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Published: June 1, 2010
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The Role of Ceramic and Glass Powder Processing in Medical Devices

Employing factorial experimental design modelling during product development can bring a number of benefits. These include consistency and improved quality of granulate coatings for metal implants as well as the ability to predict possible osteoblast–osteoclast behaviour following implantation.

By: P. Jackson, CERAM, Penkhull, Stoke-on-Trent, UK

Taking control
Ceramic, glass and glass-ceramic powders are employed in a variety of medical device applications. For example, they are used directly as bone granulate for coating metal implants, or as shaped bone replacements, bone cements and dental crowns. They also have a number of important peripheral roles such as a core or shell material in investment casting of alloy implants. In the area of sensing, ceramics have an important role to play; for example, the piezoelectric components essential in ultrasound scanning are ceramic.
 
Understanding and then controlling the variables at play during the production of primary ceramic or glass powders (and the subsequent use of these powders to shape or coat components) is critical to medical device manufacturers. Failure to do so will mean batch-to-batch variations in chemistry and granulate size/strength; this in turn will deliver variable bioactivity and component durability. This article explains how the utilisation of testing and modelling informs research and development (R&D) and enables product development.
 
The control of variables starts with the production of primary powders. These are typically produced in two ways.
  • Precipitation, which involves combining solutions (often at a controlled pH) to generate a solid powder product that is suspended in the remaining liquid phase. The solid product is then recovered by filtration or centrifuge. It is then washed, dried and perhaps further processed, for example, by sintering.
  • Calcination of two or more precursor powders.
With both methods of production, there are a large number of variables with potential to affect subsequent processing and the final product quality. For calcination, for example, variables include:
  • purity, crystallinity, particle size distribution of precursor powders
  • method of mixing (dry or as a wet suspension)
  • density of packing in the refractory box (sagger) containing the powders that are inside the kiln
  • the temperature-time profile experienced by the powder mix on heating
  • type or level of milling performed on end powder
  • storage conditions, such as humidity, which affect powder quality over time.
Figure 1: Visualisation of results from FED analysis.

Using trial and error to alter variables and thereby optimise product quality can be time consuming and expensive for medical device manufacturers. Factorial experimental design (FED) modelling studies can help to eliminate this trial and error. By listing variables and defining the end properties that are desired, modelling software will suggest a reduced number of experiments. Those experiments will feature different combinations of variables, which are assigned a high, low and perhaps intermediate value. Once the experiments have been performed, the results can be depicted in a suite of three-dimensional (3D) graphs that illustrate the combined impact of any two variables. The graph in Figure 1 shows the effect of heating time and the maximum temperature experienced. Particularly useful is the ability to have a single z-axis component in terms of “desirability.” This single entity can be calculated from any number of target end properties, which are assigned a particular weighting according to their importance.

 
FED can also be useful for precipitation processes. Knowledge of the surface charge (zeta potential) versus pH of the phase being precipitated, and the pH at which precipitation takes place, can affect the particle shape and particle size of the powder that is created after drying/heating.
 
Suspension rheology
In terms of using primary ceramic or glass powders to create a secondary powder or shaped product, processing usually follows one of three generic routes (Figure 2). It will be appreciated from Figure 2 that, although components are ultimately pressed from granulate powder or extruded from a high solids paste, processing almost always initially proceeds via a fluid suspension. Controlling the rheology of a given suspension is, therefore, important if yields associated with the next process step are to be high. In addition, there are implications in terms of the final microstructure and, therefore, in-service properties. For example, retaining a good fluid rheology prior to spray drying whilst maximising the solids content in the fluid suspension will ensure granulates are more homogeneous. This in turn leads, for example, to more consistent coatings on metal implants or pressed components that are free from voids that affect strength or electrical properties.
 
Figure 2: Processing of primary ceramic powders.

