Patent Grant
8715340
The invention concerns an endovascular device for insertion into a body vessel to treat a diseased, damaged or weakened portion of a vessel wall.
Vascular diseases include aneurysms causing hemorrhage, atherosclerosis causing occlusion of blood vessels, vascular malformation and tumors. Vessel occlusion or rupture of an aneurysm within the brain can result in stroke. Aneurysms fed by intracranial arteries can grow within the brain to a point where their size can also cause a stroke or the symptoms of a stroke, requiring surgery to remove the aneurysm, or other remedial intervention.
Occlusion of coronary arteries is a common cause of heart attack. Diseased and obstructed coronary arteries result in restricted blood flow in the heart which can lead to ischemia or necrosis. While the exact etiology of sclerotic cardiovascular disease is still in question, the treatment of narrowed coronary arteries is more defined. Surgical construction of coronary artery bypass grafts (CABG) is often the method of choice when there are several diseased segments in one or multiple arteries. Conventional open-heart surgery is of course highly invasive and traumatic for patients undergoing such procedures. Therefore, less invasive procedures that accomplish the same goals are highly desirable.
One alternative method of treatment involves the use of balloon angioplasty as a way in which to reopen the lumen of an occluded vessel. In this procedure a folded balloon is inserted via a catheter into a stenosed region that is either partially or fully occluding the vessel lumen. Inflation of the balloon physically expands the lumen, reopening the occluded region, and restoring normal or at least significantly improved blood flow through the vessel. Alternatively, occlusive atheromas may be cut from the inner surface, a procedure known as atherectomy. In both methods, a certain incidence of restenosis (resealing) occurs resulting in a loss of the benefit of the procedure, and potentially the need for additional rounds of therapy. Restenosis also results in reversion back to the original occluded condition, such that the vessel no longer conducts a normal flow volume, which can lead to ischemia or infarct depending on the particular location and function of the vessel in question.
A recent preferred therapy for repairing vascular occlusions involves placement of an expandable metal wire-frame (i.e. a stent) within the occluded region of a blood vessel in order to keep the lumen of the vessel open. Stents are generally delivered to the desired location within a vascular system by an intraluminal route, usually via a catheter. Advantages of the stent placement method over conventional vascular surgery include obviating the need for surgically exposing, removing, replacing, or by-passing the defective blood vessel, including heart-lung bypass, opening the chest and in some cases general anaesthesia.
When inserted and deployed in a vessel, duct or tract (all of which can be conveniently referred to as a vessel) of the body, for example, a coronary artery after dilation of the artery by balloon angioplasty, a stent acts as a prosthesis to maintain the vessel in an open state, thus providing a fluid pathway in the previously occluded vessel. The stent usually has an open-ended tubular form with interconnected struts as its sidewall to enable its expansion from a first outside diameter which is sufficiently small to allow the stent to traverse the vessel lumen and be delivered to a site where it is to be deployed, then expanded to a second outside diameter sufficiently large to engage the inner lining of the vessel for retention at that site. The stent may be expanded via the use of a mechanical device, for example a pressurizable balloon, or alternatively the stent may be self-expanding. Self-expanding stents can be manufactured at a to be deployed size, and then compressed to a smaller size to enable delivery, or may be manufactured from shape memory materials that are deformable to a memorized shape in response to an externally applied energy.
Usually a stent suitable for successful interventional placement should be hypoallergenic, or preferably non-allergenic, have good radio-opacity to permit radiographic visualization, free from distortion during magnetic resonance imaging (MRI), plastically deformable, resistant to vessel recoil, and be as thin as possible to minimize obstruction to blood flow (or other materials or fluids in vessels other than those of the cardiovascular system), and be relatively non-reactive in terms of eliciting thrombogenic responses.
The typical reaction when a foreign body is implanted in a body vessel is generally negative. Foreign bodies frequently cause an inflammatory response, and in the case of blood vessels, neointimal proliferation which results in narrowing and occlusion of the body vessel, obviating the benefit of the implant. As a result, both selection of the materials from which the stent is composed, as well as the design of the stent, play an important role in influencing the final suitability of the device in practice. Therefore, in addition to the structural requirements for a stent to maintain a previously occluded vessel in a substantially open conformation, stents must also be biologically compatible, and must be chemically stable when exposed to a biological environment.
A variety of materials have been tested and used in stents to address the issues of biocompatibility and material stability. For example, polyurethanes have been used in long term implants, but are not always suitable for use in endovascular treatments, especially in small blood vessels. Small blood vessels are considered to be those with an inner diameter of 2.0 to 5.0 mm. In addition, many commercially available polymers are with additives, or have impurities, that are surface-active and so reduce their usefulness in some biological applications.
More recently, polymers have been developed which can be further modified by the covalent attachment of various surface-modifying end groups, these end groups reducing the reactivity of the material with cells and other factors that function in the immune response. End groups can also be useful in providing greater chemical stability to the material, reducing degradation and improving the longevity of the prosthesis. For example, U.S. Pat. No. 5,589,563 (Ward & White) discloses a series of biomedical base polymers with covalently attached end groups that give the polymer certain desirable properties. These modified polymers possess surface properties that improve the biocompatibility and overall performance of objects fashioned from them.
In addition to their biomechanical functionality, implantable medical devices like stents have been utilized for delivery of drugs or bioreagents for different biological applications. U.S. Pat. No. 5,891,108 (Leone et al.) discloses a hollow tubular wire stent with holes through which an active substance can be delivered to a site in a vessel. In some cases the drugs or bioreagents can be coated directly onto the surface of the implantable medical devices or mixed with polymeric materials that are then applied to the surface of the devices. For example, U.S. Pat. No. 5,599,352 (Dinh et al.) discloses a drug eluting stent comprising a stent body, a layer of a composite of a polymer combined with a therapeutic substance, overlaid by a second layer comprising fibrin.
However, each of these methods suffers from one or more problems including poor control of release or limitations of the form of drug or other reagent that can be applied. Also, these methods are unsuitable for situations where it would be desirable to maintain the bioactive molecule on the device rather than having it be released, in order to maintain a relatively high local activity of the reagent of interest.
As a result, in practice, the design and use of stents in the repair of aneurysms or other vessel defects or diseases typically represents a compromise among competing factors. First, the stent must adequately support the diseased or weakened region in order to prevent rupture of the aneurysm or vessel, either of which could lead to serious complications including death, depending on the size, location and nature of the aneurysm or defect. Second, in the case of stents use in the repair of aneurysms, the stent must permit sufficient blood to maintain the patency of both the parent and perforator vessels, while at the same time limiting flow to the aneurysm proper. Generally speaking, flow of material through the framework of a stent is achieved by regulating the porosity of the stent.
