Mirza Saquib Sarwar

Bend, stretch, and touch: Locating a finger on an actively deformed transparent sensor array

A stretchable, transparent touch pad and proximity sensor made using silicone and gel operates while being bent and stretched. The development of bendable, stretchable, and transparent touch sensors is an emerging technological goal in a variety of fields, including electronic skin, wearables, and flexible handheld devices. Although transparent tactile sensors based on metal mesh, carbon nanotubes, and silver nanowires demonstrate operation in bent configurations, we present a technology that extends the operation modes to the sensing of finger proximity including light touch during active bending and even stretching. This is accomplished using stretchable and ionically conductive hydrogel electrodes, which project electric field above the sensor to couple with and sense a finger. The polyacrylamide electrodes are embedded in silicone. These two widely available, low-cost, transparent materials are combined in a three-step manufacturing technique that is amenable to large-area fabrication. The approach is demonstrated using a proof-of-concept 4 × 4 cross-grid sensor array with a 5-mm pitch. The approach of a finger hovering a few centimeters above the array is readily detectable. Light touch produces a localized decrease in capacitance of 15%. The movement of a finger can be followed across the array, and the location of multiple fingers can be detected. Touch is detectable during bending and stretch, an important feature of any wearable device. The capacitive sensor design can be made more or less sensitive to bending by shifting it relative to the neutral axis. Ultimately, the approach is adaptable to the detection of proximity, touch, pressure, and even the conformation of the sensor surface.

Cone-shaped forest of aligned carbon nanotubes: An alternative probe for scanning microscopy

A scanning microscopy probe based on three-dimensionally shaped carbon nanotube (CNT) forests and its application to atomic-force microscopy (AFM) are reported. Micro-scale CNT forests directly grown on silicon cantilevers are patterned into cone shapes with the tips of a few individual nanotubes. The CNT-forest-based probes provide significantly higher mechanical stability/robustness than the common single-CNT probes. AFM imaging using the fabricated probes reveals their imaging ability comparable to that of commercial probes. The patterning process also improves the uniformity of the CNT forests grown on each cantilever. The results suggest a promising future for CNT scanning probes and their production approach.

Transparent and conformal 'piezoionic' touch sensor

A polyurethane hydrogel based touch sensor with high transparency and conformability is demonstrated. Polyurethane hydrogels swollen with various electrolytes were compressed at a pressure of 30 kPa, simulating a fingertap on a conventional touch screen device. Unlike ionic polymer metal composite and conducting polymer trilayer sensors, where electrodes render the sensors opaque and relatively rigid, the electrodes used in this work are metal wires or strips, separated from each other by regions of transparent film, enabling transparency and compliance. The voltages and currents observed when the perturbation is above one electrode are on the order of 10-2 V and 10-7 A, relative to a second electrode that is approximately 1 cm away. The sign of voltage and current signals detected from perturbations made between electrodes is determined by relative proximity to each electrode, and the magnitude appears to decrease with increasing distance from the electrodes. These observations suggest that it may be possible to discriminate the location of touch based on signals transmitted to the edges of an ionically conductive film. A model to describe the inhomogeneous ionic distribution and predict the resultant voltage and current is presented to qualitatively explain the sensing, based on the Donnan potential.

Proximity and touch sensing using deformable ionic conductors (Conference Presentation)

There is increasing interest in creating bendable and stretchable electronic interfaces that can be worn or applied to virtually any surface. The electroactive polymer community is well placed to add value by incorporating sensors and actuators. Recent work has demonstrated transparent dielectric elastomer actuation as well as pressure, stretch or touch sensing. Here we present two alternative forms of sensing. The first uses ionically conductive and stretchable gels as electrodes in capacitive sensors that detect finger proximity. In this case the finger acts as a third electrode, reducing capacitance between the two gel electrodes as it approaches, which can be detected even during bending and stretching. Very light finger touch is readily detected even during deformation of the substrate. Lateral resolution is achieved by creating a sensor array. In the second approach, electrodes placed beneath a salt containing gel are able to detect ion currents generated by the deformation of the gel. In this approach, applied pressure results in ion currents that create a potential difference around the point of contact, leading to a voltage and current in the electrodes without any need for input electrical energy. The mechanism may be related to effects seen in ionomeric polymer metal composites (IPMCs), but with the response in plane rather than through the thickness of the film. Ultimately, these ionically conductive materials that can also be transparent and actuate, have the potential to be used in wearable devices.

Micro glow plasma for localized nanostructural modification of carbon nanotube forest

This paper reports the localized selective treatment of vertically aligned carbon nanotubes, or CNT forests, for radial size modification of the nanotubes through a micro-scale glow plasma established on the material. An atmospheric-pressure DC glow plasma is shown to be stably sustained on the surface of the CNT forest in argon using micromachined tungsten electrodes with diameters down to 100 μm. Experiments reveal thinning or thickening of the nanotubes under the micro glow depending on the process conditions including discharge current and process time. These thinning and thickening effects in the treated nanotubes are measured to be up to ∼30% and ∼300% in their diameter, respectively, under the tested conditions. The elemental and Raman analyses suggest that the treated region of the CNT forest is pure carbon and maintains a degree of crystallinity. The local plasma treatment process investigated may allow modification of material characteristics in different domains for targeted regions or patterns...

Field-emission from carbon nanotube cones fabricated by micro-electro-discharge machining

Field-emitters based on patterned carbon nanotube (CNT) arrays have promising properties. For example, they operate at low voltages and produce significant current. To pattern carbon nanotube arrays into various shapes, the typical approach consists of using lithography to pattern the catalyst prior to nanotube growth. However, this technique enables only two-dimensional patterning, where the height of the nanotubes remains unchanged. It is highly desirable to tailor the shape of CNT arrays in three dimensions (3D) in order to optimize the field emission performance of the arrays, including height control and creating angled surfaces. Here, we report on the polishing of the top surface of a CNT forest pillar and creation of cone-type structures in CNT arrays, similar in shape to the emitters in a conventional field-emitter array based on bulk metals. For this, we use dry micro-electro-discharge machining (μEDM) in oxygen ambient. We also report the results of field-emission experiments from them and show that the beam resulting from the CNT cone produces a sharp, uniform emission spot on the phosphor screen in field-emission microscopy.