This proof-of-concept study used ultrasonic methods to quantify irreversible changes in the bulk, microstructural, and micro-viscoelastic properties of compressed pharmaceutical tablets after controlled moisture exposure and dry-down. The approach was non-destructive and targeted microstructure-sensitive responses that are not fully captured by simple geometric inspection.
Moisture exposure can alter critical quality attributes (CQA) of tablets by affecting interparticle bonding, chemical state, porosity, and residual stress distribution in compressed granular materials. Such changes may degrade mechanical integrity and affect performance-related attributes even when gross tablet geometry appears to recover. The authors position ultrasonic characterization as a sensitive modality for detecting these microstructural and micro‑viscoelastic alterations.
Samples underwent uniform vapor diffusion to introduce moisture without direct liquid contact. Following the moisture exposure, tablets experienced a controlled dry‑down phase during which ultrasonic waveforms were repeatedly acquired. A custom rig operating in pitch‑catch mode with paired pressure transducers captured the transmitted ultrasonic signals throughout the moisture cycle. The study design enabled monitoring of temporal evolution from initial dry state, through moisture uptake, and during subsequent drying.
Waveforms collected across the moisture cycle were analyzed in three complementary domains: temporal (time‑domain characteristics), spectral (frequency content), and wave‑dispersion (frequency-dependent propagation behavior). These analyses produced metrics sensitive to both bulk mechanical behavior and microstructural features, allowing comparison of geometric recovery versus functional recovery of elastic and viscoelastic properties.
Moisture uptake increased tablet mass by approximately 8.3 ± 0.77%, confirming vapor diffusion into the samples. After controlled dry-down, geometric changes relative to the initial dry state were modest: mean diameter increased by about 0.97 ± 0.25%, thickness by 1.83 ± 0.13%, and calculated mass density by 3.69 ± 0.49%. These results indicate that observable dimensional recovery was nearly complete but not fully restituted to the baseline.
Despite only minor residual geometric changes, ultrasonic-derived mechanical properties exhibited substantial irreversible reductions after the moisture cycle and dry-down. Specifically, the study reports reductions in pressure wave speed (~28.87%), group velocity (~13.20%), and apparent modulus of elasticity (~47.34%). These declines reflect persistent alterations in microstructure and micro‑viscoelastic response that are not revealed by geometric measurements alone.
Analysis of equivalent residual stresses indicated anisotropic recovery: equivalent axial residual stress after drying remained approximately 3–4 times higher than equivalent radial residual stress. This finding implies non-uniform recovery of deformation and stress redistribution across tablet axes following moisture exposure and drying.
The results demonstrate that ultrasonic characterization can sensitively detect irreversible moisture‑induced changes in tablet microstructure and viscoelastic properties that escape conventional geometric inspection. Because the method is non‑destructive and responsive to microstructural and mechanical endpoints, it has potential utility within Process Analytical Technology (PAT) frameworks, Quality by Design (QbD) approaches, Continuous Manufacturing (CM), and real‑time release testing (RTRT) for oral solid dosage forms.
The abstract provides key quantitative outcomes and the overall experimental approach, but several experimental details are not reported in the abstract. Specifics such as exact sample composition, number of tablets tested, vapor exposure conditions (e.g., relative humidity and exposure duration), the frequency range of ultrasonic measurements, signal processing algorithms, and statistical analysis methods were not described in the abstract and therefore cannot be summarized here.
Ultrasonic, pitch‑catch evaluation with temporal, spectral, and dispersion analyses identified substantial, irreversible reductions in wave propagation metrics and apparent modulus despite near-complete geometric recovery after moisture uptake and drying. The approach shows promise as a non‑destructive technique to monitor moisture-induced microstructural and micro‑viscoelastic changes in tablets and could inform CQA monitoring, PAT implementation, and RTRT strategies in pharmaceutical manufacturing.