Wearable bioelectronic skin patches are a busy research area: part 2

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

This part continues exposing the remaining 3 wearable biometric patches left out of part 1. Wearable patches are receiving considerable research attention for many obvious and not-so-obvious reasons.

#4 Smart bandage promotes and monitors healing

A smart bandage developed at the California Institute of Technology (Caltech) may make treatment of wounds that will not go away and become infected, and fester instead become easier, more effective, and less expensive. Unlike a typical bandage, which might only consist of layers of absorbent material, smart bandages are made from a flexible and stretchy polymer containing embedded electronics and medication. The electronics allow the sensor to monitor for the presence of molecules such as uric acid or lactate and conditions like pH level or temperature in the wound that may indicate inflammation or bacterial infection, as seen in Figure 1.

Figure 1. Wireless stretchable wearable system for chronic wound care. (a) Schematic of a soft patch on a diabetic foot ulcer for real-time monitoring and therapy. (b) The layered design includes a SEBS substrate, biosensor array, drug-releasing electrodes, and drug-loaded hydrogel. (c) Sensor layout with temperature, pH, NH₄⁺, glucose, lactate, uric acid sensors, and drug-release electrodes. (d–e) Images of the compact, flexible patch. (Scale bars, 1 cm.) (f–g) Miniaturized wireless electronics diagram and image (Scale bar, 1 cm). (h) Patch applied to a diabetic rat wound (Scale bar, 2 cm). (Image: California Institute of Technology via *Nature Communications*)

The bandage can respond in one of three ways: First, it can transmit the gathered data from the wound wirelessly to a nearby computer, tablet, or smartphone for review by the patient or a medical professional. Second, it can deliver an antibiotic or other medication stored within the bandage directly to the wound site to treat the inflammation and infection. Third, it can apply a low-level electrical field to the wound to stimulate tissue growth, resulting in faster healing.

The wearable patch monitors a panel of wound biomarkers, including temperature, pH, ammonium, glucose, lactate, and uric acid (UA), which were chosen because they are important in reflecting the infection, metabolic, and inflammatory status of chronic wounds.

It consists of a multimodal biosensor array for simultaneous and multiplexed electrochemical sensing of wound exudate biomarkers, a stimulus-responsive electroactive hydrogel loaded with a dual-function anti-inflammatory and antimicrobial peptide (AMP), and a pair of voltage-modulated electrodes for controlled drug release and electrical stimulation. The multiplexed sensor array patch is fabricated via standard microfabrication protocols on a sacrificial layer of copper followed by transfer printing onto a SEBS thermoplastic elastomer substrate.

The power-management circuitry consists of a magnetic reed switch and a voltage. The electrical stimulation and drug delivery circuitry used a voltage reference, op am square-wave generator circuit, and a switch array. The potentiometric, amperometric, and temperature sensor interface circuitry consists of a voltage buffer array, switch array, voltage divider, and electrochemical analog front-end. A programmable system on-chip Bluetooth Low Energy (BLE) module was used for data processing and wireless communication

The work is discussed in their paper “A stretchable wireless wearable bioelectronic system for multiplexed monitoring and combination treatment of infected chronic wounds” published in Nature Communications; the paper hard to follow due to its stilted, formal tone, and is heavily skewed towards the biochemistry and medical implication rather than the fabrication and electronics. However, the 28-page Supplementary Materials file provides more details on the device construction and includes a schematic diagram and bill of materials.

#5 Bioelectronic patches

Researchers at Cambridge University (UK) have developed a way of making high-performance bioelectronics that can be customized to a wide range of biological surfaces, from a fingertip to the fluffy seedhead of a dandelion, by printing them directly onto that surface. Their technique is inspired partly by spiders, who create sophisticated and strong web structures adapted to their environment using minimal material.

The researchers spun their bioelectronic ‘spider silk’ from PEDOT:PSS (a biocompatible, conducting polymer widely used in these projects), hyaluronic acid, and polyethylene oxide. The high-performance fibers were produced from a water-based solution at room temperature, which enabled the researchers to control the fibers’ ‘spinnability.’ The researchers then designed an orbital spinning approach to allow the fibers to morph to living surfaces, even down to microstructures such as fingerprints.

Despite its apparent fragility, the fiber fabrication process does not require a clean room, and the exposed fiber is mechanically stable and surprisingly rugged, as shown in Figure 2.

