BIOCERAMICS: Materials Successfully Characterized
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rom outerspace to innerspace, 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 supports (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" (for bioactive implants, see later), 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 [1]. 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-weing 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 (see side panel). The volume of pores thus penetrated and filled is monitored quite simply by the depletion of an external mercury reservoir (not dissimilar from a buree). The principle of Mercury Intrusion Porosimetry was first touted over eighty years ago! [2]. The technique lives on today in the form of fully automatic state-of-the-art porosimeters, the PoreMaster ® series from Quantachrome Instruments (Boynton Beach, FL), complete with familiar Windows ® user interface. THE FUTURE OF PORE SIZE DETERMINATION - NOW! The PoreMaster has been used in detailed studies of bioactive glass foam [3], 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 soware ensures that today's bioceramic engineers have a viable and valuable laboratory technique for pore structure characterization. References 1. D. Keys, "The Mummies of Cladh Hallan" hp://www.bbc.co.uk/history/archaeology/mummie s_cladhhallan_03.shtml 2. E.W. Washburn (1921) "Note on a Method of Determining the Distribution of Pore Sizes in a Porous Material", Proc. Natl. Acad. Sci. USA. 7, 115-116. 3. J. R. Jones and L. L. Hench (2003) "Effect of surfactant concentration and composition on the structure and properties of sol-gel-derived bioactive glass foam scaffol ds for tissue engineering" J. Mat. Sci. 38, 3783-3790. * Images provided by: Dr. Julian R. Jones at Imperial College London. For more information about pore size and related measurement instruments, contact Quantachrome Instruments by phone: (561) 731.4999, fax: (561) 732.9888, email: qc.support@ quantachrome.com or visit www.quantachrome.com. F TO AUTOGRAFTS AND ALLOGRAFTS Pore size is calculated using the Washburn equation [2]: Pore diameter = - (4 γ cos θ )/ P Where γ = surface tension, P = pressure and θ = contact angle. MIP Pore Diameter Range: 950µ m- 35Å Target Pore Diameters: Initial osteoblast activity: 100 µ m? Osteoconduction: 150 µ m - 350 µ m Fibrovascular development: ~500+µ m Materials Successfully Characterized: Hydroxyapatite Alumina (aluminum oxide) Zirconia (zirconium dioxide) Titania (titanium dioxide) coating Bioactive glass (e.g. SiO 2 /HAP) MIP is a recognized method: ASTM International’s Standard Guide F2150-02e1 for “Characterization and Testing of Biomaterial Scaffolds Used in Tissue-Engineered Medical Products”. Mercury Intrusion Porosimetry Highlights BIOCERAMICS: BIOCERAMICS: by: Martin A. Thomas Ph.D., Director of Business Development Quantachrome Instruments ([email protected]) The PoreMaster GT ® uses the extremely effective, rapid mercury intrusion technique. With automatic pore size and pore volume distribution analyzers, you will never have to guess what lies beneath. F o r a n s w e r s t o y o u r p o r o s i t y r e l a t e d q u e s t i o n s a n d t o s p e a k t o a Q u a n t a c h r o m e c o n s u l t a n t , c a l l u s a t 5 6 1 . 7 3 1 . 4 9 9 9 o r e m a i l u s a t q c . s u p p o r t q u a n t a c h r o m e . c o m . Advanced Features: -Automatic air purge -Vapor trap -Extremely quiet Three Operating Modes: -Pressure Scanning -Autospeed -Step-wise PoreMaster GT ® w w w . q u a n t a c h r o m e . c o m SYNTHETIC OPTIONS SYNTHETIC OPTIONS Optimised Foam Sca old* Sol-gel derived bioactive glass foam scaffolds* GET UNDER THE SKIN OF IMPLANTABLE CERAMICS: KNOW THE PORE VOLUME AND PORE SIZE DISTRIBUTION IN 15 MIN! Part #07152-Rev A. © 2008-2010 Quantachrome Corporation @
Transcript of BIOCERAMICS: Materials Successfully Characterized
rom outerspace to innerspace, ceramics areperhaps the most fascinating group ofmaterials. They protect space vehicles against
the remarkable heat of reentry into the earth'satmosphere, electrically insulate against thousands ofvolts, and protect the environment by providing vitalsupport to automotive emission catalysts. And theyare used to repair broken bodies. Not as mere externalsupports (though one might consider the humbleplaster-of-Paris cast a primitive ceramic), but asinternal prosthetics, integrated into the organism,performing fully the function of the skeletal materialthey replace. Of course, that's onlytrue if the foreign agent trulymodels the original!
A BONE MODELSo apart from the (usually)necessary property of "bio-inertness" (for bioactive implants,see later), what makes a ceramiccompatible with the host? For theanswer to that, we only need lookat the structure of bone. It is itsporosity that is the trademark of bone; it isremarkably strong given its light weight. How elsecould birds fly? How else could it live and grow? It isnot unreasonable therefore to design and manufacture abioceramic with a pore structure that is not unlikethat of real bone to promote cell adhesion, boneingrowth and vascularisation. Good design anddevelopment begins with good observation andmeasurement, and ends with accurate and detailedmonitoring of process and product. And for that, therecommended technique of pore structurecharacterization is one that relies on liquid penetration.Why? Because such a technique can be used on a fully3-dimensional specimen (not thin section) with minimalpreparation, it is sensitive to the arrangement ofpores, and is not unlike the mode by which bodyfluids and tissues penetrate and grow into thenetwork of pores. Indeed, such a technique is powerfulenough to monitor bacterial action on cadaver decay, tothe point of providing evidence of mummification [1].
ANALYZING PORE SIZEThe liquid of choice might at first be surprising,mercury. Familiar to dentists as an amalgam, it is alsoan extremely useful analytical tool. Not a reagent,since we will rely on its inertness: its non-we�ingbehavior on almost all surfaces (save a few metals).In order to penetrate a pore network, in bone orceramic, an external pressure must be applied to force
the mercury through pore openings. Fortunately,there exists a remarkably simple relationshipbetween pore size and the pressure required (see sidepanel). The volume of pores thus penetrated andfilled is monitored quite simply by the depletion of anexternal mercury reservoir (not dissimilar from abure�e). The principle of Mercury IntrusionPorosimetry was first touted over eighty years ago![2]. The technique lives on today in the form of fullyautomatic state-of-the-art porosimeters, the PoreMaster®
series from Quantachrome Instruments (Boynton Beach,FL), complete with familiar Windows® user interface.
THE FUTURE OF PORE SIZEDETERMINATION - NOW!The PoreMaster has been usedin detailed studies of bioactiveglass foam [3], a surgeon-friendly bone-replacement becauseof its ease of shaping at theoperating table, and the abilityto release ionic biologicalstimuli that promote bone cellp r o l i f e r a t i o n b y g e n e
activation. The speed of analysis, 15-20 minutes ontwo samples, combined with powerful data reductionand reporting so�ware ensures that today's bioceramicengineers have a viable and valuable laboratorytechnique for pore structure characterization. �
References1. D. Keys, "The Mummies of Cladh Hallan"h�p://www.bbc.co.uk/history/archaeology/mummies_cladhhallan_03.shtml
2. E.W. Washburn (1921) "Note on a Method ofDetermining the Distribution of Pore Sizes in a PorousMaterial", Proc. Natl. Acad. Sci. USA. 7, 115-116.
3. J. R. Jones and L. L. Hench (2003) "Effect of surfactantconcentration and composition on the structure andproperties of sol-gel-derived bioactive glass foamscaffolds for tissue engineering" J. Mat. Sci. 38, 3783-3790.
* Images provided by: Dr. Julian R. Jones at ImperialCollege London.
For more information about pore size and relatedmeasurement instruments, contact QuantachromeInstruments by phone: (561) 731.4999, fax: (561) 732.9888,email: [email protected] or visit www.quantachrome.com.
F
TO AUTOGRAFTS AND ALLOGRAFTS
Pore size is calculated using theWashburn equation [2]:Pore diameter = - (4 γ cosθ )/ PWhere γ = surface tension, P = pressure and θ = contact angle.
