Laser Engineered Bioceramic Coatings for Load Bearing Implant Applications

Engineered artificial scaffold biomaterials with appropriate mechanical properties, surface chemistry and surface topography are in a great demand for enhancing cell attachment, cell growth and tissue formation at the defect sites. Most of these engineering techniques are aimed at mimicking the natural organization of the bone tissues and thereby create a conducive environment for bone regeneration. Hence, there has been a great thrust towards development of Ca-P based surface coatings on various metallic and non-metallic substrates for load bearing implant applications such as hip joint prosthesis, knee joint prosthesis and dental implants. In our group we aim towards synthesizing such textured Ca-P based coating with appropriate topographical cues and surface chemistry by a laser based coating technique. The advantages associated with the above technique are that both a 3-dimensional topographic cue and appropriate chemistry suitable for load bearing implant applications can be achieved by a single step process.

The Hydroxyapatite (Ca10(PO4)6(OH)2, HA) precursor powder was mixed in a water based organic solvent sprayed on the thoroughly cleaned Ti-6Al-4V plate. The sprayed components can then be scanned using a laser beam to achieve the textured coating. The actual processing window and the schematic of the coating process is illustrated in Figure 1. As the laser beam can be programmed to have different lateral spacing within the track, coatings with different textures (Figure 2) can be synthesized.

Figure 1. Laser processing

Figure 2. Textured coatings

Further, with varying laser processing parameters (energy, average power, laser beam residence time, etc.) surfaces with appropriate chemistry (HA, CaTiO3, Ca3(PO4)2, TiO2 (Anatase), TiO2 (Rutile)) as observed from the X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) spectra (Figure 3) of the coated samples can also be synthesized.

When an implant material is placed inside a human body, among the plethora of events that takes place, the first and the foremost one is the wetting of the implant material by thephysiologicalfluids. This further controls the absorption of proteins followed by attachment of cells to the implant materials. Also, as the surface chemistry and the surface morphology of the coatings influence the wetting behavior and there by the protein adsorption and cell attachment, an understanding of the wettability of these surfaces by contact angle measurements is also major focus of the group. Accordingly, these textured coatings have shown enhanced hydrophilicity of a test liquid (Figure 4a) compared to the untreated Ti-6Al-4V (Figure 4b).

Figure 3. XRD and EDS analysis of textured coatings

Figure 4. SBF drop wetting test

Since, HA is a naturally occurring mineral component of human bone laying alongside the collagen fibrils, the precipitation of such a phase on the coatings following immersion in a simulated body fluid (SBF), a solution with ion concentrations and pH value similar to those of human blood plasma, provides information pertaining to the in vitro bioactivity of the coatings. Further, as this layer provides the appropriate chemistry and there by acts as a bone bonding interface, where the cells can preferentially proliferate and differentiate into complex tissues such as bone, an understanding of the mineralizing ability of the coatings following immersion in SBF, has attracted us as it is a process similar to biological mineralization. The surfaces of textured coatings following immersion in SBF demonstrated precipitation of whisker-like apatite crystals (Figure 5 a) following 1 day immersion in SBF. As the immersion time was increased to 3 days the whisker-like feature is further covered by another mixed layer of whisker-like and nano HA crystals (Figure 5b). After 5 day immersion, the layer is completely covered with nano crystallites of HA (Figure 5c) and at the end of 7 days there is a thick mineralized layer of HA on the surface of the sample (Figure 5d). XRD studies of the SBF immersed samples further confirmed the mineralized layer to be of HA (Figure 6).

Figure 5. Precipitates after SBF immersion

Figure 6. XRD analysis of precipitates

The in vitro biocompatibility of the coatings can be studied by the culture of mouse MC3T3-E1 osteoblast-like cells. The MC3T3-E1 osteoblast-like cells show a triangular morphology (Figure 7a) on the control (untreated Ti-6Al-4V) samples and have a rectangular and elongated morphology (Figure 7b) on the laser processed samples. Their sound adhesion, especially on the laser textured samples is characterized by the lamellipodia trying to extend and adhere along the grooves. In contrast, no such adhesion is observed on the control (untreated Ti-6Al-4V) sample (Figure 7). The cytoskeletal organization of the cells after 7 day culture demonstrated more confluence with well stressed actin filaments on the laser textured samples (Figure 7d) compared to the control (Figure 7c).

Figure 7. Cell culture on laser textured coatings

Laser Engineered Bioceramic Coatings for Load Bearing Implant Applications

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