A. VASANTHANATHAN, S. M. KENNEDY: BIO-PRINTING OF FEMUR MODEL: A BONE SUBSTITUTE FOR ... 283–289 BIO-PRINTING OF FEMUR MODEL: A BONE SUBSTITUTE FOR BIOMEDICAL RESEARCH BIOTISK MODELA STEGNA: KOSTNI NADOMESTEK ZA BIOMEDICINSKE RAZISKAVE Vasanthanathan A 1 , Senthil Maharaj Kennedy 2* 1 Department of Mechanical Engineering, Mepco Schlenk Engineering College, Sivakasi – 626005, India 2 Department of Mechanical Engineering, AAA College of Engineering and Technology, Sivakasi – 626123, India Prejem rokopisa – received: 2023-03-21; sprejem za objavo – accepted for publication: 2023-04-25 doi:10.17222/mit.2023.831 This paper deals with the development of a medical support model that can be used as a prototype to study the anatomy of the femur and for biomechanical research experimentation related to bone plates. CT scan data of the femur bone are converted into a 3D model using MIMICS software and imported into a finite-element model for analysis. The materials selected for the fabri- cation of the femur model were PEEK and CF PEEK (infused with chopped carbon fibre). The femur bone model was analysed using ANSYS® WORKBENCH® 2021 R2 with different material properties. By conducting a subsequent FE analysis, the opti- mal material was finally arrived at. Using 3D-printing technology, the 3D model of the femur was fabricated by using a material spool with better properties suited for the femur bone. The FE results were compared with the experimental results of the fabri- cated femur model and the results of the CF PEEK bone model closely matched the properties of real human femur, and it can be used as a femur bone substitute for biomechanical investigations of bone plates instead of using a real femur. Keywords: Femur bone, 3D printing, FE analysis, PEEK, ANSYS V ~lanku je opisan razvoj modela medicinske podpore. Uporabi se ga lahko kot prototip za {tudij anatomije stegna in eksperi- mentalne biomedicinske raziskave, ki se nana{ajo na kostne plo{~ice. Podatke iz posnetkov ra~unalni{ke tomografije (CT)stegenske kosti so pretvorili v model 3D z uporabo programskega orodja MIMICS in ga nato uvozili v programsko orodje na osnovi metode kon~nih elementov (MKE), s pomo~jo katerega so izvedli kon~no analizo. Za izdelavo modela stegna je bil izbran polietereterketon (PEEK)in PEEK z vlitimi nasekanimi ogljikovimi vlakni. Model kosti stegna so analizirali s programskim orodjem ANSYS® WORKBENCH® 2021 R2 z uporabo podatkovnih baz za lastnosti razli~nih materialov. Z nadaljnjo analizo MKE so dolo~ili optimalni material. Na osnovi analiz so izdelali modelno stegensko kost iz najprimer- nej{ega materiala. Rezultate analize MKE so primerjali z eksperimentalnimi rezultati izdelanega modela stegna. Ugotovili so, da se model kosti izdelane iz materiala CF PEEK dobro ujema z lastnostmi realnega ~love{kega stegnain se ga zato lahko tudi uporabi kot nadomestek za biomehanske raziskave kostnih plo{~ic. Klju~ne besede: stegenska kost, 3D tisk, o metoda kon~nih elementov, polietereterketon ,programsko orodje ANSYS 1 INTRODUCTION Bones are hard tissues that make up the main part of human vertebrates. Bones protect different parts of the body, support loads, produce red blood cells and white blood cells, store minerals, and provide body structure and support to enable movement. 1,2 The femur is the largest bone in the body and is present in the thigh. Dif- ferent kinds of trauma can damage the femur, causing it to fracture into two or more pieces. This might happen to the part of the femur head, the femur shaft, or the condylar part of the femur. In certain types of femur fractures, even though the femur has broken, its pieces would still line up correctly. In other types of fractures, the injury moves the bone fragments 3 out of position. Rapid Prototyping (RP) is a scientific advance that creates models out of 3D software systems (CAD). Un- like the subtractive method, where material is removed to fabricate the product, 3D printing (3DP) relies on an ad- ditive-level correlation method that adds material layer by layer to the substrate to build a complete model. Since it takes a long time to make models, molds, and proto- types in the production field, various complicated pro- cesses are used to reduce the production time. The indus- try has initiated 3DP victimisation technology to provide exquisite models, molds, and prototypes. 4 In the subtract- ive method, tool movements are planned for material re- moval from the workpiece to achieve the specified form. Compared to subtractive methods like turning and ma- chining, AM technology has the greatest capability to in- duce complicated geometries like anatomical structures. RP provides cost-effective models of the styles that will be used to understand the merchandise before the fabri- cation process of high-priced prototypes. RP techniques include stereolithography (SLA), selective optical maser sintering (SLS), fused-deposition modeling (FDM), and laminated object manufacturing (LOM). 