Stent porosity can be managed in a number of ways. The simplest way is to manufacture the stent so that the framework itself defines the porosity of the device. However, in biological applications, regulating movement of materials on cellular or subcellular scale is required, and it is difficult and costly to manufacture stents that have such fine effective pore size. Other approaches have been to cover the stent framework for example with a membrane, where the membrane is either impermeable or porous as desired. U.S. Patent Application No. 2006/0217799 (Mailander et al.) discloses a stent comprising a grid or mesh structure in which one or more cells of the grid are covered with a membrane. Similarly, U.S. Patent Application No. 2006/0200230 (Richter) discloses a covering for an endoprosthetic device that comprises a sheath with holes of varying size and varying frequency disposed in different areas of the sheath.
However, a problem inherent with these designs is that they are not easily adapted for effecting vessel wall repairs where the area of disease, damage or weakness can vary in size. Thus, in order to optimally treat an aneurysm, it would be necessary to tailor the stent and its covering to more or less the precise size of the damaged area, in order to properly occlude the aneurysm site, while maintaining vessel patency in the parent vessel and any perforator vessels. Furthermore, these designs are not optimized such that they will generally provide flow to perforator vessels that are part of the collateral circulation in the area of the diseased, damaged, or weakened vessel, while blocking flow to an aneurysm.
Thus, in some embodiments an endovascular device for insertion into a body vessel to treat an aneurysmal portion of the body vessel, the endovascular device comprises: an expandable member, expandable from a first position to a second position, said expandable member being expandable radially outwardly to the second position such that an outer surface of said expandable member engages with an inner surface of the vessel so as to maintain a fluid pathway in said vessel through a lumen in the expandable member; a membrane covering at least a portion of an outer surface of said expandable member; a plurality of pores in a porous section of the membrane, the porous section having a substantially uniform porosity over a length extending from a proximal end to a distal end of the porous section, porosity being determined by a pore spacing and a pore size; wherein the proportion of the total area of an outer surface of the porous section that consists of membrane material defines a material ratio; wherein the substantially uniform porosity is selected such that, when the expandable member is positioned in the body vessel, the membrane permits a flow of blood from within the lumen of the expandable member, through at least one of the pores, and into at least one branch vessel that branches off of the body vessel; and wherein the substantially uniform porosity is further selected such that, when the expandable member is positioned in the body vessel, the membrane reduces blood flow to the aneurysmal portion of the vessel, promoting thrombosis at or in the aneurysmal portion.
In some embodiments, the porosity of the porous section is selected such that it enables enhanced endothelial cell migration and tissue in-growth for endothelialization while substantially preventing blood circulation to the diseased, damaged or weakened portion of the vessel wall.
In some embodiments, the pore size is between about 1 μm and about 150 μm.
In some embodiments, the pore size is between about 10 μm and about 50 μm.
In some embodiments, the pore spacing is between about 40 μm and about 100 μm.
In some embodiments, the pore spacing is between about 60 μm and about 75 μm.
In some embodiments, the material ratio in an as-manufactured state is between about 85% and about 96%.
In some embodiments, the material ratio in a deployed state is between about 25% and about 90%.
In some embodiments, the material ratio in the deployed state is between about 70% and about 80%.
In some embodiments, the material ratio in the deployed state is about 75%.
In some embodiments, a diameter of the device in the deployed state is between about 2 mm and about 5 mm.
In some embodiments, a thickness of the membrane is between about 25 μm to about 125 μm.
In some embodiments, the thickness of the membrane is measured in an as-manufactured state.
In some embodiments, a thickness of the membrane is between about 5 μm to about 25 μm.
In some embodiments, the thickness of the membrane is measured in a deployed state.
In some embodiments, the device further comprises at least one surface-modifying end group that promotes healing of the body vessel after the device is inserted into the body vessel.
In some embodiments, the surface-modifying end group comprises at least one of a fluorocarbon and the combination of polyethylene glycol and silicon.
In some embodiments, the device further comprises at least one agent, permanently attached the membrane, that promotes healing of the aneurysm.
In some embodiments, at least one permanently attached agent comprises at least one of a peptide, a protein, an enzyme regulator, an antibody, a naturally occurring molecule, a synthetic molecule, a nucleic acid, a polynucleotide, L-PDMP, and D-PDMP.
In some embodiments, each pore has a diameter between about 30 μm and about 40 μm, and a distance between adjacent pores is between about 60 μm and about 70 μm.
In some embodiments, the aneurysmal portion of the vessel is located at or near at least one of an intracranial aneurysm, a saccular aneurysm, a wide-neck aneurysm, a fusiform aneurysm, a caroticocavenous fistula, an arteriovenous malformation, a carotid artery stenosis, a saphenous vein graft, a small vessel stenosis, and a renal artery repair.
In some embodiments, the porous section can be divided into n porous regions, and wherein an outer surface area of each of the n porous regions is substantially 1/n of a total outer surface area of the porous segment, and wherein each one of the n porous regions has substantially the same porosity as each of the other n−1 porous regions.
In some embodiments, n=2.
In some embodiments, n=3.
In some embodiments, n=4.
In some embodiments, n=5.
In some embodiments, the pore size is in a range between about 1 μm and about 150 μm, and pore spacing is between about 10 μm and about 50 μm.
In some embodiments, the pore size is between about 10 μm and about 50 μm, and the pore spacing is between about 60 μm and about 75 μm.
In some embodiments, an endovascular device system for insertion into a body vessel to treat an aneurysmal portion of the vessel, the endovascular device comprises: an expandable member, expandable from a first position to a second position, said expandable member being expandable radially outwardly to the second position such that an outer surface of said expandable member engages with an inner surface of the vessel so as to maintain a fluid pathway in said vessel through a lumen in the expandable member; a membrane covering at least a portion of an outer surface of said expandable member; a plurality of pores in a porous section of the membrane, the porous section having a substantially uniform porosity over a length extending from a proximal end to a distal end of the porous section, porosity being determined by a pore spacing and a pore size; wherein the proportion of the total area of an outer surface of the porous section that consists of membrane material defines a material ratio; wherein the substantially uniform porosity is selected such that, when the expandable member is positioned in the body vessel, the membrane permits a flow of blood from within the lumen of the expandable member, through at least one of the pores, and into at least one branch vessel that branches off of the body vessel; and wherein the substantially uniform porosity is further selected such that, when the expandable member is positioned in the body vessel, the membrane reduces blood flow to the aneurysmal portion of the vessel, promoting thrombosis at or in the aneurysmal portion; and a delivery device, operable to deliver the expandable member to the aneurysmal portion of the vessel, onto which the expandable member is loaded prior to delivery.
