers. It is not required, however, that in such case the holes fully penetrate the conductive and dielectric layers, and either or both of those layers may instead cover the entire surface of that side of one or more of the membranes. In such cases, the permeability of the conductive and/or dielectric layer can influence the overall permeability of the membrane structure to the passage of the chemical, biological and/or other particulate species for which the system is designed. 
     Yet another function of the dielectric layer can be to adsorb, absorb, or react with harmful and undesired species that may diffuse into the membrane structure when the membrane is in the unactuated state. Hence, the dielectric layer may include activated carbon, high surface silica, molecular sieves, xerogels, ion exchange materials, powdered metal oxides, powdered metal hydroxides, antimicrobial agents, and the like, which may be in the form of nanoparticles if so desired. 
     Dielectric layers, if they are incorporated in the structure as described above, also need not cover the entire surface of the substrate membrane. In particular, if a patterned electrode is used in a membrane structure, a patterned dielectric layer may be used which covers the patterned electrodes to electrically isolate them from their surroundings, but the dielectric layer need not cover all of the remaining substrate surface, which is not covered by the patterned electrode layer.  FIG. 12  shows an example of a patterned dielectric layer  11  applied to a patterned electrode layer  7  applied to a substrate membrane  2  in a similar design. 
     Note that the materials and thickness of substrate membranes  2 ,  2 ′ and  2 ″ may be the same or different. Likewise, the materials and thickness of conducting layers  7 ,  7 ′, and  7 ″ may be the same or different. Likewise, the materials and thickness of the dielectric layers  11 ,  11 ′, and  11 ″ may be the same or different. Furthermore, the choice of which side of the substrate membrane to use for placement of the dielectric layer, for any or all of the three membranes, may be reversed as previously disclosed above. 
     The adaptive membrane structures disclosed herein are not limited to having adaptive membrane structures only three membranes. The design disclosed in  FIGS. 3 ,  4 ,  10 , and  11  can be extended to four or more membranes as well by addition of appropriate component substrate membranes, conducting layers, dielectric layers, hole arrays, potential sources, switches and conductors. The embodiments disclosed above may thus include, for example, in addition to first, second and third membranes ( 2 ,  2 ′ and  2 ″), a fourth membrane  2 ′″ having holes, and means to respond to an actuating stimulus that moves the fourth membrane away from a second or third membrane into a position in which it is separated from the second or third membrane by a gap between them. Furthermore, the conducting and dielectric layers may cover the entire substrate membrane surface to which they are applied or they may be patterned following, for example, the designs previously disclosed. Multiple membrane systems enable adaptive barrier systems which can selectively impede the passage of different chemical, biological and/or other particulate species by actuating different combinations of adjacent membrane closures. Such systems may include, in addition to one or membranes in addition to two that are brought into contact, one or more layers of fabric. Also, layers and/or other membranes can be interposed between the membranes with holes that come into contact and the place where direct application of the actuating stimulus occurs. 
     As the adaptive membrane structure may contain one or more membranes and/or layers in addition to two membranes that are separated by the actuating stimulus, it is not required that more than two membranes have a conductive coating, or that the membranes that have a conductive coating are the membranes that also have holes. That is, the structure can contain one or more layers and/or membranes in addition to a membrane with holes that is moved by the application of the actuating stimulus occurs. Whatever is required to make at least one of the membranes with holes move away from another membrane with holes to form a gap is, however, part of the adaptive membrane structure. The permeability of the structure is thus determined with respect to all such components, be they just the two membranes with holes or additional layers, membranes and/or other materials or components. 
     Although an applied electric field is a preferred form in which the actuating stimulus will operate, there are numerous other types of actuating stimuli that are useful for the purpose of causing the movement of membranes in the structure. Other possible actuating stimuli include without limitation a magnetic force, hydrostatic force, or hydrodynamic force. Two or more different kinds of actuating stimuli may be used on an adaptive membrane structure. 
     For example, certain polymers can absorb considerable amounts of water and other solvents, and can thereby swell to volumes that are significantly greater than the original dry volume. In so doing, the expansion and change of dimension of such a swellable polymer can transmit a hydrostatic force that would cause membrane movement. 
