Thin Film Phosphate Based Bioactive Substrates
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ABSTRACT Calcium phosphate based thin films prepared by colloidal sol-gel slow draw dipping of quartz substrates are bioactive to both osteoclast resorption and osteoblast deposition. The bioactivity is related to the presence of mixed crystalline phases in the films generated under different sintering conditions. As the sintering temperature was increased from 800'C to 1000'C, a transition in film composition from calcium hydroxylapatite to alpha-tricalcium phosphate is achieved. INTRODUCTION Calcium hydroxylapatite (HA), CaI 0(PO 4)6(OH) 2, is the primary inorganic component of bone. It is commonly known that calcium phosphate based ceramics are biocompatible with osteogenic activity. Over the past ten years, metal implants used in joint replacements have been plasma sprayed with HA to increase the ingrowth of natural bone and to better cement the implant [1]. The goal of research has been to produce coatings of high quality, stoichiometric materials (such as HA). However these types of coatings have low resorption levels and if the plasma sprayed coating detaches, the resultant powder can cause inflammation in adjacent tissues. Calcium phosphate based thin films prepared by colloidal sol-gel methods on quartz substrates have been found to be bioactive to both resorption and deposition [2, 3]. Such substrates are now commercially available under the tradename OsteologicTM from Millenium Biologix. These films are formed on quartz plates by coating from a particulate sol gel suspension. The basic reaction is: 5 Ca(NO
2
+ 3 (NH4) 3PO4
+
NH40H
--
Ca5 (OH)(PO4 ) 3 +
10
NH4OH
(1)
Films prepared from an aqueous suspension have a thickness of the order of less than 0.5 jm and a grain size of approximately 0.5 pm. In order to determine the films' chemical and phase composition, a series of samples was generated and their composition determined using glancing angle x-ray diffraction (XRD). Results from this series indicated a phase change from HA to alpha-tricalcium phosphate (a-TCP) not observed in the bulk material. Assuming that the HA dehydrated to form a-TCP, samples were sintered in a humid environment in an attempt to suppress the reaction. Scanning electron microscopy (SEM) in concert with cross-sectional transmission electron microscopy (TEM) was undertaken to determine the interaction with the quartz substrate. This interaction was further investigated with electron diffraction and energy dispersive x-ray spectroscopy (EDX).
87 Mat. Res. Soc. Symp. Proc. Vol. 414 01996 Materials Research Society
EXPERIMENTAL Air dried films were provided by Millenium Biologix Inc. Films were prepared by dipping and slow withdrawal of 12.7 mm quartz disks in and from a colloidal sol-gel suspension (described by equation 1 above). The dip coated quartz discs were then air dried and inserted into a furnace at 800'C. The temperature was increased to the sintering temperature at a rate of approximately 20°C/min. The disks remained at the sintering temperature for a given hold time (sixty, twenty and five minutes), cooled
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