Patent Abstract:
A delayed degradability drip irrigation pipe including a water conduit at a water conduit pressure and a plurality of drip irrigation outlets, each communicating with the water conduit and providing a water output at a pressure below the water conduit pressure, at least the water conduit being formed at least partially of a degradable material and also including a degradability delayer which provides a desired delay prior to failure of the water conduit but permits eventual degradation of the degradable material under predetermined conditions.

Full Description:
FIELD OF THE INVENTION 
     The present invention relates to drip irrigation and more particularly to drip irrigation pipes which are biodegradable in situ. 
     BACKGROUND OF THE INVENTION 
     The following patent publications are believed to represent the current state of the art:
     U.S. Pat. No. 4,474,330;   U.S. Patent Publications 2008/0191464 and 2008/0072480.   

     SUMMARY OF THE INVENTION 
     The present invention seeks to provide drip irrigation pipes having desired biodegradable characteristics. 
     There is thus provided in accordance with a preferred embodiment of the present invention a delayed degradability drip irrigation pipe including a water conduit at a water conduit pressure and a plurality of drip irrigation outlets, each communicating with the water conduit and providing a water output at a pressure below the water conduit pressure, at least the water conduit being formed at least partially of a degradable material and also including a degradability delayer which provides a desired delay prior to failure of the water conduit but permits eventual degradation of the degradable material under predetermined conditions. 
     In accordance with a preferred embodiment of the present invention, the degradable material includes biodegradable material. 
     Preferably, the degradability delayer includes a bacterial growth delayer. 
     In accordance with a preferred embodiment of the present invention, the degradability delayer includes a generally non-biodegradable material which is mixed with the biodegradable material. 
     In accordance with a preferred embodiment of the present invention, the degradability delayer is mixed with the biodegradable material. 
     Preferably, the degradability delayer is formed as a co-extruded layer alongside the biodegradable material. Additionally, the degradability delayer is formed as a co-extruded inner layer of the drip irrigation pipe. Alternatively or additionally, the degradability delayer is formed as a co-extruded outer layer of the drip irrigation pipe. 
     In accordance with a preferred embodiment of the present invention the degradability delayer is formed as strips along the length of the drip irrigation pipe. 
     In accordance with a preferred embodiment of the present invention the water conduit includes at least one first layer formed of a biodegradable material, the biodegradable material being mixed with a biodegradability delayer and at least one second layer formed of a non-biodegradable, UV degradable material. Additionally, the at least one second layer also includes an oxo-biodegradable material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: 
         FIG. 1  is a simplified illustration of part of a delayed degradability drip irrigation pipe constructed and operative in accordance with a preferred embodiment of the present invention; 
         FIG. 2  is a simplified comparative illustration of the relative degradability characteristics of a biodegradable drip irrigation pipe and of the delayed degradability drip irrigation pipe, constructed and operative in accordance with a preferred embodiment of the present invention, illustrated in  FIG. 1 ; 
         FIG. 3  is a simplified illustration of part of a delayed degradability drip irrigation pipe constructed and operative in accordance with another preferred embodiment of the present invention; 
         FIG. 4  is a simplified comparative illustration of the relative degradability characteristics of a biodegradable drip irrigation pipe and of the delayed degradability drip irrigation pipe, constructed and operative in accordance with another preferred embodiment of the present invention, illustrated in  FIG. 3 ; 
         FIG. 5  is a simplified illustration of part of a delayed degradability drip irrigation pipe constructed and operative in accordance with a further preferred embodiment of the present invention; 
         FIG. 6  is a simplified comparative illustration of the relative degradability characteristics of a biodegradable drip irrigation pipe and of the delayed degradability drip irrigation pipe, constructed and operative in accordance with a further preferred embodiment of the present invention, illustrated in  FIG. 5 ; 
         FIG. 7  is a simplified illustration of part of a delayed degradability drip irrigation pipe constructed and operative in accordance with yet another preferred embodiment of the present invention; 
         FIG. 8  is a simplified comparative illustration of the relative degradability characteristics of a biodegradable drip irrigation pipe and of the delayed degradability drip irrigation pipe, constructed and operative in accordance with yet another preferred embodiment of the present invention, illustrated in  FIG. 7 ; 
         FIG. 9  is a simplified illustration of part of a delayed degradability drip irrigation pipe constructed and operative in accordance with still another preferred embodiment of the present invention; 
         FIG. 10  is a simplified comparative illustration of the relative degradability characteristics of a biodegradable drip irrigation pipe and of the delayed degradability drip irrigation pipe, constructed and operative in accordance with yet another preferred embodiment of the present invention, illustrated in  FIG. 9 ; 
         FIG. 11  is a simplified illustration of part of a delayed degradability drip irrigation pipe constructed and operative in accordance with still a further another preferred embodiment of the present invention; and 
         FIG. 12  is a simplified comparative illustration of the relative degradability characteristics of a biodegradable drip irrigation pipe and of the delayed degradability drip irrigation pipe, constructed and operative in accordance with yet another preferred embodiment of the present invention, illustrated in  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is now made to  FIG. 1 , which is a simplified illustration of part of a delayed degradability drip irrigation pipe, constructed and operative in accordance with a preferred embodiment of the present invention, and to  FIG. 2 , which is a simplified comparative illustration of the relative degradability characteristics of a biodegradable drip irrigation pipe and of the delayed degradability drip irrigation pipe constructed and operative in accordance with the preferred embodiment of the present invention illustrated in  FIG. 1 . 
