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Journals Conferences Posters Patents

Journal Articles

2020

  1. Flexible Fringe Effect Capacitive Sensors with Simultaneous High-Performance Contact and Non-Contact Sensing Capabilities Sara Rachel Arussy Ruth, Vivian Rachel Feig, Min-gu Kim, Yasser Khan, Jason Khoi Phong, and Zhenan Bao Small Structures, 2020 n/a, n/a.

    To mimic human touch sensing, robotics must be able to leverage multiple sensory inputs. Previously, to achieve both proximity and pressure sensing, most approaches have required using two separate sensors, each with their corresponding electronics, limiting the achievable density. More recently, sensors with multifunctional pressure and proximity capabilities have been realized at the cost of compromised pressure sensing. Presented here is a new design for a multifunctional interdigitated fringe field capacitive pressure sensor with a pyramid microstructured dielectric layer that has proximity-sensing capabilities (noncontact mode) while also sensing pressure (contact mode) as strongly as an equivalent parallel plate capacitive sensor of the same size. In contact mode, both sensors have a response time of less than 20 ms and can respond to loads lighter than 0.5 Pa. Further, the interdigitated fringe field sensor can clearly distinguish between the two sensing modes, as well as between conductive and nonconductive materials in the noncontact mode. Finally, we use the interdigitated fringe field sensor to demonstrate both proximity and high-sensitivity pressure sensing on a robotic gripper.

    @article{ruth2020flexible, author = {Ruth, Sara Rachel Arussy and Feig, Vivian Rachel and Kim, Min-gu and Khan, Yasser and Phong, Jason Khoi and Bao, Zhenan}, title = {Flexible Fringe Effect Capacitive Sensors with Simultaneous High-Performance Contact and Non-Contact Sensing Capabilities}, journal = {Small Structures}, volume = {n/a}, number = {n/a}, pages = {2000079}, year = {2020}, keywords = {electrode designs, multifunctional sensors, pressure sensors, proximity sensors, robotics}, doi = {10.1002/sstr.202000079}, thumbnail = {ruth2020flexible.png}, url = {http://dx.doi.org/10.1002/sstr.202000079}, pdf = {ruth2020flexible.pdf} }

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  2. Optimization of printed sensors to monitor sodium, ammonium, and lactate in sweat Alla M. Zamarayeva, Natasha A. D. Yamamoto, Anju Toor, Margaret E. Payne, Caleb Woods, Veronika I. Pister, Yasser Khan, James W. Evans, and Ana Claudia Arias APL Materials, 2020 8, 10.

    We describe the optimization of a flexible printed electrochemical sensing platform to monitor sodium ion (Na+), ammonium ion (NH4+), and lactate in human sweat. We used previously reported material systems and adapted them to scalable fabrication techniques. In the case of potentiometric Na+ and NH4+ sensors, ion-selective electrodes (ISEs) required minimum optimization beyond previously reported protocols, while a reference electrode had to be modified in order to achieve a stable response. We incorporated a carbon nanotube (CNT) layer between the membrane and the silver/silver chloride (Ag/AgCl) layer to act as a surface for adsorption and retention of Cl−. The resulting reference electrode showed minimal potential variation up to 0.08 mV in the solutions with Cl concentration varying from 0.1 mM to 100 mM. Increasing the ionophore content in the NH4+ ISE sensing membrane eliminated an offset in the potential readout, while incorporating CNTs into the sensing membranes had a marginal effect on the sensitivity of both Na+ and NH4+ sensors. Na+ and NH4+ sensors showed a stable near-Nernstian response with sensitivities of 60.0 ± 4.0 mV and 56.2 ± 2.3 mV, respectively, long-term stability for at least 60 min of continuous operation, and selectivity to Na+ and NH4+. For the lactate sensor, we compared the performance of the tetrathiafulvalene mediated lactate oxidase based working electrode with and without diffusion-limiting polyvinyl chloride membrane. The working electrodes with and without the membrane showed sensitivities of 3.28 ± 8 A/mM and 0.43 ± 0.11 μA/mM with a linear range up to 20 mM and 30 mM lactate, respectively.

    @article{zamarayeva2020optimization, author = {Zamarayeva, Alla M. and Yamamoto, Natasha A. D. and Toor, Anju and Payne, Margaret E. and Woods, Caleb and Pister, Veronika I. and Khan, Yasser and Evans, James W. and Arias, Ana Claudia}, title = {Optimization of printed sensors to monitor sodium, ammonium, and lactate in sweat}, volume = {8}, number = {10}, journal = {APL Materials}, pages = {100905}, year = {2020}, doi = {10.1063/5.0014836}, thumbnail = {zamarayeva2020optimization.png}, url = {http://dx.doi.org/10.1063/5.0014836}, pdf = {zamarayeva2020optimization.pdf}, publisher = {AIP Publishing} }

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  3. A potentiometric mechanotransduction mechanism for novel electronic skins Xiaodong Wu, Maruf Ahmed, Yasser Khan, Margaret E. Payne, Juan Zhu, Canhui Lu, James W. Evans, and Ana C. Arias Science Advances, 2020 6, 30.

    Human skin perceives external mechanical stimuli by sensing the variation in the membrane potential of skin sensory cells. Many scientists have attempted to recreate skin functions and develop electronic skins (e-skins) based on active and passive sensing mechanisms. Inspired by the skin sensory behavior, we investigated materials and electronic devices that allow us to encode mechanical stimuli into potential differences measured between two electrodes, resulting in a potentiometric mechanotransduction mechanism. We present here a potentiometric mechanotransducer that is fabricated through an all-solution processing approach. This mechanotransducer shows ultralow-power consumption, highly tunable sensing behavior, and capability to detect both static and low-frequency dynamic mechanical stimuli. Furthermore, we developed two novel classes of sensing devices, including strain-insensitive sensors and single-electrode-mode e-skins, which are challenging to achieve using the existing methods. This mechanotransduction mechanism has broad impact on robotics, prosthetics, and health care by providing a much improved human-machine interface.

    @article{wu2020potentiometric, author = {Wu, Xiaodong and Ahmed, Maruf and Khan, Yasser and Payne, Margaret E. and Zhu, Juan and Lu, Canhui and Evans, James W. and Arias, Ana C.}, title = {A potentiometric mechanotransduction mechanism for novel electronic skins}, volume = {6}, number = {30}, elocation-id = {eaba1062}, year = {2020}, doi = {10.1126/sciadv.aba1062}, thumbnail = {wu2020potentiometric.png}, url = {https://advances.sciencemag.org/content/6/30/eaba1062}, pdf = {wu2020potentiometric.pdf}, publisher = {American Association for the Advancement of Science}, eprint = {https://advances.sciencemag.org/content/6/30/eaba1062.full.pdf}, journal = {Science Advances} }

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  4. A low-cost, helmet-based, non-invasive ventilator for COVID-19 Yasser Khan, Hossain Mohammad Fahad, Sifat Muin, and Karthik Gopalan , 2020

    Coronavirus disease 2019 (COVID-19) has created an unprecedented need for breathing assistance devices. Since the demand for commercial, full-featured ventilators is far higher than the supply capacity, many rapid-response ventilators are being developed for invasive mechanical ventilation of patients. Most of these emergency ventilators utilize mechanical squeezing of bag-valve-masks or Ambu-bags. These "bag squeezer" designs are bulky and heavy, depends on many moving parts, and difficulty to assemble and use. Also, invasive ventilation requires intensive care unit support, which may be unavailable to a vast majority of patients, especially in developing countries. In this work, we present a low-cost (<200), portable (fits in an 8"x8"x4" box), non-invasive ventilator (NIV), designed to provide relief to early-stage COVID-19 patients in low-resource settings. We used a high-pressure blower fan for providing noninvasive positive-pressure ventilation. Our design supports continuous positive airway pressure (CPAP) and bilevel positive airway pressure (BiPAP) modes. A common concern of using CPAP or BiPAP for treating COVID-19 patients is the aerosolization of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). We used a helmet-based solution that contains the spread of the virus. Our end-to-end solution is compact, low-cost (<400 including the helmet, viral filters, and a valve), and easy-to-use. Our NIV provides 0-20 cmH2O pressure with flow rates of 60-180 Lmin−1. We hope that our report will encourage implementations and further studies on helmet-based NIV for treating COVID-19 patients in low-resource settings.

    @article{khan2020lowcost, title = {A low-cost, helmet-based, non-invasive ventilator for COVID-19}, author = {Khan, Yasser and Fahad, Hossain Mohammad and Muin, Sifat and Gopalan, Karthik}, year = {2020}, eprint = {2005.11008}, archiveprefix = {arXiv}, primaryclass = {physics.med-ph}, thumbnail = {khan2020lowcost.png}, url = {https://arxiv.org/abs/2005.11008}, pdf = {khan2020lowcost.pdf} }

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  5. Pulse Oximetry Using Organic Optoelectronics under Ambient Light Donggeon Han, Yasser Khan, Jonathan Ting, Juan Zhu, Craig Combe, Andrew Wadsworth, Iain McCulloch, and Ana C. Arias Advanced Materials Technologies, 2020 n/a, n/a.

    Light absorption in oxygenated and deoxygenated blood varies appreciably over the visible and near-infrared spectrum. Pulse oximeters use two distinct wavelengths of light to measure oxygen saturation SpO2 of blood. Currently, light-emitting diodes (LEDs) are used in oximeters, which need additional components to drive them and negatively impact the overall size of the sensor. In this work, an ambient light oximeter (ALO) is demonstrated, which can measure photoplethysmography signals and SpO2 using various kinds of ambient light, avoiding the use of LEDs. Spectral filters are combined with organic photodiodes to create the ALO with sensitivity peaks at green (525 nm), red (610 nm), and near-infrared (740 nm) wavelengths. Finally, the wearable ALO is used to measure photoplethysmography signals and SpO2 on the index finger in different indoor and outdoor lighting conditions and the measurements are validated with commercial pulse oximeters under normal and ischemic conditions.

