1. Vascular filter with improved strength and flexibility
Peter Besselink1
, Zameed Saheb Ali2
, Anthony Don Michael3
,Ton van Roermund1
1
Memory Metal Holland BV, Gronausestraat 1220, 7534 AT Enschede, The Netherlands
2
Occam International BV, Beemdstraat 23, 5653 MA Eindhoven, The Netherlands
3
Advanced Heart and Medical Center,5343 Truxtun Avenue, Bakersfield, CA 93309, USA
ABSTRACT
A distal protection filter can be used as a safety device during the performance of a procedure
like angioplasty and/or stenting.
During such procedures there is a risk that emboli detach from the artery wall and cause
problems downstream, for example stroke. Therefore such a filter is manoeuvred through the
lesion site and deployed in order to catch such emboli. After the procedure the filter is
collapsed again and removed, together with the trapped emboli, through the treated (stented)
location.
Memory Metal Holland BV and Occam International have developed a filter based upon a
self-expandable Nitinol frame with a polyurethane membrane attached thereto. In the
polyurethane a series of 100 micron diameter holes are cut with a laser. Particles in the blood,
which are larger than 100 micron are trapped, while the normal blood flow remains intact.
The membrane is reinforced with fibers, made of filaments of a material with high strength in
the longitudinal direction, but high flexibility upon bending, to prevent rupture or crack
propagation of the extremely thin membrane. These fibers further give a secure attachment of
the membrane to the expandable Nitinol frame, work as a hinge between membrane and frame
and also enable a smooth delivery and retrieval of the device by means of a sheath. The
Nitinol frame has a stent-like shape with an open proximal mouth, which allows full flow of
the blood. At the proximal side additional high-strength fibers are used to collapse the stent
frame upon retrieval. This design gives the filter full freedom of movement in relation to the
attached guide wire in radial, tangential and axial direction, meaning that the filter stays
undeformed during the procedure. Further this design allows the operator to keep the filter
empty by flushing and suction.
INTRODUCTION
Distal protection filters are used today in most procedures for carotid angioplasty and stenting.
For other procedures they will also become more and more standard instruments to prevent
embolization. Besides filters there are also occlusion balloons and retrograde flow devices.
However, not all patients can tolerate a total occlusion, and even fewer patients withstand the
cerebral steal resulting from the use of retrograde flow devices. In filters the pores permit
distal blood flow, which allows the use of contrast imaging for continued intraprocedural
visualization in addition to reducing the risk of ischemic events [1].
Filters are generally mounted near to the distal end of a guidewire. Distally of the filter the
guidewire has a floppy tip for easier manoeuvring through the tortuous artery. The filter itself
is always made from an expandable structure plus a fine mesh structure which can collect the
debris. Before insertion the filter has to be brought into a collapsed state with a minimized
profile, in order to enable the operator to advance the guide wire plus filter through the narrow
lesion, without the release of particles from this lesion site. Therefore it is extremely important

2. to make use of a smooth delivery device, mostly a delivery catheter that keeps the expandable
structure in its collapsed state upon insertion.
The guide wire not only serves as a delivery device for the filter, but also serves as the means
to bring the treatment device to the lesion site. Therefore it is desirable that the guide wire
dimensions have a standardized size for angioplasty and stenting. In most filters the
expandable structure is made of a superelastic nitinol frame, which keeps the actual filter
mesh in an open state. The filter mesh is mostly made of either a braided wire mesh (e.g. fine
nitinol wires) or a thin polymer with a series of drilled holes (e.g. polyurethane with 100
micron laser drilled holes).
Retrieval of the filter through the expanded and/or stented lesion site is achieved by collapsing
the expandable structure with a retrieval catheter. In some devices an internal actuation wire is
moved through the hollow guide wire to expand or collapse the filter.
PROBLEMS IN EXISTING FILTERS
Several typical problems can occur in using filters. First of all there is the risk of a decreasing
volume during collapsing the filter, where trapped particles pass the filter, because they leave
the filter through the proximal entrance. Another problem is that parts are squeezed through
the fine holes of the mesh. Dimensions of the expandable frame are critical. Most filters are
made from superelastic nitinol tubing, which is connected directly to the guidewire. The use of
such monolithic basket frames, cut from tubing and than shape set to its final expanded state,
is well known [2]. In some filters the total length of the frame is so large, that the axial
flexibility is not very good. When the filter is placed in a strong bent artery, it may not fit well
to the wall or it may be deformed by the interaction with the guidewire. A long filter also
needs more parking space in general, so it has to be placed further behind the lesion. The
direct connection to the guidewire causes another problem: during the procedure of
angioplasty and stenting it is almost impossible for the operator to hold the guidewire
completely still. Movements in axial, radial, tangential and rotational direction can influence
the position and geometry of the filter. This may cause an insufficient wall apposition and thus
leakage, but also a spasm or even permanent damage of the sensitive artery inner wall.
Direct connection of the frame to the guidewire at the proximal side further makes it
impossible to enter the filter with an additional device for a treatment like flushing and
suction. Further, the surface area of the strut structure at the filter entrance may be so large
that emboli do not enter the filter, but collect at the struts. Later they may move further when
the filter is removed. Therefore the proximal surface area of the structure should be
minimized. Finally there is the risk of tearing or disconnection of the thin, fragile polymer
filter mesh from the frame. The mesh is extremely thin and must have numerous small holes,
so the strength of such a filter bag is very limited.
Above mentioned problems were enough reason to start developing a filter with improved
characteristics.
GOALS FOR A NEW FILTER
The following goals should be achieved:
1. Improved strength of the filter bag, thus allowing more holes at the same surface.
2. Short length of the frame to reduce parking length and enable placement in curved arteries.
3. Reduced mechanical interaction between guidewire and filter.
4. Optimized surface area for proximal opening.
5. One type of filter should cover a wide range of artery sizes.

