Bone-like apatite coating on Mg-PSZ/Al2O3 composites using bioactive systems

A biomimetic method was used to promote bioactivity on zirconia/alumina composites. The composites were composed of 80 vol% Mg-PSZ and 20 vol% Al₂O₃. Samples of these bioinert materials were immersed in simulated body fluid (SBF) for 7 days on either a bed of wollastonite ceramics or bioactive glass. After those 7 days, the samples were immersed in a more concentrated solution (1.4 SBF) for 14 days. Experiments were also performed without using a bioactive system during the first stage of immersion. A bone-like apatite layer was formed on the surface of all the materials tested, using wollastonite the bioactive layer was thicker and its morphology was close to that observed on the existing bioactive systems. A thinner apatite layer consisting of small agglomerates was obtained using bioactive glass. The thickness of the ceramic layers was within the range of 15 to 30 μm.     

Electrophoretic deposition of porous CaO–MgO–SiO<Subscript>2</Subscript> glass–ceramic coatings with B<Subscript>2</Subscript>O<Subscript>3</Subscript> as additive on Ti–6Al–4V alloy

The sub-micron glass–ceramic powders in CaO–MgO–SiO₂ system with 10 wt% B₂O₃ additive were synthesized by sol–gel process. Then bioactive porous CaO–MgO–SiO₂ glass–ceramic coatings on Ti–6Al–4V alloy substrates were fabricated using electrophoretic deposition (EPD) technique. After being calcined at 850°C, the above coatings with thickness of 10–150 μm were uniform and crack-free, possessing porous structure with sub-micron and micron size connected pores. Ethanol was employed as the most suitable solvent to prepare the suspension for EPD. The coating porous appearance and porosity distribution could be controlled by adjusting the suspension concentration, applied voltage and deposition time. The heat-treated coatings possessed high crystalline and was mainly composed of diopside, akermanite, merwinite, calcium silicate and calcium borate silicate. Bonelike apatite was formed on the coatings after 7 days of soaking in simulated body fluid (SBF). The bonding strength of the coatings was needed to be further improved.     

Apatite formation from simulated body fluid on various phases of TiO2 thin films prepared by filtered cathodic vacuum arc deposition

Hydroxyl (OH⁻)-free TiO₂ thin films with amorphous and crystalline phases were deposited onto (100) silicon substrates using filtered cathodic vacuum arc deposition in order to investigate the in vitro apatite formation in simulated body fluid (SBF). The surface morphology, composition and structure of the TiO₂ thin films were characterized. The X-ray photoelectron spectroscopy results confirmed the presence of calcium and phosphorus on all TiO₂ thin film surfaces after immersion in SBF at 37 °C. Fourier transform infra red results showed the presence of carbonated apatite on the surface of these films. Amorphous structured TiO₂ thin film showed poor ability to form apatite on its surface in SBF. Apatite formation was more pronounced on the surfaces of the anatase films in comparison to those of rutile. The carbonated apatite deposition rate increased significantly when the TiO₂ film was illuminated with UV light prior to immersing in the SBF. In particular, the UV-treated anatase and rutile films showed increased rates of carbonated apatite formation on their surfaces in comparison to samples not treated with radiation. The increase in hydrophilicity due to UV treatment appears beneficial for the apatite growth on these surfaces.     

Apatite formation abilities and mechanical properties of hydroxyethylmethacrylate-based organic–inorganic hybrids incorporated with sulfonic groups and calcium ions

Apatite formation in the living body is an essential requirement for artificial materials to exhibit bone-bonding bioactivity. It has been recently revealed that sulfonic groups trigger apatite nucleation in a body environment. Organic–inorganic hybrids consisting of organic polymers and the sulfonic groups are therefore expected to be useful for preparation of novel bone-repairing materials exhibiting flexibility as well as bioactivity. In the present study, organic–inorganic hybrids were prepared from hydroxyethylmethacrylate (HEMA) in the presence of vinylsulfonic acid sodium salt (VSAS) and calcium chloride (CaCl₂). The bioactivities of the hybrids were assessed in vitro by examining the apatite formation in simulated body fluid (SBF, Kokubo solution). The hybrids deposited on the apatite after soaking in SBF within 7 days. Tensile strength measurements showed a tendency to increase with increases in VSAS and CaCl₂ content. It was assumed that this phenomenon was attributed to the formation of cross-linking in the hybrids.     