    @article{han2020pulse, author = {Han, Donggeon and Khan, Yasser and Ting, Jonathan and Zhu, Juan and Combe, Craig and Wadsworth, Andrew and McCulloch, Iain and Arias, Ana C.}, title = {Pulse Oximetry Using Organic Optoelectronics under Ambient Light}, journal = {Advanced Materials Technologies}, volume = {n/a}, number = {n/a}, year = {2020}, pages = {1901122}, keywords = {flexible electronics, organic photodiodes, oximeters, photoplethysmography, wearable sensors}, doi = {10.1002/admt.201901122}, url = {http://dx.doi.org/10.1002/admt.201901122}, thumbnail = {han2020pulse.png}, pdf = {han2020pulse.pdf}, eprint = {https://onlinelibrary.wiley.com/doi/pdf/10.1002/admt.201901122} }

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  6. Large-Area Fabrication of High-Performance Flexible and Wearable Pressure Sensors Xiaodong Wu, Yasser Khan, Jonathan Ting, Juan Zhu, Seiya Ono, Xinxing Zhang, Shixuan Du, James W. Evans, Canhui Lu, and Ana C. Arias Advanced Electronic Materials, 2020 n/a, n/a.

    Flexible pressure sensors with high sensitivity, broad working range, and good scalability are highly desired for the next generation of wearable electronic devices. However, manufacturing of such pressure sensors still remains challenging. A large-area compliant and cost-effective process to fabricate high-performance pressure sensors via a combination of mesh-molded periodic microstructures and printed side-by-side electrodes is presented. The sensors exhibit low operating voltage (1 V), high sensitivity (20.9 kPa−1), low detection limit (7.4 Pa), fast response/recovery time (23/18 ms), and excellent reliability (over 10 000 cycles). More importantly, they exhibit ultra-broad working range (7.4–1 000 000 Pa), high tunability, large-scale production feasibility, and significant advantage in format miniaturization and creating sensor arrays with self-defined patterns. The versatility of these devices is demonstrated in various human activity monitoring and spatial pressure mapping as electronic skins. Furthermore, utilizing printing methods, a flexible smart insole with a high level of integration for both foot pressure and temperature mapping is demonstrated. The scalable and cost-effective manufacturing along with the good comprehensive performance of the pressure sensors makes them very attractive for future development of wearable smart devices and human–machine interfaces.

    @article{wu2020large, author = {Wu, Xiaodong and Khan, Yasser and Ting, Jonathan and Zhu, Juan and Ono, Seiya and Zhang, Xinxing and Du, Shixuan and Evans, James W. and Lu, Canhui and Arias, Ana C.}, title = {Large-Area Fabrication of High-Performance Flexible and Wearable Pressure Sensors}, journal = {Advanced Electronic Materials}, volume = {n/a}, number = {n/a}, year = {2020}, pages = {1901310}, keywords = {electronic skin, periodic microstructures, pressure sensors, side-by-side electrodes}, doi = {10.1002/aelm.201901310}, url = {http://dx.doi.org/10.1002/aelm.201901310}, thumbnail = {wu2020large.png}, pdf = {wu2020large.pdf} }

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2019

  1. A New Frontier of Printed Electronics: Flexible Hybrid Electronics Yasser Khan, Arno Thielens, Sifat Muin, Jonathan Ting, Carol Baumbauer, and Ana C. Arias Advanced Materials, 2019 n/a, n/a.

    The performance and integration density of silicon integrated circuits (ICs) have progressed at an unprecedented pace in the past 60 years. While silicon ICs thrive at low-power high-performance computing, creating flexible and large-area electronics using silicon remains a challenge. On the other hand, flexible and printed electronics use intrinsically flexible materials and printing techniques to manufacture compliant and large-area electronics. Nonetheless, flexible electronics are not as efficient as silicon ICs for computation and signal communication. Flexible hybrid electronics (FHE) leverages the strengths of these two dissimilar technologies. It uses flexible and printed electronics where flexibility and scalability are required, i.e., for sensing and actuating, and silicon ICs for computation and communication purposes. Combining flexible electronics and silicon ICs yields a very powerful and versatile technology with a vast range of applications. Here, the fundamental building blocks of an FHE system, printed sensors and circuits, thinned silicon ICs, printed antennas, printed energy harvesting and storage modules, and printed displays, are discussed. Emerging application areas of FHE in wearable health, structural health, industrial, environmental, and agricultural sensing are reviewed. Overall, the recent progress, fabrication, application, and challenges, and an outlook, related to FHE are presented.

    @article{khan2019new, author = {Khan, Yasser and Thielens, Arno and Muin, Sifat and Ting, Jonathan and Baumbauer, Carol and Arias, Ana C.}, title = {A New Frontier of Printed Electronics: Flexible Hybrid Electronics}, journal = {Advanced Materials}, volume = {n/a}, number = {n/a}, year = {2019}, pages = {1905279}, keywords = {environmental sensors, flexible electronics, printed electronics, structural health monitoring, wearable health monitoring}, doi = {10.1002/adma.201905279}, url = {http://dx.doi.org/10.1002/adma.201905279}, thumbnail = {khan2019new.png}, pdf = {khan2019new.pdf} }

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  2. Organic Multi-Channel Optoelectronic Sensors for Wearable Health Monitoring Yasser Khan, Donggeon Han, Jonathan Ting, Maruf Ahmed, Ramune Nagisetty, and Ana C. Arias. IEEE Access, 2019 , .

    Recent progress in printed optoelectronics and their integration in wearable sensors have created new avenues for research in reflectance photoplethysmography (PPG) and oximetry. The reflection-mode sensor, which consists of light emitters and detectors, is a vital component of reflectance oximeters. Here, we report a systematic study of the reflectance oximeter sensor design in terms of component geometry, light emitter and detector spacing, and the use of an optical barrier between the emitter and the detector to maximize sensor performance. Printed red and near-infrared (NIR) organic light-emitting diodes (OLEDs) and organic photodiodes (OPDs) are used to design three sensor geometries: (1) Rectangular geometry, where square OLEDs are placed at each side of the OPD; (2) Bracket geometry, where the OLEDs are shaped as brackets and placed around the square OPD; (3) Circular geometry, where the OLEDs are shaped as block arcs and placed around the circular OPD. Utilizing the bracket geometry, we observe 39.7% and 18.2% improvement in PPG signal magnitude in the red and NIR channels compared to the rectangular geometry, respectively. Using the circular geometry, we observe 48.6% and 9.2% improvements in the red and NIR channels compared to the rectangular geometry. Furthermore, a wearable two-channel PPG sensor is utilized to add redundancy to the measurement. Finally, inverse-variance weighting and template matching algorithms are implemented to improve the detection of heart rate from the multi-channel PPG signals.

    @article{khan2019organic, author = {Khan, Yasser and Han, Donggeon and Ting, Jonathan and Ahmed, Maruf and Nagisetty, Ramune and Arias., Ana C.}, title = {Organic Multi-Channel Optoelectronic Sensors for Wearable Health Monitoring}, journal = {IEEE Access}, volume = {}, number = {}, pages = {}, year = {2019}, publisher = {IEEE}, url = {http://dx.doi.org/10.1109/ACCESS.2019.2939798}, doi = {10.1109/ACCESS.2019.2939798}, thumbnail = {khan2019organic.png}, pdf = {khan2019organic.pdf} }

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  3. Reliability Challenges in Fabrication of Flexible Hybrid Electronics for Human Performance Monitors: A System Level Study Varun Soman, Yasser Khan, Madina Zabran, Mark Schadt, Paul Hart, Michael Shay, Frank Egitto, Konstantinos Papathomas, Natasha AD Yamamoto, Donggeon Han, Ana C Arias, Kanad Ghose, Mark D Poliks, and James N Turner IEEE Transactions on Components, Packaging and Manufacturing Technology, 2019 , .

    Flexible hybrid electronics (FHE) interface rigid electronic components with flexible sensors, circuits, and substrates. This paper reports the reliability improvement of a FHE Human Performance Monitor (HPM), designed to monitor electrocardiography (ECG) signals. The 50.8 mm × 50.8 mm HPM is fabricated on Kapton® HN polyimide (PI) substrate with flexible gold (Au) ECG electrodes on one side of the substrate and rigid electronic components for signal processing and communication mounted on the other side of the substrate. Our previously reported HPMs demonstrated reliability issues due to (1) cracking of the copper (Cu) circuitry, and (2) thinning and lack of adhesion at the printed Au and plated Cu interface that connected printed sensors to the Cu circuitry. Both failure mechanisms resulted in electrical opens in the circuit, which caused device failure. We explored effect of different design parameters, such as PI substrate thickness (50 μm vs 125 μm), Cu circuit thickness (2 μm vs 6 μm), solder reflow temperature (205 °C for Tin-Lead (Sn-Pb) vs 175 °C for Tin-Bismuth (Sn-Bi) solder), solder pad design, and optimized inkjet printing (printing on bare Cu vs Au plated Cu) on improving FHE reliability. Test vehicles (TVs) with different combinations of these factors were fabricated and bend tested to determine the most robust configuration. TVs with 50 μm thick PI substrate, 6 μm thick Cu circuit, Sn-Bi solder, redesigned solder pads with rounded corners, and printed Au traces on Au plated Cu pads demonstrated the best reliability results.

