CA1170404A

CA1170404A - Vascular prosthesis and method in producing it

Info

Publication number: CA1170404A
Application number: CA000384711A
Authority: CA
Inventors: Sven E. Bergentz; Kjell Bockasten; Kurt Strid
Original assignee: Astra Meditec AB
Current assignee: Astra Meditec AB
Priority date: 1980-08-28
Filing date: 1981-08-27
Publication date: 1984-07-10
Also published as: AU7393981A; FI812659L; AU552482B2; EP0047231A2; NO153793B; DK377981A; EP0047231A3; NO812800L; IE52733B1; ES260102Y; IE811936L; NO153793C; ES260102U; EP0047231B1; ATE7357T1; DE3163505D1; US4729766A; SE8006024L; SE444640B; JPS5772638A

Abstract

ABSTRACT

The invention relates to a vascular prosthesis in the form of a flexible tube which can be implanted in animals or human beings, with a regular pattern of blind holes in the external surface of the tube, which are regular with regard to direction, transverse dimensions and depth and are produced by laser treatment.
The invention also relates to a method of producing such a vascular prosthesis consisting in that the holes are produced by means of pulsated laser beams along one or more helical lines, the tube being rotated and displaced acially at the same time.

Description

1~7~34~9~
,, VASCULAR PROSTHESIS AND METHOD IN PRODUCING IT

The invention relates to a vascular prosthesis in the form of a flexibile tube which can be implanted in animals or human beings, wherein at least one outer sur-face layer consists of a soft synthet;c material with microdepressions in the external surface of the tube.
Furthermore, the invention relates to a method in producing a vascular prosthesis of this kind.
It is an indispensible requirement that such a vascular prosthesis should be able to fulfil for a long time, i.e. for several years, its vitally important function in the organism in which it is implanted. This presupposes that the vascular prosthesis is of a mate-rial which is not rejected by the organism and which has such mechanical properties that the vascular prosthesis can take up the Yarying internal pressure to which it may be exposed5 that it can bend with a small radius without becoming occluded and that it can be firmly sewn to joining blood vessels, producing a satisfactory anastomosis, without the suture thread cutting through the material. The lumen must be bounded by a surface which does not tend to give rise to throm-bosis and thromboem~Dlisms, and the external surface of the vascular prosthesis should be compatible with tissue and allow the prosthesis to be anchored by the tissue growing or infiltrationing into the wall of the vessel so that no oedema is formed or inflammation occurs through displacement or twisting of the vascular pros-thesis in relation to adjacent tissues. Furthermore, it is necessary for the material in the vascular pros-thesis to be able to stand some form of sterilization.
Finally, it should be added that the properties listed above must be present at the temperature at which the , vascular prosthesis is to be used, namely at body tem-perature.
The~vascular prosthesis which has been used for the longest time consists of porous textile material, usually a crimped or resiliently circular-knitted stocking of polymerized ethyleneglycolterephthalate (Dacron). It also occurs in embodiments with a velour covering on the outside or both on the outside and the inside to promote the growing of the surrounding tissue into the implanted vascular prosthesis, even into the boundary surface of the lumen, so that a pseudointimal coating is obtained on the boundary surface, which is more resistant to blood coagulation than the actual surface of the vascular prosthesis. Deposits of fibrin and cellular material from the blood also contribute to the formation o~ this coating. In this manner, the vascular prosthesis is covered internally, within a few ' days, with a biological lining whi'ch can be tolerated 'by the blood and prevents the occurrence of thrombosis or thromboembolisms. The textile vascular prostheses must~ however, be treated with biood, so-called pre--~lotting, before they are implanted,'which involves a time-consuming and lengthy pre-treatment. The textile vascular prostheses are accepted by the orga~ism and fulfil their function well if they have a limited length and a sufficiently large internal diameter and work with a rapid flow o'f blood and under high pressure.
If these conditions are not-fulfilled, the woven vas-cular prostheses have a short operative life.
In recent years, vascular prostheses have been made of expanded polytetrafluorethylene (Teflon) which forms a network of nodes and flexible fibrils. The advantage of this type of vascular prosthesis is above all that it can be given the desired porosity and the desired flexibility and that no pretreatment with blood `` 1~7~

is necessary. Moreover, it is easy to suture and is resistant to occlusion. A subskantial disadvantage, however, is that the vascular prosthesis has an uneven surface at the inside and cannot be heparinized to prevent particles in the blood from coating the inside of the vascular prosthesis. It is still too early to judge whether this type of vascular prosthesis is better, with regard to its operation, than the vascular prosthesis of textile material.
According to the German laid open specification