In some embodiments, the pore size is between about 1 μm and about 150 μm.
In some embodiments, the pore size is between about 10 μm and about 50 μm.
In some embodiments, the pore spacing is between about 40 μm and about 100 μm.
In some embodiments, the pore spacing is between about 60 μm and about 75 μm.
In some embodiments, the material ratio in an as-manufactured state is between about 85% and about 96%.
In some embodiments, the material ratio in a deployed state is between about 25% and about 80%.
In some embodiments, the material ratio in the deployed state is between about 70% and about 80%.
In some embodiments, the material ratio in the deployed state is about 75%.
In some embodiments, a diameter of the expandable member in the deployed state is between about 2 mm and about 5 mm
In some embodiments, a thickness of the membrane is between about 25 μm to about 125 μm.
In some embodiments, the thickness of the membrane is measured in an as-manufactured state.
In some embodiments, a thickness of the membrane is between about 5 μm to about 25 μm.
In some embodiments, the thickness of the membrane is measured in a deployed state.
In some embodiments, the system further comprises at least one surface-modifying end group that promotes healing of the body vessel after the device is inserted into the body vessel.
In some embodiments, the at least one surface-modifying end group is at least one of a fluorocarbon and the combination of polyethylene glycol and silicon.
In some embodiments, the system further comprises at least one permanently attached agent to promote healing of the aneurysmal portion.
In some embodiments, the at least one permanently attached agent comprises at least one of a peptide, a protein, an enzyme regulator, an antibody, a naturally occurring molecule, a synthetic molecule, a nucleic acid, a polynucleotide, L-PDMP, and D-PDMP.
In some embodiments, each pore has a diameter between about 10 μm and about 50 μm and the distance between adjacent pores is between about 60 μm and about 75 μm.
In some embodiments, the aneurysmal portion of the body vessel is located at or near at least one of an intracranial aneurysm, a saccular aneurysm, a wide-neck aneurysm, a fusiform aneurysm, a caroticocavenous fistula, an arteriovenous malformation, a carotid artery stenosis, a saphenous vein graft, a small vessel stenosis, and a renal artery repair.
In some embodiments, an endovascular device for insertion into a body vessel to treat an aneurysmal portion of a body vessel, the endovascular device comprises: means for maintaining a fluid pathway in the body vessel; means for covering at least part of the means for maintaining, the means for covering having a substantially uniform porosity in a porous segment of the means for covering; and wherein, when the means for maintaining is positioned in a body vessel, the means for covering permits blood flow from the fluid pathway to at least one branch vessel branching off the body vessel, while reducing blood flow to the aneurysmal portion, and the means for maintaining supports the body vessel in the region of the aneurysmal portion and provides a fluid pathway in the body vessel.
In some embodiments, a method of treating a body vessel having an aneurysmal portion comprises the steps of: providing an endovascular device, comprising: an expandable member, expandable from a first position to a second position, said expandable member being expandable radially outwardly to the second position such that an outer surface of said expandable member engages with an inner surface of the body vessel so as to maintain a fluid pathway in said body vessel through a lumen in the expandable member; a membrane covering at least a portion of an outer surface of said expandable member; a plurality of pores in a porous section of the membrane, the porous section having a substantially uniform porosity over a length extending from a proximal end to a distal end of the porous section, porosity being determined by a pore spacing and a pore size; wherein the proportion of the total area of an outer surface of the porous section that consists of membrane material defines a material ratio; wherein the substantially uniform porosity is selected such that, when the expandable member is positioned in the body vessel, the membrane permits a flow of blood from within the lumen of the expandable member, through at least one of the pores, and into at least one branch vessel that branches off of the body vessel; and wherein the substantially uniform porosity is further selected such that, when the expandable member is positioned in the body vessel, the membrane reduces blood flow to the aneurysmal portion of the body vessel, promoting thrombosis at or in the aneurysmal portion; and positioning the expandable member in the body vessel.
In some embodiments, the porosity of the membrane is selected such that it enhances endothelial cell migration and tissue in-growth.
In some embodiments, the pore size is between about 1 μm and about 150 μm.
In some embodiments, the pore size is between about 10 μm and about 50 μm.
In some embodiments, the pore spacing is between about 40 μm and about 100 μm.
In some embodiments, the pore spacing is between about 60 μm and about 75 μm.
In some embodiments, the material ratio in an as manufactured state is between about 85% and about 96%.
In some embodiments, the material ratio in a deployed state is between about 25% and about 80%.
In some embodiments, the material ratio in the deployed state is between about 70% and about 80%.
In some embodiments, the material ratio in the deployed state is about 75%.
In some embodiments, a diameter of the expandable member in the deployed state is between about 2 mm and about 5 mm.
In some embodiments, a thickness of the membrane is between about 25 μm to about 125 μm in the as-manufactured state.
In some embodiments, a thickness of the membrane is between about 5 μm to about 25 μm in the deployed state.
In some embodiments, the method further comprises providing a membrane having at least one surface-modifying end group that encourages healing of the body vessel after the device is inserted.
In some embodiments, the at least one surface-modifying end group is at least one of a fluorocarbon and the combination of polyethylene glycol and silicon.
In some embodiments, the membrane further comprises at least one permanently attached agent to promote healing of the aneurysm.
In some embodiments, the at least one permanently attached agent comprises at least one of a peptide, a protein, an enzyme regulator, an antibody, a naturally occurring molecule, a synthetic molecule, a nucleic acid, a polynucleotide, L-PDMP, and D-PDMP.
Examples of embodiments of the invention will now be described with reference to the following drawings.
Implantable medical devices include physical structures for delivering drugs or reagents to desired sites within the endovascular system of a human body. These devices may take up diversified shapes and configurations depending upon specific applications. Common implantable medical devices include stents, vena cava filters, grafts and aneurysm coils.
The endovascular system of a human body includes blood vessels, cerebral circulation system, tracheo-bronchial system, the biliary hepatic system, the esophageal bowel system, and the urinary tract system. Although exemplary stents implantable in blood vessels are described, they are applicable to the remaining endovascular system. Embodiments of the invention, some of which are described herein are readily adaptable for use in the repair of a variety of vessels, including but not limited to, treatment or repair in cases of aneurysm, ischemic stroke, carotid artery stenosis, saphenous vein graft, small vessel stenosis, or renal artery repair.
