Biomed. Glasses 2019; 5:1–12 Research Article Roland Wetzel, Leena Hupa, and Delia S. Brauer* Glass ionomer bone cements based on magnesium-containing bioactive glasses https://doi.org/10.1515/bglass-2019-0001 Received Sep 25, 2018; revised Dec 16, 2018; accepted Jan 14, 2019 1 Introduction Abstract: Glass ionomer cements (GIC) are used in restora- Cements for prosthetic stabilisation or spinal corrective tive dentistry and their properties (low heat during setting, surgeries (vertebroplasty, kyphoplasty) typically consist adhesion to mineralised tissue and surgical metals) make of polymethylmethacrylate [1, 2]. They exhibit a number them of great interest for bone applications. However, den- of drawbacks which include high curing temperatures tal GIC are based on aluminium-containing glasses, and or the presence of unreacted and toxic methacrylic acid the resulting release of aluminium ions from the cements monomers. They also do not bind to bone and are held in needs to be avoided for applications as bone cements. Re- place by mechanical interlocking only [2–6]. As a result, placing aluminium ions in glasses for use in glass ionomer there is a demand for alternative non-toxic cements with cements is challenging, as aluminium ions play a critical bone bonding capability. role in the required glass degradation by acid attack as Glass ionomer cements (GIC) have been used in well as in GIC mechanical stability. Magnesium ions have restorative dentistry as filler or luting materials for been used as an alternative for aluminium in the glass decades [7, 8]. They are formed by an acid-base reaction component, but so far no systematic study has looked into between a polymeric acid and an acid-degradable fluoro- the actual role of magnesium ions. The aim of the present aluminosilicate glass [9]. Owing to their direct adhesion study is therefore the systematic comparison of the ef- to mineralised tissue these cements have aroused inter- fect of magnesium ions compared to calcium ions in GIC est as bone cements. However, while Al3+ ions play a key glasses. It is shown that by partially substituting MgO for role in cement setting, stability and performance, they are CaO in simple SiO2-CaO-CaF2 glasses, ion release from the known to be neurotoxic [10] and impede bone mineralisa- glass and, subsequently, GIC setting behaviour can be ad- tion [11–13]. Bioactive glasses may present an alternative justed. Magnesium ions act as typical network modifiers to these aluminosilicate glasses owing to their fast ion re- here but owing to their larger field strength compared to lease when in contact with aqueous solutions [14–16]. calcium ions reduce ion release from the glasses signifi- Efforts to substitute Al3+ ions with other multivalent cantly. By choosing an optimum ratio of magnesium and ions, e.g. Fe2+/3+ or Zn2+, have already been reported but calcium ions in the glass, GIC setting and subsequently the resulting cements exhibited drawbacks regarding ce- compressive strength can be controlled. ment stability or even cell toxicity [17–19]. By contrast, studies showed that GIC formed using Mg-containing Keywords: bioactive glasses; magnesium; dissolution; glasses revealed more promising properties [17, 20, 21]. continuous flow; glass ionomer cement Magnesium is an important element found e.g. in bone ap- atite. In addition, it is necessary in preventing osteoporo- sis [22, 23]. The aim of this study was to investigate the effect of MgO for CaO substitution on glass properties and particu- larly on GIC formation and properties. *Corresponding Author: Delia S. Brauer: Otto Schott Institute of Materials Research, Friedrich Schiller University, Fraunhoferstr. 6, 07743 Jena, Germany; Email: [email protected]; Tel.: +49-3641-948510 Roland Wetzel: Otto Schott Institute of Materials Research, Friedrich Schiller University, Fraunhoferstr. 6, 07743 Jena, Germany Leena Hupa: Johan Gadolin Process Chemistry Centre, Åbo Akademi University, Piispankatu 8, 20500 Turku, Finland Open Access. © 2019 R. Wetzel et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 License 2 Ë R. Wetzel et al. the dilatometric softening point (Td) and the thermal ex- 2 Materials and methods ∘ pansion coefficient (α; temperature range 100 to 400 C). Fine glass particles were heated in a furnace 2.1 Glass synthesis and basic (LT3/11/B410, Nabertherm GmbH, Lilienthal, Germany) characterisation to Tx, and the temperature was held constant for either 10 min or 3 hours before switching off the furnace and A series of glasses in the system SiO2-CaO-MgO-CaF2 was leaving the samples to cool down in the furnace over night. prepared via a melt-quench route by replacing one third Samples were analysed by powder X-ray diffraction (Mini- (glass 15Mg) or two thirds (glass 30Mg) of CaO with MgO flex 300, Rigaku Corporation, Tokio, Japan; CuKα, 30 kV, ∘ (Table 1). Mixtures of SiO2, CaCO3, CaF2 and MgCO3 were 20 mA, 5-75 2θ). ∘ sintered together in a platinum crucible at 1400 C using an inductively heated furnace and afterwards melted at Table 1: Nominal and analysed (in brackets) glass composition ∘ 1500 C for 1 hour. To prevent crystallisation the melts were (mol%). rapidly quenched in water. Glass powders were prepared using ball milling (KM1, Janetzki, milling time 30 min) and SiO2 CaO MgO CaF2 sieving with analytical sieves to obtain fine (≤ 38 µm) or 0Mg 45 45 0 10 coarse (38 µm ≤ x ≤ 63 µm) powders. (42.05 ± (47.18 ± - (10.77 ± Glass monoliths were prepared by re-melting the glass 1.54) 3.46) 1.41) ∘ frits at 1500 C. The melt was then cast into brass moulds, 15Mg 45 30 15 10 which for annealing were placed into a pre-heated fur- (43.76 ± (31.55 ± (15.27 ± (9.41 ± nace set to 30 K below the glass transition temperature 1.53) 2.42) 0.76) 1.14) (Tg). Glasses were cooled down to room temperature in the 30Mg 45 15 30 10 switched-off furnace overnight. Compositions of prepared (43.64 ± (16.79 ± (29.09 ± (10.45 ± glasses were analysed using energy-dispersive X-ray spec- 1.59) 2.38) 1.32) 1.32) troscopy (EDX; JOEL JSM-7001F scanning electron micro- scope equipped with an EDAX Trident analysing system) on polished monoliths. Glass density was measured us- ing helium pycnometry (AccuPyc 1330-1000, Micromeritics GmbH). Furthermore, the molar volume (Vm) of the glass 2.3 Static and dynamic dissolution series was calculated as described previously [24]. Glass experiments powders were characterised by powder X-ray diffraction (XRD; D5000, Siemens) and attenuated total reflectance For static dissolution tests 75 mg of coarse glass powder Fourier transform infrared spectroscopy (ATR-FTIR; Fron- was immersed in 50 mL cell culture medium (Minimum Es- ∘ tier IR/NIR, Perkin Elmer; ATR sample holder, Specac). sential Medium Eagle M4526, Sigma-Aldrich) at 37 C for ei- In addition, particle size distribution of fine and coarse ther 24 hours or 7 days. Glass powders were analysed using powder was determined by dynamic light scattering (DLS; ATR-FTIR after each immersion time point. Malvern Mastersizer 2000 equipped with HYDRO 2000S Early stage ion release was investigated by dynamic module). dissolution experiments, where a dynamic flow cell was connected to an inductively coupled plasma optical emis- sion spectrometer (Optima 5300 DV, Perkin Elmer) as de- 2.2 Glass thermal analysis and scribed previously [25–27]. The flow cell contained glass crystallisation powder (200 to 245 mg; normalised to 243 mg) with a grain size between 315 and 500 µm. Random packing was as- Glass transition temperature (Tg) and crystallisation peak sumed. On-line measurement and experimental set-up are temperature (Tx) were determined by differential ther- shown in detail elsewhere [25, 27, 28]. mal analysis (DTA; in-house made device, heating rate: 10 K min−1) using fine glass powder. Dilatometry (DIL 402 PC, Netzsch, heating rate: 2.4 GIC preparation 5 K min−1; measurements performed on glass rods 5 mm in diameter and 20 mm in length) was used to determine Tg, GICs were prepared by weighing and hand mixing glass powder and poly(vinylphosphonic acid-co-acrylic acid) Glass ionomer bone cements based on magnesium-containing bioactive glasses Ë 3 (PVPA-co-PAA; 40 wt% copolymer solution, weight aver- lost during melting. Substitution of MgO for CaO resulted age of chain length between 40 and 70 kDa according to in a decrease in density and molar volume (Figure 1). manufacturer, First Scientific Dental GmbH). A glass to polymer ratio of 1:3 by weight was used to form cements. The amount of Mg containing glasses was adjusted ow- ing to lower molecular weight of Mg compared to Ca. After mixing, the cement was filled into PTFE moulds (7 mmin ∘ height, 4 mm in diameter) and set for 1 hour at 37 C. Sam- ples were stored for up to 28 days either inside the mould, in deionised water or in 100% relative humidity (rH) as de- scribed elsewhere [21]. 2.5 GIC setting rate and mechanical testing Changes in cement structure during setting were analysed using ATR-FTIR. Spectra were acquired every 60 seconds Figure 1: Density and molar volume (V , blue, right axis) of the for the first 10 minutes, every 150 seconds for the following m glasses. 10 minutes and one spectrum at 25 minutes. The ratio of the peak heights of the carboxylate band at 1550 cm−1 and −1 the carboxyl band at 1700 cm were calculated to char- Results of particle size distribution measurements for acterise the setting rate of the different cement composi- fine and coarse glass powder are shown in Table 2. Nosig- tions [29–33]. nificant variation with glass composition was observed. For mechanical testing, specimens were stored for 1 ∘ day either inside the PTFE mould at 37 C or for 1 hour in- Table 2: Glass particle size distribution: cumulative volume percent- side the mould followed by 23 hours in either deionised age (D10, D50, D90, in µm) for fine and coarse powder.