Microwave Dielectric Analysis on Porous Hydroxyapatite/Starch Composites with Various Ratio of Hydroxyapatite to Starch

This study aims to investigate the dielectric response of the porous hydroxyapatite/starch composites by varying the starch proportion in determining the feasibility of the microwave sample characterization technique in bone tissue engineering. The porous hydroxyapatite/starch composites were fabricated by using natural starch (gelatinization and retrogradation) through the solvent casting and particulate leaching technique. The dielectric constant (ε′) and loss factor (ε′′) of the complex permittivity of the porous hydroxyapatite/starch composites were measured in the Ku band frequency of 12.4-18.0 GHz. ε′ and ε′′ of the porous composites increase with frequency. The highly porous composite that due to higher starch proportion exhibit higher ε′ and ε′′, resulting in the significant dielectric responses.


Introduction
Bone trauma and disease have become a global health issue with the population aging and socioeconomic problem. Thereby, the development of bone tissue engineering is promoted to create the implantable bone substitutes as the temporary biomimic framework for bone regeneration [1,2]. In bone tissue engineering, hydroxyapatite is commonly used in the bone scaffold fabrication due to its significant bioactivity, biocompatibility, and osteoconductivity. However, hydroxyapatite is a brittle material in single-phase scaffold fabrication and limits the load-bearing application [1,3]. Meanwhile, starch is one of the most promising natural polymeric materials for various industrial fields, especially biomedical engineering application. Starch exhibits the prominent biocompatibility, biodegradability, processability, bioresorbability and renewability properties [1,4]. Hence, hydroxyapatite can be used as the reinforcement fillers in the starch based scaffolds to reinforce mechanical properties and improve bioactivity for bone regeneration [1][2][3]. Starch is consists of the linear amylose and the branched amylopectin molecules. The amylose and amylopectin contents of starch are normally made up of 20-25% and 75-80% by weight. The starch granules swell and burst when it heats with water during the gelatinization process. The semi-crystalline arrangement of the starch melt into gelatinized starch matrix and then the rearrangement occurs between the starch molecules. The reinforced hydroxyapatite fillers  [1,2,5]. The composite with appropriate architectures is obtained via the bone scaffolding techniques. The porous structure of the composite plays an important role in the bone regeneration process which provides cell infiltration, nutrients/waste transport, and capillary ingrowth [1,2]. The rapid development of bone tissue engineering motivate advent of a rapid and non-invasive sample characterization technique which might bring massive convenience to reveal the bone scaffold properties, e.g. microwave characterization techniques. The result of the sample polarization in response to the microwave electric field would illustrate the morphological and physicochemical properties of the bone scaffold. The polarisation and energy dissipation within the composite are induced under exposure to the time-varying electromagnetic field. Polarization takes place as the orientation motions of the composite's dipolar molecules, while the direction of polarization switch leads to storage of energy [6][7][8]. In Maxwell-Wagner's theory, it stated that the heterogeneous system of the porous composite exhibits the interfacial polarization mechanisms that the charge accumulation at the interface which contributed to the different permittivity [8,9]. The complex permittivity is function of microwave propagation. In this study, an attempt has been made to investigate the effect of the starch proportion and the average porosity on the dielectric properties of the porous composites over a broadband frequency range of 12.4-18.0 GHz.

Material
The porous hydroxyapatite/starch composites were prepared by using commercial rice starch (Rs) and potato starch (Ps), hydroxyapatite nanopowder (HAp) and Sodium Chloride (NaCl) porogen particles.

Sample fabrication
The porous hydroxyapatite/starch composites were fabricated by using solvent casting/particulate leaching (SCPL) technique. The starch solution was prepared by dissolving the starch in distilled water. Then, it went through heat-moisture treatment (temperature range of 45-65°C) for about 30-60 minutes. temperature range from 80-90°C for about 20-24 hours and then further drying process was conducted at higher temperature (110-140°C for 2-3 hours). Finally, the porous composite were dried (85-95°C for 3-4 hours) after porogen leaching process.