Stents are expandable prostheses employed to maintain vascular and endoluminal ducts or tracts of the human body open and unoccluded. For example, stents are now frequently used to maintain the patency of a coronary artery after dilation by a balloon angioplasty procedure. A stent is a typically a tubular meshwork structure having an exterior surface defined by a plurality of interconnected struts and spaces between the struts. The tubular structure is generally expandable from a first position, wherein the stent is sized for intravascular insertion, to a second position, wherein at least a portion of the exterior surface of the stent contacts and engages the vessel wall where the stent has been placed.
The expanding of the stent is accommodated by flexing and bending of the interconnected struts throughout the structure. The force for expansion can be applied externally as from a inflated balloon onto which the stent is loaded prior to placement, or the stent itself may be self-expanding. A myriad of strut patterns are known for achieving various design goals such as enhancing strength, maximizing the expansion ratio or coverage area, enhancing longitudinal flexibility or longitudinal stability upon expansion, etc. One pattern may be selected over another in an effort to optimize those parameters that are of particular importance for a particular application.
Illustrated in
Illustrated in
While a stent 112 may be deployed by radial expansion under outwardly directed radial pressure exerted, for example, by active inflation of a balloon 114 of a balloon catheter on which the stent is mounted, the stent 112 may be self-expandable. In some instances, passive spring characteristics of a preformed elastic (i.e., self-opening) stent serve the purpose, while in others shape memory materials are used, such that upon activation by the appropriate energy source, the stent deforms into a pre-determined memorized shape. Regardless of design, in all cases the stent is expanded to engage the inner lining or inwardly facing surface of the vessel wall with sufficient resilience to allow some contraction, but also with sufficient stiffness to largely resist the natural recoil of the vessel wall following deployment.
Referring to the delivery system depicted in
In
In some embodiments, the delivery of the stent is accomplished as follows. The stent is first mounted onto an inflatable balloon on the distal extremity of the delivery catheter, and the stent is mechanically crimped onto the exterior of the folded balloon. The catheter/stent assembly is then introduced into the vasculature through a guiding catheter. A guide wire is disposed across the diseased arterial section and then the catheter/stent assembly is advanced over the guide wire that has been placed in the vessel until the stent is substantially located at the site of the diseased or damaged portion of the vessel. At this point, the balloon of the catheter is inflated, expanding the stent against the artery. The expanded stent engages the vessel wall, which serves to hold open the artery after the catheter is withdrawn.
Due to the formation of the stent from an elongated tube, the undulating component of the cylindrical elements of the stent is relatively flat in transverse cross-section, so that when the stent is expanded, the cylindrical elements are pressed into the wall of the vessel and as a result do not significantly interfere with the blood flow through the lumen. The cylindrical elements of the stent, which are pressed into the wall of the vessel, will eventually be overgrown with a layer of endothelial cells, further minimizing interference with blood flow that could be caused by the presence of the stent in the lumen. The closely spaced cylindrical elements, located at substantially regular intervals, provide uniform support for the wall of the artery, and consequently are well adopted to tack up and hold in place small flaps or dissections that may exists in the vessel wall.
Resilient or self-expanding prostheses can be deployed without dilation balloons. Self-expanding stents can be pre-selected according to the diameter of the blood vessel or other intended fixation site. While their deployment requires skill in stent positioning, such deployment does not require the additional skill of carefully dilating the balloon to plastically expand the prosthesis to the appropriate diameter, as the final diameter will be primarily a function of the stent design itself. Further, the size of the self-expanding stent is chosen such that when in place it remains at least slightly elastically compressed, and thus has a restoring force which facilitates acute fixation. By contrast, a plastically expanded stent must rely on the restoring force of deformed tissue, or on hooks, barbs, or other independent fixation elements included as part of the stent structure.
Self-expanding stents can be fashioned from resilient materials such as stainless steel, and the like, wherein the stent is loaded onto the delivery device in a compressed state, and upon placement at the desired location is allow to naturally elastically expand. Expandable stents can also be fashioned from shape memory materials such as nickel-titanium alloys and the like, wherein the stent is expanded from a first shape to a second shape by activation with an energy source such as heat, magnetic fields or an RF pulse for example.
The presence of a foreign object in a vessel, like a stent, can promote thrombus formation as blood flows through the vessel, and platelets contact the stent surface. This is a well-recognized problem in other areas of cardiovascular treatment, such as when artificial heart valves are implanted. In serious instances, clot formation can lead to acute blockage of the vessel. In addition, as the outward facing surface of the stent in contact or engagement with the inner lining of the vessel, tissue irritation can lead to an inflammatory reaction, further exacerbating restenosis due to localized hyperplasia. Stent design and use must take into account all these myriad factors.
In one embodiment, illustrated in
The stent 202 also has longitudinal flexibility equal to or better than what is provided by a delivery catheter. This means that the stent does not add increased rigidity to the device. The trackability of the stent 202 depends on the mechanical properties of the catheter and is not restricted by stent 202 alone. The longitudinal flexibility of the stent 202 can be measured by force in grams to deflect the stent from neutral line. This force brings stent deflection to 1 mm for less than 8 grams. Existing catheters can provide 20-22 grams per 1 mm deflection. This condition is also extremely important when creating stent compliance to particular vessels and saves the vessel from possible traumatic reaction.
The structure of the stent 202 is designed to provide a normalized radial force of 18-19 grams/mm of length and may reach values close to the ones found in existing coronary stents. Stent structural support provides 3-4% of deflection of the stent structure together with intracranial vessel wall natural pulsing. This leads to greater stent conformity and a reduced vessel injury score.
The intracranial stent 202 has profile in compressed delivery mode .020″
The intracranial stent 202 is designed to be compressed onto delivery catheter with a profile as low .014″-.016″ having stent profile .020″-.022″.
The intracranial stent 202 has even material distribution and wall coverage, creating needed vessel support. The material ratio is in the range of 10-17% depending on deployment diameter.
The intracranial stent 202 has a strut thickness and width not larger than .0028″. Strut dimensions are selected which make the least intrusive stent material volume and to reduce the vessel injury score. The stent surface to length ratio is set to be 1.1-1.3 mm2/mm to provide minimal vessel injury score.
