Characterization of Polylactide Layer Deposited on Ni-Ti Shape Memory Alloy

Figure 1 shows set of the x-ray diffraction patterns registered for the as-quenched NiTi alloy (Fig. 1a), as-received PLA (Fig. 1b), and NiTi alloy after PLA deposition (Fig. 1c).

The x-ray diffraction pattern measured for as-received PLA (before layer deposition) reveals two broaden diffraction peaks which is a proof of the amorphous state of the sample (Fig. 1b). Contrary to that, pattern measured for the NiTi alloy confirmed the presence of the parent phase B2 (Fig. 1a). However, in the measured range there was no evidence of the diffraction line 100_B2 as well as 200_B2. It was caused by the presence of texture. In order to prove that, the {100}_B2, {110}_B2, and {211}_B2 pole figures were measured (Fig. 2). In general, the analyzed texture is not strong. The maximum of the pole density does not exceed five levels. Identification of the crystallographic orientation revealed, that pole density was concentrated around orientation {111}〈−1 −1 2〉. From ODF calculation it was found that 18% of the grains at the surface was oriented along the identified sheet texture.

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.

Surface of as-quenched NiTi alloy reveals quite rough character (Fig. 3) as a consequence of the etching applied after quenching. It can be seen that the process caused appearance of craters and pinholes. This procedure enabled for an increase of the contact surface between the alloy and PLA. It is worth noting that at the bottoms of the pinholes as well as in some places at surface, precipitates of the Ti₂Ni phase can be distinguished. The average value of the roughness parameters R _t, R _a, R _z measured for the as-quenched NiTi alloy were: 3.43, 0.51, and 2.8 µm, respectively. After PLA deposition the surface became smoother. The measured parameters were as follows: 1.52, 0.19, and 1.0 µm.

In medical application, wettability of material surface plays an important role. It determines its hydrophobic or hydrophilic character. Figure 4 shows results of the contact angle measurements. At the surface of the as-quenched sample, the drop was spread over a larger area. Value of measured contact angle was 75.1°. It proves hydrophilic character of the as-quenched NiTi alloy. In contrast to that, sample with deposited PLA layer shows a decrease of the contact area of the drop. The contact angle was 117.3°. As regards of that, deposited PLA layer reveals hydrophobic character. In general, the natural PLA shows hydrophilic character with the contact angle varied between 70° and 76° (Ref 20, 21). However, following a technology of the PLA production authors in Ref 21 proved that the PLA can change hydrophilic character to the hydrophobic one.

In order to study course of the transformations occurring in NiTi alloy as well as in the PLDL copolymer, differential scanning calorimetric analysis was done. Figure 5 shows comparison of the thermograms measured for samples after steps, which were done for surface covering. From thermograms characteristic transformation temperatures (M _s, M _f, M _p, A _s, A _p, A _f, R _s, R _p, R _f) as well as enthalpies were calculated and set in Tables 1 and 2, respectively. As-received NiTi alloy revealed three maxima during cooling and one broaden minimum on heating curve (Fig. 5—blue line). The first maximum, situated at a thermal range between 10 and −10°C on cooling DSC curve, corresponds to the B2 → R-phase transition. Lowering temperature down to −44°C triggers two-stage transformation: the R-phase → B19′. Description of mechanism for two-step transformation occurrence can be found at (Ref 22-24) During heating both transformations B19′ → R-phase and R-phase → B2 overlap at thermal region between −15 and 15°C. From the point of view of the medical application temperatures: R _s (10.4°C) and A _f (14.7°C) are too close to temperature of the human body. In order to lower temperature of the martensitic transformation NiTi alloy was annealed at 850°C for 20 min and quenched into ice-water. First, the thermal treatment changed course of the martensitic transformation. The transition is reversible and occurs in one step B2 ↔ B19′ without any additional presence of the R-phase (Fig. 5—red line). The martensitic transformation starts at −17°C and finishes at −31°C. The reverse transformation starts at about −4°C and finishes at 8°C (Table 1). Also, quenching increased thermal gap between transformation temperatures and temperature of the human body. Also, it increased transformation enthalpy from 14 up to 19 J/g. After the PLA deposition, thermal behavior of the martensitic transformation remained unchanged. The transformation occurs as a one-step. However, its temperatures decreased by about 2°. Also, transformation enthalpy decreased by about 4 J/g. However, this effect was not caused by circumstances of the layer deposition. Lowering of the enthalpy can originate in a lowering of the sample amount, which undergoes martensitic transformation. Weight of the sample was comparable (about 14 mg). However in the covered sample, some amount of the sample was PLA, which does not transform to the martensite as the parent phase B2 does. In comparison to the NiTi alloy, thermal effect of the transformation occurring in the PLA is several times lower. In order to study thermal behavior of the PLA, a part of the layer was removed and separately measured in DSC. Figure 6 shows results of the measurement. Following x-ray diffraction results, the PLA is amorphous at room temperature (Fig. 2). During the PLA heating, the polymer undergoes three transitions. First, the glass transition starts at about 59°C and goes with the highest speed at 62°C. It is followed by crystallization, which occurs at 129°C with enthalpy 6.8 J/g. Finally PLA melts at 154°C (enthalpy −8.9 J/g). Obtained values of the temperature transition are in a good agreement with ones given in Ref 25. It is worth noticing, that thermal behavior of the as-quenched NiTi alloy as well as deposited the PLA, shows ±30°C distance from the temperature of the human body.

The response of the covered NiTi alloy for applying external stress was studied by a cyclic axial loading and unloading up to 4.5% elongation of the sample. The covered NiTi alloy reveals typical behavior observed in the shape memory alloys with the superelastic phenomena (Fig. 7). First, the sample undergoes elastic response for the stress. Exceeding value of the critical stress (about 400 MPa) causes start of the forward martensitic transformation. The evidence of that is almost constant value of the stress when strain increases. Initially, unloading of the sample is elastic up to 200 MPa. Further unloading causes appearance the other plateau, which is related to the reverse martensitic transformation occurrence. During strain decreasing, with the almost constant stress, the martensite is transformed to the parent phase. After unloading, sample came back to the original dimension. Observation of the surface, done for the strengthen sample, showed that PLA surface did not reveal any cracks (Fig. 8). Moreover, PLA layer was elastic enough, that it followed NiTi alloy stretching and did not limit shape memory effect. Following mechanical properties of the PLA, it can stand up elongation up to 6%.