Binder Saturation, Layer Thickness, Drying Time and Their Effects on Dimensional Tolerance and Density of Cobalt Chrome-Tricalcium Phosphate Biocomposite

Traditional metals such as stainless steel, titanium and cobalt chrome are used in biomedical applications (implants, scaffolds, etc.) but suffer from issues such as osseointegration and compatibility with existing bone. One way to improve traditional biomaterials is to incorporate ceramics with these metals so that their mechanical properties can be similar to cortical bones. Tricalcium phosphate is such a ceramic with properties such that it can be used in the human body. This research explores the use of the Binder Jetting based additive manufacturing process to create a novel biocomposite made of cobalt chrome and tricalcium phosphate. Experiments were conducted and process parameters were varied to study their effect on the printing of this biocomposite. Layer thickness, binder saturation and drying time affected the dimensional tolerance and the density of the brown samples. This effect is important to understand so that the material can be optimized for use in specific applications iii TABLE OF CONTENTS INTRODUCTION................................................................... 1 BINDER JET-BASED ADDITIVE MANUFACTURING.................... 4 PROCESS PARAMETERS........................................................ 5 EXPERIMENTAL PLAN.......................................................... 5 RESULTS............................................................................. 7 DISCUSSION......................................................................... 11 CONCLUSION....................................................................... 12 REFERENENCES................................................................... 14


Introduction
A biomaterial is any material, natural or man-made, that can perform a bodily function or can replace a body part or tissue. Depending on their application, biomaterials can be made from polymers, metals, ceramics, or composites [1][2][3].
Biomedical implants can be classified into three major categories: external to the body (non-clinical, which includes surgical instruments, prosthetics, etc.), internal to the body and permanent (includes hip implants, knee implants, stents, etc.), and internal to the body & temporary (includes scaffolds, degradable screws and drug delivery systems) [1][2][3]14].
According to FDA, a ''permanently implantable device is a device that is intended to be placed into a surgically or naturally formed cavity of the human body for more than one year to continuously assist, restore, or replace the function of an organ system or structure of the human body throughout the useful life of the device. '' Some examples include knee and hip implants.
Temporary implants are commonly used in sports surgeries, such as in shoulder and knee ligamentous reconstruction and spinal reconstructive surgery [5,6,14].
The most common biomaterials for implants are metals, their alloys, ceramics, and polymers. In the past few years, cobalt alloys have gained popularity for being used as biomaterials for implants. Bones [3,4,7].
These differences lead to stress shielding resulting in loosening of the implant due to degradation of human tissues around implants and, consequently, further surgeries to replace the implants. Ceramics are inorganic materials with high compressive strength and biological inertness that make them suitable for scaffolds used in strengthening or replacing damaged bones and tissues. The most commonly used bioceramics are metallic oxides (e.g. Al2O3, MgO), calcium phosphate (e.g. hydroxyapatite (HA), tricalcium phosphate (TCP), octacalcium phosphate (OCP)), and glass ceramics (e.g. Bioglass, Ceravital).
Calcium phosphates have the best biocompatibility and properties closest to natural bones: elastic modulus 7-13 GPa, compressive strength 350-450 MPa, tensile strength 38-48 MPa, and flexural strength 100-120 MPa. However, they have poor fracture toughness and tensile strength that limits their application for bioimplants. Several in vitro and in vivo works have shown that calcium phosphates support the adhesion, differentiation, and proliferation of osseogenesis-related cells (e.g. osteoblasts, mesenchymal stem cells), and induce gene expression in bone cells. The most important calcium phosphate is hydroxyapatite (HA, Ca10(PO4)6(OH)2). With chemical characteristics similar to hard tissues such as bone and teeth, they promote hard tissue in growth and osseointegration when implanted into the human body. The porous structure of this material can be tailored to suit the interfacial surfaces of the implant. As a bulk material, HA lacks sufficient tensile strength and is too brittle to be used in most load bearing applications. In such cases, HA is coated onto a metal core or incorporated into polymers as composites. The ceramic coating on the titanium implants improves the surface bioactivity but often fails as a result of poor ceramic/metal interface bonding [8][9][10][11].
α-TCP and β-TCP are the two crystalline varieties of HA of interest in biological applications. β-TCP is the thermodynamically stable form at low temperature. It transforms into α-TCP in the temperature range 1120-1170 o C. β-TCP is generally preferred in sintered ceramic implants, while α-TCP is more commonly used in bone graft cements because of its hydrolysis properties. The requirements that allow bone ingrowth are a porosity of 30-70 vol%, a pore diameter between 300 and 800 µm, and mechanical properties of 0.5-15 MPa. These being similar to cancellous bone.
Thus, there is a drive in the biomedical industry to create novel materials that behave very similar to bone and can be used for multiple applications from permanent to temporary implants.
Furthermore, these materials need to be manufactured in a manner that would create porosity in situ for biological applications.
Due to the versatility of additive manufacturing, it is gaining a lot of popularity in the field of bone implants. Selective Laser Sintering, Selective Laser Melting, Electron Beam Melting and Binder jet manufacturing have all been used to create various porous structures for biomedical implants [7,9]. To accomplish the various requirements of the implants it is necessary to create biocomposites that can have the strength properties of metals as well as the biological properties of bioceramics.
This research explores the use of the Binder Jet based additive manufacturing process to create a novel biocomposite made of cobalt chrome (CC) and tricalcium phosphate (TCP).
Experiments were conducted and processing parameters were varied to study their effect on the printing of this biocomposite. Layer thickness, binder saturation, and drying time affected the dimensional tolerance and the density of the brown samples. This effect is important to understand so that the material can be optimized for use in specific applications.

