effect of encapsulation coatings on strength

Abstract: Stents, that are covered with encapsulation coatings often present radial and/or longitudinal resistance owing to the polymer membrane that is formed between the struts. This concern is more pronounced with self-expanding braided stents and may reduce the handling properties during stent deployment or insertion of the stent into its sheath.

Author: Dr. Amir kraitzer

Background: Polyurethanes and silicones are often used in medical device applications such as covered stents also known as encapsulation coatings (Figure 1). These coatings form a membrane between the struts and are generally applied to induce durability, or to prevent the detrimental impact of the device on tissue and/or body fluids. These coatings serve is cases such as protective coatings, embolic protection, thrombus retrieval, aneurysm protection, etc..

Polyurethanes are biocompatible, have high tear strength, low coefficient of friction to be used in blood contacting device to refrain from turbulence and reduce friction forces during delivery to the site.  Mechanically, polyurethanes have higher tensile strength of up to 7,000psi, better tear strength 500psi, and better abrasion resistance (Taber abrasion of 25 mg lost per 1000 cycles) than silicone. On the other hand, the mechanical properties of polyurethane including modulus, may increase the recoil force obtained by the covering membrane, which would increase the force during deployment and the recoil force by the stent struts.

Figure 1- Encapsulation coating of a braided stent

One of our customers is using self expendable device based on braided nitinol mesh stent. The customer requested to use polyurethane having a recoil force similar to silicone, meaning a considerable low recoil force during deployment and insertion into its sheath. The force acts to resist deployment is the radial force of the membrane while the force acts during inserting of the stent into the sheath is the longitudinal force of the membrane, both act in tension. When the stent is inserted into the sheath it is compressed and the longitudinal direction is stretched. The coating resists this stretch. The extent of resistance is influenced both by the material properties and the coating’s profile (cross section thickness) which is orthogonal to the stretch direction.

Method

The stent was coated with a thin profile coating (Figure 2) using a low young’s modulus material to present low recoil properties (Table 1). The membrane’s recoil force is influenced by two parameters, one is the material properties or type of polyurethane selected and the other is the cross-section area in the direction orthogonal to the stretching direction. The recoil force is calculated by the stress at a given elongation multiplied by the cross section area. Thus, to reduce recoil force the coating profile was reduced to 40µm (Figure 3) and the material chosen had low modulus.

Figure 2- Left: Medibrane encapsulation Stent coating, Right: cross sectional stent profile

Figure 3 – Membrane thickness, T

The Young’s modulus is a material property.  Usually with elastomers the modulus is not linear thus at each elongation the stress (or force) is measured. The 200% point of elongation is relevant at this case (Table 1).

Table 1: Silicon and Thermoplastic polyurethane properties
Material	Manufacturer	Grade	Stress (psi) at 200% elongation
TPU-10% silicone	Advansource	ChronoSil 75A 10% Si	834
TPU- aliphatic polycarbonate	Advansource	Chronoflex AL 75A	800
TPU-Silicone	Biomerics	Quadrasil™Elast-EON E5-130	725
TPU-Silicone	Advansource	ChronoSil adjusted	570
Silicon	Applied silicone	Dispersion 4000	170

The material of choice was Advansource Chronosil adjusted to have 570psi stress at 200%. Indeed, it presented lower resistance to the insertion of the stent into the sheath compared to coatings having higher profiles, or coated with types of polyurethanes having higher recoil strength. During insertion into the sheath the stent presented lower radial resistance.

Conclusions

The radial resistance of encapsulation coating with polyurethane membrane may be optimized to match required resistance so as to match the medical device requirements. Both the coating profile as well as the material can be selected can together present low redial resistance during deployment and the insertion into the sheath.