As shown in
In
Stents may also be used to treat a number of different types of aneurysms, including bifurcation aneurysm, as shown in
Table 1 provides a table with exemplary dimensions for an intracranial stent 202 designed for use with a membrane 203. The membrane 203 is biocompatible, has good adhesion to stent struts made from a variety of materials including, but not limited to stainless steel, titanium and nickel alloys and the like. The membrane forms an ultra-thin film that is porous as opposed to being a solid film, having holes or pores included during the process of manufacturing the membrane. In some embodiments, the pore size and material coverage area are selected to prevent blockage of perforator vessels, and while restricting blood flow to the aneurysm.
In some embodiments, the membrane 203 is made from a thin film generally in a range of from about 25 μm to about 125 μm in thickness, measured in the as-manufactured state, and is from about 5 μm to about 25 μm thick, as measured in the deployed state (expanded state). The film has good expandability, and can be expanded up to about 400% using relatively low force. The membrane 203 also has good chemical stability at ambient conditions allowing for extended storage prior to use, and is stable under sterilization conditions (ethanol). Examples of physical properties of the membrane are a hardness of about 75A (measured with a Shore durometer), tensile strength up to about 7500 p.s.i., and elongation of up to about 500%.
In other embodiments, the film is blood “permeable” rather than being a solid film. The covered sections, that is, the borders between pores or holes do not exceed 75 μm so as to prevent any part of the stent 202 or the membrane 203 from blocking perforators. Several options can be undertaken to achieve this. The membrane 203 is made from a thin film that does not exceed 0.001″ in width. The film has good expandability, and can expand up to 400% at a low force. The membrane 203 also has a shelf life or chemical stability at ambient conditions and is stable in sterilization conditions (Eto).
Conveniently, membranes can be made porous, and if desired uniformly porous, by drilling holes into a solid film. In this way a stent 202 covered by a uniformly porous membrane 203 can be provided as illustrated in
Where the stent is covered with membrane strips, as shown in
Where a mesh or woven membrane is used, a sheet of woven membrane 203 can be wrapped circumferentially around the stent 202, as illustrated in
The membrane may be made of a biocompatible and elastomeric polymer. The membrane may have a thickness of about 0.001 to 0.005″ with pore or hole sizes of about 20 to 100 microns. The membrane may be made from polymeric material or biodegradable material. The biodegradable material may form multiple sub-layers mixed with drugs or reagents.
In some embodiments, the membrane 203 completely surrounds the stent struts, and forms a stable film between the struts, as shown in
The membrane 203 is elastic to allow its own expansion five to six times without disintegration and detachment from the stent structure. The thickness of the membrane 203 is expected to be not more than .002″ in crimped position and .001″ in expanded form. The mechanical properties do not introduce extra rigidity to the intracranial stent 202 and have no resistance to stent expansion. The membrane material also allows an expanded membrane 203 to endure normal blood pressure.
Implantable medical devices can also be used to deliver drugs or reagents to specific locations within the vascular system of a human body. As shown in
The spaces surrounded by the first layer, and the circumferential strips and the second layer form the pockets 208 that serve as receptacles for drugs or reagents. In other embodiments, the intermediate layer includes at least one opening so that the pockets can be formed within the openings. The shapes and sizes of the openings can be varied in accordance with specific applications. The stent 202 can be partially covered by a membrane 203 that comprises a first layer 206 and a second layer 207.
In some embodiments, the membrane 203 can cover the entire stent, or portions of the stent 202, as is shown in
Many polymeric materials are suitable for making the layers of the membrane 203. Typically, one first layer is disposed onto the outer surface of a stent. The first layer has a thickness of about 50-125 μm, with pore sizes of about 20-30 μm as a nominal initial diameter. In certain embodiments, the first layer can serve as an independent membrane 203 to mechanically cover and seal the aneurysm 201. The first and/or second layers can be comprised of biodegradable material, and function as a drug or reagent carrier in order to provide sustained release functionality.
It is desirable that the intermediate layer be formed of a material which can fuse to the first and second layers or attached to the first layer in a different manner. In certain embodiments, the intermediate layer may be merged with the first layer to form a single layer with recessions within the outer surface of the merged layer. The second and intermediate layers can be made of biodegradable material that include drugs or other reagents for immediate or sustained release. After the biodegradable material is dissipated through the degradation process, the membrane 203 is still intact, providing vessel support. The second layer can also be composed of a polymeric material. In some embodiments, the second layer has a thickness of about 25-50 μm, with pore sizes ranging from about 70-100 μm.
The polymeric layers may be fashioned from a material selected from the group consisting of fluoropolymers, polyimides, silicones, polyurethanes, polyurethanes ethers, polyurethane esters, polyurethaneureas and mixtures and copolymers thereof. Biodegradable polymers can include polylactide, poly(lactide-co-glycolide), poly-orthoesters, polyphosphazenes, polyanhydrides, or polyphosphoesters. The fusible polymeric layers may be bonded by adhering, laminating, or suturing. The fusion of the polymeric layers may be achieved by various techniques such as heat-sealing, solvent bonding, adhesive bonding or the use of coatings.
Types of drugs or reagents that may prove beneficial include substances that reduce the thrombogenic, inflammatory or smooth muscle cell proliferation response due to the implanted device. For example, cell proliferation inhibitors can be delivered in order to reduce or inhibit smooth muscle cell proliferation. In intracranial or some other applications fibrin sealants can be used and delivered to seal aneurysm neck and provide fibroblasts and endothelial cells growth. Specific examples of drugs or reagents include heparin, phosporylcholine, albumin, dexamethasone, paclitaxel and vascular endothelial growth factor (VEGF). This list is not exhaustive, and other factors known to regulate inflammatory responses, cellular proliferation, thrombogenesis and other processes related to reaction to foreign bodies are contemplated to be useful within the scope of the disclosure.
The drug or reagents can be incorporated into the implantable medical devices in various ways. For example the drug or reagent can be injected in the form of a gel, liquid or powder into the pockets. Alternatively the drug or reagent can be supplied in a powder which has been formed into a solid tablet composition, positioned in receptacles placed in the device.
It is at times desirable to provide a stent that is highly flexible and of small profile in order to effect treat vessels of very small caliber, for example, intracranial vessels with lumen diameters ranging in size from about 1.5 mm to about 5.0 mm. High flexibility allows the stent to be advanced along the anatomy of the intracranial circulation.