3. 6. Perfusion should be as high as possible.
7. There must be proximal access to the filter for additional treatment.
CONSTRUCTION
The new filter is based on a 7 mm long self-expanding cylindrical Nitinol stent, acting as the
frame that keeps the filter in position. In Figures 1 and 2 an animation and picture are given.
At the distal (at right in the Fig. 1) side the frame is connected to a conical polymer filter bag
with about 1800 holes with a diameter of 110 microns (See detail in Fig. 4). The tip of the
membrane bag is connected to a ring that slides on the 0.35 mm diameter guidewire, which
has a flexible, radio opaque distal tip.
Fig. 1: Filter with fiber-reinforced membrane Fig. 2: Picture of reinforced
filter (Occam)
At the proximal (at left in Fig. 1) side the stent has elongated struts, provided with anchor-like
strut connectors for the attachment of fine ultra high molecular weight polyethylene fibers
with high flexibility and extreme tensile strength.
When expanded, the elongated struts fit well to the artery wall and leave the proximal entrance
completely open. Typical dimensions of the frame are a length of 7 mm (expanded) and an
external diameter of 0.7 mm (collapsed, see Fig. 3) up to 7 mm (expanded). The only parts
that hinder the free entrance of the blood stream into the filter are the six free-moving fibers
with a thickness of about 50 microns, running from the strut connectors to a sliding ring,
mounted on the guidewire. Mechanical interaction between guidewire and frame is only
possible if a mechanical stop on the guidewire is moved far enough forward or backward to
engage with the (radio opaque) sliding rings, which are connected to the filter via the fibers.
Fig. 3: Gold plated frame (Occam) Fig. 4: Detail of membrane with holes(Occam)

4. During use the guidewire may move in any direction without influencing the geometry and
position of the filter, as long as the stop on the guidewire stays within the range of free
movement between the two sliding rings. Although the operator may not be able to hold the
guidewire completely still during treatment, the complete lack of mechanical interaction
between filter and guidewire will guarantee an ideal wall apposition of the frame, even if the
filter is placed in a strongly curved artery. Of course this also means that the filter frame must
hold the filter in its axial position without the help of the guidewire, in contrary to most other
devices. The maximum axial force, exerted on the filter, would occur in case of full occlusion
and the frame dimensions were designed so that it can hold a filter without perfusion holes in
place against full blood pressure.
Free movement of the proximal sliding ring, even beyond the frame, further enables the
operator to advance an additional device, like a suction tube, into the filter in order to suction
it empty (see Fig. 5) and thus preventing a pile-up of debris. This also makes it possible to
leave the filter in place for a longer period without the problem of full occlusion, or to use it in
cases where extreme amounts of debris are expected. Radio opacity of the nitinol frame is
enhanced by a gold-coating of 3 microns thickness, so that the status of deployment of the
filter is well visible on X-ray.
REINFORCEMENT FIBERS
Normally a filter membrane is very thin and therefore it may tear accidentally. Crack
propagation is not easily stopped and eventually the polymer may disconnect from the frame.
Therefore it is investigated if the membrane could be improved by embedding a pattern of
reinforcement fibers. Such fibers have different advantages. They can be directly wrapped
around the frame struts and then embedded, so that accidental detachment from the frame
becomes impossible. Furthermore these embedded fibers prevent crack propagation through
the membrane. Several types of fibers have been tested and the best results were obtained with
fine multifilament fibers of high molecular weight polyethylene. The tensile strength of the
used fibers is well over 3000 MPa, while the material is flexible as silk, except in length
direction (maximum elongation only 3%). Bending of the fibers does not influence the
properties, even after thousands of cycles.
Fig. 5: Suction device entering the filter Fig. 6: Detail of strut connectors

