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Skin-interfaced wearable systems with integrated microfluidic structures and sensing capabilities offer powerful platforms for monitoring the signals arising from natural physiological processes. This paper introduces a set of strategies, processing approaches, and microfluidic designs that harness recent advances in additive manufacturing [three-dimensional (3D) printing] to establish a unique class of epidermal microfluidic (“epifluidic”) devices. A 3D printed epifluidic platform, called a “sweatainer,” demonstrates the potential of a true 3D design space for microfluidics through the fabrication of fluidic components with previously inaccessible complex architectures. These concepts support integration of colorimetric assays to facilitate in situ biomarker analysis operating in a mode analogous to traditional epifluidic systems. The sweatainer system enables a new mode of sweat collection, termed multidraw, which facilitates the collection of multiple, independent sweat samples for either on-body or external analysis. Field studies of the sweatainer system demonstrate the practical potential of these concepts.

Important insights into human health can be obtained through the non-invasive collection and detailed analysis of sweat, a biofluid that contains a wide range of essential biomarkers. Skin-interfaced microfluidic platforms, characterized by soft materials and thin geometries, offer a collection of capabilities for in situ capture, storage, and analysis of sweat and its constituents. In ambulatory uses cases, the ability to provide real-time feedback on sweat loss, rate and content, without visual inspection of the device, can be important. This paper introduces a low-profile skin-interfaced system that couples disposable microfluidic sampling devices with reusable 'stick-on' electrodes and wireless readout electronics that remain isolated from the sweat. An ultra-thin capping layer on the microfluidic platform permits high-sensitivity, contactless capacitive measurements of both sweat loss and sweat conductivity. This architecture avoids the potential for corrosion of the sensing components and eliminates the need for cleaning/sterilizing the electronics, thereby resulting in a cost-effective platform that is simple to use. Optimized electrode designs follow from a combination of extensive benchtop testing, analytical calculations and FEA simulations for two sensing configurations: (1) sweat rate and loss, and (2) sweat conductivity, which contains information about electrolyte content. Both configurations couple to a flexible, wireless electronics platform that digitizes and transmits information to Bluetooth-enabled devices. On-body field testing during physical exercise validates the performance of the system in scenarios of practical relevance to human health and performance.

Skin-interfaced wearable systems with integrated colorimetric assays, microfluidic channels, and electrochemical sensors offer powerful capabilities for noninvasive, real-time sweat analysis. This Perspective details recent progress in the development and translation of novel wearable sensors for personalized assessment of sweat dynamics and biomarkers, with precise sampling and real-time analysis. Sensor accuracy, system ruggedness, and large-scale deployment in remote environments represent key opportunity areas, enabling broad deployment in the context of field studies, clinical trials, and recent commercialization. On-body measurements in these contexts show good agreement compared to conventional laboratory-based sweat analysis approaches. These device demonstrations highlight the utility of biochemical sensing platforms for personalized assessment of performance, wellness, and health across a broad range of applications.

Acoustic forces are an attractive pathway to achieve directed assembly for multi-phase materials via additive processes. Programmatic integration of microstructure and structural features during deposition offers opportunities for optimizing printed component performance. We detail recent efforts to integrate acoustic focusing with a direct-ink-write mode of printing to modulate material transport properties (e.g., conductivity). Acoustic field-assisted printing, operating under a multi-node focusing condition, supports deposition of materials with multiple focused lines in a single-pass printed line. Here, we report the demonstration of acoustic focusing in concert with diffusive self-assembly to rapidly assembly and print multiscale, mm-length colloidal solids on a timescale of seconds to minutes. These efforts support the promising capabilities of acoustic field-assisted deposition-based printing to achieve spatial control of printed microstructures with deterministic, long-range ordering across multiple length scales.