2,149,027 it has been proposed to produce microdepressions in the external surface of an implant by a procedure wherein particles, granulate or fibres of predetermined size and shape are embedded into the material of the external surface of the implant and are then removed again so that depressions remain in the surface, which correspond to the size and shape of the particles, granulate or fibres at the embedding locations. The method produces a random distribution of the depressions, and certain parts of the surface may be supplied to a less extent than others. It is mentioned in the specification that the implant may be in the form of a vascular prosthesis but it does not appear to have reached any practical use as such, probably because the microdepressions produced in the manner indicated do not permit a satisfactory growing-in of surrounding tissue to achieve a reliable anchoring of a vascular prosthesis in the organism.
The invention is a contribution to the efforts made in order to provide a vascular prosthesis which satisfies the requirements discussed above.
The invention provides a vascular prosthesis in the form of a flexible tube which can be implanted in animals or human beings, wherein at least one outer surface layer consists of a soft synthetic material with microdepressions in the exter-. 3 .

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nal surface of the tube, character.ized in that the micro-depressions consist of a regular pattern of blind holes which are regular with regard to direction, transverse dimensions and depth and are produced by laser treatment.
This vascular prosthesis has a non-permeable `\
117(~
. ` 4 wall, which is an important difference from the vascular prostheses now used of a permeable construction. No preclotting is therefore required.
The vascular prosthesis preferably consists of polyurethane with blind holes set obliquely to the radial direction alternately in one direction and the other. Then, in a particularly advantageous manner the holes may be inter-connected in pairs at their inner ends, so that they form a duct from the surface of the vascular prosthesis which ex-tends through the material of the vascular prosthesis and then goes out again to emerge at the surface of the vascular prosthesis.
In order that the vascular prosthesis can be sutured more easily and reliably when connected to adjacent blood vessels, i.t is a further advantage to reinforce the wall of the vascular prosthesis with a stocking of textile material embedded in the material.
Embodiments of the invention will be described in more detail below together with a method of producing the vascular prosthesis, reference being made to the accompanying drawings in which Figure 1 is a greatly enlarged perspective view of a part of a vascular prosthesis according to the invention.
Figure 2 is a further enlarged fragmentary perspective sectional view of the vascular prosthesis in Figure 1.
Figure 3 is a perspective view of a simple laboratory apparatus for producing blind holes in the external surface of the vas-cular prosthesis as shown in Figures 1 and 2.
~ Figure 4 ~as the same sheet as Figure 1) is a perspective view, with parts broken away, of a vascular prosthesis as shown in Figures 1 and 2, but with a reinforcing stocking of textile material embedded in the wall of the vascular prosthesis, Figure 5 is a view corresponding to Figure 2 of yet another embodiment of the invention.
Figure 6 is a view corresponding to Fi.gure 2 of a still ~ 17(~40~

-further embodiment of the invention.
Figure 7 (on the same sheet as Figures 16 and 17) is a perspective view of an apparatus for producing the vascular prothe-sis as shown ln Figure 6, Figure 8 is a diagrammatic view, which shows the con-struction of the apparatus in Figure 7, Figure 9 is a diagrammatic plan view which shows the geometric relationships during the production of a vascular prosthesis in the apparatus shown in Figure 7, 10 Figure 10 is a diagrammatic cross-sectional view, which likewise illustrates the geometric relationships during th0 production of a vascular prosthesis in the apparatus shown in Figure 7, Figure 11 is an enlarged part of the plan view in Figure 9, namely the part which is marked by a square in chain line, Figure 12 is a fragmentary plan view of the external surface of a vascular prosthesis according to the invention in yet another modified embodiment, Figure 13 is a sectional view on the line XIII-XIII in Figure 12, Figure 14 is a greatly enlarged fragmentary perspective view of a further embodiment of the vascular prosthesis according to the invention, Figure 15 is a side view of an apparatus for producing the vascular prosthesis according to the invention, adjusted to produce the embodiment as shown in Figure 14, Figure 16 is an end view of the apparatus in Figure 15, and Figure 17 is a diagrammatic cross-sectional view, which illustrates how the laser beams are directed during the production of -`- the vascular prosthesis as shown in Figure 14 in ~he apparatus shown in Figures 15 and 16.

