Abstract:
A method of activating a micro-cell in which the micro-cell includes a first compartment, a second compartment, a fluid in the first compartment, an element in the second compartment and a porous barrier separating the first compartment from the second compartment. The porous barrier, in a first state, is operable to prevent the fluid from entering the second compartment whereas the porous barrier, in a second state, is operable, in response to an event, to allow the fluid to enter the second compartment and interact with the element in the second compartment so as to generate an activation signal.

Description:
BACKGROUND 
       [0001]    This disclosure relates to event activated micro control devices. In general, event activated control devices generate a signal that can be measured and recorded, in response to a particular event or action. The signal generated by the control device may be used, for example, to activate a system or other device as a result of the event. In some implementations, the signal generated by the control device may be used as a means to detect the occurrence of a particular event or to detect tampering for security purposes. 
         [0002]    In particular applications, event activated devices can be used to sense changes in the environment such as pressure, acceleration, gravitational, force, temperature, voltage, current, magnetic fields, electric fields, light and acoustic changes or, alternatively, to detect biological and chemical agents. Examples of systems that use event activated control devices include, for example, air bags, which are deployed in response to a change in acceleration; and chemical sensors, which emit warning alarms in response to detection of toxic chemicals. 
       SUMMARY 
       [0003]    The details of one or more embodiments of the invention are set forth in the description below, the accompanying drawings and in the claims. 
         [0004]    For example, in one aspect, a micro-cell includes a first compartment that has a fluid, a second compartment that has an element, and a porous barrier separating the first compartment and the second compartment, in which the barrier, in a first state, is operable to prevent the fluid from entering the second compartment and in which the barrier, in a second state, is operable, in response to an event, to allow the fluid to enter the second compartment and interact with the element to generate an activation signal. 
         [0005]    In another aspect, a method for activating a micro-cell includes causing an increase in pressure difference between a fluid in a first compartment of the cell and a vapor in a second compartment of the cell above a critical pressure, in which the increase in pressure difference allows the fluid to flow from the first compartment through a porous barrier into the second compartment. 
         [0006]    In another aspect, a method for activating a micro-cell includes applying an external stimulus to the micro-cell to at least partially collapse a barrier in which the collapse of the barrier allows a fluid in a first compartment of the cell to pass into a second compartment of the cell. 
         [0007]    In yet another aspect, a method of detecting an event or stimulus applied to a micro-cell includes detecting a signal representing the event or stimulus, in which the event or stimulus causes a fluid in a first compartment of the micro-cell to pass through a porous barrier and into a second compartment of the micro-cell and in which, upon entering the second compartment, the fluid interacts with an element in the second compartment to generate the signal. 
         [0008]    In another aspect, a method of activating a device includes applying an event or stimulus to a micro-cell coupled to the device, in which the event or stimulus causes a fluid in a first compartment of the micro-cell to pass through a porous barrier and into a second compartment of the micro-cell such that an activation signal is generated when the fluid interacts with an element in the second compartment and in which the activation signal activates the device. 
         [0009]    In another aspect, a method for activating a micro-cell includes applying a voltage across a fluid and a porous barrier, in which application of the voltage causes the fluid to flow from a first compartment, through micro-pores in the barrier, to a second compartment and in which an activation signal is generated when the fluid interacts with an element in the second compartment. 
         [0010]    In some implementations, the micro-cell includes micro-pores that extend from the first compartment to the second compartment to allow passage of the fluid from the first compartment into the second compartment when the difference in pressure is greater than the critical pressure. The pores can have respective sidewalls, in which the critical pressure is a function of the fluid-vapor surface tension, the barrier pore size and a contact angle between the fluid and a pore sidewall. The interface between the fluid and vapor can be located at an opening of a pore in the barrier. Alternatively, the interface between the fluid and vapor can be located in a pore of the barrier. 
         [0011]    In some implementations, the micro-cell includes a sub-structure supporting the barrier, wherein the sub-structure is arranged to break in response to the event such that the barrier at least partially collapses and allows the fluid to enter the second compartment. 
         [0012]    In some implementations, the barrier includes a non-wetting surface. In some cases, the element is an electrode and the fluid is an electrolyte solution. 
         [0013]    The activation signal can include an electrical signal, a magnetic signal, a visible signal, an auditory signal, or a thermal signal. The event can include acceleration or deceleration of the cell, a change in pressure applied to the cell, shaking of, vibrating or an impact applied to the cell, or application of an electric potential. 
