Prototyping Braided Textile Stents

 

SWAGAT IRSALE1, SABIT ADANUR1*, STEVE WARNER2, and ELLIOT CHAIKOF3

 

1Department of Textile Engineering, Auburn University, Auburn-AL 36849

2Textile Sciences Department, University of Massachusetts, Dartmouth- MA 02747

3Department of Surgery, Emory University School of Medicine, Atlanta- GA 30322

 

ABSTRACT

        Stents are implanted inside the arteries and vessels to act as a scaffold and keep the vessel lumen open. Braided textile structures are explored for their application as endovascular implants. The object of the study was to analyze the compression properties of braided textile stents. Various braided structures were manufactured with polyester monofilaments and tested for their compression force on Instron materials tester. Denier of monofilament showed statistically significant effect on the compression force of braided textile stents. Factors affecting the in vivo performance of braided textile stents are presented. A novel compression force test is suggested and geometrically compared with the ideal conditions experienced by the braided textile stents during placement on the catheter as well as in vivo applications. A diamond trellis unit is defied as a repeating unit of the braided structures.

 

 

 

 

 

 

Keywords: Coronary Artery Disease (CAD), stent, braiding, monofilaments, compression force, modeling.

 

 

Evolution of Endovascular Implants

 

Endovascular stents are being implanted successfully for more than a decade to combat coronary artery disease (CAD), keep artery lumen open, and avoid abrupt closure of arteries. Stents have proven very effective and less invasive, hence grown recently up to $ 1.5 billion industry [1]. Dotter [2] pioneered the ‘catheter deliverable prosthesis’ implantation by deploying open centered stainless steel coil springs in canine model which he found very advantageous in compressing the plaque and opening the artery lumen. Sigwart et al. [3] developed and successfully implanted an intravascular metal stent in human arteries with the aim of preventing restenosis and sudden closure of diseased arteries after angioplasty. Thus the stents exhibited very promising role in combating CAD and restoring blood flow thorough a diseased artery.

Various textile structures contribute to several diverse biomedical applications starting from wound sutures to cardiovascular implants. The use of textile structures for vascular applications started with the successful implantation of vinyon N cloth tubes for bridging arterial defects in fifteen dogs by Voorhees et al. [4]. Hufnagel and Rabil [5] implanted orlon, nylon, polyester and glass fiber tubes for vascular applications in dogs. Wesolowski et al. [6] conducted a three year study involving over 350 growing pigs and dogs with implantation of thirty seven different synthetic fabrics for vascular applications.  Several textile fibers, yarns, and fabric structures have also shown excellent biocompatible properties. King et al. [7] provided detail information about a wide variety of _____________________________________________________________________________________________

To whom correspondence should be addressed: Phone: 1-334-844-5497, Fax: 1-334-844-4068, Email: sadanur@eng.auburn.edu

polyester filaments, yarns and fabrics used in commercial biomedical devices. He mentioned polyester as the most preferred fiber for biomedical applications. Shumacker and King [8] successfully implanted blood impervious nylon tubes as arterial substitutes in humans. In the present study prototypes of braided textile stents were manufactured with the available textile machinery and tested for their compression properties. The aim of the current study was to provide detail analysis of compression properties of prototype braided textile stents.

 

Materials and Methods

 

Polyester monofilaments of 150 and 1100 denier were selected and braided structures were manufactured for 0.4, 0.6, 0.8, 1.2, 1.6, 1.9, and 2.54 cm diameters and 60˚ braid angle. All braided structures were heatset for 60 minutes. Compression force of all structures was measured and the results were analyzed with ANOVA. Denier of monofilament showed statistically significant effect (p = 0.000538) on compression force of braided structures. The 150 denier monofilament braided stent structures were selected due to their flexibility.

All prototype braided textile stents were manufactured with 150 denier polyester monofilament yarn. Each stent had a total of 64 individual filaments braided as regular braid on Wardwell composite braiding machine. Best heatset temperature (182 ˚C), braid angle (60˚) and braid heatset time (60 minutes) were selected based on the previous studies carried by the authors [9].  The test apparatus used for testing compression force of braided textile stents is shown in Figure 1. With this arrangement, it was possible to have variable gauge lengths (GLs) for different diameter braided textile stent structures. As the test started, the lower jaw moved upwards and compressed the braided structure (Figure 1-C). A specific distance between the upper and lower jaw was maintained as the end point of the test (test end distance). The ‘test end distance’ between the upper and lower jaw was kept the same (0.2 cm) for all braided textile stent compression tests. This test end distance was further used in compression force modeling.

 

 

 

A

 

B

 

C

Figure 1: Compression force testing of braided textile stents on Instron material tester

 

 

 

 

 

 

 

 

Why Compression Force of Braided Textile Stents Is Important?