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With reference to Figures 1 and 2, the vascular prosthesis according'to the inventio~ shown therein ^~ consists o~ a moulded, injection-moulded or extruded cylindrical, flexible t'ube iO of a synthetic material, for example plastics, preferably polyurethane, which is compatible with tissue and is available'in various degrees of hardness. In order to obta'in a good con-nection to the body's own blood vessels at the anas-tomosis the material should be soft. Shore nu~bers of-20 to 25 have proved suitable. In itself, this mate-rial has a negative electrical surface charge, whic'h is an advantag'e, as mentioned in the introduction, since - the material repels thrombocytes as a result thereof.
Ho~lever, materials which do not have such a negative electrical surface charge, for example polyvinylacetate, can also be used'in the vascular prosthesis according to the invention, if they are suitable for heparinizing, i.è. the incorporation of heparin in the vessel wall at the inside of the tube. As a result of the heparin-izing, the inside becomes repellant to thrombocytes andparticles in the blood are prevented from coating the inside of the vascular prosthesis, and so the occurrence of thro'mboses and thromboembolisms 'is prevented. Even ' materiais which already have a negative surface charge can be heparinized at the inside of the tube if it is desired to increase the thrombocyte-repelling effect.
Polyethyl:ene, polytetrafluorethylene and silicone rubber'may be mentioned as further examples of materials which are suitable for the vascular prosthesis according to the invention. On the other hand, polyvinyl chloride ' for example, is unsuitable,' since it changes during the laser treatment. The changes are manifested by dis-colouring of the material. Materials which undergo - structural changes during laser treatment, which are -manifested as carbonization of the surface or discoloura-, ~ 4 - tion of the material, should not be used for the vascular prosthesis according to the invention.
- Summing up, it is required of the material which is used for the vascular prosthes~is, that it should be accepted by the organism and should not be rejected by the organism after the implantation, and that it should ` have-a negative electrical surface charge or be able to be heparinized. Furthermore, the material should be able to be treated with a laser without undergoing structural changes, and it should have such mechanical - properties that it can withstand the stresses to which it is exposed not only during manufacture but also when the vascular prosthesis is implanted in the organism. The material should be soft so that the vascular prosthesis can bend with a small radius without a tendency to so-called cracking. The fact that the material is soft is also a prerequisite for the vascular prosthesis to seal against joining blood vessels, which have often grown stiff due to calcification, duriny the connection to these blood vessels. At the same time, -' however, the material should afford adequate securing means for the suture thread which should not tend to cut through the material. As will be shown later, in connection with another embodiment of the invention, the anchoring of'the suture thread'can, however, be improved by inserting a'reinforcement in the tube wall.
Finally, it is, of course, necessary for the material :~
' of the vascular prosthesis to be able to stand some form of sterilization, for example beta or gamma steri- ~
lization. It may be added that the properties listed t must be present at the temperature at which the vascular prosthesis is to be used, namely at body temperature.
'' In order that the vascular prosthesis may be able to fulfil its purpose, it must be able to be anchored ~!
~5 in the body by the growing-in and infiltration of the .. ~

