Temperature activated adhesive for releasably attaching stents to balloons

A delivery catheter, with or without an inflation balloon, and coated with a heat activated adhesive to secure a stent thereon is disclosed. The adhesive has a phase transformation temperature just above the temperature of human blood, so that below the transformation temperature the adhesive is tacky, and above the transformation temperature the adhesive is non-tacky. In a stenting procedure, when the stent is mounted to the balloon catheter and the catheter is introduced into a body lumen, the adhesive is below the transformation temperature and remains tacky to hold the stent to the catheter. Once the stent-catheter assembly is positioned at the deployment site, a warm saline or dye solution is injected to heat the adhesive to above the transformation temperature. The adhesive becomes non-tacky and releases the bond between the catheter and deployed stent allowing the former to be withdrawn.

BACKGROUND OF THE INVENTION 
The present invention relates to an apparatus for loading a tubular graft, 
such as a stent, onto a catheter assembly. Such a catheter assembly can 
be, for example, of the kind used in typical percutaneous transluminal 
coronary angioplasty (PTCA) procedures. 
In typical PTCA procedures, a guiding catheter is percutaneously introduced 
into the cardiovascular system of a patient through the brachial or 
femoral arteries and advanced through the vasculature until the distal end 
of the guiding catheter is in the ostium. A guide wire and a dilatation 
catheter having a balloon on the distal end are introduced through the 
guiding catheter with the guide wire sliding within the dilatation 
catheter. 
The guide wire is first advanced out of the guiding catheter into the 
patient's coronary vasculature and the dilatation catheter is advanced 
over the previously advanced guide wire until the dilatation balloon is 
properly positioned across the arterial lesion. Once in position across 
the lesion, a flexible and expandable balloon is inflated to a 
predetermined size with a radiopaque liquid at relatively high pressures 
to radially compress the atherosclerotic plaque of the lesion against the 
inside of the artery wall, thereby dilating the lumen of the artery. The 
balloon is then deflated to a small profile, so that the dilatation 
catheter can be withdrawn from the patient's vasculature and the blood 
flow resumed through the dilated artery. As should be appreciated by those 
skilled in the art, while the above-described procedure is typical, it is 
not the only method used in angioplasty. 
In angioplasty procedures of the kind referenced above, restenosis may 
occur in the artery, which may require another angioplasty procedure, a 
surgical bypass operation, or some other method of repairing or 
strengthening the area. To reduce the likelihood of restenosis and to 
strengthen the area, an intravascular stent is implanted for maintaining 
vascular patency. The stent is typically transported through the patient's 
vasculature where it has a small delivery diameter, and then is expanded 
to a larger diameter, often by the balloon portion of the catheter. The 
stent also may be of the self-expanding type. 
Since the catheter and stent will be traveling through the patient's 
vasculature, and probably through the coronary arteries, the stent must 
have a small, delivery diameter and must be firmly attached to the 
catheter until the physician is ready to implant it. Thus, the stent must 
be loaded onto the catheter so that it does not interfere with delivery, 
and it must not come off of the catheter until it is implanted in the 
artery. 
In conventional procedures where the stent is placed over the balloon 
portion of the catheter, it is necessary to crimp the stent onto the 
balloon portion to reduce its diameter and to prevent it from sliding off 
the catheter when the catheter is advanced through a patient's 
vasculature. Non-uniform crimping can result in sharp edges being formed 
along the now uneven surface of the crimped stent. Furthermore, 
non-uniform stent crimping may not achieve the desired minimal profile for 
the stent and catheter assembly. Where the stent is not reliably crimped 
onto the catheter, the stent may slide off the catheter and into the 
patient's vasculature prematurely and embolize as a loose foreign body, 
possibly causing thrombosis. Thus, it is important to ensure the proper 
crimping of a stent onto a catheter in a uniform and reliable manner. 
