Abstract:
The display device ( 10 ) capable of irreversibly switching from a first indicating state to a second indicating state comprises a chamber ( 14 ) containing an electrolytic liquid ( 16 ) and having at least one exit opening ( 46 ), and at least two electrodes ( 22,24 ) located in said chamber ( 14 ) and in contact with said electrolytic liquid ( 16 ) as well as subjectable to electric voltage, wherein, upon application of electric voltage of a predetermined level to said electrodes ( 22,24 ), gas is generated in said chamber ( 14 ) causing the pressure within said chamber ( 14 ) to exceed a threshold value such that liquid ( 16 ) irreversibly exits said chamber ( 14 ) through said at least one exit opening ( 46 ) thereby switching from the first indicating state to the second indicating state.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a non-reversible display device capable of irreversibly switching from a first indicating state to a second indicating state.  
         [0003]     1. Related Prior Art  
         [0004]     Several indicating or displaying devices of different types and technologies are known in the prior art. Among these types of indicating or displaying devices there are electrochrome display devices (see e.g. DE-A-25 45 391 and DE-A-198 25 371), electroluminescent display devices (see e.g. DE-A-100 42 500), and the widely spread LCD display devices. Moreover, indicating devices are known in which the change of an indicating state results from a chemically induced change of colour (see e.g. EP-B-0 081 031 and DE-A-44 43 470).  
         [0005]     The problem with all of the above-identified types of display devices is that maintaining one of the two indicating states requires the provision of energy of a power supply. As soon as power is no longer supplied (e.g. due to an exhaust of the power supply like a battery or the like) the display device automatically switches to the indicating state which requires no power.  
         [0006]     In U.S. Pat. No. 4,156,559 as well as in DE-B-27 27 854 there is disclosed an electrolytic display cell comprising two parallel plates whereof one is covered by a semi-transparent electrode and whereof the other supports a counter-electrode. Between the electrode and the counter-electrode an electrolyte is arranged containing a metallic salt dissolved in a solvent. The electrode and counter-electrode are connected to the positive and negative terminals of a d.c. voltage source via a switch permitting the connection of each terminal to any one of the electrodes. Connecting the electrode of the electrolytic display device to the negative terminal of the d.c. voltage source results in a electrolytic deposition of the metallic salt of the electrolyte on the semi-transparent electrode resulting in a change of its transmissivity defining a first indicating state. If the electrode is connected to the positive terminal of the d.c. voltage source, the metallic salt disposed on the semi-transparent electrode dissolves into the electrolyte. That means an indication on the known electrolytic display device can be erased again which is a disadvantage if the switch can be operated inadvertently so that valuable information may get lost.  
       SUMMARY OF THE INVENTION  
       [0007]     It is an object of the present invention to provide a non-reversible bistable display device which works reliably within a wide temperature range and with low activation energy consumption.  
         [0008]     Another object of the present invention is to provide a non-reversible bistable display device which can be manufactured with low production costs.  
         [0009]     According to the invention, a non-reversible bistable display device capable of irreversibly switching from a first indicating state to a second indicating state is provided, wherein the display device comprises 
        a chamber containing an electrolytic liquid and having at least one exit opening, and     at least two electrodes located in said chamber and in contact with said electrolytic liquid as well as subjectable to electric voltage,     wherein, upon application of electric voltage of a predetermined level to said electrodes, gas is generated in said chamber causing the pressure within said chamber to exceed a threshold value such that liquid irreversibly exits said chamber through said at least one exit opening thereby switching from the first indicating state to the second indicating state.        
 
         [0013]     The function of the display device according to the invention is based on the discharge of liquid through at least one exit opening of a chamber by increasing the pressure within the chamber to exceed a threshold value. A specific feature of the invention is that the increase of pressure results from the generation of gaseous components caused by an electrolysis performed within the chamber and initiated upon the application of electrical energy.  
         [0014]     Accordingly, the chamber comprises an electrolytic liquid and at least two electrodes subjectable to electric voltage. When applying electric voltage to the at least two electrodes, current flows through the electrolytic liquid resulting in an electrolysis-generated gas. The electrolytic liquid comprises conductivity modulators such as for example a salt or other materials basically known to the those skilled in the art. In particular, the electrolytic liquid can be ink having a sufficient electrical conductivity so as to function as a electrolyte.  
