Biomedical skin patches integrating multiple sensing, power, and reporting disciplines are receiving significant R&D effort.
This part continues exposing the remaining three 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 treating wounds that won’t heal and become infected, and fester, easier, more effective, and less expensive. Unlike a typical bandage, which may only consist of layers of absorbent material, smart bandages are made from a flexible and stretchy polymer that contains embedded electronics and medication. The electronics enable the sensor to monitor the presence of molecules such as uric acid or lactate, as well as conditions like pH level or temperature in the wound, which may indicate inflammation or a bacterial infection, as shown in Figure 1.
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 the University of Cambridge (UK) have developed a method for creating high-performance bioelectronics that can be customized for a wide range of biological surfaces, from a fingertip to the fluffy seed head 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, allowing the researchers to control the fibers’ spinnability. The researchers then designed an orbital spinning approach that allows the fibers to morph into living surfaces, even down to microstructures like 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.
The tests they ran with volunteers (humans, of course) included electrocardiogram and pulse assessments, as seen in Figure 3.
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, San Diego, engineers have developed a wearable ultrasound patch that offers continuous, non-invasive monitoring of blood flow 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 remaining layers consist of stretchable electrodes, as shown in Figure 4.
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 remaining layers consist of stretchable electrodes, as shown in Figure 5.
The 2 MHz ultrasound waves reduce attenuation and phase aberration caused by the skull, and the copper-mesh shielding layer provides conformal contact to the skin, 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 are 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.
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