Processing of bioceramics
Technical ceramics are composed of raw materials generally as powder and of
natural or synthetic chemical additives, favoring either compaction (hot, cold or
isostatic), or setting (hydraulic or chemical) or accelerating sintering processes.
According to the formulation of the bioceramic and the shaping process used, we
can obtain ceramics, dense or with variable porosity, cements, ceramic depositions
or ceramic composites.
12.4.1. Massive ceramics
Molding, traditional or under pressure, and pressing, unidirectional or isostatic,
is part of the ceramist’s range of tools for obtaining the final form of the piece to be
made.
The heads of hip prothesis in zirconia or alumina are formed by isostatic pressing
(approximately 1,200 bars) from micronized powders. The sintering operation which
follows is carefully defined by heating and cooling parameters (temperatures and
rates), so as to obtain a finished product with a nil total porosity, especially in the
case of prostheses heads [MOC 93]. The choice of a micronized and reactive powder
514 Ceramic Materials
combined with isostatic pressing leads to this result [SED 93]. More recent
manufacturing methods borrowed from the electronic industry like strip molding can
also be used for making bone substitutes [RIC 98]. Pieces obtained are of small
thicknesses (from 100 µm to 2 mm) and of large surface (many tens of cm2).
Porosity is an important parameter for cell rehabilitation. It can be obtained by
different methods. The utilization of a pore forming agent is the method most widely
used. The pores generated by the release of a gaseous compound (H2O2, CO2),
combined either with slurry or a powder, are generally irregular and we prefer
calibrated organic substances (naphthalene, polystyrene, polyethylene), added to the
powder, which decompose slowly during heating. These techniques can be
employed for different types of ceramics. The use of calibrated polymer foams,
impregnated with slurry, has also been proposed and achieved successfully.
In parallel, more specific techniques have been sought for calcium phosphates
using smaller temperatures. Thus, porous ceramics in calcium phosphates
resembling coral skeletons can be obtained by the “replamineform” technique
(Interpore) in aqueous media [RIC 98]. Moreover, a hydrothermal process helping to
obtain apatitic ceramics at temperatures clearly lower than those used by ceramists
has been perfected and developed: this is the way of hydrothermal cements [DON
98]. Other methods using hydraulic cements have also been proposed [DRI 95].
These methods allow easy preparation, molding and machining of the sintered piece,
as well as a more flexible check of the composition and the additives. All these
methods lead, however, to porous compounds with weak mechanical performance.
Another way of material processing is developing; it is based on the biomimitic
processes aiming at imitating natural and biological processes and offers the
possibility of making these bioceramics at a temperature lower (ambient
temperature) than that of conventional or hydrothermal processes [GRO 96]. The
prospect of using these very low forming temperatures opens up possibilities for
mineral organic combinations with improved biological properties by the addition of
proteins and biologically active molecules (growth factors, antibiotics, anti-tumor
agents, etc.). However, these materials have poor mechanical properties which can
be improved, partially, by combinations with bonding proteins.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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