<p>A novel method for rapid prototyping and heat treatment of nitinol wires is introduced that integrates cost-effective wire bending techniques with a selective wet etch process adapted from the microfabrication technology. Miniaturized self-expanding frames, widely used in minimally invasive surgeries, are typically made from nitinol wires, sheets, or tubes that are heat treated to maintain desirable shapes. In the process of shape setting, nitinol structures are placed inside a furnace at ~500 °C, while they are confined by a fixture.&nbsp; This heat treatment process induces large stresses; therefore, the fixtures should be capable of maintaining a high mechanical strength at the high temperature. This requirement is often met by using traditionally manufactured or 3D printed metallic fixtures, that in turn impose a high cost for prototyping and testing nitinol structures. Alternatively, winding nitinol wires on metallic mandrels or plates with reconfigurable pins enables a cost-effective process, however its capability in producing complex geometries is limited. &nbsp;Ceramic and wooden fixtures with local heating are another alternative; &nbsp;however their inferior dimensional precision&nbsp;imposes limitations for shape setting nitinol wires with a small form-factor. Here we introduce a novel technique for shape setting nitinol wires that eliminates the need for fixtures operating at a high temperature. This technique is adapted from the microfabrication techniques by using sacrificial copper structures and then selective etching. In this process, nitinol wires are preassembled inside a sacrificial copper sheath or thin hypotube (~350 micron thick). &nbsp;The copper sheath allows for efficient handling of the small form-factor nitinol (&lt;200 microns diameter) and the wire assembly can be shaped using wire bending hand tool or inexpensive plastic mandrels. Upon bending the assembled wire, the sacrificial sheath deforms with plastic deformations at the room temperature. Subsequently, the sheath forms a supporting sacrificial fixture, which restricts the elastic deformation of the inner nitinol. We demonstrated that the stiffness of this sacrificial sheath is sufficient to withstand the forces induced in the nitinol heat treatment. However, after the treatment, the sheath could be etched in a&nbsp; solution of ammonium persulphate in water, and the&nbsp;nitinol wires could be released. The etchant was shown to be capable of etching copper hypotubes without etching/affecting the nitinol wires (~200 micron diameter). The technique was capable of producing complex geometries with radii of curvature ranging from ~2 mm &nbsp;to ~5 cm, with the aid of inexpensive fixtures or traditional hand tools. This technique is ideal for rapid prototyping of self-expanding nitinol frames, that are suitable for transcatheter implants such as septal defects occluders, stent grafts, and implantable sensors. The nitinol wires reinforced by copper hypotubes can be integrated with commercially available wire bending machines to fabricate complex structures with a small form-factor.</p>

Rapid Prototyping of Self-Expanding Nitinol Frames With a Small-Form Factor Using Sacrificial Hypotubes in the Shape Setting Process

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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