Rong Long

A mechanical metamaterial made from a DNA hydrogel

Metamaterials are artificial substances that are structurally engineered to have properties not typically found in nature. To date, almost all metamaterials have been made from inorganic materials such as silicon and copper, which have unusual electromagnetic or acoustic properties that allow them to be used, for example, as invisible cloaks, superlenses or super absorbers for sound. Here, we show that metamaterials with unusual mechanical properties can be prepared using DNA as a building block. We used a polymerase enzyme to elongate DNA chains and weave them non-covalently into a hydrogel. The resulting material, which we term a meta-hydrogel, has liquid-like properties when taken out of water and solid-like properties when in water. Moreover, upon the addition of water, and after complete deformation, the hydrogel can be made to return to its original shape. The meta-hydrogel has a hierarchical internal structure and, as an example of its potential applications, we use it to create an electric circuit that uses water as a switch.

Effect of backing layer thickness on adhesion of single-level elastomer fiber arrays

The effect of the backing layer thickness on adhesion was investigated for single-level elastomer fibrillar adhesives. Polyurethane microfiber arrays with spatulated tips on a 160μm thick backing layer show nine times greater adhesion strength (around 22N∕cm2) than those with a 1120μm thick backing. A theoretical model is proposed to explain this difference in which very thin backing layers promote equal load sharing, maximizing adhesion, while very thick backings can lead to reduced adhesion due to edge stress concentration. Therefore, backing layer thickness should be considered as a significant parameter for design of high performance fibrillar adhesives.

Mechanics of Adhered, Pressurized Graphene Blisters

We study the mechanics of pressurized graphene membranes using an experimental configuration that allows the determination of the elasticity of graphene and the adhesion energy between a substrate and a graphene (or other two-dimensional solid) membrane. The test consists of a monolayer graphene membrane adhered to a substrate by surface forces. The substrate is patterned with etched microcavities of a prescribed volume and when they are covered with the graphene monolayer it traps a fixed number (N) of gas molecules in the microchamber. By lowering the ambient pressure, and thus changing the pressure difference across the graphene membrane, the membrane can be made to bulge and delaminate in a stable manner from the substrate. Here we describe the analysis of the membrane/substrate as a thermodynamic system and explore the behavior of the system over representative experimentally-accessible geometry and loading parameters. We carry out companion experiments and compare them to the theoretical predictions and then use the theory and experiments together to determine the adhesion energy of graphene/SiO2 interfaces. We find an average adhesion energy of 0.24 J/m2 which is lower, but in line with our previously reported values. We assert that this test, which we call the constant N blister test, is a valuable approach to determine the adhesion energy between two-dimensional solid membranes and a substrate, which is an important, but not well-understood aspect of behavior. The test also provides valuable information that can serve as the basis for subsequent research to understand the mechanisms contributing to the observed adhesion energy. Finally, we show how in the limit of a large microcavity, the constant N test approaches the behavior observed in a constant pressure blister test and we provide an experimental observation that suggests this behavior.

Effect of rate on adhesion and static friction of a film-terminated fibrillar interface.

A film-terminated fibrillar interface has been shown to result in significant enhancement of adhesion and static friction compared to a flat control. This enhancement increases with interfibril spacing. In this, the first of a two-part study, by studying the effect of rate on adhesion and static friction, we show that both adhesion and static friction enhancement are due to a crack-trapping mechanism. For adhesion, as measured by an indentation experiment, an analytical model is used to relate the applied indenter displacement rate and measured forces to contact line velocity and energy release rate, respectively. The two mechanisms for adhesion enhancement--varying rate and crack-trapping--are found to be coupled multiplicatively.

Modeling the soft backing layer thickness effect on adhesion of elastic microfiber arrays

A recent experiment has shown that the force required to pull off a flat circular rigid punch in adhesive contact with an array of elastic fibrils is sensitive to the thickness of the elastic backing layer to which these fibrils are attached. This result motivates us to study the effect of sample compliance on the adhesion of fibril arrays. A closed form expression for the compliance of such arrays attached to a backing layer of finite thickness is derived. Our model is based on the assumption that the adhesive strength of a fibril is deterministic. In addition, we show that the normalized pull-off force is inversely proportional to the square root of a single dimensionless parameter β. For large β, the pull-off force is low as it is governed by the stress concentration at the punch edge. For small β, this pull-off force reaches a theoretical limit that is governed by the ability of fibrils to share load equally [equal load sharing (ELS) limit]. The pull-off force predicted by our model is compared with n...

Toward single cell traction microscopy within 3D collagen matrices.

