The basic calcium phosphate known as hydroxyapatite (HAp) is used in a number of essential applications. Probably the most well known is its use in the biomaterial field. It is essentially the hard component of bone, so products made of HAp are widely used in-vivo. Porous HAp shapes can be used as bone substitutes in reconstruction. The porosity is essential to allow access to fluids and tissue and bone ingrowth. HAp also exhibits ion-exchange (IE) properties and is used in IE and protein affinity chromatography. Substituted HAp can be used as an oxidative coupling catalyst, or straight HAp can be used as a catalyst support. Apart from the chemistry of HAp’s surface, therefore, it is important to know and to control surface area, pore size and pore volume since these parameters control adsorption capacity and biocompatibility.
"Adsorption on and Surface Chemistry of Hydroxyapatite
Fifteen Years of Clinical Experience with Hydroxyapatite Coatings in Joint Arthroplasty"
"Hydroxyapatite and Related Materials"
"CRC Handbook of Bioactive Ceramics, Volume II"
"Bioceramics (Advanced Ceramics)"
"Bioceramics: Materials, Properties, Applications"
"Progress In Bioceramics"
"Bioceramics: Materials and Applications III"
The preferred method of measurement is gas adsorption. Given the surface irregularity of real powder particles, not withstanding the presence of pores, the practice of calculating surface area from particle size normally resulting in a severe underestimation of the true surface area. Furthermore, gas adsorption is the only technique that can properly access internal surface, i.e. that which is due to the presence of pores. Modern automatic instruments like the NOVA make this determination rapid and user-friendly
Not only do pores express surface area and internal volume, their size controls accessibility to that space. Both cells and proteins range from nm- to micron size therefore biocompatible HAp’s tend to have pores on this scale. The most appropriate technique for measurement of these pore sizes is mercury intrusion porosimetry. Porosimeters such as the PoreMaster rapidly (< 30 mins.) measure a complete pore size distribution from ca. 900 microns to as small as 0.4nm. Gas sorption (see surface area, above) is also widely used for pore size measurement, especially for pores <0.4nm, but cannot be usefully employed for pores > ca. 0.4 microns.
It may be sufficient to know just the porosity of a sample, i.e. the pore volume expressed as a fraction, or percentage, of the total bulk volume. The total pore volume can be determined directly by mercury intrusion, or can be calculated as the difference between bulk volume and true (skeletal) volume. Bulk volume can be calculated from external dimensions for simple geometric shapes, or by dry powder pycnometery (displacement) for irregular objects or particles that are too small to determine geometrically. Dry powder pycnometery is performed using a powder-tapping device like the Autotap. This instrument is also used to determine the bulk density and compression behavior (under vertical oscillation) of loose powders. True density is measured cleanly and quickly by gas pycnometry, fully automatically.
Abbreviated Quantachrome Density Reference List
These papers cite the use of Quantachrome's products and the list represents a fraction of all such papers. If you would like more examples, please contact us.
"Calcium phosphate and fluorinated calcium phosphate coatings on titanium deposited by Nd:YAG laser at a high fluence" D.Ferro, S.M.Barinov, J.V.Rau, R.Teghil and A.Latini (2005) Biomaterials, 26, 805-812.
"Bioactive coatings prepared by sol–gel on stainless steel 316L" C.García, S.Ceré and A.Durán (2004) Journal of Non-Crystalline Solids, 348, 218-224.
"Strength and toughness of tape cast bioactive glass 45S5 following heat treatment" D.C.Clupper, L.L.Hench and J.J.Mecholsky (2004) Journal of the European Ceramic Society, 24, 2929-2934.
"Fused deposition modeling of novel scaffold architectures for tissue engineering applications" I.Zein, D.W.Hutmacher,K.C.Tan and S.H.Teoh (2002) Biomaterials, 23, 1169-1185.
"Fabrication of hydroxyapatite bodies by uniaxial pressing from a precipitated powder L.M.Rodriguez-Lorenzo, M.Vallet-Reg and J.M.F.Ferreira (2001) Biomaterials, 22, 583-588.
"Sol-gel derived carrier for the controlled release of proteins E.M. Santos, S.Radin and P.Ducheyne (1999) Biomaterials, 20, 1695-1700
Due to copyright restrictions, Quantachrome cannot supply copies of the above papers but we will be pleased to direct you the the appropriate authors so you may make your request to them. Ask here.
Standards (methods, specifications etc.)
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Note: provision of these links does not imply that Quantachrome's products are suitable for all of stated methods. This feature is provided as a convenience to those interested in hydroxyapatite and bioceramics. Quantachrome cannot entertain any questions regarding proper use of those standards not appropriate for Quantachrome's products. In such cases, you should contact the publishers.
Implants for surgery. Hydroxyapatite. Ceramic hydroxyapatite, BS ISO 13779-1:2000
Implants for surgery.
Hydroxyapatite. Coatings of hydroxyapatite, BS ISO 13779-2:2000
Standard Specification for Composition of Hydroxylapatite for Surgical Implants, ASTM F1185-03 (2003), ASTM International