Date of Award
Master of Science in Materials Science and Engineering (MSMSE)
Dr. Adel Hamammi
Prof. Mohd Fahim
Shape memory alloys are one class of the family of smart materials. They are considered as biomaterials, since they are well accepted by the human body. One special feature of shape memory alloys (SMA) is their ability to revert to their previous shape. This material is deformed at low temperature and when it is heated, it returns (remember) to its original form. Although, the implant binary NiTi SMA is particularly successful as a biomaterial, our NiTi SMA consist of more that 50% (55.7%) of nickel. The main purpose of this thesis was to evaluate the biological response of the shape memory alloys.
In this project, two classes of shape memory alloys were investigated: Nitinol family (NiTi SMA) and capper based alloys (CuAINi SMA). A comparison was made between standard NiTi, oxide NiTi, non-oxide NiTi, and CuAINi SMA. These four materials were subjected to the following experiments. Electrochemical techniques such as potentiodynamic polarization, tafel experiments, and polarization resistance were evaluated to identify the corrosion performance for these alloys. Test specimens from the four materials had been implanted into rat's left legs and near the sciatic nerve to evaluate the tissue response regarding toxicity of nickel release and biocompatibility in vivo. Muscle contraction measurements were evaluated to study muscle response. The electrodes of SMA were removed after a period of 4, 8, 12, and 16 weeks. The muscles were taken for ultrastructure examination under TEM. In these two main experiments, the materials being used were tested via SEM to analyze the surface conditions and microstructure and EDAX to find chemical composition of the surface. Blood analysis, which was collected from rats, was prepared to evaluate the amount of metal release in the blood sample using ICP-AES.
Electrochemical test was evaluated in the physiological medium to investigate corrosion resistance and therefore corrosion rate of the tested SMA. The main finding was that oxide NiTi SMA has the lowest corrosion rate, which indicated that it has the best corrosion resistance. This is due to the oxide layer acting as a protective layer that defends the NiTi from melting. The resulting micrographs agreed with the electrochemical test. Also, it was found that the surface of non-oxide NiTi SMA had pitting corrosion on its surface. It is founded from blood analysis that the nickel release from Nitinol SMA family was less than CuAINi SMA regardless of the time of removal of the electrodes from animals.
The effects of these SMA alloys were considered in flexor muscle of implanted animals as a model for studying muscle contraction and neuromuscular ultrastructure. Comparative analysis of in muscle contractile characteristics has been studied (at 1 Hz, 5 Hz, and 30 Hz nerve stimulation) in control and animals that were implanted with tested SMA at four points: 4, 8, 12, and 16 weeks. Twitch tension (evoked directly by muscle stimulation and indirectly by nerve stimulation) and synaptic delay time were recorded in rats via a transducer connected to a computer system. There was a significant increase in synaptic delay time in animals that were implanted with standard NiTi and the electrode was pushed out earlier at the end-of- test point. Compared to control, the muscle of animal that kept non-oxide NiTi showed reduced twitch tension at all three frequencies. However, there was a significant decrease in twitch tension at the end-of-test point in muscle response of animals that were implanted with CuAINi SMA. This occurred regardless of whether the electrode was left inside the body.
Ultrastructural alterations were determined via TEM. There was impairment in neuromuscular junctions; nerve terminal, nerve axon, and blood pial microvessels. It seems that standard NiTi did not show significant degree of muscle or nerve impairment. Non-oxide NiTi caused significant damage in blood pial microvessels and intramuscular nerves, where it caused the appearance of thrombi that consisted of platelet aggregates, compared with control. Furthermore, the myelin sheath of the intramuscular nerve was disrupted with the degeneration of microtubules and neurofilaments with mitochondria. This finding demonstrated that non-oxide NiTi caused the most damage in the muscle tissue among other tested alloys. Moreover, CuAINi altered the nerve terminal and intramuscular nerve. Furthermore, oxide NiTi showed less synaptic vesicles and degenerated mitochondria in the nerve terminal in neuromuscular junction. These changes are possibly related to alteration in Ca2+ mobilization across muscle membrane. The SMA electrodes, which were removed from the animal's body, showed a small degree of corrosion but this relates to a small pits.
It can be concluded from most of the experimental results that oxide NiTi SMA could be represented as superior when compared to the other three tested SMA. Unfortunately at the end of this thesis, the tested SMA was not biocompatible, as one would expect. The reasons could be from the absence of a good surface treatment and high nickel concentration.
Al-Khalifi, Noura Ali, "Assessment of the Performance of Shape Memory Alloys as Implant Devices" (2003). Theses. 409.