Surface mechanical properties, corrosion resistance, and cytocompatibility of nitrogen plasma-implanted nickel-titanium alloys: a comparative study with commonly used medical grade materials.

Stainless steel and titanium alloys are the most common metallic orthopedic materials. Recently, nickel-titanium (NiTi) shape memory alloys have attracted much attention due to their shape memory effect and super-elasticity. However, this alloy consists of equal amounts of nickel and titanium, and nickel is a well known sensitizer to cause allergy or other deleterious effects in living tissues. Nickel ion leaching is correspondingly worse if the surface corrosion resistance deteriorates. We have therefore modified the NiTi surface by nitrogen plasma immersion ion implantation (PIII). The surface chemistry and corrosion resistance of the implanted samples were studied and compared with those of the untreated NiTi alloys, stainless steel, and Ti-6Al-4V alloy serving as controls. Immersion tests were carried out to investigate the extent of nickel leaching under simulated human body conditions and cytocompatibility tests were conducted using enhanced green fluorescent protein mice osteoblasts. The X-ray photoelectron spectroscopy results reveal that a thin titanium nitride (TiN) layer with higher hardness is formed on the surface after nitrogen PIII. The corrosion resistance of the implanted sample is also superior to that of the untreated NiTi and stainless steel and comparable to that of titanium alloy. The release of nickel ions is significantly reduced compared with the untreated NiTi. The sample with surface TiN exhibits the highest amount of cell proliferation whereas stainless steel fares the worst. Compared with coatings, the plasma-implanted structure does not delaminate as easily and nitrogen PIII is a viable way to improve the properties of NiTi orthopedic implants.

Corrosion resistance, surface mechanical properties, and cytocompatibility of plasma immersion ion implantation-treated nickel-titanium shape memory alloys.

Nickel-titanium shape memory alloys are promising materials in orthopedic applications because of their unique properties. However, for prolonged use in a human body, deterioration of the corrosion resistance of the materials becomes a critical issue because of the increasing possibility of deleterious ions released from the substrate to living tissues. We have investigated the use of nitrogen, acetylene, and oxygen plasma immersion ion implantation (PIII) to improve the corrosion resistance and mechanical properties of the materials. Our results reveal that the corrosion resistance and mechanical properties such as hardness and elastic modulus are significantly enhanced after surface treatment. The release of nickel is drastically reduced as compared with the untreated control. In addition, our in vitro tests show that the plasma-treated surfaces are well tolerated by osteoblasts. Among the three types of samples, the best biological effects are observed on the nitrogen PIII samples.

Bioactivity of titanium following sodium plasma immersion ion implantation and deposition.

Bio-activation of titanium surface by Na plasma immersion ion implantation and deposition (PIII and D) is illustrated by precipitation of calcium phosphate and cell culture. The bioactivity of the plasma-implanted titanium is compared to that of the untreated, Na beam-line implanted and NaOH-treated titanium samples. Our data show that the samples can be classified into two groups: non-bioactive (untreated titanium and beam-line Na implanted titanium) and bioactive (Na-PIII and D and NaOH-treated titanium). None of the four types of surfaces exhibited major cell toxicity as determined by lactate dehydrogenase (LDH) release. However, the LDH release was higher on the more bioactive PIII and NaOH-treated surfaces. From a morphological point of view, cell adherence on the NaOH-treated titanium is the best. On the other hand, the cell activity and protein production were higher on the non-bioactive surfaces. The high alkaline phosphatase activity per cell suggests that the active surfaces support an osteogenic differentiation of the bone marrow cells at the expense of lower proliferation. The use of Na-PIII and D provides an environmentally cleaner technology to improve the bioactivity of Ti compared to conventional wet chemical processes. The technique is also particularly useful for the uniform and conforming treatment of medical implants that typically possess an irregular shape and are difficult to treat by conventional ion beam techniques.

Investigation of nickel suppression and cytocompatibility of surface-treated nickel-titanium shape memory alloys by using plasma immersion ion implantation.

Nickel-titanium (NiTi) shape memory alloys are increasingly being used in orthopedic applications. However, there is a concern that Ni is harmful to the human body. We have recently investigated the use of nitrogen, or oxygen plasma immersion ion implantation to mitigate this deleterious effect. Our results reveal that the near-surface Ni concentration in all the treated samples is significantly suppressed. In addition, our in vitro tests show that the plasma-treated surfaces are cytologically compatible allowing the attachment and proliferation of osteoblasts. Among the two types of samples, the best biological effects are found on the samples with nitrogen implantation.

Carbon plasma immersion ion implantation of nickel-titanium shape memory alloys.

Nickel-titanium (NiTi) shape memory alloys possess super-elasticity in addition to the well-known shape memory effect and are potentially suitable for orthopedic implants. However, a critical concern is the release of harmful Ni ions from the implants into the living tissues. We propose to enhance the corrosion resistance and other surface and biological properties of NiTi using carbon plasma immersion ion implantation and deposition (PIII&D). Our corrosion and simulated body fluid tests indicate that either an ion-mixed amorphous carbon coating fabricated by PIII&D or direct carbon PIII can drastically improve the corrosion resistance and block the out-diffusion of Ni from the materials. Our tribological tests show that the treated surfaces are mechanically more superior and cytotoxicity tests reveal that both sets of plasma-treated samples favor adhesion and proliferation of osteoblasts.