Materials Letters 282 (2021) 128843
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Materials Letters
journal homepage: www.elsevier.com/locate/mlblue
Preparation and characterization of sintered bioactive borate glass tape
Susanta Sengupta a,⇑, Martin Michalek a,⇑, Liliana Liverani b, Peter Švančárek a,
Aldo R. Boccaccini b, Dušan Galusek a,c
a
Centre for Functional and Surface Functionalized Glass, Alexander Dubcek University of Trenčín, Slovakia
Institute of Biomaterials, Department of Materials Science and Engineering, University of Erlangen-Nuremberg, Erlangen, Germany
c
Joint Glass Centre of the IIC SAS, TnUAD and FChFT STU, FunGlass, Trenčín, Slovakia
b
a r t i c l e
i n f o
Article history:
Received 27 August 2020
Received in revised form 4 October 2020
Accepted 11 October 2020
Available online 16 October 2020
Keywords:
Tape casting
Borate bioactive glass
Bioactivity
Densification
a b s t r a c t
Non-aqueous tape casting was used to prepare flexible green tapes from borate glass (1393B3) particles.
The tapes were sintered at 550, 575, 600 °C: dense tapes were prepared at 600 °C. In-vitro bioactivity in
simulated body fluid (SBF) of tapes sintered at different temperatures was evaluated. SEM, FTIR and XRD
results showed slow degradation of dense borate glass tape in SBF compared to the non-dense tapes sintered at different temperatures. Formation of HCA (hydroxy carbonated apatite) on sample surfaces was
confirmed after 7D in SBF.
Ó 2020 Elsevier B.V. All rights reserved.
1. Introduction
Bioactive glasses are investigated due to their ability to form
bone-like mineral, HCA on their surface and integrate with bone
in-vivo for bone tissue engineering applications [1]. Along with
the well known 45S5 bioactive glass (BG), other glass systems
like borates and borosilicates have also proven to be bioactive
[2–4]. Borate glass degrades faster than silicate bioactive glasses
and converts completely to HA [2]. Moreover, boron is an essential element for bone growth and supports new bone formation
by enhancing osteoblast proliferation [5]. Borate glass scaffolds
also facilitate vascularization by inducing angiogenesis along
with new bone formation at defected bone site [6]. Moreover,
H3BO3 formed by degradation of borate-based glasses acts as
an effective antiseptic aiding the wound healing process. Borate
glass scaffolds can be thus considered suitable for both bone and
soft tissue engineering applications [7,8]. However, they degrade
rapidly and a sudden increase in ionic concentration (BO33 ) can
be cytotoxic [9].
Tape casting is a simple alternative to additive manufacturing
for shaping bioactive glass into a desired form. In previous studies, 45S5 BG tapes were prepared and the laminated tapes were
sintered to study the effect of sintering conditions on bioactivity
⇑ Corresponding authors.
E-mail addresses: susanta.sengupta@tnuni.sk (S. Sengupta), martin.michalek@
tnuni.sk (M. Michalek).
https://doi.org/10.1016/j.matlet.2020.128843
0167-577X/Ó 2020 Elsevier B.V. All rights reserved.
[10,11]. Sintering profile was found to be a key factor for cell survival, proliferation, and bone formation [12]. Dense glass tapes
can be used to moderate glass degradation in body fluids. Due
to its versatility, tape casting offers additional opportunities,
considering the role of ordered porous structures in tissue
engineering. Preparation of porous ceramic thin films by using
sacrificial pore-forming agents have been reported [13].
Complex-shaped parts with variable compositions in layered
form can be obtained by building up stacked layers of green
tapes followed by advanced cutting through laser or computeraided machining [14].
Here we report on the use of non-aqueous tape casting for
preparation of borate glass tapes. Their sintering profile was optimized to obtain a fully dense structure to decrease their degradability. The bioactivity of sintered tapes was evaluated by invitro tests in SBF. Reported sintering profiles could be useful for
large scale preparation of 3D structures of tape cast borate glass
with superior mechanical strength.
2. Materials and methods
1393B3 glass with a composition of (53B2O3, 6Na2O, 20CaO,
12K2O, 5MgO, 4P2O5 in wt. %) was melted from the pre-mixed analytical grade purity (99%, CentralChem, Slovakia) raw materials:
H3BO3, Na2CO3, CaCO3, K2CO3, MgO, and P2O5 in a Pt-10%Rh crucible at 1100 °C for 2 h. The glass was annealed at 550 °C for
30 min, crushed and sieved through 25 mm sieve.
S. Sengupta, M. Michalek, L. Liverani et al.
Materials Letters 282 (2021) 128843
The thermal analysis confirmed that organic additives are
removed by thermal treatment of green tapes at T < 500 °C so
any organic or carbon residue could be eliminated before sintering
(Fig. 1B). The thickness of the green tape was 215 ± 7 mm. Shrinkage values in sample thickness 33, 44, and 54% were observed after
sintering at 550, 575 and 600 °C, respectively.
Fig. 2A shows SEM micrographs of sintered glass tapes (crosssectional views); the surfaces of the tapes are shown in insets.
The microstructures indicate that viscous flow contributed to densification of the glass matrix at T 575 °C. Its contribution
increased with the temperature, and at 600 °C almost dense glass
tapes with few isolated spherical pores were obtained.
