Snakeskin-Inspired Elastomers with Extremely Low Coefficient of Friction under Dry Conditions

Friction-reduction effect of the hierarchical surface microstructure of carrion beetle by controlling the real contact area

The discovery and elucidation of the surface microstructure functions of living organisms are crucial to resolving issues, such as friction. We newly discovered that Necrophila japonica, a type of carrion beetle that lives on the ground surface, exhibited a hierarchical surface microstructure comprising a submicron-sized wrinkle structure on top of a micron-sized microstructure. The surface microstructure of this beetle improved wettability but did not exhibit superhydrophobicity, a well-known function of hierarchical structures, so it was expected to have a different function. By combining the insights in the field of structural mechanics that avoidance of stress concentration by the structural geometry affects deformation with the basic principles of friction, the frictional properties and mechanisms of the hierarchical surface microstructure of carrion beetle were investigated. The measurements of frictional force indicated that the mimicked structure exerted lower frictional forces than flat and single-layer microstructure surfaces. Analysis of finite element method simulations showed that even though the mimicked structure was prone to pressure concentration due to small contact points, the surface contact pressure was reduced more than that of the single-layer structure by hierarchical load dispersion like that of metamaterials. As a result, the suppression of the increase in the real contact area due to deformation suppression contributed to effective friction-reduction. The effective friction-reduction by the hierarchical structure provides not only new insight into the surface microstructure function of various organisms, but also new design for the lubricant-free friction reduction that has been the focus of attention in carbon neutrality and other fields.

Temperature Dependence of Kinetic Friction: A Handle for Plastics Sortation?

Sortation is a crucial step in mechanical recycling of post-consumer plastics (PCR) whereby properties such as density or spectral signature are used to separate plastics. However it is difficult to sort polyolefin flakes at high throughput by these properties. We ask whether the frictional properties of plastics as a function of temperature may be used as an alternate sorting property. However, fundamental studies of friction at temperatures near their melting points are limited. Here we measure the temperature dependence of kinetic friction for three common polyolefins (high and low den- sity polyethylene and polypropylene) as well as polyethylene terephthalate (PET), focusing on the softening/melting regime. The results are augmented by differential scanning calorimetry and temperature dependence measurements of both dynamic modulus and and probe tack. For the polyolefins, we find strong increases in the coefficients of kinetic friction during temperature ramps in the melting/softening regime. For the PET, we report a notable peak in the kinetic friction which we associate with the glass transition and cold-crystallization. We discuss the enhanced friction in the context of rub- ber friction, which exhibits comparable coefficients of kinetic frictions.

Durable hydrogel-based lubricated composite coating with remarkable underwater performances

HYPOTHESIS

Hydrogel coatings have received great attention in the field of such as medical devices, water treatment membranes, flexible electronics, and marine antifouling. However, when it comes to lubrication of hydrogel materials, though it has great potential applications in the field of industrial and medical drag reduction, some restrained properties are urgently needed to overcome for releasing the practical potential.

EXPERIMENTS

Durability of high lubrication was revealed from the sliding test during the long-term storage, as well as the long-distance sliding. Some variables which possibly affect the lubrication performance were examined to demonstrate that excellent lubricity of the coating would not be easily influenced by load, frequency, friction pair and temperature. The microstructure and mechanical characterization of the lubricative coating indicate that the resistance to harsh running conditions is premised on enough hydration extent and robustness. The formulae of Possion ratio and ball-on-disk contact stress which apply to soft matter were used for calculating contact stress values in tribology tests. Anti-swelling and bio-compatibility are also verified.

FINDINGS

This work found a route of achieving superior lubrication and coexisting with stability in lubrication, which can be used for drag reduction in medical devices and shipbuilding industry.

Influence of Surface Texturing on the Dry Tribological Properties of Polymers in Medical Devices

There is a constant need to improve patient comfort and product performance associated with the use of medical devices. Efforts to optimise the tribological characteristics of medical devices usually involve modifying existing devices without compromising their main design features and functionality. This article constitutes a state-of-the-art review of the influence of dry friction on polymeric components used in medical devices, including those having microscale surface features. Surface tribology and contact interactions are discussed, along with alternative forms of surface texturing. Evident gaps in the literature, and areas warranting future research are highlighted; these include friction involving polymer Vs polymer surfaces, information regarding which topologies and feature spacings provide the best performing textured surfaces, and design guidelines that would assist manufacturers to minimise or maximise friction under non-lubricated conditions.

Contact Lens Sensor with Anti-jamming Capability and High Sensitivity for Intraocular Pressure Monitoring

Contact lens sensors provide a noninvasive approach for intraocular pressure (IOP) monitoring in patients with glaucoma. Accurate measurement of this imperceptible pressure variation requires highly sensitive sensors in the absence of simultaneously amplifying IOP signal and blinking-induced noise. However, current noise-reduction methods rely on external filter circuits, which thicken contact lenses and reduce signal quality. Here, we introduce a contact lens strain sensor with an anti-jamming ability by utilizing a self-lubricating layer to reduce the coefficient of friction (COF) to remove the interference from the tangential force. The sensor achieves exceptionally high sensitivity due to the strain concentration layout and the confined occurrence of sympatric microcracks. The animal tests prove our lens can accurately detect IOP safely and reliably.