Connected wearable biosensors are a critical part of athletic performance analysis, injury and recovery time assessment, and hydration analytics, enabling elite athletes, trainers, and coaches to characterize the daily demands of sports. However, existing classes of wearable biosensors are constrained to a few body locations and tend to limit mobility due to their bulky size and weight. Recent advances in soft and stretchable skin-interfaced wearable sensors capable of real-time physiological monitoring and in situ sweat collection provide capabilities for real-time continuous motion, physiology, and biochemical analysis in an imperceptible mode from any location on the body. This review presents an overview of the latest developments in skin-interfaced wearable sensor technologies with an emphasis on soft materials and stretchable designs most suitable in sports. We conclude with a summary of unresolved challenges, opportunities, and future directions facing the field of sports science and analytics.

Bio-integrated wearable systems can measure a broad range of biophysical, biochemical, and environmental signals to provide critical insights into overall health status and to quantify human performance. Recent advances in material science, chemical analysis techniques, device designs, and assembly methods form the foundations for a uniquely differentiated type of wearable technology, characterized by noninvasive, intimate integration with the soft, curved, time-dynamic surfaces of the body. This review summarizes the latest advances in this emerging field of "bio-integrated" technologies in a comprehensive manner that connects fundamental developments in chemistry, material science, and engineering with sensing technologies that have the potential for widespread deployment and societal benefit in human health care. An introduction to the chemistries and materials for the active components of these systems contextualizes essential design considerations for sensors and associated platforms that appear in following sections. The subsequent content highlights the most advanced biosensors, classified according to their ability to capture biophysical, biochemical, and environmental information. Additional sections feature schemes for electrically powering these sensors and strategies for achieving fully integrated, wireless systems. The review concludes with an overview of key remaining challenges and a summary of opportunities where advances in materials chemistry will be critically important for continued progress.

Recently developed ultrasmall, fully implantable devices for optogenetic neuromodulation eliminate the physical tethers associated with conventional set-ups and avoid the bulky head-stages and batteries found in alternative wireless technologies. The resulting systems allow behavioural studies without motion constraints and enable experiments in a range of environments and contexts, such as social interactions. However, these devices are purely passive in their electronic design, thereby precluding any form of active control or programmability; independent operation of multiple devices, or of multiple active components in a single device, is, in particular, impossible. Here we report optoelectronic systems that, through developments in integrated circuit and antenna design, provide low-power operation, and position- and angle-independent wireless power harvesting, with full user-programmability over individual devices and collections of them. Furthermore, these integrated platforms have sizes and weights that are not significantly larger than those of previous, passive systems. Our results qualitatively expand options in output stabilization, intensity control and multimodal operation, with broad potential applications in neuroscience research and, in particular, the precise dissection of neural circuit function during unconstrained behavioural studies. An optoelectronic platform that operates at low power and uses position- and angle-independent wireless power harvesting can provide multimodal programmable control over optogenetic stimulation parameters.

Sweat is a promising, yet relatively unexplored biofluid containing biochemical information that offers broad insights into the underlying dynamic metabolic activity of the human body. The rich composition of electrolytes, metabolites, hormones, proteins, nucleic acids, micronutrients, and exogenous agents found in sweat dynamically vary in response to the state of health, stress, and diet. Emerging classes of skin-interfaced wearable sensors offer powerful capabilities for the real-time, continuous analysis of sweat produced by the eccrine glands in a manner suitable for use in athletics, consumer wellness, military, and healthcare industries. This perspective examines the rapid and continuous progress of wearable sweat sensors through the most advanced embodiments that address the fundamental challenges currently restricting widespread deployment. It concludes with a discussion of efforts to expand the overall utility of wearable sweat sensors and opportunities for commercialization, in which advances in biochemical sensor technologies will be critically important.

