Bioinspired mechanical mineralization of organogels

Mineralization in biological systems provides mechanical resistance and structural support through hierarchically organized mineral growth that adapts to mechanical stress (e.g., bone, shells, exoskeletons). Inspired by this, synthetic systems have sought adaptable mineralization, especially in hydrogels for tissue engineering (e.g., hydroxyapatite growth) and via advanced manufacturing such as 3D printing. Mechanically mediated mineralization in synthetics has been rarely demonstrated; a notable example is a piezoelectric polymer scaffold that mineralized apatite from surrounding media under load, enabling spatial control via stress distribution. Mineralization methods in synthetic composite materials within organic solvents are scarce, with prior work including metal/inorganic nanoparticle growth in organogels and CO2-induced carbamate crosslinking in polyethyleneimine organogels. To translate biomineralization concepts into organic, manufacturing-relevant environments, the authors sought in situ mineral fabrication within polymer matrices to tailor composite mechanics. Prior work from the authors and others used piezoelectric nanoparticles to mediate polymerizations via the piezoelectrochemical effect, including thiol-ene and thiol–disulfide reactions using ZnO under controlled vibration/ultrasound. Extending this chemistry with a redox-sensitive thiol, 2-mercapto-5-methyl-1,3,4-thiadiazole (McMT), they discovered that ZnO nanoparticles react under mechanical stimulation to form rod-shaped crystalline microparticles (microrods) composed of a Zn/McMT coordination compound. The study aims to elucidate microrod formation and composition, examine growth mechanisms and rheology, and demonstrate in situ mineralization within polymer solutions and organogels to achieve mechanically mediated hardening.

The paper situates its work within biomineralization and adaptive materials literature: extensive studies on hydroxyapatite and other calcium minerals in hydrogels for tissue engineering, including strategies for controlling mineral morphology and 3D printing approaches for complex architectures. A rare prior example of mechanically mediated mineralization is a piezoelectric polymer scaffold that induces apatite deposition from media under load (Kang group), enabling spatially controlled mineralization. In organic media, reported strategies include the growth of metal/inorganic nanoparticles within organogels and CO2 fixation to form carbamate crosslinks in PEI organogels. Mechanoredox/piezoelectrochemical catalysis has been used to drive polymerizations (e.g., ATRP, RAFT) and thiol-based reactions using piezoelectric nanoparticles like ZnO under ultrasound or vibration. Prior coordination chemistry shows McMT forms insoluble Zn(II) coordination compounds (often described as Zn(McMT)2), though crystal structures were not previously determined. These strands motivate exploring a mechanically mediated, coordination-driven mineralization route in organogels/organic matrices.

• Microrod synthesis: McMT dissolved in DMF (typically 0.50 mmol in 400 µL) with dispersed ZnO nanoparticles (18 nm, e.g., 20 mg, ~0.25 mmol Zn), sonicated in an ultrasonic bath (40 kHz) for 4–6 h in the dark. Post-reaction, dilute with methanol, isolate by centrifugation (3000 xg, 10 min), wash with methanol, dry under vacuum at 50 °C. Representative yield: 71 mg (86%).
• Controls and variables: Omitting ZnO prevents rod formation. Magnetic stirring at 400 rpm yields rods at lower yield (~40%). Simple vortexing (400 rpm) produces large irregular aggregates. Replacing ZnO with ZnBr2 (soluble Zn salt) or replacing McMT with its disulfide dimer prevents rod formation. Stoichiometry studies varied ZnO (10–40 mg; 0.13–0.50 mmol Zn2+) at fixed McMT (0.50 mmol) and varied McMT (0.25–1.00 mmol) at fixed ZnO (20 mg). Optimal formation at ZnO:McMT ≈ 1:2, with yields up to ~80% and best morphology/viscosity.
• Growth kinetics and rheology: Time-course sonication (0, 1, 2, 4, 6 h) with shear viscosity measurements revealed rapid viscosity increases (to 10^3 Pa·s by 6 h). SEM at timepoints tracked morphology transition from spherical ZnO to mixed particles to predominant microrods.
• Composition/structure characterization: XPS (Zn 2p, O 1s shift from 530 eV in ZnO to 532 eV in microrods), STEM-EDS (uniform Zn, N, S; low O), combustion elemental analysis (consistent with ZnC6H8N4S4 ~ Zn(McMT)2), pXRD (matches Zn(McMT)n with minor ZnO residual phases), FTIR and modulated DSC supporting coordination compound formation. Microrods dissolve in TCEP/DMF consistent with ligand substitution/chelation; insoluble in most solvents except polar aprotic under heat.
• In situ growth in polymer solution: Prepared a polyurethane-forming solution (PPG diisocyanate, tetraethylene glycol, catalyst) in DMF, then added McMT and ZnO; sonicated 6 h (40 kHz). Light microscopy/SEM confirmed microrods; dimensions increased relative to plain solution. Rheology measured shear viscosity and shear-thinning. Controls without McMT or without ZnO were processed identically.
• In situ growth in organogels: Synthesized azido-functionalized polyurethane (Mn 21.6 kDa, Đ 1.5). Formed organogels via strain-promoted azide–alkyne cycloaddition (DBCO-PEG5-DBCO crosslinker) in DMF containing McMT and ZnO; gelled within minutes at ~4 °C, then sonicated 6 h (40 kHz), dried under vacuum. Dynamic mechanical analysis (DMA) under compression (amplitude and frequency sweeps) performed in triplicate versus controls. Microscopy (cross-polarized light) and TEM confirmed embedded crystalline microrods/bundles.

