Kai-Tak Wan

Direct measurement of graphene adhesion on silicon surface by intercalation of nanoparticles

We report a technique to characterize adhesion of monolayered/multilayered graphene sheets on silicon wafer. Nanoparticles trapped at graphene-silicon interface act as point wedges to support axisymmetric blisters. Local adhesion strength is found by measuring the particle height and blister radius using a scanning electron microscope. Adhesion energy of the typical graphene-silicon interface is measured to be 151±28 mJ/m2. The proposed method and our measurements provide insights in fabrication and reliability of microelectromechanical/nanoelectromechanical systems.

A theoretical and numerical study of a thin clamped circular film under an external load in the presence of a tensile residual stress

Tensile residual stress in a plate or membrane clamped at the perimeter can be measured by either applying a uniform hydrostatic pressure or a central load via a cylindrical punch (with several different loading configurations). Analytical constitutive relations are derived here based on an average membrane stress approximation and are compared to finite element analysis results. The thickness and flexural rigidity of the film are not confined to a small range but will span a wide spectrum. The elastic responses of the blistering films are shown to be linear when the film is thick, relatively rigid, or subjected to a large residual stress, and cubic when the film is thin, flexible, or under a small residual stress. The linear-to-cubic transition is formulated.

Crack velocity functions and thresholds in brittle solids

Abstract A unifying treatment of environment-sensitive crack velocity functions for intrinsically brittle solids is presented. The formalism is soundly based on the concept of thermal activation barriers, but is phenomenological in that it does not attempt to identify the explicit underlying physical and chemical processes responsible for these barriers. Equations prescribing the v-G (crack velocity versus mechanical energy release rate) characteristics at specified chemical concentrations (partial pressures) and temperatures are thereby presented. These equations incorporate the familiar three velocity regions into a composite function: region I, chemically assisted fluctuations over stress-enhanced energy barriers; region III, similar but in the absence of environmental species; region II, a connecting flow-limited transport branch. In addition, the equations include provision for healing and repropagation branches in unloading-reloading cycles. Central to the argument is the assertion that zero-velocity thresholds are quiescent configurations, defined by appropriate Dupre work of adhesion terms, W. These W terms serve as reference baselines for the entire v-G function, such that changes in chemical concentration (relative humidity) or interface type (virgin versus healed) may be considered in terms of simple curve shifts along the G axis. Data for selected brittle solids, principally mica but also glass and sapphire, in moist environments are used to illustrate the formalism.

Fracture mechanics of a shaft-loaded blister of thin flexible membrane on rigid substrate

A shaft-loaded blister test has been developed to measure the interfacial energy W of a thin flexible polymeric film adhered to a rigid substrate. A theoretical analysis is given of an axisymmetric debond (‘blister’) in terms of an external applied load P, tensile stretching modulus E and thickness h of the adhering layer. The fracture mechanics model presented considers both elastic and elastoplastic deformations in the thin film. The intrinsic stable interface debonding process provides an attractive alternative to the conventional adhesion measurement techniques.

Fracture mechanics of a new blister test with stable crack growth

Abstract A new blister test is proposed to measure the specific work of adhesion W between a thin flexible film on a rigid substrate. In contrast to the conventional blister loaded by constant fluid pressure which leads to catastrophic crack propagation, the new test is driven by an internal expansion of a fixed mass of working gas which leads tostable crack growth. The new technique is demonstrated by measuring W of an interface with a commercial sticky tape serving as the thin film and aluminium as the rigid substrate.

Contact mechanics of a thin-walled capsule adhered onto a rigid planar substrate.

A thin-walled capsule, modelled as an incompressible liquid droplet contained in a thin flexible membrane, was allowed to adhere onto a rigid substrate. The contact mechanics were formulated, based on linear elasticity, to portray quantitatively the relationships between osmotic inflation, contact area and angle, membrane stretching and adhesion strength. The predicted results shed light on fundamental adhesive contact mechanics in a cell-substrate system.

