Optically- and thermally-responsive programmable materials based on carbon nanotube-hydrogel polymer composites.

A simple approach is described to fabricate reversible, thermally- and optically responsive actuators utilizing composites of poly(N-isopropylacrylamide) (pNIPAM) loaded with single-walled carbon nanotubes. With nanotube loading at concentrations of 0.75 mg/mL, we demonstrate up to 5 times enhancement to the thermal response time of the nanotube-pNIPAM hydrogel actuators caused by the enhanced mass transport of water molecules. Additionally, we demonstrate the ability to obtain ultrafast near-infrared optical response in nanotube-pNIPAM hydrogels under laser excitation enabled by the strong absorption properties of nanotubes. The work opens the framework to design complex and programmable self-folding materials, such as cubes and flowers, with advanced built-in features, including tunable response time as determined by the nanotube loading.

Hybrid core-shell nanowire forests as self-selective chemical connectors.

Conventional connectors utilize mechanical, magnetic, or electrostatic interactions to enable highly specific and reversible binding of the components (i.e., mates) for a wide range of applications. As the connectors are miniaturized to small scales, a number of shortcomings, including low binding strength, high engagement/disengagement energies, difficulties with the engagement, fabrication challenges, and the lack of reliability are presented that limit their successful operation. Here, we report unisex, chemical connectors based on hybrid, inorganic/organic nanowire (NW) forests that utilize weak van der Waals bonding that is amplified by the high aspect ratio geometric configuration of the NWs to enable highly specific and versatile binding of the components. Uniquely, NW chemical connectors exhibit high macroscopic shear adhesion strength (approximately 163 N/cm(2)) with minimal binding to non-self-similar surfaces, anisotropic adhesion behavior (shear to normal strength ratio approximately 25), reusability (approximately 27 attach/detach cycles), and efficient binding for both micro- and macroscale dimensions.

Sliding-induced adhesion of stiff polymer microfibre arrays. I. Macroscale behaviour.

Gecko-inspired microfibre arrays with 42 million polypropylene fibres cm(-2) (each fibre with elastic modulus 1 GPa, length 20 microm and diameter 0.6 microm) were fabricated and tested under pure shear loading conditions, after removing a preload of less than 0.1 N cm(-2). After sliding to engage fibres, 2 cm2 patches developed up to 4N of shear force with an estimated contact region of 0.44 cm2. The control unfibrillated surface had no measurable shear force. For comparison, a natural setal patch tested under the same conditions on smooth glass showed approximately seven times greater shear per unit estimated contact region. Similar to gecko fibre arrays, the synthetic patch maintains contact and increases shear force with sliding. The high shear force observed (approx. 210 nN per fibre) suggests that fibres are in side contact, providing a larger true contact area than would be obtained by tip contact. Shear force increased over the course of repeated tests for synthetic patches, suggesting deformation of fibres into more favourable conformations.

Sliding-induced adhesion of stiff polymer microfibre arrays. II. Microscale behaviour.

The adhesive pads of geckos provide control of normal adhesive force by controlling the applied shear force. This frictional adhesion effect is one of the key principles used for rapid detachment in animals running up vertical surfaces. We developed polypropylene microfibre arrays composed of vertical, 0.3 microm radius fibres with elastic modulus of 1 GPa which show this effect for the first time using a stiff polymer. In the absence of shear forces, these fibres show minimal normal adhesion. However, sliding parallel to the substrate with a spherical probe produces a frictional adhesion effect which is not seen in the flat control. A cantilever model for the fibres and the spherical probe indicates a strong dependence on the initial fibre angle. A novel feature of the microfibre arrays is that adhesion improves with use. Repeated shearing of fibres temporarily increases maximum shear and pull-off forces.

Variable Stiffness Fiber with Self-Healing Capability.

A variable stiffness fiber made of silicone and low melting point alloys quickly becomes >700 times softer and >400 times more deformable when heated above 62 °C. It shows remarkable self-healing properties and can be clamped, knitted, and bonded, as shown in a foldable multi-purpose drone, a wearable cast for bone injuries, and a soft multi-directional actuator.

Versatile Soft Grippers with Intrinsic Electroadhesion Based on Multifunctional Polymer Actuators.

A highly versatile soft gripper that can handle an unprecedented range of object types is developed based on a new design of dielectric elastomer actuators employing an interdigitated electrode geometry, simultaneously maximizing both electroadhesion and electrostatic actuation while incorporating self-sensing. The multifunctionality of the actuator leads to a highly integrated, lightweight, fast, soft gripper with simplified structure and control.