POLARIZED YTTRIA-STABILIZED ZIRCONIA IMPROVES DURABILITY FOR DEGRADATION AND APATITE FORMATION IN SIMULATED BODY FLUID

Abstract

Yttria-stabilized zirconia is currently used as an orthopedic and dental material, because of its excellent mechanical properties. In this study, we have improved the bioactivity of yttria-stabilized zirconia by a combination of electrical polarization and chemical treatment. The phase transformation from tetragonal to monoclinic ZrO2 after alkaline treatment was inhibited on positively charged yttria-stabilized zirconia surfaces compared with negatively charged and conventional surfaces. During polarization, some oxide ions move from the positively charged surface to the negatively charged surface, leading to an increase in oxygen vacancies on the positive surface and hence greater formation of Zr-OH when this surface was exposed to alkaline solution. The bioactivity was assessed by immersing the samples in simulated body fluid and evaluating the growth of apatite on the surfaces. The combination of polarization and alkaline treatment increased the bioactivity in vitro. (Received January 11, 2012; Accepted February 3, 2012) INTRODUCTION Since the late 1960s, yttria-stabilized zirconia (YSZ), especially 3 mol% Y2O3-doped zirconia, has been shown to be a biomaterial with excellent mechanical properties, and YSZ ceramics are currently used as ball heads in artificial hip joints and post-crowns in dental applications [1-2]. YSZ is however bioinert, meaning that no direct bonding of YSZ to natural bones occurs in vivo. In the present study, we have addressed this shortcoming by employing chemical treatments with the aim of increasing the bioactivity of YSZ. We have recently demonstrated that the ceramic hydroxyapatite (HA) [3] may be polarized via proton migration [4], and that the polarized surfaces of HA enhanced tissue regeneration of both hard [5] and soft [6] tissues in vivo. Additionally, the polarized HA also accelerated protein adsorption [7] and cell adhesion through improved wettability of water [8]. Thus electrical polarization can have excellent chemical, biological, and biomedical effects. Another serious problem with YSZ lies in its instability due to a tetragonal (t) to monoclinic (m) phase transformation at relatively low temperatures of (< 400C) in moist air or hot water [9]. These phenomena are usually termed low temperature degradation (LTD). Kobayashi et al. have reported that the annealing treatment of zirconia at 65-400C in water for a long time spontaneously gave rise to the transformation of tetragonal into monoclinic phase [10]. This t-m phase transformation in YSZ is reportedly accompanied by cracking on the surface due to the volume expansion of about 4-5% and degradation of mechanical strength, which can cause critical accidents in medical applications. It is crucial to inhibit the LTD of YSZ in medical and structural applications and also to understand the role of moisture in LTD. We observed that our polarization process successfully inhibited LTD in the chemical treatments of YSZ ceramics. This report details the bioactivation and LTD-inhibition of polarized YSZ ceramics and discusses the polarization mechanisms. MATERIALS AND METHODS Yttria-partially stabilized zirconia powder (TZ-3Y, Tosoh, Japan) was pressed into a mold at 120 MPa. The YSZ compacts were sintered in air at 1400 C for 1 h. The relative density values of the sintered YSZ compacts were ca. 99.2  0.1 %. The sintered YSZ samples were electrically polarized as previously described [4] with a pair of platinum electrodes at 200 C in a direct-current (d.c.) electric field of 20 Vcm -1 for 30 min in air. The electrically polarized YSZ samples were treated with alkaline (5 moldm -3 NaOH at 95 C for 24 h) solution in commercial glass tubes. After chemical treatment, the samples were washed with deionized water and dried at 60 C for 3 h. The unpolarized YSZ with and without alkaline treatment were designated as O and YSZ, as the controls, respectively. The negatively charged YSZ and positively charged YSZ surfaces after the alkaline treatment were designated as N and P,