Patent Publication Number: US-2011071495-A1

Title: Shape memory embolic particles having a high packing fraction

Description:
FIELD 
     The present disclosure relates generally to embolic particles and methods of using such particles to occlude the flow of fluid in a body vessel. More specifically, the present disclosure relates to the use of an embolization system having a high packing fraction of embolic particles. 
     BACKGROUND 
     Embolization may be defined as the therapeutic introduction of various substances into a body vessel to fully occlude the vessel, to arrest or prevent hemorrhaging or gastrointestinal bleeding, or to reduce blood flow to an arteriovenous malformation, such as fibroids, tumors, or aneurysms. The use of embolic particles leads to the formation of an occlusion that is not substantially absorbed by the body or easily removable therefrom. 
     The ability to stop the flow of fluid through the pores established in and around embolic particles that are compacted together in a body vessel is critical to ensure the complete occlusion of fluid through the body vessel. Since this fluid flow depends on diffusivity through the compacted particles, the properties associated with these particles, such as packing fraction, are important for limiting or reducing said flow. Accordingly, there exists a continual desire to provide embolic particles that will effectively compact together leading to a high particle packing fraction in order to ensure occlusion of the body vessel. 
     SUMMARY 
     The present disclosure provides an embolization system having a high packing fraction of embolic particles for use in occluding the flow of body fluid through a body vessel. One embodiment of an embolization system, constructed in accordance with the teachings of the present disclosure, generally comprises a mixture of embolic particles made from a shape memory material that exhibit both a collapsed state and an expanded state. The mixture of embolic particles, which is delivered into the body vessel in the collapsed state, exhibits at least a bimodal particle diameter distribution in such collapsed state. This bimodal distribution, which has a first mean diameter (D 1C ) and a second mean diameter (D 2C ), exhibits a ratio of D 1C /D 2C  that is less than about 1/7. The mixture of embolic particles maintains at least a bimodal particle diameter distribution upon transitioning to its expanded state. In such an expanded state, the bimodal distribution has a first mean diameter (D 1E ) and a second mean diameter (D 2E ) that defines the ratio of D 1E /D 2E  to be less than about 1/7. The embolic particle mixture is adapted such that the ratio of D 1E /D 1C  and D 2E /D 2C  is greater than about 1.5 with the expanded state of the mixture having a particle packing fraction in the body vessel that is at least about 0.85. The particle packing fraction of the mixture in its expanded state causes the occlusion of the flow of body fluid through the body vessel. 
     According to another aspect of the present disclosure, the mixture of embolic particles may include a tri-modal or quad-modal distribution of particle sizes leading to an increase in the particle packing fraction for the embolic particles when in their expanded state. The embolization system may further include some particles that are not made from a shape memory material. These particles exhibit a similar mean particle diameter in both the collapsed state and expanded state. 
     Another objective of the present disclosure is to provide a method for occluding the flow of body fluid through a body vessel of a patient using the embolization system described herein. This method generally comprises preparing the mixture of embolic particles made from a shape memory material having a collapsed state and an expanded state; delivering said mixture of embolic particles to a targeted site in the body vessel of the patient; causing said mixture of embolic particles to transition from their collapsed state to their expanded state; and occluding the flow of body fluid through the body vessel by the expanded state of the mixture having a particle packing fraction in the body vessel that is at least about 0.85. 
