Yasser Khan is a postdoctoral scholar in the Department of Chemical Engineering at Stanford University, in Prof. Zhenan Bao’s Group. Yasser completed his Ph.D. in Electrical Engineering and Computer Sciences from the University of California, Berkeley, in Prof. Ana Claudia Arias’ Group. He received his B.S. in Electrical Engineering from the University of Texas at Dallas, and M.S. in Electrical Engineering from King Abdullah University of Science and Technology. Yasser’s research focuses mainly on wearable medical devices, with an emphasis on flexible bioelectronic and biophotonic sensors. Additionally, he worked on projects ranging from “electrochemical etching of ultra-sharp SPM tips” to “energy harvesting in complex systems.” His job experience includes internships at UC Berkeley, Oxford University, Stanford University, and Zyvex Labs in Texas.
Doctor of Philosophy, University of California Berkeley (2018)
A flexible organic reflectance oximeter array
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
2018; 115 (47): E11015–E11024
Transmission-mode pulse oximetry, the optical method for determining oxygen saturation in blood, is limited to only tissues that can be transilluminated, such as the earlobes and the fingers. The existing sensor configuration provides only single-point measurements, lacking 2D oxygenation mapping capability. Here, we demonstrate a flexible and printed sensor array composed of organic light-emitting diodes and organic photodiodes, which senses reflected light from tissue to determine the oxygen saturation. We use the reflectance oximeter array beyond the conventional sensing locations. The sensor is implemented to measure oxygen saturation on the forehead with 1.1% mean error and to create 2D oxygenation maps of adult forearms under pressure-cuff-induced ischemia. In addition, we present mathematical models to determine oxygenation in the presence and absence of a pulsatile arterial blood signal. The mechanical flexibility, 2D oxygenation mapping capability, and the ability to place the sensor in various locations make the reflectance oximeter array promising for medical sensing applications such as monitoring of real-time chronic medical conditions as well as postsurgery recovery management of tissues, organs, and wounds.
View details for DOI 10.1073/pnas.1813053115
View details for Web of Science ID 000450642800006
View details for PubMedID 30404911
View details for PubMedCentralID PMC6255203
- Local electrochemical control of hydrogel microactuators in microfluidics JOURNAL OF MICROMECHANICS AND MICROENGINEERING 2018; 28 (10)
- Emission Area Patterning of Organic Light-Emitting Diodes (OLEDs) via Printed Dielectrics ADVANCED FUNCTIONAL MATERIALS 2018; 28 (37)
Flexible Blade-Coated Multicolor Polymer Light-Emitting Diodes for Optoelectronic Sensors
2017; 29 (22)
A method to print two materials of different functionality during the same printing step is presented. In printed electronics, devices are built layer by layer and conventionally only one type of material is deposited in one pass. Here, the challenges involving printing of two emissive materials to form polymer light-emitting diodes (PLEDs) that emit light of different wavelengths without any significant changes in the device characteristics are described. The surface-energy-patterning technique is utilized to print materials in regions of interest. This technique proves beneficial in reducing the amount of ink used during blade coating and improving the reproducibility of printed films. A variety of colors (green, red, and near-infrared) are demonstrated and characterized. This is the first known attempt to print multiple materials by blade coating. These devices are further used in conjunction with a commercially available photodiode to perform blood oxygenation measurements on the wrist, where common accessories are worn. Prior to actual application, the threshold conditions for each color are discussed, in order to acquire a stable and reproducible photoplethysmogram (PPG) signal. Finally, based on the conditions, PPG and oxygenation measurements are successfully performed on the wrist with green and red PLEDs.
View details for DOI 10.1002/adma.201606206
View details for Web of Science ID 000402963400007
View details for PubMedID 28394455
- Flexible Hybrid Electronics: Direct Interfacing of Soft and Hard Electronics for Wearable Health Monitoring ADVANCED FUNCTIONAL MATERIALS 2016; 26 (47): 8764–75
Monitoring of Vital Signs with Flexible and Wearable Medical Devices
2016; 28 (22): 4373–95
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.
