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Understanding Composition–Structure–Bioactivity Correlations in Bioactive Glasses
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).ORCID iD: 0000-0003-3242-0205
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Bioactive glasses integrate with bone/tooth tissues by forming a layer of hydroxy-carbonate apatite (HCA), which mimics the composition of bone mineral. In the current thesis, we investigated composition–structure–bioactivity correlations of phosphosilicate and borophosphosilicate (BPS) glasses. Bioactive phosphosilicate glasses extend the compositional space of the ”45S5 Bioglass®”, which has been in clinical use for decades. Recently developed bioactive BPS glasses with SiO2→B2O3 substitutions transform more completely into HCA and their glass dissolution behaviors can be tuned by varying the relative contents of B and Si. 

It is known that the average silicate network connectivity NSi and the phosphate content (x(P2O5)) affect the apatite formation (in vitro bioactivity) of phosphosilicate glasses, but the details remain poorly explored. Three series of phosphosilicate glasses were designed by independently varying NSi and x(P2O5). After immersion of the glasses in a simulated body fluid (SBF) for 24 hours, different degrees of their apatite formation were quantified by Fourier-transform infrared (FTIR) spectroscopy. The results revealed that a high P content widened the NSi range that generated optimum amounts of apatite and also mitigated the detrimental effects associated with using glass particles with < 50 μm. The amounts of apatite derived from FTIR agreed with those from 31P magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The growth of apatite at bioactive glass surfaces was found to follow a sigmoidal growth model, in which the precursor phase, amorphous calcium phosphate (ACP), formed in the induction period and then crystallized into HCA in the following proliferation period, with an improvement in the structural ordering of HCA in the maturation period. This formation process closely resembles the apatite precipitated spontaneously from supersaturated Ca/P-containing solutions. The simultaneous growth of ACP and HCA is discussed in conjunction with a previously proposed mechanism for explaining in vitro bioactivity and apatite growth from bioactive glasses. 

The short- and medium- range structures of bioactive borophosphosilicate (BPS) glasses were investigated by solid-state MAS NMR. Two series of BPS glasses were designed by gradually replacing SiO2 with B2O3 in the 45S5 glass, as well as another base glass featuring a more condensed glass network. As the B2O3 content is increased, the glass networks become more polymerized, together with decreased fractions of the dominating BO3 and orthophosphate units. Borate groups are homogeneously mixed with the isolated orthophosphate groups, while the remaining phosphate groups exhibit a slight preference for bonding to BO4 over SiO4 units. Linkages among borate groups are dominated by B[3]–O–B[4] linkages at the expenses of B[3]–O–B[3] and B[4]–O–B[4] linkages, with the latter B[4]–O–B[4] motifs disfavored yet abundant. A similar fashion of borate mixing was observed in P-free Na/Ca-based borosilicate glasses that span a large compositional space. The content of B[4]–O–B[4] linkages was found to be controlled by the relative fractions of BO4 groups and non-bridging oxygen ions.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry, Stockholm University , 2018. , p. 76
Keywords [en]
Bioactive glasses, Phosphosilicate glasses, Borophosphosilicate glasses, Solid-state NMR spectroscopy, Glass structure, Fourier-transform infrared spectroscopy, Hydroxyapatite, Amorphous calcium phosphate, Apatite formation, In vitro bioactivity testing
National Category
Physical Chemistry
Research subject
Physical Chemistry
Identifiers
URN: urn:nbn:se:su:diva-161477ISBN: 978-91-7797-458-1 (print)ISBN: 978-91-7797-459-8 (electronic)OAI: oai:DiVA.org:su-161477DiVA, id: diva2:1259225
Public defence
2018-12-12, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2018-11-19 Created: 2018-10-29 Last updated: 2018-11-20Bibliographically approved
List of papers
1. Quantitative composition–bioactivity relationships of phosphosilicate glasses: Bearings from the phosphorus content and network polymerization
Open this publication in new window or tab >>Quantitative composition–bioactivity relationships of phosphosilicate glasses: Bearings from the phosphorus content and network polymerization
2018 (English)In: Journal of Non-Crystalline Solids, ISSN 0022-3093, E-ISSN 1873-4812, Vol. 502, p. 106-117Article in journal (Refereed) Published
Abstract [en]