This crimping is sometimes done by hand, which can be unsatisfactory due to 
the uneven application of force, again resulting in non-uniform crimps. In 
addition, it is difficult to judge when a uniform and reliable crimp has 
been applied. Some self-expanding stents are difficult to load by hand 
onto a delivery device such as a catheter. Furthermore, the more the stent 
is handled, the higher the likelihood of human error which would be 
antithetical to crimping the stent properly. Hence, there is a need in the 
art for a device for reliably crimping a stent onto a catheter. 
There have been mechanisms devised for loading a stent on to a catheter. 
For example, U.S. Pat. No. 5,437,083 to Williams et al. discloses a 
stent-loading mechanism for loading a stent onto a balloon delivery 
catheter of the kind typically used in PTCA procedures. The device 
comprises an arrangement of plates having substantially flat and parallel 
surfaces that move in rectilinear fashion with respect to each other. A 
stent carrying catheter can be crimped between the flat surfaces to affix 
the stent onto the outside of the catheter by relative motion between the 
plates. The plates have multiple degrees of freedom and may have 
force-indicating transducers to measure and indicate the force applied to 
the catheter while crimping of the stent. 
Williams et al. also discloses a stent-loading device comprising an 
elongated tubular member having an open end and a sealed off end. The 
tubular member houses an elastic bladder which extends longitudinally 
along the inside of the tubular member. The tubular member and bladder are 
designed to hold a stent that is to be loaded onto a balloon catheter 
assembly. Upon placement of the stent over the balloon portion of the 
catheter, a valve in the loading device is activated to inflate the 
bladder. The bladder compresses the stent radially inward onto the balloon 
portion of the catheter to a reduced diameter to thus achieve a snug fit. 
Although the above-described methods by which stents are crimped are 
simple, there is a potential for not crimping the stent sufficiently tight 
to prevent it from loosening in the tortuous anatomy of the coronary 
arteries. Because the amount of compression needed to be applied by the 
fingers will vary with the (a) strength of the operator, (b) day-to-day 
operation, (c) catheter and balloon material and configuration, (d) 
experience of the operator in crimping, and (e) other factors, the 
tightness in which the stent is crimped onto a balloon catheter may vary 
considerably. 
Indeed, because of these factors, the tightness follows a normal or Chi 
square distribution. At the lower tail end of the distribution, the stents 
will be loose and susceptible to movement on the balloon during insertion. 
At the higher tail end, the stent will be too tight and will affect the 
expansion characteristics (i.e., a dog bone effect) of the balloon. 
Currently, a majority of stents are crimped onto the balloons of PTCA 
delivery catheters by deformation of the stent. In these cases, there is 
no adhesive on the balloons. As a result, a one to three percent stent 
loss has been observed. If the stent detaches from the balloon, the 
patient may require surgery to retrieve the stent. 
To minimizes the stent loss problem, some manufacturers premount or 
precrimp their stents onto the PTCA balloons to ensure that the stents are 
securely attached to the catheter. This is an extra cost to the 
manufacturer, and does not give the cardiologist the choice to use any 
type of PTCA delivery catheters. It is also an added cost to the cath lab 
which pays for the extra PTCA delivery catheter. 
One solution has been to coat the PTCA balloons with a pressure sensitive 
adhesive. The adhesive anchors the stent onto the balloon of the PTCA 
delivery catheter. Stent loss during stenting is reduced as a result of 
this approach. 
However, several problems arise when it is necessary to remove the balloon 
from the stented artery at the deployment site. First, stent apposition 
against the arterial wall may be affected as the catheter balloon is 
deflated after stent expansion. Second, as the balloon is deflated, the 
stent struts which are still adhered to the balloon retract with the 
balloon. As a result, the struts can be bent, deflected, or deformed 
towards the lumen of the artery. This may result in deleterious effects on 
the blood compatibility of the stent and affect the stent's ability to 
support the artery. 
Third, the stent can be displaced from the lesion as the catheter is pulled 
out of the patent. This leads to mispositioning of the stent with respect 
to the lesion. The stent is no longer supporting a lesion as it was 
intended, but after it has shifted, is supporting a healthy portion of the 
artery. 
