A Novel Method for Nano-Polishing TEM Samples Out of a Ni-Ti Endodontic File

Transmission electron microscopy (TEM) provides critical analyses of nano-scale structures [1,2] in especially nanomaterials science. To obtain reliable results from the TEM analysis, the samples must be thinned to electron transparency. This can be achieved conventionally by a combination of mechanical polishing and focused ion beam (FIB) [3] or ion milling [4]. The FIB is common equipment usually attached to a high-end scanning electron microscope (SEM) for TEM specimen preparation with its advantages and disadvantages [57]. For example, arte facts can be introduced during the sample preparation which may jeopardize the interpretation of the TEM data [8]. Besides the trouble in maintaining and using the expensive FIB-SEM, the techniques require special skills, and are time-consuming [8]. In particular, the regular mechanical polishing and FIB are both not good to analyze the different hue-color of oxides on the surface of the Ni-Tialloy endodontic file, since during the processes the oxide-layer’s structure and composition can be altered. Thus, finding a user-friendly alternative for preparing especially the Ni-Tialloy sample with the much reduced cost and time can greatly benefit the users with the limited budget and time. In this opinion, we demonstrated a new nano-polishing method for preparing the Ni-Tiendodontic file for TEM analysis by employing a low-cost mechanical polisher called Tripod Polisher (Model 590, South Bay Technology).In particular, the thin Ni-Tiendodontic file has aspiral (twisted) and tapered sharp tip see Figure 1A at which the TEM-sample’s preparation is quite difficult.


Opinion
Transmission electron microscopy (TEM) provides critical analyses of nano-scale structures [1,2] in especially nanomaterials science. To obtain reliable results from the TEM analysis, the samples must be thinned to electron transparency. This can be achieved conventionally by a combination of mechanical polishing and focused ion beam (FIB) [3] or ion milling [4]. The FIB is common equipment usually attached to a high-end scanning electron microscope (SEM) for TEM specimen preparation with its advantages and disadvantages [5][6][7]. For example, arte facts can be introduced during the sample preparation which may jeopardize the interpretation of the TEM data [8]. Besides the trouble in maintaining and using the expensive FIB-SEM, the techniques require special skills, and are time-consuming [8]. In particular, the regular mechanical polishing and FIB are both not good to analyze the different hue-color of oxides on the surface of the Ni-Tialloy endodontic file, since during the processes the oxide-layer's structure and composition can be altered. Thus, finding a user-friendly alternative for preparing especially the Ni-Tialloy sample with the much reduced cost and time can greatly benefit the users with the limited budget and time.
In this opinion, we demonstrated a new nano-polishing method for preparing the Ni-Tiendodontic file for TEM analysis by employing a low-cost mechanical polisher called Tripod Polisher (Model 590, South Bay Technology).In particular, the thin Ni-Tiendodontic file has aspiral (twisted) and tapered sharp tip see Figure 1A at which the TEM-sample's preparation is quite difficult.
In experiment, this sample was first of all embedded in a mold filled with a self-setting polymeric resin, as showed in Figure 1A. There is in-molded sample was then cross-sectioned using a low-speed diamond saw. Next, the cross-sectioned file was mounted onto the polymer stub of the tripod polisher. Then, the stub was attached to the screw-micrometer for being gradually lowered down until the sample touches the abrasive surface ( Figure 1B).
On this basis, sandpapers of ascending grit-sizes (400, 800, and 1200) were iteratively applied onto the specimen for grinding, which produced a smooth surface. Then, likewise using diamond-polish paper with the grit-size ranging from 6 to 0.1 microns, the surface of the sample was polished further. The final Juniper Online Journal Material Science polishing step on the same side of the specimen was performed using a mechano-chemical process with a water-solution of colloidal silica. After finishing this side, the sample was flipped over for grinding and polishing, as described above, to finally obtain a thin slab (~0.1mm). Thereafter, the angle of the holder was calculated (see equations below), and physically adjusted. Then, the thin-slab (on the tripod-polisher) was further polished by repeating the aforementioned steps ( Figure 1B), until the tip was thinner than 100nm for the ease of electron transmission for the TEM analysis [9]. To avoid overheating and dehydrating the sample to induce unwanted reactions on the sample-surface during the polishing, a water-cooling was used all the time.
The results from the SEM (FEI XL-30, Philips) and TEM study on the Ni-Ti sample were outlined in Figure 1C-1F. The SEM micrograph ( Figure 1C) showed morphology of Ni-Ti endodontic file, and that of Figure 1D showed a transparent edge from the middle of the Ni-Ti sample slice. The images from the TEM analysis (FEI, TITAN 80-300) ( Figure 1E & 1F) showed a clear crystal lattice-structure of the Ni-Ti endodontic file. These results implied that the use of the low-cost tripod polisher for preparing the TEM sample out of the Ni-Ti endodontic file's spiral tip was effective.
The angular adjustment of the holder can be calculated based on the thickness (d) and length (l) of the specimen ( Figure  1G) via trigonometric function. The equations are shown in the (1) and (2) below, where, d=the specimen's thickness (measure by screw micrometer); l=the specimen's length (measure by rule); θ=angle between holder base and horizontal axis; D=the height the micrometer screw gauge need to lowered; L=the triangle's height from three screw micrometers see Figure 1G.

Summary
In summary, a user-friendly and simple angular adjustment using the low-cost tripod polisher enabled an effective and efficient method for preparing Ni-Ti endodontic file TEM samples. This method has the proven versatility for reliably, reproducibly, and efficiently producing specimen of many types for TEM nano-analysis, which is ideal for new users with limited funds and time.