Mechanics of fragmentation of crocodile skin and other thin films

Crocodile-Skin-Inspired Omnidirectionally Stretchable Pressure Sensor

Stretchable pressure sensors are important components of multimodal electronic skin needed for potentializing numerous Internet of Things applications. In particular, to use pressure sensors in various wearable/skin-attachable electronics, both high deformability and strain-independent sensitivity must be realized. However, previously reported stretchable pressure sensors cannot meet these standards because they exhibit limited stretchability and nonuniform sensitivity under deformation. Herein, inspired by the unique sensory organ of a crocodile, an omnidirectionally stretchable piezoresistive pressure sensor made of polydimethylsiloxane (PDMS)/silver nanowires (AgNWs) composites with microdomes and wrinkled surfaces is developed. The stretchable pressure sensor exhibits high sensitivity that changes negligibly even under uniaxial and biaxial tensile strains of 100% and 50%, respectively. This behavior is attributed to the microdomes responsible for detecting applied pressures being weakly affected by tensile strains, while the isotropic wrinkles between the microdomes deform to effectively reduce the external stress. In addition, because the device comprises all PDMS-based structures, it exhibits outstanding robustness under repeated mechanical stimuli. The device shows strong potential as a wearable pressure sensor and an artificial crocodile sensing organ, successfully detecting applied pressures in various scenarios. Therefore, the pressure sensor is expected to find applications in electronic skin for prosthetics and human-machine interface systems.

Cracking of Colloidal Films to Generate Rectangular Fragments

Cracks are common in nature. Cracking is known as an irreversible and uncontrollable process. To control the cracking patterns, many researchers have proposed methods to prepare notches for stress localization on films. In this work, we investigate a method of controlling cracks by making microscale pyramid patterns that have notches between the pyramids. After preparing pyramid patterns consisting of colloidal particles with organic residue, we annealed them to induce volume shrinkage and cracking between the pyramids. We studied the effect of film thickness on cracking and the generation of rectangular fragments consisting of multiple pyramids. The area of rectangular fragments was in good agreement with the results of scaling analysis. The concept of controlling cracks by imprinting notches on a film and the relationship with the film thickness can guide the study of cracking phenomena.

An anti-infective hydrogel adhesive with non-swelling and robust mechanical properties for sutureless wound closure

An anti-infective TA/hydrogel with non-swelling and adhesion properties could close wounds in a non-invasive way.

Jamming and tiling in fragmentation of rectangles

We investigate a stochastic process where a rectangle breaks into smaller rectangles through a series of horizontal and vertical fragmentation events. We focus on the case where both the vertical size and the horizontal size of a rectangle are discrete variables. Because of this constraint, the system reaches a jammed state where all rectangles are sticks, that is, rectangles with minimal width. Sticks are frozen as they cannot break any further. The average number of sticks in the jammed state, S, grows as S≃A/sqrt[2πlnA] with rectangle area A in the large-area limit, and remarkably, this behavior is independent of the aspect ratio. The distribution of stick length has a power-law tail, and further, its moments are characterized by a nonlinear spectrum of scaling exponents. We also study an asymmetric breakage process where vertical and horizontal fragmentation events are realized with different probabilities. In this case, there is a phase transition between a weakly asymmetric phase where the length distribution is independent of system size and a strongly asymmetric phase where this distribution depends on system size.

Flexible Transient Phototransistors by Use of Wafer-Compatible Transferred Silicon Nanomembranes

Flexible transient photodetectors, a form of optoelectronic sensors that can be physically self-destroyed in a controllable manner, could be one of the important components for future transient electronic systems. In this work, a scalable, device-first, and bottom-up thinning process enables the fabrication of a flexible transient phototransistor on a wafer-compatible transferred silicon nanomembrane. A gate modulation significantly restrains the dark current to 10^-12 A. With full exposure of the light-sensitive channel, such a device yields an ultrahigh photo-to-dark current ratio of 10⁷ with a responsivity of 1.34 A W^-1 (λ = 405 nm). The use of a high-temperature degradable polymer transient interlayer realizes on-demand self-destruction of the fabricated phototransistors, which offers a solution to the technical security issue of advanced flexible electronics. Such demonstration paves a new way for designing transient optoelectronic devices with a wafer-compatible process.

