Glucose Sensing Using Surface-Enhanced Raman-Mode Constraining.

Diabetes mellitus is a chronic disease, and its management focuses on monitoring and lowering a patient's glucose level to prevent further complications. By tracking the glucose-induced shift in the surface-enhanced Raman-scattering (SERS) emission of mercaptophenylboronic acid (MPBA), we have demonstrated fast and continuous glucose sensing in the physiologically relevant range from 0.1 to 30 mM and verified the underlying mechanism using numerical simulations. Bonding of glucose to MPBA suppresses the "breathing" mode of MPBA at 1071 cm-1 and energizes the constrained-bending mode at 1084 cm-1, causing the dominant peak to shift from 1071 to 1084 cm-1. MPBA-glucose bonding is also reversible, allowing continuous tracking of ambient glucose concentrations, and the MPBA-coated substrates showed very stable performance over a 30 day period, making the approach promising for long-term continuous glucose monitoring. Using Raman-mode-constrained, miniaturized SERS implants, we also successfully demonstrated intraocular glucose measurements in six ex vivo rabbit eyes within ±0.5 mM of readings obtained using a commercial glucose sensor.

Surface-Enhanced Raman Spectroscopy-Based Label-Free Insulin Detection at Physiological Concentrations for Analysis of Islet Performance.

Label-free optical detection of insulin would allow in vitro assessment of pancreatic cell functions in their natural state and expedite diabetes-related clinical research and treatment; however, no existing method has met these criteria at physiological concentrations. Using spatially uniform 3D gold-nanoparticle sensors, we have demonstrated surface-enhanced Raman sensing of insulin in the secretions from human pancreatic islets under low and high glucose environments without the use of labels such as antibodies or aptamers. Label-free measurements of the islet secretions showed excellent correlation among the ambient glucose levels, secreted insulin concentrations, and measured Raman-emission intensities. When excited at 785 nm, plasmonic hotspots of the densely arranged 3D gold-nanoparticle pillars as well as strong interaction between sulfide linkages of the insulin molecules and the gold nanoparticles produced highly sensitive and reliable insulin measurements down to 100 pM. The sensors exhibited a dynamic range of 100 pM to 50 nM with an estimated detection limit of 35 pM, which covers the reported concentration range of insulin observed in pancreatic cell secretions. The sensitivity of this approach is approximately 4 orders of magnitude greater than previously reported results using label-free optical approaches, and it is much more cost-effective than immunoassay-based insulin detection widely used in clinics and laboratories. These promising results may open up new opportunities for insulin sensing in research and clinical applications.

Cuff-Less Blood Pressure Estimation Using Pulse Waveform Analysis and Pulse Arrival Time.

Using the massive MIMIC physiological database, we tried to validate pulse wave analysis (PWA) based on multiparameters model whether it can continuously estimate blood pressure (BP) values on single site of one hand. In addition, to consider the limitation of insufficient data acquirement for home user, we used pulse arrival time (PAT) driven BP information to determine the individual scale factors of the PWA-BP estimation model. Experimental results indicate that the accuracy of the average regression model has error standard deviations of  mmHg (PAT),  mmHg (PWA) for SBP and  mmHg (PAT),  mmHg (PWA) for DBP on 23 subjects over a 1 day period. We defined a local-model which is extracted regression model from sparsely selected small dataset, contrast to full dataset for 24h (average-model). The limit of BP estimation accuracy from the local-model of PWA is lower than that of PAT-BP average-model. Whereas the error of the BP estimation local-model was reduced using more data for scaling, it required more than four times the 1 min data extracted over the 12 h calibration period to predict BP for 1 day. This study shows that PWA has possibility to estimate BP value and PAT-driven BP information could be used to determine the individual scale factors of the PWA-BP estimation model for home users.

Fully Stretchable Optoelectronic Sensors Based on Colloidal Quantum Dots for Sensing Photoplethysmographic Signals.

Flexible and stretchable optoelectronic devices can be potentially applied in displays, biosensors, biomedicine, robotics, and energy generation. The use of nanomaterials with superior optical properties such as quantum dots (QDs) is important in the realization of wearable displays and biomedical devices, but specific structural design as well as selection of materials should preferentially accompany this technology to realize stretchable forms of these devices. Here, we report stretchable optoelectronic sensors manufactured using colloidal QDs and integrated with elastomeric substrates, whose optoelectronic properties are stable under various deformations. A graphene electrode is adopted to ensure extreme bendability of the devices. Ultrathin QD light-emitting diodes and QD photodetectors are transfer-printed onto a prestrained elastomeric layout to form wavy configurations with regular patterns. The layout is mechanically stretchable until the structure is converted to a flat configuration. The emissive and active area itself can be stretched or compressed by buckled structures, which are applicable to wearable electronic devices. We demonstrate that these stretchable optoelectronic sensors can be used for continuous monitoring of blood waves via photoplethysmography signal recording. These and related systems create important and unconventional opportunities for stretchable and foldable optoelectronic devices with health-monitoring capability and, thus, meet the demand for wearable and body-integrated electronics.