• Mechanically mediated mineralization: Under ultrasound (40 kHz), ZnO nanoparticles react with McMT to form crystalline rod-shaped microparticles comprised of a Zn/McMT coordination compound, consistent with Zn(McMT)2.
• Microrod morphology: From DMF synthesis, average length 4.3 ± 1.2 µm, width 0.4 ± 0.1 µm (SEM). In polymer solution, rods are 74% longer and 50% thicker: 7.5 ± 3.2 µm length, 0.6 ± 0.2 µm width.
• Composition evidence: XPS shows O 1s peak shift (ZnO 530 eV to microrods 532 eV), indicating absence of crystalline ZnO and presence of bound water/hydroxide; STEM-EDS shows uniform Zn, N, S with low O; elemental analysis consistent with ZnC6H8N4S4; XRD matches Zn(McMT)n with minor ZnO traces; FTIR/DSC consistent with coordination compound.
• Mechanistic inferences: McMT associates with ZnO surface, mechanical vibration promotes Zn2+ release/complexation, ZnO serves as both Zn2+ source and nucleation sites for crystal growth; growth requires reactive thiol and particulate ZnO (soluble Zn salts ineffective).
• Reaction conditions: Optimal at ZnO:McMT ≈ 1:2; yields up to ~80% regardless of excess; ultrasound superior to stirring (stirring yield ~40%); vortexing yields irregular aggregates.
• Rheology during growth: Slurry viscosity rises from ~1 Pa·s (0 h) to thousands of Pa·s by 6 h, peaking between 4–6 h; correlates with rod formation observed by SEM.
• Polymer solution reinforcement: In situ microrods increase low-shear viscosity to ~100 Pa·s with shear-thinning; controls (no McMT or no ZnO) show ~3 Pa·s and near-Newtonian behavior.
• Organogel reinforcement (DMA): Complex modulus E′ (mean ± σ) in linear region: All components 3.47 ± 0.37 MPa; No McMT 1.65 ± 0.35 MPa; No ZnO 1.01 ± 0.04 MPa. Doubling McMT/ZnO loading (2X) yields ~4× increase in elastic modulus versus 1X; halving gives modulus comparable to 1X.
• Spatial/material compatibility: Microrods form in bulk within polymer matrices, enabling mineralization-driven hardening in organic environments via a piezo-responsive route.

The study demonstrates a bioinspired, mechanically mediated mineralization pathway within organic media that converts spherical piezoelectric ZnO nanoparticles into crystalline Zn/McMT microrods in situ. This addresses the challenge of achieving adaptive, stress-responsive strengthening in synthetic composites by leveraging mechanical energy to drive coordination-driven crystal growth. Evidence from control experiments establishes that both particulate ZnO and the reactive thiol are essential, and that mechanical agitation (especially ultrasound) substantially enhances yield and morphology. Spectroscopic and diffraction data consistently support formation of a Zn(McMT)n (likely 1:2) coordination compound, with ZnO acting both as Zn2+ source and as nucleation sites, rationalizing residual ZnO peaks in XRD. The resultant microrods substantially alter rheology: rapid viscosity increases during growth and pronounced shear-thinning when grown in a polymer solution, suggesting network-like interactions or rod entanglement. Critically, in crosslinked organogels, in situ mineralization increases the composite modulus by factors of ~2–3 relative to controls, and modulus scales with precursor loading, confirming that rod formation within the matrix is the main source of reinforcement. This mechanochemical mineralization is compatible with different polymer chemistries provided orthogonality to the coordination reaction, and it offers a route to guide mineral deposition via stress concentration patterns, akin to biological systems, albeit with limited precision compared to templated methods. The findings open a path for fabricating stress-directed, mechanically adaptive organogel composites in manufacturing-relevant solvents.

The work introduces a simple, scalable method to induce inorganic mineralization within organic polymer matrices by mechanically triggering the reaction of ZnO nanoparticles with McMT to form crystalline microrods, likely Zn(McMT)2. The approach enables bulk transformation of nanoparticles into reinforcing microrods, markedly modifying rheology and enhancing mechanical properties of both viscous polymer solutions and crosslinked organogels. Mechanistic and compositional analyses indicate that ZnO provides both Zn2+ and nucleation sites under vibration/ultrasound. Future directions include detailed studies of rod growth and morphology control under defined mechanical fields (e.g., continuous shear, electrodynamic shaker), improving interfacial bonding between microrods and polymer matrices to further boost mechanical performance, and developing strategies to spatially pattern mineral deposition with greater precision.

• Precise control over spatial mineral deposition is limited; growth is driven by local mechanical stress without fine templating.
• Low solubility of products precluded single-crystal XRD, leaving the exact crystal structure and stoichiometry (though likely 1:2 Zn:McMT) unconfirmed.
• Mechanism remains inferential based on controls and spectroscopy; direct observation of Zn2+ release and surface intermediates is lacking.
• Reinforcement depends on orthogonality between polymer crosslinking chemistry and microrod formation; not all polymer systems may be compatible.
• Residual ZnO phases can be trapped in microrods, as indicated by XRD.
• Process involves prolonged ultrasound and specific solvent conditions (DMF), potentially limiting throughput or solvent compatibility.
• Yield appears capped near ~80% under tested conditions; vortex/stirring give significantly lower yields and poorer morphology.