Submicrometer Hollow Bioglass Cones Deposited by Radio

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Sub-micrometre hollow bioglass cones deposited by radiofrequency magnetron sputtering: Formation mechanism, properties and prospective biomedical applications Adrian-Claudiu Popa, George E. Stan, Cristina Besleaga, Lucian Ion, ValentinAdrian Maraloiu, Dilshat U Tulyaganov, and José Maria da Fonte Ferreira ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00606 • Publication Date (Web): 02 Feb 2016 Downloaded from http://pubs.acs.org on February 4, 2016

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ACS Applied Materials & Interfaces

Sub-micrometre hollow bioglass cones deposited by radio-frequency magnetron sputtering: Formation mechanism, properties and prospective biomedical applications A. C. Popa a,b, G. E. Stan a,*, C. Besleaga a, L. Ion c, V. A. Maraloiu a, D. U. Tulyaganov e, J. M. F. Ferreira d,* a

National Institute of Materials Physics, 077125 Magurele–Ilfov, Romania b

c d

Army Centre for Medical Research, 010195 Bucharest, Romania

University of Bucharest, Faculty of Physics, 077125 Magurele–Ilfov, Romania

Department of Materials and Ceramic Engineering, CICECO, University of Aveiro, 3810–193 Aveiro, Portugal e

Turin Polytechnic University in Tashkent, 100095 Tashkent, Uzbekistan

Abstract This work reports on the unprecedented magnetron sputtering deposition of submicrometric hollow cones of bioactive glass at low temperature in the absence of any template or catalyst. The influence of sputtering conditions on the formation and development of bioglass cones was studied. It was shown that larger populations of welldeveloped cones could be achieved by increasing the argon sputtering pressure. A mechanism describing the growth of bioglass hollow cones is presented, offering the links for process control and reproducibility of the cone features. The composition, structure and morphology of the as-synthesized hollow cones were investigated by EDS, FTIR, GIXRD, SEM and TEM-SAED. The in vitro biological performance, assessed by degradation tests (ISO 10993-14) and cytocompatibility assays (ISO 10993-5) in endothelial cell cultures, was excellent. This allied with resorbability and the unique morphological features make the sub-micrometre hollow cones interesting candidate material devices for focal transitory permeabilization of the blood-brain barrier in the treatment of carcinoma and neurodegenerative disorders. Keywords: Bioglass cones; sputtering; surfaces; structure; sub-micrometre needles. *Corresponding authors: E-mail: [email protected] (G.E. Stan), Tel: +40–724–131131; Fax: +40–21–3690177. [email protected] (J.M.F. Ferreira); Tel.: +351–234–370242; Fax: +351–234–370204.

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1. Introduction Bioactive glasses (BGs) were discovered in late 1969s and demonstrated the ability to bond to living bone1 and soft tissues2. The formulation [(wt.%) 45SiO2– 24.5Na2O–24.5CaO–6P2O5] with the highest bonding ability to hard and soft tissues has been approved by the USA Food and Drug Administration (FDA) for clinical use in 1985, and is trade marketed as 45S5 Bioglass®. This pioneering work opened a new and very active area of research and worldwide discussion about the immense benefactor potential of BGs. Nowadays BGs are available in a variety of forms (bulk materials, porous scaffolds, pellets, particulates, etc.)3–7 and applications in healthcare range from cochlear implants to drug delivery systems3,8. Due to their ability to gradually resorb after implantation, BGs were originally proposed as transient non-load-bearing bone reconstruction materials.2–9 The resorption rate can be engineered by compositional and structural design, enabling BGs to act as ion sources for specific genes activation. Recent research efforts on this topic aimed at developing BG implant coatings with reliable mechanical and biological properties. To achieve this goal several coating techniques have been attempted, some well-established, such as enamelling10, plasma spray11, electrophoretic deposition,12,13 pulsed laser ablation,14,15 radio-frequency magnetron sputtering (RFMS),16–18 and sol-gel19; and novel approaches, including matrix assisted pulsed laser evaporation,20 laser cladding,21 high velocity suspension flame spraying,22 and sponge replication23. There is also a growing interest towards the fabrication of amorphous24,25 and crystalline26,27 nano- or sub-micrometre sized organic or inorganic objects. The plethora of reported shapes includes tubes, fibres, wires, whiskers, grasses, belts, domes, pillars, cones, spheres, sheets, needles or cones.24–32 Their realm of applications expanded greatly


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in the recent period, encompassing now almost all facets intersecting with modern life, including optics, electronics, photovoltaics, photonics, sensing, spintronics and biotechnology.24–32 It the biomedical field, the recent development of bioglass nanospheres33 and nano-fibers34–36 with potential use in drug delivery systems and biodegradable scaffolds can be mentioned. The fabrication methods of low dimensional objects can be divided according to the condensation state of the deposition environment in: (i) gas phase reaction processes [e.g., chemical vapour deposition (CVD),26,27,37 microwave plasma evaporation,29 ultra-high vacuum evaporation,38 and (ii) solution reaction processes (e.g., electro-deposition,30,39,40 dip coating,41 electrospinning34–36). Micro-needles constitute presently one of the hottest topics of biomedical research.42–47 They allow painless transdermal delivery of vaccines and drugs, and avoid the emotional trauma of multiple injections of large doses of active substances with potential irritating or even toxic outcomes.43,44 The materials of choice for the fabrication of micro-needles are mainly silicon, various metals and biodegradable polymers.43,44 Their design evolves continuously along with their potential applications, and four varieties have been identified43−45: (i) in solid form – for dermal treatment prior to the medicine administration; (ii) drug-coated devices – for the dissolution of medication into the skin; (iii) hollow micro-needles – for the injection or active substances; (iv) resorbable micro-needles fabricated from polymeric materials embedded with medicine – for a controlled release. The micro fabrication methods often rely on the adaptation of technological protocols currently used in micro-nano-electronics, such as masking and dry or wet etching, lithography, or micro-moulding.43–47 CVD is also prone to produce various


