specific embodiments of the present invention are now described with reference to the figures , wherein like reference numbers indicate identical or functionally similar elements . the terms “ distal ” and “ proximal ” are used in the following description with respect to a position or direction relative to the treating clinician . “ distal ” or “ distally ” are a position distant from or in a direction away from the clinician . “ proximal ” and “ proximally ” are a position near or in a direction toward the clinician . fig1 and 2 illustrate a side view of a guidewire 100 in accordance with an embodiment of the present invention . guidewire 100 includes an elongate shaft 102 formed from an appropriate material , such as stainless steel , nitinol , an alloy of tungsten - rhenium , or , a work - hardenable cobalt chromium superalloy such as 35nlt . shaft 102 has a proximal end 104 , a distal end 205 , and a distal region 106 . proximal end 104 of shaft 102 may be provided with a tubular socket ( not shown ), or may be otherwise adapted for connection with a guidewire extension , as such guidewire extension systems would be apparent to one of ordinary skill in the art . a flexible helical coil 108 surrounds distal region 106 of shaft 102 . as would be apparent to one of ordinary skill in the relevant art , coil 108 may comprise a flexible tubular sheath instead of , or in combination with a coiled filament . a proximal end of coil 108 may be attached , for example by soldering , brazing , or by adhesive at a proximal end of distal segment 106 and may also be attached at a point or points along its length within distal segment 106 . a distal end of coil 108 is secured to distal end 205 of shaft 102 within a hemispherical tip weld 217 . as would be apparent to one of ordinary skill in the art , tip weld 217 may comprise a weld with or without added filler material , or a joint including braze , solder or adhesive . in an embodiment , flexible coil 108 may be formed from a radiopaque alloy , such as a stainless steel - platinum , gold - platinum or a platinum - tungsten alloy . distal region 106 of shaft 102 may include a continuous or stepped taper , for example , as disclosed in u . s . pat . no . 4 , 922 , 924 to gambale et al ., which is incorporated by reference herein in its entirety . as illustrated in the embodiment of fig2 - 4 , distal region 106 of shaft 102 includes a tapered core wire 110 having a distal tip segment 212 . core wire tip segment 212 is provided with proximal and distal flat drops 314 , 316 , which are axially - spaced flattened portions of core wire 110 that are wider than an adjacent outer diameter of core wire 110 . as mentioned above , distal end 205 of core wire 110 is fixed within tip weld 217 at a distal tip of coil 108 . in another embodiment , distal flat drop 316 of core wire 110 may extend to and be fixed within tip weld 217 . elongate shaft 102 may be a unitary shaft from proximal end 104 to distal end 205 , wherein distal region 106 of shaft 102 undergoes a centerless grinding process to fabricate reduced - diameter core wire portion 110 . various centerless grinding steps may be implemented to achieve a stepped - down taper in core wire portion 110 to thereby incrementally increase its flexibility as it extends distally . in an alternate embodiment , shaft 102 may have a constant diameter proximal shaft region of a harder material , such as cobalt chromium superalloy , stainless steel or titanium , which is connected to a reduced or tapered diameter distal region of a softer more flexible material , such as a softer grade of stainless steel or nitinol . in such an embodiment , the proximal and distal shaft regions may be joined by a coupling sleeve , a weld or solder as would be apparent to one of ordinary skill in the art . in an embodiment , the proximal shaft region may be a hollow tube coupled at its distal end to a proximal end or the distal shaft region . in various embodiments , core wire flat drops 314 , 316 may be formed in succession , for example , by stamping a first flat drop , rotating and translating core wire 110 and then stamping the second flat drop . alternatively , one or more platens , or punch and die sets could be used to concurrently form one or more flat drops . with reference to fig5 and 6 , distal flat drop 316 has a first planar surface 518 substantially in parallel with an opposing second planar surface 520 . similarly , proximal flat drop 314 has a first planar surface 522 substantially in parallel with an opposing second planar surface 524 . in the embodiment shown in fig5 and 6 , planar surfaces 518 , 520 , 522 , 524 are substantially parallel to a core wire center axis l x . planar surfaces 518 , 520 of distal flat drop 316 are disposed substantially perpendicular to planar surfaces 522 , 524 of distal flat drop 314 . in other embodiments , planar surfaces 518 , 520 of distal flat drop 316 may be disposed at an angle of less than 90 ° to planar surfaces 522 , 524 of distal flat drop 314 . distal flat drop 316 is axially spaced from proximal flat drop 314 by a linking portion 526 , which may be frusto - conical or cylindrical in shape . length , width and thickness dimensions of proximal and distal flat drops 314 , 316 may be the same or different . in an embodiment , a thickness of proximal and / or distal flat drops 314 , 316 may taper in a distal direction . in another embodiment , distal flat drop 316 may be wider and / or thinner than proximal flat drop 314 . proximal flat drop 314 provides increased flexibility of distal tip section 212 for mono - axial bending in a z - direction l z perpendicular to the plane of flat drop 314 , as represented by dashed arc line a z in fig6 . similarly , distal flat drop 316 provides increased flexibility of distal tip section 212 for mono - axial bending in a y - direction l y perpendicular to the plane of flat drop 316 , as represented by dashed arc line a y in fig5 . making proximal flat drop 314 and nearby distal flat drop 316 in relatively perpendicular planes increases the bi - axial or omni - axial flexibility of distal tip