Ex-vivo parametric study of laser ablation-based drilling of cortical bone.

Frequently orthopedic surgeries require mechanical drilling processes especially for inserted biodegradable screws or removing small bone lesions. However mechanical drilling techniques induce large number of forces as well as have substantially lower material removal rates resulting in prolong healing times. This study focuses on analyzing the impact of quasi-continuous laser drilling on the bone's surface as well as optimizing the drilling conditions to achieve high material removal rates. An ex-vivo study was conducted on the cortical region of desiccated bovine bone. The laser-based drilling on the bovine bine specimens was conducted in an argon atmosphere using a number of laser pulses ranging from 100 to 15,000. The morphology of the resulting laser drilled cavities was characterized using Energy dispersive Spectroscopy (EDS) and the width and depth of the drills were measured using a laser based Profilometer. Data from the profilometer was then used to calculate material removal rates. At last, the material removal rates and laser processing parameters were used to develop a statistical model based on Design of Experiments (DOE) approach to predict the optimal laser drilling parameters. The main outcome of the study based on the laser drilled cavities was that as the number of laser pulses increases, the depth and diameter of the cavities progressively increase. However, the material removal rates revealed a decrease in value at a point between 4000 and 6000 laser pulses. Therefore, based on the sequential sum of square method, a polynomial curve to the 6th power was fit to the experimental data. The predicted equation of the curve had a p-value of 0.0010 indicating statistical significance and predicted the maximum material removal rate to be 32.10 mm3/s with 95%CI [28.3,35.9] which was associated with the optimum number of laser pulses of 4820. Whereas the experimental verification of bone drilling with 4820 laser pulses yielded a material removal rate of 33.37 mm3/s. Therefore, this study found that the carbonized layer formed due to laser processing had a decreased carbon content and helped in increasing the material removal rate. Then using the experimental data, a polymetric equation to the sixth power was developed which predicted the optimized material removal rate to occur at 4820 pulses.

Laser fabrication of structural bone: surface morphology and biomineralization assessment.

The current work explores the surface morphology of the laser-ablated bone using Yb-fiber coupled Nd:YAG laser (λ = 1064 nm) in continuous wave mode. As the laser-ablated region contains physiochemically modified carbonized and nonstructural region, it becomes unknown material for the body. Thus, biomineralization on such a laser-ablated region was assessed by in vitro immersion test in noncellular simulated body fluid. The presence of hydroxyapatite was detected in the precipitated mineral product using scanning electron microscopy equipped with energy dispersive spectroscopy, and X-ray diffraction analysis. The effect of varying laser parameters on distribution of surface morphology features was identified and its corresponding effect on biomineralization was studied.

Evolution of surface morphology of Er:YAG laser-machined human bone.

The extensive research on the laser machining of the bone has been, so far, restricted to drilling and cutting that is one- and two-dimensional machining, respectively. In addition, the surface morphology of the laser machined region has rarely been explored in detail. In view of this, the current work employed three-dimensional laser machining of human bone and reports the distinct surface morphology produced within a laser machined region of human bone. Three-dimensional laser machining was carried out using multiple partially overlapped pulses and laser tracks with a separation of 0.3 mm between the centers of consecutive laser tracks to remove a bulk volume of the bone. In this study, a diode-pumped pulse Er:YAG laser (λ = 2940 nm) was employed with continuously sprayed chilled water at the irradiation site. The resulting surface morphology evolved within the laser-machined region of the bone was evaluated using scanning electron microscopy, energy dispersive spectroscopy, and X-ray micro-computed tomography. The distinct surface morphology involved cellular/channeled scaffold structure characterized by interconnected pores surrounded by solid ridges, produced within a laser machined region of human structural bone. Underlying physical phenomena responsible for evolution of such morphology have been proposed and explained with the help of a thermokinetic model.

Machine learning based de-noising of electron back scatter patterns of various crystallographic metallic materials fabricated using laser directed energy deposition.

A novel machine learning (ML) method of refining noisy Electron Back Scatter Patterns (EBSP) is proposed. For this, conditional generative adversarial networks (c-GAN) have been employed. The problem of de-noising the EBSPs was formulated as an image translation task conditioned on the input images to get refined/denoised output of EBSPs which can be indexed using conventional Hough transform based indexing algorithms. The ML model was trained using 10,000 EBSPs acquired under different settings for additively manufactured FCC, BCC and HCP alloy samples ensuring enough diversity and complexity in training data set. Pairs of noisy and corresponding optimal EBSPs were acquired by suitable tweaking of the EBSP acquisition parameters such as beam defocus, pattern binning and EBSD camera exposure duration. The trained model has brought out significant improvement in EBSD indexing success rate on test data, accompanied by betterment of indexing accuracy, quantified through 'pattern fit'. Complete automation of the EBSP refinement was demonstrated where in entire EBSD scan data can be fed to the model to get the refined EBSPs from which high quality EBSD data can be obtained.

