Effect of Sintering Time on the Microstructure and Properties of Inorganic Polyphosphate Bioceramics

Sintering is an important step in the fabrication process of ceramic bodies, which can significantly affect the microstructure and properties of materials. In this article, calcium based inorganic polyphosphate (CPP) bioceramics were synthesized by gravity sintering. Effects of the sintering time (30 minutes, 1 hour, 3 hours and 5 hours) on the microstructure, physicochemical degradation and mechanical property were investigated. It was found that all prepared CPP samples for various sintering times showed a β-CPP phase at the temperature of 800 oC. The sample morphology changed to more compact with extending the sintering time from 30 minutes to 5 hours. Moreover, the grain size increased with the increase of sintering time, from 1.59 μm for 30 minutes to 3.40 μm for 5 hours. The in vitro degradation test revealed that the degradation velocity had an inverse relationship with the sintering time. The CPP samples sintering for 30 minutes showed the fastest degradation, while CPP sintering for 5 hours was the slowest one. Compression test results showed that longer sintering times led to improved mechanical properties.

Degradability and mechanical properties play important roles in scaffolds for bone tissue engineering, which can be signifiacntly affected by preparation conditions.After studying the effect of the polymerization degree on the mechanical performance and degradation behavior of CPP in vitro, Ding and Qiu [6,16] found that by increasing the polymerization degree, the compressive strength was promoted but the degradability was weakened.However, at present there is no report about the effect of the sintering time on the properties of CPP.In this paper, we focus on the physicochemical degradation and mechanical properties of CPP affected by the sintering time.The crystal structure, relative density and grain size were also evaluated and compared.

Materials and methods
Pure CPP glass was prepared as previously described [17].Briefly, Ca(H 2 PO 4 ) 2 •H 2 O (calcium phosphate monobasic monohydrate) powders were synthesized using calcium carbonate and phosphoric acid.Then the powders were calcined at 500ºC for ten hours, followed by melting at 1200ºC for one hour to form amorphous CPP.After screening to yield powders in a size range of <75 μm, the amorphous powders (0.6g) were mixed with stearic acid (0.4g) to produce cylindrical green bodies of 10 mm in diameter and 10 mm thickness in a cylindrical mold pressed by a compressive stress of 1 MPa.After drying, these CPP green bodies were then heated to 800 ºC at a heating rate of 5 ºC /min.After holding for 30 minutes, 1 hour, 3 hours and 5 hours, the samples were cooled naturally to room temperature.
Powder X-ray diffraction was performed to identify the crystalline phases of samples obtained using an X'Pert Pro MPD X-ray diffractometer (Philips, Netherlands), at 40 kV 40mA using Cu Kα.The microstructure was examined by a scanning electron microscope (SEM, Hitachi S2400) and the average grain size was determined by the lineal intercept method [18].Using the Archimedes method [19], the density of CPP was measured.Relative density is defined as the ratio between bulk density and the theoretical density of a CPP sample.Compression tests were conducted with an Instron 4302 material testing system (American) (n=5).The in vitro degradation test was carried out at 37 ºC for up to 15 days in a Tris-HCl buffer solution (PH=7.4) according to ISO 10993-14 (n=3).By using the colorimetric method [20] and calcein titration method [21], orthophosphate and calcium ion concentration in degraded medium was measured, respectively.

Results and discussion
Fig. 1 shows the XRD patterns and SEM images of CPP samples prepared at 800 • C for (a) 30 min; (b)1 h; (c) 3 h and (d) 5 h, respectively.From the XRD patterns, it is found that there is no difference among these four patterns except some small differences in peak width and absolute intensity of the diffraction patterns.The results of XRD indicated that all samples reveal a single-phase CPP structure and the effect of sintering time on the CPP is not obvious.SEM analysis demonstrates the strong influence of sintering time on the grain size and morphology.It can be seen that the higher sintering time lead to a more amorphous region and the grain size increased with increase of the sintering time.
The process of sintering has three stages: an initial, intermediate, and final stage [22,23].In the initial stage, the CPP green body has a low-density and is generally lacking in physical integrity.There is a small degree of adhesion between adjacent particles.Then, the necks begin to form at the contact points between the particles in the intermediate stage (Fig. 2).The final stage of sintering begins when most of the pores are closed.As shown in Fig. 1, the samples sintered for 30 minutes consist of approximately the same sized grains (the initial stage).With increase of the sintering time, some necks are formed and grain shapes change.Besides, grains show obvious growth and some isolated pores are formed (the intermediate stage).With further increase of the sintering time, the grain size of sample increases greatly to 3.4 mm and pores are mostly closed (the final stage).Grain growth can be expressed by the following equation [24]: where G and G 0 are the average grain size after and before sintering, a is the kinetic grain growth exponent, T is the sintering temperature, t is the sintering time, Q is the apparent activation energy for grain growth, K 0 and R are constants.According to this grain-growth model, CPP grains have more time to grow for a longer sintering time.As a result, both the average grain size and relative density of CPP increased with increase of the sintering time.For bone tissue engineering, the scaffold should have controllable degradation to match the rate of new tissue regeneration.In a previous study, it was proved that the crystal structure [16], sintering temperature [25], polymerization degree [6,16], porosity [26], ion doped [27] and degradation media [8] can affect the degradation rate of CPP bioceramic, respectively.Besides, research by Pilliar [13] demonstrated that the degradation media can easily attack the grain boundary (or amorphous) regions of CPP.From the SEM micrographs shown in Fig. 1, it can be clearly observed that CPP sintered for 30 minutes exhibits a larger area of grain boundary regions, suggesting that it could be more easily attacked by a degradation solution, thus having a higher degradation rate.As known, bioceramic materials with significantly different degradation rates may be prepared covering several potential clinical applications.In this work, we can obtain CPP with different degradation velocity by controlling the sintering time, which may provide an approach to achieve a controllable degradability scaffold and explore more biomedical applications.2, it can be observed that for the same sintering temperature (800°C), the relative density increased from 70.5% to 79.4% when the sintering time increased from 30 minutes to 5 hours.As a consequence, the compressive strength of CPP sintered 30 minutes is lower than that of samples sintered for 5 hours.
Bone tissues are the main load-bearing tissues of the human body, so the design of bone implant materials must have reasonable mechanical properties.Generally, a bone tissue engineering scaffold should have mechanical strength similar to the natural bone to maintain integrity until the new tissue regenerates.In this study, the compress strength of porous CPP specimens ranged from 2.5 MPa to 8.1 MPa.Hench reported that the compressive strength of human cancellous bone was in the range from 2 to 12 MPa [28].The results obtained for compress strength suggest that maybe the porous CPP scaffold could meet the mechanical requirements and support new bone tissue regeneration when implanted in the body.

Conclusion
The effect of sintering time on the microstructure, degradability and mechanical property of CPP bioceramics synthesized by gravity sintering was investigated in this work.The results indicated that the crystal structure of obtained CPP are not significantly influenced by the sintering time.The relative density, grain size and mechanical property of CPP increased according to increasing sintering time.However, with the time increased from 30 minutes to 5 hours the degradation velocity of CPP decreased.Consequently, it is expected that CPP with different microstructures and properties would be useful in several potential clinical applications in the future.

Fig. 3 .
Fig. 3.The relative density and grain size of CPP with different sintering times

Fig. 4 .
Fig. 4. Phosphate (a) and calcium (b) ion release from the CPP into Tris-HCl buffer solution against time of immersion

Fig. 5 .
Fig. 5. Compressive strength of CPP with different sintering times