 

Coronary artery disease (CAD) is the most common type of heart disease causing from atherosclerosis- the gradual build up of plaques inside blood vessels. Plaques both narrow and harden the arteries. Stents are implanted through catheters to compress the plaque and open the artery lumen for efficient flow of blood. After implantation, stents undergo various forces due to continuous blood flow, vessel wall movements, as well as external force applied by the plaque itself. The braided textile stents must have sufficient compression force so as to withstand the external forces without compressing itself. Once the braided textile stent is compressed the vessel will close and this may ultimately lead to a heart attack. Therefore, the compression force of braided textile stents plays an important role in studying their in vivo performance. Before discussing the compression behavior of braided textile stents it is necessary to understand the factors which affect their in vivo performance. Figure 2 shows a comprehensive collection of factors affecting the in vivo performance of braided textile stents. All possible and known factors are classified into five major categories such as material related factors, design related factors, delivery and implantation related factors, implantation location related factors, and patients health related factors. Analytical model representing performance of braided textile stents may be somewhat difficult due to involvement of large number of different factors, their interaction between each other and variability from patient to patient depending on the patient’s age, sex, health status, eating habits, lifestyle, etc. Also all the factor groups are inter-related to each other in a complex manner. Several researchers from various fields contributed significantly probably on each factor. But a comprehensive theoretical chart was not observed in the cited references, which is given in Figure 2. This study is specifically concentrated on the compression force of braided textile stents, which comes in delivery and implantation related factors category.

 

 

Figure 2: Factors affecting the in vivo performance of the braided textile stents

(Inspired from literature on metal stents in reference [9])

 

The braided textile stents must exert sufficient pressure on the vessel wall to reestablish patency, but not enough to cause damage or rupture to the vessel. The final radius into which the braided textile stent expands within a vessel depends on the pressure exerted by the vessel on the stent. The braided textile stents must be designed to withstand the normal pressures encountered in an artery without occluding or permanently deforming during the application. The braided textile stents should not fail under pressure peaks generated by arterial spasms. The braided textile stents can not be made bulky to achieve this; they have to be flexible to be delivered via catheter, and must offer minimum resistance to blood flow.  Thus braided textile stents must have an optimum balance between material properties and structural design. The factors exclusively affecting the compression force of braided textile stents are braid angle, braid diameter, monofilament denier, total number of monofilaments, and heatset time.

A Novel Compression Force Test Method for Braided Textile Stents

 

The compression force measured in this test was the maximum compression force that can be sustained by the braided textile stents (Figure 1). During the test, the braided textile stents were elongated longitudinally and compressed radially. In vivo, the braided textile stent has to sustain various forces acting longitudinally as well as radially, so the braided textile stent must have sufficient compression force to avoid in vivo compression and ultimate blockage of the artery.

Two similar compression tests were observed in the literature. In the first, the stents were supported in shallow troughs and clamped to each end and compressed. This method was used in an attempt to emulate in vivo conditions [10]. Rieu et al. [11] performed in vitro mechanical tests to assess the stent radial force. The test measured the deformation of coronary stents in a V-stand by using a deformation controlled by a dynamometer. Both authors used somewhat similar compression tests as the one defined in this study. But the validation or comparison of these in vitro tests with the proposed in vivo compression of stents was not observed. In the present study, the defined compression test is geometrically compared with the proposed implantation and in vivo compression.

Assumptions for geometrical analysis of compression test:

  • The braided stent structure is uniformly compressed and the compression force is not up to the level where the monofilaments in the braided structure will deform.
  • The cross-section of a braided textile stent is similar throughout the structure. The braided stent structure is made up of 64 monofilaments.
  • All the monofilament intersections are distributed uniformly in four quadrants.
  • The distance traveled by the monofilament intersections will be similar to those traveled by metal stent struts, if they were tested in this method. After compression all the monofilament intersections travel in perpendicular direction.

 

The braided textile stent structure is divided equally in four quadrants. During compression the monofilament intersections are compressed. Therefore the work done by all monofilament intersections during compression test equals to the product of total displacement of all intersections and force applied. The total displacement of all monofilament intersections is compared with the proposed total displacement of monofilament intersections during implantation. It is assumed that the stents are compressed up to test end distance, which was 0.2 cm.

                In the present study there were a total 64 monofilaments, which made 32 monofilament intersections. The ideal total displacement by all intersections during compression of the braided textile stent while implanting on catheter can be given as

     

Total displacement = 32 × (r – 0.1) cm                                                                                                          (1)

where r = radius of the braided textile stent in cm.

 

The total distance traveled by all the monofilament intersections can be given as

 

Total displacement = [ 2 (Q2+…+Q8) + Q9 + 17 (r – 0.2) ] + [ 14 (r – 0.2) –2 (Q2+…..+ Q8) ]              (2)                              

where Qi  is the distance traveled by monofilament intersections during compression.                                                                                                                                                                                  

From equations 1 and 2, the values of displacements were calculated for 0.4, 0.6, 0.8, 1.2, 1.6, 1.9, and 2.54 cm braided textile stent diameters.