     Changes in temperature can also serve as another form of an actuating stimulus. Certain synthetic materials, naturally-occurring materials and engineered structures can generate significant forces as they change their dimensions in response to changes in temperature. Such a gain or loss of thermal energy can thus also be used to cause the movement of membranes herein, working through the material as its size is changed thereby. 
     In another embodiment, an electrostrictive material can be used to transmit a force derived electrically. An electrostrictive material, when subjected to electrical voltage, can undergo size deformation, with a consequent change in dimension, which can produce a force that will transmit the effect of the actuating electrical stimulus and move a membrane. 
     An embodiment based on the use of a magnetic force as the actuating stimulus can be configured by incorporating a spiral or helical winding of a conducting wire (e.g. copper wire) in the adaptive membrane structure so that the winding is adjacent to the membranes in the structure and oriented such that the axis of the winding is normal to the plane of the membranes. The winding is electrically connected in series with a switch and a source of electrical potential such as a battery. A magnetic material is incorporated in one or more of the membranes in the structure, and the membranes are appropriately located within the structure such that their motion under action of the force of magnetic attraction will cause them to come in contact with each other or with one or more other membranes, or be moved away and separated from other membranes. The magnetic material can be incorporated within the bulk of a membrane or as a coating on a membrane surface. Possible magnetic materials include carbonyl iron particles dispersed within the bulk of a membrane or within a matrix comprising a coating on a membrane surface. Upon actuation of the system by closure of the switch, a magnetic field will develop in the vicinity of the winding, and this field will generate a force on the magnetic material incorporated in one or more membranes thereby causing the membrane(s) containing the magnetic materials to come in contact with, or be moved away from, one or more adjacent membranes. 
     The embodiments disclosed above also illustrate a corresponding variety in the means that is provided to respond to the actuating stimulus, examples of which included above are a swellable polymer, a material that changes size in response to temperature change, an electrostrictive material and a magnetic material. Also suitable for use as means responsive to an actuating stimulus is a thermoelectric material, which can generate electrical energy when subjected to a change in temperature, and thus transmit to membranes the force of a useful voltage that is representative of a gain in thermal energy. 
     The means responsive to the actuating stimulus are typically located in, on, within or adjacent to the adaptive membrane structure in close enough physical proximity to enable application of the force of the actuating stimulus to move at least one membrane. A conductor or magnetic particles can, for example, be printed on a membrane that has holes, can be printed on another membrane or layer that does not have holes, or can be formed itself as a separate membrane or layer. Further, a polymer or layer that changes shape and/or size can be located adjacent to a membrane that has holes, although other membranes or layers that do not have holes can be located there between provided that the mission of the polymer or material to apply a moving force to the membrane with holes is not hindered. 
     In view of the variety of forms of the actuating stimulus, as disclosed above, in some embodiments the adaptive membrane structure comprises first, second, and third membranes; an actuating stimulus and a means to respond to an actuating stimulus, wherein the actuating stimulus and the means to respond to an actuating stimulus are at least one selected from the group consisting of:
         (i) an electrostatic force is the actuating stimulus, and the means to respond to an actuating stimulus is an electrically conductive material that is incorporated in, on, or adjacent to the adaptive membrane structure,   (ii) a magnetic force is the actuating stimulus, and the means to respond to an actuating stimulus is a magnetic material that is incorporated in, on, or adjacent to the adaptive membrane structure; and   (iii) an electrostrictive force is the actuating stimulus, and the means to respond to an actuating stimulus is an electrostrictive material that is incorporated in, on, or adjacent to the adaptive membrane structure and       

     a source of electrical potential; 
     wherein in an unactuated state, the first membrane is in contact with the second membrane, but not in contact with the third membrane such that there is a gap between the first membrane and the third membrane;
 
wherein in an actuated state, the source of electrical potential, the actuating stimulus, and the means to respond to an actuating stimulus are configured to move the first membrane, or a portion thereof, into contact with the third membrane by an attractive force, thereby creating a gap between the first and second membranes; and the means to respond to an actuating stimulus is in physical proximity to the adaptive membrane structure to enable application of the force of the actuating stimulus to move the first membrane or a portion thereof.
