Plant-available nitrogen, often in the form of nitrate, is an essential nutrient for plant growth... more Plant-available nitrogen, often in the form of nitrate, is an essential nutrient for plant growth. However, excessive nitrate in the environment and watershed has harmful impacts on natural ecosystems and consequently human health. A distributed network of nitrate sensors could help to quantify and monitor nitrogen in agriculture and the environment. Here, we have developed fully printed potentiometric nitrate sensors and characterized their sensitivity and selectivity to nitrate. Each sensor comprises an ion-selective electrode and a reference electrode that are functionalized with polymeric membranes. The sensitivity of the printed ion-selective electrodes was characterized by measuring their potential with respect to a commercial silver/silver chloride reference electrode in varying concentrations of nitrate solutions. The sensitivity of the printed reference electrodes to nitrate was minimized with a membrane containing polyvinyl butyral (PVB), sodium chloride, and sodium nitrat...
The sensors and microprocessor-radios of the Internet of Things require power and one possibility... more The sensors and microprocessor-radios of the Internet of Things require power and one possibility is a hybrid source consisting of energy harvesting coupled with a rechargeable battery. This paper concerns a system with a low, but near constant, supply of harvested energy that is sufficient to slowly charge a battery, which buffers that energy and can then supply the higher power level necessary at data transmission. The cycle-life of the battery is then significant and this paper describes the cycling of a commercial lithium coin cell, Panasonic ML2020, for small depth of discharge. Initial work was on a Bio-Logic battery tester and cells were successfully tested, without failure, for a few million cycles at very shallow depth of discharge. Thereafter a battery tester was developed, based on the Particle Photon®, that was much less expensive than a channel of a commercial battery tester. Four cells have been cycled, using this tester, for more than 50 million times with no deterioration in performance evident. Capacitors and a NiMH cell are being similarly tested.
Medical electrophysiological sensors that can study the body and diagnose diseases depend on cons... more Medical electrophysiological sensors that can study the body and diagnose diseases depend on consistently low impedance electrode-skin interfaces. Clinical-standard wet electrodes use hydrogels and skin abrasion to improve the interface and thus the recorded signal quality. These electrodes are challenging to self-administer and impede in-home care. Wearable dry electrodes are more practical; however, they show higher impedances than wet electrodes and are costly to customize. This work presents a fabrication method for rapidly producing low impedance, anatomically fit dry electrodes that do not require hydrogels. By using electroless copper and gold plating with 3D printing, biocompatible electrodes can be optimized for individuals at a fraction of the cost of existing vacuum deposition-based techniques. Example 3D dry electrodes made with this process are evaluated alongside clinical-standard devices in typical scenarios to compare electrical performance and comfort. The resulting dry electrodes exhibited an average electrode-skin impedance of 66.7 kΩ at 50Hz and DC offset of-20 mV without any hydrogel, which, when area normalized, is within the range achieved by wet electrodes without skin abrasion.
Stretchable strain sensors with well-controlled sensitivity and stretchability are crucial for ap... more Stretchable strain sensors with well-controlled sensitivity and stretchability are crucial for applications ranging from large deformation monitoring to subtle vibration detection. Here, based on single-metal material on the elastomer and one-pot evaporation fabrication method, we realize controlled strain sensor performance via a novel programable cracking technology. Specifically, through elastomeric substrate surface chemistry modification, the microcrack generation and morphology evolution of the strain sensing layer is controlled. This process allows for fine tunability of the cracked film morphology, resulting in strain sensing devices with a sensitivity gauge factor of over 10 000 and stretchability up to 100%. Devices with a frequency response up to 5.2 Hz and stability higher than 1000 cycles are reported. The reported strain sensors, tracking both subtle and drastic mechanical deformations, are demonstrated in healthcare devices, human−machine interaction, and smarthome applications.
We have developed a process for fabricating patient specific Magnetic Resonance Imaging (MRI) Rad... more We have developed a process for fabricating patient specific Magnetic Resonance Imaging (MRI) Radio-frequency (RF) receive coil arrays using additive manufacturing. Our process involves spray deposition of silver nanoparticle inks and dielectric materials onto 3D printed substrates to form high-quality resonant circuits. In this paper, we describe the material selection and characterization, process optimization, and design and testing of a prototype 4-channel neck array for carotid imaging. We show that sprayed polystyrene can form a low loss dielectric layer in a parallel plate capacitor. We also demonstrate that by using sprayed silver nanoparticle ink as conductive traces, our devices are still dominated by sample noise, rather than material losses. These results are critical for maintaining high Signal-to-Noise-Ratio (SNR) in clinical settings. Finally, our prototype patient specific coil array exhibits higher SNR (5 × in the periphery, 1.4 × in the center) than a commercially ...
