What Are Sweat Biosensors?
Sweat biosensors are devices that monitor a given physiological quantity in sweat, often being worn on the arm or the back shoulder as these locations experience high sweat production compared to the rest of the body. Oftentimes, sweat biosensors are designed for analyzing biomarkers such as glucose, lactate, or sodium, providing information on glycemic content, muscle activity, or hydration.
Sweat biosensors have seen increased interest in the medical device industry and in academia as they present a unique opportunity for non-invasively monitoring important biomarkers for health and fitness applications as well as for medical diagnosis and treatment.
Researchers at Northwestern University Center for Bio-Integrated Electronics, led by Professor John Rogers, are among those championing the investigation of wearable sweat biosensors. One of their recent publications titled “Battery-free, skin-interfaced microfluidic/electronic systems for simultaneous electrochemical, colorimetric, and volumetric analysis of sweat” highlights their efforts.
Figure showing different facets of the sweat biosensor particularly highlighting its multilayered construction as well as its battery-less operation. Image from the Rogers Research Group
Voltage Buffer for Potentiometric Glucose and Lactate Sensors
The analog front-end is fairly simple requiring only a simple voltage follower with integrated RF filters. The glucose and lactate sensors are potentiometric sensors, meaning they output a small voltage proportional to the concentration of glucose or lactate present in the sweat sample being analyzed. This same operation is the basis of common laboratory pH probes and requires fairly simple hardware, consisting of a low-noise voltage buffer, to implement.
Schematic of glucose and lactate potentiometric sensors. Each signal is buffered and filtered before digitizing to remove extraneous noise. Image content recreated from Science Advances.
The research group included a capacitor in the feedback network of the follower circuit to decrease bandwidth and reduce noise. The signal is then read by a 14-bit analog-to-digital converter.
Wireless Power through NFC
One of the key features highlighted by the research team is the battery-less operation of their device. Instead of powering the device with a standard primary or secondary cell battery, the Rogers Research Group instead chose to employ a wireless powering scheme craftily leveraging NFC (near-field communication) for both power and communication.
This technique is famously utilized in consumer NFC tags for short-range wireless communication between the tag and an NFC-enabled smart device. Even though NFC has not completely taken over the consumer electronics industry like many previously predicted, NFC is finding growing use in enabling contactless payment using mobile phones and smartwatches.
NFC provides a very modest amount of power, so it is necessary that the circuit operates at extremely low power. They utilized the RF430FRL152H sensor transponder from Texas Instruments which is designed for operating on a small battery or, more interestingly, on a magnetic field.
The functional block diagram of the Texas Instruments RF430FRL152H. Image from the RF430FRL152H datasheet.
The RF430FRL152H has an incredibly low operating voltage of 1.45 V and is designed to handle the unregulated, variable power provided by an intermittent magnetic field.
The RF430FRL152H includes Texas Instruments’ popular low power MSP430 microcontroller architecture, which boasts one of the lowest operating voltages in the industry. The Rogers group mention buffering the sensor signals with the ADA4505-2, a small footprint, zero-crossover, low noise, low operating voltage amplifier. Minimal footprint is critical for ensuring the sensor remains inconspicuous when worn on the body. Zero-crossover and low noise are necessary to minimize distortion since the signals from the glucose and lactate sensors have a very small dynamic range and are not amplified (because an amplifier, rather than a unity-gain buffer, would require additional passive components).
Data showing sensor glucose and lactate measurements from the sensor compared to traditional laboratory techniques. Image from the Rogers Research Group
Some In-House Fabrication
The NFC antenna fabrication was performed in-house using photolithography to pattern conductive traces onto a flexible printed circuit board material (DuPont Pyralux AP8535R) separated by a layer of polyimide and encapsulated by a silicone material for waterproofing. Developing the NFC antenna in-house presents the research group with the greatest flexibility in quality control resulting in excellent antenna performance with high Q, even at modestly high bend radii.
Early Feasibility Studies
The researchers demonstrated their device’s ability to measure different biomarkers while powering the device using an NFC-enabled smartphone as well as from a custom designed large-scale antenna for continuous monitoring while riding a stationary bike. The results are, of course, preliminary, but are promising nonetheless.
Sweat biosensors are notorious for baseline drift, variability due to temperature, and variability due to corroding of the sensing elements leading to lower signal-to-noise ratio over time with increasing wear. Though this paper does not address these issues specifically, other concerns such as the hypoallergenic adhesive and the miniaturization of the analog front-end presented interesting solutions to very challenging issues facing sweat biosensors.
The Case for Lower Power and Miniaturization
This paper demonstrates the need for miniaturization and for more flexible powering options in order to develop advanced biomedical technology as well as the need for wafer scale integrated circuits for reducing device footprint. Miniaturization and decreased power consumption are particularly critical for wearable sensors as such sensors rely on being discreet and long-lasting. This paper also presents key opportunities for industry-academia relations that would streamline the development of new medical technology by leveraging each partner’s particular specialties.
Semiconductor companies can provide chipsets with advanced feature sets and low power, fabrication houses can provide academics tape out space for turning their prototypes into custom integrated circuits, and academia can provide the testing centers for the high-risk work that companies would like to explore.