Raju, Rashmi Mariam; (2020) Joining Technologies for Cardiovascular Implants. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Nitinol (NiTi) is widely used in the medical field, due to its unique superelastic and shape memory properties. Nitinol undergoes thermo-mechanical processing, such as thermosetting, laser-cutting and joining for medical application, however these processes can alter these properties. The Temperature-dependant characteristics of nitinol can work as a disadvantage to fully exploit this alloy for biomedical applications. Three different themes are explored in this thesis, a) the properties of nitinol-nitinol bonds compared to base metal b) understanding how welding method affects bond properties and joint strength and c) investigating the optimum method for bonding nitinol to non-metal such as biopolymers. In order to overcome the above-mentioned limitations, different joining techniques for nitinol was investigated in this study. Initially, the effect of different joining techniques on nitinol properties, in comparison to base metal was investigated. Experimental results mainly focus on the nitinol to nitinol joining techniques, including laser micro welding (Ytterbium-fiber laser and quansi continuous wave laser), capacitor discharge welding, percussive arc welding, adhesive bonding, PEEK shrink tubes and crimping. The effects of these joining techniques on the microstructure, superelastic properties and strength of a joint are analysed and compared. The mechanical tests and analytical results suggest that the use of laser welding should be employed for the development of new and improved medical implants. However, long-term studies are required which are currently underway to further develop this technique. For various medical applications, metal components are mainly used to provide the strength and stiffness whereas polymeric biomaterials can provide unique chemical properties and manufacturing, benefits as they can be moulded and shaped into complex designs depending on the application. However, due to the massive difference in physical and chemical properties of metal and polymers, joining them generates new challenges. Hence, the third part of the thesis explores different approaches to join nitinol to polymeric biomaterials including Polyetheretherketone (PEEK), and polyethylene terephthalate (PET) fabric, commercially known as Dacron®. Due to the difference in chemical and physical properties of nitinol to PET, it was difficult to join them without using an interlayer. Thus, a polyurethane coating was used as an interlayer to bond nitinol to PET fabric. This study also provides an insight into different surface treatments used to improve the bonding between nitinol and polyurethane coating including chemical etching and cold atmospheric plasma treatment. The study shows how an appropriate selection of the joining technique can enhance the exploitation of properties of nitinol alloy in the clinical area, resulting in improvements in the safety and durability of medical devices.

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