referring to fig1 - 4 , a vaso - occlusive device 10 , in accordance with a preferred embodiment of the invention , comprises three elongate , coaxial elements : an inner element 11 , a non - metallic intermediate element 12 , and a non - expansile outer element 13 that covers at least a portion of the intermediate element . the intermediate element 12 is in intimate contact with both the inner element 11 and the outer element 13 . the inner element 11 is formed of a flexible , elongate filament or wire that is preferably made of a material that allows visualization under various medical imaging means , such as x - ray , mri , or ultrasound . preferably , the inner element 11 is formed from a length of wire made of any of various biocompatible , radiopaque metals , such as platinum , tantalum , tungsten , gold , titanium , nitinol , stainless steel , elgiloy ( cobalt - chromium - nickel ), or other suitable alloys known in the art . alternatively , it can be made from or include non - metallic materials , such as polymers , collagen , proteins , drugs , biologic materials ( e . g ., cellular material and genes ), bioactive agents , therapeutic compounds , or combinations of these materials . if made of a non - radiopaque material , it should advantageously be doped or impregnated or chemically modified to be visible with one or more imaging techniques . alternatively , it can be made of a material that is highly visible by means of mri or ultrasound . the inner element 11 can be formed in various configurations , including , but not limited to , coils , rods , tubes , cables , braids , cut tubes , or other elongate , flexible forms . as shown , it is in the form of a helical coil , which may be preferred . in one specific embodiment , it is formed at least in part of a multi - filar coil configuration , as described in the co - owned us patent application publication no . 2004 / 0006362 , the disclosure of which is incorporated herein by reference . the intermediate element 12 may be formed as a coating , wrapping , tubular sleeve , or other construction to create a substantially continuous surface coaxially around the inner element 11 . alternatively , it can be formed into a cylinder and then skewered onto the inner core element 11 , as described in the co - owned and co - pending us patent application publication no . 2002 / 0177855 , the disclosure of which is incorporated herein by reference . the intermediate element 12 preferably covers all of the length of the inner element 11 , except for short proximal and distal sections . the intermediate element 12 may be made of any of various suitable , substantially non - metallic , biocompatible materials , including polymers , biopolymers , biologic materials , and combinations of these materials . suitable polymers include cellulose , polypropylene , polyvinylpyrrolidone , polyacrylics , polylactides , polyamides , polyvinyl alcohol ( pva ), polyester , polyurethane , polyglycolic acid , polyfluorocarbons ( e . g ., ptfe ), nylon , polymethylmethacrylate ( pmma ), hydrogels , and silicones . exemplary biologic materials include alginates , hyaluronic acid , fibrin , collagen and silk . optionally , the intermediate element 12 can be impregnated , grafted , bound , or modified to deliver therapeutic compounds , drugs , collagen , proteins , genes , bioactive agents , or cellular material . see , e . g ., u . s . pat . no . 5 , 658 , 308 and international publications nos . wo 99 / 65401 and wo 00 / 27445 , the disclosures of which are incorporated herein by reference . in one preferred embodiment , the intermediate element 12 is made of a state - of - the - art bioabsorbable or biodegradable polymer , such as , for example , those described in us patent application publications nos . 2002 / 0040239 and 2002 / 0020417 , the disclosures of which are incorporated herein by reference . in another preferred embodiment , the intermediate element 12 is made of a soft conformal material , and more preferably of an expansile material such as a hydrogel . the most preferred material is an environmentally responsive hydrogel , such as that described in us patent application publication no . 2002 / 0176880 , the disclosure of which is incorporated herein by reference . specifically , the hydrogels described in us patent application publication no . 