Simple, Large-Scale Fabrication of Uniform Raman-Enhancing Substrate with Enhancement Saturation.

It is well-known that gold nanoparticle (AuNP) clusters generate strong surface-enhanced Raman scattering (SERS). In order to produce spatially uniform Raman-enhancing substrates at a large scale, we synthesized vertically perforated three-dimensional (3D) AuNP stacks. The 3D stacks were fabricated by first hydrothermally synthesizing ZnO nanowires perpendicular to silicon wafers followed by repetitively performing liquid-phase deposition of AuNPs on the tops and side surfaces of the nanowires. During the deposition process, the nanowires were shown to gradually dissolve away, leaving hollow vestiges or perforations surrounded by stacks of AuNPs. Simulation studies and experimental measurements reveal these nanoscale perforations serve as light paths that allow the excitation light to excite deeper regions of the 3D stacks for stronger overall Raman emission. Combined with properly sized nanoparticles, this feature maximizes and saturates the Raman enhancement at 1-pM sensitivity across the entire wafer-scale substrate, and the saturation improves the wafer-scale uniformity by a factor of 6 when compared to nanoparticle layers deposited directly on a silicon wafer substrate. Using the 3D-stacked substrates, quantitative sensing of adenine molecules yielded concentrations measurements within 10% of the known value. Understanding the enhancing mechanisms and engineering the 3D stacks have opened a new method of harnessing the intense SERS observed in nanoparticle clusters and realize practical SERS substrates with significantly improved uniformity suitable for quantitative chemical sensing.

Phagocytosis dynamics depends on target shape.

A complete understanding of phagocytosis requires insight into both its biochemical and physical aspects. One of the ways to explore the physical mechanism of phagocytosis is to probe whether and how the target properties (e.g., size, shape, surface states, stiffness, etc.) affect their uptake. Here we report an imaging-based method to explore phagocytosis kinetics, which is compatible with real-time imaging and can be used to validate existing reports using fixed and stained cells. We measure single-event engulfment time from a large number of phagocytosis events to compare how size and shape of targets determine their engulfment. The data shows an increase in the average engulfment time for increased target size, for spherical particles. The uptake time data on nonspherical particles confirms that target shape plays a more dominant role than target size for phagocytosis: Ellipsoids with an eccentricity of 0.954 and much smaller surface areas than spheres were taken up five times more slowly than spherical targets.

Biophotonic techniques for the study of malaria-infected red blood cells.

Investigation of the homeostasis of red blood cells upon infection by Plasmodium falciparum poses complex experimental challenges. Changes in red cell shape, volume, protein, and ion balance are difficult to quantify. In this article, we review a wide range of optical techniques for quantitative measurements of critical homeostatic parameters in malaria-infected red blood cells. Fluorescence lifetime imaging and tomographic phase microscopy, quantitative deconvolution microscopy, and X-ray microanalysis, are used to measure haemoglobin concentration, cell volume, and ion contents. Atomic force microscopy is briefly reviewed in the context of these optical methodologies. We also describe how optical tweezers and optical stretchers can be usefully applied to empower basic malaria research to yield diagnostic information on cell compliance changes upon malaria infection. The combined application of these techniques sheds new light on the detailed mechanisms of malaria infection providing potential for new diagnostic or therapeutic approaches.

Optical trapping of colloidal particles and cells by focused evanescent fields using conical lenses.

We demonstrate advantages in terms of trapping force distribution and laser efficiency that come from using a telescopic pair of conical lenses ('axicon') to generate a ring-like beam, that in conjunction with a high NA objective is used for direct optical trapping with a focused evanescent field near a surface. Various field geometries are considered and compared. First, a Gaussian beam and a laser beam focused on the back focal plane of the objective are compared with each other, and they are scanned across the inlet aperture of the objective. This allows to detect the point of total internal refraction, and to study the trapping power near the surface. We confirm that the hollow beam generated by the conical lenses can generate an evanescent field after a high NA objective lens, and that micron-sized particles can be trapped stably. Finally, we apply the focused evanescent field to erythrocytes under flow, showing that cells are trapped against the flow and are held horizontally against the surface. This is a different equilibrium condition compared to conventional single beam traps, and it is particularly favorable for monitoring the cell membrane. We foresee the integration of this type of trapping with the imaging techniques based on total internal refraction fluorescence (TIRF).

Flickering analysis of erythrocyte mechanical properties: dependence on oxygenation level, cell shape, and hydration level.

