A comparison of bioactive glass scaffolds fabricated by robocasting from powders made by sol-gel and melt-quenching methods

Bioactive glass scaffolds are used in bone and tissue biomedical implants, and there is great interest in their fabrication by additive manufacturing/3D printing techniques, such as robocasting. Scaffolds need to be macroporous with voids ≥100 μm to allow cell growth and vascularization, biocompatible and bioactive, with mechanical properties matching the host tissue (cancellous bone for bone implants), and able to dissolve/resorb over time. Most bioactive glasses are based on silica to form the glass network, with calcium and phosphorous content for new bone growth, and a glass modifier such as sodium, the best known being 45S5 Bioglass®. 45S5 scaffolds were first robocast in 2013 from melt-quenched glass powder. Sol-gel-synthesized bioactive glasses have potential advantages over melt-produced glasses (e.g., greater porosity and bioactivity), but until recently were never robocast as scaffolds, due to inherent problems, until 2019 when high-silica-content sol-gel bioactive glasses (HSSGG) were robocast for the first time. In this review, we look at the sintering, porosity, bioactivity, biocompatibility, and mechanical properties of robocast sol-gel bioactive glass scaffolds and compare them to the reported results for robocast melt-quench-synthesized 45S5 Bioglass® scaffolds. The discussion includes formulation of the printing paste/ink and the effects of variations in scaffold morphology and inorganic additives/dopants

Robocasting: Prediction of ink printability in solgel bioactive glass

Bioactive glass powders synthesized by solgel are usually porous and exhibit high specific surface areas, conferring them poor ability for scaffolds fabrication using colloidal processing approaches. The difficulties associated with colloidal processing of solgel glass have hindered so far the processing of 3-D scaffolds by robocasting. This research paper investigates the importance of calcination temperature (CT) and balls to powder ratio (BPR) used upon wet milling on the maximum achievable solid loading in aqueous media. The effects of CT, BPR, and solid loading on the flow behavior and viscoelastic properties of the suspensions/pastes were evaluated in this preliminary work. The aim is to disclose the sets of experimental variables that are most promising for the formulation of printable inks, and open the way for the future fabrication of porous scaffolds by robocasting and other 3-D additive manufacturing techniques

The role of calcium (source & content) on the in vitro behaviour of sol–gel quaternary glass series

To highlight the effect of salt precursors on the final properties, bioactivity and biocompatibility, five quaternary (Si–Ca–P–Na) glass compositions were successfully prepared through two distinct rapid sol–gel routes; one using acetate salt precursors (A) catalysed by nitric acid, and the other using nitrate salts (N) and citric acid as a catalyst. The sols dried rapidly, and stabilised at 550 & 800 °C to be characterised by X–ray diffraction (XRD), Magic angle spinning–Nuclear magnetic resonance (29Si MAS–NMR) and Fourier transform infra–red spectroscopy (FTIR). Upon immersion in simulated body fluid (SBF), hydroxyapatite (HAp) formation was initially enhanced by increasing Ca–content up to 40 mol%, but the formation of calcite was favoured with further increments of Ca to 45 and 48 mol%. The A–glasses exhibited lower density and lower network connectivity compared with N–glasses. The chemical surface modifications after 4 h in SBF were more evident for N–glasses in comparison to A–glasses. The biocompatibility is favoured for the samples treated at 800 °C and for the samples of the higher silica contents

Robocasting of ceramic glass scaffolds: Sol–gel glass, new horizons

This article reports the first robocasting of a sol–gel based glass ceramic scaffold. Sol–gel bioactive glass powders usually exhibit high volume fractions of meso– and micro–porosities, bad for colloidal processing as this adsorbs significant portion of the dispersing medium, affecting dispersion and flow. We circumvent these practical difficulties, to achieve pastes with particle size distributions, high solids loading and appropriate rheological properties for extrusion through fine nozzles for robocasting. Scaffolds with different macro-pore sizes (300–500 μm) with solid loadings up to 40 vol.% were robocast. The sintered (800 °C, 2 h) scaffolds exhibited compressive strength of 2.5–4.8 MPa, formed hydroxyapatite after 72 h in SBF, and had no cytotoxicity and a considerable MG63 cells viability rate. These features make the scaffolds promising candidates for tissue engineering applications and worthy for further in vivo investigations

Robocasting of Cu 2+ & La 3+ doped sol–gel glass scaffolds with greatly enhanced mechanical properties: Compressive strength up to 14 MPa