2002 / 0176880 are of a type that undergoes controlled volumetric expansion in response to changes in such environmental parameters as ph or temperature . these hydrogels are prepared by forming a liquid mixture that contains ( a ) at least one monomer and / or polymer , at least a portion of which is sensitive to changes in an environmental parameter ; ( b ) a cross - linking agent ; and ( c ) a polymerization initiator . if desired , a porosigen ( e . g ., nacl , ice crystals , or sucrose ) may be added to the mixture , and then removed from the resultant solid hydrogel to provide a hydrogel with sufficient porosity to permit cellular ingrowth . the controlled rate of expansion is provided through the incorporation of ethylenically unsaturated monomers with ionizable functional groups ( e . g ., amines , carboxylic acids ). for example , if acrylic acid is incorporated into the crosslinked network , the hydrogel is incubated in a low ph solution to protonate the carboxylic acids . after the excess low ph solution is rinsed away and the hydrogel dried , the hydrogel can be introduced through a microcatheter filled with saline at physiological ph or with blood . the hydrogel cannot expand until the carboxylic acid groups deprotonate . conversely , if an amine - containing monomer is incorporated into the crosslinked network , the hydrogel is incubated in a high ph solution to deprotonate amines . after the excess high ph solution is rinsed away and the hydrogel dried , the hydrogel can be introduced through a microcatheter filled with saline at physiological ph or with blood . the hydrogel cannot expand until the amine groups protonate . more specifically , in a preferred formulation of the hydrogel , the monomer solution is comprised of ethylenically unsaturated monomers , an ethylenically unsaturated crosslinking agent , a porosigen , and a solvent . at least a portion , preferably 10 %- 50 %, and more preferably 10 %- 30 %, of the monomers selected must be ph sensitive . the preferred ph sensitive monomer is acrylic acid . methacrylic acid and derivatives of both acids will also impart ph sensitivity . since the mechanical properties of hydrogels prepared exclusively with these acids are poor , a monomer to provide additional mechanical properties should be selected . a preferred monomer for providing mechanical properties is acrylamide , which may be used in combination with one or more of the above - mentioned ph sensitive monomers to impart additional compressive strength or other mechanical properties . preferred concentrations of the monomers in the solvent range from 20 % w / w to 30 % w / w . the crosslinking agent can be any multifunctional ethylenically unsaturated compound , preferably n , n ′- methylenebisacrylamide . if biodegradation of the hydrogel material is desired , a biodegradable crosslinking agent should be selected . the concentrations of the crosslinking agent in the solvent should be less than about 1 % w / w , and preferably less than about 0 . 1 % w / w . the porosity of the hydrogel material is provided by a supersaturated suspension of a porosigen in the monomer solution . a porosigen that is not soluble in the monomer solution , but is soluble in the washing solution can also be used . sodium chloride is the preferred porosigen , but potassium chloride , ice , sucrose , and sodium bicarbonate can also be used . it is preferred to control the particle size of the porosigen to less than about 25 microns , more preferably less than about 10 microns . the small particle size aids in the suspension of the porosigen in the solvent . preferred concentrations of the porosigen range from about 5 % w / w to about 50 % w / w , more preferably about 10 % w / w to about 20 % w / w , in the monomer solution . alternatively , the porosigen can be omitted and a non - porous hydrogel can be fabricated . the solvent , if necessary , is selected based on the solubilities of the monomers , crosslinking agent , and porosigen . if a liquid monomer ( e . g . 2 - hydroxyethyl methacrylate ) is used , a solvent is not necessary . a preferred solvent is water , but ethyl alcohol can also be used . preferred concentrations of the solvent range from about 20 % w / w to about 80 % w / w , more preferably about 50 % w / w to about 80 % w / w . the crosslink density substantially affects the mechanical properties of these hydrogel materials . the crosslink density ( and hence the mechanical properties ) can best be manipulated through changes in the monomer concentration , crosslinking agent concentration , and solvent concentration . the crosslinking of the monomer can be achieved through reduction - oxidation , radiation , and heat . radiation crosslinking of the monomer solution can be achieved with ultraviolet light and visible light with suitable initiators or ionizing radiation ( e . g . electron beam or gamma ray ) without initiators . a preferred type of crosslinking initiator is one that acts via reduction - oxidation . specific examples of such red / ox initiators that may be used in this embodiment of the invention are ammonium persulfate and n , n , n ′, n ′- tetramethylethylenediamine . after the polymerization is complete , the hydrogen is washed with water , alcohol or other suitable washing solution ( s ) to remove the porosigen ( s ), any unreacted , residual monomer ( s ) and any unincorporated oligomers . preferably this is accomplished by initially washing the hydrogel in distilled