Measuring mechanical properties of thin flexible films by a shaft-loaded blister test

Mechanical properties of a thin circular flexible membrane are directly measured by applying an external load to the film center via a rigid spherical capped shaft. Constitutive equations are proposed for the shaft-loaded blister configuration based on linear elasticity. The dependence on the ratio of shaft radius to blister dimension is reflected in the load versus shaft displacement relation.

Reduced Graphene Oxide-GelMA Hybrid Hydrogels as Scaffolds for Cardiac Tissue Engineering

Biomaterials currently used in cardiac tissue engineering have certain limitations, such as lack of electrical conductivity and appropriate mechanical properties, which are two parameters playing a key role in regulating cardiac cell behavior. Here, the myocardial tissue constructs are engineered based on reduced graphene oxide (rGO)-incorporated gelatin methacryloyl (GelMA) hybrid hydrogels. The incorporation of rGO into the GelMA matrix significantly enhances the electrical conductivity and mechanical properties of the material. Moreover, cells cultured on composite rGO-GelMA scaffolds exhibit better biological activities such as cell viability, proliferation, and maturation compared to ones cultured on GelMA hydrogels. Cardiomyocytes show stronger contractility and faster spontaneous beating rate on rGO-GelMA hydrogel sheets compared to those on pristine GelMA hydrogels, as well as GO-GelMA hydrogel sheets with similar mechanical property and particle concentration. Our strategy of integrating rGO within a biocompatible hydrogel is expected to be broadly applicable for future biomaterial designs to improve tissue engineering outcomes. The engineered cardiac tissue constructs using rGO incorporated hybrid hydrogels can potentially provide high-fidelity tissue models for drug studies and the investigations of cardiac tissue development and/or disease processes in vitro.

Microfluidics-Assisted Fabrication of Gelatin-Silica Core–Shell Microgels for Injectable Tissue Constructs

Microfabrication technology provides a highly versatile platform for engineering hydrogels used in biomedical applications with high-resolution control and injectability. Herein, we present a strategy of microfluidics-assisted fabrication photo-cross-linkable gelatin microgels, coupled with providing protective silica hydrogel layer on the microgel surface to ultimately generate gelatin-silica core–shell microgels for applications as in vitro cell culture platform and injectable tissue constructs. A microfluidic device having flow-focusing channel geometry was utilized to generate droplets containing methacrylated gelatin (GelMA), followed by a photo-cross-linking step to synthesize GelMA microgels. The size of the microgels could easily be controlled by varying the ratio of flow rates of aqueous and oil phases. Then, the GelMA microgels were used as in vitro cell culture platform to grow cardiac side population cells on the microgel surface. The cells readily adhered on the microgel surface and proliferated over time while maintaining high viability (∼90%). The cells on the microgels were also able to migrate to their surrounding area. In addition, the microgels eventually degraded over time. These results demonstrate that cell-seeded GelMA microgels have a great potential as injectable tissue constructs. Furthermore, we demonstrated that coating the cells on GelMA microgels with biocompatible and biodegradable silica hydrogels via sol–gel method provided significant protection against oxidative stress which is often encountered during and after injection into host tissues, and detrimental to the cells. Overall, the microfluidic approach to generate cell-adhesive microgel core, coupled with silica hydrogels as a protective shell, will be highly useful as a cell culture platform to generate a wide range of injectable tissue constructs.

Fracture Mechanics of a Shaft-loaded Blister Test – Transition from a Bending Plate to a Stretching Membrane

Abstract A linear elastic solution is proposed for a circular disc in transition from a plate-like (pure bending) to a membrane-like behavior (pure stretching) under a central point load. The strain energy release rate for film delamination is found to be G = χ(Pw 0/πa 2) with χ a numerical constant varying from 1/2 for a plate-like disc to 1/4 for a thin flexible membrane.