This research details the successful fabrication of scaffolds by robocasting from high silica sol–gel glass doped with Cu 2+ or La 3+ . The parent HSSGG composition within the system SiO 2 –CaO–Na 2 O–P 2 O 5 [67% Si – 24% Ca – 5% Na – 4% P (mol%)] was doped with 5 wt% Cu 2+ or La 3+ (Cu5 and La5). The paper sheds light on the importance of copper and lanthanum in improving the mechanical properties of the 3–D printed scaffolds. 1 h wet milling was sufficient to obtain a bioglass powder ready to be used in the preparation of a 40 vol% solid loading paste suitable for printing. Moreover, Cu addition showed a small reduction in the mean particle size, while La exhibited a greater reduction, compared with the parent glass. Scaffolds with macroporosity between 300 and 500 µm were successfully printed by robocasting, and then sintered at 800 °C. A small improvement in the compressive strength (7–18%) over the parent glass accompanied the addition of La. However, a much greater improvement in the compressive strength was observed with Cu addition, up to 221% greater than the parent glass, with compressive strength values of up to ∼14 MPa. This enhancement in compressive strength, around the upper limit registered for human cancellous bones, supports the potential use of this material in biomedical applications. Statement of Significance: 3D porous bioactive glass scaffolds with greatly improved compressive strength were fabricated by robocasting from a high silica sol–gel glasses doped with Cu 2+ or La 3+ . In comparison to the parent glass, the mechanical performance of scaffolds was greatly improved by copper-doping (>220%), while a modest increase of ∼9% was registered for lanthanum-doping. Doping ions (particularly La 3+ ) acted as glass modifiers leading to less extents of silica polymerisation. This favoured the milling of the glass powders and the obtaining of smaller mean particle sizes. Pastes with a high solid loading (40 vol%) and with suitable rheological properties for robocasting were prepared from all glass powders. Scaffolds with dimensions of 3 × 3 × 4 mm and macro-pore sizes between 300 and 500 µm were fabricated

BIONANOSCULP, an ongoing project in biotechnology applications for preventive conservation of outdoor sculptures

The objective of this paper is the presentation of the research strategies adopted and results of the ongoing BIONANOSCULP research project that is aiming to develop solutions in the area of sustainable nanomaterials, which are non-invasive and high-performance in their preventive conservation approach. An integrated methodology was designed as a holistic strategy to the characterisation of the microbiota present on the surface of public outdoor sculptures. Gathering objective data in the characterization of the surface microbiota of public outdoor sculptures is important, in order to design strategies for the preventive conservation of these objects that make use of biotechnology innovative coatings. Such is one of the objectives of the project BIONANOSCULP. Methodologies applied include conservation reports, surface sampling methodologies using gels, 3D modeling, SEM, flow cytometry and metagenomics. The project is already significantly contributing to create a bridge between the experts from different areas: the skills of biotechnologists, microbiologists, materials scientists, art historians and conservators-restorers to assess the state of conservation, biodeterioration and biocontamination of a selected number of sculptures, and to design the appropriate materials to pursue preventive conservation through coatings with anti-microbial activities

BIONANOSCULP, an ongoing project in biotechnology applications for preventive conservation of outdoor sculptures

The objective of this paper is the presentation of the research strategies adopted and results of the ongoing BIONANOSCULP research project that is aiming to develop solutions in the area of sustainable nanomaterials, which are non-invasive and high-performance in their preventive conservation approach. An integrated methodology was designed as a holistic strategy to the characterisation of the microbiota present on the surface of public outdoor sculptures. Gathering objective data in the characterization of the surface microbiota of public outdoor sculptures is important, in order to design strategies for the preventive conservation of these objects that make use of biotechnology innovative coatings. Such is one of the objectives of the project BIONANOSCULP. Methodologies applied include conservation reports, surface sampling methodologies using gels, 3D modeling, SEM, flow cytometry and metagenomics. The project is already significantly contributing to create a bridge between the experts from different areas: the skills of biotechnologists, microbiologists, materials scientists, art historians and conservators-restorers to assess the state of conservation, biodeterioration and biocontamination of a selected number of sculptures, and to design the appropriate materials to pursue preventive conservation through coatings with anti-microbial activities.This research work is supported by national funding from FCT- Fundação para a Ciência e a Tecnologia, I.P. for project BIONANOSCULP- Desenvolvimento de BIONANOmateriais para revestimento anti-microbiano de eSCULturas exteriores metálicas e de Pedra PTDC/EPHPAT/6281/2014. This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID /CTM /50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. R.C. Pullar thanks the FCT for funding under grant IF/00681/2015.publishe

Layer-by-layer technique to developing functional nanolaminate films with antifungal activity

