Abstract This work draws inspiration from totipotent cellular systems to design smart materials whose compositions and properties can be learned or evolved. Totipotency refers to the inherent genetic potential of a single cell to adapt and produce all types of differentiated cells within an organism. To study this principal and apply it synthetically, tissue-like compartmentalized assemblies are constructed via lipid membrane-separated aqueous droplets in a hydrophobic medium through the droplet interface bilayer (DIB) method. Within our droplets, we explore synthetic totipotency via cell-free reactions including actin polymerization and cell free protein synthesis (CFPS). The transcription and translation of our CFPS reactions are controlled by stimuli-responsive riboswitches (RS). Via this scheme, adaptable material properties and functions are achieved in vitro via protein production from cell-free machinery administered through RS governance. Here, we present thermally or chemically-triggered riboswitches for orthogonal production of representative fluorescent protein products, as well functional proteins. To characterize the material properties of target proteins, we study the formation of polymerized actin shells to stabilize organically-encased droplets and span DIBs. We present a modified protocol for chemically-triggered actin polymerization as well as a thermally triggered actin RS. We characterize theophylline (TP)-triggered production of alpha hemolysin (α-HL) through CFPS and synthesized an organic-soluble trigger that can be sensed from the oil phase by a RS in an aqueous bioreactor droplet. We also demonstrate increased droplet conductivity when CFPS α-HL products are incorporated in DIBs. This interdisciplinary work involves cell culture, gene expression, organic synthesis, vesicle formation, protein quantification, tensiometry, droplet aspiration, microplate fluorescence/absorption experiments, fluorescent microscopy, and electrophysiology. This project is an essential design analysis for creating smart, soft materials using synthetic biology and provides motivation for artificial tissues capable of adapting in response to external stimuli.

The demand for clean and sustainable energy sources continuously increases. One of the promising ways to provide electrical power is using fuel cells. Polymer electrolyte membrane fuel cell (PEMFC) represents the most common type of fuel cells. However, PEMFCs have not yet been fully commercialized because of the high cost and low performance. A main part of PEMFC, which significantly contributes to the cost and weight is the bipolar plate (BPP). The US Department of Energy (DOE) has recommended some physical properties for BPP for sustainable commercialization of PEMFC. Those set properties have yet to be met. Conductive polymer composites (CPCs) use conductive fillers such as carbon nanotube (CNT), carbon fiber (CF), and graphite (Gr) to impart electrical and thermal conductivities and can potentially provide an optimum combination of weight, cost, mechanical properties and conductivity characteristics for BPPs. In the current work, CPCs of polycarbonate (PC) filled with singular filler of CNT, binary fillers of CNT and CF and ternary fillers of CNT, CF and Gr were fabricated using melt mixing method followed by compression molding. The through-plane and in-plane electrical conductivities of the CPCs were investigated. The results showed that the electrical percolation thresholds for the PC-CNT is ∼1 wt. % CNT in both the through-plane and in-plane directions. Addition of 3 wt. % CNT to PC composites with 10 - 30 wt. % CF improved the conductivity performance. It was noticed increasing CF content from 20 to 30 wt. % did not yield a big change in conductivity, so that at 20 wt. % CF, the through-plane and in-plane electrical conductivities are 0.11 S.cm −1 and 6.4 S.cm −1 respectively. Moreover, using 20 wt. % CF will allow for higher loading of graphite. To further enhance the conductivities towards the DOE recommendations, 30 wt. % Gr was introduced to the PC composite with binary filler (i.e., 3 wt. % CNT and 20 wt. % CF). The results showed that the through-plane and in-plane electrical conductivities were increased to 1.5 S.cm −1 and 13.5 S.cm −1 , respectively. These properties recommend a potential application of polycarbonate based CPCs for BPP manufacturing.