Sample characterization
The porosity of the porous composite samples was determined using the liquid displacement technique that based on Archimedes principle. The porosity of the porous composite was calculated by using the equation (1)

Results and Discussions
The complex permittivity can be expressed in terms of dielectric constant (ε′) and loss factor (ε′′). The variation of ε′ and ε′′ with frequency for various porous hydroxyapatite/starch composites are shown in figure 1 and figure 2, respectively. In the porous composites, the average porosity (as shown in table 2) play the vital role in developing interfacial polarization when expose to the operating frequency range of applied field. In table 2, the average porosity of the both porous hydroxyapatite/starch composites increase with the starch proportion. The average porosity of the porous hydroxyapatite/potato starch composites is higher than the porous hydroxyapatite/rice starch composites. In figure 1 and figure 2, the ε′ and ε″ of the porous hydroxyapatite/starch composites increase along the frequency range. The peaks of permittivity spectrum are caused by various polarization relaxations. When the proportion of the starch from 50 to 70 wt% increases, the ε′ (figure 1) of the porous hydroxyapatite/rice starch composites increase from 0.06 to 0.28 at 13.8 GHz and from 0.14 to 0.29 at 15.2 GHz whereas the porous hydroxyapatite/potato starch composites increase from 0.20 to 1.02 at 13.8 GHz and from 0.05 to 0.61 at 15.2 GHz. This might due to the charge density between the welldispersed hydroxyapatite nanoparticles in the starch matrix and cause the formation of an abundant micro capacitors in the porous composites. The nanoparticle structures of the hydroxyapatite might also cause the formation of the polarization centers. Thus, the micro capacitors and polarization centers increase when the starch proportion increase (or the hydroxyapatite proportion decrease) to perform well-distribution of hydroxyapatite nanoparticles in order to enhance the ε′ [9]. The ε′ of 16.6 GHz is different from the variation trend of ε′ at 13. 8   Overall, the ε′′ (figure 2) of the both porous hydroxyapatite/starch composites increase at 13.8, 15.2 and 16.6 GHz when the proportion of the starch increase from 50 to 70 wt%. It shows that the higher proportion of the starch in the porous hydroxyapatite/starch composites enhance the ε′′. When the frequency increases, the ε′′ of the porous hydroxyapatite/rice starch composites increase from -1.39 to -0.90 for HAp-Rs50, -1.34 to -0.84 for HAp-Rs60 and -1.04 to 1.97 for HAp-Rs70. The ε′′ values of the porous hydroxyapatite/potato starch composites increase from -1.50 to -0.33 for HAp-Ps50 and -1.38 to -0.37 for HAp-Ps60 when frequency increases. However, the ε′′ of the HAp-Ps70 decrease from 0.90 to 0.51 as the frequency increase from 13. 8-16.6 GHz. This might due to the highest average porosity of the HAp-Ps70 (67.74%) that leads to the highly interconnected porous structure which possesses various complicated interfaces, resulting in poor electromagnetic radiation dissipation efficiency in the porous composites to the applied field when frequency increases [9,10].

Conclusions
The porous hydroxyapatite/starch composite with higher average porosity was fabricated through incorporated the hydroxyapatite nanoparticles and starch matrix by varying the starch proportion. The average porosity of the porous hydroxyapatite/starch composite increases when the starch proportion increases. The investigation on the effect of the starch proportion and average porosity to the dielectric of the porous hydroxyapatite/starch composites was conducted. It can be concluded that the average porosity and starch proportion lead to the polarization and dielectric dissipation behaviors of the porous composites, and resulting the significant responses in the permittivity spectrum. The increments of the starch proportion in the porous hydroxyapatite/starch composites enhance the ε′ and ε′′. However, the increment of the average porosity induces the variation of the permittivity spectrum. The non-invasive microwave sample characterization technique has the potential in characterizing bone scaffold.