Wearable bioelectronic skin patches are a busy research area: part 3

Biomedical skin patches integrating multiple sensing, power, and reporting disciplines are receiving significant R&D effort.

This final part concludes with an exposition of three more wearable biometric patches, a technology receiving considerable research attention for many obvious and not-so-obvious reasons.

#7 Sensing stress levels

Employing a somewhat different approach, a group at the California Institute of Technology (Caltech) has developed a multimodal sensor that targets assessing an individual’s stress levels. Their small, thin, adhesive sensor is worn on the wrist and dubbed CARES (consolidated artificial-intelligence-reinforced electronic skin). It allows users to participate in all normal daily activities with minimal interference during testing, which allows for measuring both baseline and acute stress levels. It uses sweat to identify and measure physiological conditions, as shown in Figure 1.

Figure 1. This microfluidic sweat sensor samples nine distinct parameters, which are then used in conjunction with machine learning algorithms to assess the wearer’s stress level. (Image: California Institute of Technology)

This patch monitors three vital signs (pulse waveform, galvanic skin response, and skin temperature) and six molecular biomarkers in human sweat (glucose, lactate, uric acid, sodium ions, potassium ions, and ammonium) using microfluidic sampling on a wearable sweat sensor. Machine-learning (ML) algorithms complete the stress analysis.

Previous materials used for sweat sensors could be efficiently produced using inkjet printing and could accurately measure even very scarce compounds. However, those materials gradually broke down in the presence of bodily fluids. The Caltech team added a nickel-based compound that helps to stabilize the enzymatic-based sensors, such as those that detect lactate or glucose, as does a new polymer added to the ion-based sensors, which detect biomarkers like sodium or potassium.

To develop the ML algorithms, they performed experiments to induce stress in subjects wearing the CARES device, demonstrating that the sensor accurately measures the interrelatedness of physiological (such as pulse) and chemical (such as glucose) biomarkers. Subjects also answered questionnaires to self-report their feelings of anxiety and psychological stress before and after exposure to stressful situations like vigorous exercise or intense video gameplay.

Using these machine learning algorithms, the researchers maintain that the platform can differentiate among the stressors with an accuracy of 98.0% and quantify psychological stress responses with a statistical (if not actual) confidence level of 98.7%. You can read about their work in their paper “A physicochemical-sensing electronic skin for stress response monitoring,” published in Nature.

#8 Biosensing and energy harvesting

Engineers at the Department of Chemical and Nano Engineering at the University of California at San Diego have developed an electronic finger wrap that monitors vital chemical levels—such as glucose, vitamin C, lactate and levodopa, a drug used for treating Parkinson’s disease — present in the same fingertip sweat from which it derives its energy (the device can be customized to detect different sets of biomarkers).

Their design uses a single wrap for bio-assessment and energy harvesting based on fingertip sweat. The researchers noted that fingertips are among the body’s most prolific sweat producers, each packed with over a thousand sweat glands. They can reliably produce 100 to 1000 times more sweat than most other body areas, even during rest. This constant trickle of natural perspiration — without any stimuli or physical activity — offers a reliable energy source, fueling the device even during periods of inactivity or sleep, as seen in Figure 2.

Figure 2. Design and principle of a fingertip-wearable microgrid. (a) Schematic of the system integrating BFCs, AgCl-Zn batteries, fPCB, sensors, and an osmotic paper fluidic sweat extraction system. The inset shows a fingertip interface with four BFCs and a central osmotic pump. (b) Working principle for energy harvesting, storage, sensing, and wireless data transmission. Fingertip sweat supplies biofuel and biomarkers for continuous monitoring. (c) Optical images of the device before folding, after folding and connecting components, and with sensors integrated. Scale bars, 1 cm. (Image: University of California San Diego via ResearchGate)

The bio-based fuel cells have been engineered to efficiently collect and convert chemicals in sweat into electricity, which is stored in a pair of stretchable, silver chloride-zinc (AgCl-Zn) batteries. The entire package is fabricated using a layer-by-layer printing process.

For the sensing function, sweat is wicked through a microgrid based on tiny paper microfluidic channels with enzymatic biofuel cells where the device analyzes the biomarker levels. Finally, a small chip processes signals from the sensors and wirelessly transmits the data via Bluetooth low energy to a custom-designed smartphone or laptop application.