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tissue into the wall of the prosthesis to eliminate oedema and inflammation. In order that this growing-in may be un-iform over the whole external surface-of the . vascular prosthesis, this is formed with a regular pattern of microdepressions in the form of blind holes 11. It is an essential requirement that the whole surface should be uniformly provided with these holes.
The holes are disposed substantially radially in the embodiment shown in Figures 1 and 2. The holes are produced by laser treatment which can be effected in the manner shown in Figure 3. According to this Figure, the tube 10 is passed onto a mandrel 12 which is secured between a chuck i3 and a tai~stock 14. The chuck and the tailstock may constitute parts of a conventional lathe. Simultaneously with the rotation of the tube 10, axial displacement of the tube is brought about. A laser, partially indicated at 15j is placed with its focussing lens system 16 above the tube 10. The laser beam is directed radially towards the tube. While the tube is rotated and displaced axially, the laser is pulsated to produce a hole in the tube wall on each pulse, the holes thus formed coming to lie on a continuous helical line round the outside of the tube.
~` During the production of a vascular prosthesis of polyurethane with a hardness of 20 to 25 Shore, the tube was rotated with a pitch of 0.30 mm. The pulse frequency of the laser and the ra~e of rotation of the tube were selected so that the spacing between the holes along the helical line was about 0.30 mm. The laser used was a 500 W C02 laser with radiation in the form of a parallel beam of infrared light with a wavelength of `~ 10.6 ~m. The diameter of the beam was about 12 mm, and this beam was focussed by the lens 16 on the surface of the stock material. In order that well-defined holes `` 35 may be obtained, though the tube is rotating at a cons-~7~

tant speed, the pulse period must be selected short in comparison with the ;nterval between two pulses; with a pulse rate of 100 - 400 pulses per second, the pulse period is of the order of 10 - 100 ~s. With correct focussing of the laser beam on the tube, the hole dia-meter is 0.-10 mm, but the hole diameter can be increased by defocussing slightly. The depth cf hole is determined by the energy of the l'aser pulse, and this depth should appropriately be selected so that the distance from the bottom of the hole to the inner wall corresponds to approximately l/3 of the thickness of the tube wall.
In an embodiment of a vascular prosthesis of polyurethane produced in this manner the hole diameter was 0.150 mm and the hole depth 0.30 mm with a wall thickness of ~.40 mm. The hole spacing was 0.30 mm and the pitch of the helical line 0.30 mm. There were about ll'holes on each square millimetre of the surface of the vascular prosthesis.
' In another embodiment, the hole diameter was 0.125 mm and the hole depth again 0.30 mm with a wall thickness of 0.40 mm. The hole spacing was 0.2~ mm and the pitch }
,likewise 0.25 mm. This correspond's to 16 holes/mm2.
The hole pattern which is obtained by the laser treatment is very regular and reproduc;ble, since the pulse energy and pulse rate of the laser can be adjusted accurately. The number of holes per unit of surface can be'altered by altering the pitch and the spacing between the holes along the helical line. The individual holes ~- will be'very regular. The method of producing micro-depressions in this manner is clean and contact-free, since the material which is removed by means of the laser is gasi~ied instantaneously and blown away. The treatment is carried out in an inert atmosphere by a stream of argon being blown towards the tube during the treatment.
.. l lo . Figure 4 shows a vascular prosthesis with holes li arranged in the same manner as in Figures 1 and 2.
Embedded in the tube wall, however, is a circular-. knitted stocking 17 of textile material, preferably .
of Dacron, so that the tube has a portion lOA at the outside of the stocking, in which the holes 11 aré
provided, and an. internal portion lOB at the inside of the stocking, which forms the interior of the vascu-lar prosthesis. The stocking 17 serves to bring about the anchoring of the suture during the connection of the.vascular prosthesis, so that it is not necessary to rely only on the material of.the tube for the anchoring of the suture, and eliminates the risk of the'suture cutting t-hrough.this material. The stocking 17 can be 15 laid in the material in the tube during the moulding, ' injectionmoulding or extrusion thereof. This embodi-ment is a'lso interesting from the point of view that the tube can be.produced of a composite material, meaning that the portion lOA is of a material different from that of the portion lOB. In this manner, a material - can be selected for the portion .lOA which is suitable for laser treatment and for growing-in and infiltration of surrounding tissue, while a material can be selected for the portion lOB which is sui'table to form the lu~en of the vascular prosthesis, i.e. a material without a tendency to form thromboses or thromboembolisms.
In the embod;ments sho~n ;n Figures 1, 2 and 4, the holes 11 are disposed radially, i.e. they are at right angles to the external surface of the vascular prosthesis and to its longitudinal axis. The l'aser may, however, be directed obliquely to the external surface of the vascular prosthesis but sti'll in a plane which is at right'angles to the longitudinal axis of the . tube lO and in such a case the holes 11 are set obliquely as shown in Figure 5. A suitable inclination to the radial direction is of the'order of 45. As a result of the inclination of the holes, these can be made longer at unaltered .

