Patent Publication Number: US-2007106364-A1

Title: Deployment system for an intraluminal medical device

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates to a deployment system for an intraluminal medical device. More particularly, the invention relates to a deployment system having a support member on an inner member of a catheter that helps distribute loads more uniformly througout the intraluminal medical device during loading and delivery.  
      2. Related Art  
      Percutaneous transluminal angioplasty (PTA) is a therapeutic medical procedure used to increase blood flow through an artery. In this procedure, an angioplasty balloon is inflated within the stenosed vessel, or body passageway, in order to shear and disrupt the wall components of the vessel to obtain an enlarged lumen.  
      More recently, transluminal prostheses, such as stents, have been used for implantation in blood vessels, biliary ducts, or other similar organs of a patient in order to open, dilate or maintain the patency thereof. Such stents are often referred to as balloon expandable stents, as in U.S. Pat. No. 4,733,665 to Palmaz, as braided or self-expandable stents as in U.S. Pat. No. 4,655,771.  
      Balloon expandable stents can be impractical for use in some vessels, such as the carotid artery, due to their proximity to the surface of a patient&#39;s skin when deployed. Braided stents, on the other hand, pose other disadvantages, such as insufficient radial strength and the risk of unraveling of some of the fibers comprising the braided stent. Other types of self-exapnding stents, such as those comprising shape-memory materials, have thus been devised to address some of these risks. Even self-expanding stents, however, are susceptible to resistive forces during delivery that may cause the stent to undesirably bend, bunch, buckle or break as the stent is attempted to be pushed to its intended treatment site. Reactive forces can accumulate at portions of a stent as a result of such bending or bunching. Non-uniform deployment of the stent can occur as a result.  
      This tendency is particularly prevalent in newer stent designs that have reduced longitudinal stiffness properties in order to provide greater in situ flexibility. Buckling of the stent can further result in undesirable out-of-plane deflections of struts or segments of the stent, which may be particularly prevalent where a series of stents or stent grafts comprise the device. Such deflections risk penetration of the struts or segments into portions of the delivery sheath, or possibly the vessel wall, that can hinder deployment of the stent. Buckling can also lead to undesirable accumulations of reactive forces in portions of the stent that results in non-uniform deployment of the stent as described above. Of course, where resistive forces are so great that breaking of the stent occurs, the stent is no longer usable for its intended purpose. Resistive forces can also damage coatings where a coating stent is used, thereby also rendering the stent unusable. Premature deployment of self-expanding stents also tends to occur in a range of prior art systems.  
      U.S. Pat. Nos. 6,302,893 and 6,077,295 discloses a catheter having an elastomer inner member tube or sleeve on which a stent is positioned and an outer member positioned over the inner member or sleeve. Prior to delivery, the inner member tube or sleeve is heated until it fills and forms attachment projections in the open lattice structure of the stent. The outer member of the catheter retains the self-expanding stent from expanding radially outwardly as the attachment projections of the heated inner member tube or sleeve keeps the stent from undergoing axial or other movement until the outer sleeve of the catheter is withdrawn from over the inner member by relative axial movement of the inner member and the outer member. Upon withdrawal of the outer member, the self-expanding stent expands and extricates itself from the inner member tube or sleeve at an intended treatment site. The heat required to produce the attachment projections of the inner member tube or sleeve of the &#39;893 or &#39;295 patents can weaken bridging, or other portions, of a stent that is to be delivered however. Moreover, the requirement of heat to produce the attachment projections and filling of the open lattice structure of a stent is an additional step that can complicate the delivery of the stent, particularly where insufficient heat is provided rendering the stent susceptible to shifting during delivery as a result of under-developed attachment projections or other open lattice filling functions. Such heat also risks premature, or other, degradation of stent coatings, where the stent to be delivered is provided with such a polymeric, drug or other bio-active agent coating.  
      Still other delivery systems, such as in U.S. Pat. No. 6,468,298, include preformed protrusions or mechanically retractable grippers that extend from an inner member of a catheter delivery system in order to assist in maintaining a stent in place during delivery thereof. Such mechanically retractable grippers, when extended, engage the struts of a stent during delivery thereof, and when retracted, permit deployment of the stent at the intended treatment site. The mechanics of such grippers can complicate the delivery and deployment of a stent, particularly where the mechanics of such grippers fail. Moreover, such retractable grippers, or such preformed protrusions, must be provided with some degree of precision so as to align properly with the strut configuration of a stent in order to reliably engage, or disengage, the same during delivery.  