    @article{soman2019reliability, author = {Soman, Varun and Khan, Yasser and Zabran, Madina and Schadt, Mark and Hart, Paul and Shay, Michael and Egitto, Frank and Papathomas, Konstantinos and Yamamoto, Natasha AD and Han, Donggeon and Arias, Ana C and Ghose, Kanad and Poliks, Mark D and Turner, James N}, title = {Reliability Challenges in Fabrication of Flexible Hybrid Electronics for Human Performance Monitors: A System Level Study}, journal = {IEEE Transactions on Components, Packaging and Manufacturing Technology}, volume = {}, number = {}, pages = {}, year = {2019}, publisher = {IEEE}, url = {http://dx.doi.org/10.1109/TCPMT.2019.2919866}, doi = {10.1109/TCPMT.2019.2919866}, thumbnail = {soman2019reliability.png}, pdf = {soman2019reliability.pdf} }

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2018

  1. A flexible organic reflectance oximeter array Yasser Khan, Donggeon Han, Adrien Pierre, Jonathan Ting, Xingchun Wang, Claire M Lochner, Gianluca Bovo, Nir Yaacobi-Gross, Chris Newsome, Richard Wilson, and Ana C Arias Proceedings of the National Academy of Sciences, 2018 115, 47. Media coverage: Physics World, UC Berkeley News Center, KCBS Radio, Innovators Magazine, The Engineer (UK), Medgadget, ScienceDaily, and many more.

    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.

    @article{khan2018flexible, title = {A flexible organic reflectance oximeter array}, author = {Khan, Yasser and Han, Donggeon and Pierre, Adrien and Ting, Jonathan and Wang, Xingchun and Lochner, Claire M and Bovo, Gianluca and Yaacobi-Gross, Nir and Newsome, Chris and Wilson, Richard and Arias, Ana C}, journal = {Proceedings of the National Academy of Sciences}, volume = {115}, number = {47}, pages = {E11015--E11024}, year = {2018}, publisher = {National Academy of Sciences}, url = {http://dx.doi.org/10.1073/pnas.1813053115}, doi = {10.1073/pnas.1813053115}, thumbnail = {khan2018flexible.png}, pdf = {khan2018flexible.pdf}, note = {Media coverage: }, media_1 = {Physics World, }, media_1_link = {https://physicsworld.com/a/flexible-sensor-maps-blood-oxygen-levels/}, media_2 = {UC Berkeley News Center, }, media_2_link = {https://news.berkeley.edu/2018/11/07/skin-like-sensor-maps-blood-oxygen-levels-anywhere-in-the-body/}, media_3 = {KCBS Radio, }, media_3_link = {https://omny.fm/shows/kcbsam-on-demand/uc-berkeley-research-shows-new-sensor-detects-oxyg}, media_4 = {Innovators Magazine, }, media_4_link = {https://www.innovatorsmag.com/wearable-monitors-blood-oxygen-levels/}, media_5 = {The Engineer (UK), }, media_5_link = {https://www.theengineer.co.uk/flexible-oximeter-blood-oxygen/}, media_6 = {Medgadget, }, media_6_link = {https://www.medgadget.com/2018/11/flexible-led-sensor-monitors-blood-oxygenation-levels-through-skin.html}, media_7 = {ScienceDaily, }, media_7_link = {https://www.sciencedaily.com/releases/2018/11/181107172917.htm}, media_8 = {and many more.}, media_8_link = {https://www.altmetric.com/details/50956419} }

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  2. Emission Area Patterning of Organic Light-Emitting Diodes (OLEDs) via Printed Dielectrics Donggeon Han, Yasser Khan, Karthik Gopalan, Adrien Pierre, and Ana C Arias Advanced Functional Materials, 2018 28, 37.

    Solution-processibility is one of the distinguished traits of organic light-emitting diodes (OLEDs) compared to existing solid-state LED technologies. It allows new opportunities which can simplify the fabrication and potentially reduce the cost of manufacturing process. Emission area patterning is one of the crucial fabrication steps and it usually involves subtractive methods, such as photolithography or etching. Here, printing techniques are used to pattern the emission area of blade-coated OLED layers. The print qualities of a number of printing schemes are characterized and compared. Spray coating and screen printing are used to deposit dielectrics with desired patterns on the OLED layers. At luminance of 1000 cd m−2 the OLEDs patterned using spray-coated and screen-printed dielectric show current density of 8.2 and 10.1 mA cm−2, external quantum efficiency (EQE) of 2.1% and 2.1%, and luminous efficacy of 5.5 and 6.3 lm W−1, respectively. The OLED characteris-tics and features of each printing scheme in depositing the dielectric layer are discussed. The printing methods are further applied to demonstrate displays with complex shapes and a seven-segment display.

    @article{han2018emission, title = {Emission Area Patterning of Organic Light-Emitting Diodes (OLEDs) via Printed Dielectrics}, author = {Han, Donggeon and Khan, Yasser and Gopalan, Karthik and Pierre, Adrien and Arias, Ana C}, journal = {Advanced Functional Materials}, volume = {28}, number = {37}, pages = {1802986}, year = {2018}, url = {http://dx.doi.org/10.1002/adfm.201802986}, doi = {10.1002/adfm.201802986}, thumbnail = {han2018emission.png}, pdf = {han2018emission.pdf} }

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  3. Local electrochemical control of hydrogel microactuators in microfluidics Leeya Engel, Chengming Liu, Nofar Mintz Hemed, Yasser Khan, Ana Claudia Arias, Yosi Shacham-Diamand, Slava Krylov, and Liwei Lin Journal of Micromechanics and Microengineering, 2018 28, 105005.

    Due to the complexity of a chemo-electro-mechanical system and the need for a wet environment, to date, few devices fully integrate hydrogels with microelectromechanical systems. In this paper, we demonstrate the use of inkjet-printed gold electrodes integrated in microfluidic channels to alter the morphology of electroactive polymer hydrogels, cross-linked in situ. Microfluidics is a convenient platform for integrating hydrogels with microsystems as it provides a means for encapsulating electrolytic environments, while maintaining UV transparency. Printed electronics provide a new method for rapid prototyping of electrodes on flexible substrates for electrical control of electroactive polymer microsystems. We attribute the observed actuation to electrochemically-induced pH variations in the vicinity of the printed anode and cathode which diffuse into the hydrogel. Response to pH was verified by exposing the hydrogel to various pH conditions in control experiments without applied electrical bias. This work demonstrates a new, integrated, polymer-based, rapid prototyping approach to building flexible electroactive hydrogel systems which can benefit microfluidic valves, biomimetics, electrochemical sensors and artificial muscles.

    @article{engel2018local, title = {Local electrochemical control of hydrogel microactuators in microfluidics}, author = {Engel, Leeya and Liu, Chengming and Hemed, Nofar Mintz and Khan, Yasser and Arias, Ana Claudia and Shacham-Diamand, Yosi and Krylov, Slava and Lin, Liwei}, journal = {Journal of Micromechanics and Microengineering}, volume = {28}, number = {105005}, year = {2018}, publisher = {IOP Publishing}, thumbnail = {engel2018local.png}, pdf = {engel2018local.pdf}, url = {http://dx.doi.org/10.1088/1361-6439/aacc31}, doi = {10.1088/1361-6439/aacc31} }

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2017

  1. Flexible blade-coated multicolor polymer light-emitting diodes for optoelectronic sensors Donggeon Han, Yasser Khan, Jonathan Ting, Simon M King, Nir Yaacobi-Gross, Martin J Humphries, Christopher J Newsome, and Ana C Arias Advanced Materials, 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, retrieved PPG and oxygenation measurements are successfully performed on the wrist with green and red PLEDs.

    @article{han2017flexible, title = {Flexible blade-coated multicolor polymer light-emitting diodes for optoelectronic sensors}, author = {Han, Donggeon and Khan, Yasser and Ting, Jonathan and King, Simon M and Yaacobi-Gross, Nir and Humphries, Martin J and Newsome, Christopher J and Arias, Ana C}, journal = {Advanced Materials}, volume = {29}, number = {22}, pages = {1606206}, year = {2017}, publisher = {Wiley Online Library}, url = {http://dx.doi.org/10.1002/adma.201606206}, doi = {10.1002/adma.201606206}, thumbnail = {han2017flexible.png}, pdf = {han2017flexible.pdf} }

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2016

  1. Flexible hybrid electronics: Direct interfacing of soft and hard electronics for wearable health monitoring Yasser Khan, Mohit Garg, Qiong Gui, Mark Schadt, Abhinav Gaikwad, Donggeon Han, Natasha AD Yamamoto, Paul Hart, Robert Welte, William Wilson, Steve Czarnecki, Mark Poliks, Zhanpeng Jin, Kanad Ghose, Frank Egitto, James Turner, and Ana C Arias Advanced Functional Materials, 2016 26, 47.

    The interfacing of soft and hard electronics is a key challenge for flexible hybrid electronics. Currently, a multisubstrate approach is employed, where soft and hard devices are fabricated or assembled on separate substrates, and bonded or interfaced using connectors; this hinders the flexibility of the device and is prone to interconnect issues. Here, a single substrate interfacing approach is reported, where soft devices, i.e., sensors, are directly printed on Kapton polyimide substrates that are widely used for fabricating flexible printed circuit boards (FPCBs). Utilizing a process flow compatible with the FPCB assembly process, a wearable sensor patch is fabricated composed of inkjet-printed gold electrocardiography (ECG) electrodes and a stencil-printed nickel oxide thermistor. The ECG electrodes provide 1 mVp–p ECG signal at 4.7 cm electrode spacing and the thermistor is highly sensitive at normal body temperatures, and demonstrates temperature coefficient, α ≈ –5.84% K–1 and material constant, β ≈ 4330 K. This sensor platform can be extended to a more sophisticated multisensor platform where sensors fabricated using solution processable functional inks can be interfaced to hard electronics for health and performance monitoring, as well as internet of things applications.