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wall thickness and an unaltered spacing between the inner ends of the holes and the boundary surface of the lumen. The inclination and the increased length of the blind holes afford a better anchoring of the vascular prosthesis during the growing-in of surrounding.tissue into the implanted vascular prosthesis.
Figure 6, to which reference is now made, illustra-tes a further development of the embodiment as shown in Figure~;5,according to which the blind holes are in-.cl;ned alt~rnately to one side and.the other but stillextend in planes which are at right angles to the longi-tudinal axis of the tube. Holes llA are situated on a helical line with a pitch of 0.60 mm substantially at 45 to the radial direction, i.e. substantially at`45 to the external surface of the tube, while holes llB
are disposed on another helical line with a pitch of 0..60 mm but offset axially by 0.30 mm in relation to the first helical line. These holes also form an angle of substantially 45 to the radial direction of the.
tube but are inclined in the opposite direction to the holes llA. All the holes haYe their axes i.n planes which are at right angles to the longitudinal axis of the tube.
Thus, in two adjacent rows of holes, the holes are directed obliquely in opposite directions, and this system of holes ~an be produced during a single feed of the tube if the laser beam is d;vided into two beams . of equal intensity by means of a beam splitter. Figures . . 7 - 9 illustrate method and apparatus for producing the 30 hole pattern as shown in Figure 6.
With reference mainly to Figure 8, the beam of light from the laser 15 is directed towards a semi-transparent mirror 17 at an angl.e.of 45, and the divided beam is transmitted further on the one hand ~ia 1~7~4~9~

two mirrors 18 and 19 to a lens 16A and on the other hand via a mirror 20 to a lens 16B. As can be seen also' from Figure 7, the lenses 16A and 16B are s'ituated at opposite'sides of the tube 10 set up in the lathe to direct their beams towards the tube from opposite dir'ections. This is also illustrated in Figures g and 10, and as can best be seen from Figure 10~ the beams' impinge on the tube below the axis at the points 21A
and 21B, which' are offset by ~5 from the horizontal plane, to produce the holes llA and llB,respectively.
As a result of the fact that the tube 10 is in- !
clined i'n relation to the position in Figure 9, where the beams' from the lenses are pérpendicular to the tube axis, so-that the tube assumes the position 10', Figure 9, the holes 'no longer come to lie with their axes in planes which are at right angles to thP longi-tudinal axis of the tube. This can be see'n more clearly from Figure 11. A hole llA on one helical line can therefore be brought to join at the bottom thereof, ti-e bottom of a hole llB on the other helical line, so that together the holes form a duct which enters the 3:
tube wall from its outside at one point and comes out again at another point on the outside of the tube wall.
This is illustrated in Figures 12 and 13 where the holes llA' and llB' are shown connected to one another, and it will be seen'apparent that there are 3 holes in the peripheral direction between the two-connected holes, which are situated one in each of two rows of holes located beside one another in the axial direction. As' a result of this arrangement, the anchoring of the implanted vascular prosthesis in the surrounding tissue can be further considerably improved by the grQwing-in and infiltration of the tiss-ue in the holes. It is, of course, conceivable to allow the holes'to be connec-ted otherwise than as shown in Figure 12. The fact that ., " ~

