Dual Setting β-tricalcium Phosphate Composite Cement Obtained by 3D Printing

Abstract:

Calcium phosphate cements (CPCs) could be employed as synthetic bone graft substitutes or in the manufacture of scaffolds for tissue engineering. The limitations of these systems are their low mechanical strength, which limits its use to places with small mechanical stresses; however, the use of polymeric additives such us acrylamide (AA) and ammonium polyacrylate (PA), reinforced the system through in situ polymerization and increase the mechanical properties of the final piece. Moreover, the fabrication of the cement scaffolds through rapid prototyping technologies at low temperatures such as 3D printing, will allows the fabrication of more complex forms and customization of implants. Thus, the objective of this work was the evaluation of the α-tricalcium phosphate/AA/PA system in the fabrication of scaffolds by rapid prototyping technology. The results showed slight differences between the porosities of the printed pieces (61% for cylindrical test bodies and 59% for rectangular); but water absorption was significantly different for each type of printed form. Mechanical strength (1.3MPa) and flexural stress (3.2MPa) were lower than expected due to the high porosity of the samples although the morphology of the final material showed the presence of homogeneous and interlinked network of hydroxyapatite crystals. Nevertheless, printed materials might be used as spongy graft substitutes or scaffolds for tissue engineering in low-mechanical solicitation.

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Calcium phosphate cements (CPCs) could be employed as synthetic bone graft substitutes or scaffolds for tissue engineering allowing the fabrication of more complex geometries and the customization of the implants mainly due to the possibility of be molded [1]. Moreover, the use of additive manufacturing technologies at low temperatures such as 3D printing permits the fabrication of pieces with enhanced performance over traditional techniques [2-3]. Usually, the obtained pieces have low mechanical strength, which limits its use to places with small mechanical stresses; however, the use of polymeric additives such as acrylamide (AA) and ammonium polyacrylate (PA), could reinforce the system through in situ polymerization and increase the mechanical properties of the final piece [4].

Some studies reports the use of calcium phosphate powders such as -tricalcium phosphate, tetracalcium phosphate, and β-tricalcium phosphate [β-Ca₃(PO₄)₂; β-TCP] as row material in the manufacture of scaffolds by means of 3D printing technology [5-8]. However, none of the reported studies refer the use of dual setting β-TCP-based cement hydraulic system as proposed by the authors.

Thus, the aim of this work was the fabrication and characterization of a dual setting composite cement based on β-tricalcium phosphate (β-TCP)/AA/PA by 3D printing technology.

A Z310 Plus Printer Prototyper was used to print the pieces. Previously shyntethized β-TCP powder [9], was mixed with ammonium persulfate [(NH₄)₂S₂O₈] and placed in the printer chamber. Powder layer thickness was set to 0.0875mm and binder liquid/powder ratio was 0.31mL/g. The binder was composed by a solution of 5%wt Na₂HPO₄, 10%wt acrylamide (AA), 1% N,N methylenebisacrylamide and 0.5% N,N,N,N- tetramethylethylenediamide [4]. Pieces in the form of cylinders and rectangles for compressive and 4 point bending assays were prototyped.

Phase composition of the samples was determined by X-Ray Diffraction (XRD) in a PHILLIPS^ diffractometer (X´Pert MPD). Morphological differences were observed by Scanning Electron Microscopy (SEM) using a JEOL microscope (JSM-6060). Compressive strength (CS) and flexure stress (FS) were measured in servohydraulic Universal Testing Machine (Instron 3369) with a load measuring cell of 2kN and a loading rate of 1 mm/min.

Figure 1 shows a photograph of the printed materials after removing powder excess. Minor differences were observed in relation to the original sample size.

Figure 2 shows the XRD patterns of β-TCP powder and prototyped cement after 7 days in water/37.5°C. After setting and aging, some β-TCP peaks (JCPDS 09-0348) could be identified in addition to the characteristics peaks of CDHA (JCPDS 46-0905).

Mechanical properties, water absorption apparent porosity and density can be observed in Table 1. Values of both compressive strength, and flexure stress were very low. Slight differences between the values of apparent porosity of the samples were found as a function of the arraignment of prototyped piece; on the other hand, apparent density values were the same regardless of the format of the piece.

Microstructural features of the prototyped material are shown in Figure 4. Typical petal-like plates distinctive of setting and hardening α-TCP-based cements can be observed both on the surface and the fracture surface. However, the size of the crystals in the inside of the material is higher (5) than those found in the surface and a greater homogeneity is observed. In addition, some unreacted α-TCP grains and macropores from about 5 microns of diameter can be observed at the outward of materials. No evidence of the presence of the hydrogel formed during the in situ polymerization of acrylamide was observed.

Usually, strength is difficult to reproduce for β-TCP-based CPCs because of the variability of β-TCP phosphate properties from different sources. Factors like mean particle size and distribution, specific surface area, wettability, and phase impurities markedly influence the properties of the resulting cement.

Since the precipitation of CHDA is responsible for the adherence and interlocking of the crystalline grains, which results in hardening; the fall of the mechanical strength can be attributed to the low transformation of β-TCP into CDHA (Eq 1) according to the results of XRD. The value of the apparent density was close to the theoretical density of β-TCP (2.86g/cm³), confirming no transformation of this phase into CHDA.

When polymerization is conducted in aqueous slurry of ceramic powder, the resulted crosslinked polyacrilamide hydrogel is able to bind the ceramic particles and provide strength to the resulting system [10]. However, the addition of acrylamide to the system apparently did not work as reinforcement of the β-TCP-based cement as expected. Moreover, the presence of the hydrogel after polymerization could be prevented the solubilization of the β-TCP particles and subsequently inhibited the precipitation of the CDHA; so the strength of materials decreases. Furthermore, the high porosity of the prototyped materials also negatively influences the mechanical strength obtained.

Water absorption values were not significantly different from those found for samples of cement without additions hydrogel [11], which reinforces the idea that it could be possible that the in situ polymerization of the acrylamide have not occurred.

SEM results showed the presence of large number of plate-like crystals of aged CDHA in both: surface and fracture surface. Differences in the sizes of the crystals are due to the mechanism of hydrolysis which is dependent on the diffusion of fluid through the layer formed and occurs from the inside to the outside of the material. However, even though the presence of this entanglement of CDHA is responsible for the mechanical strength, the existence of unreacted β-tricalcium phosphate and the high porosity of the prototyped materials are critical factors in the final properties of the cement.

In conclusion, it is possible to obtaining scaffolds of dual setting hydraulic cement by 3D printing. The mechanical properties thereof are low for applications where high mechanical stresses were required. Nevertheless, the obtained pieces were high porosity and could be used as scaffolds for cellular growth and cancellous bone replacement.

Acknowledgment

The authors acknowledge the financial support from CNPq-the National Science and Technology Development Council (Research Grant 190005/2013-0).

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