Stretchable strain sensors with large strain range, high sensitivity, and excellent reliability are of great interest for applications in soft robotics, wearable devices, and structure-monitoring systems. Unlike conventional template lithography-based approaches, 3D-printing can be used to fabricate complex devices in a simple and cost-effective manner. In this paper, we report 3D-printed stretchable strain sensors that embeds a flexible conductive composite material in a hyper-plastic substrate. Three commercially available conductive filaments are explored, among which the conductive thermoplastic polyurethane (ETPU) shows the highest sensitivity (gauge factor of 5), with a working strain range of 0%–20%. The ETPU strain sensor exhibits an interesting behavior where the conductivity increases with the strain. In addition, an experiment for measuring the wind speed is conducted inside a wind tunnel, where the ETPU sensor shows sensitivity to the wind speed beyond 5.6 m/s.

Piezoresistive strain sensors can be manufactured by embedding carbon nanotubes (CNTs) in an insulating polymer matrix, by taking advantage of CNTs superior electromechanical properties. In particular, the electromechanical properties find their roots in the conductive network formed by the randomly dispersed CNTs, through which the current can flow. When a mechanical strain occurs the conductive network configuration varies, changing the overall material conductivity. In this study this concept is being exploited to form a CNTs-based functional paint that allows to monitor ultra-large structural areas, in multiple directions, with an easy to assemble and processing approach. In particular, CNTs are dispersed in a PolymethylMethacrylate (PMMA) matrix following a carefully designed process to achieve a proper viscosity for direct painting onto a large in scale structure. Electromechanical tests are performed to characterize the piezoresistive behaviour of the coating in static and dynamic loading conditions. The results showed the great sensitivity of the coating to strain. The proposed approach to directly paint a sensitive coating onto the structure to be monitored has the advantages of: ultra-low weight, direct contact with the structure to be monitored and an extremely simple installation procedure.

The focus of this study was to apply a robust inspection technique for monitoring damage nucleation and propagation in 7075 aluminum alloy specimens exposed to cyclic loading. A previously developed specimen, linearly tapered in width along the length, was subjected to a sinusoidal tension-tension load while conductivity and strain were measured in-situ. Ex-situ measurements of modulus, hardness, surface potential, digital image correlation strain field, and neutron diffraction were made as a function of fatigue cycles. It is hypothesized that varying levels of induced stress along the length due to equal-force but varying area along the length will create a record of damage which can be probed to intuit a temporal history for the specimen. Baseline, intermediate, and failure sensor measurements for several specimens were compared and analyzed as a function of applied stress (varied linearly along the length) and fatigue cycles (constant). Mechanisms of damage nucleation and propagation due to fatigue cycling are discussed with an emphasis on which inspection methods are most promising for improving structural durability and state monitoring.

To continue to meet spacecraft systems ever increasing thermal management requirements, new control methods need to be developed. While advances in metamaterials have provided the ability to generate materials with a broad range of material properties, relatively little advancement has been made in the development of adaptive metamaterials. This paper is focused on the development of a thermal management metamaterial that enables the active and passive control of a metamaterial’s thermal conductance. This variable conductivity is achieved through the application of internally or externally applied loads that induce internal contact resulting in changes in the conductive path length and the effective conductive area. This capability enables active or passive control of a metamaterial’s effective thermal conduction through the application of mechanical and thermal strain. Passively applied thermal strains can be used to design a highly nonlinear material thermal conductivity as a function of temperature. Actively, this can be used to precisely control the temperature of an interface through dynamically changing the instantaneous heat flux through the metamaterial. This work expands on the field of thermal switches by enabling a non-binary configuration where the initial air gap is slowly closed as contact sequentially introduced into the metamaterial. As internally or externally developed loading is applied, contact is introduced with an increasing contact area until full contact is achieved. This intermediate step of partial contact enables unique design capabilities that enable highly nonlinear thermal conductivity as a function of temperature as well as stability regions that allow passive thermal control. An example metamaterial was developed and evaluated to quantify the potential of this concept. The specific metamaterial configuration assessed in this paper uses offset flat and curved copper plates that are connected at the edges of the plate using a low conductivity epoxy. To evaluate the metamaterial performance, the stiffness and thermal conductivity are calculated as a function of the resulting contact area and the required applied loading. This work is focused on determining the potential of this metamaterial concept by evaluating this initial concept confirmation to establish the magnitude of the thermal conductance change, and the design of the conductivity change a function of applied loading.

In this paper, we present characterization results for thermal, mechanical, and electrical properties of a 3D-printed conductive polylactic acid (PLA) composite material. The material exhibits electrically controllable stiffness, allowing for the fabrication of novel robotic and biomedical devices. In particular, an applied voltage induces a Joule heating effect, which modulates the material stiffness. Dumbbell samples are 3D-printed and loaded into a universal testing machine (UTM) to measure their Young’s moduli at different temperatures. The conductive PLA composite shows 98.6% reduction of Young’s modulus, from 1 GPa at room temperature to 13.6 MPa at 80 °C, which is fully recovered when cooled down to its initial temperature. Measurements with differential scanning calorimeter (DSC) and thermal diffusivity analyzer are conducted to investigate the thermal behavior of this material. Electrical conductivity of the material is measured under different temperatures, where the resistivity increases about 60% from 30 °C to 100 °C and hysteresis between the resistivity and the temperature is observed. These tests have shown that the conductive PLA composite has a glass transition temperature (Tg) of 56.7 °C, melting point (Tm) of 153.8 °C, and thermal conductivity of 0.366 W/(mK). The obtained results can be used as design parameters in finite element models and computational tools to rapidly simulate multi-material components for several applications such as object manipulation, grasping, and flow sensing.

Additive manufacturing (AM) offers a new and unique method for the fabrication of functional and smart material and structures. In this method, parts are fabricated directly from a 3D computer model layer by layer. Fused deposition modeling (FDM) is the most widely adapted AM method. In this method, the feedstock is usually a thermoplastic-based material. In recent years, flexible smart materials have gained unflagging interests due to their promising applications in health monitoring, sensing, actuation, etc. However, the 3D printing of flexible materials is recent with its own challenges and limited sources of feedstock. Conductive polymer nanocomposites (CPNs) have many promising uses within sensing filed including liquid sensing. Sensing chemical leakage is one the important capabilities of liquid sensors. There is a good number of studies on the fabrication and sensitivity characterization of CPN-based liquid sensors. However, the sensitivity and response time of CPN-based liquid sensors do not yet meet the industrial demands and should be further enhanced for their practical and widespread applications. This study presents an attempt to integrate the tunability of CPN’s conductivity behavior and the design flexibility of 3D printing to explore the benefits that their coupling may offer toward more sensitive and/or faster liquid sensing. Thermoplastic polyurethane/multiwalled carbon nanotube (TPU/MWCNT) nanocomposites were selected as a model material system and their filaments were first fabricated using melt-mixing by twin-screw extruder at 1, 2 and 3 wt.% of MWCNT. Flexible U-shaped TPU/MWCNT specimens were designed and successfully 3D-printed as a liquid sensor. Specimens fabricated at three different raster patterns of linear, 0–90, and 45/−45 and three infill percent levels of 100, 75, and 50%. Ethanol was used as the model chemical and the resistivity change of the sensors was measured as a function of time when immersed in ethanol. The results revealed that the printed sensors greatly outperformed the pressed bulk counterparts. This enhancement in the 3D printed sensors was primarily due to the increased surface area, and thus higher surface/volume ratio, enabling faster liquid uptake. In addition, MWCNT content, raster pattern, and infill percent all affected the overall response time as well as the sensor sensitivity. This work suggests that highly sensitive liquid sensors can be developed by material and structure optimizations via FDM 3D printing.

This paper builds on previous work done [1, 2] to explore the effective piezoresistive response of polymer bonded explosive (PBX) materials where the polymer medium is reinforced with carbon nanotubes (CNTs). In the present work, the nanocomposite binder is modeled explicitly as a piezoresistive material whose properties are determined from the nanoscale through a micromechanics based 2-scale hierarchical model connecting the nanoscale to the microscale grain structure. Electromechanical cohesive zones are used to model the interface between the grains and nanocomposite binder in order to characterize interface separation and the resulting piezoresistive effect. The overall microscale piezoresistive effect is measured by using the volume averaged properties of the microscale RVE. The hierarchical framework developed here is used to explore key features of the NCBX microstructure such as the effect of grain conductivity, weight percentage of CNTs used and nanocomposite gage factor.