Relationship between apatite-forming ability and mechanical properties of bioactive PMMA-based bone cement modified with calcium salts and alkoxysilane

Polymethylmethacrylate (PMMA)-based bone cement is used for the fixation of artificial joints in orthopaedics. However, the fixation is liable to loosen in the body, because the cement does not bond to living bone. So-called bioactive ceramics bond directly to living bone through the apatite layer formed on their surfaces in the body. We previously revealed that modification using γ-methacryloxypropyltrimethoxysilane (MPS) and water-soluble calcium salts such as calcium acetate and calcium hydroxide was effective for providing the PMMA-based bone cement with apatite-forming ability in a simulated body fluid (SBF, Kokubo solution) that closely reproduces the body environment. However, the effect of the chemical reaction forming the apatite on the mechanical properties of the cements has not been clarified. The present work aimed to investigate this issue from the viewpoint of the interface structure between the apatite and the cement. The surface of the cement modified with calcium acetate and MPS was fully covered with newly formed apatite after soaking in Kokubo solution within 7 days, while half of the surface area of the cement modified with calcium hydroxide and MPS was covered with the apatite. The bending strength of the modified cements decreased after soaking in Kokubo solution. Porous structure was observed in the region about 50–100 μm in depth from the top surface because of release of the Ca²⁺ ions by both modified cements after soaking in Kokubo solution. The decrease in bending strength of the modified cements could be attributed to the formation of the pores. In addition, the pores on the top surfaces of the cements were filled with the newly formed apatite. The apatite formation would be effective not only for bioactivity but also for decreasing the reduction of mechanical strength.     

Apatite deposition on polyamide films containing carboxyl group in a biomimetic solution

The development of organic–inorganic hybrids composed of hydroxyapatite and organic polymers is attractive because of their novelty in being materials that show a bone-bonding ability, i.e. bioactivity, and because they have mechanical properties similar to those of natural bone. The biomimetic process has received much attention for fabricating such a hybrid, where bone-like apatite is deposited under ambient conditions on polymer substrates in a simulated body fluid (SBF) having ion concentrations nearly equal to those of human extracellular fluid or related solutions. It has been shown that the carboxyl group is effective for inducing heterogeneous nucleation of apatite in the body. In the present study, apatite deposition on polyamide films containing various numbers of carboxyl groups was investigated in 1.5 SBF, which had ion concentrations 1.5 times those of a normal SBF. The effect of incorporation of calcium chloride on the formation of apatite was examined. Polyamide films containing 33 mol % CaCl₂ did not form apatite, even after soaking in 1.5 SBF for 7 days, and even when the polymer film contained 50 mol % carboxyl group. On the other hand, those modified with 40 mass % CaCl₂ formed apatite on their surfaces in 1.5 SBF. The ability of the modified film to form an apatite layer increased, and the adhesion of the apatite layer bonded to the film improved, with increasing carboxyl group content. It is concluded that novel apatite–polyamide hybrids can be prepared by a biomimetic process.     

Bioactivity of CaSiO<Subscript>3</Subscript>/poly-lactic acid (PLA) composites prepared by various surface loading methods of CaSiO<Subscript>3</Subscript> powder

Mixing bioactive ceramic powders with polymers is an effective method for generating bioactivity to the polymer-matrix composites but it is necessary to incorporate up to 40 vol% of bioactive ceramic powder. However, such a high mixing ratio offsets the advantages of the flexibility and formability of polymer matrix and it would be highly advantageous to lower the mixing ratio. Since surface loading of ceramic powders in the polymer is thought to be an effective way of reducing the mixing ratio of the ceramic powder while maintaining bioactive activity, CaSiO₃/poly-lactic acid (PLA) composites were prepared by three methods; (1) casting, (2) spin coating and (3) hot pressing. In methods (1) and (2), a suspension was prepared by dissolving PLA in chloroform and dispersing CaSiO₃ powder in it. The suspension was cast and dried to form a film in the case of method (1) while it was spin-coated on a PLA substrate in method (2). In method (3), CaSiO₃ powder was surface loaded on to a PLA substrate by hot-pressing. The bioactivity of these samples was investigated in vitro using simulated body fluid (SBF). Apatite formation was not observed in the samples prepared by method (1) but some apatite formation was achieved by mixing polyethylene glycol (PEG) with the PLA, producing a porous polymer matrix. In method (2), apatite was clearly observed after soaking for 7 days. Enhanced apatite formation was observed in method (3), the thickness of the resulting apatite layers becoming about 20 μm after soaking for 14 days. Since the amount of CaSiO₃ powder used in these samples was only ≤0.4 vol%, it is concluded that this preparation method is very effective in generating bioactivity in polymer-matrix composites by loading with only very small amounts of ceramic powder.     