      In view of the above, a need exists for an intraluminal medical device delivery system that more simply and reliably delivers a stent or stent graft to an intended treatment site while more uniformly distributing forces throughout the stent or stent graft during loading and delivery thereof.  
     SUMMARY OF THE INVENTION  
      Various aspects of the systems and methods of the invention comprise an intraluminal medical device delivery system that simply and reliably delivers a stent or stent graft to an intended treatment site while distributing forces more uniformly throughout the stent or stent graft during loading and delivery.  
      In a preferred embodiment, the intraluminal medical device delivery system comprises a catheter system having an outer member, an inner member, a support member on the inner member of a catheter, and a self-expanding stent or stent graft fitted onto the support member. The stent or stent graft may comprise continuous or discontinous segments forming the stent or stent graft, or combinations thereof. At least one of the inner member and the outer member is axially movable relative to one another and the stent or stent graft is longitudinally supported by the support member as delivery of the stent or stent graft to an intended treatment site occurs.  
      The support member is preferably comprised of a material having a low modulus of elasticity that increases in modulus when the material is compressed. The stent or stent graft is crimped, i.e., reduced to a non-expanded state, onto the support member so as to mechanically compress the support member. The support member provides increased longitudinal stability to the stent or stent graft by more uniformly distributing resistive forces throughout the stent or stent graft during loading onto the inner member and support member of the delivery system and during delivery thereof to an intended treatment site. Because the stent or stent graft is crimped onto the support member, the stent or stent graft may be more readily moved forward or backward, i.e., towards or away from an intended treatment site, during delivery thereof, which aids re-positioning of the stent or stent graft to effect even more precise placement thereof, when desired or deemed medically preferred. The crimping of the stent or stent graft onto the support member also enables a series of two or more stents or stent grafts, or a series of continuous or discontinous segments of a single stent or stent graft, or a combination thereof, to be moved more readily in unison to effectuate emplacement thereof in an intended treatment site, as desired. The crimping of the stent or stent graft onto the support member in this manner also helps to maintain the stent or stent graft in place during delivery thereof to the intended treatment site, which helps minimize, or ideally eliminates, premature deployment of the stent or stent graft from the delivery catheter.  
      The support member can be comprised of various materials and morphologies provided it exhibits a soft, flexible, compliant nature. The materials can be any known or later developed version of a mechanically compressible material having a modulus of elasticity that increases upon compression, and that recovers some or all of its expanded state more slowly than does the stent or stent graft, or segments thereof, upon deployment at the intended treatment site. The morphologies of the support member can be any of a tube, a sleeve, a coating, or a film applied to the surface of the support member and onto which the stent, stent graft, or segments thereof are crimped prior to and during delivery to an intended treatment site.  
      Generally, the stent, stent graft, or segments thereof, is crimped onto the support member in order to aid stability of the stent, stent graft, or segments thereof, during loading and delivery. In some embodiments, however, portions of the support member protrude through open or accessible areas of the stent or stent graft, or segments thereof, so as to releasably grip the stent or stent graft, or segments thereof. Such gripping by protruding portions of the support member secures the stent, stent graft, or segments thereof, to the inner member of the delivery system even more than otherwise occurs by only crimping until delivery is effected, although crimping alone provides sufficient longitudinal stability to the stent, stent graft or segments thereof, to distribute forces effectively throughout the intraluminal medical device during loading and delivery thereof. Where a stent, stent graft or segments thereof is provided with a coating, the portions of the support structure that protrude through the open or accessible areas of the stent, stent graft, or segments thereof, also provide a barrier to the coating. Such barriers can extend the shelf life of the intraluminal medical device, and helps minimize undesirable or premature delamination of a coating from the device, in addition to the gripping function described above.  
      In some embodiments, the support member is attached to the inner member of the delivery system using adhesives. In other embodiments, the support member is attached to the inner member by marker bands crimped or swaged over the support member, thereby securing the support member to the inner member. Preferably, one marker band is located at a distal end of the support member, and another marker band is located at a proximal end of the support member. Radiopaque materials may comprise some or all of the marker bands to enhance visualization of the stent, stent graft or segments thereof, and the catheter during delivery and deployment of the intraluminal medical device.  
