Accurate measurement of elastic constants in thin films is still an important issue to understand the scale behavior of nanosized materials. In the present study, we introduced an advanced non-destructive method, picosecond ultrasonics (PU), for measuring the out-of-plane elastic modulus of thin chromium (Cr) films. The femtosecond light pulse is focused on the Cr film to excite the longitudinal acoustic phonons (LAP), which propagate along the thickness direction and repeat reflections inside the Cr film. Then, the propagation/distribution of LAP is detected by the time-delayed probe light pulse through the photoelastic effect. Therefore, we can determine the out-of-plane modulus by measuring the periodic pulse echoes or the breathing mode vibrations within the Cr film. For most Cr films, the determined modulus is smaller than the corresponding bulk value and decreases with the decreasing thickness, while for some Cr films, it closes and may exceed the bulk value. This work describes the thickness-dependent elasticity of thin Cr films and provides evidence of the stiffness enhancement in Cr films on the Si substrate. In addition, since LAP with central frequency up to 310 GHz is excited in Cr films on the SiO₂ substrate, we also demonstrate the potential of Cr films as high-frequency photoacoustic transducers.

With the development of flexible electronic technology, lately, there has been an increase in demand for flexible electronic devices based on soft polymer-substrate metal film structures in challenging applications. These soft polymer-substrate metal film structures must tolerate bending, folding, stretching, and even deformation into any shape without failing to be used successfully. As a result, research into the fracture behavior of soft polymer-substrate metal film structures is essential. The purpose of this study was to investigate how fractures develop in Cr film attached to a polyimide (PI) substrate under biaxial stress. A fracture development model was built to determine the fracture propagation law of soft polymer-substrate metal film structures under biaxial stress. Experiments and finite element methods were applied to verify the correctness of the model. The theoretical analysis and finite element simulation results showed that fractures appeared initially at the perimeter of the film and then propagated to the center under biaxial stress. The theoretical and experimental results indicated that the crack propagation direction was related to the ratio of biaxial loading, which became progressively parallel to the direction of small loading as the biaxial loading ratio increased. The theoretical results were in line with the experiment results, which could be used as a preliminary step for further research on the fracture behavior of film-substrate structures.

Flexible tactile sensors have emerged as vital transduction interfaces for human–machine symbiosis and soft robotics. Despite significant advances, two persistent bottlenecks have hindered their practical viability. At the material level, the intrinsic viscoelasticity of polymeric matrices enforces stringent trade-offs among pressure sensitivity, output linearity, and loading–unloading hysteresis. At the system level, the conventional spatial separation of sensory acquisition from signal processing incurs considerable bandwidth and energy overheads when handling high-dimensional, unstructured analog data. To address these limitations, this review proposes a vertically integrated co-design framework that systematically couples material engineering, device architecture, and neuromorphic algorithm design. At the material-device level, we survey structural strategies, including dynamic network construction, rigid-phase anchoring, and topological interlocking, aimed at suppressing microscale viscoelastic relaxation and preventing interfacial delamination at mechanically heterogeneous junctions. Complementarily, alternating current excitation protocols have been evaluated to mitigate the electrochemical degradation intrinsic to iontronic architectures. We argue that the concurrent resolution of these mechanical and electrochemical instabilities provides the drift-free analog baseline essential for downstream neuromorphic computation, a cross-domain connection that has received limited systematic attention in the existing literature. Building on this stabilized platform, we further elucidate how intrinsic material dynamics, notably ferroelectric polarization switching and ion migration, can serve as native computational primitives, effectively linking material engineering and algorithm-design layers within the proposed framework. The coupling of these material-embedded dynamics with spiking neural networks enables in situ temporal encoding, whereby continuous mechanical stimuli are transduced into sparse event-driven spike trains, circumventing the von Neumann bottleneck. This co-design perspective offers a pathway for flexible tactile systems to evolve beyond passive structural biomimicry toward autonomous cognitive interfaces with real-time edge-localized adaptability.

Residual stress in brittle films on compliant substrates is routinely inferred from fragmentation experiments by combining an elastic stress-transfer model with a fracture strength criterion. This inversion is inherently coupled because the observed crack spacing depends jointly on the residual stress and the film fracture strength. Conventional closed-form estimators typically rely on a single feature, such as the cracking onset strain, and prescribe the fracture strength a priori, often at its bulk value. This practice discards most of the information encoded in the full crack-spacing evolution. It also obscures two sources of uncertainty: the intrinsic variability of thin-film fracture strength and the limited sensitivity of any single observable to individual parameters. Here, we recast the inversion as a Bayesian physics-informed neural network (B-PINN) in which the entire measured curve of the mean crack spacing versus applied strain is likely to occur. Stochastic gradient Langevin dynamics then sample the joint posterior of residual stress and fracture strength. A central finding is that crack-spacing data alone constrain only the difference between fracture strength and residual stress, confining the posterior to a one-dimensional manifold in parameter space and leaving each quantity individually unresolved. A single substrate curvature measurement, which, through the Stoney relation, depends on the residual stress but not on the fracture strength, provides the missing orthogonal constraint and collapses the posterior to a tight, well-resolved region. We further derive an identifiability condition under which buckle-wavelength observations serve as a third independent channel for recovering interface adhesion energy, and provide a synthetic proof-of-concept of this three-channel extension on DLC/Si and Mo/Si datasets; an experimental validation of the adhesion channel is identified as the natural next step but lies beyond the present scope. Requiring only standard fragmentation measurements and a single non-destructive curvature scan, the framework converts a point-estimate procedure into a posterior-quantified inverse method that makes explicit what can, and cannot, be learned from thin-film mechanics experiments.