Adrien Pierre

Monitoring of Vital Signs with Flexible and Wearable Medical Devices

Advances in wireless technologies, low-power electronics, the internet of things, and in the domain of connected health are driving innovations in wearable medical devices at a tremendous pace. Wearable sensor systems composed of flexible and stretchable materials have the potential to better interface to the human skin, whereas silicon-based electronics are extremely efficient in sensor data processing and transmission. Therefore, flexible and stretchable sensors combined with low-power silicon-based electronics are a viable and efficient approach for medical monitoring. Flexible medical devices designed for monitoring human vital signs, such as body temperature, heart rate, respiration rate, blood pressure, pulse oxygenation, and blood glucose have applications in both fitness monitoring and medical diagnostics. As a review of the latest development in flexible and wearable human vitals sensors, the essential components required for vitals sensors are outlined and discussed here, including the reported sensor systems, sensing mechanisms, sensor fabrication, power, and data processing requirements.

All-organic optoelectronic sensor for pulse oximetry

Pulse oximetry is a ubiquitous non-invasive medical sensing method for measuring pulse rate and arterial blood oxygenation. Conventional pulse oximeters use expensive optoelectronic components that restrict sensing locations to finger tips or ear lobes due to their rigid form and area-scaling complexity. In this work, we report a pulse oximeter sensor based on organic materials, which are compatible with flexible substrates. Green (532 nm) and red (626 nm) organic light-emitting diodes (OLEDs) are used with an organic photodiode (OPD) sensitive at the aforementioned wavelengths. The sensors active layers are deposited from solution-processed materials via spin-coating and printing techniques. The all-organic optoelectronic oximeter sensor is interfaced with conventional electronics at 1 kHz and the acquired pulse rate and oxygenation are calibrated and compared with a commercially available oximeter. The organic sensor accurately measures pulse rate and oxygenation with errors of 1% and 2%, respectively.

All-Printed Flexible Organic Transistors Enabled by Surface Tension-Guided Blade Coating

A combination of surface energy-guided blade coating and inkjet printing is used to fabricate an all-printed high performance, high yield, and low variability organic thin film transistor (OTFT) array on a plastic substrate. Functional inks and printing processes were optimized to yield self-assembled homogenous thin films in every layer of the OTFT stack. Specifically, we investigated the effect of capillary number, semiconductor ink composition (small molecule-polymer ratio), and additive high boiling point solvent concentrations on film fidelity, pattern design, device performance and yields.

Empirically based device modeling of bulk heterojunction organic photovoltaics

We develop an empirically based optoelectronic model to accurately simulate the photocurrent in organic photovoltaic (OPV) devices with novel materials including bulk heterojunction OPV devices based on a new low band gap dithienothiophene-DPP donor polymer, P(TBT-DPP), blended with PC70BM at various donor-acceptor weight ratios and solvent compositions. Our devices exhibit power conversion efficiencies ranging from 1.8% to 4.7% at AM 1.5G. Electron and hole mobilities are determined using space-charge limited current measurements. Bimolecular recombination coefficients are both analytically calculated using slowest-carrier limited Langevin recombination and measured using an electro-optical pump-probe technique. Exciton quenching efficiencies in the donor and acceptor domains are determined from photoluminescence spectroscopy. In addition, dielectric and optical constants are experimentally determined. The photocurrent and its bias-dependence that we simulate using the optoelectronic model we develop, wh...

Solution-processed image sensors on flexible substrates

Image sensors are ubiquitous and used in awide variety of applications ranging from consumer products to healthcare and industrial applications. The signal-to-noise ratio (SNR) of an image increases with larger pixels, which is costly to scale using silicon andwafer-basedmicrofabrication. On the other hand, the performance of solution-processed photodetectors and transistors is advancing considerably. The printability of these devices on plastic substrates can enable low-cost scaling of large-pixel, high SNR image sensors. In addition, theflexibility of the substrates can enable new imaging systems never possible with the rigidity of conventional sensors. In this workwe review the progressmade towards solution-processed image sensors on flexible substrates. The fundamental operation of image sensors using intra-pixel charge integration isfirst explained to introduce the figures ofmerit for these systems. The physics, figures ofmerit, and state of the art for solutionprocessed photodiodes and phototransistors is also overviewed. A literature survey is done on solution-processed passive and active pixel image sensors with emphasis on active-switching for intrapixel charge integration. Finally, optics compliant with large area andflexible image sensors are reviewed.

High-detectivity printed organic photodiodes for large area flexible imagers

The recent increase in utilization of image sensors and significant development in the field of wearable and disposable sensors not only demands a decrease in photodiode fabrication costs, but also requires new functional abilities such as operating at extremely low light intensities, narrowband and broadband spectral selectivity, lightweight and mechanical flexibility. Today, most of these demands cannot be fulfilled by conventional workhorse silicon photodetectors without a significant amount of processing steps to achieve heterogeneous integration. The chemical tunability, mechanical properties and printable processing of organic materials make them promising candidates for fabrication of low-cost, highly scalable and flexible organic photodiodes (OPDs). In addition, the large area scalability of printing techniques is beneficial for photodiodes since the signal-tonoise ratio (SNR), at a given light intensity, increases with detector size. Since the SNR depends on multiple parameters, such as the external quantum efficiency (EQE), the dark current density, sampling bandwidth and photoactive area, it significantly complicates the comparison of photodetectors. For this reason a useful figure of merit to consider is specific detectivity. This paper shows that state of the art all-printed, flexible OPDs with average specific detectivities as high as 3.45×1013 cm-Hz 0.5·W−1 can be printed using industrially scalable techniques (i.e., blade coating and screen printing) by controlling charge selectivity of organic electrodes. In addition to the high specific detectivity needed to resolve low light intensity, these OPDs are able to sustain high reverse biases. High reverse biases are used to maximize the capacity of the photodiode to capacitatively store photogenerated charge, which is essential for increasing the dynamic range of image sensors. Through optimization and characterization of the properties of each printed layer, device yield is maximized with minimal variability over large areas. The performance of all-printed photodiodes achieved here becomes comparable to silicon photodiodes used in image sensors, which have a specific detectivity of approximately 6–7 × 1013 cm-Hz 0.5·W−1. These results show that along with advantages such as inexpensive fabrication and mechanical flexibility, all-printed OPDs can have performance competitive with their Si-counterparts.

System design for organic pulse oximeter

Wearable medical devices that would benefit from mechanical flexibility and new form factors represent a great shift in direction of research in the field of printed electronics. The minimal functionality desired from wearable medical devices is the monitoring of vital signs. Pulse rate and blood oxygenation are considered primary vital signs that help to evaluate the general physical health of a person. The methods used to measure pulse rate and blood oxygenation with sensors based on organic light-emitting diodes (OLEDs) and organic photodiodes (OPDs) are reported here. Departing from the conventional practice of using red (630 nm) and infrared (940 nm) light for measuring pulse oxygenation, we have successfully implemented solution processed red (626 nm) and green (532 nm) OLEDs fabricated from polyfluorene blends in an all-organic optoelectronic pulse oximeter sensor. The red and green OLEDs operate at 9 V, 1 kHz, and transmit light through a human index finger. The transmitted light is sensed by an OPD placed on the opposite side of the finger. After filtering and amplification, the photoplethysmogram (PPG) signal is obtained and used to accurately measure pulse rate and blood oxygenation.