View details for DOI 10.1002/adma.201504366
View details for Web of Science ID 000377123500011
View details for PubMedID 26867696
High-performance flexible energy storage and harvesting system for wearable electronics
2016; 6: 26122
This paper reports on the design and operation of a flexible power source integrating a lithium ion battery and amorphous silicon solar module, optimized to supply power to a wearable health monitoring device. The battery consists of printed anode and cathode layers based on graphite and lithium cobalt oxide, respectively, on thin flexible current collectors. It displays energy density of 6.98 mWh/cm(2) and demonstrates capacity retention of 90% at 3C discharge rate and ~99% under 100 charge/discharge cycles and 600 cycles of mechanical flexing. A solar module with appropriate voltage and dimensions is used to charge the battery under both full sun and indoor illumination conditions, and the addition of the solar module is shown to extend the battery lifetime between charging cycles while powering a load. Furthermore, we show that by selecting the appropriate load duty cycle, the average load current can be matched to the solar module current and the battery can be maintained at a constant state of charge. Finally, the battery is used to power a pulse oximeter, demonstrating its effectiveness as a power source for wearable medical devices.
View details for DOI 10.1038/srep26122
View details for Web of Science ID 000375980400002
View details for PubMedID 27184194
View details for PubMedCentralID PMC4869018
- Identifying orthogonal solvents for solution processed organic transistors ORGANIC ELECTRONICS 2016; 30: 18–29
- Inkjet-Printed Flexible Gold Electrode Arrays for Bioelectronic Interfaces ADVANCED FUNCTIONAL MATERIALS 2016; 26 (7): 1004–13
Impedance sensing device enables early detection of pressure ulcers in vivo
2015; 6: 6575
When pressure is applied to a localized area of the body for an extended time, the resulting loss of blood flow and subsequent reperfusion to the tissue causes cell death and a pressure ulcer develops. Preventing pressure ulcers is challenging because the combination of pressure and time that results in tissue damage varies widely between patients, and the underlying damage is often severe by the time a surface wound becomes visible. Currently, no method exists to detect early tissue damage and enable intervention. Here we demonstrate a flexible, electronic device that non-invasively maps pressure-induced tissue damage, even when such damage cannot be visually observed. Using impedance spectroscopy across flexible electrode arrays in vivo on a rat model, we find that impedance is robustly correlated with tissue health across multiple animals and wound types. Our results demonstrate the feasibility of an automated, non-invasive 'smart bandage' for early detection of pressure ulcers.
View details for DOI 10.1038/ncomms7575
View details for Web of Science ID 000352720700010
View details for PubMedID 25779688
- All-organic optoelectronic sensor for pulse oximetry NATURE COMMUNICATIONS 2014; 5
- Enhanced energy storage in chaotic optical resonators NATURE PHOTONICS 2013; 7 (6): 474–79
Two-step controllable electrochemical etching of tungsten scanning probe microscopy tips
REVIEW OF SCIENTIFIC INSTRUMENTS
2012; 83 (6): 063708
Dynamic electrochemical etching technique is optimized to produce tungsten tips with controllable shape and radius of curvature of less than 10 nm. Nascent features such as "dynamic electrochemical etching" and reverse biasing after "drop-off" are utilized, and "two-step dynamic electrochemical etching" is introduced to produce extremely sharp tips with controllable aspect ratio. Electronic current shut-off time for conventional dc "drop-off" technique is reduced to ∼36 ns using high speed analog electronics. Undesirable variability in tip shape, which is innate to static dc electrochemical etching, is mitigated with novel "dynamic electrochemical etching." Overall, we present a facile and robust approach, whereby using a novel etchant level adjustment mechanism, 30° variability in cone angle and 1.5 mm controllability in cone length were achieved, while routinely producing ultra-sharp probes.
View details for DOI 10.1063/1.4730045
View details for Web of Science ID 000305833100028
View details for PubMedID 22755635