The rheological behaviour of aqueous suspensions is governed by the extent to which the suspended particles want to come together (agglomerate) or stay apart. For aqueous suspensions, this is dictated by the surface charge, that is, the zeta potential of powder particles. A low charge encourages agglomeration and viscous suspensions. A high charge (positive or negative) ensures de-agglomeration and low viscosity. Figure 3 indicates that different ceramic powders have different zeta potential versus pH plots. Where different powders are to be mixed together, this information is invaluable for ensuring that the correct conditions exist so that the powders retain homogeneity with each other. Where unacceptable zeta potential conditions exist, altering pH and conductivity, employing surfactants to change the surface charge, and paying attention to the order of powder/additive that is introduced to suspensions will optimise suspension behaviour.

 
Application examples
Zeta potential has been applied successfully in a number of medical devices. For example, rheological quality control based on initial zeta potential versus pH measurements has ensured that spray drying leads to a consistent ceramic granulate for coating metal implants.
 
In the field of investment casting of implants, faults associated with poor control of ceramic slurries that are used to fabricate cores and shells have been overcome. An example of this is the avoidance of pimpling in cast alloys arising from unwanted air bubbles present in the ceramic shell.
 
Figure 3: Representative zeta potential behaviour recorded for a three ceramic powder employed in medical applications (lines represent polynomial best fit curves). 

Finally, leaching of inorganic ions from suspended bioactive glass powders can alter rheological behaviour versus time prior to processing. This can be important in the fabrication of bone replacement components via powder suspensions; any glass powder has the ability to lose ions to the water it is suspended in. This can result in changes in pH, conductivity and so forth that alter the suspension rheology as a function of time. Zeta potential measurements made as a function of time can provide an insight into this unwanted behaviour prior to processing. Conversely, ion leaching can be beneficial once implants are placed in the body. In this instance, zeta potential measurements made on calcined powders in simulated body fluid can be used to predict possible osteoblast–osteoclast behaviour following implantation.

 
A sound knowledge of powder suspension behaviour is also a valuable tool in R&D. For example, nano-yttria stabilised zirconias have the potential to deliver greater longevity to zirconia hip replacements through enhanced resistance to hydrothermal ageing. Novel sintering to retain the nano-structure in the final component is important, yet so is the ability to maximise the solids content in suspensions prior to granulation. Research is currently being conducted to demonstrate the importance of re-optimising granulates containing nano-zirconia against production demands associated with ease of die filling, ease of pressing and avoiding die sticking. Suspension control is crucial to this process. Other areas of ceramic research involving the optimisation of powder suspensions include the following:
 
  • Granulate development for additive layer manufacture where, for example, ink jet printing of a fluid, possibly a fine powder suspension, is used to “glue” granulate powder particles together layer by layer to create 3D dental or implant structures that are sintered. As an aside, ink jet printing could also be used to deliver drugs, polymeric equivalents of soft tissues or stem cells within scaffold structures, thus providing a route to co-fabrication of the organic and inorganic components required in the human body.
  • Casting of high solids content suspensions into nonporous moulds; direct
    consolidation casting is the generic name given to the process whereby a pourable     slurry can be converted into a solid without de-watering; optimised packing of particles via particle size distribution control helps ensure a near net shape after drying/sintering; this could have applications in the dental field where the ability to maintain near net shape avoids the need for predicting shape contraction on drying/sintering.
  • Granulate optimisation associated with thermal or plasma spraying of ceramic coatings on metal implants, or of titania coatings as antibacterial components on metallic objects used in hospitals.
Longevity is today’s requirement
Increased human lifespan and a demand for associated mobility and quality of life mean that there are huge pressures to create bespoke products with enhanced longevity. It can be seen that, because powder preparation and processing with powder suspensions represents the early stages of production, understanding and then optimising important variables will have a huge impact on yields and the ability to consistently deliver the enhanced in-service performance that is demanded.
 
Ongoing research in this area continues to drive advances in medical device manufacturing processes. For today’s manufacturers, consultation with materials analysis specialists can help to optimise manufacturing processes and ensure products are of the highest quality and are created in the most efficient manner possible. 
 
Phil Jackson, PhD,
is Business Development Manager, Medical Devices, at CERAM
Queens Road, Penkhull, Stoke-on-Trent, UK
tel. +44 1782 764 444
e-mail: phil.jackson@ceram.com
www.ceram.com/medical
 
 

 

Published in European Medical Device Technology, June 2010, Volume 1, No. 6



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