Natural articular cartilage has ultralow friction even at high squeezing pressure. Biomimicking cartilage with soft materials has been and remains a grand challenge in the fields of materials science and engineering. Inspired by the unique structural features of the articular cartilage, as well as by its remarkable lubrication mechanisms dictated by the properties of the superficial layers, a novel archetype of cartilage-mimicking bilayer material by robustly entangling thick hydrophilic polyelectrolyte brushes into the subsurface of a stiff hydrogel substrate is developed. The topmost soft polymer layer provides effective aqueous lubrication, whereas the stiffer hydrogel layer used as a substrate delivers the load-bearing capacity. Their synergy is capable of attaining low friction coefficients (order 0.010) under heavily loaded conditions (order 10 MPa contact pressure) in water environment, a performance incredibly close to that of natural articular cartilage. The bioinspired material can maintain low friction even when subjected to 50k reciprocating cycles under high contact pressure, with almost no wear observed on the sliding track. These findings are theoretically explained and compounded by multiscale simulations used to shed light on the mechanisms responsible for this remarkable performance. This work opens innovative technology routes for developing cartilage-mimicking ultralow friction soft materials.
Constructing tubular hydrogel materials with desirable structures based on their functional application is a big challenge. Here, we report a simple but effective method to prepare tubular hydrogels with complex geometries by surface radical polymerization, in which an iron wire acts as both catalyst and template for the formation of a gel layer with controllable thickness. The formed hydrogel layer can be easily peeled off from the template after secondary cross-linking to obtain hollow hydrogel tubes which exhibit extraordinary and tunable tensile strength, good elasticity, and pressure-bearing capability. The method can be generalized to construct a series of complex three-dimensional hydrogel tubes with versatile components for building up fluidic channels or biocompatible 3D cell culturing platform for tissue engineering. Such a method is a great advance in the field of hydrogel materials. It is anticipated that this innovation would open up the door for developing functional 3D tubular hydrogel materials suitable for multiple applications.
The ability to tolerate large strains during various degrees of deformation is a core issue in the development of flexible electronics. Commonly used strategies nowadays to enhance the strain tolerance of thin film devices focus on the optimization of the device architecture and the increase of bonding at the materials interface. In this paper, we propose a strategy, namely elasto-plastic design of an ultrathin interlayer, to boost the strain tolerance of flexible electronics. We demonstrate that insertion of an ultrathin, stiff (high Young’s modulus) and elastic (high yield strain) interlayer between an upper rigid film/device and a soft substrate, regardless of the substrate thickness or the interfacial bonding, can significantly reduce the actual strain applied on the film/device when the substrate is bent. Being independent of existing strategies, the elasto-plastic design strategy offers an effective method to enhance the device flexibility without redesigning the device structure or altering the material interface.
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