 

                The values of displacement for the proposed compression force test of braided textile stents showed closeness to displacements during the in vitro stent compression. Therefore it is possible that this test can be used in future to correlate the in vitro compression of braided textile stent to in vivo compression. For the purpose of calculation it was assumed that the filament intersections travel in straight line during compression. Due to friction between the filaments in a single intersection, they will certainly deviate outwards and this will add to the total distance which is not considered in the calculation of total displacement. Calculating the actual total displacement by all the intersections may be difficult due to merging of yarn intersections after certain compression during the test. Because of this, the calculated total displacement of all intersections in braided textile stents is less than those of the ideal during actual compression. The proposed compression test can also be correlated to the in vivo compression of braided textile stents as well as metal stents. If the compression force of the artery or plaque is known, then the expected in vivo compression of the stents can be predicted with the proposed test.

 

Modeling of the Compression Force

 

Compliance of textile stents is ultimately the behavior of braided textile stents, when they are radially compressed to place on the catheter and again decompressed while implanting them inside the arteries or vessels (Figure 3). As far as braided structures are concerned, their compliance is mainly the radial resistance of the structure to the force applied. When the stents are compressed, they reach their extended jamming position, then placed on the catheter and again inflated to their original heatset diameter. Longitudinal and radial deformation of textile stents cause changes in stent diameter, stent length as well as braid angle. But the textile stents reach their original heatset dimensions after deploying inside the arteries.

 

Figure 3: Representation of radial and longitudinal deformation of textile stents

 

Assumptions:

  • The braided textile stents are perfectly round, porous tubes with uniform surface and when compressed, reach their extended jamming position and are not compressed after their jamming position so as to avoid permanent deformation in monofilaments.
  • Each monofilament is perfectly circular and has the same diameter, which remains uniform throughout the length in the structure. All monofilaments are equally spaced throughout the braided structure.
  • The defined diamond trellis repeats and generates the braided structure. There is no monofilament slippage or variation in braid take up speed which can disturb the braid uniformity. The design is the same throughout the structure.
  • When the structure is cut and opened, it forms a rectangular structure and the length of braid formed in one carrier rotation always remains the same (Figure 4).

 

Figure 4: Opened braided structure showing the length of braid in single carrier rotation and the length and width of one diamond trellis unit

The cut and opened braided structure as shown in Figure 4 has a total of m number of yarns, m/2 heading in one direction and m/2 in the other direction. From the geometry of the structure, it is clear that 4/m th part of braided structure will have only 4 yarns and form a series of diamond shaped trellises. The total number of trellises along the length 2π r will be m / 4.

                                                                    2 π r

            Width of each diamond trellis = ________ cm

 

                                                                   m / 4                                                                                                        (3)

 

where r = radius of the braided structure in cm.

 

                In Figure 4, ‘l’(cm) is the length of fabric braided in one carrier rotation, the total number of yarns from both directions constituting the edge of length l are m / 2, forming m / 4 number of diamond trellises on the edge of the structure. So the distance between two yarns traveling in the same direction will be the length of a diamond trellis.

                                                                     l

Length of each diamond trellis = _________   cm

                                                                                 m / 4                                                                                                     (4)

 

Discussion

 

The compressive forces applied in this novel compression force test were uniform and unidirectional. Also the effect of friction between the monofilaments was not included. The in vivo forces may be different from those considered in the test method. Also this method did not give the compressive effects due to a localized sharp spur of calcified plaque which is very important as the stents are generally compressed at some point which further spreads throughout the structure. This was the limitation of the test method.

The purpose of a stent is to sustain various forces exerted by the lesions; so the fundamental requirement is to withstand that loading. As the stents are permanent implants it is very important to ensure that they sustain a cyclic load without any damage. Once positioned, the stent may be expanded by inflated balloon and due to structural properties it is very much possible that once implanted the stent diameter can not be changed. A stent implanted in an artery of middle aged person endure (1.5 to 2) ×109 cardiac cycles [12]. The choice of the specific diameter had a major impact on the amount of compressive force, experienced by stent after deployment. The unequal distribution of calcium rich and non-calcium rich plaque inside the artery may lead to eccentric stent dilatation with negative aspects for long term behavior [13]. Palmaz et al. [14] concluded that high radial strength and low elastic recoil are important requirements of any stent design. A large stent diameter and a large open or non-metal surface may cause less intimal hyperplasia, but non-turbulent, fast arterial flow is probably the most important factor in ensuring the long term patency of the vessel. The length of the stent decreases as the diameter increases. The force applied externally on the stents, by the wall pressure can be resolved into two components, one directed along the struts of the stent and one perpendicular to them. The larger the stent diameter, the greater is the component along the struts, therefore, the longer diameter stents are longitudinally stiffer than those of smaller ones [15]. Thus the literature also supports that compression force plays an important role in understanding the in vivo performance of braided structures.  

