ARTICLE Hydroxyapatite with Permanent Electrical Polarization: Preparation, Characterization and Response against Inorganic Adsorbates Manuel Rivas, [a] Luis J. del Valle, [a,b] Elaine Armelin, [a,b] Oscar Bertran, [c] Pau Turon,* [a,d] Jordi Puiggalí,* [a,b] and Carlos Alemán* [a,b] Dedication ((optional)) Abstract: Permanently polarized hydroxyapatite (HAp) particles have been prepared by applying a constant DC of 500 V at 1000 ºC for 1 h to the sintered mineral. This process causes important chemical changes, as the formation of OH – defects (vacancies), the disappearance of hydrogenophosphate ions at the mineral surface layer, and structural variations reflected by the increment of the crystallinity. As a consequence, the electrochemical properties and electrical conductivity of the polarized mineral increase noticeably compared with as prepared and sintered samples. Moreover, these increments remain practically unaltered after several months. In addition, permanent polarization favours significantly the ability of HAp to adsorb inorganic bioadsorbates in comparison with as prepared and sintered samples. The adsorbates cause a significant increment of the electrochemical stability and electrical conductivity with respect to bare polarized HAp, which may have many implications for biomedical applications of permanently polarized HAp. Introduction Hydroxyapatite (HAp), Ca10(PO4)6(OH)2, is the major inorganic component of biological hard tissues such as bone and tooth. [1] Synthetic HAp, which shows excellent ability to interact with living systems, has been investigated for biomedical applications, as for example drug and gene delivery, tissue engineering and bone repair. [2] An important difference between amorphous calcium phosphate (ACP) and crystalline synthetic HAp (cHAp) is the alignment of the OH – ions along the c-axis in the latter. The crystal structure of stoichiometric cHAp, which contains no OH – defects, is monoclinic at room temperature. [3] However, the monoclinic cHAp changes to hexagonal phase at about 210 ºC, which means a change from an ordered to a disordered distribution of OH – ions along the c-axis. The hexagonal phase becomes the most stable form of cHAp in the pH range of 4-12 because of the disorder caused by the presence of vacancies and presence of O2 – ions in the columns of OH – groups. [3] Although the properties of cHAp were altered by thermally-induced changes in the positions of OH – ions, [4] the observed effects were not stable at room temperature (i.e. the OH – reorientation has a short relaxation time). ACP and cHAp interact with different phosphates and a biophosphonate (BPs), [5] which is a very relevant topic in the field of biomaterials for biomedical applications. Thus, polyphosphate, which is an orthophosphate polymer found in mammalian organisms, [6] promotes mineralization and bone regeneration when adsorbed onto HAp by stabilizing basic cell growth and differentiation. [7] On the other hand, the oxygen atom that links the phosphate groups of pyrophosphates is replaced by a carbon atom in BPs, which results in the inhibition of both hydrolytic and enzymatic degradations. [8] The affinity of BPs towards HAp has been associated with the formation of strong interactions between the two species. [9] Indeed, BPs are primary agents in the current pharmacological arsenal against different bone diseases (e.g. osteoporosis, Paget disease of bone and metastatic bone malignancies). [10] In the last decades, Yamashita and co-workers [11] caused polarization effects in HAp samples by applying a constant DC electric field of 1.0-10.0 kV/cm at elevated temperature (300-850 ºC) to samples previously sintered at 1250 ºC for 2 h. Results indicated that the polarization effects were consequence of the electrical dipoles associated with the formation of defects inside crystal grains and of the space charge polarization originated in the grain boundaries. The slow relaxation of such dipoles suggested that polarization was only partially maintained ( semi- permanently), even though this effect was not quantified. In addition, the above mentioned thermally stimulated polarization process (TSP) was found to affect some surface properties as for example the wettability and adhesion of osteoblastic cells, which were higher onto semi-permanently polarized samples than onto non-polarized ones. [12] In a very recent work, we used an alternative TSP strategy to prepare an electrophotocatalyst based on permanently polarized HAp particles. [13] More specifically, the new catalyst [a] M. Rivas, Dr. L. J. del Valle, Dr. E. Armelin, Dr. P. Turon, Prof. Dr. J. Puiggalí, Prof. Dr. C. Alemán Department of Chemical Engineering Universitat Politecnica de Catalunya EEBE, C/ Eduard Maristany 10-14, Ed. I2, 08019, Barcelona, Spain E-mail: jordi.puiggali@upc.edu and carlos.aleman@upc.edu [b] M. Rivas, Dr. L. J. del Valle, Dr. E. Armelin, Dr. P. Turon, Prof. Dr. J. Puiggalí, Prof. Dr. C. Alemán Barcelona Research Center for Multiscale Science and Engineering, Universitat Politecnica de Catalunya Institution EEBE, C/ Eduard Maristany 10-14, Ed. I2, 08019, Barcelona, Spain [c] Dr. O. Bertran Department of Physics Universitat Politecnica de Catalunya EEI, Av. Pla de la Massa, 8, 08700 Igualada, Spain [d] Dr. P. Turon B. Braun Surgical, S.A Carretera de Terrassa 121, 08191 Rubí (Barcelona), Spain E-mail: pau.turon@bbraun.com Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))