Why nitinol is used in stents?

1. Classification of nitinol stent

• According to the surface state, nitinol stent can be divided into bare stent, drug coated stent, polymer coated stent, metal coated stent, radioactive stent and artificial blood vessel covered stent;
• According to the implanted parts, the stents were esophageal stents, intestinal stents, tracheal stents, biliary stents, urethral stents, peripheral vascular stents and intracardiac vascular stents;
• According to the time of stent implantation, they were divided into permanent stent and temporary stent.
• According to its design, nitinol stent can be divided into five types.

(a)Bent wire

This kind of scaffolds can be sewn, pressed or welded together by bending round or flat wires into repeated sinusoidal flower cavities, rolling into spiral rings. The performance of the scaffold can be changed by changing the sinusoidal geometry, the number of repeating units, the geometric size of wires and the connection mode between rings. This bracket usually provides moderate flexibility, radial ring strength, and minimal size reduction.

(b)Braid wire

The bracket is formed by knitting a plurality of round or flat wires on the mandrel. The filament forms a helix on the mandrel, in which the clockwise filament and the counterclockwise filament are braided together. The performance of the scaffolds can be changed by changing the number of strands, the geometric size, the weaving cavity and the weaving angle. This bracket usually provides good flexibility, lower radial ring strength and larger size reduction.

(c)Coild wire

The bracket is formed by gradually winding single or multiple strands of round or flat wires on the mandrel. The performance of the bracket can be changed by changing the number of wires, the geometric size of wires and the coil spacing. This kind of bracket usually provides good flexibility, high radial ring strength (coil is easy to collapse under some loading conditions) and large size shortening.

(d)Laser cut tube

This kind of scaffold is made by laser cutting “cells” with complex arrangement of coloratures on a small diameter tube. These “cells” are expanded to the required geometric size and heated to shape. The diameter of the tube is required to be as close as possible to the geometric size of the stent in order to ensure that the characteristics of the designed lumen will not be lost in the process of expansion. The performance of scaffolds can be changed by changing the thickness of tube wall, the geometry of single cell and the connectivity between cells. These stents usually provide moderate flexibility, high radial strength, and small or no size shortening.

(e)Laser cut sheet

This kind of scaffolds is made by laser cutting out “single cells” arranged in complex flower cavities on the plate, rolling them into tubes, welding or pressing them all the way. The final size of the scaffold is only limited by the width of the plate, the shape of the cell, and the connectivity between cells. These stents usually provide moderate flexibility, high radial / ring strength, and small or no size shortening.

• Nitinol stents can be divided into three types according to their transformation temperature.

（a) Self expanding stent with AF temperature around body temperature. When the implant is cold, the shape of the implant will be restored by hyperelasticity at body temperature.

（b) Self expanding stent with AF temperature above body temperature. The shape of the implant does not change when it is implanted under the body temperature, but when it is heated above the body temperature, the shape will recover.

（c) Balloon expandable stent with AF temperature above body temperature. When the stent was implanted at body temperature, the NiTinol alloy was in martensite state, and the stent diameter was enlarged by balloon dilation. When it is necessary to remove the stent, the stent is heated locally, the shape of the stent is reduced, the stent is clamped tightly on the sheath, and the stent is taken out of the body.

2. Characteristics of nitinol stent

In recent ten years, NiTinol alloy stent has been widely used in the treatment of narrow human lumen. In general, the development history of various stents is similar, which has experienced improvements from spiral coil structure to mesh woven structure, laser cutting tubular structure, from bare support to polymer coating or polymer tube package, from shape memory effect type to super elastic self expansion type, from long-term implantation to short-term implantation and recovery.

The initial support products are spiral coil structure, which is made of Nitinol alloy round wire, flat wire or thin thin sheet. It is divided into two structures: dense winding and sparse winding, mostly dense winding structure, which belongs to shape memory type. The diameter of coil is reduced by using ice salt water and other cooling supports. The stent is implanted in the body and heated by temperature, which makes the shape of the stent recover.

The advantages of nitinol stent:

• Easy to take out, spray cooling liquid by spray tube, make the stent soft, clamp the bracket coil with clamp, pull out in vitro, suitable for temporary implantation;
• The dense winding structure can avoid the tumor growth inward, causing the tumor to be narrowed again.

The disadvantages include:

• The spiral coil can not be wound too thin, which causes the diameter of its input device to be large;
• The shortening of length direction is too large when shape is restored, and the shape of each part is not synchronized, which makes the coil not closed completely, bending or twisting easily;
• The flexibility of the stent is poor, and the adaptability to the curved cavity is poor;
• The expansion and pulling out of the stent need special conduit to spray cold and hot brine, which is difficult to operate;
• The anti-corrosion performance of this kind of structure stent

The support products developed later are nickel titanium alloy wire woven grid support. It is mainly made of single silk, which belongs to self expansion type. Compared with the Ni Ti alloy coil support, the braided grid support is:

• Flexible and flexible, easy to pass through narrow segments;
• It can make large pipe diameter and ensure sufficient support strength;
• The diameter of the stent becomes smaller, the system is simple to operate and the expansion rate is high.

The disadvantages are:

The processing process is complex, the technical requirements are high, the cost is higher.

The quality of the NiTinol alloy coated stent has the advantages of:

• It can greatly reduce the probability of tumor endogenous growth;
• It can block all kinds of fistula.

The disadvantages are:

• The surface of the covered stent is greasy and difficult to adhere to the inner wall of the cavity, so the covered stent is displaced,There is a great danger in the management of the disease;
• It affects the function of the inner surface of the lumen, such as the secretion of mucus.