Formation of Hydroxyapatite on the Surface of Portland Cement in the Built Environment.
This week, in California, I had the distinct pleasure of attending a lecture by the Materials Science Engineer/Orthopedic Surgeon/Professor Cato T. Laurencin, M.D., Ph.D, a child of the inner city schools who grew up to study Chemical Engineering at Princeton, and to attend Harvard Medical School while simultaneously obtaining a Ph.D at MIT in biochemical engineering.
Here is a photograph of Dr. Laurencin receiving the National Medal of Technology and Innovation from his friend, Barack Obama, who he mentioned during his talk while speaking volumes in omission of the mention of the orange idiot:
(I also had the great privilege of speaking alone briefly to Dr. Laurentin, and even had the opportunity of telling him about my son, who is studying materials science engineering, whereupon the great man gave me his card and told me to be sure to have my son look him up when applying to graduate school...swoon...something that is several years off.)
Anyway.
Dr. Laurentin is an advocate, given his unique polymathic expertise of "convergent science" whereby physicians, chemists, physicists, mathematicians and engineers work together to solve important problems.
Dr. Laurentin's lecture concerned his development of stem cells supported in biopolymers and or synthetic matrices in order to grow new bone where it has been lost by trauma or disease - his discoveries are now commercially available to patients - as well as his current efforts to grow new connective tissue, tendons and ligaments in damaged knees and shoulders, thereby reducing the need for artificial (and somewhat problematic) implants. His long term goal, which he discussed at some length is develop a means by which people might regrow limbs and organs that they have lost, again, either to trauma or disease.
A remarkable man.
From a materials science perspective, part of his discussion involved the chemistry of hydroxyapatite, a calcium hydroxyphosphate mineral which in slightly modified form represents the bulk of bones and teeth.
Here is one structure of hydroxyapatite:
The lecture, particularly with the reference to convergent science reminded me of a paper I stumbled across recently on the protection and restoration of portland cement by use of biologically deposited hydroxyapatite surfaces, this one: Biogenic Hydroxyapatite: A New Material for the Preservation and Restoration of the Built Environment (Ronald J. Turner, Joanna C. Renshaw, and Andrea Hamilton, ACS Appl. Mater. Interfaces, 2017, 9 (37), pp 31401–31410)
(By the way, the abstract of the paper refers to the use of hydroxyapatite as a "waste form" for so called "nuclear waste," and before someone points it out, I'd like to state that I oppose the "disposal" of so called "nuclear waste" on the grounds that every single constituent of used nuclear fuel is extremely valuable and worthy of use for things that no other materials can do as well.)
Anyway. As mentioned in the abstract, portland cement is the single most produced material in the world, and its manufacture is responsible for a huge releases of the dangerous fossil fuel waste carbon dioxide. Because of the acidification of the planetary atmosphere and hydrosphere by the aforementioned carbon dioxide as well as the dangerous fossil fuel wastes sulfur dioxide, sulfur trioxide, sulfuric acid, nitrogen dioxide and nitric acid, portland cement has a tendency to degrade significantly, requiring its replacement.
This paper proposes to prevent or slow this degradation via the deposition on cement surfaces a thin layer or coating of hydroxyapatide, as it less susceptible to acid degradation than the mixed calcium carbonate and silicate comprising Portland cement.
An excerpt from the introductory text of the paper.:
Hydroxyapatite (HAP) is a calcium phosphate mineral with the general chemical formula Ca10(PO4)6(OH)2.1 HAP is an example of a bioceramic,2 a novel class of materials with potential applications such as bone implants3 and drug delivery.4 Hydroxyapatite coatings have been used to reinforce bone cements, enhancing load-bearing capacity5 and promoting proliferation and differentiation of human osteoblast-like cells.6,7 In addition, there is an emerging interest in the use of hydroxyapatite coatings for the consolidation and preservation of marble and limestone buildings and sculptures.8?12
Cement and concrete have been commonly used building materials throughout the 20th century, and there is significant interest in repairing concrete structures to reduce the amount of cement consumed worldwide. Cement is also used as an encapsulant for radioactive waste storage, which links with research on the use of hydroxyapatites for the remediation of radionuclide13,14 and heavy metal15 contaminants from the environment.
The crystal structure of naturally occurring HAP is too complex to be accurately mimicked by synthetic crystalline apatites and the use of synthetic apatites has been observed to result in poor adhesion and low mechanical strength in the field of dental treatments...
...Hydroxyapatite produced through the direct actions of a living organism (termed biogenic hydroxyapatite) may offer a solution to these limitations via intrinsic properties such as reduced solubility and comparable particle size to natural hydroxyapatites. 16?18
Here, we identify and describe a new method for deposition of a biogenic hydroxyapatite onto the surface of OPC using the Gram negative bacterium Pseudomonas fluorescens.
The authors utilize a common bacteria growth medium, LB (Lysogeny) Broth in a phosphate buffer to form the hydroxyapatite layer in an in situ bioreactor that utilizes free calcium from the cement to reform as the apatite layer.
The rest of the paper is rather technical, and probably would not be meaningful for readers here, referring to the structural characterization of the apatite layers by XRD, XRF and FT-IR, as well as a cool discussion of the mechanism of the deposition and the interactions of various constituents of the growth media, phosphate and carbon dioxide.
They remark that what they call "OPC" (Ordinary Portland Cement) is actually a superior material on which to grow biological hydroxyapatite than other more elaborate efforts to do pretty much the same thing.
Pseudomonas and other bacteria are capable of producing nanohydroxyapatite with physico-chemical properties very similar to those of natural bone material.31 Research on the bacterial production of hydroxyapatite has generally required immobilization of the bacteria on a matrix such as sol?gels or alginate beads prior to mineral formation31 and the addition of a specific calcium source to stimulate hydroxyapatite production.32 It is significant that these steps are not necessary when biohydroxyapatite is generated on an OPC substrate material, as shown in this study. OPC has several properties which may have contributed to this. The OPC block provides a fixed substrate for hydroxyapatite deposition, negating the requirement for a sol?gel or similar matrix.
In previous studies, calcium phosphate loading was found to be necessary when generating biohydroxyapatite on polyurethane or titanium,31 and the addition of calcium and phosphorus donors was also necessary when using sol?gel or alginate substrates.32 In the case of OPC, calcium is present in sufficient quantities (Table 1) both on the OPC surface and in the surrounding solution.
Cool, I think.
I wish you a very pleasant Friday.
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