Erythrocytes (red blood cells) play an essential role in the respiratory functions of vertebrates, carrying oxygen from lungs to tissues and CO(2) from tissues to lungs. They are mechanically very soft, enabling circulation through small capillaries. The small thermally induced displacements of the membrane provide an important tool in the investigation of the mechanics of the cell membrane. However, despite numerous studies, uncertainties in the interpretation of the data, and in the values derived for the main parameters of cell mechanics, have rendered past conclusions from the fluctuation approach somewhat controversial. Here we revisit the experimental method and theoretical analysis of fluctuations, to adapt them to the case of cell contour fluctuations, which are readily observable experimentally. This enables direct measurements of membrane tension, of bending modulus, and of the viscosity of the cell cytoplasm. Of the various factors that influence the mechanical properties of the cell, we focus here on: 1), the level of oxygenation, as monitored by Raman spectrometry; 2), cell shape; and 3), the concentration of hemoglobin. The results show that, contrary to previous reports, there is no significant difference in cell tension and bending modulus between oxygenated and deoxygenated states, in line with the softness requirement for optimal circulatory flow in both states. On the other hand, tension and bending moduli of discocyte- and spherocyte-shaped cells differ markedly, in both the oxygenated and deoxygenated states. The tension in spherocytes is much higher, consistent with recent theoretical models that describe the transitions between red blood cell shapes as a function of membrane tension. Cell cytoplasmic viscosity is strongly influenced by the hydration state. The implications of these results to circulatory flow dynamics in physiological and pathological conditions are discussed.

Non-constrained blood pressure monitoring using ECG and PPG for personal healthcare.

Blood pressure (BP) is one of the important vital signs that need to be monitored for personal healthcare. Arterial blood pressure (BP) was estimated from pulse transit time (PTT) and PPG waveform. PTT is a time interval between an R-wave of electrocardiography (ECG) and a photoplethysmography (PPG) signal. This method does not require an aircuff and only a minimal inconvenience of attaching electrodes and LED/photo detector sensors on a subject. PTT computed between the ECG R-wave and the maximum first derivative PPG was strongly correlated with systolic blood pressure (SBP) (R = -0.712) compared with other PTT values, and the diastolic time proved to be appropriate for estimation diastolic blood pressure (DBP) (R = -0.764). The percent errors of SBP using the individual regression line (4-11%) were lower than those using the regression line obtained from all five subjects (9-14%). On the other hand, the DBP estimation did not show much difference between the individual regression (4-10%) and total regression line (6-10%). Our developed device had a total size of 7 x 13.5 cm and was operated by single 3-V battery. Biosignals can be measured for 72 h continuously without external interruptions. Through a serial network communication, an external personal computer can monitor measured waveforms in real time. Our proposed method can be used for non-constrained, thus continuous BP monitoring for the purpose of personal healthcare.

The nonlinear mechanical response of the red blood cell.

We measure the dynamical mechanical properties of human red blood cells. A single cell response is measured with optical tweezers. We investigate both the stress relaxation following a fast deformation and the effect of varying the strain rate. We find a power-law decay of the stress as a function of time, down to a plateau stress, and a power-law increase of the cell's elasticity as a function of the strain rate. Interestingly, the exponents of these quantities violate the linear superposition principle, indicating a nonlinear response. We propose that this is due to the breaking of a fraction of the crosslinks during the deformation process. The soft glassy rheology model accounts for the relation between the exponents we observe experimentally. This picture is consistent with recent models of bond remodeling in the red blood cell's molecular structure. Our results imply that the blood cell's mechanical behavior depends critically on the deformation process.

Nonlinear characteristics of heart rate time series: influence of three recumbent positions in patients with mild or severe coronary artery disease.

The aim of this study was to investigate the nonlinear characteristics of heart rate variability (HRV) for three recumbent positions: the supine, left lateral and right lateral decubitus positions. Recently, using a linear analyses method (for time and frequency domains), the effect of the right lateral decubitus position on vagal modulation has been found to increase parasympathetic activity and decrease sympathetic modulation. Little is known about the nonlinear dynamics of HRV for the three recumbent positions. Therefore, we studied the correlation dimension (CD), the largest Lyapunov exponent (LLE), the sample entropy (SampEn), the approximate entropy (ApEn) and the exponent alpha of the 1/falpha power spectrum as nonlinear characteristics of HRV. In response to the right lateral decubitus position, the CD, LLE, SampEn and ApEn increased significantly in both coronary artery disease (CAD) and control groups. In the linear analyses, the normalized high-frequency power (nHF) increased in the right lateral decubitus position. The CD, LLE, ApEn and SampEn correlated positively to the nHF. The alpha exponent did not correlate to either linear measure or CD, but correlated negatively to LLE, ApEn and SampEn. Among the three recumbent positions, it was found that the right lateral decubitus position can increase the complexity of the human physiological system and the vagal modulation of the cardiac autonomic nervous system the most.