* Corresponding author. Tel.: #34-91-394-1861; fax: #34-91-394- 1786. E-mail address: vallet@eucmax.sim.ucm.es(M. Vallet-Regm H ). Biomaterials 22 (2001) 1847}1852 Colloidal processing of hydroxyapatite L.M. Rodrm H guez-Lorenzo, M. Vallet-Regm H *, J.M.F. Ferreira Departamento de Qun & mica Inorga & nica y Bioinorga & nica, Facultad de Farmacia, Universidad Complutense de Madrid, 28040 Madrid, Spain Departamento de Engenharia Cera L mica e do Vidro, UIMC, Universidade de Aveiro, 3810-193 Aveiro, Portugal Received 5 April 2000; accepted 30 October 2000 Abstract Reliable bioceramics are needed to implement the high requirements that living tissues demand. This work focuses on the processingstepsnecessarytomanufactureadvancedceramicsthatcanbeusedasimplantdevices.Thein#uenceoftheheattreatment temperatureonthecharacteristicsofaprecipitatedhydroxyapatite(OHAp)powderwasevaluatedinordertoobtainanappropriate speci"csurfaceareaforcolloidalprocessing.Ballmillingofthecalcinedpowdersfor20hwasrequiredtoimprovetherheological propertiesofthesuspensionsandthepackingabilityduringconsolidation.Di!erentdispersingagentsweretestedandthe "rsttrial wasmadebasedontheire!ectonthezetapotential.Themostpromisingoneswerethenselectedandtheire$ciencywasevaluated from rheological measurements and slip-casting performance of suspensions prepared at di!erent solids loading. Targon 1128 was revealedtobethemoste$cientdispersant,enablingtoprepare #uidsuspensionscontaining50vol%solidsandthehighestgreenand sintered density values to be obtained.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Hydroxyapatite; Bioceramics; Colloidal processing; Slip casting 1. Introduction There is an escalating interest in calcium phosphates, particularlyapatites,whichseemstobedrivenmainlyby the requirements for the development, understanding andmanufactureofbiomaterials[1].Appropriatehard- tissue replacement implants should be bioactive, have modulusequaltothatofbone,andbeeventougherthan the bone. In the case of trauma, bone should fracture rather than the implant. The bone would heal naturally but an implant should be removed, which sometimes could be rather di$cult [2]. Hydroxyapatite (OHAp) seems to be the most appropriate ceramic material for hard-tissuereplacement.OHApceramicsdonotexhibit any cytotoxic e!ects, show excellent biocompatibility withhardtissuesandalsowithskinandmusclestissues [3,4]. Unfortunately, due to low mechanical reliability, especiallyinwetenvironments[5],OHApceramicscan- not be used for heavy load-bearing applications [6]. Thus, in recent years, a huge amount of literature has beenpublishedfocusingonnewroutesormodi"cations of old methods to produce more reliable calcium phos- phates ceramics. This literature has been conveniently reviewed by several authors [1,4,6}14]. Although large emphasis has been put on synthesising methods, the ultimate properties of any material depend also on the processing technique and e!orts to achieve more reliable bioceramic bodies should be made. Techniques based on colloidal approach could be promising for achieving this objective. Colloidal ap- proachhasthepotentialforeliminatingdetrimentalhet- erogeneities, common to all kinds of ceramic powders, and avoiding their reintroduction during the successive processing steps [15]. The "rst step of a colloidal method is to mix powder with water to prepare a #uid system. Good dispersion conditionsareusuallyachievedbyselectinganappropri- atedispersantandexertingsomemechanicalaction(stir- ring,milling),orultrasonicagitation,tobreakdownthe agglomerates[16].Particlesizeandparticlesizedistribu- tionarealsorelevantfactorsthatdeterminetherheologi- calpropertiesofthesuspensionsandthepackingability of the powders both in suspension and in the green bodiesobtainedbyslipcasting[17].Viscosityoftheslips shouldbelowenoughtoadequately "llthemoulds,while 0142-9612/01/$-see front matter  2001 Elsevier Science Ltd. All rights reserved. PII:S0142-9612(00)00366-5