A metal alloy of nickel (Ni) and titanium (Ti), nitinol (NiTi) exhibits the properties of shape memory and superelasticity, which make it very useful for adaptation to conformal coatings. However, like parylene, nitinol is often difficult and expensive to produce; the extreme reactivity of the alloy’s titanium component requires exceptionally tight compositional control during combination and manufacture.
NiTi’s vacuum physical deposition (VPD) application process allows numerous applications for implantable medical devices, often of microelectricalmechanical system (MEMS) or nano-technology scale.
Shape Memory, Superelasticity, and Other Specialized Properties
Nitinol’s shape memory properties encompass the capacity of structural deformation at one temperature, followed by return to its original shape after reheating to a point above the transformation temperature. Occupying a narrower temperature range, superelasticity is achieved at temperatures just above nitinol’s transformation level. NiTi then displays a range of elasticity reaching 10-30 times that of ordinary metal. Also, no heating is required to initiate recovery to the original, undeformed shape.
These unique properties result from the martensitic transformation, a reversible solid-state, diffusionless conversion of two different martensite crystal phases. Mechanical stress between 10,000–20,000 psi (69–138 MPa) is required for this transformation to be generated. Nitinol’s parent phase occurs at higher temperatures; called the austenite phase, nitinol then exists in simple cubic structure. Lower temperatures stimulate conversion to a more complex body-centered tetragonal crystal structure; this is nitinol’s martensite (daughter phase).
Transformation in both directions is instantaneous, providing the substance an exceptional malleability very valuable for implant and similar medical uses, where device insertion can be complicated by variations in the physical dimensions of the inner-body areas requiring treatment.
Nitinol has been approved by the Federal Drug Administration (FDA), as a generally a safe material for numerous long-term implant applications. It offers a wide range of material applications for biocompatible medical devices, particularly when used in conjunction with resilient, micron-thin parylene conformal coatings, similarly adaptable to MEMS/nano uses.
Nitinol guide-wire cores and stents are among its most useful applications for specific medical devices. Chemically joining nickel with titanium creates a strong intermetallic bond, limiting the risk of a negative medical reaction, even for patients with nickel-sensitivity. Its unique properties encompass products ranging from surgical tools to permanent implants, including implants within the bloodstream. NiTi also exhibits excellent cytocompatibility and corrosion resistance, characteristics that are useful for adaptation to the purposes of conformal coating, especially when combined with parylene.
In general, soft tissue responses to the insertion of nitinol devices are non-toxic and non-irritating; neural and perineural responses are similarly non-affected. Overall inflammatory and muscular response in the presence of nitinol devices equals or surpasses that of other coating materials, such as stainless steel (StSt) or titanium-aluminum-vanadium alloy Ti with 6% Al and 4% V (Ti-6Al-4V). Development of foreign body giant cells, immune cells or macrophages also compared at similar levels to other coating/implant materials. The combination of NiTi with biocompatible parylene coating materials further diminishes the incidence of negative response to the implanted device.
Larger-scale manufacture of nitinol medical devices is confronted by a number of challenges unique to the alloy. Among the most pertinent of these are:
- the availability of appropriate standard product forms;
- the difficulties of machining nitinol components;
- the extreme precision and cleanliness required of the melting process combing the metals;
- reliable methods for joining nitinol components;
- standardizing techniques for hot and cold working nitinol; and
- uncovering persistently reliable methods for coating or finishing the surfaces of nitinol devices.
These conditions remain the subject of research and experiment for solutions that improve the performance of parylene coated NiTi.
Nitinol in Conjunction with Parylene: A Multifaceted Coating Solution for Biomedical Devices
Nitinol shape memory metal alloy offers a relatively new and generally reliable material for conformal coatings. A primary strength is its ability to prepare functional implants activated at body temperature that withstand device/component kinking better than conventional metals. Combination with similarly resilient and adaptable materials, like parylene, can significantly increase NiTi’s effectiveness and utility.
As surface treatments, conformal coatings are expected to ensure a biomedical device’s ease-of-use, operating function, and service-life, while improving the patient’s overall comfort and welfare. Applied to biomedical substrates through gaseous chemical vapor deposition CVD processing, parylene’s ultra-thin coating capacities allow application into the tightest of spaces, within both
- the device itself and
- the interior of the human body
CVD coating application enables an exceptional range of MEMS/nano uses for devices such as cannulas, guide wires, needles, stents and tubing. These products benefit from surface solutions that combine
- nitinol’s shape memory and superelasticity with
- parylene’s capacity for lubricious, resilient, fully conformal coating,
- making it a good partner coating material for NiTi.
Parylene is effective in reducing intervascular friction occurring with corrosion resistant stainless steel/nitinol coiled or straight guide wires. Micron-thin coating properties enhance parylene’s use with nitinol for smallest-diameter balloons, catheters, continuous-lengths wire, stents and tubing. Technological superiority at Diamond MT allows coating application to surgical blades at 0.0001 in. (2.5µ) without interfering with their function.
In conjunction with a parylene conformal coating, nitinol’s VPD application process can generate MEMS and nano level films. When used in conjunction with parylene’s CVD process, NiTi’s sputtering technique provides a valuable coating option for such medical components or devices as:
- bioMEMS of all types,
- connective materials following surgery,
- corrective bone deformities devices,
- drug delivery systems,
- endontic cleansing,
- filling bone defects in weight-bearing locations,
- intramedullary nails for bone treatment,
- neurovascular implants,
- orthodontic brackets /wires,
- orthopedic implants
- shape-changing stents for arteries/veins
- tendon suture material,
- tubing for catheters, stents, and superelastic needles, and
- wires for locating and denoting the presence of breast and other tumors, so that subsequent surgeries are more accurately informed.
Further developmental research should increase the number and quality of medical device applications for the combined use of nitinol and parylene.
To learn more about parylene coating with medical devices, download our whitepaper: