Effect Investigation of Ammonium Bicarbonate and Polyethylene Wax as Pore-Forming Agents on the Physicochemical Properties of Macroporous Biphasic Calcium Phosphate Bioceramics Processed by Space-Holder Technique

Abstract

The effect of ammonium bicarbonate (AB) and polyethylene wax (PW) on the physicochemical properties of macroporous biphasic calcium phosphate bioceramics (BCPs) processed by space-holder technique was investigated. BCP powder was mixed with AB and PW and, then, uniaxially cold-pressed at 300 MPa. AB and PW were eliminated by heat treatment at 170°C for 2 hours or 550°C for 4 hours, respectively. Subsequently, the samples were air-sintered at 1070ºC for 2 hours. The results revealed that AB is more attractive than PW due to its lower degradation temperature, which has avoided the formation of undesirable phases. In addition, it favored an adequate consolidation of the particles with the formation of sintering necks that provided a better mechanical resistance to handling. The samples with AB also presented the formation of uniformly distributed macro and microporosity in great extension, which is shown by literature as a fundamental combination to stimulate cells/scaffold interaction.

Keywords:
Biphasic calcium phosphate; pore-forming agent; space-holder technique; macroporous bioceramics; biomaterials

1. Introduction

Rehabilitation after bone tissue damage due to traumatic injuries remains a challenging issue in clinical applications and biomaterial engineering. Biocompatibility, porosity, mechanical strength, osteoconductivity and osteoinduction are some criteria to be considered during biomaterials design¹. In this context, bioceramics based on calcium phosphate (CaP) are good candidates for dentistry and orthopedic applications due to their similar chemical composition and affinity to bone tissue²-⁵.

Among different types of CaP, biphasic calcium phosphate (BCP) bioceramics composed of hydroxyapatite (HA) and beta-tricalcium phosphate (β-TCP) have been successfully used in bone replacement owing to the combination of lower solubility of HA and higher resorbability of β-TCP, which leads to a material with tailorable biodegradability that favors the osteointegration process⁶,⁷. However, there is still no consensus about the optimal HA/β-TCP ratio for BCPs, but the literature has reported that ratios of 50/50, 60/40 and 30/70 promoted more rapid cell proliferation⁸,⁹. Moreover, the reactivity of BCPs increases with the HA/β-TCP ratio decreasing¹⁰.

In addition to the HA/β-TCP ratio, the physicochemical properties of BCPs can also be optimized by introducing porosity into their structure. Porous bioceramics can be processed by different techniques, such as gel casting, laser sintering, rapid prototyping, space-holder, and polymeric sponge methods. Among these techniques, the space-holder method has considerable advantages such as low cost; ability to produce scaffolds in final form, independent control of porosity, pore size, and pore morphology; capacity to provide a random and irregulars pore distribution with different sizes, which may perform significantly better in bone regeneration applications; and potential to reduce amounts of waste. This method involves mixing ceramic powder and spacer, compressing the mixture, and removing the spacer before, during, or after the sintering process (depending on the type of spacer used)¹¹,¹².

The control of pore characteristics (size, morphology, and distribution) can be reached by the space-holder technique using different types of pore-forming agents, such as paraffin, naphthalene, sucrose, sodium bicarbonate, ammonium bicarbonate, poly(methyl methacrylate), hydrogen peroxide, cellulose derivatives, polyethylene wax, gelatin, sodium chloride and several other compounds⁷,¹⁰-²⁰. Among these different types of pore-forming agents, ammonium bicarbonate and polyethylene wax have been reported as good options for obtaining high porosity¹⁶,¹⁹. However, the literature has shown few works on the processing of macroporous calcium phosphate bioceramics using these pore-forming agents, indicating the necessity of more studies to elucidate the influence of their characteristics on the physicochemical properties such as porosity, pore shape, pore distribution homogeneity, undesired phase formation, defect generation, and particle sinterability.

Although mechanical tests are not covered by the present work, it is worth mentioning that the presence of pores in the bioceramic structure reduces its mechanical strength. Nevertheless, it can be improved by optimizing the processing parameters for promoting refinement and homogenization of microstructure, and a higher quality of sintering necks, which will help to offset the loss of mechanical strength inherent in increased porosity²¹. In the other hand, the low mechanical resistance is not a limitation for bioactive ceramics, since they have demonstrated excellent features as bone grafts in non-load bearing applications, where the bone regeneration kinetic is more important than mechanical properties²².

In the present work, in order to identify the most suitable pore former, it was investigated the effect of two different types of pore-forming agents, ammonium bicarbonate and polyethylene wax, on the physicochemical properties of macroporous biphasic calcium phosphate bioceramics processed by space-holder technique.

2. Materials and Method

2.1. Synthesis of BCP powders

Biphasic calcium phosphate powders with 20-217 μm particle or agglomerate size range were synthesized by alcoholic sol-gel technique following previous work methodology⁸, as schematized in Figure 1.

Figure 1
Schematization of biphasic calcium phosphate powder synthesis.

2.2. Processing of sintered macroporous samples

For macroporous (Ma) samples preparation, ammonium bicarbonate – AB (CAS number 1066-33-7) and polyethylene wax – PW (Licowax® PE 190 powder) with 250-300 µm and 106-125 µm particle size ranges were used as pore-forming agents. BCP powders were mixed with 25 wt% or 40 wt% of each pore-forming agent in a Turbula mixer (Glen Mills Inc.) for 20 min. The mixtures were uniaxially cold-pressed at 300 MPa, using a cylindrical tool steel pressing die (Ø = 6.17 mm) in an electric hydraulic press (Marconi, MA098/20EL). Oleic acid was used to lubricate the inner wall of the die to facilitate the removal of green samples. After compaction, the green samples were submitted to a heat treatment at 170°C for 2 hours (ammonium bicarbonate, Ma-AB) or 550°C for 4 hours (polyethylene wax, Ma-PW) with a heating rate of 0.5ºC/min to eliminate porogenic agents. Posteriorly, they were sintered at 1070ºC for 2 hours with a heating rate of 5ºC/min in a chamber furnace (Jung, Model 0212) under an air atmosphere and naturally cooled down in the furnace.

The proportion and particle size of pore-forming agents were chosen from the 2^3-1 experimental design, since this tool was successfully applied in previous work⁸. Table 1 displays the variables/factors studied in the 2^3-1 experimental design, where the proportions and particle sizes of porogenic agents were based on the literature data²³. The processing parameters, such as compaction load and sintering conditions, were defined from the results reported by Balbuena et al.⁸, which revealed that such conditions were able to promote suitable particle consolidation without significantly affecting the porosity. The elimination parameters of pore-forming agents were determined from thermogravimetric analyses and previous work, as described in section 3.2²⁴.

Table 1
Parameters used in the 2^3-1 experimental design²³.

Sample	Pore-forming agent	Particle size	Amount
		μm	wt%
1Ma-AB	Ammonium bicarbonate	106-125	25
2Ma-AB	Ammonium bicarbonate	250-300	40
3Ma-PW	Polyethylene wax	106-125	40
4Ma-PW	Polyethylene wax	250-300	25

The results of 2^3-1 experimental design showed that 1Ma-AB and 4Ma-PW samples exhibited unsatisfactory features in terms of porosity and mechanical resistance to handling, respectively. On the other hand, 2Ma-AB and 3Ma-PW samples displayed suitable characteristics and were selected for the present work. More details are presented in section 3.1.

2.3. Characterization of samples

Thermal behavior of AB and PW pore-forming agents was determined by thermogravimetric analysis (TGA) in a SDT Q600 V20.9 build 20 thermal analyzer (TA Instruments), under a synthetic air atmosphere flowing at 100 ml/min, at a temperature range from 40 to 700°C with heating rate of 20°C/min.

Crystalline phase identification of BCP powder and macroporous sintered samples were obtained by X-ray diffraction (XRD). X-ray data were collected in the 2θ from 20 to 80°, in 2 s and 0.05° steps, using a Phillips X’Pert diffractometer with Cu K-α radiation (λ = 1.5406 Å). The diffractometer was set at 40 kV and 40 mA.

Fourier-transform infrared spectroscopy (FTIR) analyses in transmission mode were performed in a Shimadzu Pestige-21 IR spectrophotometer, operated in the range of 400-4000 cm^-1, with a resolution of 4 cm^-1 and 128 scans. The samples were macerated and mixed with KBr and then compacted into pellets.

The morphology of 2Ma-AB and 3Ma-PW samples were observed by scanning electron microscopy (SEM, FEI Inspect S). A thin gold (Au) film was deposited on all samples by sputtering technique (Emitech SC7620 Sputter Coater) under an argon atmosphere to make them conductive. From images with magnifications of 50X or 20,000X, the macro and micropore sizes were measured by the calibrated scale bar from the image software of FEI Inspect S SEM. The average macro and micropore sizes correspond to the random measurement of approximately 34 macropores and 42 micropores, respectively.

Bulk density (${\text{d}}_{\text{bulk}})$of microporous samples were determined by Archimedes’ principle (ASTM B962-17)²⁵ in distilled water, using Equation 2⁸,²⁶:

${\text{d}}_{\text{bulk}}=\frac{{\text{m}}_{\text{dry}}}{\left({\text{m}}_{\text{wet}}-{\text{m}}_{\text{in liquid}}\right)}\times {\text{d}}_{\text{liquid}}$ (2)

where ${\text{d}}_{\text{liquid}}$ is the density of water at room temperature (${\text{d}}_{\text{H}2\text{O}}$ = 0.9971 g/cm³ at 25ºC)²⁷; ${\text{m}}_{\text{dry}}$, ${\text{m}}_{\text{wet}}$ and ${\text{m}}_{\text{in liquid}}$ are, respectively, the dry mass, the mass soaked in water and the mass immersed in water, measured in an analytical balance with a density determination kit (Shimadzu model AUY220). Before measurements, the samples were immersed in distilled water at 100°C for 1 h in order to fill the pores with water.

The relative density was obtained by Equation 3, considering the theoretical density of BCP. In the case of the biphasic materials, the theoretical density (d_theoretical) must be determined considering the volume fraction of each phase in the sample (Equation 4).

${\text{d}}_{\text{relative}}=\frac{{\text{d}}_{\text{bulk}}}{{\text{d}}_{\text{theoretical}}}\times 100$ (3)

${\text{d}}_{\text{theoretical}}={\text{x}}_1{\text{d}}_{\text{t}1}+{\text{x}}_2{\text{d}}_{\text{t}2}$ (4)

where x is the volume fraction of each phase and d_ti is the theoretical density of the i phases determined from the Rietveld refinement.

The porosity (P) was estimated by Equation 5:

$\text{P}=1-{\text{d}}_{\text{relative}}$ (5)

3. Results and Discussion

3.1. 2^3-1 experimental design

The results of 2^3-1 experimental design (Table 1) showed that 1Ma-AB sample exhibited unsatisfactory pore distribution and porosity level (43%) for biomedical applications, since the literature has reported a minimum porosity higher than 50% to stimulate new bone tissue ingrowth to the porous space²⁸-³⁰. 2Ma-AB and 3Ma-PW samples presented similar and satisfactory porosity characteristics (>50%), and 4Ma-PW sample displayed low mechanical resistance to handling, preventing its evaluation.

The probable reasons that explain why 4Ma sample was less performing from the mechanical properties view point may be associated with the fact that the presence of hard or coarse agglomerates in the powder (see section 3.5) causes difficulties on compaction, such as non-uniform particle packing and the occurrence of inter-agglomerate pores within the compacts, and consequently non-uniform and incomplete densification on sintering with the formation of a weak neck inter-agglomerate bonding³¹,³². This undesirable behavior is intensified by the addition of pore-forming agents to produce scaffolds with high porosity level. Further, pore-forming agents with viscoelastic property, such as polyethylene wax, tend to recover original shape after the release of the applied pressure, and the undergone expansion during the recovery leads to the collapse of the compacts, turning them useless or difficult to handle³³. Then, the larger particle size range of PW (250-300 µm) may has undergone a significant shape recovery that generated a high defect amount inside the sample, weakening its mechanical properties.

Based on this evidence, 2Ma-AB and 3Ma-PW samples were selected for the present work. In the following sections, the selected samples will be referred to as Ma-AB and Ma-PW, respectively.

3.2. Thermogravimetric analysis (TGA)

Figure 2 shows the thermogravimetric (TGA) and derivative thermogravimetric (DTG) curves for ammonium bicarbonate (AB) and polyethylene wax (PW). TGA curves revealed a pronounced weight loss in the 100-165ºC and 222-572ºC temperature ranges for AB and PW, respectively. Although the ammonium bicarbonate (NH₄HCO₃) presents low decomposition temperature (~36-60ºC)³⁴, its DTG curve displayed two weight loss peaks, at 102.08ºC which is assigned to water evaporation, since ammonium bicarbonate is very hygroscopic³⁵, and at 139.01ºC which is attributed to the thermal decomposition in NH₃, CO₂, H₂O (all species in gas phase)³⁶.

Figure 2
TGA and DTG curves of AB and PW pore-forming agents.

For PW, DTG curve exhibited a multistage decomposition process, which indicates that the thermal degradation of polyethylene in air occurs via reaction with oxygen, that is, oxidative degradation. Degradation in air leads to the formation of several different products including peroxides, acids, and alcohols. The peak at 222.08ºC has been attributed to the formation of polymeric oxides, while that one at 370.23ºC is related to the major degradation step followed by a smaller weight loss step with peak at 411.86ºC. The degradation becomes exothermic around ~400ºC that leads to an unsteady degradation process. This temperature is associated with the ignition temperatures of many carboxylic acids, which predominate at the later stages of degradation with a peak at 554.17ºC³⁷,³⁸.

From thermograms, it is observed that the degradation mechanism of AB is much less complex than that of PW, demonstrating that the inorganic compound is easier removed by heat treatment when compared to the polymer. The lower degradation temperature of AB offers advantages in terms of energy consumption and processing time, which makes this material more attractive to be used as pore-forming agent.

3.3. X-ray diffraction (XRD)

Figure 3 displays XRD diffractograms of BCP powder and Ma-AB and Ma-PW sintered samples. It is observed that BCP powder and Ma-PW sample presented characteristic peaks of HA (JCPDS card #22059), β-TCP (JCPDS card #6191) and calcium pyrophosphate (CPP or Ca₂P₂O₇) (JCPDS card #09–0346) crystalline phases, whereas Ma-AB sample indicated the presence of HA and β-TCP phases only.

Figure 3
XRD diffractograms of BCP powder, Ma-AB and Ma-PW samples.

In Table 2, it is seen that the sintering conditions favored β-TCP→HA partial phase transformation from BCP powder, being the Ma-AB sample with a higher percentage of HA than the Ma-PW sample. Despite this result, both macroporous samples presented suitable HA/β-TCP ratio for biomaterial applications, since the literature has reported no statistically significant differences (p > 0.05) between ratios of 30/70 vs. 50/50, 60/40 vs. 0/100, 60/40 vs. 100/0 and 70/30 vs. 30/70 in terms of new bone formation, bone volume, volume stability, bone mineral density and osteoconductive capacity in the defect site. However, some authors have concluded that the amount of bone formation and biodegradation of BCP is inversely proportional to the HA/β-TCP-TCP ratio⁹, suggesting that the Ma-PW sample (ratio = 3.35) presented better phase composition than the Ma-AB sample (ratio = 4.56).

Table 2
Fraction of HA/β-TCP/CPP and HA/β-TCP ratio determined by Rietveld method for BCP powder and macroporous samples.

Sample	HA/β-TCP/CPP (%)	HA/β-TCP ratio
BCP powder	69/23/8	3.05
Ma-AB	82/18	4.56
Ma-PW	77/23	3.35

The higher amount of HA in the sintered samples can be attributed to the thermal decomposition of HA to β-TCP at temperatures above 800ºC³⁹. Then, during cooling down step, part of β-TCP converts to HA by rehydration⁴⁰, considering the pre-existing β-TCP phase in the BCP powder as verified in Figure 3 with the presence of the peak (31°), corresponding to the β-TCP.

3.4. Fourier-transform infrared spectroscopy (FTIR)

FTIR spectra of BCP powder, Ma-AB and Ma-PW samples are shown in Figure 4 and the absorptions bands are summarized in Table 3. For BCP powder, FTIR spectrum had been reported elsewhere⁸. Typical functional groups of HA, β-TCP, and CPP were identified. It is observed that the characteristic bands of PO₄^-3 groups for Ma-AB and Ma-PW samples are like those found for BCP powder, with absorptions at 1091, 1043, 962, 602, 570, and 473 cm^-1 for HA phase, 945 cm^-1 for β-TCP phase⁴¹. In Figure 4a, when grouping the individual spectra together at the 4000-400 cm^-1 interval, the bands at 962 and 945 cm^-1 were grouped because of proximity. Then, in Figure 4b, the spectra limited at the 1500-500 cm^-1 interval evidence these bands. Moreover, the absorptions at 3570 and 632 cm^-1 are assigned to the OH^- stretching mode of the crystalline HA. The bands at 1450, 1370, and 876 cm^-1 correspond to the substitution of PO₄^-3 ions with CO₃^-2 ions in the HA lattice due to the presence of atmospheric carbon dioxide in the reactional and calcination media⁴². The adsorbed H₂O content is attributed to the bands at 3440 and 1641 cm-1 ⁴³. Additionally, Ma-PW sample exhibited a slight absorption band at 3713 cm^-1 corresponding to the OH^- group of carbonated HA⁴⁴, bands at 1210 and 720 cm^-1 for pyrophosphate (Ca₂P₂O₇^-4) phase, and at 1735 cm^-1 for C=O vibration related to the oxidation of polyethylene wax³⁷.

Figure 4
FTIR spectra of Ma-AB and Ma-PW samples: (a) at the 4000-400 cm^-1 interval and (b) at the 1500-500 cm^-1 interval showing the grouped bands at 962 and 945 cm^-1.

Table 3
Assignments of the observed vibrational frequencies of BCP powder, Ma-AB and Ma-PW sintered samples (adapted from reference⁸,³⁷,⁴¹-⁴⁴).

Assignments	Observed vibrational frequencies (cm-1)
	BCP powder8	Ma-AB	Ma-PW
v1 stretching mode, PO4-3 HA	962	962	962
Vibrational mode, PO4-3 β-TCP	−	945	945
v2 bending mode, PO4-3 HA	−	473	473
v3 stretching mode, PO4-3 HA	1089, 1041	1091, 1043	1091, 1043
v4 stretching mode, PO4-3 HA	600, 560	602, 570	602, 570
Vibrational mode, OH− HA	632	632	632
Stretching mode, OH− HA	3572	3570	3570
v2 bending mode, CO3-2	873	−	876
v3 stretching mode, CO3-2	1425	1450, 1370	1450, 1370
v2 bending mode, H2O	1641	1630	1630
v3 stretching mode, H2O	3440	3440	3440
Stretching mode, H2O	3640	−	−
OH-derived band, carbonated HA	−	−	3713
Vibrational mode, P2O7-4	723	−	1210, 720
C=O vibrations, PE wax oxidation			1735

FTIR results corroborate with XRD diffractograms in Figure 3, indicating that the conditions of heat treatment for AB and PW elimination and sintering were adequate. However, the long heat treatment time (550°C for 4 hours) for PW elimination favored the formation of pyrophosphate undesired phase, once studies have reported that bioceramics processed from biphasic material with the presence of CPP phase have shown low mechanical strength. This may be related to the lack of coalescence of the particles due to the difference in the thermal coefficients of the phases present. Further, the degree of densification and the resulting microstructure also depend on the proportion of each phase present and that the presence of other components affects the sinterability of β-TCP⁴⁴-⁴⁸.

The formation of pyrophosphate during PW elimination can be associated to the impurities resulting from the thermal degradation, such as peroxides, acids, alcohols, and polymeric oxides, as mentioned in section 3.2. According to literature, the presence of impurities leads to polyphasic bioceramics in which phase composition and proportions are poorly controlled. The impurities also influence the sintering behavior, causing a sinterability increasing⁴⁸.

3.5. Scanning electron microscopy (SEM)

For BCP powder, SEM results had been reported elsewhere⁸. In Figure 5, it is observed that the synthesized powder is formed by oval or spherical agglomerates of small particles⁸. Figure 6 exhibits SEM images of the Ma-AB (Figures 6a, 6b, 6c) and Ma-PW (Figures 6d, 6e, 6f) samples. From Figures 6a and 6d, it was possible to evaluate the average macropore size, which was estimated at 307.5 ± 181.6 µm for Ma-AB and 370.1 ± 137.6 µm for Ma-PW. In addition, it was noted that Ma-PW presented macropores with spherical shape and non-uniform distribution when compared to Ma-AB sample, which disclosed uniformly distributed macroporosity.

Figure 5
SEM images of synthesized BCP powder⁸.

Figure 6
SEM images of Ma-AB (a, b, c) and Ma-PW (d, e, f) samples.

The uniform and non-uniform macropore distributions of Ma-AB and Ma-PW samples, respectively, may have induced by density and/or size differences of BCP, AB and PW particles during mixing and compaction steps⁴⁹. From Table 2 and Equation 4, the theoretical density for BCP powder was estimated at 3.32 g/cm³. The densities of ammonium bicarbonate (CAS 1066-33-7) and polyethylene wax (Licowax® PE 190 powder) are 1.59 and 0.97 g/cm³, respectively. The density between the materials of the powder mixture to be processed is one of the main problems that can generate segregation. Low-density materials tend to “float” and settle toward the top of the mixture, while high-density components tend to sink toward the bottom of the mixture⁵⁰. Moreover, in powders randomly mixed with different particle size ranges (or with the presence of agglomerates), the larger particles migrate to the top of the die cavity when compacted and/or submitted to vibrating movements. During compaction procedure, the smaller particles tend to segregate to the bottom of die through the interstices formed between the larger particles⁵¹. The mixing, compaction and segregation of components are driven by shear-induced breaking of agglomerates or fragile particles, whereby the size-reduced agglomerates or fragile particles sift through the interstitial volume between coarse particles, promoting a better distribution of materials in the mixture⁵². Then, the uniform macropore distribution in Ma-AB sample may be related to the breaking of BCP agglomerates and/or AB particles in the compaction step, leading to uniform result. For Ma-PW sample, the non-uniform macropore distribution may be attributed to the elasticity of PW that prevents the breaking of its particles, avoiding a uniform particle distribution in the mixture and contributing for segregation by density and/or size difference.

Figures. 6b and 6e revealed cracks for Ma-AB and Ma-PW samples, respectively. For Ma-AB sample, in Figure 6b, the microcrack generation may be associated with the formation of micropores from the accelerated release of gases generated during the thermal treatments for AB elimination and sintering⁸,³⁶. On the other hand, for Ma-PW sample, in Figure 6e, the presence of CPP phase may have induced a sudden dilatometric expansion due to the difference in the thermal coefficients of phases present, which creates mechanical stresses leading to crack formation⁴⁸.

In Figure 6c, the Ma-AB sample displayed a regular microstructure with uniformly distributed microporosity. The average micropore size was estimated at 0.47 ± 0.21 μm. Such microporosity may be considered closed and inherent in the processing of ceramics. The image suggests that sintering parameters were effective for particle consolidation since neck formation was observed⁵³. It is also verified that the microstructure was formed by grains with smaller sizes than BCP powder particle size (20-217 μm), reinforcing the previous findings that precursor powder is mainly composed of agglomerates of small particles (Figure 5)⁸. For Ma-PW sample (Figure 6f), a lower level of microporosity is noted in an irregular surface with roughness. Further, the neck formation was not noticed, which suggests a poor particle consolidation due to the CPP presence, corroborating with the discussion of section 3.4.