Shape memory alloy (SMA) materials, such as Nickel Titanium (NiTi), can generate stress and strain during phase transformation induced by thermomechanical stimulation. Therefore, they may be used to construct active actuating devices for various biomedical applications such as smart surgical tools. Since temperature rise during the operation of SMA devices may damage the surrounding tissue, it is important to thermally shield such devices. We propose to use polydopamine (PDA) as an insulating coating for NiTi-based smart needles. PDA is a biomolecule (dopamine) derived polymer that can form conformal coating on various materials including NiTi. It is hypothesized that the surface temperature of the PDA coated needle can be reduced by the low thermal conductivity of PDA and the thermal resistance of the PDA/NiTi interface. Our experiments conducted in ambient air at room temperature showed that the coating reduced the surface temperature by as much as 45%. In this paper, we will present the thermal insulating performance of the PDA coating on NiTi wires. An experimental setup where the wire is embedded inside a gel phantom/tissue has been developed to simulate needle-tissue interaction. Effects of the coating thickness (material thermal resistance) and the number of layers (interfacial thermal resistance) will be discussed. 2D finite element analyses (FEA) were performed using ABAQUS to investigate the thermal distribution around the coated NiTi wires and the tissue gel phantom. In addition, using thermal distribution, potential tissue damage was assessed.

A theoretical investigation of the dynamic response of a pair of interacting carbon nanotubes (CNTs) dispersed in a liquid medium under the presence of an alternating current (AC) electric field is presented. The proposed modeling strategy is based on the dielectrophoretic (DEP) theory and classical electrodynamics, and considers the effect of an applied AC electric field on the rotational and translation motion of interacting CNTs represented as electrical dipoles. The mutual interaction between a pair of adjacent CNTs stems from the presence of DEP-induced charges on the CNTs and, as such, contributes to the rotational and translational dynamics of the system. Based on experimental evidence, the parameters which are expected to cause a major contribution to the CNTs motion are investigated for different initial configurations. Based on the obtained results, it is here predicted that high electric field frequencies, long CNTs, high values of electrical permittivity and conductivity of CNTs immersed in solvents of high polarity promote faster rotational and translational motion and therefore faster equilibrium conditions (CNT tip-to-tip contact and horizontal alignment). The results incorporate important knowledge towards a better understanding of the complex mechanisms involved in the efforts of tailoring CNT networks by electric fields.

Structural supercapacitors are very interesting multifunctional devices combining the properties of an electrical energy storage device and a structural component simultaneously. These types of supercapacitors are mostly equipped with solid state electrolytes, instead of traditional liquid electrolytes, avoiding leakage and safety problems and supporting the mechanical performance of the composite materials. In the present study, the Lithium-ion based solid ceramic electrolyte Li 1.4 Al 0.4 Ti 1.6 (PO 4 ) 3 was successfully synthesized by sol-gel method. Its electrical properties were characterized by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Results show that Li 1.4 Al 0.4 Ti 1.6 (PO 4 ) 3 possesses a conductivity of 2.94×10 −4 S/cm at room temperature and a specific capacity of 55.57 μF/g. The as-prepared samples were embedded into fiber composite material using the aviation approved resin RTM6 with an injection process making the composite structure flexible. Subsequently, the specific capacity and conductivity were tested getting values of 53.44μF/g and 2.00×10 −4 S/cm respectively. The reason for electrical properties loss was investigated by computerized tomography (CT) and EIS tests and the results provide reference for the future research.

Energy harvesting is widely used in terrestrial and aerial sensor applications but is conspicuously absent in the marine environment despite several possible harvesting modalities and numerous applications. One such energy harvesting modality is to use magnetohydrodynamic (MHD) power generators to directly produce electricity from flowing seawater. Fundamentally, MHD generators convert the kinetic energy of a conductive fluid directly into electricity by separating charged particles, thereby generating an electric field transverse to the direction of fluid flow and the magnetic field. The electric field is then accessed with an external circuit to provide power to a load. Since the power output from an MHD generator is linearly related to the conductivity of the flowing fluid and to the square of both the magnetic field strength and the fluid velocity, strong magnets and high fluid velocity are desirable. Thus, there are a myriad of possible MHD generator configurations available to maximize power output under various conditions and constraints. These include configurations of permanent magnets that offer localized high magnetic fields or geometries of the fluid duct that can be used to increase the fluid velocity through the magnetic field. One novel application for MHD generators is to power sensors and bio-loggers used in marine animal telemetry. The animal sensors are designed to take time-series measurements and store the data on the logger for transmission to satellite networks or human retrieval. These sensors and loggers are often battery-limited which constrains either the data fidelity or the longevity, or both. An MHD generator attached to a marine animal can help to supplement some of the sensor or bio-logger power requirements, thereby increasing sensor lifetimes and data fidelity. Thus, MHD generators will enable new research in the marine sciences, climatology, and biology, among others. The MHD generator can be positioned above the fluid boundary-layer so that the fluid flow around the animal is channeled through the MHD generator, producing electricity. In this work, we will develop some of the fundamental equations that describe the physics of an MHD generator and use them to make estimates of the potential power outputs that could be expected from various marine animals. We will also investigate several electrical configurations of the MHD to determine the most suitable MHD generator for different flow regimes. Initial studies suggest that MHD generators are viable power sources in the marine environment and can easily supplement the entire energy budget of a bio-logger under certain conditions.

In this research, we developed a skin-like tactile sensor array to measure the contact pressure of curved surfaces. The sensor array is laminated into a thin film with 3mm in thickness and can easily be wrapped around a pencil without damaging its skin-like structure. So far we have achieved the array containing 8×16 sensor elements. Its spatial resolution is 1 element per 9mm 2 area and it can measure the pressure up to 360kPa. The sensor-array patch contains three layers. The upper and lower layers are polydimethylsiloxane (PDMS) thin films embedded with conductor strips formed by PDMS-based silver nanowires (AgNWs) networks. The middle layer is formed by the mixing of nickel powder with liquid PDMS for contact force measurement. Experimental tests have demonstrated that conductor strips on the upper layer can maintain their resistances around 23Ω with less than 4Ω increase when the tensile strain is up to 50%. Noted is that conductors made with carbon nanotubes can keep its conductivity unchanged for up to only 40% tensile strain. Through fatigue tests, it is observed that the measured AgNWs/PDMS conductor strip exhibits low and stable resistances. This is one of the desired behaviors of the stretchable interconnects for signal transmission. The integrated sensor system can successfully measure the contact pressure induced by objects of different shapes. It can be applied on curved or non-planar surfaces in robots or medical devices for force detection and feedback.

This paper introduces a technique of inducing bulk conductivity in a polymer. The technique uses coiled copper ‘cells’ embedded into a polymer during fabrication which can subsequently create highly redundant series-parallel networks. The preceding body of work aimed to improve the conductivity of non-conducting polymers by embedding particulates (of metal, carbon, etc.) into the polymer, or by altering the polymerization chemistry to incorporate conductive elements. The technique described here keeps the process independent of the specific polymer chosen by not relying on the polymerization chemistry to aid in the incorporation of the cells. The embedding drastically lowers the resistivity of the polymer, from 10 12 Ω -cm (approx.) for pure silicone rubber to less than 50 Ω -cm for the composite at room temperature: a drop of 12 orders of magnitude. A secondary consideration of this paper is the mechanical stiffness changes brought about by the embedding of metal inside a flexible polymer. Although the connected network of copper cells allows the rubber to be highly conductive in bulk, the cells are themselves compliant and thus have minimal effect on the stiffness of the cured silicone rubber.

Direct write (DW) technology offers a simple method of rapid manufacturing technology for printing electronic, optoelectronic devices, and complex functional devices. The key component of DW technology is the functional inks, which are colloidal suspensions of functional nanoparticles in various solvents such as aerosol or liquid form. With a DW approach, patterns or structures can be easily deposited on flexible substrates such as paper, plastics, and composites, once the solvent volatilizes or is driven off via conventional, laser, or microwave sintering. In this work, polymer-assisted silver (Ag) nanoinks were synthesized by silver salt and polymer in the water solution at relatively high silver precursor concentrations and relatively low concentration of polymers. The silver nanoparticle dispersion and morphology was examined by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The results showed that the size of Ag nanoparticles was in nanoscale (∼20 nm) with a narrow distribution of Ag nanoparticle sizes. The viscosity and thermal properties of synthesized silver nanoinks were characterized to determine their applicability and the lifetime. It has been shown that the synthesized silver nanoink can be printed on a flexible plastic substrate or glass substrate. The morphology of the Ag nanoink line printed on the substrate was observed by optical microscopy and scanning electron microscopy (SEM) to understand the relationship between the microstructure and wettability. Uniaxial tension tests of silver nanoink line on a Kapton film indicate that the ink can be stretched ∼20% without failure. The resistance of silver nanoink line printed on the Kapton films was also measured by four probe conductivity measurement system to assess the electrical performance. The resistivity is about 7.5 × 10 −5 Ω-cm by thermal treatment at 250°C for 30 min, which is about half that of bulk silver (1.6 × 10 −6 Ω-cm). Overall, the performance of the synthesized silver nanoink is comparable to a commercially available ink with lower Ag weight content at relatively low cost.

