Nickel titanium, also known as Nitinol (part of shape memory alloy), is a metal alloy of nickel and titanium, where the two elements are present in roughly equal atomic percentages e.g. Nitinol 55, Nitinol 60.
Nitinol alloys exhibit two closely related and unique properties: shape memory effect (SME) and superelasticity(SE; also called pseudoelasticity, PE). Shape memory is the ability of nitinol to undergo deformation at one temperature, then recover its original, undeformed shape upon heating above its “transformation temperature”. Superelasticity occurs at a narrow temperature range just above its transformation temperature; in this case, no heating is necessary to cause the undeformed shape to recover, and the material exhibits enormous elasticity, some 10-30 times that of ordinary metal.
There are four commonly used types of applications for nitinol:
- Nitinol is deformed at a low temperature, and heated to recover its original shape through the Shape Memory effect.
- As for free recovery, except that recovery is rigidly prevented and thus a stress is generated.
- Here the alloy is allowed to recover, but to do so it must act against a force (thus doing work).
- Nitinol acts as a super spring through the Superelastic effect.
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Thermal and electrical actuators
- Nitinol can be used to replace conventional actuators (solenoids, servo motors, etc.), such as in the Stiquito, a simple hexapod robot.
- Nitinol springs are used in thermal valves for fluidics, where the material both acts as a temperature sensor and an actuator.
- It is used as autofocus actuator in action cameras and as an Optical Image Stabilizer in mobile phones.
- It is used in pneumatic valves form comfort seating and has become an industry standard.
- The 2014 Chevrolet Corvette incorporates nitinol actuators, which replaced heavier motorized actuators to open and close the hatch vent that releases air from the trunk, making it easier to close.
Biocompatible and biomedical applications
- Nitinol is highly biocompatible and has properties suitable for use in orthopedic implants. Due to Nitinol’s unique properties it has seen a large demand for use in less invasive medical devices. Nitinol tubing is commonly used in catheters, stents, and superelastic needles.
- In colorectal surgery, the material is used in devices for reconnecting the intestine after removing the pathology.
- Nitinol is used for devices developed by Franz Freudenthal to treat Patent ductus arteriosus, blocking a blood vessel that bypasses the lungs and has failed to close after birth in an infant.
- In dentistry, the material is used in orthodontics for brackets and wires connecting the teeth. Once the SMA wire is placed in the mouth its temperature rises to ambient body temperature. This causes the nitinol to contract back to its original shape, applying a constant force to move the teeth. These SMA wires do not need to be retightened as often as other wires because they can contract as the teeth move unlike conventional stainless steel wires. Additionally, nitinol can be used in endodontics, where nitinol files are used to clean and shape the root canals during the root canal procedure. Because of the high fatigue tolerance and flexibility of nitinol, it greatly decreases the possibility of an endodontic file breaking inside the tooth during root canal treatment, thus improving safety for the patient.
- Another significant application of nitinol in medicine is in stents: a collapsed stent can be inserted into an artery or vein, where body temperature warms the stent and the stent returns to its original expanded shape following removal of a constraining sheath; the stent then helps support the artery or vein to improve blood flow. It is also used as a replacement for sutures – nitinol wire can be woven through two structures then allowed to transform into its preformed shape, which should hold the structures in place.
- Similarly, collapsible structures composed of braided, microscopically-thin nitinol filaments can be used in neurovascular interventions such as stroke thrombolysis, embolization, and intracranial angioplasty.
- A more recent application of nitinol wire is in female contraception, specifically in intrauterine devices.
Damping systems in structural engineering
- Superelastic Nitinol finds a variety of applications in civil structures such as bridges and buildings. One such application is Intelligent Reinforced Concrete (IRC), which incorporates Ni-Ti wires embedded within the concrete. These wires can sense cracks and contract to heal macro-sized cracks.
- Another application is active tuning of structural natural frequency using Nitinol wires to dampen vibrations.
Other applications and prototypes
- Demonstration model heat engines have been built which use nitinol wire to produce mechanical energy from hot and cold heat sources. A prototype commercial engine developed in the 1970s by engineer Ridgway Banks at Lawrence Berkeley National Laboratory, was named the Banks Engine.
- Nitinol is also popular in extremely resilient glasses frames.It is also used in some mechanical watch springs.
- Boeing engineers successfully flight-tested SMA-actuated morphing chevrons on the Boeing 777-300ER Quiet Technology Demonstrator 2.
- It can be used as a temperature control system; as it changes shape, it can activate a switch or a variable resistor to control the temperature.
- It is used in cell-phone technology as a retractable antenna, or microphone boom, due to its highly flexible and mechanical memory nature.
- It is used in some novelty products, such as self-bending spoons which can be used by amateur and stage magicians to demonstrate “psychic” powers or as a practical joke, as the spoon will bend itself when used to stir tea, coffee, or any other warm liquid.
- It can also be used as wires which are used to locate and mark breast tumours so that the following surgery can be more exact.
- Due to the high damping capacity of Superelastic nitinol, it is also used as a golf club insert.
- Nickel titanium can be used to make the underwires for underwire bras.
- It is used in some actuation-bending devices, such as those developed by Finnish technology company Modti Inc.