         [0014]    In some cases, the signal representing detection of the event or stimulus includes a color change in the fluid. In addition, the signal representing detection of the event or stimulus can include an electrical signal. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]      FIGS. 1A-1B  are examples of an electrochemical cell. 
           [0016]      FIG. 1C  is a block diagram of an electrochemical cell coupled to a system. 
           [0017]      FIG. 2  is an illustration of a liquid contact angle. 
           [0018]      FIG. 3  is an illustration of a capillary. 
           [0019]      FIG. 4  is an example of an electrochemical cell. 
           [0020]      FIG. 5A  is an example of a barrier. 
           [0021]      FIG. 5B  is a top view of a barrier. 
           [0022]      FIG. 5C  is a top view of a pore. 
           [0023]      FIG. 5D  is a side view of a pore. 
           [0024]      FIGS. 6A-6B  are examples of an electrochemical cell. 
           [0025]      FIGS. 7A-7B  are examples of an electrochemical cell. 
           [0026]      FIG. 8  is an example of a package. 
           [0027]      FIG. 9  is an example of a package. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    An example of an event activated micro control device in a first embodiment is presented in the context of an electrochemical cell  2  as illustrated in  FIGS. 1A and 1B . The cell  2  is configured in a reserve state, in which electrodes  4  located in a first compartment  6  are isolated by a barrier  8  from a second compartment  12  containing a fluid  10  such as a liquid electrolyte. The term fluid, as used herein, refers to any liquid, vapor, gas or mixture thereof that is supportable on the barrier  8  and able to pass through openings in the barrier  8 . The barrier  8  prevents the electrolyte solution  10  from contacting, and subsequently reacting with, the electrodes  4  in the first compartment  6 . While the electrolyte is separated from the electrodes (see  FIG. 1A ), the cell  2  does not generate electricity. Upon activation of the cell  2 , the electrolyte solution  10  passes through the barrier  8  and is introduced into the first compartment  6  (see  FIG. 1B ), such that the electrolyte  10  and electrodes  4  chemically react and produce a current or a potential difference V across the electrodes  4 . The potential difference V can be detected using external terminals  14  which are connected to the electrodes  4 . Alternatively, the cell  2  can be coupled to another device or system  11  such that the potential difference V generated across terminals  14  serves to activate the device or system (see  FIG. 1C ). Depending on the implementation, activation of the cell  2  can occur in response to external conditions or events such as excess vibrations, shock, pressure and/or acceleration, such as gravitational acceleration. Other external events or conditions also can induce activation of the cell  2 . 
         [0029]    The potential difference V produced across electrodes  4  is characteristic of the particular electrode and electrolyte combination used. Accordingly, the voltage generated may serve to provide confirmation of the activation event, in contrast to spurious signals in the environment. For example, a 1.5 volt difference can be generated across Zn/MnO 2  electrodes when the electrodes come into contact and electro-chemically react with a ZnCl 2  electrolyte solution. Other electrode and electrolyte combinations may be used as well to provide alternate potential differences or to supply electrical current. The potential difference or current is detected and measured on external terminals  14  which are connected electrically to the electrodes  4 . In addition to detection, the potential difference or current generated by the electro-chemical reaction also can be used as a power source to activate other devices or systems. For example, in the context of automobiles, if the cell  2  is activated as a result of rapid deceleration, the potential generated across the electrodes can trigger deployment of an automobile air-bag. 
         [0030]    In some embodiments, the electric potential or current produced by the interaction of the fluid and the electrodes can be used to generate activation or notification signals. For example, the electrodes may be coupled to an audio circuit that produces an audible alarm or signal indicating that a triggering event has occurred when the electric potential is produced. In another example, the electrodes may be coupled to one or more heater elements that serve to heat the device or provide an increase in ambient temperature upon generation of the electric potential. In another example, the electrodes may be coupled to a device that produces a magnetic field, such as a solenoid. In some applications, the electrodes may be coupled to a light emitting device such as a light emitting diode. 
         [0031]    Alternatively, in some embodiments, the fluid  10  reacts with a corresponding chemical or biological agent upon entering the second compartment to produce a color change in the fluid  10  that is visible to a human. Such color changes may be used as a simple means of detection or threshold analysis. For example, the fluid  10  can be an acid-base indicator solution. In other embodiments, the fluid  10  chemically reacts with biological or chemical agents in the second compartment  12  to produce a color change in the fluid  10 . The biological or chemical agents can be in solution form or, alternatively, they can be bound to the interior walls of the second compartment  12 . 
