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Rapid Formation and Testing of Self-expanding NiTi Frames with a Small Form Factor Suitable for Minimally Invasive Implants
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Summary
This work illustrates a low-cost fabrication technique for shape-setting nitinol wires/frames with a small form factor using sacrificial fixtures. The technique is demonstrated for the fabrication of self-expanding frames designed for minimally invasive implants with complex shapes.
Abstract
NiTiNOL (commonly referred to as nitinol or NiTi) wires feature exceptional shape memory and super-elastic characteristics, while shape-setting is often a costly process. Among the steps for this process, heat treatment requires exposure to high temperatures for shape-setting. Traditionally, metal fixtures are used for this purpose; however, their manufacturing costs can be significant, which is not ideal for iterating prototypes. This work demonstrates a recently introduced approach using sacrificial fixtures made of copper tubes, which eliminates the need for expensive fixtures. These copper tubes allow for the formation of complex geometries, and they offer a scaffold for various phases of the fabrication process. Moreover, ammonium persulfate is used as the agent for selective copper etching, which simplifies the production of NiTi frames. This work's findings confirm the effectiveness of this technique, as demonstrated in the successful shape-setting of NiTi wires for self-expanding frames. This methodology paves the way for future research, allowing for rapid prototyping of NiTi wireframes for various applications especially those in medical devices.
Introduction
NiTi wires are widely used in medical implants but require an initial shape-setting process during device fabrication1. Various devices are made from NiTi, including catheter tubes, guidewires, stone retrieval baskets, filters, needles, dental files, as well as other surgical instruments2. NiTi's biocompatibility, superelasticity, and fatigue resistance make it suitable for these applications. Additionally, it has applications in the automotive and aerospace industries3.
Despite the broad applications of NiTi, its usage is limited due to its high cost and complex pr....
Protocol
NOTE: See the Table of Materials for details related to all materials used in this protocol. Figure 1A shows an example of the copper/NiTi frame. Use safety gloves.
1. Iteration of a design of a NiTi frame/prototype
- Align NiTi wire inside copper tubes (or brass tubes; Figure 2A).
- Select NiTi wire (0.008 in) and a copper tube (OD 1.00 mm x 400 mm).
- Turn on the stereos.......
Representative Results
NiTi frames were shape-set into various topologies using low-cost plastic fixtures and hand tools (Figure 1). In protocol steps 1.1 to 1.4 (Figure 1A), NiTi/Cu frames were formed into complex topologies. Following protocol step 1.5, Cu was etched to release the NiTi frames (Figure 1B). Here, the Cu fixture was completely etched away, allowing the NiTi frame to be released using low-cost jigs/fixtures that were 3D printed (step 1.2)........
Discussion
In this protocol, multiple steps require meticulous attention such as the heat treatment (annealing), etching, and design of 3D-printed fixtures. Large variations in temperature from 500 °C 17 or the annealing time of NiTi can be detrimental to the superelasticity of the NiTi wire and to achieving the desired shapes18. Heat treatment with inaccurate conditions (temperature and time) can also lead to a loss of the superelastic property19. The etc.......
Acknowledgements
Research reported in this publication was supported by the National Institute of Biomedical Imaging and Bioengineering of the National Institutes of Health under Award Number R21EB030654. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. S. Alaie and J. Mata also thank the Department of Mechanical and Aerospace Engineering and the College of Engineering at New Mexico State University for their support. The authors thank Oscar Lara and Angel de Jesus Zuniga Ramirez for their contributions in generating Figure 2 and editing the references.....
Materials
| Name | Company | Catalog Number | Comments |
| 304 SS Hypotubes Generic Name: Needle | Tegra Medical | ||
| 3D printed frame for testing Generic Name: PLA filament | R3D | ||
| 3D printed polymeric part for heat press Generic Name: PLA filament | R3DÂ | ||
| Ammonium Persulfate Generic Name: Ammonium Persulfate | Sigma-Aldrich | ||
| Chronoflex AR 22% Generic Name: Polyurethane | AdvanSource biomaterials | aromatic polycarbonate urethane elastomer | |
| Copper Web Type Electrodes (1.00 mm x 400 mm) Generic Name: Copper Tube | Holepop edm supplies &electrodes | ||
| Dilator Generic Name: Dilator | QOSINA | ||
| Ecoflex 00-30 Generic Name: Ecoflex 00-30 | Smooth-on | silicone | |
| Fr 12 or 13 Catheter Generic Name: Sheath | QOSINA | ||
| Nickel Titanium Wire (0.008) Generic Name: NiTi Wire | Malin Co. | ||
| PTFE Teflon rod 1/8" Diameter x 36" Generic Name: Polytetrafluoroethylene | Sterling Seal & Supply, Inc. (STCC) | ||
| Tecoflex Generic Name: Thermoplastic Polyurethane | Lubrizol | aliphatic polyurethane elastomer | |
| Trichloro(1H,1H,2H,2H-tridecafluoro-n-octyl)silane Generic Name: C8H4Cl3F13Si | Sigma-Aldrich | ||
| Dimethylacetamide (DMAC) Generic Name: Dimethylacetamide | Sigma-Aldrich | ||
| SOLIDWORKS Generic Name: Proprietary CAD software | Dassault Systèmes | ||
| FreeCAD Generic Name: Open Source CAD software | freecad.org | ||
| ABS Like Photopolymer Resin Generic Name: Photopolymer Resin | ELEGOO |
References
- Smith, S., Hodgson, E. Shape setting nitinol. Proc of the Mater Process Med Devices Conf. , 266-270 (2004).
- Kapoor, D. Nitinol for medical applications: A brief introduction to the proper....
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