Characterization of Polylactide Layer Deposited on Ni-Ti Shape Memory Alloy
Polylactide (PLA) thin layer was deposited on the surface of the as-quenched NiTi shape memory alloy. First, NiTi alloy was quenched from the 850°C, then its surface was covered with PLA. Deposited PLA is in an amorphous state, whereas the as-quenched NiTi alloy stays in the B2 structure. PLA deposition caused smoothing of the surface and changed its hydrophilic character to hydrophobic one. In general, procedure of PLA deposition does not influence the course of the reversible martensitic transformation. After deformation of NiTi sample covered with PLA up to 4%, its surface does not reveal any cracks and still remains continuous.
KeywordsNiTi polylactide shape memory alloy shape memory effect superelasticity
The Ni-Ti shape memory alloy is frequently use in medical applications. Number of its practical use for implants and medical devices still grows from year to year (Ref 1, 2). However, in respect to the nickel content there still exists discussion over its biocompatibility and corrosion resistance (Ref 3-7). In order to eliminate any nickel diffusion to a human body as well as limit contact between metallic implant and tissue, the surface of the shape memory alloys can be modified by formation of a protective layer. One of the well-known coatings applied for protection of NiTi surface can be titanium nitrides (Ref 8-10), titanium oxides (Ref 11-13), or diamond-like layers (Ref 14-16). Additionally, surface smoothing reduces adhesion of bacteria as well as a biofilm formation. However, too thick and/or stiff layer can limit or completely block shape memory effect. That is why, biocompatible polymers became at the point of interest when covering of Ni-Ti alloys is considered. One of them can be polylactide (PLA). The PLA is biodegradable polymer with degradation time from 1 to 24 months (dependently on composition) and well metabolized inside of the human body (Ref 17). Using its properties it found application in medicine as material for drug delivery, bone fixing, stitching, etc. (Ref 18). In respect to the degradation time, it can be used as a protection layer for short-time Ni-Ti implants. One of them can be: clamps for bone fracture fixing (healing time shorter than 3 months), cranial reshaping springs (healing time shorter than 6 months), or compression clips for anastomoses (healing time about 1 month), etc. (Ref 19).
In presented work the PLA was used as a protective layer deposited on the surface of NiTi shape memory alloy. Structure of obtained layer, its influence on the course of the martensitic transformation as well as the shape memory effect in the covered alloy was studied.
The commercial NiTi alloy with nominal chemical composition: Ti-50.6 at.% Ni was used as a substrate for PLA deposition. Rectangular samples were cut off, with dimension of 10 mm × 8 mm × 0.8 mm. Before PLA deposition, samples were quenched from 850°C to the iced water. Surface was cleaned in ultrasonic cleaner following by chemical etching in HNO3 + HF + H2O solution for 30 s.
In order to cover the NiTi surface, the l-lactide/dl-lactide copolymer 80/20 Purasorb PLA 8038 (PURAC Biochem, The Netherlands) was used. The samples were immersed with a rate of 10 mm/s and dried at room temperature for 24 h. Thickness of the copolymer layers was about 2-3 µm.
Phase identification of the materials was carried out using x-ray diffraction patterns measured in classical Bragg-Brentano geometry in X’Pert Pro diffractometer. Also for the layer characterization x-ray diffraction grazing incident beam technique (GIXD) was applied. All measurements were done at room temperature in 2θ range: 5-140°. Cu Kα1 and α2 radiation was used. Pole figures were measured with use of Philips PW 1138 diffractometer equipped with texture goniometer. Pole figures were registered in reflective mode with α angle up to 80°. Calculation of orientation distribution function (ODF) was done with use of LaboTex computer program.
Observation of surface morphology as well as studies of chemical composition was carried out with use of a scanning electron microscope (SEM) JEOL JSM 6480 with energy-dispersive x-ray spectroscopy (EDS). Topography of the surface was characterized with use of profilograph Hommel Tester T500, Hommelwerke.
Phase transformation was studied by use of differential scanning calorimeter Mettler Toledo DSC-1 (DSC). Thermograms were measured during sample cooling as well as heating with a rate of 10°C/min at thermal range between −125 and 200°C. Weight of the sample was about 14 mg.
Shape memory effect was studied by means of stress-strain curve measurement. The measurement was done on a testing machine Zwick 7000. Tensile test was performed with a rate of 20 mm/min. Length of the sample was 20 mm.
Results and Discussion
In order to verify phase components in the NiTi alloy with deposited PLA layer, the x-ray diffraction patterns were measured with use of the GIXD technique. The measurements were done at the constant alpha angle equals 1°, 0.5°, 0.4°, and 0.3°. An example of obtained result is shown in Fig. 2(c). It can be clearly seen that after deposition PLA still remains amorphous and the NiTi alloy reveals diffraction lines belonging to the B2 parent phase. Decreasing of the alpha angle enables a decrease of the x-ray penetration depth. Going further, it enables determination of the layer sequence as well as phase identification. The decrease of the alpha angle caused lowering of intensities of the B2 phase. Simultaneously, intensities of the broaden peaks belonging to the PLA increased. Such dependence is an evidence that the PLA layer was deposited at the top of the NiTi surface.
Properties of the Surface
Transformation temperatures determined from DSC cooling/heating curves
After PLA deposition
Enthalpy calculated for NiTi alloy before and after PLA deposition
ΔHB2 → R, J/g
ΔHR → B19, J/g
ΔHB19 → B2, J/g
After PLA deposition
Shape Memory Effect
l-Lactide/dl-Lactide copolymer deposited on the NiTi surface changed its character from the hydrophilic to hydrophobic.
Deposited PLA layer with thickness of 2-3 µm, follows elongation, caused by shape memory effect, up to 4% keeping continuous surface without any cracks.
The presence of PLA coatings on NiTi alloy does not influence on temperature of martensitic transformation and shape memory effect.
The PLA coatings can be useful way for NiTi surface protection, when it is applied in medicine for short-time implants.
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