Daniel A. Steingart

Power Sources for Wireless Sensor Networks

Wireless sensor networks are poised to become a very significant enabling technology in many sectors. Already a few very low power wireless sensor platforms have entered the marketplace. Almost all of these platforms are designed to run on batteries that have a very limited lifetime. In order for wireless sensor networks to become a ubiquitous part of our environment, alternative power sources must be employed. This paper reviews many potential power sources for wireless sensor nodes. Well established power sources, such as batteries, are reviewed along with emerging technologies and currently untapped sources. Power sources are classified as energy reservoirs, power distribution methods, or power scavenging methods, which enable wireless nodes to be completely self-sustaining. Several sources capable of providing power on the order of 100 μW/cm3 for very long lifetimes are feasible. It is the authors’ opinion that no single power source will suffice for all applications, and that the choice of a power source needs to be considered on an application-by-application basis.

3D Printed Quantum Dot Light-Emitting Diodes

Developing the ability to 3D print various classes of materials possessing distinct properties could enable the freeform generation of active electronics in unique functional, interwoven architectures. Achieving seamless integration of diverse materials with 3D printing is a significant challenge that requires overcoming discrepancies in material properties in addition to ensuring that all the materials are compatible with the 3D printing process. To date, 3D printing has been limited to specific plastics, passive conductors, and a few biological materials. Here, we show that diverse classes of materials can be 3D printed and fully integrated into device components with active properties. Specifically, we demonstrate the seamless interweaving of five different materials, including (1) emissive semiconducting inorganic nanoparticles, (2) an elastomeric matrix, (3) organic polymers as charge transport layers, (4) solid and liquid metal leads, and (5) a UV-adhesive transparent substrate layer. As a proof of concept for demonstrating the integrated functionality of these materials, we 3D printed quantum dot-based light-emitting diodes (QD-LEDs) that exhibit pure and tunable color emission properties. By further incorporating the 3D scanning of surface topologies, we demonstrate the ability to conformally print devices onto curvilinear surfaces, such as contact lenses. Finally, we show that novel architectures that are not easily accessed using standard microfabrication techniques can be constructed, by 3D printing a 2 × 2 × 2 cube of encapsulated LEDs, in which every component of the cube and electronics are 3D printed. Overall, these results suggest that 3D printing is more versatile than has been demonstrated to date and is capable of integrating many distinct classes of materials.

Highly Stretchable Alkaline Batteries Based on an Embedded Conductive Fabric

Recent progress in the fabrication of ultrathin silicon ribbons and novel architectures have enabled devices that can stretch, bend, and twist without mechanical fatigue or changes in operational performance. [ 1–5 ] These advances have lead to compliant, conformable electronics for health monitoring and sensing purposes. [ 6 , 7 ] For true autonomous operations, these devices require an equally accommodating power source. Existing commercially available power sources are too bulky and negate the advantages of these compliant/fl exible devices. We demonstrate a stretchable battery with electrochemically active materials embedded in a compliant conductive fabric, which acts as a support for the material. The assembled manganese dioxide (MnO 2 ) zinc (Zn) stretchable cell with a polyacrylic acid (PAA) based polymer gel electrolyte (PGE) had an open circuit potential (OCV) of 1.5 V and a capacity of 3.875 mAh/cm 2 . The capacity remained constant when tested under strain as high as 100%. Two cells connected in series continuously powered an LED when stretched to 150% and twisted by 90 degrees. In the past decade, stretchable electronics with a wide variety of functionality such as biological sensors, [ 8 ] solar cells, [ 9 ] polymer light-emitting devices, [ 10 ] transistors, [ 11 , 12 ] active matrix displays, [ 13 , 14 ] and photo-detectors [ 15 ] have been demonstrated. While there has been progress on power sources with similar mechanical properties, there is still a signifi cant gap. Previously, stretchable supercapacitors based on SWNT deposited on stretched PDMS, [ 16 ] CNTs embedded in fabric [ 17 ] and conducting polymer on compliant substrates [ 18 ] have been demonstrated, but these devices are suited to short term energy storage and cannot be used to power standalone devices. A stretchable MnO 2 -Zn primary battery with a stretchable carbon oil current collector was demonstrated, however the discharge capacity decreased by ∼ 55% when the battery was stretched by 50%. The discharge profi le showed a high ohmic potential drop (I × R) at the start of discharge for the strained battery. Drop in potential can be accounted by decrease in electrical conductivity of the carbon oil current collector and loss in electrical contact in the electrode when stretched. [ 19 ]

A flexible high potential printed battery for powering printed electronics

Mechanically flexible arrays of alkaline electrochemical cells fabricated using stencil printing onto fibrous substrates are shown to provide the necessary performance characteristics for driving ink-jet printed circuits. Due to the dimensions and material set currently required for reliable low-temperature print processing of electronic devices, a battery potential greater than that sourced by single cells is typically needed. The developed battery is a series interconnected array of 10 low resistance Zn-MnO2 alkaline cells, giving an open circuit potential of 14 V. This flexible battery is used to power an ink-jet printed 5-stage complementary ring oscillator based on organic semiconductors.