         [0032]    In the illustrated implementation, the barrier  8  is a porous micro-structure that includes a series of holes  16  extending from the first compartment  6  to the second compartment  12 . The holes allow the electrolyte  10  to flow through the barrier  8  into the first compartment  6  under specific, pre-designed conditions. The surface of the barrier and/or the holes  16  can be formed such that they have super-lyophobic properties. As used herein, a lyophobic surface is a surface upon which a drop of liquid has a contact angle CA greater than 90°, the contact angle CA being measured between the solid-liquid interface and the liquid-vapor interface as shown in  FIG. 2 . Accordingly, the liquid drop appears to “bead up” on the lyophobic surface. A lyophobic surface discourages wetting of any fluid, including, for example, aqueous solutions or organic liquids such as hexane, methanol, and glycerol. Also, as used herein, a super-lyophobic surface is a surface upon which a liquid drop has a contact angle greater than 150°. A subset of lyophobic surfaces, which pertains to water and aqueous solutions, includes both hydrophobic and super-hydrophobic surfaces. A hydrophobic and super-hydrophobic surface refers to a surface upon which droplets of water have contact angles greater than 90° and 150°, respectively. In the absence of any external force or stimuli to drive the electrolyte through the pores  16 , the super-lyophobic barrier surface substantially prevents the electrolyte  10  from flowing through pores  16  and into the first compartment  6 . 
         [0033]    The stability of the electrolyte  10  on the porous super-lyophobic barrier  8  in this example is determined by the pressure stability of the portion of electrolyte  10  that enters each individual pore  16 . For example, the electrolyte  10  and lyophobic pore  16  may be modeled as a capillary system as shown in  FIG. 3 . In this system, the pressure difference across the liquid-vapor interface at equilibrium is given by Δp c =(2*γ)/R, where γ is the surface tension of the liquid at the liquid-vapor interface and R is the radius of the capillary. The critical pressure difference Δp c  is the minimum pressure needed to ensure that liquid flows through a pore  16  having a radius R to the opposite end of the barrier  8 . When the pressure difference across the interface is equal to or less than Δp c , however, the liquid cannot flow through the pore  16 . 
         [0034]    Accordingly, the pore size can be designed such that there is a critical pressure above which a liquid is forced through the pores. For example, if the cell  2  experiences an event which causes the critical pressure to be exceeded, the electrolyte  10  in the second compartment  12  flows through the pore  16  and exits on the opposite side of the barrier  8 , where it reacts with the electrodes  4  in the first compartment  6  to generate a specified voltage across terminals  14 . The voltage across terminals  14  then can be measured, detected or used to activate another device or system. Thus, any event which causes the critical pressure to be exceeded may be detected by measuring the voltage across terminals  14 . 
         [0035]    Events or stimuli which lead to the increase in pressure include, but are not limited to, vibration of or impact with the cell  2 , a change in pressure in either the first compartment  6  or the second compartment  12 , or an acceleration or deceleration of the cell  2 . For example, the cell  2  can have flexible walls that move in response to an applied force such as vibrations or a change in atmospheric pressure. The movement of the cell walls then can lead to a pressure increase in the first compartment  6  or a pressure decrease in the second compartment  12  so that the critical pressure is exceeded and the electrolyte passes through the pores  16 . In another example, the cell  2  can include an orifice on an outer wall through which pressure or vacuum can be applied externally. In another example, the cell  2  can undergo rapid acceleration such that the fluid  10  experiences high gravitational forces that increase the pressure difference above the critical pressure. 
         [0036]    Alternatively, techniques known in the art as “electrowetting” or “electrowetting-on-dielectric” can be used to transfer the fluid through the pores  16 . For example, an external voltage pulse  15  can be applied between the electrolyte  10  and the barrier surface to reduce the contact angle of the electrolyte  10  on the pore surface (see  FIG. 4 ). Depending on the surface tension properties of the fluid  10 , the properties of the pore  16 , and the applied voltage, the liquid contact angle can be reduced enough such that the fluid  10  spreads easily through the pores  16  and into the second compartment  12 . The fluid  10  then can react with the electrodes  4  in the second compartment. Accordingly, in some implementations, the cell  2  can be used to detect a change in voltage above or below a specified threshold voltage. 