      A method of delivering an intraluminal medical device according to the invention generally comprises providing a catheter based delivery system with an outer member, an inner member, a mechanically compressible support member and a self-expanding intraluminal medical device, wherein at least one of the inner member and the outer member are longitudinally movable relative to one another, and the mechanically compressible support member is located along a distal portion of the inner member. The method further comprises loading the intraluminal medical device onto the support member by crimping so as to mechanically compress the support member, restraining the intraluminal medical device in its crimped state and the support member in its compressed state within the outer member, delivering the intraluminal medical device to an intended treatment site by one of pushing the inner member beyond a distal end of the outer member or by withdrawing the outer member while maintaining the inner member in place relative to the intended treatment site, resuming the expanded state of the intraluminal medical device at the intended treatment site and withdrawing the inner member and support member before the support member resumes its fully non-compressed state. The intraluminal medical device may be a stent, a stent graft, segments thereof, or a series of such stents, stent grafts or segments thereof.  
      The above and other features of the invention, including various novel details of construction and combinations of parts, will now be more particularly described with reference to the accompanying drawings and claims. It will be understood that the various exemplary embodiments of the invention described herein are shown by way of illustration only and not as a limitation thereof. The principles and features of this invention may be employed in various alternative embodiments without departing from the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and other features, aspects, and advantages of the apparatus and methods of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:  
       FIG. 1  illustrates a conventional catheter delivery system for delivery of self-expanding stents.  
       FIG. 2  illustrates a conventional catheter delivery system having a heat-softened inner member with attachment projections for securing a stent thereto.  
       FIG. 3  illustrates schematically a support member compressed by a stent crimped thereon in accordance with the invention.  
       FIG. 4  further illustrates a stent crimped onto a support member of a catheter delivery system in accordance with the invention.  
       FIG. 5  illustrates the stent of  FIG. 4  emerging from the catheter delivery system of  FIG. 4  in accordance with the invention.  
       FIG. 6  illustrates the imprint remaining on the support member after delivery, deployment and expansion of the stent has been effected according to the invention.  
       FIG. 7  illustrates another embodiment of a support member of a catheter delivery system in accordance with the invention.  
       FIG. 8  illustrates a stent graft mountable to a delivery system support member by crimping for eventual delivery to an intended treatment site according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  illustrates a conventional catheter delivery system  10  for delivering a stent  10  into a patient&#39;s vasculature, such as into the coronary artery, the carotid artery, the renal artery, peripheral arteries or veins, and the like. As shown in  FIG. 1 , a self-expanding stent  20  having an open lattice structure is inserted between an inner member  11  and an outer member  12  at a distal end of the catheter delivery system  10 . The stent  20  is self-expandable such that the stent  20  expands as the inner member  11  on which the stent is mounted moves beyond the outer member  12 . Because the stent  20  is not secured to the inner member  11  prior to delivery, other than by the overlying outer member  12 , expansion of the stent occurs rapidly as the stent  20  emerges from the catheter delivery system  10 .  
       FIG. 2  illustrates a prior art catheter delivery system  110  that is similar to that described above with respect to  FIG. 1 , except that a distal end of the inner member  111  is made from a polymeric material that is heated to become soft. The outer surface of the heat-softened inner member  111  thus forms attachment projections  113  that fill the open lattice of the stent  120 . The attachment projections  113  help to keep the stent  111  from moving axially as delivery of the stent  120  is performed. The attachment projections  113  also help to slow the expansion of the stent  120  after the stent  120  emerges from the constraint of the outer member  112 . Better positioning of the stent  120  at an intended treatment site ideally can occur as a result. The heating required to soften the inner member  111  is provided by various methods including by directly heating the inner member  111 , or by indirectly heating the inner member  111  through the outer member  112 .  