    @article{khan2016flexible, title = {Flexible hybrid electronics: Direct interfacing of soft and hard electronics for wearable health monitoring}, author = {Khan, Yasser and Garg, Mohit and Gui, Qiong and Schadt, Mark and Gaikwad, Abhinav and Han, Donggeon and Yamamoto, Natasha AD and Hart, Paul and Welte, Robert and Wilson, William and Czarnecki, Steve and Poliks, Mark and Jin, Zhanpeng and Ghose, Kanad and Egitto, Frank and Turner, James and Arias, Ana C}, journal = {Advanced Functional Materials}, volume = {26}, number = {47}, pages = {8764--8775}, year = {2016}, publisher = {Wiley Online Library}, url = {http://dx.doi.org/10.1002/adfm.201603763}, doi = {10.1002/adfm.201603763}, thumbnail = {khan2016flexible.png}, pdf = {khan2016flexible.pdf} }

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  2. High-performance flexible energy storage and harvesting system for wearable electronics Aminy E Ostfeld, Abhinav M Gaikwad, Yasser Khan, and Ana C Arias Scientific reports, 2016 6,

    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/cm2 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.

    @article{ostfeld2016high, title = {High-performance flexible energy storage and harvesting system for wearable electronics}, author = {Ostfeld, Aminy E and Gaikwad, Abhinav M and Khan, Yasser and Arias, Ana C}, journal = {Scientific reports}, volume = {6}, pages = {26122}, year = {2016}, publisher = {Nature Publishing Group}, url = {http://dx.doi.org/10.1038/srep26122}, doi = {10.1038/srep26122}, thumbnail = {ostfeld2016high.png}, pdf = {ostfeld2016high.pdf} }

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  3. Monitoring of vital signs with flexible and wearable medical devices Yasser Khan, Aminy E Ostfeld, Claire M Lochner, Adrien Pierre, and Ana C Arias Advanced Materials, 2016 28, 22.

    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.

    @article{khan2016monitoring, title = {Monitoring of vital signs with flexible and wearable medical devices}, author = {Khan, Yasser and Ostfeld, Aminy E and Lochner, Claire M and Pierre, Adrien and Arias, Ana C}, journal = {Advanced Materials}, volume = {28}, number = {22}, pages = {4373--4395}, year = {2016}, publisher = {Wiley Online Library}, url = {http://dx.doi.org/10.1002/adma.201504366}, doi = {10.1002/adma.201504366}, thumbnail = {khan2016monitoring.png}, pdf = {khan2016monitoring.pdf} }

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  4. Inkjet-printed flexible gold electrode arrays for bioelectronic interfaces Yasser Khan*, Felippe J Pavinatto*, Monica C Lin, Amy Liao, Sarah L Swisher, Kaylee Mann, Vivek Subramanian, Michel M Maharbiz, and Ana C Arias Advanced Functional Materials, 2016 26, 7. Cover article.

    Bioelectronic interfaces require electrodes that are mechanically flexible and chemically inert. Flexibility allows pristine electrode contact to skin and tissue, and chemical inertness prevents electrodes from reacting with biological fluids and living tissues. Therefore, flexible gold electrodes are ideal for bioimpedance and biopotential measurements such as bioimpedance tomography, electrocardiography (ECG), electroencephalography (EEG), and electromyography (EMG). However, a manufacturing process to fabricate gold electrode arrays on plastic substrates is still elusive. In this work, a fabrication and low-temperature sintering (≈200 °C) technique is demonstrated to fabricate gold electrodes. At low-temperature sintering conditions, lines of different widths demonstrate different sintering speeds. Therefore, the sintering condition is targeted toward the widest feature in the design layout. Manufactured electrodes show minimum feature size of 62 μm and conductivity values of 5 × 10 6 S m−1. Utilizing the versatility of printing and plastic electronic processes, electrode arrays consisting of 31 electrodes with electrode-to-electrode spacing ranging from 2 to 7 mm are fabricated and used for impedance mapping of conformal surfaces at 15 kHz. Overall, the fabrication process of an inkjet-printed gold electrode array that is electrically reproducible, mechanically robust, and promising for bioimpedance and biopotential measurements is demonstrated.

    @article{khan2016inkjet, title = {Inkjet-printed flexible gold electrode arrays for bioelectronic interfaces}, author = {Khan*, Yasser and Pavinatto*, Felippe J and Lin, Monica C and Liao, Amy and Swisher, Sarah L and Mann, Kaylee and Subramanian, Vivek and Maharbiz, Michel M and Arias, Ana C}, journal = {Advanced Functional Materials}, volume = {26}, number = {7}, pages = {1004--1013}, year = {2016}, publisher = {Wiley Online Library}, url = {http://dx.doi.org/10.1002/adfm.201503316}, doi = {10.1002/adfm.201503316}, thumbnail = {khan2016inkjet.png}, pdf = {khan2016inkjet.pdf}, note = {Cover article.} }

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  5. Identifying orthogonal solvents for solution processed organic transistors Abhinav M Gaikwad, Yasser Khan, Aminy E Ostfeld, Shishir Pandya, Sameer Abraham, and Ana Claudia Arias Organic Electronics, 2016 30, Solvents visualization program is available in the Downloads section: http://arias.berkeley.edu/downloads/

    Identification of solvents for dissolving polymer dielectrics and organic semiconductors is necessary for the fabrication of solution-processed organic field effect transistors (OFETs). In addition to solubility and printability of a solvent, orthogonality is particularly important when forming multilayer structure from solutions. Currently, the process of finding orthogonal solvents is empirical, and based on trial-and-error experimental methods. In this paper, we present a methodology for identifying orthogonal solvents for solution-processed organic devices. We study the accuracy of Hildebrand and Hansen solubility theories for building solubility boundaries for organic semiconductor (Poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene (PBTTT) and polymer dielectrics (Poly(methyl methacrylate) (PMMA), Polystyrene (PS)). The Hansen solubility sphere for the organic semiconductor and polymer gate dielectrics are analyzed to identify solvents that dissolve PMMA and PS, but are orthogonal to PBTTT. Top gate/bottom contact PBTTT based OFETs are fabricated with PMMA gate dielectric processed with solvents that are orthogonal and non-orthogonal to PBTTT. The non-orthogonal solvents swell the semiconductor layer and increase their surface roughness.

    @article{gaikwad2016identifying, title = {Identifying orthogonal solvents for solution processed organic transistors}, author = {Gaikwad, Abhinav M and Khan, Yasser and Ostfeld, Aminy E and Pandya, Shishir and Abraham, Sameer and Arias, Ana Claudia}, journal = {Organic Electronics}, volume = {30}, pages = {18--29}, year = {2016}, publisher = {Elsevier}, url = {http://dx.doi.org/10.1016/j.orgel.2015.12.008}, doi = {10.1016/j.orgel.2015.12.008}, thumbnail = {gaikwad2016identifying.png}, pdf = {gaikwad2016identifying.pdf}, note = {Solvents visualization program is available in the Downloads section: http://arias.berkeley.edu/downloads/} }

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2015

  1. Impedance sensing device enables early detection of pressure ulcers in vivo Sarah L Swisher, Monica C Lin, Amy Liao, Elisabeth J Leeflang, Yasser Khan, Felippe J Pavinatto, Kaylee Mann, Agne Naujokas, David Young, Shuvo Roy, Michael R Harrison, Ana C Arias, Vivek Subramanian, and Michel M Maharbiz Nature communications, 2015 6, Media coverage: BBC News, UC Berkeley News Center, Futurity, NSF News, ACM Communications, and many more.

    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.

    @article{swisher2015impedance, title = {Impedance sensing device enables early detection of pressure ulcers in vivo}, author = {Swisher, Sarah L and Lin, Monica C and Liao, Amy and Leeflang, Elisabeth J and Khan, Yasser and Pavinatto, Felippe J and Mann, Kaylee and Naujokas, Agne and Young, David and Roy, Shuvo and Harrison, Michael R and Arias, Ana C and Subramanian, Vivek and Maharbiz, Michel M}, journal = {Nature communications}, volume = {6}, pages = {6575}, year = {2015}, publisher = {Nature Publishing Group}, url = {http://dx.doi.org/10.1038/ncomms7575}, doi = {10.1038/ncomms7575}, thumbnail = {swisher2015impedance.png}, pdf = {swisher2015impedance.pdf}, note = {Media coverage: }, media_1 = {BBC News, }, media_1_link = {http://www.bbc.com/news/health-31903367}, media_2 = {UC Berkeley News Center, }, media_2_link = {http://newscenter.berkeley.edu/2015/03/17/smart-bandages-detect-bedsores/}, media_3 = {Futurity, }, media_3_link = {http://www.futurity.org/smart-bandage-bedsores-876942/}, media_4 = {NSF News, }, media_4_link = {https://www.nsf.gov/news/news_summ.jsp?cntn_id=134610}, media_5 = {ACM Communications, }, media_5_link = {https://cacm.acm.org/news/184717-smart-bandage-detects-bedsores-before-they-are-visible-to-doctors/fulltext}, media_6 = {and many more.}, media_6_link = {https://www.altmetric.com/details/3798805} }

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2014

  1. All-organic optoelectronic sensor for pulse oximetry Claire M Lochner*, Yasser Khan*, Adrien Pierre*, and Ana C Arias Nature communications, 2014 5, *Equal contribution. Media coverage: UC Berkeley Grad News, NSF Science 360 News, UC Berkeley News Center, Phys.Org, ScienceDaily, MSN News, Yahoo News, and many more.