         [0015]     An advantage of the display device according to the invention is the comparatively low energy consumption for activating the display device i.e. for discharging liquid out of the chamber thereby irreversibly switching from a first indicating state in which no liquid is discharged, to a second indicating state in which liquid is or has been discharged. Generating the gaseous components using an electrolysis process can already be initiated by the application of relatively low electrical voltage (e.g. in the range of up to about 5 V) and, accordingly, the electrical energy required is rather low. In particular, if the speed of switching from the first indicating state to the second indicating state is of less relevance, the electrolysis can take place over a time of several seconds which further reduces the electrical power required.  
         [0016]     By way of experiments it was found out that for example a voltage of around 3V was sufficient when applied to a volume of 15 nanoliter of a aqueous solution containing a salt (e.g. NaCl) and conventional ink particles (e.g. as used in ink jet printers), causing a current of around 0.7 mA and a switching time of about 20 seconds for discharging electrolytic liquid out of the chamber.  
         [0017]     Moreover, the design of the display device according to the invention need not be complex and, accordingly, its function and operation can be rather reliable also within a wide temperature range because the electrolysis is relatively independent of the temperature of the electrolytic liquid.  
         [0018]     Depending on the content of the chamber either the electrolytic liquid or another indicating liquid can be discharged. In one embodiment of the present invention the chamber is completely filled with electrolytic liquid so that this liquid is discharged out of the at least one exit opening. In another embodiment of the present invention the chamber is partially filled with electrolytic liquid within the area of the two electrodes, but within the area of the at least one exit opening is filled with another indicating liquid. If these two liquids are immisible, the two liquids directly can contact each other. As an alternative of such a design, between both liquids there can be arranged a membrane or the like flexible separating film of wall dividing the chamber into two parts wherein in the one part the at least two electrodes and the electrolytic liquid are arranged and in the other part the at least one exit opening and the indicating liquid are located.  
         [0019]     Moreover, in another embodiment of the present invention, the discharged liquid is received by a receiving space which at least partially is visible from outside. The receiving space can be open to the environment so that liquid can exit or the receiving space can be closed. In the latter case a venting opening for exiting gas from the receiving space to the environment when liquid enters the receiving space, can be arranged in the receiving space. However, a venting opening is not necessary, because the gas contained in the receiving space can be compressed when the liquid enters the receiving space. Finally, the receiving space can have an expansion wall so that the receiving space can expand when liquid enters. Also a receiving space can be used employing the mechanisms of venting and/or compression and/or expansion in combination.  
         [0020]     In a preferred embodiment of the present invention a porous element is arranged within the receiving space for sucking-up the liquid discharged from the at least one exit opening and entering the receiving space. Preferably, the sucking-up of the liquid takes place by capillary forces generated within the porous element. Most preferably the porous element is a fabric and in particular a non-woven fabric.  
         [0021]     It is preferred to have the at least two electrodes and each of the exit openings be spaced from each other as far as possible. By such a design, when gas is generated and the pressure is increased, during at least the first phase of the discharge process the gas pushes liquid out of the at least one exit opening, before perhaps the gas as such flows through the at least one exit opening.  
         [0022]     Different shapes of the chamber between the at least two electrodes and the at least one exit opening are possible. For example the chamber can be designed like a channel comprising two opposite ends, the one end being provided with the electrodes and the other end being provided with the at least one exit opening. Moreover, such a channel can be substantially straight or corrugated or spirally wound.  
         [0023]     In another aspect of the present invention the at least one exit opening comprises a means for preventing unintended exit of liquid. This means can be a capillary stop i.e. a narrowed area through which the liquid within the chamber will not exit due to surface tension phenomena. Moreover, a capillary stop can be provided by a hydrophobic surface of the chamber within the area of the at least one exit opening. Suitable different designs of capillary stops are known to those skilled in the art of microfluidic systems.  
         [0024]     According to another aspect of the present invention, the chamber of the display device comprises at least one weakened location which will break if the pressure within the chamber exceeds a threshold value. The broken weakened location provides an exit opening of the liquid.  
         [0025]     One alternative for providing a weakened location is the formation of a thin portion within the wall of the chamber i.e. a hole in the chamber covered by a film which breaks when the pressure within the chamber exceeds a threshold value. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]     The present invention will be described in more detail referring to specific embodiments as depicted in the drawing in which  
         [0027]      FIG. 1  shows a cross-sectional view of a display device according to a first embodiment of the invention in its first indicating state,  
         [0028]      FIG. 2  shows a top view of the display device of  FIG. 1  with its cover removed,  
         [0029]      FIG. 3  shows a cross-sectional view of the display device when liquid is transferred from one chamber to an adjacent chamber switching from the first to the second indicating state,  
         [0030]      FIG. 4  shows a top view of a display device with its cover removed, according to a second embodiment of the invention,  
         [0031]      FIG. 5  shows a top view of a display device with its cover removed, according to a third embodiment of the invention,  
         [0032]      FIG. 6  shows a cross-sectional view of a display device according to a fourth embodiment of the invention,  
         [0033]      FIG. 7  shows a top view of a display device with its cover removed, according to a fifth embodiment of the invention, and  
         [0034]      FIG. 8  shows a cross-sectional view of a display device according to a sixth embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     In the Figures different display devices according to the invention are shown wherein like parts of the individual embodiments are referred to by the same reference numerals.  
