Network interactions simultaneously enhance stiffness and lubricity of triple-network hydrogels.

Synthetic hydrogels displaying cartilage-mimetic bulk and surface properties may serve as cartilage substitutes. Multi-network, electrostatic hydrogels that leverage intra- and inter-network repulsive and attractive forces represent a promising approach. Herein, triple network (TN) hydrogels were prepared to obtain a combination of desired characteristics (, hydration, stiffness, shear stress, and friction properties).

Adhesion Mechanics and Detachment Dynamics of Vanishing Surface Layers on Hydrogels.

Cross-linked hydrogel surfaces exhibit reduced stiffness when polymerized against polymeric hydrophobic surfaces. As such, these layers play a critical role in contact mechanics, particularly exhibiting strong relative adhesion with colloidal probes when the contact area is small. This prevents the use of continuum models of adhesive soft contact.

Fewer polymer chains but higher adhesion: How gradient-stiffness hydrogel layers mediate adhesion through network stretch.

The presence of gradient softer outer layers, commonly observed in biological systems (such as cartilage and ocular tissues), as well as synthetic crosslinked hydrogels, profoundly influences their interactions with opposing surfaces. Our prior research demonstrated that gradient-stiffness hydrogel layers, characterized by increasing elasticity with depth, control contact mechanics, particularly in proximity to the layer thickness. We postulate that the distribution of polymers within these gradient layers imparts extraordinary stretch and adhesion characteristics due to network adaptability and stress-induced reorganization.

Composition controls soft hydrogel surface layer dimensions and contact mechanics.

Hydrogels are soft hydrated polymer networks that are widely used in research and industry due to their favorable properties and similarity to biological tissues. However, it has long been difficult to create a hydrogel emulating the heterogeneous structure of special tissues, such as cartilage. One potential avenue to develop a structural variation in a hydrogel is the "mold effect," which has only recently been discovered to be caused by absorbed oxygen within the mold surface interfering with the polymerization.

Soft Contact Mechanics with Gradient-Stiffness Surfaces.

The stiffness in the top surface of many biological entities like cornea or articular cartilage, as well as chemically cross-linked synthetic hydrogels, can be significantly lower or more compliant than the bulk. When such a heterogeneous surface comes into contact, the contacting load is distributed differently from typical contact models. The mechanical response under indentation loading of a surface with a gradient of stiffness is a complex, integrated response that necessarily includes the heterogeneity.

Nonlinear elasticity and damping govern ultrafast dynamics in click beetles.

Many small animals use springs and latches to overcome the mechanical power output limitations of their muscles. Click beetles use springs and latches to bend their bodies at the thoracic hinge and then unbend extremely quickly, resulting in a clicking motion. When unconstrained, this quick clicking motion results in a jump.

Cartilage-like tribological performance of charged double network hydrogels.

A synthetic hydrogel material may offer utility as a cartilage replacement if it is able to maintain low friction in different sliding environments and achieve bulk mechanical properties to withstand the severe environment of the joint. In this work, we compared the tribological behavior of four double network (DN) hydrogels to that of fresh porcine cartilage in both water and fetal bovine serum (FBS). The DN hydrogels were comprised of a negatively charged 1st network and a 2nd network wherein comonomers of varying charge (i.

Precise Correlation of Contact Area and Forces in the Unstable Friction between a Rough Fluoroelastomer Surface and Borosilicate Glass.

Stick-slip friction of elastomers arises due to adhesion, high local strains, surface features, and viscous dissipation. In situ techniques connecting the real contact area to interfacial forces can reveal the contact evolution of a rough elastomer surface leading up to gross slip, as well as provide high-resolution dynamic contact areas for improving current slip models. Samples with rough surfaces were produced by the same manufacturing processes as machined seals.

An indentation-based approach to determine the elastic constants of soft anisotropic tissues.

Characterization of the mechanical properties of tissue can help to understand tissue mechanobiology, including disease diagnosis and progression. Indentation is increasingly used to measure the local mechanical properties of tissue, but it has not been fully adapted to capture anisotropic properties. This paper presents an indentation-based method to measure elastic constants of soft anisotropic tissues without additional mechanical tests.

Self-regenerating compliance and lubrication of polyacrylamide hydrogels.

