Overview of a wearable sensor platform, where flexible sensors are utilized for real-time health monitoring.

A skin-like flexible and wearable sensor patch, seamlessly measuring body’s vital signs - realizing this device is a major goal of my doctoral work. My research focuses mainly on the system-level implementation of wearable medical devices, with an emphasis on flexible and printed bioelectronic and biophotonic sensors.

Wearable and flexible sensors are promising for medical sensing because they provide an improved signal-to-noise ratio (SNR) by establishing a conformal skin-sensor interface [1]. Moreover, in my work, printing techniques are used to fabricate the sensors, which ensures large-area scaling of the devices. Additionally with the rapid prototyping capability of printing, the sensors can be designed in different sizes and shapes, accommodating the needs of a diverse population. The sensors, sensing schemes, and application areas I focused on are listed in the sections below:

#### 1. Printed biophotonic sensors for blood and tissue oximetry

Organic optoelectronic probe for transmission-mode pulse oximetry.

Reflection-mode pulse oximetry using an organic oximeter probe on a volunteer's wrist.

Oximetry, the technique for determining oxygen saturation, optically measures the light absorption of oxygenated and deoxygenated blood and tissue at two different wavelengths. In 2014, we demonstrated the first all-organic optoelectronic oximeter sensor composed of organic light-emitting diodes (OLEDs) and an organic photodiode (OPD) to accurately measure pulse rate and oxygenation with errors of 1% and 2%, respectively [2]. This transmission-mode probe demonstrated that oximetry can be performed with organic optoelectronics. However, to realize the true potential of organic optoelectronics for oximetry, a reflection-mode operation is essential that allows sensor placement on different parts of the body. After optimizing the sensor design and the printing process, in 2017, we reported a reflection-mode organic oximeter probe and performed blood oxygenation measurements on the wrist [3].

#### 2. Printed bioelectronic sensors for BIA (Bioimpedance Analysis), ECG (Electrocardiography), and EMG (Electromyography)

Inkjet-printed flexible gold electrode array consisting of 31 electrodes, which is used for impedance mapping of conformal surfaces.

Non-invasively mapping pressure-induced tissue damage with inkjet-printed flexible gold electrode array.

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. Since a manufacturing process to fabricate gold electrode arrays on plastic substrates is elusive, we devised a fabrication and low-temperature sintering ($\approx 200 ^\circ C$) technique to print gold electrodes. Utilizing the versatility of printing and plastic electronic processes, electrode arrays were fabricated and used for impedance mapping of conformal surfaces at 15 kHz [4].

A more direct application of the array was to non-invasively map pressure-induced tissue damage. Here, the array was used to detect pressure ulcers in an animal model, even before the damages were observed visually. Our results demonstrated the feasibility of an automated, non-invasive “smart bandage” for early detection of pressure ulcers [5]. Moreover, for a different project, we used the gold electrodes printed on a wearable sensor patch as electrocardiography (ECG) electrodes.

#### 3. Integration of printed sensors to flexible hybrid electronics for wearable health monitoring

An overview and system design of the wearable sensor patch (WSP) enabled by flexible hybrid electronics. The sensor side faces down toward the skin, and the component side faces up. The printed gold ECG electrodes and the thermistor are shown.

A flexible power source integrating a lithium-ion battery and amorphous silicon solar module for powering wearable health monitoring devices.

Flexible hybrid electronics (FHE) are a fundamental enabling technology for system-level implementation of novel printed and flexible devices. FHE bring together soft and hard electronics into a single platform, where the soft devices are used for conformal sensor interfaces, and the hard silicon-based devices provide the computational backbone and compatibility with existing electronic systems and standards. The interfacing of soft and hard electronics is a key challenge for flexible hybrid electronics.

For a project in collaboration with Binghamton University, i3 Electronics, Lockheed Martin, and American Semiconductor, we demonstrated a single substrate interfacing approach, 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 was fabricated composed of inkjet-printed gold ECG electrodes and a stencil-printed nickel oxide thermistor [6]. In another project, for powering wearable health monitoring devices, we demonstrated a flexible power source by integrating a lithium-ion battery and amorphous silicon solar module [7].

#### References

1. Monitoring of vital signs with flexible and wearable medical devices Advanced Materials, 2016 28, 22. [Abstract] [Bibtex] [PDF]
1. All-organic optoelectronic sensor for pulse oximetry Nature communications, 2014 5, [Abstract] [Bibtex] [PDF] *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.
1. Flexible blade-coated multicolor polymer light-emitting diodes for optoelectronic sensors Advanced Materials, 2017 29, 22. [Abstract] [Bibtex] [PDF]
1. Inkjet-printed flexible gold electrode arrays for bioelectronic interfaces Advanced Functional Materials, 2016 26, 7. [Abstract] [Bibtex] [PDF] Cover article.
1. Impedance sensing device enables early detection of pressure ulcers in vivo Nature communications, 2015 6, [Abstract] [Bibtex] [PDF] Media coverage: BBC News, UC Berkeley News Center, Futurity, NSF News, ACM Communications, and many more.
1. Flexible hybrid electronics: Direct interfacing of soft and hard electronics for wearable health monitoring Advanced Functional Materials, 2016 26, 47. [Abstract] [Bibtex] [PDF]
1. High-performance flexible energy storage and harvesting system for wearable electronics Scientific reports, 2016 6, [Abstract] [Bibtex] [PDF]