         [0036]     The working principal of the invention will be explained hereinbelow referring to FIGS.  1  to  3  showing a first embodiment of a non-reversible bistable display device  10 . The display device  10  comprises a substrate  12  made of a non conductive material such as molded synthetic material, a foil, or glass. Most preferably the substrate  12  comprises an electrically insulating material. Within the substrate  12  there is provided a chamber  14  containing an electrolyte  16 . As can be seen from  FIGS. 1 and 3  the chamber  14  is open to the top side  18  of the substrate  12 . Within the chamber  14  and preferably at the bottom  20  thereof, there are arranged two electrodes  22 , 24  connected to a voltage source  26  via a switch  28 .  
         [0037]     Beside the chamber  14  within the top side  18  of the substrate  12  there is arranged another recess  30  filled with a porous element  32  provided as a non-woven fabric  34 . This non-woven fabric comprises synthetic fibers, for example fibres of polypropylene or polyethylene. However, other fibers are also possible. Also other porous elements  32  are possible like for example foamed elements. As described below, the porous element  32  is selected to provide a structure that, by capillary action, is capable of sucking-up liquid from the chamber  14  and distributing it evenly within the recess  30 .  
         [0038]     Typically, the chamber  14  has a diameter of less than 10 mm and the sides of the recess  30  have a length in the range of from about 5 to 12 mm.  
         [0039]     The top side  18  of the substrate  12  is covered by a film  36  covering the chamber  14  and the recess  30 . The film  36  is transparent within its area covering the recess  30  and is opaque within its remaining area and, accordingly, in particular within the area covering the chamber  14 . Within its opaque areas the film  36  can be provided with a coating  38 . The film  36  is mounted to the substrate  12  by means of an adhesive layer  40 . The film  36  and adhesive layer  40  may, for example, be provided by a conventional transparent adhesive tape applied over the substrate  12 .  
         [0040]     The chamber  14  and the recess  30  are separated by a curved narrow edge  42  to which the film  36  is also adhered by the adhesive layer  40 . If the pressure within the chamber  14  increases, the film  36  will be subjected to a peeling force around the edge of the recess  30 . The peeling force will act preferentially at the edge  42  because of the curvature of the latter and, as soon as it exceeds the adhesive force between the film  36  and the adhesive layer  40  or between the edge  42  and the adhesive layer  40  at this point, the film  36  will peel away from the edge  42 .  
         [0041]      FIG. 1  shows the device  10  in its first indicating state in which the chamber  14  completely filled with electrolytic liquid  16  is closed by the film  36 . In  FIG. 3  the situation is shown in which the display device  10  is in its second indicating state which will be explained now.  
         [0042]     In order to transfer the device  10  from its first indicating state of  FIG. 1  to its second indicating state of  FIG. 3 , an operating voltage is applied to the electrodes  22  and  24  by closing the switch  28 . Since the electrolytic liquid  16  is electrically conductive, a current is flowing there through. The flow of current through the electrolytic liquid  16  results in an electrolysis which in turn generates gas bubbles  44 . The generation of gas of O 2  and H is a normal process in the electrolysis. These gaseous components increases the pressure within the chamber  14 . As a result thereof, the force acting on the film  36  at its interface to the edge  42  decreases the adhesive force therebetween so that the film  36  pops up to a little extent providing an exit opening  46  for the chamber  14 . In other words the edge  42  together with the film  36  provides a weakened location  48  which breaks when the pressure in the chamber  14  exceeds a threshold value. Due to the narrow design of the edge  42  the film  36  will release within the area of the edge  42  and not within another area of the top side  18  of the substrate  12 . It is possible that several small passages over the edge  42 , i.e. several exit openings  46  are provided upon the increase of the pressure within the chamber  14 .  
         [0043]     As a result of the formation of the exit opening  46 , liquid  16  from the chamber  14  exits into the recess  30  which functions as a receiving space  50  for the liquid  16 . Liquid  16  entering the non-woven fabric  34  will be sucked thereby so that the fabric  34  is filled with the liquid  16 . In case of a coloured liquid  16  i.e. an electrolytic ink, the non-woven fabric  34  will change its colour when soaked with the ink, thereby making visible the switching from the first indicating state in which the non-woven fabric  34  has the colour of the fibers thereof, to the second indicating state in which the non-woven fabric  34  is coloured by the ink.  