    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 sensor’s 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.

    @article{lochner2014all, title = {All-organic optoelectronic sensor for pulse oximetry}, author = {Lochner*, Claire M and Khan*, Yasser and Pierre*, Adrien and Arias, Ana C}, journal = {Nature communications}, volume = {5}, pages = {5745}, year = {2014}, publisher = {Nature Publishing Group}, url = {http://dx.doi.org/10.1038/ncomms6745}, doi = {10.1038/ncomms6745}, thumbnail = {lochner2014all.png}, pdf = {lochner2014all.pdf}, note = {*Equal contribution. Media coverage: }, media_1 = {UC Berkeley Grad News, }, media_1_link = {http://grad.berkeley.edu/news/headlines/engineering-team-invents-medical-sensor/}, media_2 = {NSF Science 360 News, }, media_2_link = {http://news.science360.gov/obj/story/d8f7fa4c-4e41-4bcb-8ccd-1939dc4af3da/organic-electronics-lead-cheap-wearable-medical-sensors}, media_3 = {UC Berkeley News Center, }, media_3_link = {http://newscenter.berkeley.edu/2014/12/10/organic-electronics-cheap-wearable-medical-sensors/}, media_4 = {Phys.Org, }, media_4_link = {http://phys.org/news/2014-12-electronics-cheap-wearable-medical-sensors.html}, media_5 = {ScienceDaily, }, media_5_link = {https://www.sciencedaily.com/releases/2014/12/141210131356.htm}, media_6 = {MSN News, }, media_6_link = {https://www.msn.com/en-us/news/technology/is-the-next-fitbit-a-tattoo/ar-BBHIYih}, media_7 = {Yahoo News, }, media_7_link = {https://in.news.yahoo.com/device-cheap-wearable-fitness-sensors-081008659.html}, media_8 = {and many more.}, media_8_link = {https://www.altmetric.com/details/2972740} }

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2013

  1. Enhanced energy storage in chaotic optical resonators Changxu Liu, Andrea Di Falco, D Molinari, Yasser Khan, Boon S Ooi, Thomas F Krauss, and Andrea Fratalocchi Nature Photonics, 2013 7, 6. Cover article. Media coverage: EurekAlert!, nanowerk, Photonics.com, ScienceDaily, Phys.Org, and many more.

    Chaos is a phenomenon that occurs in many aspects of contemporary science. In classical dynamics, chaos is defined as a hypersensitivity to initial conditions. The presence of chaos is often unwanted, as it introduces unpredictability, which makes it difficult to predict or explain experimental results. Conversely, we demonstrate here how chaos can be used to enhance the ability of an optical resonator to store energy. We combine analytic theory with ab initio simulations and experiments in photonic-crystal resonators to show that a chaotic resonator can store six times more energy than its classical counterpart of the same volume. We explain the observed increase by considering the equipartition of energy among all degrees of freedom of the chaotic resonator (that is, the cavity modes) and discover a convergence of their lifetimes towards a single value. A compelling illustration of the theory is provided by enhanced absorption in deformed polystyrene microspheres.

    @article{liu2013enhanced, title = {Enhanced energy storage in chaotic optical resonators}, author = {Liu, Changxu and Di Falco, Andrea and Molinari, D and Khan, Yasser and Ooi, Boon S and Krauss, Thomas F and Fratalocchi, Andrea}, journal = {Nature Photonics}, volume = {7}, number = {6}, pages = {473}, year = {2013}, publisher = {Nature Publishing Group}, url = {http://dx.doi.org/10.1038/nphoton.2013.108}, doi = {10.1038/nphoton.2013.108}, thumbnail = {liu2013enhanced.png}, pdf = {liu2013enhanced.pdf}, note = {Cover article. Media coverage: }, media_1 = {EurekAlert!, }, media_1_link = {http://www.eurekalert.org/pub_releases/2013-05/uoy-cps050713.php}, media_2 = {nanowerk, }, media_2_link = {http://www.nanowerk.com/news2/newsid=30367.php}, media_3 = {Photonics.com, }, media_3_link = {http://www.photonics.com/Article.aspx?AID=53827}, media_4 = {ScienceDaily, }, media_4_link = {https://www.sciencedaily.com/releases/2013/05/130507060852.htm}, media_5 = {Phys.Org, }, media_5_link = {https://phys.org/news/2013-05-chaos-superior.html}, media_6 = {and many more.}, media_6_link = {https://www.altmetric.com/details/1447982/} }

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2012

  1. Two-step controllable electrochemical etching of tungsten scanning probe microscopy tips Yasser Khan, Hisham Al-Falih, Yaping Zhang, Tien Khee Ng, and Boon S Ooi Review of Scientific Instruments, 2012 83, 6.

    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.

    @article{khan2012two, title = {Two-step controllable electrochemical etching of tungsten scanning probe microscopy tips}, author = {Khan, Yasser and Al-Falih, Hisham and Zhang, Yaping and Ng, Tien Khee and Ooi, Boon S}, journal = {Review of Scientific Instruments}, volume = {83}, number = {6}, pages = {063708}, year = {2012}, publisher = {AIP}, url = {http://dx.doi.org/10.1063/1.4730045}, doi = {10.1063/1.4730045}, thumbnail = {khan2012two.png}, pdf = {khan2012two.pdf} }

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Conference Proceedings

2020

    2019

      2018

      1. System Design for Flexible All-Organic Reflectance Oximeter Yasser Khan, Donggeon Han, Adrien Pierre, Jonathan Ting, Xingchun Wang, Claire M. Lochner, and Ana C. Arias In MRS Spring Meeting, Phoenix, AZ, USA, 2018

        Existing techniques for measuring oxygen concentration in blood heavily relies on non-invasive transmission-mode pulse oximetry - a ratiometric optical sensing method, where light absorption in oxygenated and deoxygenated blood is interpreted to a person’s oxygen saturation (SpO2). Since transmitted light through tissues is used to generate the signal, transmission-mode pulse oximetry is restricted to only tissues that can be transilluminated, such as the ear and the fingers. Here, we present a reflection oximeter, which uses printed organic light-emitting diodes (OLEDs) and organic photodiodes (OPDs) to sense reflected light from tissues to determine the oxygen concentration in blood and tissues. Using the reflection-mode, the sensor can be used beyond the conventional sensing locations. We used the reflection-mode sensor to measure SpO2 on the forehead with 1.1% mean error. We also demonstrate a method to determine oxygen saturation in the absence of pulsatile blood. Additionally, printing techniques are utilized to fabricate the sensor on flexible plastic substrates, making the sensor both comfortable to wear and efficient at extracting high-quality biosignal.

        @inproceedings{khan2018system-c, author = {Khan, Yasser and Han, Donggeon and Pierre, Adrien and Ting, Jonathan and Wang, Xingchun and Lochner, Claire M. and Arias, Ana C.}, title = {System Design for Flexible All-Organic Reflectance Oximeter}, booktitle = {MRS Spring Meeting, Phoenix, AZ, USA}, year = {2018} }

      2. Emission area patterning of blade-coated organic light-emitting diodes (OLEDs) via printed dielectrics Donggeon Han, Yasser Khan, Karthik Gopalan, and Ana C. Arias In MRS Spring Meeting, Phoenix, AZ, USA, 2018

        @inproceedings{han2018emission-c, author = {Han, Donggeon and Khan, Yasser and Gopalan, Karthik and Arias, Ana C.}, title = {Emission area patterning of blade-coated organic light-emitting diodes (OLEDs) via printed dielectrics}, booktitle = {MRS Spring Meeting, Phoenix, AZ, USA}, year = {2018} }

      3. An EMG Gesture Recognition System with Flexible High-Density Sensors and Brain-Inspired High-Dimensional Classifier Ali Moin, Andy Zhou, Abbas Rahimi, Simone Benatti, Alisha Menon, Senam Tamakloe, Jonathan Ting, Natasha Yamamoto, Yasser Khan, Fred Burghardt, and others In Circuits and Systems (ISCAS), 2018 IEEE International Symposium on, 2018

        EMG-based gesture recognition shows promise for human-machine interaction. Systems are often afflicted by signal and electrode variability which degrades performance over time. We present an end-to-end system combating this variability using a large-area, high-density sensor array and a robust classification algorithm. EMG electrodes are fabricated on a flexible substrate and interfaced to a custom wireless device for 64-channel signal acquisition and streaming. We use brain-inspired high-dimensional (HD) computing for processing EMG features in one-shot learning. The HD algorithm is tolerant to noise and electrode misplacement and can quickly learn from few gestures without gradient descent or back-propagation. We achieve an average classification accuracy of 96.64% for five gestures, with only 7% degradation when training and testing across different days. Our system maintains this accuracy when trained with only three trials of gestures; it also demonstrates comparable accuracy with the state-of-the-art when trained with one trial.

