Z. Lekston

Surface Structure of NiTi Alloy Passivated by Autoclaving

Sterilization of the NiTi alloy in boiling water or steam causes passivation, which results in an amorphous 3.5 nm thick TiO2 layer on the surface. Between the surface and the matrix a transition layer of Ni2O3 and NiO was observed, using the X-ray photoelectron spectroscopy. Differences in sterilization conditions affect the amount of metallic nickel on the surface.

Structure and Properties of Cold-Worked and Annealed Ti-Ni-Co Shape Memory Wires Designed for Medical Application

The effect of plastic deformation by drawing and rolling in the martensite state and final annealing on the structure, grain size, martensitic transformations and shape memory properties of TiNiCo wires is described in the paper. The wires were cold worked by 30 and 40% and then annealed at the temperature range 300-600oC. The structure, phase composition and transformation behaviour of samples after processing and annealing were studied using transmission electron microscopy (TEM), X-ray diffraction (XRD) method and differential scanning calorimetry (DSC) technique. The marformed and annealed at 400 and 450oC wires exhibited a very good superelastic behaviour. The obtained wires were used for production of the shape memory staples with the recovery temperatures close to the human body temperature and as well as for small superelastic staples for fixation of mandible bone fractures.

Superelasticity of NiTi Ring-Shaped Springs Induced by Aging for Cranioplasty Applications

This paper concerns the application of titanium-nickel rings in modeling the cranium. After being fixed to the osseous margins, the ring’s expansion at the same time broadens and shortens the cranium vault. The rings formed from a straight superelastic wire, flattened to an ellipse, do not show the presence of a typical force plateau but rather a pseudoelastic loop during loading-unloading in the relationship between the force and the deflection. Based on the idea that superelasticity in more complex shape-springs may be induced by the precipitation hardening process, the further studies were carried out on alloys with higher nickel contents (51.06 at.% Ni). The rings that had been formed were welded and aged at an optimal temperature and time. The improved superelastic behavior during compression and unloading the rings was obtained by introducing small deformation by drawing the quenched wires before forming the rings and aging. Very positive clinical reshaping by long-term distraction with the superelastic ring-shaped springs was achieved in young children under one year and a less spectacular effect was observed in the group of older children.

Low-Temperature XRD Study of Phase Transformations in NiTi and TiNiCo Shape Memory Alloys Used for the Preparation of the Prototypes of Medical Implants

The NiTi shape memory alloys with ternary additions of Co are being considered for future applications in the construction of medical products. In this study the commercially available Ni50.8Ti49.2 medical alloy and Ti50Ni48.7Co1.3 alloy obtained by the conventional VIM technique, were used to prepare shape memory and superelastic staples. The phase transformations in the wires of those alloys after various thermo-mechanical and thermal treatments have been defined by the differential scanning calorimetry (DSC) method and three-point bending and free recovery ASTM F2082-06 tests. In this work the courses of phase transformations in the studied alloys were investigated by means of the low-temperature X-ray powder diffraction method. In both alloys after cold working and annealing during cooling two phase transformations occur: from parent B2-phase to rhombohedral R-phase and monoclinic B19’ martensite phase. Such phase transformations are fully reversible during heating and the obtained characteristic temperatures from DSC and X-ray powder diffraction measurements are in good agreement.

Influence of Low Temperature Glow Discharge Nitriding and/or Oxiding Process on Structure and Shape Memory Effect in NiTi Alloy

In order to increase corrosion resistivity of the NiTi alloy the surface is covered by layers. Layers can be made from such as titanium nitrides and/or oxides as well as their mixture. Recently, a glow discharge technique has been applied for coatings formation. However, the deposition process requires to be done at elevated temperature. Therefore, it may have a negative effect on the structure, which is responsible for the shape memory phenomena. The results obtained from studies, done over the influence of the glow discharge nitriding and combination of nitriding and oxidizing process on the structure, the kinetics of martensitic transformation, the one-way shape memory effect and the superelasticity effect of the NiTi alloys are reported. The results showed that during deposition process, curried out at temperatures above 250°C and for time up to 30 minutes, the precipitation of dispersive particles of Ni4Ti3 phase already starts and has a positive effect on the superelasticity phenomena. The applied deposition technique does not affect also negatively the shape memory effect.

Phase Transformation in Ti-Ni-Ta Shape Memory Alloy

A new Ti50Ni48.7Ta1.3 shape memory alloy was designed for medical application. In order to influence the martensitic transformation temperature the alloy was solution treated and additionally aged at 400oC for various time. Phase transformation was studied applying differential scanning calorimeter (DSC) and X-ray diffraction techniques. The ageing causes that the martensitic transformation occurs in two steps: B2↔R↔B19’ during cooling and heating. During cooling the transformations: B2→R and R→B19’ are well separated whereas on heating they are overlapped. Also ageing causes a shift of temperatures of the martensitic transformation into the higher region. It is due to the precipitation process. Precipitates of the Ni4Ti3 phase were observed. Applied thermal treatment leads to shift of the transformation temperatures below temperature of a human body. This makes the Ti-Ni-Ta alloy attractive for application in medicine.