 
     Whatever form the actuating stimulus takes, it operates in one embodiment to a substantially uniform extent on all portions of at least one membrane. In such embodiments, it is preferable for the entire membrane to move as a result of the application of the actuating stimulus. 
     In other embodiments, the actuating stimulus does not operate to a uniform extent on all portions of the membrane, and one or more portions of one membrane are moved into contact with, or away from, a corresponding portion or portions of another membrane in a position in which the holes of each portion of the first membrane are substantially out of registration, or are out of registration, with the holes of the corresponding portion of the second membrane. 
     In particular, the adaptive membrane structure disclosed herein can be designed to display multiple states of gas, vapor and/or liquid permeability in addition to and different from those exhibited when the adaptive membrane structure is in the fully actuated, fully unactuated or fully deactuated state. In one embodiment, an adaptive membrane structure can be formed to have two or more portions or subsections, where each subsection of the structure is itself an adaptive membrane structure that displays some or all the features disclosed herein. The permeability of the structure as a whole can be altered by changing the permeability of some or all of the subsections of the structure, and by doing so at different times. An actuating stimulus can be applied to each subsection of the membrane structure independently of all the other subsections. Hence, several different states of permeability can be obtained for the structure as a whole by moving membranes in some of the subsections, while not moving membranes in other of the subsections, that together make up the adaptive membrane structure as a whole. In another embodiment, however, all membranes in all subsections can be moved at the same time. 
     One example of an adaptive membrane structure that has several such subsections is illustrated in  FIG. 13 . The figure shows a plan view of a membrane that has four subsections, and each subsection consists of an array of holes. Two or more membranes such as the membrane illustrated in  FIG. 13  can be provided in the structure such that the array of holes in each subsection of one membrane are substantially out of registration, or are out of registration, with the array of holes of the corresponding subsection on another adjacent membrane. A separate actuating stimulus, and means responsive thereto, can be provided for each subsection of a membrane. By assembling the membrane illustrated in  FIG. 13  with at least one and possibly more corresponding membranes, and by connecting the resulting adaptive membrane structure to an appropriate electrical circuit, it is possible to apply an actuating stimulus to any one, any two, any three or all four of the subsections of the membrane structure. At least 5 different states of permeability for such an adaptive membrane structure with four subsections. The membrane of  FIG. 13  is shown having four subsections that are similar in shape and area. However, such similarity is not required. 
     Articles comprising the adaptive membrane disclosed herein provide environmental protection to a human, animal, or object enclosed by the article. As used herein, the term “enclosed by the article” indicates that the article is being worn as a garment or is an enclosure (e.g., a tent or building or container) large enough to contain that which is to be protected; it does not require that the protected being or object is completely encased. The article will have “exposed area,” i.e., the area of the article that is exposed to (equivalently, in contact with) the surrounding environment. The size of the exposed area will vary based on the application. For example, the exposed area of a protective garment for a human adult may be in the range of about 30-40 ft 2  (2.8-3.7 m 2 ). A collective protection enclosure such as a tent may have an exposed area greater than 200 ft 2  (18.6 m 2 ). For a residential building application, the exposed area could be as small as 500 ft 2  (46.5 m 2 ) or as large as 10,000 ft 2  (929 m 2 ). The exposed area will depend on the size of the home and whether whole building or only parts need to be protected. For a commercial building application, the exposed areas could be 100,000 ft 2  (929 m 2 ) and possibly higher. The exposed area will depend on the size of the building and whether it is desired to protect parts of the building (e.g., the roof) or the entire building. As one can see, there is a very large range of exposed areas that could be protected by using adaptive membranes. Generally, the adaptive membranes structures disclosed herein are used to protect such exposed areas are greater than or equal to about 1 ft 2  (0.09 m 2 ) in area. 
     The entire exposed area or only a part of the exposed area can comprise one or more adaptive membranes as disclosed herein.  FIG. 13  illustrates an embodiment where only part of the square exposed area includes adaptive membranes. The four circular regions represent four adaptive membrane structures. The percent of the exposed area that is protected (or, equivalently, “covered”) by adaptive membranes can be calculated by summing up the areas of the individual adaptive membrane structures, dividing by the total exposed area, and multiplying the result by 100. 
