We report recent progress in the development of low modulus, highly electrically conducting thin film sheet and fabric materials and devices formed by molecular-level self-assembly processing methods and their use in flexible circuits.
Recent work in the fabrication of self assembled quantum dot (QD) detectors on active structural fibers and
for the implementation of optical fiber sensors is reported in this paper. The ability to develop the QD
based devices and materials via the electrostatic self-assembly (ESA) process has been demonstrated by
Hand and Kang in prior work. The QD precursor nanocluster materials involved in ESA have been
designed and synthesized to proper size, stabilized in an aqueous-based solution, and functionalized to
allow self-assembly. Optical fiber sensor instrumentation has been developed to monitor the reflected
optical power with the buildup of the QD layers on the fiber endface during the ESA process. The results
are confirmed by observing the effects of low-finesse QD Fabry-Perot interferometric cavities formed via
such processes on the ends of optical fibers. The photocurrent-voltage characteristics show a diode-like
behavior with linear photocurrent in the reverse bias and non-linearity in the forward bias. It is suggested
that fast response times can be achieved due to the high carrier mobilities that arise in part due to structure
of the materials formed via the solution-based ESA process. This paper reviews this prior work and shows
examples of deposition of devices on both fiber endfaces and cladding surfaces.
This paper describes the use of Metal Rubber<sup>TM</sup>, which is an electrically-conductive, low modulus, highly-flexible, and optically transparent free-standing or conformal coating nanocomposite material that is fabricated via Electrostatic Self-Assembly (ESA), as a polymer MEMS sensor for actuator materials. ESA is an environmentally-friendly layer-by-layer fabrication technique in which Metal Rubber<sup>TM</sup> can be tailor designed at the molecular level to function as a sensor and/or electrode for active polymer devices. With its controllable and tailorable properties (such as mechanical modulus [from less than 0.1 MPa to greater than 500 MPa], electrical conductivity, sensitivity to flex and strain (tension and compression), thickness, transmission, glass transition, and more), Metal Rubber<sup>TM</sup> exhibits massive improvements over traditional stiff electrodes and sensors (with bulky/heavy wire components) that physically constrain the actuator device motion and thus limit productivity. Metal RubberTM shows exceptional potential for use as flexible sensors, electrodes, and interconnect components for many active polymer applications. One example of such is NanoSonic's Metal Rubber<sup>TM</sup>-Polymer MEMS (MR<sup>TM</sup>-PMEMS) nanocluster-based corrosion sensor for aircraft coatings that was developed for an Air Force SBIR program. MR<sup>TM</sup>-PMEMS was tailored via ESA for use as an in-situ sensor of chemical modifications and the breakdown of surface coatings via micro-strain measurements.
Two different optical fiber-based sensor approaches are compared for the detection of hydrogen gas. The two sensors both use Fabry-Perot techniques that have been investigated for some time for other applications. One involves the use of an Extrinsic Fabry-Perot Interferometric (EFPI) sensor scheme, and the other uses a nanoFabry-Perot (nanoFP) cavity that is formed on the distal end of a fiber endface. It is found in general that the sensitivity of the EFPI sensor is higher than that of the nanoFP, but that its speed of response is slower.
Molecular-level self-assembly processes allow the formation of novel materials with properties that are not
achievable using conventional fabrication methods. For example, nanostructured metals and polymers may be combined
to form inorganic/organic materials that exhibit properties typically associated with each of these species separately,
namely high electrical conductivity and low Young's modulus. The combination of such properties is of interest for a
number of engineering applications. For example, methods to form stretchable metal conductors, either on elastomeric
substrates or as free-standing materials, have been investigated for some time, in part as a way to overcome the high
modulus of sensor and actuator electrode materials, and more generally to address the need for mechanically flexible
interconnections in polymer electronic devices, flex circuits, electronic textiles and similar electrical circuit applications.
Of particular recent interest for example is summarized in  where a process to form electrical connectivity using 100
nm-wide gold stripes evaporated onto polydimethylsiloxane (PDMS) is reported, and where non-zero electrical
conductivity was observed for strains up to 22%.
This paper describes the use of free-standing electrically conductive ultra-low modulus materials that withstand elongations up to 1000% as sensors for the measurement of large strains. NanoSonic has developed novel, high performance, multifunctional polymers for use in self-assembly processing that result in durable free-standing conductive films - with both controlled nominal conductivity and Young's modulus. Such films exhibit a change in electrical conductivity as a function of tensile strain; whereby the magnitude of the change is controlled via chemical processing.
This paper summarizes nanostructured optical fiber sensors fabricated by molecular self-assembly chemistry. Strain, pressure, vibration and chemical sensors are described which are based on selfassembled fiber cores, claddings, distal endface coatings and free-standing membranes.
We report recent improvements of Metal RubberTM strain sensors formed by electrostatic self-assembly (ESA) processing. The sensors may be used to measure strains from approximately 1 microstrain to several hundred percent strain, over gauge lengths ranging from approximately 1 millimeter to several tens of centimeters.
We discuss recent improvements of Metal RubberTM materials formed by electrostatic self-assembly (ESA) processing. Free-standing and mechanically robust sheets of Metal RubberTM have been synthesized with electrical conductivities approximately one order of magnitude lower than those of bulk noble metals and with moduli from 1 to 100 MPa.
This paper describes the use of Metal Rubber, which is an electrically conductive, low modulus, and optically transparent free-standing nanocomposite, as an electrode for active polymer devices. With its controllable and tailorable properties [such as modulus (from ~ 1 MPa to 100 MPa), electrical conductivity, sensitivity to flex and strain, thickness, transmission, glass transition, and more], Metal Rubber exhibits massive improvements over traditional stiff electrodes that physically constrain the actuator device motion and thus limit productivity. Metal Rubber shows exceptional potential for use as flexible electrodes for many active polymer applications.
This paper describes the commercial applications of Metal Rubber, the first material of its kind, a self-assembled free-standing electrically conductive elastomer in biomedical, aerospace and microelectronic areas. Metal Rubber is a novel nanocomposite formed via the self-assembly processing of metal nanoparticles and elastomeric polyectrolytes. This type of processing allows for control over bulk mechanical and electrical properties and requires only ppm quantities of metal to achieve percolation. The use of nanostructured precursors also results in transparent, electrically conductive nanocomposites. Metal Rubber elastomers are being developed as electrodes, for biomedical applications; flexible interconnects for microelectronics, and sensors to detect fatigue, impact and large strain for aerospace applications. This novel material may be formed as a conformal coating on nearly any substrate or as free standing films.
This paper presents an update concerning the properties of a new class of nanostructured materials that exhibit the combined properties of low mechanical modulus and high electrical conductivity. Such "Metal Rubber<sup>TM</sup>" materials are formed by molecular-level self-assembly processes. Material synthesis and properties are described. Potential applications for space-based photonics and electronics are in flexible polymer-based electrodes and opto-electronic devices.
We report the development of low modulus, highly conducting thin film electrodes formed by molecular-level self-assembly processing methods. The electrodes may be used on sensor or actuator materials requiring large strain.
We report the development of nanostructured strain sensors formed by electrostatic self-assembly (ESA) processing. The sensors may be used to measure strains from 1 microstrain to more than 100% strain, over gauge lengths ranging from approximately 1 millimeter to tens of centimeters.