       FIG. 1  illustrates part of a delayed degradability drip irrigation pipe  100  which includes discrete emitter units  102  distributed along the length thereof in communication with the interior thereof. It is appreciated that the present invention is not limited in its applicability to this type of drip irrigation pipe and also applies to other types of drip irrigation pipes wherein the emitters are fully or partially defined by the pipe. The present invention applies to drip irrigation pipes which are formed by extrusion and equally to drip irrigation pipes that are formed by welding of elongate sheets. 
     The term “biodegradable” is used throughout to refer to degradation as the result of biological activity. When applied to irrigation pipes, it is not limited to pipes which do not leave any residue whatsoever in the ground. 
     The irrigation pipe  100  is preferably formed of a biodegradable plastic material, such as PBAT (polybutylene adipate/teraphthalate), PTMAT (polymethylene adipate/teraphthalate), naturally produced polyester, such as PHA polyesters (polyhydroxyalkanoates), PHBH polyesters (poly-hydroxybutyrate-co-polyhydroxy hexanoates) and PLA polyesters (polylactic acid), which is biodegradable by bacterial and/or fungal action. 
     In accordance with a preferred embodiment of the present invention delayed degradability functionality is provided by the addition of an active anti-bacterial and anti-fungal agent which demonstrates activity against a wide range of bacteria, mold and yeast, such as CIBA® IRGAGUARD® B-1000, B-5000 or B-7000, HYGATE® 4000 or 9000 and ALPHASAN®. 
     Preferably, the active anti-bacterial and anti-fungal agent is mixed with the biodegradable plastic material prior to formation of the pipe, for example, prior to extrusion of the pipe or of a sheet from which the pipe is formed. Alternatively, the active anti-bacterial and anti-fungal agent is co-extruded onto one or more surface of the pipe or sheet or coated thereon. 
     As seen in  FIG. 1 , the active anti-bacterial and anti-fungal agent may appear throughout the thickness of the pipe  100 . 
     Turning to  FIG. 2 , degradable plastic drip irrigation pipes  100 , which include an active anti-bacterial and anti-fungal agent, are shown alongside biodegradable plastic drip irrigation pipes  110 , which do not include an active anti-bacterial and anti-fungal agent, at the same point in time. 
     It is seen that at a given point in time, typically six months following installation, biodegradable plastic drip irrigation pipes  110 , which do not include an active anti-bacterial and anti-fungal agent, are in the process of biodegradation, typically under bacterial and fungal action. In contrast, in accordance with a preferred embodiment of the present invention, delayed degradability drip irrigation pipes  100 , constructed and operative in accordance with a preferred embodiment of the present invention, include active anti-bacterial and anti-fungal agents, thereby delaying biodegradation under bacterial and fungal action, for a time duration, until such agents are no longer released or they become ineffective. 
     Reference is now made to  FIG. 3 , which is a simplified illustration of part of a delayed degradability drip irrigation pipe, constructed and operative in accordance with another preferred embodiment of the present invention, and to  FIG. 4 , which is a simplified comparative illustration of the relative degradability characteristics of a biodegradable drip irrigation pipe and of the delayed degradability drip irrigation pipe constructed and operative in accordance with the preferred embodiment of the present invention illustrated in  FIG. 3 . 
