Recent advances in lab-on-a-chip technology establish solid foundations for wearable biosensors. These newly emerging wearable biosensors are capable of non-invasive, continuous monitoring by miniaturization of electronics and integration with microfluidics. The advent of flexible electronics, biochemical sensors, soft microfluidics, and pain-free microneedles have created new generations of wearable biosensors that explore brand-new avenues to interface with the human epidermis for monitoring physiological status. However, these devices are relatively underexplored for sports monitoring and analytics, which may be largely facilitated by the recent emergence of wearable biosensors characterized by real-time, non-invasive, and non-irritating sensing capacities.
Miniaturization of laboratory apparatus into microscale devices is a promising technology called lab-on-a-chip (LOC) . About 30 years ago the concept of micro total analysis systems (μTAS) emerged from the field of semiconductor fabrication and was enhanced by microelectromechanical systems (MEMS) technologies . The μTAS concept is to shrink an entire analytical procedure, such as cell sorting, single-cell capture, captured-cell transport, cell lysis, and intracellular analysis, into a miniaturized multifunctional chip , and nowadays its well-known synonym is called lab-on-a-chip (LOC) . This growing field has garnered considerable attention since scaled-down biochemical analysis has several key advantages over both conventional and current laboratory benchtop methods . These advantages are consistently demonstrated in clinical medicine, engineering, biology, and life science, etc., for example, to expedite the experimental process by embracing automation and parallelization ; to lower the cost by reducing the volume of expensive reagents ; to yield better interpretation of experimental results by gleaning vital information at cellular even molecular levels .
Interest in device miniaturization , combined with advances in bio-microfabrication and enabling materials , is motivating various microfluidic methods in which microchips can be mass-manufactured at extremely low cost via polymers (e.g., polydimethylsiloxane, PDMS) and soft lithography for microfabrication . Microfluidics is the science of microscale devices that process and manipulate extremely low (10−9 to 10−18 L) amounts of fluids in microchannels with dimensions of tens of micrometers . Conventional macroscale experimental technologies meet difficulties to deal with such low amounts of fluids, impeding their development in various fields. Conversely, microfluidic technologies begin to address numerous tough challenges, because fluid phenomena at the microscale are dramatically different from those at the macroscale . For instance, capillary forces and surface tension are more dominant than gravitational forces , allowing for passively pumping fluids in opposition to gravity . Flows at the microscale are laminar instead of turbulent, resulting in more predictable liquid handling and diffusion kinetics . Based on the different phenomena behaving at the microscale, microfluidic technologies offer a sensitive, predictable, and controllable avenue for bioanalysis .
Despite all the attractive capacities of LOC/microfluidics devices that have enabled the widespread implementation of microchip-based systems in biology and life science , microfluidic technologies often only improve the performance of existing macroscale assays or provide equivalent alternatives . Conversely, they have not reached their full potential due to the lack of essentially new capacities . In recent years, however, LOC/microfluidics technologies begin to address some problems that have not yet been solved by current laboratory benchtop methods. An excellent example can be found in wearable/ambulatory healthcare monitoring and sports analytics harnessing skin-interfaced wearable biosensors . Although this field is still in its infancy, the fundamentals of it are exceptionally strong: in the past decade, the wearable LOC devices gradually integrated with well-established techniques, including biocompatible materials , flexible electronics , optical/electrochemical sensors , microfluidics , near-field communications (NFC) , pain-free microneedles , as well as big data and cloud computing .
These above-mentioned enabling techniques establish the foundations for a new generation of wearable biosensors that directly interfaced with the human epidermis instead of rigid packages embedded in wrist straps or bands . The distinguishing characteristics of the emerging wearable biosensors, lightweight, flexibility, and portability , have made them especially suitable for point-of-care testing (POCT). Therefore, brand-new wearable biosensors capable of real-time physiological monitoring quickly emerge, as shown in Figure 1. However, these wearable biosensors are mainly designed for health monitoring , especially, some of them are only developed to measure the physical strain/stress bending change . Although many wearable devices have been deployed in sports, they are used to monitoring biophysical markers , such as movement  and cardiovascular information (e.g., blood oxygenation) .
Figure 1. Representative examples of wearable biosensors for both healthcare and sports monitoring. (a) Contact lens sensors in ocular diagnostics . Copyright 2015, Wiley. (b) Google glass for immunochromatographic diagnostic test analysis . Copyright 2014, American Chemical Society. (c) A wearable microsensor array for multiplexed heavy metal monitoring . Copyright 2016, American Chemical Society. (d) A hybrid sensor for simultaneous electrochemical, colorimetric, and volumetric analysis of sweat . Copyright 2019, American Association for Advancement of Science. (e) A wearable sensor for autonomous sweat extraction and analysis . Copyright 2017, National Academy of Sciences of United States of America (NAS). (f) A microfluidic device for colorimetric sensing of sweat . Copyright 2018, American Association for Advancement of Science. (g) Binodal, wireless epidermal electronic systems with in-sensor analytics for neonatal intensive care . Copyright 2019, American Association for Advancement of Science. (h) Wearable textile-based self-powered sensors . Copyright 2016, Royal Society of Chemistry. (i) A microfluidic system for real-time tracking of sweat loss and electrolyte composition . Copyright 2018, Wiley. (j) A microfluidic system for colorimetric analysis of sweat biomarkers and temperature . Copyright 2019, American Chemical Society. (k) A smartwatch for continuous sweat glucose monitoring . Copyright 2019, American Chemical Society. (l) A miniaturized battery-free wireless sensor for wearable pulse oximetry . Copyright 2017, Wiley. (m) An epidermal stimulation and sensing platform for sensorimotor prosthetic control, management of lower back exertion, and electrical muscle activation . Copyright 2016, Wiley. (n) A wearable electrochemical sensor for noninvasive simultaneous monitoring of Ca2+ and pH . Copyright 2016, American Chemical Society. (o) A wearable salivary uric acid mouthguard sensor . Copyright 2015, Elsevier. (p) A microfluidic system for time-sequenced discrete sampling and chloride analysis . Copyright 2018, Wiley. (q) Skin-mounted microfluidic networks for chrono-sampling of sweat . Copyright 2017, Wiley.