Review
Non-invasive wearable electrochemical sensors: a review

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Highlights

  • Major advances in the development of wearable electrochemical sensors and biosensors.

  • Non-invasive monitoring of chemical constituents in sweat, tears, or saliva.

  • Monitoring of wearer's health or fitness.

Wearable sensors have garnered considerable recent interest owing to their tremendous promise for a plethora of applications. Yet the absence of reliable non-invasive chemical sensors has greatly hindered progress in the area of on-body sensing. Electrochemical sensors offer considerable promise as wearable chemical sensors that are suitable for diverse applications owing to their high performance, inherent miniaturization, and low cost. A wide range of wearable electrochemical sensors and biosensors has been developed for real-time non-invasive monitoring of electrolytes and metabolites in sweat, tears, or saliva as indicators of a wearer's health status. With continued innovation and attention to key challenges, such non-invasive electrochemical sensors and biosensors are expected to open up new exciting avenues in the field of wearable wireless sensing devices and body-sensor networks, and thus find considerable use in a wide range of personal health-care monitoring applications, as well as in sport and military applications.

Section snippets

Why continuous non-invasive chemical sensing?

Chemical sensors and biosensors have been widely used as attractive alternatives to the bulky, expensive, and complex analytical instruments used in the health-care sector [1] (Box 1). Over decades, several of these devices have been developed for detecting vital analytes using optical, piezoelectric, and electrochemical transducers. Of these, electrochemical sensors have gained a dominating role in clinical diagnostics owing to their high performance, portability, simplicity, and low cost [2].

Wearable non-invasive electrochemical biosensors

Similar to their in vitro counterparts, wearable non-invasive electrochemical sensors can detect target analytes in tears, saliva, sweat, and skin interstitial fluid. Researchers have recently made considerable efforts to develop wearable chemical sensors that can conveniently monitor these biofluids (Table 1).

Saliva-based sensors

Saliva is a complex biofluid comprising numerous constituents permeating from blood via transcellular or paracellular paths. Hence, sialochemistry offers an excellent non-invasive alternative to blood analysis for monitoring emotional, hormonal, nutritional, and metabolic state of the human body [6]. Saliva is also readily available compared to blood and requires fewer pretreatment steps. These virtues of saliva have attracted the attention of several researchers to develop portable in vitro

Tear-based sensors

Tears are a complex extracellular fluid containing proteins/peptides, electrolytes, lipids, and metabolites from lacrimal glands, ocular surface epithelial cells, Meibiomian glands, goblet cells, and blood. Because blood is one of the sources of these constituents, tears can be used as an attractive fluid for non-invasive monitoring. This is especially true for continuous diabetes management due to a correlation between glucose levels in tears and blood [14]. Conventional bench-top instruments

Sweat-based sensors

Human sweat contains abundant information about a person's health status and thus is an excellent biofluid for non-invasive chemo-sensing [31]. For example, sodium, lactate, ammonium, and calcium levels in sweat are indicators of electrolyte imbalance [32] and cystic fibrosis (CF) [33], physical stress [34], osteoporosis [35], and bone mineral loss [36], respectively. Continuous detection of the above mentioned analytes is highly desired for optimal physiological balance. Sweat has also been

Skin interstitial fluid-based sensors

A rich variety of vital information can be obtained from skin interstitial fluid (ISF) in a completely non-invasive and continuous fashion. Over the past three decades, researchers have used the ISF for non-invasive detection of inherited metabolic diseases [57], organ failure [58], and drug efficacy [59]. However, most of the activity in this field has focused on non-invasive glucose sensors in connection to efficient diabetes management [60]. The correlation between ISF and blood glucose has

Challenges in implementing wearable sensors

There are still several challenges that need to be addressed before the realization of wearable non-invasive chemical sensors. Particular attention should be given to technological issues such as the resiliency of these devices, their long-term stability, and their biocompatibility. For example, epidermal sensors face complex mechanical deformations during normal bodily movements. The extent of such deformations increases when the wearer is performing intense physical activity. Additional

Concluding remarks and future perspectives

It is expected that with attention to key challenges, such as those discussed above, wearable electrochemical sensors will bring many exciting opportunities for continuously monitoring the human body across a broad range of biomedical and fitness applications. With the entry of big multinational companies, such as Google, and several smaller companies, such as NovioSense, OrSense, and Electrozyme, the nascent field of wearable, non-invasive electrochemical sensors is expected to grow rapidly,

Acknowledgments

The authors would like to acknowledge the support of the US National Science Foundation (Awards CBET-1066531).

Glossary

Amperometric sensors
devices that measure the current produced during the oxidation or reduction of an electroactive species at a constant applied potential. This current is proportional to the concentration of the electroactive product.
Biocompatibility
is a condition of being harmless to living tissue or a living system by not being toxic or injurious and not causing immunological rejection.
Biofouling
the accumulation and growth of undesired biomaterials on a surface.
Biofluid
a biological fluid.

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