Different routes for electrode processing which fulfill the requirements of piezoelectric transducer will be presented. One attempt is the electrode deposition via inkjet printing, another is the deposition via an air-brush technique. For the preparation of electrodes via inkjet printing, different inks such as a silver composite or the semiconducting poly(3,4-ethylene-dioxythiophene): polystyrenesulfonate (PEDOT:PSS) are used. A further attempt is the deposition of carbon nano tubes (CNT’s) via an air-brush technique. For all three systems the ink or solution formulation, the deposition techniques, suitable parameter and partly additional encapsulation steps will be discussed in detail accompanied by a description of electrode properties, e.g. the conductivity, as well as by the characterization of the materials poling behavior in particular.

This study focuses on the characterization of a porous multifunctional elastomer-CNT nanocomposites for potential use as pressure sensors. A thermoplastic polyurethane (TPU) was chosen as an elastomeric matrix, which was reinforced with multiwall carbon nanotubes (0–10 wt%) by high shear twin screw extrusion mixing. Porosity was introduced to the composites through the phase separation of a single TPU-CO 2 solution. Interactions between MWNT and TPU were elucidated through calorimetry, gravimetric decomposition, conductivity measurements and microstructure imaging. The piezoresistance (pressure-resistance) behavior of the nanocomposites was investigated and found to be dependent on MWNT concentration and nanocomposite microstructure.

Dielectric Electroactive Polymers belong to a new class of smart materials, whose functional principle is based on electrostatic forces. They can either be used as actuators to provide considerable stretch ratios or as generators to convert mechanical strain energy into electrical energy by use of an initial amount of energy. Since the polymer material and also the covering compliant electrodes show non-ideal electrical properties, like finite resistivity and conductivity respectively, design rules have to be derived, in order to optimize the devices. The electrode conductivity in connection with the polymer resistivity causes a voltage drop along the electrode surface, resulting in a reduced actuation strain or energy conversion. To minimize its parasitic effects, the influence of this effect is studied by the in-plane field propagation based on a model obtained with the equivalent network method. It is shown that the proposed model provides accurate results, which can be used to study the effect of contacting electrodes, especially in case of point contacts.

Fiber reinforced composites (FRP) for industrial applications face constantly increasing demands regarding efficiency, reliability and economy. Furthermore, it was shown that FRP’s with tailored reinforcements are superior to metallic or monolithic materials. However, a trustworthy description of load-specific failure behaviour and damage evolution of composite structures can hardly be given, because these processes are very complex and are still not entirely understood. Amongst other things, several research groups have shown that material damages like fiber fracture, delamination, matrix cracking or flaws can be discovered by analyzing the electrical properties of conducting composites, e.g. carbon fiber reinforced plastics (CFRP). Furthermore, it was shown that this method could be used for structural health monitoring or non-destructive testing (NDT) [8–12].Within this work, Magnetic Induction Tomography (MIT), which is a new imaging approach, is introduced into the topic of NDT of CFRP’s. This non-contacting imaging method gains the inner spatial distribution of conductivity of a specimen and depicts material inhomogeneity, like damages, in 2D or 3D images. Numerical and experimental investigations are presented and give a first impression of the performance of this technique. It is demonstrated that MIT is a promising approach for NDT and could be used for fabrication quality control of conductive FRP’s and could potentially be used as a health monitoring system using an integrated setup.