water . as discussed above , the control of the expansion rate of the hydrogel is achieved through the protonation / deprotonation of ionizable functional groups present on the hydrogel network . once the hydrogel has been prepared and the excess monomer and porosigen have been washed away , the steps to control the rate of expansion can be performed . in embodiments where ph sensitive monomers with carboxylic acid groups have been incorporated into the hydrogel network , the hydrogel is incubated in a low ph solution . the free protons in the solution protonate the carboxylic acid groups on the hydrogel network . the duration and temperature of the incubation and the ph of the solution influence the amount of control on the expansion rate . generally , the duration and temperature of the incubation are directly proportional to the amount of expansion control , while the solution ph is inversely proportional . it has been determined that the water content of the treating solution also affects the expansion control . in this regard , the hydrogel is able to expand more in the treating solution and it is presumed that an increased number of carboxylic acid groups are available for protonation . an optimization of water content and ph is required for maximum control on the expansion rate . after the incubation is concluded , the excess treating solution is washed away and the hydrogel material is dried . the hydrogel treated with the low ph solution has been observed to dry down to a smaller dimension than the untreated hydrogel . this is a desired effect since delivery of these hydrogel materials through a microcatheter is desired . if ph sensitive monomers with amine groups were incorporated into the hydrogel network , the hydrogel is incubated in high ph solution . deprotonation occurs on the amine groups of the hydrogel network at high ph . the duration and temperature of the incubation , and the ph of the solution , influence the amount of control on the expansion rate . generally , the duration , temperature , and solution ph of the incubation are directly proportional to the amount of expansion control . after the incubation is concluded , the excess treating solution is washed away and the hydrogel material is dried . for the embodiment of the vaso - occlusive device having an intermediate element formed of an expansile polymeric hydrogel , when the intermediate element 12 expands , the areas of the soft , conformal intermediate element 12 that are not covered or constrained by the outer element 13 extend radially outward through the openings or gaps , or between the coils of the outer element 13 ( as described below ) to form an undulating outer surface comprising a chain of arcuate segments , as a result of the constraint imposed by the outer element 13 . because the arcuate segments of the undulating outer surface contact the interior wall surface of a microcatheter through which the device is deployed only at or near tangential contact points proximate the apex of each segment , this undulating or arcuate configuration provides reduced friction as compared to a continuous or smooth surface of the same material . the outer element 13 is a flexible , elongate , substantially tubular member , at least a substantial portion of the length of which , and preferably most of the length of which , includes or defines at least one opening or gap to allow the exposure and / or protrusion of the intermediate element 12 . suitable configurations for the outer element 13 include helical coils , braids , and slotted or spiral - cut tubes . the outer element 13 may be made of any suitable biocompatible metal or polymer , including those listed above for the inner element 11 . for those embodiments using a soft , conformal intermediate element 12 , the outer element 13 should have sufficient radial strength to compress or restrain the intermediate element 12 . preferably , the openings in the outer element 13 are such that , along a substantial portion of the length of the device , the open area of the outer element 13 is at least about 20 %, and more preferably , more than about 40 %, of the total external surface area of the device between the proximal and distal limits of that portion of the device length . for example , near the ends of the device , the outer element 13 may have a relatively low percentage of open area ( i . e ., less than about 20 %), while for the greater part of the length of the device between the end portions thereof , a greater percentage ( i . e ., at least about 20 %, and preferably more than about 40 %) of the outer element 13 may be open , allowing a greater exposure of the intermediate element 12 through the outer element 13 . preferably , the portion of the outer element 13 covering at least about 75 % of the overall length of the device will have the greater percentage of open area . in the most preferred embodiment , the device comprises an inner element 11 formed of a tightly - wound (“ close - wound ”) helical coil of a biocompatible metal wire ( e . g ., platinum alloy ), an intermediate element 12 of a hydrophilic expansile polymer ( e . g ., hydrogel ), and an outer element 13 in the form of a biocompatible metal or polymer helical coil that is open - wound for most of its length , with a close - wound proximal end section 14 and a close - wound distal end section 15 . the open - wound portion of the outer element 13 defines a single , continuous , helical opening or gap . the coil is preferably made from a wire that has a diameter of no more than about 0 . 