The research objective of this award is to apply Laser Engineered Net Shaping (LENS(TM)), a rapid prototyping based advanced manufacturing technology, to create porous structures of nitinol. The physical, mechanical and biological properties of nitinol structures of varying porosity will be measured. Use of porous materials in load bearing implants can reduce the stiffness mismatches due to porosity and solve long-standing problems like stress shielding. Designed porosity will also achieve stable long-term biological fixation due to bone-tissue in-growth into interconnected porosity from the surface to the inside. Nitinol is a shape memory alloy that shows up to 8 percent recoverable strain, similar to bone in which about 1 percent recoverable strain is observed. This similarity in the deformation behavior between Nitinol and bone contribute to identical performance of load bearing implants under loading-unloading conditions in the body ensuring excellent biomechanical compatibility. Nitinol is also a biocompatible material and currently in use in several biomedical devices. The approach is to use flexible LENS(TM) parameters to fabricate porous nitinol implants with designed macro and micro porous structures to achieve desired performance. Process-property relationships, for both mechanical and biological properties, for LENS(TM) fabricated nitinol will then be established.
If successful, results from this program will offer design and manufacturing flexibility of simple and complex shaped implants with novel shape memory alloy, nitinol, that can match mechanical properties of bone. Novel porous implants can potentially double or triple the lifetime of load bearing implants. Being a computer-aided, design-based process, LENS(TM) can also be used to fabricate patient-specific implants from computed tomography and magnetic resonance imaging data. The project will also educate and train graduate and undergraduate students in mechanical engineering, materials science and engineering and bioengineering.
The first objective of this research was to apply Laser Engineered Net Shaping (LENS™), a rapid prototyping based advanced manufacturing technology, to create porous structures of Nitinol to measure effects of porosity on physical, mechanical and biological properties to solve long-standing problems like stress shielding in load bearing implants such as total hip and knee prosthesis. We have fabricated porous NiTi (50:50 at.%) samples with laser powers (W) 150 and 200, scan speeds (mm/s) 10, 12 and 20, powder feed rates (g/min) 15, 20 and 30 at hatch distance of 0.762 mm. The influence of both open and closed porosity on density, compression strength and modulus and shape memory effect has been evaluated. The second objective of research was to modify the surface of laser processed NiTi alloy via anodization to enhance its biocompatibility and also to reduce the Ni ion release during service. Anodization of NiTi alloy can increase the corrosion resistance by forming oxide layer and by forming microtextured layer (with micro porosity) the cell-materials interactions can be significantly enhanced. LENS™ processed fully dense NiTi alloy samples were anodized in 1N sulfuric acid (H2SO4) electrolyte at three different pHs, 4.5, 2.0 and 1.5. All anodization experiments were performed at 20V, 30°C for different durations at 15 minute increments up to the 1h. Fabrication, Mechanical & Shape Memory Properties: Typical LENS fabricated, net-shape porous NiTi alloy samples are shown in Fig. 1a. The total porosity of NiTi alloy samples can be tailored by changing the LENS processing parameters such as laser power (P), scan speed (n), hatch distance or scan line spacing (h) and Z-increment or layer thickness (t). We have unified all of these parameters into one single factor, i.e., total energy input per volume of each track/scan, to understand their influence on the density of LENS processed parts. Fig. 1b shows the influence of specific energy input (E) on the relative bulk density of laser processed NiTi alloy samples. The porosity decreased with a decrease in the scan speed, powder feed rate and by increasing the laser power. In other words, the bulk density increased with increasing specific energy input up to 50 J/mm3. Further increase in the energy input had small effect on increasing the density. The open pore volume in present samples varied between 36% to 62% of the total volume % porosity. Maximum open pore volume was observed at a scan speed of 15 mm/s with a powder feed rate of 20 g/min and 200 W laser power. The pore diameter in these samples varied between 60 and 800 mm with 95% of the pores below 500 mm in diameter. Pore connectivity increased with either decreasing the laser power or increasing the scan speed or powder feed rate. Laser parameters which increased sample porosity via decreasing specific energy input, also increased the overall grain size. From the compression tests following observations were made: (i) increasing the total strain from 2% to 6% decreased the recoverable deformation for all densities; and (ii) despite large variation in the recoverable deformation, samples with higher density showed high recoverable deformation. In all cases, a minimum of 70% of applied deformation can be recovered in laser processed porous NiTi alloy samples. Nitinol samples with density > 90% showed a maximum recoverable strain of 6%. Experimental data show that the modulus of laser processed Nitinol samples can be tailored in the range of 2 – 18 GPa by controlling the LENS process parameters. The compressive strength of these porous samples was in the range of 174 to 1240 MPa. Fatigue Response of Laser Processed Porous NiTi Alloy: Rotating bending fatigue behavior of laser processed NiTi alloy samples with 100% and 90% relative densities was evaluated. The tests carried out at various stress amplitudes ranging from 30 to 270 MPa. As expected, the fully dense samples showed higher fatigue resistance than the samples with 10% porosity. Compression-compression fatigue tests were carried out on porous NiTi alloy cylindrical samples, having ~ 0, 10 and 20% porosity, up to failure or 106 cycles (whichever occurred first) at 15 Hz with a stress ratio (R) of 0.1. Each sample with varying porosity was stressed under sinus wave function at several values of cyclic maximum compressive stress, i.e., 100%, 120%, 140% and 150% of respective 0.2% proof strength of NiTi alloy. We have shown that LENS processed NiTi samples can sustain stresses in excess of their respective yield strengths with minimal amount of plastic deformation. Anodization of Laser Processed NiTi Alloy We have shown that anodization process can modify the surface morphology/roughness by creating micro/nano textured surfaces which can potentially enhance cell material interactions of nitinol. Both the electrolyte pH and anodization duration found to influence the surface morphologies. This process produced highly uniform porous surfaces on the order of micron and nanometer sized pores.
