Hydroxyapatite and Other Calcium Phosphates for the Conservation of Cultural Heritage: A Review

The present paper reviews the methods and the performance of in situ formation of calcium phosphates (CaP) for the conservation of materials belonging to cultural heritage. The core idea is to form CaP (ideally hydroxyapatite, HAP, the most stable CaP at pH > 4) by reaction between the substrate and an aqueous solution of a phosphate salt. Initially proposed for the conservation of marble and limestone, the treatment has been explored for a variety of different substrates, including sandstones, sulphated stones, gypsum stuccoes, concrete, wall paintings, archaeological bones and paper. First, the studies aimed at identifying the best treatment conditions (e.g., nature and concentration of the phosphate precursor, solution pH, treatment duration, ionic and organic additions to the phosphate solution, mineralogical composition of the new CaP phases) are summarized. Then, the treatment performance on marble and limestone is reviewed, in terms of protective and consolidating effectiveness, compatibility (aesthetic, microstructural and physical) and durability. Some pilot applications in real case studies are also reported. Recent research aimed at extending the phosphate treatment to other substrates is then illustrated. Finally, the strengths of the phosphate treatment are summarized, in comparison with alternative products, and some aspects needing future research are outlined

Mineral consolidants

Inorganic consolidants, such as ethyl silicate, nanolime and ammonium oxalate, have proven to be effective for certain materials, but each has its drawbacks. This has recently led to the investigation of hydroxyapatite (HAP) as a novel inorganic consolidant, which demonstrated excellent performance on carbonate stones. Considering that a mineral that matched calcite crystal lattice parameters even more closely than HAP would be expected to provide a consolidating action even greater than HAP, in this study aluminum phosphate (AP) was investigated as a potential new consolidant. Indeed, AP has lattice parameters differing from those of calcite by only 1%. The consolidating ability of AP was preliminarily investigated here in comparison with HAP. Both treatments were tested on artificially weathered marble samples, in the view of their application for conservation of sugaring marble. A novel method is also proposed for producing samples with near-surface damage similar to that of sugaring marble in the field. The results of the study point out that the novel weathering method is able to provide samples with tailored gradient in dynamic elastic modulus, closely resembling naturally sugaring marble. The AP treatment was found to significantly improve the dynamic elastic modulus of weathered marble, at least as efficiently as the HAP treatment investigated in this study. This confirmed the high potential of AP as a new inorganic consolidant

Consolidation of porous carbonate stones by an innovative phosphate treatment: Mechanical strengthening and physical-microstructural compatibility in comparison with TEOS-based treatments

For preservation of stones used in Cultural Heritage, affected by weathering processes that threaten their cohesion and mechanical properties, the application of consolidants is a common practice. However, available consolidating products generally exhibit some drawbacks that hinder their performance, in terms of either mechanical efficacy, compatibility with the substrate and/or durability. Ethyl silicate is currently the most widely used among stone consolidants; nevertheless, its reduced efficacy on calcitic substrates, together with its temporary hydrophobicity, its tendency to crack and its common formulation with volatile organic solvent, make the research for alternative consolidants for carbonate stones necessary. In this paper, a recently proposed new consolidation treatment based on the formation of hydroxyapatite inside the stone was tested on two different porous carbonate stones (Globigerina Limestone and Giallo Terra di Siena), and compared with TEOS-based treatments, frequently used for the consolidation of these lithotypes. The results show that the hydroxyapatite treatment exhibits a good efficacy in terms of mechanical properties and, compared to TEOS, it causes less pronounced alterations in open porosity and water transport properties. This makes the new treatment a potentially valid alternative to TEOS, especially in those situations where the possible presence of water behind the consolidated layer (e.g. in case of rising damp, condensation or infiltration) might threaten the durability of the consolidation intervention

Preliminary study on the use of ammonium phosphate for the conservation of marble-imitating gypsum-stuccoes

In this study, a novel method for consolidation and improvement of resistance to water of gypsum-stuccoes was preliminarily investigated. The idea is treating gypsum with an aqueous solution of diammonium hydrogen phosphate (DAP, (NH4)2HPO4) to form hydroxyapatite (HAP, Ca10(PO4)6(OH)2), which has much lower solubility than gypsum. Tests carried out on gypsum paste samples, manufactured to resemble historic stuccoes, showed that, after treatment with the DAP solution, a significant improvement in mechanical properties was achieved and brushite (CaHPO4∙2H2O) was formed (alongside some other by-products, that can be removed by an additional poultice treatment). Even if brushite is more soluble than HAP, still its formation is expected to be beneficial for stuccoes conservation, as brushite is significantly less soluble than gypsum