Microstructure enhanced biocompatibility in laser additively manufactured CoCrMo biomedical alloy.

The present work investigated biocompatibility of the unique nanostructural surface morphology inherently evolved in laser-based additively manufactured CoCrMo after biocorrosion in simulated body fluid at physiological temperature (37 °C). The extremely rapid thermokinetics intrinsically associated with the laser-based additive manufacturing technique resulted in heterogeneous cellular dendritic solidification morphologies with selective elemental segregation along the cell boundaries within CoCrMo samples. Consequently, a selective and spatially varying electrochemical response resulted in generation of a nanoscale surface morphology (crests and troughs) due to differential localized electrochemical etching. Also, depth of the trough regions was a function of the applied potential difference during potentiodynamic polarization which resulted in samples with varying morphological ratio (depth of trough/width of cell wall). CoCrMo with such nanoscale surface undulations were proposed for enhanced biocompatibility in terms of viability, spreading, and integration of MT3C3 pre-osteoblasts cells elucidated via MTT assay, immunofluorescence, and microscopy techniques. Furthermore, the influence of the morphological ratio, characteristic to the additively deposited CoCrMo after electrochemical etching (biocorrosion) on biocompatibility of MT3C3 pre-osteoblasts cells was qualitatively and quantitatively compared to a mirror-polished flat CoCrMo surface.

A multi modal approach to microstructure evolution and mechanical response of additive friction stir deposited AZ31B Mg alloy.

Current work explored solid-state additive manufacturing of AZ31B-Mg alloy using additive friction stir deposition. Samples with relative densities ≥ 99.4% were additively produced. Spatial and temporal evolution of temperature during additive friction stir deposition was predicted using multi-layer computational process model. Microstructural evolution in the additively fabricated samples was examined using electron back scatter diffraction and high-resolution transmission electron microscopy. Mechanical properties of the additive samples were evaluated by non-destructive effective bulk modulus elastography and destructive uni-axial tensile testing. Additively produced samples experienced evolution of predominantly basal texture on the top surface and a marginal increase in the grain size compared to feed stock. Transmission electron microscopy shed light on fine scale precipitation of Mg[Formula: see text]Al[Formula: see text] within feed stock and additive samples. The fraction of Mg[Formula: see text]Al[Formula: see text] reduced in the additively produced samples compared to feed stock. The bulk dynamic modulus of the additive samples was slightly lower than the feed stock. There was a [Formula: see text] 30 MPa reduction in 0.2% proof stress and a 10-30 MPa reduction in ultimate tensile strength for the additively produced samples compared to feed stock. The elongation of the additive samples was 4-10% lower than feed stock. Such a property response for additive friction stir deposited AZ31B-Mg alloy was realized through distinct thermokinetics driven multi-scale microstructure evolution.

A Review of Diagnostics Methodologies for Metal Additive Manufacturing Processes and Products.

Additive manufacturing technologies based on metal are evolving into an essential advanced manufacturing tool for constructing prototypes and parts that can lead to complex structures, dissimilar metal-based structures that cannot be constructed using conventional metallurgical techniques. Unlike traditional manufacturing processes, the metal AM processes are unreliable due to variable process parameters and a lack of conventionally acceptable evaluation methods. A thorough understanding of various diagnostic techniques is essential to improve the quality of additively manufactured products and provide reliable feedback on the manufacturing processes for improving the quality of the products. This review summarizes and discusses various ex-situ inspections and in-situ monitoring methods, including electron-based methods, thermal methods, acoustic methods, laser breakdown, and mechanical methods, for metal additive manufacturing.

Crystallographic texture dependent bulk anisotropic elastic response of additively manufactured Ti6Al4V.

Rapid thermokinetics associated with laser-based additive manufacturing produces strong bulk crystallographic texture in the printed component. The present study identifies such a bulk texture effect on elastic anisotropy in laser powder bed fused Ti6Al4V by employing an effective bulk modulus elastography technique coupled with ultrasound shear wave velocity measurement at a frequency of 20 MHz inside the material. The combined technique identified significant attenuation of shear velocity from 3322 ± 20.12 to 3240 ± 21.01 m/s at 45[Formula: see text] and 90[Formula: see text] orientations of shear wave plane with respect to the build plane of printed block of Ti6Al4V. Correspondingly, the reduction in shear modulus from 48.46 ± 0.82 to 46.40 ± 0.88 GPa was obtained at these orientations. Such attenuation is rationalized based on the orientations of [Formula: see text] crystallographic variants within prior columnar [Formula: see text] grains in additively manufactured Ti6Al4V.

Computational Assessment of Thermokinetics and Associated Microstructural Evolution in Laser Powder Bed Fusion Manufacturing of Ti6Al4V Alloy.