Mechanical interaction between the cell and its extracellular matrix (ECM) regulates cellular behaviors, including proliferation, differentiation, adhesion, and migration. Cells require the three-dimensional (3D) architectural support of the ECM to perform physiologically realistic functions. However, current understanding of cell-ECM and cell-cell mechanical interactions is largely derived from 2D cell traction force microscopy, in which cells are cultured on a flat substrate. 3D cell traction microscopy is emerging for mapping traction fields of single animal cells embedded in either synthetic or natively derived fibrous gels. We discuss here the development of 3D cell traction microscopy, its current limitations, and perspectives on the future of this technology. Emphasis is placed on strategies for applying 3D cell traction microscopy to individual tumor cell migration within collagen gels.

Cohesive Zone Models and Fracture

Basic concepts on cohesive models and their usage in fracture are reviewed. These included potential based cohesive zone models and the concept of an anisotropic failure surface. Some new results are presented for history-dependent cohesive zone models. In particular, a class of cohesive zone models where damage is represented by a state variable which evolves according to loading history is studied. The connection between cohesive zone model and crack nucleation is explored.

Finite Strain Crack Tip Fields in Soft Incompressible Elastic Solids

A finite element model (FEM) is used to study the behavior of the large deformation field near the tip of a crack in a soft incompressible plane stress fracture specimen loaded in mode I. Results are obtained for the case of a neo-Hookean solid (ideal rubber) and a hyperelastic solid with exponentially hardening behavior. In contrast to the predictions of linear elastic fracture mechanics (LEFM), the near tip stress fields are dominated by the opening stress which shows a 1/R singularity for the neo-Hookean material and a -1/(R ln R) singularity for the exponential hardening solid. We found very similar qualitative behavior in the near tip stress fields despite the very large difference in strain hardening behavior of the two material models. Our result shows that the near tip opening stress is controlled by the far field energy release rate for large applied loads.

Graphene blisters with switchable shapes controlled by pressure and adhesion.

We created graphene blisters that cover and seal an annular cylinder-shaped microcavity in a SiO2 substrate filled with a gas. By controlling the pressure difference between the gas inside and outside of the microcavity, we switch the graphene membrane between multiple stable equilibrium configurations. We carried out experiments starting from the situation where the pressure of the gas inside and outside of the microcavity is set equal to a prescribed charging pressure, p0 and the graphene membrane covers the cavity like an annular drum, adhered to the central post and the surrounding substrate due to van der Waals forces. We decrease the outside pressure to a value, pe which causes it to bulge into an annular blister. We systematically increase the charging pressure by repeating this procedure causing the annular blister to continue to bulge until a critical charging pressure pc(i) is reached. At this point the graphene membrane delaminates from the post in an unstable manner, resulting in a switch of graphene membrane shape from an annular to a spherical blister. Continued increase of the charging pressure results in the spherical blister growing with its height increasing, but maintaining a constant radius until a second critical charging pressure pc(o) is reached at which point the blister begins to delaminate from the periphery of the cavity in a stable manner. Here, we report a series of experiments as well as a mechanics and thermodynamic model that demonstrate how the interplay among system parameters (geometry, graphene stiffness (number of layers), pressure, and adhesion energy) results in the ability to controllably switch graphene blisters among different shapes. Arrays of these blisters can be envisioned to create pressure-switchable surface properties where the difference between patterns of annular versus spherical blisters will impact functionalities such as wettability, friction, adhesion, and surface wave characteristics.

A microfluidic system enabling continuous characterization of specific membrane capacitance and cytoplasm conductivity of single cells in suspension

This paper presents a microfluidic system enabling continuous characterization of specific membrane capacitance (Cspecific membrane) and cytoplasm conductivity (σcytoplasm) of single cells in suspension. In this study, cells were aspirated continuously through a constriction channel while cell elongations and impedance profiles at two frequencies (1kHz and 100kHz) were measured simultaneously using microscopy imaging and a lock-in amplifier. 1kHz impedance data were used to evaluate cellular sealing properties with constriction channel walls and 100kHz impedance data were translated to quantify equivalent membrane capacitance and cytoplasm resistance of single cells, which were further translated to Cspecific membrane and σcytoplasm. Two model cell lines (kidney tumor cell line of 786-O (n=302) and vascular smooth muscle cell line of T2 (n=216)) were used to evaluate this technique, producing Cspecific membrane of 3.67±1.00 and 4.53±1.51μF/cm(2) and σcytoplasm of 0.47±0.09 and 0.55±0.14S/m, respectively. Compared to previously reported techniques which can only collect Cspecific membrane and σcytoplasm from tens of cells, this new technique has a higher throughput, capable of collecting Cspecific membrane and σcytoplasm from hundreds of cells in 30min immediately after cell passage.