        @inproceedings{moin2018emg-c, title = {An EMG Gesture Recognition System with Flexible High-Density Sensors and Brain-Inspired High-Dimensional Classifier}, author = {Moin, Ali and Zhou, Andy and Rahimi, Abbas and Benatti, Simone and Menon, Alisha and Tamakloe, Senam and Ting, Jonathan and Yamamoto, Natasha and Khan, Yasser and Burghardt, Fred and others}, booktitle = {Circuits and Systems (ISCAS), 2018 IEEE International Symposium on}, pages = {1--5}, year = {2018}, organization = {IEEE}, url = {http://dx.doi.org/10.1109/ISCAS.2018.8351613}, doi = {10.1109/ISCAS.2018.8351613}, thumbnail = {moin2018emg-c.png}, pdf = {moin2018emg-c.pdf} }

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      2016

      1. A wearable flexible hybrid electronics ECG monitor Mark Poliks, James Turner, Kanad Ghose, Zhanpeng Jin, Mohit Garg, Qiong Gui, Ana Arias, Yasser Kahn, Mark Schadt, and Frank Egitto In Electronic Components and Technology Conference (ECTC), 2016 IEEE 66th, 2016

        Flexible hybrid electronics (FHE) integrate both traditional printed circuits, solder assembled standard and thinned silicon chips along with printable electronic materials and sensors. The combination results in high performance from thin, light weight, flexible devices that potentially could be manufactured at low cost. In this paper, flexible hybrid electronics technology is being used to develop a wearable ECG and skin temperature monitor. All components and materials were commercially available, and all fabrication processes were executed in manufacturing environments. The monitor is composed of a flexible polyimide substrate with printed ECG electrodes, a printed thermistor, and connecting traces printed on one surface, and the electronic components mounted on other. Both sides have copper metal circuits connected by copper plated through hole vias (THV). ECG signals are amplified, preconditioned and wirelessly transmitted via Bluetooth to a nearby handheld mobile phone or computer. The wearable monitor is 2x2 inches in size and has been demonstrated to produce high fidelity ECG signals at the host from both certified archived human ECG signals and ECG signals from human volunteers. The monitor reproduced the archived signals at the host from which a set of clinical parameters were calculated that closely matched those of the archived signals. Manufacturing challenges and device reliability will be discussed. Current work includes building upon this platform and integration of other monitoring and sensor devices included those that monitor for biomarkers in sweat. This work was sponsored by the NanoBio Manufacturing Consortium administered by the Flextech Alliance and funded by the US Air Force Research Laboratory.

        @inproceedings{poliks2016wearable-c, title = {A wearable flexible hybrid electronics ECG monitor}, author = {Poliks, Mark and Turner, James and Ghose, Kanad and Jin, Zhanpeng and Garg, Mohit and Gui, Qiong and Arias, Ana and Kahn, Yasser and Schadt, Mark and Egitto, Frank}, booktitle = {Electronic Components and Technology Conference (ECTC), 2016 IEEE 66th}, pages = {1623--1631}, year = {2016}, organization = {IEEE}, url = {http://dx.doi.org/10.1109/ECTC.2016.395}, doi = {10.1109/ECTC.2016.395}, thumbnail = {poliks2016wearable-c.png}, pdf = {poliks2016wearable-c.pdf} }

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      2. Flexible Electrode Arrays for Bioelectronic Interfaces Y. Khan, and A. C. Arias In Flexible and Printed Electronics Conference, CA, USA, 2016

        Bioelectronic interfaces require electrodes that are mechanically flexible and chemically inert. Flexibility allows pristine electrode contact to skin and tissue, and chemical inertness prevents electrodes from reacting with biological fluids and living tissues. Therefore, flexible gold electrodes are ideal for bioimpedance and biopotential measurements such as bioimpedance tomography, electrocardiography (ECG), electroencephalography (EEG), and electromyography (EMG). However, a manufacturing process to fabricate gold electrode arrays on plastic substrates is still elusive. In this work, we demonstrate a fabrication and low temperature sintering (≈ 200 ⁰C) technique to fabricate gold electrodes. At low-temperature sintering conditions, lines of different widths demonstrate different sintering speeds. Therefore, the sintering condition is targeted towards the widest feature in the design layout. Manufactured electrodes show minimum feature size of 62µm and conductivity values of 5 x 106 Sm⁰1. Utilizing the versatility of printing and plastic electronics processes, electrode arrays consisting of 31 electrodes with electrode to electrode spacing ranging from 2 mm to 7 mm are fabricated, and used for impedance mapping of conformal surfaces at 15 kHz. Overall, we show the fabrication process of an inkjet-printed gold electrode array that is electrically reproducible, mechanically robust, and promising for bioimpedance and biopotential measurements

        @inproceedings{khan2016flexible-c, title = {Flexible Electrode Arrays for Bioelectronic Interfaces}, author = {Khan, Y. and Arias, A. C.}, booktitle = {Flexible and Printed Electronics Conference, CA, USA}, year = {2016} }

      2015

      1. Inkjet-Printed Sensors for Wearable Health Monitoring Yasser Khan, Mark Schadt, Mohit Garg, Qiong Gui, Paul Hart, Robert Welte, Stephen Cain, Bill Wilson, Zhanpeng Jin, Mark Poliks, Kanad Ghose, Steve Czarnecki, Frank Egitto, James Turner, and Ana Claudia Arias In MRS Fall Meeting, Boston, MA, USA, 2015 Best Oral Presentation Award

        Wearable physiological sensors fabricated on flexible substrates using low-cost, additive printing of functional inks is ideal for sensing human vital signs. The conformal interface to human body improves the signal to noise ratio, and the flexible nature of the materials allows sensor designs in wearable form-factor. Furthermore, the printing-based manufacturing process enables high volume production. Here, we report on a wearable human monitor based on flexible hybrid electronics (FHE) for recording and transmitting biometric parameters, specifically, electrocardiogram (ECG) and body temperature. We employ a hybrid manufacturing process where printed sensors and conventional rigid electronics are interfaced on a flexible 50 μm thick polyimide substrate. The ECG electrodes and connecting traces are printed using gold nanoparticle ink, and the thermistor is printed using nickel oxide (NiO) nanoparticles. ECG electrodes demonstrate minimum feature size of 70 μm with sheet resistance of 0.35 Ω/sq. Printed thermistors provide linear response from 25 °C to 50 °C with a controllable beta of 1000. Filtered ECG signals are amplified and recorded with a 12-bit resolution analog to digital converted (ADC). Preliminary human trials show the ability to record ECG signals comparable to those recorded using clinical Ag/AgCl electrodes and commercial ECG equipment. Overall, we report on the fabrication and implementation of a wearable sensor patch with inkjet-printed sensors, which accurately provide ECG signal and body temperature.

        @inproceedings{khan2015inkjet-c, title = {Inkjet-Printed Sensors for Wearable Health Monitoring}, author = {Khan, Yasser and Schadt, Mark and Garg, Mohit and Gui, Qiong and Hart, Paul and Welte, Robert and Cain, Stephen and Wilson, Bill and Jin, Zhanpeng and Poliks, Mark and Ghose, Kanad and Czarnecki, Steve and Egitto, Frank and Turner, James and Arias, Ana Claudia}, booktitle = {MRS Fall Meeting, Boston, MA, USA}, year = {2015}, thumbnail = {award_2015_fall_mrs_best_presentation.jpg}, note = {Best Oral Presentation Award} }

      2. System design for organic pulse oximeter Yasser Khan, Claire M Lochner, Adrien Pierre, and Ana Claudia Arias In Advances in Sensors and Interfaces (IWASI), 2015 6th IEEE International Workshop on, 2015

        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.

        @inproceedings{khan2015system, title = {System design for organic pulse oximeter}, author = {Khan, Yasser and Lochner, Claire M and Pierre, Adrien and Arias, Ana Claudia}, booktitle = {Advances in Sensors and Interfaces (IWASI), 2015 6th IEEE International Workshop on}, pages = {83--86}, year = {2015}, organization = {IEEE}, url = {http://dx.doi.org/10.1109/IWASI.2015.7184975}, doi = {10.1109/IWASI.2015.7184975}, thumbnail = {khan2015system-c.png}, pdf = {khan2015system-c.pdf} }

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      3. Interfacing Printed Sensors to Conventional Electronics for Wearable Sensor Patch Y. Khan, M. Garg, M. Schadt, Q. Gui, P. Hart, Z. Jin, M. Poliks, R. Welte, S. Czarnecki, F. Egitto, K. Ghose, J. Turner, and A. C. Arias In Flexible and Printed Electronics Conference, CA, USA, 2015

        Mechanical flexibility, low-cost processing, and large-area scalability make printed sensors ideal for wearable physiological sensing. Although printed sensors can efficiently extract physiological signals from conformal surfaces, complete system level implementation of these sensors require silicon-based electronics for data processing and transmission. In this work, we report a flexible and wearable sensor patch composed of printed electrodes and thermistors capable of measuring electrocardiogram (ECG) signal and body temperature. We employ a hybrid manufacturing process where printed sensors and conventional rigid electronics are interfaced on a flexible 50 µm thick polyimide substrate. Several surface energy modification steps are used to inkjet print sensors in conjunction with flex-pcb manufacturing steps. Biopotential electrodes are inkjet-printed using gold nanoparticle ink where minimum feature size of 80 µm was achieved with a sheet resistance of 0.4 ohm/sq. Thermistors are inkjet-printed using a blend of PEDOT:PSS and nickel oxide nanoparticles. Printed thermistors provide linear response from 25 °C to 150 °C with a controllable ? of 500 to 1000. Finally, the sensors were interfaced with an analog front-end, a microcontroller, and a Bluetooth chip, to provide ECG signal and accurate body temperature.