5 The FDM tech- nique is preferred for the current work, as mostly strong polymers can replicate the femur bone and the easy availability. So far, the most widely used polymer filaments are Acrylonitrile Butadiene Styrene (ABS) and polylactic Materiali in tehnologije / Materials and technology 57 (2023) 3, 283–289 283 UDK 004.356:616.718.4 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(3)283(2023) *Corresponding author's e-mail: maharaj@aaacet.ac.in (Senthil Maharaj Kennedy) acid (PLA). 6 PLA filaments have a better prospect than ABS because they are perishable, bio-absorbable, and re- newable thermoplastic polyesters with great mechanical strength and methodability. 7,8 Polycarbonate is also widely used in 3D printing due to its good strength and stiffness properties. 9,10 PEEK materials are now widely used in a variety of surgical and medical fields. PEEK is biomechanically similar to that of human bone, and it is becoming more useful and provides more accurate and stable results. 11,12 Lately, carbon-fiber-reinforced 3D-printing filaments have been introduced into the bio- medical field as the carbon fiber provides an increase in strength and mechanical properties of the filament. 13 Spi- nal cages, bone-fixation screws, and cardiac and neuro- logical leads have all used carbon-fiber-reinforced PEEK in the past. Carbon-fiber-reinforced PEEK has recently been used in orthopedic implants and it is ideal for artic- ulating implants, such as knee and hip replacements and bone plates. 14 3D printing is a perfect technology for manufacturing implants and medical devices. The major reasons are low-cost additive manufacturing and that the medical im- plant producers have great independence in designing new implants and prototypes, allowing them to custom- ise new medical implants based on the market’s needs in a much shorter time. Medical 3D printing was once imagined to be a dream project. But time and investment brought it to reality. Today, 3D printing represents a huge specific implant, enabling the opportunity for pharma- ceutical or healthcare companies to help create more rapid production of medical implants and change the way doctors and surgeons plan procedures. 15,16 Femur CT scan data referred by a physician is used for the femur modeling. The finite-element analysis of the femur is carried out using thr material test data re- sults of the 3D-printing filament. Despite the abundance of research in 3D modeling and finite-element simula- tions of the femur, this paper presents a new approach to the 3D printing of femur models and provides ample suggestions for bone-plate research. There is a lot of re- search going on right now in the field of orthopedic im- plants. There are many computerized methods for ana- lyzing the implants, but the experimental analysis needs real people or animal specimens. The 3D-printed femur bone provided in this can be used to experiment with the bone plates. 17,18 This present work is implemented in the following four phases. • Phase 1: 3D Modelling of Femur Bone using CT scan data via. MIMICS Software and SOLIDWORKS®2020 software. • Phase 2: Finite-element modelling and simulation of femur bone. • Phase 3: 3D printing of femur-bone model using CF PEEK. • Phase 4: Experimentation on femur-bone model to validate the results 2 MATERIALS AND METHODS The main objective of the present paper is to fabricate a femur-bone model with the nearest similar property of the human femur. CT scan, MIMICS Software, SOLID- WORKS ® 2020, ANSYS WORKBENCH ® 2021 R2 and CURA slicing software are used during the course of this work. 2.1 Materials Carbon-fiber-reinforced 3D filaments are being used in the work as they possess better strength when com- pared to the normal filaments. The materials 19 that are best suited for 3D printing and have better property re- semblance to human bone, i.e., PEEK and CF PEEK (re- inforced with chopped carbon fibers), are preferred here. The filaments were purchased from the filament manu- facturer 3DXTECH Additive Manufacturing and the technical data sheet of the material characteristics was also provided by the filament manufacturers. 3D printing was chosen as the femur structure was very complex and difficult to fabricate using the conven- tional manufacturing processes. Table 1 indicates the material properties of the materials in the present study. The material properties were collected from the filament manufacturer. 