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Enabling Sweat-Based Biosensors: Solving the Problem of Low Enabling sweat-based biosensors: Solving the problem of low biomarker concentration in sweat A dissertation submitted to the Graduate School of the University of Cincinnati in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Biomedical Engineering of the College of Engineering & Applied Science by Andrew J. Jajack B.S., Biology, Wittenberg University, 2014 Committee Chairs: Jason C. Heikenfeld, Ph.D. and Chia-Ying Lin, Ph.D. Abstract Non-invasive, sweat biosensing will enable the development of an entirely new class of wearable devices capable of assessing health on a minute-to-minute basis. Every aspect of healthcare stands to benefit: prevention (activity tracking, stress-level monitoring, over-exertion alerting, dehydration warning), diagnosis (early-detection, new diagnostic techniques), and management (glucose tracking, drug-dose monitoring). Currently, blood is the gold standard for measuring the level of most biomarkers in the body. Unlike blood, sweat can be measured outside of the body with little inconvenience. While some biomarkers are produced in the sweat gland itself, most are produced elsewhere and must diffuse into sweat. These biomarkers come directly from blood or interstitial fluid which surrounds the sweat gland. However, a two-cell thick epithelium acts as barrier and dilutes most biomarkers in sweat. As a result, many biomarkers that would be useful to monitor are diluted in sweat to concentrations below what can be detected by current biosensors. This is a core challenge that must be overcome before the advantages of sweat biosensing can be fully realized. The objective of this dissertation is to develop methods of concentrating biomarkers in sweat to bring them into range of available biosensors. This dissertation will encompass both the physiological understanding of how biomarkers enter sweat as well as two strategies for increasing concentration: technological and biological. The technological strategy involves a novel microfluidic-based biofluid preconcentration device. The biological strategy takes advantage of paracellular permeability enhancers in combination with reverse iontophoresis to increase biomarker flux into sweat, thereby increasing the concentration in sweat. Increasing the concentration of biomarkers in sweat is expected to improve the detection of previously hard-to-detect biomarkers, making sweat biosensing a more viable option for health monitoring. ii Copyright Page iii Acknowledgements To my committee—thank you for your thoughtful feedback and direction, challenging me to grow as a researcher. To my friends and colleagues of the Novel Device Lab—I have greatly appreciated working with each one of you. Our supportive, collaborative culture could be summed up as simply: work hard, play harder. Thanks for making the journey a blast. To my advisor and mentor, Dr. Heikenfeld—you have taught me to think big, to challenge myself, and to trust my gut as I plunge into new adventures. Thanks for believing in me. To my friends and family—thank you for helping me get to this point, supporting me as I grow, and keeping me grounded by reminding me of what matters most. To my loving wife, Doreen—my rock, this journey would not have been possible without your love and support, and so it is only fitting that I dedicate this work to you. Thank you, I love you, and I look forward to our next adventure with you by my side. iv Table of Contents Abstract....................................................................................................................................... ii Copyright Page .......................................................................................................................... iii Acknowledgements .................................................................................................................... iv Table of Contents ........................................................................................................................ v List of Figures ........................................................................................................................... vii List of Tables ............................................................................................................................. ix List of Acronyms ......................................................................................................................... x Chapter 1: Introduction ............................................................................................................... 1 1.1 Background ...................................................................................................................... 1 1.2 Sweat as an information-rich biofluid ................................................................................ 1 1.3 Catalytic and affinity biosensors ....................................................................................... 2 1.4 Research aims.................................................................................................................. 4 Chapter 2: Understand how biomarkers enter sweat and describe sweat-rate dependence of biomarkers ................................................................................................................................10 2.1 Introduction ......................................................................................................................10 2.2 The epidermis as an information barrier ..........................................................................10 2.2.1 Epidermal structure ...................................................................................................11 2.2.2 Chemical impedance .................................................................................................13 2.2.3 Chemical contamination ............................................................................................13 2.3 Biofluid partitioning ..........................................................................................................15 2.3.1 From blood to ISF .....................................................................................................15 2.4 Barrier to biomarker entry into sweat ...............................................................................17 2.5 Experimental methods .....................................................................................................19 2.5.1 Custom testing device ...............................................................................................19 2.5.2 Sweat generation ......................................................................................................20 2.5.3 Avoiding sweat contamination ...................................................................................23 2.5.4 Sweat collection ........................................................................................................24 2.5.5 Sweat glucose ...........................................................................................................25 2.5.6 Blood glucose ...........................................................................................................25 2.5.7 Sweat rate .................................................................................................................25 2.5.8 Glucose flux under normal conditions ........................................................................26 2.5.9 IRB protocol ..............................................................................................................27 2.6 Results and discussion ....................................................................................................27 2.7 Conclusions .....................................................................................................................28 Chapter 3: Continuously concentrate sweat samples within real-time device ............................30 3.1 Introduction ......................................................................................................................30 3.2 Experimental methods .....................................................................................................34 3.2.1 Membrane and draw molecule optimization ..............................................................34 v 3.2.2 Preconcentration device construction, characterizing, and modeling .........................39 3.3 Results and discussion ....................................................................................................43 3.3.1 Membrane and draw molecule optimization ..............................................................43 3.3.2 Preconcentration device ............................................................................................46 3.4 Conclusions .....................................................................................................................50 Chapter 4: Increase concentration of target biomarkers in sweat using chemical permeability enhancement and reverse iontophoresis...................................................................................51 4.1 Introduction ......................................................................................................................51 4.2 RI likely does not induce electroporation under study conditions .....................................54 4.3 Experimental methods .....................................................................................................56
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