We describe the optimization of a flexible printed electrochemical sensing platform to monitor so... more We describe the optimization of a flexible printed electrochemical sensing platform to monitor sodium ion (Na+), ammonium ion (NH4+), and lactate in human sweat. We used previously reported material systems and adapted them to scalable fabrication techniques. In the case of potentiometric Na+ and NH4+ sensors, ion-selective electrodes (ISEs) required minimum optimization beyond previously reported protocols, while a reference electrode had to be modified in order to achieve a stable response. We incorporated a carbon nanotube (CNT) layer between the membrane and the silver/silver chloride (Ag/AgCl) layer to act as a surface for adsorption and retention of Cl−. The resulting reference electrode showed minimal potential variation up to 0.08 mV in the solutions with Cl concentration varying from 0.1 mM to 100 mM. Increasing the ionophore content in the NH4+ ISE sensing membrane eliminated an offset in the potential readout, while incorporating CNTs into the sensing membranes had a marg...
Flexible and biodegradable sensors are advantageous for their versatility in a range of areas fro... more Flexible and biodegradable sensors are advantageous for their versatility in a range of areas from smart packaging to agriculture. In this work, we characterize and compare the performance of interdigitated electrode (IDE) humidity sensors printed on different biodegradable substrates. In these IDE capacitive devices, the substrate acts as the sensing layer. The dielectric constant of the substrate increases as the material absorbs water from the atmosphere. Consequently, the capacitance across the electrodes is a function of environmental relative humidity. Here, the performance of polylactide (PLA), glossy paper, and potato starch as a sensing layer is compared to that of nonbiodegradable polyethylene terephthalate (PET). The capacitance across inkjet-printed silver electrodes is measured in environmental conditions ranging from 15 to 90% relative humidity. The sensitivity, response time, hysteresis, and temperature dependency are compared for the sensors. The relationship between...
earable sensors are increasingly prevalent in health monitoring and human-machine interface appli... more earable sensors are increasingly prevalent in health monitoring and human-machine interface applications as a result of improvements in their size and comfort 1-4. Recent advances in flexible electronics have enabled the fabrication of wearable sensors that can mechanically bend and conform to non-planar and dynamic surfaces of the human body, allowing physiological signals of low bandwidth to be measured 5-10. Hybrid systems that combine flexible sensors with rigid computational components on a separate substrate have also been developed 8,10. Such systems are useful for applications that require local signal processing and miniaturized form factors. To provide a real-time analysis of physiological signals, wearable biosensors can implement machine-learning models for signal processing. Local (in-sensor) processing of signals from biosensors has advantages over wirelessly streaming raw data to an external computational device, including reduced communication link bandwidth and radio power requirements. Processing the signals locally can also offer improved latency and security. Machine learning models for in-sensor processing are, however, typically trained offline before they are implemented in low-power embedded processors 11,12. Modern, lightweight machine-learning algorithms perform well when the training data align with the conditions that are expected during deployment, but, when the initial training of a classification model fails to capture a wide set of conditions, the classification accuracy of the model degrades, resulting in suboptimal performance or poor user experience 13-18 (Supplementary Fig. 5). Therefore, the ability to train and update an in-sensor classification model during practical application is desirable. Gesture recognition using surface electromyography (sEMG) 19-24 could benefit from a hybrid system with in-sensor processing. sEMG-based gesture recognition devices can measure electrical muscle activity from electrodes placed on the surface of the skin and perform pattern recognition on features that are extracted from these signals 11,25. These devices should ideally have a small standalone form factor for autonomy, a high channel count and high-density electrode placement for more comprehensive spatial coverage and improved classification accuracy 26,27 , and in-sensor intelligence that is generalizable or adaptable to various wear conditions. Existing devices are either in non-wearable form factors 26 , dependent on external devices for computation 28 or reliant on a small number of precisely positioned, bulky electrodes 12,29,30. Furthermore, the systems that are capable of in-sensor (online) training do not support in-sensor model updates 29,30. These systems thus cannot adapt to signal