Fourth, residual adhesives may be transferred onto the stent's inner 
surface as the balloon is physically peeled away from the stent. The 
presence of the residual adhesive on the metal surface may affect 
crossability of other catheters and guidewires. Residual adhesives left on 
the stent may affect the blood compatibility of the stent as well. 
Another approach is disclosed in U.S. Pat. No. 5,100,429 to Sinofsky et al. 
This approach suggests anchoring an endovascular stent to a balloon 
catheter with a photodegradable adhesive. Once the stent is delivered and 
expanded against the arterial wall, light is directed onto the adhesive 
resulting in degradation of the adhesive. 
A problem with using a photodegradable adhesive is that it may be released 
into the blood stream and tissue when the adhesive breaks down in the 
presence of UV light. There is also an added engineering requirement to 
integrate an optical fiber into the delivery catheter or some other means 
to expose the adhesive to light. Without this light source, the adhesive 
is not degraded and the stent cannot be detached from the PTCA balloon 
after stent expansion. Including an optical fiber to the delivery catheter 
not only increases costs to the manufacturer, but also makes the delivery 
catheter profile larger and ungainly. 
In view of the foregoing, there is a need for a catheter having a facility 
to secure a stent thereon, yet easily releases the stent from the catheter 
on command at the deployment site. 
SUMMARY OF THE INVENTION 
The present invention is directed to a stent delivery catheter for 
delivering a stent, the catheter comprising a catheter body having a 
deflated balloon portion, a layer of heat sensitive adhesive disposed on 
the balloon portion, wherein the adhesive is tacky at and below a 
temperature T, and is non-tacky above T; and wherein the stent is disposed 
on the layer of heat sensitive adhesive. In the preferred embodiment, 
temperature T is greater than 38.degree. C. but less than 47.degree. C. 
Again in the preferred embodiment, the heat sensitive adhesive is made 
from a crystallizable polymer. 
Because blood temperature is typically 37.degree. C. or below, the adhesive 
anchors a crimped stent onto the catheter during stent delivery, thus 
preventing stent movement or detachment from the catheter at an 
inopportune or unforseen instant. Once the stent is delivered and deployed 
against the arterial wall, warm saline or dye solution can be injected 
into the blood stream to heat the adhesive to above 38.degree. C., or 
preferably 40.degree. C. to accommodate for biological variances in the 
blood temperature. 
The rise in temperature leads to phase transformation; i.e., melting of the 
adhesive and subsequent changes from a tacky state to a non-tacky state. 
Consequently, the stent no longer adheres to the catheter. The catheter 
can then be withdrawn without interfering with the deployed stent. 
In an alternative embodiment, the catheter includes an inflation balloon 
used to expand the stent. The balloon is coated with the heat sensitive 
adhesive while deflated and the stent is mounted thereon. Once at the 
deployment site, the balloon is inflated to seat the stent at the lesion 
in the vessel. Again, warm saline or a dye solution is injected into the 
blood stream to heat the adhesive to above the phase transformation 
temperature of the adhesive, melting the adhesive. The adhesive changes 
from a tacky state to a non-tacky state. The stent then no longer adheres 
to the balloon. The balloon can then be deflated and removed from the 
artery; the balloon detaches from the stent without residual bonding or 
adherence. 
The ability of the temperature sensitive adhesive to change from tackiness 
to non-tackiness addresses all of the problems inherent in prior art 
devices. Naturally, one main function of the adhesive is to prevent 
movement and loss of the stent during its delivery. 
After the stent is positioned at the lesion and expanded, the adhesive is 
no longer useful. As mentioned above, it can be made non-tacky by the 
cardiologist with a slight elevation in blood temperature. The slight 
change in blood temperature, up to 10.degree. C. above normal body 
temperature, does not affect the health of the patient. 