        @inproceedings{khan2015interfacing-c, title = {Interfacing Printed Sensors to Conventional Electronics for Wearable Sensor Patch}, author = {Khan, Y. and Garg, M. and Schadt, M. and Gui, Q. and Hart, P. and Jin, Z. and Poliks, M. and Welte, R. and Czarnecki, S. and Egitto, F. and Ghose, K. and Turner, J. and Arias, A. C.}, booktitle = {Flexible and Printed Electronics Conference, CA, USA}, year = {2015} }

      4. Impedance sensing device for monitoring ulcer healing in human patients Amy Liao, Monica C Lin, Lauren C Ritz, Sarah L Swisher, David Ni, Kaylee Mann, Yasser Khan, Shuvo Roy, Michael R Harrison, Ana C Arias, Vivek Subramanian, David Young, and Michel M Maharbiz In Engineering in Medicine and Biology Society (EMBC), 2015 37th Annual International Conference of the IEEE, 2015

        Chronic skin wounds affect millions of people each year and take billions of dollars to treat. Ulcers are a type of chronic skin wound that can be especially painful for patients and are tricky to treat because current monitoring solutions are subjective. We have developed an impedance sensing tool to objectively monitor the progression of healing in ulcers, and have begun a clinical trial to evaluate the safety and feasibility of our device to map damaged regions of skin. Impedance data has been collected on five patients with ulcers, and impedance was found to correlate with tissue health. A damage threshold was applied to effectively identify certain regions of skin as “damaged tissue”.

        @inproceedings{liao2015impedance-c, title = {Impedance sensing device for monitoring ulcer healing in human patients}, author = {Liao, Amy and Lin, Monica C and Ritz, Lauren C and Swisher, Sarah L and Ni, David and Mann, Kaylee and Khan, Yasser and Roy, Shuvo and Harrison, Michael R and Arias, Ana C and Subramanian, Vivek and Young, David and Maharbiz, Michel M}, booktitle = {Engineering in Medicine and Biology Society (EMBC), 2015 37th Annual International Conference of the IEEE}, pages = {5130--5133}, year = {2015}, organization = {IEEE}, url = {http://dx.doi.org/10.1109/EMBC.2015.7319546}, doi = {10.1109/EMBC.2015.7319546}, thumbnail = {liao2015impedance-c.png}, pdf = {liao2015impedance-c.pdf} }

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      2014

      1. All-Organic Green Light Pulse Oximeter for Wearable Medical Sensing Yasser Khan, Adrien Pierre, Claire Lochner, and Ana C. Arias In MRS Fall Meeting, Boston, MA, USA, 2014

        Pulse oximetry, a facile technique for noninvasively measuring pulse rate and arterial blood oxygenation, is a ubiquitous medical sensing method. However, rigid sensors and expensive optoelectronic components restrict the true potential of pulse oximetry by limiting sensing locations to finger tips or ear lobes. If an oximeter probe is realized in a flexible and wearable form-factor, the recent thrust towards wearable medical sensing can be advanced by putting oxygenation sensors on different parts of the body. In this work, we report an all-organic green light pulse oximeter - capable of measuring heart rate and arterial oxygen saturation. Green (532 nm) and red (626 nm) organic light emitting diodes (OLEDs) were used with an organic photodiode (OPD) sensitive at the mentioned wavelengths. We chose green light excitation because of the well-developed solution-processable green emitters, as opposed to the early development stage near-infrared emitter materials. The organic probe was compared to a commercially available oximeter, and we acquired pulse rate and oxygenation that lied within 1% and 2% error respectively during simultaneous measurements. To the extent of our knowledge, this is the first optoelectronic sensor fabricated using all-organic materials that works in conjunction with conventional electronics at 1KHz, and provides accurate pulse rate and blood oxygenation.

        @inproceedings{khan2014all-c, title = {All-Organic Green Light Pulse Oximeter for Wearable Medical Sensing}, author = {Khan, Yasser and Pierre, Adrien and Lochner, Claire and Arias, Ana C.}, booktitle = {MRS Fall Meeting, Boston, MA, USA}, year = {2014} }

      2011

      1. Fabrication of tuning-fork based AFM and STM tungsten probe Hisham Al-Falih, Yasser Khan, Yaping Zhang, Damain Pablo San-Roman-Alerigi, Dongkyu Cha, Boon Siew Ooi, and Tien Khee Ng In High Capacity Optical Networks and Enabling Technologies (HONET), 2011, 2011

        We compare the sharpness of tungsten probe tips produced by the single-step and two-step dynamic electrochemical etching processes. A small radius of curvature (RoC) of 25 nm or less was routinely obtained when the two-step electrochemical etching (TEE) process was adopted, while the smallest achievable RoC was  10 nm, rendering it suitable for atomic force microscopy (AFM) or scanning tunneling microscopy (STM) applications.

        @inproceedings{al2011fabrication-c, title = {Fabrication of tuning-fork based AFM and STM tungsten probe}, author = {Al-Falih, Hisham and Khan, Yasser and Zhang, Yaping and San-Roman-Alerigi, Damain Pablo and Cha, Dongkyu and Ooi, Boon Siew and Ng, Tien Khee}, booktitle = {High Capacity Optical Networks and Enabling Technologies (HONET), 2011}, pages = {190--192}, year = {2011}, organization = {IEEE}, url = {http://dx.doi.org/10.1109/HONET.2011.6149815}, doi = {10.1109/HONET.2011.6149815}, thumbnail = {al2011fabrication-c.png}, pdf = {alfabrication-c.pdf} }

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      2. Facile Method for Fabricating Reproducible Tungsten Probe Tips with Varying Cone Angles Yasser Khan, Josh Ballard, Yaping Zhang, Justin Alexander, Miles Larkin, and Boon Ooi In International Conference on Materials for Advanced Technologies (ICMAT), 2011

        In scanning tunneling microscopy, sharper tips offer higher performance. Consistence in tip shape and reproducibly are extremely important for both commercial applications and research. In this work, electronic current shutoff time for conventional DC “drop-off” technique is reduced to  36ns; static DC etching results in undesirable variability tip shapes, so “dynamic etching,” where electrolyte level moves up and down during etching, was introduced, resulting smooth controllable cone angles. It has been observed that natural potential difference between counter electrode and the W tip causes tip blunting even after fast current shut off, reverse biasing after “drop-off,” takes care of the mentioned issue. In this paper, we present a facile and robust approach that incorporates synergy of “drop-off” method, dynamic electrochemical etching, and reverse biasing after “drop-off” yielding highly reproducible tips with controllable cone angles. Reproducibility was confirmed by scanning electron microscopy, and 5-40 nm radius tungsten probe tips were produced with more than 80% success rate.

        @inproceedings{khan2011facile-c, title = {Facile Method for Fabricating Reproducible Tungsten Probe Tips with Varying Cone Angles}, author = {Khan, Yasser and Ballard, Josh and Zhang, Yaping and Alexander, Justin and Larkin, Miles and Ooi, Boon}, booktitle = {International Conference on Materials for Advanced Technologies (ICMAT)}, year = {2011} }

      3. ZnO nanorods for simultaneous light trapping and transparent electrode application in solar cells Yasser Khan, Yaping Zhang, Muhammad Amin, A Bayraktaroglu, Tien Khee Ng, H Bağcı, J Phillips, and Boon S Ooi In Photonics Conference (PHO), 2011 IEEE, 2011

        Efficacy of using vertically grown ZnO nanorod array in enhancing electromagnetic field intensity and serving as the top contact layer (transparent electrodes) for solar cells was investigated.

        @inproceedings{khan2011zno-c, title = {ZnO nanorods for simultaneous light trapping and transparent electrode application in solar cells}, author = {Khan, Yasser and Zhang, Yaping and Amin, Muhammad and Bayraktaroglu, A and Ng, Tien Khee and Ba{\u{g}}c{\i}, H and Phillips, J and Ooi, Boon S}, booktitle = {Photonics Conference (PHO), 2011 IEEE}, pages = {619--620}, year = {2011}, organization = {IEEE}, url = {http://dx.doi.org/10.1109/PHO.2011.6110700}, doi = {10.1109/PHO.2011.6110700}, thumbnail = {khan2011zno-c.png}, pdf = {khan2011zno-c.pdf} }

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      4. Wireless embedded control system for atomically precise manufacturing Yasser Khan, and John Randall In Information Technology: New Generations (ITNG), 2011 Eighth International Conference on, 2011

        This paper will explore the possibilities of implementing a wireless embedded control system for atomically precise manufacturing. The manufacturing process, similar to Scanning Tunneling Microscopy, takes place within an Ultra High Vacuum (UHV) chamber at a pressure of 10-10 torr. In order to create vibration isolation, and to keep internal noise to a minimum, a wireless link inside the UHV chamber becomes essential. We present a MATLAB simulation of the problem, and then demonstrate a hardware scheme between a Gumstix computer and a Linux based laptop for controlling nano-manipulators with three degrees of freedom.

        @inproceedings{khan2011wireless-c, title = {Wireless embedded control system for atomically precise manufacturing}, author = {Khan, Yasser and Randall, John}, booktitle = {Information Technology: New Generations (ITNG), 2011 Eighth International Conference on}, pages = {965--969}, year = {2011}, organization = {IEEE}, url = {http://dx.doi.org/10.1109/ITNG.2011.165}, doi = {10.1109/ITNG.2011.165}, thumbnail = {khan2011wireless-c.png}, pdf = {khan2011wireless-c.pdf} }

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      Poster Presentations

      2020

      1. Mentaid: A skin-like sensor system for decoding mental health Yasser Khan, Boris Murmann, and Zhenan Bao In 2020 eWEAR Annual Meeting, Stanford, CA, USA, 2020

        @inproceedings{khan202mentaid-p, author = {Khan, Yasser and Murmann, Boris and Bao, Zhenan}, title = {Mentaid: A skin-like sensor system for decoding mental health}, month = feb, year = {2020}, booktitle = {2020 eWEAR Annual Meeting, Stanford, CA, USA} }

      2019

        2018

        1. Fully Screen-Printed NiO Thermistor Arrays Jonathan Ting*, Natasha Yamamoto*, Yasser Khan*, Abhinav Gaikwad, and Ana Claudia Arias In Flexible Electronics Conference and Exhibition - 2018 FLEX, Monterey, CA, USA, 2018 Best Poster Award.

