ORIGINAL PAPER

Bioactivity Characterization of Amorphous Silica CeramicsDerived from Rice Husk Ash

J. P. Nayak & J. Bera

Received: 27 July 2010 /Accepted: 5 October 2010 /Published online: 19 October 2010# Springer Science+Business Media B.V. 2010

more bioactive and degradable than calcium phosphatebiomaterial [10, 11].

Bioactive silica ceramics are generally prepared usingalkoxysilane precursors [12], which are expensive. Rice huskash (RHA) may be a low cost precursor for the synthesis ofamorphous silica bioceramics because the ash is the cheapestsource for amorphous silica of bio-origin. RHA is generallyprepared by burning husk in the temperature range 500 to600 °C in order to get amorphous silica in the ash [13]. Thisamorphous silica of the ash is very easy to dissolve inalkaline solution [14] as well as in aqueous alcohol withmethanolic choline hydroxide solution [15]. Crystalline silicacan also be prepared from husk by burning at temperatures atand above 700 °C [13]. Also crystalline nano particles ofsilica can be prepared at room temperature by the fungus-mediated biotransformation of amorphous silica present inrice husk [16]. Rice husk also has been used to prepare Si-based ceramic materials [17, 18]

To the best of our knowledge, there is no report onbioactivity of amorphous silica that is derived from RHA.In this study, amorphous silica has been synthesized fromRHA. The in vitro bioactivity and degradability of thematerial have been evaluated and both the properties weregood for its bio-application.

Three types of amorphous silica namely brown ash(BA), white ash (WA) and silica gel (SG) were preparedfrom rice husk. The preparation procedure and character-istics of BA, WA and SG have been reported earlier by theauthors [14]. Briefly, BA was prepared by burning husk at700 °C and it contains about 96% silica. WA contains99.78% silica and was prepared by burning acid washedhusk. The SG was prepared from BA through an alkalineextraction of silica and an acid neutralization process.

The three silica materials were ground into fine powders.The powders were made into pellets. The pellets were

J. P. Nayak : J. Bera (*)Department of Ceramic Engineering,National Institute of Technology,Rourkela, Odisha 769008, Indiae-mail: jbera@rediffmail.com

Silicon (2012) 4:57–60DOI 10.1007/s12633-010-9058-3

Abstract Rice husk ash has been used to prepare amorphoussilica bioactive ceramics. Three varieties of silica powders,namely brown ash, white ash and silica gel containing 96.0,99.8 and 99.9% silica respectively, were used to prepare silicaceramics. The bioactivity and biodegradability properties ofthese ceramics were evaluated. The formation of crystallineapatite was observed on all the specimens in simulated bodyfluid. The phase composition, morphology and calcium/phosphorous ratio of the apatite layer formed were evaluatedby X-ray Diffraction, Scanning Electron Microscopy andEnergy Dispersive Spectroscopy. Controlled biodegradabilityof amorphous silica in Tris buffer solution was found. Theseresults suggest that the amorphous silica derived from ricehusk ash is a promising and cost effective biomaterial.

Keywords Rice husk ash . Amorphous silica . Bioactivity .

Apatite . Biodegradability

Bioactive materials such as glass [1], glass-ceramics [2],hydroxyapatite [3], calcium phosphate [4], calcium silicate[5] and composite materials [6], have been widely studiedfor clinical applications. One significant characteristic of abioactive material is that a bone-like carbonated hydroxyl-apatite (HAp) layer is formed on their surface whenimplanted in the body and it bonds to living bone throughthat apatite layer [7]. In addition to the above biomaterials,amorphous and nano silica have also been proven to bebiocompatible and biodegradable in living tissue [8, 9]. Ithas been proposed that some silica containing ceramics are

sintered at 900 for BA, 1100 for SG and 1200 °C for WArespectively. The specific sintering temperature for eachwas selected on the basis that the silica was amorphous upto that temperature [14].

Bioactivity was evaluated by immersing the pellet insimulated body fluid (SBF). The SBF was prepared as perthe method mentioned by Cuneyt Tas [19]. Pellets weresoaked for 7, 14 and 21 days at 37 °C. The pellet surfacewas then investigated by X-ray Diffraction (XRD, PhilipsPW 1830, Holland) for phase analysis, Scanning ElectronMicroscopy (SEM, Jeol JSM-6480LV) for morphology andEnergy Dispersive Spectroscopy (EDS, Oxford Instrument,INCA) for elemental analysis.

The dissolution feature of the ceramics was evaluated inTris buffer solution (pH 7.4). 300 mg powder was stirred(60 rpm) in 200 ml Tris solution of 6.1 g/l strength at 37 °Cfor different time periods [20]. The pH of the Tris solutionwas measured after each soaking interval. The dissolved Siconcentration was estimated through molybdenum bluecomplex absorbance at 820 nm [21] using a Perkin ElmerLambda 35 visible spectrophotometer.