2.2 Rapid prototyping Rapid prototyping is a set of techniques used to rap- idly manufacture scale models of physical parts or as- semblies using three-dimensional computer-aided design A. VASANTHANATHAN, S. M. KENNEDY: BIO-PRINTING OF FEMUR MODEL: A BONE SUBSTITUTE FOR ... 284 Materiali in tehnologije / Materials and technology 57 (2023) 3, 283–289 Table 1: Material Characteristics Properties Standard Carbon Fiber PEEK PEEK Density (g/cc) ISO 1183 1.39 1.32 Tensile Strength (MPa) ISO 527 126 105 Tensile Modulus (MPa) ISO 527 10,100 3980 Tensile Elongation (%) ISO 527 1.9 7 Flexural Strength (MPa) ISO 178 145 141 Flexural Modulus (MPa) ISO 178 11,200 2850 Glass Transition Temperature (Tg) (°C) DSC 143 143 Deflection Temperature at 0.45 MPa (66psi) (°C) ISO 75 305 140 (CAD) data. The construction of parts or components is usually done with 3D printing or additive layer fabrica- tion technology. Figure 1 represents the entire methodol- ogy incorporated into the present study for the 3D print- ing of the femur-bone model. 2.3 3D modeling using CT scan data The CT scan data of the femur were used to create a 3D model of the femur bone. A CT scan using CT equip- ment was performed on a 33-year-old male. The knee in the neutral position was scanned where the least tension or pressure on tendons, muscles, and bones was felt. The scans were performed with a slice distance of 2 mm and were made up of 1816 cross-sectional cuts. The images were exported in DICOM format from the CT equip- ment. The DICOM images from the CT scan were then processed with the Materialize Interactive Medical Im- age Control System (Mimics) 10.01 software to generate the primary 3D model using the density-segmentation techniques. The primary 3D models that were generated were then processed and assembled as geometrical data files. Finally, the model was saved as a .stl file. Loads, boundary conditions, material constitutive models, kine- matic constraints, and mesh discretization processes were then used to prepare the model for analysis. 20–22 The SOLIDWORKS® 2020 software package was used to smooth the 3D model. Figure 2 shows the smoothened bone model in the SOLIDWORKS ® 2020 environment after being imported from MIMICS soft- ware and the cross-sectioned model of the femur bone to view the cavity in the femur bone. CURA slicing software was used to slice the model to enable 3D-printing capabilities. Cura software con- verts digital 3D models into printing instructions for a given 3D printer to build an object. 2.4 Finite-Element Analysis of Femur-Bone Model The 3D model of the femur bone was imported into the FE model and the entire finite-element computations 23 were made using the ANSYS WORK- BENCH ® 2021 R2 software package. A. VASANTHANATHAN, S. M. KENNEDY: BIO-PRINTING OF FEMUR MODEL: A BONE SUBSTITUTE FOR ... Materiali in tehnologije / Materials and technology 57 (2023) 3, 283–289 285 Figure 1: Schematic of the methodology Figure 2: a) 3D model and b) cross-sectioned model of femur 2.4.1 Preprocessing In the preprocessing stage, the 3D modeling of the femur bone model, assigning material models, material properties, meshing, applying appropriate loads and boundary conditions were carried out in chronological order. Separate finite-element models are developed with different material models and material properties, i.e., fe- mur bone, 2 PEEK and CF PEEK (Table 1). Different materials were taken into account in the FE model for the purpose of analyzing a better material for the femur bone. The femur bone modeled using SOLID- WORKS ® 2020 software was imported into ANSYS WORKBENCH ® 2021 R2. The model was assumed to be isometric for the analysis 24 . The material properties were imported to ANSYS from the data available in Ta- ble 1. A fine mesh was considered for the femur-bone analysis with 8-node tetrahedral elements. A mesh-con- vergence study was conducted to confirm that that FEA model converged to a solution. 25 The mesh convergence was done by increasing the number of elements from 12000 to 4716336. The maximum deformation results were compared for the mesh convergence. The maxi- mum deformation results were almost constant for all the material properties after the fine meshing. The mesh size that was selected for this work was 5 mm and the num- bers of elements was 206,216 (Figure 3a). 2.4.2 Solving After completing the meshing, boundary conditions were applied to the femur similar to the loads acting on the human femur while standing and walking. The lower end of the femur model was distally fixed and the load was given from the femoral head. ISO 7206-4:2010 was used for the loading conditions of the femur model. ISO 7206-4:2010 specifies the test parameters and the re- quirements of the endurance limit of stemmed femoral components tested experimentally. 