Abstract:
Systems and techniques are described for constructing a gamma container. In general, the techniques include sandwiching a material between layers of cooling pads that conduct heat energy laterally away from the material being irradiated to a cold sink.

Description:
[0001]     This application claims priority of U.S. Provisional Application No. 60/498,941, filed Aug. 29, 2003, the disclosure of which is hereby incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD  
       [0002]     The following description relates to systems and techniques for conducting energy from materials.  
       BACKGROUND  
       [0003]     Gamma radiation transfers energy to material primarily by scattering, which involves elastic collisions between incident photons and unbound (or weakly bound) electrons in which the incident energy is shared between the scattered electron and the deflected photon. These electrons recoil a short distance as unbound electrons, giving up energy to the molecular structure of the material as they collide with other electrons, causing ionization and free-radical formation. The scattered gamma ray carries the balance of the energy as it moves off through the material, possibly to interact again with another atomic electron. Gamma rays typically penetrate relatively deeply into the tissue before scattering occurs. Gamma radiation typically requires a low dose rate in combination with a high exposure period.  
         [0004]     The physical or physiological properties of active compounds may be altered by variations in the compounds&#39; surrounding environment. For example, changes in pH, ionic strength, or temperature can result in reversible or irreversible changes in the character of compounds.  
         [0005]     Radiation sterilization is widely used in industry for a variety of products and both dosage levels and its biological effects are well known. Gamma sterilization can be effective in killing microbial organisms. Gamma radiation sufficient to effectively kill microorganisms also may alter the structure of biological and other compounds.  
       SUMMARY OF THE DISCLOSURE  
       [0006]     The invention relates to systems, apparatus and methods for conducting energy away from materials.  
         [0007]     Gamma radiation is often utilized to inactivate undesirable organisms, so that growth of the undesirable organisms is minimized or eliminated.  
         [0008]     Inactivation of undesirable organisms from allograft tissue may be accomplished by using gamma rays from Cobalt 60. A sufficient accumulated absorbed dose level to reduce or eliminate a given spectrum of these organisms may also have a damaging effect upon the structure and biochemical nature of unprotected tissue. An aspect of the invention is to provide a system that will allow gamma rays to have the desired effect upon micro-organisms while protecting allograft or other tissue from degradation.  
         [0009]     The invention includes an apparatus into which soft human or animal tissue is placed between layers of energy conductive medium that provides support and thermal management during exposure to gamma irradiation at temperatures below −20° Celcius. The apparatus provides a static mechanism to drain pure energy effects absorbed by the tissue(s). Energy is withdrawn in a controlled direction to a cold sink at the side of the apparatus without significant attenuation that would interfere with the microbe-killing effects of gamma rays penetrating the front of the apparatus. The rate of energy loss through the conductive layers results in a diminished level of energy available for degenerative biochemical reactions that would otherwise occur within the tissue during gamma exposure.  
         [0010]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0011]      FIG. 1  is a section through loaded container according to one aspect.  
         [0012]      FIG. 2  is an example of a cooling pad layer of  FIG. 1 .  
         [0013]      FIG. 3  is an example of a biological layer of  FIG. 1 .  
         [0014]      FIG. 4  is an example of a hinge arrangement for the container. 
     
    
       [0015]     Like reference symbols in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0016]      FIG. 1  illustrates an example of an apparatus useful for exposing tissue to gamma rays while simultaneously protecting the tissue.  