15 mm . the pitch of the coil of which the outer element 13 is comprised may be up to ten times the diameter of filament from which the coil is wound , and preferably between about 5 percent and about 100 percent greater than the diameter of the filament . a coupling element 16 is advantageously attached to the proximal end of the inner element 11 for detachable attachment to a deployment device ( not shown ). a rounded obturator tip 17 may be attached to the distal end of the inner element 11 . in the above - described most preferred embodiment , the hydrogel of the intermediate element 12 expands or swells upon exposure to an aqueous environment ( e . g ., blood ). preferably , the hydrogel expands to between about two times and about 20 times its original volume . as shown in fig3 and 4 , the swollen or expanded intermediate element 12 protrudes through the helical opening or gap defined between the coils of the open - wound section of the outer element 13 to form an undulating , convexly - curved surface defining a chain of arcuate or rounded segments , each having a diameter that is substantially greater than the diameter of the outer element 13 . the helical outer element 13 described above may be considered as defining a single , helical opening or gap , or it may be viewed as defining a plurality of connected openings or gaps , each defined between an adjacent pair of windings of the coil of the outer element 13 . alternatively , if the outer element 13 is formed as a slotted tube , for example , the outer element 13 will be seen to define a plurality of discrete openings or gaps in its axial middle section that are functionally equivalent to the helical opening ( s ) defined in the illustrated embodiment . the device 10 can be constructed with various radial thickness of each coaxial element to provide different handling characteristics . preferably , the inner element 11 has a diameter of between about 0 . 075 mm and 0 . 75 mm ; the intermediate element 12 has a thickness of between about 0 . 025 mm and 1 . 00 mm ; and the outer element 13 has a thickness of between about 0 . 025 mm and 0 . 25 mm . for the embodiments that use an expansile intermediate element 12 , these thicknesses are measured in the non - expanded state . preferably , the outer diameter of the outer element 13 is actually somewhat less than the expanded or swollen diameter of the intermediate element 12 , so that the latter will readily expand through the openings or gaps in the outer element 13 . fig5 and 6 illustrate variants of the preferred embodiment of the invention , with multilayer structures for the inner element . specifically , fig5 shows a first variant having an inner element comprising first and second coaxial coils 110 a , 110 b , respectively . fig6 shows a second variant having an inner element comprising first , second , and third coaxial coils 210 a , 210 b , and 210 c , respectively . suitable coaxial coil structures and their methods of manufacture are disclosed in commonly owned and co - pending us patent application publication no . 2004 / 0006363 , the disclosure of which is incorporated herein by reference . the inner element may , in fact , comprise four or more coaxial layers . similarly , the outer element 13 may comprise multiple coaxial helical coil layers , provided that a suitable percentage of the surface area of the outer element remains open for the exposure of the intermediate element , as explained above . fig7 illustrates a third variant of the present invention , in which the inner element comprises a helical coil 310 defining an axial lumen , at least a substantial portion of the length of which is filled with a solid core member 320 . the solid core member 320 may be impregnated with a therapeutic agent that can be absorbed into the bloodstream . fig8 illustrates a fourth variant of the invention , in which the inner element comprises a helical coil 410 defining an axial lumen , at least a substantial portion of which contains a hollow , tubular core member 420 . the tubular core member 340 provides additional strength , and it may be filled with a liquid therapeutic agent ( not shown ). fig9 shows a vaso - occlusive device 10 ′ in accordance with an alternative embodiment of