In some embodiments, as illustrated in
The membrane 203 is part of the stent structure and is effective to occlude the aneurysm neck and “recanalize” a diseased, damaged, or weakened vessel, leading to healing of the vessel and elimination of the aneurysm. The use of a stent as shown in
In some embodiments, there is provided a porous membrane as part of the device. The membrane 203 has a system of holes or pores 25 with pore diameter 21 on the order of about 1 to 100 μm, and borders 23 between the pores have a width generally less than about 100 μm, as shown in
It has been further discovered that a membrane having uniform porosity can be effective in blocking blood flow to an aneurysm while maintaining flow to perforator vessels.
In some embodiments, pore spacing (the distance between adjacent pores) can be in a range of from about 40 to 100 μm. To produce a membrane of uniform porosity, pore diameter 21, and interpore spacing 22, will be generally equidistant, as in
Furthermore, the characterization of a membrane as either macro-or microporous is not limiting to the disclosure. The functionality of the membrane is dependent on pore diameter and pore spacing, which are described in terms of physical measurement units, and how the particular physical dimensions of the membrane pores operate in situ to regulate blood flow. In either case, membranes having porous sections of uniform porosity can be fashioned by selecting a desired pore diameter and pore spacing combination. As is seen in the data presented below, various combinations of pore diameter and pore spacing are effective to provide a membrane of optimal porosity over a range of deployed sizes. Thus, a porous membrane 203 is able to significantly improve hemodynamics around the aneurysm 201, since it has a lower delivery profile and is more flexible, as compared to a stent 202 with a solid membrane.
One application for a device having a macroporous membrane is to treat aneurysms within close proximity of branches or perforators. Another specific application is the treatment of an intracranial saccular or wide neck aneurysm located above the ophthalmic artery where perforators extend from the parent artery within close proximity of the aneurysm. Microporous devices are suitable for use in areas where perfusion of perforators is of less immediate concern. Thus, the micro-porous device is used for conditions which require total coverage to immediately block blood flow, for example, a caroticocavenous fistula, or where there is little or no risk of blocking perforators, for example, below the ophthalmic artery.
The device may be used for the treatment of endovascular disease such as aneurysms, arteriovenous malformations (AVM's) and caroticocavenous fistulas. The device may also be useful in other vessel related applications such as treatment or repair in cases of ischemic stroke, carotid artery stenosis, saphenous vein graft, small vessel stenosis, or renal artery repair. The pore patterns are designed with consideration of factors such as specific flow conditions of blood vessels, and the location of the vessel being repaired.
The design of the porous section of a membrane is therefore initially determined according to the intended application of the device, and three main factors, pore size 21, bridge dimensions 22, 23, and material ratio of the membrane. Pore size 21 can be measured in the “as designed and manufactured” (i.e. unexpanded) and “as deployed” (i.e. expanded) states. Typically, pore size in the unexpanded state is about 1.5 to 2.5 times smaller than pore size after the membrane has been expanded to its deployed size. This is depicted in
Bridge dimensions 22, 23 refer to the shortest distance separating one pore 25 from its adjacent pores, as shown in
Membrane Porosity
The relative porosity of a porous section of a membrane will be dictated by the size of individual pores and the number of pores per unit area (i.e. pore density). As used herein, the term “porous section” refers to that area of a membrane that includes substantially all the pores of the membrane. Coverage and porosity can both be described in terms of a relationship between the area of the apparent area of the porous section of the membrane corresponding to membrane material, versus that corresponding to the pores. Thus, the material ratio is the fraction of a membrane area that corresponds to membrane material, or in other terms, total apparent area or the porous section (100%) —pore area (%)=material ratio (%). As used herein, the term “material ratio” refers in particular to the membrane material versus pore area in a porous section of a membrane.
As indicated, material ratio is conveniently expressed as a percentage. So, for example, a membrane lacking pores has a material ratio=100%, while in a membrane with 20% of its total area encompassed by pores, the material ratio=80%. Likewise, porosity can also be expressed as a percentage, where porosity (%)=total area of the porous section of the membrane (100%)—material ratio (%). A membrane having a material ratio of 75% would have a porosity of 25%. Both material ratio and porosity can be described in membranes in the “as manufactured” and “as deployed” stages. In some embodiments, the overall material ratio in the deployed state can range between about 25% to about 80%.
It has been discovered that a membrane of uniform porosity can be effective to promote healing of an aneurysm if the material ratio of the porous section of the membrane is within a certain range when the membrane is in the deployed state. Thus, in some embodiments the material ratio of the porous section of the membrane is preferably in a range between about 70% to 80%, with the optimal material ratio considered to be about 75%, when the membrane is deployed. Uniformity is achieved by maintaining the variance in the size of pores, as well as the spacing between pores in a porous section of the membrane, while an optimal material ratio is achieved on the basis of particular pore diameters and spacing.
The porous section can also be conceptually divided into a number (n) of porous regions, wherein the area of each of the n regions is substantially 1/n of the total area of the porous section of the membrane. For example, in some embodiments, there can be 2, 3, 4, 5 or more porous regions, where each of the regions has substantially the same porosity as each of the other porous regions existing with the porous section of the membrane. The porosity of either a region or the porous section as a whole is determined by the combination of pore size and pore spacing.
While the interpore size variance will be substantially uniform over the area of a porous section within each individual membrane, it is to be recognized that it is possible to provide different membranes with different numbers of pores, or different pore spacing as a way in which to provide a set of membranes of varying porosity. In this way it is possible to have a set of membranes with a range of porosities, any one of which can be chosen based on the requirement in a particular application. Thus depending on a variety of factors, a membrane could be produced with properties that would make it particularly well-suited for use in aiding in the stabilization and repair of a particular vessel, while for another application a membrane of a different porosity might be preferable, and could be fashioned accordingly.
Porosity of the membrane is considered optimal when the membrane permits blood supply to perforators of main arteries while reducing blood circulation to the diseased, damaged or weakened portion of the vessel wall being repaired. In addition, a further benefit may be realized by selecting a membrane having a porosity that enables enhanced endothelial cell migration and tissue ingrowth for faster endothelialization. The membrane as disclosed may be used in devices designed for a variety of vessel repair applications other than aneurysms. These may include, but are not limited to, use in the treatment of ischemic stroke, carotid artery stenosis, saphenous vein graft, small vessel stenosis, or renal artery repair.
As indicated above, part of the novelty described in the present disclosure lies in the discovery that a stent having a uniformly porous membrane is capable of supporting a vessel wall at the site of an aneurysm, maintaining the patency of parent and perforator vessels, while restricting blood flow to the aneurysm itself. In prior art devices these functionalities were achieved using membranes with non-uniform porosity, or regions of varying porosity. By providing these features the device promotes more rapid and more effective healing of an aneurysm, while at the same time providing a device that is more universally adaptable for use in a wider variety of in vivo locations than previously possible, and simpler to manufacture and use.