SEM micrographs after immersion of samples in SBF indicate
complete coverage of B550 and B575 tapes by HCA (Fig. 2B). Isolated HCA crystals form at the surface of the dense B600 tape,
due to lower available surface area to react with SBF.
Two diffraction lines at 2H ~ 26 and 32° attributed to HCA
phase were observed for B550 and B575 after 3 days of immersion
in SBF [2,7]. No diffraction lines corresponding to HCA formation
were detected for B600 after 7 days in SBF (Fig. 3A).
FTIR spectra (Fig. 3B) revealed some changes in chemical bonding of borate glass matrix and formation of hydroxyapatite after
immersion in SBF. The main vibration bands at 850–1100 cm 1,
1330–1450 cm 1, and 680–750 cm 1 respectively correspond to
the B-O stretching mode of BO4 and BO3, and bending mode of
BO3 [7]. The bands at ~ 1020 and 550–600 cm 1 belong to asymmetric stretching and bending modes of the PO34 [4,17]. After
3 days immersion of B550 and B575 in SBF the bands attributed
the BO4 and BO3 almost disappeared, while the band attributed
to BO4 is noticeable even after 7 days for B600, indicating slower
degradation of the dense tape. After immersion in SBF the PO34
bands dominate over other resonances in all samples. However,
for B550 and B575, the resonance at 550–600 cm 1 becomes more
intense with immersion time and splits into two peaks at 560 and
600 cm 1, confirming the formation of crystalline HCA [7]. No
splitting was observed for B600, indicating poor crystallinity of
HCA or nanocrystals formation. The results indicate moderate formation of HCA with time due to the slow degradation of dense
glass matrix of B600.
The suspension for tape casting was prepared following the procedure described in [15] and contained 25 vol% of 1393B3 glass
powder, 9.3 vol% of PEG-300 (Sigma-Aldrich) dispersant/plasticizer
and 7.7 vol% of PVB (Mowital B-30, Kuraray Europe, Germany) binder dissolved in abolute ethanol.
The suspension of glass powder with ethanol was homogenized
in PE bottle on rollers with SiO2 milling balls (5 mm diameter).
After 4 h binder and plasticizer were added and milled for another
20 h.
The suspension was tape cast on a 75 mm silicon-coated carrier
foil (MylarÒ, Germany) at a speed of 0.12 cm/s and a 400 mm gap of
the doctor blade. The tape was dried for 48 h at room temperature,
and sintered in air at 550, 575, and 600 °C with heating rate 1 °C min 1 without dwell time. The samples are labeled as B550,
B575, and B600.
The rheological behavior of the suspension was analyzed with a
rotational rheometer (Rheometer-viscometer- Haake Mars III) at
22 °C, measuring 15 points in the shear rate range 5–125 s 1. Thermal analysis measurements were performed using the Perkin
Elmer DSC 8500 calorimeter in the temperature range 25–800 °C
at a heating rate of 3 °C min 1 in air. The microstructure and thickness of the tapes was examined by scanning electron microscopy
(JEOL, JSM-7600F, Japan). The phase composition was determined
by X-ray powder diffraction (XRD) (Rigaku, MiniFlex 600, Japan)
in the 2b range of 20–80°. The borate glass network was characterised by Fourier transform infrared spectroscopy (FTIR;
IRAffinity-1S, SHIMADZU, Japan) in the wavenumber range 4000
to 400 cm 1, averaging 40 spectral scans with a resolution of
4 cm 1.
Sintered borate glass tapes (discs of 12 mm in diameter) were
immersed in 23 ml of SBF prepared according to Kokubo et al.
[16], placed in an incubator at 37 °C for 3 and 7 days, and washed
twice with deionized water and EtOH before drying.
3. Results and discussion
Rheological study confirmed pseudoplastic behaviour of the
suspension (Fig. 1A), indicating the suitability of the suspension
for tape casting [15].
Fig. 1. A) Viscosity and shear stress of non-aqueous slurry of borate glass particulates, B) TG/DSC curves of green 1393B3 glass tape. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article.)
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Materials Letters 282 (2021) 128843
S. Sengupta, M. Michalek, L. Liverani et al.
Fig. 2. A) Microstructure of borate glass tapes after sintering at 550 °C (B550), 575 °C (B575) and 600 °C (B600), and B) B550-7D, B575-7D, B600-7D after 7 days immersion in
SBF.
Fig. 3. A) XRD patterns, and B) FTIR spectras of borate glass tapes before and after (3 and 7 days) immersion in SBF (The peaks are discussed in the text).
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Materials Letters 282 (2021) 128843
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4. Conclusions
1393B3 borate glass tapes with various porosities were prepared by tape casting and viscous flow sintering at 550 – 600 °C.
The formation of HCA during in vitro tests in SBF confirmed the
influence of porosity on degradation rate and HCA formation, with
significant reduction of degradation in dense tapes. In future, cell
culture with a quantitative degradation study can establish the link
between bioactivity and degradation of sintered borate tape cast
materials.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgments
This paper is a part of dissemination activities of the project
FunGlass. This project has received funding from the European
Uniońs Horizon 2020 research and innovation programme under
grant agreement No 739566. Financial support of this work by
the grants SAS-MOST JRP 2018/02, and VEGA 1/0098/19 is gratefully acknowledged.
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