     According to another aspect of the present disclosure, the step of preparing a mixture of embolic particles in the collapsed state may further include a particle diameter distribution that is tri-modal or quad-modal in nature. Such tri-modal or quad-modal distributions are maintained upon transitioning of the embolic particles from their collapsed state to their expanded state. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1A  is a graphical depiction of the volume concentration of embolic particles in an embolization system according to one embodiment of the present disclosure plotted as a function of particle diameter. This graph shows a mixture of embolic particles having a bimodal particle size distribution in the collapsed state and in the expanded state; 
         FIG. 1B  is a cross-sectional schematic representation of the embolic particles of  FIG. 1A  in their collapsed state as delivered into the vasculature of a patient; 
         FIG. 1C  is a cross-sectional schematic representation of the embolic particles of  FIG. 1B  shown in their expanded state; 
         FIG. 2A  is a graphical depiction of the volume concentration of embolic particles in an embolization system according to another aspect of the present disclosure depicting a tri-modal particle size distribution in the collapsed state and in the expanded state when plotted as a function of particle diameter; 
         FIG. 2B  is a cross-sectional schematic representation of the embolic particles of  FIG. 2A  in their collapsed state as delivered into the vasculature of a patient; 
         FIG. 2C  is a cross-sectional schematic representation of the embolic particles of  FIG. 2B  shown in their expanded state; 
         FIG. 3A  is a graphical depiction of the volume concentration of embolic particles in an embolization system according to another aspect of the present disclosure depicting a quad-modal particle size distribution in the collapsed state and in the expanded state when plotted as a function of particle diameter; 
         FIG. 3B  is a cross-sectional schematic representation of the embolic particles of  FIG. 3A  in their collapsed state as delivered into the vasculature of a patient; 
         FIG. 3C  is a cross-sectional schematic representation of the embolic particles of  FIG. 3B  shown in their expanded state; 
         FIG. 4A  is a graphical depiction of the volume concentration of embolic particles in an embolization system according to yet another aspect of the present disclosure depicting a quad-modal particle size distribution in the collapsed state and in the expanded state when plotted as a function of particle diameter; 
         FIG. 4B  is a cross-sectional schematic representation of the embolic particles of  FIG. 4A  in their collapsed state as delivered into the vasculature of a patient; 
         FIG. 4C  is a cross-sectional schematic representation of the embolic particles of  FIG. 4B  shown in their expanded state; and 
         FIG. 5  is a schematic depiction of a process according to another embodiment of the present disclosure for using an embolization system comprising embolic particles that exhibit a collapsed configuration for delivery into the body vessel and an expanded configuration to occlude the flow of fluid in a body vessel. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is in no way intended to limit the present disclosure or its application or uses. It should be understood that throughout the description and drawings, corresponding reference numerals indicate like or corresponding parts and features. 
     The present disclosure generally provides an embolization system having a high packing fraction of embolic particles for use in occluding the flow of body fluid through a body vessel near a targeted site. Such an occlusion is useful in the treatment of a tumor, an aneurysm, or to stop the occurrence of hemorrhaging or gastrointestinal bleeding, among other reasons. For example, an aneurysm typically occurs in blood vessels at a location where the strength of the blood vessel has been compromised or weakened. In this situation, the flow of fluid through the blood vessel can cause the blood vessel wall to lose its shape, resulting in a ballooning or stretching of the blood vessel at the weakened location. If left untreated, the blood vessel wall may continue to deform until it fails, the result of which is often fatal. Embolic particles delivered to a targeted site near the aneurysm can pack together and block the flow of fluid to and around the aneurysm, thereby, reducing the chance that the aneurysm will continue to deform. 
     The ability to stop the flow of fluid through pores established in the packed embolic particles is critical to the complete occlusion of fluid through the body vessel. Since the fluid flow depends on diffusivity through the packed particles, many of the properties of the packed particles, such as packing fraction and pore size, may be adapted to limit or reduce such an occurrence. 
     According to one embodiment of the present disclosure, the embolization system comprises a mixture of embolic particles made from a shape memory material. Referring to  FIGS. 1A-1C , the mixture of embolic particles  10 ,  20  in the embolization system  1  exhibits both a collapsed state  15  and an expanded state  16 . This mixture of particles  10 ,  20  exhibits at least a bimodal particle diameter distribution in the collapsed state  15 . This bimodal distribution is defined by a first mean diameter (D 1C ) and a second mean diameter (D 2C ) giving rise to a ratio of D 1C /D 2C  that is less than about 1/7. The embolic particles  10 ,  20  are delivered into the body vessel  5  in their collapsed state  15 . Upon transitioning from their collapsed state  15  to their expanded state  16 , the embolic particles  10 ,  20  maintain at least a bimodal particle diameter distribution. The bimodal distribution in the expanded state  16  is defined by a first mean diameter (D 1E ) and a second mean diameter (D 2E ) with the ratio of D 1E /D 2E  being less than about 1/7. The embolic particles  10 ,  20  are adapted such that the ratio of D 1E /D 1C  and D 2E /D 2C  is greater than about 1.5 with the expanded state  16  of the mixture of particles having a particle packing fraction (P F ) in the body vessel that is at least about 0.85. The particle packing fraction of the embolic particles  10 ,  20  in their expanded state  16  causes the occlusion of the flow  7  of body fluid through the body vessel  5 . 