         [0037]    An example of a porous barrier  8  is illustrated in  FIGS. 5A and 5B . The illustrated barrier  8  includes a series of hexagonally shaped pores  16  arranged in a lattice. As shown in  FIG. 5A , each pore  16  extends from a first side  17  of the barrier  8  to a second side  19 . As discussed above, the surfaces of the barrier  8  and the pores  16  can be coated with a super-lyophobic layer to help prevent fluid from entering the pores  16 . The shape of each pore  16  is not limited to a hexagonal design. Other pore shapes can be formed in the barrier  8  as well. For instance, the pores  16  can be circular, square, or amorphous in shape. Preferably, the pore size is small enough that fluid cannot flow from the first side  17  to second side  19  without the application of an external force or stimulus. As an example, the pore opening can be formed to have a width d of approximately 10-40 microns, a height h of approximately 10-40 microns and a wall thickness t of approximately 1-2 microns as illustrated in the pore top view (see  FIG. 5C ) and side view (see  FIG. 5D ). However, other pore dimensions also can be used. 
         [0038]    The super-lyophobic porous barrier  8  can be made, for example, of silicon using semiconductor and micro-electro-mechanical systems (MEMS) processing technologies. Alternatively, the barrier  8  can be formed of metal foils. For example, tantalum foil can be machined to create an array of through-holes using laser machining, chemical etching, or by stamping holes through the foil. The barrier  8  then can be oxidized using, for example, electrochemical oxidation or anodization, and coated with a lyophobic layer. 
         [0039]    In some implementations, the super-lyophobic barrier  8  is supported by sub-structures  20  as shown in  FIG. 6A . In the illustrated example, the sub-structures  20  are tabs or columns that serve to support the barrier  9  and may be broken off by a particular event or stimulus. For example, the sub-structures  20  can be designed to break off when a predetermined stress or frequency of vibration is applied to the cell  2 . Once the sub-structures have broken, the barrier  8  collapses and releases the electrolyte  10  from the second compartment  12  into the first compartment  6  where the electrolyte  10  and electrodes  4  chemically react to produce a potential difference V across terminals  14  as shown in  FIG. 6B . The sub-structures  20  can include, for example, portions of the cell walls  22  that protrude from the walls. Events which lead to the collapse of the sub-structures include, but are not limited to, acceleration or deceleration of the cell  2  and vibration of or impact with the cell  2 . 
         [0040]    In some embodiments, the lyophobic porous barrier itself can collapse either partially or completely in response to a particular event or stimulus, without the use of sub-structures. As an example,  FIGS. 7A and 7B  show a porous barrier  8  fixed to walls  22  of the cell  2  before and after a specified event occurs. Prior to the event, the barrier is fixed in place and the electrolyte  10  cannot pass through the pores  16  (see  FIG. 7A ). After the event occurs, the barrier  8  partially or completely collapses exposing regions  24  large enough to allow the electrolyte to pass into the second compartment  12  and react with the electrodes  4  (see  FIG. 7B ). Alternatively, the barrier  8  can partially or completely dissolve in response to the event or stimulus. 
         [0041]    An exploded view of a package  800  that includes the electrochemical power cell  2  is shown in  FIG. 8 . The package  800  has a base  801  for holding external terminals  14 . The external terminals  14  are electrically connected to electrode  4  inside the package base  801 . The electrode  4  can be formed, for example, as a series of interdigited electrodes having alternating polarity. Other electrode designs may be used as well. A compliant sheet  804  can be provided beneath the electrode  4  to absorb shock and excessive force on the package  800 . A spacer  815  between the electrode  4  and barrier  8  has an opening  814  in which a filter paper stack  808  can be placed. The filter paper stack  808  allows the electrolyte solution to spread evenly across the electrode  4 . A reservoir  820  having an opening  810  is positioned above the barrier  8  and is used to hold the electrolyte solution. A second filter paper stack  822  can be placed in the opening  810  to facilitate even distribution of the electrolyte on the barrier  8 . A metal cap  824  is secured to the package base  801  to confine the components and seal the electrolyte solution in the reservoir  820 . In the illustrated example, the cap  824  includes a window  826  that allows a user to observe operation of the cell. For example, should the electrolyte solution change color upon reacting with the electrode  4 , the color change can be viewed through the window  826 .  FIG. 9  illustrates an example of the package  800  fully assembled. 
         [0042]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Other implementations are within the scope of the claims.