       FIG. 3  illustrates part of a delayed degradability drip irrigation pipe  200  which includes discrete emitter units (not shown) distributed along the length thereof in communication with the interior thereof. It is appreciated that the present invention is not limited in its applicability to this type of drip irrigation pipe and also applies to other types of drip irrigation pipes wherein the emitters are fully or partially defined by the pipe. The present invention applies to drip irrigation pipes which are formed by extrusion and equally to drip irrigation pipes that are formed by welding of elongate sheets. 
     The irrigation pipe  200  is preferably formed of a biodegradable plastic material, such as PBAT (polybutylene adipate/teraphthalate), PTMAT (polymethylene adipate/teraphthalate), naturally produced polyester, such as PHA polyesters (polyhydroxyalkanoates), PHBH polyesters (poly-hydroxybutyrate-co-polyhydroxy hexanoates) and PLA polyesters (polylactic acid), which is biodegradable by bacterial and/or fungal action. 
     In accordance with a preferred embodiment of the present invention, delayed degradability functionality is provided by provision of an active anti-bacterial and anti-fungal agent, which demonstrates activity against a wide range of bacteria, mold and yeast, such as CIBA® IRGAGUARD® B-1000, B-5000 or B-7000. 
     Preferably, the active anti-bacterial and anti-fungal agent is coextruded with the biodegradable plastic material during formation of the pipe or of a sheet from which the pipe is formed. Alternatively, the active anti-bacterial and anti-fungal agent is coated onto one or more surface of the pipe or sheet. 
     As seen in  FIG. 3 , the active anti-bacterial and anti-fungal agent may appear as strips  204  along the length of the pipe  200 . 
     Turning to  FIG. 4 , biodegradable plastic drip irrigation pipes  200 , which include an active anti-bacterial and anti-fungal agent, are shown alongside biodegradable plastic drip irrigation pipes  210 , which do not include an active anti-bacterial and anti-fungal agent, at the same point in time. 
     It is seen that at a given point in time, typically six months following installation, biodegradable plastic drip irrigation pipes  210 , which do not include an active anti-bacterial and anti-fungal agent, are in the process of biodegradation, typically under bacterial and fungal action. In contrast, in accordance with a preferred embodiment of the present invention, delayed degradability drip irrigation pipes  200 , constructed and operative in accordance with a preferred embodiment of the present invention, include active anti-bacterial and anti-fungal agents, thereby delaying biodegradation under bacterial and fungal action, for a time duration, until either such agents are no longer released or they become ineffective. 
     Reference is now made to  FIG. 5 , which is a simplified illustration of part of a delayed degradability drip irrigation pipe, constructed and operative in accordance with yet another preferred embodiment of the present invention, and to  FIG. 6 , which is a simplified comparative illustration of the relative degradability characteristics of a biodegradable drip irrigation pipe and of the delayed degradability drip irrigation pipe constructed and operative in accordance with the embodiment of the present invention illustrated in  FIG. 5 . 
       FIG. 5  illustrates part of a delayed degradability drip irrigation pipe  300  which includes discrete emitter units (not shown) distributed along the length thereof in communication with the interior thereof. It is appreciated that the present invention is not limited in its applicability to this type of drip irrigation pipe and also applies to other types of drip irrigation pipes wherein the emitters are fully or partially defined by the pipe. The present invention applies to drip irrigation pipes which are formed by extrusion and equally to drip irrigation pipes that are formed by welding of elongate sheets. 
     The irrigation pipe  300  is preferably formed of a biodegradable plastic material, such as PBAT (polybutylene adipate/teraphthalate), PTMAT (polymethylene adipate/teraphthalate), naturally produced polyester, such as PHA polyesters (polyhydroxyalkanoates), PHBH polyesters (poly-hydroxybutyrate-co-polyhydroxy hexanoates) and PLA polyesters (polylactic acid), which is biodegradable by bacterial and/or fungal action. 
     In accordance with a preferred embodiment of the present invention, delayed degradability functionality is provided by the addition of a generally non-biodegradable material, such as polyethylene, to the biodegradable plastic material. 