     According to another aspect of the present disclosure, the embolization system  1  comprises a mixture of embolic particles  10 ,  20 ,  30  having a tri-modal particle diameter distribution as shown in  FIGS. 2A-2C . In other words, the bimodal mixture of embolic particles  10 ,  20 , which has a first mean diameter (D 1C ) and a second mean diameter (D 2C ), may further include another embolic particle  30  distribution that gives rise to a third mean diameter (D 3C ) in a collapsed state  25 . This tri-modal mixture of embolic particles  10 ,  20 ,  30  is such that the ratio of D 2C /D 3C  is less than about 1/7. Upon transitioning from its collapsed state  25  to its expanded state  26 , the mixture of embolic particles  10 ,  20 ,  30  maintains its tri-modal particle distribution in the expanded state  26 . The tri-modal distribution in the expanded state  26  exhibits a first mean diameter (D 1E ), a second mean diameter (D 2E ), and a third mean diameter (D 3E ). The ratio of D 2E /D 3E  is less than about 1/7 with the particle packing fraction (P F ) of the mixture  10 ,  20 ,  30  in the expanded state being greater than about 0.90. The particle packing fraction of the mixture of embolic particles  10 ,  20 ,  30  in its expanded state  16  causes the occlusion of the flow  7  of body fluid through the body vessel  5 . 
     According to yet another aspect of the present disclosure, the embolization system  1  comprises a mixture of embolic particles  10 ,  20 ,  30 ,  40  that exhibits a quad-modal particle distribution as shown in  FIGS. 3A-3C . In other words, the mixture of embolic particles  10 ,  20 ,  30 ,  40  exhibits not only a first mean diameter (D 1C ), a second mean diameter (D 2C ), and a third mean diameter (D 3C ), but also a fourth mean particle diameter (D 4C ) in the collapsed state  35 , This quad-modal mixture of embolic particles  10 ,  20 ,  30 ,  40  is such that the ratio of D 3C /D 4C  is less than about 1/7. Upon transitioning from its collapsed state  35  to its expanded state  36 , the mixture of embolic particles  10 ,  20 ,  30 ,  40  maintains its quad-modal particle distribution in the expanded state  36 . The quad-modal distribution in the expanded state  36  exhibits a first mean diameter (D 1E ), a second mean diameter (D 2E ), a third mean diameter (D 3E ), and a fourth mean diameter (D 4E ). The ratio of D 3E /D 4E  is less than about 1/7 with the particle packing fraction (P F ) of the embolic particle mixture  10 ,  20 ,  30 ,  40  in the expanded state being greater than about 0.95. 
     According to another aspect of the present disclosure, the embolization system  1  may comprise a combination of particles made from a shape memory material and particles that are not made from a shape memory material. In this case, the diameter of the embolic particles made from the shape memory material will increase upon transitioning from a collapsed state to an expanded state. However, the diameter of the embolic particles that are not made from a shape memory material will remain substantially the same between the collapsed state and the expanded state. For example, referring now to  FIGS. 4A-4C , an embolization system  1  having a mixture of embolic particles  10 ,  20 ,  30  made from a shape memory material that exhibits a tri-modal particle distribution with particle diameters given in the collapsed state by D 1C , D 2C , &amp; D 3C  and in the expanded state of D 1E , D 2E , &amp; D 3E  may further incorporate particles  50  that are not made from a shape memory material, which gives rise to a fourth particle diameter distribution of D 4  in both the collapsed state and expanded state. Although this example describes an embolization system  1  that has a quad-modal particle size distribution of embolic particles  10 ,  20 ,  30 ,  50  with the mean particle diameter, D 4 , of one of the particle distributions being unaffected by transitioning between a collapsed state to an expanded state, one skilled-in-the-art will understand that a tri-modal particle size distribution of embolic particles where one of the mean particle diameters, i.e., D 3 , of one of the particle distributions is unaffected by transitioning between a collapsed state to an expanded state, may also be utilized. 