This has been confirmed experimentally in an animal aneurysm model. In this model system, aneurysms are induced by infusion of elastase into the lumen of a vessel by way of a catheter, as diagrammed in
In the illustrated experiment, a stent was deployed at the site of the aneurysm shown in
Results from a series of studies like these have suggested that the material ratio of the membrane for optimal efficacy should be about 75%, or at least in the range of about 70-80%. In order to achieve this optimal porosity, several factors are considered. For example, the size as manufactured relative to the deployed size will be important, as the change in pore area occurs at a different rate than does the overall area of the membrane.
The material ratio has therefore been determined for membranes of varying pore diameter, pore spacing, and degree of expansion from the manufactured size to various deployment sizes, in order to evaluate what pore spacing and pore size can provide a material ratio in the range of about 70-80%, at deployed sizes ranging from 2.5-5.0 mm. In the examples described, material ratio in the unexpanded state ranged from 86-96% depending on the pore size and spacing. To determine the material ratio in the expanded state, membranes were expanded as they would be during deployment, and the pore diameter measured at selected areas. The material ratio was then determined as follows:
A=total area of porous section of membrane; P=total area of pores; Porosity=(P÷A)×100%; Material Ratio=(1−(P÷A))×100%
In the data shown in Table 2, two membranes having porous sections with different pore size and pore spacing were evaluated. Macroporous 30/70 (30/70 membrane) refers to a membrane manufactured with 30 μm pores with an interpore spacing of 70 μm in the unexpanded state; likewise, Macroporous 40/60 (40/60 membrane) refers to a membrane with 40 μm pores and an interpore spacing of 60 μm, again, in the unexpanded state.
As the data in Table 2 shows, when a membrane is expanded from its manufactured size (here 2.0 mm) to various deployed sizes, ranging from 2.5 to 4.0 mm, the material ratio decreases. Thus, depending on the initial pore size and density, the optimal material ratio of about 70-80% will be achieved at different degrees of expansion, analogous to the various deployment diameters of the stent being covered by the membrane.
For example, in a 30/70 membrane, material ratios within the optimal desired range of about 70-80% are substantially achieved at deployment diameters of about 3.0 to about 4.0 mm, when starting with a manufactured size of 2.0 mm. For a 40/60 membrane the optimal material ratio is achieved at a point between 2.0 to 2.5 mm, up to about 3.0 to 3.5 mm.
By extending this analysis it is possible to determine the number of different stent pore patterns, the pattern being the combination of pore size and interpore spacing, necessary to provide about a 70-80% material ratio over wide range of stent diameters. The goal is to know beforehand, the combination of pore size and spacing that, when the membrane is expanded to its deployed size, will provide a material ratio within the desired range of about 70-80% and preferably about 75%.
For example, the calculations in Table 3 show that with three different membrane patterns, it is possible to achieve a material ratio in the range of about 70-80% using a stent with a manufactured size of 2.2 mm, expanded to deployment sizes ranging from 2.5-5.0 mm. In these cases, the material ratio of the membrane in the unexpanded state ranges from 86-96%.
These results are further exemplified in
As shown in
In practice, and as shown in
For example, consider an aneurysm 50 with aneurysm neck diameter of about 6 mm and height of about 10 mm. If the aneurysm neck is covered by a 25% material ratio macro-porous device 10, a reduction of 25% blood flow into the aneurysm sac is possible, with higher material ratios, for example 70-80%, or preferable 75%, even greater inhibition of blood flow to the aneurysm is achieved. It is expected that the percentage reduction in blood flow will exceed the simple percentage material ratio due to the viscosity of blood, as well as further reduction of blood flow due to flow disruption and dispersion. The geometry of the aneurysm can also play a role in the effectiveness and operation of the device.
Lubricious Coating for Membrane and/or Stent
A lubricious layer can be optionally applied onto the outer surface of the stent to improve trackability during delivery of the device to the surgical site. This coating may be applied after the device is fabricated and placed onto a delivery system or before placement onto the delivery system. Alternatively, this layer may be introduced in combination with the membrane material as an additional surface property, by modifying the chemical structure or surface properties of the device to provide a device with a low surface coefficient of friction.
In some embodiments, the membrane polymer may further comprise surface-modifying end groups such as those disclosed in U.S. Pat. No. 5,589,563 (Ward & White), the entire contents of which are hereby incorporated by reference.
Lubricious coatings have been used previously in devices designed to access target sites in small vessels. For example, U.S. Pat. No. 5,312,356 (Engelson et al.) discloses a catheter comprising anti-friction materials to prevent an internal guide wire from sticking against the internal tubular surface of the catheter.
The lubricious layer may be made from, for example, hydrophilic polyvinylpyrrolidone (PVP), and hydrophilic polymers like polyacrylate or polymethacrylate, as well as hydrogels like polyethylene oxide (PEO) may also be used. Gelatin may also be used. Preferably the layer is also biocompatible. It is also desirable that the layer have the optimal balance of stability and durability to maintain integrity during tracking.
Chemical Properties of the Membrane
The membrane is preferably made from biocompatible, highly elastomeric polymer. Polyether urethane (PEU) or polycarbonate urethane (PCU) may be used.
Trade names for PEU include Tecoflex, Tecothane, Hapflex, Cardiothane, Pellethane, and Biospan. Trade names for PCU include ChronoFlex, Carbothane, and Corethane.
In some embodiments the membrane is made from BioSpan F, a material developed by Polymer Technology Group (PTG), Berkeley, Calif., USA. BioSpan F is a polyurethane based material with fluorocarbon surface-modified end groups. In studies performed both in vitro and in vivo, this material has been shown to possess excellent compatibility properties matching the environment of small blood vessels. The selection of BioSpan F for the membrane of the device in treating small vessels is preferred due to resistance to thrombogenesis as compared with PET or e-PTFE membranes. Preferably, the membrane fashioned from BioSpan F will include a specific pore pattern as described earlier to obtain better resolution and healing of the aneurysm.
Table 4 shows initial results from in vitro biocompatibility tests comparing three materials; BioSpan, BioSpan F, and ePTFE. As can be seen, BioSpan F was the least thrombogenic of the three. The results of animal studies, shown in
Additional animal studies, shown in
In some embodiments, membranes can be fashioned from materials of the BioSpan family using the same surface modifying end group technique, but with application of different end groups. BioSpan PS, for example, is a surface modified material with PEO and silicon end groups.
