(Invited) Wearable Microfluidic and Electronic Frameworks for Biomedical Applications
Authors: Gray Bonnie L. , Chung Daehan 
Published: February 2020

I. Introduction
Advances in wearable electronics, functional nanocomposite polymers, flexible microfluidics, and commercial polymer microfluidics are reported on a daily basis. However, much of this research is field-specific, despite the enormous potential that the convergence of these research areas has for the future of health care, worker safety, food safety, consumer devices, and commercial microfluidics. We present an innovative approach of powerful, general-purpose frameworks that facilitate interfacing of nanocomposite polymer-based, microfluidic, and electronic devices on flexible polymer, textile, or industrial polymer platforms. Such approaches offer practical solutions for areas such as safety lighting and health monitors for safety vests; heart monitors and perspiration monitors for athletic clothing; and other applications in real-time wearable bioelectric and biochemical monitoring.
II. Wearable Microfluidic and Electronic Frameworks
A. Wearable bioelectric sensors
Intense research into wearable electronics has resulted in many innovative devices and systems, e.g.: flexible polyimide or Kapton printed circuit boards (PCBs) [1]; roll-to-roll foil and other printed devices [2]; printing and weaving of textiles [3]; and special geometries for flexible interconnect between rigid components [4]. We have investigated conductive nanocomposite polymers and metal transfer processes for wearable bioelectric sensors [5, 6]. These technologies can be employed in, for example, conformable and wearable systems for electrocardiogram (ECG) [5], tissue impedance [7] and pressure [8] sensors. Unlike many other techniques, the technology and materials employed for our devices are highly compatible with clothing-based textiles.
B. Wearable microfluidics
Much less research has been performed in the emerging area of wearable microfluidics. Traditionally, microfluidic devices are fabricated in rigid substrates or flexible materials such as polydimethylsiloxane (PDMS) that are bonded to rigid substrates. Free-standing PDMS devices are developed for, e.g., perspiration sensors [9]; however, such processes typically require long fabrication times, equipment in a cleanroom facility, and difficulty with integration onto textiles. Other devices employ porous materials or microneedles to deliver biofluids, e.g., perspiration or interstitial fluid, by capillary forces [10]; however, such devices are limited in fluid collection or are too invasive. Wearable textile-based devices have been recently demonstrated that use the textile itself as a fluidic “channel” [11]; however, such devices are dependent on the fluidic characteristics of the fabric. To overcome these limitations and facilitate the development of fully flexible, wearable, and durable microfluidic devices that can be used for a wider variety of applications, we have developed a printing-based fabrication process that employs screen printable plastisol ink [12]. This process is demonstrated for simple multi-level microfluidic devices toward the goal of fully wearable microfluidic systems.
III. Summary
Many methods have been developed for microelectronic and microfluidic devices in rigid or flexible packages (e.g., [13, 14, 15]). However, such technologies suffer from difficult integration and/or limited flexibility, limited geometries, limited microfluidic sample size, and/or difficult integration, especially on textile-based substrates. The technologies we present offer simple and compatible printing-based and transfer processes for microelectronic and microfluidic frameworks that facilitate cost-effective development of durable wearable biomedical systems.
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