     Additionally, in accordance with a preferred embodiment of the present invention, delayed degradability functionality may be enhanced by the addition of an active anti-bacterial and anti-fungal agent which demonstrates activity against a wide range of bacteria, mold and yeast, such as CIBA® IRGAGUARD® B-1000, B-5000 or B-7000, HYGATE® 4000 or 9000 and ALPHASAN®. Preferably, the active anti-bacterial and anti-fungal agent is mixed with the biodegradable plastic material prior to formation of the pipe, for example, prior to extrusion of the pipe or of a sheet from which the pipe is formed. 
     Preferably, the generally non-biodegradable material is mixed with the biodegradable plastic material prior to formation of the pipe, for example, prior to extrusion of the pipe or of a sheet from which the pipe is formed. The resulting pipe or sheet includes relatively long linked plastic molecules which define a net or screen type structure  302 , which resists and delays degradation, such as failure due to bursting of the drip irrigation pipe  300 , notwithstanding early stage biodegradation of the biodegradable plastic material thereof. 
     As seen in  FIG. 5 , the generally non-biodegradable material preferably is distributed generally throughout the thickness of the pipe  300 . 
     Turning to  FIG. 6 , biodegradable plastic drip irrigation pipes  300 , which include an internal net or screen type structure  302  formed of a generally non-biodegradable material, are shown alongside biodegradable plastic drip irrigation pipes  310 , which do not include an internal net or screen type structure formed of a generally non-biodegradable material, at the same point in time. 
     It is seen that at a given point in time, typically six months following installation, biodegradable plastic drip irrigation pipes  310 , which do not include an internal net or screen type structure formed of a generally non-biodegradable material, are in the process of biodegradation, typically under bacterial and fungal action. In contrast, in accordance with a preferred embodiment of the present invention, delayed degradability drip irrigation pipes  300 , constructed and operative in accordance with a preferred embodiment of the present invention, are mechanically strengthened against bursting by net or screen type structure  302 , thereby delaying degradation under bacterial and fungal action for a desired time duration. 
     Reference is now made to  FIG. 7 , which is a simplified illustration of part of a delayed degradability drip irrigation pipe, constructed and operative in accordance with still another preferred embodiment of the present invention, and to  FIG. 8 , which is a simplified comparative illustration of the relative degradability characteristics of a biodegradable drip irrigation pipe and of the delayed degradability drip irrigation pipe constructed and operative in accordance with the preferred embodiment of the present invention illustrated in  FIG. 7 . 
       FIG. 7  illustrates part of a delayed degradability drip irrigation pipe  400  which includes discrete emitter units  402  distributed along the length thereof in communication with the interior thereof. It is appreciated that the present invention is not limited in its applicability to this type of drip irrigation pipe and also applies to other types of drip irrigation pipes wherein the emitters are fully or partially defined by the pipe. The present invention applies to drip irrigation pipes which are formed by extrusion and equally to drip irrigation pipes that are formed by welding of elongate sheets. 
     The irrigation pipe  400  is preferably formed of a biodegradable plastic material, such as PBAT (polybutylene adipate/teraphthalate), PTMAT (polymethylene adipate/teraphthalate), naturally produced polyester such as PHA polyesters (polyhydroxyalkanoates), PHBH polyesters (poly-hydroxybutyrate-co-polyhydroxy hexanoates) and PLA polyesters (polylactic acid), which is biodegradable by bacterial and/or fungal action. 
     In accordance with a preferred embodiment of the present invention, delayed degradability functionality is provided by the addition of an active anti-bacterial and anti-fungal agent, which demonstrates activity against a wide range of bacteria, mold and yeast, such as CIBA® IRGAGUARD® B-1000, B-5000 or B-7000, HYGATE® 4000 or 9000 and ALPHASAN®, to at least one of, and preferably all of, an outer coextruded biodegradable plastic layer  404 , an innermost coextruded biodegradable plastic layer  405  and a middle biodegradable plastic layer  406  of pipe  400 . 
     Preferably, the active anti-bacterial and anti-fungal agent is mixed with the biodegradable plastic material used to form layer  404 , layer  405  and/or layer  406 , prior to coextrusion of the pipe or of a sheet from which the pipe is formed. 