Consolidation of sugaring marble by hydroxyapatite: some recent developments on producing and treating decayed samples

Consolidation of sugaring marble (i.e., marble affected by granular disaggregation) still lacks fully effective solutions. Consequently, the use of an innovative phosphate-based treatment, aimed at bonding calcite grains by formation of hydroxyapatite at grain boundaries, has recently been proposed. In this paper, firstly a novel method for producing artificially decayed marble samples, by contact with a heating plate, is proposed. Then, some results are presented about the effectiveness and the compatibility of two different formulations of the phosphate treatment, differing in terms of concentration of the phosphate precursor (3.0 M or 0.1 M aqueous solutions of diammonium hydrogen phosphate, DAP), possible ethanol addition to the DAP solution and number of DAP solution applications (1 or 2). The results of the study point out that the new weathering method allows to obtain specimens with a gradient in microstructural and mechanical properties with thickness, just like naturally weathered samples. Both phosphate treatments were able to significantly improve marble cohesion, without causing significant changes in thermal behaviour and aesthetic appearance after treatment. The addition of small quantities of ethanol to the DAP solution seems to be a very promising method for favouring HAP formation and improving the treatment performance

Durable self-cleaning coatings for architectural surfaces by incorporation of TiO2 nano-particles into hydroxyapatite films

To prevent soiling of marble exposed outdoors, the use of TiO2 nano-particles has been proposed in the literature by two main routes, both raising durability issues: (i) direct application to marble surface, with the risk of particle leaching by rainfall; (ii) particle incorporation into inorganic or organic coatings, with the risk of organic coating degradation catalyzed by TiO2 photoactivity. Here, we investigated the combination of nano-TiO2 and hydroxyapatite (HAP), previously developed for marble protection against dissolution in rain and mechanical consolidation. HAP-TiO2 combination was investigated by two routes: (i) in series application of HAP followed by nano-TiO2 (“H+T”); (ii) simultaneous application by introducing nano-TiO2 into the phosphate solution used to form HAP (“HT”). The self-cleaning ability was evaluated before and after prolonged exposure to simulated rain. “H+T” and “HT” coatings exhibited much better resistance to nano-TiO2 leaching by rain, compared to TiO2 alone. In “H+T” samples, TiO2 nano-particles adhere better to HAP (having flower-like morphology and high specific surface area) than to marble. In “HT” samples, thanks to chemical bonds between nano-TiO2 and HAP, the particles are firmly incorporated in the HAP coating, which protects them from leaching by rain, without diminishing their photoactivity and without being degraded by them

Experimental study on the physical-mechanical durability of innovative hemp-based composites for the building industry

For reducing the environmental impact of the building sector, novel sustainable composites have recently been developed, by bonding hemp hurds with a new hybrid organic-inorganic binder. These composites, designed as substitutes for traditional insulating materials or as substitutes for formaldehyde-bonded wood particle boards, exhibit very promising thermal, physical and mechanical properties. To ensure that the panel performance is maintained during the building operation phase, durability needs to be specifically evaluated as well. Therefore, in this study three composite types with low, medium and high density (LD, MD and HD, respectively) were subjected to accelerated ageing and the alterations in their physical-mechanical properties were evaluated. Composite resistance to accelerated ageing is strongly correlated with bulk density. HD composites, the only ones actually designed to be directly exposed to rainfall, exhibited almost negligible decreases in mechanical properties and hence a substantially satisfactory behavior. MD and LD composites, designed to provide thermal insulation and hence to be sheltered by HD panels, were affected to a larger extent by accelerated ageing, which however was definitely more severe than the real exposure conditions of the composites during their service life. Further studies are currently in progress to optimize the composites formulation and physical-mechanical durability

Consolidation of porous carbonate stones by an innovative phosphate treatment: mechanical strengthening and physical-microstructural compatibility in comparison with TEOS-based treatments