     The required percent covered area will be dictated by the needs of the specific application. In some embodiments, the covered area will be at least about 10%. Where a particularly high level of protection is required, the covered area may approach 100%. For example, for chemical and biological protective apparel, it would be desirable to provide a high level of breathability and water vapor permeability for comfort to the wearer. Therefore the designer of the protective apparel may strive to maximize the percent covered area to enhance comfort of the suit when it is in the unactuated state. Alternatively, the designer may prefer to use adaptive membranes to cover a portion of the exposed area, enough to provide breathability, and a more rugged barrier material (for example, butyl rubber) to cover the remaining exposed area. Similarly, if the adaptive membranes were to be used to control the water vapor transport in and out of building envelopes, the optimal percent covered area would depend on the nature of specific building application. 
     The adaptive membranes may or may not be uniformly distributed over the entire exposed surface area. In addition, the adaptive membrane areas need not be all of the same size. In  FIG. 13 , the four circular adaptive membranes are of the same area and are uniformly distributed. However, for protective apparel, for example, it may be desirable to have the adaptive membrane areas of different sizes and distributed non-uniformly over the entire exposed area.  FIG. 14  is a schematic diagram of a protective garment, wherein the rectangular shaded regions represent adaptive membrane structures. In this embodiment, the apparel designer chose not to place the adaptive membranes over certain parts of the body because of the complexity in the curvature of a human body. For example, a designer may choose not to place active areas over the shoulder regions, and over the knees and elbows and behind the knees and elbows. The adaptive membranes also vary in size and shape to conform to the needs of the garment design. In contrast, an architect who is designing a building enclosure may choose to place adaptive membrane structures uniformly throughout the entire surface area because of the ease of fabrication and installation over the walls of the building. 
     Those skilled in the art will recognize that the transport properties of the adaptive membranes disclosed herein are not only affected by the percent coverage and the uniformity of distribution of the adaptive membranes but also to a great extent by the total porosity of the membrane. In an embodiment of the adaptive membrane structures disclosed herein, the porosity is distributed. If, as in  FIG. 13 , the exposed surface includes the holes and the holes have a uniform cross-sectional area (e.g., are right cylinders), then the porosity of the article simply equals the total cross sectional area of the holes divided by the total exposed surface area. 
     The diffusive mass transfer across any porous membrane is directly proportional to the porosity of the membrane. Likewise, for the adaptive membranes disclosed herein, mass transfer under diffusion conditions is affected by the porosity of the membrane. However, diffusion is not the only mode for heat and mass transfer across membranes. Under certain conditions, mass transfer by convection can outweigh mass transfer by diffusion and in those cases convection can become the dominant mode of transport. When convection becomes dominant, as in a garment, small amounts of porosity can have a very large effect on the amount of mass transferred across a membrane. This was observed in a recent research study undertaken to relate the effect of garment porosity on critical heat stress (Thomas E. Bernard, Candi D. Ashley, Joseph D. Trentacosta, Vivek Kapur, and Stephanie M. Tew, “Critical heat stress evaluation of clothing ensembles with different levels of porosity,”  Ergonomics , in press). Thomas Bernard and coworkers showed that at low garment porosity values, ranging between 0 and 2%, there is a directly proportional relationship between critical wet bulb globe temperature, which is a measure of evaporative cooling, and garment porosity. As evaporative cooling increases, heat stress decreases. Thus, Bernard et al. showed that increasing garment porosity, even only from 0% to 2%, leads to a statistically significant reduction in heat stress. Beyond a porosity of 2% up to a porosity of 20% there is no further increase in evaporative cooling. 
     Thus, in one embodiment, the protective articles disclosed herein will have a total porosity that is greater than or equal to about 0.5%. When the membrane structure is actuated, the porosity will be reduced to a lower level to provide the desired protection. 
     The adaptive membrane structures can be used as components of a variety of articles of manufacture, including without limitation articles of apparel, enclosures, and sensor devices. 
     The adaptive membrane structures can be used as components of articles of apparel, especially for clothing intended to protect against chemical and biological toxins and pathogens. Such articles include without limitation those selected from the group consisting of protective suits, protective coverings, hats, hoods, masks, gowns, coats, jackets, shirts, trousers, pants, gloves, boots, shoes, shoe or boot covers, and socks. 