       FIGS. 3-8  more specifically detail aspects of the intraluminal medical device delivery system according to various embodiments of the invention, wherein the intraluminal medical device is understood to be either a stent, a stent graft, or segments thereof, or a series of such stents, stent grafts, or segments thereof. For descriptive purposes only, the delivery system set forth herein is generally described in terms of a single stent, although any of the various intraluminal medical devices described immediately above are contemplated by this description, unless specifically described otherwise herein. Where stent or stent graft segments are referred to, such are understood to be either continuous, i.e., connected, or discontinous, i.e., disconnected, segments of a stent or stent graft, or a combination thereof, including those segments that are continuous when loaded onto the delivery system wherein some or all of the segments become discontinuous during or after delivery to the intended treatment site.  
       FIG. 3  illustrates schematically a self-expanding stent  1120  crimped onto a distal end of a support member  1111  so as to mechanically compress the support member  1113  according to the invention. No heat is provided to the support member, and no preformed protrusions or other mechanical members are provided on the support member to effectuate the mechanical compression of the support member  1113  or the crimping of the stent  1120  thereon. An outer member  1112  surrounds the inner member  1111  and maintains the stent  1120  and the support member  1113  in their respective crimped and compressed states until delivery is effected.  FIG. 4  similarly illustrates a self-expanding stent  1120  crimped onto a distal end of the mechanically compressed support member  1113  of an otherwise conventional catheter delivery system according to the invention, wherein only that portion of the distal end of the inner member of the catheter delivery system is evident in  FIG. 4 .  
       FIG. 5  illustrates a self-expanding stent  1120  and inner member  1111  including the support member  1113  emerging from the outer member  1112  of the catheter delivery system  1110 . Upon emerging from the distal end of the catheter delivery system  1110 , as by pushing the inner member  1111  beyond the distal end of the outer member  1112  or by withdrawing the outer member  1112  from over the inner member while maintaining the inner member  1111  in place relative to an intended treatment site, the stent  1120  expands from its crimped state on the support member  1113 , to its expanded state at the intended treatment site.  
       FIG. 6  illustrates the imprint of the crimped stent  1120  that remains on the support member  1113  after the stent has been delivered and deployed to its intended treatment site. Ideally, the support member  1113  is comprised of a material that takes longer to resume its non-compressed, i.e., expanded, state than it takes the self-expanding stent  1120  to resume its expanded state upon delivery thereof to the intended treatment site. In this way, the expanded stent  1120  overcomes any frictional or other retaining effect of the support member  1113 , and the support member  1113  may be withdrawn from the body of the patient so as to minimize damage to or interference with the deployed stent  1120  or vessel in which the stent is emplaced from the support member  1113 . Thus, e.g., where the original non-compressed state of the support member  1113  exceeds the original non-compressed state of the stent  1120 , then ideally the support member  1113  is withdrawn after the stent  1120  has fully expanded and before the support member  1113  has resumed its fully non-compressed state. Of course, as the artisan will readily appreciate, where the original non-compressed state of the support member  1113  is less than the original non-compressed state of the stent  1120 , then withdrawal of the support member  1113  may occur even before full expansion of the stent  1120  at the intended treatment site has occurred without detrimentally impacting the stent  1120  or vessel provided the retaining effect of the support member  1113  is sufficiently overcome by the expansion of the stent  1120 . Where the original non-compressed states of the support member  1113  and the stent  1120  are equal or substantially so, then expansion of the stent  1120  to overcome the retaining effects of the support member  1113  is preferable before withdrawal of the support member  1113  occurs.  