Investigation of boundary conditions for biomimetic HA deposition on titanium oxide surfaces

To improve the clinical outcome of metal implants, i.e. earlier loading and reduction of the incidence of revision surgery, better bone bonding ability is wanted. One method to achieve this is to change the surface chemistry to give a surface that facilitates bone bonding in vivo, i.e. a bioactive surface. Crystalline titanium oxide has recently been proven to be bioactive in vitro and is an interesting option to the more common hydroxylapatite (HA) coatings on implants. A materials possible in vitro bioactivity is tested through soaking in simulated body fluid and studies of possible HA formation on the surface. For bioactive materials, the formed HA layer can also be used as a coating. The aim of the current paper is to investigate some boundary conditions for HA formation on crystalline titanium oxide surfaces regarding influence from coating thickness, soaking time and soaking temperature. The influence from soaking time and temperature on the HA growth were investigated on oxidised Ti samples, (24 h at 800°C) resulting in a rutile surface structure. The oxidised samples were tested at three temperatures (4, 37 and 65°C) and four times (1 h, 1 day, 1 week and 4 weeks). The influence from titanium coating thickness on the HA growth was investigated via depositing thin films of crystalline titanium dioxide on Ti plates using a reactive magnetron sputtering process. Four different PVD runs with coating thicknesses between 19 and 74 nm were tested. The soaking temperature had an effect on the HA formation and growth on both rutile surfaces and native oxide on Ti substrates. Higher temperatures lead to easier formation of HA. It was even possible, at 65°C, to grow HA on native titanium oxide from soaking in PBS. The coating quality was better for HA formed at 65°C compared to 37°C. All PVD-coatings showed HA growth after 1 week in PBS at 37°C, thus even very thin coatings of crystalline titanium oxide coatings are bioactive.

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Bioactivity of wollastonite/aerogels composites obtained from a TEOS–MTES matrix

Organic–inorganic hybrid materials were synthesized by controlled hydrolysis of tetraethoxysilane (TEOS), methyltrimethoxysilane (MTES), synthetic wollastonite powders and polydimethylsiloxane (PDMS) in an ethanol solution. Aerogels were prepared from acid hydrolysis of TEOS and MTES with different volume ratio in ethanol, followed by addition of wollastonite powder and PDMS in order to obtain aerogels with 20 wt% of PDMS and 5 wt% of CaO of the total silica. Finally, when the wet gels were obtained, they were supercritically dried at 260 °C and 90 bar, in ethanol. In order to obtain its bioactivity, one method for surface activation is based on a wet chemical alkaline treatment. The particular interest of this study is that we introduce hybrid aerogels, in a 1 M solution of NaOH, for 30 s at room temperature. We evaluate the bioactivity of TEOS–MTES aerogel when immersed in a static volume of simulated body fluid (SBF). An apatite layer of spherical-shaped particles of uniform size smaller than 5 microns is observed to form on the surface of the aerogels after 25 days soaking in SBF.     

Bioactivity of CaSiO<Subscript>3</Subscript>/poly-lactic acid (PLA) composites prepared by various surface loading methods of CaSiO<Subscript>3</Subscript> powder

