Ceramics are perhaps the most fascinating group of materials. They protect space vehicles against the remarkable heat of reentry into the earth’s atmosphere, electrically insulate against thousands of volts, and protect the environment by providing vital support to automotive emission catalysts. And they are used to repair broken bodies. Not as mere external guards (though one might consider the humble plaster-of-Paris cast a primitive ceramic), but as internal prosthetics, integrated into the organism, performing fully the function of the skeletal material they replace. Of course, that’s only true if the foreign agent truly models the original!
A Bone Model
So apart from the (usually) necessary property of “bio-inertness”, what makes a ceramic compatible with the host? For the answer to that, we only need look at the structure of bone. It is its porosity that is the trademark of bone; it is remarkably strong given its light weight. How else could birds fly? How else could it live and grow? It is not unreasonable therefore to design and manufacture a bioceramic with a pore structure that is not unlike that of real bone to promote cell adhesion, bone ingrowth and vascularisation. Good design and development begins with good observation and measurement, and ends with accurate and detailed monitoring of process and product. And for that, the recommended technique of pore structure characterization is one that relies on liquid penetration. Why? Because such a technique can be used on a fully 3-dimensional specimen (not thin section) with minimal preparation, it is sensitive to the arrangement of pores, and is not unlike the mode by which body fluids and tissues penetrate and grow into the network of pores. Indeed, such a technique is powerful enough to monitor bacterial action on cadaver decay, to the point of providing evidence of mummification.
Figure 1. Porous scaffold in bioceramic material.
Analyzing Pore Size
The liquid of choice might at first be surprising, mercury. Familiar to dentists as an amalgam, it is also an extremely useful analytical tool. Not a reagent, since we will rely on its inertness: its non-wetting behavior on almost all surfaces (save a few metals). In order to penetrate a pore network, in bone or ceramic, an external pressure must be applied to force the mercury through pore openings. Fortunately, there exists a remarkably simple relationship between pore size and the pressure required. The volume of pores thus penetrated and filled is monitored quite simply by the depletion of an external mercury reservoir (not dissimilar from a burette).
Mercury Intrusion Porosimetry
The principle of Mercury Intrusion Porosimetry was first touted over eighty years ago! The technique lives on today in the form of fully automatic state-of-the-art porosimeters, the PoreMaster® series from Quantachrome Instruments, complete with familiar Windows user interface.
Figure 2. PoreMaster mercury intrusion porosimeter.
The Future Of Pore Size Determination – Now!
The PoreMaster has been used in detailed studies of bioactive glass foam, a surgeon friendly bone-replacement because of its ease of shaping at the operating table, and the ability to release ionic biological stimuli that promote bone cell proliferation by gene activation. The speed of analysis, 15-20 minutes on two samples, combined with powerful data reduction and reporting software ensures that today’s bioceramic engineers have a viable and valuable laboratory technique for pore structure characterization.
This information has been sourced, reviewed and adapted from materials provided by Quantachrome Instruments.
For more information on this source, please visit Quantachrome Instruments.