Although Ma-AB and Ma-PW samples have displayed suitable macropore size for scaffold application⁵⁴, the SEM images suggest that ammonium bicarbonate was more effective and advantageous in the processing of samples with uniformly distributed macro and microporosity, since this pore-forming agent allowed a more appropriate particle consolidation with neck formation, which provided a better mechanical strength to handling, and a highest degree of microporosity in the pore walls which favors cell adhesion and differentiation⁵⁵.

3.6. Bulk density, theoretical density, relative density, and porosity

Table 4 presents bulk, theoretical and relative densities, and porosity results of Ma-AB and Ma-PW samples determined by Archimedes’ principle. It is possible to notice that the theoretical density, estimated from Equation 4 and Table 2, was similar for both samples. Also, PW promoted a slightly higher porosity level than AB. Such behavior can be related to the low particle consolidation verified in Figure 6f, which resulted in lower bulk and relative densities for the Ma-PW sample. However, although the porosity values are close, the results indicated that polyethylene wax, with a smaller particle size (106-125 µm), was able to generate a porosity level near ammonium bicarbonate with a larger particle size (250-300 µm), suggesting that the particle size of pore-forming agents did not significantly influence on the porosity of materials.

Table 4
Bulk density, relative density and porosity of Ma-AB and Ma-PW samples determined by Archimedes’ principle.

Sample	dbulk (g/cm3)	dtheoretical (g/cm3)	drelative (%)	Porosity (%)
Ma-AB	1.36	3.25	42	58
Ma-PW	1.23	3.28	37	63

The porosity level and pore size (section 3.5) are in agreement with bioceramic scaffolds obtained by other literature studies, which have reported porosity in the range of 60-80% and pore size ≥ 300 µm⁵⁴-⁵⁷, indicating that Ma-AB and Ma-PW samples have the potential for bone regeneration application¹,⁵⁴. However, Ma-AB sample displayed advantages in terms of the presence of uniformly distributed macro and microporosity in great extension. A combination of macro and microporosity have demonstrated to be fundamental for stimulating cells/scaffold interaction, once microporosity favors protein adsorption and retention, and macroporosity allows cell infiltration and migration through the scaffold⁵⁸,⁵⁹.

4. Conclusions

Macroporous biphasic calcium phosphate bioceramics were successfully processed by space-holder technique, using ammonium bicarbonate and polyethylene wax as pore-forming agents. The conditions of heat treatment for AB and PW elimination and sintering were adequate. However, the long heat treatment time for PW elimination favored the formation of pyrophosphate undesired phase, which prevented an appropriate particle consolidation during the sintering. Ammonium bicarbonate demonstrated to be more attractive than polyethylene to be used as pore-forming agent, since it did not induce the formation of undesirable phases due to its lower degradation temperature, allowed a suitable particle consolidation with neck formation, which provided a better mechanical strength to handling, and promoted the production of uniformly distributed macro and microporosity in great extension, which is shown by literature as a fundamental combination for stimulating cells/scaffold interaction.

5. Acknowledgements

The authors thank to Carlos Chagas Filho Research Foundation of the State of Rio de Janeiro (FAPERJ/Brazil), Coordination for the Improvement of Higher Education Personnel (CAPES/Brazil) and National System of Nanotechnology Laboratories (MCTI/SisNANO/INTCENANO-CNPq Process number 442604/2019–0) for financial support, Brazilian Center for Research in Physics (CBPF/Brazil) for FTIR analyses.

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Publication Dates

• Publication in this collection
  11 Nov 2024
• Date of issue
  2024

History

• Received
  17 June 2024
• Reviewed
  19 Sept 2024
• Accepted
  19 Oct 2024