Plant-inspired TransfOrigami microfluidics

The healthy functioning of the plants' vasculature depends on their ability to respond to environmental changes. In contrast, synthetic microfluidic systems have rarely demonstrated this environmental responsiveness. Plants respond to environmental stimuli through nastic movement, which inspires us to introduce transformable microfluidics: By embedding stimuli-responsive materials, the microfluidic device can respond to temperature, humidity, and light irradiance. Furthermore, by designing a foldable geometry, these responsive movements can follow the preset origami transformation. We term this device TransfOrigami microfluidics (TOM) to highlight the close connection between its transformation and the origami structure. TOM can be used as an environmentally adaptive photomicroreactor. It senses the environmental stimuli and feeds them back positively into photosynthetic conversion through morphological transformation. The principle behind this morphable microsystem can potentially be extended to applications that require responsiveness between the environment and the devices, such as dynamic artificial vascular networks and shape-adaptive flexible electronics.

Travelling Wave Generation of Wrinkles on the Hydrogel Surfaces

The dynamic and static properties of structured surfaces have important functions in nature. In particular, wrinkles have important static roles, for example, increasing surface area, but dynamic roles of wrinkles remain poorly understood. Specifically, to understand and utilize the dynamic functions of wrinkles, it is necessary to observe wrinkle formation directly. In this study, a polyion complex (PIC) is formed on a hydrogel surface by electrophoresis, and the process of wrinkle formation through a transparent electrode is directly observed. By quantitative analysis of the wavelength and amplitude of wrinkles, it is found that the wrinkles move randomly in a wavy pattern in the initial stage of growing process. Furthermore, the direction of wavy motion of wrinkles is controlled by the compression of hydrogels in the in-plane direction. The present study provides important insights into the fabrication of wrinkled surfaces with a controlled flow direction; opening the possibility for active wrinkles used in the development of functional surface structures as actuators that are capable of transporting small objects in water.

Co-doping optimized hydrogel-elastomer micro-actuators for versatile biomimetic motions

Hydrogels can respond to changes in humidity or temperature, while elastomers can resist structural collapse due to dehydration or external force application. A hybrid bilayer of hydrogel-elastomers while retaining the merits of both the hydrogels and elastomers has emerged as a promising stimuli-responsive micro-actuator. However, the preparation of a hydrogel-elastomer micro-actuator requires multiple steps, mainly due to the differences in the surface properties of these two materials. Among them, the steps to surface-treat the elastomer and functionalize the material of each layer involve intricate processes and excessive consumption of resources. In this work, we introduce a co-doping method to optimize the preparation of a stimuli-responsive hydrogel-elastomer micro-actuator. The surface treatment and functionalization processes are combined into one step by directly doping the polymerization initiator and functional nanomaterials into the hybrid bilayer. The thermo-responsive hydrogel is combined with a photothermal elastomer to fabricate a soft micro-actuator that can bend and unbend in response to changes in humidity and light. Based on this actuator, a set of biomimetic soft micro-robots were developed, demonstrating a series of motions, such as grabbing, crawling, and jumping. This strategy of stimuli-responsive micro-actuator preparation can benefit the hydrogel-elastomer hybrid micro-robot designs for applications ranging from self-locomotive robots in environmental monitoring to drug delivery in biomedical engineering.

A Bioinspired Hydrogel-Elastomer Hybrid Surface for Enhanced Mechanical Properties and Lubrication

Developing surfaces that realize lubrication and durable wear resistance under high pressure has great implications in areas ranging from electromechanical systems to advanced biomedical devices but has proven challenging. Inspired by the zonal and transitional structure of articular cartilage, we fabricate a hydrogel-elastomer hybrid surface, where the hydrogel interpenetrates into the polymer elastomer substrate as a transitional and bonding zone, that exhibits a low coefficient of friction and wear resistance under a high load. First, we entrap benzophenone within the surface of polymer substrates such as polydimethylsiloxane, polyvinylchloride, and polyurethane. The hybrid surface is then achieved through initiating polymerization of the acrylamide monomer on the polymer surface upon ultraviolet irradiation. We observe an interpenetration area of the hydrogel and the polymer substrate. The hybrid surface shows a low coefficient of friction (∼0.05) under a very high load (over 100 atm contact pressure). It conserves the lubrication property over 100,000 cycles under a 10.9 MPa pressure and shows slight wear. This work brings a new perspective on designing surfaces with a lubrication property and wear resistance, showing broad applications.

Spontaneous Wrinkle Formation on Hydrogel Surfaces Using Photoinitiator Diffusion from Oil-Water Interface

Patterning wrinkles on three-dimensional curved or enclosed surfaces can be challenging due to difficulties in application of uniform films and stresses on such structures. In this study, we demonstrate a simple one-step wrinkle-formation method on various hydrogel structures utilizing the oil-water interfaces. By diffusion of the photoinitiator from the oil phase to the prepolymer solution in water through the interface, a characteristic cross-linking gradient is set up in the hydrogel. Then, after photopolymerization, we observe diverse patterns of wrinkles upon changing the concentration of the hydrogel or photoinitiator. As the wrinkle formation via photoinitiator diffusion through the interface requires only UV exposure for polymerization, while taking advantage of the oil-water interfacial tension, wrinkles can be developed easily on various curved structures. In addition, we illustrate the formation of wrinkles on surfaces underneath another layer of polymer or on completely enclosed surfaces, which is difficult with conventional methods. We expect that our results will lead to production of novel microstructures and provide a platform for studying the morphogenesis of wrinkles found in nature such as in curved substrates and multilayers.