     The adaptive membrane structure can also be used in consumer apparel to protect against the natural elements. In one embodiment, the structure can be used as an inner liner in responsive outerwear apparel used for recreational and other outdoor activities, such that the liner could be made to change its permeability depending upon external temperature and wind conditions, so as to increase the comfort of the wearer. Examples of such outerwear include without limitation coats, jackets, ski pants, gloves, hats, hoods and masks. In another embodiment, a membrane structure could be used as a responsive liner in raingear. In dry external conditions, the liner would be highly permeable, thus breathable, but in wet and rainy conditions, the liner would be made impermeable to external precipitation. 
     The adaptive membrane can be used for various medical applications. In one embodiment, the structure could be used to fabricate items of apparel for health care workers, including without limitation surgical masks, medical or surgical garments, gowns, gloves, slippers, shoe or boot covers, and head coverings. 
     For some of the aforementioned applications, the adaptive membrane structures can be used in the absence of any additional porous material layers, while for some other applications a multi-layered system can be created where the adaptive membrane structure forms only one component in the multi-layered system. Examples of porous layers that could be used in conjunction with the adaptive membrane structure are woven fabrics, non-woven films and porous membranes. Additional porous layers can be used with the objective of (i) creating a composite system that protects the adaptive membrane structure from an environment that can degrade its performance, and (ii) creating a composite system that has more features than those that can be offered by the adaptive membrane structure itself. 
     For example, for the purpose of creating fire retardant apparel that also protects a firefighter from noxious fumes and vapors, the adaptive membrane structure can be layered with or sandwiched between fire retardant fabrics. In this case, the outer fire retardant fabric protects the wearer and the adaptive membrane structure from the fire. For the purpose of creating commercial apparel that protects against the natural elements, the structure can be sandwiched between woven fabrics. The outer and the inner fabric can be chosen to impart a comfortable feel as well as to provide a fashionable appearance to the apparel. Colored and patterned fabrics can also be used as outer layers to introduce additional camouflage feature to chemical and biological protective apparel for the soldier. In some cases, microporous membranes can be used to protect the adaptive membrane structure from dust and liquids. 
     An adaptive membrane structure as disclosed herein can be incorporated into an article of apparel by any of the knitting, sewing, stitching, stapling or adhering operations known in the art. It is common in the art to use fabrics or other materials having multiple layers from which to make apparel, and the structure can be incorporated therein by conventional methods. 
     The potential uses of the adaptive membrane structures disclosed herein are numerous and are not limited to protective apparel for humans. In other embodiments, the adaptive membrane structure can be used to create or construct an enclosure for the occupancy of humans, animals or perishables. The term “perishables” as used herein includes not just edible materials but any material that is sensitive to, or can be damaged or degraded by exposure to, the environment. Such enclosure would include for example collective shelters, such as tents, that protect groups of individuals against chemical and biological warfare agents. In another embodiment, safe rooms can be provided in commercial and residential buildings. For example, the safe rooms assembled using the adaptive membrane would be permeable under non-threatening conditions but would become impermeable when toxic agents are released in the external environment. In another embodiment, a tarpaulin comprising the adaptive membrane structure can be used to protect stored equipment. 
     The adaptive membranes can also be used to create an external water barrier layer in the construction of commercial and residential buildings such as dwellings and office buildings. The vapor barrier, or vapor-retardant layer, in a building should be impermeable enough to prevent precipitation from outside of the building to permeate inside, but yet should be breathable enough to allow excess moisture in the walls to permeate to the outside. Therefore, in one embodiment, the adaptive membrane can be used as a responsive vapor barrier in commercial and residential buildings such that the barrier layer can exist in multiple states. When there exists excess moisture in the building walls, the barrier layer would be made vapor permeable, and when there is high humidity in the external environment, the barrier layer would be made impermeable. 