         [0017]     As shown in  FIG. 1 , a container  100  has a bottom  108  and upstanding sides  101  forming right angles with the bottom  108 , at edges  110 . Bottom  108  can be rectangular or square in shape. As will be appreciated, there are also upstanding sides along at least one of the other edges (not shown) of bottom  108 . In the embodiment illustrated, three of the sides  101  are fixed in relation to bottom  108 . A fourth side  101  can be hingedly connected to the two sides  101  that are adjacent the fourth side  101 . If fourth side  101  is hingedly attached, it can be rotated between an open orientation that is approximately parallel to bottom  108 , and a closed orientation approximately perpendicular to bottom  108 . Hinge connections on the sides  101  that are adjacent to fourth side  101 , and near bottom  108 , could be used. As will be readily appreciated, these connections can be rotatable rivets, screws or bolts  118  which hold flanges  116  that are on either side of fourth side  101  to each of the two adjacent sides  101 . (See  FIG. 4 .) The fourth side  101  can rotate as shown by arrow A in  FIG. 4 .  
         [0018]     As shown in  FIG. 1 , at least two of the sides  101  have a retaining flange  109  along the top edges of sides  101 . Each such retaining flange extends out approximately perpendicularly to sides  101  (and therefore approximately parallel to bottom  108 ).  
         [0019]     As shown in the  FIG. 1  example, when in use, container  100  can be loaded with a plurality of alternating layers  102 ,  104 ,  102 ,  104 ,  102 ,  104 ,  102 ,  104 ,  102 . For simplicity of illustration, five layers  102  and four layers  104  are shown, but either more or fewer layers can, of course, be used depending on the height of sides  101 , the thickness of the layers  102 ,  104 , the tissue being gamma irradiated, etc. For example, four layers  102  alternating with three layers  104  can be used.  
         [0020]      FIG. 2  illustrates an example of cooling pad layer  102 . Each cooling pad layer  102  comprises a support  202  whose surface can be coated with a heat conduction material  203 . Support  202  and heat conduction material  203  are sealed in a sleeve  201 .  
         [0021]     Support  202  comprises a hydrophilic substance, such as open-cell polyurethane foam, fiberglass mesh, leather, felt or terrycloth. Heat conduction material  203  is coated on the outer surfaces of support  202 . Suitable heat conduction materials  203  include boron nitride powder and beryllium copper.  
         [0022]     In preparing cooling pad layer  102 , a heat conduction material  203  forms a coating on a dry support  202  and the coated support is put into a sleeve  201 . A small amount of water is added to the sleeve (say 100-150 mls.). All the water added should be absorbed by the coated support and the support should not be fully saturated. Almost all the air is then removed from sleeve  201  and the sleeve is then sealed to be airtight such as by heat-sealing. Sleeve  201  can be any suitable plastic, such as 3-mil thick propylene, high density polyethylene or mylar.  
         [0023]      FIG. 3  illustrates an example of biological layer  104 . Each layer  104  comprises tissue  302  and sleeve  301 . Tissue  302  can be soft human or animal tissue, such as tendon or human sheet dermis or any material which is to be subjected to gamma radiation, such as a synthesized polymeric. Sleeve  301  can be a polypropylene bag, high density polyethylene or mylar. Sleeve  301  is sealed after tissue  302  is placed therein and almost all air has been removed from sleeve  301 . Tissue  302  can be wetted tissue.  
         [0024]     Referring back to  FIG. 1 , retention layer  106  can be slightly flexible so that it can be flexed and its edges then inserted under retaining flanges  109 . Upon release, retention layer  106  can resume a more planar configuration and its edges can extend beyond the width of the opening formed by retaining flanges  109  in the top of container  100 . Retention layer  106  can comprise a stainless steel screen, for example. Retention layer  106  is useful to keep alternating layers  102 ,  104 ,  102 , etc. in place prior to closing container  100 . Retention layer  106  also can provide a smooth surface along which cover layer  107  can slide.  
         [0025]     Cover layer  107  is relatively stiff and inflexible compared to retention layer  106  and can slide along the top of retention layer  106 , below retaining flanges  109  which are on sides  101 . Once container  100  is loaded with alternating layers  102 ,  104 ,  102 , etc., and once retention layer  106  is in place, cover layer  107  is slid into place above retention layer  106 . It is sized so that it closes the top of container  100  once cover layer  107  is in place. Now, fully loaded, container  100  is further closed by moving fourth side  101  to its closed position, perpendicular to bottom  108 .  