variations from sweating, fatigue, varying muscle contraction effort, and electrode displacement due to changing situational contexts such as limb and body position or device doffing and donning 13-18. In this Article, we report a wearable, high-density sEMG biosensing system (Fig. 1a) that uses hyperdimensional (HD) computing to implement in-sensor adaptive learning and real-time inference for hand gesture classification. HD computing is an emerging computing paradigm that supports fast and simple learning and is inherently robust against noise and errors 31. HD computing takes advantage of information being represented by very high-dimensional vectors (hypervectors) to perform otherwise complex tasks, such as classification or reasoning, using simple computational operations 31. This approach has already shown promising results in classification tasks
A battery design and fabrication process is demonstrated to make Lithium-ion (Li-ion) microbatter... more A battery design and fabrication process is demonstrated to make Lithium-ion (Li-ion) microbatteries with high capacity to power IoT devices. The battery consists of printed anode and cathode layers based on graphite and lithium cobalt oxide (LCO) respectively. The active area of the electrodes is scaled down to 1 mm 2 and the resulting electrochemical performance is evaluated. These miniature batteries demonstrate a significantly higher discharge capacity (6.4 mAh/cm 2) and energy density (23.6 mWh/cm 2) than thin-film and thick-film, and 3D microbatteries. This work shows a miniaturized Li-ion battery capable of powering a MEMS-based wireless sensor system with peak current requirements as high as 4 mA, demonstrating its effectiveness as a power source for integrated electronics.
Human skin is equipped with slow adapting (SA) and fast adapting (FA) capabilities simultaneously... more Human skin is equipped with slow adapting (SA) and fast adapting (FA) capabilities simultaneously. To mimic such functionalities, elaborately designed devices have been explored by integrating multiple sensing elements or adopting multimode sensing principles. However, the complicated fabrication, signal mismatch of different modules, complex operation, and high power‐consumption hinder their widespread applications. Here, a new type of single‐mode and self‐powered mechanoreceptor that can mimic both SA and FA via seamless fusion of complementary while compatible potentiometric and triboelectric sensing principles is reported. The resultant potentiometric–triboelectric hybridized mechanoreceptor exhibits distinctive features that are hard to achieve via currently existing methods, including single‐mode output (only voltage signal), greatly simplified operation (single‐measurement setup), ultralow power‐consumption (<1 nW), self‐adaptive response behavior, and good capability for ...
Flexible pressure sensors with high sensitivity, broad working range, and good scalability are hi... more Flexible pressure sensors with high sensitivity, broad working range, and good scalability are highly desired for the next generation of wearable electronic devices. However, manufacturing of such pressure sensors still remains challenging. A large‐area compliant and cost‐effective process to fabricate high‐performance pressure sensors via a combination of mesh‐molded periodic microstructures and printed side‐by‐side electrodes is presented. The sensors exhibit low operating voltage (1 V), high sensitivity (20.9 kPa−1), low detection limit (7.4 Pa), fast response/recovery time (23/18 ms), and excellent reliability (over 10 000 cycles). More importantly, they exhibit ultra‐broad working range (7.4–1 000 000 Pa), high tunability, large‐scale production feasibility, and significant advantage in format miniaturization and creating sensor arrays with self‐defined patterns. The versatility of these devices is demonstrated in various human activity monitoring and spatial pressure mapping a...
Wound healing is a complex process involving diverse changes in multiple cell types where the app... more Wound healing is a complex process involving diverse changes in multiple cell types where the application of electric fields has been shown to accelerate wound closure. To define the efficacy of therapies based on electric fields, it would be valuable to have a platform to systematically study the effects of electrical stimulation (ES) upon the inflammation phase and the activation of signaling mediators. Here, an in vivo ES model in which flexible electrodes are applied to an animal model for monitoring inflammation in a wound is reported on. Subcutaneous implants of polyvinyl alcohol sponges elicit inflammation response as defined by the infiltration of leukocytes. The wound site is subjected to electric fields using two types of additively fabricated flexible electrode arrays. The sponges are then harvested for flow cytometry analysis to identify changes in the phosphorylation state of intracellular targets. This platform enables studies of molecular mechanisms, as it shows that ...
