**1. Introduction**

Oral cancer, infection, tumors, and trauma may lead to structural defects in the mandible either aesthetically or functionally, which needs to be treated through reconstructive surgery. In this surgery, the defective bone is removed and replaced with a bone graft using bone fixation plates and screws. Graft mispositioning causes serious problems and so the reconstruction of segmental bony defects needs to be accurate to provide facial contour with proper height and width of the bone and restore jaw continuity [1]. To this end, fixation plates, most commonly made of Ti-6AL-4V (Ti64), or generally known as titanium), are used to provide stabilization and immobilization of the bone graft. Although Ti64 bone fixation plates provide a high level of immobilization immediately after the surgery, they may lead to stress shielding (bone resorption) or stress concentrations (device failure). Due to the stiffness-mismatch between the Ti64 bone plates and the bone, stress shielding occurs on the adjacent bone tissue, based on the bone remodeling theory, which leads to bone resorption. Followed by continuous bone resorption on the adjacent bone, the stress levels on the bone plates increase and this may lead to the failure of the plate, bone, and the surgery.

As a solution, our group has proposed and introduced a new generation of bone fixation plates that are stiffness matched to the bone tissue and are fabricated via additive manufacturing [2–7]. Stiffness modulation and the matching is achieved via replacing the Ti64 with Nitinol, as well as incorporating engineered porosity to the bone fixation plates. Nitinol (NiTi), is a biocompatible, low stiffness, shape memory alloy, which benefits from interesting features such as superelasticity and shape memory. Nitinol has been already used in many industrial applications as well as biomedical applications [8–14]. In addition to the inherent low stiffness, NiTi superelastic behavior can be made to be very similar to bone tissue. This makes it a grea<sup>t</sup> candidate for bone fixation and other skeletal reconstruction applications. Although NiTi has a relatively lower level of stiffness, it still provides a higher level of stiffness in comparison to the bone tissue. We have shown that by introducing an engineered level and type of porosity to a bone fixation plate, one is able to further reduce the stiffness of NiTi bone fixation plates and reach the level of bone tissue [15,16]. We have also shown that the additive manufacturing method, in the form of selective laser melting (SLM), can be used for the fabrication of porous NiTi bone fixation plates [17,18]. Although we have shown successful results in different sections of this novel approach, in this article with a focus on the fabrication approach (i.e., additive manufacturing), we characterized the fabricated parts and updated the design methodology based on that.

Advances in AM techniques and process development, we have been encouraged by many researchers to study the specific fabrication of biomedical components, which were hard or impossible to fabricate by conventional methods (e.g., machining, forging, etc.) in some cases. The use of AM allows many opportunities not open to conventional methods, such as patient-specific implants with tailorable properties [19–22], complex geometries [16], and better accuracies [23]. Rana et al. fabricated a titanium patient-specific implant for reconstruction of the unilateral orbital fracture using selective laser melting and compared it with a pre-bent titanium mesh, which was manually bent to the desired shape. The results of 34 cases showed a higher degree of accuracy of reconstruction in the case of SLM fabricated implants [23]. A custom made SLM titanium implant used in [24] to treat post-traumatic zygomatic deficiency showed no sign of infection after one year with good integration, suggesting SLM implants are an effective approach for alloplastic craniomaxillofacial bone reconstruction. A clinical study of ten patients who used 3D-printed patient-specific Ti plates showed facilitation in jaw reconstructive surgery as well as higher accuracies [25]. Most of the research in this area has been conducted on Ti-6AL-4V plates and/or implants. In recent years, additive manufacturing of NiTi alloys are getting more attention for the fabrication of complex shapes and geometries [26–30]. This is mostly due to the freedom of fabrication and the superior properties of NiTi, as mentioned earlier. However, most of the research is fundamental, aimed at finding optimal fabrication process parameters and their effects on the part's properties [31], lattice structures [18], corrosion behavior [3], modeling [32], and biocompatibility [33]. It should also be noted that all the SLM fabricated porous structures in the literature have been evaluated only in compression mode and no study, as far as we know, has been done on a realistic stiffness-matched porous bone fixation plate, which is under tension.

