Andrew G. Gillies

Nanowire active-matrix circuitry for low-voltage macroscale artificial skin

Large-scale integration of high-performance electronic components on mechanically flexible substrates may enable new applications in electronics, sensing and energy. Over the past several years, tremendous progress in the printing and transfer of single-crystalline, inorganic micro- and nanostructures on plastic substrates has been achieved through various process schemes. For instance, contact printing of parallel arrays of semiconductor nanowires (NWs) has been explored as a versatile route to enable fabrication of high-performance, bendable transistors and sensors. However, truly macroscale integration of ordered NW circuitry has not yet been demonstrated, with the largest-scale active systems being of the order of 1 cm(2) (refs 11,15). This limitation is in part due to assembly- and processing-related obstacles, although larger-scale integration has been demonstrated for randomly oriented NWs (ref. 16). Driven by this challenge, here we demonstrate macroscale (7×7 cm(2)) integration of parallel NW arrays as the active-matrix backplane of a flexible pressure-sensor array (18×19 pixels). The integrated sensor array effectively functions as an artificial electronic skin, capable of monitoring applied pressure profiles with high spatial resolution. The active-matrix circuitry operates at a low operating voltage of less than 5 V and exhibits superb mechanical robustness and reliability, without performance degradation on bending to small radii of curvature (2.5 mm) for over 2,000 bending cycles. This work presents the largest integration of ordered NW-array active components, and demonstrates a model platform for future integration of nanomaterials for practical applications.

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.

Carbon nanotube active-matrix backplanes for conformal electronics and sensors.

In this paper, we report a promising approach for fabricating large-scale flexible and stretchable electronics using a semiconductor-enriched carbon nanotube solution. Uniform semiconducting nanotube networks with superb electrical properties (mobility of ∼20 cm2 V(-1) s(-1) and ION/IOFF of ∼10(4)) are obtained on polyimide substrates. The substrate is made stretchable by laser cutting a honeycomb mesh structure, which combined with nanotube-network transistors enables highly robust conformal electronic devices with minimal device-to-device stochastic variations. The utility of this device concept is demonstrated by fabricating an active-matrix backplane (12×8 pixels, physical size of 6×4 cm2) for pressure mapping using a pressure sensitive rubber as the sensor element.

Shear adhesion strength of thermoplastic gecko-inspired synthetic adhesive exceeds material limits.

Natural gecko array wearless dynamic friction has recently been reported for 30,000 cycles on a smooth substrate. Following these findings, stiff polymer gecko-inspired synthetic adhesives have been proposed for high-cycle applications such as robot feet. Here we examine the behavior of high-density polyethylene (HDPE) and polypropylene (PP) microfiber arrays during repeated cycles of engagement on a glass surface, with a normal preload of less than 40 kPa. We find that fiber arrays maintained 54% of the original shear stress of 300 kPa after 10,000 cycles, despite showing a marked plastic deformation of fiber tips. This deformation could be due to shear-induced plastic creep of the fiber tips from high adhesion forces, adhesive wear, or thermal effects. We hypothesize that a fundamental material limit has been reached for these fiber arrays and that future gecko synthetic adhesive designs must take into account the high adhesive forces generated to avoid damage. Although the synthetic material and natural gecko arrays have a similar elastic modulus, the synthetic material does not show the same wear-free dynamic friction as the gecko.

CLASH: Climbing vertical loose cloth

CLASH is a 10cm, 15g robot capable of climbing vertical loose-cloth surfaces at 15 cm per second. The robot has a single actuator driving its six legs which are equipped with novel passive foot mechanisms to facilitate smooth engagement and disengagement of spines. These foot mechanisms are designed to be used on penetrable surfaces and offer improved tensile normal force generation during stance and reduced normal pull-off forces during retraction. Descended from the DASH hexapedal robot, CLASH features a redesigned transmission with a lower profile and improved dynamics for climbing. CLASH is the first known robot to climb loose vertical cloth and is able to climb surfaces when surface rigidity is not guaranteed.