     Adaptive membranes disclosed herein, when constructed from transparent polymer films, could also be used to construct agricultural and horticultural greenhouses. Temperature control in a greenhouse is an important issue for optimum plant growth. Existing greenhouses are constructed from polymer films of low gas and vapor permeability. Since such polymer films are not breathable, the temperature in a greenhouse is conventionally controlled by the opening and closing of engineered vents. This often leads to undesirable temperature gradients in the greenhouse. If an adaptive membrane structure is used to construct the greenhouse, the internal temperature could be more evenly controlled by changing the permeability of the membrane that envelops the greenhouse. As the temperature in the greenhouse rises, the membrane could be made more permeable, thereby allowing the process of free convection to reduce the temperature in the greenhouse. Similarly, as the temperature in the greenhouse falls, the membrane could be made less permeable, allowing the temperature in the greenhouse to rise. 
     In yet another embodiment, an adaptive membrane structure could be used in temporary, soft-walled construction, or in permanent construction, to create a clean room in which to perform surgical procedures, or in which to conduct activities requiring high air purity such as computer chip fabrication. 
     The adaptive membranes can also be used for managing the environment in small and large storage areas and containers such as those used for storing perishables. The term “perishables” as used herein includes not just edible materials but any material that is sensitive to, or can be damaged or degraded by exposure to, the environment. For example, edible materials such as fresh fruits and vegetables may need to be stored under optimum humidity levels to maintain freshness and enhance their shelf life. Adaptive membranes disclosed herein can be used to create storage areas or storage containers that respond to the local environment conditions. For example, when the local water vapor concentration in the stored area is above the desired level, the adaptive membranes will actuate to release excess water vapor to the surrounding environment, and will deactuate once the water vapor drops below the desired level. Such responsive storage devices could be used to ship edible materials or other perishables from one place to another or to store them in commercial and residential settings such as cold storage areas and refrigerators. 
     The adaptive membrane structures can also be used to enhance the life and performance of a sensor device, and in this sense a sensor device can be viewed as a perishable. The active components in a sensor device are very sensitive to their environment and can be poisoned by liquid or vapor or particulate species in the environment. Such devices can also be corrupted when exposed to high concentrations of the species they are designed to sense. In one embodiment, an adaptive membrane structure, by its ability to have different states of permeability in the actuated and the deactuated states, can be used to control the flow of species to an enclosure housing the active component of a sensor. In another embodiment, an adaptive membrane structure can be used as a protective layer or a shroud around the active component. For this application, when it is desired that the sensor be in the active state for sensing, the adaptive membrane structure can be left in the actuated state allowing the active component of the sensor to come in contact with species in the environment that need to be sensed. But when the sensor is no longer in the active or sensing state, the adaptive membrane structure can be deactuated to provide closure between membranes thereby protecting the active component of the sensor and enhancing its life. 
     The use of the adaptive membrane structure in connection with physical assets or devices such as enclosures, buildings, and sensors can be achieved by fabrication and construction methods known in the art. The adaptive membrane structure can be interleaved between other layers or structural elements such as when a building wrap is installed between the interior and exterior portions of a wall. Or when the adaptive membrane structure is used in an essentially free-standing application such as in a tent, tarpaulin, greenhouse, valve or protective cover for a sensor, installation can be achieved by anchoring it to a suitable frame. 
     Where an apparatus or method herein is stated or described as comprising, including, containing, having, being composed of or being constituted by certain components or steps, one or more components or steps other than those explicitly stated or described may be present in the apparatus or method. In an alternative embodiment, however, an apparatus or method disclosed herein may be stated or described as consisting essentially of certain components or steps, in which embodiment components or steps that would materially alter the principle of operation or the distinguishing characteristics of the apparatus or method would not be present therein. In a further alternative embodiment, an apparatus or method may be stated or described as consisting of certain components or steps, in which embodiment components or steps other than those as stated or described would not be present therein. 
     Where the indefinite article “a” or “an” is used with respect to a statement or description of the presence of a component in an apparatus, or a step in a method, the use of such indefinite article does not limit the presence of the component in the apparatus, or of the step in the method, to one in number. 
     EXAMPLES 
     The present invention is further illustrated in the following examples. The examples, while indicating preferred embodiments, are given by way of illustration only. From the above discussion and these examples, one skilled in the art will be able to ascertain the essential characteristics of this invention, and, without departing from the spirit and scope thereof, will be able to make various changes and modifications to adapt it to various uses and conditions. 
