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Advanced capabilities in noninvasive, in situ monitoring of parameters related to sweat serve as the basis for obtaining real‐time insights into human physiological state, health, and performance. Although recently reported classes of soft, skin‐interfaced microfluidic systems support powerful functions in this context, most are designed as single‐use disposables. As a result, associated waste streams have the potential to create adverse environmental impacts. Here, we introduce materials and fabrication techniques that bypass these concerns through biodegradable microfluidic systems with a full range of features, including measurement of sweat rate and total loss, and colorimetric analysis of biomarkers. The key components fully degrade through the enzymatic action of microorganisms in natural soil environments, or in industrial compost facilities, to yield end products with beneficial uses as fertilizers and species to improve soil health. Detailed characterization of the constituent materials, the fabrication procedures, the assembly processes, and the completed devices reveal a set of essential performance parameters that are comparable to, or even better than, those of non‐degradable counterparts. Human subject studies illustrate the ability of these devices to acquire accurate measurements of sweat loss, sweat rate, pH, and chloride concentration during physical activities and thermal exposures. Soft, skin‐interfaced microfluidic systems exploit biodegradable thermoplastic copolyester elastomers for the microfluidic layers, cellulose films and pressure sensitive adhesives as sealing layers, and carefully selected chemical reagents as colorimetric assays for monitoring sweat loss, sweat rate, pH, and chloride concentration. The resulting platforms can fully degrade in natural soil or composting facilities to organic compounds that can act as plant nutrients, thereby eliminating environmental stresses from discarded devices.

Sound-evoked vibrations transmitted into the mammalian cochlea produce traveling waves that provide the mechanical tuning necessary for spectral decomposition of sound. These traveling waves of motion that have been observed to propagate longitudinally along the basilar membrane (BM) ultimately stimulate the mechano-sensory receptors. The tectorial membrane (TM) plays a key role in this process, but its mechanical function remains unclear. Here we show that the TM supports traveling waves that are an intrinsic feature of its visco-elastic structure. Radial forces applied at audio frequencies (2-20 kHz) to isolated TM segments generate longitudinally propagating waves on the TM with velocities similar to those of the BM traveling wave near its best frequency place. We compute the dynamic shear storage modulus and shear viscosity of the TM from the propagation velocity of the waves and show that segments of the TM from the basal turn are stiffer than apical segments are. Analysis of loading effects of hair bundle stiffness, the limbal attachment of the TM, and viscous damping in the subtectorial space suggests that TM traveling waves can occur in vivo. Our results show the presence of a traveling wave mechanism through the TM that can functionally couple a significant longitudinal extent of the cochlea and may interact with the BM wave to greatly enhance cochlear sensitivity and tuning.

Real-time monitoring of hydration biomarkers in tandem with biophysical markers can offer valuable physiological insights about heat stress and related thermoregulatory response. These metrics have been challenging to achieve with wearable sensors. Here we present a closed-loop electrochemical/biophysical wearable sensing device and algorithms that directly measure whole-body sweat loss, sweating rate, sodium concentration, and sodium loss with electrode arrays embedded in a microfluidic channel. The device contains two temperature sensors for skin temperature and thermal flux recordings, and an accelerometer for real-time monitoring of activity level. An onboard haptic module enables vibratory feedback cues to the wearer once critical sweat loss thresholds are reached. Data is stored onboard in memory and autonomously transmitted via Bluetooth to a smartphone and cloud portal. Field studies conducted in physically demanding activities demonstrate the key capabilities of this platform to inform hydration interventions in highly challenging real-world settings.

Soft microfluidic systems that capture, store, and perform biomarker analysis of microliter volumes of sweat, in situ, as it emerges from the surface of the skin, represent an emerging class of wearable technology with powerful capabilities that complement those of traditional biophysical sensing devices. Recent work establishes applications in the real-time characterization of sweat dynamics and sweat chemistry in the context of sports performance and healthcare diagnostics. This paper presents a collection of advances in biochemical sensors and microfluidic designs that support multimodal operation in the monitoring of physiological signatures directly correlated to physical and mental stresses. These wireless, battery-free, skin-interfaced devices combine lateral flow immunoassays for cortisol, fluorometric assays for glucose and ascorbic acid (vitamin C), and digital tracking of skin galvanic responses. Systematic benchtop evaluations and field studies on human subjects highlight the key features of this platform for the continuous, noninvasive monitoring of biochemical and biophysical correlates of the stress state.

Remarkable sensitivity and exquisite frequency selectivity are hallmarks of mammalian hearing, but their underlying mechanisms remain unclear. Cochlear insults and hearing disorders that decrease sensitivity also tend to broaden tuning, suggesting that these properties are linked. However, a recently developed mouse model of genetically altered hearing ( Tectb −/− ) shows decreased sensitivity and sharper frequency selectivity. In this paper, we show that the Tectb mutation reduces the spatial extent and propagation velocity of tectorial membrane (TM) travelling waves and that these changes in wave propagation are likely to account for all of the hearing abnormalities associated with the mutation. By reducing the spatial extent of TM waves, the Tectb mutation decreases the spread of excitation and thereby increases frequency selectivity. Furthermore, the change in TM wave velocity reduces the number of hair cells that effectively couple energy to the basilar membrane, which reduces sensitivity. These results highlight the importance of TM waves in hearing. Mutation of the Tectb gene reduces auditory sensitivity but increases frequency selectivity. Here the authors show that Tectb mutation reduces both the spatial and temporal propagation of travelling waves along the tectorial membrane, explaining the unexpected auditory abnormalities in this mutant.

We present motions of individual freestanding hair bundles in an isolated cochlea in response to tonal sound stimulation. Motions were measured from images taken by strobing a light source at the tone frequency. The tips and bases of hair bundles moved a comparable amount, but with a phase difference that increased by 180° with frequency, indicating that distributed fluid properties drove hair bundle motion. Hair bundle rotation increased with frequency to a constant value, and underwent >90° of phase change. The frequency at which the phase of rotation relative to deflection of the bundle base was 60° was comparable to the expected best frequency of each hair cell, and varied inversely with the square of bundle height. The sharpness of tuning of individual hair bundles was comparable to that of hair cell receptor potentials at high sound levels. These results indicate that frequency selectivity at high sound levels in this cochlea is purely mechanical, determined by the interaction of hair bundles with the surrounding fluid. The sharper tuning of receptor potentials at lower sound levels is consistent with the presence of a negative damping, but not a negative stiffness, as an active amplifier in hair bundles.