     The particle packing fraction (P F ) of the embolic particles in the embolization system  1  where all of the particles are made from a shape memory material may be greater than the particle packing fraction (P F ) exhibited by an embolization system  1  having a similar distribution of particle sizes in the collapsed state, but includes some particles that are not made from a shape memory material. For example, an embolization system  1  comprising a mixture of embolic particles  10 ,  20 ,  30  made entirely from a shape memory material and giving rise to a tri-modal particle size distribution will exhibit a packing fraction (P F ) greater than about 0.90. In comparison, an embolization system  1  comprising a mixture of embolic particles  10 ,  20  made from a shape memory material and embolic particles  50  that are not made from a shape memory material that also gives rise to a tri-modal particle size distribution will exhibit a packing fraction (P F ) greater than about 0.85. Similarly an embolization system  1  exhibiting a quad-modal particle size distribution will exhibit a particle packing fraction (P F ) in the expanded state of greater than about 0.95 when all particles are made from a shape memory material and greater than about 0.90 when one of the particle distributions (i.e., D 4 ) comprises particles that are not made from a shape memory material as shown in  FIGS. 4A-4C . 
     The embolization system  1  when in its collapsed state may contain a mixture of embolic particles dispersed and suspended within a suitable, biocompatible carrier medium, such as a saline solution. A mixture of embolic particles suspended in a carrier medium can be prepared in calibrated concentrations in order to ease its delivery into the vasculature of the patient. The density of the composition may range from about 1.1 to 1.5 g/cm 3  with the weight ratio of the embolic particles to the carrier medium being about 0.01 to 20% by weight. The physician may determine the concentration of embolic particles suspended in the carrier medium by adjusting the weight ratio of the embolic particles to the carrier medium. The suspension of embolic particles in a carrier medium should be stable for between about 1 to 10 minutes in order for the physician to deliver the embolic particles to the targeted site within the vasculature of the patient. 
     The embolization system  1  comprises embolic particles that are either entirely comprised of a shape memory material or a mixture of particles made from a shape memory material with particles that are not made from a shape memory material. Any biocompatible material known to one skilled-in-the-art may be utilized as the embolic particles that are not made from a shape memory material. Examples of biocompatible materials include polyesters, nylon, polytetrafluoroethylene (PTFE), or polypropylene, among others. The embolization system  1  is configured in a collapsed state and may expand to a predetermined shape capable of occluding the body vessel. The shape memory material induces the embolization system  1  to transition from its collapsed state to an expanded state upon exposure to a predetermined stimulus. 
     The embolic particles made from a shape memory material may be deformed from a memorized or unconstrained shape (expanded state) into a different collapsed shape. This collapsed shape will remain stable for a period of time before the shape memory material is activated and returns to its unconstrained shape. The process of creating a memorized shape may include, but not be limited to, deforming the shape memory material into a deformed shape and cooling it to a storage temperature. Cooling the deformed shape memory material to a storage temperature serves to keep the shape memory material in its deformed shape and to avoid activation of the shape memory material. Activation may include subjecting the shape memory material to a stimulus such as heat, radiation, pH, a chemical, or other stimuli known to one skilled-in-the-art. The shape memory material may provide a permanent occlusion, i.e., the occlusion is not substantially absorbed by the body and/or is not intended to be removed from the body when in its expanded state. 
     The shape memory material can be a polymer or a metal alloy. Examples of shape memory alloys include, but are not limited to, TiNi (Nitinol), CuZnAl, and FeNiAl alloys. The metal alloy may also be a superelastic or pseudo-elastic alloy known to one skilled-in-the-art. Examples of shape memory polymers include but are not limited to polyvinyl alcohol (PVA), polyethers, polyether esters, polyether amides, polyacrylates, polyamides, polysiloxanes, polyurethanes, polyurethane-ureas, and urethane-butadiene copolymers. The shape memory polymer may be cross-linked or crystalline in nature. The polymer may be set to its expanded state during the occurrence of cross-linking. The collapsed state may subsequently be formed by heating the polymer beyond its a softening point where it is compacted into the collapsed state, and subsequently cooled below its softening point to maintain the collapsed state. When the polymer is subjected to a temperature above its softening point, the polymer will transition from its collapsed state to its expanded state. 
