Electrical Polarization of b-Tricalcium Phosphate Ceramics

Wei Wang,z Soichiro Itoh,y Naoki Yamamoto,z Atsushi Okawa,z Akiko Nagai,z and Kimihiro Yamashitaw,z

zDepartment of Inorganic Materials, Institute of Biomaterials and Bioengineering, Tokyo Medical andDental University, Tokyo 113-8519, Japan

yAffiliated Facility for Clinical and Fieldwork Practices, International University of Health andWelfare, Chiba 272-0827,Japan

zDepartment of Orthopaedic and Spinal Surgery, Tokyo Medical and Dental University, Tokyo 113-8519, Japan

The b-tricalcium phosphate (b-TCP)-sintered pellets and pow-ders were prepared and the ability of b-TCP to store an elec-trical charge by electrical polarization was studied. Theprepared b-TCP was characterized by X-ray diffractometryand Fourier transform infrared spectroscopy. Alternating-current impedance conductivity and the energy of activation re-quired for the ionic migration of b-TCP were calculated. Afterelectrical polarization of synthesized b-TCP, the thermally stim-ulated depolarization current was measured, and the total storedcharges in the samples were calculated. The energy of activationfor the relaxation of electrical polarization in the synthesizedb-TCP pellets and powders approximated the energy of activa-tion required for ionic migration in the b-TCP pellets. It wasconfirmed that b-TCP has a suitable composition and structurefor ion conduction and a large electrical charge is stored in boththe b-TCP pellets and powders. Instead of proton transport po-larization in hydroxyapatite, other charge carrier, such as Ca

21

or O2�, is assumed in b-TCP, in which proton is not contained.

I. Introduction

THE calcium phosphate ceramics, especially hydroxyapatite(HA), Ca10(PO4)6(OH)2, and b-tricalcium phosphate

(b-TCP), Ca3(PO4)2 have been generally used in the skeletalsurgery. Their superior osteoconductivity and bioactivity, sup-porting the formation of mechanically and chemically bondedbone directly on their surface, are clinically approved.1,2 Wehave recently reported that bone bonding on the negativelycharged surface (N-surface) of electrically polarized HA ceram-ics was enhanced when compared with that on the positivelycharged surface (P-surface) and the surface of nonpolarized HAceramics (0-surface) after transplantation into canine femora.3 Itis supported that surface bioreactivity and polarization-inducedsurface charge of HA ceramics synergistically promote osteo-conduction.4 Polarization technique is thus under further devel-opment for apatite-based ceramics and the other biomaterialssuch as bioactive glasses.5

In the development of advanced bioresorbable bioceramics, itwas observed that the enhanced activities of both osteoblastsand osteoclasts that progressed predominantly on the negativelycharged surface of the polarized HA/b-TCP blocks comparedwith the nonpolarized composites.6 These results were inter-preted that synergic effects of superior osteoconductivity of HA/

b-TCP composites, adsorption of charge-compensatory Ca21

ions on the N-surface,7,8 and activated osteogenic cell activitiesinduced by large surface charges enhance new bone growth. Thefact has strongly indicated the possibility of electrical polariza-tion of b-TCP. In this study, the ability of b-TCP to store anelectrical charge by electrical polarization was studied by meansof the thermally stimulated depolarization current (TSDC) andelectrical conduction measurements.

II. Materials and Methods

(1) Preparation of b-TCPb-TCP was manufactured from a commercially available(NH4)2HPO4 and CaCO3 powder (Wako Pure Chemical Indus-tries Ltd., Osaka, Japan).9 The initial weight ratio of(NH4)2HPO4:CaCO3 was 1.0:1.5. After mixing these powderswith 70% (v/v) ethanol at room temperature for 48 h, ethanolwas removed completely by a rotary evaporator. The mixedpowder was sintered at 11001C for 24 h in the air to obtain ab-TCP powder, which was mixed into a slurry with 3.0% (v/v)polyvinyl alcohol (Wako Pure Chemical Industries Ltd.), driedat 601C for 24 h, and grounded under 100 mesh. The powderwas uniaxially pressed at 140 MPa and sintered in the air at11001C for 3 h to form a pellet.

(2) Characterization of b-TCPX-ray diffractometry (XRD) was performed on b-TCP pellets aswell as powders using a diffractometer control (PW-1710; Phil-lips, Holland, the Netherlands) fitted using a 4 kW X-ray gen-erator, copper target, and a graphite monochromator (eachN5 3). Infrared absorption spectrum was measured on b-TCPpellets using the KBr pellet method using a Fourier transforminfrared spectrophotometer (FTIR-500; JASCO, Tokyo, Japan)in the range from 4000 to 400 cm�1 (N5 3).

Alternating-current (AC) impedance conductivity of b-TCPpellets was calculated from the complex impedance measured ina frequency range of 100 Hz–10 MHz using an impedance/gain-phase analyzer (SI 1260; Solartron, Analytical, Hampshire,U.K.) (N5 3).10,11 Samples of b-TCP pellet were sputtered byplatinum (Pt) on bilateral surfaces, clamped between a pair of Ptelectrodes, and tested from room temperature to 8501C at 501Cintervals using a voltage of 0.1 Vrms (Cycle 1). A heating rate of51C/min was used with a dwell of 10 min at each measurementtemperature. AC impedance conductivity of b-TCP pellets wascalculated from the following formula:

s ¼ L

RS(1)

where s is the impedance conductivity (S/cm), R is the electricresistivity (O), L is the thickness of the pellet (cm), and S is thesurface area of the pellet (cm2).

T. B. Troczynski—contributing editor

No benefit of any kind will be received either directly or indirectly by the authors.wAuthor to whom correspondence should be addressed. e-mail: yama-k.bcr@tmd.ac.jp

Manuscript No. 26976. Received October 23, 2009; approved February 2, 2010.

Journal

J. Am. Ceram. Soc., 93 [8] 2175–2177 (2010)

DOI: 10.1111/j.1551-2916.2010.03710.x

r 2010 The American Ceramic Society

2175

The Arrhenius equation indicating the relationship betweenthe energy of activation and impedance conductivity in theb-TCP samples is shown in the following formula:

s ¼ A� exp�Ea

RT

� �(2)

where A is the frequency factor (S/cm), Ea is the energy ofactivation (kJ/mol), R is the gas constant (J � (K �mol)�1), and Tis the absolute temperature (K).

The following equation is derived from this formula:

lns ¼ lnA� Ea

RT(3)

Arrhenius equation was plotted from the graph with log10s(S/cm) on the y-axis and T (K�1) on the x-axis, and the gradientwas calculated by the method of least squares.

(3) Electrical Polarization of b-TCPElectrical polarization was carried out with the system setup.4

The b-TCP pellets were clamped between a pair of Pt electrodes,and electrically polarized in a direct-current field of 4 kV/cm at4001C for 1 h in the air. The samples were cooled to room tem-perature under the electric field polarization. In the case ofb-TCP powders, they were packed in alumina rings (innerdiameter of 2.0 cm and thickness of 0.5 cm) and electricallypolarized under the same condition as the pellets.

The TSDC measurement of the electrically polarized b-TCPpellets as well as powders was performed (each N5 5).7,8 Theb-TCP samples were clamped between a pair of platinum elec-trodes and heated to approximately 7001C at a ramp rate of51C/min in the air and measured with a galvanometer (4140B;Hewlett Packard, Tokyo, Japan). The total stored charges in theb-TCP samples were calculated from the following formulae:

Q ¼ 1

b

ZJ tð Þdt (4)

whereQ is the total stored charge (nC/cm2), b is the ramp rate oftemperature (K/s2), and J(t) is the electric current density (nA/cm2).

The Arrhenius equation is applicable between the orientationtime and temperature in the relaxation process of electrical po-larization, as follows:

t Tð Þ ¼ t0 expEa

kT

� �(5)

where t is the orientation time (s), t0 is the frequency factor (s),Ea is the energy of activation for the relaxation of electrical po-larization (S/cm), and k is the Boltzmann constant.

The following equation is derived from this formula

ln t ¼ Ea

kTþ ln t0 (6)

Arrhenius equation was plotted from the graph with lnt(T)on the y-axis and T (K�1) on the x-axis, and the gradient wascalculated by the method of least squares giving the energy ofactivation for relaxation of electrical polarization.

III. Result

(1) Characterization of b-TCPThe XRD patterns of synthesized b-TCP pellets as well as pow-ders had representative peaks of b-TCP. Their lattice parametersmatched well with those of b-TCP. The FTIR patterns of thesesynthesized ceramics had also representative peaks of b-TCP.

The AC impedance of synthesized b-TCP pellets plotted asemicircular Nyquist locus. Representative Nyquist locus of b-TCP drew a real part of a perfect circle (Fig. 1), having a goodselectivity of impedance response. This result indicates that theb-TCP samples consist of a single semicircle corresponding tothe total resistivity of grains and grain boundaries; the trans-granular resistivity or grain boundary resistivity. The ionic con-ductivity of b-TCP pellets exhibited the Arrhenius pattern, andtheir values of resistance decreased as their temperature in-creased (Fig. 2), showing their ionic conductivities increased.The average energy of activation required for ionic migration inthe b-TCP pellet was 1.2570.08 eV.

(2) Electrical Polarization of b-TCPThe TSDC curves in b-TCP pellets and powders plotted a sim-ilar pattern, and a representative TSDC spectrum of polarizedb-TCP pellets is shown in Fig. 3. The average stored charge inthe b-TCP pellets and powders were 72.579.6 and 17.674.8mC/cm2, respectively. A representative relationship of logt and1000/T obtained by the TSDC measurement of these b-TCPsamples is shown in the Fig. 4. The average energy of activationfor the relaxation of electrical polarization in these synthesizedb-TCP pellets and powders was 1.2370.12 and 1.6370.08 eV,respectively.

Fig. 1. The Nyquist locus and Arrhenius plot of synthesized b-TCPpellet. The Nyquist locus of b-TCP pellet draws a real part of a perfectcircle, having a good selectivity of impedance response.

Fig. 2. The ionic conductivity of b-TCP pellet. The ionic conductivityof b-TCP pellet exhibits the Arrhenius pattern, and the value of resis-tance decreases as the temperature increases, indicating that the ionicconductivity increases. The energy of activation (Ea) required for ionicmigration in this b-TCP pellet is 1.3 eV.

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IV. Discussion

Acceleration of the growth of the bone-like apatite layer onthe surface of the electrically polarized HA in simulated bodyfluid was reported.12 To recognize this phenomenon, it washypothesized that the poling power could stem the orientationof OH� ions in HA structure. Yamashita et al.13 indicatedthat dehydroxylation during measurement of electrical conduc-tivity in the air led to an increase in conductivity due to anincrease in the initial concentration of free protons. Protontransport polarization, however, cannot affect the b-TCPbecause the proton is not contained in it. For this reason, othercharge carrier, such as Ca21 or O2�, must be assumed in b-TCP.The oxide ion conduction has already been proved with anoxygen concentration cell in ceramic oxyhydroxyapatite[Ca10�xYx(PO4)6(OH)2�xOx], where Ca21 is partially substi-tuted with Y31, giving vacancies at OH� lattice sites for chargecompensation and carbonate apatite.14 As the lattice structureof b-TCP contains some amount of Ca21 vacancies, the con-duction of Ca21 ions is also possible; however, a direct proof ofthe conduction is difficult to date because of no availableCa21—conductive electrode.

The stored charge in b-TCP pellets was larger than that inb-TCP powders. The result presumably arose from the coarsesize of the powder; the powders of b-TCP used were homoge-neously ball milled to 70 mm in diameter and then packed inalumina ring loosely. This procedure may bring the differenceof the surface roughness of samples between the pellets andpowders. Because the measured surface charges are actually theelectrons evoked on the directly contacting metal electrodes withthe counter ions displaced during the electrical polarizationtreatment, the measured surface charges were largely differentbetween the pellets and powders. In fact, the stored charges maybe larger in b-TCP powders than in b-TCP pellets because thesurface of powders is larger than that of pellets. It was confirmedthat b-TCP has a suitable composition and structure for ion

conduction and a large electrical charge is stored in both theb-TCP pellets and powders.

References

1R.W. Bucholz, A. Carlton, and R. E. Holmes, ‘‘Hydroxyapatite and TricalciumPhosphate Bone Graft Substitute,’’ Orthop. Clin. North. Am., 18 [2] 323–34 (1987).

2S. R. Radin and P. Ducheyne, ‘‘Effect of Bioactive Ceramic Composition andStructure on In Vitro Behavior, III: Porous Versus Dense Ceramics,’’ J. Biomed.Mater. Res., 28 [11] 1303–9 (1994).

3T. Kobayashi, S. Nakamura, and K. Yamashita, ‘‘Enhanced Osteobonding byNegative Surface Charges of Electrically Polarized Hydroxyapatite,’’ J. Biomed.Mater. Res., 57 [4] 477–84 (2001).

4S. Nakamura, T. Kobayashi, and K. Yamashita, ‘‘Extended Bioactivity in theProximity of Hydroxyapatite Ceramic Surfaces Induced by Polarization Charges,’’J. Biomed. Mater. Res., 61 [4] 593–9 (2002).

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7S. Nakamura, H. Takeda, andK. Yamashita, ‘‘Proton Transport Polarization andDepolarization of Hydroxyapatite Ceramics,’’ J. Appl. Phys., 89 [10] 5386–92 (2001).

8M. Ueshima, S. Nakamura, and K. Yamashita, ‘‘Huge Millicoulomb ChargeStorage in Ceramic Hydroxyapatite by Bimodal Electric Polarization,’’ Adv.Mater., 14 [8] 591–5 (2002).

9M. Vallet-Regı, J. Pena, and I. Izquierdo-Barba, ‘‘Synthesis of b-TricalciumPhosphate in Layered or Powdered Forms for Biomedical Application,’’ SolidState Ionics, 172 [1–4] 445–9 (2004).

10K. Yamashita, H. Owada, H. Nakagawa, T. Umegaki, and T. Kanazawa,‘‘Trivalent-Cation-Substituted Calcium Oxyhydroxyapatite,’’ J. Am. Ceram. Soc.,69 [8] 590–4 (1986).

11K. Yamashita, H. Owada, T. Umegaki, T. Kanazawa, and T. Futagami, ‘‘IonicConduction in Apatite Solid Solutions,’’ Solid State Ionics, 28 [1] 660–3 (1988).

12K. Yamashita, N. Oikawa, and T. Umegaki, ‘‘Acceleration and Decelerationof Bone-Like Crystal Growth on Ceramic Hydroxyapatite by Electric Poling,’’Chem. Mater., 8 [12] 2697–700 (1996).

13K. Yamashita, K. Kitagaki, and T. Umegaki, ‘‘Thermal Instability and Pro-ton Conductivity of Ceramic Hydroxyapatite at High Temperature,’’ J. Am.Ceram. Soc., 78 [4] 1191–7 (1995).

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Fig. 3. Thermally stimulated depolarization current (TSDC) spectrumof polarized b-TCP pellet. The TSDC curve increased at 3301C in b-TCPpellet and then reached its maximum at 5901C. The stored charge in thisb-TCP pellet is 73.6 mC/cm2.

Fig. 4. The relationship of logt and 1000/T obtained by the thermallystimulated depolarization current measurement of b-TCP pellet. Theenergy of activation for the relaxation of electrical polarization in thisb-TCP pellets is 1.5 eV.

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