Figure 1 shows the XRD patterns of the pellet surfacesafter SBF immersion for 7, 14 and 21 days. 7 days BAspecimen shows calcium hydrogen phosphate hydrate;Ca8(HPO4)2(PO4)4 (H2O)5 phase (Fig. 1a). Whereas, 14and 21 days BA specimen show HAp; Ca10(PO4)6(OH)2phase on their surfaces. SG specimen shows similarbehavior of hydrated calcium phosphate precipitation at7 days and conversion of phosphate phase into HAp at14 days and above (Fig. 1b). In the case of WA (Fig. 1c),hydrated calcium phosphate phase appears only at 14 daysand HAp appeared at 21 days.

The results showed that BA and SG ceramics are morebioactive than WA ceramics. The mechanism of apatiteformation in amorphous silica body is believed to bethrough the formation of Si–OH functional groups on thesurface due to the hydration and dissolution of the silicanetwork [22]. The hydrolysis of the silica network leads tothe release of soluble Si(OH)4 into the body fluid.Simultaneously, Si–OH groups are formed at the solid–fluid interface as follows:

�Si�O�Si� þ H2O ! �Si�OHþ OH�Si� ð1Þ

These Si–OH silanol groups undergo acid/base reactionsdepending on the pH of the medium. Silica is an acidic oxide.It undergoes base (OH−) reaction in SBF solution (pH 7.4)forming a negatively charged surface as follows [23]:

�Si�OHþ OH� ! �Si�O�þ H2O ð2ÞThen the Ca+2 and the PO4

3− group migrates to thesurface, forming Ca–PO4

3− clusters on the surface, followedby growth of amorphous calcium phosphate (CaP). Next,

Fig. 1 XRD patterns of the silica ceramics (a) BA, (b) SG and (c)WA after SBF incubation

58 Silicon (2012) 4:57–60

amorphous CaP crystallizes to form an apatite layer byincorporation of OH− and CO3

2− anions from solution [22].A. V. Lluch et al. [24] also mentioned that the silanol groupsprovide favorable sites for apatite nucleation.

The greater bioactivity of BA and SG is due to easyhydrolysis and the formation of surface Si–OH groups inthem. The hydrolysis of the silica network in BA might beeasy due to the dissolution of impurity cations as it has acontent of about 4 wt.% impurities like CaO, K2O and F2O3

etc [14]. In case of SG, the hydrolysis might be easier dueto its inherent gel history of the material. The slow responseof WA might be due to its very low impurity content andhigh temperature heat treatment (1200 °C) of the ceramics.

Figure 2 shows the microstructure and EDS spectra ofBA, SG and WA ceramic surfaces before and afterimmersion in SBF. It shows a cauliflower like microstructureof deposited HAp on the specimens. Table 1 shows theelemental analysis of EDS spectra for different specimen.14 days specimens shows a relatively higher percentage of Sicompared to 21 days specimens. Similarly, 21 days specimen

shows relatively higher percentage of Ca and P. This is dueto the greater amount of HAp formation in the 21 daysspecimen. The Ca/P ratio of HAp in each of the ceramicswas found to increase with incubation time.

Degradation of bioceramics in body fluid is anotherimportant characteristic for practical scaffold applications.Figure 3 shows the change in Si concentration and pH in Trisbuffer solution with incubation time during dissolution. TheSi concentration increases as well as pH decreases withincreasing time. It is known that silica dissolves into bodyfluids as silicic acid; hence, pH decreases. The silica networkdissolution was rapid up to about 6 h after which the ratedecreased due to the saturation of medium with respect tosilicic acid. The Si concentration became almost saturated at3-days (72 h). For this reason, the Si concentrations were notmeasured after 72 h. BA ceramics shows a faster dissolutionthan the other two ceramics due to its higher impuritycontent as stated earlier. The presence of other cations insidethe silica network facilitates the hydrolysis of silica. This isbecause network modifier cations like Ca+2 and K+ dissolve

Fig. 2 SEM image and EDS spectra of the BA, SG and WA ceramics without incubation and after incubation in SBF for 14 and 21 days

Specimens Without incubation 14 days 21 days

Si Si Ca P Si Ca P

BA 45.4 45.1 2.9 1.8 13.3 21.5 12.8

SG 43.5 11.1 25.3 14.2 6.9 29.1 14.9

WA 43.4 16.8 20.5 11.8 9.8 25.6 12.5

Table 1 Si, Ca, and P atomweight% analysis of EDS spec-tra of without and with SBFincubated specimen. Otheratomic weight percentages arenot shown for simplicity

Silicon (2012) 4:57–60 59

quickly and thus open up the silica network for itsdissolution. Silica dissolution of WA was lowest due to itslower impurity content.

In summary, amorphous silica based bioceramics wereprepared using rice husk ash as a cost effective raw material.An in vitro bioactivity test showed the formation of an apatitelayer on the silica specimen surface. A biodegradability testshowed a substantial dissolution of the silica network within6 h. The rate of silica network dissolution and apatite phaseformation was dependent on the impurity cation content inamorphous silica which was inherited from the husk compo-sition. The properties, in vitro bioactivity and degradation,suggest that rice husk ash is a promising low cost raw materialfor the preparation of bioactive amorphous silica ceramics.

Acknowledgement The authors are thankful to the Ministry ofEnvironment and Forests, Government of India, New Delhi forproviding the research grant vide sanction no. 19/50/2004 RE.

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Fig. 3 Change of silica concentration and pH (inset) with incubationtime in Tris medium

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