     As seen in  FIG. 7 , the active anti-bacterial and anti-fungal agent may appear throughout the thickness of the outer layer  404 , innermost layer  405  and/or middle layer  406  of pipe  400 . It is appreciated that the active anti-bacterial and anti-fungal agents included in outer layer  404 , innermost layer  405  and middle layer  406  may be the same for each layer or may be different for each layer to provide different time delays for the delayed degradability functionality of delayed degradability drip irrigation pipe  400 . 
     Turning to  FIG. 8 , biodegradable plastic drip irrigation pipes  400 , in which at least one of outer layer  404 , innermost layer  405  and/or middle layer  406  include an active anti-bacterial and anti-fungal agent, are shown alongside biodegradable plastic drip irrigation pipes  410 , which do not include an active anti-bacterial and anti-fungal agent, at the same point in time. 
     It is seen that at a given point in time, typically six months following installation, biodegradable plastic drip irrigation pipes  410 , which do not include at least one layer including an active anti-bacterial and anti-fungal agent, are in the process of biodegradation, typically under bacterial and fungal action. In contrast, in accordance with a preferred embodiment of the present invention, delayed degradability drip irrigation pipes  400 , constructed and operative in accordance with the preferred embodiment of the present invention of  FIG. 7  include active anti-bacterial and anti-fungal agents, thereby delaying biodegradation under bacterial and fungal action for a time duration until either such agents are no longer released or they become ineffective. 
     It is appreciated that, although in the illustrated embodiment shown in  FIG. 7 , pipes  400  include three layers  404 ,  405  and  406 , in accordance with the present invention pipes  400  may include any number of layers, including two or more layers, of which at least one layer includes active anti-bacterial and anti-fungal agents. In a most preferred embodiment of the present invention, at least the outermost layer of pipes  400  includes active anti-bacterial and anti-fungal agents. 
     Reference is now made to  FIG. 9 , which is a simplified illustration of part of a delayed degradability drip irrigation pipe, constructed and operative in accordance with still another preferred embodiment of the present invention, and to  FIG. 10 , which is a simplified comparative illustration of the relative degradability characteristics of a biodegradable drip irrigation pipe and of the delayed degradability drip irrigation pipe constructed and operative in accordance with the preferred embodiment of the present invention illustrated in  FIG. 9 . 
       FIG. 9  illustrates part of a delayed degradability drip irrigation pipe  500  which includes discrete emitter units  502  distributed along the length thereof in communication with the interior thereof. It is appreciated that the present invention is not limited in its applicability to this type of drip irrigation pipe and also applies to other types of drip irrigation pipes wherein the emitters are fully or partially defined by the pipe. The present invention applies to drip irrigation pipes which are formed by extrusion and equally to drip irrigation pipes that are formed by welding of elongate sheets. 
     The irrigation pipe  500  is preferably formed of a biodegradable plastic material, such as PBAT (polybutylene adipate/teraphthalate), PTMAT (polymethylene adipate/teraphthalate), naturally produced polyester such as PHA polyesters (polyhydroxyalkanoates), PHBH polyesters (poly-hydroxybutyrate-co-polyhydroxy hexanoates) and PLA polyesters (polylactic acid), which is biodegradable by bacterial and/or fungal action. 
     In accordance with a preferred embodiment of the present invention, delayed degradability functionality is provided by the addition of at least one of an outer coextruded biodegradable plastic layer  504  and an innermost coextruded biodegradable plastic layer  505 , containing an active anti-bacterial and anti-fungal agent which demonstrates activity against a wide range of bacteria, mold and yeast, such as CIBA® IRGAGUARD® B-1000, B-5000 or B-7000, HYGATE® 4000 or 9000 and ALPHASAN®. 
     Preferably, the active anti-bacterial and anti-fungal agent is mixed with the biodegradable plastic material used to form layer  504  and/or layer  505 , prior to coextrusion of the pipe or of a sheet from which the pipe is formed. 
     It is appreciated that the active anti-bacterial and anti-fungal agent may appear throughout the thickness of the outer layer  504  and/or innermost layer  505  of pipe  500 . 
     Turning to  FIG. 10 , biodegradable plastic drip irrigation pipes  500 , which include a coextruded outer layer  504  and/or inner layer  505 , including an active anti-bacterial and anti-fungal agent, are shown alongside biodegradable plastic drip irrigation pipes  510 , which do not include an active anti-bacterial and anti-fungal agent, at the same point in time. 