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miniaturized objects of various shapes.26,27,37 The high electrical power consumption, and the high costs of equipment, and the used of potential toxic gaseous reagents or catalysts that might originate hazardous by-products42, are the main drawbacks of CVD relative to other coating techniques such as RFMS. Here we unveil the unprecedented fabrication of bioglass hollow sub-micrometre cones with inner nano-sized channels onto titanium substrates by RFMS at low temperature, in the absence of any specially designed template or catalyst. This technique enabled the deposition of strongly adherent BG coatings16 with tailored resorbability in body fluids. We realised that the morphological features of the BG coatings could be controlled by manipulated the experimental sputtering parameters, justifying RFMS to be proposed as an alternative manufacturing technique of hollow sub-micrometre BG needles. Such needles could be thought to deliver through the bloodstream a drug or vaccine to specific regions, determine by their low size, the piercing and permeabilization of biological “barriers” of interest, thus allowing directional access of medication and treatment until their full solubilisation. The main goal of this study is to demonstrate the ability of RFMS as an alternative cheap method for producing hollow sub-micrometre BG needles to stir the interest of scientific community and thus create the premises for further insightful investigations. The biological features were assessed by degradation tests and cytocompatibility assays, and the structural and compositional data are discussed in detail.


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2. Materials and methods 2.1 Magnetron cathode target preparation The cathode targets (110 mm diameter, 3 mm thick) were manufactured by mildpressing at RT into tantalum dishes a novel BG powder48,49, having the following composition (mol.%): SiO2–45.45, CaO–30.3, MgO–12.99, P2O5–2.6, CaF2–4.33, and Na2O–4.33. Such a target preparation technique is both time and cost efficient, and preserved the target’s consistency during the sputtering processes.16,50,51

2.2 RFMS deposition procedure Square titanium (Ti, Mateck GmbH) substrates were used with five different surface roughnesses: as-manufactured, grinded with abrasive papers (SiC P400, P1200, P2500), and mirror polished. The substrates were firstly degreased with acetone in an ultrasonic cleaner, subsequently washed in ethanol and in distilled water, and finally mechanically fixed into the deposition chamber. The BG deposition was performed by RFMS using a UVN-75R1 (1.78 MHz) system. The sputtering chamber was firstly evacuated to a pressure of ~2–3×10−3 Pa. Then argon and oxygen of high purity grade (99.9999%) were admitted at various flow rates in order to obtain the designed deposition atmospheres and total pressures, described in Table 1. Prior to deposition, the Ti substrates were plasma etched in argon atmosphere for 10 min, aiming at improving the BG adhesion. A long target presputtering time (60 min) under the same atmosphere conditions used for coatings was employed to grant reproducible composition transfer. Constant sputtering power (75 W), target-to-substrate distance (25 mm), and room temperature (RT) were used. Due to thermal radiation derived from the plasma bombardment during the deposition processes,


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the substrate heated up to a maximum temperature of ~100 ºC. The deposition rates were previously determined by spectroscopic ellipsometry measurements for films deposited on silicon wafers. The deposition time in this study was adjusted to obtain an equivalent thickness of ~600 nm. The same sputtering parameters assured an optimised target-tosubstrate atomic transfer by RFMS as thoroughly discussed in a recent paper.16 The mass of deposited BG was assessed by weighting each Ti substrate before and after deposition. Table 1: Sputtering conditions for BG coatings.

Sample batch code BG2 BG3 BG4 BG3O10 BG3O15 BG3O20

Working atmosphere (volumetric per cents)

Total pressure (Pa)

Deposition rate (nm/min)

100% Ar 100% Ar 100% Ar 90%Ar+10%O2 85%Ar+15%O2 80%Ar+20%O2

0.2 0.3 0.4 0.3 0.3 0.3

~8.0 ~6.2 ~5.5 ~5.6 ~5.2 ~4.5

2.3 Physico-chemical characterization of sputtered BG structures The characteristics of the structures deposited were assessed by a broad array of techniques. Morphology, microstructure and composition of BG films were analysed by scanning electron microscopy (SEM, Hitachi SU-70 analytical microscope), transmission electron microscopy (TEM, probe corrected JEM ARM 200F analytical microscope), and energy dispersive spectroscopy (EDS, Bruker QUANTAX 400 model for SEM apparatus, and JEOL JED-2300T unit for TEM machine), respectively. Samples for TEM analysis were prepared by mechanical scraping the surface of the coatings with a diamond blade on an ethanol wetted grid. This extraction procedure also enabled analysing the structure, composition and size of individual broken cones, conical frustums and small fragments of the coating matrix. Quintuplicate EDS analyses were performed in randomly selected


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regions onto the surfaces of the coatings. The results are presented as mean ± standard deviation. Statistical analysis was performed by using the unpaired Students t-test and differences were considered significant at p

Submicrometer Hollow Bioglass Cones Deposited by Radio

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