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- the connected holes cut through other adjacent holes . does not matter.in itsel~. In other words by disposing the holes as shown in Figuresl2 and 13, it is possible to:achieve that the surface layer of the vascular pros-: i thesis acquires.the character of.'a porous material with - -~
communicating'.cells. It is desirable that the surface layer should be as porous as possible without jeopard-izing the strength. '-. In the embodiment shown in Figure 14, the holes disposed on one and the same helical line are connected ' ~ to one another, a hole llA substantially at an angle of 4'5 to the external surface of the tube communicating at its inner end with a`nother hole llB which is situated on the same helical line as the hole llA and.is directed substantially at an angle oi 45 to the external surface of the tube but is inclined in the opposite direction to the hole llA. Thus,the holes llA and llB meet at their inner ends substantially at a right angle. The hole llA
has its mouth at the external surface of the tube in' common with another hole llB', inclined in the opposite direction to the hole llA,-and the hole liB has.its mouth on the external surface of the tube in common wlth another hole 11 A-' inclined i'n the opposite direction'to the hole llB. Between each.pair of holes which communi- i cate with one another at the inner ends, there lie two hole mouths a'nd each hole cuts through two holes which originate from these mouths. In ~igure 14, the tube wall has a reinforcing stocking 17.
The apparatus shown in Figures 15 and.l6 is set up for the production of the embodiment as shown in Figure 14 but can also be used for the production of other described embodiments of the vascular prosthesis according to the invention. It comprises an elongated stand 22 with,parallel, guide bars 23 on which a carriage 24 is guided for displacement. Mounted on this carria~ge '` ' ;s an electric motor 25 with a change speed gear 26 to the outpu~ shaft of which there is coupled a shaft 27 which extends parallel to the guide bars 23 and ca~ be rotated,by means of the motor at an accurately adjusted 5 rate of,rotation. The output shaf,t of the change spe.ed gear is also coupled to a shaft 28 which is parallel to , the.shaft 27 and is mounted 'in a headstock 29 mounted ' on the carriage, by means of a play-free transmission 30 with the ratio 1:1; this transmission may consist, 10 for example, of a so-called Daimler-Benz gear. Mounted on, the shaft 28 is a chuck 31 for securing the tube 10 ` which is to form the vasc,ular prosthesis and which is passed onto a mandrel.
Fixedly mounted in, the stand 22 ;s a linear feeder 32, 15 the bar 27 being engaged therewith. By the rotation of the bar, this linear feeder causes an axial displace-ment of the bar accurately adjusted to the speed of . rotation, and the linear feeder which is adjustable with re-qard to the relationship between speed ~f rotation and displace-20 - ment. On axial displacement of the shaft to the left , with regard to Figure 15, during the rotation of the shaft', the shaft ,is taken up in a protective tube 33 which i,s,disposed..in the stand in parallel to.the guide bars 23.
The tube 10 extends through a guide bushing 34 which 'is disposed on a carriage 35 which can travel freely on the guide bars 23. This carriage -is initially placed substantially in .the middle between the change speed gear 26 and the linear feeder 32. The shaft 27 passes freely through the carriage. Two fixed guide bushings 35 are also disposed on the stand. When the - shaft 27 is rotated and at the same time is dispiaced axially, the carriage 24 is pulle,d towards the left displacing the tube 10 axial'ly through the guide bush-ings 34 and 35 into a guide tube 36 which is fixedly mounted in the stand,in parallel to the guide bars Z3, the tube 10 being rotated at the same time and at the same s~eed as.the shaft 27 via the tra'nsmission 30.
A laser of the kind indicated,.ea'rlier, partially indicated at 15, is stationarily mounted associated with the stand 22. The bundle of rays from the laser i,s directed towards a semi transparent mirror 17 at an-angle of 45, and the divided beam is transmitted .. ..
further on the'one hand via a mirror 18 to a lens 16A
and on the other hand via a mirror 20 to a lens 16B.The lenses are adapted to direct their bPams towards ' the tube 10 set up in the apparatus at an anglé of 90 ~ ¦~
to one.another; see Figure 17. The mirror 17 is disposed ~¦
- in a,unit 37 which is mounted o,n the iaser 15, while !i .15 the mirror 18 and the lens 16A are disposed in a unit 38 which is connected to the unit 37 by means of a ' ~l rotary coupling 39, and the mirror 20 and the lens 16B ~1, are disposed in a unit 40 which is connected to the unit.
37 by means of a rotary coupling 41 so that it is 20 possible to direct the l'aser,beams in various directions ,~
towards the tube 1Ø For the production.of the vascular -' prosthesis as shown in Figure 14, th'e lase.r beams are ..
directed,radially 'towards the tube to produce holes on a helical line when the laser is pu.lsated with simultaneous .~
25 rotation and axial displacement of the tube 10 on rota- 'I
tion of the motor 25 The guide bushing 34 provides a support for the tube so that it does not hang down or shake during the treatment. The carriage 25 gradually entrains the carriage 35 in its movement.