Pristine hydrogel surfaces typically have low friction, which is controlled by composition, slip speeds, and immediate slip history. The stiffness of such samples is typically measured with bulk techniques, and is assumed to be homogeneous at the surface. While the surface properties of homogeneous hydrogel samples are generally controlled by composition, the surface also interfaces with the open bath, which distinguishes it from the bulk.

Latching of the click beetle (Coleoptera: Elateridae) thoracic hinge enabled by the morphology and mechanics of conformal structures.

Elaterid beetles have evolved to 'click' their bodies in a unique maneuver. When this maneuver is initiated from a stationary position on a solid substrate, it results in a jump not carried out by the traditional means of jointed appendages (i.e.

Compliant and stretchable thermoelectric coils for energy harvesting in miniature flexible devices.

With accelerating trends in miniaturization of semiconductor devices, techniques for energy harvesting become increasingly important, especially in wearable technologies and sensors for the internet of things. Although thermoelectric systems have many attractive attributes in this context, maintaining large temperature differences across the device terminals and achieving low-thermal impedance interfaces to the surrounding environment become increasingly difficult to achieve as the characteristic dimensions decrease. Here, we propose and demonstrate an architectural solution to this problem, where thin-film active materials integrate into compliant, open three-dimensional (3D) forms.

Mechanically-Guided Deterministic Assembly of 3D Mesostructures Assisted by Residual Stresses.

Formation of 3D mesostructures in advanced functional materials is of growing interest due to the widespread envisioned applications of devices that exploit 3D architectures. Mechanically guided assembly based on compressive buckling of 2D precursors represents a promising method, with applicability to a diverse set of geometries and materials, including inorganic semiconductors, metals, polymers, and their heterogeneous combinations. This paper introduces ideas that extend the levels of control and the range of 3D layouts that are achievable in this manner.

Poroelasticity-driven lubrication in hydrogel interfaces.

It is widely accepted that hydrogel surfaces are slippery, and have low friction, but dynamic applied stresses alter the hydrogel composition at the interface as water is displaced. The induced osmotic imbalance of compressed hydrogel which cannot swell to equilibrium should drive the resistance to slip against it. This paper demonstrates the driving role of poroelasticity in the friction of hydrogel-glass interfaces, specifically how poroelastic relaxation of hydrogels increases adhesion.

Publisher's Note: Multicellular density fluctuations in epithelial monolayers [Phys. Rev. E 92, 032729 (2015)].

This corrects the article DOI: 10.1103/PhysRevE.92.

Soft hydrated sliding interfaces as complex fluids.

Hydrogel surfaces are biomimics for sensing and mobility systems in the body such as the eyes and large joints due to their important characteristics of flexibility, permeability, and integrated aqueous component. Recent studies have shown polymer concentration gradients resulting in a less dense region in the top micrometers of the surface. Under shear, this gradient is hypothesized to drive lubrication behavior due to its rheological similarity to a semi-dilute polymer solution.

Kinetics of aqueous lubrication in the hydrophilic hydrogel Gemini interface.

The exquisite sliding interfaces in the human body share the common feature of hydrated dilute polymer mesh networks. These networks, especially when they constitute a sliding interface such as the pre-corneal tear film on the ocular interface, are described by the molecular weight of the polymer chains and a characteristic size of a minimum structural unit, the mesh size, ξ. In a Gemini interface where hydrophilic hydrogels are slid against each other, the aqueous lubrication behavior has been shown to be a function of sliding velocity, introducing a sliding timescale competing against the time scales of polymer fluctuation and relaxation at the surface.

Multicellular density fluctuations in epithelial monolayers.

Changes in cell size often accompany multicellular motion in tissue, and cell number density is known to strongly influence collective migration in monolayers. Density fluctuations in other forms of active matter have been explored extensively, but not the potential role of density fluctuations in collective cell migration. Here we investigate collective motion in cell monolayers, focusing on the divergent component of the migration velocity field to probe density fluctuations.

A novel method for low load friction testing on living cells.

A low load tribology technique for studying the effects of friction on living cells was developed. Results show a direct relationship between the coefficient of friction (COF) and the extent of cell damage. The COF, mu, for a glass pin on an intact layer of human corneal epithelial cells is determined to be on the order of mu = 0.

A biphasic model for micro-indentation of a hydrogel-based contact lens.

The stiffness and hydraulic permeability of soft contact lenses may influence its clinical performance, e.g., on-eye movement, fitting, and wettability, and may be related to the occurrence of complications; e.