section 212 of guidewire 100 , as compared to a guidewire distal tip segment having a single flat - drop or flat safety / shaping ribbon construction . the axial flexibility of distal tip section 212 may not be perfectly uniform in all directions , however the flexibility in the y - or z - directions l y , l z , may be only slightly lower than the flexibility in other directions . when distal tip segment 212 is bent in an axial direction other than the y - or z - directions l y , l z , the bending stress is divided between flat drops 314 , 316 such that each flat drop 314 , 316 resiliently bends to a degree that is substantially proportional to the extent that each flat drop is aligned with the bend . for example , if distal tip segment 212 is bent only slightly off - axis to y - direction l y , then distal flat drop 316 will accommodate most of such a deflection , and proximal flat - drop 314 will accommodate only a small portion of such a deflection . in another embodiment ( not shown ), an intravascular guidewire according to the present invention may include an elongate shaft having a core wire region with a distal tip segment that includes a plurality of flat drops spaced from each other by short frusto - conical or cylindrical linking portions . planar surfaces of adjacent flat drops are disposed along the distal tip segment at an angle to each other , such that the surfaces are not in the same plane . as in the previous embodiment , a flexible coil may surround and be attached to at least the core wire region of the guidewire shaft . further , the core wire region may include a planar surface of at least one flat drop that is substantially perpendicular to a planar surface of at least one other flat drop and / or one or more linking portions that are tapered . having a plurality of orthogonal or out - of - plane flat drops in accordance with the disclosure may provide good torque transmission from proximal end 104 to distal tip 205 of guidewire 100 , possibly due to the flat drops being axially separated by short linking portion ( s ) 526 . good torque transmission in core - to - tip construction may enhance the rotational steering or so - called steerability of steerable medical guidewires having small diameters of , e . g ., 0 . 46 mm ( 0 . 018 in ) or less . because the flat drops of the disclosure each tend to bend in a single direction , flat drops 314 , 316 may be compared to the orthogonal hinges in a conventional cardan or hooke &# 39 ; s driveshaft universal joint , although flat drops 314 , 316 and y - and z - directions l y , l z are distinctly not within the same plane . having a plurality of orthogonal or out - of - plane flat drops in accordance with the disclosure may also increase the rotational strain limit of guidewire 100 . such rotational strain limits are useful design measures for predicting and / or preventing material failure during clinical use , when the guidewire &# 39 ; s tip may be trapped while the clinician is rotating the guidewire in an attempt to steer it . a typical bench test for rotational strain limit involves clamping the guidewire distal end in a fixture , and counting the number of rotations of shaft proximal end 104 before material failure , which typically occurs adjacent the guidewire distal end , either in the core wire or in a safety / shaping ribbon , if the device is so equipped . the average number of turns - to - failure in examples made according to the disclosure have been found to exceed the number of turns - to - failure typical of core - to - tip guidewire constructions , and have approached the number of turns - to - failure typical of safety / shaping ribbon constructions . having a plurality of orthogonal or out - of - plane flat drops in accordance with this disclosure may also reduce the potential for vessel perforations with the distal end of guidewire 100 . as mentioned above , known core - to - tip constructions have tip stiffness suitable for crossing tight stenoses , but such tip stiffness may require additional care to avoid perforating a vessel wall when advancing the guidewire tip through undiseased sections of a patient &# 39 ; s vasculature . in comparison to known core - to - tip or single flat - drop constructions , the plurality of out - of - plane flat drops in accordance with this disclosure provide multiple locations for buckling or bending when the guidewire distal tip abuts an obstruction such as a vessel wall . thus , the instant guidewire disclosure provides embodiments having , in a single device , an improved combination of features not found in known guidewire designs having either core - to - tip , single flat drop , or ribbon - tip constructions . while various embodiments according to the present invention have been described above , it should be understood that they have been presented by way of illustration and example only , and not limitation . it will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention . thus , the breadth and scope of the present invention should not be limited by any of the above - described exemplary embodiments , but should be defined only in accordance with the appended claims and their equivalents . it will also be understood that each feature of each embodiment discussed herein , and of each reference cited herein , can be used in combination with the features of any other embodiment . all patents and publications discussed herein are incorporated by reference herein in their entirety .
a medical guidewire having a core - to - tip construction that includes a core wire region surrounded by a flexible coil . the core wire having a distal tip segment that includes a proximal flat drop axially separated from a distal flat drop by a cylindrical or frusto - conical linking portion . the proximal and distal flat drops each having a pair of parallel planar surfaces , wherein the planar surfaces of the proximal flat drop are at an angle to the planar surfaces of the distal flat drop . the tip construction provides improved flexibility while maintaining columnar strength and providing excellent torsional characteristics .