Poroviscoelastic relaxations and rate-dependent adhesion in gelatin.

Hydrogels, polymeric networks swollen with water, exhibit time/rate-dependent adhesion due to their poroviscoleastic constitution. In this study, we conducted probe-tack experiments on gelatin and investigated the influence of dwelling times and unloading rates on pull-off forces and work of adhesion. We utilized in situ contact imaging to monitor separation kinematics and interfacial crack velocities. We found that the crack velocities scaled nonlinearly with the unloading rate, in a power law with an exponent of 0.8 and were independent of dwelling time. At maximum unloading rates corresponding to subsonic interfacial crack speeds, we observed an order of magnitude enhancement in the apparent work of adhesion. The enhancement of adhesion and the crack velocities were related by a power law with an exponent of 0.39. The maximum vertical extension during unloading, a measure of crack opening, exhibited linear correlation with the enhancement of adhesion. Both correlations were in line with the rate-dependent work of fracture modeled for viscoelastic solids (e.g., Persson and Brener model). We explored the links between dwelling times corresponding to varying degrees of poroelastic diffusion and the adhesion. We found 40% additional enhancement in adhesion at the highest unloading rate. This enhancement is due to the unbalanced osmotic pressure, also known as the suction effect. The influence of dwelling times on adhesion was negligible for the interfacial cracks propagating slower than the diffusive time scales. These results identify viscoelastic relaxations as the dominant mechanism governing the rate-dependent enhancement of adhesion, and hence pave the way for tuning rate-dependent adhesion in soft multiphasic materials.

Characterization of drying-induced changes in moduli and internal stresses in a constrained gel using laser vibrometry.

Hydrogels, water-saturated polymer networks find widespread use in soft robotics, biomedical, pharmaceutical and food industries. Both solid and water constituents of hydrogels are sensitive to external stimuli such as temperature, humidity, osmolarity, and light. For instance, common hydrogels swell or shrink in the presence of chemical potential gradient between the sample and surrounding environment. Corresponding changes in internal water content lead to significant changes in mechanical properties of hydrogels. Besides, internal stresses build up if the gel samples are constrained during swelling or dehydration. In the present research, we utilize modal analyses technique on drying hydrogels to identify dehydration-induced changes in elastic moduli and internal stresses. In particular, natural frequencies and damping ratios of the first two axisymmetric transverse vibration modes are measured on clamped gelatin disks using non-contact laser vibrometry at various water loss states. Experimental modal frequencies are then compared to the predictions of a pre-stressed thick plate model. The evolutions of elastic moduli and internal stresses for water losses up to 80% are identified. The broadband loss capacity of gelatin is also determined from the measured modal damping ratios. Highly transient mechanical response observed on the gelatin disks further demonstrates the need for non-contact and rapid mechanical characterization of hydrogels. As illustrated in this work, vibration and wave-based techniques are promising candidates to fulfill that need.

EPHA3 Contributes to Epigenetic Suppression of PTEN in Radioresistant Head and Neck Cancer.

EPHA3, a member of the EPH family, is overexpressed in various cancers. We demonstrated previously that EPHA3 is associated with radiation resistance in head and neck cancer via the PTEN/Akt/EMT pathway; the inhibition of EPHA3 significantly enhances the efficacy of radiotherapy in vitro and in vivo. In this study, we investigated the mechanisms of PTEN regulation through EPHA3-related signaling. Increased DNA methyltransferase 1 (DNMT1) and enhancer of zeste homolog 2 (EZH2) levels, along with increased histone H3 lysine 27 trimethylation (H3K27me3) levels, correlated with decreased levels of PTEN in radioresistant head and neck cancer cells. Furthermore, PTEN is regulated in two ways: DNMT1-mediated DNA methylation, and EZH2-mediated histone methylation through EPHA3/C-myc signaling. Our results suggest that EPHA3 could display a novel regulatory mechanism for the epigenetic regulation of PTEN in radioresistant head and neck cancer cells.