5. Optionally a polyurethane layer can be applied around the fine filaments to improve the
adhesion to the polyurethane filter membrane into which the reinforcement fibers are
integrated by a dipping process. It was found that the laser drilling parameters for making the
pattern of 110 micron holes, could be adjusted in such a way that the membrane could be cut,
without cutting the embedded filaments. This opened the possibility of using a randomly
oriented network of reinforcement fibers, without taking care of where the holes were cut.
In the present filter membrane most fibers run from the distal slide ring to and around the strut
ends of the nitinol frame and than back to the distal ring. Delivery of the filter from a delivery
sheath becomes easy, and the membrane itself is almost free from stresses when it is pulled
out of the sheath. Delivery takes place by pushing the stop on the guidewire in distal direction
until it engages with the distal slide ring. By pushing the guidewire further, the embedded
fibers take all the tensile stresses and actually pull the stretched filter membrane plus nitinol
frame out of the sheath. Therefore delivery is much smoother than if the filter had to be
pushed out, like in alternative systems, where the membrane dimensions may become larger
due to the friction in the sheath.
WITHDRAWAL OF FILTER
On contrary to most other filters, the proximal section of the frame of this filter can be
collapsed without changing the shape and volume of the actual filter section. As can be seen in
the Figures 6-8, the proximal fibers start pulling at the strut connectors of the elongated struts,
thus creating a conical section in the frame. This conical section finally enters the withdrawal
sheath, while the strut connectors are smoothly guided into the catheter by the connected
fibers, which are located at the outside surface of the strut ends (see Fig. 6). This placement of
the fibers at the outside ensures that the edges of the strut end do not collide (and get stuck)
against the distal edge of the retrieval sheath. In Fig. 7 can be seen that even while the
elongated struts are pulled further inside the sheath, the filter section itself is still in its fully
deployed state and gives still full distal protection. Meanwhile the gaps between the struts of
the frame are closed more and more (see Fig. 8) and the closing frame acts as a cap that closes
the proximal entrance of the filter, thus preventing any loss of captured debris. In fact the
closing frame starts acting as an additional proximal filter [3].
Fig. 7: Filter at start of retrieval Fig. 8: Proximal view on collapsing frame

6. OTHER APPLICATIONS
Besides the described filter, the principle of a composite structure using embedded
reinforcement wires in membranes can be useful in numerous products. In some of those
products the membrane will be perforated, in others not. The reinforcement fibers can only be
used for their high flexibility and tensile strength, but may also be combined with embedded
fibers (e.g. nitinol) to control the shape of the membrane. Often this can be combined with
expandable or deformable frames. The attachment of additional proximal fibers to such
devices can make them removable. Examples are removable temporary stents, occlusion
devices, grafts, valves, clips, retrieval bags, inflatable members, devices for body tissue
replacement and delivery platforms for drugs, radiation or gene therapy [4].
CONCLUSION
Fiber reinforcement opens new possibilities in improving the strength of membranes in
medical devices. This allows a further miniaturization of devices, because membranes can be
made much thinner without loss in strength. Detachment from the frame and tearing of
membranes can be prevented.
The reinforced filter meets the requirements that were formulated. The strength of the
membrane is highly improved and the filter can be placed in strongly curved arteries, due to
the frame length of only 7 mm. Wall apposition and thus safety are optimized by the lack of
interaction with the guidewire, which would normally deform the frame in other devices, if
they were placed in strong bends.
The fine proximal fibers almost allow full opening of the frame and a single filter wit 3.4
French delivery profile fits in arteries of a wide diameter range from 4 to 6.5 mm. Due to the
improved strength of the membrane this design allows an increased number of perfusion holes
and a smaller thickness compared to non-reinforced membranes, so the blood flow is
optimized. Finally, the full proximal opening and the flexible proximal fibers allow the
operator to advance an additional device, like a catheter for flushing and suction, into the filter
in order to keep it empty.
1. Karthikeshwar Kasirajan; Peter A. Schneider; K.Craig Kent: “Filter Devices for
Cerebral Protection During Carotid Angioplasty and Stenting”. Journal of
Endovascular Therapy 2003; 10; 1039-1045
2. Rohit C.L.Sachdeva and Petrus A. Besselink; “Medical Instrument with Slotted
Memory Metal Tube”. US Patents 5,885,258 and US 6,780,175.
3. T.Anthony Don Michael and Peter Besselink; “Vascular embolism prevention device
employing filters”. US Patent 6,485,502.
4. Petrus Besselink; “Vascular filter with improved strength and flexibility”. PCT Patent
application WO2004/026175.