Journal of Micromechanics and Microengineering, 2018
than field-activated EAPs for implementation in microsystems for their comparatively low efficien... more than field-activated EAPs for implementation in microsystems for their comparatively low efficiency, low actuation forces, and slow response times [9], which have mainly limited their industrial applications to drug delivery systems. The general mechanism of actuation in EAP hydrogel systems is change of dimension in response to electricallyinduced osmotic concentration gradients [10]. EAP hydrogels that are stimulated electrostatically in electrolytic environments experience coupled chemical, electrical, and mechanical fields, and are thus challenging to model [11]. Deformation of
Solution-processed phototransistors can substantially advance the performance of image sensors. P... more Solution-processed phototransistors can substantially advance the performance of image sensors. Phototransistors exhibit large photoconductive gain and a sublinear responsivity to irradiance, which enables a logarithmic sensing of irradiance that is akin to the human eye and has a wider dynamic range than photodiode-based image sensors. Here, we present a novel solution-processed phototransistor composed of a heterostructure between a high-mobility organic semiconductor and an organic bulk heterojunction. The device efficiently integrates photogenerated charge during the period of a video frame then quickly discharges it, which significantly increases the signal-to-noise ratio compared with sampling photocurrent during readout. Phototransistor-based image sensors processed without photolithography on plastic substrates integrate charge with external quantum efficiencies above 100% at 100 frames per second. In addition, the sublinear responsivity to irradiance of these devices enables a wide dynamic range of 103 dB at 30 frames per second, which is competitive with state-of-the-art image sensors.
at Berkeley, in partial satisfaction of the requirements for the degree of Master of Science, Pla... more at Berkeley, in partial satisfaction of the requirements for the degree of Master of Science, Plan II.
The increasing miniaturization and complexity of emerging electronics products and systems brings... more The increasing miniaturization and complexity of emerging electronics products and systems brings many technical challenges but also excellent promise for improving capabilities and reducing costs through printing-like manufacturing processes and through combining of functionalities in what are called hybrid flexible electronics systems. Important application areas include energy, consumer electronics, healthcare, communications, and national defense. This report is a review of hybrid flexible electronics research and development activities in Western Europe, conducted by a panel of leading U.S. experts in the field, as a window to view what opportunities and challenges exist for U.S. researchers, educators, and manufacturers in this highly interdisciplinary field. The report covers materials development, device challenges, systems opportunities, and processing and manufacturing topics. It also includes a section that reviews several of the large European innovation centers that are bringing a multidisciplinary and practical commercial focus to R&D in the field. The report details the strengths of the basic research being conducted in Europe and the vitality of the close partnerships between university groups, basic research laboratories, industry, and innovation centers, sustained by sizeable research grants that specifically promote such interactions and efficiently support development. Appendix C summarizes the results of the WTEC bibliometric study of world research in hybrid flexible electronics, 1994?2008, which includes a view of the status of R&D in this field in Asia as well as in Europe and the United States.
Printing is a promising manufacturing approach for the fabrication of flexible, large-area, custo... more Printing is a promising manufacturing approach for the fabrication of flexible, large-area, custom sensor systems for applications such as wearable electronics and sensor labels. While printed examples of many physical and chemical sensors have been demonstrated, functionality useful for stand-alone systems such as high-resolution analog-to-digital conversion and wireless communication to address these sensors is not currently available in circuits printed entirely from solution. In order to retain the benefits of print manufacturing without sacrificing performance, a hybrid approach can be followed where printed and prefabricated devices are used together. In this report a mostly-printed wireless light and temperature sensor system that operates at 3.6 V and reports light and temperature readings wirelessly with a sensitivity of 0.7°C and 2.5 μW cm −2 at 500 nm respectively is demonstrated. The number of printed components was maximized and off-the-shelf microfabricated devices were only introduced where functionality is not currently achievable through printing. Onto a flexible polyester substrate, print processes were used to fabricate the bulk-heterojunction photodiode, thermistor, low-voltage operation multiplexing transistors, antenna, passives and interconnects, and silicon ICs are used for signal processing, memory and wireless communication. The printed components (sensors and transistors) were specifically developed to facilitate electrical integration into the hybrid system.
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Papers by Ana Arias