Mixing bioactive ceramic powders with polymers is an effective method for generating bioactivity to the polymer-matrix composites but it is necessary to incorporate up to 40 vol% of bioactive ceramic powder. However, such a high mixing ratio offsets the advantages of the flexibility and formability of polymer matrix and it would be highly advantageous to lower the mixing ratio. Since surface loading of ceramic powders in the polymer is thought to be an effective way of reducing the mixing ratio of the ceramic powder while maintaining bioactive activity, CaSiO₃/poly-lactic acid (PLA) composites were prepared by three methods; (1) casting, (2) spin coating and (3) hot pressing. In methods (1) and (2), a suspension was prepared by dissolving PLA in chloroform and dispersing CaSiO₃ powder in it. The suspension was cast and dried to form a film in the case of method (1) while it was spin-coated on a PLA substrate in method (2). In method (3), CaSiO₃ powder was surface loaded on to a PLA substrate by hot pressing. The bioactivity of these samples was investigated in vitro using simulated body fluid (SBF). Apatite formation was not observed in the samples prepared by method (1) but some apatite formation was achieved by mixing polyethylene glycol (PEG) with the PLA, producing a porous polymer matrix. In method (2), apatite was clearly observed after soaking for 7 days. Enhanced apatite formation was observed in method (3), the thickness of the resulting apatite layers becoming about 20 μm after soaking for 14 days. Since the amount of CaSiO₃ powder used in these samples was only ≤0.4 vol%, it is concluded that this preparation method is very effective in generating bioactivity in polymer-matrix composites by loading with only very small amounts of ceramic powder.     

Anti-washout carboxymethyl chitosan modified tricalcium silicate bone cement: preparation,mechanical properties and in vitro bioactivity

Anti-washout CaF₂ stabilized C₃S (F-C₃S) bone cement was prepared by adding water-soluble carboxymethyl chitosan (CMCS) to the hydration liquid. The setting time, compressive strength and in vitro bioactivity of the CMCS modified F-C₃S (CMCS–C₃S) pastes were evaluated. The results indicate that CMCS–C₃S pastes could be stable in the shaking simulated body fluid (SBF) after immediately mixed. The addition of CMCS significantly enhances the cohesion of particles, at the same time restrains the penetration of liquid, and thus endows the anti-washout ability. The setting times of the pastes increase with the increase of CMCS concentrations in the hydration liquid. Besides, the compressive strengths of CMCS–C₃S pastes after setting for 1–28 days are lower than that of the pure F-C₃S paste, but the sufficient strengths would be suitable for the clinical applications. The crystalline apatite deposited on the paste surface is retarded from 1 to 2 days for the addition of CMCS, but the quantities of deposited apatite are same after soaking in SBF for 3 days. As the result that pure C₃S paste has shorter setting times than pure F-C₃S paste, CMCS modified pure C₃S pastes would have better anti-washout ability. Our study provides a convenient way to use C₃S bone cement with excellent anti-washout ability when the pastes are exposed to biological fluids. The novel anti-washout CMCS–C₃S bone cement with suitable setting times, sufficient strengths and in vitro bioactivity would have good prospects for medical application.     

Chitosan reinforced apatite–wollastonite coating by electrophoretic deposition on titanium implants

A novel bioactive porous apatite–wollastonite/chitosan composite coating was prepared by electrophoretic deposition. The influence of synthesis parameters like pH of suspension and current density was studied and optimized. X-ray diffraction confirmed crystalline phase of apatite–wollastonite in powder as well as composite coating with coat crystallinity of 65%. Scanning electron microscope showed that the porosity had interconnections with good homogeneity between the phases. The addition of chitosan increased the adhesive strength of the composite coating. Young’s modulus of the coating was found to be 9.23 GPa. One of our key findings was sheet-like apatite growth unlike ball-like growth found in bioceramics. Role of chitosan was studied in apatite growth mechanism in simulated body fluid. In presence of chitosan, dense negatively charged surface with homogenous nucleation was the primary factor for sheet-like evolution of apatite layer. The results suggest that incorporation of chitosan with apatite–wollastonite in composite coating could provide excellent in vitro bioactivity with enhanced mechanical properties.     

In vitro bioactivity and osteoblast-like cell test of zinc containing fluoridated hydroxyapatite films

Zinc containing fluoridated hydroxyapatite (ZnFHA) films on Ti6Al4V substrates was prepared using sol–gel dip-coating method. The release of zinc ions from ZnFHA film was controlled mainly by the zinc content in the film. The release behavior showed an initial rapid increase release followed by a tapering-off and directed to a constant value at longer time. After soaking in SBF for 8 days, a layer was deposited and completely covered the original surface of the ZnFHA film, indicating good in vitro “bioactivity.” The osteoblast-like MG63 cells were seeded on the ZnFHA films; FHA film and Ti6Al4V substrate were used as control. The cell culture result showed that cell adhesion and proliferation on ZnFHA films were significantly increased compared with the controls. The results in this work suggest that ZnFHA films on Ti6Al4V substrates can function as an implant with good bioactivity and cytocompatibility.     