35 ' ' 5 1l ~i 7~D4C~4 Various modifications are possible within the scope of the invention and the invention is not limited to the specific embodiments shown in the drawings. For example, the outer surface of the vascular prosthesis can be covered with holes in a much more dense pattern than that iliustrated in the drawings, and even so densely that the entrances of adjacent holes partly overlaps each other. Furtherl it is possible to cover the surface with holes having rather a conical shape, that is that the diameter of the holes is gradually decreasing inwards. Such holes are easily obtained by suitable focusation of the lense system of the laser. With regard to possible coatings on the inside of ~he prosthesis one may, as a supplement or alternative to possible heparinization, provide the prosthesis with a thin hydrophilic coating containing e.g. polyvinylpyrrolidone or other innocuous hydrophilic polymers known to prevent the formation of blood-clots and giving a slippery surface with low frictional resistance against water-based liquids such as blood. Methods ~or obtaining such hydrophilic coatings are known in connection with production of catheters; see e.g. German laid open specification 2,828,617.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A vascular prosthesis in the form of a flexible tube which can be implanted in animals or human beings, wherein at least one outer surface layer consists of a soft synthetic material with microdepressions in the external surface of the tube, characterized in that the microdepressions consist of a regular pattern of blind holes which are regular with regard to direction, transverse dimensions and depth and are produced by laser treatment.

2. A vascular prosthesis as claimed in claim 1, characterized in that the holes have a circular or elliptical cross-sectional shape.

3. A vascular prosthesis as claimed in claim 1 or 2, characterized in that the holes are disposed radially.

4. A vascular prosthesis as claimed in claim 1, characterized in that the holes are disposed obliquely to the radial direction.

5. A vascular prosthesis as claimed in claim 4, characterized in that holes situated axially beside one another are disposed oppositely at an angle to the radial direction.

6. A vascular prosthesis as claimed in claim 5, characterized in that the holes are inclined at an angle to the radial direction of the order of 45°.

7. A vascular prosthesis as claimed in claim 4, characterized in that the holes are disposed obliquely to the radial direction both peripherally and axially.

8. A vascular prosthesis as claimed in claim 7, characterized in that holes which are oppositely inclined are interconnected at their inner ends to form a duct which extends from the external surface of the tube into the tube wall and then out of this again to emerge at the external surface of the tube.

9. A vascular prosthesis as claimed in claim 1, characterized in that the holes are disposed on one or more helical lines.

10. A vascular prosthesis as claimed in claim 9, characterized in that holes which are inclined in one direction, are disposed on one helical line, while holes which are inclined in the other direction, are disposed on another helical line.

11. A vascular prosthesis as claimed in claim 1, characterized in that the material in the tube consists of polyurethane with a hardness between 20 and 25° Shore.

12. A vascular prosthesis as claimed in claim 1, characterized in that the hole diameter is in the region of 50 - 200 µm.

13. A vascular prosthesis as claimed in claim 1, characterized in that the hole diameter is in the region of about 100 µm.

14. A vascular prosthesis as claimed in claim 1, characterized in that the spacing of the holes is in the region of 9 - 16 holes/mm2.

15. A vascular prosthesis as claimed in claim 1, characterized in that the thickness of the material between the bottom of the holes and the inside of the tube wall amounts to at least 25% of the wall thickness, preferably 30 to 35%.

16. A vascular prosthesis as claimed in claim 1, characterized in that the wall thickness of the tube is 0.75 -1.00 mm.

17. A vascular prosthesis as claimed in claim 1, characterized in that a reinforcing stocking of textile material is embedded in the tube wall in a part thereof which is situated between the bottom of the holes and the inside of the tube wall.

18. A vascular prosthesis as claimed in claim 1, characterized in that the internal surface of the tube wall is heparinized.

19. A method of producing a vascular prosthesis as claimed in claim 1, comprising a flexible tube with a regular pattern of blind holes which are regular with regard to direc-tion, transverse dimensions and depth, in the outside of the tube wall, characterized in that the holes are produced by means of a pulsated laser beam along one or more helical lines, the tube being rotated and displaced axially at the same time.

20. A method as claimed in claim 19, characterized in that the laser beam is directed towards the tube along a chord of the circular cross-section of the tube.

21. A method as claimed in claim 19 or 20, characterized in that two laser beams are directed towards the tube from opposite sides thereof.

22. A method as claimed in claim 19 or 20, characterized in that two laser beams are directed towards the tube sub-stantially at right angles to one another.