MIP Pore Diameter Range: 950µm- 35Å
Target Pore Diameters:Initial osteoblast activity: 100µm?Osteoconduction: 150µm - 350µmFibrovascular development: ~500+µm
Materials Successfully Characterized:HydroxyapatiteAlumina (aluminum oxide)Zirconia (zirconium dioxide)Titania (titanium dioxide) coatingBioactive glass (e.g. SiO2/HAP)
MIP is a recognized method:ASTM International’s Standard GuideF2150-02e1 for “Characterization andTesting of Biomaterial Scaffolds Usedin Tissue-Engineered Medical Products”.
MercuryIntrusionPorosimetryHighlights
BIOCERAMICS:BIOCERAMICS:
by: Martin A. Thomas Ph.D., Director of Business DevelopmentQuantachrome Instruments ([email protected])
The PoreMaster GT® uses the extremelyeffective, rapid mercury intrusion technique.With automatic pore size and pore volumedistribution analyzers, you will never have toguess what lies beneath.
For answers to your porosity related questionsand to speak to a Quantachrome consultant,call us at 561.731.4999 or email us atqc.support quantachrome.com.
Advanced Features:-Automatic air purge
-Vapor trap
-Extremely quiet
Three Operating Modes:-Pressure Scanning
-Autospeed
-Step-wise
PoreMaster GT®
www.quantachrome.com
SYNTHETIC OPTIONSSYNTHETIC OPTIONS
Optimised Foam Sca old*
Sol-gel derived bioactive glass foam scaffolds*
GET UNDER THE SKIN OF IMPLANTABLECERAMICS: KNOW THE PORE VOLUME AND PORE SIZE DISTRIBUTION IN 15 MIN!
Part # 07152-Rev A. © 2008-2010 Quantachrome Corporation
@
he technique of mercury intrusion porosimetry(MIP) is well understood to be of value in thefield of bioceramics and rigid high-densitypolyethylene porous surgical implants. One might be
hesitant to use the same pressure-based technique on soft,deformable polymers e.g. polyhydroxyalkanoates(PHAs), chitosan, polylactic acid (PLA), polyglycolicacid (PGA) and their co-polymers (PLGA). However, sincethe pore sizes involved are typically quite large (usuallyabove 100 microns1), the pressures involved are rathersmall (less than 10 psi). Furthermore, the pressure isapplied "isometrically", not transversely as in bubble-point(permeametry) techniques, so the polymer will not ruptureeven if in sheet form. MIP is therefore a viable technique.
Thin Sheets or Regular ShapesNevertheless, if detailed pore size measurements are notrequired, but just a measurement of overall porosity, theanalysis is remarkably straightforward. Pore volumeneed not be measured directly (by pore filling withmercury2) but can be calculated as the difference betweenbulk volume and "skeletal" volume. Bulk volume can bedetermined from consideration of geometric dimensions.Many biopolymers are formed as thin sheets or regularshapes. It is a therefore a trivial matter to takemicrometer or caliper readings. Alternatively, andcertainly for irregularly shaped pieces, bulk volume canbe determined by Dry Powder Pycnometry. Thistechnique entails measuring the increase in volume of apacked bed of powder when the sample is immersed in it,Archimedes style. One simply needs a powder,reasonably free flowing, whose particle size precludes itpenetrating those pores connecting to the polymer surface.
Limited Only By Ones Imaginat ionOver the years a number of different materials have beenemployed: graphite [1], aluminum hydroxide [2], metalpowders [2,3], rapeseed [4], glass beads [5,6]. The list ofpossible materials is only limited by one's imagination.To complete the testing a reproducible means of packingthe powder, by tapping, is employed. The Autotap(Quantachrome Instruments, Boynton Beach, FL) is sucha device. The powder and sample volumes are simplyread from the graduations on the sample container. Bothrapid and inexpensive, the Autotap (or its two-stationsibling the Dual Autotap) is a valuable addition to anymaterial development lab.