Preparation and characterization of biomimetically and electrochemically deposited hydroxyapatite coatings on micro-arc oxidized Ti–13Nb–13Zr

Surface properties and corrosion resistance analyses of Ti–13Nb–13Zr coated by an oxide film (obtained by micro-arc oxidation at 300 V) or an oxide/hydroxyapatite (HA) film are reported. HA films were biomimetically or electrochemically deposited on the alloy/oxide surface, and their properties compared. Both the biomimetic and the electrochemical method yielded rough and globular apatite surfaces (10–20 μm globules for the former and 1–2 μm for the latter). As inferred from XRD data, the electrochemical method yielded more biologic-like HA films, while the biomimetic method yielded films containing a mixture of calcium phosphate phases. Coated Ti–13Nb–13Zr samples were immersed in an aerated PBS solution and continuously analyzed during 49 days. Considering that, after immersion, the biomimetically deposited films presented smaller variations in thickness and morphology and higher electric resistance (determined by electrochemical impedance spectroscopy), they clearly provide significantly better protection to the Ti–13Nb–13Zr alloy when in PBS solution.     

Hydroxyapatite/Bioactive Glass Films Produced by a Sol–Gel Method: In Vitro Behavior

Hydroxyapatite (HA) and HA/bioactive glass (49S) films were deposited on Si(100) substrates by a sol–gel dip‐coating method. The microstructure and in vitro bioactivity of the films were investigated by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X‐ray photoelectron spectroscopy (XPS). Polycrystalline HA and amorphous bioactive glass films were obtained after annealing at 600 and 400 °C, respectively. The crystallization temperature of HA was determined to be around 568 °C. The surfaces of the HA films were covered with an apatite layer consists of spherulites formed by nanosized needle‐like aggregates after the soaking in simulated body fluid (SBF) for 10 days, while amorphous HA/bioactive glass surface was covered with larger spherical crystallites. Both XPS and EDS results obtained from HA/bioactive glass film, after soaking in SBF, showed increasing P amounts on the surface at the expense of Si. The higher density of the newly formed layer on HA/bioactive glass surface than that of the HA surface after 10 days of soaking was evidence of increased reaction rate and apatite forming ability when bioactive glass layer is present on the HA films.     

Effect of water aging on the apatite formation of a low-modulus Ti–7.5Mo alloy treated with aqueous NaOH

The objective of this experiment was to develop biomimetic calcium phosphate coatings on low-modulus Ti–7.5Mo substrates treated with NaOH aqueous solutions and subsequent water aging before soaking them in simulated body fluid (SBF). Specimens of commercially pure titanium (c.p. Ti) and Ti–7.5Mo were initially treated with 5 M NaOH at 60 °C for 24 h, resulting in the formation of a porous network structure composed of sodium titanate (Na₂Ti₅O₁₁). Afterward, the specimens were aged in distilled water at 80 °C for 12, 24, or 48 h, and subsequently immersed in 1.5SBF at 37 °C for either 1 or 13 days. The calcium phosphate-forming abilities of the c.p. Ti and Ti–7.5Mo achieved by a single NaOH treatment were low, but were significantly increased by the water aging. The amount of calcium phosphate deposited on the Ti–7.5Mo after NaOH treatment and subsequent water aging for 12 or 24 h was much greater than other conditions. The calcium phosphate-coated Ti–7.5Mo has strong potential as an artificial bone substitute or in other hard tissue-replacement materials with heavy load-bearing requirements due to a favorable combination of bioactivity, low elastic modulus, and low processing costs.     