the invention . this embodiment includes an outer element 13 ′ with a distal section 15 ′ that is not close wound , but is , instead , made with small gaps of approximately 5 % to 100 % of the diameter of the wire or filament of which the outer element 13 ′ is made . these gaps make the distal section 15 ′ of the device 10 ′ more flexible in the area where the outer element 13 ′ overlaps the inner element 11 ′. in the embodiment shown in fig9 , the proximal ends of both the inner element 11 ′ and the outer element 13 ′ are both advantageously attached to a coupling element 16 ′ by soldering or welding . the attachment of both the inner element 11 ′ and the outer element 13 ′ to the coupling element 16 ′ makes the proximal end of the device 10 ′ more resistant to deformation during deployment and implantation . in any of the embodiments described above , a portion of the outer element and / or a portion of the intermediate element may be modified ( as by coating , for example ) to include structure that promotes the adhesion of beneficial cells or growth factors . an exemplary coating that be used for this purpose is disclosed in us patent application publication no . 2002 / 0049495 , the disclosure of which is incorporated herein by reference . as indicated above , the present invention provides good trackability in a microcatheter . in other words , it is easily advanced through a catheter without binding against or moving the catheter . this advantage is achieved through reduced friction and reduced buckling at the ends of the device . the force required to advance the device through a typical microcatheter would normally be less than about 0 . 7 lbs . the device is preferably detachable from a flexible , elongate delivery apparatus ( not shown ), such as a wire , a pusher tube , or the like . exemplary detachment systems known in the art include electrolytic , mechanical , electromechanical , thermal , ultrasonic , and hydraulic detachment mechanisms . the device may be formed into a secondary configuration , such as a helical coil , a sphere , an ovoid , or any other suitable two - or three - dimensional shape known in the art of vaso - occlusive devices . alternatively , the device can be left in a relatively straight configuration with or without a curvature at the end ( such as a “ j ” configuration ). in any of the above - described embodiments , either the inner element or the intermediate element , or both , may be formed so as to be substantially stretch - resistant , so as to limit the stretching of the entire device ( including the outer element ) if the device needs to be partially withdrawn for repositioning or the like . it will be appreciated that a number of the materials discussed above will be stretch - resistant to varying degrees . the device is useful for the occlusion and / or embolization of blood vessels , other vascular spaces such as aneurysms , and other tubular or saccular organs or spaces throughout the body . specific applications where it may be useful include the occlusion of cerebral aneurysms , aortic aneurysms , fistulas , fallopian tubes , cardiac septal defects , patent foramen ovale , and the left atrial appendage of the heart . for some of these applications , it may be preferable to use devices with dimensions larger than those specified above . although preferred embodiments of the invention have been described in this specification and the accompanying drawings , it will be appreciated that a number of variations and modifications may suggest themselves to those skilled in the pertinent arts . thus , the scope of the present invention is not limited to the specific embodiments and examples described herein , but should be deemed to encompass alternative embodiments and equivalents , as determined by a fair reading of the claims that follow .
a vaso - occlusive device includes inner , intermediate , and outer elements arranged coaxially . the inner element is a filamentous element , preferably a microcoil . the intermediate element is made of a non - metallic material , preferably an expansile polymer . the outer element is substantially non - expansile and defines at least one gap or opening through which the intermediate element is exposed . in a preferred embodiment , when the intermediate element is expanded , it protrudes through the at least one gap or opening in the outer element and assumes a configuration with an undulating , convexly - curved outer surface defining a chain of arcuate segments , each having a diameter significantly greater than the diameter of the outer element . the expanded configuration of the intermediate element minimizes friction when the device is deployed through a microcatheter , thereby reducing the likelihood of buckling while maintaining excellent flexibility . the result is a device with enhanced pushability and trackability when deployed through a microcatheter .