26 The boundary con- ditions (Figure 3b) are considered by fixing the lower end of the femur model and a compressive load is given from the top. The load value as per ISO standard is 2300 N, and it is applied from the top of the femoral head and the load type is a compression load. A static structural analysis was carried out in three parts: femur bone, 2 PEEK, and CF PEEK. 2.4.3 Post Processing The post-processing capabilities of ANSYS ® were utilized for generating the equivalent stress plot and total deformation plot for the femur bone with various mate- rial properties. A detailed post-processing result was generated in the FE analysis. Figure 4 represents the fi- nite-element method results of the equivalent stress, equivalent strain and total deformation of femur bone model using the femur bone property, PEEK and CF PEEK material properties, respectively. From the results it is inferred that CF PEEK had good compression prop- erties as per the ISO 7206-4:2010 standard FE analysis, and moreover the compression property closely matched A. VASANTHANATHAN, S. M. KENNEDY: BIO-PRINTING OF FEMUR MODEL: A BONE SUBSTITUTE FOR ... 286 Materiali in tehnologije / Materials and technology 57 (2023) 3, 283–289 Figure 4: Equivalent stress, equivalent strain and total deformation of femur model with different material properties Figure 3: Processing of femur model in ANSYS WORKBENCH ® 2021 R2 to that of the human femur. So, a CF PEEK filament was used to 3D print the femur-bone model. 2.5 3D Printing The femur model was 3D printed using a Pratham 5.0 3D printer and 3DXTECH CF PEEK filament spool was used for the fabrication based on the FEM results. The Pratham 5.0 3D printer was used because of its larger bed size of (500 × 500 × 500) mm. The Fusion Deposi- tion Modelling method is used in the 3D printer to fabri- cate the bone model. FDM 27 is a method of 3D printing in which layers of materials blend together in a pattern to fabricate an object. Using the FDM technique the layers of CF PEEK were deposited layer by layer from the model obtained using the CURA ® software (Version: Cura LulzBot Edition v3). The time taken for the fabri- cation of femur model with 100 % fill density was 12 h. The 3D printer has a single extruder, 100 μm to 500 μm / 0.1 mm to 0.5 mm layer resolution, 400 micron / 0.4 mm supported extruder nozzle diameter, 572 °F / 300 °C Ex- truder nozzle temperature and 248 °F / 120 °C build- plate temperature. The bonding between the pre-laid layer and the upcoming layer was good because the tem- perature held the next layer and the previous layer in a rigid form so that high strength and the mechanical prop- erties required could arrive. This made the fabricated carbon-fibre PEEK 3D model stand firm without any flexible deformation. A CF PEEK spool was used as a fabrication material. 2.6 Compression Test on Femur Model A servo computerized universal testing machine was used for the compression test on the 3D-printed femur bone model. The load capacity of the machine is 100–2000 kN and the displacement resolution is 0.01 mm. The results were obtained from the data-acquisition system provided with the machine. The experimental test up as per ISO 7206-4:2010 with the vertical load applied to the femur-bone model from the top of the femoral head (Figure 5). 3 RESULTS AND DISCUSSION 3.1. FEA Results The finite-element method results of the equivalent stress, equivalent strain and total deformation of the fe- mur bone model using the femur-bone property, PEEK and CF PEEK material properties for ISO 7206-4:2010 loading conditions. The maximum total deformation of the femur-bone model with the properties of a real femur carries a value of 10.68 mm, which is smaller, followed by the CF PEEK material with 17.813 mm and PEEK material with 21.376 mm. The lowest deformation of the material yields a higher strength to the material. In this regard, it is numerically investigated that CF PEEK fe- mur model is better than the other counterparts. The maximum equivalent stress for the femur-bone model with the properties of a real femur carries an equivalent stress of 202.62 MPa, CF PEEK with a value 212.252 MPa and PEEK with 192.73 MPa. From these predic- tions it is inferred that the CF PEEK femur-bone model has a better strength than the PEEK model. The maxi- mum equivalent strain values of the femur-bone model with properties of real femur, CF PEEK, PEEK materials are (0.019573, 0.021041 and 0.024467) mm/mm. From these strain values it is evident that the strain values of a. real human femur and the femur model made of CF PEEK match closely when compared to that of PEEK material, making CF PEEK a better material for the fab- rication of fthe emur model with the properties close to that of a human femur. Table 2 shows the comparison of maximum equiva- lent stress, maximum equivalent strain and maximum to- tal deformation of the materials that were used in the present FEA study. Table 2: Finite-element snalysis results Femur model Maximum equivalent stress (MPa) Maximum equivalent strain (mm/mm) Total deforma- tion (mm) Human Femur Bone 202.62 0.019573 10.688 CF PEEK 212.5 0.021041 17.813 PEEK 192.73 0.024467 21.376 3.2 3D Printed Femur-Bone Model From the finite-element results, the femur bone was fabricated using CF PEEK filament material by 3D print- ing technology. 