      Referring still to  FIGS. 3-6 , the support member  1113  is comprised of a mechanically compressible material. The mechanically compressible material may be a foam, gel, or other material, including polymers or elastomers, that are normally soft, flexible and mechanically compressible without the assistance of property altering inputs such as heat. In its non-compressed state, the support member preferably has a low modulus of elasticity. When the self-expanding stent  1120  is crimped onto the support member  1113 , the material comprising the support member is mechanically compressed and becomes stiffer or more rigid. In this compressed state, the support member  1113  exhibits an increased modulus of elasticity as compared to its modulus of elasticity it the non-compressed state. The increased modulus of elasticity of the support member provides increased longitudinal stability to the stent  1120  during loading of the stent onto the inner member  1111  and support member  1113  of the delivery system  1110  and during delivery of the stent to an intended treatment site. For example, when the stent  1120  is loaded or pushed onto the inner member  1111  and support member  1113  of the delivery system  1110 , or when the stent  1120  is otherwise manipulated to be deployed at an intended treatment site, resistive forces experienced by the stent are more uniformly distributed throughout the stent because of the friction fit mounting of the stent  1120  over the support member  1113 , rather than such resistive forces being concentrated at a proximal edge of the stent, as occurs in many conventional delivery systems. The more uniform distribution of forces throughout the stent, as a result of its mounting on the support member  1113 , helps minimize undesirable bending, buckling, bunching or breaking of the stent during loading and delivery thereof. The support member  1113  is fitted to the inner member  1111  of the delivery system by compression, or friction, fitting a sleeve of the mechanically compressible material onto the inner member  1111 , by directly overmolding the mechanically compressible material onto the inner member  1111 , or by co-extruding the mechanically compressible material as a layer onto the inner member  1111 . Moreover, because the friction fit of the stent  1120  on the support member  1113  ideally secures the stent so well thereon, movement of the stent forwards and backwards, i.e., towards or away from the intended treatment site, can occur with minimal, or ideally no, movement of the stent during such delivery tactics. Repositioning of the stent  1120  to achieve a more precise or desirable emplacement of the stent is also more reliably performed as a result of the friction fitting of the stent to the support member  1113  according to the invention. Thus, the friction fitting of the stent to the support member provides this movement and repositioning capacity without the need for any other attachment members or projections from the support member  1113 .  
      The support member  1113  is comprised of one or more materials that are processable into a mechanically compressible structure that exhibits a higher modulus of elasticity in its compressed state than in its non-compressed state. Such materials include, but are not limited to: polyurethane, polytetrafluoroethylene (in the form of expanded PFFE), silicone, rubbers (such as natural rubbers, EPDM rubbers, epichlorohydrin rubbers, or the like), polyamides, polyimides, fluoropolymers, hydrogels (such as hydroxyethylmethacrylate, polyvinylpyrrolidone, polysulfopropylacrylate, and the like), polyolefins, polyacrylates, methacrylates, or blends and co-polymers thereof, or any other known or later developed material in which its modulus of elasticity increases when in a mechanically compressed state, and that recovers its expanded state more slowly than does the intraluminal medical device, upon deployment of the device at the intended treatment site. The material comprising the support member is also deformable without heat so as to achieve fully the mechanically compressed state thereof solely by the crimping of the intralumnial medical device thereon. Where the material comprising the support member is a foam, the foam may be either an open cell foam or a closed cell foam. The morphologies of the support member can be any of a tube, a sleeve, a coating, or a film, any of which can be applied to the surface of the support member.  
      As a result of the mechanically compressible materials and morphologies comprising the support member  1113  according to the various embodiments of the invention described herein, stabilization and securement of the stent  1120  on the inner member  1111  and support member  1113  of the catheter delivery system  1110  occurs without heating of the inner member or the support structure, and without providing any additional mechanical members or formations on the surface of the inner member  1111  or support member  1113 . Simpler, more reliable, loading and delivery of the stent  1120  to an intended treatment site is thus effected according to the invention.  
      In some embodiments, when the support member  1113  is mechanically compressed by crimping of the stent  1120  thereon, portions of the support member  1113  protrude between the open lattice-like areas of the stent  1120  so as to releasably grip portions of the stent  1120  with the protruding portions of the support member  1113 . Such gripping can provide even more stability to the stent  1120  during loading or delivery thereof, although such gripping is not essential to achieving sufficiently increased stability of the stent to perform loading and delivery thereof to an intended treatment site. Where gripping of the stent  1120  occurs by protruding portions of the support member  1113 , such gripping also acts as a barrier that minimizes damage to, and increase the shelf life of, the stent  1120 . In particular, such gripping of the stent by the protruding portions of the support member  1113  can also comprise a barrier that minimizes damage to coatings, where such coatings are provided on the stent. Coatings can comprise polymers, drugs or other bio-active agents applied to the stent, as the artisan will readily appreciate, which coatings could more easily delaminate from the stent were the gripping barriers of the protruding portions of the support member not in place. In any event, expansion of the stent  1120  upon delivery to the intended treatment site releases the stent  1120  from the grip of the protruding portions of the support member  1113 .  