With the adhesive no longer sticking to the stent, the balloon on the 
delivery catheter can be easily deflated and removed without affecting the 
stent's apposition against the arterial wall or positioning relative to 
the lesion. Little or no adhesive residuals are transferred onto the stent 
surface, because the balloon is not pulled or peeled away from the stent. 
The present invention is not limited to attaching coronary stents on 
catheters. Other stents used to maintain body lumens open can be attached 
to a balloon or a balloonless delivery catheter in the same manner. The 
present invention can also be applied to delivery and removal of temporary 
vena cava filters used to trap embolus. 
The degree of tackiness or stickiness of the present invention heat 
activated, pressure sensitive adhesive can be varied. It may be adjusted 
to have greater adhesion towards the balloon and/or stent, but preferably, 
towards the balloon. It may be formulated to covalently bond the balloon 
material which are typically made from polyethylene (PE), polyethylene 
terephthalate (PET), or Nylon. 
The phase transformation temperature of the adhesive can be adjusted 
depending on the adhesive formulation and chemistry. A higher temperature 
prevents premature change from tackiness to non-tackiness of the adhesive. 
The present invention heat sensitive adhesive can be applied onto the 
balloon surface by various methods including brushing, wiping, spraying, 
dipping, or other techniques known in the art. Application of the heat 
sensitive adhesive to the catheter can be performed by the manufacturer or 
by the cardiologist in the cath lab. 
In an exemplary embodiment, the thickness of the heat sensitive adhesive on 
the balloon can vary. It may range from 0.000010 inch to 0.005 inch thick. 
In various alternative embodiments, the heat sensitive adhesive can fully 
coat or partially coat the balloon surface. It can be applied on the 
distal or proximal ends of the balloon depending on the strength of 
adhesion required between the stent and the balloon. If a delivery sleeve 
or sheath covers the balloon, such as with a C-flex sleeve covering the 
balloon, the coating can be applied onto the surface of the C-flex sleeve. 
In an alternative method of the invention, a stent having interstitial 
spaces between its struts is crimped on to a catheter or a balloon portion 
of the catheter. Next, a heat sensitive adhesive is at least partially 
filled in to the interstitial spaces and bonds to the surface of the 
balloon or catheter. The presence of the adhesive helps maintain the 
stent's position on the balloon or catheter. Also, with the interstitial 
spaces at least partially filled in, the stent profile is much smoother. 
The smoother profile minimizes "fish scaling" of the stent as it is 
advanced through a patient's tortuous arteries. 
These and other advantages of the invention will become apparent from the 
following detailed description thereof when taken in conjunction with the 
accompanying exemplary drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is directed to a stent delivery catheter that employs 
a temperature sensitive adhesive to bond the stent to the catheter. While 
the invention is described in detail as applied to use in the coronary 
arteries, those skilled in the art will appreciate that it can be applied 
to devices for use in other body lumens as well, such as peripheral 
arteries and veins. Also, although the invention is described with respect 
to mounting a stent on the balloon portion of a catheter, the invention is 
not so limited and includes mounting stents or grafts on any type of 
catheter used to deliver and implant such stents. Where different 
embodiments have like elements, like reference numbers have been used. 
As shown in FIGS. 1 and 2, the present invention in a preferred embodiment 
is directed to a catheter with a temperature activated adhesive that is 
applied to the surface of the catheter. The performance characteristics of 
an exemplary temperature activated adhesive is plotted graphically in FIG. 
2, where the horizontal axis represents adhesive temperature and the 
vertical axis represents the tackiness of the adhesive. As seen in FIG. 2, 
the adhesive has a sticky or tacky surface at or below, preferably, 
38.degree. C. but transforms to a non-sticky surface at temperatures above 
38.degree. C. Accordingly, the phase transformation temperature of 
38.degree. C. is represented by the vertical dashed line. 
Because human blood temperature is approximately 37.degree. C. or below, 
the adhesive is in its tacky state and can attach the stent on the 
catheter during stent delivery to prevent stent movement or detachment. 