Important insights into human health can be obtained through the non-invasive collection and detailed analysis of sweat, a biofluid that contains a wide range of essential biomarkers. Skin-interfaced microfluidic platforms, characterized by soft materials and thin geometries, offer a collection of capabilities for in situ capture, storage, and analysis of sweat and its constituents. In ambulatory uses cases, the ability to provide real-time feedback on sweat loss, rate and content, without visual inspection of the device, can be important. This paper introduces a low-profile skin-interfaced system that couples disposable microfluidic sampling devices with reusable 'stick-on' electrodes and wireless readout electronics that remain isolated from the sweat. An ultra-thin capping layer on the microfluidic platform permits high-sensitivity, contactless capacitive measurements of both sweat loss and sweat conductivity. This architecture avoids the potential for corrosion of the sensing components and eliminates the need for cleaning/sterilizing the electronics, thereby resulting in a cost-effective platform that is simple to use. Optimized electrode designs follow from a combination of extensive benchtop testing, analytical calculations and FEA simulations for two sensing configurations: (1) sweat rate and loss, and (2) sweat conductivity, which contains information about electrolyte content. Both configurations couple to a flexible, wireless electronics platform that digitizes and transmits information to Bluetooth-enabled devices. On-body field testing during physical exercise validates the performance of the system in scenarios of practical relevance to human health and performance.

The intricate microarchitecture of tissues – the “tissue microenvironment” – is a strong determinant of tissue function. Microfluidics offers an invaluable tool to precisely stimulate, manipulate, and analyze the tissue microenvironment in live tissues and engineer mass transport around and into small tissue volumes. Such control is critical in clinical studies, especially where tissue samples are scarce, in analytical sensors, where testing smaller amounts of analytes results in faster, more portable sensors, and in biological experiments, where accurate control of the cellular microenvironment is needed. Microfluidics also provides inexpensive multiplexing strategies to address the pressing need to test large quantities of drugs and reagents on a single biopsy specimen, increasing testing accuracy, relevance, and speed while reducing overall diagnostic cost. Here, we review the use of microfluidics to study the physiology and pathophysiology of intact live tissues at sub-millimeter scales. We categorize uses as either in vitro studies – where a piece of an organism must be excised and introduced into the microfluidic device – or in vivo studies – where whole organisms are small enough to be introduced into microchannels or where a microfluidic device is interfaced with a live tissue surface (e.g. the skin or inside an internal organ or tumor) that forms part of an animal larger than the device. These microfluidic systems promise to deliver functional measurements obtained directly on intact tissue – such as the response of tissue to drugs or the analysis of tissue secretions – that cannot be obtained otherwise., Microengineering: Microfluidic systems for physiological study of live, intact tissues Microfluidics constitutes an invaluable tool for precisely stimulating, manipulating, and analyzing the intricate microarchitecture in live tissues and optimizing mass transport. Microfluidic systems are becoming an essential tool for studying intact tissues with in vitro and, increasingly, in vivo applications. A team headed by Albert Folch at the University of Washington, USA conducted a review of the use of microfluidics to study the physiology and pathophysiology of live, intact tissues at sub-millimeter scales. These studies show that microfluidic applications to intact systems have enabled more sophisticated study of animal and plant development, physiology, and pathophysiology, as well as drug delivery, to different tissues in larger animals, including humans. They believe that the wide-ranging applications of microfluidics to intact tissues have considerable future potential in research and health care.

International audience; The concentration of chloride in sweat remains the most robust biomarker for confirmatory diagnosis of cystic fibrosis (CF), a common life-shortening genetic disorder. Early diagnosis via quantitative assessment of sweat chloride allows prompt initiation of care and is critically important to extend life expectancy and improve quality of life. The collection and analysis of sweat using conventional wrist-strapped devices and iontophoresis can be cumbersome, particularly for infants with fragile skin, who often have insufficient sweat production. Here, we introduce a soft, epidermal microfluidic device ("sweat sticker") designed for the simple and rapid collection and analysis of sweat. Intimate, conformal coupling with the skin supports nearly perfect efficiency in sweat collection without leakage. Real-time image analysis of chloride reagents allows for quantitative assessment of chloride concentrations using a smartphone camera, without requiring extraction of sweat or external analysis. Clinical validation studies involving patients with CF and healthy subjects, across a spectrum of age groups, support clinical equivalence compared to existing device platforms in terms of accuracy and demonstrate meaningful reductions in rates of leakage. The wearable microfluidic technologies and smartphone-based analytics reported here establish the foundation for diagnosis of CF outside of clinical settings.

