Beyond biomimicry - next generation applications of bioinspired adhesives from microfluidics to composites.

In this perspective article, Professor Dan Sameoto outlines his opinion on future opportunities in the field of biomimetic adhesives. Despite over twenty years of excellent academic work by groups all around the world in this subfield, the economic value and impact of these materials is somewhat underwhelming. The question for the field is whether it should have a scientific and engineering focus to create every greater performance and understanding of the materials and hope that "if we build it, they will come".

The thermal 'Buddha Board'-application of microstructured polyolefin films for variable thermal infrared transparency materials.

In this study, we explore the innovative application of biological principles of scattering foams and structural colouration of white materials to manipulate the transmission properties of thermal infrared (IR) radiation, particularly within the 8-14 μm wavelength range in polyolefin materials. Inspired by the complex skin of organisms such as chameleons, which can dynamically change colour through structural alterations, as well as more mundane technologies such as Buddha Boards and magic water colouring books, we are developing methods to control thermal IR transmission using common thermoplastic materials that are semi-transparent to thermal IR radiation. Polyethylene and polypropylene, known for their versatility and cost-effectiveness, can be engineered into microstructured sheets with feature sizes spanning from 5 to 100 μm.

Towards High Efficiency and Rapid Production of Room-Temperature Liquid Metal Wires Compatible with Electronic Prototyping Connectors.

We present in this work new methodologies to produce, refine, and interconnect room-temperature liquid-metal-core thermoplastic elastomer wires that have extreme extendibility (>500%), low production time and cost at scale, and may be integrated into commonly used electrical prototyping connectors like a Japan Solderless Terminal (JST) or Dupont connectors. Rather than focus on the development of a specific device, the aim of this work is to demonstrate strategies and processes necessary to achieve scalable production of liquid-metal-enabled electronics and address several key challenges that have been present in liquid metal systems, including leak-free operation, minimal gallium corrosion of other electrode materials, low liquid metal consumption, and high production rates. The ultimate goal is to create liquid-metal-enabled rapid prototyping technologies, similar to what can be achieved with Arduino projects, where modification and switching of components can be performed in seconds, which enables faster iterations of designs.

A Review of Electrically Driven Soft Actuators for Soft Robotics.

In recent years, the field of soft robotics has gained much attention by virtue of its aptness to work in certain environments unsuitable for traditional rigid robotics. Along with the uprising field of soft robotics is the increased attention to soft actuators which provide soft machines the ability to move, manipulate, and deform actively. This article provides a focused review of various high-performance and novel electrically driven soft actuators due to their fast response, controllability, softness, and compactness.

Smart Textiles for Visible and IR Camouflage Application: State-of-the-Art and Microfabrication Path Forward.

Protective textiles used for military applications must fulfill a variety of functional requirements, including durability, resistance to environmental conditions and ballistic threats, all while being comfortable and lightweight. In addition, these textiles must provide camouflage and concealment under various environmental conditions and, thus, a range of wavelengths on the electromagnetic spectrum. Similar requirements may exist for other applications, for instance hunting.

Durability and Recoverability of Soft Lithographically Patterned Hydrogel Molds for the Formation of Phase Separation Membranes.

Hydrogel-facilitated phase separation (HFPS) has recently been applied to make microstructured porous membranes by modified phase separation processes. In HFPS, a soft lithographically patterned hydrogel mold is used as a water content source that initiates the phase separation process in membrane fabrication. However, after each membrane casting, the hydrogel content changes due to the diffusion of organic solvent into the hydrogel from the original membrane solution.

Editorial for the Special Issue on Polymer Based MEMS and Microfabrication.

Polymers are becoming increasingly important in MEMS and microfabricated products [...

Fluorosilicone as an Omnimold for Microreplication.

Soft lithography and replica molding have been an integral part of polymer basic microfabrication for over 20 years. The use of silicone rubber materials as either molds or directly molded parts are well described in the literature and have provided researchers with an easily accessible technique to reproduce complex micro and nanostructures with minimal costs and technical challenges. Yet, for many applications, the use of standard silicones may not necessarily be the best choice, either as a mold material or as a replicated surface.

