Materials Science and Engineering C 27 (2007) 546 – 550 www.elsevier.com/locate/msec

Preparation of porous hydroxyapatite scaffolds E. Saiz ⁎, L. Gremillard, G. Menendez, P. Miranda, K. Gryn, A.P. Tomsia Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley CA 94720, USA Available online 27 June 2006

Abstract In this work, the sintering and grain growth of hydroxyapatite green bodies are analyzed in order to identify the optimum heat treatments for the preparation of porous hydroxyapatite scaffolds. Sintering in air at temperatures ranging between 1100 and 1200 °C yields dense materials with narrow grain-size distributions. The scaffolds are formed by the infiltration of polymer foams with hydroxyapatite slurries or by robocasting, a novel rapid-prototyping technique. Examples of the microstructures achieved with each approach are presented. It is observed that both techniques can be used to fabricate scaffolds with adequate pore size to promote bone ingrowth. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydroxyapatite; Scaffolds; Sintering; Robocasting

1. Introduction The ideal bone substitute is a material that will form a secure bond with the tissues by allowing, and even encouraging, new cells to grow and penetrate. One way to achieve this is to use a material that is osteophilic and porous, so that new tissue, and ultimately new bone, can be induced to grow into the pores and help to prevent loosening and movement of the implant. As a consequence, a great deal of effort has been placed in the development of porous scaffolds for bone replacement and tissue engineering [1–3]. Synthetic hydroxyapatite (HA) has been one of the materials of choice for the fabrication of inorganic scaffolds due to its close relationship with the mineral component of the bone and its excellent osteophilic properties [4–8]. Klawitter and Hulbert [9] established a minimum pore size of ∼100 μm for bone growth into porous ceramic structures and a similar conclusion was reached by Simske et al. [10]. More recently, Itala claimed that bone ingrowth occurred in pores as small as 50 μm [11]. The porosity must be interconnected to allow the ingrowth of cells, vascularization, and diffusion of nutrients. Tamai determined that pore interconnections < 10 μm will not allow cell migration from pore to pore [12]. It now seems accepted that a minimum interconnect size of ∼100 μm is needed for mineralized tissue ingrowth. These results underline ⁎ Corresponding author. MS 62R0203 Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. E-mail address: [email protected] (E. Saiz). 0928-4931/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2006.05.038

the need for developing new technologies for the fabrication of strong HA scaffolds with controlled porosity. This work focuses on the sintering of porous hydroxyapatite scaffolds fabricated using two different technologies based on the manipulation of HA slurries: infiltration of polymer foams and robocasting. The first method involves the infiltration of a polymer sponge with ceramic slurry until the inner polymer walls are completely coated by the ceramic powders. Subsequently, the sample is fired to remove the polymer and form a ceramic skeleton that is strengthened by sintering at high temperature. Recently, the use of computer-driven rapidprototyping techniques to produce porous ceramic with anisotropic microstructures has been investigated by several groups [13]. Robocasting is a simple technique to produce porous ceramic parts with complex shapes [7,14]. In robocasting, a ceramic ink is extruded through a thin nozzle to build a part layer-by-layer following a computer design. The ink must exhibit a controlled viscoelastic response. It should flow through the nozzle and subsequently settle very fast, bonding to the previous layer so that the part maintains its shape while printing. 2. Experimental 2.1. Characterization of the starting HA powders The grain size and crystalline structure of the commercial hydroxyapatite powders (Alfa Aesar, USA) used in this work were analyzed using scanning electron microscopy (SEM), X-ray