Heat treatment affects the material properties of metals and metal alloys by heating them to a predetermined temperature, maintaining that temperature for a certain period, and then cooling to room temperature. Heat treatment improves material ductility, fatigue strength and corrosion resistance while reducing brittleness. These material enhancements enable medical devices to accommodate concentrations of stress without fracturing and improve their biocompatiblity.
In the manufacturing of metal implants and surgical tools, heat treatment is the only process that influences material properties directly, whereas all other processes determine the geometry, dimensional accuracy and surface quality. STI’s heat treatment capabilities include annealing of hard metal alloys such as stainless steel and cobalt chrome and shape setting of shape-memory alloys, primarily nitinol.
Shape setting configures the transformational and mechanical properties of shape memory alloys, particularly Nitinol (Nickel Titanium). A machined NiTi component is mounted on a special jig or mandrel and then inserted into the furnace for heat treatment, which sets the shape. STI designs and manufactures these mandrels and special jigs according to the exact specifications of the product.
Good bio-compatibility combined with shape-memory and super-elastic capabilities makes Nitinol the material of choice for a wide variety of medical devices, e.g.: Stents, endoscopic and arthroscopic tools, anastomotic devices, guidewires, and embolic coils.
Self-expanding stents, for example, use the stress-strain behavior (stress hysteresis) exhibited by super-elastic Nitinol to exert low chronic outward force on the walls of the target vessel thereby keeping it open while resisting external deformation (crush and kink recovery).
STI optimizes the shape setting process per each product according to the specific mechanical properties required by the product. Shape setting procedures involve several heat treatment steps run in succession. The number of steps and exact heating and cooling profile depend on the original dimensions (as cut), the final dimensions of the part (as specified) and the desired 3D geometry. The last step fixes the final shape of the component.
STI uses state of the art equipment to induce specific thermal cycles of NiTi from austenite (As, Af temperatures) to martensite (Ms, Mf temperatures) using controlled heating and cooling processes respectively (≤ 700°C). STI adjusts the entire process to meet desired Af temperature and RF values within small tolerances and with highly repeatable results.
Shape memory (thermo-elastic transformation)
The video presentation shows the ability of shape memory alloys to change their shape as a result of heating or cooling. At first, a Nitinol spring is placed on a table at room temperature. The transformation temperature and mechanical properties of the spring were previously set by a shape setting process.
Stretching the spring beyond its self-expanding or superelastic capability (elastic memory), causes the spring to undergo plastic deformation. Putting the spring inside hot water heats the spring above its Af temperature thereby causing a thermal transformation which returns the spring to its original shape (thermal memory).
Radial Force (RF) and OD recovery testing
RF / OD recovery test performed on a Nitinol stent verifies that the superelastic properties of the manufactured stent adhere to the exact specifications defined by the customer.
In order to exert gentle outward force on the walls of a vessel, self-expanding stents are manufactured in a larger diameter than the target vessel. Since superelastic or self-expanding stents operate at body temperature their transformation temperature must be set below (Af < 37°C), typically around 30°C. At room temperature or colder, Nitinol stents can be crimped down to a seventh of their original diameter and fitted into narrow delivery systems.
Consider an illustrative example- Nitinol stent designed for 8-10mm biliary conduit tested at 25°C. This particular biliary stent has a crimped diameter of 2mm and an expanded diameter of 10mm. The upper line plots the radial forces measured during crimping, whereas the lower line shows measurements taken during OD recovery. The most important data, from a manufacturing perspective, relates to the end points measured at the crimping target diameter and at full expansion.
Annealing is a heat treating process that modifies the material properties of metals that have different crystal structures at low and high temperatures. Annealing softens the metal, increases fatigue strength and relieves internal stresses in the metal introduced earlier in the laser machining stage. Temperature and time determine the particular thermal cycle. The cycle begins by controlling the heating rate until the metal reaches the annealing temperature, holding that temperature for a certain period, and adjusting the cooling rate according to the desired properties.
Material purity and grain size affect the physical properties of the material. The quality of the original ingot determines material purity, whereas grain size can be influenced by the annealing process, aiming at achieving homogeneous, fine grains.
Homogenous composition and distribution of the grains together with fine grain size improve the material’s break elongation, tensile strength, fatigue strength, corrosion resistance and surface smoothness. These physical properties enable smaller struts, thinner walls, and improved surface quality.
STI uses annealing to improve the mechanical properties of metals, such as: Stainless steel (316LVM), titanium and cobalt-chrome alloys (L605/MP35N). STI performs annealing at various controlled heating and cooling temperatures (≤ 1,250°C). The specific parameters for performing a particular annealing cycle depend on the material being treated and the intended application.
STI uses three furnace systems and has developed specific heat treatment profiles for each of the metal alloys commonly applied for implants manufacturing. To achieve optimal results, STI fine tunes the annealing profile for each product by measuring tensile strength and break elongation in-house and determining grain size through the services of the Technion’s Metallurgic Department.