         [0044]     Several different designs of the electrodes  22 , 24 , the edge  42  separating the chamber  14  from the receiving space  50  and the arrangement of these two chambers are shown in FIGS.  4  to  9 .  
         [0045]     In particular, the embodiment 60 of  FIG. 4  is provided with a substrate  12  manufactured in molded interconnect device (MID) technique. This technique is known to those skilled in the art for forming three dimensional printed circuit boards. The electrodes  22 , 24  can be formed as electrically conductive traces along the top surface  18  of the substrate  12  and down into the chamber  14 . As in FIGS.  1  to  3 , the edge  42  between the chamber  14  and the receiving space  50  (recess  30 ) at the top side  18  of the substrate  12  is curved towards the chamber  14  resulting in a concentration of the forces for releasing the film  36  from the edge  42 .  
         [0046]     In  FIG. 5  an embodiment of a display device  70  is shown in which an annular channel  72  is arranged around the chamber  14  wherein the annular channel  72  is in fluid communication with the receiving space  50 . The edge  42  between the chamber  14  and the receiving space  50  is annular. Due to the surrounding channel  72  irrespective of where the film  36  will release from the edge  42 , liquid  16  from the chamber  14  via the channel  72  will be received by the receiving space  50 . The substrate  12  of the display device  70  is again made in MID-technique.  
         [0047]     In  FIG. 6 a  display device  80  is shown which, adjacent the edge  42 , is provided with a channel  82  leading to the receiving space  50 . The benefit of this design is that through the channel  82  which is dimensioned accordingly, liquid  16  can flow due to capillary forces. This supports the exit of liquid  16  from the chamber  14  even if the film  36  is merely slightly separated from the edge  42 .  
         [0048]      FIG. 7  shows a design of a device  90  wherein the receiving space  50  is arranged as an annular space surrounding the chamber  14 . Also the edge  42  separating the chamber  14  from the receiving space  50  is annular.  
         [0049]     In  FIG. 8  another design of a display device  100  is shown. The chamber  14  of this device  100  is in the form of a straight channel with two opposite ends wherein the electrodes  22 ,  24  are arranged near one end and the exit opening  46  is arranged at the other end. Distancing the electrodes  22 ,  24  from the exit opening  46  in this way can be useful in certain circumstances in preventing gas bubbles generated during electrolysis in the liquid  16  from entering the receiving space  50 . If it is required to increase the length of the channel-shaped chamber  14 , it may be necessary to use a channel that follows a corrugated or a spiral path (rather than a straight path) to avoid an undesirable increase in the length of the device  100 .  
         [0050]     The exit opening  46  of the chamber  14  of device  100  is designed such that liquid  16  of the chamber  14  does not exit there through under normal pressure conditions. In other words the exit opening  46  forms a capillary stop preventing liquid  16  from flowing out of the chamber  14 . The capillary stop function can be overcome in the device  100  upon the increase of the pressure within the chamber  14  when the electrolysis takes place. In this case, liquid  16  is discharged through the exit opening  46  into the adjacent receiving space  50 .  
         [0051]     In connection with  FIG. 8  another alternative of the present invention will be described which alternative can also be used in the embodiments of FIGS.  1  to  7 .  
         [0052]     As can be seen in  FIG. 8 , the chamber  14  is filled with two different liquids  16  and  102 . Liquid  16  which is in contact with the electrodes  22  and  24  is an electrolyte as described above. The other liquid  102  is not necessarily an electrolyte and is located within that part of the volume of the chamber  14  which is located between the electrolyte  16  and the exit opening  46 . Increasing the pressure of the electrolyte  16  due to electrolysis, will result in a force acting on the volume of liquid  102  causing liquid  102  to exit from the chamber  14  into the receiving space  50 .  
         [0053]     The interface between both liquids  16  and  102  is referred to in  FIG. 8  by  104 . These two liquids can contact each other directly if the liquids are not immisible. In order to guarantee no mixing-up of the two liquids, at the interface  104  thereof a flexible membrane (not shown) can be arranged.  
         [0054]     Although the invention has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those illustrative embodiments. Those skilled in the art will recognise that variations and modifications can be made without departing from the true scope of the invention as defined by the claims that follow. It is therefore intended to include within the invention all such variations and modifications as fall within the scope of the appended claims and equivalents thereof.