A Good FitThe second half of indirect pore volume determination isthe measurement of Skeletal Volume (true volume ofpolymer excluding pores---not the patient!). Nowadaysthis is normally achieved using a gas expansionpycnometer (dry, non destructive) rather than liquid
displacement (solubility problems, solvent disposal etc).The one-sample Ultrapycnometer 1000 and the uniquefive-station Pentapycnometer (both QuantachromeInstruments) feature simple programming of automaticrepeat measurements, balance and PC interfaces, and automaticprinted report generation. The Ultrapycnometer is availablein a special version, the UltraFoam Pycnometer, whichadditionally calculates open porosity and closed porevolume (as percentages). Already used for a number ofyears in non-surgical fields (packaging, insulation, wickingetc), the technology is well established and seems like agood fit to the objectives of the bio-materials community.
References1. S. Davis, A. Dolle and E. Moreau (1988) Prog. Batt.
Sol. Cells, 7, 342-348 2. B. Buczek and D. Geldart (1986) Powder Technol,45, 173-1763. A. Korta and J. Klinik (1975) Chem. Anal. 20, 10794. Approved Methods of the American Association of
Cereal Chemists, 10-05 Guidelines for Measurement of Volume by Rapeseed Displacement.
5. M. P. Hwang, and K.-I. Hayakawa, (1980) J. Food Sci, 45, 1400-1402.
6. S. L. Wilkison and M. S. Robinson. (2000) Meteorit.Planet. Sci., 35, 1203-1213.
Footnotes1. Wound dressings have considerably smaller pores.2. ...although mercury is an excellent contrast medium to
assist in visualizing pore shape and connectivity by X-ray tomography and/or confocal light microscopy.
* featured in article “Options: MEDPOR® Biomaterialand Surgical Implants” (www.porexsurgical.com)
For more information contact Quantachrome Instrumentsby phone: (561) 731.4999, fax: (561) 732.9888, email:[email protected] or visitwww.quantachrome.com.
The Role of PorosityA look at the application of porosimetry in reconstructive surgery and wound care biopolymers.
H e a t h c a r e T r e n d s
Mart in A. Thomas Ph.D. , Direc tor of Bus iness D evelopment Quantachrome Instruments (mar t in . [email protected])
T
Craniofac ia l reconstruct ion is a typica l appl icat ion forporous biopolymers .
C A L C U L A T I O N S
1. Porosity =Pore VolumeBulk Volume
x 100%
3. Porosity =Bulk Volume - Skeletal Volume
Bulk Volumex100%
2. Porosity = Bulk Volume - Skeletal Volume
4. Porosity = x100%Skeletal VolumeBulk Volume
1-( )
5. Porosity = x100%Bulk DensitySkeletal Density
1-( )
and since
then
therefore
or
How You Measure Porosity Depends On Which Direction You’re Coming From.
For more information, call us at561.731.4999 or email us at
V b u l k
A U T H O R ’ S N O T E : D E L V I N G F U R T H E R I N T O V - B U L K
The diagram shows the principle of simpleArchimedes-style displacement in order to determinethe bulk volume of a three-dimensional object. Whenusing dry powder, the trick is to cause reproduciblepacking behavior of the powder around the sample. Itis for this reason that the powder bed should be tappeduntil it reaches its maximum packing density (at theso-called “jamming transition”). External force suppliedby a ram, or tamping, should be avoided so as not tocompress a porous polymer.
Displacement of liquid mercury can also beemployed. Non-wetting mercury will not enter a poreunless an external pressure is applied. In this casethe determination is usually performed gravimetrically,using a mercury intrusion porosimeter to fill the sampleholder (pycnometer) with mercury. The Gay-Lussacdensity-bottle principle is then used to calculate thesample volume from a series of weighings.
If you already know the true skeletal density of your polymer(for example if you are using sintered polyetheylene) or thespecific pore volume (from liquid uptake), then all you needis bulk volume.The AutotapTM series inexpensively allowsyou to determine bulk volume of irregularly shapedbiomaterials using any reasonably free-flowing granular or powdered medium.
If all you have is the bulk volume (say of a sheet, orother regular geometric shape), then you need toknow either pore volume directly (by some liquidfilling method),or calculate it from bulk volume andskeletal volume. The UltrafoamTM gas pycnometerquickly and accurately reports open and closedpercent porosity, non-destructively using dry gas.
Part# 07152 Rev A © 2004-2010 Quantachrome Corporation