Acoustic forces can align and consolidate particles in fluids, enabling microstructural control of two-phase materials at time-scales compatible with direct-write printing of composites. This paper presents key scaling relationships for acoustically-assisted direct-write printing that describe characteristic time-scales for assembly and alignment of particles during printing. Critical combinations of system parameters (including particle and nozzle dimensions, acoustic excitation amplitude, viscosity, and flow rate) are defined that govern particle focusing and assembly in the print stream. The results can be used to identify combinations of printing protocols and nozzle configurations that control particle packing parallel and transverse to the print direction. Impact statement We present theory and experiments demonstrating acoustic focusing in conjunction with direct-write printing for ‘on-the-fly’ control of two-phase microstructures, and a design framework for printing arbitrary material combinations.

The rich composition of solutes and metabolites in sweat and its relative ease of collection upon excretion from skin pores make this class of biofluid an attractive candidate for point of care analysis. Wearable technologies that combine electrochemical sensors with conventional or emerging semiconductor device technologies offer valuable capabilities in sweat sensing, but they are limited to assays that support amperometric, potentiometric, and colorimetric analyses. Here, we present a complementary approach that exploits fluorometric sensing modalities integrated into a soft, skin-interfaced microfluidic system which, when paired with a simple smartphone-based imaging module, allows for in situ measurement of important biomarkers in sweat. A network array of microchannels and a collection of microreservoirs pre-filled with fluorescent probes that selectively react with target analytes in sweat (e.g. probes), enable quantitative, rapid analysis. Field studies on human subjects demonstrate the ability to measure the concentrations of chloride, sodium and zinc in sweat, with accuracy that matches that of conventional laboratory techniques. The results highlight the versatility of advanced fluorescent-based imaging modalities in body-worn sweat microfluidics platforms, and they suggest some practical potential for these ideas.

New methods to 3D print multiphase materials with tailored microstructures could expand additive manufacturing capabilities to include structures with unprecedented complexity. One promising technique to control microstructure is acoustic focusing, a method by which particles in a fluid are manipulated by acoustic waves. By combining acoustic focusing with direct ink writing (where fluid inks are solidified after extrusion), the distribution of particles in fluid matrices can be modified throughout print lines. Here, we describe the design space for acoustic focusing with direct ink writing and explore the associated trade-offs, for inks composed of epoxy resin, silica, acetone, and glass microspheres. Increasing silica content increases the viscosity, widening the spatial distribution of particles, but it improves the preservation of printed shapes. Increasing the amplitude of the acoustic wave and decreasing the printing speed allow for narrower distributions of particles in printed lines but increase the time and energy needed for the print. To quantify these trade-offs, we conducted a combinatorial study of four factors: fumed silica and acetone loadings in epoxy-based matrices (which control ink rheology), print speed, and acoustic wave amplitude. We show that after deposition, particle distributions within focused lines are preserved; particle-poor regions on the edges of focused lines spread out farther than the particle-rich regions in the middle. The present study relating device operating parameters, ink properties and printed microstructures structure provide key insights to future designs of actuated print nozzles, which target new microstructures enabled by field-assisted control.

