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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.

A spin‐contrast‐variation neutron powder diffractometry technique that extracts the structure factor of hydrogen atoms, i.e. the contribution of hydrogen atoms to a crystal's structure factor, has been developed. Crystals of l‐glutamic acid were dispersed in a d‐polystyrene matrix containing 4‐methacryloyloxy‐2,2,6,6,‐tetramethyl‐1‐piperidinyloxy to polarize their proton spins dynamically. The intensities of the diffraction peaks of the sample changed according to the proton polarization, and the structure factor of the hydrogen atoms was extracted from the proton‐polarization‐dependent intensities. This technique is expected to enable analyses of the structures of hydrogen‐containing materials that are difficult to determine with conventional powder diffractometry. [ABSTRACT FROM AUTHOR]

To investigate the mechanism of structural changes of water and polymer networks with drying and swelling, we measured the Raman spectra of a physically cross-linked poly(vinyl alcohol) (PVA) hydrogel synthesized using the freezing–thawing method. The results show that the vibrational frequencies of the O–H and C–H stretching modes decrease with dehydration. The frequency shifts observed are attributed to reduction of free water inside the polymer network. The C–H bonds elongate as the water density decreases, and the average length of the O–H bonds increases with increasing proportion of bound water to the total amount of water. On the basis of the dependence of the frequency shifts on the PVA concentration of the original solution, it was found that the structure of the polymer network in the reswollen hydrogel becomes inhomogeneous due to shrinkage of the polymer network with drying. Furthermore, to investigate the effects of the crosslinking structure on the drying process, these results were compared with those of a chemically cross-linked PVA hydrogel synthesized using glutaraldehyde as a cross-linker. The result shows that the vibrational frequency of the O–H stretching mode for the chemically cross-linked hydrogel increases with dehydration, whereas that of the C–H stretching mode decreases. The opposite trend observed in the O–H stretching mode between the physically and chemically crosslinked hydrogels is due to the difference in the shrinkage rate of the polymer network. Because the rate of shrinking is slow compared with that of dehydration in the chemically cross-linked hydrogel, water density in the polymer network decreases. For the physically cross-linked hydrogel, the polymer network structure can be easily shrunken, and the average strength of hydrogen bonds increases with dehydration. The results show that the structures of the polymer network and water change with the gel preparation process, cross-linking method, and drying and reswelling processes. The structure of the polymer network and the behavior of water accommodated in the network are important factors governing the chemical and physical properties of gel materials. [ABSTRACT FROM AUTHOR]

To investigate the structure and dynamics of water in mixed solutions including laponite clay particles and poly(ethylene oxide) (PEO), we measured the Raman spectra of the mixed solutions in the temperature range 283-313 K. The results show that the vibrational energies of the O-H stretching modes in the mixed solutions depend on the water content and temperature. The energy shifts of the O-H stretching modes are attributed to changes in the water structure. By applying a structural model of bulk water to the spectra in the O-H stretching region, the local structures of water in the solutions were analyzed. The result shows that the formation probability of hydrogen bonds in the solutions decreases as the water content decreases. Laponite and PEO have effects to disrupt the network structure of hydrogen bonds between water molecules. Further, it was found that laponite and PEO cause increase in the strength of hydrogen bonds of surrounding water,although the strength of the hydrogen bonds increases with the order water-laponite < water-water < water-PEO. It is concluded that water in laponite-PEO mixed solutions has a less-networked structure with strong hydrogen bonds compared with bulk water. [ABSTRACT FROM AUTHOR]

Raman spectra of poly-N,N,-dimethylacrylamide hydrogel were measured in order to investigate the mechanism of the structural changes in water and the polymer network during dehydration. The results show that the vibrational energies of the C==O and the O–H stretching modes increase with the extent of dehydration, whereas that of the CH3 rocking mode decreases. The energy shifts observed in the C==O stretching and CH3 rocking modes indicate that the polymer network shrinks with the dehydration and undergoes a glass transition at some point. The energy shifts of the O[Single_Bond]H stretching modes are attributed to changes in the water structure with the structural change of the polymer network. By applying a structural model of bulk water to the spectra of the O[Single_Bond]H stretching region, the local water structures in the gel and the dried glassy polymer were analyzed. The result shows that a tetragonal water structure consisting of four hydrogen bonds increases in the residual water of the dried glassy polymer, suggesting that the residual water forms a two-dimensional hydrogen-bonded network. The local water structure in a polymer might have important implications for the interpretation of properties of localized water, for instance, water in a mineral crack. [ABSTRACT FROM AUTHOR]

ABSTRACT To investigate the effects of crosslinker density on the properties of hydrogels, compression tests, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and Raman measurements were performed on poly- N, N-dimethylacrylamide hydrogels. The results of the compression tests showed that the Young's modulus increases as the crosslinker density increases. To understand the mechanism of the change in the mechanical properties, the structures of the polymer networks and water and the molecular vibrations were analyzed using SEM, DSC, and Raman methods. From the SEM images, it was found that the porosity estimated from the mesh size and cell density increases with increasing crosslinker density. In addition, the DSC and Raman results show that the thickness of the bound water increases as the porosity increases, although the density of the polymer chains in the porous wall remains nearly constant. The increase in the number density of polymer chains can be one of the mechanisms contributing to the increase in the mechanical strength of the hydrogels at lower crosslinker density below 5 mol %, as proposed by previous studies. At higher crosslinker density, however, the number density of polymer chains does not increase with increasing crosslinker density. The present results suggest that the bound water plays an important role in strengthening the hydrogel. The water structure may be one of the dominant factors governing the chemical and physical properties of hydrogels. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1017-1027 [ABSTRACT FROM AUTHOR]