For preservation of stones used in Cultural Heritage, affected by weathering processes that threaten their cohesion and mechanical properties, the application of consolidants is a common practice. However, available consolidating products generally exhibit some drawbacks that hinder their performance, in terms of either mechanical efficacy, compatibility with the substrate and/or durability. Ethyl silicate is currently the most widely used among stone consolidants; nevertheless, its reduced efficacy on calcitic substrates, together with its temporary hydrophobicity, its tendency to crack and its common formulation with volatile organic solvent, make the research for alternative consolidants for carbonate stones necessary. In this paper, a recently proposed new consolidation treatment based on the formation of hydroxyapatite inside the stone was tested on two different porous carbonate stones (Globigerina Limestone and Giallo Terra di Siena), and compared with TEOS-based treatments, frequently used for the consolidation of these lithotypes. The results show that the hydroxyapatite treatment exhibits a good efficacy in terms of mechanical properties and, compared to TEOS, it causes less pronounced alterations in open porosity and water transport properties. This makes the new treatment a potentially valid alternative to TEOS, especially in those situations where the possible presence of water behind the consolidated layer (e.g. in case of rising damp, condensation or infiltration) might threaten the durability of the consolidation intervention

Bone on-a-chip: a 3D dendritic network in a screening platform for osteocyte-targeted drugs

Age-related musculoskeletal disorders, including osteoporosis, are frequent and associated with long lasting morbidity, in turn significantly impacting on healthcare system sustainability. There is therefore a compelling need to develop reliable preclinical models of disease and drug screening to validate novel drugs possibly on a personalized basis, without the need of in vivo assay. In the context of bone tissue, although the osteocyte (Oc) network is a well-recognized therapeutic target, current in vitro preclinical models are unable to mimic its physiologically relevant and highly complex structure. To this purpose, several features are needed, including an osteomimetic extracellular matrix, dynamic perfusion, and mechanical cues (e.g. shear stress) combined with a three-dimensional (3D) culture of Oc. Here we describe, for the first time, a high throughput microfluidic platform based on 96-miniaturized chips for large-scale preclinical evaluation to predict drug efficacy. We bioengineered a commercial microfluidic device that allows real-time visualization and equipped with multi-chips by the development and injection of a highly stiff bone-like 3D matrix, made of a blend of collagen-enriched natural hydrogels loaded with hydroxyapatite nanocrystals. The microchannel, filled with the ostemimetic matrix and Oc, is subjected to passive perfusion and shear stress. We used scanning electron microscopy for preliminary material characterization. Confocal microscopy and fluorescent microbeads were used after material injection into the microchannels to detect volume changes and the distribution of cell-sized objects within the hydrogel. The formation of a 3D dendritic network of Oc was monitored by measuring cell viability, evaluating phenotyping markers (connexin43, integrin alpha V/CD51, sclerostin), quantification of dendrites, and responsiveness to an anabolic drug. The platform is expected to accelerate the development of new drug aimed at modulating the survival and function of osteocytes

Customized biofilm device for antibiofilm and antibacterial screening of newly developed nanostructured silver and zinc coatings

Background Bacterial colonisation on implantable device surfaces is estimated to cause more than half of healthcare-associated infections. The application of inorganic coatings onto implantable devices limits/prevents microbial contaminations. However, reliable and high-throughput deposition technologies and experimental trials of metal coatings for biomedical applications are missing. Here, we propose the combination of the Ionized Jet Deposition (IJD) technology for metal-coating application, with the Calgary Biofilm Device (CBD) for high-throughput antibacterial and antibiofilm screening, to develop and screen novel metal-based coatings. Results The films are composed of nanosized spherical aggregates of metallic silver or zinc oxide with a homogeneous and highly rough surface topography. The antibacterial and antibiofilm activity of the coatings is related with the Gram staining, being Ag and Zn coatings more effective against gram-negative and gram-positive bacteria, respectively. The antibacterial/antibiofilm effect is proportional to the amount of metal deposited that influences the amount of metal ions released. The roughness also impacts the activity, mostly for Zn coatings. Antibiofilm properties are stronger on biofilms developing on the coating than on biofilms formed on uncoated substrates. This suggests a higher antibiofilm effect arising from the direct contact bacteria-coating than that associated with the metal ions release. Proof-of-concept of application to titanium alloys, representative of orthopaedic prostheses, confirmed the antibiofilm results, validating the approach. In addition, MTT tests show that the coatings are non-cytotoxic and ICP demonstrates that they have suitable release duration (> 7 days), suggesting the applicability of these new generation metal-based coatings for the functionalization of biomedical devices.Conclusions The combination of the Calgary Biofilm Device with the Ionized Jet Deposition technology proved to be an innovative and powerful tool that allows to monitor both the metal ions release and the surface topography of the films, which makes it suitable for the study of the antibacterial and antibiofilm activity of nanostructured materials. The results obtained with the CBD were validated with coatings on titanium alloys and extended by also considering the anti-adhesion properties and biocompatibility. In view of upcoming application in orthopaedics, these evaluations would be useful for the development of materials with pleiotropic antimicrobial mechanisms