During periods of activity, sweat glands produce pressures associated with osmotic effects to drive liquid to the surface of the skin. The magnitudes of these pressures may provide insights into physiological health, the intensity of physical exertion, psychological stress factors and/other information of interest, yet they are currently unknown due to absence of means for non-invasive measurement. This paper introduces a thin, soft wearable microfluidic system that mounts onto the surface of the skin to enable precise and routine measurements of secretory fluidic pressures generated at the surface of the skin by eccrine sweat glands (surface SPSG, or s-SPSG) at nearly any location on the body. These platforms incorporate an arrayed collection of unit cells each of which includes an opening to the skin, an inlet through which sweat can flow, a capillary bursting valve (CBV) with a unique bursting pressure (BP), a corresponding microreservoir to receive sweat and an outlet to the surrounding ambient to allow release of backpressure. The BPs systematically span the physiologically relevant range. The set of unit cells is designed such that the BP difference between any two unit cells is greater than the combined uncertainty. Human studies demonstrate measurements of s-SPSG under different conditions, from various regions of the body. Average values in healthy young adults lie between 2.4 and 2.9 kPa. Sweat associated with vigorous exercise have s-SPSGs that are somewhat higher than those associated with sedentary activity. For all conditions, the forearm and lower back tend to yield the highest and lowest s-SPSGs, respectively.

At the intersection of the outwardly disparate fields of nanoparticle science and three-dimensional printing lies the promise of revolutionary new "nanocomposite" materials. Emergent phenomena deriving from the nanoscale constituents pave the way for a new class of transformative materials with encoded functionality amplified by new couplings between electrical, optical, transport, and mechanical properties. We provide an overview of key scientific advances that empower the development of such materials: nanoparticle synthesis and assembly, multiscale assembly and patterning, and mechanical characterization to assess stability. The focus is on recent illustrations of approaches that bridge these fields, facilitate the design of ordered nanocomposites, and offer clear pathways to device integration. We conclude by highlighting the remaining scientific challenges, including the critical need for assembly-compatible particle-fluid systems that ultimately yield mechanically robust materials. The role of domain boundaries and/or defects emerges as an important open question to address, with recent advances in fabrication setting the stage for future work in this area.

Real-time measurements of the total loss of sweat, the rate of sweating, the temperature of sweat, and the concentrations of electrolytes and metabolites in sweat can provide important insights into human physiology. Conventional methods use manual collection processes (e.g., absorbent pads) to determine sweat loss and lab-based instrumentation to analyze its chemical composition. Although such schemes can yield accurate data, they cannot be used outside of laboratories or clinics. Recently reported wearable electrochemical devices for sweat sensing bypass these limitations, but they typically involve on-board electronics, electrodes, and/or batteries for measurement, signal processing, and wireless transmission, without direct means for measuring sweat loss or capturing and storing small volumes of sweat. Alternative approaches exploit soft, skin-integrated microfluidic systems for collection and colorimetric chemical techniques for analysis. Here, we present the most advanced platforms of this type, in which optimized chemistries, microfluidic designs, and device layouts enable accurate assessments not only of total loss of sweat and sweat rate but also of quantitatively accurate values of the pH and temperature of sweat, and of the concentrations of chloride, glucose, and lactate across physiologically relevant ranges. Color calibration markings integrated into a graphics overlayer allow precise readout by digital image analysis, applicable in various lighting conditions. Field studies conducted on healthy volunteers demonstrate the full capabilities in measuring sweat loss/rate and analyzing multiple sweat biomarkers and temperature, with performance that quantitatively matches that of conventional lab-based measurement systems.

Wearable sweat sensors rely either on electronics for electrochemical detection or on colorimetry for visual readout. Non-ideal form factors represent disadvantages of the former, while semiquantitative operation and narrow scope of measurable biomarkers characterize the latter. Here, we introduce a battery-free, wireless electronic sensing platform inspired by biofuel cells that integrates chronometric microfluidic platforms with embedded colorimetric assays. The resulting sensors combine advantages of electronic and microfluidic functionality in a platform that is significantly lighter, cheaper, and smaller than alternatives. A demonstration device simultaneously monitors sweat rate/loss, pH, lactate, glucose, and chloride. Systematic studies of the electronics, microfluidics, and integration schemes establish the key design considerations and performance attributes. Two-day human trials that compare concentrations of glucose and lactate in sweat and blood suggest a potential basis for noninvasive, semi-quantitative tracking of physiological status.