     A variety of techniques can be used to form an embolic particle from a shape memory material. Examples of suitable techniques include, but are not limited to, micro- or nano-machining, nanoetching, nanoassembly, and micro-electromechanical (MEM) techniques. The embolic particles can be formed by polymer extrusion, polymer molding, or by stamping a sheet of a shape memory alloy, as well as by any other method known to one skilled-in-the-art. The particles may be sterilized prior to being packaged using, for example, electron-beam irradiation. 
     A complete description of particles includes a discussion regarding preferred diameters (size), shapes, and surface reactivity in regards to agglomeration, as well as the presence of surface moieties that may sterically or electrostatically affect agglomeration. In order to achieve high packing densities, it is desirable to limit the physical interaction that may occur between particles. This can be accomplished via any means known to one skilled-in-the-art, including but not limited to the following examples. First, the embolic particles may be spherical in nature in order to limit the area that will be in contact with neighboring particles. However, one skilled-in-the-art will understand that non-spherical particles may also be utilized. 
     Large differences in particle sizes can improve the packing density by filling the interstitial voids between embolic particles. A broad particle size distribution assists in determining a high packing fraction for the particles. When small particles are allowed to fill the voids between larger particles, a higher packing fraction will result. Spheres of a single size are known to provide a packing fraction on the order of only about 0.50 to 0.75, which means that the packing of such particles contains about 25-50% voids. Preferably, at least a bimodal distribution of particle sizes is used in the embolization system in order to increase the particle packing fraction. The ratio of the mean particle diameter for each of the particle size distributions should be on the order of about 1:7. 
     The particle size is an important variable for use in controlling the overall packing fraction or density. Preferably the largest particle diameter in its expanded state is about an order of magnitude less than the diameter of the vasculature in order to achieve a high packing fraction. Thus the largest mean particle diameter for the embolic particles in its expanded state is about 10% of the diameter for the vasculature in which the particles are delivered. 
     Vibration of the embolic particles during the introduction of the particles into the body vessel is preferable in order to enhance the packing of said particles. Such vibration may arise from moving the catheter or guide wire back and forth in a proximal-distal direction or a side to side direction during the delivery of the embolic particles to the targeted site or immediately thereafter. 
     A negative consequence related to the clustering of the embolic particles is a decrease in the packing fraction or density. The packing fraction associated with single size spheres may decrease to about 0.35 with the occurrence of particle agglomeration. Agglomeration or clustering of small particles arises due to cohesion between particles. The attractive forces that exist between particles may become larger in direct relationship to an increase in the surface area associated with the particles. In order to overcome these attractive forces, the use of additives or surface treatments that either sterically hinder the particles from coming close to one another or that electrostatically induce like charges on the surface of the particles to repel the particles from one another can be used to reduce or eliminate agglomeration. Examples of surface active agents include, but are not limited to, polyvinyl alcohol, stearic acid, sodium oleate, glycerine, and oleic acid. Generally, a surface active agent having a backbone of at least about eight carbon atoms can function to inhibit agglomeration. 
     The embolic particles may optionally be delivered with other materials, such as a contrast agent, a radiopaque agent, or a therapeutic agent, among others, as well as mixtures thereof. The embolic particles may be delivered along with other liquid embolic materials, for example, n-butyl cyanoacrylates (NBCA), polyvinyl alcohol (PVA) foam, or other hydrogel embolic materials. Several examples of therapeutic agents include, but are not limited to, anti-thrombogenic agents, anti-proliferative agents, anti-inflammatory agents, anti-cancer agents, anesthetic agents, anti-coagulants, and vascular cell growth promoters. 
     Another objective of the present disclosure is to provide a method of occluding the flow of body fluid through a body vessel of a patient using the embolization system described above. In general, the method comprises the delivery of an embolization system in which the embolic particles are in their collapsed state to a targeted site in the body vessel. The embolic particles are then allowed to transition from their collapsed state to their expanded state. The transition from a collapsed state to an expanded state causes the embolic particles to compact, thereby, leading to a high packing fraction. In a collapsed configuration, the interparticle attraction between particles is dominated by weak van der Waal forces. However, upon transitioning of the particles of the present disclosure to an expanded configuration, the number of contacts and the interfacial attractive area for the embolic particles increases, thereby, increasing the packing fraction. 