 

Summary

 

                Braided textile structures are explored for their potential application as endovascular implants. Braided textile stents come in self-expanding stents category and do not need a balloon to inflate. The compressed braided textile stent structure springs back at the site of implantation, when the balloon is retracted. The proposed compression test showed closeness in results to those of ideal compression test. Parameters obtained from the diamond trellis geometry will be used in further quantification of compression force of braided textile stents.

 

Disclaimer

 

The authors make no representation, promise, or imply warranty concerning the suitability of ‘braided textile stents’ for implantation in any living organism. These prototypes were strictly developed for this specific research study and the results and applications are valid and limited only to this study. The results do not approve or endorse the implantation of such prototype braided textile stents devices. The authors have no control over the information given in the references and can not be held responsible for their content and authenticity.

 

Acknowledgements

 

The authors are thankful to the US Department of Commerce through the National Textile Center (NTC) for financial support, Dr. Peter Schwartz, Dr. Lewis Slaten, Dr. Ramsis Farag, of Auburn University- Textile engineering department for their suggestions.

 

References

 

1.        Charonko, J., Ragab, S., and Vachos, P., A Numerical and Experimental Analysis of Cardiovascular Stent Design Considerations, American Society of Mechanical Engineers, Bioengineering Division 55, 63-64 (2003).

2.        Dotter, C., Transluminally Placed Coil Spring Endarterial Tube Grafts, Investigative Radiology 4, 329-332 (1969).

3.        Sigwart, U., Puel, J., Mirkovitch, V., Joffre, F., and Kappenberger, L., Intravascular Stents to Prevent Occlusion and Restenosis after Transluminal Angioplasty, The New England Journal of Medicine 316(12), 701-706 (1987).

4.        Voorhees, A., Jaretzki, A., and Blakemore, A., The Use of Tubes Constructed From Vinyon N Cloth in Bridging Arterial Defects, Annals of Surgery 135(3), 332-337 (1952).

5.        Hufnagel, C., and Rabil, P., Replacement of Arterial Segments, Utilizing Flexible Orlon Prosthesis, A.M.A. Archives of Surgery, 105-110 (1954).

6.        Wesolowski, S., Fries, C., Karlson, K., Bakey, M., and Sawyer, P., Porosity: Primary Determinant of Ultimate Fate of Synthetic Vascular Grafts, Surgery 50(1), 91-96 (1961).

7.        King, M., Guidoin, R., Gunasekera, K., and Gosselin, C., Designing Polyester Vascular Prosthesis for the Future, Medical Progress through Technology  9, 217-226 (1983).

8.        Shumacker, H., and King, H., The Use of Pliable Plastic Tubes as Aortic Substitutes in Man, Surgery, Gynecology and Obstetrics, 287-294 (1953).

9.        Irsale, S., Textile Prosthesis for Vascular Applications, Masters Thesis- Auburn University (Spring 2004). http://graduate.auburn.edu/auetd/

10.     Dyet, J., Watts, W., Ettles, D., and Nicholson, A., Mechanical Properties of Metallic Stents: How do These Properties Influence the Choice of Stents for Specific Lesions? Cardiovascular and Interventional Radiology 23, 47-54 (2000).

11.     Rieu, R., Barragan, P., Masson, C., Fuseri, J., Garitey, V., Silvestri, M., Roquebert, P., and Sainsous, J., Radial Force of Coronary Stents: A Comparative Analysis, Catheterization and Cardiovascular Interventions 46, 380-391 (1999).

12.     Dumoulin, C., and Cochelin, B., Mechanical Behavior Modeling of Balloon Expandable Stents, Journal of Biomechanics 33, 1460-1470 (2000).

13.     Salunke, N., Topoleski, L., Humphrey, J., and Mergner, W., Compressive Stress-relaxation of Human Atherosclerotic Plaque, Journal of Biomedical Materials Research 55, 236-241 (2001).

14.     Palmaz, J., Kopp, D., Hayyashi, H., Schatz, R., Hunter, G., Tio, F., Garcia, O., Alvarado, R., Rees, C., and Thomas, S., Normal and Stenotic Renal Arteries: Experimental Balloon Expandable Intraluminal Stenting, Radiology 164(3), 705-708 (1987).

15.     Grenacher, L., Deutscii, J., Lubienski, A., and Richter, G., Resistance of Hoop Stresses in Balloon Expandable Stents: Evaluation in an Ex Vivo Model, Investigative Radiology 38(2), 65-72 (2003).