Characterization of surface oxide films on titanium and bioactivity

Biological properties of titanium implant depend on its surface oxide film. In the present study, the surface oxide films on titanium were characterized and the relationship between the characterization and bioactivity of titanium was studied. The surface oxide films on titanium were obtained by heat-treatment in different oxidation atmospheres, such as air, oxygen and water vapor. The bioactivity of heat-treated titanium plates was investigated by immersion test in a supersaturated calcium phosphate solution. The surface roughness, energy morphology, chemical composition and crystal structure were used to characterize the titanium surfaces. The characterization was performed using profilometer, scanning electronic microscopy, ssesile drop method, X-ray photoelectron spectroscopy, common Bragg X-ray diffraction and sample tilting X-ray diffraction. Percentage of surface hydroxyl groups was determined by X-ray photoelectron spectroscopy analysis for titanium plates and density of surface hydroxyl groups was measured by chemical method for titanium powders. The results indicated that heat-treatment uniformly roughened the titanium surface and increased surface energy. After heat-treatment the surface titanium oxide was predominantly rutile TiO₂, and crystal planes in the rutile films preferentially orientated in (1 1 0) plane with the highest density of titanium ions. Heat-treatment increased the amount of surface hydroxyl groups on titanium. The different oxidation atmospheres resulted in different percentages of oxygen species in TiO₂, in physisorbed water and acidic hydroxyl groups, and in basic hydroxyl groups on the titanium surfaces. The immersion test in the supersaturated calcium phosphate solution showed that apatite spontaneously formed on to the rutile films. This revealed that rutile could be bioactivated. The analyses for the apatite coatings confirmed that the surface characterization of titanium has strong effect on bioactivity of titanium. The bioactivity of the rutile films on titanium was related not only to their surface basic hydroxyl groups, but also to acidic hydroxyl groups, and surface energy. Heat-treatment endowed titanium with bioactivity by increasing the amount of surface hydroxyl groups on titanium and its surface energy.     

Apatite wollastonite–poly methyl methacrylate bio-composites

Bio-composites consisting of sol–gel processed apatite wollastonite (AW) glass ceramics and poly methyl methacrylate (PMMA) were prepared by hot compaction method. Density of the composites decreased with increase in PMMA content, while, biaxial flexural strength (BFS) was 21 MPa for 20 wt.% PMMA and beyond which it decreased. A correlation between phase compositions of AW glass ceramics with BFS was attempted from the XRD results. In vitro bioactivity of the composites in a simulated body fluid (SBF) showed the formation of spherical globules on the surface within 7 days of soaking as observed by environmental SEM. Thin film XRD and EDX measurement confirmed these globules to be bone like apatite with Ca/P ratio 1.53 and FTIR measurement showed the corresponding peaks for phosphates. Results indicated the bone bonding ability of the composites by forming a surface apatite (calcium phosphate) layer in SBF and the growth increased with increase in soaking durations. ICP measurement of the remaining SBF after 7, 14 and 21 days soaking of samples was found to be in good agreement with the EDX analysis results.     

PLD bioactive ceramic films: the influence of CaO–P2O5 glass additions to hydroxyapatite on the proliferation and morphology of osteblastic like-cells

This work consists on the evaluation of the in vitro performance of Ti6Al4V samples PLD (pulsed laser deposition) coated with hydroxyapatite, both pure and mixed with a CaO–P₂O₅ glass. Previous studies on immersion of PLD coatings in SBF, showed that the immersion apatite films did not present the usual cauliflower morphology but replicated the original columnar structure and exhibited good bioactivity. However, the influence of glass associated to hydroxyapatite concerning adhesion, proliferation and morphology of MG63 cells on the films surface was unclear. In this study, the performance of these PLD coated samples was evaluated, not only following the physical–chemical transformations resulting from the SBF immersion, but also evaluating the cytocompatibility in contact with osteoblast-like MG63 cells. SEM and AFM confirmed that the bioactive ceramic PLD films reproduce the substrate’s surface topography and that the films presented good adherence and uniform surface roughness. Physical–chemical phenomena occurring during immersion in SBF did not modify the original columnar structure. In contact with MG63 cells, coated samples exhibited very good acceptance and cytocompatibility when compared to control. The glass mixed with hydroxyapatite induced higher cellular proliferation. Cells grown on these samples presented many filipodia and granular structures, typical features of osteoblasts.     

Chemical deposition of silver shells on the surface of hollow glass microspheres

This article reports the chemical deposition of silver shells on the surface of hollow glass microspheres. This was accomplished by using titanium dioxide, derived from titanium (IV) tetra-n-butoxide, as the surface modifier. Two routes of formation of thin titanium dioxide layers on the substrate were compared in terms of composition and properties: photochemical approach and atmospheric hydrolysis. The structure and composition of the samples were characterized by atomic force microscopy, scanning electron microscopy, IR spectroscopy, electron paramagnetic resonance spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The resulting thickness of uniform silver shells on the surface of microspheres averaged out at 1.0–1.25 μm.