The deposition of multiphase materials with microstructural control would enable new fabrication modalities, such as spatial variation in composition and the integration of microstructure and structural features. In this work, acoustically-excited microfluidic print nozzles were used to tailor the microstructure of printed composite filaments consisting of SiC fibers, solid BaTiO3 spheres, or hollow SiO2 spheres in an epoxy matrix. The results demonstrate that acoustic focusing is a promising technique to control microparticles and deposit two-phase ordered structures using a single nozzle. In addition to tuning the microstructure within a single print line, the overall concentration of particles can be increased by focusing the particle stream in a branched geometry, enabling the deposition of material with higher particle volume fractions than the initial ink composition. We show that even modest volume fractions of acoustically-focused and concentrated SiC fibers can produce printed composite filaments with unprecedented control over microstructural ordering that exhibit strengths rivaling polymer-matrix composites with higher volume fractions of stiffer fibers.

Highly-organized particle assembly at the microscale offers the potential to transform fields ranging from medical diagnostics to materials-by-design. Here, we demonstrate tunable ordering and alignment of microscale particles, including ∼10×50 μm glass rods and SU-8 brick and bowtie particles, via acoustic excitation, which produces columns with controlled spacing or highly-regular ‘brick-and-mortar’ packing. The method does not require surface functionalization and is broadly agnostic to colloidal chemistry or particle properties. We show that particle anisotropy promotes alignment and ordering even in the presence of flow, and can be used to increase the efficiency of particle delivery and sorting. Acoustically-assembled microstructures are preserved via photopolymerization of hydrogel matrices or binding with DNA ’mortar;’ the latter provides sufficient stability and flexibility to fold ribbon-like structures in solution by tuning the direction of the acoustic waves. Significant insight into pattern spacing and stability is provided by models which incorporate primary acoustic focusing and secondary acoustic scattering forces. These results suggest that acoustic excitation may be a powerful complement to diffusion-controlled self-assembly and enable hierarchical fabrication; e.g., micron-scale anisotropic crystals formed via self-assembly of smaller particles can be rapidly transported large distances, aligned and packed together to quickly assemble much larger objects.

Waterproof epidermal microfluidics enable collection and analysis of sweat during aquatic exercise., Noninvasive, in situ biochemical monitoring of physiological status, via the use of sweat, could enable new forms of health care diagnostics and personalized hydration strategies. Recent advances in sweat collection and sensing technologies offer powerful capabilities, but they are not effective for use in extreme situations such as aquatic or arid environments, because of unique challenges in eliminating interference/contamination from surrounding water, maintaining robust adhesion in the presence of viscous drag forces and/or vigorous motion, and preventing evaporation of collected sweat. This paper introduces materials and designs for waterproof, epidermal, microfluidic and electronic systems that adhere to the skin to enable capture, storage, and analysis of sweat, even while fully underwater. Field trials demonstrate the ability of these devices to collect quantitative in situ measurements of local sweat chloride concentration, local sweat loss (and sweat rate), and skin temperature during vigorous physical activity in controlled, indoor conditions and in open-ocean swimming.