     It is seen that at a given point in time, typically six months following installation, biodegradable plastic drip irrigation pipes  510 , which do not include a coextruded outer layer including an active anti-bacterial and anti-fungal agent, are in the process of biodegradation, typically under bacterial and fungal action. In contrast, in accordance with a preferred embodiment of the present invention, delayed degradability drip irrigation pipes  500 , constructed and operative in accordance with the preferred embodiment of the present invention of  FIG. 9  include active anti-bacterial and anti-fungal agents, thereby delaying biodegradation under bacterial and fungal action for a time duration until either such agents are no longer released or they become ineffective. 
     Reference is now made to  FIG. 11 , which is a simplified illustration of part of a delayed degradability drip irrigation pipe, constructed and operative in accordance with still a further preferred embodiment of the present invention, and to  FIG. 12 , which is a simplified comparative illustration of the relative degradability characteristics of a biodegradable drip irrigation pipe and of the delayed degradability drip irrigation pipe constructed and operative in accordance with the preferred embodiment of the present invention illustrated in  FIG. 11 . 
       FIG. 11  illustrates part of a delayed degradability drip irrigation pipe  600  which includes discrete emitter units  602  distributed along the length thereof in communication with the interior thereof. It is appreciated that the present invention is not limited in its applicability to this type of drip irrigation pipe and also applies to other types of drip irrigation pipes wherein the emitters are fully or partially defined by the pipe. The present invention applies to drip irrigation pipes which are formed by extrusion and equally to drip irrigation pipes that are formed by welding of elongate sheets. 
     The irrigation pipe  600  is preferably formed with an outer layer  604  of a biodegradable plastic material, such as PBAT (polybutylene adipate/teraphthalate), PTMAT (polymethylene adipate/teraphthalate), naturally produced polyester such as PHA polyesters (polyhydroxyalkanoates), PHBH polyesters (poly-hydroxybutyrate-co-polyhydroxy hexanoates) and PLA polyesters (polylactic acid), which is biodegradable by bacterial and/or fungal action. 
     In accordance with a preferred embodiment of the present invention, delayed degradability functionality is provided by the addition to the outer layer  604  of an active anti-bacterial and anti-fungal agent which demonstrates activity against a wide range of bacteria, mold and yeast, such as CIBA® IRGAGUARD® B-1000, B-5000 or B-7000, HYGATE® 4000 or 9000 and ALPHASAN®. 
     In accordance with a preferred embodiment of the present invention, additional delayed degradability functionality is provided by the provision of an inner layer  606  formed of a plastic material which is not-biodegradable but is degradable in response to exposure to another degradability initiator, such as UV. A suitable UV degradable plastic material is polyethylene. Layers  604  and  606  are preferably co-extruded. 
     Additionally, inner layer  406  may also include an oxo-biodegradable material, such as EPIcor™ 2058, commercially available from EPI Environmental Products, Inc., of Vancouver, B.C., Canada, which enhances breakdown of inner layer  606 . 
     Preferably, the active anti-bacterial and anti-fungal agent is mixed with the biodegradable plastic material used to form layer  604 , prior to co-extrusion of the pipe or of a sheet from which the pipe is formed. 
     It is appreciated that the active anti-bacterial and anti-fungal agent may appear throughout the thickness of the outer layer  604 . 
     Turning to  FIG. 12 , biodegradable plastic drip irrigation pipes  600 , which include a coextruded outer layer  604  and inner layer  606  as described hereinabove, are shown alongside biodegradable plastic drip irrigation pipes  610 , which do not include an active anti-bacterial and anti-fungal agent, at the same point in time. 
     It is seen that at a given point in time, typically six months following installation, biodegradable plastic drip irrigation pipes  610 , which do not include a coextruded outer layer including an active anti-bacterial and anti-fungal agent, are in the process of biodegradation, typically under bacterial and fungal action. In contrast, in accordance with a preferred embodiment of the present invention, delayed degradability drip irrigation pipes  600 , constructed and operative in accordance with the preferred embodiment of the present invention of  FIG. 11 , remain intact and functional for a predetermined, desired duration. 
     It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes combinations and subcombinations of the features described above as well as modifications and variations which would occur to persons skilled in the art upon reading the foregoing description and which are not in the prior art.

Technology Classification (CPC): 8