Once the stent is delivered and deployed against the arterial wall, warm 
saline or dye solution can be injected into the blood stream to heat the 
adhesive to above 38.degree. C., preferably 40.degree. C. to accommodate 
for biological variances in blood temperature. The rise in temperature 
leads to phase transformation, and melting of the adhesive. With the rise 
in temperature, the adhesive changes from a tacky state to a non-tacky 
state. Without the tackiness in the adhesive, the stent no longer adheres 
to the catheter. The catheter can then be cleanly separated from the 
deployed stent and withdrawn from the artery with equal effectiveness as a 
PTCA catheter without any adhesive. 
FIG. 1 depicts a preferred embodiment of the present invention showing a 
cross-section of a portion of catheter 10 having catheter body 12 that 
includes folded balloon 14. An exterior surface of balloon 14 is coated 
with a layer of heat sensitive adhesive 16. Stent 18 is mounted on heat 
sensitive adhesive 16. 
Although catheter 10 is shown having balloon 14, in an alternative 
embodiment, the present invention contemplates a catheter without an 
inflation balloon. To be sure, the present invention is not limited to 
attaching coronary stents onto catheters. Other stents used to maintain 
bodily lumens open can be attached to a balloon or a balloonless delivery 
catheter in the same manner. The present invention also can be applied to 
delivery and removal of temporary vena cava filters used to trap embolus. 
Stent 18 represents a typical stent design. Stents such as that shown in 
FIG. 1 are disclosed in, for example, U.S. Pat. No. 5,421,955 issued to 
Lau et al., U.S. Pat. No. 5,514,154 to Lau et al., or U.S. Pat. No. 
5,649,952 to Lam. 
In a preferred method of applying heat sensitive adhesive 16, the present 
invention contemplates applying the adhesive by various methods such as 
brushing, wiping, spraying, dipping, or like techniques known in the art 
onto the surface of catheter 10. Application of heat sensitive adhesive 16 
to catheter 10 can be performed by the manufacturer or by the cardiologist 
in the cath lab. 
Some stent manufacturers use a C-flex sleeve over the balloon on which the 
stent is crimped. One such manufacturer is Advanced Cardiovascular 
Systems, Inc., Santa Clara, Calif. If there is a C-flex sleeve (not shown) 
over the balloon, the adhesive can be applied to the C-flex prior to 
crimping the stent on the C-flex. 
The layer of heat sensitive adhesive 16 preferably ranges between 0.000010 
inch to 0.005 inch inclusive. In the preferred embodiment, it is less than 
0.001 inch thick, but sufficiently thick to ensure stent attachment. 
When heat sensitive adhesive 16 is applied and stent 18 is crimped onto 
balloon 14, it is important to assure that the local temperature is below 
the transition or phase transformation temperature of heat sensitive 
adhesive 16 so that it is in a tacky state. The tackiness bonds stent 18 
to balloon 14. But various permutations in the process steps for applying 
heat sensitive adhesive 16 to balloon 14 are contemplated. 
For example, it is possible to coat balloon 14 with heat sensitive adhesive 
16 below the transition temperature, heat the adhesive to above the 
transition temperature, then mount stent 18 onto balloon 14. This permits 
easy mounting, alignment, and adjustment during the crimping process when 
stent 18 is seated to a precise location on the balloon 14. After the 
crimping process, the local temperature is decreased to below the phase 
transformation temperature of the adhesive to render it tacky again. This 
secures stent 18 to balloon 14 for the delivery process. 
In another variation of the procedure, stent 18 may be crimped onto balloon 
14 while the local temperature is below the transformation temperature so 
that heat sensitive adhesive 16 is tacky during the crimping process. If 
the adhesive is already tacky during the crimping step, there is less of a 
chance of introducing new stresses into the stent due to phase 
transformation in the adhesive. 