Hydrocephalus is a common and costly neurological condition caused by the overproduction and/or impaired resorption of cerebrospinal fluid (CSF). The current standard of care, ventricular catheters (shunts), is prone to failure, which can result in nonspecific symptoms such as headaches, dizziness, and nausea. Current diagnostic tools for shunt failure such as computed tomography (CT), magnetic resonance imaging (MRI), radionuclide shunt patency studies (RSPSs), and ice pack–mediated thermodilution have disadvantages including high cost, poor accuracy, inconvenience, and safety concerns. Here, we developed and tested a noninvasive, skin-mounted, wearable measurement platform that incorporates arrays of thermal sensors and actuators for precise, continuous, or intermittent measurements of flow through subdermal shunts, without the drawbacks of other methods. Systematic theoretical and experimental benchtop studies demonstrate high performance across a range of practical operating conditions. Advanced electronics designs serve as the basis of a wireless embodiment for continuous monitoring based on rechargeable batteries and data transmission using Bluetooth protocols. Clinical studies involving five patients validate the sensor’s ability to detect the presence of CSF flow ( P = 0.012) and further distinguish between baseline flow, diminished flow, and distal shunt failure. Last, we demonstrate processing algorithms to translate measured data into quantitative flow rate. The sensor designs, fabrication schemes, wireless architectures, and patient trials reported here represent an advance in hydrocephalus diagnostics with ability to visualize flow in a simple, user-friendly mode, accessible to the physician and patient alike.

Novel methods to control the microstructures of multiphase extruded materials are needed to control spatial variations in structural and functional properties in additively manufactured components. One promising method is acoustic focusing, wherein microparticles suspended in fluids flowing through a silicon microfluidic channel are manipulated by establishing an acoustic wave in the channel. Acoustic focusing quality depends on factors including nozzle and particle geometries, acoustic pressure, matrix rheology, and flow characteristics. Using these factors, we experimentally verified scaling relationships governing acoustic focusing based on classical acoustic forces, using ceramic microspheres in epoxy resin. Further experiments characterized focusing behavior in more complex shear-thinning matrices composed of epoxy resin, fumed silica, and acetone. We designed image processing methods to quantify particle focusing and form holding (the ink’s ability to maintain shapes), and we used those methods to identify the trade-offs associated with matrix formulation, printing speed, and acoustic pressure.

3D printing mechanically flexible composite materials with high electrical conductivity is currently hindered by the need to use high loading of conductive filler, which severely limits flexibility. Here, microstructural patterning of composite materials via acoustophoresis imparts these materials with high conductivity and flexibility simultaneously, filling a technology gap in the field. Acoustophoresis patterns filler particles into highly efficient percolated networks which utilize up to 97\% of the particles in the composite, whereas the inefficient stochastic networks of conventional dispersed-fiber composites utilize $$500 bending cycles without losses in conductivity and changing conductivity only $5$\% within cycles on average (for 2.6v\% composites). In contrast, conventional unpatterned composites with the same conductivity require such high loading that they're prohibitively brittle. Finally, modulating the shape of the applied acoustic fields allows control over the anisotropy of the conductive networks and produces materials which are either 2-D conductive, 1-D conductive, or insulating, all using the same nozzle and ink., 10 pages; 5 figures

Precise, quantitative measurements of the thermal properties of human skin can yield insights into thermoregulatory function, hydration, blood perfusion, wound healing, and other parameters of clinical interest. The need for wired power supply systems and data communication hardware limits, however, practical applicability of existing devices designed for measurements of this type. Here, a set of advanced materials, mechanics designs, integration schemes, and wireless circuits is reported as the basis for wireless, battery-free sensors that softly interface to the skin to enable precise measurements of its temperature and thermal transport properties. Calibration processes connect these parameters to the hydration state of the skin, the dynamics of near-surface flow through blood vessels and implanted catheters, and to recovery processes following trauma. Systematic engineering studies yield quantitative metrics in precision and reliability in real-world conditions. Evaluations on five human subjects demonstrate the capabilities in measurements of skin hydration and injury, including examples of continuous wear and monitoring over a period of 1 week, without disrupting natural daily activities.