Slit Tubes for Semisoft Pneumatic Actuators.

This article describes a new principle for designing soft or 'semisoft' pneumatic actuators: SLiT (for SLit-in-Tube) actuators. Inflating an elastomeric balloon, when enclosed by an external shell (a material with higher Young's modulus) containing slits of different directions and lengths, produces a variety of motions, including bending, twisting, contraction, and elongation. The requisite pressure for actuation depends on the length of the slits, and this dependence allows sequential actuation by controlling the applied pressure.

Adhesion Circle: A New Approach To Better Characterize Directional Gecko-Inspired Dry Adhesives.

The number of different designs of directional gecko-inspired adhesives has proliferated over the past 15 years, but some basic characterization tools are still nonstandardized, which can make direct comparisons of different adhesives in the literature difficult. By far the most common type of test for directional adhesives, the load-drag-pull (LDP) test is useful but can miss substantial information on the exact behavior of gecko-inspired adhesives in a variety of loading conditions. Other test techniques, including angled approaches and pull-offs, have been employed by a few groups but they are not as widely adopted; peel tests can be employed but require a larger amount of adhesive material to use in the test, which is not always practical given some current manufacturing constraints.

Switchable Dry Adhesion with Step-like Micropillars and Controllable Interfacial Contact.

Dry adhesives have attracted much attention because of their repeatable and reversible attachment. Many research groups have made fruitful achievements in fabricating and designing various dry adhesives. However, most of these studies focus on imitating bioinspired geometry to achieve this smart adhesion, neglecting the contact interface control through their peeling motion.

Strong, reversible underwater adhesion via gecko-inspired hydrophobic fibers.

Strong, reversible underwater adhesion using gecko-inspired surfaces is achievable through the use of a hydrophobic structural material and does not require surface modification or suction cup effects for this adhesion to be effective. Increased surface energy can aid in dry adhesion in an air environment but strongly degrades wet adhesion via reduction of interfacial energy underwater. A direct comparison of structurally identical but chemically different mushroom shaped fibers shows that strong, reversible adhesion, even in a fully wetted, stable state, is feasible underwater if the structural material of the fibers is hydrophobic and the mating surface is not strongly hydrophilic.

Fabrication and characterization of thermoplastic elastomer dry adhesives with high strength and low contamination.

Polydimethylsiloxane (PDMS) and polyurethane elastomers have commonly been used to manufacture mushroom shaped gecko-inspired dry adhesives with high normal adhesion strength. However, the thermosetting nature of these two materials severely limits the commercial viability of their manufacturing due to long curing times and high material costs. In this work, we introduce poly(styrene-ethylene/butylene-styrene) (SEBS) thermoplastic elastomers as an alternative for the manufacture of mushroom shaped dry adhesives with both directional and nondirectional performance.

Anisotropic dry adhesive via cap defects.

We demonstrate how introducing a deliberate defect on the overhanging caps of strongly adhering mushroom shaped dry adhesive fibers can produce directional adhesion behavior. We find that the shape and location of this defect controls both the total adhesion force and the degree of directionality for these bio-inspired adhesives. Linear beam theory is used to demonstrate how the application of a shear load to a fiber in tension can create a small compressive load to an asymmetric crack, thereby delaying adhesion failure and producing directional adhesion, and the theory is confirmed with finite element models and empirical data.

Controllable interfacial adhesion applied to transfer light and fragile objects by using gecko inspired mushroom-shaped pillar surface.

Gecko-inspired surfaces are smart dry adhesive surfaces that have attracted much attention because of their wide range of potential applications. However, strong frictional force, rather than adhesive force, is frequently targeted in most of research in this area. In this study, the interfacial adhesive and frictional properties of a gecko-inspired mushroom-shaped polyurethane pillar array surface have been systematically characterized to design and control the interfacial adhesion of the surface by considering the nanoscale interfacial adhesion, the microscale structural compliance and deformation, and the macro-scale actuation.