Skin-Based Sensing and Energy Harvesting: Flexible and Non-Invasive Technologies

What you’ll learn:

Why skin-based sensing and energy harvesting is an attractive research topic.
How a device harvests power, while also assessing and reporting on biomarker data.
How a very flexible, puncture- and abuse-tolerant thermoelectric device for harvesting was designed and optimized.

Using a person’s skin for non-invasive bio-data sensing as well as energy harvesting makes sense: It’s available, accessible, risk-free, and likely to receive lots of attention (which leads to future grants). Even if the project isn’t revolutionary or a breakthrough—and let’s face it, they rarely are—innovative aspects still could be further exploited or leveraged. Two recent examples clearly show this situation.

Fingertip Biofuel Cells: Simultaneous Biomarker Assessment and Energy Harvesting

Engineers at the Department of Chemical and Nano Engineering at the University of California, 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).

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Their design uses a single wrap for two functions: bio-assessment and energy harvesting, both 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 and can reliably produce 100X to 1,000X more sweat than most other areas of the body, 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 (Fig. 1).

1. Principle and design of integrated fingertip-wearable microgrid: (a) Schematic of the fingertip-wearable microgrid system, which includes BFCs, AgCl-Zn batteries, flexible printed circuit board (fPCB), and wearable sensors with an osmotic sweat extraction assisted paper fluidic system. The combination of biofuel cells (BFCs) and AgCl-Zn batteries is constructed as an energy module, which comprises a serial connection of two AgCl-Zn batteries, each being charged by two serial-connected BFCs. The inset (in the red circle) is zoomed in on the components interfaced with the fingertip, which includes four BFCs and a center osmotic pumping system for sweat extraction: (i) main schematic, (ii) ventral side and (iii) dorsal side. (b) Schematic of the fingertip-mounted energy microgrid working principle for energy harvesting, energy storage, and electrochemical sensing with wireless data transition and smartphone display. Fingertip perspiration provides biofuel and biomarkers for passive energy harvesting and continuous sensing, respectively. The powered MCU supports four sensors for metabolic monitoring, dietary supplements, and drugs monitoring, which are associated with various physiological and metabolic conditions. (c) Optical images of the fingertip-wearable microgrid. Outlooks of the expanded structure (i) before folding, (ii) after folding and connecting the BFC-battery to the fPCB, and (iii) after inclusion of a paper fluidic channel and sensors. Scale bars, 1 cm.

The bio-based fuel cells were 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 (Fig. 2).

2. Optical images of the layer-by-layer printing of the wearable fingertip microgrid. Scale bar: 1 cm. (sAg: stretchable Ag)

For the sensing function, sweat is wicked through a microgrid based on tiny paper microfluidic channels to enzymatic biofuel cells, where the device analyzes the biomarker levels (Fig. 3). 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.

3. Photo of laser-engraved paper microfluidic channel.

They performed tests under a variety of operating conditions with associated biomarker results (Fig. 4).

4. Sensor operation with osmotically withdrawn sweat: (a) (i) Schematic highlighting the hydrogel interfacing the skin to facilitate sweat withdrawal with biomarkers, and the (ii) orientation of the osmotic platform with multiple sensor arrays on the fingertip. (b) Optical image of sweat flow in the paper channel with the interfaced enzymatic sensors, and mechanical properties of sensors under stretching and twisting. Scale bar, 5 mm. (c) Dye flow rate on the finger using different hydrogel variants. (d) Schematic illustration of biomarker sensing with related daily activity. (e) Schematic of (i) potentiometric sensors of vitamin C and l-dopa and (ii) enzymic sensors of glucose and lactate. (f) Plots showing in vitro sensor performance with an inset of the calibration curve for (i) vitamin C, (ii) l-dopa, (iii) lactate, and (iv) glucose detection in sweat. (g) On-finger response of the vitamin C sensor after pill intake at t = 0 min from two participants. (h) On-finger response of the l-dopa sensor and blood l-dopa chronoamperometry signal from two participants after fava beans consumption. (i) On-finger response of the lactate sensor measurements at rest using (i) a completely sedentary participant and (ii) on the same participant after 30 min of outdoor walking. (j) On-finger response of the glucose sensor and measured blood lactate concentration after a meal intake at t = 0 min from two participants. BG, blood glucose concentration.

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

Skin-Heat Harvesting: Designing Stretchable Thermoelectric Devices

Harvesting of skin/ambient temperature differential isn’t a new development—it’s been done with various thermoelectric-generator materials and construction specifics. Now, a team based at the University of Washington has developed a skin-based thermoelectric device (TED) that’s notable for its high levels of 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.

To provide stretchability, conductivity, and electrical self-healing, the semiconductors are connected via printed liquid-metal traces. In addition, 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 (Fig. 5). Except for the semiconductors, everything was designed and developed as part of this project.

5. High‐performance, stretchable thermoelectric device (TED) with 3D soft architectures. (a) Schematic illustration of the device design and its multifunctional layers. (b) Images of the TED when it’s bent by a finger (left) and worn on the wrist (right). (c) Comparison of stretchable TED tensile test specimens with respect to strain at mechanical failure and output voltage at equilibrium. (d) Scattered plot for comparison of soft and stretchable TEDs. (e) A wearable thermoelectric generator is worn on the upper arm with enhanced energy harvesting during running.

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 both simulated and real-sample tests of the effect of different material combinations as well as “infill” ratios on thermoelectric energy conversion and structural integrity.

This was achieved by 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, thereby resulting in higher output voltage and improved heating and cooling performance (Fig. 6).

6. Electromechanical response and self‐healing: (a) Photograph of folded TED with air pockets. (b) Maximum strain at mechanical failure and (c) measured stiffness of four different designs under uniaxial tension. (d) Measured internal resistances of four different TE tensile specimens over 2,000 cycles of 0% to 50% uniaxial strain. (e) Measured open circuit voltages of air pocket TE tensile specimen under 0% to 50% uniaxial strain at the temperature gradient of 20°C. (Inset: schematics of TED cross‐section when uniaxially strained.) (f) Measured internal resistances of air pocket TE tensile specimen with electrical disconnection at 230% strain on average. (g) A photograph of the TE specimen after being punctured by a pushpin (top) and the schematic of electrical self‐healing (bottom). (h) Measured internal resistance and open‐circuit voltage responses at peak and thermal equilibrium of the TED tensile specimen before and after being pierced by a push pin. (i) Measured internal resistances before and after being punctured for over 2,000 stretching cycles of 50% uniaxial strain. Inset: IR images of active heating (left) and cooling (right) before and after damage.

Furthermore, these thermoelectric generators showed remarkable damage tolerance, remaining fully functional even after multiple punctures and 2,000 stretching cycles at 50% strain. These factors were evaluated using a universal load frame to obtain the stress-strain data, under ASTM D638-14 Standard Test Method (Fig. 7).

7. 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 pocket TE tensile specimens. (d) A stress-strain plot of composite TE tensile specimens. (e) A stress-strain plot of elastomer TE tensile specimens.

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