Atomic and vibrational origins of mechanical toughness in bioactive cement during setting

Abstract

Bioactive glass ionomer cements (GICs) have been in widespread use for ∼40 years in dentistry and medicine. However, these composites fall short of the toughness needed for permanent implants. Significant impediment to improvement has been the requisite use of conventional destructive mechanical testing, which is necessarily retrospective. Here we show quantitatively, through the novel use of calorimetry, terahertz (THz) spectroscopy and neutron scattering, how GIC's developing fracture toughness during setting is related to interfacial THz dynamics, changing atomic cohesion and fluctuating interfacial configurations. Contrary to convention, we find setting is non-monotonic, characterized by abrupt features not previously detected, including a glass-polymer coupling point, an early setting point, where decreasing toughness unexpectedly recovers, followed by stress-induced weakening of interfaces. Subsequently, toughness declines asymptotically to long-term fracture test values. We expect the insight afforded by these in situ non-destructive techniques will assist in raising understanding of the setting mechanisms and associated dynamics of cementitious materials.

Figure 1. GICs properties and glass nanoscopic structure.

(a) GIC restorative, including occlusal view. (b) G338 fluoro-alumino-silicate glass powder and dangling aqueous polymer (acrylic acid). (c) Fracture toughness K_C versus strength σ_Y for dental materials compiled as an Ashby plot: dentin, glass and polymers, GICs and amalgam. (d) Fracture energy G_c versus Poisson's ratio ν and the brittle–ductile transition, expanded for a wide range of materials; ν values for dental materials combined with G_C values; toughness decline during setting indicated by the dashed arrow; trends for annealing and polymerization (solid arrows). a and b courtesy Faculty of Dentistry, Semmelweis University.

Figure 2. GIC characterization.

(a) TEM image of individual glass particle (above, scale bar, 100 nm) with three distinguishable glass phases: GP1, GP2 and GP3 seen on an expansion of the white frame area (below, scale bar, 20 nm, see text for details). (b) Three DSC upscan curves for the fresh G338 glass (red), the G338 glass subjected to the first up- and downscans (blue), and the GIC sample (green) subjected to 62 h setting and the subsequent up- and downscans, respectively. The red curve exhibits a water-loss endothermic response, followed by an exothermic enthalpy-release response; the blue upscan-2 curve reveals three sharp glass transitions T_g1, T_g2 and T_g3, which we associate with the glass phases GP1, GP2 and GP3, respectively; the green curve reveals how glass transitions are modified by setting. Both up- and downscan rates are 20 K min⁻¹. (c) Coherent terahertz spectroscopy: changing sub-THz relative reflectance during setting, differentiating gel formation (Ca⁺² and Al³⁺release) from chelation (Al⁺³ release); minimum (CP) identifies the point where glass and polymer couple dynamically.

Figure 3. Non-monotonic advancement in atom cohesion and fracture toughness during GIC solidification.

(a) Overall GIC NCS peak width Δp^av variations with setting time, measuring different stages in atomic cohesion during setting: CP, ISP and ISZ (see text for details). (b) Inverse relationship between atomic cohesion Δp^av and fracture toughness K_C^av of GIC, polymer and glass (asterisks) and associated materials (see text for details). (c) Non-monotonic fall in overall K_C^av with setting time at 300 K obtained from overall Δp^av a using b, identifying CP and reaction points ISP and ISZ. (d) K_C^av(280 K), K_C^av(300 K) and K_C^av(320 K), showing shifts in ISP and ISZ with setting temperature. (e) Fluctuations in K_Cⁱ for H and F, showing evidence for hydration and fluorination. (f) K_Cⁱ for O and Al through the various setting stages. Elemental fracture toughness K_Cⁱ were derived from Δp_i values (Supplementary Fig. 3a), with K_C^av (280, 300 and 320 K) obtained from Δp^av (280, 300 and 320 K) values (Supplementary Fig. 3b), in each case using b.

Figure 4. Neutron scattering measurements.

Time-averaged (a) S(Q) and (b) G(r), including FSDP and expected locations for nearest- and next-nearest-neighbour pair correlations NN and NNN, respectively. (c) In situ time-resolved S(Q) over 24 h revealing changes in small-angle Porod scattering (SANS) at interfaces, and in the position of FSDP for the glass. (d) Fluctuations with setting in the FSDP position and the integrated SANS from c (Methods). Abrupt changes around 15 h coincide with the Δp^av minimum and K_C^av maximum (Fig. 4a,c). (e) In situ time-resolved G(r) with NN distances for GIC related to glass (upper) and aqueous polymer (lower). (f) Trends in ΔG(r) obtained by differencing G(r) across the ISZ (from d)—splines for guiding the eye. a,b and e data are all offset vertically for clarity of presentation.