         [0026]     Cover layer  107  can be made of high-density polyethylene (HDPE) or any suitable rigid plastic material.  
         [0027]     It should be noted that all three sides  101  and fourth side  101  can have retaining flanges  109  so that all four edges of cover layer  107  will be retained thereby when fourth side  101  is in its closed position.  
         [0028]     In another example, fourth side  101  is not hingedly connected to the two adjacent sides  101 , but can be completely removable from container  100 . In this example, after cover layer  107  has been slid into place, fourth side  101  can be placed on container  100 . In this example, retaining flange  109  can have an opposing flange  109 , arranged so that one retaining flange  109  frictionally fits over cover layer  107  and the opposing retaining flange  109  frictionally fits the outside of bottom  108  of container  100 .  
         [0029]     When container  100  is loaded and is in use, cover layer  107  faces toward a gamma ray source, such as Cobalt 60. The gamma rays can penetrate through cover layer  107 , and the other layers, namely, retention layer  106  and alternating layers  102 ,  104 ,  102 , etc. and finally through the bottom layer  108  of container  100 . Before the tissue to be exposed to gamma irradiation is treated, the loaded container  100  is cooled to − 60  to −80° C. and maintained at that temperature. When container  100  is rectangular, container  100  is positioned with one of the two shorter sides  101  down and the other short side up, and dry ice is placed along and in contact with both longer sides  01 , which are vertically arranged in this example. The alternating layers  102 ,  104 ,  102 , etc. are prepared as described above and are firmly held in contact with one another in container  100 . It is important that a cooling pad layer  102  be in contact with each side of each biological layer  104  in loaded container  100 . Cooling pad layers  102  can, and should, compress somewhat around the tissue in biological layer  104  to maintain a snug fit as container  100  is being loaded.  
         [0030]     Bottom  108 , sides  101  and fourth side  101  can all be 0.033 in. thick aluminum. Alternatively, a 0.005 in. thick formed aluminum food tray has been found to be a useful substitute for the 0.033 in. thick aluminum.  
         [0031]     In one example, a rectangular container having bottom and sides made of 0.033 inches thick aluminum is used. Each of five cooling pad layers is made of hydrophilic polyurethane open-cell foam sheet, roughly 0.2 inches thick and 6.5 by 9.5 inches in plan view. Each side of each cooling pad is saturated with boron-nitride powder (from Saint-Gobain, CTL40, 3 to 5 grams) while dry, then is placed within a 3 mil thick heat-sealed polypropylene sleeve. Before final vacuum sealing, the bag is filled with approximately 100 ml of RO-purified or demineralized water. Sufficient air is withdrawn from the cooling pad at final sealing to remove most excess air from between the polyurethane foam pad and the polypropylene sleeve. The tissue used in each of the four biological layers is tendon. The retention layer and the cover layer are added and the fourth side is closed. The loaded container with the tendon layers present can be regarded as a single conducting unit. The calculated heat conductance coefficient (k) for this unit is determined to be 80 W/m−K from one end of the unit to the other. When the container is fully loaded and closed, the five cooling layers are in firm contact with and support the four adjacent biological layers. The loaded container is chilled to about −60 to −80° C. The container is placed with one of its short sides down, with its plastic cover layer facing the gamma ray source.  
         [0032]     The gamma ray source can release radiation at a controlled rate between about 2 and about 10 kilo-Grays/hour. Tissue can typically absorb about 10 to about 40 kilo-Grays of energy with little or no energy-caused degradation.  
         [0033]     In use, the container is kept at about − 60  to −80° C. by contacting both long sides of the container with a cold sink, such as dry ice, while being exposed to gamma radiation.  
         [0034]     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. Accordingly, other embodiments are within the scope of the following claims.