Bioactive phosphosilicate glasses integrate with bone/tooth tissues by forming a bone-mineral mimicking surface layer of calcium hydroxyapatite (HA). The HA formation (“in vitrobioactivity”) in a simulated body fluid (SBF) solution is known to depend both on the P content (nP) and silicate network connectivity () of the glass, but the precise bioactivity–composition relationships remain poorly understood. We present a comprehensive study that clarifies the dependence of the in vitro bioactivity on the {nP, } parameters for Na2O–CaO–SiO2–P2O5 glass powders (2.6–6.0 mol% P2O5) exposed to SBF for 24 h, using infrared (IR) and solid-state 31P nuclear magnetic resonance (NMR) spectroscopy in conjunction with measured Ca and P concentrations in the solution. IR-derived relative apatite amounts reveal that an increase in the P content of the pristine glass promotes apatite formation by gradually reducing its dependence on the silicate network polymerization: increasing nP widens the  range that provides a high and nearly constant amount of HA, which scales roughly linearly with nP; these properties assist future design of P-rich bioactive glasses. All glasses provide significant HA formation for increasing  values up to ≈2.6, above which the in vitro bioactivity is lost due to insufficient glass dissolution. We also discuss the complex dependence of the SBF-testing outcome on the mass concentration and composition of the glass powder, as well as on its particle sizes, highlighting critical concerns that may guide developments of improved in vitro bioactivity-testing protocols. A strong dependence of the HA formation on the particle sizes is observed for glass powders with low P2O5 content (2.6 mol%), as opposed to their P-richer counterparts that reveal no pronounced particle-size effects.

Keywords
Bioactive glass, Composition-bioactivity relationships, Simulated body fluid, In vitro apatite formation, FTIR, 31P MAS NMR
National Category
Chemical Sciences
Research subject
Physical Chemistry
Identifiers
urn:nbn:se:su:diva-161475 (URN)10.1016/j.jnoncrysol.2018.07.060 (DOI)
Available from: 2018-10-27 Created: 2018-10-27 Last updated: 2018-11-27Bibliographically approved
2. Contrasting In Vitro Apatite Growth from Bioactive Glass Surfaces with that of Spontaneous Precipitation
Open this publication in new window or tab >>Contrasting In Vitro Apatite Growth from Bioactive Glass Surfaces with that of Spontaneous Precipitation
2018 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 11, no 9, article id 1690Article in journal (Refereed) Published
Abstract [en]

Body-fluid-exposed bioactive glasses (BGs) integrate with living tissues due to the formation of a biomimetic surface layer of calcium hydroxy-carbonate apatite (HCA) with a close composition to bone mineral. Vast efforts have been spent to understand the mechanisms underlying in vitro apatite mineralization, as either formed by direct precipitation from supersaturated solutions, or from BG substrates in a simulated body fluid (SBF). Formally, these two scenarios are distinct and have hitherto been discussed as such. Herein, we contrast them and identify several shared features. We monitored the formation of amorphous calcium phosphate (ACP) and its crystallization into HCA from a Na2O–CaO–SiO2–P2O5 glass exposed to SBF for variable periods out to 28 days. The HCA growth was assessed semi-quantitatively by Fourier transform infrared spectroscopy and powder X-ray diffraction, with the evolution of the relative apatite content for increasing SBF-exposure periods evaluated against trends in Ca and P concentrations in the accompanying solutions. This revealed a sigmoidal apatite growth behavior, well-known to apply to spontaneously precipitated apatite. The results are discussed in relation to the prevailing mechanism proposed for in vitro HCA formation from silicate-based BGs, where we highlight largely simultaneous growth processes of ACP and HCA.