      In still other embodiments, as shown in  FIG. 7 , the support member  2113  is additionally secured to the inner member  2111  of the catheter delivery system. The support member  2113  may be additionally secured to the inner member  2111  by adhesives, as the artisan should readily appreciate, or by marker bands crimped or swaged over ends of the support member.  FIG. 7  illustrates, for example, a support member  2113  having a first marker band  2113   a  securing the distal end of the support member  2113  to the inner member  2111  of the catheter delivery system, and a second marker band  2113   b  securing the proximal end of the support member  2113  to the inner member  2111  of the catheter delivery system. Of course, more or less marker bands than as shown may be used to secure the support member  2113  to the inner member  2111  of the catheter delivery system. In any event, radiopaque materials may comprise some or all of the marker bands in order to enhance visualization of the stent and catheter during loading, delivery and deployment of the stent to the intended treatement site. As shown also in  FIG. 7 , the inner member  2111  may further comprise a tapered distal end  2111  la in order to ease insertion and emplacement of the stent to its intended treatment site. The inner member may also comprise a stop member  2111   b  proximally of the proximal end of the support member  2113  so as to better position the support member  2113  relative to the inner member  2111  of the delivery system.  
       FIG. 8  illustrates one example of a stent graft  300  usable with the various embodiments of the delivery system described herein. As the artisan should appreciate, the stent graft  300  is comprised of a stent  301  having an interior surface  302  and an exterior surface  303 . Graft material  304  is provided on either or both surfaces, i.e., the interior surface  302  and the exterior surface  303  of the stent graft  300 . The graft material  304  can cover all or only portions of the stent graft  300 , as is evident in  FIG. 8 . Where portions of a support member  1113  or  2113  are intended to grip portions of the stent graft  300 , such gripping would most easily occur in areas where graft material  304  has been omitted in the interior surface  302  of the stent graft  300 . However, even where graft material  304  is provided over the entire interior surface  302  of the stent graft  300 , protruding portions of the support member  1111  or  2114  may releasably grip portions of the stent  301  and the graft material  304  until deployment of the stent graft  300  is achieved as discussed elsewhere herein. Although such stent grafts  300  are most typically employed with respect to treatment of abdominal aortic aneurysms, stent grafts may be delivered for treatment of smaller or peripheral vessels as well using the catheter-based intraluminal medical device delivery system described herein.  
      In practice, delivery of an intraluminal medical device according to the invention generally comprises providing a catheter based delivery system having an outer member, an inner member, a mechanically compressible support member and a self-expanding intraluminal medical device, wherein at least one of the inner member and the outer member is longitudinally movable relative to one another and the mechanically compressible support member is located along a distal portion of the inner member. The method further comprises loading the intraluminal medical device onto the support member by crimping so as to mechanically compress the support member, restraining the intraluminal medical device in its crimped state and the support member in its compressed state within the outer member, delivering the intraluminal medical device to an intended treatment site by one of pushing the inner member beyond a distal end of the outer member or by withdrawing the outer member while maintaining the inner member in place relative to the intended treatment site, resuming the expanded state of the intraluminal medical device at the intended treatment site and withdrawing the inner member and support member so as to minimize damage to or interference with the device or vessel in which the device is emplaced from the support member as described hereinabove. In some embodiments, the method further comprises releasably gripping portions of the intraluminal medical device with protruding portions of the support member until delivery of the device is effected at the intended treatment site, whereafter expansion of the device releases the device from the grip of the protruding portions of the support member. Releasably gripping portions of the intraluminal medical device in this manner can further provide a barrier that minimizes undesirable, or premature delamination of coatings, where such coatings are provided on the intraluminal medical device. The intraluminal medical device may be a stent or stent graft, or segments thereof. The support member is comprised of materials and morphologies as described hereinabove.  
      The various exemplary embodiments of the invention as described hereinabove do not limit different embodiments of the systems and methods of the invention. The material described herein is not limited to the materials, designs or shapes referenced herein for illustrative purposes only, and may comprise various other materials, designs or shapes suitable for the systems and methods described herein, as should be appreciated by the artisan.  
      While there has been shown and described what is considered to be preferred embodiments of the invention, it will, of course, be understood that various modifications and changes in form or detail could readily be made without departing from the spirit or scope of the invention. It is therefore intended that the invention be not limited to the exact forms described and illustrated herein, but should be construed to cover all modifications that may fall within the scope of the appended claims.