In stretchable electronics, the island-bridge structure on a soft substrate plays an important role in achieving large stretchability. A key issue in developing such a system is to prevent the island-bridge structure from breaking during use because it is composed of brittle semiconductor materials (e.g., silicon) which withstand very small strains (∼1%). In the following, an analytical mechanics model of the island-bridge structure is established and the accurate solution is obtained. A validated scaling law is found to reveal the dependence of the normalized maximum strain in the island on the prestrain of the substrate, which controls the mechanical failure of the island-bridge structure and provides a theoretical basis for fracture-safe design of stretchable electronics.

Flexible and wearable electronics integrated with various sensors have great potential for applications in monitoring human activities and personal health. Bending and tension/compression dominate the deformation modes yielded by flexure of joints and diverse body gestures. A key challenge now is to sense the curvature/bending angle, while much research has been focused on the strain sensors for tension/compression. Alternative approaches by strain sensors or noncontact optical methods for curvature sensing are not practical for wearable electronics. A novel adhesion‐free thin‐film‐like curvature sensor that can monitor bending activities is introduced for flexible and wearable electronics. This study presents comprehensive design, fabrication, mechanism, structural analysis, performance characterization, and device‐level demonstrations for bending of joints, gesture recognition, and real‐time sitting posture correction. The most prominent advantage of the present sensor is that the measurement is independent of the strain of the target surface and the interfacial slippage, thus the perfect adhesion between the sensor and the surface is unnecessary. The features of adhesion‐free, simple mechanical principle, low cost, and satisfactory monitoring results highlight the superiority of the present curvature sensors for practical applications to flexible and wearable electronics.

Multi-layer structures with soft (compliant) interlayers have been widely used in flexible electronics and photonics as an effective design for reducing interactions among the hard (stiff) layers and thus avoiding the premature failure of an entire device. The analytic model for bending of such a structure has not been well established due to its complex mechanical behaviour. Here, we present a rational analytic model, without any parameter fitting, to study the bending of a multi-layer structure on a cylinder, which is often regarded as an important approach to mechanical reliability testing of flexible electronics and photonics. For the first time, our model quantitatively reveals that, as the key for accurate strain control, the splitting of the neutral mechanical plane depends not only on the relative thickness of the middle layer, but also on the length-to-thickness ratio of the multi-layer structure. The model accurately captures the key quantities, including the axial strains in the top and bottom layers, the shear strain in the middle layer and the locations of the neutral mechanical planes of the top and bottom layers. The effects of the length of the multi-layer and the thickness of the middle layer are elaborated. This work is very useful for the design of multi-layer structure-based flexible electronics and photonics.

Monitoring soft-tissue biomechanics is of interest in biomedical research and clinical treatment of diseases. An important focus is biointegrated strain gauges that track time-dependent mechanics of targeted tissues with deforming surfaces over multidirections. Existing methods provide limited gauge factors, tailored for sensing within specific directions under quasi-static conditions. We present development and applicability of implantable/wearable strain gauges that integrate multiple ultrathin monocrystalline silicon–based sensors aligned with different directions, in stretchable formats for dynamically monitoring direction angle–sensitive strain. We experimentally and computationally establish operational principles, with theoretical systems that enable determination of intensities and direction of applied strains at an omnidirectional scale. Wearable evaluations range from cardiac pulse to intraocular pressure monitoring of eyeballs. The device can evaluate cardiac disorders of myocardial infarction and hypoxia of living rats and locate the pathological orientation associated with infarction, in designs with possibilities as biodegradable implants for stable operation. These findings create clinical significance of the devices for monitoring complex dynamic biomechanics.

A unified analytic solution approach to both static bending and free vibration problems of rectangular thin plates is demonstrated in this paper, with focus on the application to corner-supported plates. The solution procedure is based on a novel symplectic superposition method, which transforms the problems into the Hamiltonian system and yields accurate enough results via step-by-step rigorous derivation. The main advantage of the developed approach is its wide applicability since no trial solutions are needed in the analysis, which is completely different from the other methods. Numerical examples for both static bending and free vibration plates are presented to validate the developed analytic solutions and to offer new numerical results. The approach is expected to serve as a benchmark analytic approach due to its effectiveness and accuracy.