28,29 From the visual inspection, the fabri- cated femur-bone model had good strength and stiffness, with a considerable degree of elasticity. Figure 6 shows the 3D-printed femur model using CF PEEK material and a composite bone plate that is to be screwed and biomechanically tested. A. VASANTHANATHAN, S. M. KENNEDY: BIO-PRINTING OF FEMUR MODEL: A BONE SUBSTITUTE FOR ... Materiali in tehnologije / Materials and technology 57 (2023) 3, 283–289 287 Figure 5: Compression test on 3D-printed CF Peek femur model 3.3 Compression-Test Results Compression tests were conducted on the fabricated CF PEEK femur-bone model (Figure 6), similar to the FE method. The femur model showed an initial linear elastic response, followed by some non-linearity and showed signs of fracture at a strain of 1.9 percent, exhib- iting a sudden drop in load. The CF PEEK femur model fails at a compressive stress of 208.58 MPa and a strain of 1.99 %. Failure occurred in the middle portion of the femur model, at the point of contact with the top roller in the case of intact bones. The compression strength of the human femur bone is 205 MPa. The FEM result obtained for the results of the maximum compressive stress of the femur bone was 202.62 MPa (Figure 5a). The result de- viation is only 1.6 %. This proves the accuracy of the FEM results. The FEM results for the maximum stress value of the CF PEEK femur model was 212.5 MPa and the compressive strength obtained from the experimental value was 208.58 MPa. These results closely match the compression strength of the real human femur, and thus this CF PEEK model can be used as prototype model to study the anatomy of the human femur and can also be used by researchers to experiment on bone plates made of advanced materials. Table 3 shows the validation of the FEM results and the experimental results with the strength of real femur. Table 3: Comparison of FEM and Experimental results with Human Femur Type Compressive stress (MPa) Real human femur 30 205 FEM result for human femur 202.62 FEM result for 3D-printed CF PEEK femur model 212.5 Experimental result for 3D-printed CF PEEK femur model 208.58 4 CONCLUSIONS The following conclusions are drawn from the pres- ent study: • The materials selected for the present study were PEEK and CF PEEK. The materials were selected based on the feasibility of 3D-printing features and their current applications for orthopedic implants. In recent PEEK-made implants are largely used due to their property resemblance with human bone. • From the finite-element analysis viewpoint, CF PEEK was found to be the better choice among the other counterparts. CF PEEK femur-bone model was fabricated using 3D printing and experimented by compression testing as per the ISO 7206-4:2010 stan- dard to find its strength. Both the FE results and the experimental closely matched to the compressive strength of the real human femur. • Though this CF PEEK 3D-printed model of the fe- mur cannot replicate the exact properties of the hu- man femur, it has better properties when compared to its other materials. • The 3D-printed CF PEEK femur model provided an ample suggestion for experimenting with the proper- ties of bone-plate research, since it is impossible for researchers to experiment using a real femur. It can also be used as a pilot model to study the femur bone, but this model is strictly for external usage only. • From the newly developed femur model, the bone plates can be experimented on for mechanical strength analysis, without regard to humans or animal ethics. Acknowledgment The authors of this paper would like to thank Dr. M. Sekar, Principal, AAA College of Engineering and Tech- nology, Dr. P. Nagaraj, Senior Professor & Head, Depart- ment of Mechanical Engineering of Mepco Schlenk En- gineering College, Dr. P. Seenikannan, Dean and Professor & Head, Department of Mechanical Engi- neering, AAA College of Engineering and Technology, for their continuous support in writing this research arti- cle. The authors express their sincere gratitude to Dr. S. Ambalatharasan, MBBS, D. Ortho, DNB, Bone and Joint clinic, Sivakasi for his consistent guidance in this re- search paper. 5 REFERENCES 1 D. Pahr, A. 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