Keywords
bioactive glass, biomimetic mineralization, apatite growth mechanism, simulated body fluid, quantification of apatite content, infrared spectroscopy, X-ray diffraction
National Category
Chemical Sciences
Research subject
Physical Chemistry
Identifiers
urn:nbn:se:su:diva-161476 (URN)10.3390/ma11091690 (DOI)000446395200206 ()
Available from: 2018-10-27 Created: 2018-10-27 Last updated: 2018-11-12Bibliographically approved
3. Structure-composition relationships of bioactive borophosphosilicate glasses probed by multinuclear B-11, Si-29, and P-31 solid state NMR
Open this publication in new window or tab >>Structure-composition relationships of bioactive borophosphosilicate glasses probed by multinuclear B-11, Si-29, and P-31 solid state NMR
2016 (English)In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 6, no 103, p. 101288-101303Article in journal (Refereed) Published
Abstract [en]

By combining B-11, Si-29, and P-31 nuclear magnetic resonance (NMR) experimental results, we present a comprehensive structural investigation of 15 borophosphosilicate (BPS) glasses of the Na2O-CaOB2O3- SiO2-P2O5 system: in two base compositions comprising 46 mol% (S46) and 49 mol% (S49) SiO2, progressive replacements of SiO2 by B2O3 were performed at a constant total Na2O and CaO content. The S46 glass members constitute B-bearing analogs of 45S5 Bioglass that is utilized extensively for bone grafting in periodontal and orthopedic surgery. Orthophosphate ions prevail throughout all structures, while the silicate network polymerization increases slightly with a growing amount of B2O3 in the glass. B-11 NMR revealed continuous BO3 -> BO4 conversions for increasing B2O3 content, with asymptotic fractions of 34% and 43% of B-[4] coordinations out of the borate speciation observed for the series of S46 and S49 glasses, respectively. While all BPS glasses are homogeneous across a mm-scale, strong preferences for B-[3]-O-B-[3] and B-[4]-O-Si-[4] bond formation lead to structures comprising (sub) nm-sized domains of BO3 groups in boroxol rings and borosilicate networks built by SiO4 and BO4 tetrahedra. These borate/ borosilicate networks are merged mainly by B-[4](3Si) and B-[3](1Si) moieties in Si-rich BPS glasses (where each value in parentheses specifies the number of bonds to Si atoms), while B-[4](3Si) and B-[4](2Si) groups are the dominant network contact points in the B-rich glasses. We discuss the partitioning of non-bridging oxygen ions between the BO3 and SiO4 groups, the relative propensities for B-[4]-O-Si-[4] and B-[4]-O-B-[3] bond formation, as well as the expected bearings of our proposed BPS structural model for the glass degradation in aqueous media, where we identify the fractional population of B-[4] coordinations and the silicate network connectivity to constitute the dissolution-controlling parameters.

National Category
Chemical Sciences
Research subject
Physical Chemistry
Identifiers
urn:nbn:se:su:diva-136912 (URN)10.1039/c6ra15275a (DOI)000387427700073 ()
Available from: 2017-01-04 Created: 2016-12-19 Last updated: 2018-10-31Bibliographically approved
4. Medium-Range Structural Organization of Phosphorus-Bearing Borosilicate Glasses Revealed by Advanced Solid-State NMR Experiments and MD Simulations: Consequences of B/Si Substitutions
Open this publication in new window or tab >>Medium-Range Structural Organization of Phosphorus-Bearing Borosilicate Glasses Revealed by Advanced Solid-State NMR Experiments and MD Simulations: Consequences of B/Si Substitutions
2017 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 121, no 41, p. 9737-9752Article in journal (Refereed) Published
Abstract [en]