In this paper, standard-shaped 4-hole bone fixation plates with modulated levels of stiffness were modeled and simulated in a finite element (FE) model. To validate the design and modeling procedure, designed bone fixation plates with different levels of porosity were fabricated via selective laser melting using Ni50.1Ti powder and mechanically tested under tension. After validation of the modeling procedure, to achieve superelastic behavior, Ni-rich Ni50.8Ti powder was used for the fabrication of the second generation of the bone fixation plates. Thermomechanical and composition analysis of the

superelastic stiffness-matched porous bone fixation plates were discussed and a proper methodology for polishing and removing the un-melted powder is then proposed.

#### **2. Materials and Methods**

In order to test the bone fixation plates using a standard tensile setup, a straight (not curved) standard 4-hole bone fixation plate geometry, as shown in Figure 1a, was used for the fabrication of the bone fixation plates in this paper. For the first series of fabrication, bone fixation plates with a 3-mm thickness profile were designed and fabricated. At the second stage of the project, after validating the FE model, to characterize the bone fixation plates with the critical level of porosity and the most complex geometry, bone fixation plates with 1.5 mm thickness were fabricated. In order to impose porosity on the bone fixation plates, a cubic pore cell as shown in Figure 1b, was used. The cube was 1 mm and by changing the thickness of the sides, the level of porosity was modulated. A thin 0.5 mm cover as shown in Figure 1 was also designed to cover the bone fixation plate and provide the required support for screws.

**Figure 1.** (**a**) Overall geometry of the fabricated bone fixation plates and (**b**) unit pore cell used for creating the porous bone fixation plates.

Two batches of Ni50.1Ti (at. %) and Ni50.8Ti (at. %) ingot were gas atomized using an electrode induction-melting gas atomization (EIGA) by TLS Technik GmbH (Bitterfeld, Germany) to make powder. The powder was sieved to produce particle sizes from 20 to 75 μm. The components of NiTi were produced using an SLM machine (Phenix Systems PXM, [3D Systems], Rock Hill, SC, USA). Our PXM SLM machine has a 300 W Ytterbium fiber laser with a wavelength of 1070 nm and a spot diameter of 80 μm that yields a Gaussian (TEM00) profile. To lessen the impurities in the manufactured samples, the oxygen level in the fabrication chamber was controlled to be less than 800 ppm. There are two series of SLM fabrication process parameters, which are shown in Table 1. The standard dog-bone samples of Ni50.8Ti with the gauge length of 20 mm, gauge the width of 3 mm, and thickness of 2 mm were fabricated for capturing the mechanical properties of as-fabricated SLM parts.



A Perkin-Elmer (Waltham, MA, USA) Pyris 1 differential scanning calorimetry (DSC) was used to identify the transformation temperatures (TTs) from −90 to 100 ◦C with a heating/cooling rate of 10 ◦C /min in a nitrogen atmosphere by separating small portions of samples ranging from 30 to 45 mg. The standard tensile samples were tested with 25 kN TestResources 910 Series Servohydraulic fatigue test machine (TestResources, Shakopee, MN, USA). A BOSE ElectroForce 3330 machine (TA Instruments, New Castle, DE, USA) was employed to test the bone fixation plates. All samples were loaded with a strain rate of 10−<sup>4</sup> (s−1) to make sure of isothermal condition. All tests were performed at room temperature (RT). A 2D digital image correlation (DIC) system (correlated solutions, Irmo, SC, USA) by Correlated Solutions, which uses a 5-megapixel camera, a Tonika 100 mm lens, and VIC\_2D software was employed to measure the strain. The samples were painted with black and white speckle pattern for DIC measurement. The chemical analyses were performed on the SLM bone fixation plates according to ASTM 2063 standards (Table 2). Two different etching solutions (Table 3) and three different exposing times (2, 4, and 6 min) were utilized to chemically polish the superelastic porous bone fixation plates [34–36]. To evaluate the polishing procedure a FEI Quanta 3D FEG scanning electron microscopy (SEM, Thermo Scientific, Waltham, MA, USA) was used.