     Referring now to  FIG. 5 , the method  100  for using the embolization system  1  described herein comprises preparing  110  a mixture of embolic particles made from a shape memory material having a collapsed state and an expanded state. This mixture has at least a bimodal particle diameter distribution in the collapsed state, the bimodal distribution having a first mean diameter (D 1C ) and a second mean diameter (D 2C ), the ratio of D 1C /D 2C  being less than about 1/7. The mixture of embolic particles is then delivered  120  to a targeted site in the body vessel of the patient. Then the mixture of embolic particles is caused  130  to transition from their collapsed state to their expanded state. The mixture of embolic particles maintains at least a bimodal particle diameter distribution in their expanded state. The bimodal distribution of the particles in their expanded state is defined by a first mean diameter (D 1E ) and a second mean diameter (D 2E ) with the ratio of D 1E /D 1C  and D 2E /D 2C  being greater than about 1.5, the ratio of D 1E /D 2E  being about equal to the ratio D 1C /D 2C , and D 2E  being less than about 10% of the diameter (D Vessel ) of the body vessel. The flow of body fluid is occluded  140  from flowing through the body vessel by the expanded state of the mixture whose particle packing fraction in the body vessel is at least about 0.85. 
     According to another aspect of the present disclosure, the step of preparing  120  a mixture of embolic particles in the collapsed state may further include a particle diameter distribution that is tri-modal in nature with the mixture having a third mean diameter (D 3C ) and the ratio of D 2C /D 3C  being less than about 1/7. In this case, the step of causing  130  the embolic particles to transition from a collapsed state to an expanded state will result in the particle diameter distribution remaining tri-modal in the expanded state, the mixture of particles having a third mean diameter (D 3E ), and the ratio of D 2E /D 3E  being less than about 1/7 with D 3E  being less than about 10% the diameter of the body vessel (D Vessel ). The step of occluding  140  the flow of body fluid in the body vessel is then accomplished by the particle packing fraction of the mixture in the body vessel in its expanded state being greater than about 0.90. 
     According to another aspect of the present disclosure, the step of preparing  110  a mixture of embolic particles in the collapsed state may include a fourth mean particle diameter (D 4C ) in the mixture, the ratio of D 3C /D 4C  being less than about 1/7. In this case, the step of causing  130  the embolic particles to transition from a collapsed state to an expanded state results in the mixture maintaining a fourth mean diameter (D 4E ) in the expanded state, the ratio of D 3E /D 4E  being less than about 1/7 with D 4E  being less than about 10% the diameter of the body vessel (D Vessel ). The step of occluding  140  the flow of body fluid in the body vessel is then accomplished by the particle packing fraction of the mixture in the body vessel in its expanded state being greater than about 0.95. 
     According to yet another aspect of the present disclosure, the step of preparing  110  a mixture of embolic particles in the collapsed state may further include particles whose diameter in the collapsed state is substantially the same as their diameter in the expanded state. In other words, embolic particles that are not made of a shape memory material may be incorporated into the embolization system. 
     The mixture of embolic particles may be delivered  120  to the target site in their collapsed state using a catheter placed near the intended site. The collapsed state of the embolic particles enables the suspended mixture to be capable of flowing through the catheter without aggregation. The mixture of embolic particles dispersed or suspended in a carrier medium can exhibit a temperature that is lower than the transition temperature in order to inhibit the shape memory material in the embolic particles from transitioning from their collapsed state to their expanded state. The particles once delivered to the targeted site can expand and aggregate to occlude the body vessel. 
     Referring once again to  FIG. 5 , the method  100  may further include a step of vibrating  150  the embolic particles during the introduction of the particles at the targeted site in the body vessel or immediately thereafter in order to enhance the packing of said particles. Such vibratory motion may arise from moving the catheter or guidewire back and forth in a proximal-distal direction or in a side to side direction. 
     A person skilled-in-the-art will recognize that any measurements described herein, such as those used to determine particle size distributions and particle packing fractions, are standard measurements that can be obtained by a variety of different test methods. For example, particle size distributions may be measured using such known standard method as ASTM B822-02. Maximum particle packing fractions can be determined using standard methods, such as those described in ASTM D4512-85 or ASTM D4781-88, among others. 
     The foregoing description of various embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Numerous modifications or variations are possible in light of the above teachings. The embodiments discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.