They performed tests under various operating conditions with associated bio-maker results, as shown in Figure 3.

skin patches — Figure 3. Sensor operation with osmotically withdrawn sweat. (a) Schematic of hydrogel–skin interface for sweat extraction and osmotic sensor array on fingertip. (b) Optical image of sweat flow with enzymatic sensors and sensor mechanics under strain. (c) Dye flow with different hydrogels. (d) Biomarker sensing during daily activities. (e) Schematics of potentiometric (vitamin C, l-dopa) and enzymatic (glucose, lactate) sensors. (f) In vitro sensor performance with calibration curves. (g–j) On-finger sensor responses to vitamin C, l-dopa, lactate, and glucose following specific activities or intake. BG, blood glucose. (Image: University of California San Diego via ResearchGate)

The work is detailed in their Nature Electronics paper “A fingertip-wearable microgrid system for autonomous energy management and metabolic monitoring”; there’s also a supplement that provides fabrication and test details along with 42 figures, including a complete schematic diagram of the circuit, PC-board photos, and more.

#9 Thermoelectric device energy harvesting

Harvesting skin/ambient temperature differential is not new; it has been done with various thermoelectric generator materials and construction specifics. Now, a team at the University of Washington has developed a skin-based, thermoelectric device (TED) notable for its high flexibility, stretchability, and puncture tolerance.

Their TED has three main layers. At the center are rigid thermoelectric semiconductors that convert heat to electricity. These semiconductors are surrounded by 3D-printed composites with low thermal conductivity, which enhances energy conversion and reduces the device’s weight.

The semiconductors are connected with printed liquid metal traces to provide stretchability, conductivity, and electrical self-healing. Additionally, liquid metal droplets are embedded in the outer layers to improve heat transfer to the semiconductors and maintain flexibility because the metal remains liquid at room temperature, as shown in Figure 4. Everything except the semiconductors was designed and developed as part of this project.

These TEDs are extremely stretchable, functioning at strain levels as high as 230%. Their unique design, veriﬁed through multiphysics simulations and actual tests, results in a fairly high power density of 115.4 μW/cm2 at a low temperature gradient of 10 °C. The project involved simulated and real-sample tests of the effect of different material combinations and “infill” ratios on thermoelectric energy conversion and structural integrity.

This was achieved through 3D printing multifunctional elastomers and examining the eﬀects of three thermal insulation inﬁll ratios (0%, 12%, and 100%). The engineered structure is lighter and eﬀectively maintains the temperature gradient across the thermoelectric semiconductors, resulting in higher output voltage and improved heating and cooling performance.

Furthermore, these thermoelectric generators showed remarkable damage tolerance, remaining fully functional even after multiple punctures and 2000 stretching cycles at 50% strain. These factors were evaluated using a universal load frame to obtain the stress-strain data, under the ASTM D638-14 Standard Test Method, shown in Figure 5.

Figure 5. Uniaxial tensile test of TE tensile specimens. (a) Photographs of TE tensile specimen with air pockets under 0%, 400%, and 600% strain without mechanical failure. (b) A stress-strain plot of Interlayer Gap TE tensile specimens. (c) A stress-strain plot of Air Pockets TE tensile specimens. (d) A stress-strain plot of Composite TE tensile specimens. (e) A stress-strain plot of Elastomer TE tensile specimens. (Image: University of Washington via Advanced Materials)

The project and the many variations they investigated as simulation and with real samples are presented in their paper “3D Soft Architectures for Stretchable Thermoelectric Wearables with Electrical Self-Healing and Damage Tolerance,” published in Advanced Materials. The paper also includes a 14-page Supporting Information file detailing fabrication, variations, test arrangements, and resultant data.

Summary

There’s a lot of innovative research in biomedical-focused electronic skin patches, incorporating and merging diverse electronic, mechanical, and material disciplines and technologies. These patches, both passive and active, address a wide variety of medical-sensing situations and actions. While they are all presently just laboratory devices and not in public use, perhaps in a few years, patients will be advised to wear short or roll-up sleeves when they visit a clinic or doctor.