A super ink jet printed zinc–silver 3D microbattery

A novel super ink jet printing (SIJP) system was used to fabricate 3D zinc–silver microbatteries directly on a substrate. The SIJP provides a simple and flexible method to deposit interesting 2D and 3D structures of varying morphologies without the waste and large energy inputs typical of standard microfabrication technologies. The system was used to print pairs of silver electrodes with arrays of pillars on glass substrates, and in the presence of an electrolyte, the battery self-assembled during the first charge. Using an aqueous electrolyte solution of KOH with dissolved ZnO, the SIJP printed structures showed similar electrochemical behavior to batteries composed of silver foil electrodes. For a sparse array of pillars (~2.5% footprint area of each electrode pad occupied by pillars), a capacity increase of 60% was achieved in comparison with a cell with planar electrodes.

PicoCube: a 1 cm 3 sensor node powered by harvested energy

The PicoCube is a 1 cm3 sensor node using harvested energy as its source of power. Operating at an average of only 6 uW for a tire-pressure application, the PicoCube represents a modular and integrated approach to the design of nodes for wireless sensor networks. It combines advanced ultra-low power circuit techniques with system-level power management. A simple packaging approach allows the modules comprising the node to fit into 1 cm3 in a reliable fashion.

Thick film thermoelectric energy harvesting systems for biomedical applications

The potential for the microfabrication of thermoelectric generators has been shown for powering autonomous wireless sensors in and around the human body. Existing bulk fabrication methods (extrusion and dicing) as well as traditional microfabrication methods (sputtering and etching) cannot create structures with the correct size factors and aspect ratios for optimal power generation. As a result, this paper describes a new promising printing method, specifically developed to additively create microscale generators. Early results show that the method is both cost effective and scalable for the mass production of thermoelectric generators to power medical devices.

Energy Harvesting - A Systems Perspective

Deployment of sensing devices in remote areas makes battery replacement virtually impossible, so harvesting available energy for replenishment of the supply is essential. The energy available is very small, therefore ultra low power design for both computation and communication devices is required. In the paper, we will discuss the need for a system- level approach to energy scavenging, and demonstrate how effective harvesting is possible even under demanding circumstances.

A Lateral Microfluidic Cell for Imaging Electrodeposited Zinc near the Shorting Condition

The morphology evolution of zinc electrodeposited from alkaline ZnO/KOH is imaged in situ using a microfluidic cell. Working and counter electrodes are in a lateral configuration, separated by a flow channel with a height of 90 m, resulting in quasi-twodimensional zinc layers. At a flow rate of 0.3 cm/s, zinc packing in the channel is highest at a current density just above the transition from porous to dense zinc, i 170 mA/cm 2 . When deposited, compact zinc is approximately 3 times as dense as porous zinc, as determined by image analysis of the layer. The dense mode invariably leads to ramifications and critical growth, causing cell shorting. Greater zinc packing is possible at a flow rate of 3.1 cm/s, although flow rates of this order are impractical for flow-assisted zinc batteries. Ramified zinc tips approach a kinetically limited rate, independent of electrolyte flow rate. Therefore, increased flow rate cannot control critical growth once it begins. Increased flow rate results in a higher density of ramified tips at equivalent cell potential. The zinc deposition reaction has a Tafel slope of 130 mV below 10 mA/cm 2 and 50 mV

Flexible and stretchable power sources for wearable electronics

Compliant battery design strategy for wearable power sources with high degree of flexibility and stretchability. Flexible and stretchable power sources represent a key technology for the realization of wearable electronics. Developing flexible and stretchable batteries with mechanical endurance that is on par with commercial standards and offer compliance while retaining safety remains a significant challenge. We present a unique approach that demonstrates mechanically robust, intrinsically safe silver-zinc batteries. This approach uses current collectors with enhanced mechanical design, such as helical springs and serpentines, as a structural support and backbone for all battery components. We show wire-shaped batteries based on helical band springs that are resilient to fatigue and retain electrochemical performance over 17,000 flexure cycles at a 0.5-cm bending radius. Serpentine-shaped batteries can be stretched with tunable degree and directionality while maintaining their specific capacity. Finally, the batteries are integrated, as a wearable device, with a photovoltaic module that enables recharging of the batteries.