Binder Jet-based Additive Manufacturing
Binder Jetting additive manufacturing (AM) is a Drop on Demand (DoD) inkjet printing process in which binder is emitted through a nozzle to form a short jet. This jet condenses into a drop and the position at which each drop lands on substrate is controlled by relative motion between drop and the substrate. The nozzle head is piezoelectric and uses the deformation of ceramic element to generate a pressure pulse needed to eject the binder. Typical drop diameters vary from 10-100 µm, drop volumes vary from 0.5-500 pl, and the drop speeds are 5-8 m/s [15].
The main technique of manufacture using the Binder Jet process is as follows: (a) The CAD model is converted to an STL file and a slicer is used to slice it into layers, (b) Each layer begins with a thin distribution of powder spread over the surface of a powder bed, (c) Using a technology similar to ink-jet printing, a binder material selectively joins particles where the object is to be formed, (d) A piston that supports the powder bed and the part-in-progress lowers so that the next powder layer can be spread and selectively joined, (e) This layer-by-layer process repeats until the part is completed, (f) Following a heat treatment, unbound powder is removed and the metal powder is sintered together. Fig. 1 shows the details of the whole process.

Process parameters
The Binder Jet process described above can be divided into 3 basic steps: 1) Binding 2) Curing and 3) Sintering. There are various process parameters that can be changed to obtain a customized part in each of these steps. These include powder size, layer thickness during binding, part orientation in bed, heater power, roller speed, curing temperature, curing time, sintering time, sintering temperature, and sintering atmosphere. In this research we concentrated on the feasibility of printing of the CC and TCP biocomposite.

Experimental Plan
The materials used in the study were cobalt chrome (Co 212-H, Sandvik Osprey), which has a mean particle size of 53 μm and an apparent density of 3.15 g/cc. The chemical composition of Co 212-H is shown in Table 1.    (Table 2). After the printing, all the samples were cured at 175 o C for 3 hours.
The print quality was measured by two means: dimensional tolerance compared to the CAD model and the density of the part after the printing. Two different size of parts were printed to study the effect of size on the dimensional tolerance. The CAD drawings of the parts with nominal dimensions is shown in Fig. 3. Eight parts of 10 mm and three parts of 30 mm were printed for the corresponding experimental run.  Table 3.  100  15  10  2  60  100  20  10  3  90  100  20  30  4  90  80  20  10  5  60  80  15  10  6  90  80  15  30  7  60  100  15  30  8 60 80 20 30

Results
After the parts were printed, they were weighed and measured for the outside X, outside Y, inside X and inside Y dimensions. Dimensional deviation from the CAD model was calculated according to the following formula for each of the four dimensions: Dimensional Deviation (%) = !"#$%&' )$#*%+$"% (-.))01*&+23*4 )$#*%+$"% !"#$%&' )$#*%+$"% (-.)) * 100% The box plots for the two different sample sets and their dimensional variations are shown in Fig. 4-6.  The response surface analysis of all the results was done using MINITAB. Only the main factors were considered in the analysis and the interaction between the factors was ignored for this study since the aim is to find the process parameters important for dimensional variations. A typical fitted response surface is shown in Fig. 9.  All the response surfaces were analyzed and the results of the P-values were summarized to identify the significant factors in each case. The summary of P-values is shown in Table 4 and 5.  g) There is a wide variation in the density of green parts depending upon the process parameters. This property can be exploited to control the porosity of the parts made by various materials.

Conclusions
Traditional metals such as stainless steel, titanium and cobalt chrome are used in biomedical applications (implants, scaffolds, etc.) but suffer from issues such as osseointegration and compatibility with existing bone. One way to improve traditional biomaterials is to incorporate TCP is such a ceramic with properties that it can be used in human body. This research explores the use of the Binder Jet based additive manufacturing process to create a novel biocomposite made of cobalt chrome and tricalcium phosphate. Experiments were conducted and processing parameters were varied to study their effect on the printing of this biocomposite.
It is found that layer thickness is significant factor for printing with 0% TCP and 20% TCP samples. However, drying time is only significant for 20% TCP samples because of the soft nature of TCP powder and a similar effect is found with binder saturation. Thus, printing parameters are crucial in manufacturing parts from Binder Jet additive manufacturing.
Further research needs to be conducted to take the best printing parameters, use the samples from this configuration, and study the effect of sintering parameters on the strength and biocompatibility of these samples.