Researchers in the US have developed a graphene-based wearable sensor that can continuously detect a wide range of chemicals in sweat without wearing out.
The team at Caltech’s Cherng Department of Medical Engineering developed the wearable electrochemical biosensor for the continuous analysis of trace levels of multiple metabolites and nutrients, including all essential amino acids and vitamins. This is key for health monitoring, as many of the target molecules are undetectable continuously by any existing wearable technology (see video below).
The ‘Nutritrek’ biosensor consists of laser-engraved graphene (LEG) electrodes with a molecularly imprinted polymer (MIP) that can be repeatedly regenerated in situ.
The key is this imprinted polymer. Previous versions of sweat sensors relied on enzymes embedded within them to detect a limited number of relevant compounds. While antibodies could be used in sensors to detect more compounds at low concentrations, that technique had a big weakness: antibodies in the sensor can only be used once, meaning the sensors will wear out.
“We’ve done wearable sweat sensors before,” said Wei Gao, assistant professor of medical engineering. “There were so many biomarkers we wanted to detect, but in the past we could not. There was no good way.”
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The imprinting process creates a shape specific to a molecule for sensing. For example, to detect the amino acid glutamine, the polymer is prepared with the glutamine molecules inside. Then, through a chemical process, the glutamine is removed to leave a polymer with holes in it that are the exact shape of molecule.
This polymer is them combined with a material that can be oxidized or reduced under an applied electrical voltage when in contact with human sweat. As glutamine molecules come into contact with the polymer, they fit into the holes. This means less sweat is able to contact the inner electrode and the electrical signal becomes weaker. By monitoring that electrical signal, the researchers can then deduce how much glutamine is present in the sweat. More glutamine means a weaker signal. Less glutamine results in a stronger signal.
“This unique strategy allows us to detect all of the nine essential amino acids and multiple vitamins,” said Gao. “We can do them all continuously.”
Sensor in use
Unlike enzyme sensors, the polymer can be easily reset using a weak electrical signal that destroys the target molecule to empty the hole.
This sensor is combined with microfluidics to allow the sensor to operate when even a miniscule amount of sweat is present, reducing the amount of current that is needed in the skin to stimulate sweating..
“This microfluidic design allows us to use very small currents,” he said. “We can stimulate four to five hours of sweat from several minutes of stimulation with tens of microamps.”
The system includes the wearable electronic patch with a wireless link to smart watch that can induce sweat via iontophoresis and monitor sweat via electrochemical methods. The sweat induction and the sweat sensing procedures are initiated and controlled by the microcontroller, a STM32L432KC from STMicroelectronics, when it receives a user command from the Bluetooth module over a UART link.
Programmable iontophoretic current is generated by a voltage-controlled current source that consists of a unity-gain difference amplifier (AD8276, Analog Devices) and a boost transistor (BC846, ON Semiconductor).
The circuit is supplied by the output of a boost converter (LMR64010) that boosts the 3.7 V battery voltage to 36 V. The microcontroller controls the digital-to-analogue converter (DAC) (DAC8552, Texas Instruments) over a serial peripheral interface to set the control voltage of the current source.
The current source output is checked by a comparator (TS391, STMicroelectronics), and the microcontroller is interrupted through its general-purpose input/output pin at output failure. The protection circuit consists of a current limiter (MMBF5457, ON Semiconductor) and analogue switches (MAX4715, Maxim Integrated; ADG5401, Analog Devices).
The microcontroller’s general-purpose input/output is also used to enable or disable the iontophoresis circuit. For the optimized design, a 100-µA current (~2.6 µA mm−2) was applied for on-body iontophoresis sweat induction using the flexible microfluidic patch.
When powered at 3.3 V, the electronic system consumes ~28 mA during an active electrochemical measurement and ~61 mA during iontophoresis. The microcontroller and Bluetooth module each consume ~12 mA; the sensor interface consumes ~4 mA; the boost converter and iontophoresis module consumes ~33 mA; and the display module consumes an additional ~8 mA when refreshing its screen.
The sensor can perform two-channel simultaneous sensing, as well as potentiometric and temperature measurements. A bipotentiostat circuit is constructed by a control amplifier (AD8605) and two transimpedance amplifiers (AD8606). A series voltage reference (ISL60002, Renesas Electronics) and a DAC (DAC8552, Texas Instruments) is used to generate a dynamic potential bias across the reference and working electrodes.
An instrumentation amplifier (INA333, Texas Instruments) is used for potentiometric measurements, and a voltage divider is used for the resistive temperature sensor. All analogue voltage signals are acquired by the microcontroller’s built-in analogue-to-digital converter (ADC) channels, processed and then transmitted over Bluetooth to a user device.
Custom mobile application design
The custom mobile application was developed with the cross-platform Flutter framework. The mobile application can wirelessly communicate with the wearable devices via Bluetooth to send commands, and to acquire, process and visualize the sweat biomarker levels. T
he application establishes a secure Bluetooth connection to the wearable sensor and plots the user’s historical biomarker levels, highlighting the most recently measured analyte concentrations. When a sweat biomarker measurement is prompted, the user can switch over to the measurement page that plots the data in real time. The app extracts the peak currents using a custom baseline correction algorithm, then converts the peak currents to corresponding biomarker concentrations. These measurement data are added to the list of historic analyte levels on the home page.
In volunteers, the biosensor enabled the real-time monitoring of the intake of amino acids and their levels during physical exercise, as well as the assessment of the risk of metabolic syndrome by correlating amino acid levels in serum and sweat.
“This approach allows us to detect a bunch of new crucial nutrients and metabolites. We can monitor when we eat and watch nutrient levels change,” he said. “It not only monitors nutrients, but also hormones and drugs. It can provide continuous monitoring for many health conditions.”
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