Although most of the near non-equilibrium microstructures of alloys produced by laser powder bed fusion (LPBF) additive manufacturing (AM) are being reported at a rapid rate, the accountable thermokinetics of the entire process have rarely been studied. In order to exploit the versatility of this AM process for the desired properties of built material, it is crucial to understand the thermokinetics associated with the process. In light of this, a three-dimensional thermokinetic model based on the finite element method was developed to correlate with the microstructure evolved in additively manufactured Ti6Al4V alloy. The computational model yielded the thermal patterns experienced at given location while building a single layer through multiple laser scans and a whole part through multiple layers above it. X-ray analysis of the resultant microstructure confirmed the presence of acicular martensitic (α') phase of (002) texture within the build-plane. Computationally predicted magnitude of the thermal gradients within the additively manufactured Ti6Al4V alloy in different directions (X, Y, and Z) facilitated the understanding about the evolution of grain morphology and orientation of acicular martensite in prior ß grains. The scanning electron microscopy observations of the alloy revealed the distinct morphology of phase precipitated within the martensitic phase, whose existence was, in turn, understood through predicted thermal history.

Thermal Assessment of Ex Vivo Laser Ablation of Cortical Bone.

As a potential osteotomy tool, laser ablation is expected to provide rapid machining of bone, while generating minimal thermal damage (carbonization) and physical attributes within the machined region conducive to healing. As these characteristics vary with laser parameters and modes of laser operation, the clinical trials and in vivo studies render it difficult to explore these aspects for optimization of the laser machining parameters. In light of this, the current work explores various thermal and microstructural aspects of laser-ablated cortical bone in ex vivo study to understand the fundamentals of laser-bone interaction using computational modeling. The study employs the Yb-fiber Nd:YAG laser (λ = 1064 nm) in the continuous wave mode to machine the femur section of bovine bone by a three-dimensional machining approach. The examination involved thermal analysis using differential scanning calorimetry and thermogravimetry, phase analysis using X-ray diffractometry, qualitative analysis using X-ray photoelectron spectroscopy, and microstructural and semiquantitative analysis using scanning electron microscopy equipped with energy-dispersive spectrometry. The mechanism of efficient bone ablation using the Nd:YAG laser was evaluated using the computational thermokinetics outcome. The use of high laser fluence (10.61 J/mm2) was observed to be efficient to reduce the residual amorphous carbon in the heat-affected zone while achieving removal of the desired volume of the bone material at a rapid rate. Minimal thermal effects were predicted through computational simulation and were validated with the experimental outcome. In addition, this work reveals the in situ formation of a scaffold-like structure in the laser-machined region which can be conducive during healing.

In-vitro biomineralization and biocompatibility of friction stir additively manufactured AZ31B magnesium alloy-hydroxyapatite composites.

The present study aims to evaluate effect of hydroxyapatite (HA, Ca10(PO4)6OH2), a ceramic similar to natural bone, into AZ31B Mg alloy matrix on biomineralization and biocompatibility. The novel friction stir processing additive manufacturing route was employed to fabricate Mg-HA composites. Various HA contents (5, 10, 20 wt%) were incorporated into Mg matrix. Microstructural observation and chemical composition analysis revealed that refined Mg grains and dispersion of HA particles at micro/nanoscales were achieved in Mg-HA composites after the friction stir processing. The biomineralization evaluation were carried out using immersion experiments in simulated body fluid followed by mineral morphology observation and chemical composition analysis. The wettability measurements were conducted to correlate the biomineralization behavior. The results showed improvement in wettability and bone-like Ca/P ratio in apatite deposit on the composites compared to as-received Mg. In addition, the increase of blood compatibility, cell viability and spreading were found in the higher HA content composites, indicating the improved biocompatibility. Therefore, friction stir processed Mg-20 wt%HA composite exhibited the highest wettability and better cell adhesion among other composites due to the effect of increased HA content within Mg matrix.

Optimization of biocompatibility in a laser surface treated Mg-AZ31B alloy.

Biodegradable bone implants can remove the need for subsequent bone-implant surgeries by controlled biomineralization and degradation. Although Mg-alloys generally possess biocompatible properties, they corrode rapidly, thereby preventing sufficient hydroxyapatite formation and biomineral growth. In an attempt to address these limitations, laser surface treatments were performed via the employment of a continuous wave Nd:YAG laser on the Mg-AZ31B alloy using laser fluences in the range of 1.06-4.24 J/mm2 (250-1000 W). The laser-treated samples were investigated for their wettability in simulated body fluid. In vitro analyses were performed in simulated body fluid to examine corrosion and biomineralization behavior on the laser-treated samples. Statistical optimization algorithms based on wettability data predicted an optimal laser fluence of 3.286J/mm2 (775 W) within the range of laser fluences used in the present study for achieving a balance between biodegradation and biomineralization. Confirmatory tests on optimized samples indicated an up to 84% grain size reduction in laser-treated surface regions, a several-fold increase in Mg17Al12 (ß) phase volume fraction, a reasonably abundant formation of hydroxyapatite, and increased rates of biomineralization that exceeded degradation. These findings indicate the potential of laser surface engineering to realize Mg-AZ31B alloy as a viable biodegradable bone implant material.