Figure 2. Performance and durability of fiber-based bioelectronic sensors. (a) Illustration and images showing fiber alignment along fingerprint ridges (scale bars: 5 mm, 500 μm). (b) Contact impedance decreases with a longer deposition time. (c) High-fidelity ECG signals from fiber vs. gel electrodes (P = 0.99). (d) (i) EMG signals detected from thumb muscle under varying loads; (ii) EMG amplitude increases with load. (e) Fiber sensors can be repaired via redeposition, restoring low impedance. (f) Exposed fiber electrodes maintain performance under wear, clicking, friction, and wet conditions. (g) Protective fibers enhance stability under wet friction. (h) Encapsulation enables function after multiple water-rinsing cycles. (Image: Cambridge University via *Nature Electronics*)

The tests they ran with volunteers (humans, of course) included electrocardiogram and pulse assessments, as seen in Figure 3.

Figure 3. (a) Augmented touch perception via dual-ECG sensing with person-(i) wearing bioelectronic fiber arrays and person-ii without. The dual-ECG signal acquired through the fiber array is compared with the reconstructed composite-ECG signal from validation gel electrodes. The red downward-facing and green upward-facing triangles indicate the R peaks of person-i and person-ii, respectively. (b) A breathable skin-gated OECT on a fingertip; the OECT displays a response time in the 60-sec range. (c) Dual-modal sensing for augmented perception of mist pulses with acidic, alkaline, and neutral compositions distinguished through colorimetric and electrical readouts. The mist pulse photographs show an example of a neutral mist pulse, and the fiber resistance change was recorded by applying three consecutive neutral mist pulses (the initial resistances of the fiber arrays are in the range of 10 kΩ). (Image: Cambridge University via Nature Electronics)

The work is detailed in their paper, “Sustainable and imperceptible augmentation of living structures with organic bioelectronic fibers,” published in Nature Electronics.

#6 Wearable ultrasound patch

At the University of California at San Diego, engineers have developed a wearable ultrasound patch that can offer continuous, non-invasive blood flow monitoring in the brain. The soft and stretchy patch can be comfortably worn on the temple to provide three-dimensional data on cerebral blood flow, which they claim is a first in wearable technology.

The current clinical standard is a Transcranial Doppler (TCD) ultrasound, which requires a trained technician to hold an ultrasound probe against a patient’s head. The process is operator-dependent so that the measurement accuracy can vary based on the operator’s skill, and it is also impractical for long-term use. In contrast, the UCSD wearable ultrasound patch offers a hands-free, consistent, and comfortable solution that can be worn continuously during a patient’s hospital stay.

The patch, roughly the size of a postage stamp, is constructed from a silicone elastomer embedded with several layers of stretchy electronics. One layer consists of an array of small piezoelectric transducers, which produce ultrasound waves when electrically stimulated and receive ultrasound waves reflected from the brain. Another key component is a copper mesh layer—made of spring-shaped wires—that enhances signal quality by minimizing interference from the wearer’s body and environment. The rest of the layers consist of stretchable electrodes, shown in Figure 4.

Figure 4. This two-dimensional array of ultrasonic piezoelectric transducers is placed on the patient’s scalp and is combined with significant post-processing to reveal specifics of blood flow in the brain, an important medical assessment consideration. (Image: University of California at San Diego)

The patch, roughly the size of a postage stamp, is constructed from a silicone elastomer embedded with several layers of stretchy electronics. One layer consists of an array of 2 MHz piezoelectric transducers, which produce ultrasound waves when electrically stimulated and receive ultrasound waves reflected from the brain. Another key component is a copper mesh layer—made of spring-shaped wires—that enhances signal quality by minimizing interference from the wearer’s body and environment. The rest of the layers consist of stretchable electrodes, seen in Figure 5.

Figure 5. Conformal ultrasound patch for brain blood flow monitoring. (a) Diagram of a wearable patch for transcranial Doppler (TCD) imaging, showing placement on the scalp to map cerebral arteries. The patch features a 16×16 piezoelectric transducer array with stretchable electrodes, copper shielding, and silicone encapsulation. (b) Simulated diverging and focused ultrasound fields using 2D beamforming. (c) Photos of the flexible patch conforming to curved surfaces, with insets showing transducer details and shielding (Scale bars: 5 mm (b,c); 1 mm (c, insets). (Image: UC San Diego via *ResearchGate*)

The 2 MHz ultrasound waves reduce the attenuation and phase aberration caused by the skull, and the copper-mesh shielding layer provides conformal contact to the skin while improving the signal-to-noise ratio. Focused ultrasound waves support continuous recording of blood flow spectra at selected locations.

Their design requires advanced algorithms and a significant amount of post-data acquisition computation, but that’s an acceptable tradeoff. Tests on 36 subjects showed close agreement with the medical-standard TCD instrumentation; note that the figures of merit they use for quantifying accuracy are quite different than what is used for ultrasound technology for non-medical and even non-cranial assessments. Details in in their paper in Nature, ““Transcranial volumetric imaging using a conformal ultrasound patch.”

Part 3 of this article looks at three more biometric skin patches.