Gecko toe and lamellar shear adhesion on macroscopic, engineered rough surfaces

The role in adhesion of the toes and lamellae – intermediate-sized structures – found on the gecko foot remains unclear. Insight into the function of these structures can lead to a more general understanding of the hierarchical nature of the gecko adhesive system, but in particular how environmental topology may relate to gecko foot morphology. We sought to discern the mechanics of the toes and lamellae by examining gecko adhesion on controlled, macroscopically rough surfaces. We used live Tokay geckos, Gekko gecko, to observe the maximum shear force a gecko foot can attain on an engineered substrate constructed with sinusoidal patterns of varying amplitudes and wavelengths in sizes similar to the dimensions of the toes and lamellae structures (0.5 to 6 mm). We found shear adhesion was significantly decreased on surfaces that had amplitudes and wavelengths approaching the lamella length and inter-lamella spacing, losing 95% of shear adhesion over the range tested. We discovered that the toes are capable of adhering to surfaces with amplitudes much larger than their dimensions even without engaging claws, maintaining 60% of shear adhesion on surfaces with amplitudes of 3 mm. Gecko adhesion can be predicted by the ratio of the lamella dimensions to surface feature dimensions. In addition to setae, remarkable macroscopic-scale features of gecko toes and lamellae that include compliance and passive conformation are necessary to maintain contact, and consequently, generate shear adhesion on macroscopically rough surfaces. Findings on the larger scale structures in the hierarchy of gecko foot function could provide the biological inspiration to drive the design of more effective and versatile synthetic fibrillar adhesives.

Dynamic climbing of near-vertical smooth surfaces

A 10 cm hexapedal robot is adapted to dynamically climb near-vertical smooth surfaces. A gecko-inspired adhesive is mounted with an elastomer tendon and polymer loop to a remote-center-of-motion ankle that allows rapid engagement with the surface and minimizes peeling moments on the adhesive. The maximum velocity possible while climbing decreases as the incline gets closer to vertical, with the robot able to achieve speeds of 10 cm second-1 at a 70-degree incline. A model is implemented to describe the effect of incline angle on climbing speed and, together with high-speed video evidence, reveals that climbing velocity is limited by robot dynamics and adhesive properties and not by power.

Simulation of synthetic gecko arrays shearing on rough surfaces

To better understand the role of surface roughness and tip geometry in the adhesion of gecko synthetic adhesives, a model is developed that attempts to uncover the relationship between surface feature size and the adhesive terminal feature shape. This model is the first to predict the adhesive behaviour of a plurality of hairs acting in shear on simulated rough surfaces using analytically derived contact models. The models showed that the nanoscale geometry of the tip shape alters the macroscale adhesion of the array of fibres by nearly an order of magnitude, and that on sinusoidal surfaces with amplitudes much larger than the nanoscale features, spatula-shaped features can increase adhesive forces by 2.5 times on smooth surfaces and 10 times on rough surfaces. Interestingly, the summation of the fibres acting in concert shows behaviour much more complex that what could be predicted with the pull-off model of a single fibre. Both the Johnson–Kendall–Roberts and Kendall peel models can explain the experimentally observed frictional adhesion effect previously described in the literature. Similar to experimental results recently reported on the macroscale features of the gecko adhesive system, adhesion drops dramatically when surface roughness exceeds the size and spacing of the adhesive fibrillar features.

A micromolded connector for reconfigurable millirobots

We have developed a micromolded connector for applications such as folded millirobots where reusability, low engaged profile and rapid assembly are required. Using laser micromachining and micromolding techniques, the microconnector is formed with a manufacturing process that can be integrated with the rapid prototyping of the millirobots. The microconnector engages through shear in one direction while forming a strong connection in orthogonal directions, with an engaged thickness of 200 µm. The microconnectors have been shown to be strong in the shear and normal pull-off directions with strengths of 42 and 27 N cm−2, respectively, as well as being robust to shear failure, maintaining over 65% of their strength after being run to failure ten times. Due to the anisotropic property of the microconnector, it can also be rapidly engaged and disengaged repeatedly in the release direction without degrading the connector strength. The microconnectors have also been successfully integrated into various millirobots for quick appendage changes, showing their applicability to millirobots.

Angled microfiber arrays as low-modulus, low Poisson"s ratio compliant substrates

This paper presents a novel fabrication method for producing high-aspect-ratio, angled polymer microfiber arrays by directional pulling of polyethylene from low-aspect-ratio, microporous polycarbonate templates. These arrays represent a novel substrate for electrostatic sensors and actuators because they are characterized by low stiffness (<24 kPa effective elastic modulus), low Poissons ratio (effectively zero at low strains), and very low density (<1% of solid polyethylene). Validation of these properties is presented by developing a model based on elastica theory, and by performing experiments using a parallel-plate electrostatic actuator fabricated with a fiber array as the dielectric and return spring.