As shown in FIG. 3, another preferred method of applying the adhesive, 
stent 18 is first firmly crimped onto balloon 14 (or C-flex if it is 
used), adhesive 16 is then applied in a conventional manner so that the 
adhesive tends to fill the interstitial space or gaps 15 in between stent 
struts 17. By applying the adhesive after crimping the stent onto the 
balloon, there is no adhesive between the stent and balloon which should 
facilitate separation of the stent from the inflated balloon. Also, the 
adhesive filling the stent gaps makes a smoother stent profile as it is 
advanced through the coronary arteries, especially where tight curves are 
encountered. Finally, the adhesive prevents "fish scaling" which is a term 
referring to slight projections on the stent which develop as the stent is 
bent while being advanced through tortuous arteries. 
The heat sensitive adhesives used in the present invention are disclosed 
in, for example, U.S. Pat. Nos. 5,412,035; 5,387,450; and 5,156,911, which 
are incorporated herein by reference thereto. Although such adhesives are 
intended for use in medical applications where the substrate is the skin, 
the adhesive compositions can be used for the present invention purposes. 
Indeed, such adhesives are commonly used in Band-Aids, transdermal 
delivery drug delivery patches, ECG electrode patches, and surgical 
dressing. 
These heat sensitive adhesives have polyacrylate or styrene/butadiene 
copolymer backbone. Different functional groups are attached to the 
backbone. Up to 50 percent of these functional groups are crystalline in 
nature and can become amorphous at the phase transformation temperature 
(i.e., melting point of the adhesive). The degree of tack of the adhesives 
is less than 25 g-cm/sec below the phase transformation temperature and 
improves to above 100 g-cm/sec above the phase transformation temperature. 
Importantly, it should be noted that the reverse can also occur whereby 
the adhesive changes from non-tacky to tacky state when it is above its 
phase transformation temperature (melting point). 
With the foregoing adhesives used in conjunction with the present invention 
stent delivery system, the ability of the temperature sensitive adhesive 
to change from tackiness to non-tackiness addresses all of the potential 
delivery problems expressed above. It is possible to use a stronger 
adhesive strength for the present invention heat activated adhesive, as 
compared to a balloon with normal pressure sensitive adhesive. The latter 
type adhesives should not be formulated to have too high of an adhesive 
strength towards the stent because it might interfere with the balloon 
removal after stent expansion. On the other hand, the present invention 
use of a heat sensitive adhesive does not have this disadvantage because 
its tackiness, regardless of strength, can be neutralized by application 
of heat. 
As mentioned above, the present invention system is preferably adapted for 
use with a PTCA balloon catheter 10 having balloon 14 at the distal end. 
Of course, the present invention can be used with a balloon catheter of 
any conventional design known in the art as well as any catheter without a 
balloon. For example, a nitinol stent can be adhered to a balloonless 
catheter and deployed when heated above 37.degree. C. at the same 
temperature the heat sensitive adhesive releases (becomes non-tacky). This 
eliminates the need for a protective sheath over the nitinol stent. 
Nitinol stents are difficult to crimp and usually require a retractable 
protective sheath unless the adhesive of the present invention is used. 
Any lubricant or lubricous coatings are removed from the exterior surface 
of catheter balloon 14 with a cleaning fluid such as isopropyl alcohol. 
The warm saline or dye used to melt the adhesive can be injected or 
delivered in any conventional manner as shown, for example, in U.S. Pat. 
Nos. 4,641,654 to Samson et al., and 5,611,775 to Machold et al., which 
are incorporated herein by reference. In addition, the adhesive can be 
melted by a balloon catheter that includes heating elements. Such 
dilatation catheters with a heated balloon are shown in, for example, U.S. 
Pat. Nos. 5,035,694 and 5,114,423 to Kasprzyk et al., whose disclosures 
are incorporated herein by reference. 
Other modifications can be made to the present invention without departing 
from the scope thereof. The specific dimensions and materials of 
construction are provided as examples, and substitutes are readily 
contemplated which do not depart from the invention.