Membranes With Permanently-Attached Agents
In some embodiments, one of which is illustrated in
The cell function being regulated can include, but is not limited to, proliferation, migration, maturation, and apoptosis. The desired target component 30 can include, but is not limited to, an endothelial progenitor cell, in which case the signal agent 22 could up regulate the rate of endothelialization, and reduce the time for inflammation and thrombosis. Conveniently it is possible to combine a membrane having uniform porosity, with one comprising agents, 1020, 1021, 1022, permanently attached to the membrane. A membrane configured in this way would thus be adapted to substantially prevent blood flow to an aneurysm, while maintaining blood flow to perforators, and in addition could provided various agents that would enhance the process of healing the aneurysm.
The pharmaceutical agents 1020, 1021, 1002, coated on the lumen side of the membrane 1015, prevent the occlusion of the original patent lumen. In some embodiments, the capture and agent 1021 is arranged in a first conformation of a single arm structure made of an organic linker anchored to the membrane 1015. The organic linker may be a short chain of organic molecules anchored on one end to the membrane 1015, and the other end bound to the agent molecule that captures specific endothelial cells from the blood to promote endothelialization. The capture and signal agents 1020, 1021, 1022 are arranged in a second conformation of a branched structure made up of an organic linker anchored to the membrane 1015. The capture agent 1021 specifically captures endothelial progenitor cells similar to the other capture agent 1020, while a signal agent 1022 enhances endothelial cell alignment and proliferation. Alternatively, the signal agent 1022 is arranged in a first conformation of a single arm structure made up of an organic linker anchored to the membrane 1015.
On the vessel wall side of the membrane 1015, a third pharmaceutical agent 1023 is permanently attached to the vessel wall surface of the membrane 1015 to enhance healing of the vessel wall 1005 from injury after the stent 1011 is deployed. Alternatively, the agents on the vessel wall side of the membrane 1015 also encourage proliferation of vessel wall components, for example, intima, elastic lamina, for enhancing the healing of the weakened, damaged or diseased portion of the vessel wall, for example, the aneurysm neck.
The agents can be effective to reduce, minimize, or prevent, immune reactions to foreign bodies. In some embodiments, agents can be effective to attract and capture endothelial cells, or endothelial progenitor cells, to aid in the formation of a healthy endothelium in the region of the aneurysm being treated. The lumen side of the membrane can be configured to generally discourage factors that are involved in thrombosis.
The capture and signal agents 1021, 1022, can include, but are not limited to, enzyme regulators tagged with antibodies or peptides, ceramides like L-PDMP, peptides, antibodies, naturally occurring molecules, and synthetic molecules, a nucleic acid, or even a polynucleotide, if desired. Specifically, the signal agent 1022 can be an endothelial cell specific L-PDMP or an smooth muscle cell-specific D-PDMP, that can bind specifically to target molecules on endothelial cells or progenitors. Peptide or antibodies have high binding affinity and specificity for endothelial cells and progenitors. Naturally occurring molecules (pure or synthesized) can mimic part of the basal lamina of the endothelium, so that endothelial cells or progenitors will preferentially bind and orient on the membrane. For example, laminin-mimetic pentapeptide immobilized on the lumen surface can be effective as a capture agent. The choice of capture agent is not considered to be a limitation of the disclosure. A number of molecules or moieties will be useful in preventing blood flow to an aneurysm, while maintaining flow to perforators, and which will promote healing and/or endothelialization, while reducing the risk of thrombosis or other injury to the vessel being treated are considered to be within the scope of the disclosure.
The signal agent 1022 can also be an anti-inflammatory agent in order to reduce recruitment and infiltration of white blood cells. Thus, through the choice of various signal agents it is possible to enhance attachment of endothelial cells to the membrane, while minimizing the inflammatory response. The capture agent 1021 and signal agent 1022 thus act cooperatively to increase the rate of endothelialization and decrease the during which thrombosis and restenosis might occur after the stent is expanded.
As shown in
In
In
Due to the particular porosity and dimensions of the membrane 203, blood circulation to the aneurysm 201 is obstructed while blood supply to perforators and microscopic branches of main brain arteries as well as larger arteries is permitted. As described earlier, obstructing blood supply to the aneurysm 201 isolates the aneurysm 201 from normal blood circulation. The aneurysm in effect “dries out.” The stent 202 and membrane 203 thus treats the aneurysm 201 by altering the hemodynamics in the aneurysm sac such that intra-aneurysmal thrombosis is initiated. At the same, blood flow into the arteries (branch, main, big or small) are not significantly affected by the implantation of the stent 202 or the membrane 203 due to the special porosity of the membrane 203. Although a bifurcation aneurysm has been described, it is envisaged that the stent 202 may be used to treat a trituration aneurysm, or other aneurysms, in a similar manner.
It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the specific embodiments disclosed herein, without departing from the scope or spirit of the disclosure as broadly described. The present embodiments are, therefore, to be considered in all respects illustrative and not restrictive of the invention, which is defined by the claims as presented herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| SG200401735-6 | Mar 2004 | SG | national |
| SG200508026-2 | Dec 2005 | SG | national |
The present application is a continuation-in-part of U.S. patent application Ser. No. 11/586,899, filed Oct. 25, 2006, which is a continuation-in-part of U.S. patent application Ser. No. 10/580,139, filed May 19, 2006, which is filed under 35 U.S.C. § 371 as a U.S. National Stage Application of PCT International Patent Application No. PCT/SG2004/000407, filed Dec. 13, 2004, which claims priority to Singapore Patent Application No. SG200401735-6, filed Mar. 31, 2004; the present application is also a continuation-in-part of U.S. patent application Ser.No. 11/637,188, filed Dec. 12, 2006, which claims priority to Singapore Patent Application SG200508026-2, filed Dec. 13, 2005; the present application is also a continuation-in-part of PCT International Patent Application No. PCT/SG2006/000028, filed Feb. 13, 2006; the contents of each of the aforementioned applications are hereby incorporated by reference herein in their entireties.
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| Number | Date | Country | |
|---|---|---|---|
| 20070255388 A1 | Nov 2007 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11586899 | Oct 2006 | US |
| Child | 11786023 | US | |
| Parent | 10580139 | US | |
| Child | 11586899 | US | |
| Parent | 11786023 | US | |
| Child | 11586899 | US | |
| Parent | 11637188 | Dec 2006 | US |
| Child | 11786023 | US | |
| Parent | PCT/SG2006/000028 | Feb 2006 | US |
| Child | 11637188 | US |