This paper introduces super absorbent polymer valves and colorimetric sensing reagents as enabling components of soft, skin-mounted microfluidic devices designed to capture, store, and chemically analyze sweat released from eccrine glands. The valving technology enables robust means for guiding the flow of sweat from an inlet location into a collection of isolated reservoirs, in a well-defined sequence. Analysis in these reservoirs involves a color responsive indicator of chloride concentration with a formulation tailored to offer stable operation with sensitivity optimized for the relevant physiological range. Evaluations on human subjects with comparisons against ex situ analysis illustrate the practical utility of these advances.

Portable and low-cost platforms for protein biomarker detection are highly sought after for point of care applications. We demonstrate a simple microfluidic device for the rapid, electrically-based detection of proteins in serum. Our aggregation analyzer relies on detecting the protein-induced aggregation of sub-micron particles, using a one-step procedure followed by a fast, particle-by-particle measurement with a very high count rate. This enables the rapid and precise quantification of C-Reactive protein levels, within the clinically relevant range, using unprocessed human serum and a disposable microfluidic device; no optics are involved in the implementation. Due to the single particle detection format and the use of microfluidics, only a small volume of serum (~50 nL) is needed to complete the analysis. The method can be easily extended to multiplexed biomarker detection by combining an assay using differently sized particles, each targeting a separate protein. We illustrate this by using two sizes of latex beads and demonstrating the simultaneous detection of two different proteins in a serum environment with minimal cross-interference. This confirms that our aggregation analyzer platform provides a simple and straightforward method for multiplexed biomarker detection in a low cost, portable design.

Plasmonic nanoparticles are used in a wide variety of applications over a broad array of fields including medicine, energy, and environmental chemistry. The continued successful development of this material class requires the accurate characterization of nanoparticle stability for a variety of solution-based conditions. Although many characterization methods exists, there is an absence of a unified, quantitative means for assessing the colloidal stability of plasmonic nanoparticles. We present the particle instability parameter (PIP) as a robust, quantitative, and generalizable characterization technique based on UV-vis absorbance spectroscopy to characterize colloidal instability. We validate PIP performance with both traditional and alternative characterization methods by measuring gold nanorod instability in response to different salt (NaCl) concentrations. We further measure gold nanorod stability as a function of solution pH, salt, and buffer (type and concentration), nanoparticle concentration, and concentration of free surfactant. Finally, these results are contextualized within the literature on gold nanorod stability to establish a standardized methodology for colloidal instability assessment.

Colloidal gold nanospheres have been used in a variety of applications since the Middle Ages, when artisans blended tissue paper thin gold sheets into molten glass, creating stained glass panels with rich ruby red hues. Despite both substantial interest and well-established procedures for producing nanoparticles of various shapes, little is known about the growth mechanisms that govern the formation of shapes such as rods, cubes, tetrahedrons, and dog-bones. Understanding these mechanisms is an important step in developing applications using nanoparticles. With more finely defined controls, metallic nanoparticles could be fabricated or grown in desired shapes with far less trial and error, offering greater potential for complex and functional nanostructures. In this work, a cellular automata model is used to model the growth of high aspect ratio gold nanorods. One mechanism that has been suggested for nanorod growth is competitive binding between the colloidal gold in solution and a surfactant, which functions as a structure-directing agent. The model incorporates experimental conditions in the framework of this competitive binding. Results suggest that cellular automata modeling can be a computationally efficient means of modeling the competitive and non-deterministic interactions involved in the growth of gold nanorods.

Quantum dots are semiconductor nanoparticles that are approximately 1-10nm in diameter, similar to small proteins, and their photoluminescence is sensitive to the presence and nature of adsorbates. We have deployed these nanomaterials as luminescent probes of DNA structure. Sequence dependent conformational flexibility of DNA is of great interest due to its implications for drug-DNA and DNA-protein interactions. The counterion atmosphere surrounding DNA plays an important role in its structure, dynamics, and packaging. In this paper, we investigate the effect that various monovalent and divalent cations have on the binding of 4.5 nm CdS quantum dots to oligonucleotides that have sequence-directed intrinsic structure.