Structure of the alloy Ti50Ni48.7Co1.3

The structure of the R phase of the alloy Ti(50)Ni(48.7)Co(1.3) was determined by the crystallogeometric analysis of martensitic transformations in titanium nickelide. This analysis is based on the concept of cooperative thermal vibrations of the (110)-like planes in the b.c.c. and B2 structures and their freezing (transformation to static displacements) at the point of the phase transition. The space group of the R phase is trigonal P31m with a hexagonal Bravais lattice. The validity of the R-phase structure determination is confirmed by an X-ray diffraction pattern that was obtained at room temperature using Cu Kalpha radiation. The Rietveld procedure was used to compare the calculated and experimental X-ray diffraction patterns. The proposed structural model of the R phase agrees satisfactorily with the experimental data.

Shape memory compression anastomosis clips in gastrointestinal surgery in dogs

This paper describes clinical experiences with the use of shape memory nickel-titanium (NiTi) clips in gastrointestinal surgery in dogs. Side-to-side small bowel anastomosis was performed in eight dogs where intestinal continuity had to be restored after bowel resection. Billroth’s operation I was performed in one case. Compression anastomosis clips with two-way shape memory were used in all surgical procedures. Intestinal and gastrointestinal anastomoses involving shape memory clips were effective in all patients. Anastomotic leaks were not observed, and all clips were expelled 5–7 days after surgery. The outcomes of surgical procedures performed on canine patients with the use of shape memory NiTi clips indicate that sutureless compression anastomosis is a safe, effective and simple method of restoring gastrointestinal continuity, which can be widely applied in veterinary practice.

Compression Anastomosis Clips Versus a Hand-Sewn Technique for Intestinal Anastomosis in Pigs.

BACKGROUND Anastomotic dehiscence and leaks are major problems in gastrointestinal surgery and result in increased morbidity and mortality. The ideal device to create anastomoses should ensure good serosal apposition without requiring either transgression of the bowel wall or the presence of foreign material for an extended period of time. OBJECTIVES The aim of this experimental study was to evaluate the safety and efficacy of a new compression anastomosis clip (CAC) for jejunojejunostomies and ileocolostomies by comparing CAC anastomoses with hand-sewn (HS) anastomoses in pigs in terms of healing, breaking strength and the time to create anastomoses. MATERIAL AND METHODS The 11 pigs in the study underwent side-to-side CAC and HS jejunojejunostomies and ileocolostomies, for a total of 88 anastomoses. The pigs were sacrificed on postoperative day 5 (5 pigs) or 7 (6 pigs). Macroscopic, histopathological and breaking-strength examinations were performed. The time to create the anastomoses was recorded. RESULTS Neither group had anastomotic complications such as leakage or obstruction. Macroscopic examination showed no statistically significant differences between the groups. In the CAC group, the healing process was characterized by a lesser inflammatory reaction (p < 0.05) and very thin scar tissue at the anastomotic line (less collagen deposition and better epithelial regeneration), while the HS group had a much thicker anastomotic line. The breaking strength was significantly greater in the CAC group compared with the HS group (p < 0.05). The anastomosis time was shorter in the CAC group than in the HS group (p < 0.01). CONCLUSIONS Anastomosis using a CAC appears to be safe and less time-consuming than HS; it was also characterized by a good healing process with little inflammatory reaction and a high breaking strength compared with HS anastomosis.

Structure and Properties of NiTi Shape Memory Alloys after Severe Plastic Deformation

The results presented here concern two NiTi alloys (near-equiatomic NiTi and Ni-rich alloy) subjected to plastic deformation by compression combined with reversion oscillating torsion. The maximal strain obtained was εc = 6.20. Finally the alloys were annealed at the temperature range 300 – 500°C for 1 hour. The structure of the as-prepared alloys was studied with the use of temperature X-ray diffraction and TEM observations. Also the DSC and bend and free recovery ASTM tests were carried out. It was found that the structure consists of a mixture of highly deformed B2 parent phase and B19’ martensite. The TEM studies revealed some amorphous areas in the most strained region of the samples. Annealing at lower temperatures caused formation of nanocrystalline structure that grew to the microcrystalline and finally well-defined polygonized structure in annealed at 500°C specimens. Multi-stage transformation was observed in the annealed at lower temperatures samples.