          The performance of commercial Lithium ion batteries is a function of both battery voltage and temperature. Temperature has a significant effect on the performance and lifetime of these batteries, and influences several different failure mechanisms. One of the most dangerous failure mechanisms involves thermal runaway, where the battery becomes progressively more permanently damaged as it continues through each stage of thermal runaway. A possible method to improve battery safety is to monitor the temperature of the whole battery surface to potentially detect local heating, as a precursor to thermal runaway. Here, we present a fully screen-printed thermistor array, which uses a NiO based ink to sense temperature over a large area. We used the 4 x 4 array of thermistors to measure the temperature of a commercial Lithium ion battery when operated at higher C-rates. By multiplexing the individual thermistor pixels, we were able to plot, in real-time, the temperature of the battery surface, and spatially resolve local heating. Additionally, the screen-printing was done on flexible plastic substrates, making the sensor array conformal to the object being measured, making the sensor more efficient at resolving surface temperatures than conventional rigid thermistors.

          @inproceedings{ting2018fully-p, author = {Ting*, Jonathan and Yamamoto*, Natasha and Khan*, Yasser and Gaikwad, Abhinav and Arias, Ana Claudia}, title = {Fully Screen-Printed NiO Thermistor Arrays}, month = feb, year = {2018}, booktitle = {Flexible Electronics Conference and Exhibition - 2018 FLEX, Monterey, CA, USA}, note = {Best Poster Award.} }

        2014

        1. Flexible Printed Circuit Board for Wearable Physiological Monitoring Yasser Khan, Felippe Pavinatto, and Ana Claudia Arias In MRS Spring Meeting, San Francisco, CA, USA, 2014 Nominated for Best Poster Award.

          The scope of wearable technology stretches beyond electronic gadgets, and has the potential to revolutionize both in-hospital and in-home health monitoring. However, wide implementations of wearable medical sensors is hindered largely due to non-conformality of conventional printed circuit boards (PCBs) and solid-state electronic components. Leveraging the recent advances in flexible electronics, an integrated sensor platform has been established, which employs solid-state electronic components and utilizes conformality of flexible electronics. In this work, we demonstrate a flexible and wearable sensor board composed of a printed electrode array capable of measuring biopotential and bioimpedance. While using a thin (35µm) PEN substrate improved conformality of the sensor board, flexure cuts further improved skin-electrode contact. Inkjet printed gold nanoparticles were used for electrodes and routing due to chemical inertness and biocompatibility of gold. Minimum feature size of 70µm was routinely achieved with sheet resistance of 0.5 Ω/sq at annealing temperature of 200° C. The sensor board was interfaced with a polyimide flex board that hosted solid-state electronic components for data processing and transmission. The flexible sensor board provided significant edge over conventional rigid PCBs by providing high resolution data from conformal surfaces. Overall, we demonstrate a wearable flexible sensor platform that can efficiently extract biomedical signals from conformal surfaces without compromising signal quality and can transmit data over “state of the art” wireless links.

          @inproceedings{khan2014flexible-p, author = {Khan, Yasser and Pavinatto, Felippe and Arias, Ana Claudia}, title = {Flexible Printed Circuit Board for Wearable Physiological Monitoring}, month = apr, year = {2014}, booktitle = {MRS Spring Meeting, San Francisco, CA, USA}, note = {Nominated for Best Poster Award.} }

        2. Printed Pulse Oximeter for Wearable Medical Sensor Patch Yasser Khan, Adrien Pierre, Claire Lochner, and Ana Claudia Arias In NASCENT IAB Meeting, Austin, TX, USA., 2014 Best Poster Award

          @inproceedings{khan2014printed-p, author = {Khan, Yasser and Pierre, Adrien and Lochner, Claire and Arias, Ana Claudia}, title = {Printed Pulse Oximeter for Wearable Medical Sensor Patch}, month = jan, year = {2014}, booktitle = {NASCENT IAB Meeting, Austin, TX, USA.}, note = {Best Poster Award}, thumbnail = {award_2014_nascent_best_poster.png} }

        2012

        1. Energy Harvesting in Complex Systems Yasser Khan, Changxu Liu, Diego Molinari, Boon Ooi, and Andrea Fratalocchi In Electrical Engineering Days, King Abdullah University of Science and Technology., 2012 Best Poster Award.

          @inproceedings{khan2012energy-p, author = {Khan, Yasser and Liu, Changxu and Molinari, Diego and Ooi, Boon and Fratalocchi, Andrea}, title = {Energy Harvesting in Complex Systems}, month = feb, year = {2012}, booktitle = {Electrical Engineering Days, King Abdullah University of Science and Technology.}, note = {Best Poster Award.} }

        2011

        1. Controllable Electrochemical Etching of Tungsten STM Tips Yasser Khan, Josh Ballard, Justin Alexander, Miles Larkin, and Boon Ooi In First WEP Research Poster Session, King Abdullah University of Science and Technology., 2011 Best Poster Award.

          @inproceedings{khan2011controllable-p, author = {Khan, Yasser and Ballard, Josh and Alexander, Justin and Larkin, Miles and Ooi, Boon}, title = {Controllable Electrochemical Etching of Tungsten STM Tips}, month = jan, year = {2011}, booktitle = {First WEP Research Poster Session, King Abdullah University of Science and Technology.}, note = {Best Poster Award.}, thumbnail = {award_2011_kaust_beacon_zoom.png} }

        2. ZnO Nanorods for Simultaneous Light Trapping and Transparent Electrode Application in Solar Cells Yasser Khan, Yaping Zhang, Muhammad Amin, Tien Khee Ng, Jamie Phillips, Hakan Bagci, and Boon Ooi In First Graduate Research Symposium, King Abdullah University of Science and Technology., 2011 Best Poster Award.

          @inproceedings{khan2011zno-p, author = {Khan, Yasser and Zhang, Yaping and Amin, Muhammad and Ng, Tien Khee and Phillips, Jamie and Bagci, Hakan and Ooi, Boon}, title = {ZnO Nanorods for Simultaneous Light Trapping and Transparent Electrode Application in Solar Cells}, month = may, year = {2011}, booktitle = {First Graduate Research Symposium, King Abdullah University of Science and Technology.}, note = {Best Poster Award.}, thumbnail = {award_2011_kaust_grs.png} }

        Patents

        2020

        1. Reflectance based pulse oximetry systems and methods Ana Claudia Arias, Claire Lochner, Adrien Pierre, and Yasser Khan 2020 US Patent 10,548,519

          @misc{arias2020reflectance, title = {Reflectance based pulse oximetry systems and methods}, author = {Arias, Ana Claudia and Lochner, Claire and Pierre, Adrien and Khan, Yasser}, year = {2020}, month = feb, publisher = {Google Patents}, note = {US Patent 10,548,519} }

        2019

        1. Methods and apparatus for monitoring wound healing using impedance spectroscopy Michel Maharbiz, Vivek Subramanian, Ana Claudia Arias, Sarah Swisher, Amy Liao, Monica Lin, Felippe Pavinatto, Yasser Khan, Daniel Cohen, Elisabeth Leeflang, and others 2019 US Patent 10,463,293

          @misc{maharbiz2019methods, title = {Methods and apparatus for monitoring wound healing using impedance spectroscopy}, author = {Maharbiz, Michel and Subramanian, Vivek and Arias, Ana Claudia and Swisher, Sarah and Liao, Amy and Lin, Monica and Pavinatto, Felippe and Khan, Yasser and Cohen, Daniel and Leeflang, Elisabeth and others}, year = {2019}, month = nov, publisher = {Google Patents}, note = {US Patent 10,463,293} }

        2. Printed all-organic reflectance oximeter array Yasser Khan, Donggeon Han, Adrien Pierre, Jonathan Ting, Xingchun Wang, Claire M Lochner, and Ana C Arias 2019 International Patent App. PCT/US20 19/033381

          @misc{khan2019printed, title = {Printed all-organic reflectance oximeter array}, author = {Khan, Yasser and Han, Donggeon and Pierre, Adrien and Ting, Jonathan and Wang, Xingchun and Lochner, Claire M and Arias, Ana C}, year = {2019}, month = nov, publisher = {Google Patents}, note = {International Patent App. PCT/US20 19/033381 } }

        2018

        1. Flexible, non-invasive real-time hematoma monitoring system using near-infrared spectroscopy Claire Meyer Lochner, Rachel Nancollas, Jacob Sadie, Yasser Khan, and Ana Claudia Arias 2018 US Patent App. 15/852,366

          @misc{lochner2018flexible, title = {Flexible, non-invasive real-time hematoma monitoring system using near-infrared spectroscopy}, author = {Lochner, Claire Meyer and Nancollas, Rachel and Sadie, Jacob and Khan, Yasser and Arias, Ana Claudia}, year = {2018}, month = may, publisher = {Google Patents}, note = {US Patent App. 15/852,366} }

        2017

          Last modified: 2020-09-21