The layer-by-layer (LbL) deposition method was used to build up alternating layers (five) of different polyelectrolyte solutions (alginate, zein-carvacrol nanocapsules, chitosan and chitosan-carvacrol emulsions) on an aminolysed/charged polyethylene terephthalate (A/C PET) film. These nanolaminated films were characterised by contact angle measurements and through the determination of water vapour (WVTR) and oxygen (O2TR) transmission rates. The effect of active nanolaminated films against the Alternaria sp. and Rhizopus stolonifer was also evaluated. This procedure allowed developing optically transparent nanolaminated films with tuneable water vapour and gas properties and antifungal activity. The water and oxygen transmission rate values for the multilayer films were lower than those previously reported for the neat alginate or chitosan films. The presence of carvacrol and zein nanocapsules significantly decreased the water transmission rate (up to 40 %) of the nanolaminated films. However, the O2TR behaved differently and was only improved (up to 45 %) when carvacrol was encapsulated, i.e. nanolaminated films prepared by alternating alginate with nanocapsules of zein-carvacrol layers showed better oxygen barrier properties than those prepared as an emulsion of chitosan and carvacrol. These films containing zein-carvacrol nanocapsules also showed the highest antifungal activity (30 %), which did not significantly differ from those obtained with the highest amount of carvacrol, probably due to the controlled release of the active agent (carvacrol) from the zein-carvacrol nanocapsules. Thus, this work shows that nanolaminated films prepared with alternating layers of alginate and zein-carvacrol nanocapsules can be considered to improve the shelf-life of foodstuffs.The authors acknowledge financial support from FP7 IP project BECOBIOCAP^. M. J. Fabra is recipients of a Juan de la Cierva contract from the Spanish Ministry of Economy and Competitivity. Maria L. Flores-López thanks Mexican Science and Technology Council (CONACyT, Mexico) for PhD fellowship support (CONACyT Grant Number 215499/310847). The author Miguel A. Cerqueira is a recipient of a fellowship (SFRH/BPD/72753/2010) supported by Fundação para a Ciência e Tecnologia, POPH-QREN and FSE (FCT, Portugal). The authors also thank the FCT Strategic Project of UID/ BIO/04469/2013 unit, the project RECI/BBB-EBI/0179/2012 (FCOMP- 01-0124-FEDER-027462) and the project BBioInd - Biotechnology and Bioengineering for improved Industrial and Agro-Food processes,^ REF. NORTE-07-0124-FEDER-000028 Co-funded by the Programa Operacional Regional do Norte (ON.2–O Novo Norte), QREN, FEDER. The support of EU Cost Action FA0904 is gratefully acknowledged

Novel route for rapid sol-gel synthesis of bioglass and bioceramics, using fast drying with a 100-fold to 200-fold reduction in processing time

It is extremely difficult to produce pure amorphous bioglasses from a quaternary system containing sodium, as they are readily prone to crystallisation, leading to inhomogeneity in the final glass matrix. Moreover, the aging time required in sol-gel synthesis is the main barrier to using the sol-gel method widely, compared with other preparation methods such as the melt quenching method. Typically, ageing times of one week or more are required to achieve an amorphous glass. Similar problems exist in the synthesis of bioceramics with sol-gel, in which unwanted phases may form irreversibly, or the desired phases may form with large crystallite size, as a result of slow ageing. To overcome this problem, we have developed an innovative, rapid sol-gel method for producing bioglass and bioceramic nanopowder, which avoids the conventional lengthy ageing and drying processes. This is 200 times quicker in comparison to conventional aqueous sol-gel bioglass preparation, and 100 times quicker than standard sol-gel bioceramic methods. In this chapter, we summarise the existing work on conventional sol-gel synthesis of such bioglasses and bioceramics, as well as some other, more rapid methods. We then give details of the novel sol-gel protocol developed by us to synthesise a quaternary glass (with incorporation of Na2O) in only 1 h. A comparative study of sol-gel derived glasses made by this novel rapid route using rotary evaporator drying, and a lengthy conventional route using oven drying and aging, revealed that the two methods produce stabilised bio-glasses with virtually identical behaviour and properties. We also expand on using this rapid method to produce pure hydroxyapatite (Ca10(PO4)6(OH)2, HAp) bioceramics, obtained after only 1 h with no prior ageing step. The rapid process favoured the formation of smaller/finer nanopowders, while producing pure HAp virtually identical to that obtained from the slow conventional drying method. Indeed, both rapidly dried powders for HAp and bioglass possessed enhanced properties such as smaller crystallite sizes, larger surface areas, and the silica network in the glass matrix exhibits a lower degree of polymerisation. All these enhanced properties should in turn result in increased bioactivity

Fast route for synthesis of stoichiometric hydroxyapatite by employing the Taguchi method

The Taguchi experimental design method is an elegant and efficient way of deriving optimum conditions for processes from the minimum number of experiments. We correlated various relevant synthesis parameters in the precipitation synthesis of single-phase pure hydroxyapatite (Ca10(PO4)6(OH)2, HAp) nanoparticles, via a rapid wet precipitation method, without any aging time. Taguchi planning was used for a systematic study of the combined effects of five different parameters: pH, synthesis temperature, synthesis time, drying temperature and calcination temperature. Using Taguchi methods, we were able to evaluate the effects of four variations (levels) in each of these five parameters, with just 16 experiments (an L16 (1024) orthogonal array). We assessed the impact of these parameters on four distinct properties, namely crystallite size, surface area, Ca/P atomic ratio and mol% of HAp. Calcination temperature exerted the greatest impact on hydroxyapatite morphology, corresponding to crystallite size and specific surface area, for which the role of other processing parameters was not significant. On the contrary, the Ca:P ratio was affected mainly by pH. These findings were confirmed by microstructural, structural and spectroscopic characterisation. FTIR spectra, revealing the conditions to retain a pure or prevailing hydroxyapatite phase, and also to indicate favourable conditions for A-type substitutions of carbonate for hydroxide groups, or B-type substitution for phosphate groups