The short and intermediate range structures of a large series of bioactive borophosphosilicate (BPS) glasses were probed by solid-state nuclear magnetic resonance (NMR) spectroscopy and atomistic molecular dynamics (MD) simulations. Two BPS glass series were designed by gradually substituting SiO2 by B2O3 in the respective phosphosilicate base compositions 24.1Na(2)O-23.3CaO-48.6SiO(2)-4.0P(2)O(5) (S49) and 24.6Na(2)O-26.7CaO-46.1SiO(2)-2.6P(2)O(5) (S46), the latter constituting the 45S5 Bioglass utilized for bone grafting applications. The BPS glass networks are built by interconnected SiO4, BO4, and BO3 moieties, whereas P exists mainly as orthophosphate anions, except for a minor network-associated portion involving P-O-Si and P-O-B-[4] motifs, whose populations were estimated by heteronuclear P-31{B-11} NMR experimentation. The high Na+/Ca2+ contents give fragmented glass networks with large amounts of nonbridging oxygen (NBO) anions. The MD-generated glass models reveal an increasing propensity for NBO accommodation among the network units according to BO4 < SiO4 < BO3 << PO4. The BO4/BO3 intermixing was examined by double-quantum-single-quantum correlation B-11 NMR experiments, which evidenced the presence of all three BO3-BO3, BO3-BO4, and BO4-BO4 connectivities, with B-[3]-O-B-[4] bridges dominating. Notwithstanding that B-[4]-O-B-[4] linkages are disfavored, both NMR spectroscopy and MD simulations established their presence in these modifier-rich BPS glasses, along with non-negligible B-[4]-NBO contacts, at odds with the conventional structural view of borosilicate glasses. We discuss the relative propensities for intermixing of the Si/B/P network formers. Despite the absence of pronounced preferences for Si-O-Si bond formation, the glass models manifest subtle subnanometer-sized structural inhomogeneities, where SiO4 tetrahedra tend to self-associate into small chain/ring motifs embedded in BO3/BO4-dominated domains.

National Category
Chemical Sciences
Research subject
Physical Chemistry
Identifiers
urn:nbn:se:su:diva-148999 (URN)10.1021/acs.jpcb.7b06654 (DOI)000413617800028 ()28876931 (PubMedID)
Available from: 2017-11-27 Created: 2017-11-27 Last updated: 2018-10-31Bibliographically approved
5. Direct Experimental Evidence for Abundant BO4–BO4 Motifs in Borosilicate Glasses From Double-Quantum 11B NMR Spectroscopy
Open this publication in new window or tab >>Direct Experimental Evidence for Abundant BO4–BO4 Motifs in Borosilicate Glasses From Double-Quantum 11B NMR Spectroscopy
2018 (English)In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 9, p. 6372-6376Article in journal (Refereed) Published
Abstract [en]

By using double-quantum–single-quantum correlation 11B nuclear magnetic resonance (NMR) experiments and atomistic molecular dynamics (MD) simulations, we resolve the long-standing controversy of whether directly interlinked BO4–BO4 groups exist in the technologically ubiquitous class of alkali/alkaline-earth based borosilicate (BS) glasses. Most structural models of Na2O–B2O3–SiO2 glasses assume the absence of B[4]–O–B[4] linkages, whereas they have been suggested to exist in Ca-bearing BS analogs. Our results demonstrate that while B[4]–O–B[4] linkages are disfavored relative to their B[3]–O–B[3]/B[4] counterparts, they are nevertheless abundant motifs in Na2O–B2O3–SiO2 glasses over a large composition space, while the B[4]–O–B[4] contents are indeed elevated in Na2O–CaO–B2O3–SiO2 glasses. We discuss the compositional and structural parameters that control the degree of B[4]–O–B[4] bonding.

National Category
Chemical Sciences
Research subject
Physical Chemistry
Identifiers
urn:nbn:se:su:diva-161473 (URN)10.1021/acs.jpclett.8b02907 (DOI)
Available from: 2018-10-27 Created: 2018-10-27 Last updated: 2018-10-31Bibliographically approved

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