Morphometric analysis of polygonal cracking patterns in desiccated starch slurries

We investigate the geometry of two-dimensional polygonal cracking that forms on the air-exposed surface of dried starch slurries. Two different kinds of starches, made from potato and corn, exhibited distinguished crack evolution, and there were contrasting effects of slurry thickness on the probability distribution of the polygonal cell area. The experimental findings are believed to result from the difference in the shape and size of starch grains, which strongly influence the capillary transport of water and tensile stress field that drives the polygonal cracking.

Thermally Triggered Mechanically Destructive Electronics Based On Electrospun Poly(ε-caprolactone) Nanofibrous Polymer Films

Electronics, which functions for a designed time period and then degrades or destructs, holds promise in medical implants, reconfigurable electronic devices and/or temporary functional systems. Here we report a thermally triggered mechanically destructive device, which is constructed with an ultra-thin electronic components supported by an electrospun poly(ε-caprolactone) nanofibrous polymer substrate. Upon heated over the melting temperature of the polymer, the pores of the nanofibers collapse due to the nanofibers' microscopic polymer chain relaxing and packing. As a result, the polymer substrate exhibits approximately 97.5% area reduction. Ultra-thin electronic components can therefore be destructed concurrently. Furthermore, by integrating a thin resistive heater as the thermal trigger of Joule heating, the device is able to on-demand destruct. The experiment and analytical results illustrate the essential aspects and theoretical understanding for the thermally triggered mechanical destructive devices. The strategy suggests a viable route for designing destructive electronics.

Mussel-Inspired Adhesive and Tough Hydrogel Based on Nanoclay Confined Dopamine Polymerization

Adhesive hydrogels are attractive biomaterials for various applications, such as electronic skin, wound dressing, and wearable devices. However, fabricating a hydrogel with both adequate adhesiveness and excellent mechanical properties remains a challenge. Inspired by the adhesion mechanism of mussels, we used a two-step process to develop an adhesive and tough polydopamine-clay-polyacrylamide (PDA-clay-PAM) hydrogel. Dopamine was intercalated into clay nanosheets and limitedly oxidized between the layers, resulting in PDA-intercalated clay nanosheets containing free catechol groups. Acrylamide monomers were then added and in situ polymerized to form the hydrogel. Unlike previous single-use adhesive hydrogels, our hydrogel showed repeatable and durable adhesiveness. It adhered directly on human skin without causing an inflammatory response and was easily removed without causing damage. The adhesiveness of this hydrogel was attributed to the presence of enough free catechol groups in the hydrogel, which were created by controlling the oxidation process of the PDA in the confined nanolayers of clay. This mimicked the adhesion mechanism of the mussels, which maintain a high concentration of catechol groups in the confined nanospace of their byssal plaque. The hydrogel also displayed superior toughness, which resulted from nanoreinforcement by clay and PDA-induced cooperative interactions with the hydrogel networks. Moreover, the hydrogel favored cell attachment and proliferation, owning to the high cell affinity of PDA. Rat full-thickness skin defect experiments demonstrated that the hydrogel was an excellent dressing. This free-standing, adhesive, tough, and biocompatible hydrogel may be more convenient for surgical applications than adhesives that involve in situ gelation and extra agents.

Ordered fragmentation of oxide thin films at submicron scale

Crack formation is typically undesirable as it represents mechanical failure that compromises strength and integrity. Recently, there have also been numerous attempts to control crack formation in materials with the aim to prevent or isolate crack propagation. In this work, we utilize fragmentation, at submicron and nanometre scales, to create ordered metal oxide film coatings. We introduce a simple method to create modified films using electroplating on a prepatterned substrate. The modified films undergo preferential fragmentation at locations defined by the initial structures on the substrate, yielding ordered structures. In thicker films, some randomness in the characteristic sizes of the fragments is introduced due to competition between crack propagation and crack creation. The method presented allows patterning of metal oxide films over relatively large areas by controlling the fragmentation process. We demonstrate use of the method to fabricate high-performance electrochromic structures, yielding good coloration contrast and high coloration efficiency.

Fracture and related processes are typically considered detrimental, but have also attracted interest in more constructive roles. Here authors demonstrate ordered fragmentation at submicron scales of a metal oxide/hydroxide thin film by introducing preferential sites for fracture on the underlying substrate.