biosensors Review Recent Advances in Portable Biosensors for Biomarker Detection in Body Fluids Brian Senf 1 , Woon-Hong Yeo 2 and Jong-Hoon Kim 1,* 1 School of Engineering and Computer Science, Washington State University, Vancouver, WA 98686, USA; [email protected] 2 Human-Centric Interfaces and Engineering Program, Wallace H. Coulter Department of Biomedical Engineering, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-360-546-9250; Fax: +1-360-546-9438 Received: 26 August 2020; Accepted: 14 September 2020; Published: 18 September 2020 Abstract: A recent development in portable biosensors allows rapid, accurate, and on-site detection of biomarkers, which helps to prevent disease spread by the control of sources. Less invasive sample collection is necessary to use portable biosensors in remote environments for accurate on-site diagnostics and testing. For non- or minimally invasive sampling, easily accessible body fluids, such as saliva, sweat, blood, or urine, have been utilized. It is also imperative to find accurate biomarkers to provide better clinical intervention and treatment at the onset of disease. At the same time, these reliable biomarkers can be utilized to monitor the progress of the disease. In this review, we summarize the most recent development of portable biosensors to detect various biomarkers accurately. In addition, we discuss ongoing issues and limitations of the existing systems and methods. Lastly, we present the key requirements of portable biosensors and discuss ideas for functional enhancements. Keywords: portable biosensor; biomarkers in body fluids; portability; point-of-care 1. Introduction Biosensors are a subset of chemical sensors, which transform chemical data into an analytical signal to monitor physiological and chemical analytes in the body. The chemical sensors are comprised of a chemical recognition system and a physicochemical transducer. Likewise, biosensors use a biochemical reaction as the recognition element [1] and convert a biological response into an electric signal [2]. The biosensor requires multiple components to measure these analytes and display information related to the analysis done. Figure1 shows the schematic diagram of a biosensor composed of a biorecognition element, transducer, and signal processing unit. The biorecognition layer is the defining component that determines the specificity of the device. It’s also called the bio receptor, which is a molecular species that utilizes a biochemical mechanism for the recognition of analytes. The biorecognition element binds the analytes of interest to the sensor’s surface for the reaction. The reaction is then converted by the transduction mechanism. The transducer converts a biological signal to a measurable signal. It converts one form of energy to another and can take many forms depending on design specifications. One common type of transduction mechanism used in biosensors is the electrochemical transducer. Many reactions with the biorecognition layer either produce or consume ions or electrons. This can cause changes in the electrical properties of a solution that can be measured and transduced to a signal processor. The signal processor is generally a computer or microprocessor which acquires the signal, then filters and amplifies the data. Electrical noise is inherent when processing data from the transducer. Thus, the signal processor typically subtracts the baseline noise from the transducer to amplify the signal of interest [2]. Biosensors 2020, 10, 127; doi:10.3390/bios10090127 www.mdpi.com/journal/biosensors Biosensors 2020, 10, 127 2 of 23 Biosensors 2020, 10, x FOR PEER REVIEW 2 of 24 FigureFigure 1. 1. 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