Wearable sensors are uniquely placed to fill the technology gap for real-time analytics at the point of need. Seamless integration of chemical sensors into wearable platforms gives the power of laboratory-based chemical analyses directly on the wearer's body. Several biosensors, based primarily on enzyme electrodes, have been incorporated recently into cutting-edge wearable devices to allow non-invasive sensing of lactate, glucose, or alcohol in sweat; uric acid in saliva; and glucose in tears. While the majority of these wearable sensor systems has focused on fitness and healthcare applications, there are growing demands for developing wearable sensor platforms for monitoring hazardous chemicals for diverse security and environmental applications.
Organophosphate (OP) nerve agents, including sarin, are highly toxic and can prevent the nervous system from working properly. OP pesticides are far less potent, but work in a similar way and can cause illness in people who are exposed to them, according to the U.S. Centers for Disease Control and Prevention. Detecting either type of these sets of compounds accurately and quickly could help improve both defense and food security measures.
The increasing use of chemical warfare agents (CWAs) represents a major security challenge; in particular, OP nerve agents represent a serious concern, as they can be weaponized for utility as CWAs, and are routinely used as pesticides in agricultural and domestic settings. These OP neurotoxins severely affect the nervous system and lead to rapid death. Due to the high toxicity of OP nerve agents and pesticides, there are urgent demands for reliable wearable sensor systems for their rapid and selective on-site analyses.
The Lab-on-a-Glove Solution
A glove-embedded printable biosensor system was developed by researchers at the University of Southern California, San Diego (UCSD) that withstands extreme mechanical deformations, and is used for detecting different OP nerve agent compounds. The new glove-based biosensor system brings the analysis of OP compounds directly to the user's fingertips. Disposable polymer gloves, in combination with screen-printable sensors, can provide a scalable, low-cost, and flexible platform to realize wearable point-of-use electrochemical screening tools.
The glove biosensor system was fabricated using large-scale, low-cost screen-printing technology (Figure 1). The stencil is positioned on top of the nitrile glove surface inserted with a planar 3D-printed mold, then a rubber squeegee is displaced, dragging the stress-enduring inks and filling the designed hole patterns of the stencil. The fabrication of this glove biosensor requires two glove fingers: the index finger, termed the “sensing finger” containing the three-electrode biosensor, and the thumb, termed the “collection finger” used for sampling the threat residues. The sensing finger is based on three different layers of elastic inks printed on the nitrile glove surface: a silver layer combined with elastomeric Ecoflex material, a flexible layer of carbon ink modified with an elastomeric styrene-isoprene copolymer that imparts intrinsic stretchability, and a top layer of transparent, flexible, stretchable insulator that covers the serpentine connections while exposing the sensing area and square contact pads.
How it Works
The new wearable, flexible glove biosensor carries out the sampling and electrochemical biosensing steps on different fingers, with the thumb used for collecting the nerve agent residues, and an enzyme immobilized on the index finger.
The on-glove assay consists of two steps: the swipe and scan (Figure 2). The scan step is the electrochemical sensing, carried out by joining the index (sensing) and thumb (collector) fingers. A conductive semisolid gel on the sensing area of the index finger provides a medium for analyte diffusion from the collection pad toward the OPH enzyme layer on the working electrode, as well as the conductivity essential for completing the electrochemical cell. Finally, the electrochemical detection of the p-nitrophenol (an OP hydrolysis product) is recorded by SWV using a wearable electrochemical analyzer. The connections to the square contact pads are made using an adjustable ring bandage for easy reusability and inter-user adjustment. The resulting voltammetric response is wirelessly transmitted to a smartphone via the built-in wireless communication feature of the handheld analyzer.
To impart the high resilience against extreme mechanical deformations expected during the glove sampling/sensing operation, a combination of stress-enduring inks and serpentine micro-structures was used. The glove biosensor platform utilizes printed serpentine patterns, with two levels of stretchability due to the intrinsic stretchability of tailor-made printable inks and the unwinding of the serpentine structure. Careful attention was given to the elastomeric, electrical, and electrochemical properties of the new inks and the printed patterns to withstand large strain deformations. Dynamic mechanical deformation, bending, and stretching studies have demonstrated the high resilience against extreme mechanical deformations expected in real-life applications of the biosensing glove. Such compliance with the complex glove surface is achieved while retaining the attractive electrical and electrochemical properties of the printed electrodes.
Testing showed that the glove could detect organophosphate pesticides methyl parathion and methyl paraoxon on various surfaces — including glass, wood, and plastic — and on produce. The researchers say the sensor could be used in both security and food safety settings. The “on-hand” detection of different OP chemical agents on a variety of surfaces demonstrates that the glove biosensor holds considerable promise for real-time, onsite screening of chemical threats for military, forensic, consumer protection, and food safety applications.
The research was done by Jospeh Wang, Rupesh K. Mishra, Lee J. Hubble, Aida Martin, Rajan Kumar, Abbas Barfidokht, and Jayoung Kim of UCSD; and Mustafa Musameh and Ilias L. Kyratzis of CSIRO Manufacturing, Australia.