     Example 1 
     A three-membrane adaptive membrane structure following the embodiment of  FIGS. 3 and 4  can be constructed as follows. 
     Membrane  2  is a relatively stiff but nevertheless, flexible sheet of magnetically receptive material (such as RubberSteel® rubber magnet, available from, Magnum Magnetics, Marietta, Ohio) coated with a vinyl polymer on both surfaces, it contains numerous 0.04 inch diameter holes punched in a square array with a hole-to-hole spacing of 0.11 inch. 
     Membrane  2 ′ is a second, more flexible sheet of magnetic material (such as DigiMag® vinyl magnetic sheeting, available from Magnum Magnetics, Marietta, Ohio) coated with vinyl polymer on both surfaces and further coated with aluminum. It is placed adjacent to membrane  2  with the aluminum-coated surface facing membrane  2 . Membrane  2 ′ contains numerous 0.04 inch diameter holes punched in a square array with a hole-to-hole spacing of 0.11 inch. The aluminum coating on the surface on membrane  2 ′ is the conductive layer  7 ′ in  FIGS. 3 and 4 . 
     Membranes  2  and  2 ′ are held by a frame at their perimeter such that they are parallel to each other. By design and assembly in the frame, the hole arrays of membranes  2  and  2 ′ are displaced laterally with respect to each other such that none of the holes in membrane  2  overlaps any of the holes in membrane  2 ′; i.e., the holes are out of registration. 
     The two membranes are attracted to each other in the unactuated state because of the magnetic properties of both materials, so that the membranes are in contact and there is no gap between their adjacent surfaces. Thus, in the unactuated state, the combination of membranes  2  and  2 ′ is impermeable to the flow of a gaseous or liquid agent normal to their surfaces because they are in contact and the hole arrays on each membrane are out-of registration. 
     Membrane  2 ″ is made of flexible polyester film and is located adjacent to membrane  2 ′. Membrane  2 ′ contains numerous 0.04 inch diameter holes punched in a square array with a hole-to-hole spacing of 0.11 inch. It is coated with aluminum coating on its surface facing away from membrane  2 ′. This aluminum coating is the conductive layer  7 ″ in  FIGS. 3 and 4 . 
     Membranes  2 ′ and 2″ are held by the frame such that they are parallel and there is a small gap of 0.01 inch between their adjacent surfaces. Furthermore, the array of holes in membrane  2 ″ is located by design and assembly in the frame such that the holes in membrane  2 ′ overlap with the adjacent holes in membrane  2 ″. 
     Conductive wires are soldered to conductive layers  7 ′ and 7″ and form a circuit further comprising a switch and a 3000 volt DC power supply. When the switch is closed, the actuated state of the system is achieved, in which an electrostatic potential is developed between conductive layers  7 ′ and  7 ″ on membranes  2 ′ and  2 ″. This causes an attractive electrostatic force between membranes  2 ′ and  2 ″ which overcomes the attractive magnetic force between membranes  2  and  2 ′ and, further, moves membrane  2 ′ into contact with membrane  2 ″. This generates a gap between membranes  2  and  2 ′, thereby allowing the flow of a gaseous or liquid agent normal to the structure via a flow path designated as  6  in  FIG. 4C . 
     The three-membrane structure is thus highly permeable when actuated and impermeable when deactuated. The device of this example can be tested for permeability in the actuated and unactuated state using the test device shown in FIG. 15 of U.S. Pat. No. 7,597,855. 
     Comparative Example A 
     This Comparative Example demonstrates that charging two aluminum coated sheets of Mylar polyester film with like charges does not produce a repulsive force between the sheets. 
     The aluminum surfaces of two-coated sheets of Mylar polyester film were connected to opposite sides of a 1000 Volt power supply, so that the aluminum layers acquired opposite charges. The polyester film acted as an insulator to keep the aluminum layers from shorting out. The two sheets were attracted to each other because they were oppositely charged. The two aluminum surfaces were then instead connected to one side of the 1000 Volt power supply, so that the aluminum surfaces were charged up with the same sign of charge. The sheets were not repulsed from each other, nor were they attracted to each other.